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Gas Engines

image og a packaged gas engine CHP

CHP Gas engines ‘travel’ the equivalent of 300,000-400,000 miles each year!

Gas-engine CHP packages are available in a range of electrical outputs – from less than 50 kW to around 1,000 kW. The electrical generating efficiency of these packages is typically around 30%, and units can be operated at reduced load with very little drop in engine efficiency. The ratio of recovered heat to electricity generated in a gas-engine package is typically around 1.5:1.

The gas engines used in CHP packages are internal combustion engines that operate on the same familiar principles as the engines in vehicles: they use spark plugs to ignite the fuel in the engine and are sometimes referred to as ‘spark-ignition engines’.  These engines have been designed for operation on a gaseous fuel, most commonly natural gas. Many engines can operate on supply pressures as low as 0.1 bar gauge (barg), the pressure at which gas is usually available from the gas supply system. In situations where the gas pressure is inadequate, a small pressure booster unit can be installed as part of the CHP package.

Since the CHP engine drives an electrical alternator, the engine must be designed to operate at constant speed and at exactly the same frequency as the mains supply, even though the fuel input and electrical output of the CHP package may be variable. The gas engines used in CHP packages typically operate at 1,500 rpm: units above 1.3 MW may operate at 1,000 rpm.

figure showing the energy distribution of spark ignition CHP

Engines and their lubricating oil must be cooled to prevent overheating. This cooling system provides heat in the form of hot water, which is produced whenever the engine is running, irrespective of whether or not it can be used. In a packaged CHP unit, the engine/lubricating oil cooling system is usually connected to a heat exchanger that also recovers heat from the engine exhaust. This helps to maximise the efficiency of the engine. Cooling system heat and exhaust heat are recovered in roughly equal proportions from a gas engine CHP package. The heat from the engine is typically at around 80°C, but some engines can operate using pressurised hot water, which delivers heat at up to 120°C.

If the recovered heat is not all required by the site, the surplus must be dissipated using a cooling system. Alternatively, the power output must be modulated to match heat demand. The cooling system is similar in principle to a vehicle engine’s radiator and needs to be of sufficient capacity to maintain the flow of water to the engine at the correct temperature. All engines are equipped with automatic controls, which shut down the engine if it starts to overheat.

Gas engines vibrate, and the package design usually incorporates supports to dampen the effect of any vibrations on the floor beneath the package and on pipework. The noise levels from gas engines can also be a nuisance, particularly if the noise resonates within a building, and nearly all CHP packages are designed to act as effective acoustic enclosures to limit this problem. The enclosure itself is ventilated to avoid overheating.

All engines have moving parts, some of which suffer gradual wear and, therefore, require maintenance or replacement at regular intervals. Some of the routine maintenance tasks may be carried out while the engine is operating, but regular shutdowns for maintenance and servicing are also required. The total downtime is not excessive and high levels of engine availability can be achieved (typically 90%).


 

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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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