Theoretical and experimental investigation of vapour absorption refrigeration VAR system for small scale applications

Abstract

Existing Technologies for the cold room application are mainly based on the electricity driven Vapour Compression Refrigeration (VCR) cycle, which leads to an increase in global energy consumption as well as greenhouse gases emission. Thus, exploration of novel techniques as proposed is much needed for the alternative of the VCR system. Small capacity absorption refrigeration systems, in spite of having several challenging issues such as the risk of crystallization, low COP through experiments and extensive volume handling of the absorber; could be an option if commercialization of the ARS at the farm level is implemented especially for small farmers. In the present work, the energy and exergy investigation of the absorption refrigeration system (ARS) in LiBr - H2O solution is modelled and analysed for each component. An optimization criterion was applied to the generator temperature in order to enhance energy and rational efficiency. Also, most exergy destruction components were identified. The results propose that generator and absorber are the essential components according to the design aspect. Likewise, it is highlighted that COP and rational efficiency are fluctuated regarding the component temperature of ARS. It is also found that the COP of heating is increased with increment in generator temperature while the circulation ratio showed the inverse pattern. Design methodology of a small scale single effect aqua LiBr (Lithium bromide) absorption refrigeration system with a capacity of 5.3 kW is presented. The system delivers cooling by using solar thermal or any other open heat source as input energy. The design of various components such as generator, condenser, absorber, evaporator and solution heat exchanger are stepwise analysed, formulated and discussed. Shell and tube heat exchanger is typically irreconcilable for small scale applications due to larger size and weight. In this context, a double-helical coil heat exchanger is deployed for the condenser and Solution heat exchanger (SHX). The absorber is designed using a vertical tube heat exchanger. The evaporator is intended to be a double pass flooded heat exchanger. Besides, the cross-section of the evaporator is set based on the hydrostatic pressure effect. In experiments, the generator temperature of the proposed unit is varied in the range of 80 to 100˚C and results indicates that the maximum COP of 0.35 is obtained at the generator temperature of 92˚C. Also, the viii Break-even point (BEP) of 3 years & 6 months is determined through a comparison with the conventional VCR chiller, which can lead to a unit as economically viable. Thermo-economic behaviour of nanoparticle seeded single effect LiBr-H2O absorption refrigeration system (ARS) is investigated for a small scale application. In the proposed method, alumina nanoparticle with a volume concentration of 3%, 5% and 7% are dispersed into the aqua lithium bromide solution. The multi-objective heat transfer search algorithm is employed to examine the design trade-off between the coefficient of performance (COP) and total annualized cost (TAC). To analyze the overall performance of the system, the influence of five design parameters, namely temperature of generator, absorber, evaporator, condenser and heat exchanger pipe diameter, are studied. It is found that with an increase in the COP, the TAC of the system is initially raised marginally and, after that raised rigorously with further increment. The comparative results indicate that COP and TAC of the nanofluid based ARS system are increased by about 7% and decreased by about 3.2%, respectively corresponding to the Pareto points of the base ARS system. The lower break-even point of about 2 years & 7 months is achieved for the ARS system containing a nanoparticle compared to the base ARS system. Overall, the ARS system containing 5% nanoparticles is the best solution from a thermodynamic and economic point of view. A case study of biomass powered ARS system having a capacity of 1 TR is also presented. The ARS uses different types of biomass (wheat straw, rice straw and soybean stalk) and produces a cooling effect at a temperature of 8˚C. The thermo-economic optimum generator temperature about 93.5˚C is obtained for the present ARS. In addition, the coefficient of performance (COP) is obtained about 0.68. The break-even point for the ARS is calculated about 10 years & 10 months using wheat straw biomass. Further, the break-even point and biomass consumption are found to be maximum for soybean stalk due to its higher cost and lower calorific value compared to wheat straw biomass. Keywords: Small Scale Absorption, Novel Generator Condenser, Double Helical coil Heat exchanger, Nanofluid based ARS, Biomass based ARS, HTS algorithm, Optimization.