Last week I wrote about the importance of renewable heat, and how heating our houses sustainably receives less attention than renewable electricity, despite being the bigger challenge. If we are to transition beyond fossil fuels, then heating our houses without gas is something of a priority. That means renewable heat, and there are several forms of it.
1. Incoming solar is one obvious source of warmth. I say ‘incoming’ because other energy sources, including fossil fuels, are ultimately solar in origin. Incoming solar uses the direct warmth of the sun to heat buildings, and there are a couple of ways to do that.
The most basic is to orient new buildings so that they are warmed by the morning sun. You can also use the principles of thermal mass to absorb and store the sun’s warmth in walls, tubes or the floor, and then release it as the sun goes down.
We all know how hot greenhouses or conservatories can get, trapping heat inside all that glass. Naturally, you can use this warm your house. A Trombe Wall is an external glass wall that captures heat in a tight cavity, and you can then circulate this into the house. These sorts of techniques are called ‘passive solar’, as they require no further intervention or energy.
Solar hot water is a technology that may be more familiar. Fitted on roofs, this provides hot water and in some cases, heat as well.
2. Geothermal heat is another avenue for heating our houses sustainably. Some places, such as Iceland, can tap volcanic activity for hot water and heat. 90% of Iceland’s homes enjoy geothermal heat piped through district heating networks.
But even without that unusual resource, the earth is a source of warmth. Below the surface, the earth maintains a fairly constant temperature. In winter, this will be warmer than the air. A network of pipes can be laid a couple of metres underground, with circulating fluid absorbing the warmth and relaying it to the surface. These ground source heat pumps work best in larger buildings, using heated floors and maintaining a constant temperature, so they aren’t for everyone. They’re also difficult to fit to existing houses, as you have to dig up a large amount of space to lay the pipes.
3. Biomass is the third main form of renewable heat, and it serves as a catch-all for anything that burns wood or waste for heat. Wood-burning stoves are the simplest form. They usually heat a single room, though with a few design considerations they can be used to heat a whole house. Wood pellet boilers are a next step up, and can be used to create a heat source for a number of houses or a district.
A further possibility is heat from biomass power stations, which may be burning wood or agricultural wastes. Heat can be piped to nearby buildings in a district heating system.
Burning wood for heat is renewable as long as new trees are being planted, and wood is not being harvested sustainably.