Main Technical and Economic Guidelines to Implement Wind/Solar-Powered Reverse-Osmosis Desalination Systems
2. Preliminary Sizing and Appropriate Range of the RO Nominal Capacity
2.2. Off-Grid PV-Driven Reverse-Osmosis Desalination Systems
2.3. Wind-Driven Reverse-Osmosis Desalination Systems
- The identification of recommendations for wind-driven desalination systems .
- The identification of the RO operation oscillations under variable frequencies of the isolated grid due to the low wind speed conditions: pressure (59–61 bar), water conductivity (900–925 µS/cm), and product flow (890–980 L/h; 89–98% of the nominal point) .
- Stability in power balance under low-wind conditions .
3. Appropriate Pre-Treatment
3.1. General Recommendations
- A direct raw water intake from the sea should be avoided in order to prevent the introduction of organic matter and suspended solids. The recommended solution for the medium and low scale is to dig a coastal well, thereby using the ground as a natural pre-filter.
- Chemical product requirements should be limited insofar as possible, in order to reduce the external dependence, as RE-driven autonomous systems are normally located in remote areas.
- We should select a medium–low RO recovery ratio in the design in order to reduce the pre-treatment requirements, selecting a value below the results from simulations: 75–80% for brackish water (the normal recovery is 85–90%), and 35–40% for seawater (the usual recovery is 43–45%).
3.2. Physical Pre-Treatment
3.3. Chemical Pre-Treatment
4. Brine Energy Recovery
5. Energy Storage Systems
6. Recommendations for Autonomous RE Desalination Plants in Remote Locations
6.1. Information to Be Gathered on Local Conditions
6.2. General Concept of the Design
- Indications regarding the O&M team include preliminary steps along the commissioning and first weeks of operation will require an active implication of external installers, including the comprehensive training of the local team who will progressively assume the O&M tasks. The simpler the operation and understanding of the system (being user friendly), the longer the successful operation the installation will have.
- Indications regarding the equipment include the inclusion of spare parts for the main components and the most common hydraulic and electric elements, as well as the use of tough and high-quality materials.
6.3. Operation and Maintenance
7. Wind-PV (Hybrid) Systems
- The low nominal capacity of the desalination plant and the associated economic scale factor, particularly in PV-driven RO installations.
- The additional investment for all of the components of the stand-alone generation system.
8.2. CAPEX Review
8.3. Calculation of the Desalinated Water Costs
- The size of the facility;
- The local wind/solar resources;
- The quality and salinity of the raw water;
- The factors associated with the location: transport, the proximity to a place with spare parts and a consumables supply, and the availability of skilled operators for corrective maintenance, among others).
8.4. Optimization of the Water Cost
9. Future Perspectives
9.1. Technical Keys
- The reduction of energy consumption: There is a progressive upgrading process in energy saving and its associated exceptional results (below 2 kWh/m3) using ultra-low-energy membrane elements, high-performance pumps, a salinity gradient, and very efficient energy recovery systems. The lower the energy demand, the greater the operation time for the same available energy; this progression will open an interesting path to optimize RE-powered RO desalination [61,63].
- The performance of RO operations: The latest tendencies in membrane technologies (intermediate stages, a higher size of modules, advances in nano- and ultra-filtration as pre-treatment options) could lead to a higher performance of RO operations, reducing the specific energy consumption space requirements and maintenance costs, and increasing the product water quality and the lifespan of membrane elements and installations;
- The latest advances in wind energy involve an increment in the unitary power, improvements in the output power control power, advance monitoring, and preventive maintenance.
- There are indications to improve the design, durability and quality of components and reduction of O&M costs by minimizing the time to repair and reinstall failed equipment, along with preventive maintenance schedules, particularly regarding inverters, a key component which is responsible for up to 36% of the energy loss and 43–70% of PV power plant service requests .
- The use of hybrid generation systems: The preferable option is mainly, but not only, based on PV and wind power. The incorporation of wind energy balances the lack of solar power during low solar radiation and nighttime periods, extending the available energy, and thus the operation time and associated water production [60,64,67,68].
- The latest advances in control systems involve the incorporation of genetic algorithms , the forecast of wind and solar resources, and the use of machine learning techniques [68,69,70,71,72,73,74] to predict the performance and to control the autonomous RO units, allowing more accurate and efficient operation.
- Integration into micro-grids: The simultaneous supply of water and electricity by RO plants coupled to hybrid micro-grids in isolated places that can both cover the demands and extend the penetration of RE sources . Furthermore, the inclusion of a RO unit, as a controllable load, in a micro-grid will contribute to a more stable supply.
9.2. Economic Keys
- The capex of RE technologies has been decreasing over the past few years; it is expected to reach a value of EUR 1 per installed watt (referred to the whole PV system) within a few years .
- There is a rising market and new commercial opportunities derived from the climate crisis, and indications from the IPCC to reduce greenhouse effect emissions.
- The volatility of oil prices, the increasing difficulty to discover new reserves, and uncertain future prices could lead to a crisis as soon as oil demand overcome the offer.
- The future water crisis derived from climate change, particularly relevant in countries with historical water shortage, will increase the demand for autonomous and RE-driven desalination systems.
- Very attractive water costs can be expected, considering the recent lowest water levelized tariffs in on-grid SWRO plants: about USD 0.3 per cubic meter (Hassyan plant) . This is possible thanks to the high capacity of the plant (more than 450,000 m3/d), the use of low-cost energy and the selection of the most efficient and reliable technologies.
9.3. Other Aspects
- Social component: There is a specific necessity for autonomous water supply in developing regions associated with cooperation projects, wherein the social issues must be specifically considered from the beginning of the project . The long-term success of the system will strongly depend on the following social aspects:
- The analysis of the social reality of the beneficiary community in order to adapt the project to the local conditions and incorporate their participation and commitment.
- The involvement of local authorities to give them a relevant role in the decisions and, ideally, to achieve a contribution to cover either part of the initial investment and/or the assumption of the O&M expenses.
- The appropriate selection of people to be trained to assume the management and maintenance tasks of the system.
- Environmental component: The integration of RE resources, such as wind and/or solar energy, in the operation of RO plants curbs (or at least, highly reduces) CO2 emissions associated with conventional on-grid RO plants. Nonetheless, brine discharge must be assessed and considered, particularly in inland locations; evaporation ponds, the blending with raw water for appropriate dilution and later disposal, or even the watering of crops adapted to high-salinity water, could be adopted as actions to curb or restrict the environmental impact of brine discharge .
- Political component: Decision makers, who are normally the key group amongst the stakeholders to start a project, suffer from an important lack of information on and knowledge of these technologies. Therefore, dissemination activities and introductory training courses are essential items for the creation of the necessary awareness. A complete vision of the barriers and suggested proposals is given in .
- The high importance of the appropriate identification of the local characteristics in order to carry out a tailor-made design and a correct O&M plan. This is particularly critical when the system is installed in developing countries.
- The suitable selection of high-quality materials and main components.
- The inclusion of an energy recovery unit for seawater, and an energy storage system.
- The use of a beach well as the feed water intake for SWRO is much more favorable.
- Wherever there is the simultaneous availability of relevant solar and wind resources, it makes sense to consider hybrid systems.
- The consideration of an integral concept of the project to include all of the elements associated with the cost.
- The latest advances in membranes and generation technologies have produced more efficient and economical products, leading to more competitive costs. The set of recommendations presented in this document may also be considered as a basic set of ideas to reduce water costs.
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
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|Country||Year||Feed Water||Permeate Production [m3/d]|
|Type of Feed Water & Associated Power Demand for a Nominal Flow of 1 m3/h||Energy Storage to Cover One Day Operation (8 h) Without Solar Energy||PV Field to Feed the Energy Storage (70% of Efficiency and 5 Peak Solar Hours)||Inverter Power (Internal Efficiency: 0.9)|
|Seawater (SW), 4 kW (3 kW for RO—with ERD—and 1 kW for the feed water pump)||667 Ah (4000 W × 8 h/48 V)||9.1 kWp (4 kW/0.7 × 8 h = 45.7 kWh; 45.7 kWh/5 h)||4.4 kW (4 kW/0.9)|
|Brackish Water (BW), 2–2.5 kW||334 Ah||4.5 kWp||2.2 kW|
|Power||Reference||Associated RO Capacity|
|24 kW||3 phase off-grid system ||5.7 m3/h (SW)|
11.36 m3/h (BW)
|Treatment||Objective||Possible Compounds||Recommended Dosing|
|Chlorination||Biocide to avoid fouling||NaClO or Ca(ClO)2 or Cl2O||Depending on the type of raw water and the physical pre-treatment|
|Acidification||pH adjustment to prevent scaling and enhance the biocide activity||H2SO4, HNO3, or HCl||The required for a pH of 5.5–6|
|Anti-scaling dosing||Avoid the solid deposits of low solubility salts (BaSO4, CaSO4, SrF2, among others)||Na hexametaphosphate, among many other compounds||Depending on the concentration of the low solubility salts and the recovery ratio|
|De-chlorination||Avoid damage to the membranes due to the presence of free chlorine||Either active carbon filter or sodium meta-bisulphite dosing||Dosing is regulated by measuring the residual free chlorine or REDOX|
|Flocculation||Increase the retention of particles in case of high levels of suspended solids||FeCl3 or Al2(SO4)3||Optimal dosing adjusted experimentally until reaching the appropriate turbidity|
|Manufacturer/Model||Brine Flow [m3/h]||Efficiency (%)||Reference||SEC * (kWh/m3)|
|Danfoss i-Save||7–52 (several models)||Up to 93–95||||n.a.|
|KSB/4 in 1 system||12.6||n.a.||||n.a.|
|Danfoss (APP-APM, Axial Piston Pump-Motor)||3.1||n.a.||||2.16|
|Clarck pump||24 L/h (product flow)||n.a.||||3.6|
|RO Kinetic||8 and 40||Close to 98||||2–2.5|
|Raw Water and Energy Data|
|Physical Properties||Chemical Properties||RE Resources||Water Demand|
|Geographic information||State of possible existent infrastructure of water & energy supply||Economic data||Social data|
|Indication||Possible Causes||Recommended Actions|
|1. Increase of the specific energy consumption||i. Loss of efficiency in pumps|
ii. Reduction in product flow
iii. Reduction in inlet pressure
iv. Increase in pressure drop through the membranes (fouling or scaling)
|i. Check the internal components of the pump (impeller, bearings, fan) to find friction points and/or overheating|
ii. See indication 3
iii. Check pressure drop in filters and clean or replace filtering material
iv. Chemical cleaning or replacement of affected modules
|2. Increase of the product water conductivity||v. Malfunction in membrane elements|
vi. Possible brine leakage inside the pressure vessels
|v. Replacement of damaged elements|
vi. Check the internal connections between elements for possible damage in O rings.
|3. Reduction of the product flow||vii. Insufficient feed flow|
viii. Insufficient inlet pressure to membranes
ix. Product flow to brine current
|vii. Check feed pump, pressure drop in filters, or level of raw water well|
viii. Check the high-pressure pump
|Synchronous machine coupled to the flywheel||Difficult starting of the stand-alone grid under low wind conditions.|
Insufficient power in specific moments (under a high decrease of wind speed).
Overheating and excessive friction losses.
|Inclusion of a variable speed starting-up motor to initiate the movement.|
Proposed replacement of the installed flywheel by a friction-less (vacuum operation) high-speed flywheel.
Selection of tough and efficient mechanical supports and bearings.
|Wind generator||Corrosion on the outside metal components.|
Failure in the blade motors due to the continuous regulation of the pitch angle to control the output power.
|Selection of specific high quality materials.|
Consider extra motors as spare parts. Improvements on the control software to minimize the stress on the blade motors.
|Feed water system||Variations in the feed water pressure due to the variable connection of the different RO units.||Extend the operation range of feed pressure to the RO units.|
|RO plants||Sudden reduction of operation pressure after stops.||Proposed installation of an automatic needle valve for appropriate pressure control.|
|Control system||Malfunction in control PC due to the simultaneous monitoring and control software in the same hardware.||Suggested use of specific control and monitoring software in different hardware.|
|PV field||Corrosion on metallic surfaces. |
Solar tracking systems were damaged due to wind load.
|Proposed use of other materials for structures.|
Installation of plastic walls as windbreakers.
|Batteries||Failure of module.||Replacement by a new module; meanwhile, operation without the damaged module was temporally implemented but at lower DC voltage (updating of control and converters setup was required).|
|Converters||High local temperature (50 °C) in Tunisia.||Construction of a partially buried building to use the soil as natural thermal isolation.|
|RO plants||High temperature of raw water (>35 °C) in Tunisia|
Reduction in the water quality and quantity in comparison with the nominal values after 8 years of operation
Water production is higher than demand.
|Installation of a feed water tank before the RO membranes to store feed water and slightly reduce the temperature.|
Proposed increase of the frequency of chemical cleanings.
Proposed use of water surplus by sending water to nearby communities, services or for watering. Another option would be to use part of the electricity to power other loads, such as lighting.
|Control system||Start/stop sequences of RO unit and batteries operation are not optimal.||New control software setup is recommended.|
|More power at all times, i.e., more energy per day.||Not all locations are appropriate, since simultaneous availability of solar and wind resources are required.|
|More operation time, i.e., more water production.||Two types of generation systems with very different variability in power production, maintenance requirements and operation performance.|
|Probable reduction in water cost, in comparison with a system based on only one RE source.||More complex control & monitoring systems to check the power balance and reach a stable operation.|
|Location||PV Power||Wind Power||RO Capacity||Batteries Storage||Reference|
|CRES facilities CRES (Lavrio, Greece)||3.96 kWp||900 W||130 L/h||1800 Ah/100 h|||
|ITC facilities ITC (Pozo Izquierdo, Gran Canaria, Spain)||600 Wp||890 W||154 L/h||868 Ah/100 h|||
|Component||Nominal Size||Specific Cost (€)||Reference Year||Location||Ref.|
|Flywheel and synchronous machine||100 kVA||224 €/kVA||1997||Gran Canaria Island, Spain|||
|Wind farm (only the wind generators)||2 × 230 kW||828 €/kW||1999||Gran Canaria Island, Spain|||
|RO plant SW (1 m3/h)||30 m3/d||1200 €/installed daily m3||2007||Gran Canaria Island, Spain|||
|Complete PV-RO system||50 m3/d||5230 €/installed daily m3||2007||Ksar Ghilène, (Tunisia)|||
|Off-grid inverter||4 kW||433 €/kW (nominal power)||2008||Sidi Ifni, (Morocco)|||
|Batteries & associated charge controllers||650 Ah||6.6 €/Ah||2008||Sidi Ifni, (Morocco)|||
|RO plant of brackish water (BW)||24 m3/d||1045 €/installed daily m3)||2008||Sidi Ifni, (Morocco)|||
|PV system (modules, batteries, converters, cabling, structure)||4 kWp||5 €/Wp||2008||Sidi Ifni, (Morocco)|||
|Complete PV-RO system||3 × 24 m3/d + 12 m3/d||4250 €/installed daily m3||2008||Four villages in Morocco (Provinces of Essaouira and Tiznit)|||
|Small wind generator||4–20 kW||2.3–6.2 €/W (nominal power)||2012||Public data prices|||
|Solar kit for off-grid supply||5 kWp||2.16 €/Wp||2018||Public data prices|||
|Maintenanceless gel batteries||41–200 Ah C100||1.6–2 €/Ah||2018||Public data prices|||
|Off-grid inverter||8 kW||404 €/kW (nominal power)||2018||Public data prices|||
|Investment Costs||Operation Costs||Incomes|
|Type of Plant||RE Power (kW)||Water Production||Total Water Cost||Place||Commissioning Year (Estimation)||Ref.|
|PV-SWRO||100||5 m3/h||6 €/m3||Lampedusa (Italy)||1990|||
|Wind-PV SWRO||0.6 (Wind) + 3.5 (PV)||125 L/h||7.53 €/m3||Maagan (Israel)||1999|||
|PV-BWRO||1.1||250 L/h||10.32 €/m3||Ceara, Brazil||2000|||
|PV-SWRO||4.8||400 L/h||9 €/m3||Pozo Izquierdo, Gran Canaria (Spain)||2000|||
|Wind-SWRO||2.5||500 L/h||1.78 €/m3||R&D test Loughboroug University (UK)||2003|||
|Wind-SWRO||15||800 L/h||3–5 €/m3||Pozo Izquierdo, Gran Canaria Island (Spain)||2004|||
|PV-BWRO||10.5||February: 3.3–8.3 m3/d August: 6.6–12.8 m3/d||3 €/m3||Ksar Ghilène (Tunisia)||2006|||
|PV-BWRO (4 units)||2.5–4||3–6 m3/d||5.45 €/m3 (average value)||4 villages in Morocco||2008|||
|PV-SWRO||2.4||3–5 m3/d||2 UK pounds/m3||Case study for Eritrea||n.a.|||
|PV-SWRO||243||250 m3/d||2.2–3.2 USD/m3||Theoretical study||n.a.|||
|Wind-SWRO||n.a.||250 m3/d||3–4 US$/m3 (case of 2000 full load hours/year)||Theoretical study||n.a.|||
|60Wind-SWRO||15||10.5 m3/d (anual average)||3.8 €/m3||Theoretical study||n.a.|||
|PV-Diesel SWRO||12 (Diesel) + 18 (PV)||10 m3/d||1.6–2.4 USD/m3||Theoretical study||n.a.|||
|Wind-PV SWRO||Several||Several||1–2 €/m3||Forecast study for 2030||n.a.|||
|Generation System||RO Plant||Energy Storage System|
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Subiela-Ortín, V.J.; Peñate-Suárez, B.; de la Fuente-Bencomo, J.A. Main Technical and Economic Guidelines to Implement Wind/Solar-Powered Reverse-Osmosis Desalination Systems. Processes 2022, 10, 653. https://doi.org/10.3390/pr10040653
Subiela-Ortín VJ, Peñate-Suárez B, de la Fuente-Bencomo JA. Main Technical and Economic Guidelines to Implement Wind/Solar-Powered Reverse-Osmosis Desalination Systems. Processes. 2022; 10(4):653. https://doi.org/10.3390/pr10040653Chicago/Turabian Style
Subiela-Ortín, Vicente J., Baltasar Peñate-Suárez, and Juan A. de la Fuente-Bencomo. 2022. "Main Technical and Economic Guidelines to Implement Wind/Solar-Powered Reverse-Osmosis Desalination Systems" Processes 10, no. 4: 653. https://doi.org/10.3390/pr10040653