Industrial & commercial heating Combined heat & power
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environmental impact. These twin aims – efficient fuel use and reduced environmental impact – have driven the development of smaller-scale energy and heat generation systems that have become variously known as combined heat and power (CHP), cogeneration or total energy systems. (See CIBSE Guide F 2004, Energy efficiency in buildings, section 5.3.) The use of CHP has grown steadily in the UK in the past
20 years, mainly in industrial plants. This growth followed the Energy Act of 1983, which encouraged the use of decentralised electricity generation. The 1986 Gas Act and the 1989 Electricity Act also opened up the market for the reselling of electricity and gas to end-users.
Uses of CHP A CHP plant would typically be sized to meet the base heat load of the buildings that it serves; it would also act as the ‘lead boiler’. The operation of the plant may be optimised with the use of appropriate controls and equipment so that heat and power can be applied usefully above the base load. The key principle for efficient operation is to ensure that there is a useful application for the heat. Large-scale CHP systems – generally defined in the UK
Analysis Challenging times for CHP specifiers
Why use a CHP plant at all to generate electricity if electricity is readily supplied by the grid? This is a reasonable question, particularly if your evaluation of the efficacy of specifying a CHP includes a financial assessment of the installation and its ongoing operations/maintenance costs; and if space requirements and carbon-footprint considerations are taken into account. The comparison on space requirement
again will be reasonably clear, with the CHP plant and distribution equipment replacing at least some of the traditional local heat generation boilers and storage. In the case of a bio-fuelled CHP plant, space and access for fuel storage and delivery would also need to be included. However, assumptions made in predicting
the ‘utilisation factor’ – the hours when a CHP may effectively be used to both supply heat as well as generate electricity – can significantly affect the assessment. The density of the heat demand will also influence how effectively the heat may be distributed. The methodology for financial calculations
– by comparing net present value (NPV) of whole lifecycle cost – is well understood. The CHP costs would include the plant and associated infrastructure of, for example, fuel supply, district heating scheme and consumer heat exchangers. Basic operating costs will be dominated by fuel prices and electrical-generation efficiency. Maintenance
also must be taken into account. Despite all these calculations, uncertainties arise from presumptions of utilisation and distribution inefficiencies – in other words, how and when the CHP plant will be operated. Determining comparative carbon footprints of centralised electricity generation compared with local CHP systems, appears to be one of the most challenging issues facing engineers. Not only is this a function of several assumed efficiencies in the fuel-to- power/heat transformation, it is also related to the utilisation factor. It is difficult to be precise as to the ‘real’
CO2 emissions of the grid as, for example, at the point of generation, the carbon impact of gas turbines is around 0.37 kg CO2/kWe, whereas coal is 0.87 kg CO2/kWe and wind and nuclear are (nominally) 0 kg CO2/kWe. The currently accepted representative value
(of annually averaged grid fuel use) for UK electricity (at the point of use) is 0.54 CO2/ kWe. Well-operated natural gas-fuelled CHP plant would have a similar value but would additionally be providing a local supply of heat. Further clouding attempts to compare
CHP installations with the average grid CO2 emissions is the mechanism to assure ‘good-quality’ CHP: the CHPQA scheme and its applicability to EU legislation. The implementation of the European Cogeneration Directive that regulates CHP
performance should involve comparing the efficiency of a CHP plant with the ‘same fuel categories’. This would appear to require that the performance of gas-fuelled CHP plant be compared directly with gas-fuelled grid generation. As seen in Huw Blackwell’s article ‘Sustainability without the hot water’ in the August 2010 issue of CIBSE Journal: ‘local CCGT CHP + end-user heat pump’ is comparable with ‘grid CCGT + end-user heat pump’ systems. This is due to the utilisation of the heat in the local plant that would otherwise be rejected back into the environment. So, if the grid was able to produce electricity
with an average CO2 footprint approaching that of CCGT, then the case for local natural gas-fired CHP could be considered as being marginal. However, the basket of fuels that are currently serving the grid produce almost 50% more CO2 than this grid CCGT example; and so it could be argued that whilst the grid is in its current state, such an application of CHP is clearly preferable in CO2 terms. Until the grid is ‘decarbonised’, appropriate application of CHP can continue to provide opportunities for effective energy production, together with potential social and economic benefits. However, this will need informed pragmatic integration into systems, and should not be driven or precluded by preconceptions and dogmatic positions.
as those above 2MWe – employ reciprocating engines or gas turbines, and can be used to serve a group of buildings, industrial applications, or a district scheme. Big industrial facilities such as factories may be powered
by steam boilers and turbines primarily providing power and steam at the temperature and pressure required by the industrial process. By contrast, high-efficiency electricity- generation plants can use condensing turbines that convert the maximum practical power to electricity by only rejecting low-grade heat. The efficiencies of electricity-generating CCGT power plants are significantly higher when used on a large scale for supplying electricity to the national grid, compared with the use of smaller, local CCGT schemes that supply (local) CHP plants: the thermodynamic efficiency of electrical generation in these smaller, localised schemes will be reduced because some of the heat will leave the process at a higher temperature in order to serve the heating loads being met by the CHP system – and this will reduce the thermodynamic efficiency. However, this does not mean that the high efficiencies of
CCGT in centralised power production cannot be achieved when CCGT is used locally for CHP systems. The use of
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CIBSE Journal February 2011
www.cibsejournal.com
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