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III.1 Combined heat and power systems

III.1.1 CHP Technology

Combined heat and power (CHP) means the simultaneous generation of thermal and electrical power in one system. In comparison to the separate generation of heat in a domestic heating system and drawing of electricity from the public electricity grid, CHP-systems have the potential to save primary energy. The main reason for this saving potential is the use of the waste heat which is normally rejected by thermal energy conversion systems. For small decentralized CHP-systems, avoiding network losses is an additional positive aspect. Figure III-1 compares the typical fuel input needed to produce 30 units of electricity and 60 units of heat using conventional separate heat and power generation on the left side and CHP technology on the right side.

Figure III-1: Conventional vs. CHP

Because of the simultaneous generation and the very limited and expensive possibilities to store electricity immediate utilisation of the electrical power is needed.

CHP systems can follow either the heat or the electrical power demand. Micro-CHP-systems which were treated in this study generally run as heating appliances, providing space heating and warm water in residential, suburban, rural or commercial buildings. Any electricity not consumed is fed into the public grid.

To describe CHP-systems some characteristic values have to be defined. The power to heat ratio is defined as:


σ : Power to Heat Ratio
Pel : Electrical Power
: Heat Flow

To get the maximum rate of energy saving, the power to heat ratio of the installation ( σi ) and of the demand ( σd ) has to be equal most of the time.

σi = σd III-2

In practical appliances such equality is rare so that an additional peak burner and heat storage for supplemental heat demand as well as a connection to the public electricity grid for peak or off-peak1 current are needed.

Another important value is the efficiency. For a CHP-system, three efficiencies need to be defined: electrical (ηel), thermal (ηth) and total (ηto) efficiency. Even though it is not correct the single overall efficiency for electricity and heat for CHP are often quoted instead of the energy for both energy forms.




E : Total Primary Energy Consumption
Eel : Electricity Generation
Q : Heat Generation

As the second law of thermodynamics implies that heat cannot be transformed completely into work, the overall efficiency does not describe a system unambiguously. To compare different CHP-systems correctly at least two of these efficiencies are needed.

III.1.2 Different Micro-CHP-Systems

For micro-CHP-systems several technologies have been developed [Simader et al., 2006].

  • Internal combustion engines are conventional combustion engines coupled with a generator and heat exchangers to recover the heat of the exhaust gas and the cooling cycle.
  • Stirling engines are thermal engines where the heat is generated externally in a separate combustion chamber (external combustion engines). They are also equipped with a generator and heat exchangers.
  • Micro gas turbines are small gas turbines belonging to the group of turbo machines up to an electric power output of 300 kWel. In order to raise the electrical output micro gas turbines are equipped with a recuperator (heat/heat exchanger). They are also equipped with a regular heat exchanger in order to use the waste heat from the exhaust gases.
  • ORC: The Organic Rankine Cycle (ORC) is similar to the cycle of a conventional steam turbine, except for the fluid that drives the turbine, which is a high molecular mass organic fluid. The selected working fluids allow low temperature heat sources to be exploited efficiently to produce electricity in a wide range of power outputs (from few kW up to 3 MW electrical power per unit).

Various other technologies, such as steam engines, thermoelectric devices, etc. are still under development.

While reciprocating units are already commercially available, Stirling engines, ORC and micro-gas turbines are at field-testing or demonstration stage.

Another competitor in this field, which is discussed in depth, are Fuel Cell CHP appliances (see chapter IV).

1 Feeding surplus electricity into the public grid.

12 Acceptable costs indicate the level at which a new technology reaches a break-even point in comparison to conventional technologies. They can be derived from the marginal costs of the optimal solution