Types Of Cogeneration System

Keywords: cogeneration system types, types of cogeneration

Cogeneration which is also known as combined heat and power, serves as a two different varieties of energy being made in one single power source. These two different varieties of energy are usually thermal and mechanical energy. Both of these types of energy are then used for different functions. The mechanical energy is usually used to create electricity via an alternator, if not to work rotating equipment such as compressors, motors or pumps. Thermal energy on the other hand can be utilized for the production of hot water, steam, heat for dryers or also another typical use is for cold water for process cooling. The primary good thing about cogeneration is the entire efficiency of energy, which in some cases can be as high as 85 %. Another benefit of cogeneration is also the fact that it helps reduce the emissions of greenhouse gases and pollutants. Therefore, cogeneration is actually the use of the heat engine or else an entire power station to generate both electricity and heat simultaneously. A fact is that all power plants emit a great deal of heat through the process of producing electricity. Therefore, by using this fact, cogeneration is used to fully capture and use some, or even in some cases all, of the emitted heat for heating purposes.

Types of cogeneration systems

These days, a number of different cogeneration systems are widely-used, namely the following
  • Steam Turbine Cogeneration System
  • Internal Combustion Engine Cogeneration System
  • Gas Turbine Cogeneration System

Steam Turbine Cogeneration System

Two different types of cogeneration systems are trusted, which are
  • The backpressure steam turbine
  • The extraction condensing type steam turbine

One would need to make a definite choice between your two. This choice depends on a number of factors, which include the quantities of power and heat, financial factors, as well as the quality of heat.

Also, another type of steam turbine cogeneration system is the extraction back pressure turbine which is generally used where in fact the end result needed in thermal energy at two different temperature levels. Alternatively, the condensing type steam turbines are usually used when the heat rejected from a process will be finally used to generate electricity.

So why use steam turbines rather than any other kind of prime mover? This would give the user a choice of using a large variety of conventional and alternate types of fuel, such as biomass, fuel oil, natural gas, as well as coal. To be able to optimize heat supply, the power generation efficiency of the cycle in some cases may not be as good. Steam turbines are therefore mostly used in cases where the demand is greater than 1MW up to a huge selection of MW. Therefore, due to the inertia in the machine, steam turbines aren't suited to areas with intermittent demand.

Internal Combustion Engine Cogeneration system

When comparing to other cogeneration systems, this technique has an increased power generation efficiency. Both resources of heat for recovery are the exhaust gas at high temperature as well as the engine jacket cooling water at a minimal temperature. This sort of system is more widely used in areas and facilities which consume a little amount of energy because heat recovery is efficient for smaller systems. It is therefore widely used in facilities which have a greater dependence on electricity than thermal energy and also in places where in fact the quality of heat required is not high.

Even though the most common type of fuel used is diesel, this type of system can also operate with gas or heavy fuel oil. Another fact about this type of system is that, unlike gas turbines, it isn't sensitive to changes in ambient temperature, therefore this makes it a perfect machine to work with for intermittent operation.

Gas Turbine Cogeneration Systems

Gas turbine cogeneration systems may be used to generate all or in some instances area of the actual dependence on energy needed by the plant. In this type of system, the power released at temperature through exhaust can be recovered and used for certain applications of heating and cooling.

Even though the most common means of fuel used is natural gas, sometimes other fuels like light fuel oil and diesel are also used in such a system. The number of gas turbines usually varies between a few MW to 100MW. Due to certain factors like a greater availability of natural gas, a major reduction in installation costs, greater environmental performance, as well as a fast and large progress in technology, gas turbine cogeneration systems are the best developed systems lately. The following are some advantages of by using a gas turbine cogeneration system
  • Gas turbines have a brief start up time
  • Gas turbines provide flexibility of intermittent operation
  • At high temperatures, more heat can recovered

Also, in case of the heat output being significantly less than what's required, and in order to ensure that the thermal output efficiency is kept high, natural gas is employed by mixing additional fuel to the exhaust gas.

In the truth of more power being required at the website, additionally it is possible to utilize what's called a combined cycle, which entails a blend of both steam turbine and gas turbine cogeneration. The steam which is being generated from the exhaust gas of the gas turbine is passed through either a backpressure steam turbine or an extraction condensing type steam turbine, which will create more power. Also, the exhaust from the steam turbine will also provide an amount of required thermal energy.

Classification of system

How does one classify a cogeneration system? These systems are usually classified according to the sequence of energy use as well as the operating procedure used. Therefore, a cogeneration system can usually be classified as either one of the next
  • A topping cycle
  • A bottoming cycle

The system is actually a topping cycle when the fuel supplied is first used to create power, and then later along the way to create thermal energy. The thermal energy in the machine is utilized to meet process heat or other thermal requirements. Topping cycle cogeneration is the hottest type and is also nowadays the most popular kind of cogeneration system.

On the other hand, in a bottoming cycle the primary fuel can be used to produce thermal energy at a higher temperature. Heat rejected along the way is then further used to generate power by way of a recovery boiler and a turbine generator. Nowadays, bottoming cycles are trusted for manufacturing processes that want heat at high temperatures in furnaces, and also reject heat as high temperatures. Even though they are used in the list mentioned below, bottoming cycle plants are less common and not used as much as topping cycle plants. A bottoming cycle is employed in the following types of plants
  • Cement industry
  • Steel industry
  • Ceramic industry
  • Gas and petrochemical industry

Topping Cycle Cogeneration Systems

Combined Cycle

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This kind of system includes a gas turbine or diesel engine which produces electrical or mechanical power followed by a heat recovery system/boiler which is used to generate steam and drive a secondary steam turbine.

Steam Turbine

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This kind of system generates electrical power as well as process steam by burning fuel to create ruthless steam, which is then passed through a steam turbine to create the power needed, as well as using the exhaust from the turbine as low pressure process steam.

Internal Combustion Engine

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This type of system includes heat recovery from an engine exhaust and jacket cooling system flowing to a heat recovery boiler, where it is converted to process steam or hot water for further use.

Gas Turbine

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This type of system is known as a gas turbine topping system. A natural gas turbine drives a generator to create electricity. The exhaust from the turbine is passed through a heat recovery boiler which is employed to generate process steam as well as process heat.

Bottoming Cycle System

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In this kind of system, fuel is burnt in a furnace to create synthetic rutile, which really is a mineral. The furnace produces waste gases which in turn are used in a boiler to create steam. This steam is then used to operate a vehicle a turbine to create electricity through the generator.

Selection of cogeneration system

The following factors are taken into account when trying to choose what type of cogeneration system should be used
  • Base electrical load matching
  • Base thermal load matching
  • Electrical load matching
  • Thermal load matching
  • Heat to power ratio
  • The quality of thermal energy needed
  • Load patterns
  • Fuels available


Trigeneration is recognized as the procedure of generating three different types of energy in a combined manner. These three different types of energy are electricity, heat and cooling. All these are simultaneously produced from a fuel source referred to as combined heat power and cooling. Therefore, in other words, trigeneration takes the process of cogeneration of heat and electricity to some other level, with the use of wasteheat for purposes of cooling by using an absorption chiller. A trigeneration system is actually the integration of two types of technology, namely the cogeneration system as well as cooling technology which is done through compression or absorption systems.

As mentioned earlier in the document, both hottest types of cogeneration are through gas reciprocating engines and combustion engines. Though, fuel cells are also being found in the integration of trigeneration. Natural gas, due to being reliable, having low environmental effects, having zero-maintenance costs, as well as being efficient, happens to be the best fuel to use for trigeneration systems. Additionally it is widely used due to the fact so it burns so successfully in the combustion chamber ensuring lower emissions of pollutants whn in comparison to heavier fuels. As natural gas consists mainly of methane, it leads to lower emissions per unit of energy stored, as methane is a gas which includes a very important characteristic, it includes a higher hydrogen to carbon ratio.

"Based on the U. S. Department of Energy in the year 2009, 2. 5 billion a great deal of CO2 were emitted by power plants in the U. S. , which match 576g of CO2 per kWh. " Therefore by using trigeneration, companies and plants worldwide can have a major impact in reducing the amount of pollutants emitted.

Trigeneration is considered a fresh type and way of creating power, which is becoming even more prevalent in a number of countries that have a warm climate. This is because of the fact that in these countries the heating required is only needed in the wintertime season. Therefore a demand of electrical energy, cooling as well as heating is necessary in a number of different entities, such as
  • Universities
  • Gyms
  • Shopping malls
  • Hospitals
  • Public Buildings
  • Manufacturing facilities
  • Data centers

A trigeneration plant is extremely similar to a cogeneration plant, the one difference being the addition of an absorption chiller, which is utilized to make a cold flow using heat recovered from the hot exhaust gases.

Absorption chillers

Absorption coolers are being used to provide cooling using a liquid refrigerant and a heat source. By using heat, usually heat recovered from hot flue gases, absorption chillers provide cooling to buildings. The main benefits of using absorption chillers are that they use significantly less energy than conventional equipment as well as cooling plants and buildings without emitting harmful pollutants.

The main distinction between conventional electric chillers and absorption chillers is that whilst conventional electric chillers use mechanical energy in a vapor compression process to provide refrigeration, absorption chillers on the other hand use heat energy. Absorption chillers can be powered by waste heat, steam as well as natural gas.

Therefore, an absorption chiller transfers thermal energy via a refrigerant from heat source to the heat sink. Absorption systems are widely used in cooling, heating and power systems. When used in combination with a micro turbine and engine driven generator, absorption chillers may use the waste from these components and put it to use to create power, whilst also producing cooling for space conditioning. Absorption chillers therefore shift cooling in a building from an electric load to a thermal load. Different types of absorption chillers are as follows
  • Direct fired
  • Indirect fired
  • Single effect
  • Double effect
  • Triple effect

Trigeneration is therefore an extremely attractive use in certain situations where all three (power, heating, cooling) needs are essential. A typical example is production processes which demand cooling requirements. It is therefore widely used in areas where electricity, heating as well as cooling are needed.

It is important to realise that cogeneration was used in some of the earliest installations of electrical generation. Industries which generated their own power used to commonly use exhaust steam for process heating. Hotels, stores as well as large offices generated their own power and also heated the building using waste steam. Cogeneration continues to be very common using industries, namely pulp and paper mills, chemical plants as well as refineries.

"In america, Con Edison distributes 66 billion kilograms of 180 C steam every year through its seven cogeneration plants to 100, 000 buildings in Manhattan, the biggest steam district in the United States. The peak delivery is 10 million pounds per hour. "

How does the New York steam system work? This technique is an area heat which uses steam from steam making stations and it is distributed under the streets of Manhattan. This steam can be used to either heat, cool or supply capacity to lots of businesses as well as apartments and other types of buildings. In my own opinion this is a great initiative and way of reducing the emission of pollutants as well as increasing the efficiency of fuel usage. Con Edison is also using trigeneration for cooling in the summer months by using absorption chillers, a system which further increases energy and pollution savings, whilst also reducing peak electrical loads.

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