Novel solar desalination system for remote areas

As summer temperatures rise and water scarcity becomes a pressing concern, researchers at the Indian Institute of Science (IISc) have developed an innovative solution to increase the availability of clean drinking water. They have created a unique thermal desalination system that can operate using solar energy. This system aims to provide a sustainable and environmentally friendly method for converting saltwater into fresh, potable water. By harnessing solar energy, the researchers have developed a promising technology that has the potential to address water scarcity issues while reducing dependence on traditional energy sources.

The most common methods for desalination are membrane-based reverse osmosis and thermal desalination. However, both consume a lot of energy.

In thermal desalination systems, the process involves heating saltwater and subsequently condensing the resulting vapor to obtain freshwater. However, the energy needed for evaporation is typically obtained from electricity or the combustion of fossil fuels, which can have negative environmental impacts. An alternative approach is the use of solar stills, where solar energy is utilized to evaporate saltwater in large reservoirs, and the condensed vapor collected from a transparent roof. However, a challenge with this method is that during condensation, a thin layer of water forms on the roof, reducing the amount of solar energy that can penetrate the reservoir and thus impacting the system’s overall efficiency.

The researchers at the Indian Institute of Science (IISc) have introduced a new and improved design for a solar-powered desalination unit. This innovative system offers several advantages over traditional solar stills. It is more energy-efficient, cost-effective, and portable, making it particularly suitable for areas with limited or intermittent access to electricity. Susmita Dash, an Assistant Professor in the Department of Mechanical Engineering and the corresponding author of the study published in Desalination, explains that the new design addresses the challenges faced by conventional solar stills and presents a more convenient and effective solution for saltwater desalination using solar energy.

The setup, designed by Dash and her PhD student Nabajit Deka, comprises a reservoir of saline water, an evaporator, and a condenser enclosed within an insulating chamber to avoid heat losses to the ambient air.

The newly developed system utilizes solar thermal energy to evaporate a small amount of water that is wicked or absorbed into an evaporator with a specially textured surface. The process takes advantage of the capillary effect of microscale textures, which enables liquids to be drawn into narrow spaces within a porous material, similar to how a sponge absorbs water. By employing this approach, the system focuses on heating only a limited volume of liquid within the evaporator, rather than the entire reservoir. This targeted heating approach significantly enhances the energy efficiency of the system, resulting in improved overall performance. Susmita Dash explains that this advancement in design leads to a notable enhancement in energy efficiency compared to traditional methods of saltwater desalination using solar energy.

To optimize the wicking process, the research team created tiny grooves on the surface of the evaporator, which was constructed using aluminum. Experimentation was conducted to find the most suitable combination of groove dimensions, spacing, and surface roughness to achieve efficient wicking. Deka, a member of the team, explains that various patterns and configurations were tested to determine the optimal design for promoting effective and reliable liquid absorption into the evaporator. Through careful experimentation and analysis, the researchers were able to identify the groove pattern that facilitated efficient wicking, ensuring the system’s overall effectiveness.

In the solar desalination system developed by the researchers, the condenser plays a crucial role, although it is often overlooked in many desalination studies. To address the issue of water film formation during condensation, which is commonly observed in solar stills, Dash and Deka designed a condenser with alternating hydrophilic and superhydrophilic surfaces. The hydrophilic patterns on the condenser attract and collect water droplets during condensation, while the superhydrophilic region pulls the condensed water towards it. This behavior allows the hydrophilic surface to remain free for a fresh batch of condensate to form. Dash explains that this unique design ensures that the condensed water is effectively collected and does not hinder the condensation process, thereby enhancing the system’s overall performance and efficiency.

To address the issue of heat loss during condensation, the researchers incorporated a heat recovery mechanism into their system. They designed the system in a way that captures and utilizes the heat released during condensation. This captured heat is then directed to a separate evaporator located at the backside of the condenser, where it is used to heat up the imbibed saltwater. By recycling and utilizing this heat, the amount of solar energy required for the desalination process is reduced, leading to increased system efficiency.

Furthermore, the research team successfully connected multiple evaporator-condenser combinations in a series, creating a multi-stage solar desalination system. This configuration allows for further enhancement of the system’s performance. When considering a footprint area of 1 square meter, this multi-stage system has the capacity to produce one liter of potable water every 30 minutes. This output is at least twice as much as that produced by a traditional solar still of the same size. The combination of heat recovery and the multi-stage setup contributes to the system’s improved efficiency and water production capabilities.

Apart from seawater, the system can also work with groundwater containing dissolved salts as well as brackish water. It can be adjusted to align with the shifting positions of the sun during the day.

The researchers are currently working on scaling up the system and improving its durability, and increasing the volume of drinking water produced, so that it can be deployed for domestic and commercial use.