## 1 Introduction  
### Water Scarcity  
The provision of potable water via brackish water desalination powered by solar energy is an attractive option for coping with the scarcity of natural freshwater resources in many regions worldwide. The International Energy Agency has reported that around 45% of the population of Sub-Saharan Africa lives without access to electricity, with this figure dropping to 26% in rural areas [1]. It is estimated by the United Nations that over 330 million people in Sub-Saharan Africa are still relying on unimproved drinking water sources (unprotected wells, springs and surface water) [2]. A direct correlation exists between the availability of electricity and drinking water, with the effect of energy poverty indicating that the population living with electricity is also very likely to have access to an improved water source (and vice versa) [3]. Thus, opportunities for decentralized technologies exist for the applications where little water and energy infrastructure exists and the population density is sparse. When examining desalination technologies, nanofiltration/reverse osmosis (NF/RO) membranes have gained the highest level of acceptance due to a modular design, easy scaling of capacity and their low specific energy consumption (SEC). While several emergingdesalination technologies--such as membrane distillation (MD), forward osmosis (FO), pervaporation and capacitive deionization (CDI)--are being developed, only bench-scale systems have been demonstrated [4]. MD technology is a thermal-driven process that operates at atmospheric pressure and high salt rejection for seawater desalination, yet it exhibits high energy consumption and low water production [5]. FO appears to be promising for desalinating extremely saline water--containing a total dissolved solids (TDS) of >100,000 mg/L--that cannot be treated with RO [5]. Pervaporation can cope with a wide range of saline water with high rejection (>99.99%), but the choices of membrane material and low flux remain as obstacles in its development [5]. CDI is an electrochemical desalination technology based on the electrosorption of ions by porous electrodes. Several challenges exist for the identification of optimum material for manufacturing electrodes [5]. Other electrically driven separation desalination processes--such as electro-deionization (EDI) and electrodialysis (ED)--transport charged ions in a solution by utilizing an electric field. EDI is an energy-efficient option for brackish water desalination. Whilst ED exhibits significant promise and is well-suited for treating lower salinity feedwater (<1700 uS/cm) [6], it remains much less developed than NF/RO membrane technology. Many remote regions of developing countries that possess a significant solar energy resource are also located far away from the coast, thus suggesting that the most energy-efficient treatment option would be the desalination of brackish groundwater. For such applications, pilot-scale NF/RO membrane technology has been widely used as an energy-efficient and robust option for the provision of clean drinking water [7; 8]. In order to make the desalination technology more sustainable, renewable energy sources are increasingly deployed for providing the energy requirements. When renewable energy-powered membrane (RE-membrane) systems are deployed in remote areas that lack an electricity grid, such decentralized technologies can provide an ideal solution. In particular, photovoltaic (PV) energy has become an affordable source of clean electricity due to steady price declines over the last decade [7] and is currently (2020 data) one of the cheapest sources of electricity [9]. When examining energy efficiency and cost estimations, many of the emerging technologies are based on bench-scale systems. In addition, the majority of these technologies are typified by having a high energy consumption. Recent investigations at the Platforma Solar de Almeria indicated that vacuum-assisted air gap MD technology could reduce the SEC [10; 11].