Power indicator

The current characteristic referred to in thermal power stations and cogeneration plants , the ratio of electric power to Nutzwärmestrom . The number thus expresses the ratio of electrical power to used waste heat output. ${\displaystyle \sigma }$ ${\displaystyle P_{\text{el}}}$ ${\displaystyle {\dot {Q}}_{\text{nutz}}}$

${\displaystyle \sigma ={\frac {P_{\text{el}}}{{\dot {Q}}_{\text{nutz}}}}}$

Alternatively, the electricity index can also be defined from electrical energy to the waste heat used. ${\displaystyle W_{\text{el}}}$ ${\displaystyle Q_{\text{nutz}}}$

${\displaystyle \!\,\sigma ={\frac {W_{\text{el}}}{Q_{\text{nutz}}}}}$

A high power figure indicates a high yield of electrical energy in relation to the useful heat. However, the power figure does not allow any conclusions to be drawn about the overall efficiency of a system. The power figure depends on the technology and can be between 0.15 and 0.3 for Stirling engines and between 0.3 and 1.2 for internal combustion engines. Fuel cells currently achieve a power index that is usually well above the engine power index and can be up to 3.0 (see SOFC ).

The power rating of a system is not a fixed value, but depends on the current operating status of the system. Extraction condensation and back pressure operation reduce the electrical efficiency, but have high differential heating figures . The current index decreases inversely proportional to the extent to which the heat extraction increases.

See also

• Combined heat and power
• Power loss index