Simple, sustainable, water straight from the sun - batteryless electrodialysis desalination

Abstract

There is a need for reliable, low maintenance off-grid desalination for drinking water in resource-constrained regions. However, current off-grid desalination systems rely on large solar arrays and battery capacity for sufficient power and energy storage - such systems greatly increase the capital costs, operating costs, complexity and maintenance. Electrodialysis is a flexible technology with significant energy and water efficiency in comparison to other thermal and membrane processes and thus provides significant reduction in solar array capacity; however, it has not been exhibited off-grid without significant energy storage. This work proposes and validates a simple, robust, and maximal water production rate control scheme which enables batteryless off-grid desalination. The control scheme proposed involves cascade control with an outer PID loop tracking power and commanding flow rate, and a coupled inner model based control loop which always produces the maximum allowable current and thus, maximum desalination rate for the real-time power. This control scheme is applicable and adaptable to any continuous power system but can be most advantageous in direct-drive variable power situations, such as with solar panels. The controller is extremely simple, computationally efficient, and robust to implement: it relies on two sensors - a flow meter and a conductivity meter, one equation, and a PID controller. We demonstrate and conduct initial validation of this capability in a field pilot using direct-drive photovoltaic batch electrodialysis. We demonstrate a battery reduction of 99.4% from comparable prior art (20 kwh to 120 wh) on a 2 kwh system at a control speed of 100 milliseconds and utilization of 79% and 91% of total solar energy on two separate days of testing. This control scheme enables significant reduction and even elimination of batteries and is a step towards minimal-maintenance, high production off-grid desalination.