1. Introduction
The push towards rapid advancement within renewable technologies has never been of so much importance as realised within the last decade. A global demand in the reduction of greenhouse gas emissions has been, and is of, great importance that has been thoroughly debated within recent times. The everyday user of energy has without a doubt heard or read about issues in relation to the fight against climate change which often makes headlines in the news.
The changing face of politics often makes it difficult to truly understand what the 'higher beings' of the political world are mapping out for the rest of us, but one thing is for sure, climate change is of the utmost priority. Many targets and interim deliverables are set and it seems as though the numbers are always changing, however, one target is prominent; the reduction in greenhouse gas emissions by the European Union member states of 80% by 2050 as set out by United Nations Framework Convention on Climate Change (UNFCCC). The initial movement towards a global stance on climate change took form as the most notable Kyoto protocol which has attempted to prevent the building of a reckless environment by legally binding signed parties to take responsibility for bringing harm to the environment. Established in 2005, the protocol aims to achieve global reduction of CO2 emissions through set procedures or mechanisms that allow for flexibility in achieving the goal. In numerical values, parties that form the group known as Annex 1 (the European Community EC along with 37 industrialised countries [2]) are expected to reduce overall GHG emissions by 5% from 2008 to 2012 compared to levels recorded in 1990 [3].
Moving on from Kyoto is the EU Renewables Directive. Commissioned in early 2008, the set proposal was for 20% of electricity demand in the EU's energy mix to come from renewable generation by 2020 [4][5][6]. In order to achieve this ambitious target, each member state has their own targets in order to meet the total of 20%. In essence, it is up to the 'individual' to decide targets for their own state, however, a significant contribution must be made towards a diverse energy mix across the EU. Within the current legislation, the ability to import electricity generated by renewable sources from out-with EU borders and include this towards national targets is allowed. In the wider aspect of the directive, it was also agreed amongst the community that a 20% reduction in both energy consumption and carbon emission shall be adhered to. Also, making use of renewables in the transport sector has been declared with a target of 10% of consumption from renewable means, mainly biofuels, within both the commercial and non-commercial sectors.
More recently, we have seen the highly anticipated Copenhagen COP15 Climate Talks which were meant to set out the next phase of actions to be taken by Kyoto signatories as the protocol comes to an end in 2012. The meeting of delegates from 192 nations was dubbed as being 'history in the making' as leaders were to take the world in to the next major phase of climate change combat. The meeting was however deemed to be a failure as no legally binding enforcement was reached and only a Copenhagen Accord was noted with no long term action sealed [7]. The meeting divided many peoples opinions with some criticising the use of "disgusting propaganda" in its attempt towards commercial awareness (see http://www.youtube.com/watch?v=NVGGgncVq-4 and note [8]).
The enforcement of legislation has, in recent years, paved the way for an emerging industry which, under the umbrella name of the renewable energy industry, has had one common purpose across all industry players; to develop and market cost effective technologies to make efficient use of available energy to prevent major climate change. As the world has caught on to the potential profits to be made from the market, a new stock index was established as the Renewable Energy Industrial Index, RENIXX, in 2006 in which 30 of the world's largest renewable energy industry companies compete based upon market capitalisation. In order to be eligible for trading under RENIXX, the company must generate at least 50% of their revenue from the renewable energy industry [9]. According to one market analysis, "the global renewable energy market grew by 20.4% in 2008 to reach a value of $310.5 billion" [10]. With continual expansion, the report expects the market to be worth over half a trillion dollars by 2013. Such strong development of the market has been brought about by awareness of the need for such a market to exist. Continual developments and a keen interest by those involved in the market has created both wealth and opportunities all over the world. Within Germany, in 2006, €21.6billion was generated through 200,000 working places within renewable energy systems RES [11]. The greatest achievement was seen in America with 450,000 jobs related to RES in 2006 [11]. With renewables offering such a diverse range of opportunities for business, such as wind, solar, hydro, wave, tidal, biomass, CHP, geothermal, fuel cells etc. an ever achieving industry is expected to gain unprecedented growth.
The constant introduction of emission targets and subsequent renewable technologies all boils down to one thing, the combat against climate change. We all know what climate change entails; the warming of the Earth's atmosphere due to the greenhouse effect. According to the IPCC, there has been a net increase in the CO2 content of the atmosphere as a direct resultant of the industrial revolution. An increase of thirty percent has been observed from 280 to 379 parts per million in 2005 [12], with an increase in atmospheric temperature of one Kelvin [12]. The main contributor to the rise in global temperatures is the rise in CO2 emissions from the developed world where a rapid increase in the burning of fossil fuels has been observed. An increase of the CO2 content in the upper-atmosphere causes a process known as the greenhouse effect, which is a natural phenomenon in itself, to become stronger and have greater effect. The atmosphere is made up of several different gases which all serve a purpose. The collaborated effect is to trap heat within our atmosphere in order to maintain a survivable temperature on Earth. Without this 'trapping' of solar radiation, the temperature on the Earth's surface would be around -14oC [13]. As the layers of our atmosphere become thicker, the greenhouse effect becomes stronger. The reflected heat from the Earth's surface is unable to fully dispense outwards into space through the atmosphere and instead, is reflected back towards the Earth's surface with the addition of more heat from the initial source, the sun.
Global warming cause's great disruption to natural processes such as the transition from season to season and is a proven factor behind the increase in ocean temperatures as well as a loss in Arctic ice mass. The Earth's oceans play a big role in regulating the climate and thus the overall temperature needed for sustainability of life on Earth. Compared to the atmosphere, oceans store around 1000 times more of heat within the natural warming and cooling system [14]. Heat is released from the oceans in the form of water vapour. As the temperatures of the oceans increase above normal levels, they release greater amounts of water vapour than is normal. The problem with this is that the vapour is a strong heat-trapping gas. Thus an increase in vapour within the atmosphere leads to an increase in the amount of heat retained hence a warming effect. On the other hand, the water vapour leads to the creation of clouds which block direct sunlight from radiating upon the surface, thus producing a shading effect leading to cooling of the Earth.
A major consequence of warming is with the rise in sea levels. This rise occurs due to two main factors; thermal warming/expansion and melting of ice. Sea levels have been seen to rise by 1.5 millimetres per year between 1961 to 2003 [15]. It is also stated that thermal warming/expansion rates are more that 50% greater than estimated by the IPCC [15]. Continuation of a rise in sea levels is expected to reach the one metre mark within the next century [15]. A shift towards the melting of ice becoming the predominant factor is also expected in-line with an increase of average temperatures from 2 to 4oC within the next century [15].
As we look for new and improved ways in which we are able to meet the demands of the future and at the same time create a sustainable environment, methods of improving technology are constantly being developed. One particular method is CHP whereby the overall efficiency of the conversion process to generate electricity is greatly improved. By utilising the waste heat produced during electrical generation, efficiencies of 85% are achievable compared to 55% from the best conventional power plants. CHP is proving to be a big great tool in the combat against climate change by the reduction of carbon emissions from the use of this waste heat as the need to burn gas to power boilers to heat water is no longer required in most applications. CHP is a technology that has been around for sometime and is set to becoming a major player in the quest towards a zero carbon economy.
2. Background Theory
The process behind the idea of combined heat and power is known as cogeneration. The concept of cogeneration has been around for more than a century and is well understood. Essentially, the main theories behind this concept are the processes of thermodynamics related to turbine/engine combustion that governs the ability of cogeneration. The thermodynamic principles are well known and are presented in many historical texts that offer the foundation behind turbine/engine operation of cogeneration. As the topic of thermodynamics is incredibly large with knowledge already available in classical textbooks, the interested reader is suggested to review these texts if they wish. The following sections offer a basis of knowledge towards a greater understanding of the various components required for any cogeneration scheme in order to give light to the idea of using cogeneration meeting for both electrical and heat demand at the chosen plant site.
2.1 Introduction to Cogeneration
The concept of cogeneration is described by many definitions. One classification in particular that best sums up the idea of cogeneration is "the production of more than one useful form of energy (such as process heat and electric power) from the same energy source" [16]. The idea behind this is simple; to achieve greater cycle efficiency through the utilisation of waste heat produced during the generation of electricity. Conventional forms of power generation, i.e. coal-fired or natural gas power plants, on average are around 35% to 45% efficient respectively [17]. Advances in technology have led to the development of combined cycle generation (CCG) that can improve cycle efficiency to around 55%.
The overall efficiency of the entire electrical generation process, from chemical conversion through to consumer consumption, must take into account losses associated with transmission and distribution which further reduces the efficiency of large-scale generation. These losses can often equate to 5% to 10% of the overall efficiency, increasing as the location of end user is situated in remote locations. Nevertheless, by improving methods of generation and further development in generation techniques, improvement of extraction of energy from primary fuel sources will eventually lead to the reduction in fuel used and thus a reduction in harmful environmental emissions. Figure 1 below illustrates in numerical terms the difference between conventional and cogeneration. It is useful in showing the increase in fuel unites required, from 100 units to 131 units, in order to meet the demand.
Cogeneration systems operate by capturing thermal energy that would have otherwise been rejected as waste heat from an electrical generation machine, such as a steam turbine or gas turbine. This thermal energy is utilised for either district heating; space or water hearting, or for industrial manufacturing process heating. The captured heat may also be utilised as a thermal source for another independent system [16]. The use of energy in this cascading way from higher temperature to lower temperature often characterises cogeneration from conventional forms of generation that have an electrical power plant and a separate thermal boiler. The key technical benefit of cogenerating schemes is their capability to improve fuel use efficiency for generation of both electrical and thermal energy. The efficiency of a cogeneration plant can reach 90% and beyond [16]. The reason behind this is that the heat from the turbine generator set is captured through a heat recovery system that develops this into useful thermal energy, usually in the form of steam, instead of being purged into the atmosphere through large cooling towers.
As mentioned, the major benefit from this is a reduction in the environmental impact caused by the releasing of harmful greenhouse gasses, mainly Carbon Dioxide CO2, into the environment. This is done in two ways; by changing inefficient equipment within homes/building complexes with a high grade efficient central generating plant, and by generating electrical power for the grid that in turn displaces a fossil fuelled power plant that emits a higher concentration of greenhouse gasses per kilowatt-hour. Reductions in greenhouse gas emissions can in some cases be up to 50% in comparison to conventional generation [16] for the same quantity export of electrical and thermal energy.
Furthermore, both the electrical and thermal energy generated by a cogeneration plant is often used locally which offers the ability to neglect both transmission and distribution losses. Consequently, cogeneration can allow energy savings of around 15% to 40% in comparison to the supply of both electrical and heat power from conventional power plants and boilers.
One drawback of cogeneration is the need to use the thermal energy in close proximity to the plant itself. This is because the efficient transport of heat over long distances requires heavily insulated pipes that are expensive, and a loss in temperature is inevitable for every unit distance travelled. This in turn means that the cogeneration plant must be situated in close surroundings to the heat load. Ideally, cogeneration plants are built to size in order to meet/match the heat demand. In perfect conditions, both the electrical and heat demand profiles are exact. This allows for the greatest achievements in terms of efficiency as no extra electricity or heat has to be imported or 'thrown away' as waste. When the electricity generated is less than required, extra will have to be purchased from the grid in order to meet the demand. However, surplus electricity can be sold to the grid or provided to another customer through the distribution network known as wheeling. In most cases, cogeneration schemes are sized in accordance to the thermal demand that produces more electricity than is required.
Nowadays, the achievements in technology has made cogeneration more cost effective at smaller scales allowing systems to be incorporated into neighbourhood housing schemes and even individual sites such as homes and business offices. The idea of small scale incorporation is known as micro-generation and is seeing a rapid increase in the number of micro-instalments around the world with over 17 million instalments in the UK alone [20].
The most meaningful use of cogeneration is in district energy systems whereby the system is used for both heating and cooling. The captured heat is distributed in the form of hot water or low pressure steam that is circulated through a network of underground pipes. District heating systems usually have a steam boiler as the source of energy that is commonly fired by natural gas. Technological advancement has led to the development of hybrid systems in which the firing of the boiler can make use of a combination of municipal solid waste, natural gas, waste heat from industrial processes and wood waste, all of which often posses greater economical benefits.
The technical process of district energy systems are standard; using waste heat from either an existing boiler that is expelling excess heat, or from electrical generating systems that produce waste heat. This is more cost effective in comparison to conventional systems making district energy systems increasingly favourable.
The concept of cogeneration has been around for over a hundred years. Before the construction of a wide spread transmission network connecting many dispersed industries, usually at large shipping ports, many industrial plants made use of cogeneration for their electrical and thermal needs. In 1882, Thomas Edison designed and built a cogenerating plan in New York that was the first commercial use of cogeneration in the USA [18]. Later in 1913, William Le Roy Emmet, an electrical engineer from New York, designed a mercury/steam combined plant in which heat rejected by the higher mercury cycle was fed into the lower steam cycle for expansion of useful steam [19].
As mentioned, the greatest use of cogeneration is in the combat against climate change. This is done by achieving greater efficiencies that can help us win the fight for future generations. Greater penetration of cogeneration technology in both the domestic and industrial sectors will prove to be beneficial in improving energy demands of the future.
2.2 Benefits of a Cogeneration Scheme
There are various benefits from the use of cogeneration in meeting both electrical and thermal demand of a particular site. One of the major benefits as described above is the increase in cycle efficiency leading to a reduction in the environmental impact of electrical generation. There are many other benefits to be had as long as the cogeneration scheme is sized in accordance to meeting the heat demand of the site. This can include [21]:
2.2.1 Energy and Cost Savings
Savings in both monitory terms and in energy terms will always be made by the introduction of a well designed and well operated cogeneration scheme compared to a conventional power plant. These savings primarily come as the benefit of energy efficiency. Cost savings are dictated by the differential in per unit price between bought electricity from the grid that the cogeneration system displaces, and the price of the primary fuel used. The productivity of cogeneration is a resultant of its ability to provide cheap electricity. However, its ultimate success depends upon effectively using the recovered heat from electrical generation without any waste. This means that the decisive factor is finding an appropriate site with a suitable heat requirement. As suggested by many, using cogeneration is profitable in situation of fairly constant heat demand for a minimum of 4500 hours per year. Another factor of great importance is the timing of the electrical demand. The cost-effectiveness of the cogeneration scheme will increase if operation is through periods of peak electrical demand which is generally throughout the day.
2.2.2 Environmental Savings
As well as the potential cost savings, cogeneration provides considerable environmental benefits by using fossil fuels (or other fuels that will be discussed later) in an efficient manner. It is a great tool in reducing CO2 emissions as well as Sulphur Dioxide SO2 emissions. Levels of many oxides of nitrogen NOx can also be reduced by use of modern combustion techniques.
2.2.2.1 Reduction of CO2
It is often difficult to establish what percentage of electrical generation a cogenerating scheme displaces as it is near impossible to know if the electricity was previously coming from a coal fired power station, a nuclear power station, or a mixture of the two. This in turn makes it difficult to assess exactly the CO2 savings that can be achieved by introduction of a cogeneration scheme. According to COGEN Europe, savings of CO2 emissions can differ between 100kg per MWh to in excess of 1000kg per MWh of generation.
A useful example carried out by COGEN Europe shows savings of 615g per kWh of CO2 from a gas turbine cogeneration scheme a waste heat recovery boiler:
Assuming the cogeneration scheme displaces electricity and thermal energy from a mix of fuels (coal and nuclear fired power generation), CO2 savings of 615g/kWh is achieved.
The European Commission EC set a target of meeting the European share of electrical generation of 18% through the use of cogeneration by 2010. This was laid out in the EC's 1997 Communities Strategy [22]. Meeting this target could in turn reduce European CO2 emissions by 65 million tonnes per year. Significant savings in SO2 and NOx are also achievable by use of a gas turbine with waste heat recovery. These can equate to 23.4g/kWh and 2.9g/kWh respectively [21]. In particular, it is the NOx emissions that are of great concern due to their high toxicity [23]. Formation of NOx depends upon the combustion temperature as well as the air to fuel ratio [24]. NOx emissions can be reduced by lowering the combustion temperature, however, this in turn reduces the power output efficiency and creates greater emissions of Carbon Monoxide CO [24].
2.3 Applications of Cogeneration
There are various applications in which the installation of a cogeneration scheme is appropriate, from small scale to large scale demands. The prospect of using cogeneration is an attractive option for various user groups such as; homeowners, local authorities, small businesses, large corporative companies, social housing, hospitals etc. Cogeneration has an established history within many industries, especially the paper manufacturing and chemical industries that require large amounts of simultaneous heat and power. Sectors that have largely benefitted from development in cogeneration technology and global mass production that has driven down equipment costs are the process industries (manufacturing) and the public sector (hospitals, schools and district heating). All of these areas have large heat demands throughout the year which is beneficial to the success of a cogeneration scheme. The following lists provide some scope as to the areas in which cogeneration can be used [25]:
The Building Sector
The Services Sector
The Industrial Sector
The Agricultural Sector
Making use of renewable fuels can help improve the value of cogeneration. However, using natural gas is most commonly the case:
Renewable Energy
2.3.1 Industrial Cogeneration
The use of cogeneration technology is very versatile and adaptable to any given site conditions, whether it be driven mainly for an electrical demand or for a heat demand. Within industrial situations, cogeneration is best suited to sites that require large amounts of heat, for processes such as drying, as well as a suitably high electrical demand all year round. Typically, industrial cogeneration schemes are large in capacity, in the order of MWe, and are specifically designed for site requirements. However, as can be seen by the table below (2008 data), the number of installed cogeneration schemes under 1MWe of capacity is far larger than that of sites using schemes of more than 1MWe of capacity. These smaller capacity sites however account for a lesser value of the total cogeneration installed capacity.
Modern applications of cogeneration in industry make use of either gas turbine(s) as the prime mover, or more recently, combined cycle turbines. This is because industrial needs tend to be for a high thermal demand and thus high pressure steam is usually the key requirement. The industrial units are built to be long lasting as they often need to operate continuously for 8000 hours or more per year. Thus within industrialised nations, the potential for recovery of thermal energy in industry is usually large enough to allow cogeneration schemes to generate a considerable share of the total base load electrical demand.
2.3.2 Residential and Commercial Cogeneration
Residential and commercial cogeneration schemes tend to be smaller in size (less than 1MWe) than their large scale industrial counterparts. These systems are often designed as a packaged unit that contains three main system components; a reciprocating engine, a generator of small capacity, and a heat recovery system. The packaged system is housed as a complete unit installation and is usually contained within an acoustic sound proofed container. The only external links are fuel line connections (typically natural gas), and output connections for electricity and heat. Their applications can be varied and used in many situations as shown in the table below. The figures of installed capacity are given for 2008.
Appropriate reciprocating engines used within packaged units usually consist of normal stationary diesel engines or other automotive engines that can operate using natural gas. These engines can also be dual-fuelled in order to take advantage of interruptible utility tariffs, thus lowering operational costs. In packaged units, the engines cooling system is used for heat recovery along side the hot exhaust gases. As always, there must be both a sizeable demand for electricity and for heat in order to take full advantage of the small scale cogeneration scheme for the residential sector. Larger uses of cogeneration in the commercial sector make use of the same technology as used in industrial cogeneration such as gas turbines or reciprocating engines. These schemes tend to be used in large office complexes as well as universities.
2.3.3 District Heating
One of the most fundamental uses of any cogeneration scheme is to provide heat to users through a district heating scheme. The design of a cogeneration scheme is influenced by the form of heat that is required by the user, whether it be space heating or hot water, and the physical transmission distance of this thermal energy from the heat recovery system to the heat user. For direct use i.e. laundry drying, the piping used for transmission of the exhaust gases is rather bulky due to insulation. The piping comprises of cylindrical plastic tubes or iron pipes that can have both inner and outer insulation in order to maintain high temperatures along the entire length of transmission. The length of the ducting/piping is limited by the available exhaust back pressure that is a resultant from the performance of the prime mover. Cogeneration schemes that have a heat recovery boiler usually make use of the existing heat distribution network from the existing central boiler house. This central boiler house is adapted to form the heat recovery system that will be connected to the turbines or engines sited adjacent to it in another plant housing. The availability of an existing boiler house and its distribution network is an essential aspect taken into consideration when designing and locating a cogeneration plant. The availability of an existing piping infrastructure helps to reduce both capital costs and installation time as major works do not have to be carried out in laying piping to form the district heating at a particular site. This, however, may not always be the case in places of new housing developments in which the heating network will be laid in conjunction with the gas piping.
The form in which the heat energy is distributed, i.e. steam, hot water, or thermal lube oil, is crucial in delivering the heat efficiently to the end user. On sites, such as housing developments where the heat demand is widely dispersed across the entire site, being able to distribute the heat over 'large' distances will be directly affected by the location of the cogeneration plant, the size of the plant and the thermally efficient medium used to transport the heat. There is a limit to the transmission distance capable in order to maintain a cost effective scheme. Over this limit, the capital costs due to fuel requirements may prove to be unaffordable to maintain. It may be advantageous on some decentralised sites to install several smaller cogeneration units in order to efficiently distribute the recovered heat to adjacent users. However, this does involve the installation of a fuel distribution system to the several cogeneration plants on site, and the accessibility of many appropriate connections to electrical systems on site.
The use of district heating is more commonly used in Northern and Eastern parts of Europe where the winter season is much more colder and longer thus a demand for a longer 'heating season'. As electricity is being generated all year round, and thus thermal energy too, it is much more economical to install district heating systems in places that would consume this thermal energy for a longer duration of time so as not to throw any heat way. In a perfect scenario, the efficiency of a cogeneration scheme can be raised as high as 90% by use of 100% of the thermal energy provided.
2.3.4 Trigeneration
The success of cogeneration has led to the wider use of trigeneration where by heat is used to create a cooling effect in replacement of electrically hungry air conditioning units. Mostly applicable in hotter climates, the need for a longer cooling season means that cogeneration technology can still be utilised in order to provide electricity, heating and cooling.
If a cogeneration scheme is to maximise its efficiency, 100% of the heat produced by the unit should be used either on site or near by. In cases where there is a higher electrical demand than heat demand, it is still the heat demand that determines the overall size of the plant. This leads to less electricity being generated than is required by the site, thus the need for importing from the grid. This type of site would typically have a large demand for cooling. The use of absorption cooling helps to lower the electrical demand from air conditioning units throughout the site, which in turn can raise the overall efficiency of the cogeneration scheme. Trigeneration can be used for district cooling as well as meeting cooling demands in industry and in individual buildings, much like heating from cogeneration.
There are several techniques used to meet the cooling demand of a site by use of conventional cogeneration systems. Two of the most efficient ways is by either using heat to create cooling by compression, or by use of absorption chillers. The most common form of technology used is the absorption chiller that simply produces cooling from heat instead of using electricity. It is very similar to refrigeration in which an evaporator and a condenser is used to cool heat by means of mechanical vapour compression. In a refrigerator, there is a compressor. However, in an absorption chiller, there is a chemical absorber, as well as a generator and a mechanical pump that is sued to supply the required change in pressure. Absorption cooling is achieved by specific pairs of chemicals dissolving into each other. Usually based upon water and a strong water-based lithium bromide solution, it works by firstly evaporating the water by the return journey of a chilled water system that cools the chilled water coils. This is done in the evaporator. The water vapour contained within the evaporator is then drawn to the absorber in which the lithium bromide solution dilutes it. This process produces an amount of heat that needs to be removed. A mechanical pump is then used to increase the pressure of the weakened solution and transfers it from the absorber to the generator through a heat exchanger that produces the cooling effect. Heat from the cogenerating unit is the fed to the generator and the weakened solution is forced to the condenser. The absorbing liquid is then returned to the absorber [29].
Absorption chillers are very reliable and a high availability is achievable as there are very few moving parts, thus maintenance requirements are infrequent. There are also dry air coolers and adiabatic coolers that produce the same cooling effect. They can however be more expensive than absorption chillers when applied on a large scale.
2.3.5 Deployment of Cogeneration
As mentioned previously, cogeneration has been used for over a century within energy intensive industries that require large amounts of both electrical and thermal energy. Traditionally, these systems made use of steam powered generation allowing for exhaust steam to be utilised directly for processes such as industrial drying and hygienic sterilising. With rapid advances in technology, coupled with an ever-increasing variety of equipment, the production of tailor made custom-built cogeneration packages is now feasible. This has allowed for precise harmonising of site requirements, further improving overall system efficiency. Also, with the introduction of tighter legislation, the movement towards installation and operation of a cogeneration scheme has never been of such importance.
The main driving factor(s) behind further development of cogeneration will undoubtedly come from tighter emission restrictions and governmental policies due to the EU Cogeneration Directive (2004/8/EC). As of February 2009, 11% of the EU electricity was generated using cogeneration with a contributory share between member states ranging from 0% to 42.8% [26]. Increasing member state share of installed cogeneration capacity by 2020 could raise the contribution of cogeneration to the wider energy mix to 455TWhe and 1000TWhth. This would come from an estimated capacity of 122GWe [27] that has been identified as the potential increase of cogeneration in those states where the percentage of cogeneration compared to their total national electricity generation is under 20%.
2.4 System Components of Cogeneration
A cogeneration scheme can be implemented in various ways using a diverse selection of fuels and prime movers. This allows cogeneration to be installed in situations where capacities of 1kWe up to many hundreds of MWe are required. The scheme can operate for over 8000 hours per year for many years, with cogeneration schemes throughout the world still operating on old technology, as well as new technology in recent installations. The basic components of a cogeneration scheme are:
2.4.1 Prime Mover
The prime mover is a key component of any cogeneration development as it directly drives the electrical generator from which electricity is produced. The prime mover is also responsible for production of thermal energy that is captured and used. Prime movers can either make use of gas or steam turbine, reciprocating engine, combined cycle turbine or fuel cell technology.
2.4.1.1 Steam Turbine
The history of the modern steam turbine dates back to 1884, when Sir Charles Parsons from London designed and built a steam turbine connected to a dynamo that generated 7.5kW of electrical power. Since then, development of the steam turbine has grown in size with 1.5MW turbines commonly found in electrical generation plants. They are the main workhorse within industrial generation and are of great importance to sustaining modern life as we know it. Within the turbine, steam, usually at a high pressure, is expanded which produces mechanical energy through contact with turbine blades. This rotational energy is in turn used to drive a generator that generates electrical power. In terms of cogeneration, the amount of power that can be produced is dependent upon one crucial factor; by how much the steam pressure can be reduced in order to still meet the local thermal demand. In essence, the high pressure steam that is fed through the turbine loses some of its energy as it is transferred to the rotational shaft of the turbine. In doing so, the steam now becomes low pressure steam that is used for hot water.
The choice of using a steam turbine is usually when cheap fuel, such as waste industrial material, is readily available. It is only useful once the energy contained within the material is released and converted into steam. Using a steam turbine is also beneficial in areas where the heat demand is relatively high compared to the electrical demand. This is because the electrical energy a steam turbine can generate in terms of per unit fuel used is less in comparison to a gas turbine or a reciprocating engine. As the electrical demand of a site increases, a gas turbine can be used in combination with the steam turbine to raise the overall electrical output of the system. When this is the case, the gas turbines high grade exhaust heat is supplied to a heat recovery boiler which then supplies its steam to the steam turbine to generate extra electricity. The low pressure steam from the steam turbine is then used to meet the sites heat demand.
Within the turbine, as the steam passes through and expands from high pressure to low pressure steam, it drives the turbine rotor by kinetic force against the turbine blades that are directly attached to the rotor. The most basic of configurations is the back pressure turbine where by the steam travels through the turbine and is exhausted at the required pressure of the sites heat demand. In this configuration, the exit pressure from the steam turbine is greater than the atmospheric pressure. This steam then continues to the exhaust generating more power and then leaves the turbine process at a lower pressure.
Another useful configuration of the steam turbine is the extraction-condensing turbine whereby condensing of steam is required and the exit pressure is lower than the atmospheric pressure. This configuration helps to maximise the output power by expanding the steam down to a vacuum that is preserved by use of a condenser and a set of ejectors. A very low grade of heat is produced by this method that in general is not a useful trait for use in cogeneration. However, pass-out steam can be obtained by this method in order to satisfy the heat demand. Essentially, the heat load of the site dictates the power that can be generated as this determines the steam turbine configuration, whether it is back-pressure or pass-out/back-pressure. A pass-out condensing configuration releases the electrical generator from the restriction of being governed by the sites heat load.
The efficiency of generation by steam turbine can be remarkably high, sometimes as high as 84%. They can operate on gaseous, solid or liquid fuels that can be either from fossil or renewable sources. Usually, steam turbines have a heat to power ratio of 6:1, although this may be 10:1 dependent upon site requirements and turbine configuration. For optimum power generation, the steam turbine does require input steam at both a high temperature and high pressure that provides heat at moderately low grade. However, a high steam pressure incurs greater costs of both the steam boiler and of continual operational costs. The best choice of pressure, and thus an influence upon plant costs, relies upon the process steam pressures required to maintain the heat demands of the site.
The steam turbine is limited in its applications compared to the gas turbine or reciprocating engine due to the need for a high pressure boiler in order to create the motive steam. They are most favourable in conditions where a cheap, low premium fuel is available and is beneficial in the fact that steam turbines are the only form of electrical generator that can utilise energy from a mixture of fuel sources, whether it be solid, liquid or gas. Steam turbines are generally used in applications where the electrical base load is over 250kWe.
2.4.1.2 Gas Turbine
The gas turbine is the most commonly used prime mover in modern large scale cogeneration schemes. Sizes range from less than 1MWe up to 200MWe plus. Gas turbines are naturally reliable and offer very low maintenance requirements. They are also capable of achieving long term availability, usually in the order of 94% to 98% when in continuous operation. Compared to the steam turbine that requires a high pressure boiler plant, gas turbines are far simpler to install into existing power generation sites. The reduction in requirement of space is also most favourable towards the choice of a gas turbine as space is typically at a premium at most applicable sites.
Exhaust gases from the gas turbine are usually captured by a heat recovery system. This captures the gas at temperatures of 400OC to 550 OC and provides either hot water or steam. In certain cases where the heat demand is greater than the available heat from the exhaust gases, a burner can be used in series between the turbine and the heat recovery system to raise the gas temperature, and hence increasing the temperature of the heat output from the cogeneration scheme. As the name suggests, the most common fuel used to operate the turbine is gas, usually natural gas. Many gases such as biogas or gas from landfill sites can be used thus improving the flexibility of fuel choice in cases where natural gas is either too expensive to run the scheme economically, or when an interruptible gas tariff from the utility supplier is chosen to take benefit or reductions in fuel costs.
In principle, the gas turbine functions much like the steam turbine. A shaft is rotated providing mechanical energy to drive the electrical generator. The shaft of the turbine has a set of blades attached to it that are 'pushed' by pressurised combustion gases fed from the combustion chamber. The fuel and air mixture is ignited, releasing very hot combustion gases under high pressure in the combustion chamber. Once this combustion gas has passed through the rotor of the turbine, it now becomes exhaust gas that is typically between 450 OC to 550 OC. These exhaust gases are used to meet the heat demand of the site. Some energy from the shaft is used to drive a compressor that supplies the high pressure intake required by the turbine. The gas turbine is especially able of providing high grade heat when used in a cogeneration application, achieving heat to power ratios of 1.5:1 to 3:1, dependent upon optimisation of the scheme.
The gas turbine operates in tough conditions of high temperature and high rotational speed. As such, the turbine should only operate with hot gases that are free from contamination that would erode the blades or cause corrosion, thus leading to fatigue due to the harsh operating conditions. Because of this, fuels that are finely refined and are at a high premium are often used, such as natural gas. Other fuels such as concentrate gas oil are highly acceptable and are used as a secondary fuel source when the turbine is set up for dual fuelling. By operation of dual fuelling, operators of cogeneration schemes are able to take advantage of interruptible gas tariffs that are often 30% cheaper in comparison to conventional gas tariffs. Under industrial application, liquefied petroleum gas LPG or Naptha can be used as a secondary fuel as they are free from contaminates. The preferred choice is usually LPG as it can come in either liquid or gaseous form. As previously mentioned, other fuels such as biogas can be used, however, hot gas exiting the combustion chamber will be different in temperature than that of natural gas due to the lower calorific value. A gas turbine plant consumes large amounts of air for appropriate cooling of each part of the entire gas path. As a result, the exhaust gases consist of great amounts of oxygen that in turn can be utilised to help combustion of additional fuel. By doing this, the useful heat to power ratio can be raised as high as 10:1 as the exhaust gases are raised in temperature, sometimes as high as 1000 OC, through a process known as supplementary firing. This process is an extremely efficient method of increasing exhaust gas temperature due to the fact that no extra combustion air is required to burn additional fuel. This is in contrast to auxiliary firing that requires extra combustion air.
In terms of shaft efficiency, dependent upon the type of gas turbine, fuel used and temperature and pressure of its inlet, efficiencies between 20% to 45% can be expected. In reality, efficiencies between 25% to 35% are commonly achieved. The electrical efficiency of the turbine can be increased by directly injecting steam into the combustion chamber that raises the volumetric flow through the turbine. By this method, it is also possible to lower the level of NOx formation in the exhaust gas. A major downside to this method is that pure high grade high pressure steam is needed as engine life can be severely shortened if the steam quality is poor.
Generally, both the turbine and the generator are mounted on the same sub-base by use of a step down gearbox in order to lower the shaft speed from the gas turbine to an appropriate speed suited for the generator. Noise pollution of the turbo generators is of importance in relation to the housing in which the plant machinery resides. They are usually contained within an acoustic house that can be then located within a factory type building to provide an extra layer of noise reduction. This housing also provides a means of fire containment, thus reducing the need for specialist fire prevention apparatus that increase the financial costs.
The gas turbine has been instrumental in developing methods of reducing levels of NOx emissions. This has been achieved by reducing temperatures within the combustion chamber by injecting water steam into the chamber that was originally used for improving the power output. National, or even local, legislation can often dictate 'acceptable' levels of NOx emissions from electrical generation, therefore a method of actively controlling these emissions is essential. This can also be done by use of a dry low NOx burner system. However, they do require additional investment and often increase operational costs of the cogeneration scheme.
2.4.1.3 Reciprocating Engine
The reciprocating engine as used in cogeneration schemes are basically internal combustion engines similar to petrol and diesel engines used in motor vehicles. They operate on the same basis, however they are slightly disadvantaged from the gas turbines in the fact that it is much more difficult to meet the heat demand as temperatures of the exhaust gases are lower. This is because the internal thermal energy that is produced is distributed between the exhaust gases and the engine cooling system. They do however generally offer a higher electrical efficiency in comparison to the gas turbine.
The heat to power ratio is usually between 1:1 to 2:1, although this can be further increased to 5:1 through use of supplementary firing that can be easily implemented on a reciprocating engine based cogeneration scheme. This is because the exhaust gases contain vast amounts of excess air. A process known as boost firing is hindered by the pulsating rhythm of the exhaust gases from the reciprocating intervals of the engine. The engine and its lubricating oil require some form of cooling. This allows an opportunity for heat recovery usually at temperatures of 80OC, however, the heat is commonly found to be of such low grade that it is not always exploitable. This heat is transferred as hot water, usually at temperatures of 120OC. This heat is always produced whether it can be used or not. Exhaust gas temperatures are roughly 400OC and is of high grade and is highly usable. Both the heat from cooling and from exhaust gases are almost of equal share making up the entire heat generated from the reciprocating engine.
The reciprocating engine can come in two distinct forms that are categorised by their technique of ignition. This can be spark ignition (up to 4MW) or compression ignition (up to 15MW).
2.4.1.3.1 Spark Ignition Engine
The spark ignition reciprocating engine allows for a lower per kWh capital cost in generation compared to its counterpart, the compression ignition reciprocating engine. However, shaft efficiencies are lower, only reaching a maximum of 35%. Modern spark ignition engines make use of a re chamber in which the fuel/gas combination is stoichiometric (ratio between two or more chemical substances undergoing a physical or chemical change [28]). Smaller engines operate as an open chamber engine as they do not have a pre chamber. Larger engines that have a pre chamber can achieve shaft efficiencies of up to 44% which is the same as large compression ignition engines. The effect of knocking reduces the output power of the spark ignition engine, thus more favourable is the compression engine. The spark engine is well approved for smaller cogeneration schemes in which the sites heat requirement is for low to medium temperature hot water.
The most common fuel used by spark engines is natural gas, although there is the possibility to use biogas and other gaseous fuels. The spark engine can only be fuelled by gas, which is often seen as its biggest disadvantage. Compared to the compression engine, spark engines offer less heat in the exhaust gases although supplementary firing is highly feasible as 'large lean burn' engines produce exhaust gases that have an oxygen content of around 12%. Reciprocating engines with out of balance forces acting against them. They require sturdy foundations that have to be specifically designed and built for every individual application in order to absorb vibrations and counter acting forces that are produced during operation. Obviously, the more advanced the foundations, the greater the financial cost, such as pneumatic damping. However, unlike the gas turbine, the requirements for acoustic sheltering is not as severe as the general operating noise is generally lower.
The thermal energy produced by the engine is usually exploited by using it for the production of hot air. By making use of appropriate heat exchangers, all remaining energies expelled from the engine can be used. The thermal energy can also be used directly for hot water production of around 100OC. Or by using heat from the exhaust gases, steam can be produced up to pressures of around 15bar. Hot water in this configuration can also be produced by using heat captured through the engine cooling system which is usually 70OC to 80OC.
Traditionally, reciprocating engines have been tuned and optimised in order to gain the highest possible output power and efficiency. This incurs very high levels of NOx emissions as it operates with an air to fuel ratio that is a little over stoichiometric. The levels of NOx emission can however be reduced by running the engine under lean burn conditions. This means that there is a large presence of combustion air. There is a major drawback to this method however; there is a reduction in the power output of the engine as well as an increase in CO emissions and other unburned hydrocarbons. This comes as a direct resultant from high levels of air in the combustion chamber. The most common option for increasing power output is by use of a turbocharger that is both reliable and economical.
Generally, two configurations; either stoichiometric or lean burn engines, are used. The stoichiometric engines are generally less than 300kWe in size and are adapted forms of vehicle engines with modified spark ignition systems. The lean burn engine is typically fitted with a turbocharger for greater power output and increased electrical efficiency. They are used for applications where more than 300kWe is required and are fitted with complex electronic controls in order to maintain high system efficiencies. For very small electrical demands, such as 0.2kWe to 9kWe, a modern Stirling engine can be used.
2.4.1.3.2 Compression Ignition Engine
The compression ignition reciprocating engine comes with system components such as turbochargers and intercoolers attached to the engine and are mostly direct injection four stroke engines used for large scale cogeneration. Unlike the spark engine that can only operate on gas, the compression engine can run on heavy fuel oil, natural gas, or gas oil and are often operated on a dual-fuel set-up with natural gas and heavy fuel oil being used as the primary and secondary fuel source. A small amount of gas oil is required to be injected in to the combustion chamber along with the natural gas in order to guarantee ignition. As with most turbines, the compression engine is suitable for use on an interruptible fuel tariff as it can perform efficiently under gas oil as the fuel source. Power outputs can be several mega-watts electrical (15MWe) with operating shaft efficiencies in the region of 35% to 45%. Heat recovery on a compression engine is poorer than its spark engine rival. Maximum temperature of exhaust gases leaving the combustion chamber are usually 85OC. As the exhaust gases generally contain high levels of air content, supplementary firing is highly applicable. The operations are similar to that of the spark engine, running at speeds of up to 1500 rev/min. They are mostly adapted versions of diesel car engines operating on natural gas or gas oil instead of diesel.
The generation of NOx emissions are still of concern, with more modern engines incorporating a delayed ignition timing system with increased compression ratios in order to minimise formation of NOx emissions. This still allows for the desired operating performance of high output power and high electrical efficiency. In order to do this, the compression engine used in cogeneration applications exploit the engine management systems and fuel injection systems that are used in every-day motor vehicles. As with all prime movers, the operating life and availability of compression engines can be very high, between 85% to 92%, if they are well maintained and sized accordingly.
2.4.1.4 Combined Cycle Turbine
The combined cycle turbine is usually applied to the gas turbine whereby a steam turbine is used in conjunction with the gas turbine with the hot exhaust gases produced from the gas turbine being fed into a heat recovery boiler producing steam. This steam is then passed through a steam turbine generating electrical power and low pressure steam on the output. The combined cycle is applied to the gas turbine as it is the gas turbine that generates the highest grade of usable heat which allows steam to be produced at an adequate pressure, maximising power output from a steam turbine whilst still supplying low pressure steam or hot water to meet the heat demand of the site. Most larger systems, typically 3MWe or above, make use of the combined cycle gas turbine or CCGT. They are commonly found in large power plants generating up to 1800MWe. This has been common due to the rising price of fossil fuels, especially natural gas. The combined cycle can convert more than 40% of the initial energy contained within the primary fuel to electricity. With the addition of supplementary firing, CCGT configuration is the most flexible cogeneration scheme of choice. CCGT is particularly applicable to sites that need steam at low pressure and high pressure, although there is the requirement of a high pressure boiler.
One of the crucial parts of any CCGT scheme is the steam turbine. As previously mentioned, it is versatile in the fact that it can use energy obtained from any fuel, whether it be gas, liquid, or solid. High pressure steam is obtained from burning fuel in a boiler which is then passed through the turbine, generating the mechanical power for the generator. This steam looses energy and now becomes low pressure steam that is used for heating or hot water. Because the energy required to turn the steam turbine comes from the hot exhaust gases of the gas turbine, the burning of fuel is used to directly generate initial electricity from the gas turbine. This means that; per unit fuel used, the proportion of electrical output is increased by use of a CCGT configuration thus improving the overall efficiency of generation.
With ever increasing fuel prices, and the need for greater extraction of energy from the fuel available, CCGT is ever increasing in popularity particularly for applications in cogeneration.
2.4.1.5 Fuel Cells
A new and emerging technology used within cogeneration is fuel cell technology. It simply generates an electrical current through chemical reactions between an oxidant and fuel. This is done directly without the need for any auxiliary forces such as combustion or mechanical work. The fuel cell is described as being an electrochemical cell as its action is the production of electrical energy by means of complex chemical reactions. It can be thought of as a modern version of the everyday battery. However, the modern fuel cell differs from the conventional battery as it constantly requires a reactant in order to initiate the chemical reaction. This reactant has to be refilled in order to maintain an electrical output. Through the advancement in technology, various combinations of both fuels, such as hydrocarbons and alcohols, and oxidants, such as chlorine and chlorine dioxide, can be used in the construction of a fuel cell. However, for use in cogeneration, hydrogen fuel cells are the most common. As the name suggests, a hydrogen fuel cell uses hydrogen as its fuel and oxygen usually from air as the oxidant.
In the hydrogen fuel cell, the fuel can be obtained from a number of different sources such as propane, coal, electrolysis, wind energy, solar energy, biomass, etc. with natural gas being the most common. The construction of large fuel cell is made up of stacks of single cells that are either connected in series to deliver a higher voltage, or in parallel to deliver a higher current. The individual cells generally deliver around 0.6V to 0.7V each ate rated power and can be stacked to deliver a sizable amount of power.
As with all reactions, emissions are inevitable. The resultant of reactions within a fuel cell is either CO2 or water, and an electrical current. The emissions however are negligible, even if the source of hydrogen is from natural gas.
The use of fuel cells in cogeneration is most favourable due to the reduction in moving parts that are susceptible to mechanical wear, as well as contamination of fuel leading to corrosion and erosion. This benefits a cogeneration scheme as it reduces the required shut down time required for maintenance and servicing. Fuel cells also offer high efficiency in terms of both electrical energy and thermal energy. For a cell operating at 0.7V, around 50% of the energy contained within the hydrogen is directly converted to electrical energy, and the other 50% is expelled as thermal energy. Modern fuel cell cogeneration makes use of solid oxide fuel cells that can produce temperatures of up to 800OC through recombination of oxygen and hydrogen. Capturing this heat for use as hot water allows operating efficiencies of the 80% to 90% to be achieved, not taking into account transmission and distribution losses. If the cogeneration scheme is placed near the load, an overall efficiency of around 85% to 90% [31] is feasible.
2.4.1.6 Stirling Engine
The Stirling engine is useful as a prime mover within cogeneration. It is an external combustion engine that operates by the application of heat from an external source. This heat causes a fixed amount of permanent working fluid inside the engine to heat up causing it to expand. The expansion of this gas causes the working piston to move as the gas is contained within a fixed size chamber. As the working piston moves, it causes a secondary piston, known as the displacer, to also move allowing the hot expanded gas to travel to a cool zone where it is cooled. The working piston now recompresses this cooled gas and the cyclic process starts again. The fixed amount of permanent gas used as the working fluid is a gaseous medium such as air, or more commonly, helium. The Stirling engine in an external combustion engine that differs greatly from the internal combustion engines described previously as instead of directly burning fuel inside the engine, the Stirling engine makes use of an external burner that feeds the hot air to the engine in order to make it operate. The Stirling engine also contains a fixed amount of gas that is recycled throughout every cyclic process of the engine.
The engine can make use of virtually any fuel as its heat source as the fuel stock is simply burned in an external burner thus increasing the flexibility of the engine. Also, the engine has a smaller number of moving parts with no ignition system or fuel injectors etc. making its operation much more quieter that in comparison to conventional internal combustion engines. As there are fewer moving parts, there is a reduction in maintenance costs thus increasing its overall availability. Further noise reduction is also achieved as there is no knocking effect as the resultant from continuous combustion of the fuel in the burner. The Stirling engine can also achieve high efficiencies, with current Stirling engine based cogeneration schemes achieving efficiencies between 65% to 85%. Their heat to power ratio is not as other forms of prime movers, only achieving between 1.2:1 to 1.7:1. The Stirling engine is generally more suited than its rivals for sizes up to 100kW, after which the cost of generated power per unit compared to the price per unit fuel used is no longer favourable. This is because the capital costs of the Stirling engine is greater than the internal engines that are mass manufactured on a global scale for the automotive industry. Emission levels of NOx and unb
