Solar chilled drinking water sourced from thin air: modelling and simulation of a solar powered atmospheric water generator

Abstract

It is expected that there will be water scarcity in the future under the changing climate. Therefore investigations of innovative and environmentally friendly ways to produce portable water are very much essential. Atmospheric water generators (AWGs) apply vapour compression refrigeration to extract water vapour from the surrounding air. They produce drinking quality water and they require moist air and electricity. The required electricity may be produced by solar photovoltaic (PV) panels. By using solar energy the environmental impacts of an AWG associated with electricity consumption could be substantially reduced. The availability of solar energy and high humidity of air coincide with the drinking water demand. As of today the performance characteristics of stand-alone solar PV powered AWG is unknown. To address this knowledge gap a simulation model of such system has been developed. The model focuses on an individual phenomenon such as solar radiation availability, solar PV electricity output, battery storage, moisture content in the air and heat and mass transfer at the vapour compression refrigeration. The aim of this paper is to present the transient model developed and the simulation results. Kasaragod district in the state of Kerala, India is selected as the study area. Drinking water availability is a major issue in this district during the months from February to early June. Daily climate variables like temperature, maximum and minimum relative humidity, wind speed and global solar radiation on horizontal plane and wind speed were obtained from a meteorological station located in Kasaragod. A simulation model has been developed in TRNSYS with the following components: PV modules, Batteries, Regulator, Inverter and Atmospheric Water Generator. By using the model developed the capacity of the AWG, the number PV modules, the capacity of the battery bank required were determined for minimum daily water production of 15 L. The water extraction efficiency of the AWG investigated varies between 5.4% and 9.3% for Kasaragod. The cooling CCOPs of the AWG investigated are between 4 and 5, which is better than conventional air-to-air heat pump. Based on the analysis it was concluded that the monthly average daily efficiency is strongly correlated to the monthly average relative humidity of the ambient air. The monthly average daily efficiency is defined as average of all the daily average efficiency for a month. The validation of the simulation model with experiments to enable practical system design optimisations, and the development of control algorithms to obtain better performance by using the validated model are recommended for the future studies.