Water scarcity is a global problem that will be of capital importance during the first half of this century, when seawater desalination will often be the only way to achieve sustainable development. Despite significant energy efficiency improvements during recent years, seawater desalination is still an intensive energy consumer; therefore, in the current instability of oil prices and environmental requirements, the sustainability of this technological solution inevitably passes through continued improvement of energy efficiency of the physical processes involved, as well as the use of renewable energy resources such as solar energy. In 2006, the “Enhanced Zero Discharge Seawater Desalination Using Hybrid Solar Technology” Project (AQUASOL) concluded with the erection of a complete solar desalination facility at the Plataforma Solar de Almeria (Spain) for the main purpose of developing an improved-cost, energy-efficient multi-effect distillation (MED) solar desalination technology. The system was designed to make the following three desalination operating modes feasible: (a) solar-only: the energy to the first distillation effect comes exclusively from thermal energy from the solar collector field, (b) fossil-only: a double-effect absorption heat pump powered by gas supplies all of the heat required by the distillation plant, and (c) hybrid: the energy comes from both the heat pump and the solar field. In this paper, solar-only mode system performance is presented and discussed. Optimum working conditions achieved in the solar-only mode were in the range of of MED first cell inlet temperature, which implies specific thermal energy consumption from around to and a performance ratio of 11.1–10.4, respectively.