Today SirEnergies looks back at the operation of cogeneration installations, the main technologies used, and the renewal of this technology with the use of biomass and the development of micro-cogeneration.

What are we talking about cogeneration?

Principle of operation of a cogeneration plant

Cogeneration is the production and the simultaneous use of two different forms of energy from a single energy source within a single installation. Its objective is to valorize the second energy resulting from the production of the first rather than wasting it.

Cogeneration systems use fossil fuels, mainly oil (diesel or fuel oil), natural gas, and biomass-energy, which is considered to be renewable energy.

The production of electrical energy generates a large quantity of heat that is very often unused. Thanks to a heat recovery unit, it can be recovered by converting it into gas or hot water, thus making it possible to supply collective buildings, hospitals and industries.

This process therefore makes it possible to transform fossil fuels in warmth And in driving force And of produce electricity which will either be self-consumed on the site, or injected into the electrical network. The heat produced is recovered, instead of being lost, and directly usable for the production of hot water or steam, or heating.

What are the advantages and disadvantages of cogeneration technology?

The main disadvantage is:

• inevitable losses of energy during transport; production sites must therefore be close to places of consumption, at the risk of making the efficiency of the installation less optimal.

The main advantages are:

• Reduction of greenhouse gases,
• lower production costs compared to separate production systems,
• possibility of producing energy from gas or fuel oil but also from local energies: geothermal energy, biomass or incineration of household waste.

Energy efficiency of a power plant with cogeneration

The essential difference between a cogeneration and a Thermal power plant Of electricity production involves the addition of a system for recovering the heat produced by combustion in the case of cogeneration, whereas in the case of the thermal power plant the heat produced is rejected into the natural environment.

Cogeneration therefore benefits from a excellent energy efficiency.

Classic power plant:

• energy efficiency: 35%
• 65% losses, mainly in the form of heat

Cogeneration:

• electrical efficiency: 35%
• thermal efficiency: 55%
• overall energy efficiency: 90%
• 10% losses

The three main types of cogeneration installations

In order to meet the demand of the various sectors and to cover a wide range of power, from a few hundred kW to several tens of electrical MW, several cogeneration technologies have been developed around 3 main engine families:

• the steam turbine,
• the heat engine,
• the combustion turbine.

Steam turbine cogeneration

Steam turbine cogeneration is the system The most used for steam cogeneration. Steam is produced by a boiler that powers a steam turbine; the turbine extracts thermal energy from the steam under pressure and uses it to produce mechanical work of rotating the output shaft.

This system allows the simultaneous production of electricity, self-consumed or injected into the electrical network, and steam to supply an urban heating network or an industrial process (stationery, sugar, chemistry, food, etc.).

Combustion engine cogeneration

Combustion engine cogeneration is a diesel or gas alternative engine. The thermal energy is recovered from the exhaust gases and from the coolants, mechanical energy is recovered on the motor shaft which drives an alternator producing electricity. This system is well suited to small powers for heating agricultural greenhouses.

La micro-cogeneration is a cogeneration system with very small electrical power (less than 36 kilowatts) using a heat engine. Likewise, we speak of mini-cogeneration for powers between 36 and 250 kVA.

This type of machine, mini-cogeneration or micro-cogeneration, is adapted to meet the needs of warming And ofsanitary hot water of a building instead of a conventional gas boiler. Since micro-cogeneration is a compact system, it can easily be integrated into a conventional boiler room.

Combustion turbine/gas turbine cogeneration

Cogeneration by combustion turbine or gas turbine is a Aircraft engine which operates on the principle of the expansion of a gaseous fluid in a turbine resulting from the combustion of a mixture of compressed air at the compressor and gas in a so-called “combustion” chamber.

Thermal energy is recovered from combustion gases to heat hot water through a heat exchanger; mechanical energy is recovered on the turbine rotation shaft which drives an alternator. This technique was developed mainly in The industry and Heating networks Or the hospitals Having significant energy needs.

The development of cogeneration in France

Obligation to purchase contracts

Starting in the 1990s, in France, the public authorities took measures to set up a legal, fiscal, economic and technical framework allowing promote the development of cogeneration.

These measures have provided a strong incentive to invest in a large number of cogeneration installations in industry, the tertiary sector and heating networks.

In order to encourage the installation of cogeneration plants requiring a larger investment than a gas boiler, the public authorities have put in place what are called obligation to purchase contracts. These contracts guarantee producers of cogenerated electricity the purchase by EDF electricity produced at an incentive price over a period of 12 years

Since network electricity is relatively cheap in France due to the nuclear rent, it was necessary to create a financial incentive in the form of an advantageous feed-in rate for cogenerators.

This feed-in rate is similar to a subvention making it possible to finance part of the cogeneration installation. These rates are available on The EDF institutional site. More than 80% of cogenerations were installed between 1997 and 2000 under the impetus of compulsory purchase contracts.

These cogeneration support systems were repealed by the decree of 27 May 2016. A system called “additional remuneration” has been put in place to compensate for the termination of compulsory purchase contracts.

The end of support for cogeneration is essential to the gradual abandonment of fossil fuels in favor of renewable energies ; in fact, only biomass cogeneration meets commitments to reduce greenhouse gas emissions.

Some figures and orders of magnitude on the cogeneration fleet installed in France

The choice of technology is dictated by the power and the level of heat recovery:

• More than 90% of equipment with a unit power of less than 4.5 MW are heat engines. These motors are concentrated around low electrical powers. Their average power is 1.5 MW and three quarters of the motors have a power of less than 2 MW. Motors represent 80% of cogeneration installations and 30% of electrical power.
• More than 90% of equipment over 4.5 MW is gas turbines. These turbines, on the other hand, are more divided between average powers (two thirds of the turbines have a power of less than 7 MW) and large powers (average power of around 14 MW, with some units around 40 MW). Turbines represent less than 20% of cogeneration installations and 70% of electrical power.

Average size of a cogeneration unit:

• 1.2 MWth in the tertiary sector,
• 4 MWth in heating networks,
• 11 MWth in industry.

This explains an unequal national distribution linked to economic and especially industrial activity: nearly 60% of cogenerations are located in 5 regions (Île-de-France, Provence-Alpes-Côte d'Azur, Rhône-Alpes, Rhône-Alpes, Haute-Normandie and Nord-Pas-de-Calais).

Conclusion and future of cogeneration in France

La biomass cogeneration, which is based on the steam turbine and the production of steam by biomass boiler, makes it possible to save between 15 and 20% of primary energy compared to the separate production of these same quantities of heat and electricity.

This technology, which combines the advantages of cogeneration and the use of biomass (considered renewable energy by the public authorities), is supported by a support mechanism in the form tenders with additional remuneration for 20 years.

Because of the 3 technologies mentioned, steam turbine cogeneration owes its salvation to biomass energy, and thermal cogeneration to micro-cogeneration. Indeed, after having demonstrated its effectiveness in industry or large tertiary buildings, micro-cogeneration is now developing in residential housing, on a smaller scale.

Conversely, gas turbine cogeneration, which had grown strongly in industry thanks to compulsory purchase contracts in the 1990s (because it required heavy investments), has declined in the last 10 years.

Today, the future of cogeneration therefore involves the use of biomass energy and the development of micro-cogeneration.