4.3.5 Combined heat and power (CHP)
Up to two thirds of the primary energy used to generate electricity in conventional thermal power plants is lost in the form of heat. Switching from condensing steam turbines to CHP (cogeneration) plants produces electricity but captures the excess heat for use by municipalities for district heating, commercial buildings (Chapter 6) or industrial processes (Chapter 7). CHP is usually implemented as a distributed energy resource (Jimison, 2004), the heat energy usually coming from steam turbines and internal combustion engines. Current CHP designs can boost overall conversion efficiencies to over 80%, leading to cost savings (Table 4.4) and hence to significant carbon-emissions reductions per kWh generated. About 75% of district heat in Finland, for example, is provided from CHP plants with typical overall annual efficiencies of 85–90% (Helynen, 2005).
Table 4.4: Characteristics of CHP (cogeneration) plants
|Technology ||Fuel ||Capacity MW ||Electrical efficiency (%) ||Overall efficiency (%) |
|Steam turbine ||Any combustible ||0.5-500 ||17-35 ||60-80 |
|Gas turbine ||Gasous & liquid ||0.25-50+ ||25-42 ||65-87 |
|Combined cycle ||Gasous & liquid ||3-300+ ||35-55 ||73-90 |
|Diesel and Otto engines ||Gasous & liquid ||0.003-20 ||25-45 ||65-92 |
|Micro-turbines ||Gasous & liquid ||0.05-0.5 ||15-30 ||60-85 |
|Fuel cells ||Gasous & liquid ||0.003-3+ ||37-50 ||85-90 |
|Stirling engines ||Gasous & liquid ||0.003-1.5 ||30-40 ||65-85 |
CHP plants can range from less than 5 kWe from micro-gas-turbines, fuel cells, gasifiers and Stirling engines (Whispergen, 2005) to 500 MWe. A wide variety of fuels is possible including biomass (Kirjavainen et al., 2004), with individual installations accepting more than one fuel. A well-designed and operated CHP scheme will provide better energy efficiency than a conventional plant, leading to both energy and cost savings (UNEP, 2004; EDUCOGEN, 2001). Besides the advantage of cost reductions because of higher efficiency, CHP has the environmental benefit of reducing 160–500 gCO2/kWh, given a fossil-fuel baseline for the heat and electricity generation.