In cogeneration units, an internal combustion engine – a diesel or petrol engine – is used to power a generator to generate power. The established engine technology originated in the industrial engine and ship building industry. The lion’s share of the waste gas heat as well as the heat accumulated in the engine's cooling water and in the oil cooler is extracted as useful heat by way of heat exchangers.
The shares of waste gas, cooling water and engine oil heat are dependent on the engine power. With large engines, the heat ratios are almost identical, while the cooling water and engine oil heat is greater with small engines.
The waste gas heat is available at a temperature level of approx. 500°C. At the outlet of the waste gas heat exchanger, the temperature of the waste gases is normally about 120°C. The waste gases in the CHP unit can be cooled further by means of an additional condensing heat exchanger. The thermal utilisation factor can be increased by using this heat of condensation.
The temperature of the accumulated heat from the engine cooling water and the oil cooler ranges between approx. 85-90°C and is known as low-temperature heat (=LT heat). Care should be taken that the return flow temperature for cooling the engine should not exceed 70°C whenever possible. Higher temperatures at the secondary cooling cycle are possible if so-called hot-cooled engines are used in combined heat and power units.
Two variants exist for the heat extraction from cogeneration units: