Low-grade waste heat recovery for simultaneous chilled and hot water generation

An investigation of heat recovery from industrial processes with large exhaust gas flow rates, but at very low temperatures, was conducted. Heat recovered from a gas stream at 120 °C was supplied to an absorption cycle to simultaneously generate chilled water and hot water to be used for space conditioning and/or process heating. With the steep increase in energy costs faced by industry, it may be possible to use previously unviable techniques. At nominal conditions, 2.26 MW of heat recovered from the waste heat stream yields a chilled hydronic fluid stream at 7 °C with a cooling capacity of 1.28 MW. Simultaneously, a second hydronic fluid stream can be heated from 43 °C to 54 °C for a heating capacity of 3.57 MW. Based on the cost of electricity to generate this cooling without the waste heat recovery system, and the cost of natural gas for heating, savings of $186/hr of operation may be realized. When extrapolated to annual operation with a 75% capacity factor, savings of up to $1.2 million can be achieved. The system requires large components to enable heat exchange over very small temperature differences, with the largest component being the waste heat driven desorber. Minor increases in heat source temperature result in substantial reductions in heat exchanger size.