This Solar Device Converts Seawater to Drinking Water
By Paul Brown
An international team of scientists has developed a cheap way to provide fresh water to thirsty communities by making seawater drinkable without using electricity.
So long as the sun is shining, they say, their device will produce enough high-quality potable water to cover a family's needs, at a cost of around $100.
The scientists, from Massachusetts institute of Technology (MIT), U.S. and Shanghai Jiao Tong University, China, believe their brainwave offers a simple solution to thirsty islands and arid coastal areas which lack a reliable electricity supply but have access to seawater. It could even help to prevent some of the mass migrations expected with climate change.
The researchers report their work in the journal Energy and Environmental Science. Testing their prototype on a roof at the Massachusetts Institute of Technology, they produced more than 1.5 gallons of fresh drinking water every hour for every square meter of solar collecting area.
Their device is cube-shaped, with multiple layers of solar evaporators and condensers piled one on top of another, surmounted with a layer of transparent insulation. Essentially it is a multi-layer solar still, similar to those used for centuries to make strong liquor and used today in many applications.
Diagram illustrates the basic structure of the proposed desalination system. Sunlight passes through a transparent insulating layer at left, to heat up a black heat-absorbing material, which transfers the heat to a layer of wicking material (shown in blue), where it evaporates and then condenses on a surface (gray) and then drips off to be collected as fresh, potable water. Images courtesy of the researchers
A solar still uses flat panels to absorb heat which it then transfers to a layer of water, which begins to evaporate. The vapor condenses on the next panel and the water is collected, while the heat from the vapor condensation is passed to the layer above.
Whenever vapor condenses on a surface, it releases heat; in typical condenser systems, that heat is simply lost to the environment. But in this multi-layer version the released heat flows to the next evaporating layer, recycling the solar heat and boosting overall efficiency.
The efficiency comes from using each of the multiple stages to remove salt from the sea water, with the heat released by the previous stage harnessed instead of wasted. In this way, the team's demonstration device achieved an overall efficiency of 385 percent in converting the energy of sunlight into evaporation.
Evelyn Wang, a co-author, said: "When you condense water, you release energy as heat. If you have more than one stage, you can take advantage of that heat."
Although adding more layers increases the conversion efficiency of the system, each layer also adds cost and bulk. The team settled on a 10-stage system for their proof-of-concept device.
It 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 twice as much as the record amount previously produced by any such passive solar-powered desalination system, Professor Wang says.
And a big advantage of the system is that it has a self-flushing mechanism which will clean out the accumulation of salt each night and return it to the sea.
One possible way of using the system would be with floating panels on a body of saltwater. The panels could deliver constant fresh water through pipes to the shore so long as the sun was shining. Other systems could be designed to serve a single household, perhaps using a flat panel on a large shallow tank of seawater.
The team estimates that a system with a roughly one-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 most expensive component of the prototype is the 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 instead. (The aerogel itself is made from very cheap silica but requires specialized drying equipment during its manufacture.)
"This new approach is very significant," says Professor Ravi Prasher of Lawrence Berkeley National Laboratory and the University of California at Berkeley, who was not involved in the research.
"One 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 vapour efficiency is dramatically improved … This increased efficiency will have an overall impact on reducing the cost of produced water."
Reposted with permission from Climate News Network.
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It is undisputed that vitamin D plays a role everywhere in the body and performs important functions. A severe vitamin D deficiency, which can occur at a level of 12 nanograms per milliliter of blood or less, leads to severe and painful bone deformations known as rickets in infants and young children and osteomalacia in adults. Unfortunately, this is where the scientific consensus ends.
Where Does the Deficiency Begin?<p>Nobody knows exactly how much vitamin D a person actually needs. The question of when a deficiency starts is correspondingly controversial. However, vitamin D is becoming increasingly popular.Not only is the pseudo-scientific literature on the "sun vitamin" experiencing an upswing, but the number of published studies has also increased enormously in recent years. For example, in 2019 <a href="https://academic.oup.com/edrv/article/40/4/1109/5126915" target="_blank">a study found that</a> Vitamin D is responsible for keeping the skeleton functional and is associated with cardiovascular diseases, type 2 diabetes and various types of cancer. <br></p>
An All-Rounder<p>Vitamin D levels in the body rise and fall according to sun exposure. If sufficient UV rays reach the skin, the body is able to produce the vitamin itself. However, the human body only derives an estimated 10 to 20 percent of its daily requirement from food.</p><p>The vitamin D that we synthesize from sunlight or food is not biologically active at first. Before the kidneys can produce the biologically active form of the vitamin, known as calcitriol, and release it into the blood, some metabolic processes must take place beforehand.</p><p>In addition, many organs have receptors to which the precursor of calcitriol binds. Further, this substance is also present in blood.</p><p>From this precursor, the organs then produce calcitriol themselves, which the body then uses for countless other processes in the body. This form of vitamin D thus regulates insulin secretion, inhibits tumor growth, and promotes the formation of red blood cells as well as the survival and activity of macrophages, which are important for the <a href="https://www.mdpi.com/2072-6643/5/7/2502/htm" target="_blank">immune system.</a></p>
Low Vitamin D, Severe COVID-19 Disease?<p>A research study carried out <a href="https://www.sciencedirect.com/science/article/pii/S2352364620300067?via%3Dihub" target="_blank">at the University of Hohenheim</a> has now established a link between vitamin D deficiency, certain previous diseases, and severe cases of COVID-19.</p><p>According to the study, "there is a lot of evidence that several non-communicable diseases (high blood pressure, diabetes, cardiovascular diseases, metabolic syndrome) are associated with low vitamin D plasma levels. These comorbidities, together with the often accompanying vitamin D deficiency, increase the risk of severe COVID-19 events."</p><p>"This statement is completely correct," said Martin Fassnacht, head of endocrinology at the University Hospital of Würzburg. However, he qualifies that it is a pure association, "i.e. a mere observation that these events occur together.</p><p>Dr. Fassnacht is very critical of the hype surrounding vitamin D, but not because he denies the vitamin serves important functions. However, studies on humans have not been able to show that vitamin D has the healing powers many often propagate.</p><p>Fassnacht says, "If you take a closer look, the hopes that the administration of vitamin D has a healing effect have not been confirmed so far."</p>
Association Versus Intervention Studies<p>Many studies on the vitamin are association or observational studies. "By definition, these studies cannot prove the causal relationship, but only point to mere correlations," said Fassnacht. The physician tries to illustrate this with an example:</p><p>"Imagine two groups of 80-year-olds. One group is spry, active and does sports. If you compare them with another group living in nursing homes, the difference in vitamin D levels will be dramatic. Life expectancy would also be extremely different."</p><p>But to try to explain the difference in fitness by vitamin D status alone is far too simplistic. "Vitamin D levels are a good measure of how sick someone is. But not more," says Fassnacht. </p><p>According to Fassnacht, none of the intervention studies carried out to date -- that specifically examined the effect of vitamin D on various diseases -- has been able to confirm the previous association and laboratory studies or the presumed positive effect of vitamin D.</p>
Further Research Is Needed<p>"If a coronavirus infection is suspected, it is therefore absolutely necessary to check the vitamin D status and quickly correct any possible deficit," said the recommendation of the paper published by the University of Hohenheim.</p><p>"Studies are underway to see whether vitamin D helps in COVID-19 infection, but I personally do not believe that this is really the case," says endocrinologist Fassnacht. Nevertheless, he says it is of course useful to carry out these studies.<br></p><p>"I don't want to rule out that there are actually subgroups of people who benefit from an additional vitamin D dose," he says. After all, this has been proven to be the case with a severe deficit.</p><p>In view of the study situation, Fassnacht does not think much of preventive, nationwide vitamin D substitutes. "My belief that the vitamin helps somewhere is very low. But, of course, I can be wrong."</p>
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