Direct-drive ocean wave-powered batch reverse osmosis
Graphical abstract
Introduction
While two-thirds of the earth are covered by water [1], only 1% of surface water is suitable for domestic and industrial purposes, and far less can be used sustainably [2]. Presently, more than a quarter of the world's population lacks access to sufficient purification facilities [3], which will only be exacerbated with population growth, climate change, and increased agricultural needs [4]. According to the United Nations World Water Development Report (2021) [5], over 40% of people will face water scarcity by 2030. As Africa in particular faces surface and groundwater depletion, the 35 African countries bordering a seafront may look to desalination as a solution [6]; however, progress in this direction has been obstructed by a lack of financial and energy resources to power traditional desalination systems, which are not widely available in most of the continent [7].
The desalination market has grown in capacity by 20% between 2016 and 2020 [8], and it will continue to grow as population increases and freshwater sources are depleted. However, rising interest in desalination has drawn attention to concerns about its high energy requirements. With the detrimental impact of fossil fuels on the environment, clean renewable energy sources (RES) are desirable alternatives for powering desalination systems. In addition to adverse environmental effects, energy-intensity is also a financial burden. Energy consumption makes up the largest section of operational expenditures for water desalination, at approximately 36% of total operational expenditures for a typical plant [8]. Off-grid communities reliant on diesel generators to drive their desalination plants could pay anywhere from $3.00 to $8.00/m3 for fresh water [9]. There is a significant need for renewable-driven desalination [10].
Batch Reverse Osmosis Desalination.
The most common desalination process is reverse osmosis (RO), which accounts for 69% of the volume of water desalinated [11]. In traditional continuous RO (CRO), seawater traverses multiple RO membrane stages at a constant high-pressure and brine is discharged at the end of the process. The specific energy consumption (SEC) to drive a CRO process with standard conditions, seawater with 35 g/kg salinity and 50% recovery ratio (RR), ranges from 1.91 kWh/m3 to 4.0 kWh/m3 depending on the capacity of the plant [12]. Innovations may allow the SEC to approach closer to the thermodynamic limits for these conditions, 1.09 kWh/m3 [13]. In contrast to CRO, batch desalination processes like batch reverse osmosis (BRO) and closed-circuit reverse osmosis (CCRO) recirculate the brine while varying the applied pressure along with the osmotic pressure. These processes greatly reduce the energy requirement as compared to CRO and provide additional benefits like biocidal salinity cycling and high recovery capabilities [12], [14], [15], [16], [17], [18], [19]. Prior work has considered practical methods of achieving BRO with conventional components like pressure exchangers and piston-cylinders, which have been modeled to achieve an SEC of 1.88 kWh/m3, even at low capacities [12]. BRO has also been shown to be staged and operated as an osmotically assisted process, called batch counter-flow reverse osmosis (BCFRO), to handle higher salinities and recoveries [15]. Therefore, there is merit in considering how BRO may be integrated in new configurations to make additional gains in efficiency.
Several methods of driving RO with RES have been studied [20], [21], [22]. Photovoltaic (PV) solar desalination with battery energy storage is dominant in RES-powered desalination due to its cost-effectiveness and flexibility for large and small systems [23], [24]. However, PV-RO is constrained to its periodic and relatively low availability as well as the large land footprint required for PV to generate adequate energy. Wind energy is relatively inexpensive and has low environmental impact but is limited by a large land footprint and intermittent availability, much like solar energy [2]. Additionally, geothermal energy is highly stable and reliable, as it produces a consistent heat flux. It has low operational costs due to its independence of atmospheric and temporal patterns but is limited by its geological availability and the high capital expenses of geothermal power plants [25]. Recent efforts have been aimed to incorporate salinity gradient energy storage and energy production in dynamic reverse osmosis processes [17], [26], [27]. These systems have reliable, long-term energy storage but have some concerns regarding economic feasibility.
A readily available RES for seawater desalination is marine energy because of its proximity to the intake of seawater RO systems. Marine energy comprises ocean thermal energy and mechanical energy from waves and currents [8]. It is more stable than solar and wind energy because of its high energy density and consistency [28]. Marine energy also provides the opportunity for direct hydraulic power take-off, or conversion of energy from water to work-consuming and work-producing devices. This increases system efficiency by eliminating several energy conversion steps and reducing the cost of materials [8]. Additionally, the land footprint used by this RES is negligible. While promising, marine energy technologies have not yet been commercialized on a large scale [29]. Their market value is not yet competitive with solar and wind energy, as the levelized cost of water (LCOW) for wave-powered RO is higher than PV-RO and wind-powered RO. However, the market need is present. Remote island and coastal communities are often reliant on the high cost of imported diesel fuel and/or water to meet their needs. Power instability is another risk in remote regions, where less-resilient grids are vulnerable to interruptions during storms [8]. Consistently available and reliable marine energy mitigates these risks.
At present, 40% of the world population lives within 100 km of a coastline [30]. Harnessing the energy-dense and locally available resource of ocean waves to power seawater RO is a sensible solution for coastal water scarcity [31], [32]. When selecting a wave-powered RO system, the mechanical and cost efficiency of different wave-powered desalination systems can be used to evaluate their performances. A leading wave-powered desalination company, Resolute Marine, estimates an LCOW of 1.30/m3 for its Wave2O™ system which uses a surge converter WEC on the seafloor to pressurize water to drive RO onshore [33]. A pressure-exchanger energy recovery device is used to reduce the energy consumption of RO [9]. Another competitor, Wavepiston, uses a chain of moving plates near the surface to pump seawater through a pipe to an onshore RO system, for an estimated LCOW of €1.5/m3 or approximately $1.76/m3 according to an exchange rate of 1.17 USD/EUR [34]. In 2017, NREL researchers conducted a baseline study of WEC desalination farms and arrived at $1.82/m3 for a system that generates 3100 m3/day of water. The specific energy consumption (SEC) for this study was estimated as 2.8 kWh/m3 [35].
Fig. 1 illustrates the key differences between each system as compared to wave-powered batch reverse osmosis (WPBRO). All three systems use surge converter WECs to drive RO, but the power take-offs of each WEC are configured differently. The surge WECs in Resolute Marine and WPBRO are very similar and harness energy from linear and rotational motion at the seafloor, while the WEC used by Wavepiston captures energy from linear motion near the surface. Wavepiston directly sends the seawater pressurized by the linear motion to an accumulator, ERD, and finally RO through an adaptive hydraulic pressure developer integrated with the WEC [36]. Like most WEC-RO systems, Resolute Marine and Wavepiston use CRO with an energy recovery device (ERD) for desalination. WPBRO is the first direct integration of a WEC with BRO. For the couplings, Resolute Marine and WPBRO use the pressurized water from the WEC to drive a turbine to treat seawater drawn from a beach well, dispensing the WEC water back to the sea. The additional advantage of the WPBRO coupling is that flow control devices (FCDs) are implemented to improve dampening of the nonlinear energy profile from the WEC. The full WPBRO system is shown in Fig. 2.
Modeling results predict that the SEC of the WPBRO system is 2.4 kWh/m3 at the lowest predicted LCOW of $1.96/m3 for a scale of 2400 m3/day (Table 1). To be comparable with NREL's prior work [35], these results were determined for the sea state conditions representative of Humboldt Bay, California (Table S2). A sea state is defined in the model by wave height, peak wave period, and specification of either regular or irregular waves. Results were also determined for sea states in Greece and the British Virgin Islands, two potentially competitive markets for wave-powered desalination. The system-level second law efficiency is calculated for each process, by dividing the reported specific energy consumption by the minimum energy consumption. The minimum energy consumption, or least work, is calculated based on the reported feed salinity and recovery. By reducing energy consumption and complexity, WPBRO is promising for increased resiliency in coastal communities.
The hydraulic converter pressurizes the feed water, which is then directly used for BRO desalination, eliminating any need for further energy conversion with pumps and motors (hydraulic - mechanical - electrical) and thus reducing energy losses and increasing overall power available. BRO desalination allows brine solution to be recirculated back into the high-pressure piston tank with the circulation pump. Recirculation increases the salinity of the concentrate over time and recycles the energy stored in the high-pressure solution.
Section snippets
Methods
The proposed WPBRO system is an integration of wave energy with BRO which includes a BRO system, a coupling (power take-off, PTO) system, and a WEC. This system was modeled and validated in MATLAB and Simulink, building off prior modeling of BRO [12] and of a wave energy to electric power system [38] created by Sandia National Laboratories and NREL. The model was developed through a series of governing equations and necessary assumptions and implemented as a time-domain simulation of
Results
The MATLAB and Simulink model for WPBRO indicated similar physical trends to PTO-Sim [55] and modeling of BRO. Flow power through different components and the building of pressure over time in BRO were especially significant findings. Furthermore, implementing wave-powered BRO with generators instead of throttling valves (WPBRO-Gen), yielded lower SEC and LCOW values. The generators increased the power take-off efficiency of the coupling. Notably, recovery ratio per pass on the BRO side also
Conclusions
This work analyzes the first WPBRO system, which includes a novel hydraulic converter to couple wave energy with BRO without electricity generation and uses seawater as an environmentally friendly working fluid. Three main conclusions were determined in this study:
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Dynamic coupling of wave energy with BRO leads to competitive system designs that can handle various sea states to produce 1700–2400 m3/day
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Generators for FCDs allow system to have a SEC ranging from 2.30–2.39 kWh/m3 and a second law
CRediT authorship contribution statement
K. Brodersen: Conceptualization, Methodology, Investigation, Supervision, Validation, Visualization, Funding acquisition. E. Bywater: Conceptualization, Methodology, Investigation, Validation, Software, Visualization. A. Lanter: Conceptualization, Methodology, Investigation, Validation, Software, Visualization. H. Schennuman: Investigation, Validation, Software, Visualization. K. Furia: Investigation, Validation, Software, Visualization. M. Sheth: Investigation, Visualization. N. Kiefer:
Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
David Warsinger reports financial support was provided by National Renewable Energy Laboratory, for the MECC competition. David Warsinger reports financial support was provided by US Bureau of Reclamation for batch reverse osmosis.
Katherine Brodersen reports a relationship with Oneka Technologies that includes: employment.
David Warsinger has patent: Batch
Acknowledgements
The authors would like to thank Abhimanyu Das, Antonio Esquivel Puentes, and Sandra P. Cordoba for their assistance with hydraulic modeling and Dr. Matt Folley for his assistance with hydrodynamic modeling. The authors are grateful for the DOE and NREL's Marine Energy Collegiate Competition for providing the structure and funding (RFX-2021-10397) that guided this work, and for awarding the Purdue team first place in this Competition. The authors would like to thank the Bureau of Reclamation
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