Integrating desalination with concentrating solar thermal power: AÂ Namibian case study

This paper reports on a feasibility study into the integration of a multi-effect distillation plant with a central receiver plant to generate electricity for the Namibian grid, and fresh water for the community and mining operations at Arandis. Arandis receives on average 2528 kWh/m2/year of solar irradiation, is only 48 km from the coast and 580 m above sea level, making it attractive for a cogeneration plant. Desalination is energy intensive, but the required energy is freely available from the waste heat rejected at the condenser of a Rankine cycle. In this study, high level thermodynamic models of a multi-effect distillation and central receiver plant were developed to better understand the economics of such a cogeneration plant. Results indicate that a 100 MWe central receiver plant combined with a multi-effect distillation plant, is capable of servicing the current water demand in the region. Despite the high capital costs of central receiver plant, as well as pumping seawater inland, the plant is economically viable within the proposed tariff structure for renewable energy in Namibia, and existing water tariffs. Profit parity between a cogeneration plant and a stand-alone, dry-cooled central receiver plant is reached for top brine temperatures above 65 °C. Under these conditions, water sales would subsidize electricity production. However, it is not price competitive with a grid-powered reverse osmosis plant on the coast. The most significant barriers in making cogeneration plant competitive against more conventional desalination methods such as reverse osmosis are the high capital cost of the cogeneration plant, and pumping seawater inland.