Operating modes of a CHP system

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


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.








Determined by the demand for heat

Highest degree of fuel utilisation


Lower power generation



Determined by the demand for power 

Highest degree of power generation


Lower degree of utilisation



Determined by the grid requirements

Contribution to the stability of the power grid
Marketing of balancing energy


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