Combined heat and power (CHP), or co-generation, is the simultaneous generation of usable heat and electricity.
Conventional power plants usually dissipate the heat generation using cooling towers. With CHP it is used for domestic or industrial space heating or for process heat. Tri-generation also includes cooling using an absorption chiller.
Current biomass CHP technologies
Above about 1 MWe (around 3 MWth of heat output) conventional superheated steam turbine technology can be
used. Alternatively biomass gasification used with a gas turbine in a number of configurations may also be
appropriate at high output levels. However at lower output requirements neither of these options is suitable.
Organic Rankine Cycle (ORC)
Both reciprocating steam engines and steam turbine use the thermodynamic Rankine Cycle. At small scales
this becomes very inefficient and expensive owing to the high temperatures and pressures required. It is
possible to replace water as the working medium with an organic compound with a lower boiling point, such as a
silicone oil, freon or organic solvent. This allows the system to work more efficiently at much lower
temperatures, pressures and at smaller scale. As the working medium may be less corrosive to components
such as turbine blades, superheating is no longer necessary and also the turbine can operate at a lower speed,
potentially improving reliability.
Biomass CHP systems based on ORC technology are now commercially available from a number of
manufacturers. Electrical outputs are typically in the range 300 kWe