Design and test of a single effect thermal desalination plant using waste heat from m-CHP units

This work refers to an innovative integrated system for the simultaneous production of fresh water and electricity. In particular, a 1 kWe Stirling engine coupled with a thermal desalination plant has been considered for the purpose. The prototype, which refers to the distributed micro cogeneration field, has the final aim of building and testing a single effect distillation plant with a fresh water production of about 150 L/d. Firstly, thermodynamic theories and numerical analysis have been carried out to define the final prototype configuration. Then, an experimental test phase has been carried out to evaluate the actual plant performance. The experimental analysis has been in good agreement with the predicted results. In particular, at nominal operating conditions (@50 °C) the maximum heat transfer rate was higher than the evaporator heat exchanger designed condition (5.5 kWt). Despite the non-ideal plant thermal insulation, fresh water production reached a maximum of about 7 L/h at best operating conditions, proving a good process efficiency. According to the behavior predicted by the model, fresh water production is strongly dependent on the temperature difference between the heating fluid and the salt water in the evaporator tank while it is weakly influenced by the salt content of the treated water. Moreover, the apparatus exhibited a very good response to varying thermal power input thus confirming the opportunity to feed the desalination plant also with different forms of waste heat. More precisely, the plant average efficiency was about 1.3 L/kWh of energy input with minimum and maximum values equal to 1.16 and 1.42 L/kWh. Definitely the proposed solution, studied for a coupling with a 1 kWe Stirling engine, can be easily applied also to the other micro-CHP technologies.