Typically, the term 'cogeneration' in power systems describes the use of steam for both power generation and heating. High-temperature, high-pressure steam first passes through a turbine to produce power. Normally in a simple-cycle power plant (i.e. NOT cogeneration), any remaining energy in the steam would be dissipated to the atmosphere or might even require additional energy to cool (it’s usually allowed to expand in the turbine to the lowest possible pressure and then discharged to a condenser to be converted back into water).

In a cogeneration system, this low-grade energy is put to use. The low-pressure steam that exits the turbine is used for such things as heating buildings, cooking food, boiling wood pulp at a mill, or even heating domestic water for showers in a dormitory. Energy efficiencies can be greatly improved with cogeneration.

Basic Power Plant (NOT Cogeneration)

Cogeneration Power Plant

Topping and Bottoming Cycles
Cogeneration technologies are classified as “topping-cycle” and “bottoming-cycle” systems. These terms are relative to the steam turbine: does the heat recovery occur 'before' the steam turbine(topping), or 'after' the steam turbine (bottoming)?

In a typical topping-cycle system, you'll find a gas turbine with a heat recovery steam generator, 'before' the steam turbine. You can increase the thermal efficiency of a steam electric generating system by increasing temperatures and interposing a device, such as a gas turbine, between the heat source and the conventional steam-turbine generator to convert some of the additional heat energy into electricity.

In a typical bottoming-cycle system, energy is recovered from the system 'after' the steam turbine. The heat engine in a bottoming cycle would be a condensing turbine (similar in principle to a steam turbine but operating with a different working fluid at a much lower temperature and pressure).

So cogeneration isn't that complicated, after all. Let's do a quick recap of this section, and then we can move onto the Generator.