Thermoeconomy, which is based on the Systems Theory and the Second Law of Thermodynamic, gives strategies for diagnoses and control of energetic systems (these are systems which consume great amounts of energy). In this paper a thermoeconomic analysis of a multi-stage flash and a multi-effect distillation plant coupled to a solar parabolic trough collector field have been made. The analysis was particularized to the climatic conditions of the Plataforma Solar de Almeria (PSA), a research centre in southeastern Spain, and could be generalized to coastal areas in southern Spain. Conclusions of this paper are useful for designing, for preliminary evaluation of distilled water cost, and for finding possible improvements in solar parabolic trough collector distillation plants. In addition, the importance of knowing in detail the climatic conditions (especially irradiant transients), for designing and for cost evaluation, is pointed out.