Topping and Bottoming Systems

Most cogeneration systems can be characterised either as topping systems or as bottoming systems. In topping systems, a high temperature fluid (exhaust gases, steam) drives an engine to produce electricity, while low temperature heat is used for thermal processes or space heating (or cooling).

In bottoming systems, high temperature heat is first produced for a process (e.g. in a furnace of a steel mill or of glass-works, in a cement kiln) and after the process hot gases are used either directly to drive a gas-turbine generator, if their pressure is adequate, or indirectly to produce steam in a heat recovery boiler, which drives a steam-turbine generator.