Brandon_edit:Outsourcing_book_temp.qxd 24/08/2009 12:05 Page 17
Industry Outlook
Carbon Dioxide Performance Assessment for
Micro Combined Heat and Power
a report by
Adam Hawkes
1
and Nigel Brandon
2
1. Grantham Institute for Climate Change; 2. Energy Futures Lab, Imperial College London
Given the mounting physical evidence and theoretical corroboration of heat and power (micro-CHP). It is shown that the ‘marginal
climate change, and recent developments in associated international emissions’ approach is an important tool for exploring the dynamics
agreements, decarbonisation of national energy systems is topical of energy system change, in contrast to existing research, which
worldwide. In the UK the government has adopted ambitious long- tends to focus on static snapshots of the reference system.
term greenhouse gas (GHG) emissions reduction targets, requiring an
80% decrease relative to 1990 levels by 2050 via the recent Climate Carbon Dioxide Performance Assessment for
Change Act 2008.
1
Achieving such targets will require radical changes Micro Combined Heat and Power
in the energy system, ranging from extensive introduction of low- Micro-CHP is a technology designed to replace conventional home
carbon technology to fundamental shifts in energy consumption heating systems. It provides the space heating and domestic hot water
behaviour. Recent whole-system modelling efforts such as the UKERC needs for the dwelling, and also produces some electricity for
Energy 2050 project
2
and the Committee on Climate Change report
3
consumption onsite or for export to the grid. A description of the main
have outlined the possible nature of this future energy system, and prime mover technologies – Stirling engines, internal combustion
gone some way to describing the rapid decarbonisation pathways by engines (ICEs) and solid oxide and polymer electrolyte fuel cells (SOFC
which it could be achieved. Framed by these efforts, current research is and PEMFC) – is available in Hawkes and Leach,
4
and a more detailed
focusing on enriching the detail in these transformational pathways, discussion of fuel-cell-based systems is available in Hawkes et al.
5
understanding underlying practicalities and, ideally, contributing to the
base of evidence that will inform technology, policy and societal As discussed in Hawkes et al.,
5
the key metric that determines the
development towards a low-carbon energy system. carbon dioxide (CO
2
) reduction achievable by micro-CHP is the system’s
ability to generate electricity. This ability is limited by thermal
Within this diverse field of research, performance assessment methods constraints, which force the system to modulate or switch off.
2
Thermal
for ‘alternative’ technologies are one of a variety of issues that shape constraints can be caused by lack of significant and consistent thermal
transformation of the energy system. Specifically relevant to this article demand in the target dwelling or high heat-to-power ratio (or
is the method by which the GHG performance credentials of a low- inappropriate size) of the prime mover. Both of these factors can
carbon intervention are examined. A crucial element of such an prevent the system operating and producing electricity. Conversely,
assessment is definition of the ‘reference’ or ‘baseline’ system that is technologies with low heat-to-power ratio (e.g. fuel cells) are more likely
displaced by the intervention. This is often called the ‘business as usual’ to be able to continue generating electricity at times of low thermal
scenario. This reference system is essentially a portrayal of what would demand, and are thereby able to displace more grid electricity, thus
not happen if the intervention were to go ahead, such as avoided gaining an advantage in terms of CO
2
reduction. These interactions are
generation from a particular power station or avoided investment in a illustrated in Figure 1, which confirms that low heat-to-power ratio
polluting technology, etc. Therefore, by definition, the reference prime movers are broadly capable of greater CO
2
reduction and are less
system can only be estimated ex ante, and is consequently challenging dependent on the presence of thermal demand in order to achieve this.
to defensibly quantify.
However – and this point is key to this article – the above analysis
Furthermore, changes to the reference system are ‘marginal’ by nature; ignores the inevitable change that occurs in the reference system. As is
low-carbon interventions do not bear upon all aspects of the energy typical in such analyses, this performance assessment used a static
system proportionally. It is the incremental change in the system that snapshot of the reference system, where grid electricity displaced is
defines their actual impact. For that reason, use of system-average assumed to embody grid-average CO
2
emissions (or a fixed estimate of
statistics in lieu of the estimated performance of the marginal reference a longer-term CO
2
rate). In the UK the grid CO
2
rate commonly applied
system can provide misleading conclusions regarding the GHG is 0.43kgCO
2
/kWh, which is the government-specified long-term rate
reduction afforded by the intervention. In the worst case this could lead for business reporting. A comparable approach was applied to the fuel
to promising low-carbon technology development being abandoned, consumed by the micro-CHP system, where piped gas is assumed to
or the introduction of poorly structured policy support (or none at all). embody 0.19kgCO
2
/kWh, which is the accepted figure for UK natural
Given these risks, it is important to develop and maintain a high-quality gas. The important observation here is that these estimates may be
understanding of the marginal reference system. This will ensure that misleading given the extremely rapid grid decarbonisation of electricity
low-carbon interventions contribute to keeping the energy system on supply suggested in recent energy system modelling efforts
2,3
centred
an appropriate transitional pathway. around achieving the UK government’s 2050 targets, and the potential
to replace a substantial portion of residential natural gas supply to bio
This article explores the significance of the marginal energy system and waste-gas alternatives, as outlined by the National Grid in
using the example of the modelled performance of micro combined reference 6.
© TOUCH BRIEFINGS 2009
17
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 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84