Small reverse osmosis units using PV systems for water purification in rural places

Large-scale reverse osmosis (RO) plants with productions of the order of several cubic meters per day are extensively used throughout the world for water desalination. These systems, using electricity for running the high-pressure pumps, are among the ones with the lowest cost per cubic meter of water produced. Nevertheless, in some cases, for instance small rural sites or during catastrophes where drinkable water is not available, small RO systems running on photovoltaic (PV) systems could also be used to obtain drinkable water for life support. These autonomous systems can be made using commercially available small RO compact units with typical daily productions of the order of 100–500 L and functioning with pressures as low as 5 bar. Running on PV modules from 50–100 Wp, these units can produce drinkable water from brackish waters containing salt concentrations of the order of 5000 ppm. This paper describes a unit of this type that has been assembled at the Renewable Energies Department of INETI and presents the results of the first laboratory tests. These first results are being used for tuning a mathematical model of the system based on the I–V characteristics of the PV modules. The model of the system will be used for predicting the yearly water production of the unit and obtaining a value for the cost of the water produced by this type of system.

This paper describes a small RO pilot unit powered by a PV system that has been assembled at the Renewable Energies Department of INETI, and presents the preliminary results of the laboratory and outdoor tests performed during the summer of 2000 in Lisbon. The commercial membrane type MP–TA50 spiral-wound cellulosic compound with dead-end filtration mode coupled with operation at low pressure (<5 bar) just allow for low power consumption, around 100 kJ/kg of permeate. However, these results must be confirmed by further operating conditions on a daily production basis to compare permeate quality and energy consumption vs. pressure. Other RO membranes will also be tested. Nevertheless, it can be con-cluded that for the small RO pilot the energy consumption decreases as feed water recovery and feed pressure increase. The results of these experiments will be used for tuning a mathematical model of the system based on the I–V characteristics of the PV modules. additional information could lead us to predict the yearly water production of the unit and obtain a value for the operation costs of the water produced by this type of system.