Stabilizing temperature with phase change materials

[svc]

Anyone tried it.
http://www.pcpaustralia.com.au/pc_25.html

It seems like a perfect solution for anyone with big difference's between day and night temperatures.
The one i linked wants to stay at 25 deg C.
temperatures above 25 will be stabilized as the solid melts
temperatures below 25 will be stabilized as the liquid solidifies

I have no experience with this, but have seen it used in unpowered incubators for 3rd world countrys.
Any experiences or thoughts?

[arachdog]

Anyone tried it.
http://www.pcpaustralia.com.au/pc_25.html

It seems like a perfect solution for anyone with big difference's between day and night temperatures.
The one i linked wants to stay at 25 deg C.
temperatures above 25 will be stabilized as the solid melts
temperatures below 25 will be stabilized as the liquid solidifies

I have no experience with this, but have seen it used in unpowered incubators for 3rd world countrys.
Any experiences or thoughts?

Interesting idea. But you'd need a heat exchanger since the material is soluble in water, and building a heat exchanger that can handle the constant freezing and thawing of the phase change material would be extremely difficult. But the biggest killer is probably going to be cost. Exotic material like this are invariably expensive and you would no doubt need quite a lot of it. Using water to store the heat and a control system to regulate it would probably work out a lot cheaper.

[kellenw]

Perhaps I'm seeing this far too simply, but really isn't this just another from of thermal storage? Why is a special chemical or compound needed really? Again, I could be completely missing the point here, but water seems to be quite good at doing this same thing.

[arachdog]

Perhaps I'm seeing this far too simply, but really isn't this just another from of thermal storage? Why is a special chemical or compound needed really? Again, I could be completely missing the point here, but water seems to be quite good at doing this same thing.

No, not quite. Because it's storing energy by changing phase instead of changing temperature it could in principal maintain a constant 25 degrees without the need for any fancy controllers. In a similar manner that you can keep your drink close to 0 for extended periods with the addition of ice.

[kellenw]

But thermal storage IS energy storage, and the example of adding ice is merely an external input which defeats the entire idea.... right?

[arachdog]

The ice in the drink example wasn't supposed to illustrate an external input. More that there is energy going into the drink but the temperature doesn't change. Instead the ice is absorbing the energy by changing phase (from solid to liquid). Its called latent heat, there probably a wiki on it that explains it better than I can.

[Johnh]

It's mostly about the size of the thermal storage required,for this material if it stays liquid one kilo will absorb or release
2.75Kj of heat energy to change one degree.
While it is changing from liquid to solid or the reverse one kilo will absorb or release 150Kj of energy without changing temperature.
Water to Ice has a similar effect but its not very useful in a greenhouse at 0 degrees unless you use a heatpump.
When the size of the thermal storage needs to be 50 times smaller for the same energy the economics start looking pretty good

John

[ksnyder]

SVC, it seems like it could work very well, but may be impractical.

Firstly, the heat transfer between the FT water and the phase change material (PCM) would have to be very efficient. Also, I could be wrong here but based on my understanding it would require a similar amount or greater amount of the PCM to regulate the temperature of water, as the heat capacity of water is greater than the PCM you have linked. I have not performed the calculations, but at first glance I would guess a PCM having half the mass of the FT water would be saturated with heat when the water absorbs enough energy to raise it only a degree or so, and vice versa. I believe PCMs work a lot better when used with a medium that has a lower heat capacity, such as air, as opposed to water.

Considering water is almost free, adding a well insulated reservoir that is linked to the FT via heat exchanger only (2 separate water masses - fish water and reservoir water) that is continuously circulated by a separate pump is likely to be the most cost effective route at minimizing temperature swings. Plus, if there is a leak from the heat exchanger, it is only water, and not something likely to compromise the whole system.