heat and electricity requirements can be matched carefully to a plant's ability to switch between maximizing electricity and maximizing heat output, CHP can be particularly effective.

The recent past has seen a considerable increase in the capacity of CHP and CHP with district heating (CHP/DH) installed in the United Kingdom, mainly stimulated by the opportunities for sale of electricity to external consumers after the privatization of the electricity supply industry. Electrical output from such facilities rose 29 percent from 1991 through 1993 (Department of Trade and Industry, 1994). There are now nearly 1,000 installations, about one-third at industrial sites and the remainder at commercial, administrative, and residential locations. Over one-half of the installations are small—under 100 kilowatts—but the large facilities (over 10 megawatts) are responsible for over 80 percent of the CHP and CHP/DH generating and heat-raising capacity.

In 1993, 5 percent of the United Kingdom's electricity was generated at CHP and CHP/DH sites; in the industrial sector, this figure is 14 percent. The industrial sector accounts for 90 percent of the total CHP and CHP/DH generating capacity, with particularly significant use in the refineries and chemical industry sectors. The older CHP and CHP/DH schemes tend to be fired by coal or fuel oil but, as in conventional electricity generation, new schemes are increasingly favoring natural-gas fuel.

There is a growing interest in CHP, particularly CHP/DH, in distributed-energy production systems, leading to discussion of whether there might be a reversal of the concentration of generating capacity into fewer, higher wattage facilities over the past 100 years. In the United Kingdom, this has in part been stimulated by the greater freedom of customer choice in the electricity and gas markets resulting from the privatization of the industries. The implications of such trends, where generation of electricity and supply of heat are conducted at the scale of the individual apartment block (or even, under the most bullish scenarios, within the individual home, with automatic sell-back of surplus electricity and so on), have not yet been thought through.

More conventional CHP and CHP/DH schemes are, however, likely to show a steady growth in application (Brown, 1994). At least 10 projects with a capacity of more than 5 megawatts are under construction in the United Kingdom. The U.K. government has also made increasing the proportion of generating capacity that is CHP or CHP/DH part of its strategy for achieving year-2000 targets for reductions in carbon dioxide emissions. This indicates a conviction in official circles that the expansion of the industry will continue, thereby enabling the government to achieve its goals. The most bullish forecasts from within the electricity supply industry predict up to 25 percent of U.K. electricity derived from cogeneration by 2020 (Harvey, 1994).