Combined Heat and Power Focus

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Glossary

Absorption Refrigeration plant: this is a refridgeration plant that uses heat instead of electricity to drive the refridgeration process.

Alternator: this is the machine which generates AC electricity. The alternator is driven by the primemover (engine).

Auxiliary firing: This is 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.

Back-pressure steam: The steam exhausting from the low-pressure end of a steam turbine.

Balance sheet  A company’s statement, at a given date, of all its assets and liabilities.

Bar: This is 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.)

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.

Capital expenditure That part of a project’s value (i.e. cost) that appears on the balance sheet and is depreciated according to the company’s depreciation policy.

Capital purchase  Purchasing an asset outright as opposed to leasing it under an operating lease.

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.

Chemical dosing  The addition of conditioning chemicals to boiler feed-water or cooling water to protect plant from scaling, blocking, corrosion etc.

CO, CO2  Carbon monoxide (CO) and carbon dioxide (CO2) are oxides of carbon produced by fuel combustion. CO represents incomplete combustion and can be burned to CO2, which represents complete combustion.

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.

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.

Current ratio A measure of liquidity that includes the value of stocks and work in progress. Defined as current assets/current liabilities.

Demand, maximum demand, demand profile  The rate at which energy is required, expressed in kilowatts (kW) or megawatts (MW). It is usually related to a time period, typically half an hour, e.g. 1 kWh used over half an hour is a demand rate of 2 kW. Maximum demand is the highest half-hourly rate at which electricity is required during a month or year. Peak load or peak demand are the terms usually used for heat energy. A graph of demand rate over a typical day, for example, is the demand profile.

Depreciation policy The company’s policy for depreciating its assets to reflect wear and tear and the passage of time.

The terms in the glossary have been defined in the CHP context. They may have broader or alternative meanings in other contexts.

Diesel engine  Takes its name from the famous German engineer Rudolf Diesel. A generic term for compression-ignition reciprocating engines, whatever the fuel used. 

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.

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.  

Discounted cash flow  A cash flow analysis where the time value of cash is taken into account.  

Dual-fuel  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). 

Energy services company (ESCO)  A company offering an integrated and wide range of energy-related services.

Equity  That part of a company’s capital that is permanent and participating in the company’s profits.  

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. 

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. 

Feed water treatment  The conditioning of water to make it suitable for use in boilers and associated systems. It is specific to the composition of the water on-site, type of boiler plant etc., and usually comprises some form of softening plus dosing. 

Finance lease  A lease that transfers substantially all the risks and rewards of ownership (other than legal title) of an asset from the lessor (i.e. the finance company) to the lessee (i.e. the business customer). 

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.

Frequency  The number of times per second that alternating current changes direction. Frequency is expressed as cycles/second or Hertz (Hz) of alternating current. The public electricity supply in the UK is 50 Hz.  

Gearing  Percentage ratio of debt to net assets.

Generator  Refers to the combination of prime mover and alternator. 

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 characterised by the method of construction or operation, e.g. shell-and-tube, plate, rotary. 

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. 

Heat to power ratio  The amounts of heat energy and electricity produced by a CHP unit, expressed as a ratio. 

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

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. 

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.  

Kilowatt electrical  One thousand electrical watts. A watt is a measure of power.

Liquidity  A measure of whether the company has sufficient cash, or assets which can readily be turned into cash, to meet its immediate liabilities.  

Load factor  The average intensity of usage of energy producing or consuming plant expressed as a percentage of its maximum rating. Weekly load factor, for example, would be: (total output or consumption) x 100(maximum hourly rating x 24 x 7) 

Loans <12 months  That part of a company’s debt that is due to be repaid during the next 12 months

Loans >12 months  That part of a company’s debt that is not due to be repaid until after the end of the next 12 months.

Long-term debt  Loans >12 months. 

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

Medium-temperature hot water (MTHW)  Pressurised hot water at 100-150°C for space heating and process use.  

Megawatt electrical  One million electrical watts. A watt is a measure of power. 

Net Present Value (NPV)  The aggregate value of all future costs/savings after they have been discounted back to the present time at an annual percentage figure equal to the discount rate.

Network  The distribution system that links energy production to energy usage. Mostly applied to electricity.  

NOx  A general term for oxides of nitrogen produced by fuel combustion, eventually discharged to atmosphere and considered harmful to the environment.

Operating lease  A rental agreement for a fixed period of time during which the risks and rewards of ownership remain with the lessor (i.e. the finance company) rather than the lessee (i.e. the business customer). At the end of the lease period, the equipment is returned to the lessor or, alternatively, it is purchased by the lessee for an agreed sum. 

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

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. 

Payback  The length of time before a project’s cumulative cost savings/revenues equal its capital cost.

Power factor  Kilowatts (kW) or megawatts (MW) divided by kilovolt amps (kVA) or megavolt amps (MVA) for a given point in an a.c. electricity network, e.g. the incoming distribution network operator's (DNO) supply to a consumer. DNO tariffs usually have a direct or indirect penalty charge for poor power factor (say below 0.95), which can be avoided by installing power factor correction equipment.  

Premium  A general term used to describe the quality of a fuel in terms of handling/storage, combustion, consistency of composition, pollutants etc. Natural gas, for example, is a high-premium fuel; heavy fuel oil is a low-premium fuel. Fuel price usually follows premium value.

Profit/loss account  A company’s statement, for the period between two given dates (usually 12 months apart), of all its revenues plus those of its expenses that were incurred in obtaining these revenues. 

Programmable logic controller (PLC)  A programmable device for the control of a system according to a predetermined logic.

Quick ratio  A measure of liquidity that excludes the value of stocks and work in progress. Defined as (current assets minus stocks)/current liabilities. 

Reciprocating engine  When mechanical power is produced by the to-and-fro (reciprocating) movement of a piston within a cylinder, machines are referred to as reciprocating to distinguish them from purely rotating machines like turbines. 

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.)

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. 

Return on Capital Employed (ROCE)  A measure of the financial success of a company or project, defined as: Annual profit x 100%Assets used 

Sankey diagram  a diagram demonstrating graphically and in true proportion the energy flows in a system, starting with the energy sources (inputs) and showing losses, heat exchange loops etc. to the degree desired.  

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.  

Short-term debt  Loans <12 months.

SOx  A generic term for oxides of sulphur produced by the combustion of sulphur in the fuel. Their presence in flue gases can restrict thermal efficiency because, if the flue gas temperature is reduced below specific levels, highly corrosive sulphurous and sulphuric acids are deposited on heat exchange surfaces and in the chimney.  

Spark-ignition  A reciprocating engine that utilises an electrical spark to ignite the compressed air/fuel mixture in the cylinders. 

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. 

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. 

Synchronism  The condition whereby generator frequency and voltage levels match those of the public supply. When operating in Parallel Mode, it is obligatory to maintain these levels within closely specified limits. 

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.


What's New

UK CHP Development Map

UK CHP Development Map

UK CHP Development Map

UK CHP Development Map Screenshot

The UK CHP Development Tool is the latest version of the map, originally developed as a tool aimed at assisting power station developers consider the opportunities for supplying heat and development of combined heat and power (CHP) as required under planning policy. However, it can also be used by both small and large organisations to help identify the locations where the supply of CHP heat would have the greatest potential, and therefore the largest positive environmental impact.

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