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Author Topic: Never use a slinky in Scotland?  (Read 13754 times)
JohnS
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« Reply #15 on: March 05, 2015, 08:42:30 PM »

For utility companies, moles have one big advantage.  They can tunnel under drives and paths, maybe even roads, without having to tamp down, do a temporary surface and then come back and do it properly in a month or so.

Thus they are needed and are on hand for jobs where the cost benefit is marginal.

For ground loops, there is less need for a mole and perhaps most contractors won't have one.
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2.1kWp solar PV
titan
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« Reply #16 on: March 05, 2015, 08:53:42 PM »

If the system is designed correctly and matches the maximum heat load it will work. Slinkies work fine anywhere if they are correctly sized including local conditions regarding recovery.  Most of the negative  reports about heat pumps is where they have been installed in inappropriate applications and/or with an under sized ground array.  They are not a replacement for gas or oil in  poorly insulated buildings where the heat loss cannot be matched by water circulating at 35 deg C.
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skyewright
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« Reply #17 on: March 06, 2015, 11:02:21 AM »

Ground temperature is the average of the local temperature, in Greece the ground is 18 degrees here in the UK the ground is 10 degrees. All at about 1 meter deep. Soil is very bad at transmitting heat it takes months to change even 1 degree.
Reaction rates underground are quite interesting, if you like that sort of thing.  Grin

The first attached image charts daily soil temp averages at 100mm, 300mm and 1000mm for 2010 & 2011 & partway into 2012.

The second just shows 2012. Again there are soil temps at 100mm, 300mm and 1000mm, but also daily air temp min & max.

The third show hourly temps for just 7 days in June 2010, with an extra 100mm sensor in a raised bed. The 100mm temps show a clear daily rhythm & react pretty much directly to the air & sun. 300mm show a much lower magnitude daily rhythm and is reacting (slowly) to yesterday. At 1000mm it takes a week to notice a change of trend at the surface & daily changes are very small.

The fourth shows a week in December 2012 where there was a sudden rise in the temps in the last couple of days. The 100mm sensors see it right away, 300mm just catches on, 1000m is unaware.

Sadly the soil temp sensors (1-wire sensors embedded in epoxy) are no longer working. Apparently it's a hard life down there & one by one they failed after a few years.

The ground we plan to put our ground loops in is not far from where these tests were made, but is wetter.





* soil-20100101-20120223.jpg (67.37 KB, 600x400 - viewed 552 times.)

* soil-20120101-20121231-100mm-and-1000mm_and-air.png (156.42 KB, 779x514 - viewed 584 times.)

* soil-20100601-20100607.png (12.05 KB, 1000x500 - viewed 574 times.)

* soil-20101203-20101210.png (8.55 KB, 1000x500 - viewed 609 times.)
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Regards
David
3.91kWp PV  (17 x Moser Baer 230 and Aurora PVI-3.6-OUTD-S-UK), slope 40°, WSW, Lat 57° 9' (Isle of Skye)
skyewright
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« Reply #18 on: March 06, 2015, 06:19:20 PM »

Ultimately GSHPs are extracting solar thermal energy from the ground 
Are they?
Just found this in a "Product Information" PDF I downloaded from the NIBE website:

Quote
Surface soil - using a surface collector

During the summer, solar heat is stored in the soil. This is
either directly absorbed as insulation or as heat from rain and
the air from the near-surface layer of the soil. The heat pump
collects this stored solar energy from a buried collector. That is,
a hose filled with anti-freeze, and buried at a depth of about
80 – 100 cm, the length of the hose varies between 250 and
400 metres, depending on the size of heat pump selected.
Using this energy for heating is a cost effective method. The
highest yield can be obtained from soil with a high water content.

Quote
Rock - using a ground probe

In the lower subsoil of the so-called “near-surface geothermal
layer” lies a heat source with an almost constant temperature
that can be utilised all year round. The heat pump collects stored
solar energy from a collector in a hole drilled into the rock. The
depth of the hole can vary between 90 – 200 metres, depending
on the size of heat pump selected. This type of system can be
used for all possible building types, large or small, public or
private. It requires little space and the ground probe can be drilled
in the smallest of gardens.
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David
3.91kWp PV  (17 x Moser Baer 230 and Aurora PVI-3.6-OUTD-S-UK), slope 40°, WSW, Lat 57° 9' (Isle of Skye)
davec
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« Reply #19 on: March 07, 2015, 09:07:02 AM »

Ground temperature is the average of the local temperature, in Greece the ground is 18 degrees here in the UK the ground is 10 degrees. All at about 1 meter deep. Soil is very bad at transmitting heat it takes months to change even 1 degree.
Reaction rates underground are quite interesting, if you like that sort of thing.
There's a diagram out there that shows stable ground temperature of ~10-12degC at -20m; above that, there are detectable seasonal variations... obviously ramping up to ambient at the surface as your charts illustrate; below that, temp should increase by ~1.5 degC per 100m. Another diagram shows how ground temp decreases as you go further north... yours looks like it will average about 8degC; here, in central Scotland, I'm seeing about 9.

Good research, skywright, it indicates "dig deep" to tap a bigger volume of 'less-seasonal' ground. Wetter ground is reckoned to be more conductive, hence more watts to your collector.
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Nickel2
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Method mixed with Madness


« Reply #20 on: March 07, 2015, 09:41:33 AM »

Where I live there are gravel pits at the back of the house, 250 yards away. In the older days they used a drag-line to dig out the gravel and empty it into a conveyor hopper.
One of the things I noticed, was that at the week end the muddiness of the water cleared from one side of the dig to the other. This says to me that the river flowing on the surface is only a tiny part of the bigger river flowing through the gravel below. That also means there is an ever-flowing supply of water at a constant temperature. If the river dried up on the surface it would still be there underground. Although the river is 150 yards from the back of the house, the gravels are under the house itself, and the water is continuously flowing. I am too old to make anything financially viable now, but if I knew all this stuff 30 years ago, I would have hatched a plan of some sort.
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1.140kW mono south-facing at 49*
EpEver 4210A at 24v
24V 400 Ah battery. (4x200Ah FLA)
EpEver STI1000-24-230 pure sine inverter
Of course it'll work. (It hasn't caught fire yet).
skyewright
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« Reply #21 on: March 07, 2015, 09:59:00 AM »

Wetter ground is reckoned to be more conductive, hence more watts to your collector.
And if there is a flow (which I believe there is) there's an element of refresh even in winter, at least that's the theory....

I see Nickel2 has just  made a similar point, though here it's will be a much slower flow through silt.

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Regards
David
3.91kWp PV  (17 x Moser Baer 230 and Aurora PVI-3.6-OUTD-S-UK), slope 40°, WSW, Lat 57° 9' (Isle of Skye)
davec
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« Reply #22 on: March 07, 2015, 10:39:17 AM »

Quote
And if there is a flow (which I believe there is) there's an element of refresh even in winter, at least that's the theory....

Yes, you're adding the volume of water passing through to the fixed ground mass, at surface temp. Apparently a flow doesn't always add value for vertical boreholes... if it's 'sinking', surface water can carry winter temperatures down towards the 20m 'stable' zone.
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desperate
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« Reply #23 on: March 07, 2015, 10:35:08 PM »

That's a really neat set of graphs Skyewright very interesting. Your graphs seem to show that below about a metre depth you are below most of the daily air temperature fluctuations, so I wonder why .........
. The heat pump
collects this stored solar energy from a buried collector. That is,
a hose filled with anti-freeze, and buried at a depth of about
80 – 100 cm, the length of the hose varies between 250 and
400 metres, depending on the size of heat pump selected.


.............Nibe bury the collector that far away from the heat source? Surely the energy flux would be far lower at that depth ..........wouldn't it?

Desp
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davec
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« Reply #24 on: March 08, 2015, 10:44:22 AM »

The paydirt for GSHP is stored solar energy at or below the ~20m 'non-seasonal' depth; i.e. the closer you get to this, the higher the temperature and the better the COP in the heating season. See http://www.bgs.ac.uk/research/energy/geothermal/; "shallow geothermal".
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skyewright
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« Reply #25 on: March 08, 2015, 01:58:44 PM »

.............Nibe bury the collector that far away from the heat source? Surely the energy flux would be far lower at that depth ..........wouldn't it?
I'm just a punter , not a GSHP expert, but perhaps it's a matter of the size & stability of the resource?

Nearer the surface than 800mm, especially in places less mild than where I am[1], I imagine there'd be a big danger of freezing the soil (& that's if it wasn't frozen already naturally)?

With a GSHP you aren't necessarily looking for "high" temps. A collector placed where there's a reliable modest temp is probably more useful than one where the temp is more variable, & lowest just when you need it most.


[1] Coldest air temp I've recorded since I started in 2008 has been -5°C & that was exceptional. Last  year we didn't record any negative air temps at all, though that was exceptional too. In 2015 the record low air temp currently stands at -1.1°C on 19/01/2015.
« Last Edit: March 08, 2015, 03:50:16 PM by skyewright » Logged

Regards
David
3.91kWp PV  (17 x Moser Baer 230 and Aurora PVI-3.6-OUTD-S-UK), slope 40°, WSW, Lat 57° 9' (Isle of Skye)
titan
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« Reply #26 on: March 08, 2015, 03:40:43 PM »

The paydirt for GSHP is stored solar energy at or below the ~20m 'non-seasonal' depth; i.e. the closer you get to this, the higher the temperature and the better the COP in the heating season. See http://www.bgs.ac.uk/research/energy/geothermal/; "shallow geothermal".

I would doubt any marginal COP gain installing any sort of ground array or multiple boreholes at these depths would be financially viable. In Scandinavia ground arrays are generally 800 - 1000mm they do not need to be any deeper. What  caused permafrost in a documented Scottish installation was not the depth or type of array but the length of it, grossly undersized. Direct sunlight, air temperature and rain recharge the ground, the deeper the array the longer this takes.   
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desperate
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« Reply #27 on: March 08, 2015, 09:12:59 PM »

The paydirt for GSHP is stored solar energy at or below the ~20m 'non-seasonal' depth; i.e. the closer you get to this, the higher the temperature and the better the COP in the heating season. See http://www.bgs.ac.uk/research/energy/geothermal/; "shallow geothermal".

So it would seem to be worth getting below the seasonal depth to realise the highest temperatures and as you say that is 15M+ but that lends weight to my feelings that 250-400m seems way too deep. Another thought I have is, the maximum temperature may be at 15+ metres but what about the energy flux, how quickly does the heat return after being extracted. As I understand it the closer you are to the surface the quicker it recharges, subject to frost depth and so on. Going deeper slows the recharge rate until getting to the sort of depths where you get deep geothermal heat coming up froom the mantle, but then you are talking about depths of 000's of metres. Unless you live on La Palma or somewhere similarly volcanic Smiley

I think

Desp
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davec
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« Reply #28 on: March 09, 2015, 08:05:08 AM »

Quote
...250-400m seems way too deep.
Ah, cross-purposes Mr. Desparate.... Nibe are talking about a horizontal collector: 250-400m of pipe along a ~1m trench. BGS are talking vertical (borehole) temperatures.
Quote
how quickly does the heat return after being extracted
depends on the thermal conductivity of the ground around the collector... hard dry rock is less conductive, hence less 'productive' for ground-source, than damp sandstone.
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djs63
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« Reply #29 on: March 09, 2015, 09:54:11 AM »

We are 8 miles from Scotland and we have a slinky (homemade) which has coils about 1 metre diameter and is buried vertically in a trench 2 metres deep, surrounded by sand in heavy clay. Thus the bottom of the coil is 2 metres deep and the top is 1 metre. The system is 6 years old and runs very well even at air temps of -19C.
 David
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6 Kw Proven wind turbine, 15 Navitron evacuated solar hot water tube array and 1.8 Kw PV, grid connected (SMA inverters) and GSHP supplying radiators and UFH. Wood burning stove (Esse 300) and oil fired Rayburn. Rainwater harvesting 4000 litre tank underground. Nissan Leaf
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