Glossary of Terms
There are many technical terms used in the description of Heat Pump systems; most of them are self-explanatory, but a glossary of terms will help to explain what they mean.
Most technical terms concern the refrigeration system found in all heat pumps:
Air Source
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 ‘Air-to-Air’ and ‘Air-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 ‘Air Source’.
Air-to-Water
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.
Closed Loop
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 This is a simple ratio used in BS EN 14511:2007 now superseded by BS EN 14511:2011 (See CoSP, EER).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).
Compressor
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.
Condenser
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.
Defrost Cycle
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 it’s 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 – 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).
Evaporator
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.
Ground Source
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 ‘Open Loop’ is used to describe heat taken from the ground by extracting the water within the ground (e.g. aquifer).
Geothermal Energy
Geothermal energy (so-called ‘hot 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.
Heat Pump
A heat pump is a device for transferring energy in the form of useful heat from one place to another. It cannot store, make or destroy heat energy it simply moves it. There are a number of techniques that exploit heat transfer; the commonest in use is the Refrigeration Cycle. A heat pump is capable of transforming a large quantity of low grade, low temperature heat. Some air source systems will operate in winter ambient conditions down to -15ºC. Heat pumps are available that can operate in a variety of media Air, Water, glycol, etc.
A European standard for testing and rating heat pump performance, EN 14511 Part 1, defines a heat pump: [a] heat pump [is an] encased assembly or assemblies designed as a unit to provide delivery of heat. It includes an electrically operated refrigeration system for heating. It can have means for cooling, circulating, cleaning, and dehumidifying the air. The cooling is by means of reversing the refrigeration cycle?
Heat Exchanger
A heat exchanger is a device for the transfer of heat energy from one medium to another. It can take a variety of different forms; the commonest in everyday use is a central heating radiator where hot water is circulated through pipes or plates and gives its heat up to the surrounding air.
Inverter or Variable Speed Drives (VSD)
Rarely are heat pumps required to run at full duty for significant periods of time, mostly due to variable load conditions such as external temperature conditions and hence varying building heat loss. By varying the speed of the compressor the output of the equipment can be varied and lead to increased efficiency because both the evaporator and condenser are sized for peak duties and able to conduct heat even more efficiently at part load. Such drives are used in most electrically driven vapour compression heat pumps, whether two piece âsplit systems? multi split or VRF and use Inverter technology. Variable speed drive is also possible with Gas Engine Heat Pumps by conventional throttling of engine speed via the carburettor.
Kilowatt (kW)
The SI unit of power. It is used to specify the thermal performance of a Heat Pump as well as the power it consumes. It is a Kilojoule of energy per second (kJ/s).
Kilowatt Hour (kWh)
The standard unit (Not SI) for the of sale of electrical energy; it is the equivalent power consumed by a purely resistive load of 1000 Watts (1kW) for 1 hour. Because a kW is kJ/s, a kWh is equivalent to 1 kJ/s for 1 hour. Therefore 1 kWh = 3600 kJ. Your electricity supplier will specify the price in your supply contract.
Open Loop
A heat collection system whereby water is extracted from ether the ground or an open water source (lake, river or sea) and is passed directly through a water source heat pump. This water may be re-injected or passed to waste, in the latter case water charges may be incurred. Environment Agency approval is required for all systems extracting more than 20m3 which restricts use to domestic small to medium size without such approval.
Refrigerant
The heat transfer fluid contained in a heat pump refrigeration circuit. Normally this is a chemical contained in a hermetically sealed circuit that has a low temperature boiling point. Refrigerants can be one of a number of man-made Fluorocarbons(e.g. HFC) or a Hydrocarbon compound (refined Propane or Isobutane). All refrigerants currently used have Zero Ozone Depletion potential, but many have quite high direct Global Warming Impacts if released to atmosphere. All refrigerant handling should be undertaken by qualified and certified personnel.
Refrigeration Cycle
All heat pumps use the refrigeration cycle in some way. Heat is extracted from a source, upgraded in temperature and delivered to a use. The most popular method is the vapour compression cycle.
Reverse Cycle Heat Pump
A reverse cycle system is a refrigeration system that can, by means of a valve that reverses the flow of the refrigerant fluid, change the operation of the system from heating to cooling. This process can also be used to facilitate defrost.
Seasonal Coefficient of Performance (SCoP)
An efficiency metric of heat pumps which describes performance of the unit over a typical season where the source temperature varies. Used mostly with Air Sourced Heat pumps where the source temperature varies considerably over the year and hence efficiency and/or output varies. Thus SCoP is dependant on the local climate. In order to compare units the EU has been divided into 3 principle regions and is defined in BSEN 14825 and will be used for the Energy Using Products directive (ErPEner Lot 1 and 10). Often referred to as SPF.
(See also CoP, CoSP, SPF and SEER)
Seasonal Energy Efficiency Ratio (SEER)
An efficiency metric of reverse cycle heat pumps which describes performance of the unit over a typical cooling season where the source temperature varies. Used mostly with Air Sourced Heat pumps where the source temperature varies over the year and hence efficiency and/or output varies. Thus SEER is dependant on the local climate. In order to compare units the EU has been divided into 3 principle regions under BS EN 14825 for both heating and cooling functions.
(See also CoP, SCoP and EER)
Seasonal Performance Factor (SPF)
Seasonal Performance Factor is similar to SCoP in that it is a ratio expressing the efficiency of a heat pump by describing heat output to total energy input taking into account variations in performance over the heating season. Under BS EN 15316, input energy includes auxiliary energy which may be all or part of pump/fan power. Care must be exercised as to whether this is intended to include any additional boost heat from other sources (e.g. electric immersion heater) and the full pump/fan power to overcome all resistances of circuits (i.e. not just the heat exchangers of the heat pump). For this reason the predictive SPF, if based on say CoSP or SCoP, may vary greatly from an empirical SPF based on actual data measured on site because this may include the total power consumed by circulating pumps/fans and direct electric additional low ambient boost heaters, all of which are bespoke to predict in advance.
The term is used in a wider sense than it’s specific definition under BS EN 15316 and is being widely adopted in UK and EU.
Water Source
A heat pump system whereby heat is extracted from water which is either directly extracted from the ground (e.g. a buried aquifer) called ‘open loop’ or from an open water source such as a lake, river or sea. These systems are invariably indirect and need careful filtration to remove particles from the water source before it enters the heat exchanger. This term is sometimes used interchangeably with Ground Source with a sub designation of âopen loop? In the UK permission is usually needed for extraction of ground water.
Water to Water
A heat pump where the collecting medium (source) is either ground water or a glycol solution and the destination medium is also water or glycol. These systems are invariably indirect.
Water to Air
A heat pump where the collecting medium (source) is either ground water or a glycol solution and the destination medium is air. Hence the source side is indirect but the delivery side is likely to be direct.