Multiple Reverse Osmosis sub-units supplied by unsteady power sources for seawater desalination

Abstract

A major drawback of Reverse Osmosis (RO) desalination units is their poor performance at off-design conditions. On the other hand, they can operate with low specific energy consumption and produce high-quality fresh water when they are supplied with almost constant power, according to their design and nominal operating conditions. This aspect brings additional effort when there is the need to desalinate seawater or even brackish water using unsteady power sources, such as renewable energy systems powered by, for example, solar or wind energy. In order to overcome this issue, an alternative small-scale RO unit is investigated here, whose major advantage is that it can operate with almost constant specific energy consumption and produce high-quality fresh water with low Total Dissolved Solids (TDS) for a wide range of power input. Such unit is suitable to be combined with small-scale energy systems, which can have a very high variety on their power output, such as PV, wind turbines, or even solar Organic Rankine Cycle, for power production. The developed RO unit includes three identical sub-units, each placed on its own skid and with a fresh water capacity of 0.7 m3/h. An energy recovery unit of axial piston motor type is equipped at each sub-unit for decreasing its power consumption. These sub-units are switched on/off, according to the power availability, in order to operate them within a range, thus with high efficiency and acceptable fresh water quality. Such small-scale RO unit, capable of producing 2.1 m3/h of fresh water in total, is examined here, focusing on its operation, performance, and fresh water production. The main parameter adjusted is the power availability, ranging from almost zero up to the maximum value of 12 kW. The analysis first focuses on a single sub-unit, exploring its performance for variable power input, while afterwards all three sub-units are synthesized, in order to form the complete RO unit, concluding to the dependence of the fresh water production and specific energy consumption from the power supply. For the single RO sub-unit at low power input, the TDS level is very high, whereas the specific energy consumption is low, since the membrane pressure is much lower than the designed one and slightly above the osmotic pressure. For available power, more than around 1 kW, the TDS level holds acceptable values lower than 400–500 mg/l, which is achieved with the high pressurizing of the feed seawater, with specific energy consumption up to 4–5 kWh/m3. For power input higher than 3 kW, the second and third sub-units are successively switched on. In this case, the specific energy consumption is almost constant and equal to 4.5 kWh/m3, including the required power of the feed pump, while the produced fresh water TDS has small variations and is approximately equal to 200–250 mg/l.