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DIY Solar Powered Window Air Conditioner: Part 5, Phase Change Material Heat Battery
Peltier-based cooling systems need some help if they’re going to be effective. Two techniques I’m investigating are evaporative cooling (liquid to gas phase change), and freeze-melt (solid to liquid) cooling.
Ice is Nice
Whenever a material changes phase (solid -> liquid -> gas) it either absorbs or releases energy. One of the best materials is water requiring 334 J of energy to melt 1 g of ice at 0°C. A room that is roughly 13 feet x 13 feet x 9 feet is a little over 40 cubic meters. It would take about 34,000–40,000 Joules to lower the temperature in that space by 1 degree Celsius. That’s the same as going from 78.8F to 77F. This would mean, if you had a block of ice and a fan, you’d need to melt a pretty small piece of ice (maybe a half a cup / 100g). That seems great! But remember, rooms aren’t perfectly insulated. You have extra heat inputs from the sun in the windows, walls and surfaces outside, wind, etc. Also, you have to freeze that water over and over if you want to re-use it. Without handy access to a source of lower temperatures, it takes quite a bit of work to remove the water, refreeze it, and bring it back into the room — with that much work, you might as well just install an air conditioner.
Phase Change Materials
There are a LOT of other materials that freeze and melt at higher temperatures, some are even used for extremely high temps. One that I’ve been playing with is Sodium Sulfate Hydrate. This is a neutral non-toxic salt. Tuned with another salt, this stuff can freeze at 65F. It’s latent heat of fusion is around 250 J/g, slightly worse than water, but 65F is an ideal temperature for me to store the heat from my Peltier which loves the “hot” side to be below 70F. If I can keep the hot side cool, the cold side can really cool the room, extracting the heat and dumping it into our salt.
A Heat Battery
The salt then becomes a “heat battery”, storing the heat until later. If I were to make a full system, we’d then vent that heat outside at…
