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Author Topic: Recharging the ground  (Read 21105 times)
dhaslam
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« Reply #30 on: January 29, 2016, 01:19:14 PM »

Storing heat extra seasonal heat underground only works on a large scale.     The system at Drakes Landing in Canada seems to work  even though they have a lot of water from snow melt in spring.   Their system may work  because the snow melt occurs  when there is already strong spring sunshine.    However in the British Isles   the ground is usually wet from about October through the winter so heat dissipates fairly quickly.   One thing I have found this year  is that the warm ground water  is heating the  bottom of the  seasonal store  quite well, the drain around the bottom of the store is  working in reverse  because  of the  flood water seeping into it.   The temperature in the store is fairly stable    by using a combination of rising heat from under the store and limited input from the solar panels.   The base are of the store is only  about 50 sq metres.    

If you are going to store seasonal heat it is best to have a   fairly small  but very well insulated heat store.   Water is good to store heat because you can  apply low temperature heat in winter without cooling the water above.    

I don't think that  a seasonal store  is a complete replacement  for  a ground loop even though I don't have a ground loop at present.     It is really only practical where a ground loop isn't possible.  
There is a benefit in having a warm source in  that heat pump,  output is increased by up to 50% with very little  extra electricity,  but with mild winters ground loops will work quite well.  They just need to be deep enough and  large enough to keep source input temperature high through the winter.        

http://www.sciencedirect.com/science/article/pii/S1876610215002398
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« Reply #31 on: January 29, 2016, 02:41:36 PM »

Indeed they have Titan,
                  Just like the wood pellet boilers. i am delighted that you have installed your own GSHP and that you understand it and that you can fix it and tweak it when you see fit, but wait for it,,There are loads of others who seem to be happy with their lot but some how, I keep meeting these quite sensible folks who have taken me through the whole process and explained why they are unhappy. The installers are UK registered and not local'.I look upon this as a new version of "Stone Soup". I am very aware that some of the better educated moderators are rather pleased with their GSHPs but again,that has not been the story here. Maybe it is the soil mechanics, maybe it is the installers fault, Perhaps the soil in some places works better. I just don,t know but I have seen the money spent and it will take a long long time to ever see it back. Not everyone is scientifically minded and able to work these things out which harks back to the renewable energy motto which should be, "Keep it simple."
                                                                          Biff
  NB, One little phrase kept cropping up among the folks that I had spoken to, "We thought it was going to be cheap to run or run itself for nothing"  so maybe it was oversold,?
« Last Edit: January 29, 2016, 02:43:45 PM by biff » Logged

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« Reply #32 on: January 29, 2016, 04:30:08 PM »

Indeed they have Titan,
                  Just like the wood pellet boilers. i am delighted that you have installed your own GSHP and that you understand it and that you can fix it and tweak it when you see fit, but wait for it,,There are loads of others who seem to be happy with their lot but some how, I keep meeting these quite sensible folks who have taken me through the whole process and explained why they are unhappy. The installers are UK registered and not local'.I look upon this as a new version of "Stone Soup". I am very aware that some of the better educated moderators are rather pleased with their GSHPs but again,that has not been the story here. Maybe it is the soil mechanics, maybe it is the installers fault, Perhaps the soil in some places works better. I just don,t know but I have seen the money spent and it will take a long long time to ever see it back. Not everyone is scientifically minded and able to work these things out which harks back to the renewable energy motto which should be, "Keep it simple."
                                                                          Biff
  NB, One little phrase kept cropping up among the folks that I had spoken to, "We thought it was going to be cheap to run or run itself for nothing"  so maybe it was oversold,?
Biff,

I think the problem with GSHP is that they are a) sometimes oversold b) often poorly sized c) often have inadequate ground collection loops and finally d) most people do not understand that they need to run at low output temperatures to maintain efficiency. In my opinion they should only ever be used to drive under floor heating and never radiators no matter how oversized they are.

I had my system installed in December after much thought and investigation and yes I think you are right Biff about the gamble part. The groundloops on my system were a gamble as we are on a shingle bank with pockets of clay. However after discussions with the installer and numerous test holes we decided to grossly oversize the loops as an insurance policy - the extra cost was not huge once the machinery was on site.

In terms of complexity my system is less complex than if I was able to have a gas boiler. On the heating side there are no zone valves, no ther,mostatic valves, no mixer valves and no internal thermostats, timers etc and a just one single circulating pump. It is an unvented system so totally sealed with little chance of contamination. The Hot water is a conventional vented system.  As to running cost,s well at the moment (and I accept it is unusually warm but we have had one cold spell) the running costs are only 25% of the predicted costs from the manufacturer/installer and 50% of my own calculations. Yesterday for instance where the early morning outside temp was 2 deg C and it only rose to 7 deg C the system used 9kWh of electricity (a proportion of which was supplied by the PV). The most ever used in a 24 hour period when it was -3 deg C outside  was 21.6kWh. The inside temp of the house was steady at around 20 deg C, 24 hours a day.  The house, well its a large  old detached farmhouse, about about 250 sq m in total and its in a very exposed position.

The heatpump itself is no different inside than a freezer in terms of bits to go wrong, and they seem to last a long time. Maintenence is virtually zero, keep an eye on the levels and clean the filter every once in a while, thats it.

So as with most things if used in the right situation, properly installed by someone who knows what they are doing and cares enough to do a good job, they do deliver the promise. I am still amazed at the performance of mine - the only complaint I have is that the domestic hot water gets too hot!

So don't be so hard on them. It's no different to solar PV installs where the installers are stupid enough to install in unsuitable locations where there are issues of shade - or as in another recent thread where they expected the sun to be in a different place. I looked at wood pellet boilers and was put of by seemingly endless threads on forums about problems with reliability, jammed feeds,  sooted up sensors, broken fans etc. Yet I am sure there are perfectly good installations where the owners are very happy. As with many things you often only hear of the complaints.

Oh, you are right about the ground being chewed up to install the loops, still in trouble with the other half over that bit of miss information - "it'll just be a few scratches across the grass dear, soon grow over" Whoops! Grin

As to the the original topic of the thread, I don't think it will happen. Any difference a "recharge" system will make will be minimal.  As far as I can work out ground loops tend to be around 200m to 300m long ( I stand to be corrected on that). Now my system (10kW pump) pumps liquid at the rate of around 7 litres per minute around each loop (I have four so a total of 28 to 30 litres per minute). On a normal pipe of 32mm internal diameter this equates to around 7 m per minute (1m of pipe holds roughly 1 litre of liquid. So it takes between 30 and 40 minutes (very roughly) for the liquid to make one circuit.  My heat pump - and again I don't know if this is typical but I would think it is, runs for 20 to 30 minutes and is then off for around 90 minutes. This obviously varies with temperature but is a good average.  This means the liquid in the loop just about makes one circuit per heating cycle. I have monitored the incoming "brine" temp during a normal 20 minutes heating cycle and it stays fairly constant, there is some variation as the pipe runs through shingle and clay (clay is around 9.5 degC and the shingle seems to be around 8.5 degC), However, on a longer cycle where the heat pump goes on to heat the domestic hotwater tank, the runtime can be nearly an hour if the tank is cold. Then I notice the in coming "brine" temperature starts to drop as the liquid starts its second trip round the loop. I assume this is because the plastic pipe is not a particularly good conductor of heat and the liquid takes time to "heat up" which it does during the off time of the pump. My point is this, if as the O.P. said, the idea is to enable shorter loops to be used then this effect will get worse. If you halve the loop length then the temperature drop from the start to the end of the run could well be several degrees, depending on the run time. This will have an effect on the power consumption of the heatpump and could well overide any gains made by any recharging that may, or may not, occur. To make matters worse - if you rely on solar thermal for the heat source for the ORC pump - then on the days when you need the heat the most in the house you will have the least heat input into the ground. As I alluded to above, one of the keys to an efficient and effective GSHP install is a good ground collector, if space is too tight for one then don't fit it - simple. The expense and complication of trying to fit some other system - what ever it is - will not in my opinion have any benefit. At best it will make a compromised system slightly less so.  As Biff said - keep it simple! (all just my opinion).

Roger
« Last Edit: January 29, 2016, 04:56:41 PM by marshman » Logged

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stannn
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« Reply #33 on: January 29, 2016, 05:38:01 PM »

That's a well written and very informative post Roger. Smiley
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« Reply #34 on: January 29, 2016, 06:41:54 PM »

Brilliant post Roger,
                 And it has given me plenty to think about.You have explained it better than anything I have read before.
                                                        Biff
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Stuart Ian Naylor
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« Reply #35 on: January 29, 2016, 11:55:36 PM »

About the only thing I can add, is often the energy of the brine pump is not part of the CoP rating.

Brilliant Post Roger, exactly what the thread needed.

Also found a really good document that contained an eye opener for me, the soil/ trench conditions can effect pipe length by up 50%.
http://www.gshp.org.uk/documents/CE82-DomesticGroundSourceHeatPumps.pdf which is just a great general document

Ignore the ORC and recharging angle as your post surprised me as I had a hunch it was mainly due to the poor thermal conductivity of the ground and never really thought about the plastic pipe itself.
I guess there is always going to be a temperature difference on the first total loop exchange and this is brilliant information.
You stated the ground substrate type, you have the length of coil. If we had temps on extended use of the brine much of that info could be used.

PE100 Thermal conductivity, W/mK, DIN 52612 0.38

I was just interested as to be honest I don't even know what the exit temperature of the brine loop is from the GSHP.

From reading I knew you can get localised freezing and overuse can actually seasonally reduce ground temperature, but there doesn't seem to be any figures out there.

Also to be honest I am slightly confused on trench depth as most geological surveys say the UK is a pretty constant 10-12'C at a depth of 15m.
When it comes to GSHP literature often it is said constant 10'C at a depth of 10m but strangely often state a minimum trench depth of 1 -2m in slinky configurations.

Often the 1 - 2m is quoted and use this an an example http://www.kensaheatpumps.com/the-technology/heat-sources-collectors/surface-soil/slinkies/

Roger made a good call by oversizing the system as what the manufactures seem to state seems to be dangerously optimistic.



I think you are right to some respects as the trench requirements for GSHP would seem to be under specced and some of the shallow trench / slinky installs are not much more than a ground solar collector.
The fact that they advise to keep trenches at least 3m apart whilst giving a minimum of 1m depth just doesn't make any sense in my book especially with the nature of heat.
But interesting thread and started to actually talk about figures and installs which is really, really great.    


I started to collect what I could on the thermal conductivity on various ground substrates.
http://www.geothermal-energy.org/pdf/IGAstandard/WGC/2010/2952.pdf
http://www.minersoc.org/pages/Archive-CM/Volume_33/33-1-131.pdf
http://pubs.usgs.gov/of/1988/0441/report.pdf
 
« Last Edit: January 30, 2016, 09:07:31 AM by Stuart Ian Naylor » Logged
todthedog
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« Reply #36 on: January 30, 2016, 07:59:24 AM »

What a cracking post Roger.
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« Reply #37 on: January 30, 2016, 10:42:27 AM »

Quote
... As I alluded to above, one of the keys to an efficient and effective GSHP install is a good ground collector, if space is too tight for one then don't fit it - simple...

Or you can always go downwards... consider a borehole: big collector, small footprint.

Quote
... some of the shallow trench / slinky installs are not much more than a ground solar collector...

Most of the illustrations seem to show ground temperatures as 'seasonal' to -20m, then ~10deg to -100m, then increasing at around 2deg per 100m. However, the heat available in any volume of ground seems to depend very much on region and soil / rock / ground-water conditions, i.e. there is no reliable broad-brush rule; you need to do the geology and, as Roger says, err on the big side while the kit is on site.

DaveC.
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billi
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« Reply #38 on: January 30, 2016, 11:26:13 AM »

Is it   over 10000  for GSHP to install ?

http://www.energysavingtrust.org.uk/domestic/ground-source-heat-pumps

Quote
Costs, savings and financial support
Costs

Installing a typical system costs around 11,000- 15,000. Running costs will depend on a number of factors including the size of your home and how well insulated it is.


I do not understand why so expensive ...



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« Reply #39 on: January 30, 2016, 11:35:19 AM »

Well the pipe is about 2/metre in Ireland Billi.
The pumps are in the 5k ballpark and the balance is civils and installation /commissioning I guess.
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« Reply #40 on: January 30, 2016, 11:39:21 AM »

I still say that a lot has to do with the soil and how it retains or repels the heat.
     I take on board everything that Roger has written and will hold my anti-GSHP rants in check in the future. Yes there is a place for such systems but some means of checking the soil or subsoil is essential to eliminate the gamble side of it.
    Around here, the bog is seen as a type of refrigeration . Indeed we have many cases of bog butter being discovered,some as old as 3,000 years and up to 100lbs in weight, in good condition.
  Then the lakes that dot these bogs are so cold that diving below 6ft will leave you numb in the grip of a frozen vice,unable to move a limb but just float to the top and wait to the top layer warms you up and this in mid summer. I know this because as kids we would set out to swim these lakes and visit the small islands in them. So you could feel like a dip, find that the water was quite acceptably warm and dive in only to discover that it was ice cold 4ft below the surface but we had all been well warned and knew not to panic and also keep an eye out for each other. Back then,we were never out of the water. Rivers ,lakes and the Sea and tales of Huckleberry Finn. We grew up with it and learned from it and luckily we all escaped our narrow squeaks.
  So I know it is dam,d cold below the surface here. I am not making excuses for my ignorance on the subject but a lot of our houses are built on the bog and deep black peat is all around. Now in many ways this is a good thing because the house owner would not have far to go to cut the turf for the fire. But it is dam,d cold a few feet down and getting heat out of that lot other than the fire,is a mystery to me.
 So maybe it is possible that the unhappy customers ,got the pipework wrong, got the soil wrong, got the whole installation wrong and were oversold the lot but the bottom line was,they were not happy.
                                                                                  Biff
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« Reply #41 on: January 30, 2016, 12:01:11 PM »

Hi Biff,
From an Irish perspective heat pumps in the early days were sold by a lot of less than proficient installers that didn't understand how to size or install them themselves. Furthermore they were installed in the wrong type of house and end users were not sufficiently informed as to how best to operate them.
HPs are at their most efficient when the temperature difference between the source (ground or air) and destination (house is minimised).
As has been mentioned this means underfloor heating is the best option. The surface area is so great that acceptable heat output can be realised with flow temps as low as 30 degrees.
Now this is all well & good but I believe the following issues have been encountered in early installs in Ireland:
1. Ground loops undersized - the installer cuts the length to boost profits or cut out the competition.
End result is that the incoming flow temperature drops & efficiency decreases -> bigger bills.
2. Poor insulation and/or air tightness in the house - heat pumps are generally set up to maintain a constant temperature in the house 24/7. If your house build isn't up to scratch & you try to increase your flow temps for a blast of heat when you come home you'll be left poor & cold!

Overall I really like the concept but nowadays with the high levels of thermal performance available in houses I think the capital is perhaps better spent on insulation & air tightness. At near passive levels a small air source unit may make more sense even if they don't last very long.

Finally it can be hard intuitively to understand that there is heat in that bog or freezing lake. We think of anything less than our body temperature as having no heat in it. In truth everything above absolute zero has heat energy in it.
If you get 1kg of beef from the butchers fridge & put it in the freezer when you get home, a portion of the heat in it can be felt a while later in the coils of the freezer. That same energy will have to be replaced by the air in your kitchen a few weeks later when you thaw it out for your dinner!
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« Reply #42 on: January 30, 2016, 12:45:08 PM »

The pipe I used  was 1.5"  hydrodare ducting and cost  186  euro for 150 metres, it is a bit thinner than the equivalent water pipe.   The leads to the heatpump are  3/4"  qualpex.  The connectors  were  between the  hudrodare were 27 each.     I am only using 150 metres total collector for a 3kW  but it seems to be adequate  even when the  source temperature is similar to ground temperatures at present.         
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Stuart Ian Naylor
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« Reply #43 on: January 30, 2016, 07:44:34 PM »

The loop once installed should be of PE100+ pipe and if vertical maybe even higher rated pipe should be sourced as the depth and pressure increases.

The pipe loop install is guaranteed for 50 years and is expected to last 100 and yeah it is expensive, but in that sense compare it to the cost of 3 or 4 Solar PV installs over that time.

http://www.gshp.org.uk/documents/CE82-DomesticGroundSourceHeatPumps.pdf is a great source of infomation.

Quote
Information on the thermal properties of the ground is needed for determining the length of heat exchanger required to meet a given energy load..
Most important is the difference between soil and rock, as rocks have significantly higher values for thermal conductivity. The moisture content of the
soil also has a significant effect as dry, loose soil traps air and has a lower thermal conductivity than moist, packed soil. Low-conductivity soil may require
as much as 50 per cent more collector loop than highly conductive soil. Water movement across a particular site will also have a significant impact on
heat transfer through the ground, and can result in a smaller ground heat exchanger. A geotechnical survey can be used to reduce the uncertainty associated with the ground thermal
More accurate information can result in a reduction in design loop length and easier loop installation. The British Geological Survey (http://shop.bgs.ac.uk/georeports/) has an on-line service offering simple, or more
detailed, GeoReports giving information on local ground conditions relevant for ground source heat pumps. (A basic GSHP report suitable for a domestic application currently costs 50 and is available online.)
For large schemes where multiple boreholes are required, a trial borehole and/or a thermal properties field test may be appropriate.  

A shallow trench especially the slinky type is a seasonal heat store that is recharged by the sun. Its probably why we get quite a few posts of problems north of the border.
Generally with shallow trench systems much can be ascertained by grabbing a spade and digging a test pit, high water content can be very beneficial, water flow can carry that heat away.

You seem to have a combination of bad and worst case soil Roger where clay is great stuff when not dry, but the shingle could be very prone to water flow.
Seems like your intuition got it perfect though Smiley

As for the OP in terms of shallow trench system we are already recharging the ground. I thought I would mention as it occurred to me in terms of cost that a secondary pipe could be used to dump waste heat.
I was just looking at ORC systems which have a cold loop which is dumped to air in all cases I know. It just occured to me that the 45'C waste heat could be dumped into a GSHP loop as its waste heat and otherwise going into the atmosphere.
Also the colder return of a ground source loop would be beneficial to the efficiency of the ORC.
Maybe its just a matter of divertor valves / heat exhanger where brine loop and pump is shared between the two for extra economy.

Roger many thanks for your brilliant post, I do disagree though as an ORC and GSHP are extremely complementary products as dumping that waste heat is the primary concern of the ORC. Returning colder than air temps is a Brucie bonus. So is any increase in seasonal or daily ground temp for the GSHP is also a bonus irrespective of 'recharging' efficiencies.
I would never suggest using a solar array to capture premium high quality heat and dump it into a heat store as poor as the ground. That would heat a dedicated heat store that is insulated and capture heat around an area of 150'C.
Its very similar to Biffs gasification thermal store but specifically a daily recharge, supplied by a low cost oversized solar thermal collector array.
As far as I know we have never had a panel that is suitable as designs have always been hampered by focus on direct temperature use and water.
The ORC solves a problem of stagnation and the heat capacity limits of the thermal store and the GSHP is there for when the sun don't shine or when the useful capacity of the heat store has been exhausted.

The biggest benefit of a Medium temperature solar thermal generation system is its energy profile that is delayed by several hours from collection and its a hugely complementary system to PV and is the only way to balance solar energy without the need of extremely expensive and short life electrical storage.
Its that generation offset and ease of short term storage that makes that energy extremely cost effective, but like PV needs much investment to gain economies of scale.
Also its capacity in terms of heat is way in excess of PV with heat being 85% of domestic energy use, 40% of overall.  

« Last Edit: January 31, 2016, 01:14:27 AM by Stuart Ian Naylor » Logged
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« Reply #44 on: January 31, 2016, 07:12:43 AM »

Quote
The biggest benefit of a Medium temperature solar thermal generation system is its energy profile that is delayed by several hours from collection and its a hugely complementary system to PV and is the only way to balance solar energy without the need of extremely expensive and short life electrical storage.
Its that generation offset and ease of short term storage that makes that energy extremely cost effective, but like PV needs much investment to gain economies of scale.
Also its capacity in terms of heat is way in excess of PV with heat being 85% of domestic energy use, 40% of overall.  

Ok ,  lets talk about those cost   of PV  , battery   and a heatpump   (without install)

A 40 kWh  battery costs 2000
A 10 kW PV          costs 12000
An Aquarea 6kW    costs 2500
HC Monoblock ASHP


The ASHP  COP is over 2.5  , so that 40 kWh  battery can deliver  (when cycled down to 50% DoD)  50 kWh  of heat  about 3000 times  so 150000 kWh  so this then means the stored energy in the battery is costing about .... under 2 p

The PV is  delivering  on average  over  10 kWh in winter in the UK  per day makes 25 kwh heat a day  (with a CoP of 2.5)


the average  UK  household consumption per year on gas is about  12,400kWh   or devided by CoP of 2.5  about 4960 kWh heating/hotwater  electricity need    ( for the whole year !!)

Quote
In the period between November 2014 and October 2015, the annual average dual fuel customer bill in the U.K. was 1,326 as can be seen in the table below (source: Ofgem).

http://www.thegreenage.co.uk/how-much-energy-does-my-home-use/  

Offer   a fair trade selling surplus PV (FIT)  or/ and  charge  the electric car 

.....





« Last Edit: January 31, 2016, 07:18:02 AM by billi » Logged

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