Brandon_edit:Outsourcing_book_temp.qxd 24/08/2009 12:07 Page 19
Carbon Dioxide Performance Assessment for Micro Combined Heat and Power
It is useful to note that a number of attempts have been made to
Figure 1: Sensitivity of Annual CO
2
Reduction – ‘Average’ Reference
quantitatively characterise the marginal reference system. Most
System (kg CO
2
/year) versus Annual Thermal Demand for Five Typical
UK Dwellings with 1kWe Micro-CHP Systems
notable among these are Marnay et al.,
7
Voorspools and
D’haeseleer,
8,9
Bettle et al.
10
and Rekkas.
11
These efforts unanimously
ICE: heat-to-power ratio = 3 PEMFC: heat-to-power ratio = 2
considered the marginal electricity system (as opposed to other
1,500 1,500
aspects of the energy system, such as piped gas, etc.). The
1,000 1,000
approaches developed generally revolve around the observation that
three time-frames are important when considering the marginal
/year)
2
500 500
energy system in a liberalised market: short-term ‘balancing’ impact,
systematic energy trading impact and long-term infrastructure
0 0
impact. The short-term impact relates to which elements of the
5,000 10,000 15,000 20,000 25,000 30,000 5,000 10,000 15,000 20,000 25,000 30,000
incumbent system, if any, respond to an incremental and
SOFC: heat-to-power ratio = 1 Stirling: heat-to-power ratio = 8
1,500 1,500
unpredictable change in demand. These can be very short-lived
Flat
responses, stemming from unpredictable events such as intermittency
Bungalow
1,000 1,000
Terrace
or unplanned power station outage, where (in the electricity system) Semi-detached
reduction wrt reference system (kg CO
the system operator would perform actions to balance the system in
2 Detached
500 500
realtime. In contrast, systematic impacts relate to changes in the
supply mix that occur after a predictable change in demand, where
Annual CO
0 0
5,000 10,000 15,000 20,000 25,000 30,000 5,000 10,000 15,000 20,000 25,000 30,000
(for example) the specific power stations online at a particular time
Annual thermal demand (kWh/year)
change due to a consistent change in aggregate demand. Finally,
systematic changes in demand can also lead to particular
ICE = internal combustion engine; PEMFC = proton-exchange membrane fuel cell; SOFC = solid oxide fuel cell.
infrastructure investment choices, where alternative technologies Figure 2: Modelled CO
2
Emissions Reduction Provided by a 1kWe Solid
may be chosen or investment deferred or avoided. An example of a
Oxide Fuel Cell Micro-CHP System Operating in a Typical UK Terrace
House – Sensitivity to Gas and Electricity Emission Rates
long-term infrastructure impact is deferment of build of a new power
station due to insufficient demand.
5,000
Approx current
Current break-even
grid-average
point for SOFC-based
While existing research efforts have gone some way in exploring and
CO
micro-CHP
2
rate for
4,000
Possible future break-
UK electricity
defining the marginal energy system, no accepted standard has even point for SOFC-based
UK government
micro-CHP if gas CO
long-term CO
emerged, and indeed no attempt has been made to extend analysis
2
2
rate is halved
rate for UK
3,000 grid electricity
beyond the electricity generation sector.
/year) compared with
2
2,000
Conclusions
For the case of residential micro-CHP, CO
2
emissions reduction is
1,000
driven by the ability to generate electricity to displace high CO
2
intensity incumbent electricity generators and/or the ability to run on
reduction (kgCO
2
0
low-carbon fuels. If national electricity systems are successfully
Gas CO
2
rate = 0kg/kWh
conventional grid/boiler energy provision Gas CO
decarbonised and the fuel of choice remains natural gas, micro-CHP
2
rate = 0.04kg/kWh
Annual CO
-1,000
Gas CO
2
rate = 0.09kg/kWh
will be unable to provide emissions reduction in the long term.
Gas CO
2
rate = 0.14kg/kWh
Gas CO
Alternatively, if micro-CHP can be controlled to displace high CO
2
rate = 0.19kg/kWh
2
-2,000
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
intensity electricity generation and/or low-carbon fuels can be
Marginal reference system CO
2
rate (kgCO
2
/kWh)
utilised, micro-CHP has the potential to be an important contributor
CHP = combined heat and power; SOFC = solid oxide fuel cell.
to a future low-carbon energy system. Moreover, considering the
scale and pace of energy system change predicted over the coming Despite admirable research efforts directed at this challenging problem,
decades, it is important that defensible assessment methods for no convincing standard for performance assessment of demand-side
‘alternative’ low-carbon interventions are developed. A key element measures exists. This has contributed to the demand side of the energy
of these methods is characterisation of the marginal reference equation being seen as the junior party when developing energy system
system. This is because this is the system actually supplanted by the decarbonisation strategy. This hampers potentially valuable
interventions. The currently dominant approach – the use of static opportunities to compound supply-side decarbonisation with
system average figures to assess credentials – can lead to promising complementary demand-side interventions. Correcting this oversight
options being ignored by technology developers or inhibited by could lead to a more resilient and economically efficient overall response
poorly formed policy instruments. to climate change mitigation. n
1. HM Government, Climate Change Act 2008, London, UK: The 4. Hawkes AD, Leach MA, Energy, 2007;32(5):711–23. 2000;25(11):1119–38.
Stationary Office (TSO), 2008. 5. Hawkes AD, Staffell I, Brett DL, Brandon NP, Energy & 9. Voorspools KR, D’Haeseleer WD, Energy Policy,
2. Skea J, Ekins P, Winskel M, et al., In: Skea J (ed.), UKERC Environmental Science (Royal Society of Chemistry), 2009; in press. 2000;28(13):967–80.
Energy 2050 Project, London, UK: UK Energy Research Centre, 6. National Grid plc, London, UK, 2009. 10. Bettle R, Pout CH, Hitchin ER, Energy Policy, 2006;34(18):
2009. 7. Marnay C, Fisher D, Murtishaw S, et al., Estimating Carbon 3434–46.
3. May R, et al. (eds); CCC, Building a low-carbon economy – the Dioxide Emissions Factors for the California Electric Power Sector, 11. Rekkas D, UK marginal power plant and emissions factors.
UK’s contribution to tackling climate change, Committee on Berkeley, CA: Lawrence Berkeley National Labs, 2002. MSc thesis, Centre for Environmental Policy, Imperial College
Climate Change: London, UK, 2008. 8. Voorspools KR, D’Haeseleer WD, Energy, London: London, UK, 2005.
MODERN ENERGY REVIEW VOLUME 1
19