Cogeneration is defined as the joint production, in a sequential process, of mechanical and/or electrical energy and useful thermal energy.

A cogeneration plant usually consists of reciprocating engines, gas or steam turbines, which convert the energy contained in the fuel into mechanical energy and residual or exhaust heat. The mechanical energy is usually transformed into electrical energy by means of an alternator (this is the most common case), and waste heat can be recovered in the form of water vapour, hot water, thermal oils or hot gases, as heat-bearing fluids suitable for thermal applications.

In addition to the equipment described above, cogeneration plants may have auxiliary equipment that uses fuel without the objective of producing mechanical or electrical energy, but only thermal energy or heat to support or replace that supplied by cogeneration systems. The most common systems of this type are:

Conventional generators or boilers, used to ensure heat production can be matched to demand at all times, or to be used in reserve in case of failure of the main equipment.

Post-combustion system, with the same objective as described above, or fresh air systems which allow the supply to be increased or secured in the event of failure of the main engine.

By-pass systems that could allow the use of the motors as emergency systems (without heat recovery), or generators to ensure some electricity-only demands can be met.