Zooming on the exergetic cost formation process of a Multiple-Effects-Evaporation (MEE) desalination plant

In this paper an advanced energetic and thermoeconomic analysis of a Multiple Effect Evaporation (MEE) process is proposed. Starting from the thermochemical fundamentals of desalination processes, a minimum theoretical work of separation is introduced, to be used as reference condition to assess the efficiency of desalination units. The attention is then focused on a forward feed MEE configuration; by abandoning the usual perspective oriented to analyse the system at an “overall plant” level, a zooming strategy is proposed to understand in depth the margins for systems’ improvement. Focusing on a single effect, different subprocesses and their mutual interactions are correctly recognized to plot a detailed thermoeconomic productive structure; also, notes on the relevance of the exergy content lost with brine disposal, on the possibility to recover it and on the best approaches to allocate this “residue cost” are given. With reference to the conclusions that could have been achieved by conventional energetic approaches, the proposed analysis provides to the designer further elements for plant optimization. The energy conversion processes taking place in the examined effect reveals very inefficient; improvements are to be sought in hybrid thermo-mechanical systems, which exploit the higher efficiency of Reverse Osmosis units.