Geothermal pool heating

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chimera

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Topic # 198713 19-Jul-2016 22:13

I'm getting a pool installed in a couple months time and am buying a heat pump to go with it (so we can use a good 8-9 months of the year) I considered solar heating instead of the heat pump as an option too, but have ruled it out for now (based in Auckland, unsure )

I figured if the ground is to be dug out anyway, then I could consider geothermal heating (or ground source heating) to supplement the heat pump which should save on running costs as GSHP has a very good COP rating (3-5)

I thought possibly a couple options, if the ground is getting excavated down up to almost 2m anyway, then either run a closed loop system buried around the pool at that depth too. Alternate option is that we will be concreting around the pool, so I could run piping in the concrete slab instead.

Could possibly use the piping as a heat sink for a hot house in summer too if plumbed up right.

Anyone done this? Recommendations, suggestions?

Cheers

chimera

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Reply # 1595343 19-Jul-2016 22:15

...(based in Auckland, unsure if even the climate up here will be sufficient over winter to keep the pool warm enough to be usable for 3/4 of the year)

Shoes2468

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Reply # 1595364 19-Jul-2016 23:24

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If you were going to pipe around the pool and extract the heat out of the ground around the pool, won't that suck the heat out of the pool therefore lowering the cop. Probably easier and cheaper to to use a more standard air based system and run when power prices are cheap by using someone like flick for electricity.

chimera

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Reply # 1595440 20-Jul-2016 08:47

Shoes2468: If you were going to pipe around the pool and extract the heat out of the ground around the pool, won't that suck the heat out of the pool therefore lowering the cop. Probably easier and cheaper to to use a more standard air based system and run when power prices are cheap by using someone like flick for electricity.

Sucking heat out of the pool??? I'm not putting piping up against the pool, putting it in the dirt around at the bottom of the pool. Earth at depth has very stable temperatures. It does not fluctuate like air temperature does.

COP for geothermal is usually at least 4 times and up to 5.5 times, air is usually up to 3 times (depending on climate) Heat pumps are grossly inefficient from 0-7 degrees celsius ambient air temperature, I know Auckland has hit a low of 9 degrees in the past week and has been lower than that in previous weeks. Granted it is the middle of winter, but even if it were 0 degrees in the air, the earth should be at least 15 degrees. I'm already going to use a heat pump, so I was thinking along the lines of running piping from the pool, through the ground source and then to the heat pump. The water in the pipe in the ground would in theory be warmer than the pool (heated by the earth, or concrete) so that would be less work needed for the heat pump to heat the water before returning it to the pool which = less energy which = cheaper to run. It just means another say 100m of piping (or whatever it calculated out at) for water to run through before it hits the heat pump and the hassle of running that piping.

I'm just trying to work out whether anyone has done this before and has any recommendations. Or would the difference in temperature generated by the earth vs the pool temperature be so small that it wouldn't be worthwhile doing.

MikeAqua

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Reply # 1595477 20-Jul-2016 09:51

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Shoes2468: If you were going to pipe around the pool and extract the heat out of the ground around the pool, won't that suck the heat out of the pool therefore lowering the cop. Probably easier and cheaper to to use a more standard air based system and run when power prices are cheap by using someone like flick for electricity.

I had the same initial reaction. The pool will equilibrate temperature with the soil close to it.

It seems to me that you would want some distance between your heat source (soil) and your heat sink (pool). What that distance is I don't know.

Mike

chimera

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Reply # 1595494 20-Jul-2016 10:33

I don't think its as simple as that (I'm theorizing here, happy to be told otherwise). The edges of the pool do not butt hard up to earth, it has scoria around it for drainage I believe so would be less of an insulator than say compacted dirt around a pipe for it to retain heat. Additionally, its not like the entire volume of water in the pool is sitting at 2m deep (assuming night time, to take the sun out of the equation) then the temperature would vary from the surface level (cooler) to 1.9m deep (warmer). The entire volume of piping for GSHP on the other hand (or at least the majority of it) is completely buried at 2m deep so should be a more stable temperature. It must be warmer than say a volumetric portion of the water sitting at half a metre pool depth. Remember here I'm considering GSHP to supplement the ASHP (air source heat pump) to have it do less work.

So alternatively, placing GSHP piping in the concrete slab around the pool - there is no considerable "depth" for the concrete, but it does retain heat quite well so could possibly work too?

jonb

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Reply # 1595526 20-Jul-2016 11:12

The ground loop systems are more common in Scandinavia, so assume the temperature of the ground is not too much of a problem?

GHANZ seem to be the people to ask for advice about it in NZ, I wish it was more common here it looks like a great system.

frankv

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Reply # 1595532 20-Jul-2016 11:24

I've seen this sort of thing on TV. They buried hundreds of metres of water pipe to heat all their house hot water.

What I'm wondering about is the thermal conductivity of the soil. You're burying the pipe under 2m of soil and expecting the temperature to be pretty stable. But if the soil is a good enough thermal insulator to not cool down overnight at (say) 2m deep, then it's not going to conduct the "cold" away from your pipe (or conduct warmth to it) very well. In which case there will be a limited amount of heat transfer possible.

I'd look into how much buried hose you need.

chimera

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Reply # 1595578 20-Jul-2016 12:33

frankv:

I've seen this sort of thing on TV. They buried hundreds of metres of water pipe to heat all their house hot water.

What I'm wondering about is the thermal conductivity of the soil. You're burying the pipe under 2m of soil and expecting the temperature to be pretty stable. But if the soil is a good enough thermal insulator to not cool down overnight at (say) 2m deep, then it's not going to conduct the "cold" away from your pipe (or conduct warmth to it) very well. In which case there will be a limited amount of heat transfer possible.

On the contrary, it is significantly more efficient (more so than normal air based heat pumps) with a COP of 5 vs 3 for air (eg: up to 5 times the energy produced for every 1 unit of power used - which would be the energy required to power the heat pump transfer unit) GSHP conducts warmth too it. It is just a very rarely used form of heating in NZL however it is used significantly overseas for decades and has been very successful. The cost of installation is the limiting factor, its a lot more expensive than a standard heat pump (probably why its not as common here, kiwi's have tight purse strings) However I thought if I'm getting a big hole dug out anyway, then I thought it might be worth considering.

http://energy.gov/sites/prod/files/guide_to_geothermal_heat_pumps.pdf

http://www.centralheating.co.nz/wp-content/uploads/Introduction-geothermal-heating.pdf

https://en.wikipedia.org/wiki/Geothermal_heat_pump

frankv:

I'd look into how much buried hose you need.

Agreed, this would most certainly be a very big factor. Much like a solar water heater, the more tubing the more surface area can be heated. That is why I said at least 100m (300ft) or more would probably be needed. But exactly how much vs the pool area, I'd need to research further.

Another alternative would be to use the pool heat pump solely to heat the pool, but then use the excavated area to do GSHP and use it to heat and cool my home instead.

dantheperson

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Reply # 1595937 21-Jul-2016 00:23

Sure ground source is more efficient at lower temperatures(<7 degrees) than air source, but how often are you going to be heating your pool when the outside temperature is below 7? Are you really going to be outside enjoying your pool in the middle of winter when the air temp is freezing? For the spring/autumn shoulder season, the air heatpump should still be efficient enough that you are never going to recoup the cost of a ground source installation.

My thoughts are are that ground source is only viable in the south where you want to be heating the house when the outside temperature is often below 7 and thus you get poor COP out of an air heatpump. The prices i've seen to have one installed is like 30K, so even then it's a long payback period. Perhaps in Northern Europe which has a far greater population than our southern lands, they are much more common and are more price competitive due to economies of scale?

Aredwood

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Reply # 1595942 21-Jul-2016 01:47

I say just stick to air source heatpumps as well. As another issue with ground source in your situation - you would be sticking alot of hose into a small area. Meaning the temp in the local area beneath the pool would drop too much and hurt the efficiency of the system. Compared to the same amount of buried hose installed in a straight trench.

Also you would have to check that the output of the ground source heatpump is suitable for direct circulation of pool water. As it may be intended for a closed loop underfloor heating circuit. Chlorine plays havoc with both copper and stainless steel. I wouldn't want to risk stuffing an expensive ground source heatpump just because the pool chemical balance is slightly wrong.

aucklander

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Reply # 1618509 27-Aug-2016 22:14

Aredwood:

Also you would have to check that the output of the ground source heatpump is suitable for direct circulation of pool water. As it may be intended for a closed loop underfloor heating circuit.

I always believed the fluid in the ground loop is different from the fluid in the other circuit (pool, underfloor heating, etc). Usually it would be a mixture of antifreeze. The heat pump consists of two heat exchangers: one between refrigerant and water and the other between refrigerant and antifreeze (ground loop). Is that incorrect? The two hydraulic circuits are completely separate (and they each must have their own circulation pump).

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Aredwood

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Reply # 1618861 28-Aug-2016 20:26

Yes heating wise they definitely will be separate. I was more referring to what kind of fluid you can run through the output circuit. Does the heatpump manufacturer only approve a water / antifreeze mixture? Or would they allow you to use pool water - which will contain chlorine. As chlorine is bad for alot of stainless steels due to stress corrosion cracking.