"Waste heat recovery" means recycling the heat that comes from industrial smokestacks. According to the company Recycled Energy Development (RED) over at the WorldChanging blog, there are 47,500 smokestacks that produce waste heat above 500 degrees (F) and they could generate at least 50,000 megawatts of power. That's almost half of the energy produced by all the nuclear energy in the U.S.
Specifically, Illinois-based RED retrofits smokestacks with waste-heat recovery boilers, then the heat from the stack to makes steam that spins a turbine and makes electricity. RED's CEO Sean Casten even said that the U.S. could continue generating the same amount of electricity that it does now with half the fossil fuel by recycling the waste heat from factories and power plants.