Desert farm, Big pump, Batteries?

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Re: Desert farm, Big pump, Batteries?

Postby davidg » Sat May 20, 2017 10:34 am

The reason for using storage of cold in a liquid is to remove the need to consume large amounts of power at night. By storing the energy in the form of cold produced during the day from solar drastically reducing the demand to store large amounts of energy in batteries. Being able to cycle an exetremely cold liquid through a cool room allows the room to be kept cold/cool 24x7 for very low electrcal energy demands when the sun is not shining, could be used in all sorts of ways having a large storage of cold or heat that can be powered purely by solar, has advantages over storing energy in batteries and then using large amounts of power to produce cold or heat later.

You dont need to freeze the storage liquid although it does not matter if it does partly freeze provided the liquid being moved through the cooling pipes does not or cannot freeze.

It does not nessarily have to be more efficient overall although by my calculations it will be similar after all, its still a form of energy storage there are losses involved. It should actually be more efficient in the longer term battery storage looses capacity this type storage should remain more stable in capacity and more repairable in the long term.
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Re: Desert farm, Big pump, Batteries?

Postby Warpspeed » Sat May 20, 2017 11:32 am

I disagree.
The enthalpy of frozen water is about EIGHTY times the specific heat of water.

In other words the amount of heat involved with freezing or melting ice is about eighty times the amount of heat required to raise the same mass of water by only one degree Celsius.
Latent heat is an extremely powerful mechanism.

Once all your floating ice is melted, the water temperature will rise very rapidly and the cooling effect is all but gone.

Looking at it another way, a one gallon block of ice would have the same cooling capacity as eighty gallons of water at 0 Celsius, if you allowed for a one degree temperature rise in the water. Its not an insignificant difference.
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Re: Desert farm, Big pump, Batteries?

Postby davidg » Sat May 20, 2017 11:43 am

Warpspeed wrote:I disagree.
The enthalpy of frozen water is about EIGHTY times the specific heat of water.

In other words the amount of heat involved with freezing or melting ice.

Are you talking about plain water or brine? There's no point in using just watet it's simply not cold enough. Brine is used in large refrigeration systems because it will not freeze until -21.2C. So running it at say -18C there is considerable cold stored for non sun hours use. Brine is a very cheap cold storage medium. The temp of the brine might get to say -5C, the amount of energy between the two temps is considerable.

The coolroom only needs +4C so once the amount of brine you need is calculated plus margin you can store cold to suit.
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Re: Desert farm, Big pump, Batteries?

Postby Gordon-Loomberah » Sat May 20, 2017 11:53 am

You are definitely wasting your time if you dont make use of latent heat of fusion, ie the phase change from solid to liquid, in a situation where you are trying to keep something cold without using external energy sources overnight.

Brine does not have to be at -21C, that's only the minimum MP acheiveable with a saturated solution. By variying the salinity, you can have a MP of anywhere between 0 and -21C. As I mentioned previously, by using CaCl2, you can go to a much colder MP if you have the ability to chill it that low.

I'm still in the dark as to what volume has to be kept cool, what weight of dates has to be cooled from outside air temp to coolroom temp each day. Does it have to be at 4C?
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Re: Desert farm, Big pump, Batteries?

Postby Warpspeed » Sat May 20, 2017 11:54 am

I am talking about plan frozen water which freezes at 0 Celsius.
Look it up for yourself.
Latent heat of water freezing/melting 79.7 calories per gram.
Specific heat of water 1 calorie per gram

https://en.wikipedia.org/wiki/Enthalpy_of_fusion

(from the above link) When liquid water is cooled, its temperature falls steadily until it drops just below the line of freezing point at 0 °C. The temperature then remains constant at the freezing point while the water crystallizes. Once the water is completely frozen, its temperature continues to fall.


Water is particularly high on the scale compared to anything else.
Adding salt or glycol or anything else makes it worse for heat storage.
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Re: Desert farm, Big pump, Batteries?

Postby jaahn » Sat May 20, 2017 12:10 pm

Hi :D
Interesting discussion, but we are all shooting in the dark as we do not know what the design parameters are ???? No idea of scale either ?
Two boxes of dates at 10 deg or two tonnes of dates at 1deg. OR WHAT ?
Day and night temperatures winter and summer range. etc etc. Cool room specs and size ?

I would point out the argument about latent heat vs sensible heat is really just peripheral. It is a storage mechanism so the heat to be removed says the same, just the convenience of 'handling' the storage medium is relevant. Or whether a more constant temperature may be useful. But the heat coming in the walls and roof and floor may not be much different. :roll:

IMHO brine is a terrible corrosive material and should be approached carefully. Material selection would be important for a trouble free long life in a largish system. :twisted: It has been extensively used in past times for ice making and similar uses. Nothing new there !! Some other substances are toxic also.
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OR just hire those girls to bring in some ice blocks ever now and again !! Made possibly with an ammonia system using brine tanks. Bloody high maintainance and high risk system by todays standards.
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Re: Desert farm, Big pump, Batteries?

Postby Warpspeed » Sat May 20, 2017 12:20 pm

I'm still in the dark as to what volume has to be kept cool, what weight of dates has to be cooled from outside air temp to coolroom temp each day. Does it have to be at 4C?


Cool rooms are a particularly interesting problem from the thermodynamics point of view.
Its not just the "R" value of the insulation and area of insulation as you might at first think.

Placing warm objects into the cool room can introduce a lot of new heat that must be removed.
Amazingly ! living things such as fresh fruit and vegetables that are alive, do have a working metabolism that creates heat until the thing dies (goes putrid).

Every time you open the door or drive in a fork lift, there is an interchange of cold and warm air.
And so on it goes.

An HVAC engineer will be familiar with some rules of thumb, and he will look at your cool room and how it is used (and guess a lot) and come up with a pretty good estimate based on industry experience.

Its not really something you or I can look up in books or the internet and come up with a meaningful heat load.
You really need a professional that has been doing this kind of thing for a long time.

Once you can estimate your heat load, choosing the actual machinery to do it is pretty straightforward.
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Re: Desert farm, Big pump, Batteries?

Postby davidg » Sat May 20, 2017 7:23 pm

jaahn wrote:brine is a terrible corrosive material and should be approached carefully.

Yes it is, but using plastic it's an option.

Brine is only very slightly less efficient compared to plain water the big advantage to it is it will stay liquid at much lower temps compared to plain water, allowing for convection movement within a tank/tanks, it can still be frozen if you want to, it's really cheap to make, keeping brine strictly within insulated tanks and using a different liquid say glycerol/glycol/water mix to move the cold/heat from one place to another or make use of the stored cold, keeps the potentially corrosive medium from ruining other parts susceptible to salt within the storage tanks.

We have had "cheap" generated electricity for many decades however that era is slowly coming to an end. Unless we want to keep on with the "business as usual track" in which case we are going to have a very hard time later on this century. What may not have been so attractive in the not so distant past is likely to be a very viable option once again, converting electricity into cold directly when the sun is available has got to worth seriously considering, yes there's some technical challenges to overcome but it's not new by any means and as long as there's pretty cheap power to be had from solar it's really more of an engineering design issue to work through as far as I understand and as long as one has the space for the storage of a cold medium and cold store. You can even have the cold storage medium in the cool room as well. How it's stored and added in is in the design.

I've discussed this with someone I know who is an engineer in renewable energy, it's certainly appears realistic to do, once the design parameters are defined.

It should have an ROI that should make it worthwhile considering, particularly for off-grid use.
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Re: Desert farm, Big pump, Batteries?

Postby Warpspeed » Wed May 24, 2017 12:39 pm

Thinking about this a bit more.

When its 3 Am and all is quiet, just a free standing tank of ice/iced water inside the cool room may be all that is required.

During the heat of the day when there is a lot of cool room activity, something a lot more powerful may be required during peak thermal loading.

How about a tank of ice with a submerged cooling circuit hooked up to the refrigeration condensing unit to make ice, as already suggested.

Then have a separate brine or glycol cooling loop submerged in the same ice storage tank, and a finned heat exchanger with a fan to circulate cold air. This fan forced heat exchanger could be thermostatically controlled to add additional cooling capacity during periods of peak heat load.

Obviously the circulating medium (either brine or glycol) for the fan heat exchanger must never freeze. But I still believe pure water would be best for the actual thermal storage.
Going below 0C for the storage offers no significant extra thermal storage capacity, all it does is lower the COP of the refrigeration unit.

It may be difficult sometimes to maintain +4C air temperature with 0C storage. But adding a forced air heat exchanger to it should fix that.
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Re: Desert farm, Big pump, Batteries?

Postby australsolarier » Sat May 27, 2017 7:29 am

https://www.youtube.com/watch?v=P4bpS7Wuj2Y

watch this bloke, he does similar things you are talking about
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