Surface water, rivers, lakes and spring water source heat pumps
Rules and regulations and the lack of available manufactured equipment have made this type of system less attractive. That said, there may be a increased interest in this method. A recent detailed CIBSE document discusses all the options – Surface water source heat pumps
The deeper ground temperature is approximately equal to the average air temperature over the year. (e.g. around 8 to 12ºC or 46 to 54ºF in Northern Europe). Ground water and springs are often at this average ground temperature, and sometimes vary very little from summer to winter. This is an ideal source of heat for a heat pump. Even in the middle of winter, this system should maintain a good energy-efficiency. The only pitfall to avoid would be any need for excessive pumping the water to a height or over a great distance. If the energy required to get the spring water to where you need it is great, then this will detract from the viability of the source.
Only very few heat pumps may be capable of taking spring water directly, and this may in any case be a little risky with respect to fouling or corrosion. A corroded heat-exchanger could result in a VERY expensive repair to the refrigeration circuit. The water purity should be tested to see if is suitable for the heat-exchanger in the heat pump. One option to protect your expensive heat pump is to use an intermediate heat exchanger with a glycol circuit between them. However, this will reduce the energy-efficiency because the heat pump would ‘see’ colder water compared to the direct type, and also an extra circulation pump is needed. Another alternative is to use a very large plastic pipe (like a ground source slinky) in a gravel bed with the spring water running across it. This should be a fit-and-forget system and can be extremely efficient.
Rivers and streams
A river or small stream can be utilised, but the water temperature can approach freezing point. For direct pumping (open loop), many heat pumps require water at temperatures above 5 to 8°C (varying depending on type). This means that the heat pump may not work in the middle of winter in colder climatic regions. The same water quality constraints apply as spring source (above) for direct pumping through a heat pump.
Stainless plate collectors or coiled plastic pipes may be an option, and could be filled with a low-toxicity glycol if local regulations permit. Such systems could operate at slightly colder river temperatures if they are able to operate with some ice build-up on the outside of the collector coils.
Whilst delivering high-efficiencies for much of the time, a system could be in risk of failing to operate completely in the middle of winter during lower temperatures, just when you need the most heat. An alternative back-up heating method will be needed for severe winters.