Date of Award

4-25-2018

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

First Advisor

Henshaw, Paul

Second Advisor

Ting, David

Keywords

advanced exergy, bubble pump, energy, exergy, residential air conditioning, vapor absorption refrigeration

Rights

CC-BY-NC-ND

Abstract

Large-scale heat-driven absorption cooling systems are currently available in the marketplace for industrial applications. The high temperature is required in the generator for driving this absorption chiller. For this reason, this type of chiller was originally designed to use direct-fired gas. However, the low efficiency of this cooling cycle restricts its use in small-scale applications. The concept of a solar-driven absorption chiller can satisfy the increasing demand for air conditioning without contributing greenhouse gases to the global environment. This research contributes to providing an efficient air conditioning driven by low temperature solar heat and independent of grid electricity, which may be useful in remote residential communities. The performance of 10 kW absorption and adsorption cooling systems were compared for the selection of a suitable cooling technology that can be driven by low temperature heat source such as a flat plate solar collector. Analysis revealed that under any operating conditions, the coefficient of performance (COP) of the absorption cooling system is higher. However, absorption chillers have a lower efficiency than traditional compression refrigeration systems, when used for small scale applications. Hence, energy and exergy analyses were conducted to evaluate the performance of a solar-driven air-cooled ammonia-water absorption chiller for residential air conditioning. Low driving temperature heat sources were optimized (70~80℃) and the efficiencies (COP=0.6, exergetic efficiency=32%) of the system were evaluated. The highest exergy losses were identified in the absorption process (63%) followed by the generator (13%) and the condenser (11%). In order to replace the only electrical component (pump) in an absorption chiller and make it independent of grid electricity, a solar-thermal-driven bubble pump was introduced in a vapor absorption refrigeration (VAR) cycle. This solar-thermal-driven pump can circulate the solution to the absorber and the generator to create the necessary refrigerant vapor for cooling. An analytical model of a bubble pump was developed and experimental work was conducted. Furthermore, a dimensional analysis was performed, considering bubble pump geometry and the solution properties. The bubble pump performance was defined in terms of non-dimensional parameters which can be used in all bubble-pump-driven absorption refrigeration systems. Experimental and theoretical results for a new refrigerant-absorbent solution (LiCl-H2O) were compared, and the flow regime (slug flow) was determined for the highest pump efficiency. Moreover, in order to employ the advantages of high performance, the bubble pump was incorporated into a simulation of a water-based vapor absorption refrigeration cycle. A new absorbent-refrigerant pair (LiCl-H2O) for a bubble-pump-operated VARS was proposed and a thermodynamic comparison was made between LiBr-H2O and LiCl-H2O systems. Finally, energy, exergy and advanced exergy analyses were performed on this proposed refrigeration cycle, and the exergy losses due to the internal irreversibilities of each component and the effect of the irreversibilities of the other components were determined. The avoidable exergy destruction was identified pertaining to the potential improvement of the overall system structure. The highest avoidable endogenous exergy losses occurred in the generator.

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