Ground source heat pumps (GSHP) transfer
heat from the ground into a building to provide space heating
and, in some cases, to pre-heat domestic hot water. For every
unit
of electricity used to pump the heat, 3-4 units of heat are
produced. As well as ground source heat pumps, air source
and water source heat pumps are also available.
How does it work?
There are three important elements to a GSHP:
1) The ground loop. This
is comprised of lengths of pipe buried in the ground, either
in a borehole or a horizontal trench. The pipe is usually
a closed circuit and is filled with
a mixture of water and antifreeze,
which is pumped round the pipe absorbing heat from the ground.
2) A heat pump. This
has three main parts, similar to a fridge:
- the evaporator - (e.g. the squiggly thing
in the cold part of your fridge) takes the heat from the water
in the ground loop;
- the compressor - (this is what makes
the noise in a fridge) moves the refrigerant round the heat
pump and compresses the gaseous refrigerant to the temperature
needed for the heat distribution circuit;
- the condenser - (the hot part at the
back of your fridge) gives up heat to a hot water tank which
feeds the distribution system.
3) Heat distribution system. Consisting
of under floor heating or radiators for space heating and
in some cases water storage for hot water supply.
What options are available?
The ground loop can be:
1)
borehole;
2)
straight horizontal - trench costs less than a borehole, but
needs more land area;
3)
spiral horizontal (or 'slinky coil') - needs a trench
of about 10m length to provide about 1kW of heating load.
How much does it cost?
Installation
A typical 8kW system costs £6,400-£9,600 plus the price
of connection to the distribution system. This can vary with
property and location.
Running costs
The efficiency of a GSHP system is measured by the coefficient
of performance (CoP). This is the ratio of units of heat output
for each unit of
electricity used to drive the compressor and pump for the
ground loop. Typical CoPs range from 2.5 to 4. The higher
end of this range is for
under-floor heating, because it works at a lower temperature
(30-35ºC) than radiators.
Based on current fuel prices, assuming a CoP of 3-4, a GSHP
can be a cheaper form of space heating than oil, LPG and electric
storage heaters. It is however more
expensive than mains gas. If grid electricity is used for
the compressor and pump, then an economy 7 tariff usually
gives the lowest running costs.
Ground source
heat pumps and your home
What to keep in mind when
considering a ground source heat pump.
- The type of heat distribution system.
GSHPs can be combined with radiators but under-floor heating
is better as it works at a lower temperature.
- Is there space available for a trench
or borehole to accommodate a ground loop?
- Is the ground suitable for digging a
trench or borehole?
- What fuel is being replaced? If it's
electricity, oil, LPG or any other conventional fossil fuel
the payback will be more favourable. Heat pumps are a good
option where
gas is unavailable.
- Want to be 100% renewable? Buy green
electricity, or install solar PV or some other form of renewable
electricity generating system to power the compressor and
pump.
- Need a back-up heating system?
- Is there also a cooling requirement?
- Is the system for a new building development?
Combining the installation with other building works can reduce
costs.
- Can you incorporate insulation measures?
Wall, floor and loft insulation will lower your heat demand.
Source of information is: The Energy Saving Trust
January 2007.
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