Comparative Assessment of Alternative Cycles for Waste Heat Recovery and Upgrade

Thermally activated systems based on sorption cycles, as well as mechanical systems based on vapor compression/expansion are assessed in this study for waste heat recovery applications. In particular, ammonia-water sorption cycles for cooling and mechanical work recovery, a heat transformer using lithium bromide-water as the working fluid pair to yield high temperature heat, and organic Rankine cycles using refrigerant R245fa for work recovery as well as versions coupled to a vapor compression cycle to yield cooling are analyzed with overall heat transfer conductances for heat exchangers that use similar approach temperatures for each cycle. Thermal-to-mechanical conversion efficiencies of ∼9%, upgraded heat delivered at 150°C, or cooling coefficients of performance of 0.5–0.7 are realized for source temperatures of 120°C, with a nominal 1 kW of heat extracted from the waste heat stream. Ambient sink temperatures of 35°C are used, as well as indoor return air temperatures of 27°C for cycles that produce cooling at 8°C. Comparative assessments of these cycles on the basis of efficiencies and system footprints will guide the selection of waste heat recovery and upgrade systems for different applications and waste heat availabilities. The increased implementation of such waste heat recovery systems in a variety of applications will lead to decreased primary source inputs and sustainable energy utilization.