Does heat availabilty in the soil reduce over time ?

[ecr]

In other words will I have a system that works well in year one , less well in year 2 and so on ?
Is this dependent on soil type as my soil has a fairly high clay content ?

[Argus]

...........

Is this dependent on soil type as my soil has a fairly high clay content ?

Very much so.

This is why you would have a full and competent ground survey carried out to determine the thermal composition of the ground, water table and local geology.

You are more likely to encounter heat saturation if you are using shallow trenches or so-called 'slinkies' than if you opted for bore holes. But it very much depends on the capacity of the system and the local ground. Bore holes will almost certainly go lower than a clay sub-strate. Shallow trenches may not go deep enough if the ground is water-logged which is what you seem to be suggesting.

From this survey your contractor would calculate the quantity, depth and optimum spread of your bore holes - or the spread of the trenches if you opt for this type. It is nigh impossible to predict ground thermal equilibrium - the best balance of heat removed to the amount of heat regained naturally - without a survey.


It's very much what you pay for. But don't overlook the fact that you may need local authority or Environment Agency consents for deep drilling in the UK.


.

[DEVIL]

It depends

If the engineer sizes your area of the loop correctly the amount you extract should be covert by the amount is regenerated in the sumer, judging that u don't have any construction on top of the loop.

But if the loop is smaller then the amount will drop in a few years until it gets stable (but it's not recommended ) because the sun is having an average amount of X w/m2 (x depending of your climate) and you shouldn't use more then it can store in the ground, and every ground is having it's own storing capacity and conductivity that u should take in consideration

[taz24]

In other words will I have a system that works well in year one , less well in year 2 and so on ?
Is this dependent on soil type as my soil has a fairly high clay content ?

Very much so..

Water plays the biggest part.

If there is free flowing water and the loop is placed in it, then the energy will be replenished continuosly.

The best type of ground is the damp type, clay can be quite dry.

Most gound conditions in the UK deliver between 30 and 50 watts of energy per mtr, but the positioning of the collector can affect that. If the loop is too close together it can remove to much energy from the ground.

taz.

[US Iceman]

It is also a fine balancing act between the ratio of heat extraction and heat rejection to the ground soil with the soils ability (including groundwater movement) to continuously reject or absorb the heat. In a larger system you can have concentrated hot or cold spots which affect the overall performance of the ground loops or bore fields.

[cadillackid]

if the system is heat and cool wouldnt you be "putting back" heat in the summer that you removed in the winter?

I also second that clay is very dry and doesnt hold moisture well.. and is also a pretty good insulator which is NOT what you want...
-Christopher

[Magoo]

The ground is the biggest solar panel you can find.

[desA]

For this kind of application, I'd suggest contacting someone who can simulate the local ground structure, using thermal diffusion, with addition of convection if there is local movement in the ground water.

This can be a pretty simple 2D CFD study. You would need to know the soil type & typical thermal conductivity.

If you really can't find someone in your neck of the woods, I could assist. I have both adequate CFD software & simulation expertise.

[DEVIL]

if the system is heat and cool wouldnt you be "putting back" heat in the summer that you removed in the winter?

Yes if the system is reversible then u are putting heat in the ground, but it's not always the case, because in some cases the heating is made with underfloor, and u can't do cooing this way.

You could get someone, to inspect your ground, it might not all be clay , and if it's proper sized any type of ground is suitable it just influences the area and the investment.

But if u have plenty of water close to the surface, u might think of a open loop system, u are probably going to get a higher efficiency judging that u have water in the ground with the temperature not lower then 10 deg C all the year around

[ecr]

Many thanks for the very interesting replies !
To expand a little on my question I should state that i am currently instructing architects in central France and am planning on building a new home on land I have purchased. I am keen to use ground source heating if at all possible and am currently gathering knowledge from guys like you... I can find many sites on the web that tell me how a heat pump system works but what I am really after is the down side of these systems so that I can speak with my supplier with some authority. I appreciate that a good supplier will give me a good system but unless I know what pitfalls to watch out for it is difficult to judge any prospective installers.
So what are the problem areas ?

[Peter_1]

ecr , read once this and most of your questions will be answered http://www.dimplex.de/fileadmin/dimp..._gb_102008.pdf

[desA]

^ Thanks Peter_1 for an excellent review article. That really makes for wonderful reading. Much obliged.

[micstiebel]

Yes if the system is reversible then u are putting heat in the ground, but it's not always the case, because in some cases the heating is made with underfloor, and u can't do cooing this way.

You could get someone, to inspect your ground, it might not all be clay , and if it's proper sized any type of ground is suitable it just influences the area and the investment.

But if u have plenty of water close to the surface, u might think of a open loop system, u are probably going to get a higher efficiency judging that u have water in the ground with the temperature not lower then 10 deg C all the year around

Recharging your bore holes are seldom very effective, for several reasons.
First of all a effective borehole usually has a flow of groundwater through it, which is positive in normal operation, and also for cooling,but not for storing energy.

There are ways of cooling with underfloor heateing systems, i have personally been involved in such an installation, with both active and passive cooling.

There are a few things to consider when planning for an open loop / ground water system, first of all the actual efficency is not that much higher since the pumps used must be so much bigger, in a cloosed loop system you only have to worry about the pressureloss in the system, in an open system you have to consider the pumpheight aswell.
Also the caracteristics of the water can change over time to become more aggresive so it is often recomended to use an extra plate heatexchanger to protect the heatpump.

[DEVIL]

Recharging your bore holes are seldom very effective, for several reasons.
First of all a effective borehole usually has a flow of groundwater through it, which is positive in normal operation, and also for cooling,but not for storing energy.

There are ways of cooling with underfloor heateing systems, i have personally been involved in such an installation, with both active and passive cooling.

There are a few things to consider when planning for an open loop / ground water system, first of all the actual efficency is not that much higher since the pumps used must be so much bigger, in a cloosed loop system you only have to worry about the pressureloss in the system, in an open system you have to consider the pumpheight aswell.
Also the caracteristics of the water can change over time to become more aggresive so it is often recomended to use an extra plate heatexchanger to protect the heatpump.

I wasn't talcking about recharging a bore hole, u might have a slinky loop, and that is not crossing any water flow.

And i wouldn't recommend any wife a underfloor cooling, they are very sensible to cold floors , so if u want to make passive cooling, u should use the ceiling or walls never the floor, not only because the wife problem, but
physics tels us that cold air stays down and hot air gos up and this might make air separation in the room.

Yes of corse there are lot's of things to consider but let's just take an example of a compressor efficiency (it is a typical heat pump compressor)if the
Tevaporation1 is 10 deg C, heat rejection is 23.8 Kw, and if
Tevaporation2 is 0 deg C, heat rejection is 17.5 Kw,

Tcondensing is the same 40 deg C, Power input is the same 3.7 Kw.

So , in the Te2 case every 2 hours and 46 minutes the compressor must run an extra hour to get to the Te1 heat rejection capacity, this means that u use 3.7 Kw extra electricity every 2:46 hours , this mens over 1.37 Kw of electricity / hour

And now let's be serios u can pump a lot of water with 1.3 Kw (not calculating the loop pump that u don't have any more).

And this is not the worst case scenario because water in the loop will go under 0 deg , and water in the earth (at list in Romania) rarely under 12-13 deg C.

And i never recommend a second heat exchanger because you are loosing around 5 deg C in a high efficiency heat exchanger.Unless the water wery aggressive (rarely the case )

[ukheatpumps]

If the ground collector is undersized, and the heat pump takes more energy out of the ground than the sun can replenish, then inevabilty the ground temp will fall year on year.Solution is to size collector correctly in first place,taking into consideration the load (heat pump,heating /hot water) and factor in the heat capacity of the soil.

[Peter_1]

[QUOTE=ukheatpumps;168074...and the heat pump takes more energy out of the ground than the sun can replenish, ...[/QUOTE]
This is only valid for horizontal loops. If you make vertical drills, even not that deep, then the most important factor for recovering becomes the energy stored in the deeper ground layers which aren't affected by the outside temperature.

[Peter_1]

I...
Tevaporation1 is 10 deg C, heat rejection is 23.8 Kw, and if
Tevaporation2 is 0 deg C, heat rejection is 17.5 Kw,

Tcondensing is the same 40 deg C, Power input is the same 3.7 Kw.

I wonder how you can evaporate on 10°C with ground water

[DEVIL]

I wonder how you can evaporate on 10°C with ground water

It was a theoretical example to demonstrate how 10 deg is a big difference, but in reality it would be around 0-1 deg C evaporating for water source, and -10 or less for close loop, using a efficient heat exchanger.

[Peter_1]

In fact, 1K difference makes +/- 2°C less or more energy consumption.

[DEVIL]

In fact, 1K difference makes +/- 2°C less or more energy consumption.

i don't realy understand this (probably because i read in English and think in Romanian )

[Peter_1]

Devil if you increase HP or decrease LP with 1°CK, then your electrical consumption will increase with +/- 2% or your COP will decrease with +/-2%

[DEVIL]

I thought that's what u meant , re read your post for 3 times before writed my post, and thought u meant "+/- 2%" (because it's logic) and not meant "+/- 2°C" but waned to make shore