CPD Programme
Boiler
Low loss header
DHW circuit
Heating circuit
Boiler
Low loss header
DHW circuit
Heating circuit
Larger heating systems CHP unit Figure 5: Mini-CHP hydraulic integration with larger heating systems CHP unit Figure 6: Mini-CHP hydraulic integration with smaller heating systems
A standardised method to compare the effectiveness of CHP systems is to determine the quality index (QI) and power efficiency as defined by the DECC CHPQA programme [4
].
Figure 7: Three mini-CHP units installed in commercial plant room
Environmental and economic case for CHP The environmental case for CHP is based around the displacement of primary heating appliance fuel and grid supplied electricity. The CO2 reductions offered by CHP are a function of the emission factors for natural gas or LPG, and grid-supplied electricity depending on the generation mix being operated in the UK. These are [3
]:
Grid-supplied electricity -
Natural gas - 0.529 kgCO2 /kWh
(displaced electricity) 0.198 kgCO2
/kWh
Hence, a 5.5kWe output commercially available mini-CHP unit would save around 8,700 kg CO2 per a year. This is on the basis of annual operational hours for the unit of around 5,600 (or 17 hours a day, 365 days a year, with a 90% availability), and displacing heat from the main boiler plant with an efficiency of around 80%. The economic case for CHP is driven by
what is often referred to as the ‘spark gap’. This is the ratio of the tariff for the imported grid-supplied electricity being displaced by the CHP unit and the cost of the fuel input. When considering CHP, the term ‘tri-generation’ or ‘CCHP’ is used to include cooling (using absorption refrigeration) as a means of extending the benefit of the heat produced by a CHP unit. However, this would not normally be economic for small or mini-CHP systems.
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This provides the principal evidence required for determining eligibility of CHP Schemes for Climate Change Levy (CCL) exemption and Enhanced Capital Allowances (which provide businesses with 100% first-year tax relief on qualifying capital expenditure). For gas powered mini-CHP, the quality index can be determined from
QI = 249 x ηpower + 115 x ηheat where: Power Efficiency (ηpower) = Total Power Output (MWhe) / Total Fuel Input (MWh)
Heat Efficiency (ηheat) = Qualifying Heat Output (MWhth) / Total Fuel Input
(MWh)
The constants 249 and 115 are related to the alternative electricity supply and alternative heat supply options that are being displaced by the CHP unit – and vary for different applications of CHP [4
]. Total Power Output
is the total annual power generation from a CHP scheme as measured at the generator terminals. Qualifying heat output is the amount of useful heat supplied annually from a scheme that can be directly shown to displace heat that would otherwise be supplied from other sources. The total fuel input is the annual fuel input to a CHP Scheme. A QI of 100 is the basic benchmark, and
QI should be at least 105 with a minimum power efficiency of 20% in order to help qualify for government incentives, enhanced capital allowances and CCL exemption. The fuel consumption to the CHP unit will require a dedicated gas meter, and the heat and electricity output will need metering.
Conclusion In a time when there seems to be huge industry focus on renewable technologies, whether such solutions are generating electricity or
2.
heat, CHP can play its part as a low carbon solution in achieving the increasingly stringent emission reduction targets, whatever the driver. By the nature of its lower electrical and thermal outputs, mini-CHP now presents the benefits offered by CHP to a much wider range of buildings.
© Yan Evans and Tim Dwyer
Further reading These publications provide comprehensive background information on CHP but tend to focus on larger systems than the mini-CHP as defined by this article. Building Applications Guide BG
1.
2/2007 CHP for existing buildings. Guidance on design and installation, BSRIA 2007 Applications Manual AM12 – Small-
scale combined heat and power for buildings, CIBSE 1999 3. Technical Guide to the Connection of Generation to the Distribution Network, DTI, 2003
References 1. Engineering Recommendation G.83/1-1, Recommendations for the connection of small-scale embedded generators (up to 16A per phase) in parallel with public low-voltage distribution networks. Ofgem, 2008
2. Engineering Recommendation G.59/1, Recommendations for the Connection of Embedded Generating Plant to the Regional Electricity Companies’ Distribution Systems. Ofgem, 1995
3. The Government’s Standard Assessment Procedure for Energy Rating of Dwellings. Building Research Establishment, 2010
4. CHP QA – Quality Assurance for Combined Heat and Power.
www.chpqa.com
August 2010 CIBSE Journal 53
Smaller heating systems
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