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CIBSE CHP Group Seminar, 16th June 2010
The following is a summary of the key points covered at this event, which included a presentation from Vimal Bhana, Technical Director of Self Energy UK.
Combined Heat and Power (CHP), alternatively called cogeneration, is the on-site generation of electricity from fossil fuels with recovery and utilisation of the heat produced. CHP is not a new application and has been used successfully for more than 100 years. The decision to implement CHP has historically been based on fuel prices, particularly the spark gap between electricity and gas prices. However, there are now additional reasons for CHP to be considered, primarily due to increased awareness of climate change and the measures being introduced to reduce carbon dioxide emissions. The Climate Change Levy (CCL) taxes electricity and fossil fuels with regard to the amount of CO2 they create, Part L of the Building Regulations specify CO2 emission limits, and planning policies are requiring at least 10% of generation to be renewable or CHP. The efficiency and environmental benefits of CHP led the government to set a target of 10,000MWe by 2010. This is unlikely to be met; however there are now around 1,500 CHP schemes in the UK having a combined capacity of over 5,000MWe. The future costs of energy are set to continue rising and security of supply is not guaranteed. CHP is an effective means of reducing 20% to 30% of energy costs and CO2 emissions and can reinforce energy supply by acting as a standby for essential services. The key to successful, or “good quality”, CHP is to have a constant heat demand that allows the CHP unit to run for the majority of the time.
In conventional electricity generation, a fuel is combusted to heat water and produce steam to drive a generator. All the excess heat of the process is exhausted to the atmosphere and the overall efficiency is limited to around 30% to 50%. By utilising the heat from combustion, as a CHP system does, the efficiency is increased to more than 65% and possibly up to 90% for condensing CHP. Fuels that can be used for CHP are: natural gas, LPG, oil, biofuels, and biomass. Natural gas is the most widely used fuel because there are a variety of issues with the other possible fuels: LPG is more expensive; oil produces soot that clogs heat exchangers; and biomass CHP systems are not yet fully perfected. The ratio of heat to electricity generated by CHP varies but is generally greater for smaller units, at around 1.5:1, than larger units which approach 1:1.
There is a range of CHP system sizes and types in use. Domestic micro CHP is the smallest available and is generally up to 1kWe. They are based on the Stirling engine and produce around 10kW of thermal output. These systems are not economically viable at present because of the absence of large continuous heat loads in households. Mini CHP of up to 5kWe and about 11kW thermal output is aimed at small commercial enterprises with constant hot water demand, particularly residential, shared home properties. They are based on reciprocating engine technology and there are over 10,000 such units installed throughout Europe. Small-scale CHP are units up to 1MWe and may utilise gas fired reciprocating engines or small gas turbines (typically around 500kWe). The part load efficiency of these units is good down to 50% of rated output. Another small-scale CHP system uses micro-turbines 30kWe to 100kWe in size. These have a slightly lower efficiency but can be boosted by using a recuperator to preheat air entering the turbine. Systems above 1MWe are large-scale CHP units, which use gas turbines to produce high temperature heat for large multi-building sites or district heating. A further type of CHP system that has great potential for the future is one based on fuel cells. Electricity, water and heat are generated in a fuel cell from highly efficient chemical reactions that have low or possibly zero CO2 emissions. A fuel cell CHP system could be any size from a micro 1kWe unit to a large-scale multi-megawatt unit.
The trend in the CHP market is towards large-scale community heating. A community will have a variety of buildings with different heating demands and therefore a good average heat load. At least one large load such as a hospital or university is preferable, but by also connecting many smaller buildings and households to a district heating scheme, access to CHP is possible that would otherwise be unviable. Well insulated pipes allow hot water to be transported throughout a community with losses of only about 1°C over one kilometre.
To determine the appropriate size of a CHP system, there are several approaches that can be taken. The base load heat demand could be the benchmark for selecting a unit so that all the heat produced is used. Alternatively, the system can be sized based on the electrical base load without regard to the heat demand. In either case, it is possible that there is a more optimal size than will meet just the base load. Therefore, an in-depth analysis of daily or hourly loads is necessary for correct sizing. It is also important that the true base load energy demand is determined before sizing a CHP system. This means that energy efficiency measures should be implemented first to reduce energy demand and thus reduce the size of CHP system required.
Buildings particularly well suited to CHP are: hospitals, hotels, swimming pools, leisure centres and universities. In all cases, the criteria for good quality CHP are: a year round demand for heat; the primary source of heat being the CHP system with backup boilers only supplying any remaining heat demand; and the CHP should run for a large proportion of the time so it is economically feasible. There are numerous examples of successful CHP installations that have achieved excellent cost and CO2 savings. Liverpool University connected a 3.6MWe CHP system to 30 of its buildings in 1986 at a cost of £2.1M. They achieved savings of £416,000 each year which resulted in a payback of 5 years. Marriott Hotels have made 23 CHP installations since 1993 and are saving over £550,000 each year. Queens Medical Centre in Nottingham installed a 4.9MWe gas turbine in 1998 under an energy services contract and is saving £350,000 annually.
A further application is Combined Cooling Heat and Power (CCHP), or trigeneration. For buildings with significant cooling loads, a portion of the heat can be fed to an absorption chiller. The performance of absorption chillers is well below that of air conditioning or refrigeration chillers but they make good use of excess CHP heat.
With future developments in technologies and a better understanding of how to utilise CHP, significant reductions in energy consumption and expenditure may be realised as well as reductions in carbon dioxide emissions. For good quality CHP having a power efficiency of at least 20% and quality index (measuring the overall power and heat efficiency) of over 105 for new schemes, CHP is exempt from the Climate Change Levy and benefits from Enhanced Capital Allowances. The CIBSE CHP group and London South Bank University are actively promoting the benefits of CHP. They are experts in CHP system applications and can provide valuable information and advice to organisations considering CHP.
01/07/2010