A completely passive solar-powered desalination system developed by researchers at MIT and in China could provide more than 1.5 gallons of fresh drinking water per hour for every square meter of solar collecting area.
Such systems could potentially serve off-grid arid coastal areas to provide an efficient, low-cost water source.
The system uses multiple layers of flat solar evaporators and condensers, lined up in a vertical array and topped with transparent aerogel insulation.
It is described in a paper appearing today in the journal Energy and Environmental Science, authored by MIT doctoral students Lenan Zhang and Lin Zhao, postdoc Zhenyuan Xu, professor of mechanical engineering and department head Evelyn Wang, and eight others at MIT and at Shanghai Jiao Tong University in China.
The key to the system’s efficiency lies in the way it uses each of the multiple stages to desalinate the water.
At each stage, heat released by the previous stage is harnessed instead of wasted.
In this way, the team’s demonstration device can achieve an overall efficiency of 385 percent in converting the energy of sunlight into the energy of water evaporation.
The device is essentially a multilayer solar still, with a set of evaporating and condensing components like those used to distill liquor.
It uses flat panels to absorb heat and then transfer that heat to a layer of water so that it begins to evaporate. The vapor then condenses on the next panel.
That water gets collected, while the heat from the vapor condensation gets passed to the next layer.
Whenever vapor condenses on a surface, it releases heat; in typical condenser systems, that heat is simply lost to the environment.
But in this multilayer evaporator the released heat flows to the next evaporating layer, recycling the solar heat and boosting the overall efficiency.
“When you condense water, you release energy as heat,” Wang says.
“If you have more than one stage, you can take advantage of that heat.”
Adding more layers increases the conversion efficiency for producing potable water, but each layer also adds cost and bulk to the system.
The team settled on a 10-stage system for their proof-of-concept device, which was tested on an MIT building rooftop.
The system delivered pure water that exceeded city drinking water standards, at a rate of 5.78 liters per square meter (about 1.52 gallons per 11 square feet) of solar collecting area.
This is more than two times as much as the record amount previously produced by any such passive solar-powered desalination system, Wang says.
Theoretically, with more desalination stages and further optimization, such systems could reach overall efficiency levels as high as 700 or 800 percent, Zhang says.
Unlike some desalination systems, there is no accumulation of salt or concentrated brines to be disposed of.
In a free-floating configuration, any salt that accumulates during the day would simply be carried back out at night through the wicking material and back into the seawater, according to the researchers.
Their demonstration unit was built mostly from inexpensive, readily available materials such as a commercial black solar absorber and paper towels for a capillary wick to carry the water into contact with the solar absorber.
In most other attempts to make passive solar desalination systems, the solar absorber material and the wicking material have been a single component, which requires specialized and expensive materials, Wang says.
“We’ve been able to decouple these two.”
The most expensive component of the prototype is a layer of transparent aerogel used as an insulator at the top of the stack, but the team suggests other less expensive insulators could be used as an alternative.
(The aerogel itself is made from dirt-cheap silica but requires specialized drying equipment for its manufacture.)
Wang emphasizes that the team’s key contribution is a framework for understanding how to optimize such multistage passive systems, which they call thermally localized multistage desalination.
The formulas they developed could likely be applied to a variety of materials and device architectures, allowing for further optimization of systems based on different scales of operation or local conditions and materials.
One possible configuration would be floating panels on a body of saltwater such as an impoundment pond.
These could constantly and passively deliver fresh water through pipes to the shore, as long as the sun shines each day.
Other systems could be designed to serve a single household, perhaps using a flat panel on a large shallow tank of seawater that is pumped or carried in.
The team estimates that a system with a roughly 1-square-meter solar collecting area could meet the daily drinking water needs of one person. In production, they think a system built to serve the needs of a family might be built for around $100.
The researchers plan further experiments to continue to optimize the choice of materials and configurations, and to test the durability of the system under realistic conditions.
They also will work on translating the design of their lab-scale device into a something that would be suitable for use by consumers.
The hope is that it could ultimately play a role in alleviating water scarcity in parts of the developing world where reliable electricity is scarce but seawater and sunlight are abundant.
“This new approach is very significant,” says Ravi Prasher, an associate lab director at Lawrence Berkeley National Laboratory and adjunct professor of mechanical engineering at the University of California at Berkeley, who was not involved in this work.
of the challenges in solar still-based desalination has been low efficiency due to the loss of significant energy in condensation.
By efficiently harvesting the condensation energy, the overall solar to vapor efficiency is dramatically improved. … This increased efficiency will have an overall impact on reducing the cost of produced water.”
Freshwater scarcity has become a daunting challenge as the increase of ever‐growing population and economic development.1, 2Currently, there are still over 1.6 billion people living in water‐stressed areas without access to clean and safe drinking water, and such a situation will grow much worse in the coming decades.3
In spite of the earth with abundant water resource, the potable fresh water only accounts for around 2.5%,4 and its fraction has been remarkably declining due to the frequent droughts and severe water pollutions.
In order to address the issue of global water problem, considerable efforts have been devoted to developing various advanced materials and techniques for obtaining high‐quality freshwater from brines or even polluted water.
Up to date, most of the existing water purification plants have adopted reverse osmosis (RO) or low‐temperature multiple‐effect distillation (MED) technologies.5, 6, 7, 8 However, they are susceptible to formidable drawbacks such as high energy consumption (i.e., 5 and 8 kWh m−3for RO and MED, respectively) and the inevitable need of large centralized infrastructure, which greatly limit their practical applications, especially in the offshore areas, small villages, or remote off‐grid regions.
Therefore, it is highly desired to develop an appealing solution to overcome the trade‐off between energy consumption and water purification productivity.
From the practical perspective, the ideal next‐generation water purification technology requires to effectively solve the water–energy nexus and possess these traits as follows: low‐energy, low‐cost, easy‐to‐implement, small cubage and portability, scalable manufacturing system, and high productivity.
As a kind of abundant, clean, and renewable energy, solar energy has received much research interest as the promising alternative to conventional energy such as fossil fuels. Motivated by the demands of both fundamental researches and practical applications, much effort has been paid to harness solar energy for diverse ever‐evolving applications ranging from power generation,9, 10photocatalysis/catalysis,11, 12, 13, 14 solar cells,15, 16 and water purification14, 17, 18 to water desalination.19, 20, 21 Among them, photothermal effect has been widely utilized because of their superior ability to convert solar energy into thermal energy.
This unique self‐heating feature can not only enhance the intrinsic properties of materials, but also endow materials with some extraordinary functions.
Despite great promise offered by photothermal effect as a powerful tool to reduce energy consumption, photothermal materials usually struggle to low conversion efficiency and thus exhibit modest utilization efficiency of solar energy.
It has been well known that the power distribution of solar radiation on the earth surface is divided into three parts as follows: ≈7% for the ultraviolet region (300–400 nm), ≈43% for the visible region (400– 700 nm), and ≈50% for the near‐infrared region (700–2500 nm).22
Therefore, in aim to make the most of solar energy, an inevitable challenge is how to design and exploit strong and broadband solar absorbers, covering the full solar spectrum range from 300 to 2500 nm.
Thanks to the emergence of various state‐of‐the‐art photothermal materials in the past decade, great progress has been made in the design and development of solar‐driven water purification/harvesting technologies.
Given that most of the regions with high water shortage have abundant solar energy resources, these emerging photothermal applications show great potential to utilize solar energy to directly solve the issue of water shortage and improve the overall resilience of the water–energy nexus.
To date, two typical approaches have been studied to produce clean water under the aid of solar energy: a) extracting freshwater from brines or polluted water (i.e., desalination or removing contaminants) and b) collecting freshwater from air (i.e., even low humidity environment).23, 24 First, solar‐driven photothermal effect can be directly used or integrated into other water purification technologies.
For example, solar‐generated thermal energy has the capability of remarkably accelerating water evaporation, as well as acting as the hot source during conventional membrane distillation (MD) for seawater desalination.
Besides, photocatalysis and physical adsorption process are also enhanced by solar‐driven photothermal effect to fast remove contaminants within water (e.g., dye and crude oil). Second, some strong water‐sorption materials can be employed to harvest freshwater from air or humidity environment powered by solar energy.
The role of solar‐driven photothermal effect is to provide low‐grade heat for fast release of captured water within water‐sorption materials. This proof‐of‐concept technology shows great potential because the water content (i.e., vapor and droplets) of atmosphere accounts for about 10% of all other freshwater resources.
Among aforementioned all photothermal applications, the interface engineering, especially the interface between solid photothermal materials and aqueous solutions, is also very crucial for deciding photothermal properties and performance of everything from evaporators to sorbents and membranes.25
For example, the solar evaporator tends to need a hydrophilic interface for water supply, whereas both photothermal MD and crude oil cleanup technologies require a hydrophobic interface toward targeted aqueous solutions.
Thus, rational design of photothermal materials/devices shows a glimmer of hope to solve the issue of water shortage, especially in remote water‐ and energy‐stressed areas.
To date, there have been many reviews related to the photothermal materials and their potential applications.23, 26, 27, 28, 29 However, these previously published review articles are limited to the specific topic of solar steam generation, and thus there is still no comprehensive review focusing on the latest progress and future development trend of all the solar‐driven low‐energy water purification/harvesting technologies.
Given significant achievements made in these technologies beyond solar steam generation, especially photothermal‐assisted MD, crude oil cleanup, and water harvesting from air, it is thus time to systematically highlight the recent progress and future trend.
Here, we first summarize the fundamental mechanism of light‐to‐heat and various categories of typical photothermal materials such as plasmonic metals, semiconductors, and carbon‐based materials.
Then, we present the basic mechanism, design principle, and recent development of diversified solar‐driven water purification/harvesting protocols, including photothermal‐assisted water evaporation, photothermal‐assisted MD, photothermal‐assisted crude oil cleanup, photothermal‐enhanced photocatalysis for dye degradation, and photothermal‐assisted water harvesting from air (shown in Figure 1a).
Finally, conclusions and remaining challenges in this field will be discussed, hoping to provide a novel insight into harnessing solar energy to produce clean and safe drinking water, with great implications for fulfilling the blue dream of low‐energy water purification/harvesting through multidisciplinary research collaborations within solar‐driven photothermal effect and other advanced technologies.
Provided by Massachusetts Institute of Technology
