CPD Programme
Ground level
Winter
-5m
-10m -15m 0˚C
>
5˚C 10˚C 15˚C
Figure 1 – Approximate UK Annual Ground Temperatures
or ‘closed’ ground loops. This would most frequently employ a vapour compression refrigeration cycle. The available heat generated (and released from the condenser of the refrigeration cycle) is then used for low temperature space heating or supporting the generation of hot water. There are two heat exchangers in a GSHP:
Source side – the evaporator: A ground loop
manifold provides the interface between the fluid that is drawing the heat from the ground and the source side exchanger as shown in the example in Figure 2.
Load side – the condenser: Transfers energy
between the high pressure, high temperature refrigerant gas (post compression) and the heating/hot water circuit. The complete heat exchange cycle is shown
in Figure 3. The heat transfer fluid is a water/glycol
mix to prevent freezing during periods of low ambient air temperatures and ground frost.
Open and closed loops
Water that naturally flows through the ground in aquifers (in spaces between gravel, sandstone, and fractured rock) will tend towards the temperature of the ground through which it is flowing. By drilling holes into the aquifer, the depth of which will vary depending on location, the water can be diverted (pumped) to provide a source
20˚C
Spring
Summer
Autumn
of heat for the heat pump evaporator. This is known as an open loop system and to determine the availability of such a resource requires a specialist hydrogeological survey to determine the extent and longevity of the available resource. As a consequence of the more complicated
survey requirements of this technique and due to greater uncertainties (such as unforeseen geological conditions, air locking and collapse of the boreholes or a falling water table), closed loop systems are more frequently employed where the heat exchanger is constructed within the ground itself. Closed loop heat exchangers usually consist of a sealed loop of polyethylene pipe containing a heat transfer fluid which is pumped around the loop. The heat exchanger may be installed vertically or horizontally, the choice of the arrangement will depend on the available land, local soil type and excavation costs. Like open loops they can still suffer from air locks and they require careful protection both during installation and after they have been installed to prevent blockages.
Closed loop ground heat extraction using a horizontal trench
Where horizontal trenches are used to extract energy from the ground, then the depth would normally be between 1.5 and 2 metres. For strip trenches (used commonly with ‘slinky’ type coils) the width of the trench is typically the same as that of a bucket on a digger. The actual area required for the ground loop will depend on the type of ground and the annual heat extraction period. Based on Central European data EN 15450 [3] provides the guidance in Table 1.
Operation period Soil type:
dry, non cohesive soil
10 W/m2 8 W/m2
1 800 h
per year
2 400 h
per year
Figure 2 – Manifold for connecting multiple ground loops to evaporator heat exchanger
moist cohesive soil
water saturated sand or gravel
20 to 30 W/m2
40 W/m2
16 to 24 W/m2
32 W/m2
Table 1 – Practical heat pump extraction rates per m2 ground
So, for example if a load of 10kW was to
be drawn from ground that is made up of wet clay for around 2,000 hours per year it might be reasonable to use a value of 25 W/ m2. Hence the ground loop should cover a ground area of 10000/25 = 400m2. The pipe should be laid no closer than
0.8m apart [4] and so this would mean a pipe length of at least 400/0.8 = 500m. To ensure that there is not excessive pressure drop in the ground loop (to both reduce operating costs and maintain reasonable pressures) the practical pipe length is related to the pipe diameter. The recommended maximum length for 25mm is 100 metres and for 32mm pipe is 200metres [4] so in this case if 25mm pipe was used then four loops would be used linked up via a manifold (as in Figure 2).
Figure 3 – The heat exchange process in a GSHP system
72
CIBSE Journal April 2010
Closed loop heat extraction via deepbore hole
Vertical bore hole based ground loops are suited to applications where there is limited ground availability as in more densely populated areas but are more expensive to install. Bore holes are required to be deep (in the order of 50 to 100 metres) to accommodate the appropriate length (and hence surface area) of pipe to extract the required heat from the ground. The bore holes would normally be no closer than 6 metres apart and the pipe would be surrounded by a grouting medium (injected at the same time as the pipe is inserted) to ensure good heat transfer between the ground and the pipe. The amount of heat that can be extracted from the borehole is determined by the type of ground and the annual operation period. Table 2 [5] provides an abstract of the data derived from applications in Central Europe giving the approximate available heat that may be extracted per metre of vertical borehole.
www.cibsejournal.com
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