Thermodynamic and thermoeconomic analysis of three cascade power plants coupled with RO desalination unit, driven by a salinity-gradient solar pond

Solar-based organic Rankine cycles (ORCs) for power supply of reverse osmosis (RO) desalination units are the most energy-efficient technologies for brackish water and seawater desalination within the small-to-medium electricity ranges (< 500kW). To collect and absorb solar energy, various collectors are devised, where among all, solar pond (SP) has been considered as a low-cost solar collector for a long working period. Meantime, conventional ORC/RO systems have a low condensing pressure and density, and consequently a turbine with large volume should be designed. The best solution is design of a desalination system powered by a cascade ORC or KC (Kalina cycle), using thermal heat of a SP for long-term applications. With this regard, three new cascade systems based on ORC and KC are proposed and thermodynamic and thermoeconomic analysis of the proposed systems are carried out. It is found that the ORC/ORC/RO system has the highest net power, power efficiency, and exergy efficiency as well as lowest SUCP (sum unit cost of product) and total exergy destruction. However, the total exergy rate of loss of the ORC/ORC/RO system was around 20% higher than that of the ORC/KC/RO system and approximately 10.81% lower than that of the KC/KC/RO system. Among all components, three components of condenser, cascade heat exchanger, and vapor generator had the highest exergy destruction in the ORC/KC/RO, ORC/ORC/RO, and KC/KC/RO systems, respectively. In addition, among all systems, the ORC/ORC/RO system had the lowest product cost rate and loss cost rate of 2185.83 $/h and 309.65 $/h, respectively. The results of case study simulation for the Bushehr Port indicated that the time between May and August can be the most appropriate time in a year in order to extract more thermal heat from the solar pond for the proposed power/ desalination systems. Furthermore, the most economical month for the proposed power/desalination was June due to the low value of SUCP (72.42 $/kWh) since more fresh water was produced in this month.