Plant Selection
The information on site heat and power demand will allow appropriate plant ratings to be identified. The value of heat and power generated, set against the cost of installation, will determine the correct size of plant for a particular building/process.
Few sites will have a heat to power ratio that perfectly matches any particular CHP plant so, when considering the most appropriate plant rating, it is important to consider the various options:
- Select a unit that matches the site’s electrical load. Make up any deficiency in heat demand using boilers. If there is too much heat, modulate the unit (i.e. reduce the engine’s output of heat and power) or dump the excess heat to atmosphere.
- Select a unit that matches the site’s heat demand. Make up any deficiency in electrical demand by importing from the public electricity supplier (PES). If too much power is generated, modulate the unit accordingly or, if viable, export the excess power (requires additional metering (export meter) and contract with the PES or other organisation purchasing the exported power).
With the introduction of the CHP Quality Assurance programme and the Climate Change Levy (CCL), the dumping of significant quantities of heat was discouraged. However, for building applications with a seasonal heating load, it is recognised that the optimum CHP unit (in terms of rating and economics) will usually involve dumping a small proportion of the heat output. CHP units that do dump a proportion of their heat output (typically up to 30%) can still meet the Good Quality requirements and provide an overall environmental benefit.
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