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Cold energy Storage MkIII

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Following a couple of discussions of Cold/ Ice  energy storage, I have been doing some more head scratching and research on the idea.

Ice while very efficient as storing cold energy is also somewhat problematic on  DIY Basis.  There is the problem of ice expansion and transferring the cold in the ice through water which may make the melting process difficult and somewhat expensive with the amount of coils needed to be put through the ice.

I'm thinking of using excess solar power to create cold energy and store it for use on hot summer Nights.
Ideally the energy could be restored through the day so an 8 Hour cycle to take advantage of the solar available would be ideal.
The previous discussion looked at using a domestic chest freezer for creating the ice and as a thermally insulated container. There was doubt as to weather a freezer had the strength for filling with water and freezing it due to a freezer is not going to have the cooling power to freeze the water fast enough.

My thoughts are to use a chest freezer or an old refrigerator turned on it's back as a cheap storage container. The size I have in mind is 500L +.
 For cooling power i'm thinking of a car AC compressor and using the metering valve to not go into the normal densely packed evaporator tubes but rather a loop of stainless steel pipe running the length of the container. The AC compressor could be driven by an electric motor to make use of the solar power through the day. There would also be potential to drive it with an IC motor and add in a RX valve for heating in winter. The heat from the engine and exhaust could be used to increase efficiency by elevating the temps going through the Condenser.

Looking into something Bob touched on being salt in the water, it seems this could be a very viable solution to get around a lot of problems associated with having ice.
If we took a 500L frezer and added 120Kg of Salt ( about $50 worth here) the freezing point of the water now instead of being 0 Oc becomes MINUS 20 OC.  That means if we have 500L of salt water at -20 and take it up to Plus 20, we have a thermal storage of 23Kwh of thermal energy.
Definitely a worthwhile amount and sufficient to cool a decent area of a Home for one night. 

Being we still have water, a lot of problems with having ice go away.
We wouldn't need all the pipework as with ice for a start.  The water could be removed from one end of the freezer, Pumped through the HE and returned the other end of the fridge or freezer.  Perhaps some circulation of the water during the cooling cycle could be beneficial and a small pond pump or air lift could be used to move the water around to prevent localised Freezing.

To get call it 20 KWH worth of cold, I guesstimate  the input would have to be around  5-6 Kwh of energy.  There should be an EER of about 4x on the AC cooling side but there would have to an account for the inefficiency of the electric motor and belt drive and also an electric fan running through the evaporator.  Using brine as the working Medium, if more power was available and more cooling wanted, an extra fridge/ freezer could easily  be fitted with some pipes and used as an extra holding tank.

Astheticaly, these could be used as benches or even a table or stacked on top of one another for more compact packaging.

I don't think having ice is necessary for a worthwhile system.  While it might be more efficient in storing the energy, this could be made up for with solt water by just increasing the volume  fo the water whoci would be remain a very practical amount.  If one could get 20 or 40 Kwh of cold storage, to me that would seem as much as most people can use in a night and also as much spare power as a lot of people might have to generate the cold in the first place. If you tried to Freeze too much ice, firstly you may not be able to throw in sufficient power to get the phase change and secondly, are you going to need more stored cold energy anyway?  If you could get 40 KWH, that would be a good 8 hours on a large whole house ducted AC running flat out.

Also with the salt water system, you have the ability to store heat and elevate the temp of the water.  Not sure what temp the plastics would take but I also know that most people tend to grossly underestimate how hot plastic can get without deforming.  I have boiled a number of plastics and they just don't care about 100oC  and I don't think anything in fridge would be worried about 120 if you could get it that high.   Unlikely a lot of people will have much if any excess solar power in winter but if one were using an IC engine it would give the ability to run it in the day and shut it off at night.

Going to see if I can get a couple of car AC systems this weekend and give this a go.  I have a space at the end of the house that would be perfect to locate a fridge/ Freezer which is out the way and close to the power board  and also at the end where the main bedroom is which is what I really want to cool at night.

I'm having the 32A circuits installed for the solar next week so hopefully I'll have less problems with voltage rise and can get more power out my systems. In any case being able to run something like this will be good to use the excess power that pushes the voltage above what the inverters are comfortable with.

I  believe an AC compressor needs around 3-4 HP so I might look at running my 5KW 3 Phase Motor on 2 Phases and see how that goes.  I can start it unloaded then have a timer kick in the AC compressor clutch after 5-10 sec.  Failing that I have a 1.5 HP single phase up the back and I'll look at putting a small pulley on that to gear it down.  Compressor may not make as much power but I'll be able to kick the smaller load in earlier and out later on the excess solar that will cover it.  Either way I am not thinking that chilling the water fast enough is going to be a problem with the sort of energy a car AC compressor can generate which is the equivalent of a mid size household split.

the most difficult thing I see about this is plumbing the AC side.
Car AC fittings tend to be Bolt on O ringed aluminium fittings while  Domestic/ Commercial AC tends to be copper flares.

Other than that seems practical, affordable and doable to me.   :)

I recall there being a huge amount of energy in the phase change state, converting water to ice and back to water, at 0c.  Simply dropping the freeze point 20 degrees with salt, may not be enough delta to match the phase change energy.
I'm the electronics guy, and we pass all the thermal calcs to the thermal engineering department.


While there is a lot of energy getting the water to phase change, there is also a lot of problems in both Freezing and then re thawing the Ice to release the energy.
In practice, I don't see the point of going to the trouble to use Ice when you might only use half it's cooling capacity and therefore still have spare Solar power through the day you can't use because you have already filled your cold bank.

As I showed, It would not be hard or take a very large container to store 20Kwh of cold.  I think for a lot of places, that would be sufficient cooling especially at night when there was no sun and cooler than daytime temps. If a person had cheap off peak power, it may be better to recharge the system at night for use in the day.  In that case Ice may be better BUT, it's also going to be a lot more expensive and complicated than salt water.

Even if one wanted 40Kwh of cooling capacity, something the size of 2 large Refrigerators is not an overly large or Impracticaly large amount of containment to handle.

Hi Glort, I think you have very easily doable project there. Fridges/freezers are not designed to hold that amount of fluid, brace them with timber and run ratchet straps around them.

The trick to maintaining a comfortable home in summer is to turn on the AC early, when it is powered by your solar. It is much cheaper to keep a house cool than to try to cool it down when you get home from work after dark. If it does start to get too warm then you kick in your DIY air conditioning system.



Hi Bob,

I agree with the early theory entirely. I have been doing the same over winter with heating.  During the day I let the AC bounce of thermostat set 20oc.
In the afternoon I turn it up to about 23 Before  the temp drops too much.  Being a sad case that has no life I monitor Generation and use every day and found this used much less power over all than letting the place go cold and trying to warm it up.

I'm getting a couple of 32A Circuits put in this week for the solar so i hope to be able to pump the power back in and reduce the voltage rise I'm getting now.

Didn't get the compressors as I hoped this weekend but its definitely something I want to try. I have the perfect location and application for it so will be practical in application as well.  I don't really see a problem with filling them with water,  just the freeze expansion I think would have been a problem but I'm sure I can get a fridge or Freezer cheap from my friendly local scrappy to try. If one bursts, I'm sure he won't mind doing an exchange for me.  :0)


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