- Thread starter mike
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Sounds like a lot of water. If every house had one of these, wouldn't we have a huge reduction in stored underground water, and a drop in the water table unless everyone also had very efficient soakaways to put it all back again rather than discharging into ditches or rivers?
Penners
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Their "How It Works" example shows water being extracted from a water course at 12 degrees C and being returned to it from the heat pump at 6 - 9 degrees.
I wonder how often the ambient temperature of river or sea water in the UK is at or above 12 degrees. I'm not expressing scepticism - I just don't know.
And if the inflow water is cooler than that, will the heat pump simply remove the same amount of heat, returning the outflow water at a colder temperature?
It would be interesting to know.
I wonder how often the ambient temperature of river or sea water in the UK is at or above 12 degrees. I'm not expressing scepticism - I just don't know.
And if the inflow water is cooler than that, will the heat pump simply remove the same amount of heat, returning the outflow water at a colder temperature?
It would be interesting to know.
Flyfisher
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I was also concerned about the amount of water apparently consumed, although I presume they would claim it must all get back into the water table eventually. But even so, it doesn't seem right somehow. Having said that, anyone with access to a nearby river ought to be able to extract a pretty constant supply of water and immediately return it a little downstream with no real 'consumption' at all.
As for their 12C input water temp, I assumed this is the constant temp of groundwater brought up from their boreholes. I'm sure ponds and rivers won't be this constant throughout the year; it's been years since I've been swimming in ponds or rivers but my kayaking-mad son does so regularly and says the temp can vary quite a bit.
Their open-loop system must be dependent on the incoming water being at least 7-8C if they're extracting 6C of heat from it, because it will freeze with anything colder. I believe closed-loop systems can overcome this limitation by using an anti-freeze mixture, albeit with the consequent additional costs and heat-exchanger losses. Horses for courses I guess.
One thing I thought was a bit disingenuous was their statement that the heat-pump would be completely green if it was powered by renewable energy such as a wind-turbine or PV panels. Well OK, yes in principle, but with a 4:1 CoP and a 25KW installation that means you'll need to generate just over 6KW to run the thing, which I understand is a bit of a tall order for a domestic wind turbine or an affordable number of PV panels.
As for their 12C input water temp, I assumed this is the constant temp of groundwater brought up from their boreholes. I'm sure ponds and rivers won't be this constant throughout the year; it's been years since I've been swimming in ponds or rivers but my kayaking-mad son does so regularly and says the temp can vary quite a bit.
Their open-loop system must be dependent on the incoming water being at least 7-8C if they're extracting 6C of heat from it, because it will freeze with anything colder. I believe closed-loop systems can overcome this limitation by using an anti-freeze mixture, albeit with the consequent additional costs and heat-exchanger losses. Horses for courses I guess.
One thing I thought was a bit disingenuous was their statement that the heat-pump would be completely green if it was powered by renewable energy such as a wind-turbine or PV panels. Well OK, yes in principle, but with a 4:1 CoP and a 25KW installation that means you'll need to generate just over 6KW to run the thing, which I understand is a bit of a tall order for a domestic wind turbine or an affordable number of PV panels.
Johneds
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Actually I think those figures are given as an example David - I don't think it really matters what the temperature of the water is. Take an airborne heat pump (the ones with the big fans you see on the sides of factories or the roofs of offices etc.) they quite happily extract heat from air temperatures well below freezing. I imagine the efficiency changes with lower temperatures but heat could still be extracted right down to zero - or even lower, except then of course it would be ice.
Penners
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At the risk of pushing my luck, John, can you explain to an ignoramus how the heat extracted by this method can be used to heat a home? If it's only a differential of - say - 6 degrees, how can this be "concentrated" to give a comfortable interior temperature in the house?
Johneds
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You've hit the nail on the head David. The heat is actually concentrated at source. So a high volume of low temperature heat is turned into a lower volume of higher temperature heat.
The way this happens is that there is a circuit of pipework filled with gas. The inward flow is pressurised (by a compressor) and the ouward flow is depressurised. (via a release valve)
The pressurised gas is in liquid form. (imagine a cannister of lighter gas) the depressurised gas is in gas form. (imagine the gas escaping from the lighter gas cannister)
Now the clever bit. If you were to hold open the nozzle of the lighter gas cannister the released gas, and the can itself, would immediately start to feel very cold - and in fact will quite quickly ice up. This is because when gas depressurises it absorbs vast quantities of heat making everything around feel cold. You can sometimes feel the effect with ordinary aerosol cans.
Now move the heat laden gas further up the circuit (i.e. inside the house) and pressurise it again. When the gas is pressurised it turns back into liquid and all that stored heat is released. Now all you have to do is trap the heat and move it to where you want it. In most systems the compressor and the release valve can be reversed as well so the system will heat in the winter and cool in the summer.
This is exactly the reverse of your refridgerator. The circuit absorbs heat from inside the cabinet and releases it via the metal grid at the back of the fridge.
These systems are typically 300% efficient. For a 10kW compressor and a few controls you can get as much as 30kW of heat out the other end. Of course you are not actually heating anything up. You are just moving heat from one place to another.
The way this happens is that there is a circuit of pipework filled with gas. The inward flow is pressurised (by a compressor) and the ouward flow is depressurised. (via a release valve)
The pressurised gas is in liquid form. (imagine a cannister of lighter gas) the depressurised gas is in gas form. (imagine the gas escaping from the lighter gas cannister)
Now the clever bit. If you were to hold open the nozzle of the lighter gas cannister the released gas, and the can itself, would immediately start to feel very cold - and in fact will quite quickly ice up. This is because when gas depressurises it absorbs vast quantities of heat making everything around feel cold. You can sometimes feel the effect with ordinary aerosol cans.
Now move the heat laden gas further up the circuit (i.e. inside the house) and pressurise it again. When the gas is pressurised it turns back into liquid and all that stored heat is released. Now all you have to do is trap the heat and move it to where you want it. In most systems the compressor and the release valve can be reversed as well so the system will heat in the winter and cool in the summer.
This is exactly the reverse of your refridgerator. The circuit absorbs heat from inside the cabinet and releases it via the metal grid at the back of the fridge.
These systems are typically 300% efficient. For a 10kW compressor and a few controls you can get as much as 30kW of heat out the other end. Of course you are not actually heating anything up. You are just moving heat from one place to another.
Penners
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Thanks for that explanation, John. To my amazement, I actually understood it!