Sustainable and renewable energy systems are the basic requirements of a modern technology society. A modern technological society makes substantial use of energy to make the life better and easier in terms of quality. On the other hand, further development of more sustainable energy supply networks and processes including nuclear renewable, clean coal technologies, and combined heat and power systems is vital for energy efficient systems. Combined heat and power (CHP) in particular has an important influence for creating energy efficient systems because CHP is able to capture the by-product heat for domestic and industrial heating purposes, either very close to the plant, or as hot water for district heating with temperatures ranging from approximately 80 to 130 Celsius. CHP uses heat that would be wasted in a conventional power plant, potentially reaching an efficiency of up to 89%, compared with 55% for the best conventional plants, meaning that less fuel needs to be consumed to produce the same amount of useful energy.
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The current levels of green house gases carbon dioxide and methane in the atmosphere are higher now than at any time in the last 650000 years. Due to these global warming and fossil fuel depletion an alternate solution should be implemented in order to produce a high efficient and pollution free energy. The CHP being one of the popular and efficient methods to generate electricity either from renewable energy resources or fossil fuels can be used as substitute for conventional energy generation methods.
CHP co generation produces heat and electricty in a highly efficient manner can meet both electricty and heat demand in small and large scale buildings.Due to the easy implementation and matching energy generation according to the energy demands, made CHP an vital solution for small scale heat and power generation .The statregies followed inorder to increase the CHP power generation was explained in this assignment.
Basic Issues and Energy Requirements
CHP or Co-Generation in other words, is the simultaneous process of capturing and then utilising the heat produced by generating electricity with in the fuel. Conventional electricity generation methods through power stations is only around 37% efficient, which indicates that a huge potential source of energy is simply released into the atmosphere as a by-product. It is widely known and shown that CHP can make use of the by-products released into the atmosphere and harness this power efficiently.
As CHP systems make use of the heat produced during the electricity generation process extensively, overall efficiencies in excess of 70% at the point of use can be achieved. In contrast, the efficiency of conventional coal-fired and gas-fired power stations, which discard this heat, is typically around 38% and 48% respectively, at the power station. Efficiency at the point of use is lower still because during transmission and distribution, the losses are inevitable.
Range of CHP:
Micro CHP : These were used for small scale commercial buildings which requires power output upto 5KWe and heat output of about 10- 15 KW.
Small CHP: These were most commonly used CHP for commercial buildings which supplies a power output of about 2 MWe .
Large CHP: These CHPs were used for applications greater than 2MWe power requirement. Large CHPs were used for industries and large community heating applications, usually the running hours and capital cost were kept high enough to maximise the savings.
Possibilities of Small Scale CHP:
The most competing prime movers in the field of small scale CHP were reciprocating engines, micro turbines, Stirling Engines and fuel cells.
Prime movers for CHP:
- Suitable for large scale power generation of about 1 to 500 MW.
- It works on steam boiler powered by fossil fuel or renewable energy source used to drive the electric generator .
- System installation is complex and expensive.
- Capable of operating various fuels and capable of varying HPR heat to power ratio according to the energy demands.
- The power plant based on reciprocating engine simply consists of a reciprocating engine( fuel ,gas or biomass) and generator linked to the engine .
- Suitable for small scale power generation of about 50 to 800 KWe.
- These type of plants were quite high efficient, large power range and versatile in assortment of multiple fuel .
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- The applications of Gas engines were used for back-up power source and diesel engines were used for continuous usage.
- Since it uses mechanical moving parts it requires a frequent maintenance.
- Heat to power ratio 1.2 : 3
Gas Engines: This form of CHP is widely used in buildings where the use of low grade heat in space and water heating systems enables a high level of waste heat recovery. Simply, they use natural gas as fuel, drive a generator and recover heat from the engine exhaust and cooling jacket.
Gas Turbines: This form of CHP is generally found in industry (chemicals, paper, etc.) which have high demands for steam. They derive from aerospace and industrial designs and produce exhaust gas at high temperature, which is usually ideal for generating steam or direct use in process of drying. The gas turbines has been widely used as prime mover for large scale CHP with the power rating above 1MWe.
Steam Turbines: This form of CHP is used in industry based on the designs found in coal fired power stations. Process industries are the main application areas. It was more popular before introduction of gas turbines.
Combined Cycle Gas Turbines: This form of CHP provides high power efficiency and found in the process industries. Basically, it combines a Gas Turbine feeding a heat recovery steam generator (HRSG) which in turn drives a Steam Turbine.
Absorption Chilling: This form of CHP provides power, heat and chilling by using heat to produce chilled water or chilled glycol. These kind of tri-generation systems are normally found in buildings and on some industrial sites.
1. Types of small-CHP and potential applications
CHP is a series combination of energy conversion processes, not just a single technology. Thus, it can be used to provide energy to a wide range of interests including a single home, commercial buildings, institutions and a large industrial plant, or even a whole city. CHP units are sited close to where their energy output is to be used, this unlikely occurs for conventional power plants because of their large size and pollution.
A wide range of sizes, applications, fuel and technologies are available for CHP systems. For instance, it may be employed to a gas turbine, an engine or a steam turbine in order to drive an alternator. The resulting electricity can be used either in whole or partially depending on the necessities on-site. The heat generated during power production is recovered, usually in a heat recovery boiler and it can be used to raise steam for a number of industrial processes such as providing hot water for space heating, or, cooling as mentioned above with appropriate equipment installed.
Depending on the quantity of the need for CHP, there are different units developed. The basic design criterions are to make the investment worthwhile and second criterion bases on the need for both the heat and electricity produced by the CHP unit. For instance, in the home, a micro-CHP unit resembling a gas-fired boiler will provide both heat for space and water heating, as does a boiler, and also electricity to power domestic lights and appliances. Micro-CHP units are highly demanded very new technology in the UK market. The potential for their usage is as large as the number of homes in the country.
The Heat demand and power demand for Commercial buildings and educational Institutions Detailed energy and power demands were important for designing and sizing of CHP .There are many software packages used to determine the energy and power demands and designing the CHP.
Building Energy Management Systems(BEMS) also aids in measuring and estimating the heat and power demands. For example heat and power demand of a typical hospital was given below.
Heat demand in August Power demand in August
For commercial buildings and institutions spaces, for another example, a factory-assembled, â€žpackagedâ€Ÿ CHP system is appropriate. In this case, an electricity generator, heat exchanger, controls and either an engine or a turbine is packaged together into a CHP unit so that the CHP unit can be simultaneously connected to the heating and electricity systems of the building.
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Some buildings which need a lot of energy such as leisure centres, hotels, hospitals and many others are also particularly suitable for CHP. With the addition of a chillier, CHP systems are able to supply cooling for air conditioning systems as well as heating. Some industrial processes are particularly well-suited to CHP, those that use lots of heat and operate around the clock - the manufacture of paper, chemicals, food and drink products, as well as refineries, are among those that can benefit most from CHP.
The main electricity grid may be connected to the homes and buildings fitted with CHP so that they are never short of an energy supply, during maintenance of the CHP plant or during periods of unusually-high energy loads. On the other hand, CHP plants for Commercial buildings and Institutions tend to be designed and built individually to fit their process they serve. These CHP plants are based on gas turbines, steam turbines or engines, together with electricity generators and control systems. The largest CHP plants rival traditional power-only plants in size and deliver huge quantities of energy - but at a much higher efficiency
In the examples of community heating systems, serving whole towns, areas of cities or, in a few cases, whole cities are possible with one or more CHP plants which supply heating to a grid of insulated hot water pipes that carry heat to a range of buildings. As well as CHP plants, boilers and other sources of heat may feed heat into the grid. Buildings taking heat from the community heating system do not need their own boilers. Meanwhile, the electricity generated is used to help run the community heating plant or is exported to the electricity grid which can be used for street lightning.
4. Performance Analysis and Optimisation
In any Commercial Building or Institution we need to consider several factors before installing a CHP unit, which includes fuel infrastructure, plant system allocation, possible noise attenuation problems, vibration problems, exhaust location and emissions, electrical connections etc. We will have to consider the overall efficiency of the plant by determining the electrical and thermal efficiency. The input is determined by the fuel consumption and the gross calorific value (GCV). The thermal output is measured using a heat meter and electrical output is measured by the CHP control panel. Another important parameter we have to consider is the CO2 emissions which can be calculated from the equation:
"CO2 emissions (tonnes) x energy consumption (KWh) x fuel emissions factor (Kg CO2/KWh)"
To analyse the performance we need to consider the variation of CHP flow temperatures before and after the bypass, the return temperatures to the condenser unit and the return water from the building. CHP is run for 7-8 hours according to the working hours of the Commercial Buildings and Institutions. CHP unit efficiency is evaluated from the useful heat transferred to pre-heat the return water from the building. We can also see that, as the temperature of the building return water is reduced the CHP unit efficiency is rising to around 84%. This is due to the increase in efficiency of the condenser unit at low temperatures. The heat recovered from the condenser is the difference between the temperature of the building return water and the temperature of the flue gas. The higher the temperature difference, the higher will be the increase in overall efficiency. The parameter which varies will be mostly the thermal efficiency while electrical efficiency almost remains the same.
Fig: Variation in flow and return water temperatures for a typical CHP system
After the analysis of the CHP unit in the industry, we need to optimise the performance for maximum efficiency and running hours. This can be done by operating the CHP as the "lead" source of heat with the boilers sequenced to operate as required. The sequencing is done depending on the boiler return water temperature. Another method we can implement to increase the efficiency is by using more number of gas turbines. This is achieved by selecting the best suitable load for each turbine, such that the fuel requirements are minimum and also without compromising the power and heat demands. CHP units installed in the industry should not be oversized or undersized.
It should must be rated correctly so that no energy is wasted, if the CHP is oversized then the system should be shut down from more time or during running mode more heat will generate, if it undersized the system will not reach the requirement of the Commercial Building or the Institution. Now the CHP module manufacturing companies are also installing advanced control techniques to monitor the load demand even for every hour of the day. By adopting these techniques, the performance of the CHP unit can be improved to great extent.
Fig: Variation of overall CHP efficiency over a particular Period
5. Advantages and Limitations
CHP contributes significantly to the UKâ€Ÿs sustainable energy goals, bringing environmental, economic, social and energy security benefits into the framework because CHP is a highly efficient. The advantage of CHP over using both fossil and renewable fuels.
Some of the advantages of using CHP are enlisted below:
Cost savings - The use of primary energy is reduced due to CHPâ€Ÿs high efficiency. Precious fuels are used much more efficiently, so less is used. And less fuel used means significantly lower energy costs to the end user. Savings vary, whereas can be between 15% and 40% compared to imported electricity and on-site boilers.
Lower emissions - CHP could provide the largest single contribution to reducing carbon dioxide emissions. Lower emissions means that less fuel burnt and it also means reduced emissions of carbon dioxide (the main greenhouse gas) and other products of combustion.
Increased security and power quality - In case of an interruption to main power supplies, CHP systems can be designed to continue to operate and serve essential loads accordingly. Hence, security of energy supplies is increased.
Grid reinforcement - siting an on-site CHP unit within the electricity grid can strengthen the network, and due to this, there is no need for network operators to upgrade the system there.
Despite its advantages CHP have some major disadvantages which will be barriers to the issues of implementation in CHP. Some of the disadvantages of using CHP can be enlisted below:
Initial capital cost: Cost of a CHP system is anticipated as twice of conventional boilers. Installation cost of the CHP system should also be more than the conventional systems owing to the need of additional natural gas line, additional venting and electrical requirements, and etc.
High maintenance cost: The maintenance cost of CHP is much higher than the conventional boiler and grid maintenance cost. The CHP plant needs an expert engineer inspection for maintenance, which costs relatively high. This will decrease the annual profit of the CHP system eventually.
Heat and power demand of a site: Economic feasibility of CHP systems has a direct relation with the heat and power demand of a site. Much more efficient systems can be issued if heat and power load demands are coincide (in phase) but the load profiles are not coinciding in real.
Price Changes in energy market: Competition in energy market between the big companies causes fluctuations in prices. Increase in demand of gas over the last years cause increase in gas prices. The annual profit of the CHP system cannot be guaranteed because of the uncertainty of the energy prices in the energy market. Hence investors are discouraged and make short term plans.
6. Future Trends
Efficiency of the power generation technologies together with environmental performance continues to improve as long as larger scale power generation system becomes available. There are several likely developments to note.
Fuel Cell CHP: offers the opportunity for higher levels of Power Efficiency in the range 50-60%.
Micro-CHP: In order to provide CHP Packages for individual homes, a range of technologies including Stirling Engines and Fuel Cells are being developed.
Renewable Energy: a number of technologies are being developed assisting the use of renewable fuels for CHP systems, such as the use of gasifiers to convert biomass fuels for use in gas engines, gas turbines or fuel cells.
As developments in CHP systems continue, they will retain or enhance their advantages of high efficiency and low environmental impact. Therefore, environmental sustainability of energy supplies becomes increasingly important.
This report explains combined heat and power (CHP) systems for creating energy efficient systems. Its insights are highlighted with their major advantages, and its disadvantages are analysed for further analysis of developed systems. Since a CHP system is able to capture the by-product heat for domestic and industrial heating purposes, it is concluded that as an alternative energy efficient system, CHP systems their varieties (small and large scale) are suitable for variety of different applications. CHP, as is in its current technology and its future trends continue to receive attention as developments in CHP system continue.