Development of a computational tool and dynamometer for optimizing variable-speed centrifugal pump selection for a containerized, direct-drive photovoltaic electrodialysis desalination system

Abstract

This thesis presents an optimized centrifugal pump selection methodology to improve the hydraulic efficiency of MIT’s Global Engineering and Research (GEAR) Center’s containerized, direct-drive photovoltaic electrodialysis desalination system capable of producing up to 300m3 of potable water per day. The novel flow-commanded current control scheme of this containerized desalination plant (CDP), which enables its minimal energy storage, also means that the centrifugal pumps used are operated at variable speeds to respond to the solar irradiance. Unfortunately, centrifugal pumps are typically designed for fixed operating conditions, and manufacturers often only report pump performance at their rated frequency. By estimating the hydraulic resistances of the CDP and testing potential pumps on a redesigned dynamometer, a MATLAB-based tool was developed to quickly and iteratively characterize pump performance at their expected operating points in the CDP. A "Compatibility Factor" metric, defined by the normalized area under a pump’s efficiency-flow curve at its operating points, was devised to quantify each pump’s efficiency across the entire operating range of flow rates achievable under the CDP’s system constraints. Using this methodology, two 7.5 kW pumps were selected per diluate and concentrate channels to the electrodialysis stacks for alternate operation use. Following testing pumps on a dynamometer, this work outlines a methodology for characterizing a pump’s variable-speed efficiency at its operating points in any modeled system. This approach facilitates informed pump selection for the CDP to increase its water production and reduce its specific energy consumption, with an estimated improvement in hydraulic efficiency from 10% in GEAR Center’s previous system to over 30%. Overall, this work is applicable to various photovoltaic pumping systems aiming to reduce carbon emissions through variable-speed operation.