Heat pumps 1 CHP
Current combined cycle gas turbine generation with heat pumps operating at COPs of 3 and 4
200 185% efficient heat 180 160 140 120
Figure 2: Each of the filled dots shows actual performance of CHP systems in the UK, grouped by type. The hollow dots marked ‘CT’ show the performances of ideal CHP systems quoted by the Carbon Trust; the hollow dots marked ‘Nimbus’ are from a manufacturer’s product specifications. The dots marked ‘ct’ are the performances quoted by the Carbon Trust for two real systems (at Freeman Hospital and Elizabeth House). The steep green lines show the combinations of electricity and heat that you can obtain, assuming that heat pumps have a coefficient of performance of 3 or 4, assuming that the extra electricity for the heat pumps is generated by an average gas power station or by a top-of-the-line gas power station, and allowing for 8% loss in the national electricity network between the power station and the building where the heat pumps pump heat. The top-of- the-line gas power station’s efficiency is 53%, assuming it’s running optimally Source: Sustainable Energy – Without the Hot Air ©David MacKay
100 80
Condensing boiler Standard boiler
60
Back pressure steam turbine
Pass out condensing steam turbine
40 ct Gas turbine Reciprocating engine Combined cycle gas turbine ct 20
CT CT
CT
Wärtsilä Nimbus
District heating is one of the ideal scenarios for CHP
(multi-MW) plant on site. Perhaps a comparison should therefore be made against combined cycle CHP. What happens if a large CCGT is brought into the city and the heat normally dumped from the cooling towers used in a district heating network? A quick comparison is included in the Sankey
0 10 20 30 40 50 60 Electrical efficiency (%) >
gas power stations.’ (Page 149.) The comparison between CHP and combined cycle gas turbine (CCGT) is not perhaps a fair one. The fundamental feature of the CCGT system is that it has a second process of energy recovery, which increases the overall efficiency. This leads to a large part of the difference between the two systems. As MacKay states: ‘The fact is, electrical energy is
more valuable than heat...’ The critical parameter in CHP implementation is electrical efficiency. This can be enhanced through using or combining different thermodynamic cycles. An example might be large reciprocating CHP with an Organic Rankine Cycle, or looking to the future, possibly fuel cells. However, building services engineers are only using small CHP systems because they cannot build a large
36 CIBSE Journal August 2010
diagrams (Figure 3). The first shows a ‘best case’ CCGT (53% electrical efficiency) with part of the electrical output supplied to a heat pump. This is comparable to an example used by MacKay: ‘
...the “best gas” power station, feeding electricity to heat pumps can deliver a combination of 30% efficient electricity and 80% efficient heat, a “total efficiency” of 110%.’ (Page 151.) This works because the heat pump moves heat energy
from another source, therefore we have not ‘created’ any energy. Moving this station into the city means it can become a CHP system. As pointed out by MacKay, there may be a slight loss in electrical efficiency for the CCGT relating to higher condensing temperatures in the steam cycle; but the efficiency of the gas turbine (first cycle) is not affected. However, there is also potentially a reduction in electrical transmission losses as the power station is now embedded in the city, where there is the greatest electrical demand. In the second Sankey example (Figure 3) it is
assumed there is a 4% loss in electrical efficiency (net 49% electrical efficiency), from reclaiming the
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30% efficient electricity, 80% efficient heat
Heat efficiency (%)
Coal
Gas Best Gas
New standard solution
Old standard solution
Heat pump, C0P=4
Heat pump, C0P=3
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