Cogeneration Meaning, History, Energy, and Principle
Cogeneration is simply the simultaneous production of two forms of energy from a single fuels source, and it works by a mechanism of concurrent energy production, conversion and conservation.
This article discusses cogeneration and how it works. The outline is given below;
-What are The Two Forms of Energy Produced by Cogeneration?
-What’s the Difference between CHP and Cogeneration?
-How does Cogeneration Work? Working Principle of Cogeneration Systems
Cogeneration Meaning
Cogeneration is a process by which multiple (usually two) forms of energy are simultaneously produced, from one energy source [1].
From a slightly different perspective, cogeneration refers to the use of integrated systems and methods, to produce and use energy simultaneously. This definition stems from the fact that cogeneration generally occurs when energy is being generated and consumed in an integrated and conservative manner.
There are a number of important concepts which are relevant to, and have played a role in the development of, cogeneration. One of these is energy conservation.
Ultimately, the goal of cogeneration is to minimize (or, if possible; prevent) the wastage and loss of energy. One of the two forms of energy ‘produced’ in the process of cogeneration is in fact conserved energy which would otherwise have been lost.
What are The Two Forms of Energy Produced by Cogeneration?
In many definitions, cogeneration is alternately referred to as Combined Heat and Power (CHP) generation [8].
The reason for this analogy is simple. The two most common forms of energy produced by cogeneration are heat and power (electricity) [7]. These two energy forms are usually derived from a singular source; which may be non-renewable (like fossil fuels) or renewable (like geothermal and solar).
Heat is a prominent form of energy with regards to cogeneration. This is because the exploitation of most energy sources usually involves the use of, and/or the production of, heat.
A typical example of this can be seen in the combustion of fossil fuels, which is made possible by the application of heat, and also produces energy in the form of heat.
The power (electricity) which is generated by cogeneration systems, is often derived by converting heat energy to mechanical energy. In the process of such conversions, some heat energy is often lost through any of various physical processes like friction, conduction, radiation or convection.
In conventional energy-production systems, a fuel is burnt, with the release of a large amount of heat energy; part of which is converted successfully (first to mechanical energy, and subsequently to electricity) by the system; and the remainder of which escapes from the system and becomes waste energy or waste heat [10].
Very often, the amount of heat which is lost from such systems can be enormous, with temperatures reaching as high as 500°C.
The significance of cogeneration arises generally from the capture and conservation of this waste heat energy. Through recovery of unused energy, cogeneration is often able to achieve significant gains in terms of both fuel efficiency and energy efficiency, relative to conventional methods.
It is not uncommon to see fuel savings of up to 35% in cogeneration systems (that is; compared to conventional energy systems), as well as efficiencies of up to 80 percent [9]. The production of multiple energy forms from one fuel source, also provides a cost-effective option.
History of Cogeneration
The concept and practice of cogeneration dates back to the late Nineteenth Century, with the completion of the first central power plant in the United States; in 1882, by Thomas Edison [4].
This power plant was known as the Pearl Street Station, and was responsible for supplying both heat energy (in the form of steam) and electricity/power, to parts of New York (in which the plant was also located).
The steam from was used mainly for industrial-heating purposes, and it is estimated that up to 58 percent of all electricity from industrial power plants in the early 1900s, was derived from cogeneration [5].
From the onset of the twentieth century, however; various discoveries and improvements in electricity; led to a decline in the level of interest in cogeneration.
Within this period, a number of industries including oil and gas, paper mills and sugar refineries, continued to practice cogeneration.
The use of centralized utility grids became prominent between 1940 and 1970, especially as a result of the convenience and relative cost-effectiveness of this mode of electricity generation-and-supply. By 1974, cogeneration had declined to 4% of total electricity production in the United States.
The late 1970s saw a resurgence in the development of cogeneration technology. This was driven mainly by a rapid rise in the cost of fossil fuels, as well as an increase in the level of awareness concerning the environmental problems (global warming, pollution, climate change) of energy generation, and the need to conserve energy.
In support of the further development of cogeneration technologies, the Public Utilities Regulatory Policies Act (PURPA) was passed in 1978 [11].
This act includes measures that encourage the practice of cogeneration, such as the sale of electricity generated by cogeneration plants, and the large-scale development of cogeneration facilities in the steel, paper, and chemical industries.
In 1992, the Energy Policy Act provided more opportunity for advancement of cogeneration practices [13]. This measure, in combination with several others, has been instrumental in bringing about rapid and expansive growth in the cogeneration industry, within and beyond the United States, from the 1990s and onward.
What’s the Difference between CHP and Cogeneration?
Cogeneration differs from Combined Heat and Power (CHP) technology with regards to the mechanism of energy conversion.
As has been acknowledged earlier in this article, cogeneration is often viewed as equivalent to CHP technology, and vice versa. However, the complexity of CHP technology is generally higher than that of cogeneration.
In CHP systems, efforts are usually made to convert the heat which is generated. This conversion is often done with the help of a steam turbine, which converts the heat to mechanical energy, that provides a rotary force used to generate electricity.
On the other hand, cogeneration technology typically involves a simple, single-cycle mechanism. Here, the primary goal of the system is to generate energy from a single fuel (energy source).
Heat is simply a by-product of the electricity-generation process in cogeneration plants. In order to achieve energy conservation, the unused fraction of this heat is captured and put to use.
Because cogeneration is relatively simple and does not involve multiple turbine cycles, the recovered heat is often used in its original form, without any conversion. Some uses include industrial and domestic heating.
Cogeneration does not necessarily make provision for a multiple conversion process, whereby energy recovered from the initial conversion process is re-converted to other forms of energy, and perhaps to electricity. This is the basic difference between cogeneration and CHP technology.
Also, while both CHP and cogeneration serve the same purpose of energy conservation and energy-resource optimization, CHP technology is generally capable of higher productivity in terms of energy efficiency and power generation, because it ensures that recovered energy is re-applied (by conversion) to generate electricity.
How does Cogeneration Work? Working Principle of Cogeneration Systems
Cogeneration works based on a three-stage process. The stages include fuel combustion, power-generation, and energy-recovery.
Stage 1: Fuel-Combustion
The component of a cogeneration system, which is used in this stage, is usually either a gas turbine or an internal combustion engine.
Fuels used include conventional fossil fuels like coal, natural gas or petroleum. Other fuels which can be used include biomass (which is a renewable fuel) and municipal waste. The burning of these fuels in the presence of oxygen generally produces large amounts of energy, most of which is used to generate power (electricity).
Due to the global efforts to limit fossil fuel combustion and other energy-production processes that contribute to environmental problems like greenhouse emission, global warming and climate change, renewable energy options like solar, are also used in some cogeneration facilities [6].
Some renewable fuels like biogas and biodiesel are also applicable in cogeneration [2].
Stage 2: Power-Generation
In order to generate energy from the fuel or renewable resource, a conversion mechanism is usually required.
This mechanism is often an integral component of the overall cogeneration system, and may be (in most cases) be referred to as a ‘Prime Mover’.
Within the context of energy technology; a prime mover is a mechanism or machine that converts energy into work [12]. It is exemplified by the stream turbine, gas turbine, hydraulic turbine, and reciprocating internal combustion engine.
Converting energy into work also implies the conversion of energy from one form to another.
In most prime movers, the heat energy derived from the energy source is converted to mechanical energy. For internal combustion engines and gas turbines, the heat energy from the fuel induces gaseous pressure which is used to move a piston or rotate a set of blades.
Ultimately, the fuel combustion and energy conversion, both result in electricity, which is often produced through electromagnetic induction.
Stage 3: Energy Recovery
There are different ways by which waste heat energy is produced in cogeneration systems and processes.
A fraction of the heat (which could be as high as 20-50%) originally produced by the fuel, is usually lost in the process of energy conversion and electricity generation.
Also, the electricity generation process itself, often leads to the production of heat energy, which, if not recovered, will be lost as waste heat.
Cogeneration systems recover energy through the mechanism of heat exchange.
This is simply the transfer of heat from the waste-energy outlet (usually an exhaust) of the system, to another medium, thereby ensuring that this heat energy is not wasted.
Generally, the medium used to recover heat energy in cogeneration, is a conductive fluid such as water. This fluid becomes heated and is converted to steam, which can be used for industrial or domestic heating purposes.
The steam produced by energy-recovery in a cogeneration system, may also be utilized for cooling, through a bilateral flow and heat-absorption mechanism. In such cases, the system itself is no longer referred to as a cogeneration system, but rather as a ‘trigeneration’ (Trigen/CCHP) system [3].
Conclusion
Cogeneration is an industrial process which produces two different forms of energy from a single fuel or energy resource, by capturing unused energy which would otherwise be lost while power is being generated.
The two most common forms of energy in a cogeneration system are heat and power. For this reason, cogeneration is often referred to as Combined Heat and Power (CHP) technology.
However, the two concepts differ in terns of their complexity and energy conversion pathways. Cogeneration is relatively simple and involves a single cycle of energy conversion, while CHP technology ensures that the recovered heat is re-introduced into the electricity-generation process.
Cogeneration serves the ultimate purpose of energy conservation (which includes energy efficiency), by minimizing the degree of energy wastage and loss.
Heat is a dominant form of energy with regards to cogeneration. This is because it is often the initial and final form of energy present in the system; produced first from the fuel, and finally from additional processes of energy conversion, friction and heat transfer.
Origins of cogeneration are traceable to the beginning of the industrial era itself. However, the Pearl Street Tower; a cogeneration plant commissioned in 1882 by Thomas Edison, represents the first documented use of this technology.
Subsequent improvements in the convenience of utility grid-based electricity led to a fall in the development and practice of cogeneration in the twentieth century.
Cogeneration became relevant again in the 1970s, mostly as a result of the rising economic and environmental concerns posed by the extensive consumption of fossil fuels.
The race to achieve sustainable development may also be said to have played a role in the development of cogeneration, with measures being established to conserve energy and make fuels more efficient.
Some cogeneration systems rely on renewable energy resources like solar thermal, to operate. However, fossil fuels are still used in majority of cogeneration systems.
The working principle of cogeneration can be itemized into three stages. These are fuel combustion, power generation, and energy recovery.
Fuel combustion, being the first stage, may occur in an internal combustion engine or gas turbine. It is absent only in cases where the energy source is a renewable like solar. When fuel combustion occurs, energy is produced in the form of heat.
Power generation is simply facilitated by converting the heat into another form of energy that can be used to generate electricity. This function is typically aided by a prime mover, which can convert the energy into mechanical energy that is used to do work.
Energy recovery ensures that waste heat (unused energy) from both fuel combustion and power generation, is conserved. The basic mechanism which is used in cogeneration to achieve this, is heat transfer.
A conductive medium (usually a fluid like water) absorbs the heat from the waste-energy outlet (or exhaust) of the system. This energy which has been recovered, can then be used for other purposed like industrial heating.
References
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