when a heat engine is used to generate electricity, a large part of the energy supplied to the engine in the form of fuel emerges as waste heat. This applies equally to gas-turbine-, steam-turbine- and piston-engine-based power plants. If this heat can be captured it can be utilised for space heating, water heating or for generating steam, thus making much more efficient use of the fuel.
The efficiency of piston-engine-based power generation varies from 25% for small engines to close to 50% for the very largest engines. Thus between 50% and 75% of the fuel energy emerges as waste heat. In the case of an internal combustion engine, there are four primary sources of waste heat, the engine exhaust, engine case cooling water, lubrication oil cooling water and, where one is fitted, turbocharger cooling.5 Each of these can be used as a source of heat in a combined heat and power system. And since the cooling systems are needed to remove heat from the engine and prevent it overheating, internal combustion engines can be quite simply converted into combined heat and power systems.
The exhaust gas contains between 30% and 50% of the waste heat from the engine. This can be used to generate medium-pressure steam if required. Otherwise it can be used to generate hot water. The main engine case cooling system can capture up to 30% of the total energy input. This will normally be passed through a heat exchanger to provide a source of hot water although in some cases it can be used to produce low-pressure steam as well. Engine oil and turbocharger cooling systems will provide additional energy that can be used to supply hot water.
If all the heat from the exhaust and the cooling systems of an engine is exploited, around 70-80% of the fuel energy can be used. However this falls dramatically where there is no use for hot water. Engine exhaust gases can also be used directly for drying in some situations.
An internal combustion engine must be fitted with cooling systems whether the waste heat is exploited or not, so the use of these systems in combined heat and power applications offers a logical extension of their application. Systems based on small engines can provide power, space heat and hot water to homes and commercial offices while large engines can produce power and process heat for small industrial operations. The economics of these systems can be quite favourable where there is a use for the waste heat. As a consequence they have become extremely popular. In the USA in 2000, for example, there were 1055 engine-based combined heat and power (CHP) systems in operation with an aggregate generating capacity of 800MW.6
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