Heat is extracted from ambient external air and transferred to another medium. The heat available reduces as the temperature of the air reduces reducing both performance duty and efficiency: See also âAir-to-Air?and âAir-to-Water?
Air to Air
A heat pump where the heat collecting medium is air and the destination medium is also air. Heat is extracted from ambient external air and transferred directly to the air circulating in the space being heated: Hence air is in direct contact with the condenser and evaporator. Generally, most commercial small split and packaged systems are direct air to air. See also âAir Source?
Describes a heat pump whereby heat is extracted from ambient external air and transferred to water which in turn is used for heating either a space or domestic hot water for sanitation purposes. This type of system uses a mixture of direct acting outdoor unit coupled with an indirect indoor section
A heat collection system whereby a pipe-work system is placed within the ground (or lake, river sea) and water/glycol mixed is circulated through it. A heat pump will then extract heat from the circulating fluid which will be replenished by the source (ground, lake, river or sea). The fluid is always contained within the system and does not come in direct contact with the source.
Coefficient of Performance (CoP)
The ratio, CoP, is an expression of the output of a machine in heating mode as a proportion of input power (compressor and fans) and hence is the rated capacity divided by the rated power input. In practice this is expressed as a single figure or sometimes as a percentage. For example, a system that is rated in heating at 6.5kW, with a rated power consumption of 1.8kW will have a CoP of 3.61 or 361%. This is a simple ratio used in BS EN 14511:2007 now superseded by BS EN 14511:2011 (See CoSP, EER).
Coefficient of System Performance (CoSP)
A ratio figure that describes the efficiency of a heat pump taking into account the input power from internal control circuits, compressor, as well as fan and pump power required to overcome any fluid resistances of its own heat exchangers (but not those external to the unit) . Therefore it is the rated capacity divided by the rated total power input and is more representative of the unitâs efficiency than the more simple CoP. In practice it has become the de facto measure of CoP and is expressed as a single figure or sometimes as a percentage. This ratio is now described in BS EN 14511:2011 (See also CoP, SCoP , SPF, EER).
The compressor is often referred to as the heart of the vapour compression heat pump system. It serves two main purposes. The first is to circulate the refrigerant fluid through the circuit like a pump, the other is to compress and raise the pressure and temperature of the refrigerant vapour so that it can easily be condensed back into a liquid to resume the heat transfer process.
In refrigeration systems the condenser is the heat exchanger where Hot, compressed refrigerant gas is condensed to a liquid and further cooled to recommence its journey around the circuit.
When air source heat pump systems operate at low ambient temperatures, the evaporator in the outdoor unit is likely to be below freezing point and moisture within the air will freeze on itâs surface forming a coat of ice. This is a function of the natural humidity outdoors and is not abnormal. This coating of ice is removed periodically with an automatic defrost cycle. The frequency of the defrost cycle is controlled automatically by a combination of time and temperature (refrigerant pressure) of the external coil. Rarely is any ice detection itself carried out. This will have a small effect on the efficiency of the device. (see SPF)
Direct Expansion (DX)
Used to describe the situation where refrigerant in a heat pump is used to directly heat (or cool) the final medium being heated. For instance to heat an occupied space the air from the room would pass directly over the condenser containing the refrigerant, or in the example of domestic hot water heating the refrigerant would have to pass through the cylinder coil (condenser) itself â this latter case being very rare. It offers increased thermal exchange efficiency by reducing the number of thermal interchanges however efficiency reduces in cooling modes if the separation between condenser and evaporator is large (>5m). VRF Air/Water to Air systems are good examples of DX systems.
Energy Efficiency Ratio (EER)
Describes the efficiency of a heat pump machine in cooling mode. The rated capacity is divided by the rated total power input. In practice this is expressed as a single figure or sometimes as a percentage.
For example, a system that is rated in cooling at 6.5kW, with a rated power consumption of 1.8kW will have an EER of 3.61 or 361%.(See CoP, SEER).
In the refrigeration cycle of heat pump systems the evaporator is the heat exchanger where refrigerant fluid is evaporated it absorbs heat from the surrounding air or water, thereby reducing its temperature.
Expansion Control Devices
In a refrigeration based heat pump system, the flow and evaporation rate of the refrigerant within the evaporator is controlled by devices at the entrance to the evaporator. These devices are available in a variety of forms; the 3 most common types in commercial systems are:
Capillary tubes: Any fluid flowing through an orifice will experience a drop in pressure. A capillary tube is a precisely measured length of a narrow tube with a pre-determined internal diameter that produces the desired drop in pressure along its length.
Thermostatic Expansion Valve (TEV): An automatic mechanical valve that is self compensated for the pressure losses in the evaporator and controls the leaving superheat temperature of the refrigerant.
Electronic Expansion Valve (EEV): A valve that is driven by a small dc stepping motor. Operates in the same way as a TEV, although the valve positioning is determined by a microprocessor. EEVs are capable of more precise metering and are found mainly in fully automatic systems.
A heat pump system whereby heat is extracted from the ground typically by passing a transportation fluid (water/glycol) through tubes buried in the ground either in horizontal arrays (looped or straight tube) or vertically through 2 or 4 pipe thermal boreholes. In these cases it is described as âclosed loop? In the UK the temperature below ground at a few tens of metres depth is stable within a small tolerance year round. Heat can be stored or obtained from shallow ground, about 2 m depth, or by deeper bore-holes where space is a consideration. Sometimes the phrase Ground Source âOpen Loop?is used to describe heat taken from the ground by extracting the water within the ground (e.g. aquifer).
Geothermal energy (so-called âhot rocks? is obtained by deep drilling in suitable geological areas where volcanic activity can provide heat from the earths core. This is a separate and distinct technology and should not be confused with ground source heat pumps.