This week we present the final thermal power plant type, the so-called combined heat and power (CHP) or cogeneration plant.

kraftwerk gersteinwerk luftbild - rwe pressebilder
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8. Combined heat and power

A CHP plant generates electrical energy as well as a second product: heat. As the transport of heat over long distances is not efficient, the prerequisite for running a CHP plant is the availability of a consumer of heat – a heat sink – in the vicinity of the plant. Industrial processes often require heat as an input. Because of that many industrial facilities also operate CHP plants.

Besides, the production of heat can also be used for ordinary heating purposes. In that case heat is generated centrally (in the CHP plant), but consumed in a decentralized way (e.g. in residential houses). Therefore a district heating grid is necessary. Via this district heating grid, the heat produced can be transported to buildings nearby. When it is used in residential buildings, it can replace local heat generation (e.g. via a gas or oil boiler). The investment costs for a district heating grid are generally quite high.

A CHP plant consists of a modified steam power process. Instead of entirely expanding the steam in the turbine, some part of the pressurized steam is conducted into a heat exchanger. Here, the water passes from its gaseous condition back to liquid water, while releasing condensation heat, which in turn is used in the heat exchanger.

The district heating grid itself transports water. The system performance for electricity generation in such a facility is reduced compared to a regular power plant. However, the efficiency for power and heat generation in such a CHP plant approaches almost 90 %. Power and heat generation are coupled. Thus only both forms of energy can be generated at the same time. The demand for heat is important for the utilization of a CHP plant. Accordingly, the utilization in summer is generally lower than the one in winter. The electricity which is generated has to be fed into the grid, because the production of heat cannot be interrupted.

Thus, electricity cannot be sold at its marginal costs, and a CHP plant has to be operated as a so-called Must-Run power plant.

When compared to a separate generation of electricity and heat, the utilization of a CHP plant may save primary energy. The exact savings however depend on a range of technical and operational details (e.g. the heat losses in the district heating grid).

In addition to primary energy savings a CHP plant also allows for the partial replacement of high quality energy carriers such as oil and gas in the boilers of individual households by the lower quality primary energy carrier coal.

With this contribution our series on thermal power plants comes to an end. From next week onwards we will deal with electricity generation through nuclear power.