Absorption Refrigeration plant

A refrigeration plant that uses heat instead of electricity to drive the refrigeration process. (DECC UK)


The machine which generates AC electricity. The alternator is driven by the primemover (engine). (DECC UK)

Auxiliary firing

Location where fuel is burnt (with its own air supply) in heat recovery boilers when the engine / gas turbine is not running. This allows the supply of heat to be maintained. (DECC UK)

Back-pressure steam

The steam exhausting from the low-pressure end of a steam turbine. (DECC UK).

Backup power

A specific industrial application for onsite power generation technology. This applies to any equipment that exists solely to provide a redundant power source in the case of a failure of the primary power source. Backup power devices are characterized by low load factors, rapid startup and high reliability (also called standby power or standby generation).(Washington State University)


A unit of pressure, equivalent to approximately 14.5 lbf/in2 or 1 atmosphere. (lbf/in2 is commonly, though less accurately, expressed as lb/in2 or psi.)(DECC UK)

Calorific value (CV)

The heat available from a fuel when it is completely burned, expressed as heat units per unit of weight or volume of the fuel. (DECC UK)

Cascade control

A system which automatically starts up or stops units in a predetermined sequence to meet variations in the energy demands being served. The sequence may be changed periodically to ensure that the running time of each unit is approximately equal.(DECC UK)

Chemical dosing

The addition of conditioning chemicals to boiler feed-water or cooling water to protect plant from scaling, blocking, corrosion etc. (DECC UK)


The simultaneous production of both useable heat or steam and electricity from a common fuel source (Platts).

Compression-ignition (CI)

System used in reciprocating engines whereby fuel is injected after compression of the air and is ignited by the increased temperature caused by compression. As preignition is thereby controlled, higher compression ratios than with spark-ignition engines can be utilised, with correspondingly high energy conversion efficiency. The best known examples of these designs are diesel engines. (DECC UK)

Condensing steam turbine

The steam turbine mode whereby steam surplus to site requirements is expanded to the lowest practicable pressure (vacuum stage) to generate more electricity, then exhausted to a condenser where the latent heat in the exhaust steam is removed by cooling water and the resulting condensate is returned to the boiler. (DECC UK).

Current ratio

A measure of liquidity that includes the value of stocks and work in progress. Defined as current assets/current liabilities. (DECC UK)

Demand diversity

A measure of how much of the potential connected load is experienced as an actual load at a given time. The actual definition is the peak demand at the central heat supply source divided by the sum of the individual heat demands. A factor of between 0.85 and 0.95 is normal for space heating systems. A similar concept also exists for the connected electrical load.(Washington State University).

Depreciation policy

The company’s policy for depreciating its assets to reflect wear and tear and the passage of time. (DECC UK)

Discount factor

The factor used to convert net annual cash flow to present value, depending on the interest rate and the number of years from present. Calculated by a derivation of the compound interest formula: DF = 1 (1 + r/100)n where r = % interest rate and n = number of years from now. (DECC UK).

Discount rate

The annual percentage figure used in discounted cash flow analyses to discount the future value of costs/savings to give a Net Present Value. (DECC UK)

Discounted cash flow

A cash flow analysis where the time value of cash is taken into account. (DECC UK)

Distributed generation

Low capacity power generation connected to the distribution or transmission network including renewable resources and combined heat and power units. The definition of low capacity varies usually between 1 and 50 MW. In this report 20 MW limit is used in some tables. (IEA/DSM/Report2010)

District Cooling

A system producing cooling services by means of a distribution network, supplying centrally produced chilled or centrally produced district heat to local chillers. District cooling is distributed on a commercial basis to a non-restricted number of customers.(What is CHP?)

District Heating

District heating means a system supplying heat produced centrally in one or several locations to a non-restricted number of customers. It is a distributed on a commercial basis by means of a distribution network using hot water or steam as a medium. District Heating systems provide also the means to exploit waste heat from a number of different sources. District Heating systems can vary substantially in size, from systems supplying only a few buildings, to system that supplies entire capital cities. District heat is produced centrally and therefore allows central removal of harmful substances, which leads to a better overall environmental protection level. (What is CHP?)

District heating & cooling

"District heating" or "district cooling" means the distribution of thermal energy in the form of steam, hot water or chilled liquids, from a central source of production through a network to multiple buildings or sites, for the use of space or process heating or cooling;(2009/28/EC)

District heating pipeline

Pipeline, including thermal insulation material and related accessories, for transport of heat by means of hot water for the purpose of district heating service.(What is CHP?)

District heating station

Installation in which exclusively heat is produced from other sources of energy. (What is CHP?)


The use of two fuels in a prime mover or boiler. They may be alternatives, e.g. with one as stand-by if the main fuel supply is interrupted, or simultaneous, e.g. as in dual-fuel compression-ignition engines, where gas plus a small proportion (approx. 5%) of diesel is used (the function of the diesel is to reduce the auto-ignition temperature to enable the engine to run essentially on gas).(DECC UK)

Excess air

Reciprocating engines and gas turbines have to operate with far more air than is needed purely for the combustion of the fuel. This excess over the minimum theoretical requirements for complete combustion forms the major proportion of the exhaust gases and is termed excess air. (DECC UK)

Exit fee

A fee that is paid by a customer leaving a utility network intended to compensate the utility in whole or part for the loss of fixed cost contribution from the exiting (Platts)

Extraction-condensing steam turbine

An extraction steam turbine system is one which includes a turbine which exhausts the steam to a lower than atmospheric pressure. This increases the efficiency of the generation but increases the cost and complexity of the plant.(Washington State University)

Fault level

The maximum prospective current that can flow under a three-phase short circuit condition. It should be noted that it can vary according to the point in the system at which the fault occurs. The magnitude of the potential fault level has a major influence on the choice and design of the equipment to be used. (DECC UK)

Fire-tube boiler

A cylindrical steam, hot water or thermal oil boiler, usually horizontal, but may be vertical. The body of the boiler contains water or oil, which is heated by the burner flame and by combustion products in a tubular combustion chamber called the furnace tube, and subsequently in convection tubes or annular flueways inside the boiler. A typical fire-tube is, essentially, a shell-and-tube heat exchanger, using combustion products from the burner. (DECC UK)

Flow temperature

Temperature of the district heating water before heat is taken. (What is CHP?)


Percentage ratio of debt to net assets. (DECC UK)

Heat accumulator

Installation that serves to compensate between the differences in the production and consumption of heat.(What is CHP?)

Heat exchanger

A device in which heat is transferred from one fluid stream to another without mixing. There must obviously be a temperature difference between the streams for heat exchange to occur. Heat exchangers are characterized by the method of construction or operation, e.g. shell-and-tube, plate, rotary. (DECC UK)

Heat grade

A classification of heat source or heat requirement according to temperature. Up to 90oC would generally be classed low grade, otherwise grade limits vary according to the context. Typically, medium grade would be about 90-150°C and high grade, 150°C upwards. (DECC UK)

Heat to power ratio

The amounts of heat energy and electricity produced by a CHP unit, expressed as a ratio. (DECC UK)

High temperature hot water (HTHW)

Pressurised hot water at 150-200°C for space heating and/or process use. (DECC UK)

Higher heating value (HHV)

The heat produced by combustion of one unit of substance at constant volume in an oxygen bomb calorimeter under specified conditions. The conditions are: initial oxygen pressure of 2.0-4.0 MPa (20-40 atm), final temperature of 20°-35°C, products in the form of ash, liquid water, gaseous CO2 and N2, and dilute aqueous HCl and H2SO4. It is assumed that if significant quantities of metallic elements are combusted, they are converted to their oxides. In the case of materials such as coal, wood, or refuse, if small or trace amounts of metallic elements are present, they are unchanged during combustion and are part of the ash. Also known as gross heat of combustion..(NREL Biomass Glossary)

In-duct burner

A burner comprising an arrangement of fuel nozzles located within a duct along which the combustion air (or oxidant) flows. The fuel nozzles may have their separate supply of cooling or stabilising air. This arrangement is commonly used for supplementary firing of additional fuel using the residual oxygen in gas turbine exhaust as oxidant to boost the exhaust gas temperature before it enters the heat recovery boiler. (DECC UK)

Internal rate of return (IRR)

That value of discount rate used in a discounted cash flow analysis that gives a Net Present Value of zero. (DECC UK)

Interruptible rate

Price paid for electricity by commercial or industrial customers that have agreed to have their power cut off by the supplier in the event of high demand due to severe weather or equipment problems. (Washington State University)

Lower heating value (LLV)

The heat produced by combusting one unit of a substance, at atmospheric pressure under conditions such that all water in the products remains in the form of vapor. The net heat of combustion is calculated from the gross heat of combustion at 20°C by subtracting 572 cal/g (1030 Btu/lb) of water derived from one unit mass of sample, including both the water originally present as moisture and that formed by combustion. This subtracted amount is not equal to the latent heat of vaporization of water because the calculation also reduces the data from the gross value at constant volume to the net value at constant pressure. The appropriate factor for this reduction is 572 cal/g. (NREL Biomass Glossary)

Low-temperature hot water (LTHW)

Hot water at up to 100°C for space heating and low-temperature process use. (DECC UK)

Medium-temperature hot water (MTHW)

Pressurised hot water at 100-150°C for space heating and process use. (DECC UK)

Nameplate rating

The full-load continuous rating of a generator or other electrical equipment under specified conditions as designated by the manufacturer, and written on the nameplate. (Washington State University)

Net metering

Allows the electric meters of customers with generating facilities to turn backwards when the generators are producing energy in excess of the customers’ demand, enabling customers to use their own generation to offset their consumption over a billing period. (Washington State University)

Overall efficiency

The overall efficiency of a power and heat supply station equals the quotient of the useful electricity generation plus the quantity of heat supplied to the distribution system and the total expended heat. (What is CHP?)


Particles of solid matter, usually of very small size, derived from the fuel either directly or as a result of incomplete combustion (DECC UK)

Pass-out steam

Also called extraction steam. Steam taken off part-way along a steam turbine to serve a requirement for that particular pressure, the remainder remaining in the turbine to the exhaust stage to generate more power. There may be more than one pass-out tapping to serve differing site requirements. (DECC UK)

Recuperated microturbine

A microturbine that includes a recuperator to recover some of the residual energy from the hot off gases exiting the expander, thereby increasing electrical efficiency. (Washington State University)

Register burner

A burner design incorporating a combustion air (or oxidant) regulator known as an air register. The register permits the amount of air to the burner to be controlled and is designed to ensure effective mixing of the air and fuel to give stable combustion and the desired flame shape. (Compare with in-duct burner, commonly used for supplementary firing.) (DECC UK)

Residual value

An estimate of any actual value left in an item of capital expenditure after it has been depreciated, over a number of years, to a notional nil value. (DECC UK)

Return temperature

Temperature of water after heat has been taken. (What is CHP?)

Shaft efficiency

The percentage of its initial energy supply that a prime mover delivers as mechanical energy at its output shaft. Note: check whether gross or net CV is used to calculate input energy; manufacturers normally use net CV.

Shell-and-tube heat exchanger

A unit having a bundle of tubes contained in a cylindrical shell. One fluid flows through the tubes, the other through the shell. (DECC UK)


A reciprocating engine that utilises an electrical spark to ignite the compressed air/fuel mixture in the cylinders. (DECC UK)

Spark-ignition gas engine

A form of reciprocating internal combustion engine burning gas in which spark plugs provide ignition. Most gas engines used for CHP are based on commercial diesel engine designs, but with lower compression ratios and other modifications. (Washington State University)

Superheated steam

Superheated steam is steam which has heat to a temperature above its saturation temperature. This saturation temperature varies with pressure. Superheating steam is carried out to ensure that there are none droplets of water which would occur if the steam was not superheated. These droplets can damage steam turbines. (DECC UK)

Supplementary firing

This is the combustion of additional fuel mixed with the engine / gas turbine exhaust gases in the CHP heat recovery boiler. This increases the heat recovered in the boiler and increases its temperature. supplementary firing gives CHP greater operational flexibility for meeting electricity and heat demands. It also means that the heat recovered can be used to high temperature applications which a CHP could not service on their own.(DECC UK)

Useful Heat

Useful heat is deliver at a market price to useful applications.(What is CHP?)

Water-tube boiler

The converse of a fire-tube boiler. Box-shaped, the enclosure acts as a combustion chamber and flueways. Water or thermal oil flows inside tubes arranged in panels around the walls of the combustion chamber (radiant tubes) and in tube bundles in the flue gas stream (convection tubes). The tubes are connected to one or more cylindrical drums which act as water reservoirs and/or steam separators. If superheated steam is required, saturated steam passes from the steam drum to a tube bundle mounted in the high-temperature zone and then out to the steam turbine. Economiser tube bundles, which preheat the boiler feed-water to the boiler, are used to maximise heat recovery from the flue gases.(DECC UK)