Experimental and numerical investigation of a low temperature thermal pump for reverse osmosis desalination

This work focuses on the development of a low temperature (<100°C) thermal water pump and examines the potential for its integration with a reverse osmosis desalination membrane. A review of the available literature in the fields of desalination, thermal energy collection and thermally driven pumps shows that there has been limited research into the use of low temperature thermal energy to drive single phase desalination (e.g. reverse osmosis) systems. Additionally, development of low temperature thermal water pumping systems (for use in irrigations systems etc.) has met with limited success, achieving both low efficiencies, flow rates and delivery pressures.

The system examined here is based on a novel thermodynamic cycle called the TPP cycle which has been shown in theory to be capable of reaching efficiencies of approximately 40% of Carnot for low driving temperature differences. The proposed implementation of the cycle uses isopentane (R601a) as the working fluid.

The original theory describing the cycle is expanded to include considerations for the potential factors that might be introduced for a practical implementation of the cycle.

Three experimental designs are described and tested, and the results analysed. Experimentally, efficiencies of 0.4% for delivery pressures of 200kPa.g are obtained (heat source at 78°C). Specific energy consumptions between 251MJ/m3 and 643MJ/m3 for an average feedwater salinity of 1,146ppm and recovery ratio between 26% and 42% are achieved when combined with a reverse osmosis membrane.

A numerical model of the system based on one of the experimental designs is developed in MATLAB-Simulink and used to further investigate the potential of a practical TPP system. An improved design is proposed based on the simulated results. The predicted performance of this system suggests that an overall efficiency of approximately 2% could be achieved with an overall delivery flow rate of 109L/hr at 200kPa.g.

Two case studies using the predicted performance of the improved design are presented which show the performance of a single TPP and TPP-RO system over the course of a year in southern Australia. The studies show that a single TPP or TPP-RO module (with a stroke volume of 2.18L and boiler and condenser surface area of 0.007m^2 and 0.008m^2 respectively) could operate almost continuously for the entire year with a thermal storage tank of 2m^3 and three commercial solar thermal collectors (total area of 6.39m^2). The expected volume delivered for the TPP system is 428m^3/year while the expected specific energy consumption and product water produced are 44.5MJ/m^3 and 130m^3/year respectively based on an assumed feedwater salinity of 2,200ppm and a 30% recovery ratio.