Thermodynamic and economic analysis of a combined plant for power and water production

This paper proposes a novel combined system, driven by industrial waste heat, to satisfy the simultaneous demand both for power and freshwater. The concept of organic Rankine cycle is applied to achieve the power generation, while a humidification dehumidification desalination unit is introduced to provide freshwater. For the purpose of efficient energy utilization, the internal energy of the discharged brine and preheated seawater is recovered. Based on the energetic and entropic analysis, the thermodynamic performance of the combined system is simulated, and the influence laws, mainly from the condensing temperature, terminal temperature difference of the recuperator and mass flow rate ratio of the feed seawater, are revealed. Finally, the economic viewpoint of the power and water combined system is also focused. The results show that maximum values of 13.1 kW for the net output power, and 208kgh⁻¹ for the water production can be acquired, when isobutane is applied as the working fluid. It is found that lower condensing temperature and terminal temperature difference of the recuperator indicate a higher water production, while the total efficiency can be further determined in combination with the energy input. It is also discovered that the mass flow rate ratio of the feed seawater is a critical parameter to influence the system performance, although the final effect will be restricted by the entropy generation rate of the system components. Through the comparison among the combined systems, the advantages of the current type are proved by the unit area and cost of production. With respect to the economic performance, a fixed investment for the entire combined system, 49934€, the cost of production with 0.0032€L⁻¹ for water and 0.063€kW⁻¹h⁻¹ for the electricity are obtained. It is also revealed that the production cost can be compressed with higher share of revenues, operation hours and design lifetime.