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Carbon Dioxide Performance Assessment for Micro Combined Heat and Power
Figure 2 considers these factors for the case of the low heat-to-power style control or via more passive operating strategies if these can be
ratio solid oxide fuel cell micro-CHP in more detail, investigating the proved to be effective. Also, as demonstrated by the remaining lines
sensitivity to the assumed composition of the reference system. The in Figure 2 (i.e. lower CO
2
intensity gas fuels), if piped gas can be
horizontal axis in Figure 2 is the deemed CO
2
intensity of the marginal decarbonised, the ‘break-even’ point moves further to the left,
grid electricity reference system, and the five plotted lines map the increasing the number of incumbent or alternative technologies with
sensitivity to varying CO
2
intensity in gas piped to the dwelling. If one which the micro-CHP system can compete. Overall, this demonstrates
were to assume that the marginal reference system were the current that proper definition of the marginal reference system is critical to
UK grid-average electricity system (~0.52kgCO
2
/kWh) and natural gas how micro-CHP technology is seen by developers, integrators,
were the only fuel option (0.19kg CO
2
/kWh), the predicted emissions suppliers and policy-makers, because its choice can change
reduction is just above 1 tonne CO
2
per year. However, if it could be conclusions by ±200%.
guaranteed that the micro-CHP system were displacing coal-fired
generation (~0.9kg CO
2
/kWh), emissions reduction would be above 3 Defining the Marginal Reference System
tonnes CO
2
per year. Furthermore, at the other end of the spectrum, if The above example established the importance of the choice of
the marginal reference system CO
2
rate was as low as 0.06kgCO2/kWh reference system for the case of micro-CHP. This argument can be
(i.e. the UKERC low-carbon core scenario for 2035, in grid-average generalised to all demand-side interventions as follows. In the long
terms), emissions reduction would be negative (i.e. an increase) by term, reduction in the average CO
2
intensity per unit of consumed
more than 1 tonne CO
2
. Clearly, an extremely wide range of outcomes energy (kg CO
2
/kWh) is required
3
to help mitigation of global
is possible depending on the CO
2
intensity of the grid electricity warming. However, as shown via the micro-CHP example, the actual
displaced. The risk faced when stakeholders consider the high CO
2
CO
2
reduction provided by an immediate intervention is not
intensity gas line plotted in Figure 2 is that it is tempting to conclude determined by the system average CO
2
intensity, but rather by the
that this micro-CHP system is at best a near- to medium-term CO
2
intensity of the specific elements of the incumbent system that
technology that has some role to play in system decarbonisation are displaced by that intervention. Therefore, the CO
2
intensity of the
while grid CO
2
rates are high. marginal reference system will always be important because it defines
the impact of each new intervention regardless of the system-average
However, this ignores the potential for the systems to interact on the CO
2
intensity. Furthermore, in a time of energy system upheaval, as
margin with high CO
2
intensity generators such as the remaining coal- is expected in the coming decades, understanding of the marginal
fired power stations, in which case CO
2
reduction would be reference system may become more important because it becomes a
substantial. This could be achieved by using virtual power plant (VPP)- moving target, evolving dynamically with each new development.
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