Evaluation and optimisation ofsolar water pumping systems for Lesotho

Abstract

Water and energy are the key drivers of sustainable development, yet the world is facing severe energy and water crisis. Photovoltaic water pumping system (PVWPS) is a mature technology that conserves both energy and water for sustainable applications. However the wider application of this technology is affected by improper system designs wider application of this technology is affected by improper system designs, high initial costs lack and of predictability . This study aims to evaluate critical factors for optimal sizing and performance prediction of PVWPS at the least cost of pumping. First objective of this study is to develop the meteorological parameters interpolated grid data base for Lesotho. Solar and ambient temperature data are recorded for 0.25 ×0.25 longitude and latitude interval for the range 27.00 East to 30.00 East and 28.00 South to 31.00 South. The range defines the extreme longitude and latitude boundaries of Lesotho. Grid data is interpolated and implemented into the computer program, hence meteorological parameters variations are automatically read at any point in Lesotho. Another objective is to develop a flow-power function, which comprehensively takes into account the instantaneous variation of ambient temperatures and solar irradiance and their effect on the pump system flow-rate and the system resistance. The flow-power function expresses the flow output of the solar pumping system as a function of the dynamic variation of the photovoltaic array power output, for a given pump and pipe parameters. The PVWPS components namely, the pump; solar photovoltaic array; pipeline system and the water storage are sized in an integrated fashion. The model is especially suitable for long pipelines where the PV array power required to deliver a demanded daily volume of water significantly decreases as the pumping main pipe diameter is increased. From the factory gate to site of installation the relative specific costs of PV array, pump and pipe differ from place to place. As a final objective an economical optimum combination of these sub system components, which meet the required daily demand of water at the least cost of pumping, is attained. Applying a time-step balance of the hourly pump flow output with the hourly water demand also enables a more precise estimation of the required balancing storage, by applying the mass-balance-curve approach. This study shows; how does the time step variation in meteorological parameters for a specified water requirement affect PVWP systems design and efficiencies; and how can the different pump-pipe combinations of PVWP systems be optimized from an integrated system perspective to arrive at the least cost of pumping. The applied method is technical accurate for sizing and also more economical thus proves to be a significant improvement to the traditional simplified approach of sizing solar pumping systems. It can result in significantly reduced unit cost of pumping. In the case study for Tosing, Lesotho (27.90 longitude 30.36 latitude) potable water demand of 350m3/ day. The design overall system efficiency is 7.1% the required PV array power was reduced by 25.8 % and the required water storage capacity reduced by 50% when compared to their respective values prescribed by the traditional sizing method.