Green Energy > Heat Exchangers
Air source heat pumps (ASHPs) work in a similar way to ground source heat pumps, but as the name suggests they use heat from the air as their energy source instead of latent ground heat. ASHPs can be used to bring heat into buildings for space and water heating in the winter, and extract it out of buildings in the summer to provide air cooling.
Energy savings, and a reduction in CO2 emissions, are potentially significant, although they tend to work best in energy efficient buildings using low temperature distribution systems such as under floor heating. While ASHPs form the largest market share of all the heat pump options, most of these installations are used in their reversed mode to provide cooling.
There are two types of air source heat pump systems: air-to-air, which is most common, and air-to-water. Air-to-air systems take heat energy from the outside air and pump it to the air inside using fan-assisted units. This is reversed in winter to provide cooling. Air-to-water systems are more advanced and can be used in homes with hydronic heat distribution systems such as radiators or under floor heating.
Systems range from 3kW-100kW and can be used in residential, commercial and institutional buildings. As well as moving heat from outside to inside and vice versa, it can also be moved around buildings as needed, further increasing efficiencies.
An air source heat pump has three main parts: the evaporator coil, which absorbs heat from the outside air; the compressor, which pumps the refrigerant through the heat pump and compresses it to the temperature needed for the heat distribution circuit; and the heat exchanger that transfers the heat from the refrigerant to either the air or water.
Unlike GSHPs, which require a lot of outside space and costly trenches or bore-holes, ASHPs are relatively small and can be fitted to the side of a building. This makes them cheaper to install and also means they can be used in densely packed urban areas.
ASHPs, like GSHPs, are best suited to new build applications so that the building can be planned with them in mind. They’re most efficient when supplying low temperature distribution systems such as under-floor heating because heat pumps have a typical operating temperature limit of 55°C. They also require larger duct sizes than other central heating systems, which should be considered.
The big drawback is that the source air temperature range can be highly variable – not only seasonally but also daily. Heat pumps work most efficiently when the source temperature is as high as possible, which is why they work efficiently as air conditioners. In the UK, however, the mean air temperature for winter is lower than the mean ground temperature resulting in lower seasonal efficiencies for ASHPs than GSHPs.
Additionally, the fan and compressor in an ASHP system can emit a lot of noise and vibrations. The pump should be located as far as possible from bedrooms and neighbouring properties. There’s also a lack of sufficient numbers of qualified and trained installers due to the industry being in its infancy in the UK and Ireland.
Air source heat pumps are a relatively cost-effective way of reducing energy consumption for both space and water heating. They’re a little less efficient than ground source heat pumps, but they don’t require a lot of space, installation is inexpensive and they can be used in dense urban areas.
Like most renewables, the biggest gains are made in areas not on the gas grid, due to the high relative cost of electricity. Air source heat pumps are worth exploring further for areas that need heating in winter and cooling in the summer. Many locations in the UK and Ireland probably don’t qualify.
The performance of air source heat pumps is measured by the coefficient of performance (CoP) and falls anywhere within the 2 to 4 range. This means that for every unit of energy put into the system, you get 2 to 4 units out. Determining an exact figure is difficult due to variable factors such as the outside air temperature and the heat distribution system in use.
On the whole, ASHPs tend to have lower efficiencies than ground source heat pumps, due to the seasonal differences in air temperature. When the air temperature decreases, it’s more difficult to extract heat from the cooler air. Using a distribution system, such as under floor heating, which requires low temperatures can help to increase efficiencies.
These efficiencies equate to reduced CO2 emissions and possible reductions of heat energy bills of around 50%, which should be enough to warrant the initial investment. According to the Energy Saving Trust, a typical domestic system will cost about £7,000-£14,000. With efficiencies of around 2.5 CoP, this would result in a payback period of between 8 and 15 years, with the service life of an ASHP being around 15 to 20 years. Click here for more information.