Industrial & commercial heating Combined heat & power >
appropriate innovative design can ensure that generated heat is utilised. But, without such innovation, there remains the risk that the loss in heat efficiency of a local CCGT system could negate the efficiency gains of the CHP installation.
Heat-to-power ratio CHP is normally ‘heat led’, operating when there is a demand for the heat. CHP systems have evolved from systems predominantly used as a heat engine supplying steam and the hot water needs of manufacturing processes. The heat-to-power ratios of these steam turbine machines would have typically been in the order of 6:1 (if not greater). This means that, for every kWe of electrical energy produced, there would have been at least 6kWth of heat. For industrial processes where there may be a need
for heat or large amounts of hot water, this can provide a suitable solution. However, when considering modern building developments that are highly insulated and have low heating demand, many require heat-to-power ratios far closer to 1:1. This shift in application of CHP, combined with the lowering of costs and improved manufacturing capabilities, has significantly altered the profile of CHP installations in the past 30 years. It is generally quoted that, to provide a return on investment in the capital plant and maintenance of a larger CHP installation, the CHP plant needs to operate for at least 4,500 hours per year (just over 50% of the hours in a year). The challenge is to ensure that there is sufficient demand for heat over that period so that the CHP plant will operate effectively. Manufacturing facilities account for about 90% of CHP installations, in terms of installed capacity. Applications such as hospitals, leisure centres with pools and manufacturing facilities where there has been a particular need for steam or heat, account for most of the remaining 10%. To extend the number of useful operational hours, installations can shift the load. The Pimlico housing scheme, mentioned earlier, now uses dedicated local CHP plant. This is combined with a 2,500 tonne water thermal store to level out the heat demand and to improve utilisation. A recent example of a large-scale CHP plant installation – comprising two, linked energy centres – is that supplying the new Olympic Park in east London. Part of the heat output for the Olympic Park scheme will be used to drive absorption refrigeration systems, making this a trigeneration scheme: combined cooling, heating and power (CCHP). The CHP plant will provide 9.3MWe of electricity using gas-fuelled CHP. High utilisation will be maintained by supplying a mix of building types (leisure, residential and retail). This provides a solution to the challenge that all CHP systems face: to find an application for the heat when there is a reduced need for local hot water or heating in specific individual building types. Well-operated modern CHP installations such as these
should be able to operate at around 35% electrical efficiency and 45% thermal efficiency. It could be argued that CHP plants predominate in new developments, and in existing ones such as hospitals and leisure centres, because these are the ‘low-hanging fruit’, the easy pickings, for CHP installations: brand-new
www.cibsejournal.com
200 400 600 800 1000 1200
Jan Feb
Boiler 2
Example of CHP sized above base load
Boiler 1
CHP
Mar April May June July Aug Sept Oct Nov Dec Month of the year
Figure 1: Sizing a CHP plant. (Source: GPG 176: Small-scale combined heat and power for buildings, BRECSU 1996)
radiation losses
flue losses 100% primary fuel
Exhaust heat
exchanger
engine engine heat exchanger generator
Useful heat
Figure 2: Simplified schematic of a CHP plant MWe
6,000 5,000 4,000 3,000 2,000 1,000 0 1977 > 2008
Figure 3: Types of CHP Installations 1977 and 2008 (60th Anniversary Digest of UK Energy Statistics, DECC 2009)
February 2011 CIBSE Journal 43
Reciprocating engine Gas turbine
Pass out condensing steam turbine
Back pressure steam turbine
Electricity
Combined cycle
Heat Load (kW)
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72