Earth

The earth (and water) can provide energy though a system called a ground source heat pump. This basically works on a similar principle to the refrigerator; coolant is expanded to cool it down, and compressed to heat it up.

The coolant is expanded (cooled) and then pumped in pipes around an area of land or water. The ground then brings the temperature up from the cooled state to the relative constant of the ground. The coolant is then compressed to release the heat and a heat exchanger then takes this heat into the house system. In effect you are using the latent temperature of the soil (or water) to heat the building.

The system is driven by electricity, so you would still need to produce / buy this from renewable sources.

The system has a measure, called the Co-efficient of Performance, that shows how much extra you are getting from it. So for every unit of energy put into the system (in the form of electricity) you will generally get between 3 and 4 units back (in the form of heat).

The system relies on having a series of pipes laid in the garden, or by installing a bore hole, this gives the coolant the area it needs to come up to ground / water temperature.

These types of system are very compatible with underfloor heating because they run at lower temperatures.

More detailed information is available from the Greenspec website