I’ll keep this short to ensure that it does get posted, but I suspect that I could rant on this till closing time on Friday night. For a recent renewable energy assessment for a client I finally took the time to review the potential for air source heat pumps to deliver carbon reductions and I don’t like what I found.
Using SAP 2005 we looked at a 3 bed end of terrace 2.5 storey house to ascertain the applicability of this technology to address the 25% DER reduction required by Level 3 of the Code for Sustainable Homes and the potential to provide a 10% or greater renewables contribution. The proposed system achieved a 31% reduction for the DER, provided a 23.9% renewable contribution (to heating & DHW - so for the Code one needs to add the power consumption for cooking and appliances) but results in a 23.5% annual increase in CO2 emissions compared to an A-Rated condensing boiler with no renewables.
There are clearly issues raised by this:
- With an electric heat source, the TER increases for a dwelling so the 25% reduction is relatively easier to achieve.
- The Code is therefore flawed for asking for a relative improvement over Building Regs as a measurement of carbon efficiency, whereas EcoHomes 2006 measures performance against specific emission rates per m2.
- By providing a ’renewables’ component of greater than 10%, the Code rewards the use of an air-source heat pump even further in category ENE7.
Now, none of this matters where the renewables assessment is completed in accordance with the London Energy Strategy as the Renewables Toolkit states that the baseline for consideration is unit-based gas. In this situation, the air source heat pump clearly does not make the grade. But what about all those planning authorities that don’t have such supporting documentation.
The results, so briefly described above, are not to be read as a definitive rubbishing of air source heat pumps, but I am concerned that there are loop holes here that could be exploited. I think we need to start discussing carbon efficiency rather than energy efficiency as there is too much scope to hide behind good looking statistics.
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April 3, 2008 at 8:52 pm
Richard McLaren
Your assessment within the context of SAP seems logical. However, SAP2005 has many issues which do not allow for reasoned assessment of heat pump technology.
The SAP COP efficiency does not take into account the designed flow temperatures for the heating system or inverter control and weather compensation control. Thus the actual seasonal performance of an air sourced heat pump has to be measured on that basis.
The degree day data that drives SAP assessment is not geographically specific. This leads to gross errors for seasonal energy requirements. The longer the heating season, as for Scotland, the better is the performance of air sourced heat pumps.
SAP unit energy costs do not take into account current energy cost relationships for 2008.
I agree, carbon efficiency is the key. However, with SAP a key indicator for the Labour government so far as energy efficiency is concerned, there will need to be some significant changes.
SAP is a very simplistic energy assessment model and unfortunatly the model is being seen as a predictive tool of actual energy consumption- this is definately not the case.