Heat pumps are mostly electrically powered, but can also be gas powered. They harvest the low-level heat energy in the earth or the environment to provide space heating. A heat pump with a performance factor of 4.0 generates 4 W of heat from one W of drive power. Some utilities offer special tariffs for heat pump operation, an added economic attraction. Even in colder climate countries such as Sweden, Switzerland and Austria, heat pumps have established themselves as a viable heating technology.
The heat pump cycle: A heat pump operates like a refrigerator in reverse. A refrigerant extracts low-temperature heat from the environment (ground, water or air), which causes the refrigerant in the system to evaporate. The refrigerant is then compressed. In a condenser heat is released. This heat is transferred to the water circulating in the heating system.
Effective distribution: Heat pumps work more efficiently when combined with distribution systems that work at lower temperatures.
Practically emission free – with renewable energy. Heat pumps can be driven from electricity produced from renewable energy sources such as wind power or photovoltaics and operate practically emission-free.
Closed-loop heat pumps: brine/water
Closed-loop heat pumps are the most common type. They use a closed loop of pipe containing a water and anti-freeze solution to extract heat from the ground or ground water. The heat is transferred to water for distribution in the building. Hence: “brine/ water”. High yearly/seasonal performance factors – 3.8 to 5.0 – can be achieved. The heat can be extracted from the ground or ground water using vertical collectors in boreholes or loops of pipes laid horizontally below the surface of the ground. They are available with or without integrated hot water storage.
Open-loop heat pumps: water/water
Open-loop heat pumps achieve the highest yearly/ seasonal performance factors – 4.0 to 5.0 – because they use the almost uniformly level temperature of bodies of water. The water from the source is pumped through the actual heat pump itself where its heat is extracted. Evaporators need to be rustproof. Water/water heat pumps come with or without hot water storage tanks. Building cooling is also possible.
Air-source heat pumps: air/water
Air-source heat pumps extract environmental heat from the air. They can achieve yearly/seasonal performance factors of 4.0–5.0. They are particularly suitable for installation in existing buildings. There are no earthworks necessary and thus they are easier to install. The air/water heat pumps can also be switched to operate in cooling mode.
Particular requirements for ground-source heat pumps: sufficient open land area must be available – to sink boreholes, but more so if horizontal collectors are to be used. Permits are often necessary.
The higher the temperature of the heat source, the more efficiently the heat pump will operate. The source temperature should vary as little as possible over the year. The easier it is to access the heat source the lower the investment costs for the system will be.
Horizontal ground collectors are thin coils/loops of poly ethylene pipe. These are buried horizontally 1.2 to 1.5 m below the ground (in a garden for example). The coils/loops are placed in rows 0.5 – 0.8 metres apart from each other. Generally, one 25 square metres of ground is required to produce a kilowatt of heat.
Vertical ground collectors are double “U” pipes made from polyethylene. They are placed in boreholes at depths of 30 – 100 metres. At these depths the year-round temperature is about a constant 10 °C – relatively high. Approximately 50 W of heat can be extracted per metre of pipe. This type of system can also be used for building cooling.
