Friday, 30 May 2014

Building a house

(Reposted from September, 2009, with pictures added. )

We're now heading fairly surely towards actually building a house. People keep asking me what kind of house it's going to be, and I'm not really sure how to answer. I wonder if they want me to say that I'm building an igloo, or a fourteenth century Venetian palace or a brick Victorian end terrace. Certainly they want some short answer and not a long description of what colour each wall will be and what the doorknobs will be made of.

It certainly seems like I should have a short answer for this question. As building is much more of a philosophical journey than a technical one I'm just going to set out what I want to do and what that means.

I want to build a house that has an energy consumption of less than zero.

An explanation of why I'd like to build a house that produces energy should not be necessary, and in my opinion in a developed country you should not be allowed to build to consume when you can build to conserve.

Following from this basic condition for building are four topics: Energy efficiency, thermal inertia, generation of electricity and collection of heat. They are of course interconnected, but I'll try to explain each one.

When it's going to be ten below zero outside, a big part of energy efficiency is thermal efficiency. First of all, this depends on the size and shape of the building. The bigger it is, the more heat it will need. The bigger the surface area, the more heat it will lose in the winter, also, the more heat it will gain in the summer when it's over thirty outside. As well as the design and layout of the house and the rooms, the materials used are important. It must be well insulated. A lot of the heat is lost through windows, so these are very important.

Thermal inertia will keep the building at a constant temperature. The more heat the building can contain, the better. This can be both active and passive. The air in rooms contains a certain amount of heat, and it will make a difference how this air moves by convection and how it is forced and fanned where it might not otherwise go, and whether heat can be recuperated from air as it leaves the building. Water, or other liquids, can also store heat and can be moved around the house to where heat is needed. In addition, building materials can store heat. While wood is a good insulator, stone can store a lot more heat. Another possibility is phase change materials: for example floor panels containing a liquid that freezes at 19 degrees. Because of the latent heat of freezing, this can absorb a lot of heat as it is melted, and release a lot of heat as if freezes, all at the ideal temperature of 19 degrees.

Photovoltaic solar cells are pretty much the only practical way of generating electricity in a small plot in the middle of an urban area. The amount of solar energy that falls on the earth in one hour is the same as the amount used by the human race in a year, so some kind of harnessing of this power is very feasible. In fact most kinds of energy production come indirectly from the sun, whether you're burning wood, using fossilised wood in the form of coal or prehistoric microbes in the form of oil, or even using wind and waves that have been generated ultimately by temperature changes and the evaporation of the oceans. Only nuclear power and tidal generation are not originally solar. The sun is, of a course, a large nuclear reactor so I suppose you could say that all energy is ultimately nuclear. Solar cells are not highly efficient, perhaps only converting 10 to 20% of the sun's energy that hit them into electricity, but that's worth a whole new blog.

The collection of heat is most simply achieved by facing windows to the south. As the winter sun is low and the summer sun high, eaves can easily be extended to allow heat in in the winter and keep it out in the summer. This, on its own, is not going to be enough to heat the whole house and of course the more windows there are, the more heat is lost. Solar walls, developed to dry grain in Canadian barns, can absorb heat from the winter sun and convert it to hot air inside the house. Roof mounted solar thermal panels can be used to heat air or water. Combined with photovoltaics these can greatly increase efficiency, also worthy of a whole new blog. Heat pumps are another way of getting heat. They use a small amount of power to defy the second law of thermodynamics and get heat from a colder body.

I'll come onto the colour of the walls and the doorknobs later, but these will probably be influenced by the points above.