Designing appropriate energy systems for low-energy is quite a complex task really. The electricity side of things, for grid connected houses, is relatively straightforward; generate what you sensibly can and export any surplus. If that generation is large then the capacity of the grid, particularly in distributed rural areas like our own, which wasn’t designed to integrate decentralised sources of ‘microgeneration’, can be an issue. Solar PV is the only electricity generating technology we can employ in our instance. That is a pity because high up in rural Northumberland our wind resource is fantastic but despite having a little bit of land we just don’t have a suitable location within our own title. We had hoped to buy a small patch of land from the farmer just big enough in the neighbouring field for the footprint of a turbine about 150m from the house on a ridge line where the turbine would have enjoyed sweeping winds from 360 degrees but ran into complications unfortunately. We now just look enviously across the valley at a neighbouring farm which has installed a micro-turbine that would appear to be harvesting energy with abandon! Wind turbines may be emotive but onshore wind remains the cheapest form of renewable electricity available in the UK and at a micro level turbines rarely have the sort of visual impact which so many find upsetting either. Instead though we’re opting for a solar photovoltaic array, keeping it below the 4kW mark so as to benefit from the maximum feed in tariff and not trouble the grid capacity.
Designing heating systems for low-energy houses would appear to be a bit more complicated. Passivhaus design, founded as it is on principles of reducing energy demand, is obviously a good place to start, aiming as it does to do away with the need for space heating altogether. Confirmation that a house will attain Passivhaus standards still requires the use of Passivhaus design software – the Passivhaus Planning Package (PHPP) – and of course good craftsmanship to ensure that the final building fabric meets the quality specified on paper. Once achieved though the space heating in essence takes care itself. More problematic are instances such as our own where factors beyond our control – our south facing elevation is primarily a very large existing solid stone wall – prevent us achieving a passive level of design, meaning supplementary space heating is required, yet the demand, relative say to a house that simply meets building regulations, is still very low.
We have used PHPP, but the further one gets away from Passivhaus levels of performance, the less reliable the model becomes. Naturally we are obliged to carry out a SAP assessment (SAP or Standard Assessment Procedure is the government endorsed methodology that underlies building regulations and the like) so the house will be modelled at that stage too but SAP is known to be a rather blunt instrument. The house has also been modelled using Integrated Environmental Solutions (IES) software, again not ideal because IES seems to be geared much more towards commercial buildings than one off houses. Depending on how exactly the model was set up IES software indicated, using Passivhaus standard u-values (i.e. 0.15W/m2K for walls and roof, and .8W/m2K for windows), that the peak heat load was between 6.6 kW and 9kW. Using similar u-values in the PHPP but alternating between an airtightness figure of 0.6 and 1.2 ach@50 Pa because we’re concerned about meeting the lower airtightness figure due to the conversion element of the build we got figures of 4.5kW and 5.2kW respectively for peak heating load. Thus relatively there’s quite a lot of variation in these figures which makes designing the heating system tricky – you don’t want to install excess capacity yet you don’t want to leave the property under-heated. This is a really important element of the house to get right though; obviously thermal comfort is dependent on it but also we don’t want to be coming back and having to make major alterations because we didn’t get it right at this stage! I’ll post more on the heating system in due course…