Energy Matters: Open Well Water Source Heat Pumps Pros and Cons

By Alan Bellamy   Posted in Issue 78
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Energy Matters: Open Well Water Source Heat Pumps Pros and Cons

by Derek Morgan

This months’ article describes an open well water source heat pump; how it works and what’s involved installation wise. The heat energy stored in underground water tables can be harvested and pumped to the surface from a well through a heat exchanger where the heat is extracted and is then deposited in a second well and absorbed back into the water table. As in a ground source the temperature extracted is between 2 and 4 degrees C. It is recommended that a safety heat exchanger be used to protect the internal exchanger in the heat pump from poor water quality. The main design criteria is that the water in the coldest months does not drop below 7 degrees on the intake and a constant flow is maintained. Typically a 10 kw unit would require a flow rate of 14 litres/ minute.
As the climate becomes warmer and wetter here in Scotland it is anticipated there will be an increase in the uptake of water source as a viable heat pump option. The shorter boreholes are cheaper to drill and costs could be spread amongst neighbouring properties who also required boreholes.

FAQ

1. An open well Water Source will run at -20 degrees ambient air temperature – is this true?

Yes as the ground water temperature would be nowhere near the air temperature and would only have a minimal loss of efficiency in the colder weather.

2. How much land is required?

A two well system could be accommodated in almost any garden providing the holes are at least 6 metres apart.

3. How deep would I have to drill?

In Scotland the water table in low lying areas is no more than 20 to 30 metres down though this can vary significantly. It is always advisable to undertake a desktop survey to find any data relevant to your plot.
A ground source system can typically be 3 to 4 times more expensive to install than an air source. A borehole can cost from £4,000 to £6,000 to drill and a trench system could cost £3,000; and there is access for the drilling rig to consider.

4. Is Water Source more efficient?

Yes, it can be one of the most efficient heat pumps depending on the temperature of your well water. An efficiency of 600% is not uncommon.

5. Do I need permission to extract the water?

Yes, SEPA would have to be contacted prior to installation and an abstraction license obtained if necessary.

6. How expensive are they to maintain?

They are cheaper than ground source to maintain and filters can be cleaned by the owners on a monthly basis to ensure smooth operation.