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Idea for solar assisted heat pump

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Posted by Mark on June 9, 2008, 2:02 am
You get a small refrigerator freezer.   You put 5 to 6  1 gallon jugs
of water in it.   Run the freezer at night.  During the night the
freezer will pump the heat out of the water and into your room.  The
water will freeze as the freezer extracts the heat of fusion out of
the water.   During the day, you take the frozen water and put it
outside in the sun.  The sun will melt the ice back to water.  Maybe
make a small glass enclosure for it.  The ice  will absorb heat from
the sun.  The freezer will extract this heat at night.  Yes, you need
to use electricity to run the freezer, but for every watt hour of
electricity you use, you will get 3 to 5 watt hours worth of heat due
to the heat that is extracted fom the water as it freezes.  This is
not a joke and is founded in true physics.  Look up the latent heat of
fusion of water/ice.  Water/ice is a good way to store thermal energy
and ice is very efficent at extracting heat from solar power as it
melts.  Yes you need a heat pump / freeezer to extract the heat fom
the water to make it usable to heat a room.   Still a good compromise.


Posted by Steve Ackman on June 9, 2008, 6:05 am
on Sun, 8 Jun 2008 19:02:23 -0700 (PDT), Mark, makolber@yahoo.com wrote:

  Didn't you just post this recently?

  Have you actually tried to fit 6 jugs in your freezer?
What's your definition of a *small* refrigerator

  WHY would you want to waste all that solar energy
melting ice, when melted ice is already available from
your faucet?

  It is a joke *even though* it's founded in physics.

  Yes, please do.  When you put ice into your solar
heater, all that solar energy is being wasted turning
ice into water (which you already have) when it could
be used turning cold water into hot water.

  Efficiency is determined by energy out divided by
energy in... nothing to do with latent heat of fusion.
Again.  Why on earth would you want to waste solar
energy melting ice when you have a plentiful supply
of melted ice already?

  Sure it is.  If the room is a 5x7 bathroom located
somewhere in the subtropics.

  Now, for the real question.  Cost effectiveness.
Given that you're running this refrigerator in a manner
it was never designed for, how much will that cut off
the lifespan of the appliance?  If your refrigerator
would normally last 10 years running at say a 30% duty
cycle, but now at 100% duty cycle, it only lasts 4
years, is the measly amount of heat worth the extra
cost of replacing refrigerators?  Not to mention the
extra cost for disposal of prematurely dead
refrigerators.  Environmental cost for building extra
fridges you wouldn't have otherwise needed.  Etc.

Posted by Tom on June 9, 2008, 7:50 am

No, last time he tried to run this experiment the other way, using the
freezer to make ice to cool a room

A small chest freezer will fit a lot more than 6 gallons with ease

He isn't trying to make water, he wants the heat from the sun warmed water

So far the OP has got it right, stand the frozen jugs of water in a
greenhouse during the day to warm up, carry them inside and use the
freezer/heat pump to extract the heat during the night, carry them back in
the morning. It should keep the chill off a bedroom if you can stand the hum
of a freezer running in your bedroom all night.

It doesn't get "wasted" It gets moved from outside the house to inside the
room he wants to heat

As I said, he's using the water/ice as a transport medium for heat

 Valid point. However older freezers are given away regularly on Freecycle.
If the OP gets 6 more months of use out of them before they burn out then I
think he has only delayed their eventual scrapping. As for cost of disposal,
here in the UK your local council will take them and dispose of them for



Posted by Mark on June 9, 2008, 2:51 pm

sure you can get the water out of the tap... what do you do with the 6
gallons of ice the next morning?  You can dump them outside and use 6
new gallons of water if you like each day.  Then you have a geothermal
system instead of solar,,ok fine.

melting ice at 32F will be more efficent (considering losses to the
outside winter air temp) at extracting heat energy from the sun than
will heating water to 90F.

A small freezer will have no problem running at 25% duty cycle. It
runs only at night and only in winter.... so it will still last 10
years by your calaculation

The solar collector in this case is 6  containers of water....
essentially free.    What is the material and installation cost for
your solar panals needed to make hot water?  I think the up front
cost, ease of obtaning a small freezer and 6 water jugs is much
cheaper/easier compared to installed solar panels.


Posted by Steve Ackman on June 9, 2008, 8:36 pm
 on Mon, 9 Jun 2008 07:51:16 -0700 (PDT), Mark, makolber@yahoo.com wrote:

  Then you have geothermal/solar.  Take the jugs of
tap water and put them in the same place you would have
put your ice blocks.  Now, your water jugs will end up
much warmer than your ice blocks would have.

  Solar energy is solar energy.  Adding X BTU is
adding X BTUs no matter whether it's to ice or to

  Ok, so go look up duty cycle.  You obviously don't
know what that is.

  At any rate, I did no calculation.  I posed a
hypothetical question.  

  Duty cycle absolutely makes a difference with some
appliances; the difference between keeping them running,
or burning them out.  Is that the case with refrigerators?
Probably depends on individual design.

  6 containers of water.

  Who ever said anything about solar panels?  Jugs of
water in the sun for a day end up warmer than jugs of
ice in the sun for a day.  

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