Steam Turbine CHP Generators
These use high-pressure steam in a constant flow which is generated in a boiler to drive the turbine.
Electrical efficiency is maximised when the steam is condensed and pumped back to the boiler as hot water just below boiling point.
The thermodynamic cycle is called the Rankine cycle. The industrial scale fully-condensing steam turbines in power plants (both nuclear and fossil fuel) have average electrical efficiencies of about 36-38%. But in CHP applications, (where the steam extraction reduces their electrical output), they have typical electrical efficiencies of 10.7-20%. However it must be noted that their overall efficiency ranges from 77.6-82.5%.
The electrical efficiency of steam turbines with CHP applied depends on the size of turbine and the pressure at which steam is extracted. Steam turbines can be deployed as the prime mover for custom-built CHP plants by recovering some of the heat at one of the following stages in the process:
- As low-grade hot water (about 30ºC) recovered from the secondary cooling circuit in the condenser with no consequent loss of power. This is the most efficient option but is not common as such low grade heat is only of use in a few applications such as liquefied natural gas vaporisation.
- As medium-pressure steam between turbine stages (‘pass-out’). This reduces power generation (electrical efficiency).
- As low-pressure steam slightly above atmospheric pressure exiting the final stage of the turbine (‘back pressure’).
Heat can then be used for process or space heating. Such turbines are particularly appropriate for CHP when steam is needed, or where the fuel available cannot be burned directly in the prime mover. They are typically suited to large-scale applications or where the amount of heat required is much greater than the amount of power.