GLOSSARY OF TECHNICAL 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:

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”.

Heating Only Heat Pump: Ground Source heat pump systems are often optimised specially to deliver heating only. These units are often capable of delivering greater overall energy efficiencies than reversible units because the system is designed for a constant unvarying load above the freezing point of water at the evaporator section and can dispense with the Defrost Cycle.

Compressor: The compressor is often referred to as the heart of the refrigeration or 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.

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. Or to facilitate defrost.

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.

Evaporator: In refrigeration 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.

Defrost Cycle: Where reversible air source systems operate at low ambient temperatures, the evaporator in the outdoor unit may attract moisture from the air at low ambient temperatures that will freeze 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 of the external coil.

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.

Metering Devices: In a refrigeration based heat pump system, the flow and evaporation rate of the refrigerant within the evaporator is controlled by metering 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.

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 or a Hydrocarbon compound (refined Propane or Isobutane).

Ground Source Heat Pump: A heat pump installation that uses the earth as a heat sink to store heat or as a source of heat. 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.

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 crust. This is a separate and distinct technology and should not be confused with ground source heat pumps.

Inverter Drive or Variable Speed Drive (VSD): Refrigeration systems in air to air heat pumps are often subject to wide variations in capacity and demand. A VSD system uses electronic speed control to vary the speed of the compressor motor. The motor speed can be increased to compensate for low ambient temperature conditions when less heat is available to use or decreased during mild weather. VSD systems are capable of high energy efficiencies because they match the unit’s capabilities to demand. Also known as VRV (Variable Refrigerant Volume) and VRF (Variable Refrigerant Flow).

Air to Air: A heat pump where the collecting medium is air and the destination medium is also air. Generally, most commercial small split and packaged systems are direct air to air.

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.

Air to Water: Air to water systems are mostly single packaged units. An outdoor unit collects heat from ambient air and delivers it to the conditioned space by means of water or glycol circulated through fan coils. This type of system uses a mixture of direct acting outdoor unit coupled with an indirect indoor section.

Water to Air: Water source systems are used where a supply of ground water, ponds or lakes are available. These systems are invariably indirect and need careful filtration to remove particles from the water source before it enters the heat exchanger. In the UK permission is usually needed for extraction of ground water.

Direct/Indirect: A direct system operates with the refrigerant in either or both heat exchangers in direct contact with the source or destination medium.

An indirect system has a secondary medium, water or glycol, circulated through the heat exchangers. By this means the refrigerant is contained in one remote place and refrigerant quantities can be minimised. This type of system is mandatory for all flammable or toxic refrigerants.

CoP: (Coefficient of Performance): Used as a ratio, CoP is an expression of the output of a machine in heating mode. The rated capacity 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 heating at 6.5kW, with a rated power consumption of 1.8kW will have a CoP of 3.61 or 361%.

Energy Efficiency Ratio (EER). The out put of a 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%.

Kilowatt: The SI unit of power. It is used to specify the thermal performance of a Heat Pump as well as the power energy it consumes.

Kilowatt Hour (kWh): The standard unit of sale of electricity; it is the equivalent power consumed by a purely resistive load of 1000 Watts (1kW) for 1 hour. Your electricity supplier will specify the price in your supply contract.

CO2 Emission Comparisons with other Technologies