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Proceedings of Symposium on Energy Engineering in the 21<sup>st</sup> Century (SEE2000) Volume I-IV

ISSN:
1-56700-132-7 (Print)

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

Bidyut Baran Saha
International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan; Kyushu University Program for Leading Graduate School, Green Asia Education Center Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga-koen 6-1 Kasuga-shi, Fukuoka 816-8580, Japan; Mechanical Engineering Department, Kyushu University 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

Atsushi Akisawa
Graduate School of Bio-Applications and Systems Engineering (BASE), Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei-shi, Tokyo 184-8588, Japan

Takao Kashiwagi
Department of Mechanical System Engineering,Tokyo University of Agriculture and Technology, 2-24-16 Nakamachi, Koganei 184-8588, Japan

Kim Choon Ng
Department of Mechanical Engineering, National University of Singapore; Water Desalination and Reuse Center, 4700 King Abdullah, University of Science and Technology (KAUST), Twuwal 23955-6900

Hui Tong Chua
University of Western Australia, Department of Mechanical and Production Engineering National University of Singapore, 10 Kent Ridge Crescent, S (119260), Singapore

Abstract

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 ΔTregen (heat source - heat sink temperature) higher than 35K. The advantage of two-stage cycle lies in its capacity to be operational with small ΔTregen 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.