Air source heat pumps

An air source heat pump is a type of heat pump which use the outside air as a heat source or heat sink to heat or cool an interior space. Air-source heat pumps are more efficient than oil, gas, and electric resistance heating in mild climates while they are less efficient than ground-source heat pumps. However, they are less costly to install than ground source heat pumps and therefore have become more widespread in use. So widespread, in fact, that the term "heat pump" is often used to refer by default to air source heat pumps, and the term "air source heat pump" is used mostly to distinguish from ground source heat pumps.

A heat pump transfers energy in the form of heat from a cooler location to a warmer location. A heat pump uses the refrigeration process and transfers low temperature energy to a refrigeration loop, compresses the refrigerant to a high temperature, and transfers this heat to the hot water and heating distribution system, or, in the summer, removes it from the home. Systems normally range from a single 4kW unit to multiple units with a single controller producing around 300kW. Domestic hot water storage can also be provided.

Key Benefits of air source heat pumps

* Typically draws approximately 1/3 to 1/4 of the electricity of a standard Electrical Heating for the same amount of heating, reducing utility bills. [cite web|title=Georgia State University Heat Pump Notes|url=http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatpump.html] This typical efficiency of 300%-400% compares to 100% for a resistance heater and 70-95% for a fuel-powered boiler.
* Few moving parts, reducing maintenance requirements.
* As an electric system, no flammable or potentially asphyxiating fuel is used at the point of heating, reducing the potential danger to users, and removing the need to obtain gas or fuel supplies (except for electricity).
* May be used to heat air, or water, as the need arises.
* In some installations the same system may be used as a cooler, as well as a heater.

How it works:In the outdoor unit the refrigerant meets the outdoor air in the evaporator (heat exchanger). The air is drawn through the evaporator by a fan located on side of the heat pump. The refrigerant, which is in a liquid state absorbs free energy from the air and evaporates in this process. A sensor in the expansion valve ensures that the liquid refrigerant collects the correct amount of the “free energy” before the refrigerant (now in a gas state) is led into the compressor. The compressor increases the pressure of the refrigerant. The temperature of the vapour reaches approximately 100°C. The warm gas is then led into the condenser. The condenser is the heat pump’s heat emitting part. In the condenser, which is a fully brazed heat exchanger in stainless steel, the refrigerant (gas state) meets the water from the heating system (radiators and/or floor coils). When the warm gas is cooled by the circulating heating water, it changes into a liquid state (condenses). Energy is emitted in this process to the heating system or the hot water. After the condenser, the refrigerant, which is now in liquid form, continues through a drying filter. The drying filter is used to collect any moisture in the system. After the filter it continues on to an expansion valve. The refrigerant pressure is lowered in the expansion valve. This also causes the temperature to drop. When the refrigerant has left the valve and passes the evaporator it changes to vapour again. This completes the refrigerant circuit. The expansion valve is equipped with a sensor (bulb) just before the compressor. The sensor controls the amount of fluid entering the evaporator.

Applications of air source heat pumps

Air source heat pumps do not incur any civil ground works cost for the installation and laying of pipe. The cost of civil ground works can be significant using alternative ground source systems, this is not a consideration that is required for Air source heat pumps. This makes this option a significant advantage over Ground Source versions.

Air source heat pumps provide an alternative low cost solution for space heating and hot water. A typical return on investment against oil and LPG fed heating systems the return can take as little as 5 years. Air source heat pumps are also suitable for hot water and comfort cooling applications which provide a comprehensive solution for both domestic and commercial applications.

Air source heat pumps also provide hot water from a pressurised system up to temperatures as high as 55°C which is very suitable for all domestic and business requirements. This will require a tank which is specified to the usage requirements of the tenants or usage of the building. The larger the tank the larger the output of water.

In some applications it may be appropriate to couple an air source heat pump with traditional heating technology to complement the required outputs. All air source heat pumps which are specified against user (domestic & commercial) requirements do not need any third party boosting system. The outside of a building is where the fan intake system (outdoor air handling unit) is fixed requiring no annual maintenance. The fan is the only moving part of the outdoor unit which requires no ongoing maintenance.

Air source heat pumps take energy from the air and raise it to a higher temperature, using a process which is similar to a reverse refrigeration process. For commercial and large spaces a row or bank of air source heat pumps (air handling units) will be required along with internal heat pump and pressured hot water tank for ongoing water usage. This is a system which utilises no external pipes and most of the working elements reside within the building. The air handling unit draws air across the water-anti freeze solution and transfers this energy into the refrigerant. The refrigerant boils and the gases from this are compressed to produce temperatures in excess of 100°C. This part of the process mirrors a ground source heat pump. Air source heat pumps can be used in many more applications including large commercial projects where land space is restricted. Air Sourced Heat Pumps can be used as a complete solution for room heating using the same distribution system as a ground source heat pump or a traditional system. Air Sourced Heat Pumps are ideal for very tight spaces and within an eco architectural design or within the design of a building which has large internal spaces such as audience halls and public places.

Air source heat pumps are unique as they have no major parts outside exposed to the elements, which means that the organs compressor and circulation pumps are all internal. The intake fan is the only feature which is external and kept at optimum working temperature with the support of an internal defrost cycle system ensuring 365 day operation in all seasons. The internal defrost system uses the hot water collected by the Air source heat pump as opposed to direct electric. These types of systems can last in excess of 20 years. The recommended maintenance for the outdoor unit is simply to hose it down once a year. In costal areas this process should be repeated each season”. [cite web|title=Frequently Asked Questions about Heat Pump Technology & Applications|url=http://www.ecoheatpumps.co.uk/heat_pump_faq.htm Frequently Asked Questions about Heat Pumps] [cite web|title=How a heat pump works, is installed and the savings to be expected. |url=http://www.fortismoore.com/pdfs/Fortismoore_domestic_low_cost_heat_pump_solutions.pdf PDF]

Notes

3. Microgeneration Certification Scheme - air source heat pumps covered by standards