Energy of air

Air at any temperature above absolute zero contains some energy. An air-source heat pump transfers (‘pumps’) some of this energy as heat from one place to another, for example between the outside and inside of a building. This can provide space heating and/or hot water. A single system can be designed to transfer heat in either direction, to heat or cool the interior of the building in winter and summer respectively. For simplicity, the description below focuses on use for interior heating.

The technology is similar to a refrigerator or freezer or air conditioning unit: the different effect is due to the physical location of the different system components. Just as the pipes on the back of a refrigerator become warm as the interior cools, so an ASHP warms the inside of a building whilst cooling the outside air.

The main components of an air-source heat pump are:

• An outdoor heat exchanger coil, which extracts heat from ambient air
• An indoor heat exchanger coil, which transfers the heat into hot air ducts, an indoor heating system such as water-filled radiators or underfloor circuits and/or a domestic hot water tank

Air source heat pumps can provide fairly low cost space heating. A high efficiency heat pump can provide up to four times as much heat as an electric heater using the same energy.

A “standard” domestic air source heat pump can extract useful heat down to about −15 °C (5 °F).

An air source heat pump designed specifically for very cold climates can extract useful heat from ambient air as cold as -20 °F or even -25 °F (-30 °C). Manufacturers include Mitsubishi and Fujitsu.  Coefficient of performance (COP) drops to 0.9, indicating that resistance heating would be more efficient at that temperature. At -30 °C, the COP is 1.1, according to the manufacturer’s data, ground source heat pumps, air source heat pumps have lower initial costs and may be the most economic or practical choice.  Natural Resources Canada found that cold climate air source heat pumps (CC-ASHPs) do work in Canadian winters, based on testing in Ottawa, Ontario in late December 2012 to early January 2013 using a ducted CC-ASHP. (The report does not explicitly state whether backup heat sources should be considered for temperatures below -30 °C. The record low for Ottawa is -36 °C.) The CC-ASHP provided 60% energy (though not energy cost) savings compared to natural gas, Alberta, Nova Scotia, and the Northwest Territories) where coal-fired generation was the predominant method of electricity generation. (The energy savings in Saskatchewan were marginal. Other provinces use primarily hydroelectric and/or nuclear generation.) Despite the significant energy savings relative to gas in provinces not relying primarily on coal, the higher cost of electricity relative to natural gas (using 2012 retail prices in Ottawa, Ontario) made natural gas the less expensive energy source. (The report did not calculate the cost of operation in the province of Quebec, which has lower electricity rates, nor did it show the impact of time of use electricity rates.) The study found that in Ottawa a CC-ASHP cost 124% more to operate than the natural gas system. However, in areas where natural gas is not available to homeowners, 59% energy cost savings can be realized relative to heating with fuel oil. The report noted that about 1 million residences in Canada (8%) are still heated with fuel oil. The report shows 54% energy cost savings for CC-ASHPs relative to electric baseboard resistance heating. Based on these savings, the report showed a five-year payback for converting from either fuel oil or electric baseboard resistance heating to a CC-ASHP. (The report did not specify whether that calculation considered the possible need for an electrical service upgrade in the case of converting from fuel oil. Presumably no electrical service upgrade would be needed if converting from electric resistance heat.) The report did note greater fluctuations in room temperature with the heat pump due to its defrost cycles.

Heating and cooling is accomplished by pumping a refrigerant through the heat pump’s indoor and outdoor coils. Like in a refrigerator, a compressor, condenser, expansion valve and evaporator are used to change states of the refrigerant between colder liquid and hotter gasstates.

When the liquid refrigerant at a low temperature and low pressure passes through the outdoor heat exchanger coils, ambient heat causes the liquid to boil (change to gas or vapor): heat energy from the outside air has been absorbed and stored in the refrigerant as latent heat. The gas is then compressed using an electric pump; the compression increases the temperature of the gas.

Inside the building, the gas passes through a pressure valve into heat exchanger coils. There, the hot refrigerant gas condenses back to a liquid and transfers the stored latent heat to the indoor air, water heating or hot water system. The indoor air or heating water is pumped across the heat exchanger by an electric pump or fan.

The cool liquid refrigerant then re-enter the outdoor heat exchanger coils to begin a new cycle.

Most heat pumps can also operate in a cooling mode where the cold refrigerant is moved through the indoor coils to cool the room air.

The ‘Efficiency’ of air source heat pumps is measured by the Coefficient of performance (COP). A COP of 3 means the heat pump produces 3 units of heat energy for every 1 unit of electricity it consumes. Within temperature ranges of -3 °C to 10 °C, the COP for many machines is fairly stable at 3-3.5.

In very mild weather, the COP of an air source heat pump can be up to 4. However, on a cold winter day, it takes more work to move the same amount of heat indoors than on a mild day.^{[10] The heat pump’s performance is limited by the} Carnot cycle and will approach 1.0 as the outdoor-to-indoor temperature difference increases, which for most air source heat pumps happens as outdoor temperatures approach −18 °C / 0 °F. Heat pump construction that enables carbon dioxide as a refrigerant may have a COP of greater than 2 even down to -20 °C, pushing the break-even figure downward to -30 °C (-22 °F). A ground source heat pump has comparatively less of a change in COP as outdoor temperatures change, because the ground from which they extract heat has a more constant temperature than outdoor air.

The design of a heat pump has a considerable impact on its efficiency. Many air source heat pumps are designed primarily as air conditioning units, mainly for use in summer temperatures. Designing a heat pump specifically for the purpose of heat exchange can attain greater COP ratings and an extended life cycle. The principal changes are in the scale and type of compressor and evaporator.

Seasonally adjusted heating and cooling efficiencies are given by the heating seasonal performance factor (HSPF) and seasonal energy efficiency ratio (SEER) respectively.

In units charged with HFC refrigerants, the COP rating is reduced when heat pumps are used to heat domestic water to over 60 °C or to heat conventional central heating systems that use radiators to distribute heat (instead of an underfloor heating array).

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