Closed Loop Ground Source Heating System
The efficiency of a geothermal heating system is dependent on the ratio of the input and output temperatures. At equilibrium water from a buried pipe collector will emerge at typically 7-10° C. When the system is operated the temperature can fall to around 0° C. This creates a temperature gradient in the ground and allows heat to flow into the collector pipe. The COP of the closed loop systems will for this reason always be less than that which would be achieved for directly pumped ground water where we would expect a temperature of 7-10° C in the UK.
| Closed Loop Coiled Collector | Coiled Trench Loop System |
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The
photograph shows a coiled loop (slinky) collector system. The
trench here was excavated to a depth of 2m with the top of the coil at
a
depth of 1m. MDPE pipe was laid directly into the ground without
preforming. For each 10m of trench 60m of MDPE pipe is used. The
length of the trench required depends on the thermal conductivity of
the ground. Wet clay soils perform better than dry porous soils. It is
practical to limit the length of each trench to about 30m
bringing all the systems together in a manifold. The collector may be
used for both heating and cooling. Passive cooling is possible using
the ground cooled water directly to provide a heat sink for underfloor
heating or ducted air systems.
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Horizontal planar
collectors can be used but require a greater area to
be excavated. The system will comprise of several equal
length
loops brought together in a manifold. It is important that equal
lengths
are used. A higher capacity pump will be required to maintain adequate
flow through the longest of the pipes if the lengths are not equal.
Typically these systems are buried to a depth between 0.6m and 1m. The shallower depth can provide greater efficiency saving at the beginning and end of the winter but may operate with reduced efficiency over the coldest months when solar warming of the ground is less.
Typically these systems are buried to a depth between 0.6m and 1m. The shallower depth can provide greater efficiency saving at the beginning and end of the winter but may operate with reduced efficiency over the coldest months when solar warming of the ground is less.