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I'm trying to understand the benefit of specialized phase-change materials vs. water for absorbing heat in a home during the day. It's my understanding that selecting phase-change materials whose melting point is in the range of human comfort (~75°F) have at least an order of magnitude more storage capacity than water.

What physical characteristics of the medium are relevant to claims like this? If I look at the latent heat of fusion for the various materials, the numbers are the same order of magnitude for water vs. phase-change materials such as Climsel, which is used in the Swerod products.

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If the phase change occurs at the temperature of interest, then the system can give off a lot of heat without cooling down very much.

Thus, melting ice is a great way to maintain something at a temperature around 0°C, and melting paraffin-18-Carbons is good if you are trying to maintain temperature around 20 °C. With a melting point of 28°C, the material will give off 244 kJ per kg without its temperature changing. If you wanted to achieve the same effect with water, with a heat capacity of 4.2 kJ/kg/K, you would have to start with water at around 80C and end up with water at 20 C. That very large excursion of temperature would mean that the system needs a sophisticated insulation to prevent rapid (over)heating in the "fully charged" state of the system.

The phase transition stores a lot of energy without changing the temperature; that makes controlling the output of the device as the storage depletes much simpler.

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  • $\begingroup$ Does it work in both directions with respect to the freezing/melting point? I wasn't clear enough in my post; I believe physicists discuss energy with respect to heat so I tried to word the question that way. My goal is for the material to solidify at night by pre-cooling the house at night, and then for the material to absorb as much heat from the house as possible during daytime heat (thereby maintaining a lower daytime temperature.) I don't care what the internal temperature of the medium is, only about its rate/capacity to absorb heat in the 8-10 hours following the rise in temperature. $\endgroup$ – Jeff Axelrod May 27 '16 at 12:41
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    $\begingroup$ Yes it does. In the case of the application you envisage, it is even more important that the phase transition occurs between the "cooling" and "heating" temperatures; that is, you want it to get "colder than freezing" at night, and "warmer than melting" during the day. This requires phase transition between night and day temperature - so the amount of heat absorbed / emitted without the material changing temperature is as large as possible. Water's melting point is too low. $\endgroup$ – Floris May 27 '16 at 12:57
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    $\begingroup$ And you say "I don't care what the internal temperature is" - except it must be in that narrow range, or it cannot spontaneously give up or absorb the heat. If you decide to add a heat pump, the argument changes (as is done with some geothermal systems, for example). $\endgroup$ – Floris May 27 '16 at 12:58

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