Open loop ground source heat pump.

[daftlad]

Hey ho everyone.
We are lucky enough to have 2 wells on our new property, any idea how much water I would need to extract to feed a 15kw heat pump?
They are approximately 20 metres apart, I was thinking one could be for abstraction and one for recharge?
Or would it just be easier to go for an air source heat pump?

Hope everyone is keeping well in these interesting times!

Peas

[nowty]

Unless the water is clean as a whistle I would not even think about an open loop system as the pipes / heat exchanger will silt up and you also have the issue of the water freezing in the heatpump as you would not have any anti freeze in it. It also takes a LOT more energy to pump the water out than to circulate it around a closed loop system. ASHPs have improved a lot recently, in fact I am thinking of adding one to my system to give me the best of both worlds.

I have a bespoke closed loop water source heat pump from an underground river so the heat is constantly replenished.

My 6kW heatpump has a circulation rate of about 1,200 litres per hour and about a 5 degree difference between flow and return. So you would need a LOT of flow for 15kW and you would probably empty the well quite quickly.




And sometimes the pipe from the heatpump freezes on the outside but the coolant inside the pipe does not freeze until around -20 degrees. Note - I use food grade glycol so won't pollute if any leaks out as recommended by the Environment Agency.

[Oldgreybeard]

I would second that view. I looked into getting two boreholes drilled to run a GSHP. Complete nightmare in terms of paperwork. Believe it or not the Environment Agency class pumping water out of a borehole and back again as water abstraction. I have no idea how their brains work that one out, but the bottom line is that if you pump more that 20,000 litres out of a well, even if you put it straight back down the well, you need a water abstraction licence. Seemed like complete madness to me, as we are sitting on top of a big aquifer that is at about 8 deg C, and it looked to be a no brainer to try and extract heat from it.

It is fair to say there were some other problems too. The power needed to pump water up from a well is substantial, as unlike a closed pipe loop you cannot just use a circulating pump, it has to be a proper well pump. This then hits the efficiency hard, so swings the balance away from a GSHP being more efficient and makes an ASHP look pretty good. Together with the price of ASHPs being so much lower, we decided that this is what we'd fit. Glad we did, as it was a fraction of the cost of the GSHP installation and has been working very well for several years now.

[daftlad]

Thanks for the replies.
Nowty, really interesting setup you have there, I'm pretty sure the water is very clean, it would have been used for drinking once upon a time without any filtering, and it does look crystal clear when you bring a bucket up. Pretty much every house had a well here but they say the water has too much phosphates and nitrates now from the farming!
Oldgreybeard.
The cost of an ASHP compared with a GSHP is certainly a factor, there would have to be a compelling reason to go with Ground source heat pump!
I would still like to know how much water would be needed, anyone know of an online calculator or even the formulas?

Peas

[nowty]

The cost of an ASHP compared with a GSHP is certainly a factor, there would have to be a compelling reason to go with Ground source heat pump!
I would still like to know how much water would be needed, anyone know of an online calculator or even the formulas?

15kW = 15000 watts = 15000 joules per second
Specific heat capacity of water is 4200 Joules per kilogram (or litre) per degree C.
So you need 15000 / 4200 = 3.57 litres a second per degree of heat extracted from the water.
So if we assume your heatpump has a 5 degree difference from inlet to outlet, that’s 3.57 / 5 = 0.714 litres per second or 42.84 litres a min or 2570 litres per hour.

But of course nothing is 100% efficient so probably need something like 3000 litres per hour.

And as a quick cross check with my own system badged at 8kW, but as I've measured it at 6kW.

15kW / 6kW = 2.5

My flow source rate is 1200 litres per hour so 1200 x 2.5 = 3000 litres per hour.

Any info on your well, like how many metres down is the water ?, what's the capacity of the well (or dimensions) ?, what's the refill rate ?

[daftlad]

Cheers nowty
Thanks for the calcs.
one of the wells is 12m deep to the water line and I have no idea how deep the water is! but based on experience of other wells in the area I have measured I would guess there is 3 to 5 metres of water in there. (I will drop a string down, although it might need a clean so could end up deeper?!)
So based on the diameter there might be 3000 litres in there? or more or less?
I don't yet have a pump so don't know how quickly it will refill itself?
But based on my gut feeling and the info from you guys I think it could be marginal and also not worth the extra investment over an ASHP.

Peas

[Andy]

If there is a well there then the ground must be nicely saturated. Depending on the ground if you have a traditional borehole with a closed loop this can work well. A friend almost had to abandon his borehole because of the amount of water trying to come out. His loop temperature is not affected anywhere near as much as ours in the colder depths of the winter.

[AE-NMidlands]

Cheers nowty
Thanks for the calcs.
one of the wells is 12m deep to the water line and I have no idea how deep the water is! but based on experience of other wells in the area I have measured I would guess there is 3 to 5 metres of water in there. (I will drop a string down, although it might need a clean so could end up deeper?!)
So based on the diameter there might be 3000 litres in there? or more or less?
I don't yet have a pump so don't know how quickly it will refill itself?
But based on my gut feeling and the info from you guys I think it could be marginal and also not worth the extra investment over an ASHP.

Peas

The thing about old wells is that they were only ever expected to supply what water people could draw manually. I spent my childhood holidays on a farm in rural Devon where all the water for the house had to be pumped by hand from a well in an open out-house by the back door. When the rainwater tank ran dry the WC was flushed with the water saved from washing. After the family fitted an electric pump they managed to pump the well dry, and that was in the 1970s! They blamed the climate, but I think it was just massively increased demand.
If you have 2 wells in close proximity I would guess that, although the water table might be quite high, the flow rate through the aquifer might not be very high so one well couldn't deliver enough, hence the second. Water in the pores of a rock isn't the same as water able to move through that rock.
A

[nowty]

Cheers nowty
Thanks for the calcs.
one of the wells is 12m deep to the water line and I have no idea how deep the water is! but based on experience of other wells in the area I have measured I would guess there is 3 to 5 metres of water in there. (I will drop a string down, although it might need a clean so could end up deeper?!)
So based on the diameter there might be 3000 litres in there? or more or less?
I don't yet have a pump so don't know how quickly it will refill itself?
But based on my gut feeling and the info from you guys I think it could be marginal and also not worth the extra investment over an ASHP.

Peas

The water pump (not heat pump) you would need to pump water out of the well at that depth and at the flow rates I was talking about would probably be completely unfeasible and / or uneconomic.

However if it were me I still might have a crack at doing it closed loop in copper and using both wells connected in parallel. I don't know if a 15kW heatpump would work, might turn the well into a block of ice because the heat replenishment rate is a bit of an unknown.

This is my heat exchanger which I prototyped in my lounge, there was a thread on it on the old forum, I might try and reinstate some of it on this forum because it gives a step by step story of how I did it.

[Andy]

Have you had much of a problem with corrosion on those pipes?