The 'water-energy nexus' is now receiving more attention as policy-and decision-makers grapple with measures to enable the transition to a sustainable, low-carbon economy. South Africa, in particular, finds itself in a polycrisis in terms of dealing with economic development, to alleviate poverty, within water and energy constraints. In the Western Cape Province of the country, desalination is suggested as one solution to the water shortage crisis, but the critical issue is that of energy supply, and the related cost implications, for water supply; concentrated solar thermal technology options are currently considered. In this paper a systems dynamics approach is used to assess the sustainability of these types of renewable energy technologies. The objectives of the paper are thus twofold. Firstly, the paper demonstrates the potential suitability of system dynamics modelling to inform policy-and decision-making in the developing country context. Secondly, the paper highlights the sustainability issues that must be addressed appropriately if concentrated solar thermal, and other renewable energy systems, are to be used in developing countries at the scale of desalination. Recommendations are made accordingly to improve the analysis, and its usefulness, to utilise this technique effectively in the future.