The cogeneration (or “Co-generation” ) is a system of production of energy to top Rendement (80% to 90% in general).

In the pointed industrial applications, the output can exceed 95% to even approach 100%. An installation of well conceived cogeneration offers outputs higher than those of any traditional boiler. The cogeneration énergétiquement belongs to the techniques more effective for the renewable energy utilization fossil and .

Principle of the cogeneration

It is contained in its name: it consists in producing, starting from an primary energy Combustible, two derived energies usable: a mechanical or electrical energy and a thermal energy.

Whereas in a powerplant, it is the maximum electric output which is required (electric output of about 40% with a simple cycle and reaching 55% with a Cycle combined), in the cogeneration aims at a total output increased by the priority use of thermal energy, either in an industrial process or in a boiler room; the Co-generation of electricity (or force) is not in this case plus the goal but a consequence, improving the economic assessment of the equipment whose total output can then reach 90%.

Electrical energy is obtained by mechanical energy transformation produced by a turbine or gas engine, or steam turbine. This conversion is obtained by coupling a dynamo (D.C. current) or an alternator (alternative course) with the turbine. Output of the mechanical/electric conversion is approximately 98%. In applications of research, the electric production can also come from a Combustible battery.

In equipment of cogeneration, electrical energy either is autoconsommée, or reinjected on the public electrical communication of transport (high voltage) or distribution (Medium Average) (in France EDF or Local companies of distribution of electricity), following economic conditions fixed by the public authorities. If the Law prohibits the resale of the energy thus produced by a private individual or an industrialist, it authorizes its transfer between sites of the same company.

Thermal energy is generally used with the heating of buildings and/or the medical production of warm water or industrial processes.

Primary energy

Primary energy is the energy contained in a fuel usable in the engines and the turbines: gasoline, fuel, wood, gas, biogas, gas " fatal" produced by certain industries (often destroyed, for example gas of the Flare S industrial chemical and oil), Hydrogen, etc

Electrical power

Electricity is produced by the mechanical energy transformation of the engine or the turbine, through a Alternateur, according to the principle of the Generator. It can also be directly produced by a combustible battery. According to the power concerned, electricity is produced with a tension going of 220-230 V (domestic application to Europe), while passing by 400 V (network low tension three-phase current) up to 15 Kv even more on some very large units with Cycle combined.

According to the needs, the tension will be possibly high through a Transformateur.

Thermal generation

An engine has an electric output from approximately 40 to 45%, a turbine, an electric output from approximately 35 to 40%, and that of a combustible battery is located at the neighborhoods from 20 to 30%. The near total of the balance of the power consumption is transformed into thermal energy.

The cogeneration consists in recovering this energy as well as possible, in order to develop it to reach a total output being able to go up to 80-90%.

The principal means of use of this energy are the production of hot air, the production of warm water and the production of vapor.

In the case of gas turbine like combustion engines, it is possible to recover part of heat at exit in the form of vapor high pressure and temperature. The use of this vapor through a Steam turbine makes it possible to strongly increase the electric production of the unit and to reach an electric output of about 55%. This technique of electric production is called the Cycle combined. It is useful in France of reference for calculation of the tariffs of resale at EDF of the electric production of the cogénérations. These tariffs of resale depend also many other criteria and are defined in France by a regularly updated decree.

These techniques of recovery are valid if there is locally an important need for heat and electricity (near city or of heavy industry).

Cogénération by engine

The engines of cogeneration are available in a range of power going from a few tens of kw to approximately 3 MW. They are thus especially the small installations and the domestic applications which are concerned with this type of technology. Their electric outputs generally range between 30 and 40%.

A produced engine leaves about equal 2 types there of thermal energy:

  • an energy " low température" (approximately 95°C), recovered on oils and cooling waters
  • an energy " high température" (approximately 450°C), on exhaust fumes

The practical use of this heat is a whole problem, in particular due to the various levels of temperature. Moreover, heat is available to rather low temperatures, with as consequence which heat is generally produced in the form of warm water. The production of vapor is however not excluded for engines, but limit the recovery of heat.

The strong proportion of energy low temperature implies, to obtain a good output, to have with provision a use in the form of air or of warm water, such as for example of the networks of district heating or industrial.

  • In the event of surplus of heat, this one can be evacuated by towers cooling, which return heat directly in the atmosphere. But this solution is not very respectful of the environment. An alternative consists in storing the surplus of heat to be able to redistribute it later according to the needs, for example by hydro-accumulation .

The engines of small power are until now not very profitable (in particular for a domestic use or in SME), their cost of purchase and maintenance not making it possible often to make profitable such investments. The continuous rise of the cost of primary energies could change the medium-term things.

There exists however a small unit of cogeneration (warm water and electricity), baptized WhisperGen, which exploits the Stirling engine, a high-output engine. WhisperGen turns to natural gas. This technology néozélandaise of the size of a dishwasher is currently distributed to the United Kingdom by Powergen E.ON, second supplier of electricity and gas. Its combined output is very high (more than 90% according to the manufacturer). Not consumed produced electricity can be redirected towards the network and repurchased by Powergen E.ON. Like any new technology, WhisperGen is still expensive (4 400 €, installation and VAT included) but its price should lower its production progressively.

Cogénération by turbine with combustion

The turbines with combustion are available in a range of power going from a few tens of kw to several tens of MW. Their electric output varies between 25 and 40% according to the power.

Traditionally, the gas turbine term does not indicate only the turbine of expansion, but the compressing unit - combustion chamber - turbine.

The thermodynamic process of a gas turbine is characterized by the Cycle of Brayton. Atmospheric air is aspired and compressed in a compressor. Into the combustion chamber, a fuel is injected into this compressed air and is flaring. The hot combustion gases and high pressure are slackened in a turbine which provides a mechanical work. This work is transformed into electrical energy using an alternator. With the exhaust, the gases always contain much heat. They are thus directed towards a waste heat boiler, where them thermal energy will be transmitted to a coolant (generally of water).

The turbines produce the largest share of their thermal energy (except losses by radiation) in the form of exhaust fumes to approximately 500°C. The exhaust fumes can be injected directly in a boiler or an industrial furnace, as if it were about gas produced by a traditional burner. When this gas comes in supplement from a burner, the complement of energy brought is entirely consumed by the system user, the output of recovery on the exhaust of the turbine is then almost of 100%.

This gas still containing a strong proportion of oxygen, it is possible according to the needs to carry out a " post-combustion" of this gas, by injecting a fuel supplement there downstream from the turbine, in order to raise its temperature until sometimes 900°C.

The turbines thus make it possible to produce vapor at very high temperature, which can be used industrially, or in the optics of a combined cycle. The use of post-combustion being independent of operation of the turbine, that makes it possible to ensure the full mode the turbine from the electric point of view, and to modulate post-combustion according to the thermal needs for the user.

The microphone-turbines of a few tens of kw, just as for the small engines, will find their full rise only with the increase in the cost of primary energies.

Cogénération by steam turbine

This technique, reserved to the industrialists being able to produce great quantities of vapor, makes it possible to produce electricity when vapor is produced in surplus, making it possible to regularize its consumption, by energy conversion. It is also the principal technique used in the large powerplants, nuclear or with fossile fuels, to convert the thermal energy of fuel into electricity.

The cogeneration by steam turbine makes it possible to use varied primary energy sources, including inter alia the various energy sources resulting from the valorization of waste of industry, such as the chips of wood in the sawmills, where vegetation wastes of agriculture.

The thermodynamic cycle of the steam turbines is based on the Cycle of Rankine. Using the heat released by the combustion of a fuel, one produces vapor with high pressure in a boiler. This vapor is then directed towards a turbine, where while slackening, the turbine involves. Left the turbine, the vapor is condensed and brought back to the boiler, where this cycle starts again. In this cycle, combustion is external: i.e. there is no direct contact between the fluid process (vapor) and the hearth. Thus the fuel does not require specifications of precise qualities and thus any fuel can be employed.

Cycle compound

One can also combine and gas turbine steam turbine. Like known as previously, the gas turbine can produce vapor through a waste heat boiler, with or without Co-combustion. This vapor can also actuate a steam turbine instead of being employed directly in a process, and with an alternator placed on the axis of the steam turbine, one can thus produce a complement of electricity. The demand for heat can be appeased by vapor on the outlet side of the turbine. It is obvious that similar configurations allow a high electric output, which results in a weaker thermal efficiency.

Trigénération

The Trigénération is an extension of the cogeneration, with production of a third category of energy, in general of the cold. This cold can be produced mechanically, by direct use of the mechanical energy of the engine or of the turbine, it can also be produced indirectly with through for example of a group with absorption.

Combustible battery

The cogeneration by Combustible battery seems to be a promising way. This technology, if it exists on its principle since 1839, invented by Sir William Grove, still did not become ripe. Technology makes it possible to consider applications so much domestic (heating and electric production of the houses) that industrial. The output is better than that of the cogeneration by engine or turbine and less pollution.

Certain combustible batteries are reversible, and can thus produce their characteristic carburizing and store it, for example starting from electrical energy of solar or different origin, when that it is not used in production.

The current brakes with the development of the combustible battery concern mainly the manufacturing costs of the piles, of the reliability of the equipment and the maintenance of the systems.

The increase in the cost of primary energies should strongly encourage the use of this technology.

Advantages

  • the output of an electric power station of production, that it runs with nuclear fuel, the fuel or coal, hardly exceeds the 40%, the remainder of produced energy being dispersed in the environment (river, atmosphere) in the form of heat. The cogeneration thus makes a better primary energy utilization contained in fuel.
  • We know that the fossile fuel reserves are limited and we must thus make use with parsimony of it. The cogeneration then brings to make savings in fossil energy thanks to its best primary education energy utilization of fuel.
  • transport on long distances of produced electrical energy in a centralized way generates important additional losses " in ligne" , by Joule effect, and implies expensive infrastructures, visually polluting.
  • the cogeneration, when it replaces an energy production d' starting from oil or gas, limit the emission of the pollutants and gas with greenhouse effect in the atmosphere.

Example of compared output

A Gas turbine equipped with a waste heat boiler can produce approximately 35 megajoules of electricity and 50 megajoules of heat starting from 100 fuel MJ. If the same quantities of heat and electricity were produced separately, one would consume approximately 90 fuel MJ for the generator (diesel engine and alternator, output of 38%), plus 55 MJ for the boiler, that is to say 145 on the whole. In this simplified example, the cogeneration makes it possible to save more than fuel 30% compared to the separate production.

Today, we can see power stations with the gas with combined cycle which reaches 58% of output. With a boiler with 95% of output (which also exists), we decrease the fuel consumption with 112,5 MJ for the same quantity of electrical energy and heat. That is to say an energy cost of production of 12,5% superior to the cogeneration.

The cogeneration in France

The electrical production by cogeneration accounted for 11% of the total production of electricity of the EU in 1998. If the share of the cogeneration in the electrical production increased until reaching 18%, of energy saving could represent about 3 to 4% of the total rough consumption of the EU. ( source: European Union )

Strong inequalities remain in Europe. If the French production of electricity by cogeneration were of 3% in 2000, the production in Denmark was then higher than 50% of the electric needs for the country. From 2000 to 2003, in the absence of clear legislation concerning the conditions of repurchase by EDF, practically any project of cogeneration the day in France did not see. The directive 2004/8/CE of the European Parliament and the Council of February 11th, 2004 concerning the promotion of the cogeneration on the basis of request for useful heat in the interior market of energy, and the flight of the cost of fossil energies, make it possible to suppose that the cogeneration will take more importance in France in the years to come.

See too

External bonds

  • the cogeneration in the European Union

  • ATEE (Technical Association Energy Environment)
  • Cogen Europe (European Association for the Promotion off Cogeneration)
  • I-Dacta
  • The World Alliance for Decentralized Energy
  • Countryside COGEN Challenge (European information Campaign for the small cogeneration)

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