How to Cool Your Car With Salt

Every year the summer heat beats down from above, and every year my car's AC happens to break just when I need it the most. However, this year I have a solution, salt.

Phase Change Materials or PCMs are materials that take large amounts of energy to melt, allowing energy to be absorbed in the phase transition. The specific PCM I will be using is a modified Glauber's salt formula developed by students at the University of Maryland that melts at 21c. This is a good temperature for a hot car and will readily re-freeze in a fridge.

Supplies

I was able to find all of the necessary materials at my local supermarket except for the sodium sulfate.

Materials

Sodium sulfate - https://www.amazon.com/Duda-Energy-5ssf-Granular-Anhydrous/dp/B005K4HU6S/
Table Salt
Xantham Gum
Gallon sized Ziplock bags
Large Bucket

Tools

Large Pot
Set of Measuring Cups
Heat source for the pot
Mixing device (I used a grill, but I would recommend a hotplate)

Do the Math

In order to figure out how much Glauber's Salt is needed we need to figure out how long we want it to cool the car. The National Renewable Energy Laboratory found that roughly 1KW of cooling is needed for a small sedan which means a 30 min trip requires around 1800KJ of cooling (1000*60*30).

Assuming a fridge can comfortably cool liquids to 5c and we want the cabin to remain below 30c, that gives us 25 degrees of temperature change and the energy to melt the Glauber's salt. Using the data from the research paper we find the solid phase specific heat to be 2.8 j/g and the latent heat of melting to be 170j/g so be combining these be get that 1 gram of Glauber's salt can cool 240j (2.8*25+170), which I independently verified with my own calorimeter setup. Dividing the amount of energy we need removed by the amount of energy 1 gram can remove shows us that we need roughly 7.5 kilograms of Glauber's salt.

In practice I found that the cooling was much more localized, and 8 kilograms was unnecessary, however, if you wanted to modify this setup to cool the entire cabin 8 kilograms would be necessary.

Measure the Materials

The recipe is inspired by NightHawkInLight who adapted the researcher's recipe for off the shelf ingredients. It consists of 5 cups of water, 1 cup of sodium sulfate, 1/4 a cup of table salt, and 4 teaspoons of xantham gum. I find this mixture makes a little over 1 kilogram of Glauber's salt.

Dissolve the Salts

Mix the sodium sulfate, table salt, and water all together heat, occasionally stirring to make sure they dissolve. There will be excess salt in the pot which is good. I used a grill for heat, but I found that it couldn't quite get the solution to boil very long, which is important for the next step, so I recommend a hot plate or stove top. You can do this step inside, just make sure to clean well afterwards.

Add the Thickener

Once the solution boils, add the xantham gum and let the solution boil for another 30 minutes. Be careful as the solution likes overboiling with the thickener, and happened to me even with my comedically large pot. The solution will turn into a jello like substance after it cools down, which is important as it prevents the Glauber's salt crystals from bunching together and falling out of solution. Since the grill I used could not heat my mixture properly this did happen to me. However the PCM still works, it just requires a bit of mixing before use to get the salt back into solution.

Freeze the Salt

After the Glauber's salt has cooled, put it in Ziplock bags. This makes it much more usable as cooling packs and helps it fit nicely into a fridge.

Cool Your Car

Now that the salt is frozen you can easily use it to cool yourself down in the car. I put my ziplock bags into a 5 gallon bucket with a fan and went on a 30 minute drive without AC and found that it cooled me quite nicely. I did find that placing the bags on my lap directly was far more effective than the fan and cooled me for much longer than I calculated. This localizes the cooling, so the salt only has to cool your body instead of the entire cabin. The cabin was pretty warm, so in the future I will find a way to improve the conductivity of the salt and the air, and maybe use PVA microencapsulation to improve performance and eliminate the need for a thickener. But in the meantime, this works as a cheap fix to a broken AC.

Sources

The research paper I used as well as the inspiration for the recipe and source on how much energy is required to cool a car down is linked below.