NON-REGENERATIVE MULTI-STAGE AND REGENERATIVE MULTI-BED ADSORPTION CYCLES FOR LOW-TEMPERATURE WASTE HEAT RECOVERY

This study aims at clarifying the possible operating temperature ranges for silica gel-water adsorption refrigeration cycles, driven by near-ambient temperature waste heat sources (between 40 and 90 °C) with relatively small regenerating temperature lifts (10 to 60K). To exploit waste heat or renewable energy of temperature below 60 °C, staged-regeneration is necessary. A two-stage cycle, which can be operated effectively with 55 °C in combination with a 30 °C cooling source is introduced and compared with a conventional (single-stage) cycle. These cycles are evaluated in terms of cooling capacity, COP and their operational viability with near ambient temperature driving heat sources. By cycle simulation, it is found that the single-stage cycle yields better cooling capacity and COP in comparison with the two-stage cycle for ΔT_regen (heat source - heat sink temperature) higher than 35K. The advantage of two-stage cycle lies in its capacity to be operational with small ΔT_regen so that low grade waste heat sources can be exploited effectively. The use of multiple beds (more than two) will increase performance. To extract as much enthalpy as possible from waste heat sources of temperature above 65 °C, a multi-bed, single-stage regenerative adsorption chiller is introduced. Simulation results show that for the same waste heat source flow rate and inlet temperature, a single-stage, 4-bed chiller generates 70% more cooling capacity than a single-stage, 2-bed chiller. A 6-bed chiller in turn generates 40% more than a 4-bed chiller. Since the beds can be triggered into operation sequentially during start-up, the risk of ice formation in the evaporator is greatly reduced compared with that of a 2-bed chiller. Another significant advantage of multi-bed regenerative adsorption chiller is that it will minimize the chilled water temperature fluctuation so that a downstream temperature-smoothing device may be downsized or even eliminated in applications where tighter temperature control may be required. Depending on solar radiation or waste heat availability, suitable multi-stage chillers or single-stage, multi-bed chillers can be selected for optimal operation.