Waste heat occurs in almost all mechanical and thermal processes. Sources of waste heat include for example hot combustion gases discharged to the atmosphere, heated water released into environment, heated products exiting industrial processes, and heat transfer from hot equipment surfaces. As such, waste heat sources differ regarding the aggregate state (mainly fluid and gaseous), temperature range, and frequency of their occurrence.

The most significant amounts of waste heat are being lost in the industrial and energy generation processes. The exact amount of industrial waste heat is difficult to quantify, but various studies have estimated that as much as 20 to 50 percent of industrial energy consumption is ultimately discharged as waste heat and that between 18 and 30 percent of this waste heat could be utilized.