Collectors

Horizontal collector

Horizontal collectors were the collection method originally introduced with ground source heat pumps. The horizontal collector recommended in this case is 40mm diameter pipe. Compared to other options this method has a lower energy yield per metre of collector. As such a large collector is required and these are often oversized.

Installation option 1

The collector should be covered in 10cm of sand where the material to be back filled contains sharp stones which may damage the collector. Sand should not be required where the earth to be backfilled contains smaller round stones.

Installation option 2

Collectors should be buried to a depth of at least 0.8m to avoid the effects of frost on the collector. A depth of 1m tends to be favoured.

This type of horizontal system typically provides between 10 and 18W of energy per metre of active collector. The lower figure in the range would apply if the heat pump is running all day whereas the higher figure would be typical of a system running a few hours per day. When sized to 75%-80% of the peak load of the house the energy drawn from the collector could be around 15W per metre.

The amount of energy extracted also depends on the amount of moisture that the collector is in contact with in the ground.

Appropriate pipework (PEM 40x2.4) is available from:

Uponor Housing Solutions Ltd, Snapethorpe House, Rugby Road, Lutterworth, Leicestershire LE17 4HN Tel: 01455 550355

Rehau Ltd, Hill Court, Walford, Ross-on-Wye, Herefordshire HR9 5QN Tel: 01989 762600

Sizing the horizontal collector to the heat pump

The sizing of the collector depends upon, in addition to other factors, on the COP (Co-efficient of Performance) of the heat pump system.

In simple terms the length of collector will be determined by a simple equation of the energy to be extracted from the ground in Watts divided by the yield from the collector in Watts per metre.

Using the example from the section of this document on the COP the following would apply. A heat pump capacity of 9kW has been taken as the example simply to show the calculations involved.

Heat pump capacity = 9kW

COP = 3.3

Energy provided by ground source = 6.3kW (6,300W)

Energy yield per metre of collector = 15W

6,300 ÷ 15 = 420

Collector required = 420m

Taking an example of 6,300W to be extracted from the ground and dividing the yield of 15w/m into this the required length of collector would be 420 metres.

As a rough guide, following the yield and COP above, a 6kW heat pump would require 280m of collector and a 9kw would require 420m. A single loop collector is generally possible in a length of up to 450m.

These lengths would normally have to be increased by 20 to 25% for a collector in sandy soil and decreased by 20 to 25% for wet soil (water permanently present).

Length of active collector (m) & no. of circuits required
	6kW	7kW	9kW	11kW
Collector length	280	325	420	260
No. of circuits	1	1	1	2

Compact collectors

The compact collector allows a heat pump to be installed with a reduction in the area of ground required compared to a typical horizontal collector.

The compact collectors measure 1.5 metres wide and 2 metres tall and are connected together using either compression fittings or fusion welding.

Reloading

Compact collectors can be used with or without reloading. Reloading is a method of returning energy to the ground around the collectors when the heat pump is not using it, thereby increasing the amount of energy stored in the ground for use when it is required.

Reloading is typically achieved using an air recovery system whereby waste heat from the air in a property is used in a heat exchanger to transfer energy back into the ground.

Compact collectors are available at stockists of Worcester Greenstore ground source heat pumps.

Sizing the compact collector array to the heat pump As a rough guide (based on a COP of 3.3) the table below shows the required number of compact collector panels with and without reloading.

Required no. of compact collector panels
	6kW Heat Pump	7kW Heat Pump	9kW Heat Pump	11kW Heat Pump
With reloading	9	11	14	17
Without reloading	18	22	28	33

Required no. of compact collector panels
	No. of collectors per kW of Heat Pump output	Energy recovery per panel approx.
With reloading	1.5	450W
Without reloading	3	250W

Installation option 1 – no reloading available

Where no reloading is available the panels should be installed in an almost horizontal manner as shown in the diagram.

The collectors should be installed so that there is a fall from the top of the collector (where the connections from one panel to the next are made) to the bottom of the collector of 300mm, equal to 150mm per metre.

A distance of 1 metre is required between multiple rows of collectors.

Where the backfill material contains sharp stones the collector should be laid in 10cm of sand and covered with another 10 cm of sand.

Installation option 2 – reloading only

Where reloading is possible the panels can be installed in a vertical format.

Multiple rows

Compact collectors can be arranged in multiple rows. A maximum of 11 collectors per row is permissible. Each individual collector provides a flow resistance of 5kPa/h. The table below provides further details.

Multiple rows
	6kW Heat Pump	7kW Heat Pump	9kW Heat Pump	11kW Heat Pump
With reloading - no. of panels	9	11	14	17
With reloading - no. of rows	1	1	2	2
Without reloading - no. of panels	18	22	28	33
Without reloading - no. of rows	2	2	3	3

Borehole (vertical) collectors

A borehole collector allows a ground source heat pump to be installed in a property where the area of land available is insufficient for compact collectors or a horizontal collector.

Vertical collectors typically provide a higher energy yield per metre of active collector when compared to horizontal collectors.

Vertical collectors can provide between 35 and 55 Watts per metre of active collector, which depends on the heat conductivity of the rock into which the hole is drilled. A hard rock such as granite has a better conductivity that a softer rock such as limestone. For the purpose of the example below a figure of 40W per metre has been used. The actual yield will depend, amongst other factors, on the geological conditions on site.

Pipework for vertical collectors may be supplied by the borehole contractor to the specification indicated in the pipework section.

Alternatively, pipework is available from: Uponor Housing Solutions Ltd, Snapethorpe House, Rugby Road, Lutterworth, Leicestershire LE17 4HN Tel: 01455 580355

Rehau Ltd, Hill Court, Walford, Ross-on-Wye, Herefordshire HR9 5QN Tel: 01989 762537

Sizing the vertical collector to the heat pump

Heat pump capacity = 9kW

COP = 3.3

Energy provided by ground source = 6.3kW (6,300W)

Energy yield per metre of collector = 40W

6,300 ÷ 40 = 158

Collector required = 158m

Length of active collector required (m)
Collector Yield	6kW Heat Pump	7kW Heat Pump	9kW Heat Pump	1kW Heat Pump
55 W/m	80	90	120	140
40 W/m	110	120	160	190
35 W/m	120	140	180	220

The term active collector is used to denote collector pipe that is in direct contact with the ground source, for example below the water table or in contact with a borehole filling material such as bentonite. Any part of the tube that is in contact with air will not generate a significant yield and must not be included when calculating the energy the collector will provide.

However it is a requirement when specifying the total length of tube required to include the active collector, any inactive portion in the borehole and the run from the property to the borehole and back.

It is considered good practice (and a requirement of the Environment Agency) to fill the bore hole with bentonite (a cement compound) in order to guarantee conduction of energy to the collector and to guard against disturbing rock strata or ground water layers.

The borehole collector should be filled before being lowered into the borehole with glycol mixture.