I imagine it's an absorption chiller. Here's a manually cycled one from the past:

https://en.wikipedia.org/wiki/Icyball

The video here shows a lot more about the fridge and a little about the cooler:

https://www.theglobalfight.org/innovation-reaching-last-mile...

I think it's the same general principle of how a propane-powered fridge in an RV works. It's much less efficient in terms of Wh or kJ consumed for a given amount of cooling than a modern high efficiency fridge of the same size running off 110-240VAC, but also does not require any form of grid power.

https://www.youtube.com/watch?v=cy1W-uWvR9U

As I understand it:

1. The whole cooler is heated, the pressure throughout rises, and the liquid water surrounding the inner chamber evaporates.

2. The water vapor moves into the (presumably much larger) other chambers, distributing itself equally.

3. The inner chamber is sealed while the others are opened to atmosphere. The device returns to normal pressure and that cooling drops the inner chamber pressure well below atmospheric.

4. Liquid water which has now re-condensed in the other chambers is allowed into the inner chamber in a controlled manner, evaporating due to the pressure and cooling the chamber.

To be clear, the inner chamber in my understanding would be a thin shell around the actual payload compartment. The vaccines or what have you obviously don't get wet and don't get exposed to low pressure.

I found these links that seem like similar tech. It's not clear to me if the Indigo is the same as the Isobar system.

https://www.theguardian.com/technology/2016/sep/08/einstein-...

https://en.wikipedia.org/wiki/Einstein_refrigerator

I suspect "water" is some other working fluid in the sealed system, ah maybe ammonia-water mentioned below:

https://www.jamesdysonaward.org/en-GB/2016/project/isobar/