There is a source of heat that can be harvested and is almost entirely free; almost entirely because you do need to put some energy into the system in order to get the heat out. The good news is whatever energy you put in is multiplied by a factor of two or three so you get far more energy out than you put in. I know this seems too good to be true, but such a system does exist and it's called a ground source heat pump.

When the sun's energy hits the earth the ground acts like a giant heat sink, storing much of that energy in the form of heat. The fact of the matter is that just 1 metre underground, the average temperature is around 9 to 15 degrees Celsius warmer than the temperature at the surface. This is the temperature difference that's utilised by a ground source heat pump.

A trench one metre deep and about 200 meters long is dug around your property and into this trench is placed a plastic tube filled with water. The water in the pipe is cooler than the surrounding ground and so it's heated up as it's pumped through. This warmer water is then transferred into a sealed vessel which contains a refrigerant under pressure. This causes the temperature of the water to be raised from 15 degrees to about 45 degrees Celsius. Not hot enough for a shower or bath but more than adequate to heat your home.

Usually this heat is diverted to underfloor heating which can easily raise the ambiant temperature of your home to a comfortable level. Often a small supplemental boiler is installed (gas or oil) to complement the ground source energy supply.

Because a small amount of energy is required to pump water around the system the heat produced isn't entirely free but for every unit of energy put into the system it will return 2-3 times as much in return. This type of system is ideal if you're trying to go off grid as the water pump could easily be supplied with power from solar panels or a small wind turbine.

Although a fantastic heating system, it does have a couple of disadvantages. It's more expensive to install when compared with traditional heating systems and it's not easy to retro fit to an existing property due to the ground disturbance required to create the trenches.

This type of system is becoming popular with new builds where the pipe work can be laid whilst digging the foundations and the under floor pipework can be laid and tested without having to rip up an existing floor.

Key words:   Ground Source Heat Pump ，water pump

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