Combined heat and power (CHP) is the site generation of electricity and simultaneous use of process heat. It is also known as Cogeneration. Appreciable cost savings can be made by substitution of grid supplies, with all by-product heat being captured for site use. The efficiency of fuel usage in CHP is much higher (around 90%) than is usually achievable in conventional power stations (40%).
CHP units range in size from 1 kW units for domestic use up to 3 MW giants for large installations such as in hospitals. Selection can be based on a single unit to meet maximum site load, or by multiple CHP units arranged to enter service progressively throughout the load range.
When it comes to planning applications for new-builds (which are subject to either SAP or BREEAM guidelines), CHP units can contribute carbon reductions of up to 30%. Certain European countries, such as Germany, operate a tariff system with significant rewards to any CHP operator exporting site-generated electricity to the grid. Currently, the UK has a limited version of this system, with tariffs available only to CHP up to 3.5 kW rating.
The fuel used most frequently on CHP projects is natural gas. For sites outside the reach of the gas mains network, LPG versions can be made available instead. Diesel powered
CHP units currently have limited appeal on account of the fuel price differential.
Electrical connections can be arranged for a CHP unit to start by itself (known as synchronous, or island-mode operation). These are ideal for remote locations, and need to have start-up battery provision. For the majority of the UK land area covered by the national grid, so-called ‘asynchronous’ units work in tandem with the grid and effectively use this supply to get going.
For cogeneration to be applied successfully, there needs to be a reasonable match between the CHP rated output and site thermal / electrical demand. This should be verified at the design / proposal stage. For certain types of project such as office and retail developments, combined heat and power can be hard to justify on account of thermal loads being sporadic. Much greater success with the matching process can be expected for applications such as schools, high density housing, care homes, leisure centres and hotels.
Thermal output from combined heat and power can be used to supply radiator or UFH wet systems, DHW calorifier coils, space heating based on fan coil units or air handling units. CHP has the highest efficiency of all plant room boilers, and is usually required to act as lead. Increasingly, combined heat and power units are being considered for boiler replacement programmes, which is not surprising: because CHP units replace relatively expensive electricity grid supplies, their normal payback period is between 4 – 8 years.