Heat Pump Systems For Sustainable Buildings Engineering Essay

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Energy use in the building sector accounts for about 40% of world's final energy use and 33% of direct and indirect greenhouse gas emissions [1]. This fact together with the problem of climate change and the growing energy resource shortage brings on Sustainable Buildings being no longer perceived as buildings of a remote future, but as a realistic solution for mitigating CO2 emissions and reducing energy use in the building sector. As everyone knows, sustainability is defined in the Brundtland commission report: Our Common Future (1987) as: "Maintaining the environmental, social and economical system in such a way as to meet the need of the present generation without compromising the need of future generation to meet their own needs"[2]. Since this conception is deeply ingrained in everyone's mind, professors, engineers and architects have been trying to find different techniques in order to meet the demand of Sustainable Architectures.

For the moment, heat pump is such a technology which could reduce Carbon Dioxide emissions, and also the energy consumption of heat pump is much less than other heating or cooling systems. Heat pumps offer the most energy-efficient and sustainable way to provide heating and cooling in many applications, as they can use renewable heat sources in our surroundings, which absorb heat from low-grade heat source to high-grade heat sink. Even at temperatures we consider to be cold, such as Norway, air, ground and water still contain useful heat that's continuously replenished by the sun.

Certainly, heating and cooling demand for a building, especially sustainable building, is quite important, but some people design buildings that protect the global warming and reduce energy cost without compromising comfortable indoor climate. Actually, buildings need to be designed for meeting basic human needs for health, well-being and comfort with maximum heating and cooling demand, and then there must be specific focus on energy efficiency and sustainable energy sources.

This paper aims to analyse the environment and economy factor of heat pumps, and then telling people thermal comfort is the preference factor that we should consider when choosing a heat pump system. At last, summarise the notice for electing and utilising heat pump systems for sustainable houses, which should meet the demand of building residents. Synthetically, The U.S. Environmental Protection Agency (EPA) has concluded that GSHP systems (ground source heat pump systems ) are the most energy efficient and environmentally clean of all the heating and cooling options for sustainable houses.

2. Heat pump

2.1. Principle

A heat pump is an electrical device that extracts heat from one place and transmits it to another, which could change low-grade energy such as coal, gas and oil into high-grade energy which can be used more efficient. By applying a little more energy, a heat pump can raise the temperature of this heat energy to the level needed. In common, heat will move from a higher temperature to a lower temperature. Therefore, a heat transfer coil must keep a low temperature compared to its surrounding in order to take away the heat from the surrounding, hence cooling the space. To transfer heat to the surrounding, the heat transfer device is heated up to a higher temperature than the surrounding.

Heat pumps transfer heat by circulating a substance called a refrigerant through a cycle of evaporation and condensation (see Figure 1). A compressor pumps the refrigerant between two heat exchanger coils. In one coil, the refrigerant is evaporated at a low pressure and absorbs heat from its surroundings. The refrigerant is then compressed en route to the other coil, where it condenses at high pressure. At this point, it releases the heat it absorbed earlier in the cycle. The heat pump cycle is fully reversible, and heat pumps can provide year-round climate control for your home heating in winter and cooling and dehumidifying in summer. This is to say, this system is designed to be used for cooling or heating the space. The heat exchanger coils at the indoor and outdoor become evaporator or condenser depending on the mode of operation. If it is operating in cooling style, the indoor coil behaves as evaporator and outdoor as condenser. In heating mode, the indoor coil becomes a condenser and the outdoor coil change into evaporator. This is completed without moving the coils around by using a reversing valve called four-way valve which reverses the flow of the refrigerant.

Figure 1: Basic Heat Pump Cycle [3]

There are different types of heat pumps, which are air source heat pumps (ASHPs), air water heat pumps (AWHPs) and ground source heat pumps (GSHPs).

Figure 2a : Components of an Air-source Heat Pump (Heating Cycle)[3]

Figure 2b: Components of an Air-source Heat Pump (Cooling Cycle)[3]

Figure 2a and Figure 2b show the heating cycle and cooling cycle of ASHP, respectively. During the heating cycle, heat is taken from outdoor air and "pumped" into indoors. To begin with, the liquid refrigerant passes through the expansion device, changing to a low-pressure liquid and vapour mixture. It then goes to the outdoor coil, which acts as the evaporator, while the liquid refrigerant absorbs heat from the outdoor air and boils, becoming a low-temperature vapour. Next, this vapour passes through the reversing valve the compressor. The vapour is then compressed, reducing its volume and causing it to heat up. Finally, the reversing valve sends the gas, which is now hot, to the condenser. The heat from the hot gas is transferred to the indoor air, and this causes the refrigerant to condense into a liquid. After that, this liquid returns to the expansion device and the cycle is repeated. For t he cooling cycle, during the summer. The unit takes heat out of the inside air and rejects it outdoor. As in the heating cycle, the liquid refrigerant moves across the expansion device, changing to a low-pressure liquid/vapour mixture. It then goes to the evaporator. The liquid refrigerant absorbs heat from the indoor air and boils, becoming a low-temperature vapour, and then this vapour comes to the compressor. The vapour is then compressed, reducing its volume and causing it to heat up. Finally, the gas, which is now hot, traverses the reversing valve to the outdoor coil, which acts as the condenser. The heat from the hot gas is transferred to the outdoor air, causing the refrigerant to condense into a liquid. This liquid returns to the expansion device, and the cycle is repeated[3].

Figure 3: Components of a Typical Ground-Source Heat Pump[3]

From the figure 3, we can see that unlike the air-source heat pump, where one heat exchanger is located outside, the entire ground source heat pump unit is located interior of the house. The outdoor piping system can be either an open system or closed loop. In the heating cycle, the ground water, the antifreeze mixture or the refrigerant is brought back to the heat pump unit inside the house. At first, the heat is transferred to the refrigerant, which boils to become a low-temperature vapour. Next, The reversing valve guides the refrigerant vapour to the compressor. The vapour is then compressed, which its volume is decreased and causes it to heat up again. Finally, the reversing valve directs the now-hot air to the condenser coil, where it gives up its heat to the air that is blowing across the coil and through the duct system to heat the home. Because of having given up its heat, the refrigerant goes through the expansion device, where its temperature and pressure are dropped further before it returns to the first heat exchanger to begin the cycle again. The cooling cycle is basically the antithesis of the heating cycle. The direction of the refrigerant flow is transformed by the reversing valve. The refrigerant picks up heat from the house air and transfers it directly. The heat is then pumped outside, into a water body or return well or into the underground piping[3].

2.2. Coefficient of performance

Heat pump systems are energy-saving and environmentally friendly devices for heating and cooling of buildings, which can be revealed by the coefficient of performance. When the heat pump is used for heating, the COP is the ratio of heat supplied to energy used. In cooling mode, the COP is the ratio of heat removed from the building to energy used. The performance of a heat pump depends on the performance of the compression-expansion cycle and on the performance of the heat exchangers.[4]

The equation is:

Where

is the change in heat at the heat reservoir of interest, and

is the work consumed by the heat pump.

For example, A geothermal heat pump operating at COPheating 3.5 provides 3.5 units of heat for each unit of energy consumed (i.e. 1 kWh consumed would provide 3.5 kWh of output heat). The COP of heat pumps (300%-350% efficient) make them much more efficient than high-efficiency gas-burning furnaces (90-99% efficient), and electric heating (100%). So compare to other heating systems, heat pump is much more energy efficient and harmless for environment, since it releases much less CO2 emission to the earth than others.

3. Environmental protection systems

Climate change stabilization requires an unparalleled effort to change our current method to energy production and consumption. As energy prices are sky rocketing every year, we need to invest in energy saving technologies where we keep the environment green and also save energy at the same time. Undoubtedly, heat pump technology is a better choice than traditional wooden furnace or electrical heating systems in contemporary society. It is gradually rising with the appearances of the global energy crisis and the increasing environmental problems. Concretely, heat pumps do not require a flue, and since there is no on-site combustion, there's less chance of fire, and no chance of carbon monoxide infiltrating the home. Further, compare other heating or cooling systems (e.g. air conditioning), heat pump produce little noise to outdoor environment.

The global CO2 emissions that amounted to 22 billion tonnes in 1997, heating in building causes 30% and industrial activities cause 35%. The potential CO2 emissions reduction with heat pumps is calculated as follows:

6.6 billion tonnes CO2 come from heating buildings (30% of total emissions).

1.0 billion tonnes can be saved by residential and commercial heat pumps, assuming that they can provide 30% of the heating for buildings, with an emission reduction of 50%.

A minimum of 0.2 billion tonnes can be saved by industrial heat pumps (estimation based on a study by Annex 21).

Along with reducing CO2 emissions, the ozone layer demolition will be deeply decreased. Therefore, heat pump systems can be regarded as environmental protection systems.

If it is further considered that heat pumps can meet space heating, hot water heating, and cooling needs in all types of buildings, as well as many industrial heating requirements, it is clear that heat pumps have a large and worldwide potential. Other advantages of GSHPs' include the fact that all components of the unit are housed inside the building, thereby reducing the wear and tear on the unit by Mother Nature, and also eliminating the fear of vandalism or theft.

4. Heat pump for sustainable houses

4.1. Definition of sustainable houses

A sustainable house is defined in this guide as: "A house with a substantially better performance in the field of energy use and the use of renewable materials than one built to the standard building requirements". Sustainable housing means sustainable living; sustainability is not just low energy. It also means that people are happy to live where they live and that they live in a healthy environment[2].

Figure 4: Three main aspects of sustainable housing[2]

Figure 4 shows that Not only require sustainable housing should harmonize with the outdoor environment, but also provide a healthy and pleasant living environment. Therefore, choosing a suitable heating and cooling system is very important for a sustainable building in terms of outdoor climate and indoor climate.

4.2. Heat pumps according to climate

When choosing a heat pump it is necessary to bear in mind the point that the climate characteristics of the place where it will be installed. Generally, ASHPs can be fitted outside a house and perform better at slightly warmer air temperatures, which is limited in Norway. According to the climate in Norway (The temperature can even get to minus 20 Celsius, Ecotect); GSHPs should be a better choice for sustainable buildings than ASHPs. That is because ASHPs lose their efficiency as external temperatures fall below 5 degrees Celsius. Firstly, when the outdoor surrounding temperature falls, the heating capacity of the set also goes down quickly. But the heat load of the building rises quickly with the dropping outdoor temperature. When the outdoor ambient temperature reaches a really low point, the heating capacity of the set will no longer to meet the heating demand in such regions in winter. Secondly, with the falling outdoor temperature, the pressure ratio of the compressor will increase and cause the exhaust temperature to rise continuously and the COP of the set falls sharply. In this case the compressor will automatically stop to prevent overheat. In additional, the lubricating oil will generally become viscous with the dropping temperature, making it difficult for the system to return oil and leading to poor lubrication. Lastly, the low temperature will boost the time needed for defrosting. Therefore, special methods shall be considered for drainage of water generated in defrosting. Otherwise this water may be iced at the bottom of the outdoor evaporator and, in worst cases, stall and damage the fan. This will consume much more energy, which is adverse to the principle of environment friendly.

However, GSHPs do not have above problems, since they use ground water of which the temperature usually stays within a narrow range year-round. Further, the energy saving attained by using GSHPs instead of ASHPs depends on the extent to which the ground is warmer than the air during the heating season and colder than air or cooling water during the cooling season.

4.3. Heat pumps for indoor climate

In summary, the indoor environment can be described by the following so-called environmental factors or (external) stressors:

Indoor air quality: an umbrella term comprising odour, indoor air pollution, fresh air supply, etc.

Thermal comfort: dependent on moisture (humidity), air velocity, temperature, etc.

Acoustical quality: noise from outside, indoors, vibrations, etc.

Visual or lighting quality: view, illuminance, luminance ratios, reflection, etc.[5]

Indoor thermal environment, especially for heating or cooling demand, is important as it affects the health and productivity of building inhabitants. Yet there is a great potential for improving the indoor climate in the world's buildings: in the EU today, we spend 90 % of our time inside. But up to 30% of the buildings neither contribute to nor provide a healthy indoor climate. "There is no point in saving energy, if it jeopardises the indoor climate" says Bjarne Olesen, head of International Centre for Indoor Environment and Energy at the Technical University of Denmark (DTU). The discussion on energy often leaves out the fact that energy consumption in buildings is used to meet human needs; to keep us warm and pleasant in winter, cool and shaded in summer, serviced with electrical equipment and happy and healthy all year round[5]. For example, in Norway, if we just consider economy cost of heat pump but not thermal comfort of the house, and then choosing a cheaper system, at last, it cannot meet the heating demand of the house in winter and it does not make any sense to use heat pump system as heating system without considering the thermal comfort of the house. Definitely, we need to notice the initial investment of system, but as least, it should meet the basic demand of human beings. In principle, the thermal influences of GSHPs and AWHPs are more or less the same, hence Norwegian utilise both of them, but refer to the energy cost and economy factor, GSHPs are much wider application compare with AWHPs.

5. Energy cost and economy factor

The higher the COP , the more efficient the heat pump, but this does not always mean they are less expensive to operate. Usually, in Norway, there are two types of heat pump of which thermal comfort effects are the same can be chosen by occupants. One is GSHP and the other one is AWHP, but the investment for both of them is totally different.

Premises - Boundary Conditions

While a heat pump can be sized to provide most of the heat required by a house, this is not generally a good idea. In Norway, heating loads are larger than cooling loads. If the heat pump is sized to match up the heating load, it will be too large for the cooling necessity, and will operate only intermittently in the cooling mode. This may reduce performance and the unit's ability to provide dehumidification in the summer. Generally, the design of heat pumps will according to the cooling load, when it comes to the winter (under the peak load), electricity heating or wooden furnace could replace the heat pump as a supplemental heating system. So this calculation contains the heat pump and peak load system.

Capital costs[6]

Additional investment costs, I0

Possible subsidies from the government, energy utility etc.

VAT or investment fee (possible exemption)

Real interest rate, r

Technical/economic lifetime (depreciation time), n

Real Interest Rate, r

House owners - the real interest rate equals the bank interest adjusted for:

Annual average rice in prices

Annuity Factor, a

a= r/ [1-(1+r)-n]

The annuity factor is calculated by using the interest rate (r) and the lifetime (n) for an investment (annuity loan)

Is being used to find the annual costs for an investment I0, i.e.

(I0∙a) = annual payment for instalments + interest costs

The annuity factor can be calculated directly or found in tables

AC= I0∙a +∑ (WE ∙e) + ∑MC

The annual costs for the heat pump system and the competing heating/cooling system are the sum of the annual capital costs, operating costs and maintenance costs.

AC Annual costs [NOK/year]

I0 Additional investment costs [NOK]

a Annuity factor a = f(r, n)

WE Energy demand(s) [kWh/year]

e Energy price(s) [NOK/kWh]

MC Maintenance costs [NOK/year]

Calculation of annual costs for GSHP and AWHP systems for a Norwegian residence-annual heating demand, Q= 20,000 kWh/y (space heating, DHW)

Electricity price e= 0.8 NOK/kWh Interest rate 5%

Energy coverage factor

Average COP

Total cost(NOK incl. VAT)

Average life time(y)

GSHP

90%

3.8

200,000

20

AWHP

70%

2.5

120,000

15

Table 1: parameters of GSHP and AWHP

GSHP

Investment costs (I0) = 200,000NOK (everything incl.)

Annuity factor (from attachment), n=20 and r=5% a=0.0802

Capital costs= (200,000Ã-0.0802) NOK/y=16,040NOK/y

Maintenance costs= (200,000Ã-0.03) NOK/y=6,000NOK/y

Heat supply from heat pump and peak load

Q gshp = (20,000Ã-0.90)kWh/y=18,000kWh/y

Q pl= (20,000Ã-0.1)kWh/y=2,000kWh/y

Energy consumption for heat pump and peak load

W gshp = (18,000/3.8)kWh/y=4,737kWh/y

W pl=2,000kWh/y (efficiency=approx. 1)

W tot= W gshp+ W pl= (4,737+2,000) kWh/y=6,737kWh/y

Operating costs= (6,737Ã-0.8)NOK/y=5,390NOK/y

Annual costs= (16,040+6,000+5,390) NOK/y=27,430NOK/y

AWHP

Investment costs (I0) = 120,000NOK (everything incl.)

Annuity factor (from attachment), n=15 and r=5% a=0.0963

Capital costs= (120,000Ã-0.0963) NOK/y=11,556NOK/y

Maintenance costs= (120,000Ã-0.03) NOK/y=3,600NOK/y

Heat supply from heat pump and peak load

Q gshp = (20,000Ã-0.70)kWh/y=14,000kWh/y

Q pl= (20,000Ã-0.30)kWh/y=6,000kWh/y

Energy consumption for heat pump and peak load

W gshp = (14,000/2.5)kWh/y=5,600kWh/y

W pl=6,000kWh/y (efficiency=approx. 1)

W tot= W gshp+ W pl= (5,600+6,000) kWh/y=11,600kWh/y

Operating costs= (11,600Ã-0.8)NOK/y=9,280krNOK/y

Annual costs= (11,556+3,600+9,280) NOK/y=24,436NOK/y

Base on above calculation, Annual costs for GSHP is 27,430NOK/y, when annual costs for AWHP are 24,436NOK/y. It seems that AWHP is much cheaper choice for residents and the GSHP which is the most expensive system, but it saves more energy than a AWHP since both the average COP during the year and the energy coverage factor are higher. In addition, it has a longer lifetime. GSHP does also carry the Environmental Protection Agency's Energy Star Label, which is used to designate energy-efficient equipment. Often homeowners may find tax benefits, lower mortgages, or utility rebates. The U.S. Department of Energy (USDOE) estimated that over two-thirds of the nation's electrical energy and greater than 40% of natural gas consumption is used inside buildings. In residential and commercial buildings, space heating and cooling and water heating consume greater than 40% of electrical power. The U.S. Environmental Protection Agency (USEPA) estimated that GSHPs can reduce energy consumption by up to 44% compared to ASHPs and up to 72% compared to conventional electrical heating and air conditioning. (USGAO, 1994) [7].

6. Drawbacks of GSHPs

GSHPs can have positive and negative environmental effects. The USDOE and

USEPA have encouraged the use of these heat pumps because of their energy efficiency, as discussed above. Increased energy efficiency for such a major use of energy will reduce the amount of fossil fuels burned, greenhouse gases such as carbon dioxide (CO2) generated, and other air pollutants (NOx and SO2 ) emitted (USEPA, 1997). However, there are unavoidable harmful influences for the environment.

Firstly, A potential negative effect of all ground source heat pumps is the release of antifreeze solutions to the environment. Antifreeze solutions are required in colder climates to prevent the circulating fluid from freezing. Antifreeze chemicals include methanol, ethanol, potassium acetate, propylene glycol, calcium magnesium acetate (CMA), and urea. These chemicals are generally mixed with water when used as a heat exchange fluid. These chemicals can be released to the environment via spills or corrosion of system components[7].

Secondly, Geothermal heat pumps with vertical boreholes may pose environmental threats. If these boreholes are not properly grouted or the grout fails, groundwater could be contaminated by surface water infiltration, inter aquifer flow, or antifreeze leakage. These boreholes are usually grouted with

bentonite, neat cement, or a mixture of these materials[7].

Finally, the thermal energy of the ground will gradually reduce year by year after using GSHPs. In summer, heat taken from the building is added to the ground by the heat pump ,providing additional heat for extraction by the heat pump in winter. During these two processes, there must be some heat losses, which makes the heating and cooling circulation lose balance. So special provisions will be needed in some climates to make sure that winter heat withdrawals and summer heat additions balance over the long term so that the performance of the heat pump dose not degrade over time[4].

7. Choosing and using a heat pump

When we choose a heat pump system for a house, we should always remember that a sustainable indoor environment that guarantees a high basic level of health and comfort, and then consider about energy cost and investment of the system.

7.1. Size and climate matter

The climate where you live has a big impact on what size and type of heat pump is appropriate for your home. As the temperature drops, so does the performance of heat pumps decrease. Therefore, getting the right sized heat pump for the job is extremely important. One that's too small or too large will use more energy, may not perform properly and may have a shorter life. Home's heating needs depend on a number of things, including the size of the area you want to heat, how well insulated your home is, how big your windows are and, of course, your climate. Particularly, Good insulation significantly reduces the rate of heat loss in a house. It makes a home easier and cheaper to heat properly, and healthier and more comfortable to live in. Even well-insulated houses can be hard to heat if draught constantly cold air instead of hot air, so draught-stopping is also very important. Good building airtightness, and controllable ventilation provisions could let you manage air replacement and avoid unnecessary heat loss.

7.2. Regular maintenance

As with other systems, heat pumps should receive periodic maintenance to ensure dependable, economic operation and long life. The maintenance schedule for heat pumps is similar to a central air conditioner. Inspect your system regularly. Keep filters, air passages and outdoor coils clear of obstructions, such as dirt, leaves, shrubbery and trash. This is because, on the one hand, dirty filters or coils will cause the decline of airflow in the system. The fall of airflow will make the compressor work harder and hence more electricity is consumed. On the other hand, Clean the outdoor coils whenever they are stuck with leaves or other materials. Ensure that there is nothing nearby that prevent the heat transfer to the surroundings.

Once you have chosen your heat pump, you can maximise its comfort and energy efficiency benefits by using it wisely. If your system is left running all day and night, or set to unnecessarily high heating (or low cooling temperatures), you could see your electricity bill increase significantly.

8. Conclusion

8.1. Conclusion

While it may seem that geothermal heat pumps are nothing except costly, there are many benefits to the system. Each pump conserves significantly more energy than a common heating system. They can also heat water for the house (lowering or possibly eliminating the need for a hot water heater) and are more efficient. They are also quieter. This paper shows that GSHPs are the best choice for sustainable houses in Norway no matter from perspective of climate, environment or energy cost.

(1) GSHPs are environmentally friendly:

Conserve natural resources by providing climate control efficiently and thus lowering emissions

Minimize ozone layer destruction by using factory-sealed refrigeration systems, which will seldom or never have to be recharged

(2) GSHPs could provide significant indoor environment for sustainable constructions.

(3) It seems that initial investment of a heat pump system is quite important for us, when we decide our heating and cooling systems. But the energy efficiency of a system is much more significant than the original investment; base on this, people could benefit much more from GSHPs than AWHPs.

Besides, choosing an excellent heat pump not equal having an energy efficient system forever, the daily maintenance of the system is also important.

8.2. Future prospect

The Department for Energy and Climate Change (DECC) have announced that the Renewable Heat Incentive (RHI) is expected to be launched in June 2011. It is designed to provide financial support to encourage the uptake of renewable and low carbon heat technologies like heat pump[10]. It is challenge for professors, engineers and architects to imrove the energy efficiency of heat pumps and additional research is needed on the topic of compatibility of antifreeze solutions and bentonite grouts, which is also necessary for sustainable architectures.

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