Development and testing of a renewable energy-based thermal desalination system

Abstract

In South Africa, membrane desalination has been the desalination method of choice in most cases to assist in attempts to alleviate water scarcity. In this study, an alternative evaporative desalination method was explored called the adsorption desalination technique. Evaporative desalination technologies are conventional distillers that are known to consume a significant amount of energy. The adsorption desalination technology emerged to address this issue of high energy consumption of the evaporative desalination technique. Adsorption desalination is an improved version that incorporates the adsorption-desorption refrigeration cycle which is positioned after the boiler for the purpose of capturing and rejecting the adsorbate, the working fluid, of this system. This emerging technology is recognised for its capability to utilise low heat energy to produce potable water. In this study, the adsorption desalination test rig was developed, constructed and tested using two locally supplied adsorbent materials: silica gel and zeolite. The test rig was at the Mechanical Engineering Department of the Cape Peninsula University of Technology, Bellville Campus, South Africa. After the test rig was constructed, the tests were conducted firstly using electrical energy followed by tests with a hydride of renewable and electrical energy. In performing electrically-driven experiments, a series of eight tests for silica gel were performed, each having varied experimental conditions. Then, the same series of tests were performed for zeolite. Additional to the first series of tests, a series of renewable energy-based tests were performed over 10 days; these tests were performed using the enhanced adsorbent sample comprised of silica gel, zeolite and copper shavings packed on the adsorbent bed. In this study, the desalination system was a hybrid energised system that was electric and with a solar-driven adsorption desalination technique. As South Africa is a country in the southern hemisphere, its winter stretches from June to the end of August. The first series of experiments were electrically powered. The second series of experiments on the test rig were solar powered, performed in July, a mid-winter month. The experimental output of the second series of experiments demonstrated that mid-winter climate conditions were capable of heating water to above 60oC. This water temperature was then used to trigger desorption of the previously captured water vapour onto the packed adsorbent sample within the test rig which produced more than 30 grams of water vapour when 200 grams of adsorbent material was tested. This was the highest regenerated adsorbate that occurred over two days when the solar irradiation was 415 W/m2 on day 3 and 410 W/m2 on day 10.