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The effects of global warming, climate change etc are well known and accepted by most governments of the world as a problem that can be attributed to carbon emissions generated by industrialisation. In 1997 the United Nations set a framework aiming to reduce these carbon emissions, known as the Kyoto Protocol, in which member states agreed that 'stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.' (United Nations Framework Convention on Climate Change: Article 2) should be adopted. As of 2009, 189 member states have ratified the principles set out in this protocol including Great Britain. To meet the principles of the Kyoto Protocol the Government have set targets for industry and government organisations to meet.
The NHS's energy consumption is increasing year on year and in order to meet government targets the NHS are required to reduce both carbon emission and energy consumption of its buildings and engineering services; NHS carbon reduction strategy for England states 'The NHS should set itself targets and trajectories to meet the provisions of the Climate Change Act' (NHS Sustainable Development Unit: NHS Carbon Reduction Strategy for England, Key Actions). As an incentive for the NHS to reduce their carbon emission the government made available funds to install viable sustainable technologies. One such technology to achieve these targets has been by using Combined Heat and Power (CHP) systems.
A study was undertaken at Royal Preston Hospital (NIFES Consulting Group, 2006) in order to find the most effective sustainable technology that could be found to reduce carbon emissions. From the findings of this study it was decided that a Combined Heat and Power unit would be the most cost effective means to reduce carbon emissions. A feasibility study was then conducted (NIFES Consulting Group, 2006) to determine the most appropriate CHP system for this site. This feasibility study gave detailed information on the type of engine to be used, the expected electrical output of the system, the heat recovery of the system, the cost savings and the expected payback period of the scheme. Following from the feasibility study detailed design was undertaken and in 2009 a gas fired reciprocating engine CHP system was installed at the Hospital, that provides an electrical output of 1800kW/h, approx 1000kg of steam/hr and low grade heat for heating and DHW purposes. The aims of this report is to determine if the installed CHP is in fact the most viable and cost effective approach to reducing carbon or if there are most suitable sustainable technologies available.
Aims and Objectives
The project will appraise this CHP system and an analysis of the CHP will be conducted in order to determine the CHP's efficiency. Following the initial analysis the project will investigate any improvements that could be made in order to further improve the efficiency in the CHP system. Finally alternative sustainable technologies will be discussed and compared against the CHP system with respect to their carbon saving and energy reduction capabilities and the viability of these technologies over CHP.
The CHP has been installed in order to reduce the hospitals carbon footprint and as a subsequent benefit to reduce the annual energy costs.
The initial phase of the project will discuss CHP systems currently in use and their relative merits and disadvantages such systems pose within this setting. This will take the form of a literary review of current ideas and practices incorporating the latest guidance and legislation. From this the choice of the CHP installed will be assessed as to its suitability for this particular site.
The second part of the project will assess the particular CHP system that has been installed and it will be determined if this system is performing to the CHP specification. In order to determine this then data will be collected over time and then assessed to determine the efficiency of the system is as anticipated and within the expected norm. Following this assessment the system will be critically appraised in order to determine if there are further enhancements/modifications to the system which will improve the overall efficiency.
Outline of disertation
What is CHP
Electrical power generation produced centrally and distributed along the national grid is inherently inefficient as the heat produced in generating the electricity is 'dumped' to the atmosphere. Combined Heat and Power also know as co-generation is the production of electricity and the recovery of the heat produced in a single process. In CHP systems the fuel used can be natural gas, oil, coal, bio gas or other bio fuels or any combination of these. The use of these fuels by the CHP is to provide the heat and electricity to a building and in doing so is more efficient than if the heat and electricity was provided separately, as is the case with centrally produced electrical power. CHP's can convert up to 80% to 90% (Combined Heat and Power association, 2010) of the fuel used into useful energy whereas electricity produced by central electricity generating stations are typically 30% to 40% fuel efficient. The production of electrical power by CHP requires high temperatures, however in order to utilise the waste heat in buildings lower temperatures are required to provide space heating. The high temperatures produced by the co-generation process are required to be converted into temperatures that can be utilised by space heating or process energy needs. This can be achieved by utilising the low grade heat for example that emitted by the cooling system of a reciprocating engine and by using the high grade heat such as that emitted from the exhaust gases of a reciprocating engine.
Figure - Energy balance for a typical gas engine (GPG388 Good Practice Guide, 2010)
History of CHP
In the late 1800's the use of electricity was becoming more widely used in both the domestic and industrial areas. Electrical generation used amongst other means reciprocating steam engines to produce this electricity which was found to be inefficient and wasted a large amount of steam. This wasted steam was used to provide steam for process which included space heating.
Principles of CHP
The principle requirement of the combined heat and power system is to utilise the fuel consumed by the system as efficiently as is possible. In order to understand the efficiency of the CHP system some understanding of applied thermodynamics is required. Applied thermodynamics is the science of the relationship between heat, work and the properties of systems (Eastop and McConkey, Applied Thermodynamics for engineering technologists, 5th Edition). That is to say that Applied Thermodynamics is concerned with the means to convert heat energy from available sources such as fossil fuels into mechanical work. In considering the cycle of the CHP system there are principally two Laws of thermodynamics which need to be considered, these are the 1st and 2nd Laws.
1st Law of Thermodynamics states that 'energy can be transformed from one state to another but cannot be created or destroyed. This simply means that the amount of fuel energy inputted into the system is equal to the heat outputted plus the amount of work performed by the system.
2nd Law of Thermodynamics postulates that over time the amount of useable energy decreases and conversely the amount of unusable energy increases. This simple put means that in a system the amount of useable energy, the amount of useful work, decreases and the amount of unusable energy, the amount of waste energy, increases. The basis of the 1st Law is not contradicted in this as the amount of energy is not created or destroyed.
Types of CHP systems
The main components of a CHP installation will consist of four basic items of equipment. These being:
The prime mover
The electrical generator
Heat recovery equipment
Control and instrumentation system
The prime mover of a CHP system can be an internal combustion engine, gas turbine, steam turbine or more recently fuel cells. The main prime mover in most building applications is the internal combustion engine which typically delivers electrical outputs up to 2MWe. The most common of the internal combustion engines used is the spark ignition engine using natural gas as the fuel source. The compression ignition engine is also used where diesel is the chosen fuel. Both of these types of engine can be used to burn environmentally friendly fuels such as bio-gas and bio-diesel. The electrical efficiencies of the internal combustion engine driven CHP are in the region of 30-40% with heat to power ratios of 1.1-1.5:1.
Larger applications usually require the use of the gas or steam turbine driven CHP systems which delivers electrical outputs over the 2MWe range. The electrical efficiencies of these are less than the reciprocating engine type and are typically in the range of 25-30% with a slightly higher heat to power ratio of 1.5-2.0:1.
There are two types of electrical generators that can be used in CHP applications. These are the synchronous alternator and the asynchronous alternator. Synchronous alternators use battery start and are suitable for standby generators. Their design is more complex than the asynchronous alternator however they do not require power factor correction. The asynchronous alternator uses the mains electrical supply for the excitation of the alternator and as such are not suitable for standby generation. They are less complex that the synchronous alternator and therefore less expensive but they do require power factor correction.
Any process in producing electrical power creates waste heat. The recovery of the waste heat is what makes the CHP process efficient therefore it is important to be aware of where the heat is being wasted. Taking the internal combustion engine there are three main sources of heat loss. These are from the exhaust gasses, from the engine coolant and from the lubricating oil. The waste heat from the sources will be at different temperatures i.e. typical exhaust gases temperatures range from 450-550oC whereas the coolant temperature range is 80-95oC. For heating of buildings the temperature required in most cases would be typically at a temperature range of 70-85oC (low temperature heating). Clearly the temperatures of these sources would require a heat exchanger to be installed in order to reduce the temperature to the required useable temperature. Some applications require higher temperatures i.e. where medium or high temperature water is required or the production of steam. In these cases the use of the exhaust gases can be utilised however the heat from the coolant would have to be rejected which would lower the overall efficiency of the system. It is not the normal practice to recover the heat from the lubricating oil and this would be rejected heat.
Figure - Heat recovery arrangement for a gas engine CHP
The CHP system may be designed such that electrical power generation can be utilised when there is insufficient heat load to recover all the waste heat from the CHP system. This will require the addition of a heat rejection system sized adequately to reject some or all of the waste heat generated. Running the CHP system in this manor will be inefficient.
The control and instrumentation system should incorporate three main functions:
To integrate the heating and electrical output of the CHP with the building systems. These would include stop/start of the engine, modulated output of the electrical output, connect/disconnect of the generator i.e. synchronisation etc.
Performance monitoring and targeting.
Safe system i.e. reverting to a safe condition in the event of any component failure.
CHP systems are generally classed by their electrical output, which are:
Micro CHP (range, up to 5kWe)
Small Scale (range below 2MWe)
Large Scale (range, above 2MWe)
Micro CHP systems
Reciprocating engines, fuel cells, combustion turbines and steam turbine generators combined with fossil fuel fired boilers are some of the technologies that can be used in CHP systems.
Review of Guidance and Legislation
System under investigation
Figure - CHP
Application of CHP in Buildings
The application of CHP systems depend upon the type of building that they are going to serve. Buildings that have extended hours of operation and a coincidental amount of heat demand and power requirement are worth considering for CHP systems. These could be for example hotels, hospitals residential homes, universities, leisure facilities etc.
In determining the suitability of a building in assessing whether or not to install a CHP system a number of factors need to be considered. These considerations are; the buildings load profile, the buildings heat demand, CHP plant sizing and integration into buildings which will include; integration with the building heating system, integration with the building cooling system and integration with the buildings electrical system.
Building Load Profile:
CHP systems need to operate for as long a period throughout the year as possible and at maximum output in order to make them viable. For this reason a load profile of the building where the CHP is to be installed needs to be conducted in order to determine the CHP plant size. The conventional procedure for plant sizing would be to determine the peak demand on the building however for determining the size of the CHP system this is not suitable. The approach to use is to determine the minimum loads and the load profile over a 12 month period. This will enable the CHP size to be calculated in order that the CHP system will be run at its optimum load for the maximum length of time. The building load profile and minimum load of both the electrical demand and of the gas demand (used for heating and DHW purses) will need to be determined. This can be found in existing buildings by analysing consumption data from the fiscal meters which may take the form of half hourly data or monthly consumption data. Thermal modelling of the building will provide valuable information to the designer in determining the CHP plant size.