Performance Prediction of a Cascade Compression and Absorption Cycle

Adnan Hafiz, Quaiser Suhail

Mechanical Engineering Department, Aligarh Muslim University, Aligarh-202002, UP, India

Email: hafizadnan3@yahoo.in

Abstract:

This paper presents the study of a cascade compression and absorption cycle. Both individual vapour absorption cycles and vapour compression cycles have long been studied. As far as vapour compression cycles are concerned, they use CFCs, HCFCs and HFCs as refrigerants which cause global warming and ozone depletion. This problem is solved by vapour absorption chillers and heat pumps. They use water as refrigerant and Li-Br as absorbent. These are environmentally benign and thus provide solution posed by vapour compression chillers. Drawback with vapour absorption cycles is that they have low coefficient of performance (0.7 to 1.5) as compared to vapour compression cycles (about 3 to 4). Also they have high initial cost. The advantage is that their running cost is low since they use low grade heat for their operation whereas compression chillers use high grade energy (electricity) for their operation. Vapor absorption chillers can also be operated on solar energy and biomass which are available in plenty in our country for a major part of the year. This paper combines the advantages of both these systems and is used for generating low temperatures in the evaporator. The topper cycle is LiBr-H₂O absorption cycle and bottoming cycle is compression cycle using CO₂ as refrigerant. Thermodynamic analysis is carried out to study the performance of the system. Parametric study is carried out to predict the performance of the system. It involves varying  the  generator temperature, evaporator temperature, condenser temperature of the topper cycle  and evaporator temperature of the bottoming cycle. It is observed that an increase in the  generator temperature results in an improved performance of the system up to a certain extent  and then the performance decreases, thus showing optima.

Keywords: Vapour Absorption cycle, cascade, CO₂, Lithium-bromide, Coefficient of Performance.

References:

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2.   Yunus Cengel, M.A. Boles, Thermodynamics, Tata McGraw Hills, 2006.