Heat energy from the air
Air Source Energy
An air source heat pump (ASHP) is a machine which absorbs low grade heat from the outside air, then under the principles of vapour compression refrigeration, transforms it into useable high grade heat.
This high grade heat can be used to run radiators, under floor heating and even provide hot bathing water, modern air source heat pumps can provide water at 60 to 65 centigrade.
Air source heat pumps can work at very low external temperatures, typically as low as -15 centigrade.
Heat energy from the ground
Ground Source Energy
A ground source heat pump (GSHP) is a machine connected to a series of pipes buried in the ground, a very cold water and anti freeze mixture (typically 0 to -5 Centigrade) is slowly pumped around the ground pipes (known as a ground loop heat exchanger) because the liquid is at a lower temperature than the surrounding ground it is naturally warmed. Using a vapour compression refrigeration cycle, the low grade heat is transformed in to high grade which can be used to heat underfloor pipes, radiators and even provide hot water for baths and showers.
Heat energy from the sun
Solar Thermal Energy
Solar thermal, or solar hot water systems as many people refer to them range from very simple to quite sophisticated systems, however the principle for both is the same. The collector, this can be a as simple as an old radiator painted black or as sophisticated as an evacuated tube, is places in a position where the sun shines on it. Usually this is on a roof.
A water and glycol mixture is circulated through the collector and is naturally heated by the sun. The system is a sealed circuit and with the addition of the glycol, it reaches very high temperatures without boiling, typically well in excess of 100 centigrade.
Heat energy from the trees
Heating systems which burn plant derived matter are collectively known as biomass boilers. They can be as simple as a little wood burner or a fully automated wood pellet boiler.
Grasses and trees absorb carbon throughout their life as they grow, when they are burnt they release the captured carbon back in to the atmosphere, so growing and then burning a tree is carbon neutral.
Although as the fuel has to be processed there is a small carbon footprint, so biomass is a low carbon fuel source.