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Geothermal heating     (Geothermal Heating)


I wrote the following articles for Koi Magazine.
Therefore they own the copyright but the Editor has given permission for them to be republished here.

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Geothermal heating - how does it work and is it possible to heat your pond for free?

When winters get as cold as the last one, many koi keepers begin considering the various options of heating their ponds.   Electric heaters are very popular for ponds up to three or four thousand gallons. If the pond is suitably covered and if the heater isnt required to maintain the temperature much above 10C, an electric heater is an easy-to-install option. They are, however, not cheap to run.  A separate gas heater or teeing off of the house central heating system with a heat exchanger is cheaper to run but there will still be the inevitable extra cost on top of the normal household gas bills.  Solar heating is the cheapest way to heat a pond in summer but there is very little warm sunshine in the winter, and obviously this is especially true when the weather is particularly cold and the heat is needed most, so solar heating in winter can be disregarded.

Heat Pumps
Heat pumps are far cheaper to run than electric heaters. An air-to-water model will draw in huge volumes of air, extract the heat from it and transfer that heat to the pond water by means of a built in heat exchanger.  They have a coefficient of performance (COP) rating.  In summer, when there is more heat in the air, this will typically be six or higher.  You can regard this as its efficiency rating. A COP of six means that the heat pump provides six times more heat to the pond water than the amount of electricity they use.  For example, a heat pump that uses, say, 1 kW of electricity will put the same amount of heat into the water in one hour as a 1 kW electric heater would if it were running for six hours, or a 3 kW heater for two hours.

As with solar heating, winter is a problem.  When a pond needs most heat, the air is colder and the COP of the heat pump falls.  A COP of 3 is typical for a heat pump on a cold day. This still means that it will only need to run for one hour to provide the same amount of heat as an equivalent rated electric heater would in three hours, but the efficiency of the heat pump will be halved because of the colder air. When the air temperature is very cold, some heat pumps will produce no heat at all.  They will just switch off and wait for the ambient temperature to rise a little. Heat pumps cannot produce heat from nowhere; they must have a source of heat to start with before they can transfer it to the water that is to be heated.

Geothermal horizontal loop

Figure 1.  Horizontal loop

Geothermal slinky loop

Figure 2.  Slinky loop

Where could a heat pump find a source of heat that is constant, even in winter?
It is common knowledge that, several feet down into the ground, the earth is a fairly constant temperature. The more inventive Koi keepers sometimes imagine digging down, burying some pipes and extracting this heat by pumping pond water through them. In theory there is no reason why this wouldnt work except for one important factor.  Imagine digging a one metre square hole about two metres deep, then laying a lattice-work of pipes at the bottom and refilling the hole.

This is the most basic of geothermal heating systems and it would be possible to pump water down through the pipework so that it could pick up heat from the ground in which it was buried.  The water could then go back to the pond and the earth will have warmed the water. Whats wrong with that? The answer is the recovery rate. As soon as the cold pond water begins to flow through the pipework it will be warmed slightly but, at the same time, it will cool the earth around it. That ground will warm again as heat flows in from the surrounding earth but this will be a very slow process meaning that the rate at which the ground can recover its original temperature will be so slow that, in effect, only small amounts of heat at a time can be transferred from the ground into the pond.

That is not to say that it cannot be done, any source of heat that is free, apart from the running cost of a small pump, is a welcome addition.  To gain a worthwhile amount of heat, a much longer length of pipe needs to be buried. Any pond keeper planning to take on such a project would need to dig a long deep trench and bury a lot of pipe which is coiled or zigzagged so as to cover as large an underground area as is possible. This is not a project that can easily be undertaken in an existing established garden, (the wife isnt likely to be out shopping for long enough!), but it is a consideration if large scale landscaping is being done in conjunction with a new pond being built.  There are two suitable underground pipe-work layouts that can be used, the horizontal loop method and the slinky loop method. These are shown in figures 1 and 2.

Geothermal method09

Figure 3.  How geothermal heat is extracted via vertical bore holes

Constant heat
Returning to those heat pumps and their constant heat source, there is a slightly different type of heat pump, known as a water-to-water heat pump.  These take their heat from one source of water and transfer it to another body of water that is to be warmed. The heat pump method of geothermal heating is possibly the most efficient way to heat a home and has the lowest carbon footprint apart from having a full sized wind turbine in the garden.  These systems are seriously expensive to set up but they can be a huge money saver and are a very green way to heat homes with a realistic payback period.  If the Koi pond is added on to a domestic geothermal heat pump heating system, it would be possible to heat the pond all year, virtually for free.

It works similarly to the method of burying pipes in the garden as described above, but six inch holes are drilled straight down into the ground.  To ensure a fast recovery rate, these holes can be anywhere from 80 metres to 110 metres deep. (See figure 3).  As the drill goes deeper it is lubricated and the spoil is blown/washed out of the deepening hole by a mixture of air and foam. This leaves a deep neat hole into which long U shaped pipes can be inserted.  These are connected to the heat pump. Water is pumped down one branch of the U and emerges from the other side at a constant temperature, typically 5C to 8C.

This water is circulated via the heat pump to have the heat extracted and the cooled water is returned back around the U to be continuously re-warmed.  Dependent on geology, one hole can be expected to supply the equivalent of a continuous 5 kW of heat and so two such holes would provide 10 kW of heat 24 hour per day. This is what would be needed to provide the average home with all year round domestic hot water and heating with enough spare heat to heat the pond.

Geothermal schematic
Figure 4.  How water to water heat pumps extract heat from U tubes in bore holes

The cost
The bottom line always comes last - the cost.  This will depend on geology but a ball park figure for two holes finished, and plumbed with the U pipes is 9,000 to 10,000. A suitable heat pump would cost almost 4,000. An initial outlay of 13,000 to 14,000 may seem like a deterrent but after that, running costs are relatively cheap and maintenance is negligible.  Clearly, this is a non starter if it were only for heating a koi pond.  But when compared with the ever rising cost of fuel, if the system is installed to supply heat and domestic hot water for the whole house with the pond being added on as a virtually free addition, it becomes a much more attractive option.


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