Conference Year

2016

Keywords

Thermally driven heat pumps, waste heat recovery, distillation, thermal compressor, desorption

Abstract

The use of small-scale thermally driven heat pumps allows for the design of innovative and efficient thermal systems. Recent developments have shown the feasibility of the implementation of ammonia-water absorption systems for small capacity applications (< 14 kW cooling capacity). The use of low-grade thermal energy in thermally driven systems of that scale has the potential to achieve significant reduction of high grade electrical or mechanical energy consumption. However, further research needs exist for the development of highly compact components that optimize the efficiency of the overall system. Analogous to the mechanical compressor in a vapor compression cycle, the thermal compressor in a vapor absorption cycle where driving energy input to the cycle is supplied. It has been shown that an optimal thermal compressor component configuration exists. The requirement of high refrigerant purity in the ammonia-water system favors the implementation of the diabatic distillation principle. It can significantly reduce exergy destruction in the thermal compressor and provides the opportunity to develop highly compact component designs. In this work, two new design solutions for the desorption stage of the thermal compressor are presented. These designs incorporate the diabatic distillation concept for a direct-gas-coupled as well as a coupling-fluid-driven thermal compressor. The designs operate in a liquid-vapor countercurrent flow configuration and utilize favorable temperature and concentration profiles of specific flow patterns. Therefore, the proposed designs were visually investigated using an air-water mixture to simulate the working fluids. Air-water flow experiments at adiabatic conditions were designed and conducted to simulate component operation. Proper vapor-liquid interaction (termed tray activation) in the purification stages was investigated for a wide range of flow rates to simulate variation within the component as well as part load operation. Countercurrent flow limitations exist at high flow rates, which can cause component flooding leading to detrimental system performance. These limitations were investigated and flooding curves were established. The effects of minor geometry adjustments on tray activation and flooding were studied. Surface tension effects were investigated by use of an ethanol-water mixture. High speed video data were used to obtain quantitative results. The total heat transfer area for single-phase and two-phase flow regions, as well as vapor-liquid interfacial area are quantified based on the flow visualization studies. Both designs were validated as effective solutions for the implementation of the diabatic distillation concept for small-capacity thermal compressors. Tray activation was achieved for part load operation, and resilience to countercurrent flow limitations could be shown. Consequently, these results provide specific and quantitative heat and mass transfer design guidelines for the desorption stage of the thermal compressor. Results from this investigation will guide the development of new prototype desorbers.

Share

COinS