Abstract
The thermal vacuum distillation of seawater, preheated by a hybrid solar heat pump, is a candidate technology for autonomous and hot-climate sites, but experimental energy and exergy data for it are scarce. The objective of this study is to determine the energy and exergy characteristics of a laboratory vacuum distiller and to describe them with a semi-empirical model whose validity limits are stated explicitly. A 5 L batch distiller was tested at four temperature/pressure regimes (75 °C/0.4 atm, 85 °C/0.6 atm, 90 °C/0.7 atm and 100 °C/1 atm). The model combines empirical correlations for heating and maintenance energies with an evaporation fraction; this is calibrated on the four regimes and restricted to physically admissible conditions. The productivity increased from 125 to 334 g/h, and the specific energy consumption was 3.8–5.7 Wh/g. The vacuum regime had the highest specific energy consumption (5.7 Wh/g against 3.8–4.1 Wh/g at 85–100 °C), so no energy advantage of vacuum operation was demonstrated. The calibrated model reproduced measured productivity and total energy values within 2.7% and 4.5%, respectively; leave-one-out cross-validation gave errors up to 94%, so the predictive capability of the model is limited and the model should be regarded as a descriptive fit of the four measured regimes. The eight model parameters were fitted to twelve measured values, and no independent validation data were available. The effect of pressure cannot be separated from that of temperature, because the two variables increase together (correlation coefficient, 0.99). The exergy efficiency was 1.6–3.2%: 93.8–95.8% of the fuel exergy was destroyed by irreversibilities and 2.6–3.1% was lost with the residual hot water; 41–44% of the exergy destruction occurred in the heating stage. Relative to the minimum work of separation, the second-law efficiency was only 0.014–0.020%. The measured energy inputs exclude the heat pump and the photovoltaic–thermal panel; therefore, the benefit of renewable-energy integration is prospective and was not quantified. Scientific novelty: This study conducts a combined experimental, semi-empirical and exergy assessment of the thermal vacuum distillation of seawater preheated by a hybrid solar-powered heat pump, with explicitly defined model validity limits. Practical contribution: This study presents laboratory-scale reference values of the specific energy consumption and exergy efficiency of the model, and identifies the main loss sources for the design of an industrial prototype. The delivery of distilled water to remote areas by unmanned aerial vehicles is considered only as a prospective application and was not studied.