We call cogeneration the combined production of thermal energy and mechanical energy or electricity (CHP for Combined Heat and Power).

The basic idea of cogeneration is that combustion takes place at very high temperatures (above 1,000 °C), while the need for heat in industry or for heating occurs at lower temperatures, generally between 80 °C and 300 °C.

In these circumstances it is quite possible, when using combustion to meet heating needs, to take advantage of this temperature difference to generate electricity through an engine cycle. The heat source of the engine cycle is the boiler or the combustion chamber, and the cold source corresponds to the heat needs.

It is also theoretically possible to produce heat at high temperature on the premises producing electricity, but this generally proves bad as heat is much less easy to transport than electricity.

The main advantage of cogeneration cycles is that they are among the most efficient in terms of energy and exergy . However, their economic interest should be assessed in each situation, particularly in a country like France where electricity prices are very attractive to industry.

Generally, the objectives pursued by CHP are twofold:

• firstly to achieve economies of operation;

• secondly to ensure security of electricity supply at least for part of the units.

Given their purposes, cogeneration plants can be grouped into three classes:

• "heat and power" installations where heat is the commodity, electricity being a byproduct allowing to give a better value to the fuel. This is the case of large plants using heat or district heating or garbage incineration facilities. Priority is given to the provision of heat, electricity, easily transportable, being valued by selling power surpluses to utilities. In the event of mains utility failure, the plant operates in island mode;

• "total energy" systems seeking to ensure electrical autonomy, heat being the byproduct. They are generally off-grid plants and ships;

• not autonomous facilities, undersized for economic reasons, for which a supplement is provided by utilities for electricity and by a conventional boiler for heat. The installation works only when electricity prices are high and heat needs high. This type of installation is quite common, and is often the one that leads to the best financial results for the company.

Technically, it is customary to classify CHP into two families, depending on the engine cycle used:

• boiler and steam turbine systems, which are widespread, as the benefits of this configuration has been known for over a century. They can use a wide variety of fuels, including coal or waste;

• internal combustion engine systems, which use either gas turbines or reciprocating engines (especially diesel and gas engines) . Heat is recovered from exhaust gases as well as coolants and lubricants. Only liquid and gaseous fuels can be used in these engines.

An excerpt of the textbook chapter is freely downloadable with the agreement of CRC Press

Diapason sessions on CHP are given in the table below. Session S45En deals with technology, while others allow you to build in Thermoptim various models of cogeneration plants:

• in session S46En, a small capacity micro-gas turbine CHP plant is studied;

• in session S47En, a cogeneration plant produces electricity and provides heat to the district heating network of a town of 30,000 inhabitants.