The temperature differential between what the hottest chips can accept, about 70C, and the environment suggests this is a highly inefficient source of energy, due to Carnot limits.

I wish people would go for the big guns, fully renewable energy, nuclear, electric transport, green concrete and steel - the major issues instead of wasting time and money recovering 10% of what some coal power plant supplied to their data centre.

It's better than nothing but very close to nothing. It's a pervasive way of thinking in green engineering, every bit helps but what we need is BIG help: https://www.withouthotair.com/c19/page_114.shtml