Sunlight can produce energy and clean water at the same time | Science News for Students

Sunlight can produce energy and clean water at the same time

Such a two-for-one system might generate electricity as it produces water clean enough to drink
Jul 25, 2019 — 6:45 am EST
solar-cell panels

This artist’s drawing suggests what a floating solar-electricity and water-cleansing system might look like. Here the solar-cell panels are mounted atop a water clean-up system.

WENBIN WANG

Engineers have constructed a device that not only generates electricity from the sun, but also distills freshwater from seawater. Solar farms that install such two-for-one systems could help meet the increasing global demand for drinking water while cranking out useful power. 

Peng Wang, Wenbin Wang and Yusuf Shi have applied for a patent on the new concept. They are part of an 11-member team that developed the new device. All work at King Abdullah University of Science and Technology in Thuwal, Saudi Arabia. They described their new device online July 9 in Nature Communications.

A solar cell forms the heart of the new system. Such cells harvest sunlight to make electricity. Some of the incoming light, however, will turn into heat. The new system harvests this waste heat to drive the evaporation of seawater. That water vapor wafts through a porous membrane made from plastic. This filters out contaminants, including salt. As such, this is a desalination system. So the liquid that condenses out on the other side will be clean freshwater. 

The removal of salt from the water “doesn’t affect the electricity production by the [solar cell],” explains Peng Wang. He’s an engineer and lead author of the new study.  “At the same time,” he adds, this system “gives you bonus freshwater.” 

Such tackling of two big challenges at once “is a great idea,” says Jun Zhou. He’s a materials scientist at Huazhong University of Science and Technology in Wuhan, China. Zhou was not involved in the new project.

The system’s prospects

So far, the King Abdullah researchers have reported on lab tests using a prototype, or early experimental version of the device. They exposed it to a lamp whose light mimics that of the sun. The new system converted about 11 percent of the light it received into electricity. That’s not bad. Solar cells sold today typically transform some 10 to 20 percent of the sunlight they soak up into usable energy. 

Wang’s team also tested how well their system cleaned up water. They fed it saltwater and dirty water that contained toxic heavy metals. Their findings suggest that a device about a meter (39 inches) across could pump out about 1.7 kilograms (3.7 pounds) of clean water per hour. For perspective, a gallon (3.8 liters) of water weighs 8.4 lbs (3.8 kg).

“It’s really good engineering work,” says George Ni of the project. He’s an engineer who was not involved in the new study. He did, however, work on water desalination while a graduate student at the Massachusetts Institute of Technology. 

“The next step is, how are you going to deploy this?” says Ni. “Is it going to be on a roof? If so, how do you get a source of water to it? If it’s going to be [floating] in the ocean, how do you keep it steady” so that it isn’t toppled by waves? Such practical issues would need to be figured out before the device could enter real-world use.

Power Words

(more about Power Words)

condense     To become thicker and more dense. This could occur, for instance, when moisture evaporates out of a liquid. Condense can also mean to change from a gas or a vapor into a liquid. This could occur, for instance, when water molecules in the air join together to become droplets of water.

contaminant     Pollutant; a chemical, biological or other substance that is unwanted or unnatural in an environment (such as water, soil, air, the body or food). Some contaminants may be harmful in the amounts at which they occur or if they are allowed to build up in the body or environment over time.

electricity     A flow of charge, usually from the movement of negatively charged particles, called electrons.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

filter     (in chemistry and environmental science) A device or system that allows some materials to pass through but not others, based on their size or some other feature. 

freshwater     A noun or adjective that describes bodies of water with very low concentrations of salt. It’s the type of water used for drinking and making up most inland lakes, ponds, rivers and streams, as well as groundwater.

graduate student     Someone working toward an advanced degree by taking classes and performing research. This work is done after the student has already graduated from college (usually with a four-year degree).

materials science     The study of how the atomic and molecular structure of a material is related to its overall properties. Materials scientists can design new materials or analyze existing ones. Their analyses of a material’s overall properties (such as density, strength and melting point) can help engineers and other researchers select materials that are best suited to a new application. 

membrane     A barrier which blocks the passage (or flow through) of some materials depending on their size or other features. Membranes are an integral part of filtration systems. Many serve that same function as the outer covering of cells or organs of a body.

online     (n.) On the internet. (adj.) A term for what can be found or accessed on the internet.

patent     A legal document that gives inventors control over how their inventions — including devices, machines, materials, processes and substances — are made, used and sold for a set period of time. Currently in the United States, this runs 20 years from the date you first file for the patent. The U.S. government only grants patents to inventions shown to be unique.

plastic     Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.

porous     The description of a substance that contains tiny holes, called pores , through which a liquid or gas can pass. 

prospect     (n.) The vista (as in what’s in view) or the future of something (such as whether it’s going to be successful). 

prototype     A first or early model of some device, system or product that still needs to be perfected.

salt     A compound made by combining an acid with a base (in a reaction that also creates water). The ocean contains many different salts — collectively called “sea salt.” Common table salt is a made of sodium and chlorine.

seawater     The salty water found in oceans.

solar cell     A device that converts solar energy to electricity.

technology     The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.

toxic     Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

Citation

Journal:​ ​​ W. Wang et al. Simultaneous production of fresh water and electricity via multistage solar photovoltaic membrane distillationNature Communications. Published online July 9, 2019. doi:10.1038/s41467-019-10817-6.