Renewable energy powered membrane technology: A review of the reliability of photovoltaic-powered membrane system components for brackish water desalination
Introduction
The lack of potable water is one of the most serious problems in developing countries, especially in remote areas [1]. The desalination of brackish groundwater is often a practical solution for the provision of clean drinking water where insufficient surface water is available. Brackish water is typically defined as exhibiting a salinity between 1000 and 10,000 mg/L in total dissolved solids (TDS) [2]. Amongst the many available technologies, nanofiltration (NF) and reverse osmosis (RO) have gained a high level of acceptance due to their low specific energy consumption (SEC, units: kWh/m3) [3], which represents the energy input required to treat 1 m3 of water [4], [5]. In addition, since an electricity grid is often non-existent in remote areas, powering NF/RO systems with photovoltaics (PV) is a practical powerful way of overcoming this lack of infrastructure. Such photovoltaic-powered membrane filtration (PV-membrane) systems are attractive due to strong synergies between the two technologies [6]. Firstly, both PV panels and NF/RO membrane elements are modular and can be scaled to meet the desired clean water demand. Secondly, the demand for drinking water is higher in arid regions of the world and these same regions typically exhibit an excellent solar resource [7]. Thirdly, the prices for PV modules have been dropping steadily and reached US$0.21 per Watt-peak (Wp) by the end of 2018 [8], while the RO module market has been rapidly growing at a compound annual growth rate of 10.5% by 2019 [9], with costs decreasing accordingly. Indeed, a recent study concluded that the cost of clean drinking water from small-scale decentralized membrane systems can be less than local water vendors charged for untreated water in several locations in sub-Saharan Africa [6]. In addition, even within Europe, the cost of delivering drinking water via tanker to Greek and Italian islands has been reported to cost as much as €7–11/m3 [10], [11], while the cost of water in Qatar now exceeds that of diesel [12]. While this paper does not focus on cost analysis, it should be noted that certain geographic locations that have an abundance of sunshine represent obvious potential markets for PV-membrane technology and that price is not a barrier to the further deployment of the technology. The use of renewable energy (RE) provides a long-term sustainable power supply option that is cost-competitive with fossil fuels, while also offering important environmental benefits, such as reduced greenhouse gas emissions and preservation of the limited fossil-fuel resources [13]. The combination of RE and desalination technologies can meet the requirements for sustainable development and the water demands of society [14], in particular in developing countries.
For the purpose of this work, a “small-scale” PV-membrane system is defined as having a motor power rating less than 1.5 kW and a daily clean water production capacity of less than 3 m3. While small-scale PV-membrane systems have received significant attention in recent years and a considerable number of demonstration systems have been established [8], the total number of systems deployed worldwide remains relatively low [6]. Many different system design configurations exist that offer choice. This makes it difficult to gain trustworthy data regarding long-term performance as well as information regarding the robustness and reliability of such systems. Many small-scale PV-membrane systems are being targeted for deployment in remote areas of developing countries, where the technical skills required for operation and maintenance (O&M) are initially not present and a supply chain for spare parts is not in place [6], [15]. Several reports of renewable-energy-powered membrane filtration (RE-membrane) systems – driven by either PV and/or wind energy – have been published [10], [11], [16], [17]. To evaluate the reliability of such systems, different aspects have been studied, including the choice of system components, system degradation, O&M, as well as the impact of environmental conditions. Schäfer et al. investigated the effects of selected different membranes and operating parameters on PV-membrane system performance [18], noting that careful membrane selection (suited to water quality) leads to a robust system with a respectable SEC [17]. An analysis by Bilton et al. mentioned that the performance characteristics of RO-based PV-membrane systems degrade significantly with the deposition of retained materials on the membranes, which is typically defined as fouling or scaling due to the dissolved organic and inorganic salts in the raw water [19]. This remains a general issue for all membrane filtration systems, including RE-membrane systems. Moreover, environmental conditions such as high ambient temperature, humidity and dust play an important role in the performance of PV-membrane systems, especially for the PV panels. This indicates how the successful deployment of PV-membrane systems depends on, amongst other things, the reliability of the components and robust system design. However, to date, a systematic study on the reliability of PV-membrane systems is lacking. Filling this knowledge gap regarding the robustness and reliability of PV-membrane systems is paramount to ensure that long-term operation can be achieved and that poor system design does not tarnish the ability of a relatively “high-tech” water treatment solution being successful deployed in a developing country.
In order to fill this gap, this paper provides an extensive assessment of which PV-membrane system components allow operation over the widest possible power input range (relevant for a fluctuating energy source), and with the highest reliability. For the sake of clarity, the following terms are defined:
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Robustness: refers to the ability of tolerating perturbations that might affect the functionality of the system.
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Reliability: defined as the ability of a system (or a component within) to successfully perform the task for which it was designed, in a defined environment and over a specified time range.
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Lifetime: implies the expected lifetime of a product, or the acceptable period of use in service.
The main components of a typical PV-membrane system are illustrated in Fig. 1. The power from the PV modules is required to drive the pump, which sucks water through a micro- or ultra-filter (in this case) from the feed tank, and increases the feed water pressure to force it through the NF/RO membrane. A portion of water becomes permeate (clean) water, leaving behind the concentrate water to be used for washing or other purposes. The details of each component are discussed in later sections, with Fig. 2 providing the reader with a graphical overview of the paper’s structure. Recommendations by the authors are given at the end of each section.
Section snippets
Photovoltaic modules
Solar cells are made of semiconductors that generate direct-current (DC) electricity by electromagnetic means when exposed to sunlight. Many individual solar cells are connected in series and parallel within a PV module to build the output voltage and current, respectively. Within the PV module, the solar cells are sealed between layers of glass or transparent polymers to protect the electrical circuit from the environment. One or more modules are then connected and mounted on the supporting
Power conditioning
Power conditioning refers to devices that serve to deliver a desired voltage to an electrical load. Examples include: DC-DC converters; maximum power point tracking (MPPT) devices; inverters that enable alternating current (AC) loads to be supplied; as well as drive controllers for motors. In a PV-membrane system, the electric motor which drives the pump is the key load. Electric motors operate most effectively within a certain voltage and current range, however, the power output of a PV panel
Energy storage
RE resources are by nature both fluctuating and intermittent. In the case of a PV system, the amount of power generated can fluctuate due to passing clouds, while there are periods of intermittency – for example, at night – where the power output is zero. Traditionally, the resulting variations in power supply have been regarded as detrimental to membrane systems that were designed to operate under constant flow and pressure conditions [45], [46]. A lack of power can result in both reduced
Pump motors
There are three types of motors commonly used for RE-powered water pumping applications: (i) brushed permanent magnet DC motors; (ii) brushless permanent magnet DC motors; and (iii) AC motors. Normally, the choice of a motor for a RE-powered system depends on the size required and the availability of electronics that go with it. DC motors are attractive for low power demand (<5 kW) [87] and hence can be paired with PV modules directly. For higher power demand applications (>10 kW) AC motors are
Pressure-driven membrane processes
Membrane technologies have become very important tools in drinking water production, wastewater treatment and reclamation, and industrial process water treatment. The efficiency has been proven from a technical and economical, as well as an ecological point of view [113]. Pressure-driven membrane processes, such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) – listed in order of decreasing membrane pore size – are standard components in RE-membrane
Sensors
The location of sensors for measuring the instantaneous system performance was shown in Fig. 1 for an example PV-membrane system developed by Schäfer and Richards. For research and development purposes, the number of sensors is high but for systems implemented in commercialization, the number of sensors can be greatly reduced and most important selection criteria to be established individually. Based on previously published works [7], [50], [79], [212], [213], the selection criteria of sensors,
System performance
The desalination of brackish water to provide clean drinking water is an established and promising industry [235]. Details of selected outdoor small-scale (motor power rating <1.5 kW, permeate capacity <3 m3/d) PV-membrane systems for brackish water desalination (1000–10,000 mg/L) are summarized in Table 7.
In the PV-membrane systems, the SEC is one of the most important criteria to determine the feasibility of these systems, which indicates the required PV power to achieve the permeate water
Conclusions
PV-membrane technology is an effective solution for the supply of clean drinking water, especially for small-scale systems deployed in remote, off-grid areas. In this paper, a review of major components of small-scale PV-membrane systems for brackish water desalination including components selection, operating range and reliability is presented. It indicated that the small-scale PV-membrane system could achieve a lifetime of 20 years by selecting robust and reliable components, and a strong
Acknowledgements
BSR and AIS would like to acknowledge the financial support provided by the Helmholtz Association via (i) the Recruitment Initiative Funding; and (ii) the Science and Technology of Nanosystems (STN) program, while SYL and YHC would both like to thank the China Scholarship Council (CSC) for financial support of their PhD scholarships.
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