The combined heat and power generation, also known as CHP, is the simultaneous conversion of primary energy (in the form of fuel) into thermal and mechanical (as basis for the extraction of electrical) useful energy in a technical system. This type of energy conversion is characterised by high degrees of utilisation, that is, the very efficient conversion of fuel into useful energy.

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:

Two independent secondary cycles in the cogeneration unit are required for the separate extraction of the two heat sources. In so doing, the low-temperature heat from the engine cooling water and oil cooler is extracted by way of a heat exchanger. As with the single-stage heat extraction, the return flow temperature must not exceed a defined limit of approx. 70°C. The flow usually has a temperature of approx. 85°C. The energy contained in the exhaust gas can be used for the generation of steam, heat transfer oil or hot water by means of a waste heat recovery boiler. The use of the exhaust gas in direct or indirect drying processes represents another possible application.

There are essentially three different control modes for CHP systems. They are the

• heat-led control
• power-led control
• line-commutated control

With the heat-led control, the operation of the CHP system is determined by the required heat. The CHP system follows the heat requirements and is therefore partly in part-load operation. The highest utilisation factors are achieved in this way, because no heat is unnecessarily released into the environment.

However, the heat-led control does not produce the greatest possible amount of electricity. A customer's energy requirements are generally greater than the installed electrical power. Because the price to purchase power from the grid is usually higher than the costs for generating power in a CHP system, it may be worthwhile to release part of the generated heat into the environment in order to increase the power generation. If the control of the CHP system is determined by the customer’s energy requirements, it is known as power-led control. In that case, the CHP system is only controlled in part-load operation if the power requirements are lower than the installed power of the CHP system. With the power-led control, the CHP system covers the greatest possible proportion of the power requirements. Still, the release of any amount of heat into the environment is not permitted with the power-led control either. Compliance with the high-efficiency criterion throughout the respective year and/or month is compulsory, based on which at least 70% of the fuel-derived energy must be converted into useful energy, that is, power and heat. Failing that, tax benefits with respect to the energy tax may not be claimed.

The line-commutated control is gaining increasing importance. With the line-commutated control, the plant operation is not controlled by the customer’s requirements, but by the grid requirements. The controls of several CHP systems are interconnected to create so-called virtual power stations. When the power demand in the grid is greater than the currently generated electrical power of the cogeneration unit, several CHP systems in the virtual power station increase their electrical power, thereby supporting the grid frequency. If the power available in the grid exceeds the demand, the CHP systems in the virtual power station reduce their output. With the line-commutated control, the CHP systems thus support the grid stability by supplying balancing energy. With the further expansion of renewable energies, the line-commutated control is expected to gain increasing relevance.