4. Concluding remarks
A modelling approach has been taken to assess the relative
performance of ASHP and SOFC-mCHP units under a range of conditions.
The emphasis of this research has been to investigate the
effect that the operational conditions have on the units and so
it has been necessary to consider a wider range of permutations
than in comparable studies. The effect of the control methodology
employed with the units has been the primary focus of the
study but other variables such as the climate and the specification
of the buildings which heat is supplied to have been considered.
These results can be used to inform the direction of more focussed
simulation and research.
The performance improvements which might be achieved by
using a proportional control methodology with the ASHPs are of
particular significance; it is possible that they could reduce emissions
by around a third. It is recommended that further research
should consider this approach in more detail in order to verify its
potential.
Single performance metrics for eitherASHP or SOFC-mCHP units
do not capture the range of energy performances which might be
observed when the units are installed within complete heating
systems. The effect of operational conditions should not be underestimated
when comparing the relative merits of either technology
to alternatives. Marginal metrics such as the cost of emissions
avoided are even more sensitive to this variation if they are calculated
from the difference in emissions between competing systems.
The effect of auxiliary heating units on overall system performance
should not be underestimated and highlights the
importance of the appropriate selection of boundary conditions
when comparing heating options. For the SOFC-mCHP example
considered in this study, the use of the auxiliary heater typically
halved the electrical efficiency achieved by the system. A larger
SOFC-mCHP unit for a given heat load would reduce this efficiency
penalty but incur an increase in system costs.
In the context of displacing electricity generated by the UK electrical
grid, operating the SOFC-mCHP units continuously at their
maximum electrical generation capacity maximises the net emissions
benefit that they achieve, despite the inevitable increase
in heat dumping. If this objective (i.e. reducing net emissions)
is adopted, then concerns regarding thermal cycling fatigue of
the units and their ramp rates become less relevant. However,
the insensitivity of this finding to the amount of heat which is
dumped also implies that the main benefit of this technology is as
efficient generation, regardless of whether it can be employed in
CHP schemes. It may be that another hybrid system including a
SOFC-mCHP unit and an alternative system to cover peak heating
demands would be effective.
The optimum configuration for ASHPs does not include buffer
tanks. In contrast, the use of larger buffer tanks improves the performance
of SOFC-mCHP units in most cases. All of configurations
studied use separate domestic hot water tanks.
Assessments of the performance which these technologies
might achieve in the future should take account of the climate
change which might occur;that expected by 2050 in the UK is likely
to reduce heating related emissions from ASHPs by around a quarter.
The reduction in net energy requirements is likely to exceed
the reduction in heat demand that a simple time-temperature difference
model would suggest. Lower heat demands decrease the
temperature at which ASHPs must deliver the heat. Lower heat
demands also reduce the need for auxiliary heating in the case of
the SOFC-mCHP systems, dramatically improving system electrical
efficiencies. More varied air temperatures tend to result in higher
heat demands for a given mean temperature but the overall trend
remains.
Both ASHP and SOFC-mCHP units have the potential to contribute
to reductions in energy use and the related emissions of
CO2. However, for this to potential to be fully realised, the effects
of operational conditions on the performance of both technologies
should be understood and thoroughly researched.