Changes to the Code for Sustainable Homes, as well as the SAP2005 revisions to Appendix M, have made it almost impossible sensibly to provide a zero-carbon home.

Carbon emission (as per David Strong’s comment, BSj 10/07), the latest BRE calculations require:

  • welling CO2 emissions rate (DER) <=0 as calculated by SAP2005
  • Heat loss parameter (HLP) to be less than 0.8
  • Net carbon emissions <=0.

The net carbon emissions allows for an offset from the DER to account for the Part L requirements for a secondary heater and only 30% low- energy lighting. It also allows for any allowable on-site generation.

The issue here is with multiple-dwelling schemes where you are looking to account for centrally generated electricity from wind turbines or PV arrays. Under the latest Appendix M, this is not entered into the SAP worksheet but added to the new zero-carbon worksheet for compliance with the stamp duty relief scheme. Therefore, the communal wind turbine and PV array have no impact on the individual property SAP rating. However, a CHP system, despite also being a communal electrical generator, is allowable under SAP and therefore does have an impact.

This must be an issue for turbine manufacturers. If wind turbines have no impact on SAP ratings unless individual building-mounted units are used, it will be hard to justify them. It is fairly simple to break up PV arrays to provide individual supply to each dwelling, but it is not feasible for turbines. It is hard enough to get planning with a single turbine. So, unless a development is looking for zero-carbon, there is no benefit (from a ratings viewpoint) from installing central renewable electrical generation.

If you exclude central PV or wind turbine electricity generation, it is difficult to get DER<=0 from SAP2005 despite the fact that after the allowable electricity generation is added back in, net carbon emissions are <=0, unless of course you have a biomass CHP or a big south-facing PV array. A zero-carbon home becomes impossible, under the definitions given, without these items.

We have seven four-storey townhouses above a commercial area in a coastal location. The heating is from a central biomass boiler with sub-meters to each dwelling. Electricity generation is from a 6kWp vertical wind turbine and a 9kWp PV array on the roof, connected to the main incoming supply which is sub-metered to each dwelling.

Because the PV and wind turbine are not accounted for in the SAP calculation, the DER is greater than zero. However, the net carbon emissions are <0 and the HLP is less than 0.8 for each dwelling. So, although we have a zero-carbon development overall, we don’t because DER>0, even though we have enough on-site generation to cover appliance loads and offset the dwellings.

We must look towards central generation of wind and PV (as well as other technologies) being included in the SAP worksheet, in the same way that CHP is accounted for. For the calculation of zero-carbon, we should be focusing on the net carbon emissions along with the HLP factor and not some theoretical calculation.

The SAP calculation concentrates too much on technology improvements and does not allow enough benefit for more passive improvements, such as solar gains. As SAP is the approved calculation for compliance with Part L1A, perhaps modelling software could also be allowed to show compliance in a similar manner to commercial buildings.

Nathan Williams, director, ndesign services

Source

Building Sustainable Design