Types Of Cogeneration System

Keywords: cogeneration system types, types of cogeneration

Cogeneration which is also known as combined heat and power, can be described as two different forms of energy being generated from one single energy source. These two different forms of energy are usually thermal and mechanical energy. These two types of energy are then used for different functions. The mechanical energy is usually used to produce electricity through an alternator, or else to work rotating equipment such as compressors, motors or pumps. Thermal energy on the other hand can be used for the production of hot water, steam, hot air for dryers or also another typical use is for chilled water for process cooling. The main advantage of cogeneration is the overall efficiency of energy, which in some cases can be as high as 85 per cent. Another advantage of cogeneration is also the fact that it helps reduce the emissions of greenhouse gases and pollutants. Therefore, cogeneration is basically the use of a heat engine or else an entire power station to generate both electricity and heat simultaneously. A fact is that all power plants emit a considerable amount of heat during the process of generating electricity. Therefore, by using this fact, cogeneration is used to capture and use some, or even in some cases all, of this emitted heat for heating purposes.

Types of cogeneration systems

These days, a number of different cogeneration systems are 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 widely used, which are:

• The backpressure steam turbine
• The extraction condensing type steam turbine

One would have to make a distinct choice between the two. This choice depends on a number of factors, which include the quantities of power and heat, economic factors, as well as the quality of heat.

Also, another type of steam turbine cogeneration system is the extraction back pressure turbine which is normally used where the end result needed in thermal energy at two different temperature levels. On the other hand, 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 and not any other type of prime mover? This would give the user an option of using a large variety of conventional and alternate types of fuel, such as biomass, fuel oil, natural gas, as well as coal. In order 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 hundreds of MW. Therefore, due to the inertia in the system, steam turbines are not suitable for areas with intermittent demand.

Internal Combustion Engine Cogeneration system

When comparing to other cogeneration systems, this system has a higher power generation efficiency. The two sources of heat for recovery are the exhaust gas at high temperature as well as the engine jacket cooling water at a low temperature. This type of system is more widely used in areas and facilities which consume a small amount of energy because heat recovery is efficient for smaller systems. Therefore it is widely used in facilities which have a greater need for electricity than thermal energy and also in places where 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 natural gas or heavy fuel oil. Another fact about this type of system is that, unlike gas turbines, it is not sensitive to changes in ambient temperature, therefore this makes it an ideal machine to use for intermittent operation.

Gas Turbine Cogeneration Systems

Gas turbine cogeneration systems can be used to generate all or in some cases part of the actual requirement of energy needed by the plant. In this type of system, the energy released at high 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, in some cases other fuels like light fuel oil and diesel are also used in such a system. The range of gas turbines usually varies between a few MW to 100MW. Due to certain factors such as a greater availability of natural gas, a big 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 in recent years. The following are some advantages of using a gas turbine cogeneration system:

• Gas turbines have a short 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 less than what is required, and in order to ensure that the thermal output efficiency is kept high, natural gas is used by mixing additional fuel to the exhaust gas.

In the case of more power being required at the site, it is also possible to use what is called a combined cycle, which entails a combination 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 in turn will generate 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 following:

• A topping cycle
• A bottoming cycle

The system is known as a topping cycle when the fuel supplied is first used to produce power, and then later in the process to produce thermal energy. The thermal energy in the system is used to satisfy process heat or other thermal requirements. Topping cycle cogeneration is the most widely used type and is nowadays the most popular type of cogeneration system.

On the other hand, in a bottoming cycle the primary fuel is used to produce thermal energy at a high temperature. The heat rejected in the process is then further used to generate power through a recovery boiler as well as a turbine generator. Nowadays, bottoming cycles are widely used for manufacturing processes that require heat at high temperatures in furnaces, and also reject heat as very 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 used 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 type 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 type of system generates electrical power as well as process steam by burning fuel to produce high pressure steam, which is then passed through a steam turbine to produce 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, in which 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 produce electricity. The exhaust from the turbine is passed through a heat recovery boiler which is used to generate process steam as well as process heat.

Bottoming Cycle System

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In this type of system, fuel is burnt in a furnace to produce synthetic rutile, which is a mineral. The furnace produces waste gases which in turn are used in a boiler to generate steam. This steam is then used to drive a turbine to produce electricity through a generator.

Selection of cogeneration system

The following factors are taken into consideration when trying to select 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

Trigeneration is known as the process 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 another level, with the utilization of wasteheat for purposes of cooling with the use of an absorption chiller. A trigeneration system is basically 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, the two most widely used types of cogeneration are through gas reciprocating engines and combustion engines. Though, fuel cells are also being used in the integration of trigeneration. Natural gas, due to being reliable, having low environmental effects, having low maintenance costs, as well as being efficient, is currently the best fuel to use for trigeneration systems. It is also widely used due to the fact that it burns so efficiently in the combustion chamber ensuring lower emissions of pollutants whn compared 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 consists of a very important characteristic, it has a high hydrogen to carbon ratio.

“According to the U.S. Department of Energy in the year 2009, 2.5 billion tons of CO2 were emitted by power plants in the U.S., which correspond to 576g of CO2 per kWh.” [1] Therefore by using trigeneration, companies and plants worldwide can have a major impact in reducing the amount of pollutants emitted.

Trigeneration is considered a new type and way of generating power, which is becoming even more common in a number of countries which have a warm climate. This is due to the fact that in these countries the heating required is only needed in the winter season. Therefore a demand of electrical power, cooling as well as heating is needed 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 only difference being the addition of an absorption chiller, which is used to produce a cold flow using the heat recovered from the hot exhaust gases.

Absorption chillers

Absorption coolers are 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 advantages of using absorption chillers are that they use much less energy than conventional equipment as well as cooling plants and buildings without emitting harmful pollutants.

The main difference 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 through a refrigerant from the heat source to the heat sink. Absorption systems are widely used in cooling, heating and power systems. When used with a micro turbine and engine driven generator, absorption chillers can use the waste from these components and use it to generate 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 a very attractive use in certain situations where all three (power, heating, cooling) needs are a must. A typical example is in production processes which demand cooling requirements. Therefore it is 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 up the building using waste steam. Cogeneration is still very common in certain industries, namely pulp and paper mills, chemical plants as well as refineries.

“In the United States, Con Edison distributes 66 billion kilograms of 180 °C steam each 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.” [2] 

How does the New York steam system work? This system is a district heating system which uses steam from steam generating stations and is distributed under the streets of Manhattan. This steam is used to either heat, cool or supply power to a number of businesses as well as apartments and other types of buildings. In my 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 through the use of absorption chillers, a system which further increases energy and pollution savings, whilst also reducing peak electrical loads.