Ground Source with Ground Loops
Ground loops are usually the best option if a suitable area of ground is available. Horizontal runs of pipe are installed at a depth of around 1.2 meters under the ground, connected back to a manhole. Sizing the ground loops is very important as it can make the difference between an excellent system and a very badly performing system.
The following table shows the maximum sustainable heat that can be extracted from various soil types over 1800 and 2400 hours per year. This information is in accordance with the internationally recognised standard VID4640.
|Underground Specific extraction output for Horizontal ground loops|
|Soil type||1800 hours per annum||2400 Hours per annum|
|Dry, non-cohesive soils||10 W/m||8 W/m|
|Cohesive soils, damp||20–30 W/m||16–24 W/m|
|Water saturated sand/gravel||40 W/m||32 W/m|
The figures of 1800 hours and 2400 hours are average figures per year based on how the heat pump is being used. 1800 hours is the average for heating only, while 2400 hours is for both heating and hot water.
Using the figures from the table, it can be seen that with a 10kW heat pump on a site that has cohesive, damp soil (giving an average of 20W/m²) would require 500m² of ground. For dry soil that has a lot of sand in it, the same heat pump would require 1250m².
At OHPS we always use linear runs of pipe, we never use slinkies or other space saving devices. As shown above it’s the area of ground that is the key factor.
Ground Source with Boreholes
Where ground loops are impractical, the use of boreholes is possible. There are two types of borehole that can be used:
Closed Loop Boreholes
Similar to ground loops but the pipe installed vertically instead of horizontally, six inch boreholes are drilled to a depth of 90m, and two lengths of pipe joined using a special connector are inserted into each borehole. The boreholes are then back filled with a special grout called bentonite. The pipes are connected to a manifold and which runs back to the heat pump.
On average, you can extract 3-4.5kW from a closed loop borehole, depending on the rock type. A 10kW heat pump would require 2-3 boreholes to operate efficiently. For domestic projects, the cost of drilling usually makes boreholes unviable, however for commercial projects as the scale of the project becomes larger, ground loops start to become impractical and therefore boreholes will be the more viable option.
Open Well Boreholes
Extracting and using the ground water directly, two boreholes are drilled a set distance apart, with the intention of hitting a good source of water. A deep borehole pump is placed in one of the boreholes and water is pumped from the borehole, through the heat pump and discharged through the second borehole.
This is by far the most efficient way to run a ground source heat pump but is potentially the most problematic. With open well boreholes, the water has to be tested as certain minerals or elements can damage the heat exchanger of the heat pump over time. There also has to be high level of confidence the water will be available long term.