3.4 Combined Heat and Power Generation
An alternative to using individual boilers and electric heaters in buildings is combined heat and power generation (CHP). This uses the waste heat from power stations, of many possible sizes, which has great efficiency benefits. The technical potential for this is enormous.
Figure 1 in Section 1 shows about 2400 PJ of energy ‘lost in conversion and delivery’. The bulk of this, about 1600 PJ, was lost as low-grade waste heat from power stations. This figure should be compared with the 2200 PJ, shown in the bottom bar of Figure 1, of low temperature heat used for space and water heating.
Denmark in particular has adopted a national policy of widespread use of CHP with district heating (DH), also called community heating, which involves distributing heat through large insulated pipes under the streets. By 2007 over 60% of Danish homes were fed by DH and 80% of the DH plants were fed by CHP (DEA, 2011). By comparison, DH supplied only about 2% of the national heat demand in the UK in 2009 (Pöyry, 2009).
District heating does not necessarily need to be linked to power stations. It can be supplied from heat-only boilers. It does allow the use of a wide range of fuels, such as energy from waste, biomass, or even solar energy.
In the UK in 2017 there was nearly 6.0 gigawatts of CHP electricity generation capacity (usually written 6.0 GWe). The bulk of this was large gas or steam turbine plant in industry. In buildings in 2017 there was nearly 600 MWe of CHP plant spread over more than 1600 separate installations (BEIS, 2018a). About a third were in hospitals. Most of these were spark-ignition gas engines, like the example shown in Figure 35.
Such units are essentially heavy-duty lorry engines fuelled by gas and driving a generator. Natural gas is a very clean fuel, so units can be designed to run for 5000 hours a year or more with minimal maintenance, equivalent to a quarter of a million miles for a lorry. Such units normally run in conjunction with an extra gas boiler which can provide more heat at times of peak demand. Typically units would range in size from about 50 kWe up to 1 MWe, with electrical efficiencies of 25% up to 40%, large units being more efficient.
Prime users are buildings with large heat and electricity loads such as hospitals and hotels. Leisure centres with swimming pools are particularly appropriate because of the very large low-temperature heat requirements of the pools, which allow CHP units to be run in a condensing mode with an almost 90% overall fuel efficiency. Other schemes have been set up specifically for large blocks of flats, such as the Pimlico scheme in central London which supplies over 3000 homes.