ground source: soil, well or 'lake'?

[chasfromnorfolk]

I have an existing 60m2 u/f system run from a 6kw Aztec electric boiler.

I'd like to reduce electricity consumption and I'm attracted to the idea of GSHP mainly because I have a suitable 'outside cupboard' complete with existing underground duct and electrics and it's handily placed for connection to the u/f system. I'd keep the Aztec connected, but would hope the GSHP would take most of the burden. I've seen Navitron's advice on their WBR09 page:

"GSHPs can extract heat from buried loops of pipe, but alternatively, can extract heat from wells, lakes, streams or rivers."

1) Is it generally the case that ground-buried pipes collect most heat from the sun-warmed soil and so are best used - if so at what depth if laid horizontally, not in trench coils?

2) I have a nearby well (we use it for all our water) about 2m dia and 25m deep with water in the bottom 3m. It's icy cold and it's difficult to believe any heat could be extracted - plus I'm sure I've read somewhere that the GSHP pipes can freeze a well? Is it a goer in a used well as distinct from a handy unused one?

3) I have a hankering for a 30m x 30m pond and could put pipes in that (or even under it, in the puddled clay or butyl liner I may use) - how deep underwater do the GSHP pipes need to be?

Any observation on these points welcome. Don't hold back...

Cheers,

Chas

[BruceB]

1,  You will find papers around discussing the temperature variation across the year with depth.  Somewhere between 1m and 2m is a standard answer.  Heat pump manufacturers websites make recommendations.  There is a HVCA document TR30 which has a load of guidance in.

2.  A well is almost certain to freeze unless it has water flowing through the bottom of it.

3.  I have an old mill pond that I extract heat from and the pipes are a meter deep which works fine.  You would need to look at the specific heat extraction rates in TR30 to get an idea whether your pond would be big enough for your house.  Mine, although smaller than that does not freeze because it has spring water flowing through it the whole time which keeps the temperature up.

[sam123]

1,  You will find papers around discussing the temperature variation across the year with depth.  Somewhere between 1m and 2m is a standard answer.  Heat pump manufacturers websites make recommendations.  There is a HVCA document TR30 which has a load of guidance in.

I have to strongly disagree:

Translation from www.sulpu.fi (Finnish hat pump association)

"Piping buried in about 0.7 - 1.2 m depth the ground so that the pipe loops spacing is approximately 1.5 meters, with a minimum of 1.2 meters."

I wouldn´t dig it deeper than 50-70cm as you got so much warmer climate than here in Finland. That 70cm is for South Finland and 1.2 meters for Lapland. Reason for that is our permanent frost - you have to dig your pipes under that, so your pipes don´t get too frosty at late winter. On the other hand you can NOT dig it any deeper, because energy from sun have to warm it ready for next winter.

Hope this helps.  Merry Christmas!

cheers, Sami

[dhaslam]

I think that the ground in the British Isles is different than  Finland because the ground doesn't normally freeze.  Also when there is a warm spell , like now, there is  rain that soaks into the ground when  the temperature  can be around  10C.    This completely refreshes the ground temperatures around the collector but works better when the pipes are deeper.       My seasonal store goes down to  about a metre  below the ground and  when there is heavy  rain  it  brings the temperature on the lowest sensor down to the temperature of the ground water  regardless of what it was before.

   

[Richard Owen]

I'd also like to add my voice to the deeper is better camp.

Ivan posted a graph a few years ago (and I can't find it now) that tracked soil temperature at various depths by month.

Pipes shallower than 1m risked getting frosted up by the end of a long winter and pipes deeper than 2m never got the full benefit of solar warming.

My pipes, for what it's worth, are 1.5m deep.

That's mostly because the digger I was using wasn't comfortable digging deeper than that.

[Solal]

because energy from sun have to warm it ready for next winter.

What about  the  ground temperature  below  the collector pipes?  Hundreds of  feet  at a higher temperature  which should start rising  naturally when  an  above  level  recieves some cooling.

[biff]

i am not a fan of gshp,
                  there is too much left to chance.the performance depends on the soil and how it attracts the heat. then there is the running costs.running an 8 horse motor is not cheap.i am aware that a lot of forum members have gshps but i still say that the returns do not justify the outlay,
        biff

[Richard Owen]

but i still say that the returns do not justify the outlay,
        biff

Hi Biff,

I think for some installations that may well be the case.

However, if you design your house to use low temperature heating, buy a cheap-ish Chinese Heat Pump, install both it and the pipes yourself and only run it on E7 leccy, the numbers don't work out that bad.

I spent about as much as I would have on a good oil condensing boiler and associated tank and pipework.

I

[ecogeorge]

but i still say that the returns do not justify the outlay,
        biff

Hi Biff,

I think for some installations that may well be the case.

However, if you design your house to use low temperature heating, buy a cheap-ish Chinese Heat Pump, install both it and the pipes yourself and only run it on E7 leccy, the numbers don't work out that bad.

I spent about as much as I would have on a good oil condensing boiler and associated tank and pipework.

I

Have to agree. if you  don't have access to mains gas then gshp is a good option. Run temps low , oversize collector pipe runs and you'll get good efficiency. At 3kw consumption - say 1kw contribution from pv = 2kw input cost for 9kw output of heat.
Bought and installed mine approx 5yrs ago - total cost 2k heatpump say 2-3k to buy and install manifolds/pipe/digger etc bargin  

[titan]

I would say Sam is right, the latest UK heat pump MCS standards MCS3005v3  www.microgenerationcertification.org/installers/installers  ( thanks to jonG for the link) also say between .8 and 1.2M. Any ground freezing problems at these depths  is going to be due to undersized loops. I would speak with Kensa they are made in the UK and very helpful and not just sales people like most of the other  UK suppliers ( that I spoke with).  I got my Kensa for not much more than a Chinese import.  that is not counting the £1250 grant I was not expecting.

[Solal]

Are you guys all on ufh then?

[baker]

Hi
do things the easy way
whack on a airsource heat pump
take a few hours
spend whats left  of PV
leave the electric boiler for weekly sterilisation and bivalence operation
almost as efficient  ,

[Richard Owen]

Are you guys all on ufh then?

I am

[sam123]

I would say Sam is right, the latest UK heat pump MCS standards MCS3005v3  www.microgenerationcertification.org/installers/installers  ( thanks to jonG for the link) also say between .8 and 1.2M. Any ground freezing problems at these depths  is going to be due to undersized loops.

In 1980´s we also buried ground loops too deep. That caused lot of problems, which almost stopped GSHP installations for decade. I´m glad that UK is also jumped to this century.

There is absolutely no reason to dig it any deeper than 70cm in UK. Another important thing is that forget slinkies (and other voodoo-snakes). Use 32mm or 40mm 10bar pipe for ground loops.

Space between pipes should be around 1-1,5 meter. More is better, if you have space.

Big question is how long ground loop you need? That depends how much your house consume energy. There is some thumb rules:
- Don´t go under 300m. Reason is that plastic pipe is good insulation and when GSHP is running, heat needs some time to transfer from soil to fluid inside the pipe.
- Don´t go over 450m loops. If you need more, just connect more loops to parallel. We try to keep flow turbulent ( http://en.wikipedia.org/wiki/Turbulent ) and not stress ground loop pump. Way better is 2x250m loops, than 1x500m loop.
- We get in Southern part of Finland max. 60kWh/meter from ground loop if soil is clay.

Example: You have house with radiators, which need 21000 kWh/year for heating and warm water. You need 7000kWh from grig to GSHP and you get 14000kWh from ground. Your ground is not clay, so let´s estimate: 40kWh/meter.

14000kWh/40kWh = 350m loop 40mm/10bar pipe with 1-1,5m space.

Same consumption with underfloor heating stress ground loop more, so you need 5250 kWh from grid and 15750 kWh from ground. 15750/40 = 393 meter ground loop.


My advice for choosing GSHP:

- Buy Nibe, Thermia etc. Swedish GSHP. Sweden have over 500 000 GSHP installed, so they know how to build one. Here you pay 6-8kW:n GSHP with 180l hot water tank around 5000-6000€.

If you are thinking about COP in GSHP we have some fresh measurements from year 2010 Nibe 6kW here in Finland (112m2, floorheating, 140m borehole, 125m in bedrock):

Power from grid:   1,43kW  
Flow to UFH:   0,669 m3   Power to UFH:   6,39   kW COP   4,47
Flow to UFH:  0,743 m3    Power to UFH:   7,10 kW COP   4,97
              
Temp to UFH: +35,3°C              
Temp return from UFH: +27,1°C              
              
Groundfluid in to GSHP: +2,1°C              
Groundfluid back to borehole: -0,9°C  

COP will go down from these numbers, but we hope that year-COP 3.5 is achieved.

As you may know, you get 3-4% better COP in every degree what you can rise ground fluid temperature or reduce your temperature to radiators.

5 degree from ground 30 degree to radiators equals to 10 degree from ground 35 degree to radiators. You get pretty good benefit from boosting your radiators with fans ( poor example http://www.radiatorbooster.com/ )  

In UK you get year-COP 3 easily with radiator (max. 50 degree water to radiators) and year-COP 4 in UFH (max. 35 degree to floor).

That means you buy 4 kilowatts at price of one including all domestic hot water.

Hope this helps.

Merry Christmas,  Sami

P.S Translation from www.sulpu.fi (Finnish hat pump association) is of course Finnish heat pump association          

[Bodidly]

Hi sam

Thanks for that post. My ground loop is  probably too short (1x200m loop) but well insulated house with UFH.

Current temps
outside air 2c
incoming ground loop 2.8c
outgoing ground loop -0.5c
UFH water  26.5c
room  20c

IVT greenline 6 and woodburner off

Beau