The house has been built on Passive House (Passivhaus) principles, which require specific standards to be met for airtightness and energy consumption among many other parameters. It requires a ‘fabric first’ approach to construction, where the house is designed from the start with these goals in mind. All components such as walls, windows, roofing, etc. have to separately meet the insulation requirements. This requires great attention to detail in order to avoid any thermal bridges (high conductivity routes from inside to outside) or air leaks so, for example, joints are taped up. This has proved successful and the house has now achieved formal Passivhaus certification
One Passive House requirement is that the annual heating demand must not exceed 15kwh per year per square metre; to put that in context, it is 5 or 6 times better than a high standard modern house and maybe 20 times better than a lot of older houses. It is equivalent to about 1 litre of heating oil per square metre, which means that our new house should require the equivalent of under 400 litres of oil to heat. In a Passivhaus, things like the heat generated by fridges and freezers and the losses from hot water tanks all contribute measurably to the heating – even the heat generated by humans is significant. Current calculations put our heating requirement at around 12.5kwh per year per square metre, but it looks like we are going to be even lower than that.
There are various myths about Passive Houses, the first of which is always ‘you can’t open the windows!’, and like all good myths it is pervasive but untrue; of course you can open the windows if you want to but you don’t need to open them. The house is more or less sealed when the windows are closed, but is equipped with a mechanical ventilation and heat recovery system (MVHR) which completely replaces all the air inside the house with outside air about every couple of hours. At the same time, heat is recovered from the outgoing air to heat the incoming air, and with a good MVHR system the efficiency should be over 90%. It makes for a very comfortable and healthy living environment.
The ‘passive’ of Passive House implies that you do not need to actively heat the house but rather rely on various secondary heat generators, including humans, with the odd heater for extra cold weather. However, we are using under floor heating powered from a low temperature air source heat pump, which should run at 3x to 4x efficiency most of the time. We felt that we would rather find out that we don’t use it much than that we needed it but didn’t install it.
Our final airtightness tests exceeded the Passivhaus standard (tested at 0.56, and the standard is 0.6) which, to put it in perspective, is around 18 times higher than is required by current building regulations. This is not overkill as it’s the key to being able to meet the low heating load requirements. There is more to Passivhaus certification than this test, including the PHPP calculations which have now been done. The upshot is that now we have formal certification, we have a nice plaque to fix to the house!
We have also received our Energy Efficiency assessment, which comes in at 101 out of 100, which is a reflection of being energy positive. We think we are probably better than this as no account has been taken of our thermal battery, which is outside the capability of the model. The ‘potential’ can be achieved by solar thermal heating for hot water (which is addressed by the thermal store) and by a wind turbine, although the latter is estimated as a £15-20k cost to achieve around £400 per annum savings!
The other main target of the house is to be energy positive, that is, to generate more than we consume. There is a 10kw solar array on the flat roof, which is on track to produce around 10,500kwh of power in a year and consumption is looking to be around 7,500kwh, so we are comfortably energy positive. There is a split of NS and EW orientated panels that gives a slightly lower peak generation but a longer capture period. Clearly solar generation is seasonal and it will be necessary to get power from the grid, but to some extent a certain amount of inter-seasonal time shifting can be done by selling energy to the grid in sunny seasons and then buying the cheapest energy in the winter by harnessing the batteries, albeit that nowadays even at the lowest tariff, import costs quite a bit more than export. Unfortunately our power company (Western Power Distribution) has restricted our capacity to upload to the grid to 6kw due to infrastructure limitations, so we will need to be careful to have a strategy that uses surplus power when we are potentially generating more than this, for example, by charging an eVehicle in future.
We have found in practice that in this our first year, we turned power positive for the year to date at the very beginning of May – so total power produced in the year up to that point exceeded total consumption at around 2.7MWh. We have run at 95%- 98% self-consumption from the last week of March until the third week of September. 100% is not achievable because the solar inverters use a trickle of AC power in converting from DC to AC. In the same period, all of our domestic hot water came via the thermal battery, which in turn is solar powered. In the summer we often exceeded the 6kw surplus over our consumption, but because of our feed-in limit this power can’t be used, so is simply wasted. This “wasted” power looks like being around 1Mwh for the year. In the July-Aug-Sept quarter, our consumption from the grid was 60kwh, while our solar generation was 3,500kwh in the same period, not counting the wasted power. We had to supply a photo of the feed-in metre to prove it!!
We are using two types of battery for time shifting energy. These can be ‘filled’ either via solar power or by the low cost bands of Economy 7 or 10 power, which provides 7 or 10 hours during the day at which electricity is much cheaper. There is an electric battery, which is a 10kwh DC battery rather than the 13kwh AC battery that we would have preferred, but because our electricity supply company, Western Power Distribution, adds the output of an AC battery to the output from the solar when considering the total generation capability of the house and has set our limit at 10kw, we couldn’t use an AC battery, even though our feed-in is limited to 6kw in any case. Ours not to reason why!
We also have a thermal battery that uses phase change material to store heat for water, which is then released in the form of latent energy. For much of the year (March to October), this battery is almost entirely charged from solar power.
Provision has also being made for Electric Vehicles, but we will be getting one of these in 2020.
We will add first year power performance figures early in 2020.