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In this video I'll be showing how to produce mild refrigeration with compounds extracted from urine. Human urine contains 1-2% urea (CO[NH2]2), which I'll be convert into Ammonia (NH3). This can be done either by thermal decomposition of solid urea after it's been distilled out of urine, or by reacting urine with sodium hydroxide (NaOH) and heating it. I opted for the second method, since it's less energy intensive. Videos showing the extraction of ammonia from urine often concentrate the urine by a factor of 10-20 before adding sodium hydroxide, but I found this method to be too time consuming and liable to overflow from excessive foam. Instead, I found it worked best to simply react un-concentrated urine with sodium hydroxide and heat it. Also, the smells produced from this approach are relatively mild. The only downside is that numerous batches of urine need to be processed.

Once Ammonia is obtained, there's several ways it can be used for refrigeration. Anhydrous ammonia can be used as the refrigerant in a vapor-compression system and has a temperature/pressure and phase behavior almost identical to propane. Aqueous ammonia can be used in a heat driven vapor-absorption system, and Ammonium Nitrate (NH4NO3) can be used as a "cold pack" by endothermic dissolution in water. In this video I'll be using the third method, because I wanted to investigate a heat-driven refrigeration system, and endothermic dissolving seemed like a simpler approach than vapor-absorption.

The Ammonium Nitrate is produced by reacting it with Nitric Acid (HNO3). I produced the nitric acid by the "Birkeland-Eyde" process, which uses a high voltage arc to ionize oxygen and nitrogen in the air, after which a certain percentage of those atoms recombine to form Nitric Oxide (NO) gas. The nitric acid quickly combines with more oxygen to form Nitrogen Dioxide (NO2), which is bubbled through distilled water. When Nitrogen Dioxide interacts with water, it forms Nitric Acid. This is a very slow and energy intensive process, but has the advantage that it allows the production of nitrates quite literally out of thin air.

I should note that in small quantities, Ammonium Nitrate is actually pretty safe to store and handle, and even if it's heated directly with a blow torch, it just melts and then decomposes into Nitrous Oxide (N2O) and water vapor.

To produce refrigeration, the ammonium nitrate is dissolved in water in a thin-walled aluminum can and placed into contact with a large water reservoir inside a foam container. The water reservoir acts as a thermal reservoir. Once the solution has absorbed as much heat as it can from the water reservoir, it's removed and then recharged by boiling off the water, cooling in a water bath, and then adding new water into the dry salt. In theory this works pretty well, but in reality i found that the boiler didn't work fast enough to keep the fridge temperature dropping, so really there need to be multiple cans on a rotation for this to work well.

By creating a saturated solution of aqueous ammonium nitrate, I was able to produce a temperature drop to near freezing (about 3.8C) in an aluminum bottle, but only reached a minimum temperature of 13.5C in the fridge. Pretty poor performance, but at least I was able to cool off a soda a little bit.

I also tested a few other substances to act as "instant cold" packs by mixing saturated solutions, and these are the temperature drops I recorded:

Sodium Chloride: -1.6 C
Potassium Nitrate: -11.1 C
Potassium Chloride: -13.1 C
Sodium Nitrate: -16.4 C
Ammonium Chloride: -17.0 C
Urea: -19.6 C
Ammonium Nitrate: -20.4C


Music Used:
Kevin MacLeod - Lobby Time
Kevin MacLeod - Hard Boiled
Kevin MacLeod - George Street Shuffle
Kevin MacLeod - Groove Groove