A paper recently published in the journal Nature Communications discussed the prospects of using passive solar-driven evaporative technologies (PSETs) for water treatment.
Study: Critical aspects to enable viable solar-driven evaporative technologies for water treatment. Image Credit: Lukasz Pawel Szczepanski/Shutterstock.com
Technological advancements in the field of water treatment and desalination are necessary to ensure extensive and efficient exploitation of seawater to alleviate the issue of water scarcity.
Effective water treatment methods are necessary to separate freshwater from contaminants such as salt. The adoption of solar energy represents a sustainable option for desalination and other water treatment processes.
PSETs are considered a suitable option for water treatment, specifically in impoverished and isolated areas, owing to their low operating and capital costs. Recent studies on PSETs have introduced several new devices and materials that can promote greater environmental and economic sustainability in water treatment. However, several unresolved issues and challenges are hindering the extensive adoption of these technologies.
In this paper, the author identified three crucial aspects and associated issues concerning the use of PSETs that must be investigated by the scientific community to develop sustainable and technologically mature solutions.
Comparison of Productivity
The specific freshwater/vapor productivity per unit of time and unit of area is the most important parameter indicating the efficiency of PSETs. The use of optical concentration and the steam generated as output compared to the liquid water used as input in the process primarily affect the productivity of these technologies.
Operating conditions with more than one kW m−2 energy flux/optical concentration must be evaluated thoroughly due to the lack of significant technical and economic advantages of using passive solutions in these conditions.
Even in conditions with lower than one kW m−2 energy flux, an effective productivity comparison between different solar-driven evaporative systems is necessary through the standardization of operating conditions and a transparent definition of the figure of merit and associated normalization parameters.
Comparing the productivity of PSETs based on the data obtained by investigating certain solar-driven evaporative systems under laboratory conditions or in-sea or in-field characterization is important.
The wind direction and speed also have to be considered to accurately determine the heat convection losses.
To satisfy these testing requirements, clear and well-defined reference testing conditions must be identified with various conditions for different cases including in-lab, in-field, or in-sea. These cases must be properly defined by a specified number of well-regulated operating conditions.
The operating conditions include average energy flux, ambient air temperature, relative humidity, solar simulator class, lamp-evaporative and wall-evaporative surface view factors, wall temperature, average air/wind speed, input water turbidity and salinity, and total target output.
Additionally, an unsteady in-lab test must be performed where the imposed energy flux adheres to a specified time-dependent law to characterize and standardize the transient system behaviors.
In the future, studies on PSETs can deliver exceptional results by directly performing comprehensive experiments under the accurate reference conditions established by the scientific community.
Recently, three-dimensional (3D) interfacial evaporators with smart designs have gained popularity over traditional two-dimensional (2D) solar evaporators. In 3D evaporators, hydrophilic stalks with a greater aspect ratio are placed vertically, touching the water at the bottom, while the top and lateral surfaces enable communication with the air.
These 3D evaporators demonstrate an exceptional yield of over 30 kg m−2 h−1 and offer superior fouling control. However, a fair comparison of 3D and 2D evaporator performance is not realizable as 3D evaporators are driven by sunlight energy and chemical energy of non-saturated ambient air, while 2D evaporators only exploit sunlight energy.
Moreover, the cold water vapor generated by 3D evaporators at sub-ambient or near sub-ambient conditions in the first evaporative step creates challenges as a much colder surface is required in the subsequent condensation to absorb the latent heat.
Effective comparison of specific water productivity 3D and 2D evaporators can be performed when the final product of both systems is liquid water. Additionally, volumetric figures of merit must be introduced for 3D and 2D evaporative systems where the occupied volume in kg m−3 h−1 and water productivity per unit of time are also evaluated.
Overcoming the Limit of Single-Stage Distillation
Systems with one complete distillation cycle are limited by a 1.47 liter kWh−1 thermodynamic limit corresponding to 1.47 liters m−2 h−1 productivity figure when operating with one kW m−2 energy flux.
Systems that are functioning at such thermodynamic limits, even without optical concentration, also require large-area installations to fulfill the average drinking water requirements of one person.
To overcome this productivity limit, heat management in passive devices is critical. Additionally, more research is required on materials that can assist in reducing the water vaporization enthalpy significantly.
Although the decrease in water vaporization enthalpy has been hypothesized to elucidate the energy balance indirectly, such reduction has not been verified by direct measurements in metal-based materials or hydrogels.
Extensive calorimetric studies must be performed to determine whether water cluster evaporation/ vaporization enthalpy reduction can be driven by light mediation. Specifically, researchers must focus on measurements under the Clausius-Clapeyron relationship-based equilibrium conditions.
Recent studies have demonstrated that light can directly drive the water cluster evaporation, which indicates a photomolecular effect in which water molecule clusters are directly cleaved off from the liquid-vapor interfaces by photons. However, more research is required to improve the theoretical understanding of vaporization enthalpy reduction at the molecular level.
Robustness and Scalability
The successful upscaling of lab-scale materials or devices will determine whether PSETs can be extensively adopted for water treatment in the future. Currently, conclusive evidence on the technological robustness of PSETs in terms of aging and extensive cyclability is insufficient.
Recent techno-economic analysis has demonstrated that low maintenance over a long duration significantly influences the adoption of passive technologies. Advancements in material science can facilitate the development of durable and cost-effective materials with anti-clogging properties.
However, researchers must develop new and robust solutions that prevent organic matter fouling and salt accumulation in PSE systems during long-term operation in wastewater or seawater.
Nanomembrane-based nanofiltration approach can be combined with highly efficient passive flow manipulation exploiting natural convection and the Marangoni effect to improve salt rejection.
The structural and functional integrity of PSETs can be impacted due to different external factors, including pollution debris, birds, crashing waves, strong winds, sand, snow, and storms. Thus, mechanical tests, including tape-peeling, knife scratching, and sanding, must be performed on these PSE systems to ensure their structural integrity in external conditions.
Scalability is the other crucial factor in achieving true economic sustainability. PSETs must be inherently scalable to large installation areas. Few studies have demonstrated the scalability of passive technologies to more than 100 m2 surface area until now.
Thus, the scientific community must conduct more studies focusing on large-scale installations of PSETs where operational and capital expenditure figures can be achieved realistically. Researchers can perform computational and modeling studies for scaling predictions, as field tests on a large scale are not always feasible.
To summarize, PSETs have demonstrated several advantages for water treatment and desalination. However, more research on the challenges and issues identified in this paper can assist in developing breakthroughs in passive solar-driven technology.
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Chiavazzo, E. Critical aspects to enable viable solar-driven evaporative technologies for water treatment. Nature Communications 2022. https://www.nature.com/articles/s41467-022-33533-0.