There is little dispute that the most rational use of wind power, tidal power, or photovoltaics is for electricity, and that for geothermal it is for heat. But the thermal technologies, where an energy source is burned to heat water and produce steam to drive generators, can be used for either heat or electricity production, or preferably both. Historically, coal has been used for heat and is still widely burned for this purpose. But that is generally an inefficient use of energy and thus adds unnecessarily to the world's CO2 burden. In most cases coal is best used for electricity production -- high efficiency can only be achieved in large power stations.
Although many of the fossil sources and some of the renewables can be used in combined heat and power plant (CHP) this does not mean that it is viable for every thermal plant to run as a CHP unit. There need to be matched demands for the heat and the electricity. Several industrial processes, including chemicals, paper and sugar refining, do have such demands -- and if they are industries which produce waste suitable for burning, biomass electricity is an obvious route. Another principal use for CHP is to provide towns with electricity and heat. But it is simplistic to say that CHP can be used in every application.
In Denmark, the government's "Energy 21" energy strategy plan aims to ease the pressure on fossil fuels and reduce carbon dioxide emissions without compromising living standards. It is succeeding: the country has already escaped from the vicious circle of economic growth needing more electricity, which raises emissions (Windpower Monthly, April 2001). To quote: "Top priority is given to developing wind turbines on land and in coastal waters, immediately followed by utilisation of unexploited resources of waste, wood, straw and biogas."
Interestingly, energy crops in Denmark do not start to make a significant contribution until after 2010, perhaps in the belief that gasification technology will not be ready earlier. By 2030 renewables will be supplying a third of Danish energy needs and wind will be supplying nearly 25% of this. Forestry and agricultural wastes will also supply about 25%, hardly increasing the volume they account for today, while the contribution from energy crops will increase to around 18% and smaller contributions will come from municipal waste, biogas, geothermal and solar sources. The plan includes strategies for energy efficiency and transport.
Of that overall energy use, electricity accounts for about one-third. Renewables will be providing 50% of Denmark's electricity by 2030, with the remainder coming from natural gas and oil. Coal is totally out of the picture. Since Denmark's thermal plant often combine electricity and heat production, the figures for 2030 are given in peta-joules (PJ): oil is to deliver nine PJ, natural gas 150 PJ and renewables will deliver the lion's share, 198 PJ.