Cogeneration is the simultaneous production of two or more forms of useful energy, usually electricity and heat, from a single fuel source. In the early 1900s, many industries employed cogeneration in the absence of economically viable alternatives for the production of process heat and electricity. With the development of large central generating stations and reliable electrical distribution systems, interest in cogeneration waned. Industries found it more economical to produce their own process heat and to purchase their electricity, rather than use self-generation.
In today's world of global competition, high costs of purchased power and concern for the environment, many industries are once again turning to cogeneration. The waste heat associated with many processes is being harnessed and used to generate electricity either for sale to a utility or for self-generation. Conversely, many industries are using a waste product as a fuel or increasing their current fuel usage for the purpose of generating electricity. The waste heat associated with this operation is then harnessed to provide process heat. The net effect of either approach has been a new source of revenue (the sale of electricity) or the lowering of operating costs (the displacement of process heat or the reduction of electricity purchases). The economic incentive in some instances is so great as to promote large electrical generating installations even in excess of the industrial user's own requirements.
This paper outlines the fundamental principles of cogeneration, with illustrations of topping and bottoming cycles, a number of common cogeneration cycles that can be employed in industrial applications and for developer applications, demonstrations of the application of heat-to-power ratios, and tabulations of the performance of typical prime movers in a number of cycles.
This information taken together will permit the reader to evaluate which cogeneration system may best serve a given application.