Internal energy and exergy recovery in high temperature application absorption heat transformers
Abstract In this paper, the design of a triple absorption heat transformer (TAHT) using the working fluids water and lithium bromide is dissected and reassembled using heat exchange network modelling in order to determine the optimum number and locations of internal heat exchange units within the system. It is found that the conventional design of the TAHT does not employ heat exchangers effectively, and that thus by rearranging these units system COP may be increased by 11.7% while exergy destruction within the system (its irreversibility) can be reduced by 21%. Strategically adding an extra one or two heat exchangers increases the COP by 16.4% and 18.8% while decreasing exergy destruction by 28% and 31.5% respectively compared to a conventional TAHT design.
Performance analysis of the single-stage absorption heat transformer using a new working pair composed of ionic liquid and water
Abstract The performance simulation of a single-stage absorption heat transformer using a new working pair composed of ionic liquids, 1-ethyl-3-methylimidazolium dimethylphosphate, and water (H2O + [EMIM][DMP]), was performed based on the thermodynamic properties of the new working pair and on the mass and energy balance for each component of the system. In order to evaluate the new working pair, the simulation results were compared with those of aqueous solution of lithium bromide (H2O + LiBr), Trifluoroethanol (TFE) + tetraethylenglycol dimethylether (E181). The results indicate that when generation, evaporation, condensing and absorption temperatures are 90 °C, 90 °C, 35 °C and 130 °C, the coefficients of performance of the single-stage absorption heat transformer using H2O + LiBr, H2O + [EMIM][DMP] and TFE + E181 as working pairs will reach 0.494, 0.481 and 0.458 respectively. And the corresponding exergy efficiency will reach 0.64, 0.62 and 0.59, respectively. Meanwhile the available heat outputs for per unit mass of refrigerant are 2466 kJ/kg, 2344 kJ/kg and 311 kJ/kg, respectively. The above excellent cycle performance together with the advantages of negligible vapor pressure, no crystallization and more weak corrosion tendency to iron-steel materials may make the new working pair better suited for the industrial absorption heat transformer.
Performance of a triple-pressure-level absorption cycle with R125-N,N′-dimethylethylurea
In the developed triple-pressure-level (TPL) single stage absorption cycle, a specially designed jet ejector was introduced at the absorber inlet. The device served two major functions: it facilitated pressure recovery and improved the mixing between the weak solution and the refrigerant vapour coming from the evaporator. These effects enhanced the absorption of the refrigerant vapour into the solution drops. To facilitate the design of the jet ejector for such absorption machines, a numerical model of simultaneous heat-and-mass transfers between the liquid and the gas phases in the ejector was developed. The refrigerant pentafluoroethane (R125) and the absorbent N,N'-dimethylethylurea (DMEU) were used as the working fluid. A computerized simulation program was used to perform a parametric study of the TPL absorption cycle. The influence of the jet ejector on the performance of the TPL absorption cycle was evaluated, and the performance of the TPL absorption cycle was compared with that of a double-pressure level (DPL) cycle. Four cases were studied that represent the improvements in the TPL absorption cycle performances as a result of the incorporation of the jet ejector. The four cases are: the ability to reduce the circulation ratio f, the ability to lower the evaporator temperature, the ability to lower the generator temperature and the ability to use higher-temperature cooling water.