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Old 11-27-2010, 09:01 AM   #1
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Hi guy's
This is my first time here and would like to pose a question.

I have been thinking (dangerous for me) about Heat Pumps and being a newbie I know virtually nothing but I had a bit of a thought.

I have a set of evacuated tube solar collectors on the roof to help heat my hot water supply. But here in the UK they are not too productive in winter so....

What if I took the slightly heated water from the collectors and compressed it by adding a pump before the coils in my storage tank and a restrictor valve after these coils I could increase the heat from the collectors in the storage tank coils and then send the cooled water back to the collectors.

Would this work, I would not need any external cooling or heating coils because the tank and the collectors would serve these purposes..

Your thoughts please.

John

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Old 11-27-2010, 07:45 PM   #2
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Main problem is that you can't compress water.

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Old 11-27-2010, 08:30 PM   #3
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Quote:
Originally Posted by beenthere View Post
Main problem is that you can't compress water.
Hi Mate
Yes I realised that after I posted but If I replace the water with a food grade anti freeze which is what these systems are supposed to have in them (I run 50/50% at present) or if that is not possible then I fill it with a refrigerent do you think the idea would work???

The coils must be well isolated from the domestic water so I dont suppose it has to be food grade, do you know if normal glycol antifreeze will act as a refrigerent?

I am sure someone else must have thought of doing this, I am not clever enough to have an original idea.
John
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Old 11-27-2010, 08:42 PM   #4
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You can't compress a liquid.

So water ever you wanted to use. Would have to be evaporated in the solar panel. And then "condensed" in the water tanks coils to release the heat.
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Old 11-27-2010, 09:12 PM   #5
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Quote:
Originally Posted by beenthere View Post
You can't compress a liquid.

So water ever you wanted to use. Would have to be evaporated in the solar panel. And then "condensed" in the water tanks coils to release the heat.
Hi Mate
I thought that the coil that is at the high pressure is where the heat is. I thought the liquid turns to gas because of the high pressure/temperature and it is only when it has been released by the restrictor valve it looses pressure and returns back to a liquid.

So the high pressure coil will be in the storage tank and release its heat to the water after that it will loose pressure, return to liquid state and be much cooler for its trip to the solar collector for re-heating.


Have I got it totally wrong or is this what you said, maybe I am using the wrong terminology?
John
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Old 11-27-2010, 09:24 PM   #6
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Your a bit off.

Its when it turns to liquid state that it gave off its heat.

Going from liquid to vapor absorbs heat(think of water boiling).

Going from vapor to liquid gives off the heat.
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Old 11-28-2010, 06:40 AM   #7
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Quote:
Originally Posted by beenthere View Post
Your a bit off.

Its when it turns to liquid state that it gave off its heat.

Going from liquid to vapor absorbs heat(think of water boiling).

Going from vapor to liquid gives off the heat.
Hi Mate
You are right of course and I was right in my own mind I made the mistake of thinking the refrigerent started as a liquid and went to a gas under pressure but I understand that it is the gas its self that is pressurised to super heat it, not the liquid.

I have just been studying the Wiki link http://en.wikipedia.org/wiki/Heat_pump and the pretty red and blue diagram is what I had in mind.
The red coil would be in my storage tank and the blue one is the solar collector.

I am sorry if I did not explain myself clearly.

Now back to my original question do you think it would work????

I have just thought of a problem, in summer the refrigerent is going to get super super hot when the sun is shining, perhaps I would have to arrange a bypass and turn off the compressor at a certain temp. The panel can withstand 270c but I am not sure of the other bits at this temp like pipes, soldered joints and compressor.

John
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Old 11-28-2010, 07:14 AM   #8
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Quote:
Now back to my original question do you think it would work????
Nope,... No way in 'ell....
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Old 11-28-2010, 08:25 AM   #9
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There are already air to water, and water to water heat pumps.

So the idea itself will work. However, The cost of making a system. Would probably out weigh any savings you might get. Using it just for domestic hot water.

270C is a higher temp then you would want a refrigerant gas to get. As its pressure would become to high.

The compressor would provide some of the heat that you would be putting into your storage tanks.
The heat from your solar panel probably won't be much though. Since you may get very little heat from the panels on many days.

You said slightly heated water. If the water from the panels is at 38C entering the heat pumps heat exchanger. then when it leaves the heat exchanger, it would only be 29C(at 3 gallons per minute per ton). So your panels would have to be able to heat that 3 GPM(180 gallons per hour of compressor run time) back up to 38C.

Solar panels sized to do that. Should be able by themselves to heat water to 54C, if slowed down to .5 GPM.

So perhaps you should check into how much water your current pump is moving through them. And experiment with lowering the water flow through the panels.
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Old 11-28-2010, 11:39 AM   #10
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Quote:
Originally Posted by beenthere View Post
There are already air to water, and water to water heat pumps.

So the idea itself will work. However, The cost of making a system. Would probably out weigh any savings you might get. Using it just for domestic hot water.

270C is a higher temp then you would want a refrigerant gas to get. As its pressure would become to high.

The compressor would provide some of the heat that you would be putting into your storage tanks.
The heat from your solar panel probably won't be much though. Since you may get very little heat from the panels on many days.

You said slightly heated water. If the water from the panels is at 38C entering the heat pumps heat exchanger. then when it leaves the heat exchanger, it would only be 29C(at 3 gallons per minute per ton). So your panels would have to be able to heat that 3 GPM(180 gallons per hour of compressor run time) back up to 38C.

Solar panels sized to do that. Should be able by themselves to heat water to 54C, if slowed down to .5 GPM.

So perhaps you should check into how much water your current pump is moving through them. And experiment with lowering the water flow through the panels.
You are dead right that I would not want anything to get anywhere near 270c, that is the absolute max the solar collectors can stand.

My water rate from the solar panels is at the slowest setting of a domestic heating system pump and on a good day my storage tank will heat up to 75/80c max that is a really good day in the UK midlands).

I think given the length of the 22mm pipe runs form the panels tp the coils I would need an awful lot of refrigerent and compressing about 10 foot of 22mm copper inside the tank might involve a rather large capacity compressor.
It is probably too expensive.

It was just a thought after all it gave a way of excerising my brain for a couple of days and I know more about heat pumps than I did before..

Maybe I will buy a second hand air source heat pump and plumb it in series with the solar panels thro the heat pumps condenser and just use it when the solar output is low.

Thanks for all your help.
John
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Old 11-28-2010, 11:45 AM   #11
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Your welcome.

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