Very low percentages can be recovered from the waste heat under lab conditions so there is little hope it would working for mining rigs. It would not pay for itself see what Marto74 tried to do at BTCJam people were too smart to invest in that losers of a scam idea.

https://btcjam.com/listings/35145-mining-equipment-heat-to-power-recuperation-project

There are some numbers of people looking at any angle to screw people over. Be very careful of anyone associated with Marto74 like projects.