Heat flows naturally from a higher to a lower temperature. Heat pumps reverse this natural flow, extracting heat energy from a cool source, such as the ground, and delivering it to a hot system, such as a building’s heating system. In the same way that a fridge uses refrigerant to extract heat from the inside, keeping your food cool, a heat pump extracts heat from a range for sources, and uses it to heat your home and hot water. An ideal heat source for a heat pump has a high stable temperature during the heating season. Typical sources of heat in the domestic setting are:
- Ground Source
- Water Source
- Air Source
Ground Source Heat Pumps
Ground Source Heat Pumps do not tap into geothermal heat. Instead they use solar heat that has been stored in the earth. The earth acts as a huge solar collector. Solar energy is absorbed at the earth’s surface. From there it can warm the air. It is also transported downwards into the earth by conduction and washed downwards by rainfall. As you dig deeper into the earth the temperature becomes more and more stable until it approaches the temperature of well water, about 8–10ºC year round. The temperature of the ground is too low for direct use to heat our homes, so it is necessary to refine or to upgrade the temperature of the heat. Electricity is used to upgrade it in the heat pump.
The Coefficient of Performance (CoP) is the key figure used with heat pump systems. It indicates the ratio of useful heat energy output to electrical energy input. If a system has a CoP of 3, for every 1kWh of electricity input you will get 3kWh of heat output. Well-designed systems can achieve a CoP of 3–4. The real bonus of taking this route is that the energy required to concentrate heat is much less than the energy that must be liberated by burning a fuel. This is how one unit of input energy can produce four times as much heat.
The Coefficient of Performance attainable with this process is limited ultimately by the temperature rise needed to make the heat useful. The smaller the temperature rise, the less energy needed to pump the heat.
Water and air source heat pumps
If at least a ½ acre by 8 ft deep pond or lake is available on your property, a water source system can be installed by laying coils of pipe in the bottom of the body of water. This system tends to be less efficient than ground source due to greater seasonal variations in water temperature.
Air source heat pumps
Air source heat pumps absorb heat from the outside air to heat buildings. Air source heat pumps present an advantage over ground source heat pumps because they require less space to install. They are also cheaper to install because they do not need the ground loop and associated works. When the outdoor temperature drops the efficiency of an air source heat pump drops much more quickly than the ground source option. The best Coefficient of Performance that can be currently achieved with an air source heat pump is CoP 3. However systems are improving.
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