A heat pump is a device used to warm and sometimes also cool buildings by transferring thermal energy from a cooler space to a warmer space using the refrigeration cycle, being the opposite direction in which heat transfer would take place without the application of external power. Common device types include air source heat pumps, ground source heat pumps, water source heat pumps and exhaust air heat pumps. Heat pumps are also often used in district heating systems.
The efficiency of a heat pump is expressed as a coefficient of performance (COP), or seasonal coefficient of performance (SCOP). The higher the number, the more efficient a heat pump is and the less energy it consumes. When used for space heating, these devices are typically much more energy efficient than simple electrical resistance heaters.
Heating and cooling of buildings and vehicles;
In heating, ventilation, and air conditioning (HVAC) applications, a heat pump is typically a vapor-compression refrigeration device that includes a reversing valve and optimized heat exchangers so that the direction of heat flow (thermal energy movement) may be reversed. The reversing valve switches the direction of refrigerant through the cycle and therefore the heat pump may deliver either heating or cooling to a building. In cooler climates, the default setting of the reversing valve is heating.
The default setting in warmer climates is cooling. Because the two heat exchangers, the condenser and evaporator, must swap functions, they are optimized to perform adequately in both modes. Therefore, the SEER rating, which is the Seasonal Energy Efficiency Rating, of a reversible heat pump is typically slightly less than two separately optimized machines. For equipment to receive the Energy Star rating, it must have a rating of at least 14.5 SEER.
Outdoor unit of air source heat pump operating in freezing conditions
Air source heat pump
Air source heat pumps are used to move heat between two heat exchangers, one outside the building which is fitted with fins through which air is forced using a fan and the other which either heats the air inside the building directly or heats water which is then circulated around the building through heat emitters which release the heat to the building.
Geothermal (ground-source) heat pump
A geothermal heat pump (North American English) or ground-source heat pump (British English) draws heat from the soil or from groundwater which remains at a relatively constant temperature all year round below a depth of about 30 feet (9.1 m).A well maintained geothermal heat pump will typically have a COP of 4.0 at the beginning of the heating season and a seasonal COP of around 3.0 as heat is drawn from the ground.
A geothermal heat pump can also be used to cool buildings during hot days, thereby transferring heat from the dwelling back into the soil.
Exhaust air heat pump
Exhaust air heat pump (extracts heat from the exhaust air of a building, requires mechanical ventilation)
Exhaust air-air heat pump (transfers heat to intake air)
Exhaust air-water heat pump (transfers heat to a heating circuit and a tank of domestic hot water)
Solar-assisted heat pump
A solar-assisted heat pump is a machine that represents the integration of a heat pump and thermal solar panels in a single integrated system. Typically these two technologies are used separately (or only placing them in parallel) to produce hot water. In this system the solar thermal panel performs the function of the low temperature heat source and the heat produced is used to feed the heat pump’s evaporator. The goal of this system is to get high COP and then produce energy in a more efficient and less expensive way.
Water source heat pump
Water-source heat-exchanger being installed
A water-source heat pump works in a similar manner to a ground-source heat pumps, other than that it takes heat from a body of water rather than the ground. The body of water does however need to be large enough to be able to withstand the cooling effect of the unit without freezing or creating an adverse effect for wildlife.
When comparing the performance of heat pumps the term ‘performance’ is preferred to ‘efficiency’, with Coefficient of performance (COP) being used to describe the ratio of useful heat movement per work input. An electrical resistance heater has a COP of 1.0, which is considerably lower than a well-designed heat pump which will typically be between COP of 3 to 5 with an external temperature of 10°C and an internal temperature of 20°C. A ground-source heat pump will typically have a higher performance than an air-source heat pump.
The ‘Seasonal Coefficient of Performance’ (SCOP) is a measure of the aggregate energy efficiency measure over a period of one year which it is very dependent on region climate. One framework for this calculation is given by the Commission Regulation (EU) No 813/2013:
In cooling mode, a heat pump’s operating performance is described in the US as its energy efficiency ratio (EER) or seasonal energy efficiency ratio (SEER), and both measures have units of BTU/(h·W) (1 BTU/(h·W) = 0.293 W/W) with a larger EER number indicating better performance. Actual performance varies, however, and depends on many factors such as installation details, temperature differences, site elevation, and maintenance.