Experimental Characterization of a Double-Effect Absorption Heat Pump-MED System Driven by Parabolic Trough Collectors

Abstract

This paper presents the experimental characterization of a double-effect absorption heat pump (DEAHP) using lithium bromide-water powered by a hybrid energy source solar/gas (by a gas boiler and a parabolic trough collector (PTC) solar field), which recovers the low-energy latent heat from the last effect of a multi-effect distillation (MED) unit. The experimental facility is located at the Plataforma Solar de Almeria and a test campaign has been executed with the purpose of studying the best operation strategies of the DEAHP-MED system that would optimize the use of the solar field and maximize the energy efficiency of the whole system. The impact of the variation of some influential parameters on the coefficient of performance (COP) of the heat pump, the MED performance ratio and on the total distillate production $(\dot {m}_{d})$ for two different coupling schemes (direct and indirect connection) has been analysed. Also, experimental characterization of a small-aperture PTC solar field (PolyTrough 1200 collectors supplied by NEP Solar Pty Ltd) has been performed at several solar field inlet temperatures in order to develop an annual simulation model in EES software that predicts the gas/solar ratio required to operate the whole system. The results revealed that in the indirect mode, the rise in the MED hot water inlet temperature) was the most influential in both parameters, total distillate production and COP, while in the direct mode, the regulation of the live steam mass flow) had the greatest impact on $\dot {m}_{d}$ the total distillate production. On the other hand, the results of DEAHP-MED plant driven by a hybrid source (solar/fuel) shows that the PTC solar field inlet temperature at $130{\circ} \mathrm {c}$ was the optimum point that allowed a maximum use of solar energy with 37.41% in the indirect mode, and of with 35.08% in the direct mode. In the case that thermal storage was considered, the annual gas ratio would decrease to about 14% 20% and 27% for a $t_{sto\mathrm {r}age}$ of 2 h, 4 h, and 6 h respectively.