The design and control of a thermal management system for a photovoltaic reverse osmosis system

Abstract

Reverse osmosis (RO) is a well-known process for desalinating seawater and brackish groundwater. Desalination is energy-intensive, so using photovoltaic (PV) panels to power the process is an attractive environmentally friendly concept, especially for community-scale systems. Increasing the system efficiency will lower the total cost of water produced, making the systems more economically competitive for a greater number of geographic locations. The thermal behaviors of PV panels and RO systems are complementary and can be exploited to improve photovoltaic reverse osmosis (PVRO) system efficiency. For a given level of solar radiation, a PV panel produces more electrical power at lower panel temperatures. For a given applied pressure, the flow of clean water across an RO membrane increases with increasing temperature. By using the RO feed water to cool the PV panels and warming the water in the process, more electrical power can be produced and higher flow rates of clean water across the RO membrane can be achieved, increasing total daily water production. The ability to cool the PV panels permits the use of low-cost, flat-plate concentrating mirrors, which further increase electrical power and clean water production. This thesis develops a thermal management system to improve the performance of a small-scale PVRO. Preliminary case studies show that the thermal exploitation concept is feasible and that a 50% increase in the total daily clean water production of a PVRO system is achievable, with an active thermal controller. A thermal controller is proposed that optimizes the PVRO system performance by minimizing the temperature of the solar panel and maximizing the temperature of the RO feed water. The control system uses a solar panel-mounted heat exchanger, circulator pump and servo valves to maximize water production while operating within the temperature limits of both the solar panel and the reverse osmosis membrane. Preliminary controller simulations show that it can successfully manage the temperatures of both the solar panel and RO feed water. The thermal management concept was experimentally validated on a small-scale, 300 L/day PVRO system. A 57% increase in clean water production was achieved using thermal management and solar concentrating mirrors, which agrees well with simulated performance predictions.