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Energy Policy 39 (2011) 5528–5534
Contents lists available at ScienceDirect
Energy Policy
0301-42
doi:10.1
n Tel.:
E-m
journal homepage: www.elsevier.com/locate/enpol
Combined heat and power considered as a virtual steam cycle heat pump
Robert Lowe n
UCL Energy Institute, London, WC1H 0NN, UK
a r t i c l e i n f o
Article history:
Received 21 January 2011
Accepted 9 May 2011Available online 8 June 2011
Keywords:
Combined heat and power
Heat pumps
Thermodynamics
15/$ - see front matter & 2011 Elsevier Ltd. A
016/j.enpol.2011.05.007
þ44 20 3108 5961.
ail address: [email protected]
a b s t r a c t
The first aim of this paper is to shed light on the thermodynamic reasons for the practical pursuit of low
temperature operation by engineers involved in the design and the operation of combined heat and
power (CHP) and district heating (DH) systems. The paper shows that the steam cycle of a combined
heat and power generator is thermodynamically equivalent to a conventional steam cycle generator
plus an additional virtual steam cycle heat pump. This apparently novel conceptualisation leads directly
to (i) the observed sensitivity of coefficient of performance of CHP to supply and return temperatures in
associated DH systems, and (ii) the conclusion that the performance of CHP will tend to be significantly
higher than real heat pumps operating at similar temperatures. The second aim, which is pursued more
qualitatively, is to show that the thermodynamic performance advantages of CHP are consistent with
the goal of deep, long-term decarbonisation of industrialised economies. As an example, estimates are
presented, which suggest that CHP based on combined-cycle gas turbines with carbon capture and
storage has the potential to reduce the carbon intensity of delivered heat by a factor of �30, compared
with a base case of natural gas-fired condensing boilers.
& 2011 Elsevier Ltd. All rights reserved.
1. Introduction
The main aim of the first part of this paper is to shed light onthe performance of combined heat and power (CHP) based onlarge steam cycle plant through a consideration of the funda-mental thermodynamics of such plant. The argument presented,though technical, is simple and accessible to a wide readership.The key insight is that the steam cycle of combined heat andpower generator is thermodynamically equivalent to that of aconventional steam cycle generator plus an additional steamcycle heat pump. The paper shows that this apparently novelconceptualisation enables rapid estimation of the energy perfor-mance of combined heat and power plant in terms of thetemperature at which they supply heat to associated districtheating systems. More importantly, it illuminates some of thestrategic issues at work in the design of combined heat and powerand district heating—particularly those around the choice ofoperating temperatures. For example, the systems that operateat flow temperatures of 90 1C and return temperatures of 35 1Ccan have COPs approaching 9.
Such performance levels would be of academic interest only ifCHP were fundamentally incompatible with the long-term goal ofmore-or-less complete decarbonisation of terrestrial infrastruc-ture in developed economies. The second part of the paper istherefore taken up with a discussion of a variety of ways in whichCHP and associated district heating (DH) systems can contribute
ll rights reserved.
to this long-term goal. This discussion is framed in terms ofengineering principles, and does not extend to an analysis ofeconomic costs. Nevertheless, all of the options considered liewithin the envelope of systems currently under discussion withinthe energy policy community.
Beyond the engineering, the discussion presented here hasimplications for policy. On the one hand, CHP appears to offer analternative to strategies of electrification of heat supply espoused,e.g. by the Committee on Climate Change in the UK, and, on theother hand, to strategies based on the use of biomass and/or pre-combustion carbon capture and storage (CCS) to extend the usefullife of existing gas grids. More immediately, the paper suggests thatthe definition of renewable heat currently included in the EURenewable Energy Directive, which includes conventional heatpumps but excludes thermodynamically equivalent combined heatand power, is inconsistent.
2. Comparing the thermodynamics of conventional andcombined heat and power steam cycles
A convenient way to understand the thermodynamics of a heatengine is to plot the state point of the working fluid at all points ofthe engine’s working cycle on a temperature/entropy or Molierdiagram.1 The temperature/entropy diagram for a standard steam
1 A more extended introduction to this approach to describing the perfor-
mance of turbines is provided in Volume C of Modern Power Station Practice
(Martin and Hannah, 1991).
Nomenclature
T temperature (K or 1C)S entropy (J/(kg K))Tcon temperature at which steam is exhausted from low-
pressure turbines in conventional, non-CHP plantQin heat input to the steam cycle of electricity generating
plantQcon heat contained in the steam exhausted at a tempera-
ture Tcon from low-pressure turbines in conventional,non-CHP plant; this steam is subsequently condensedand the latent heat it contains is rejected to theenvironment
Tchp temperature at which steam is exhausted from low-pressure turbines in CHP plant—the latent heat con-tained in this steam is supplied to a district heatingnetwork
Qchp heat contained in the steam exhausted, at a tempera-ture Tchp, from low-pressure turbines in CHP plant
W work output from the turbines in a conventionalpower plant; in this paper, no distinction is madebetween work and electrical output—in practice, theywill not differ by more than a few percent
Wchp work output from the turbines in a CHP plantDWchp reduction in work output from CHP plant, measured
with respect to equivalent non-CHP plant, as a resultof raising the condensing temperature from Tcon toTchp
Whp work input to a real heat pumpCOP coefficient of performance of real or virtual heat
pumpsCOPcarnot Carnot limit to the COP of a heat pump operating
between two defined temperatures
3 A Carnot cycle consists only of processes at constant temperature or
constant entropy, and its Moiier diagram is therefore rectangular. The main
departure for this virtual cycle is the oblique line at the left-hand end that links
the horizontal condenser and evaporator lines.4 But note that practical CHP plant may or may not omit the final stages of the
low-pressure turbine. Plant that does is referred to as back-pressure plant. The
alternative is to incorporate a full low-pressure turbine, with provision for
exhausting steam from the cycle at one or more intermediate points—a so-called
R. Lowe / Energy Policy 39 (2011) 5528–5534 5529
turbine cycle is shown, in sketch form, in Fig. 1. The steam cycleshown is based on that of Drax power station in the UK,2 but thespecifics are not important to the argument presented in thispaper. The line labelled Tcon denotes the temperature at whichsteam is exhausted from the low-pressure turbines andcondensed.
Because in a reversible process dQ¼T dS, the total heat flowinginto the steam is the integral
RT dS around the upper part of the
curve and the heat rejected to the environment isR
T dS along thebottom (condenser) line. Assuming that the first law holds,W¼Qin�Qcon, and therefore the work is simply the differencebetween the two integrals, which in turn is equal to the areainside the line on the diagram above.
The Molier diagram in Fig. 1 represents what happens ina standard condensing steam turbine, with a condensingtemperature of perhaps 35 1C (equivalent to a water vapourpressure of roughly 50 mbar). The starting point for thisanalysis of the thermodynamics of combined heat and power isto split this cycle diagram into two parts at a temperature of Tchp
(Fig. 2).The first thing to notice is that by converting the conventional
power station to combined heat and power operation, we haveproduced some useful heat but sacrificed some work. The CHPcycle for a given Qin rejects Qchp (4Qcon) to the district heatingnetwork, at a temperature Tchp (4Tcon), and produces a poweroutput of Wchp¼W�DWchp.
This is precisely what a heat pump does. The question is whatis the relationship between the power lost, DWchp and thequantity of useful heat gained, Qchp? Inspection of Fig. 2 revealsthat we can transform the split Molier diagram for the conven-tional power station into the Molier diagram for a CHP station byadding a virtual steam cycle exactly opposite to the bottom partof the split diagram—the equal and opposite cycles cancelalgebraically. This transformation is shown in Fig. 3.
This virtual cycle has a simple shape and rather simplethermodynamic properties. To a good approximation, all heatabsorption takes place along the lower horizontal line at aconstant temperature Tcon, and all heat rejection takes place alongthe upper horizontal line at a constant temperature Tchp. The cyclethat we have added is effectively a virtual Carnot engine operat-ing between temperatures Tcon and Tchp. Moreover, because thedevice absorbs work to move heat from a lower temperature to a
2 Drax is a coal fired power station in the North of England, completed in 1986
(http://www.draxpower.com).
higher temperature, it is a heat pump3. The limiting coefficient ofperformance (COP) of this device is
COPcarnot ¼Qchp=DWchp ¼ Tchp=ðTcon2TchpÞ
For those unfamiliar with the principles of heat pumps, briefinspection of this equation reveals the most important factor to bethe difference between the condensing temperatures of the conven-tional generator, Tcon, and of the CHP generator, Tchp. The smaller thedifference, the larger the limiting COP of the steam cycle heat pump.
It would be difficult to build a real steam cycle heat pumpoperating between these temperatures. The main drawbackwould be the very large specific volume of steam. But this is nota problem for engineers of CHP systems, because our steam cycleheat pump is only a virtual one. In principle, we do not have tomake a real steam cycle heat pump. We merely have to choosenot to make a real steam cycle heat engine (consisting of the lastfew stages of a conventional low-pressure turbine).4
3. Further consequences
The fact that the heat pump that we are discussing is a virtual onehas further consequences. A virtual heat pump reverses the directionof heat and other losses. Such a heat pump is able to compress itsworking fluid without heat flowing from the working fluid to theenvironment—in fact heat flows the other way, up the temperaturegradient and into the virtual working fluid. In a similar vein, virtualturbulence in the working fluid coheres and blows the turbine bladesaround, virtual friction turns the shaft of the compressor and virtualelectrical and magnetic losses in the alternator produce useful power.
A virtual heat pump has the additional advantage that it effec-tively pumps heat from a source at �40 1C, rather than fromanywhere between �10 and 10 1C for most conventional heat pumpsoperating in the heating season in temperate regions of the world.5
intermediate take-off condenser. This provides the flexibility to operate like
conventional non-CHP plant with condensation taking place at �30 1C, or to
operate in CHP mode with condensation taking place at a higher temperature.5 Ground source heat pumps pump from the temperature of the ground, which in
temperate regions of the world is around 10 1C. Air source heat pumps pump from
R. Lowe / Energy Policy 39 (2011) 5528–55345530
This conveniently high temperature arises in part because of thepractical difficulty of building real low-pressure steam turbines thatcondense at pressures much below 50 mbar, and in part because,as noted above, the temperature drop across the condenser in aconventional steam cycle turbine turns into a temperature rise acrossthe virtual evaporator in the equivalent steam cycle heat pump.
Fig. 2. Conceptual decomposition of steam cycle for a conventional power station (left h
virtual, very low-pressure cycle (lower right hand side).
Fig. 3. Equivalence of a CHP power station steam cycle (bottom) to a conventional powe
Fig. 1. Temperature–entropy diagram for steam with a sketch of a single reheat
steam cycle for a typical late 20th century fossil-fired power station super-
imposed. Heat input takes place in the upper parts of the cycle, which correspond
to the boiler, superheater and reheater. Work output to electrical generators takes
place in the vertical parts of the cycle, which correspond to the turbines. Heat
rejection to the environment takes place along the horizontal line at the bottom of
the cycle, which corresponds to the power station’s condensers.
The virtual cycle that we have had to add to convert ourconventional, condensing plant into a CHP plant, appears tocontravene the laws of thermodynamics. We are in the slightlybizarre situation of describing a (virtual) device that has apractical performance higher than the Carnot limit for theequivalent real device. Nothing, of course, in practice contravenesthe laws of thermodynamics. That a virtual steam cycle heatpump appears to do so is just the consequence of the consistentapplication of a sign convention.
The overall effect is shown schematically in Fig. 4. To constructthis figure, estimates of practical performance have been derivedby dividing the Carnot COP by a combined efficiency of 90% forthe turbo-alternator. This is broadly in line with estimates in theliterature for alternators (efficiencies �97%) and modern low-pressure steam turbines (isentropic efficiencies �94%)—see e.g.Bhatt (2011). Older and smaller turbines may have significantlyworse performance, which would lead to higher CHP perfor-mance, but this has been discounted.
As with any real heat pump system, overall performance of avirtual steam cycle heat pump can be improved by replacing asingle stage heat pump with a two-stage heat pump. The effect ofthis is shown in Fig. 5, which has been calculated on theassumptions that
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and
r st
the return temperature of water from the district heatingsystem is 35 1C;
side) into a steam cycle for a CHP power station (upper right hand side) plus a
ation steam cycle plus a virtual steam cycle heat pump (upper right hand side).
0
2
4
6
8
10
12
14
16
50 70 90 110 130
CO
P of
virt
ual h
eat p
ump
CHP condensing temperature TCHP (°C)
Carnot
practical
Fig. 4. Carnot limit and indication of practical limit to the COP for a virtual steam
cycle heat pump.
0
2
4
6
8
10
12
14
16
50 70 90 110 130
CO
P of
virt
ual h
eat p
ump
CHP condensing temperature TCHP (°C)
practical, 1 stagepractical, 2 stagepractical, 3 stage
Fig. 5. Indicative effect of two- and three-stage heating on performance of virtual
steam cycle heat pump. Increasing numbers of stages result in increasing
complexity and declining marginal improvements in efficiency.
R. Lowe / Energy Policy 39 (2011) 5528–5534 5531
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each stage of heating involves a heat exchanger with atemperature difference of 5 K across it, and
�6 The readers should note that the normal laws of thermodynamics re-apply
as soon as one crosses the boundary between a virtual steam cycle heat pump and
the real district heating infrastructure to which it is connected. The rate of loss
quoted is the 2007 average for Danish district heating systems (Danish Energy
the temperature of the second stage of heating is arranged sothat the two stages contribute equal quantities of heat to thedistrict heating system.
tnote continued)
ient air temperature, which ranges from �10 to 10 1C in temperate regions in the
er half of the year. Air source heat pumps pump from higher temperatures in
mer leading in principle to higher COPs during that season, but heat demand is
ller and demand temperatures are also higher. COP weighted according to heat
and is dominated by winter conditions in most buildings.
The effects of two- and three-stage heating are shown sche-matically in Fig. 5. Qualitatively, the move to multi-stage heatingmakes the system sensitive to the return temperature of the DHnetwork. If two-stage heating is combined with a district heatingsupply temperature of 90 1C and a return temperature of 35 1C,the analysis suggests that a COP approaching 9 can achieved. Evenallowing for �20% losses6 in the (real) district heating systemoutside the CHP power station, the effective COP of such a systemwould approach 7—significantly higher than could be achievedwith a real heat pump. Recent work in Denmark suggests thatmuch lower operating temperatures are possible—a flow tem-perature of just 55 1C has recently been demonstrated in adevelopment of 122 low energy dwellings (Anon, 2011). Whileit would not be possible to operate at such low temperatures overa large urban area, supply temperatures of the order of 65 1C maybe possible, potentially leading to COPs, including losses indistribution, in excess of 10.
Note that a virtual heat pump takes its electricity directly fromthe power station it is part of, and therefore avoids the transmis-sion and distribution losses that would be incurred by a system ofconventional heat pumps in individual dwellings. Averaged overthe year, transmission and distribution losses in DH systems aretypically more than twice those in an electricity distributionsystem—annual average transmission and distribution losses inthe UK are of the order of 7% (DECC 2010). But during periods ofmaximum demand the relationship is reversed, because losses inDH systems that operate at constant temperatures remainroughly constant in absolute terms, and are therefore lower as afraction of heat supplied, but losses in electricity systems increaseroughly as the square of the current carried.
What are the limits to this analysis? It seems to the author thatit is useful in the case where electricity is currently or will in theforeseeable future be produced by steam turbines, and wherethere is a heat load at some temperature Tchp, which needs to bemet. What the above analysis shows is that from a system point ofview, supplying such a heat load from CHP is directly analogous tointroducing electric heat pumps. Seen from outside the systemboundary (represented in each case by the large rectangle), Fig. 6is thermodynamically equivalent to Fig. 7.
The main difference is that, provided conventional and CHPplants are indistinguishable with respect to the upper part of thecycle shown in Fig. 2,7 the practical performance of systems basedon CHP is necessarily higher than that of systems based onconventional heat pumps and conventional power stations—toreiterate, the COP of a virtual steam cycle heat pump operating atan output temperature of 95 1C (suitable for district heating with asupply temperature of �90 1C) is likely to be of the order of 9. COPsfor real heat pumps sized for individual dwellings are typicallyaround 50% of the Carnot limit, typically �3. The performanceadvantage for CHP is therefore of the order of a factor of 3.
Some readers will note that not all the heat supplied by districtheating systems comes from CHP. In the current Danish system,around 20% comes from heat-only boilers (Danish Energy Agency,2008). Historically these were fossil fired, and would have been
Agency, 2008) and is defined as a fraction of heat sent out. Other things being
equal, losses from district heating systems rise somewhat faster than operating
temperatures. Systems that operate at higher temperatures will have correspond-
ingly higher heat distribution losses, in addition to a lower COP.7 This criterion is an important one, and in a more general form applies to all
CHP systems, not just those that use steam turbines. Not all currently available
technologies satisfy it and some, for example Stirling engine-based micro-CHP,
do not come close (Carbon Trust, 2007, 2011).
Fig. 6. System diagram of a conventional power station driving a real heat pump.
Fig. 7. System diagram for CHP power station.
R. Lowe / Energy Policy 39 (2011) 5528–55345532
responsible for a half or more of total emissions. The period from1980 to 2007 has seen a strategic shift in the source of non-CHP heatin Danish district heating systems away from oil and coal (86% in1980 falling to 25% in 2007) and towards gas (zero in 1980, rising to26% in 2007) and renewable energy (10% in 1980 rising to 40% in2007) (Danish Energy Agency, 2008). This has significantly reducedthe use of conventional primary energy in such systems. Peak heatload boilers (a sub-set of heat-only boilers) play a similar part in thethermodynamics of CHP/DH to the peak load electric resistanceheating characteristic of single dwelling heat pump systems, but byburning fossil fuel or biomass directly, they make a lower contribu-tion to total emissions from CHP/DH systems than if the equivalentamount of heat were supplied electrically.
(footnote continued)
4. Discussion
The fact that the ability of CHP systems to produce heat at auseful temperature requires a loss of electrical output has beenknown for many years. The nature of this loss has been exploredin detail (see e.g. Postlethwaite, 1980). The original contributionof the present paper is to reframe an otherwise complex set ofengineering facts in terms of a simple thermodynamic argument.
The main consequence of the thermodynamic argument set outhere is that there are significant overall energy efficiency advantagesto operating CHP systems at the lowest possible supply tempera-tures.8 These advantages are over and above a number of practical
8 One of the more interesting features of the development of combined heat
and power, particularly in Denmark, is the historical trend towards lower
operating temperatures. The earliest large-scale example that the author is aware
benefits from low temperature operation, which include reducedheat losses, simplification of engineering and operation of thedistribution network (Robinson, 1980), greater ease of integrationof renewable energy (Lund et al., 2010) and the ability to use districtheating in conjunction with highly insulated dwellings (Moller andLund, 2010; Zinko et al., 2008).
The analysis presented here applies directly to any power stationin which heat is exhausted to the environment from a steamturbine. It thus applies directly to combined-cycle gas turbine(CCGT) and nuclear stations—the potential importance of both forthe medium-to-long term is obvious. The differences between CCGTand conventional steam turbines concern the high temperature endof the respective cycles; the low temperature technology (which isthe only part of the system affected by CHP) is identical across allgenerating plants in which the final stage in electricity generation isa steam turbine. The analysis can be extended more or less looselyto other classes of CHP generators, for example systems based oninternal combustion engines, but the power of the analysis for steamcycle power stations lies in the clear identification of the virtualsteam cycle heat pump in the CHP system with actual physicalcomponents in real conventional power stations.
Thermodynamic efficiency is not the sole criterion for assessingthe parts that technologies such as CHP may play in energy systemsof the future. Limiting the extent of global climate change to close to2 K will require deep decarbonisation of industrialised societies. Inthe UK, a reduction target of 80% was made legally binding by thepassing of the Climate Change Act (2008) and the subsequentCarbon Budgets Order (2009). As the Committee on Climate Changehas recently confirmed, meeting such stringent overall targets willrequire the almost complete decarbonisation of the stationarysectors of the economy, including the power generation sector(Committee on Climate Change, 2010). Apart from their higherthermodynamic performance, the main difference between thevirtual heat pumps of combined heat and power and real heatpumps is that a virtual heat pump is permanently connected to adedicated power plant. This lock-in raises the question of whetherCHP is a medium-term rather than a long-term generation option.Unless it is possible to establish that the thermodynamic gains fromCHP need not be won at the expense of a significant overall increasein carbon emissions from electricity generation, the analysis pre-sented here would be of academic interest only.
It is not the main function of the present paper to presentdetailed analysis in this area, but an outline of reasons for believingthat CHP can satisfy the above condition must be put. These arethreefold. First, conversion of thermal power stations that would belikely to continue to operate for several decades provides a medium-term route to CHP and district heating, even where infrastructure iscurrently underdeveloped. A potential example of such a conversionis provided by the existing CCGT power station at Barking, at theeastern edge of London (Poyry Energy, 2007). Provided that suchconversion does not significantly extend the life of existing powerstations, the condition is satisfied.
Second, both new and converted CHP generators can inprinciple be combined with a number of other technologies toprovide a spectrum of technology options with carbon emissionsthat range from low to negative. These include the combination ofCHP with pre- or post-combustion CCS9, fired either with naturalgas or biomass. One or more of these combinations may allowcombustion-based thermal generation of electricity and heat to
of is the scheme at Odense, which has operated at a flow temperature of 90–95 1C
for many years (Rimmen, 2002). There are counter examples, e.g. in the operation
of the Greater Copenhagen scheme, where higher operating temperatures stem
from the need to link together a number of earlier schemes.9 The author has previously suggested this option (Lowe, 2007).
0.10
0.15
0.20
0.25
inte
nsity
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eat (
kg (C
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/kW
h)
R. Lowe / Energy Policy 39 (2011) 5528–5534 5533
persist into the second half of the current century, even against abackground of deep cuts in emissions from the electricity gen-eration sector. The key would appear to ensure that any futureCHP plant is either built with technology to allow it to operatewith very low carbon emissions or designed to facilitate subse-quent retrofit. Since current commissioning policy in the UK andelsewhere envisages precisely these options for non-CHP genera-tion, it would appear once again that the condition stated above issatisfied.
Third, the heat distribution networks that must necessarily bebuilt downstream of CHP generators significantly extend therange of future low and zero carbon options for providing heatand electricity by
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CHP,CCGT
CHP,CCGT+CCS
supporting the development of large thermal storage facilities,harnessing the obvious economies of scale, and capable ofabsorbing significant inputs of electricity from variable gen-eration sources such as renewables and
Fig. 8. Comparisons of carbon intensity of delivered heat for a range of possible
� heat supply technologies. Carbon intensity for unabated coal, and emissionreduction factor for CCGT with CCS from Hammond et al. (2011).
providing a platform for the development of large-scale heatpumps (Blarke and Lund, 2007; Lund and Mathiesen, 2009),which would in principle be capable of achieving technical andoperational economies of scale with respect to building-scalesystems, and of harnessing sources of heat that would be lessvulnerable to low ambient temperatures than air source heatpumps (Østergaard and Lund, 2011).
In the long run therefore, CHP/DH systems have the ability toprovide very low carbon heat and electricity through the additionof CCS to conventional fossil-fired power stations, and throughthe ability of district heating to facilitate the use of renewableenergy in heating and electricity production (Dyrelund and Lund,2010; Poyry Energy, 2010). The carbon intensity of heat delivered(assuming supply and return temperatures of 90/35 1C with two-stage heating and 20% distribution losses) would be of the orderof 0.065 g(CO2)/kW h for systems based on CCGT and in theregion of 0.008 g(CO2)/kW h for future CCGT CCS systems, assum-ing an 88% reduction in CO2 emissions due to CCS (Hammondet al. 2011). These figures represent reductions of the order of�70% and �97% in the carbon intensity of delivered heatcompared with natural gas burnt in a condensing gas boiler10—-
see Fig. 8. For CHP from CCGT with CCS, the estimate is lowenough to be vulnerable to a range of second order effectsincluding methane leakage from natural gas production anddistribution, secondary products of combustion such as nitrogenoxides, and leakage of refrigerant from conventional heatpumps—at this level, direct CO2 emissions from the power stationare in the noise. It is also important to acknowledge the limita-tions of any analysis of individual technologies that treats them asindependent of the wider energy supply, transmission and end-use system.11
10 This brief analysis suggests that the almost complete decarbonisation of
t supply can be achieved without measures at the dwelling level. In the context
he growing awareness of the difficulty and high cost of achieving reductions in
t demand for existing dwellings in excess of 80% through measures applied to
ividual dwellings, this is an important conclusion. It must, however, be
lified. Insulation and other energy efficiency measures undertaken at the
elling level reduce the capital costs of all upstream energy supply infrastruc-
, and are therefore likely to be crucial to achieving deep decarbonisation at
rall minimum economic cost.11 In connection with this discussion of CHP from potentially capital expen-
combinations of technology, it is worth noting the effects of CHP on load
ors. The load factor of a CHP power station may be lower than that of an
ividual conventional power station, as a result of the need to follow seasonal
As important as the long term is, it is also necessary to look atthe short term. Within the current UK discourse on the decarbo-nisation of heat, the key technology is seen as the single dwellingelectric heat pump (Committee on Climate Change, 2010; Lowe,2007). This approach is reinforced by the wording of the EURenewable Energy Directive (EU, 2009), which defines heatpumps, but not combined heat and power, as renewable. Thecriticisms of this approach include
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To reduce CO2 emissions, the COP of heat pumps needs toexceed the ratio of the carbon intensity of grid electricity tothe carbon intensity of heat supplied by conventionalmeans—in the UK mostly natural gas burnt in gas boilers.Recent UK experience with electric heat pumps reveals acomplex picture (Energy Saving Trust, 2010), but one thatprovides no grounds for believing that this criterion is likely tobe satisfied before the end of the current decade. Because ofthe high COP of virtual steam cycle heat pumps, CHP fromlarge power stations can, in principle, meet it immediately.
� The performance of single dwelling, electrically driven, air-source heat pumps during periods of low external temperature(which, in winter-peaking grids, coincide with periods of peakelectricity demand), is poorly defined but lower than theannual average (Energy Saving Trust, 2010). This in turn meansthat the additional electricity generation, transmission anddistribution infrastructure needed to support them are alsopoorly defined.12 Together with ground source and geothermal
tnote continued)
iations in the demand for heat. But shifting heat supply from conventional heat
ps to CHP will improve the overall load factor for the electricity system, as a
lt of better cold weather performance of CHP compared to conventional heat
ps, the ability of DH systems to switch to sources of heat other than CHP
ing cold weather and the routine incorporation of heat storage in DH systems.12 It appears that in the UK the capacity of the low voltage electricity
eration system is of the order of 1 kV A per dwelling. This suggests that
e-scale uptake of single dwelling electric heat pumps would require significant
forcement of this system. Estimating the additional capacity required is
plicated because most air source heat pumps switch to resistance heating in
y cold weather, doubling or trebling the load imposed on the grid. The
struction of new district heating networks avoids the need to reinforce the
tricity distribution system and associated uncertainties. Intermittent heating
l also be a significant factor.
R. Lowe / Energy Policy 39 (2011) 5528–55345534
heat pumps, the much higher source temperature of virtualsteam cycle heat pumps (see above under ‘‘Further conse-quences’’) means that their performance is unaffected byperiods of low external temperature.
The analysis presented here suggests that it would be appro-priate to consider alternatives to a strategy of complete electri-fication of heat supply.
5. Conclusions
There are three main conclusions from this paper. The first,which comes from a consideration of thermodynamics of CHP andconventional steam turbines, is that the comparison of CHP withconventional heat pumps is not coincidental, but fundamental.CHP is most conveniently conceived of as a virtual steam cycleheat pump with a performance that exceeds the Carnot limit forreal heat pumps operating under equivalent conditions. Practi-cally, the performance advantage with respect to heat pumps forindividual dwellings is of the order of a factor of 3.
The second, more qualitative conclusion is that the need toachieve deep cuts in CO2 emissions by the middle of the currentcentury does not appear to preclude the option of CHP forprovision of space and water heat to buildings.
The third is that the equivalence of CHP and heat pumps at thethermodynamic level might be expected to lead to a similarity oftreatment at the policy level, but this appears not always to be thecase. The arguments presented here may provide grounds forreconsidering such differences.
Acknowledgement
This paper is based on a working paper written by the authorcirca 1980. The author gratefully acknowledges the assistanceprovided by William Orchard and David Olivier during the writingof the current version of this paper, and the helpful and con-structive comments of two anonymous referees.
References
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