Exergy analysis has been utilized in the optimization of thermal processes in power plants and in industry. However, energy systems in buildings are designed based solely on the energy conservation principle. This principle alone does not provide a full understanding of important aspects of energy use in buildings, e.g. matching the quality levels of energy supply and end-use; fully expressing the advantages of using passive (e.g. thermal insulation, window design) and ambient energy (e.g. heat pumps) in buildings. From this viewpoint, exergy analysis is an important link in understanding and designing energy flows in buildings.
Recently, the exergy concept has been applied to the built environment as well (Shukuya 1994, Gertis 1995, Asada and Shukuya 1999, Nishikawa and Shukuya 1999, Jenni and Hawkins 2002, and Schmidt and Shukuya 2003). Some researchers (Rosen 2001 and Wall 2001) have also used the exergy concept in a context of sustainable development. In the last few years, a working group of the International Energy Agency has been formed within the Energy Conservation in Buildings and Community Systems programme: "Low Exergy Systems for Heating and Cooling of Buildings" (Annex 37, 2002 and Ala-Juusela, 2004). The overall objective of the Annex was to promote the rational use of energy by means of low valued and environmentally sustainable energy sources. This annex is being followed up by the international LowExNet group, which works towards providing knowledge on and tools for exergy analyses to be applied in the built environment (LowExNet 2004).
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This paper presents an outline and case study of a spreadsheet-based exergy analysis tool (Schmidt, 2004) and a new graphic input 'Casanova' interface being developed to enhance its user-friendliness for a residential building situated in Toronto, Ontario. The tool is meant to facilitate the practical application of exergy into building design. It does so by helping building and building-services designers develop insight into combinations of design options that can lower the total exergy consumption of a building and its associated building services. The interface is structured so that a building designer could focus more on varying building size and orientation, and /or building envelope configuration. A building services designer may like to concentrate on building occupancy schedules, indoor and outdoor air temperatures, and building service configurations.
The three equations of exergetic efficiencies for steady state processes are:
1. The conventional or simple exergetic efficiency:
This is an explicit definition and can be used for all process plants and units. It is an ideal thermodynamic system when all the components of the incoming exergy flow are transformed to other components, e.g., in the case for power stations or for building heating and cooling systems.
2. Rational exergetic efficiency and the utilizable exergy coefficient
The rational exergetic efficiency is defined by Kotas (1985) as a ratio of the desired exergy output to the exergy used or consumed which is the sum of all exergy transfers from the system, which must be regarded as constituting the desired output, plus any by-product, which is produced by the system. The desired output is determined by examining the function of the system .
Utilizable exergy coefficient
Brodyansky, Sorin and LeGoff (1994) introduced this form of exergetic efficiency, called utilizable exergy coefficient.
The total exergy input () of a real system is always higher than its exergy output () because a certain amount of exergy is irreversibly destroyed within the system. This exergy, generally referred to as the internal exergy losses or exergy destruction, is directly linked to the thermodynamic irreversibilities in the system. The rest of the exergy that leaves the system with the utilizable exergy stream is a part of the exergy input, which has simply gone through the system without undergoing any transformation and is the transiting exergy, . is the produced utilizable exergy rate and is the consumed exergy rate.
This form of efficiency is an improvement on the traditional exergetic efficiency, because it subtracts the untransformed components from the incoming and outgoing streams. To any material, heat and work stream can be associated as an exergy content, which is completely defined by temperature, pressure and composition of the stream itself and of a reference state, which is normally the environment in which the system operates. It is, therefore, possible to compute the exergy content of all incoming and outgoing streams to and from a system and to establish an overall exergy balance over any system, as shown in Fig. 1.
Always on Time
Marked to Standard
As illustrated in Fig. 1, part of the exergy output from the system may dissipate into the environment as heat losses, sewage waste or exhaust. This wasted exergy, no longer usable by subsequent processes, constitutes the external losses, Iext. It is more appropriate, from the standpoint of downstream operations, to consider the exergy that remains utilizable, Eu, rather than the total output, . Only part of the utilizable exergy is produced by the system through the physicochemical phenomena that take place within its boundaries. The rest of the exergy that leaves the system with the utilizable exergy stream is a part of the exergy input, which has simply gone through the system without undergoing any transformation and is named transiting exergy, by Kostenko (1983).
Energy, Exergy and Sustainability
The first principle of thermodynamics is that of energy conservation. It states that the sum of all energy put into a system is equal to the sum of the increase in internal energy within the system and the energy rejected by the system. Taken literally, this means that saving energy is not possible, as energy is never destroyed.
In every real process, however, something is destroyed, and that is the quality of the energy, also called exergy. This is the subject of the second principle of thermodynamics. Energy produced at higher temperatures is of higher quality, meaning that more work can be produced with this energy. Electricity is of maximum quality, as it can be fully converted into power. During this conversion, heat at lower temperatures will be rejected. On the other hand, heat at a low outside air temperature (less than 7 deg C) can be in equilibrium with its surroundings, and can therefore no longer be converted into electricity or power. This is why burning gas in a boiler in order to heat a building is very inefficient; the potential of the gas is not fully used. With the same quantity of gas, it would have been possible to produce electricity and power. Exergy is therefore a good measure for the sustainability of a system. Dincer 2000, Wall et al. 2001, Rosen et al. 2001 and Boelman et. al 2003.
Energy and Exergy Demands of Buildings
In order to analyze the energy and exergy demands of buildings which are purely based on energy balances between the building maintained at a defined level of comfort and its environment, they have to be studied in detail. When defining the energy or exergy demand, it is important to consider both the physical aspects of a building and its uses. This is because the ways in which a building is used influence the internal heat load and the lighting and power demand considerably, and therefore the building's overall energy demand as well. All relevant energy consuming items should be taken into account to avoid focusing on a single aspect of the demand, which could lead to erroneous assumptions about energy savings. For instance, adding insulation decreases heat demand but increases cooling demand, while having fewer windows decreases heat demand but increases lighting demand.
By applying exergy analysis to buildings it can be shown that the greatest fraction of the total supplied exergy for heating in buildings is consumed when heat is generated from other sources, e.g. fossil fuels like natural gas. Parts of these losses occur during energy transformation, extraction, and transformation in power stations or in heat generation, e.g. in a boiler. Only a small fraction of the exergy consumption happens within the buildings (Schmidt and Shukuya 2003).
To use the exergy most efficiently, we have to design heating systems that will keep the supply temperatures as low as possible. In most cases, low exergy consumption within a component coincides with a low inlet temperature; that means that the energy is supplied at a low temperature level. The examples of such systems already are thermally activated building constructions, floor-heating systems or waterborne systems where heating or cooling pipes are inserted into the concrete slab construction, thereby heating or cooling the rooms, to be subsequently released as fresh supply air to the rooms (Johannesson 2004). There are many more system alternatives, which are showcased in the LowEx Guidebook (Ala-Juusela et al 2004 and Annex 37 2004).
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The system studied is as follows: Heat is added to the building by lighting, people and appliances, and air flows into and out of the building through infiltration and ventilation. Ventilation air can be treated initially in an air-handling unit, where it is chilled or preheated. The total energy demand consists of seven items: (Itard 2003 and Itard 2005).
- Demand for heat in the building, Qheat
- Demand for cold in the building, Qcold
- Demand for heat in the air-handling system, Qheat, AHU
- Demand for cold in the air handling system, Qcold,AHU
- Demand for lighting, Qlight
- Demand for ventilators when using mechanical ventilation, Qventil
- Demand for appliances, such as computers and servers, Qappl.
The model for the heat and cold balances within a building envelope is based on hourly energy balances that take into account transmission, ventilation, infiltration losses and heat accumulation in the construction, as well as heat load through sun, appliances, people and artificial lighting.
The heat and cold balances in air-handling systems are simple enthalpy balances based on the temperature of the outdoor air and the specified temperature of the air-supply into the building. These balances are needed only when a mechanical ventilation system is used. The calculations for appliances and lighting are based on a specified electrical load per square meter of gross floor area. The energy demand for ventilators is calculated assuming known pressure losses in the ducts.
Exergy of electrical energy and mechanical energy: By means of the concept of exergy, the mechanical work and electrical energy is directly transferred into exergy, that is E=W
Actually, both the mechanical work and electrical energy are higher than the thermal energy in their energy quality. And all of them can be fully converted into useful work.
Exergy of heating/cooling capacity: The exergy of heating capacity is defined as the maximum useful work attainable from a heat transfer process due to temperature difference between the system and the reference environment and similarly defined for exergy of cooling capacity. The exergy demand for cold and heat in the building is calculated using the method described in Schmidt 2004. If refers to the indoor air temperature, and to the temperature of the surroundings (outside air temperature), the exergy demand for heat or cold in the building expressed in J/K is:
Exergy demand for cold and heat in the air-handling unit: This exergy demand is calculated using the method described in Shukuya 2002. In the following equation, Tblin refers to the temperature of the air that is supplied to the building's rooms.
Exergy demand for electrical equipment: Lighting, appliances and ventilators are electrical equipment. For all electrical equipment, an exergetic efficiency of one is applied, and equated as
Primary Consumption of Energy and Exergy
Primary energy consumption
Buildings need equipment in order to meet their energy demands. Boilers or heat pumps can be used to meet the heating demand. Compression cooling machines can be used to meet the cooling demand. The electricity that is needed must be produced by a power plant. Regardless of the type of equipment that is used, it will always be subject to conversion efficiency. This means that the amount of energy needed by the conversion equipment is different from the overall energy demand.
Example for heating: If the heating demand is 1MJ, and a gas boiler with an overall efficiency of 0.85 is used, the primary energy consumption to meet the heating demand is 1/0.85 = 1.18 MJ.
Example for cooling: If the cooling demand is 1 MJ, and a compression cooling machine which has an efficiency of 3 is used (this is possible because a heat pump also uses free energy from the surroundings), the heat pump needs 1/3 = 0.33 MJ of electricity to meet this demand. This electricity, however, is produced in a power plant. If the efficiency of the power plant is 0.4, the primary energy consumption to meet the cooling demand becomes 0.33/0.4 = 0.83 MJ.
Primary exergy consumption
This Equation calculates the primary exergy consumption, where is the exergetic quality factor of the entire energy conversion process:
For example, if waste heat at the temperature =50 °C is used for heating applications, and if the outside temperature is 1 °C, the quality factor will be 0.16 .
Example of energy and exergy calculation results
Residential Building Case Study
The Model Building
To perform the calculations, a base model of an average one-family house in downtown Toronto has been taken for the case study. The pre WWII built house has four person household, has five rooms (one living room, four bedrooms), a kitchen, such as heel combined with a dining room, a bathroom on the first floor and a toilet on the ground floor. The attic and cellar are not heated. Some key figures of the model building are shown in Table 1.
The storey height with its 2.9 m is higher than than newer homes, which allow the warm air to float up during the hot summer months. The disadvantage of high ceiling is that the heat energy demand in winter is higher.
The calculations were done with the programme CASAnova, an educational software for calculating the heating and cooling energy demand as well as the temperature behaviour in buildings. The programme is freely available for educational purposes by the Group for Building Physics & Solar Energy in the Department of Physics at the University of Siegen. It can be used to show the relations between building geometry, orientation, thermal insulation, glazing, solar heat gains, heating demand, heating and primary energy as well as overheating in summer.
CASAnova uses building shapes of rectangular form for which in a monthly balance transmission and ventilation losses as well as solar and internal gains are calculated. Therefore it was suitable to show the results as calculated on the model building of a simple one-family house. In addition to that, CASAnova also contains climate-data for Toronto, ON in its programme structure, which was another reason to choose it for the calculations.
To determine the number of hours during which a building is overheated, CASAnova uses a single-zone dynamical thermal model. Based on hourly data of the outside temperature and the solar heat gains through windows and walls, CASAnova calculates the usable solar heat gain as well as the transmission and ventilation losses of this zone. Together with the internal gains the balance of energy for an effective thermal mass is determined (i.e. energy losses and gains for the room-air including the heat which is stored up in an active part of the wall).
According to the amount and the sign of this balance zone temperatures change with time. Finally, the number of hours is counted for which room-air temperatures exceed a comfort temperature limit given by the user.
Results - Heat Demand Reduction for Several Renovation Options
Before Renovation - The Base Case
For the initial situation it was assumed that the house has been built post war construction. Houses older than 35 years make up more than 60 % of the downtown Toronto building stock and use 230 kWh/m2 and up. This building stock, together with buildings constructed prior to the 1990s has a notable impact on the local energy consumption.
While designing the model building it has been taken care to have more windows on the northern façade and less on the south. The window areas on the respective directions are as shown in Table 2.
For the initial situation windows with single glazing have been assumed. single glazed windows are in older Torontonian buildings. Thus the U-value (rate of heat loss through a surface) of the glassing is as high as 5.8 W/(m2K), the one of the wooden frames is 3.5 W/(m2K) and the g-value (total energy admission value) 0.92.
The exterior walls have common medium weight exterior construction (bricks) with U value of 1.2 W/(m2K). The windows has the U-value of 5.8 W/(m2K).
The first floor towards the partly-insulated roof has an U-value of 1.2 W/(m2K) and the ground floor towards the non-heated cellar without insulation an U-value of 1.0 W/(m2K). The door's U-value is 1.8 W/(m2K). Indoor temperature has been set to 21°C and overheating occurs when the temperature rises above 27°C. The internal gains which stem from a four person household and average household appliances assumed to be up to 44 kWh/m2a i.e. 5 W/m2.
All the calculations have been done for the location of Toronto, Ontario, 43°40' N 79°22' W. Toronto has summer temperature ranging from 23°C to 31°C and winter temperature to lowest -22°C as minimum temperature of the year. Natural gas is the most common energy source in Toronto for both heating and cooking since it is also much cheaper than oil fuel and electricity. Therefore the heating system of the model building has been defined as a condensing boiler, with both boiler and distribution being inside the thermal zone. The heat transfer occurs through with a system temperature of 70/55°C.
These features and the previously mentioned features of the model building result in a heat energy demand of 639 kWh/m2a and a primary energy demand for natural gas of 763.9 kWh/m2a. The final energy demand of the household amounts to 9616 m3/a of natural gas.
As can be seen from the results in Figures 2 and 3, the model house correctly reflects the current situation of old Torontonian buildings showing a high heat energy demand of 639.4 kWh/m2a. Due to bad insulation which for example may let the indoor temperatures drop down to below -15°C, the following construction leads to 323 effective heating days. According to Figure 4, most heat is lost through walls (41 %), roof (20 %) and windows (27 %), which are offering the biggest potential for a renovation that would lead to energy savings.
All renovation options were calculated using data for materials that can be easily available in Toronto.
In the first option only the windows were changed to double glazed heat protected windows with U value equal to 1.0 W/m2 K, in the second option the house walls get a better insulation, while the third renovation option is a combination of the first two. The other properties of the building have not been changed. The detailed calculations can be viewed in Annex I. Technical data for construction and building services are for a typical residential building (see Table 1). Detailed construction data were entered to the tool's input interface. On the other hand, the details for the selected building services components were provided by the interface to the calculation module as default values. The case has been taken for a residential building base case which has nominal insulations and needs retrofits (option 1 and option 2).
3 THE METHOD
For the following study of heating or cooling steady state conditions are assumed. Energy and matter are supplied into the system to make it work. Inputs and outputs are the same, according to the laws of energy and mass conservation. The energy flow through the building envelope is constant in time under steady state conditions. In the case of heating, heat transmission occurs from the warm interior to the cold ambient environment, across the building envelope. This is accompanied by an increasing flow of entropy [The entropy of a substance is a function of the temperature and pressure]. A certain amount of entropy is generated by this process, due to irreversible processes inside the building envelope.
This generated entropy has to be discarded to the surroundings, i.e. the outdoor environment. It is important to recognise that the energy flowing out of the building envelope is not only accompanied by a destruction of exergy, but also by an increased flow of entropy. Disposition of generated entropy from a system allows room for feeding on exergy and consuming it again. This process, which underlies every working process, can be described in the following four fundamental steps. Heating and cooling systems are no exception here [ 11 ]:
Table I: Four steps of the exergy-entropy process.
- Feed on exergy
- Consume exergy
- Generate entropy
- Dispose entropy
Educational Tool for Energy and Exergy analyses of
Heating and Cooling Applications in Buildings
To increase the understanding of exergy flows in buildings and to be able to find possibilities for further improvements in energy utilisation in buildings, an analysis tool has been produced during ongoing work for the IEA ECBCS Annex 37. Throughout the development, the aim was to produce a "transparent" tool, easy to understand for the target group of architects and building designers, as a whole. The Microsoft excel tool is built up in different blocks of subsystems for all important steps in the energy chain
(see Figure 2). All components, building construction parts, and building services equipment have advanced input options. Heat losses in the different components are regarded, as well as the required auxiliary electricity for pumps and fans. The electricity demand for artificial lighting and for driving fans in the ventilation system is included. On the primary energy side, the inputs are differentiated between fossil and renewable sources. The calculation is made under steady state conditions. This tool results are summarised on with diagrams as well as numbers. All steps of the energy chain - from the primary energy source, via the building, to the sink (i.e. the ambient environment) - are included in the analysis.
5 DESCRIPTION OF THE EXAMINED CASE
In order to clarify the method for this analysis, a typical residential building has been taken as a case study. For this base case model, a number of variations in the building envelope design and in the building service equipment have been calculated.
The base case has been chosen so that the building standards in North America could be met in general terms. The insulation standard is moderate and the building service systems are representative of the building stock in Toronto. To enhance the understanding of the exergy analysis method and to see the impacts of building design changes on the result, variations in the design have been calculated. For the base case, a number of different improvements and changes in the system design have been analysed:
Numerical examples are shown for the whole process of space heating, based on a system design and the sub-systems shown in Figure 2.
Results of the analysis of the base case are shown in Figure 3 and Figure 4. These figures, which indicate where losses occur, are quantified by the sub-systems/components in Figure 4.
In Figure 3 ,the system is fed with primary energy/exergy, shown on the left side of the diagram. Because of losses and system irreversibility and inefficiencies in the heat and mass transfer processes in the components, energy, as well as exergy, dissipates to the environment. At the same time, exergy is consumed in each component. When the flow of energy leaves the building through the building envelope there is still a remarkable amount of energy left over (i.e. the sum of all building heat losses), but the same is not true for exergy. At the ambient environment level, energy has no potential of doing work and all exergy has been consumed. The exergy flow on the far right side of the diagram is equal to zero. This kind of diagram helps in comprehending the flow of exergy through building systems and enables further optimisations in the overall system
To achieve improvements in the system design, it is mandatory to know where losses and inefficiencies occur (Fig 4). Major losses occur in both transformation processes. This happens namely in the primary energy transformation, where a primary energy source is transformed into an end-energy source, such as LNG, and in the generation, where the named end-energy source is transformed into heat by, for example, a boiler.
The difference between an energy and an exergy analysis becomes clear when observing the losses in the generation sub-system. The energy efficiency of this system is high, but the exergy consumption within the boiler system is the largest of all regarded subsystems. When using a combustion process, consuming a lot of exergy is indispensable in the extraction of thermal exergy from the chemical exergy contained in LNG. As for the process in the generation, the supply of energy is of a high quality factor, as it is for LNG, with 0.95. The core inside the generation is a combustion process with flame temperatures of some thousand degrees celsius, leading to the output of the process being a heat carrier medium of about 80°C. Even at this point, the temperature levels indicate a great loss.
6.1 Impact of improvements in the building envelope versus improvements in the service equipment (Base case+ HVAC options)
Starting with the base case described above, improvements on the design have been made and calculated. As already shown, exergy consumption within the heat generation is the largest among all sub-systems. This is unavoidable when generating heat for space heating through the use of a combustion process. Because of this, it may be considered that it is essential to improve the efficiency of the boiler. Thus, an increase in boiler efficiency from ?G = 0.8 to 0.95 has been reached with improvement (see Table III). However, The decrease in exergy consumption is marginal.
To increase the exergy output of the boiler, an increase of the outlet water temperature can be taken into consideration. This, however, results in the consumption of more exergy within the following systems, from the storage to the emission system. Also, the exergy consumption within the room air would be higher because the desired room temperature is just 21°C. These facts imply that an extremely highly efficient boiler alone can not necessarily make a significant contribution to the reduction of exergy consumption in the whole process of space heating.
This can change if the building envelope insulation is considered when realizinf the heating exergy load of the room. This has been done with the improved insulation of the walls and the windows have been improved. The heating exergy load, (the exergy output from the room air and the exergy input to the building envelope - 4 % of the chemical exergy input to the condensing boiler) is considered. This reduction measure could be regarded as marginal, or as having a limited impact on the total exergy consumption of the system. But, as can be seen by the difference between the whole exergy consumption profile of the base case and the base case with improvement (5), in order to decrease the rate of total exergy consumption, it is more feasible to reduce the heating exergy load by installing well-insulated exterior walls and glazings than to install thermally, extremely highly, efficient boilers.
6.2 System flexibility and the possible integration of renewable sources into building systems
The flexibility in the utilisation of different energy sources is of great imposrtance in sustainable building design along with possible use of renewable sources, and also flexibility in satisfying broad variations from the demand side. Utilising exergy analyses could help to quantify the degree of system flexibility. As already stated, a reduction in the exergy load of the room is important. However, it is equally important to consider how to satisfy the remaining demand. This is done in the analysis shown in Figure 7.
Three system solutions have been chosen to satisfy the heat demand for the same room. The base case represents a high temperature condensing boiler and high temperature radiators. The improvements represents a system where a heat pump supplies a low temperature floor heating system along with improvement options as in table III. The options satisfy the same heat demand, but with totally different exergy needs as can be seen from Exe. Thirgy /energs difference can not be clearly shown in an energy analysis, see annex II for exergy/energy graphs generated from excel tool.
The results of the exergy analysis suggest that long-term increases in the sustainability of buildings can be achieved only by reducing the energy demand for electrical appliances considerably and by either improving the efficiency of the electricity production process or applying sustainable electricity generation based on sun or wind. The reduction of the lighting demand is possible by designing buildings that make maximal use of day lighting and by developing efficient lighting. The energy demand for appliances, such as computers and televisions, should also be decreased considerably.
The improvement of the exergetic efficiency of heating and cooling systems by applying low-temperature heating and high-temperature cooling will also have positive effects on sustainability, but further reductions in the heating and cooling demand through the application of passive building physics measures will have more long-term effects.
As set out in this paper, the energy conservation concept alone is not enough to gain full understanding of all the important aspects of energy utilisation processes. From this aspect, the method of exergy analyses facilitates clearer understanding and improved design of energy flows in buildings. The test method allows for the possibility of choosing energy sources according to the quality needed for a certain application. One of these options is energy cascading, where the flow of energy is used several times, despite a quality decrease in each step.
From this general statement, a number of conclusions can be drawn from the cases analysed. The following design guidelines for building designers can be extracted from the recommendations:
- Reducing the loads on building service equipment is an efficient and mandatory step towards good, exergy-saving design, as shown by the analyses in Figure 2 and Figure 3. Utilising passive means - like good insulation standards, tight building envelopes and passive gains (solar or internal) - is an excellent starting point for optimised design. All measures offered by modern building physics in this field are highly efficient in this process and generally accepted. In a second step, building services appliances should be taken into consideration. Use of these appliances should be kept to a minimum and be restricted to cases in which passive means are insufficient. This decision depends on the building owner's preferences and on the standards or limits considered acceptable for indoor environments. Related problems (such as overheating or increased cooling needs due to excessive solar gains, for instance) must also be taken into account. Even in the case of cooling, which has not been especially addressed in this paper, the reduction of loads by e.g. efficient solar shadings is mandatory.
- Flexibility in system configurations is important for future "more sustainable" buildings. Exergy analysis can help in quantifying the degree of flexibility in a system design. Low exergy loads from the enclosed spaces and from emission, distribution and storage systems enable an open configuration of the generation and the possible supply of the building, utilising a number of different energy sources, see (Schmidt 2004) for a more detailed analysis. Here, the possibility of integrating all kinds of renewable sources of heat and coolness should be kept in mind. All renewable sources are utilised more efficiently at low temperature levels. In the case of heating, this is true for thermal solar power, generated by simple flat-plate collectors or solar walls, for instance. If these sources are efficiently used to cover the heating-energy demand of a building, the entire service system will run with decreased amounts of environmental loads, such as CO2 emissions and
- other greenhouse gases. High exergy sources like electrical power should be left to special appliances that require a high exergy content, such as artificial lighting, computers and machines. These sources should not be used for heating purposes. Even though some advantages (like low installation costs for direct electrical heating) may seem beneficial, exergy analysis proves the opposite. High primary energy transformation factors in a lot of countries can explain the same fact, through an energy analysis. If high exergy sources are to be used nonetheless, efficient processes are needed, for example heating with heat pumps in combination with low-temperature emission systems (Schmidt 2004). · Other systems that will reduce exergy loads in simple components are beneficial, too. The integration of a mechanical ventilation system (preferably a balanced ventilation system with heat recovery in the air-handling unit) will reduce the exergy consumption, equal to measures like those specified in higher insulation standards. Storing heat during summertime, and utilising these gains when they are needed in wintertime, might be another possibility. Most of these measures imply larger investment costs, hence they are not always applicable. Most of the effects due to these additional measures to increase energy efficiency can also be shown by the energy approach.
- It is already possible to build a "low-exergy house" using today's technology, as the presented examples of demonstration building projects show. Careful planning and good design of all systems are mandatory in achieving this goal, since some of the methods implemented are not yet everyday building practice. More emphasis should be placed on the importance of exergy and on preventing its destruction in the energy utilisation processes in our homes and working places. In the same sense, communities could limit the exergy consumption of buildings and specify requirements for low-exergy buildings, by analogy with limits for primary energy use that already exist. The proposed analysis method offers the background for doing this.
Exergy effecicncy by using passive systems
Shukya has described the general characteristics of six passive systems from the viewpoint of exergy-entropy process (see (Shukuya, 1998) and (Shukuya, 2000)). The rational passive (bio-climatic) design would be prerequisite to realize low-exergy systems for heating and cooling.
Daylighting: this is to consume solar exergy for indoor illumination. Exergy consumption occurs as solar exergy is absorbed by the interior surfaces of building envelopes. "Warm" exergy is produced as a result of solar exergy consumption for lighting; this may be consumed for space heating (Asada and Shukuya, 1999). The entropy generated in the course of solar exergy consumption for lighting must be discarded into the atmosphere by ventilation cooling or mechanical cooling, hopefully by a low-exergy system for cooling.
Passive heating: this is to control the rate of solar exergy consumption during daytime and nighttime by forming the built-environmental space with the appropriate materials that have low thermal conductivity and high thermal-exergy storage capacity. It is also to consume, during nighttime, the thermal exergy produced during daytime. Most of the entropy generated is discarded spontaneously through the building envelopes into the atmosphere (Shukuya and Komuro, 1996).
Shading: this is to let the excess solar exergy, namely the rest of exergy necessary for daylighting, be consumed before it enters the built environment. It is also to reduce the entropy generated within the built environment so that mechanical equipment for cooling is required to consume less exergy to remove the entropy generated within the built environment. Exterior shading devices are very much attractive in this regard, since the entropy generated at the devices is effectively discarded into the atmosphere by convection (Asada and Shukuya, 1999).
Ventilation cooling: (Free cooling) this is to consume kinetic exergy of atmospheric air, which is produced by the exergy-entropy process of the global environmental system described later (Shukuya and Komuro,1996), for removing the entropy generated within the built environment, such as the entropy discarded from the body surface of the occupants and that from the lighting fixtures, electric appliances and others, into the near-ground atmosphere.
Water spraying: this is to consume the "wet" exergy contained by liquid water, which is very large compared to thermal exergy, namely "warm" or "cool" exergy, to decrease the "warm" exergy produced by solar exergy consumption and possibly to produce "cool" exergy (See (Nishikawa and Shukuya, 1999), and (Saito and Shukuya, 1998)). Roof spraying and uchimizu, which is to scatter rainwater on the road surface, are also due to this process. The consumption of "wet" exergy to produce "cool" exergy or to decrease "warm" exergy play a very important role in photosynthetic system of leaves (Saito and Shukuya, 1998) and the temperature-regulating system of human body (Saito and Shukuya, 2000).
Composting: this is to let micro organisms consume actively a large amount of exergy contained by garbage and hence turn it into fertilizer. The "warm" exergy produced as a result of micro-organisms consuming chemical exergy can be rationally consumed for maintaining the temperature inside the container at a desired level. This is realized by making the walls of a container thermally well insulated (Takahashi and Shukuya, 1998). The entropy generated in the process of composting is discarded into the surrounding of the container and finally into the near-ground atmosphere.
With the view of passive (bio-climatic) design as exergy-entropy process, passive design is to design a route in which the exergy available from our immediate surroundings is rationally consumed and the generated entropy is rationally discarded into the atmosphere. Again, low-exergy systems for heating and cooling would be such systems consistent with passive design described above.
DIN 4701-10. 2001. Energy Efficiency of
Heating and Ventilation Systems in Buildings -
Part 10: Heating, Domestic hot Water,
Ventilation. German national standard. Berlin:
Deutsches Institut für Normung e.V.
Shukuya, M. 1998. Bioclimatic design as
rational design of exergy-entropy process.
Proceedings of PLEA '98, pp. 321-324.