> Now if you can get a miniaturized mechanical heat pump that has higher efficiency (day a coefficient of performance of 2-4) and you need to last for much long

> Now if you can get a miniaturized mechanical heat pump that has higher efficiency (day a coefficient of performance of 2-4) and you need to last for much longer than an hour, I think you can significantly beat the weight of an ice pack... Adam Savage did a video where he and someone who appears ridiculously well-qualified built one of these https://www.youtube.com/watch?v=z_Ti4GP0ntE https://www.youtube.com/watch?v=z_Ti4GP0ntE