Heat pumps are used for space heating and cooling, as well as water heating. They operate on the fact that the earth beneath the surface remains at a constant temperature throughout the year, and that the ground acts as a heat source in winter and a heat sink in summer. They can be used in both residential and commercial or institutional buildings. Other heat pump types are available such as air and water source. These operate on the same principle indoors but the method of collecting heat is different for each type.
How it works
The earth’s surface acts as a huge solar collector, absorbing radiation from the sun. In this country the ground maintains a constant temperature between 11oC and 13oC, several metres below the surface. Heat pumps take advantage of this by transferring the heat stored in the earth or in ground water to buildings in winter and the opposite in summer for cooling. Through compression, heat pumps can ‘pump up’ heat at low temperature and release it at a higher temperature so that it may be used again. A heat pump looks similar and can perform the same functions as a conventional gas or oil boiler, i.e. space heating and sanitary hot water production. For every unit of electricity used to operate the heat pump, up to four units of heat are generated. Therefore for every unit of electricity used to pump the heat, 3-4 units of heat are produced. Ground source heat pumps use a 'closed loop' system of water/anti-freeze to collect the soil heat. Air/water heat pumps collect heat from the outside air. Generally, air temperatures are moderate in Ireland but due to natural frosting of the air heat exchanger during heat collection, it is necessary that these pumps use a small amount of energy to defrost. This leads to a marginal decrease in performance which is offset by a low installation cost. Water/water heat pumps use water from a well/river directly as a heat source. These are generally 'open loop' collectors, i.e. the water is passed through and discarded, unlike the 'closed loop systems'.
Installation in the Home
The system has three main components: a series of pipes in the ground, a heat pump and a heat distribution system. Lengths of plastic pipes are buried in the ground, either in a borehole or a horizontal trench near the building to be heated or cooled. Fluid, normally water with anti-freeze, absorbs or emits heat to the soil, depending on whether the ambient air is colder or warmer than the soil. In winter, the heat pump removes the heat from the fluid, upgrades it to a higher temperature for use in the building, typically in under-floor heating. A distribution system is needed to transfer the heat extracted from the ground by the heat pump.The heat is often in the form of hot water and is distributed around the dwelling by radiators or a low temperature underfloor heating system.
Payback and Maintenance
The initial capital costs of installing a geothermal heat pump system is usually higher than other conventional central heating systems. A large proportion of the outlay will be for the purchase and installation of the ground collector. The system is among the most energy efficient and cost effective heating and cooling systems available.
Typically, 3-4 units of heat are generated for every unit of electricity used by the heat pump to deliver it, and the payback is typically about 8-10 years. The life expectancy of the system is around 20 years. Once installed a heat pump requires very little maintenance and anyone installing a heat pump should speak with their installer regarding a maintenance agreement. Heat pumps operate optimally when a system design approach is taken. It is important that the heat collector and heat distribution systems are correctly sized/installed.