Energy behaviour and economic analysis of a photovoltaic-thermal (PV/T) collector coupled with a reverse osmosis (RO) desalination unit in Tunisian climatic conditions: a feasibility study

Water scarcity affects about one billion people in the world. Around two billion people could be living in water-stressed areas by 2050. For this reason, the desalination is always evolving due to the importance of the water resources found in the seas and brackish water. As these systems are generally energy intensive, the use of a renewable energy source is among the most appropriate solution. In this paper, both experimental and numerical investigations have been conducted to evaluate the performances and the economic viability of a photovoltaic-thermal collector intended to supply a reverse osmosis (RO) unit. Experimental study is based on the input–output and dynamic system testing (DST) according to ISO 9459-5 standard method and computations use the energy and mass balances of the PV/T collector and the RO plant. Results of DST testing showed that the loss coefficient of the PV/T, the tank loss coefficient and the total tank heat capacity are 10.46 W.m−2.K−1, 1.596 W.K−1 and 388 MJ.K−1, respectively. The ability to couple the RO technology to PV/T systems has been demonstrated. The complete system has been simulated for a water salinity of 10,000 ppm and climatic data of Borj-Cedria (Tunisia) site (longitude 10° 25′ 41″ E and latitude 36° 43′ 04″ N). Numerical investigations showed that the electricity needs of a small off-grid desalination unit could be met by using a 6.48 m2 PV/T panel surface area. In this case, the purified water produced has a salinity of 1500 ppm and the flow rate is 24,000 l/day. For a grid connected site, the produced and auxiliary powers are found to be equal to 54% and 21%, respectively. Moreover, the economic cost of adding a PV/T system into an existing RO unit has been evaluated and the results showed that the payback period is 6 years.