Literature review

Nowadays, an important factor for economic and social development is energy sufficiency. Energy is the fuel of growth. Scientist's predictions show that by the year 2050, energy demand will increase significantly due to the fact of the increasing population of the earth and that more buildings are going to be constructed. (Ref: Facts and trends, energy and climate, world business).

A lot of predictions are published about how fast the population, the economy and the energy consumption of the world will increase in the years and decades to come. In reference to the matter of growth, development and energy demand, most of the predictions were wrongly made. Most predictions are reciprocally dependant on each other, and each one relies on many other factors. However, the only prediction that can be securely made is for the population and that "the growth will be larger in the less development countries than the developed countries". (UNITED NATIONS) Developed countries are managing to improve the living conditions and decrease the death rates, but at the same time the birth rates have been decreasing at about the same rate over the last century. By this way the population growth is around 0.4% per year, in the industrialized world. On the other hand, less developed countries are managing their development and as a result have increased birth rates and decreased death rates. Consequently, their average population growth has increased from about 1% per year, from fifty years ago to about 2.1% per year today. At the moment, the world's population is increasing at an annual rate of 1,7%, whereas the population in developed countries is around 1,2 billion (25% of the total) and in less developed countries is around 4 billion (75% of total world population). (United Nations)

Population increases are directly connected with the energy demand and the building sector. It is therefore essential to develop new energy technologies on a massive scale for everyone to be able to survive on this planet. Ordinary buildings are unable to contribute to these essential needs, and cover the gush of the energy demand which is going to follow over the next decade.

Energy use and climate impacts

Power plants use fossil fuels for their energy productions and therefore this way cover the energy demands of the people. As a consequence though, from the burning of the fossil fuels, green house gases are produced and emitted into the atmosphere. As mentioned in the introduction, these anthropogenic activities have a significant contribution to the green house effect and the climate changes.

Generally, in reference to the climate changes issues, scientist's opinions are split into two. On the one hand, it is believed that the changes are part of the earth's life and it is something normal which has been accelerated by our human activities and there is a possibility to stabilize the climate changes. On the other hand, it is believed that these changes are not normal and are going to make the world uninhabitable. For this reason, fast and immediate actions should be taken by all countries, targeting to reduce the energy demands and green house gases. It is almost definite that any of these actions will have a deep impact on the economy of each country.

Many people believe that energy saving, means diminishing the current quality of living and reducing economy activity. In addition, economists believe that without economic growth, investments on technology will be reduced as it will difficult to confront climate changes. On the other hand, scientists argue that technological development is the key to the solution in reference to the climate changes problem. The truth is that, any solution in reference to climate changes will need an effort from everyone and investments on technological research and development, giving us this way a chance for a better future!

IPCC's fourth assessment report further concluded that the building sector is not only the largest potential for significantly reducing greenhouse gas emissions, but also that this potential is relatively independent of the cost per ton of CO2 eqv. achieved. With proven and commercially available technologies, the energy consumption in both new and old buildings can be cut by an estimated 30-50 percent without significantly increasing investment costs. Energy savings can be achieved through a range of measures including smart design, improved insulation, low-energy appliances, high efficiency ventilation and heating/cooling systems, and conservation behaviour from the buildings users. (Reference- IPCC's fourth assessment report)

Summarising the above it is obvious that the population growth, economic development, human habits, way of living and environmental restrictions influence the energy demand around the world. Scientists and in general, the governments who are trying to give solutions to the big problem of the growing energy demands and its consequences, have to take into account all of these factors.

Reshaping the energy future

It is necessary for all countries to reshape the future of energy, as all scientific researches show. The actual word reshape, includes new innovation technologies and sources which are going to contribute to the energy needs of the world. It is necessary to find new paths which are further environmental friendly and will permit a better future.

A lower carbon world is feasible in the next decade even during the next few years, if all countries can realize that significant changes that should be done. This especially applies to the developed countries as they have to reconsider and find a link between the quality of life and their energy consumption. It is necessary for everyone to understand that a high standard of living does not demand a high consumption of energy and to adapt to the new energy sources.

The good news is that small changes in the energy scenery are now visible as many have started to be influenced. For example, the raised use of gas, the use of renewable energy on buildings, everyday life and high efficiency cars are some of the small steps that have been offered to people due to technological development. As figure three shows, the IPCC scenarios (A1B-AIM and B2-AIM) were based on the new technological achievements in the energy sector. It is definite that this evolution is not enough for our earth's climate but the two scenarios predict a possible CO2 stabilization. Finally, efforts to create an energy efficient world are starting, in reference to low carbon technologies and effective measures. (Reference-world business ...facts and trends on climate change)

As stated in the report of the World Business Council for Sustainable Development (WBCSD) 'a lower carbon world would require a marked shift in the energy/development relationship, such as similar development levels to be achieved with an average of 30% less energy use. Both energy conservation through behavioural changes and energy efficiency via technology plays a role. Such a trend is a feature of the IPCC B1 storyline, which sees a future with a globally coherent approach to sustainable development. It describes a fast-changing and convergent world toward a service and information economy, with reductions in material intensity and the introduction of clean and resource efficient technologies. The scenario leads to relatively low GHG emissions, even without explicit interventions to manage climate change.'(Reference Energy and climate change, world business)

A Sustainable World Energy Perspective

An important key to the world's energy problem is sustainable development. Sustainability includes the economic and technological development, which respect and protect the environment. Searching literature for an exact definition of sustainable development, guided us to the 'The Brundtland Report' of the UN World Commission on Environment and Development. In this report a definition of sustainable development, is given: 'Humanity has the ability to make development sustainable – to ensure that it meets the needs of the present without compromising the ability of future generations to meet their own needs'

However it is difficult to find exact definitions which represent the sustainable development accurately, due to the fact that it is an idea which involves too many parameters. (Reference Engineering_for_Sustainable_Development)

It is amazing to see how the sustainable development concept, stays on important issues of discussion even with the passing of tweedy years from the Brundtland report. In this concept, development faces three important paths: the economic, the social and the environmental (figure 4). If governments want to meet these targets it is necessary to carry out innovative technologies and a socio-economic approach.

Nevertheless, sustainable development is not the only problem and therefore it is always important to consider the three major paths. Protection of the environment, economic success and improvement of social conditions, will be the achievements of a flourishing sustainable development. These three paths are linked together for a sustainable development and their integration must be equal without any compromises.

The goal of sustainable development is, to point out the importance of the environment to the public who are now alive and for the coming generations. It is important for everyone to understand that our existence depends on the global environment and every decision of this generation is going to affect the lives of our future generations. Thus for this goal to be achievable, it is necessary to take measures for low green house emissions, use renewable sources and improve the energy consumption of our current lives. Governments and engineers are searching for the best way to come within reach of this goal as it is very difficult for developed and developing countries to achieve it.

Presently, the building sectors involvement is essential because of its deep impact on energy consumption, its significant emissions and its use of huge natural sources. The buildings that currently exist will continue to exist, for more than 30 years and therefore this influences the lives of future generations. A sustainable approach of this sector is necessary due to its rapid growth. The new approach for the buildings sector will include buildings which will need less energy to operate, produce low carbon emissions, use environmental friendly materials and produce their own energy from renewable sources. It is almost definite that the sustainable green development of the building sector will help countries accomplish the targets of the Kyoto Protocol, whilst also guarantee at the same time, the future for coming generations.

Evolution of the buildings and the opportunity for change.

As believed by many, 'buildings are our third skin' and this plays an important role for humans to survive. From the beginning of human history, human's always aimed to try and protect themselves from all weather conditions and all changes, developing due to this, different kinds of shelters. Over the years, human's adapted and managed to survive all the different changes that have happened on earth. The question now, is what will happen whilst we are facing the rapid climate changes and what will be the future consequences?

Hundreds of thousands year ago, people moved from place to place and tried to create the best conditions to live in. Depending on the place, whether hot or cold, human's developed different kind of shelters to protect themselves from the heat of the sun in the deserts, or the cold of the northern climates. Studies of these people movements over the years, shows us a big variety of shelters and developments of different ways in order to face the climate conditions.

Other factors, which determine the human's survival techniques in extreme conditions from the past, like the lower attitude of the Arctic Circle, were the design of the buildings, the quality of clothes and the behavioural adaptations, like changing posture, activity level sand choosing the most comfortable space to occupy, by moving around rooms and buildings and landscapes; and then of course the use of energy from the burning of fossil fuels or the careful use of stored energy in heat or cold stores. (Adapting building & cities for climate change)

Another extraordinary point from past decades is the energy issue. People mainly used coal, wood and water to provide themselves with enough energy, whether in a passive or active manner and covered in this way, their need for heating or cooling. By taking advantage of the natural and available energy resources, humans managed to develop houses which were ready for all extreme weather conditions. All these extreme weather conditions made humans innovate new approaches for buildings, and provide them with a more comfortable life.

An interesting approach of surviving all the climate changes is to move to different areas at the respective time of the year, which is when they are comfortable, and to leave them again when they are not – to migrate. (Adapting building & cities for climate change) This approach is an impossible one to apply, in the modern way of life and the new cities. Nevertheless what could be extracted from the past is the expertise of the ancient people and the way they faced the climate changes. In our day and age, engineers and scientists use the knowledge from the past whilst at the same time search for new innovative approaches for the buildings.

The evolution of the buildings sector involves the innovation of new technologies whilst the same time, protecting the environment and its natural sources. It is not just a matter of how to build or what to build but it is also a matter to make the buildings adaptable to the new challenges of the climate changes and energy efficient. This evolution is directly connected with the world surviving because buildings are part of the global environment which at the moment is in danger.

As written in the book titled 'Adapting Buildings and Cities for Climate Change' 'the risk of not surviving in a particular building type and region will be largely dependent on the nature of that building and on how much the climate changes. Both are crucial in the challenge of designing buildings today in which people can be comfortable in 50 years' time'.

At the point where the evolution of the building sectors began, there are great opportunities to change the current negative predictions of the climate changes. Significant reductions on energy consumption, better design, adequate technology and appropriate behaviours are some of the keys points which could accomplish the transformation of the buildings sector (figure 7). This transformation needs the participation and contribution from the businesses, the markets, the politicians and engineers. All together, they must act right away because the use of renewable sources is slowly growing and the energy demand is rapidly increasing, setting this way, tight deadlines in order to transform the sector. As it is mentioned in the Energy Efficiency, Buildings report and the IPPC 4th Assessment report, Residential and commercial buildings, 'action is essential as part of the world's response to climate change because energy use in buildings is 30-40% of final energy consumption and carbon dioxide emissions in most countries'. (Reference- Energy Efficiency in the Buildings report and the IPPC 4th Assessment report, Residential and commercial buildings)

There are many opportunities to transform the buildings sector into the new era, as well as being feasible and applicable for old and new buildings. Significant energy reductions can be achieved by using new technologies, e.g. energy efficiency appliances, low consumption cooling systems etc, use of renewable sources, better design and operation and use of environmental friendly materials. Using these methods it will be possible to reduce the energy demand of up to two-thirds. 'Low-energy buildings must become the norm rather than the novelty project'. (Reference- Energy Efficiency in the Buildings report)

Beyond the opportunities given to change the buildings sector and stabilize the climate changes, this transformation will additionally contribute to the economy growth by giving new opportunities for jobs and businesses. (Reference- Energy Efficiency in the Buildings report)As already mentioned, the transformation will only succeed in the case where, building energy becomes a high priority to the governments and businesses leaderships, whilst cooperation between engineers, businesses and authorities is also established in reference to this issue.( Reference- Energy Efficiency in the Buildings report)

Buildings types: characteristics and profiles

Around the world, a vast variety of different types of buildings can be found, and each different type covers multiple and different needs. It is therefore essential at this point, to present the different types of buildings, as this report will focus on the buildings sector and the energy demands. Despite the fact that in the literature review, it is possible to find a plethora of terminology of the building types, nevertheless the general idea of this separation, of the buildings into categories is the same. Usually the separation of the buildings is a result of its use.

It is very important to additionally mention at this point, that in most countries, many of the buildings were built before any energy regulation and these buildings will be around for at least the next 40 years. As figure 8 shows, in Europe, 50% of the buildings were built before 1975.

Residential Buildings

Residential buildings are commonly found all over the world. However, big and small differences can be found in all of them depending on the climate varieties of each country. For example, in hot climates the important need is for cooling and keeping the temperatures comfortable all over the house. This is achieved by the use of control systems, high insulation materials, shading systems and double or triple glazing. Additionally, this way, the energy demands and cost stays under control. In addition, a high use of passive or active solar systems is found in these hot climate countries. On the other hand, buildings in the cold climates have different needs to achieve temperature comfort. In these climates, the need for heating is essential but this is directly related with other parameters, such as low emissivity windows, good insulation materials and good design. It is very important in these climates, whilst designing, to consider the thermal mass of the building, as this may contribute during the night to the heating. (Low-Energy Building Design Guidelines)

Where residential buildings are concerned, it is easy to use renewable sources and cover the energy needs of a house because the demand is not so big. For example, photovoltaic systems can be used as the main source of energy, minimizing the CO2 emissions and the operation costs of the building.

Non-Residential Buildings

Non-Residential Buildings are also commonly located all over the world. In contrast with the Residential buildings, these kinds are appropriate for extreme hot or cold climates, without any access to utilities. As it is described in the Low-Energy Building Design Guidelines report of the U.S. Department of Energy these building types 'have a natural connection with the outdoors; and the structures present an opportunity to interpret the resource-conservation mission of the agency to the visiting public. These structures typically combine a need for window area, massive construction, and a tolerance for temperature swings—all of which are highly compatible with low-energy building design. Day lighting is another key strategy for deployment in these building types'. (Low-Energy Building Design Guidelines)

However, the energy balance of a Non-Residential building is almost independent, from lighting and internal gain. A great opportunity on these kinds of buildings, is to apply the low energy methods and design, due to the fact that such buildings have low energy consumption. A visitor centre is a good example, of this kind of building, and usually they have big budgets allowing the choice of high tech materials and technologies. (Low-Energy Building Design Guidelines)

Urban Office Buildings

Urban office buildings are usually located in the city centres because these buildings offer public services, to the people. As known, urbanization in most countries carries negative consequences for the city centres, for example, expensive land prices.

Due to this fact, the design and use of these buildings must be compact and offer the maximum possible. The use of the building is generally defined by the services that are offered, and the space is then separated into offices and support facilities. (Low-Energy Building Design Guidelines)

Quite frequently, another characteristic of office buildings is their old style, as well as other restrictions, due to the fact that many countries conserve the old buildings in the city centres. Thus the changes for energy conservation or better energy performance on these buildings are limited and therefore it is difficult to apply low energy strategies. In addition, the development of the surrounding area and the high tower new buildings are an important factor, which influence the energy performance of an office building due to the shade provided. (Low-Energy Building Design Guidelines)

On the other hand, new urban office buildings have a great opportunity to save energy as new technologies and design can be afforded and are significant potentials. Another point which helps low energy designs to be applied on office buildings is the wide use of curtain walls, mainly in most of the downtown buildings. The problems which can occur from the use of this kind of buildings is lack of thermal comfort, lack of orientation and the overuse of glass enhance low energy buildings design. New approaches on the office buildings, has started to be applied and they are getting transformed into high technology buildings, which offer better services to the people who work there.

A key factor of successful low energy office buildings is the placement of the private office at the back side of the building. As a result of this design, the artificial lighting will be reduced as natural lights are directed further into the buildings. This will have a significant impact not only for energy demands but also to the HVAC systems. Nevertheless, Urban Office Building's demand a careful design which takes into account the climate, the orientation, the facade design, the HVAC, shading from the surrounding buildings and the complex interactions amongst lighting. (Low-Energy Building Design Guidelines)

All the above types of buildings constitute the common categories that serve the different human needs. However, there are many subcategories which are adapted specifically for each different climate and different needs.

Energy impacts of the buildings

The energy impacts of a building, is a very important factor to consider, in order to succeed with the design of low energy buildings. The different types of buildings and the differences between their energy demands, is the key for the development of zero energy buildings. As mentioned before, each type of building is designed for a specific use and to cover different needs.

Starting with the residential buildings, studies show us, greater energy consumption than the commercial buildings. The report includes six different regions which are Brazil, China, Europe, India, Japan and the United States. During this report the residential sector is divided into two categories, consisting of the single family and the multi-family buildings, this way being able to focus on the energy performance for each case. (Reference- energy efficiency in buildings –market)

Consumption Survey; Federcasa, Italian Housing Federation (2006), Housing Statistics of the European Union 2005/2006; Statistics Bureau, Ministry of Internal Affairs and Communications (2003), 2003 Housing and Land Survey (Japan); EEB core group research) (Reference- energy efficiency in buildings –market)

As the above figure shows (figure 9), single family buildings are more common in Brazil, India and the United States, in contrast with China, Europe and Japan where the single family buildings are at the same level as multifamily buildings. It is possible that in a few years, this global scenery will change and more multifamily buildings will be required, due to an increasing population of the earth and the growing urbanization in big countries. On the other hand, the development of the countries and economies will allow more people to get richer and own a single family house. (Reference- energy efficiency in buildings –market)

Generally, the residential buildings tend to increase the energy demands all over the world. Unfortunately, the modern way of life "includes" extra comforts which are offered by the high technological appliances and the bigger buildings. As the quality of life increases, the energy consumption grows and more natural sources are needed to cover these human needs. Nevertheless, the energy demand is changing from country to country, as the climate and economy growth, are affecting peoples habits. (Figure 10)

The above graph shows us that in six different regions, the economic growth and the climates have different impacts on the energy consumption of each country. For example, space heating is essential in Europe and China, in contrast with Japan and India where the use is low. Additionally in Japan, water heating is very important, whilst in other countries not so much. Another important point to notice on this graph, is cooking in India, as many areas do not have access to electricity therefore their main energy use, is cooking. (Reference- energy efficiency in buildings –market)

Amongst the residential buildings, one big subcategory is the single family buildings. (Figure 11) All around the world, single family buildings have the greatest impact on energy consumption and CO2 emissions. In the developed countries, people tend to consume more energy at their homes, as they demand more comfort and have bigger spaces, better heating and cooling systems, artificial lighting and use more appliances. For example, whereas in Japan people tend to heat only one of the rooms instead of the whole house, but in Europe they tend to install central heating systems and heat the whole building. All these factors reflect the changes of people's behaviour, as they become wealthier from the economic growth. It is a fact, that as more people will become wealthier the demand for single family homes will also increase, and the demand will then be greater than today, therefore increasing the energy consumptions. (Reference- energy efficiency in buildings –market)

The issue of reducing consumption in single family buildings is not so simple. In general, all countries encounter serious barriers when it comes to taking effective measures for lower energy consumption. In Europe, many of the buildings that already exist, have an enormous challenge to retrofit these old buildings and apply low energy building principles. Definitely, these changes will cost money and everyone is interested in getting financial backing from the governments. Another issue at hand is to raise awareness, about all the changes that everyone needs to know about, especially with regards to the green technology and the proposed energy solutions which will cover their needs, and be easy to install. Unfortunately until now, the lack of information and financial measures has not helped the development of green technologies and designs for single family houses.

The World Business Council for Sustainable Development mention that 'there are two key barriers to transforming what is currently a refurbishment market into an energy-efficient market: the first one is that people do not know where to find the relevant information on options, prices and suppliers; there are no "one-stop shops" for retrofitting and the second one is that homeowners base decisions largely on the first cost rather than overall financial returns'. (Reference- energy efficiency in buildings –market)

In developing countries, the biggest problem is the lack of regulations and mechanisms which would then force the people and the market to change. For example, in China the building codes are not effectively applied and in Brazil, 75% of the single homes are "illegally" built. In addition, developing countries as mentioned before have different needs to the developed countries, so the need to provide houses is more essential that the need to reduce energy consumption. (Reference- energy efficiency in buildings –market)

In Japan and the US, the growing population is followed by high rates of constructions. This rapid development of the market causes huge problems to also then apply the green principles on a big scale. Another major problem in these countries is the big differences between the submarket which block, in some ways, the measures of low energy design. The key to the solution in these countries is strengthening their regulations, by giving more information to the public and changing their behaviour. (Reference- energy efficiency in buildings –market)

In the cases of the multifamily buildings, which belong in Residential buildings sector, another approach is necessary for energy efficiency. These types of buildings are commonly located in cities where the urbanization problems are huge. In Europe, the US and Japan these buildings vary from very small to luxury apartments, so the energy demand is also varied. As referred to before, many of the buildings in the centre of the towns were built many years ago, so to achieve energy efficiency and apply the low energy principle is a great issue. In developing countries, incomes influence the preference for bigger houses and more energy consumption, therefore making a multifamily building a key factor for lower energy demand. (Reference- energy efficiency in buildings –market)

Still, comparing single family homes with apartments, obviously the energy needs in an apartment are less due to their small size and space and lower exterior wall area. Looking at the example of the US (figure 12), apartments use almost half the heating energy and lighting energy than a single family house. In general, the energy profile of a single family house is much higher than that of the multifamily building. It is almost definite, that due to the increasing population the living standards in developing countries are growing fast which influences the energy demand. (Reference- energy efficiency in buildings –market)

The office sector in most countries has a significant impact on the energy consumption. These kinds of buildings belong to the commercial buildings sector and they are one of the biggest categories, as they use large amounts of space and energy! The actual buildings, depending on their use, can be found having a great variety, which are from small single buildings to skyscrapers. Usually though, due to the rapid world development which demands more public services, the office buildings are newer rather than older buildings. In China, where technological developments and services increase rapidly, the office sectors are growing rapidly. Additionally, the technological developments influence and change one's working life as with new high technology, it is easier for some people to work from their homes. The results of these new trends, is the reduction of the floor space needed per person, having fewer large offices and more flexible space. All these factors influence the energy consumption of an office building.

Some other factors that affect the energy demand in office buildings are the same ones as the ones for residential buildings, such as the climate, the type and the size of the building and its use. Usually the biggest percentages of energy consumption come from heating, cooling and lighting needs. The biggest challenge for this sector is the growing demand of energy, due to the use of more technological and office equipment. As mentioned in the World Business Council for Sustainable Development report the "total greenhouse gas emissions from IT equipment (including data centers) are growing at about 6% a year" (reference- The Climate Group (2008), "Smart 2020: Enabling the Low Carbon Economy in the Information Age", a report on behalf of the Global e-Sustainability Initiative, with analysis by McKinsey & Company.) In addition, the use of extra office equipment affects the temperature comforts in that space, as more heat is enfranchised, demanding then the use of cooling and ventilation systems.

In general the energy consumption of the office sector in six regions of EEB report varies, but generally heating has the largest percentage. For example, in the US, heating requires 25% of all the office energy and in Japan its 29 % (figure 13). It is important therefore that the efforts made for a significant energy reduction, to continue, from all manufactures who try and develop energy efficient products for office use.

The office sector is the most difficult one to apply the low energy principles on, but it has great potentials. Those who use an office don't care about saving energy or using energy efficient systems plus developers and investors prevent the development of green design technologies and methods. Everyone just cares about their direct profit and the costs related to the green investments rather than the life cycle costs and the future benefits from green technologies. Due to this outlook, transforming the office sectors and changing the behaviours is extremely difficult.

Another vital barrier for energy efficiency in the office sector is the complexity of the sector as it involves many different types of players, such as the developers, the construction companies, the material and equipment suppliers, the agents and the different owners. (Figure 14) Also the lack of experts and engineers who can afford, support and monitor the low energy design and principles is another serious barrier for energy efficiency in the office buildings. Finally, you can find the actual physical restrictions applicable for a low energy office due to its design and the construction of the building. For example, the installation of photovoltaic panels is difficult because of the small roof space compared with the building size and the energy demand. (Reference- energy efficiency in buildings –market)

The concept of Zero Energy Building (ZEB)

The concept of Zero Energy Buildings might not be something new. Over the past years, different engineers and scientists have attempted to give new approaches for the contraction and designs of the buildings, proposing new ideas and technologies. Unfortunately up to today, these new concepts have just been thoughts which have been supported from only a few.

However phrases like 'a zero energy house', 'a neutral energy autonomous house' or 'an energy-independent house' have appeared in articles as from the late seventies, due to the fact that the oil crisis consequences started to become visible. It was then, where the bug issue started in reference to the natural sources and fossil fuels which caused researches to look for alternative energy sources and started energy use discussions. Despite the fact that the energy problems were serious, the first approaches of the buildings sector were only to talk about, energy efficient technologies and passive solutions. Additionally, the word 'zero' included only the energy for heating, cooling and domestic water. (Zero Energy Building (ZEB) definitions – A literature review)

Throughout the years, ZEB's definitions varied due to the different approaches from all the engineers and scientists. Many articles and papers were published around this issue of ZEB, but lack of a common understanding and definition caused wide discussions. During of all these attempts, many definitions were given including each time different factors and approaches such as 'how the zero energy goal is achieved', 'what is the building grid interaction', 'unequal energy qualities in the energy balance', 'what are the project boundaries for the balance?' (Zero Energy Building (ZEB) definitions – A literature review)

In general, all the approaches from all over the world target to create new generation buildings which are going to be energy efficient with a low operation energy profile and energy that needs to be supplied from renewable sources. The issue of Zero Energy Buildings is still under research, as scientists are trying to combine into words what energy efficiency, renewable sources technology, low CO2 emissions and the environmental impacts are. As it is said in the 'Zero Energy Buildings: A critical look at the definition' report ' the heart of the ZEB concept is the idea that buildings can meet all their energy requirements from low-cost, locally available, non-polluting, renewable sources.' (Zero Energy Buildings: A critical look at the definition' report)

The common definition of Zero Energy Buildings is essential and very important for all the governments so that it will be possible to develop common strategies for the buildings sector and for all to follow the same principles. The transformation of the building sector, energy savings and the stabilization of the climate will be possible only when a global action and cooperation is achieved.

Zero Energy Buildings Definitions

The Zero Energy Building definition presents a wide diversity through the publications and reports that came out over the last few years. Each time a different definition is given relating to the project goals and values of the design team. For example, the owners care about the energy costs, the organizations are interested in the primary and sources energy, the architectures care about a sites energy use for energy code requirements and the environmentalists care about the CO2 emissions and the environmental impacts of the buildings.

However, an important issue is to focus on the actual word 'zero' and how this is defined within the ZEB definition. This word may include CO2 emissions, primary energy, available energy (exergy) or energy cost. The Torcellini report which was published in 2006 make use of the U.S. Department of Energy (DOE) definition which says that "A net zero energy building (ZEB) is a residential or commercial building with greatly reduced energy needs through efficiency gains such that the balance of energy needs can be supplied with renewable technologies." In the same report the authors refer to "zero" by saying "Despite the excitement over the phrase "zero energy," we lack a common definition, or even a common understanding, of what it means". (Reference-Torcellini, P., Pless, S. & Deru, M. (2006). Zero Energy Buildings: A Critical Look at the Definition. National Renewable Energy Laboratory (NREL), USA Web address: http://www.nrel.gov/docs/fy06osti/39833.pdf) In addition the Torcellini report attempted to cover all the different approaches by giving the most frequently used definitions:

  • Nett Zero Site Energy: A site where ZEB produces at least as much energy as it uses in a year, when accounted for at the site.
  • Nett Zero Source Energy: A source where ZEB produces at least as much energy as it uses in a year, when accounted for at the source. Source energy refers to the primary energy used to generate and deliver the energy to the site. To calculate a building's total source energy, the imported and exported energy is multiplied by the appropriate site-to-source conversion multipliers.
  • Nett Zero Energy Costs: The cost of ZEB, is the amount of money the utility pays the building owner for the energy that the building exports to the grid which is at least equal to the amount the owner pays the utility for the energy services and energy used over the year.
  • Nett Zero Energy Emissions: A net-zero emissions building produces at least as much emissions-free renewable energy as it uses from emissions-producing energy sources.

It was expected that the Torcellini report would create new discussions around the issue of the ZEB's definition. Another author, Kilkis, in 2007 referred to the Torcellini report but with a new approach at that time. The new definition for ZEB given by Kilkis said that "Nett-Zero Exergy Building is a building, which has a total annual sum of zero energy transfer across the building-district boundary in a district energy system, during all electric and any other transfer that is taking place in a certain period of time". (Reference- Kilkis, S. (2007). A new metric for net- zero carbon buildings. Proceedings of ES2007. Energy Sustainability 2007, Long Beach, California, pp. 219-224).In another report, also in 2007, the authors in their definition paid attention to the balance of energy use and energy production of the building. As they said the "energy use should be equal to the energy production" (Refernce- Laustsen, J. (2008). Energy Efficiency Requirements in Building Codes, Energy Efficiency Policies for New Buildings. International Energy Agency (IEA).Web address: http://www.iea.org/g8/2008/Building_Codes.pdf Mertz, G.A., Raffio, G.S. & Kissock, K. (2007). Cost optimization of net-zero energy house. Proceedings of ES2007. Energy Sustainability 2007, Long Beach, California, pp. 477-488 )

Nevertheless, another interesting approach is facing the problem on the site of the buildings, which is energy demand and balance. Over the past few years, the biggest energy consumption in the buildings has been from the space heating and hot water. Due to this fact, many publications were affected and tried to set the problems around the heating demands. Esbensen's definition, in 1977, is one example of this approach which reported that "With energy conservation arrangements, such as high insulated constructions, heat-recovery equipments and a solar heating system, the Zero Energy House is dimensioned to be self-sufficient for space heating and hot-water supply during normal climatic conditions in Denmark. Energy supply for the electric installations in the house is taken from the municipal mains." (Reference- Esbensen, T.V. & Korsgaard, V. (1977). Dimensioning of the solar heating system in the zero energy house in Denmark. Solar Energy Vol. 19, Issue 2, 1977, pp. 195-199)

In many cases, the ZEB definition is focused on the electricity consumption of the building and its relation with the total energy. The definitions which were given by Gilijamse and Iqbal, respectively, represent this approach and refer to it as "A zero energy house is defined here as a house in which no fossil fuels are consumed, and the annual electricity consumption equals annual electricity production. Unlike the autarkic situation, the electricity grid acts as a virtual buffer with annually balanced delivers and returns" and "Zero energy home is the term used for a home that optimally combines commercially available renewable energy technology with the state of the art energy efficiency construction techniques. In a zero energy home no fossil fuels are consumed and its annual electricity consumption equals annual electricity production. A zero energy home may or may not be grid connected" (reference-Gilijamse, W. (1995). Zero-energy houses in the Netherlands. Proceedings of Building Simulation '95. Madison, Wisconsin, USA, August 14–16; 1995, pp. 276–283. Web address: http://www.ibpsa.org/proceedings/BS1995/BS95_276_283.pdf Iqbal, M.T. (2003). A feasibility study of a zero energy home in Newfoundland. Renewable Energy Vol. 29, Issue 2 February 2004, pp. 277-289)

Another definition, given by Lausten, attempts to include the total energy demand, heating and electricity demand. The result was the new definition of ZEB's which said that "Zero Nett Energy Buildings are buildings that over a year are neutral, meaning that they deliver as much energy to the supply grids as they use from the grids. Seen in these terms they do not need any fossil fuel for heating, cooling, lighting or other energy uses although they sometimes draw energy from the grid." (reference-Laustsen, J. (2008). Energy Efficiency Requirements in Building Codes, Energy Efficiency Policies for New Buildings. International Energy Agency (IEA). Web address: http://www.iea.org/g8/2008/Building_Codes.pdf)

However, it is important in literature that the definitions consider the embodied energy of the construction and the materials. This approach has been developed lately, and there is a great living example close to London city, called the Bed ZED project. As mentioned from Morbitzer " the BedZED is built from natural, recycled or reclaimed materials. All the wood used has been approved to be sourced from sustainable resources, and construction materials were selected for their low embodied energy and were sourced within 35-mile radius of the site if possible." (reference- Morbitzer, C. (2008). Low energy and sustainable housing in the UK and Germany. Open House International. Vol. 33, Issue 3, 2008, pp. 17-25)

Amongst wide discussions in reference to the definition of ZEB another issue raised was regarding electricity grids. This issue has to do with the off grid and on grid ZEB. The on grid Zero energy building is the one that is connected with the grid but also produces its own energy. These kinds of buildings can purchase energy or feed some back to the grid if they produce more energy than demanded. On the other hand, off grid Zero energy buildings are not connected with the grid and produce their own. These kinds of buildings, most of the time, cover their energy needs from renewable energy sources. An example of the off grid ZEB is given in the Laustsen's definition: "Zero Stand Alone Buildings are buildings that do not require connection to the grid or as they have the capacity to store energy for night-time or wintertime use." At the same time, Lautsen gave another definition for the on grid ZEB: "Zero Nett Energy Buildings are buildings that over a year are neutral, meaning that they deliver as much energy to the supply grids as they use from the grids. Seen in these terms they do not need any fossil fuel for heating, cooling, lighting or other energy uses although they sometimes draw energy from the grid" (reference- Laustsen, J. (2008). Energy Efficiency Requirements in Building Codes, Energy Efficiency Policies for New Buildings. International Energy Agency (IEA). Web address: http://www.iea.org/g8/2008/Building_Codes.pdf)

Last but not least, is the fact that some ZEB definitions focus on the renewable energy that is used to cover the needs of the buildings. It is important to remember that the main concept of ZEB is the replacement of the fossil fuels with renewable sources which include solar energy, wind energy, wave energy, biomass energy and geothermal energy. Until now, the most known technologies are the solar thermal and photovoltaic technologies. An example of this kind of definition is given by Charron: "Homes that utilise solar thermal and solar photovoltaic (PV) technologies to generate as much energy as their yearly load are referred to as nett-Zero Energy Solar Homes (ZESH)." (Reference Charron, R. (2008). A review of design processes for low energy solar homes. Open House International Vol. 33, Issue 3, 2008, pp. 7-16)

To conclude, from the above definitions it is obvious that the Zero Energy Buildings and in general the Building sector involves many different aspects and factors that influence the energy performance of the buildings. For that reason it is very difficult to adopt a general definition for Zero energy buildings, which will be able to include all the different aspects. The best approach for a Zero Energy Building is to consider during the designing phase all the parameters which are going to affect the energy performance of the building.

The definition impacts in ZEB design

The definition of a Zero Energy Building is essential during the design procedure. Each definition includes different aspects, as mentioned before, and the results can vary significantly. Later on you can find a description of all the differences, the advantages and the disadvantages of a Nett Zero Site Building, Net Zero Source Energy Building and Nett Zero Energy Emissions Building.

Nett Zero Site Building

The Nett Zero Site Building is the building that produces energy on a site and the production is equal to the consumption. Usually the energy production comes from the renewable energy sources like photovoltaic systems (roof or parking mounted PV), solar water collectors, wind power, low impact hydro and geothermal energy. (Zero Energy Buildings: A critical look at the definition' report)

A site ZEB produces as much energy as it uses, when accounted for at the site. Generation examples include roof-mounted PV or solar hot water collectors (Table 1, Option 1). Other site-specific on-site generation options such as small-scale wind power, parking lot-mounted PV systems, and low-impact hydro (Table 1, Option 2), may be available. As discussed earlier, having the on-site generation within the building footprint is preferable.

A limitation of a site ZEB definition is that the values of various fuels at the source are not considered. For example, one energy unit of electricity used at the site is equivalent to one energy unit of natural gas at the site, but electricity is more than three times as valuable at the source. For all-electric buildings, a site ZEB is equivalent to a source ZEB. For buildings with significant gas use, a site ZEB will need to generate much more on-site electricity than a source ZEB. As an example, the TTF would require a 62-kWDC PV system to be a site ZEB, but only a 45-kWDC PV system for a source ZEB (Table 2); this is because gas heating is a major end use. The net site definition encourages aggressive energy efficiency designs because on-site generated electricity has to offset gas use on a 1 to 1 basis.

A site ZEB can be easily verified through on-site measurements, whereas source energy or emissions ZEBs cannot be measured directly because site-to-source factors need to be determined. An easily measurable definition is important to accurately determine the progress toward meeting a ZEB goal.

A site ZEB has the fewest external fluctuations that influence the ZEB goal, and therefore provides the most repeatable and consistent definition. This is not the case for the cost ZEB definition because fluctuations in energy costs and rate structures over the life of a building affect the success in reaching net zero energy costs. For example, at BigHorn, natural gas prices varied 40% during the three-year monitoring period and electricity prices varied widely, mainly because of a partial shift from coal to natural gas for utility electricity production. Similarly, source energy conversion rates may change over the life of a building, depending on the type of power plant or power source mix the utility uses to provide electricity. However, for all the ZEB definitions, the impact of energy performance can affect the success in meeting a ZEB goal.

A building could be a site ZEB but not realize comparable energy cost savings. If peak demands and utility bills are not managed, the energy costs may or may not be similarly reduced. This was the case at Oberlin, which realized a 79% energy saving, but did not reduce peak demand charges. Uncontrolled demand charges resulted in a disproportionate energy cost saving of only 35%.

An additional design implication of a site ZEB is that this definition favors electric equipment that is more efficient at the site than its gas counterpart. For example, in a net site ZEB, electric heat pumps would be favored over natural gas furnaces for heating because they have a coefficient of performance from 2 to 4; natural gas furnaces are about 90% efficient. This was the case at Oberlin, which had a net site ZEB goal that influenced the design decision for an all-electric ground source heat pump system.

Nett Zero Source Energy Building

A source ZEB produces as much energy as it uses as measured at the source. To calculate a building's total source energy, both imported and exported energy are multiplied by the appropriate site-to-source energy factors. To make this calculation, power generation and transmission factors are needed. Source Energy and Emission Factors for Energy Use in Buildings (Deru and Torcellini 2006) used a life cycle assessment approach and determined national electricity and natural gas site-to-source energy factors of 3.37 and 1.12. Site gas energy use will have to be offset with on-site electricity generation on a 3.37 to 1 ratio (one unit of exported electricity for 3.37 units of site gas use) for a source ZEB. This definition could encourage the use of gas in as many end uses as possible (boilers, domestic hot water, dryers, desiccant dehumidifiers) to take advantage of this fuel switching and source accounting to reach this ZEB goal. For example, the higher the percent of total energy used at a site that is gas, the smaller the PV system required to be a source ZEB. At BigHorn, for a source ZEB, 18,500 ft2 of PV are required; however, 31,750 ft2 of PV are required for a site ZEB (Table 2).

This definition also depends on the method used to calculate site-to-source electricity energy factors. National averages do not account for regional electricity generation differences. For example, in the Northwest, where hydropower is used to generate significant electricity, the site-to-source multiplier is lower than the national number. In addition, national site-to-source energy factors do not account for hourly variations in the heat rate of power plants or how utilities dispatch generation facilities for peak loading. Electricity use at night could have fewer source impacts than electricity used during the peak utility time of day. Further work is needed to determine how utilities dispatch various forms of generation and the corresponding daily variations of heat rates and source rates. Using regional time-dependent valuations (TDVs) for determining time-of-use source energy is one way to account for variations in how and when energy is used. TDVs have been developed by the California Energy Commission to determine the hourly value of delivered energy for 16 zones in California (CEC 2005). Similar national TDVs would be valuable to accurately calculate source energy use to determine a building's success in reaching a source ZEB goal. A first step in understanding regional site-to-source multiplier differences is available (Deru and Torcellini 2006); multipliers are provided for the three primary grid interconnects and for each state.

There may be issues with the source ZEB definition when electricity is generated on site with gas from fossil fuels. The ZEB definitions state that the building must use renewable energy sources to achieve the ZEB goal; therefore, electricity generated on site from fossil fuels cannot be exported and count toward a ZEB goal. However, this is unlikely, because buildings are unlikely to need more heat than electricity and the inefficiencies of on-site electricity generation and exportation make this economically very unattractive. The best cost or energy pathways will determine the optimal combination of energy efficiency, on-site cogeneration, and on-site renewable energy generation.

The issue of unmanaged energy costs in a site ZEB is similar for a source ZEB. A building could be a source ZEB and not realize comparable energy cost savings. If peak demands and utility bills are not managed, the energy costs may or may not be similarly reduced.

Nett Zero Energy Emissions Building

An emissions-based ZEB produces at least as much emissions-free renewable energy as it uses from emissions-producing energy sources. An on-site emission ZEB offsets its emissions by using supply-side options 1 and 2 in Table 1. If an all-electric building obtains all its electricity from an off-site zero emissions source (such as hydro, nuclear, or large scale wind farms), it is already zero emissions and does not have to generate any on-site renewable energy to offset emissions. However, if the same building uses natural gas for heating, then it will need to generate and export enough emissions-free renewable energy to offset the emissions from the natural gas use. Purchasing emissions offsets from other sources would be considered an off-site zero emissions building.

Success in achieving an emissions ZEB depends on the generation source of the electricity used. Emissions vary greatly depending on the source of electricity, ranging from nuclear, coal, hydro, and other utility generation sources. One could argue that any building that is constructed in an area with a large hydro or nuclear contribution to the regional electricity generation mix would have fewer emissions than a similar building in a region with a predominantly coal-fired generation mix. Therefore, an emissions ZEB would need a smaller PV system in areas with a large hydro or nuclear contribution compared to a similar building supplied by a utility with a large coal-fired generation contribution.

The net zero emissions ZEB definition has similar calculation difficulties previously discussed with the source ZEB definition. Many of these difficulties are related to the uncertainty in determining the generation source of electricity. Like the source definition, one would need to understand the utility dispatch strategy and generation source ratio to determine emissions from each of these sources.

Factors influencing ZEB design

The complexities of Zero Energy Buildings demand during the design phase, careful studying and understanding of the different factors. It is very important for an engineer to understand and predict the impact of each factor to the energy performance of the building.

As the following picture shows (figure 15) there are too many factors that affect the energy consumption of a ZEB, such as the building envelope, the orientation, the insulation, the shading, the lights, the air-conditioning systems, the heating systems, peoples behaviour, the technical installation and the daylight control.

However some factors are more important than others and influence the operation and consumption energy much more.

The location and the climate play an important role in the energy consumption, especially when the external environment is too severe for comfort. The more extreme, the external climate is, the more energy is used, to create a comfortable internal environment. The building placement can be used in combination with landscaping to produce mild microclimate. For instance, in cold climates the heating demands are more therefore this must be combined, with passive solar heating strategies, as they are essential. On other hand, hot climates means cooling demands are more important, and the high degree of sin control is significant for the building. Also in these climates, a good design can offer better lighting via the building spaces. Another point that is affected from the location and the climate is the natural ventilation of the building which is directly related with the current climate conditions of the place. (Reference- low energy building design guidelines AND Richard Nicholls (2002), Low Energy Design)

The building size of a Zero Energy Building is related with the operation energy consumption. Usually, apart from the outdoor climate, the indoor climate has a significant contribution on all the aspects of the building design too. In some cases, the outdoor climate supplements the indoor climate, for instance, the very cold climate will have a building which has a lot of internal heat gains sited. Otherwise, when the two climates are antagonistic, hot climate will also have a building which also has a lot of internal heat gains. The building size for these cases determine is some ways, the indoor climate, and the implications of the current outdoor climate. The size of the surface area and volume are prescribed by the form of the building. These two parameters are directly proportional to the fabric and ventilation heat loss rates, respectively. For example in hot climates "buildings with large footprints and a large amount of floor space far from the exterior of the building will require heat removal in the interior zones (generally by mechanical cooling) all or much of year." In addition, form plays an important role to the capability of the building to collect and use natural energy such as solar heat, light and natural ventilation. (Reference- low energy building design guidelines AND Richard Nicholls (2002), Low Energy Design)

The buildings fabric relates to the fabric heat loss rates which are affected by the buildings envelope. To slow down the heat losses, low density materials and insulators can be used. In contrast dense, thermally massive materials can be applied in the interior to assist cooling the building in the summer time. (Reference- low energy building design guidelines AND Richard Nicholls (2002), Low Energy Design)

The buildings ventilation is a result of the mechanical ventilation, that consumes energy and in case of an uncontrolled infiltration, it is possible to have cold air admissions or warm air losses from the building. It is thus very important to minimize the infiltration, whilst at the same time, mechanical systems should operate efficiently and only in the case of necessity.

The heating and cooling systems demand a thorough analysis so as to operate correctly. Mechanical heating systems consume a huge amount of energy to produce heat and also fossil fuels, so it is important to operate them, only when required. Good insulation of the buildings makes it possible to minimize the uses of the systems. Mechanical cooling systems also demand a huge amount of electricity but with passive methods, such as shading and exposing mass, in combination with night time ventilation, the replacement of the system could be achieved. . (Reference- low energy building design guidelines AND Richard Nicholls (2002), Low Energy Design)

Another important factor of the Zero Energy design is the internal heat gains. Most of the electric appliances such as lights, TV, computers, refrigerator etc., in addition with the building occupants, irradiate heat affecting the indoor climate. At the early stages of a design, it is very important to calculate the possible heat gains and take the necessary measures for this problem. Sometimes, heat gains in combination with hot climates can affect the HVAC system design and the energy consumption. For example, cafeterias, restaurants and laundry buildings are strongly influenced by the heat gain factor as the use of big appliances irradiate significant amounts of heat. The identification of these kinds of factors should be at an early stage of the design, giving therefore, the opportunity of appropriate design strategies. (Reference- low energy building design guidelines AND Richard Nicholls (2002), Low Energy Design)

The lighting requirements play an important to the energy performance of the building. Today's designers, tend to use artificial lightning as a decoration in modern buildings, which is in contrast with the Low or Zero Energy principles. Lighting must only be used where it is necessary and cover serious needs. Also these needs vary from building to building, depending on what is the buildings use. At the stage when one is designing the quantitatively and qualitatively lighting needs must be identified and designed with the ZEB principles. The design team is responsible to choose the suitable electrical lighting systems with integrated occupancy sensors. In addition, the concept of ZEB is to use a good combination of daylight via a good design of the glazed openings which is essential. Natural daylight entering the building means that energy consumption is reduced, minimizing this way the energy demands. (Reference- low energy building design guidelines AND Richard Nicholls (2002), Low Energy Design)

Additionally, the design procedure is also affected by the hours the building actually operates. Engineers have to take into account the use of the building and the hours of operation. These two factors influence the general energy performance of the building and this varies from building to building. For example, hospitals, airports and stations are in constant use and the energy demands they may have, has a significant difference with an office building which has standard hours of operation. The operation hours of a building can influence the needs necessary for it to be controlled well and have a high efficiency lighting system; can enhance the cost effectiveness of low-energy design strategies, such as a day's lighting in a border station or weather station and lastly can influence the heating and cooling demands. (Reference- low energy building design guidelines AND Richard Nicholls (2002), Low Energy Design)

Lastly but in no means less important, is the cost of energy, mainly electrical energy, which determines the conserve energy strategies and the cost effectiveness of them. In many countries the cost of electricity is three or four times higher than natural gas per Btu. In this case, the designer must be very careful because for example, it is easy to increase the glazing area and give more daylight to the building reducing by this way the electricity demand but on the other hand the heat losses will be bigger and heat demand will increase. It is obvious that during the energy cost analysis the designer will take into account a lot of aspects of the building and try to bring them all to a balanced situation. Moreover the energy that comes from fossil fuels, and pricing have an uncertain future. As a result, the investigation of the different energy cost scenarios and proposals for new energy sources are necessary during the design stage. During this approach the installation of green power sources seem to be achievable and offer great opportunities to decrease the environmental impacts of the buildings. "Minimizing electrical load requirements, and then meeting these requirements with clean electricity resources, is at the core of a whole-building design strategy."

Integration and development of available technologies to use in Zero Energy Buildings

As mentioned before, the building sectors can be characterized as complex system where most of the parts involved are related to its energy performance. Thus it is important to take into account all these components as an integrated package. The goal that has to be set by the engineers, the researchers and the designers is for the best optimization, and combination of the different components resulting, in lower energy consumption.

Most think that the integration and development of technology towards a Zero Energy Building is just the improvement of the HAVC and the electric appliances. The new approaches involve and expect savings not only from the HVAC and appliances, but also from the buildings envelope, air permeability, appropriate design, insulation, etc.

Building envelope

The building envelope can be described as the separation between the internal and external space of a building. Through the building envelope, the development and integration of an indoor climate is achievable. The building envelope works as a protection, or rather as a shell for someone to be able to face through the years the climate conditions in their respective countries. During the design of the building envelope, the target is to get structural integrity, moisture control, temperature control and control of air pressure boundaries of sorts. The building envelope involves different components such as the building foundation, roof, walls, doors and windows which play an important role to the energy performance of the building.

Windows of the building

Windows are made up of two different components, the glazing and the frame and they are responsible for a significant amount of heat and cooling losses. For example, in the United States the losses can reach up to 30% of the energy uses for heat or cooling in an average house. (reference- Fisette, P. (2005), "Understanding Energy Efficient Windows", Fine Homebuilding Magazine, www.taunton.com/finehomebuilding/pages/h00029.asp.)

The windows are part of the buildings envelope and offer it different functions such as access to the building, view, entrance of daylight and sometimes windows are also involved in the ventilation and fresh air inlet. Generally, a windows main use is for natural light to enter the space and during the ZEB design, windows must be used in such a way so as to give as much light as possible. Also as per the ZEB concept, passive principles windows can be used as a heat gain source (passive heating) during the winter time, and as a heat loss source in the summer time when they are open. (Efficient Windows Collaborative, Washington, DC, www.efficientwindows.org/)

Nonetheless, apart from the main role that windows have, which is to offer natural lighting, another important point which is still under research is the optimization of heat flow depending on the season. For example, during the winter whilst the heating systems in the building are in use and the outdoor temperatures are low, windows should conserve heat in the building space, have the lowest losses possible and allow solar radiation to get in. In the summer time, exactly the opposite is needed. For example, during the summer time the cooling demand in the building is essential and the cooling losses have to be at the minimum. The windows play an important role, as they should keep out the sun's heat and wherever possible to also discard the heat of the building. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

It is obvious that windows are a complex factor in reference to the building envelope and have multiple uses. This makes their construction difficult, as it is necessary to take into account more than one factor as it will influence the design, the materials used for the frames and the glass instate in multilayer windows. The new types of windows are focused on heat and cooling losses (energy losses) which directly relates to the number of layers, types of glasses and the fillings between the layers. In addition, different coatings on the glasses are used to provide protection from the heat loss and solar gain. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Apart from the optimization of a sheet of glass, the cavity width, the low emissivity coatings, the cavity gas and the frame materials, the biggest challenge is to optimise the window characteristics which are affected from previous construction factors. These characteristics are the main factors which give a window the ability to keep heat in during the winter time, to keep heat out during the summer time and to let light in all the year round. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

In cold climates, the windows should be able to keep heat in the building which means that they are then efficient with a high resistance to heat flow (low R value). An example of a high tech design is a window which has three glazing layers filled with argon or krypton. These kinds of windows offer a building, low solar gain and low emission glass resulting in low heat losses during the winter and low heat gains in the summer. The type of glazing material, the number of layers of glass and coating, the thermal resistance or conductance of the frame and the "tightness" of the installation are some of the factors that influence the window's resistance to heat flow (figure 17). To explain further, as many layers of glass that are included in the windows, the better and higher will the R value of the window be. (reference- Fisette, P. (2005), "Understanding Energy EfficientWindows",FineHomebuildingMagazine,www.taunton.com/finehomebuilding/pages/h00029.asp.)Up to now, energy efficiency of a window can improve by increasing the glass layers or using low remittance coatings. However during the last few years, multi layered windows are filled between the gaps with argon or krypton gas offering this way even better performance to a building. The losses from double or triple glazing windows are reduced significantly and therefore contribute further to the energy reduction of the building. "Low-emittance glass coatings reflect up to 90% of long-wave heat energy while allowing short wave, visible light to enter) (reference- Fisette, P. (2005), "Understanding Energy Efficient Windows", Fine Homebuilding Magazine, www.taunton.com/finehomebuilding/pages/h00029.asp. And Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

In contrast to cold climates, windows in hot climates need to prevent solar heat and minimize the cooling losses. In these cases, multi layered windows are useful but in addition the use of coatings is also essential. This way a window offers protection to the indoor space from high energy waves in the sunlight which penetrate and heat the building. However for even better results, in countries where there is sunshine for long periods, the use of shutters or shadowing technologies is also essential. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

The technology of windows is still under research and development, as they have a significant impact to the energy performance of a building. Engineers and companies are trying to develop new windows which are going to correspond to the different weather conditions and different buildings. "One promising area is that of coatings with optical properties that can be changed reversibly across a wide range, e.g. electro-chromatic or gas-chromatic glazing." (Reference - WBGU (2004), World in Transition: Towards Sustainable Energy Systems, German Advisory Council on Global Change, Earthscan.)

As shown in the above figure (figure 18) significant energy reductions can be made through the changed technological design of a window. The savings can reach between 27 to 39% depending from the weather and glazed layers of the window. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

The development of energy efficient windows has great potentials and it is very important to optimize the design and materials of frames for better results in Zero Energy Buildings.

Insulation of the building

Insulation is integrated into part of the buildings envelope and has an important role to the energy consumption of it. There are different types of insulations which depend on the current climate conditions of a building placement. In hot climates, the insulation reduces the cooling losses and keeps out excess heat and in cold climates it reduces the heat losses. Usually the installation of the insulation can be applied on all parts of the buildings such as the roof, the floors, the walls and the foundations. Proper installation of the insulation is part of the concept of Zero energy Buildings which has as one of its targets to minimize losses.

Recent studies conclude that if one was to use some forms of insulation, even in existing buildings of Europe, this would reduce the energy consumption more than 50%. In some cases, extra insulation can give reductions of the energy consumption up to 78%. Additionally, the insulation producers claim that in Europe the correct insulation type can give a reduction of 400 million tonnes of CO2 emissions. (Reference-European Mineral Wool Manufacturers Association (EURIMA), Insulation in a Nutshell, EURIMA, Brussels, www.eurima.org.)

Insulation has great advantages in both hot and cold climates, but the latest studies show that in regions where the cooling demand is high the insulation can offer a 24% reduction to this demand. (Reference- ECOFYS (2004), Mitigation of CO2 Emissions from the Building Stock, report for EURIMA and EuroACE, Utrecht.) It is additionally amazing that studies have also proven that a well insulated roof of a house, in hot climates can give reductions of up to 85% on the cooling loads.

Though insulation is really effective on the energy performance of a building, in some cases, better results can be given in combination with other measures such as the building orientation, the building construction materials and high tech windows. Nevertheless, it is necessary during the ZEB design to optimize all the factors which are related to insulation performances. For the theory of Low/Zero Energy Buildings, insulation has a great potential to save energy and contribute to lower CO2 emissions. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Insulation technology, like all other products which offer efficiency is still under research and development, and has been for a long time now. Technological and science progress encourage the development of new materials, which results in the improvements of insulation methods and types. For example, advanced insulation is a result of this kind of progress on these issues and is now three times more effective than the initial normal insulations. It is important for new buildings to use doubled insulation, compared to what was initially used over the last 25 years. The (IEA's) Implementing Agreement on Energy Conservation in Buildings and Community Systems "has a specific work programme on high-performance thermal insulation systems. There is considerable attention being paid to improving insulation quality as standards for buildings become more rigorous." (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

A living example of effective insulation measures can be found in Denmark. "Denmark has had a strong policy for improved insulation and the general energy efficiency of housing since the 1970s." Most importantly are the results of these measures as the reduction of the heat demands in buildings and houses is clearly visible despite the increases of the building sector.

As shown in the above figure (figure 19), heating energies consumption per square meter in all the existing buildings stock is only 70% of the consumptions in 1980. It is very important to look at these results from the new reports, as they are very promising due to the new energy reductions from the necessary heating needs which are up to 40% of the average for all the existing buildings. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Heating, Cooling and Ventilation

In a building the heating, cooling and ventilation systems consume the biggest percentage of energy. In general, they have a significant impact on the energy demands of the buildings influences of its energy performance. The text that follows refers to the different and available technologies such as the conventional heating systems, advanced heating systems (heat pumps), advanced heating and cooling systems (active solar), central heating and cooling, thermal energy storage and wood heating.

Conventional Heating Systems:

Despite technological progress, there are still many countries that still use individual heating systems due to their vast demands of heating. All these systems use conventional fuels such as oil, natural gas and electricity. The technology of oil fuel heaters, since the middle part of the twentieth century has made unbelievable improvements in efficiency. The development of flame-retention head burners and high-static pressure burners influence the efficiency of the oil heaters and the efficiencies are increased from 60% to 80%. As for the natural gas heaters technology, the manufactures have made progress by using a secondary heat exchanger. The second heat exchanger recovers the "latent heat of the water in the combustion exhaust gases and then condensing it."The result is the release of 10-20% more heat. The technology of condensing gas boilers is appropriate for space and water heating as the manufactures promised efficiencies of 90-97%. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ ) However, under the ZEB principles it is better to replace the conventional heaters as the potential of lower energy consumptions can be improved then. For example, lower energy consumptions of 30 to 35% can be achieved just by replacing an old boiler. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Another heating system that is used in many countries is an electric one. Firstly, this system consumes electricity that is more expensive when compared to other heating systems and additionally the heat efficiency of these systems is very low. As the efficiency of the system is low, more energy is needed to cover heating demands which influences the energy profile of a building. However, in some countries the electricity cost during the night is lower than during the day. In these cases, the use of thermal storage heaters is appropriate, and is quite common in many countries. "The resistance heater with elements encased in the heat storing ceramic" is one of the widely used system. In addition, there is another thermal storage option, which is electrically heated hot water being stored in an insulated storage tank. In all of these cases, the big issue is the big heat losses from the different systems and during the last few years there has been a significant progress with new innovative methods and materials which can reduce them. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Nowadays, all these systems, the individual space heaters and thermal storage devices can be improved but as technology is advancing these systems are pointless due to their use of conventional fuels. Also the ZEB concept, demands more efficient systems which are independent from the conventional fuels. The new methods and the new technologies of heating, promise to be more efficient than the initial conventional heating systems. Intelligent control systems, automation features, heat pumps and passive methods have great potentials for Zero Energy Buildings.

Advanced Heating Systems: Heat Pumps

The heating demands in combination with the human needs for more comfort, followed by the new advanced technologies available have increased over the last few years. A heat pump is one of these new technologies which as a concept, transforms low temperature heat into high temperature heat which is then able to cover the heating demands. The air, the water, the soil or the bedrock can be used as a heat source for a heat pump. In addition, the reverse of a heat pump operation is able to convert it to cooling unit. In the market, there are different types of heat pumps available, like those that are driven by an electric motor or an internal combustion engine. It is amazing to see that electric heat pumps use approximately one-fourth to one-half of the energy needed by electric heaters. Moreover, comparing heat pumps with fossil fuel boilers, heat pumps can achieve reductions of primary energy consumption of up to 50%. Besides the geothermal heat pumps give better results than air sourced systems and are in accordance with the US Environmental Protection Agency reduction of energy consumption, as up to 44% is achievable with geothermal heat pumps. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Heat pumps are not a new technology but until now, their use has been limited. The latest heat pumps have started to become more known, as energy efficiency is becoming a well known issue. Also, as the building sectors search for new approaches and lower energy demand technologies, the heat pumps involvement became necessary. In some OECD countries, for instance, in Sweden, heat pumps are widely used in electrical heated homes (48% of the homes). Most of these systems operate as cooling and heating devices. In Sweden and Switzerland, heat pumps are just used for heating purposes, and they cover a big fraction of these countries. Heat pumps have great potentials, as even more improvements can be done. Already you can find in the market a wide variety of heat pumps but their extensive use in buildings is still a matter of time. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Advanced Heating and Cooling Systems: Active Solar

Yet another great technology which can be used for heating and cooling is the active solar systems, as they use the solar gain to heat a building. This technology is based on solar collectors which absorb the solar radiation and transform it to thermal energy. The use of this energy varies depending always from the occupants and the buildings needs. For example, this energy is useful for heating the water and can be used for both heating and cooling needs. In markets today, there are already different types of collectors such as unglazed, glazed flat plate and evacuated tubes. However, the general technological improvements achieved show important changes on the active solar technology. Nowadays, they use aluminium instead of copper which is more expensive and heavier, and they also use laser welding which gives a smoother absorbing surface. The technology of the active solar system is not yet fully developed but there it has great potentials for development for Zero Energy Buildings. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

The main aspect in all of these technologies is efficiency and cost. The active solar system's efficiency is "calculated as heat is delivered and divided by incident solar energy and this depends on the design of the collector and on the system of which the collector is a part of." (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ ) Yet, there are active solar systems that combine more than one operation and afford space and water heating. The efficiencies of these systems can reach 40 to 50% for domestic hot water. In central and northern European climates, efficiencies of 10 to 60% can be achieved for the need of hot water and heating demands. These efficiencies are influenced by the size of the panels and the storage tanks, the insulation of the building, the window and door types and in general from the buildings thermal envelope. It is used to combine these systems as a supporting or back-up system which covers the extra heating demands. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

The active solar system technology uses solar collectors which "have a nominal peak capacity of about 0.7 kWth(kilowatt thermal capacity) per m2." As mentioned in the International Energy Agency report "estimated annual yields for glazed flat plate collectors are 1 000 kWhth(kilowatt thermal capacity) per m2 in Israel, 700 kWhth per m2 in Australia; 400 kWhth per m2 in Germany and 350 kWhth per m2 in Austria, where they reach 550 kWhth per m2 for vacuum collectors and 300 kWhth per m2 for unglazed collectors." (reference- Philibert, C. (2005), The Present and Future Use of Solar Thermal Energy as a Primary Source of Energy, Inter Academy Council, www.iea.org/textbase/papers/2005/solarthermal.pdf.)

Another important aspect of this technology is the cost which varies amongst different countries. This variation is influenced by the climate conditions of each country and especially from solar radiation. For instance in Greece, the cost of a thermo siphon system which covers the needs of one family is 700 Euro and includes 2,4m2 collectors and a 150 litre tank. On the other hand in Germany, a comparable system costs 4.500 Euro and includes 4 to 6 m2 and a 300 litre tank. The remarkable thing is that in Germany, solar radiation is lower than Greece and usually, these systems are more effective in hot and sunny climates. However, in these climates the heating demands are smaller than in cold countries so a big part of the output is wasted. In contrast to hot climates, the use of the systems in cold climates is more efficient due to the higher demands for heating. "The same solar system that provides 40% savings on heating expenses in northern France, i.e. EUR 730 to 900 per year; in southern France it provides 80% savings, i.e. only EUR 120 to 180 per year." (Reference- Philibert, C. (2005), The Present and Future Use of Solar Thermal Energy as a Primary Source of Energy, Inter Academy Council, www.iea.org/textbase/papers/2005/solarthermal.pdf.)

Last but not least is the fact that these systems can be used as cooling systems. "A standard, single-effect absorption chiller can be driven with temperatures of around 90°C. This can be generated with standard flat plate solar collectors. Cooling technologies include single- and double-effect absorption chillers, adsorption chillers, and solid or liquid desiccant systems." In Europe, there are around 45 solar air conditioning systems which have a total capacity of 4,8MW cooling power and a total solar collector area of 19000 m2. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Until now, active solar technology is available in markets for homes just to use for hot water. The capabilities of this technology, drives manufacturers and engineers to research this further and develop it further. The research is focused on issues like advanced materials, advanced solar thermal collectors, large scale solar heating systems, solar industrial processes systems, solar cooling systems, combined solar heating and cooling systems, and standards, regulations and test procedures. All of these aspects of the active solar technology in combination with ZEB research and development can influence and change the future energy consumption.

Advanced Heating and Cooling Systems: District Heating and Cooling

District heating can be described as a system where the heat is generated in central areas and distributed to different spaces of the building. The produced heat is delivered to spaces through a network of pipes which cover the heating demands of the occupants. In some cases, possible wasted energy can be used by the district heating helping this way the energy savings of the building. However some of the common use sources are the "combined heat and power systems, heat-only boilers using conventional fuels or biomass, industrial waste heat, waste-to-energy plants and geothermal heat." Additionally, in many cases the combination of different heat sources is acceptable and results in better efficiencies. The concept of green energy and low carbon emissions always look for new sources which are renewable and available for use. The size of a district heating system varies from a single building to a thousand residential and commercial buildings. A district heating plant can provide higher efficiencies and better pollution control than localized boilers. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Furthermore, energy from the sun is useful for district heating and cooling systems. Sometimes it is possible to combine the use of these systems with thermal energy storage systems. In Germany, during 2003, eight solar district heating systems were constructed. These systems are able to cope with the German conditions and cover the needs of 30% to 95% of the total annual heating and hot water demands. Austria, Sweden and the Netherlands are some of the countries that already use similar systems to provide their buildings with heat and hot water. However, the largest solar systems for heating and cooling exist in Denmark and cover the needs of 1300 houses.

So far, this technology is being used in some countries but the potential for development in many other countries is significant. The "problem" as mentioned before is the fact that this technology is affected by climate conditions. One good example is Russia, which is a cold climate country, and the development of district heating systems covers the needs of 70% of the houses and the total length of this network is about 1.8milion km. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ ). The technology of a district system is under continuous research and development. Already pre-insulated pipes and 'clever' control systems are used in new installations of district heating systems. Current research focuses on issues like system optimisation, use of thermal storage, provisions of cooling as well as heating, applying the technology to low heat density areas and on optimising network size. In addition as referred to in the International Energy Agency (IEA) report "the current work of the IEA Implementing Agreement on District Heating and Cooling centres on the evaluation of a new all-plastic piping system, assessment of the actual annual energy efficiency of building-scale cooling systems, new materials and construction for improving the quality and lifetime of district heating pipes including joints, thermal, mechanical, and environmental performance." (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Advanced Heating and Cooling Systems: Thermal Energy Storage

The energy efficiency of many systems which cover the heating and cooling needs of the buildings is strongly influenced by thermal energy storage. It is possible that in the future the "stored solar or waste heat from combined heat and power plants" to replace the fossil fuels. Combined heat and power (CHP) plants are plants designed to produce both heat and electricity from a single heat source. The technology of thermal storage systems can be described as "the heat is transferred to or from the adjacent ground by heat exchangers, which consist of vertical pipes and ducts, or energy pillars that also serve as the building foundation. Heat storage in aquifers under suitable hydro-geological conditions can help to exploit the potential of surplus low temperature heat from combined heat and power plants." Furthermore, during summertime solar heating can be stored and retrieved for use in the winter time when the heating demands are essential. Thermal storage is available for use in the district heating of small areas (figure 20). (Energy Conservation through Energy Storage Organization, Underground Thermal Energy Storage, http://www.iea-eces.org/energy-storage/storage-techniques/underground-thermal-energy-storage.html)

In addition to everything that has already been mentioned, pilot and demonstration plants of new energy storage systems achieve significant efficiencies which also gives great potentials to this technology. As already stated, thermal storage systems are using soil and aquifers as a natural heat and cold source but it is possible to use them as storage 'tanks' for solar heating. This way the systems take advantage of free and environmental friendly sources of the earth. The extensive use of these systems in many countries is due to the fact that they are meeting the green energy principles and contribute to the efforts for lower CO2 emissions. The main use of these thermal storage systems in these counties is for heating and cooling. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

"Integrating phase change materials (PCM) into the building materials" offer opportunities for improvement on the thermal capacity of the building structures. These improvements are result in better and higher performance of the thermal capacity. Lately the researchers are focusing on the use of "micro-encapsulated paraffins in plaster or gypsum boards or in the ceiling for heating and cooling." This gives the ability of a material to absorb heat energy (thermal mass) in a building so that it can be increased, avoiding this way to heat excessively a building space. This method helps to reduce the cooling demands and energy savings of a building. In addition "demand for cooling can be shifted to the night when the PCM material is then discharged by natural ventilation of cold night air." The concrete of the floors which has high thermal mass can be used in combination with air channels as "thermal buffer storage for air conditioning." (Energy Conservation through Energy Storage Organization, Underground Thermal Energy Storage, http://www.iea-eces.org/energy-storage/storage-techniques/underground-thermal-energy-storage.html) AND (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Another method, which presents a significant potential to save energy in air conditioning systems is the thermo-chemical reaction. With this method "ventilation air is dried and heated by the adsorption of water vapour in zeolites or silicagel nano-porous granulates. The absorption process can be combined with indirect evaporation cooling to bring the air to the desired temperature. The absorption process of water vapour in silicagel or zeolites can also be used to construct solid-adsorption heat-pump systems with a high annual performance." (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Sweden and the Netherlands already use this technology of thermal storage systems to cover heating and cooling needs. Instead, in other countries the technology of phase changing materials and thermo-chemical storage systems is still under research and the development phase. The installation of advanced storage systems in Low Energy Buildings is already under study by the IEA Solar Heating and Cooling Implementing Agreement project. (Energy Conservation through Energy Storage Organization, Underground Thermal Energy Storage, http://www.iea-eces.org )

Passive Solar

The technology of passive solar heating and cooling (passive cooling load reduction) has been in use for more than 30 years. This technology was used in Passive or Green houses and from then, it has been under continuous development. Whilst designing it, engineers have to 'prepare' and design the building for passive solar technology. Usually this design includes large south facing windows which effectively offer passive solar heating, natural days lighting and materials that absorb and release the sun's heat. During the adjustments of this technology it was found that the use of "extensive glazing, wall or roof mounted solar air collectors, double-facade wall construction, air-flow windows, thermal mass walls and preheating of ventilation air through buried pipes" is essential. However the adjustments of passive solar technology on a building can influence the heating cost and in some cases it is possible to have significant reductions. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

In addition with passive solar designs a building has to take advantage of the solar energy which can be used for lighting and ventilation. These methods use "interior light through a variety of simple devices that concentrate on getting the direct sunlight deep into a building and the ventilation through the temperature, which gives resulting pressure differences that are created between different parts of a building when the sun shines. The building facade can be used to generate and channel airflows that remove heat that otherwise adds to the cooling load, or which can be used to preheat ventilation air when heating is required." (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

The concept of passive design windows is the most commonly used component of a building. As already mentioned in the subsection building envelope, windows are the most inefficient component of a building as the energy loss is bigger than the energy gain. Fortunately, the new types of windows that have been created achieve significant reductions of these losses and a better energy profile for the windows. Also the new windows increase the sunlight penetration in the buildings, decrease the use of artificial lighting and minimize increased cooling demands from solar radiation. This kind of technology can be very useful on the concept of Zero Energy Buildings. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ ) Electro chromatic windows are one of the most promising future technologies where "a small voltage causes the window to change from clear to a transparent coloured state or vice versa." The effect of this technology is reducing heating needs in the winter and cooling needs in the summer. Recent research on this issue proves that this technology can achieve energy savings of up to 60% in New York climate conditions, depending on the buildings envelope and window size. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Passive solar technology is directly related with the building's design, building's materials, building envelope and the integrated building technologies. Despite the fact that this technology has been used for more than 30 years, the techniques are still being researched and developed. However "there is no recipe for one size that will fit all" in this sector. Each building is a separate case and it is necessary to analyze all the aspects that are involved such as the natural environment, climate and the needs of its inhabitants.

Air Conditioning

Air conditioning systems are commonly used in hot climate countries. The technology they use is similar with the refrigerator's operating principles and its basic components. The operation of an air condition can be described as follow: "A cold indoor coil is used as an evaporator, and a condenser releases the collected heat outside. A compressor moves a heat-transfer fluid (or refrigerant) between the evaporator and the condenser. A pump forces the refrigerant through the circuit of copper tubing and fins in the coils. The liquid refrigerant evaporates in the indoor evaporator coil, pulling heat out of the indoor air and thereby cooling the interior. The hot refrigerant gas is pumped outdoors into the condenser where it reverts back to a liquid and gives its heat to the air flowing over the condenser's tubing and fins." (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Presently, this technology is available in all markets and has different types such as a single unit space air condition, a split unit space air condition and central unit air condition such as, rooftop air conditioning. Also there are bigger air condition systems in the market which are used in some cases, like the "chillers which are linked to air or water based heat distribution systems." For the heat transfer vector the air condition systems use air or water. The systems which use water to operation are more expensive but at the same time, they are essentially more efficient. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

In cases where the condensation builds up, the installation of a central cooling or chilled beam or chilled ceiling system would be appropriate and more effective. The central cooling systems operate by circulating chilled water through cool panels. However, there are significant efficiency variations between the air to air room air conditioners. As the International Energy Agency (IEA) report stated "the least-efficient portable air conditioner might have an energy efficiency ratio of less than 1.5 W/W (watts cooling output per watts power input), whereas the most efficient split room air conditioners can achieve more than 6.5 W/W." The optimization of a partial load performance in a room and central air conditioning systems is possible makes it possible to achieve more energy savings. In addition, a enhanced energy performance for the commonly used air conditions is possible by the optimization of the refrigerant and the control, utilisation of an efficient compressor and improvement of the heat transfer at the heat exchangers. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Furthermore, in hot and dry climates the evaporative coolers are more effective. These evaporative coolers are part of the air conditioner systems which remove heat by evaporating water and cool the outside air. This way the building space is supplied by cool air whilst at the same time the hot indoor air goes out from the open windows. The installation of evaporative coolers as part of the central air condition does not cost as much and consumes only a quarter of energy. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

The technology of an air conditioning is under continuous development due to the new technologies and the new standards from the manufactures and governments. For example, you can already see that in the market there are too many models of air conditioners and each one has different energy efficiency. However, with the help of future technologies that is are getting developed, such as the heat pumps and solar power cooling systems, everything will change the aspect of an air conditioning system (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Ventilation

The ventilation of a building space is an important factor, not only due to the comfort conditions of spaces but also for health reasons. There are two types of ventilation, the natural and the artificial. The design and operation of natural ventilation is based on windows opening and infiltration. Also, during the design of natural ventilation it is essential to consider the microclimatic conditions, the buildings shape and orientation and the windows size and position. In general, the design of this concept can be characterised as a sophisticated process. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

In some cases, like the kitchen or bathroom spaces, ventilation by extraction is useful and helps to reduce moisture and odours. Usually this type of ventilation demands mechanical extraction fans which are controlled manually or automatically. Another ventilation system that is more efficient for these cases is the "passive-stack ventilation (PSV), which does not require power to operate and is "on" all the time. PSV can be controlled by humidity sensors, so that ventilation occurs only when needed." (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

However, for many years now in commercial and industrial building have used conventional mechanical ventilation, air distribution and heat recovery systems. Furthermore, the new upgraded systems have more operations now such as the treatment of the outside air resulting in the removal of pollutants and better air distribution and airflow. The treatment of the outside air and the removal of the pollutants are usually accomplished by filtration. One big issue raised with these systems is energy savings as this is possible with good insulation, energy efficient fans, conventional or more innovative heat recovery systems and preconditioning of the supply air. Apart from a few OECD countries, the installation of systems that offer mechanical supply and treatment of the outside air is rare. New houses now tend to install mechanical ventilation as it gives more comfort. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Unfortunately the development of modern methods of ventilation is advancing very slowly. Nevertheless, new innovative methods in reference to the treatment and energy savings of these systems are under research and development stages. It is possible to achieve energy reductions of 10 to 15% with the widely use of new ventilation systems. At the moment, it is obvious that the current technology has low total energy efficiency but with new methods and technology the potentials of these systems are great. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

The European Union mentioned that further development is possible in quite a few areas, such as the development of sensors to help preventive maintenance, cost reduction through optimisation and mass production of efficient ventilators and fans, monitoring of various types of ventilation (natural mainly) to ascertain their running characteristics and dynamic insulation through the development of new applications and through demonstrations. (European Commission, Energy-European strategies, http://ec.europa.eu/energy/strategies/index_en.htm )

Hot water heating demands:

Admittedly another human need which demands high energy consumption is hot water heating. There are many ways and methods to provide hot water and cover the inhabitants needs. In the following paragraphs two common methods of use are analyzed, one being hot water heating with conventional fuels and other one is with solar energy.

Hot Water Heating: Conventional Fuels

The particular aim of water heating systems is the optimization of energy and water use. Due to the wide use of this all over the world, it is very important for these systems to be energy efficient. Usually the use of these systems is for domestic hot water where it is then being used for different purposes. For instance, in America and Australia the energy consumption is higher than in Europe and Japan, as the dishwashers and washing machines use hot water from the hot water systems. In comparison, Europe and Japan have machines which use their own systems to heat the water which consumes less energy than the individual systems. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Until now, the market provides a big variety of systems which are able to operate either by electricity or by natural gas. The systems that use electricity to heat the water, by using electrical resistance, are more efficient but are not the best choice as this concerns primary energy and CO2 emissions. On the other hand, systems that use natural gases are more "environmental" friendly compared to the electric systems. This depends from the fuel that is used for electrical generation. ( Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Hot water that is heated by electricity is widely used in OECD countries (Organisation for Economic Co-operation and Development). In these cases the hot water is stored in special tanks with insulation but the losses are up to 30% of the input energy. From the tank, the hot water is distributed to the different places via a piping system. It is obvious that the improvement of the insulation of the tank will give better performance to the system. OECD regions have already made positive steps through measures for efficiency standards and industrial agreements which promise to reduce these losses. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

Furthermore the "small point of demand of electric water heaters" is widely used in central Europe. These systems are more efficient as they have lower losses in distribution of the hot water but on the other hand they have higher storage losses. As referred to in the International Energy Agency report "finding the right balance between peak and off-peak heating and losses through the distribution system is one of the main areas for optimisation of conventional water heating systems." (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )

In OECD countries, the average consumption of the year 2000 was 3 189 kWh per unit per year. In particular, this average was 3 977 kWh per year per unit in the OECD Pacific, 3 823 kWh in North America and 2 492 kWh in OECD Europe (IEA (2003), Cool Appliances, Policy Strategies for Energy Efficient Homes, OECD/IEA, Paris.)

The water heating systems which use conventional fuels need more development. There are more energy efficient technology stages presently under research, development and demonstration which are going to achieve better energy performances. Obviously improving the insulation of tanks, the distribution piping and the optimization of the points of demand against the choices given of heating technologies are some of the aspects which influence the performance of the systems. Additionally, another significant factor is the replacement of the electric systems with gas operating systems which are more efficient and emit lower CO2. In the future, it will be possible to replace the heat pump water heaters with all of the above systems and at the same time reduce the electricity demands. The only problem of this technology is to become fully commercial. Last but not least, is the option to use a closed loop water return systems. During the operation of this system, the hot water returns to the tank instead of being put in the distribution pipes which have significant losses. All these efficient technologies have great potentials, principles and designs for Zero Energy Buildings. (Reference-International Energy Agency (IEA), Energy technology perspectives 2006: Scenarios & Strategies to 2050 web page: http://www.iea.org/ )