ASHP 24

Running Head: ASHPExperimental Analysis of Air Source Heat Pumps

[Name of the writer]

[Name of the Institution]

Experimental Analysis of Air Source Heat PumpsBackground of the StudyBuildings significantly contribute to overall energy use and electricity consumption. The use of energy by the building sector continues to rise mainly due to rapid construction of new buildings. The heat pumps that are mainly used for domestic purposes make use of vapour compression cycle. They take up low grade heat from the atmosphere and discharge it at comparatively high temperature inside the house. These domestic heating pumps are capable of providing extremely efficient system for supplying water and space heating. Moreover these pumps are capable enough of supplying thrice as much as energy in the form of heat when compared to the energy they require in the form of electricity for functioning. The designed and technology behind air-source heat pumps have significantly enhanced over the past time.In United Kingdom the market of air-source heat pumps is relatively larger than ground source heat pumps. The primary factor that contributes to high demands of air-source heat pump is the fact that these pumps can be easily installed. Additionally these pumps are seen as a direct replacement for the gas boilers that are the main heat source for around 80% of the housings in UK.These pumps have potential to minimize resource intensity, emissions and costs of UK heating. Despite of the fact that ground source heat pumps have higher efficiency then air source pumps but due to their high installation cost and limited applicability they are not used much commonly now. However the government of UK encouraged the installation of air-source heating pumps by presenting various grants headed for its installation costs. Nevertheless as the air-source pumps generate fewer emissions, they are rapidly attracting peoples interest. As stated in the start of the discussion that buildings are the high consumers of energy and electricity therefore in order to minimize the energy consumption associated with buildings and attain net-zero energy target, several smart strategies can be employed. Next to the strategy of decreasing energy consumptions of buildings, another approach is to focus on energy efficiency. One strategy that can lower the energy consumption of buildings is the use of mechanical equipments. Some of the most energy efficient mechanical equipments are ground and air source heat pumps. These heating pumps can be used for space heating and cooling. This thesis will primarily focus on air-source heating pumps. It will highlight the benefits of the pump and its working mechanism. Nowadays air-source heat pumps are widely used in housing associations for the purpose of heating as well as cooling mainly because there installation cost is less than that of ground source heating pumps. Air-source heat pump systems employ ambient air as the medium of providing heat in winter. They pump heat inside the house with the help of refrigerant filled coils. At heating mode; the liquid refrigerant absorbs heat via an outdoor evaporator and converts it into vapour. The compressor then compresses the vapour, generating high temperature and pressure gas. The gas is then discharged into the condenser where usually a fan blows indoor air over the coils to deliver hot air to the zone while condensing the refrigerant. In cooling mode the cycle is reversed, the heat inside the building is discharged into the ambient air by following the same mechanism. The greatest drawback of this system is that in colder climates its heating output and coefficient of performance decreases. Hence to retain its heating and coefficient of performance most of air-source heat pumps are coupled along with the auxiliary heating source (Bertsch,Groll, 2008,p.1282-1292). Warming prerequisites in atmospheres like United Kingdome give a test to the air source heat pump since the outside temperature in Canada can reach beyond -25oC.Therefore due to extreme cold climates; larger sized heating pumps are usually employed in the buildings to meet the heating demands of the households. Due to the requirements of such huge heat pump, the compressor will frequently work at part load to take care of the building demand at milder winter temperatures. This causes a decrease in proficiency and solace because of the requirement of heat pump cycling. Manifold and modulating compressors are often used to deal with the mismatched heating loads by the means of altering the capacity of the compressor to withstand the heating loads at maximum capacity. Yet the setback of reduced efficiency of the heating cycle still persist (Roth et.al, 2009, p.69-72).The effectiveness and consistency of the operation can be improved by using potential air-source heat pumps that offer variable speeds. The efficiency and consistency of operation is enhanced because of the cut back of cyclic working time resulting in enhanced performance at lower working speeds (Erbs et.al, 1986,p.696-705).In England since 2011, the air-source heat pumps acquired for domestic purposes are classified as permitted development subjected to certain terms and conditions. The criterions needed to be fulfilled include: there must be no wind turbine present at the location where the air-source heat pump is required to install plus the size of external unit must be less that 0.6m3.In addition the size of installing unit should be greater that 1m from the edge of customers residency. It is essential to see that air-source heat pump is not installed closer to the edge of a flat roof or on a pitched roof.Introduction to Air Source Heat Pump (ASHP)An ASHP is basically a space-conditioning domestic device that is used for the purpose of residential heating and cooling. Some models of air-source heat-pumps can offer both purpose simultaneously. On the other hand models exist that can only offer one purpose or the other. In the heating mode, the heat pump utilizes electricity to extract heat from the ambient air and discharge it in homes space (Kerr Jr. et al. 1934). Although it sounds contradictory that in the winter season atmosphere can provide heat but it is true that outside air do consist of heat despite of cold season. An air-source heat pump system makes use of refrigeration cycle to step-up the heat to a temperature that is appropriate for space heating. On the other hand in the cooling mode the heat pump operates in a similar manner to a refrigerator. It removes heat from the house interior and discharges it in the atmosphere.Mechanism followed by Air-Source Heat Pump System The Air source heat pump system transfer heat in the similar manner to the refrigeration cycle. Heat is transferred by the means of refrigerant or the fluid that circulates through the refrigeration cycle. In the heating mode the refrigerant gains heat from the ambient air as it circulates across the heat exchanger coils that are located in exterior unit. The set of these coils is known as evaporator for the reason that they are responsible of converting heat absorbed from outside into vapour. The vapour then travels towards the compressor .The compressor make use of electricity to increase the temperature of the refrigerant vapour until appropriate space heating temperature is attained. The refrigerant vapour then travels towards the indoor heat exchanger that is commonly known as condenser. The function of the condenser is to reduce the vapour temperature so that it converts back to liquid. In an air-to air ducted heat pump, a fan is employed to circulate the heat across the entire space of the room. At last the liquid refrigerant goes into the expansion valve which decreases its pressure and repeats the cycle again. The air source-heat pumps use electricity to supply power to the fans, compressor and pumps. Since these types of heating pumps are not using electricity to generate heat, instead they use it to run refrigeration cycle for transferring heat; their efficiencies go beyond 100 percent. On the whole this implies that residents of a house are getting more heat energy than utilized by heat pump through electricity to run the system. The amount of current drawn by air-source heat pump system depends upon the type model. However they utilize the same amount of current as the refrigerators do. Smaller models of ASHP utilize a minimum of 10amperes of current; whereas the larger models may draw up to 20amperes of current.

An ASHP refrigeration cycle in heating mode

Source:RetScreen,2012Factors Affecting the Performance of Air-Source Heat PumpsThere are number of factors that affects the performance of air-source heat.Some of the potential factors that might affect the heating or cooling performance of air-source heat pumps include the behaviours and attitudes of its consumers, the quality of installation, the control system. In addition, how well a building that incorporates air-source heat pumps is insulated also affects the overall performance of the pump. The relative temperature of the heating source and flow temperature plus the sizing of heat pump in relation to heating demands greatly influence the performance of heat pumps.Types of Air-Source Heat PumpsThe central heat pumps can be utilized either for heating or for cooling purpose. Majority of the air-source heat pumps are split system. This means that the coils are present on both exterior sides as well in the interior part of the system. The central fan is placed indoor and it is connected to supply and return tubes. This fan is commonly known as blower or air handler and its basic function is to circulate air across the entire vicinity of home. There are electric resistance coils also present inside the fans. The cooled or heated air is transferred through the fan to the supply tubes and consequently to openings in the house known as supply registers. The cooled air is then sent back through the return registers for reheating. Beside the split systems, some air source heat pumps are packaged systems also. In such system the fans as well as the ducts are located outdoors. However the cooled or heated air is supplied to the interior of the home through the ductworks that obtrudes through a roof or a wall. An advance form of packaged system is known as ductless room heat pump. This type of packaged system is more commonly used in hotel rooms or flats as compare to houses because the efficiency of these pumps of heating or cooling is limited to a room or small home. These heating pumps can be installed across a hole in the wall or in a windowpane. However the wall-wall installations are frequently not properly insulated resulting in infiltration problems. In order to resolve these issues mini split systems can be used.

For heating purpose

For cooling purpose

Selecting a Heat Pump

There are three major factors that should be considered while selecting air-source heat pumps. These factors include the components of the system, energy efficiency rating and the sizing.

Energy Efficiency RatingThe energy efficiency of the air-source heat pumps is rated by comparing the amount of British thermal units of heat it had supplied in one hour with the amount of power it had consumed in each watt-hour. Each of the domestic heat pumps is sold with an Energy Guide Label. This guide comprises of the main features of the heat pump such as its cooling efficiency performance rating and its sheathing. The efficiency of the electric resistance components and compressor of the system is rated in terms of Heating Seasonal Performance factor. It provides the number of British thermal unit supplied per watt-hour used. The heating seasonal performance factor ranges from 8-10 for most efficient air-source heat pumps. Furthermore the cooling efficiency of the ASHP is measured in terms of seasonal efficiency performance ratio. Generally speaking, higher the value of seasonal efficiency performance ratio, higher will be the cost of the heat pump. Nevertheless, high energy savings of ASHP can compensate the initial investment cost of the pump. In the era of 1970s the SEER of the traditional heat pumps used was 6.These heat pumps were replaced with the new advance ASHPs which have SEER of 12.This denotes that these new advance heat pumps will utilize half the amount of the energy to supply the same of heating or cooling. Thus reducing the cost air-conditioning or centralized heaters to half. The Seasonal energy efficiency ratio of the most efficient ASHP ranges from 14-18.The heat pumps with a Heating Seasonal Performance factor of minimum 7 and the seasonal efficiency performance ratio of minimum 12 are labelled with a Energy Star label. This label is sponsored by the Environmental protection agency of United States and the US Department of Energy. However most of the latest heat pumps surpass these values but searching for this label while selecting a heat pump is a smart way of purchasing it.Sizing

At the time of purchasing a heat pump it is very essential to consider its size. Usually large sized heat pumps are not appropriate for small homes. Out-sizing frequently results in unusual starts and stops which decreases systems efficiency. This also builds excess pressure on its components. On the other hand an appropriate sized ASHP provides better comfort and humidity control as compared to over outsized. The cooling and heating capacity of ASHPs is rated in terms of Btu per hour. The unit for representing cooling capacity is tons and each ton equals to 12000 Btu/hour. The procedure for correct sizing typically entails complex calculation which is mainly performed by an expert contractor. He incorporates the sizing methods that are widely accepted in the heat pump industry. On contrary there are certain contractors also that simply guess the size of Air-source heat pump required. However this is not an accurate way of determining the size of heat pump because the rule-of-thumb sizing methods are not acceptable in the heat pump industry. The value of the size determined from this technique is usually higher than the required. Therefore the contractors who incorporate this technique instead of doing complex calculations should not be employed. System ComponentsIt is significant to discuss the most suitable selections for heat pumps before purchasing them. This approach not only facilitates in making the homes comfort much better but it also improves the economy of the selected air source heat pump. The most significant factor that should be taken into account while selecting an ASHP is the size and design of the air-ducts and the materials they are made up of. In addition, the amount of space they require should also be considered. Meanwhile, the design of the home must also be checked for sufficient space for installing the coils and air handler. It is an essential approach to check the blueprints of the home before selecting the type of ducts because the contractors often complain that due to restricted space they have to squeeze the size of air-source heat pump system. As a result the ducts are constricted resulting in an inadequate air flow. Moreover most of ASHPs systems excluding packaged systems also require additional appropriate indoor duct for removing plenty of summer moisture.Performance Measurements System Efficiency

There are various methods to compute the efficiency of an air-source heat pump. Generally the efficiency is measured in terms of ratio between the quantity of cooling or heating supplied to a room and the amount of energy in form of electricity that is being consumed by the heat pump. The efficiency of heat pump is expressed as follows:Heat pump efficiency=

In general the efficiency of space heating domestic devices is less than 100percent.Indeed most of the combustion appliances such as boilers and furnaces cannot achieve an efficiency of 100percent.This is because a little amount of heat might unavoidably escape from the chimney. Typically the maximum seasonal efficiency that can be attained by the most efficient combustion furnace or boiler is 90%.On the other hand the efficiency of electric driven appliances such as electric base boards, boilers and furnaces is exactly 100% for the reason that the entire quantity of electricity consumed by the appliance is converted into heat. In fact the efficiency of air-source heat pumps is more than 100%.Such high efficiency is attained by the electric heat pumps because the amount of heat energy supplied to the buildings is greater than the amount of energy consumed by the heat pumps in order to function.

Heating efficiency of heat pumps is rated by using coefficient of performance (COP) and heat seasonal performance factor (HSPF).Heat Seasonal Performance Factor The heat seasonal performance factor is the ratio of the total amount of heat supplied i.e. rate of heat output of the ASHP to the total amount electrical energy consumed by the heat pump throughout the course of heating i.e. power drawn. The heat supplied is computed in terms of British thermal units (BTU) where as the electricity is computed in Watt-hours. The heating seasonal performance factor ranges from 8-10 for most efficient air-source heat pumps. However to qualify for the Energy Star label the HSPF should be minimum 7. HSPF=Coefficient of Performance (COP)The COP is similar to heat seasonal performance factor but employs different unit for computing electrical energy used by the ASHP. Moreover COP can be measured over short time intervals.COP is the ratio of total space heating delivered to the electrical energy required. In order to calculate the coefficient of performance, the electrical energy should first be converted into BTUs by employing appropriate conversional tool.

COP=

Seasonal Coefficient of Performance

Unlike the HSPF the COP can also be calculated over short time intervals, in other words an instantaneous value of COP can be calculated by employing units of BTU/hr for components. Calculating the COP values on daily basis can assist in determining the fluctuations in efficiency with respect to the outside temperature. The only difference between COP and HSPF is of units. Indeed the value of Seasonal COP can be derived from HSPF by dividing by the conversion factor between Watt-hours and BTU. Seasonal COP=By employing the above formula, Energy Star label criteria of minimum of 7 HSPF can be converted into a seasonal COP of 2.05.This value of COP corresponds to an efficiency of 205% over a heating season.

Energy Efficiency RatioThe cooling efficiency of air-source heat pumps is measured in terms of energy efficiency ratio (EER) or the seasonal efficiency ratio(SEER).The EER is the ratio of the heat removed in BTU/hr to the electrical energy consumed by the heat pumps to remove the respective quantity of heat.EER=In order to calculate EER, the heat removed is quantified in BTU/hr and electrical power consumed by the heat pump for the heat extraction is quantified in Watts. The seasonal energy efficiency ratio(SEER) is merely the EER calculated over the cooling season. It is denoted by the ratio of heat removed during the cooling season to the electrical power consumed by the heat pump in doing so. Therefore the units of SEER are same as that of HSPF. The Seasonal energy efficiency ratio of the most efficient ASHP ranges from 14-18.Advantages and Disadvantages of ASHPs

Like any other domestic heating appliances, the air-source heat pumps also exhibit certain advantages as well as disadvantages. The most significant advantage of ASHP is that they can be used for both space heating and cooling purposes. In fact in extremely cold climate there might be numerous situations that require cooling. For example a cold environment is desired in offices or in computer server room because most of the electronic equipments placed there produce a large amount of heat that can damage the components of the equipments. Therefore cooling appliances are necessary to maintain an adequate temperature within the computer server rooms. In such cases air source heat pumps present dual responsibilities. The commercial buildings and housing associations that require heating as well as cooling, the fact that a single domestic device can fulfil both the purposes is quite appealing for them because they have to pay installation and maintenance cost for only one device rather for two. Moreover there are some devices of air-source heat pumps that offer cooling and heating simultaneously. These devices move heat from a warmer area, possibly from a computer server room in an office building towards a colder zone in the same building. Moving on to the cost of operating an ASHP, it can be sometimes considered as an advantage as well as a disadvantage. The nature of operating cost totally depends on local cost of electricity and fuel. The air source-heat pumps do not generate heat energy by employing burning fuels instead they make use of electrical energy to transfer heat from one place to another. In fact they utilize less amount of electrical energy as compare to other electrical appliances such as electric baseboards and electric furnaces. However operating an ASHP in regions where the cost of electricity is high, can be prohibitively costly. Nevertheless the maintenance cost of an ASHP is comparatively lower than that of a heating combustion appliance.It is imperative that the filter on ASHPs should be cleaned after every 3months and a maintenance check-up should be performed on annual basis in order to check the connections and adjustments required in the control system.(Northwest Ductless Heat Pump Project, 2013).The form of electricity generation acquired by a specific region significantly contributes in assessing the environmental impact of ASHPs. The areas that generate electricity by the means of hydropower or other low emission sources result in air-source heat pumps with extremely low carbon emissions. In contrast, the regions that generate electricity by the means of fossil fuels result in ASHP with high carbon emissions. It is quite hard to determine the impact of greenhouse gas emissions on the environment relative to on-site fuel use and widely varies according to the region.Coming on to the disadvantages of air-source heat pumps, the major disadvantage of these pumps is that they become less efficient in supplying heat once the outdoor temperature drops to very low values.

The graph shows that how the value of coefficient of performance for a certain heat pump declines as the temperature outside drops.

Although some air-source heat pumps are specifically designed to effectively meet the heat energy requirements of cold climates, the buyers of ASHPs should have an insight about which model is appropriate for their design temperature. Since different models are rated for peak heat output at different temperatures. If the outdoor temperature declines beyond a certain limit, the ASHPs will not function. As a result some form of back up heating will be required. Majority of the cold regions of Alaska are not considered as a good candidate for ASHPs as the technology stands today. In addition, majority of ASHPs systems currently available in the market represent a decreasing rate of heat output on encountering lower outdoor temperatures. It is observed that at a certain cut-off temperature, the efficiency of ASHPs decreases below the standard value as a result the system shuts down. Hence, in colder regions it is recommended to keep a backup heating device that can be made available when the ASHP operates at relatively lower efficiencies or is incompetent to meet the heat load. Due to this additional backup heating system that is essential in colder climates, the overall cost also increases. Moreover the issue regarding how to implement a suitable control system to effectively manage the interface between the domestic devices also arises.Heat Pumps in Cold Climates Bertsch, et al put forward four main issues associated with the operation of air-source heat pumps at low ambient temperatures (Bertsch, 2005) .This issues are as follows:

In colder climates the ASHP delivers inadequate heat although it is required the most in colder climates If the discharge temperature of the compressor is very high at relatively low ambient temperature, it can result in the overheating of the compressor. Thus the manufactures have to switch off the compressor and rely on other electric resistance heat appliances.

In colder climates, the ASHPs posses low COP values.

An ASHP that is designed for lower temperature will not operate efficiently at higher temperatures as they cycle on and off.

In 2009 Roth et al address the issues associated with the operation of ASHPs at lower temperatures. They proposed advance technological methods that can be incorporated to maximize the efficiency of ASHP in colder climates (Roth et al. 2009,p.69-72). Some of the strategies are as follows: Sizing of the ASHP according to the heating load, not the cooling load maximizes its efficiency.

Employing multiple compressors or variable speed compressors also result in improved efficiency(Bertsch,et al,2008,p.1282-1292) Increasing the outdoor capacity is also considered as an effective strategy to enhance the efficiency.

Using carbon dioxide as the refrigerant is considered vital.

The optimization of outdoor as well as indoor supply and return ducts also helps in resolving the issues associated with ASHP

The use of mechanical liquid sub-cooling in the refrigerant cycle greatly improves the efficiency of ASHPs. Mechanical sub-cooling is basically the cooling of the refrigerant below its condensing temperature so that only liquid refrigerant enters the expansion device.Factors Affecting Heat PumpsWorking Cycles of Heat PumpsHeat pumps comprises of several cycles depending upon the working category, for example domestic purpose heat pumps mainly consist of vapour compression cycle. They work by following some properties of thermodynamics along with the use of more than one different chemical called refrigerant.

In figure above a simplest air-water heat pump has been shown. At low pressure in evaporator a refrigerant as a liquid-vapour mixture is entered. Different types of evaporators are used in different heat pumps operating in different working category. In air-water heat pump a coiled tube is used as an evaporator. The temperature of air is higher than temperature of refrigerant, so heat flows inside from external air, as a result the refrigerant is vaporized. The refrigerants temperature is raised when it is compressed at a higher pressure. Now the hot refrigerant is passed to other portion of heat exchanger called condenser. The water at lower temperature is transferred to other side. Space heating is done through this water. Now we have warmed water as well as cooled and condensed refrigerant. The condensed refrigerant after expansion to low temperature and pressure is given back to the evaporator. Vapour compression cycle has its roots in the standards improved by Carnot in the early nineteenth century. He improved an articulation for the greatest conceivable efficiency of a machine which, working in a cycle, processed work through the exchange of heat from a hot to a cold source. A heat pump is exactly the opposite of this; However Carnot hypothesis is just as appropriate in determining a statement for the greatest conceivable efficiency that might be accomplished by a heat pump.

Defrost cycleA significant variable unconventional to air source heat pump performance is freezing of the evaporator. The most practical necessity for the evaporator coil is to be some degrees beneath the outer air temperature. This implies that build-up has a tendency to structure on it (for sure water run-off is an imperative consideration when introducing an ASHP outdoor unit). Moreover the edges of coil of evaporator are almost below 0 degree, when temperature of air is lower than 5C. This decreases air flow through the coil as well as coefficient of heat transfer from surface of the coil. As a result the overall efficiency of the heat pump declines. This can completely block the air flow, if the frosting continues for longer time interval.

ASHP runs a process to heat the evaporator coil intermittently, dissolving the ice. This procedure is regarded as a defrost cycle. The most well-known system for defrosting is to switch the refrigerant stream, so the evaporator coil briefly turns into the condenser. A negative impact of this strategy is that the heat is evacuated from the building, and options are at times utilized, for example, a sensible heat defrosting technique or immediate electric heating. The recent design, instead of switching the cycle, includes neglecting condenser and set directly from compression to extension. This results in a great and intense super heated vapour which is utilized to raise the temperature of the coil in evaporator. The strategy abstains from uprooting heat from the building, rather all the heat is supplied by compressor.The frosting and its effects are directly proportional to the various environmental factors such as humidity, air pressure and temperature. In case of dry weather a sufficient amount can be drawn in order to reach the moist point. While for moist weather, there is no need to provide sufficient amount of heat for condensation. For instance, tests of a particular ASHP found that at an outside air temperature of 0c and a relative humidity of 85%, 18% of frosting decreased the output of heat to one-fifth after interval of 35mins.On the other hand at 65% humidity and keeping all the environmental factors unchanged, the same performance decrease just happens 145 minutes later (Guo et al, 2008,p.2267-2278). Performance of Air-source heat pumps in UK

A study was conducted to assess the impact of air-source heat pumps on Carbon dioxide emission conclude that new advance air-source heat pumps produced lower emissions as compare to conventional gas boilers as long as their seasonal performance factor is minimum 2(Cockroft ,Kelly, 2008,p.239-245).Kelly and Cockroft further conducted a study in this regard. They modelled the integrated performance of an air-source heat pumps installed in a UK building and estimated a seasonal performance factor of 2.7.Eventhough the incorporation of air-source heat pump resulted in a increase in energy costs by 10% ,there was a reduction in emissions by 12percent noted (Kelly,Cockroft, 2008, p.239-245).In order to confirm the results concluded by Kelly and Cockroft ,Jenkins et al modelled the integrated performance of air-source heat pumps with respect to heating and cooling of a larger office building. Their findings also pointed out that the use of air-source heat pumps had greatly reduced the emissions (Jenkins et al, 2008 p. 1901-1919).Additionally, a field trial of air-source heat pumps also provided significant advantages of using air-source heat pumps particularly to environment. The field trial of air-source heat pump involved retrofit installation and monitoring of 28 air-source heat pumps in homes across United Kingdom. The results of the test concluded that a seasonal factor of 2.2 was observed which signifies that an average performing air-source heat pump can reduce carbon dioxide emission up to 9% compared to gas boilers .In converse to Cockroft and Kelly predictions, the field trial of air-source heat pump indicated that an average cost of mid-performance pump is relatively better as compared to the estimated costs of gas boiler. The findings and discussions of the test highlighted that an air-source heat pump possessing average efficiency is capable of compensating its installation cost in 31 years as compare to gas boiler that are expected to take longer pay back period. The chief reason for these varying results can be the fact that majority of air-source heat pumps employed in the field test were supplied with electricity at off-peak rate. On the other hand the study of Cockroft and Kelly presume a standard rate of electricityChapter SummaryThis chapter has discussed the relevant literature related to the topic. The literature review was designed to develop an understanding about the Air-source heat pumps. Initially the chapter had outlined the background on the study. It presented comprehensive information about the air-source heat pumps and discussed its mechanism in detail. The chapter also highlighted some of the key factors that should be considered while selecting a ASHPs. In addition, the literature review on air-source heat pumps also presented various methods that can employed to rate the efficiency of ASHPs. Some of the major advantages and disadvantages of ASHPs were also discussed in the chapter following by the issues associated with the operation of ASHPs in colder climates and the appropriate recommendation to address the issues.ReferencesArif, H. and K. Yildiz, (2009). A review of heat pump water heating system. Renew. Sustain. Energ. Rev.,13(6-7): 1211-1229.

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Erbs, D., Bullock, C., & Voorhis, R. (1986). New Testing and Rating Procedures for Seasonal Performance of Heat Pumps with Variable Speed Compressors. ASHRAE Transactions , 696-705

Guo, X., Chen, Y., Wang, W. and Chen, C.(2008). Experimental study on frost growth and dynamic performance of air source heat pump systems. Applied Thermal Engineering 28 (2008) 2267-2278

Guoyuan, M., Qinhu, C., & Yi, J. (2003). Experimental Investigation of Air-Source Heat Pump For Cold Regions. International Journal of Refrigeration , 12-18.

Jenkins, D., Tucker, R., Ahadzi, M. and Rawlings, R.( 2008). The performance of air-source heat pumps in current and future offices. Energy and Buildings 40 pp. 1901-19

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Kelly, N.J. and Cockroft, J. (2011) Analysis of retrofit air source heat pump performance : results from detailed simulations and comparison to field trial data. Energy and Buildings, 43 (1). pp. 239-245. ISSN 0378-7788Kerr Jr., C., Stotz, E., and Stotz, J. K. (1934). Reversible-Cycle Heating and Cooling Systems,United States Patent Office, Patent number: 1942295

Miller, W.A. (1982). Laboratory Evaluation of the Heating Capacity and Efficiency of a High-Efficiency Air-to-Air Heat Pump with Emphasis on Frosting/Defrosting OperationRETScreen. (2012). Heat Pump.

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Roth, K., Dieckmann, J., & Brodrick, J. (2009). Heat pumps for cold climates. ASHRAE Journal , 69-72.

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Tudor, V., Ohadi, M., Salchi, M.A., and Lawler, J.V.(2005). Advances in control of frost on evaporator coils with an applied electric field, International Journal of Heat and Mass Transfer 48p. 44284434