Monday, 2 June 2014

Net-zero houses going mainstream

Some news from Proud Green Home about net-zero homes going mainstream. 

I think the key development to make this possible has been the reduction in cost of solar panels. Of course it would be very difficult without all the low-energy building technology, but without a way to generate electricity all bets are off.

The definition of net zero is also a little suspect. If a building is off-grid and not drawing any power from the outside, then it can certainly be said to be net zero. If it is connected to the grid, taking power from the grid and sending power back, then there should be a certain amount of daylight between the generation and the consumption. This is necessary to account for line losses and grid inefficiency.

The electricity that a house draws from the grid is a lot less than the power fed into the grid from thermal or nuclear power stations, because energy is lost in the wires carrying it and in the step-up and step-down transformers that convert the voltage to travel over the long distances we like to keep between out houses and those forms of dirty power. This is called the primary energy factor and varies from country to country. We used the number 2.7 to evaluate our house.

Even if a house uses fossil fuels directly, for example with a gas cooker or oil-fired boiler, you have to take into account the energy used getting the fossil fuels out of the ground, so you can't simply look at the energy use as the amount of energy going in to the house, but you have to add the amount of energy used to get that energy out. This is something like 10% of the energy you get out of them.

Similarly, when we supply our electricity to the grid, a certain amount of it is going to be lost, inadvertently heating up the wires between our house and wherever the electricity is used.

Also we have to address the issue of embodied carbon. In other words, how much energy was used in building the house, and how much carbon did that release? With a net-zero house, the question is: How many years will it take to pay back the carbon released during construction? 

Anyway, there tends to be a trajectory of good ideas from pipe-dream to realm-of-nutters to community-hobby-horse to common practice. Net-zero houses now seem to be breaking through from realm-of-nutters to community-hobby-horse.