Cheap beads offer alternative solar-heating storage

Dec 02, 2011

A cheap material that can store heat energy collected from the sun during the day that can be released slowly over night has been developed by researchers in the India. The material based on paraffin wax and stearic acid is described in a forthcoming issue of the International Journal of Renewable Energy Technology and could help keep homes warm in sunny parts of the world that get very cold at night without burning wood or fossil fuels.

Mechanical engineer Meenakshi Reddy of Sri Venkateswara College of Engineering and Technology, in Chittoor, Andra Pradesh, and colleagues explain how certain materials, known as (PCM) can store a large amount of in the form of latent heat in a small volume. PCMs have a high heat of fusion and melt/freeze at a certain temperature. Heat is absorbed when the material melts and released when it freezes. Heated in the sun, the mixture of paraffin wax (which melts at about 37 Celsius) and stearic acid (a fat commonly used to make soap) becomes entirely liquid. However, as it solidifies it slowly releases the stored heat. The process is akin to the phase changing heating that occurs in hand-warmers that contain a PCM but in this case the material does not need to be boiled in a pan or heated in a microwave oven to absorb latent heat.

The team has now tested spherical capsules just 38 millimetres in diameter containing a blend of paraffin and stearic acid, which can be floated on the top of water in a tank. Stearic acid is a lot cheaper on the Indian market than paraffin and more readily available. The team found that costs could be held down without reducing the overall heating efficiency of the capsules by lowering the proportion of paraffin wax.

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More information: Int. J. Renewable Energy Technology, 2012, 3, 11-23

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User comments : 16

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JaySeattle
1 / 5 (3) Dec 02, 2011
Using phase-changing materials to store solar heat is an old idea, nothing new here.
dschlink
5 / 5 (1) Dec 02, 2011
But what is new is using fats and very small beads. Something like glaubers salts tends to separate after a few cycles, making it useless. The small bead size and the fact that they are made of a single material will probably make for a much longer lifecycle.
that_guy
not rated yet Dec 02, 2011
actually, forget india, this looks like it would be a useful addition to american solar water heaters.

They should design a system that works with a hot water pipe floor heating system as well.
Vendicar_Decarian
1.8 / 5 (5) Dec 02, 2011
The heat of fusion for Paraffin is 200 Joules per gram.

A 1 cubic meter block of paraffin will hold about 160,000,000 Joules of energy as latent heat. In comparison one cubic meter o f natural gas contains about 40,000,000 joules of energy.

So in my case, one cubic meter of paraffin would store enough energy as latent heat to provide just enough heat to heat my house for a full average winter day.

3 kilos of parafin seems to cost about 30 bucks. So a meter cubed block will cost approximately $24,000
CKeizer
not rated yet Dec 02, 2011
Vendicar, where did you find that price for parafin?

I see $2USD per pound at www.amazon.com/Wa...sbs_ac_1
and at http://www.candle...-wax.php

That is of course a retail price for a craft item.

Does this reduce your meter cubed block cost by roughly a factor of 10 or am I missing something?

Colin Keizer
CKeizer
not rated yet Dec 02, 2011
Looking at http://www.cajunc...wax.html

With 34 cases per pallet and roughly $100USD per case, that would be about $3400USD plus shipping per pallet. OK, that's not exactly a cubic meter, but close, so not really a factor of 10 improvement, but something like one quarter.

I hope an industrial source for less than pristine pure parafin would give a cost lower than $2USD per pound, but I have not found that, yet.

Of course, I don't need a pallet load to experiment at home.

Must remember that liquid parafin is about as spark friendly as vaporized gasoline . . . .

Colin Keizer
Newbeak
not rated yet Dec 02, 2011
I recall reading about eutectic salts years ago,and just googled it.Check out this site: http://www.allans...dex.html Sounds like a safer alternative to paraffin
Callippo
not rated yet Dec 02, 2011
Paraffin is expensive, flammable and of low thermal conductivity at solid state.
CKeizer
not rated yet Dec 02, 2011
Looking at http://www.soapgo...odPmzmIg

The stearic acid won't be cheap, either.

Colin Keizer
Newbeak
not rated yet Dec 02, 2011
I recall reading about eutectic salts years ago,and just googled it.Check out this site: http://www.allans...dex.html salts
Or this old Popular Science article:http://books.goog...eutectic salts solar&source=bl&ots=jjFmBESsFR&sig=Kv_L7oHmBzPW09NLfd0UKx1DOlw&hl=en&ei=U2nZTtvgD8iJ2AWht5C5Dg&sa=X&oi=book_result&ct=result&resnum=8&sqi=2&ved=0CGsQ6AEwBw#v=onepage&q=eutectic%20salts%20solar&f=false
Wonder why it hasn't been resurrected?
that_guy
5 / 5 (1) Dec 02, 2011
I think Vendicar was estimating the cost based on how much his candle making kits cost.

Also, He forgot that the amount of water in the system will still be be primary heat carrier.

For example, heating 250 gallons of water 40 degrees celsius will also provide 160 million joules of energy.
Vendicar_Decarian
3.4 / 5 (5) Dec 02, 2011
I was just considering the heat of fusion of the wax, not the heat content from max to min temperature.

As for the cost, ya, I did check an on line source, but it was for small quantities. Bulk is 1/10th the cost apparently and that is great because it reduces the price for a cubic meter of wax down to $3,000.

So realistically a system with a wax heat reservoir about half a cubic meter can store enough heat for my house at a cost of $1,500 for parafin.

I might actually do this.

Msean1941
5 / 5 (1) Dec 03, 2011
The $24,000 figure is correct. It's $1500 for the wax and $22,500 for your homeowner's insurance when that snooty gecko finds out you've got a vat of liquid paraffin in the basement.
Vendicar_Decarian
3 / 5 (2) Dec 03, 2011
Better to distribute it through the house with panels hanging on the walls or sitting on the floor.

If the freezing point is properly tuned, the wax will freeze and radiate it's heat of fusion at say 70'C, tending to keep the room at that temp.
Dug
not rated yet Dec 03, 2011
Did not anyone read the article regarding he necessity of encapsulating the paraffin mixture? This is what makes this system functional at all. Encapsulating paraffin in durable capsules isn't going to be cheap and it will produce some other related problems. Depending on system design - not the least of which is degradation of the capsule over time and the ultimate growth of algae and bacteria on the capsules - especially if the mixture leaks - which it will. The system will also require the use of open impeller (food pump) type recirculation pumps - not an inherently energy efficient pump design adding to the operating costs of the this system - reducing it's overall energy efficiency.

As someone pointed out eutectic salt solutions are more stable, not flammable, cheaper in the long term (never have to be replaced, are biocidal in general, and probably lend themselves to cooling tasks in the summer more so than oil based products.
unknownorgin
not rated yet Dec 06, 2011
The problem with phase change material is that above or below the critical phase change temperature they do not any more energy than water more or less so a vault of rocks under a building seems very cost effective and a simple blower and air ducts are the only equipment needed.

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