New Device Harnesses Sun and Sewage to Produce Hydrogen Fuel

New Device Harnesses Sun and Sewage to Produce Hydrogen Fuel

  9:24:00 PM    Science Lover

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A novel device that uses only sunlight and wastewater to produce hydrogen gas could provide a sustainable energy source while improving the efficiency of wastewater treatment.

The new hybrid solar-microbial device is self-driven 
and self-sustained, because the combined energy from 
the organic matter (harvested by the MFC) and sunlight 
(captured by the PEC) is sufficient to drive electrolysis 
of water. (Credit: Image courtesy of University 
of California - Santa Cruz)

A research team led by Yat Li, associate professor of chemistry at the University of California, Santa Cruz, developed the solar-microbial device and reported their results in a paper published in the American Chemical Society journal ACS Nano. The hybrid device combines a microbial fuel cell (MFC) and a type of solar cell called a photoelectrochemical cell (PEC). In the MFC component, bacteria degrade organic matter in the wastewater, generating electricity in the process. The biologically generated electricity is delivered to the PEC component to assist the solar-powered splitting of water (electrolysis) that generates hydrogen and oxygen.

Either a PEC or MFC device can be used alone to produce hydrogen gas. Both, however, require a small additional voltage (an "external bias") to overcome the thermodynamic energy barrier for proton reduction into hydrogen gas. The need to incorporate an additional electric power element adds significantly to the cost and complication of these types of energy conversion devices, especially at large scales. In comparison, Li's hybrid solar-microbial device is self-driven and self-sustained, because the combined energy from the organic matter (harvested by the MFC) and sunlight (captured by the PEC) is sufficient to drive electrolysis of water.

In effect, the MFC component can be regarded as a self-sustained "bio-battery" that provides extra voltage and energy to the PEC for hydrogen gas generation. "The only energy sources are wastewater and sunlight," Li said. "The successful demonstration of such a self-biased, sustainable microbial device for hydrogen generation could provide a new solution that can simultaneously address the need for wastewater treatment and the increasing demand for clean energy."

Microbial fuel cells rely on unusual bacteria, known as electrogenic bacteria, that are able to generate electricity by transferring metabolically-generated electrons across their cell membranes to an external electrode. Li's group collaborated with researchers at Lawrence Livermore National Laboratory (LLNL) who have been studying electrogenic bacteria and working to enhance MFC performance. Initial "proof-of-concept" tests of the solar-microbial (PEC-MFC) device used a well-studied strain of electrogenic bacteria grown in the lab on artificial growth medium. Subsequent tests used untreated municipal wastewater from the Livermore Water Reclamation Plant. The wastewater contained both rich organic nutrients and a diverse mix of microbes that feed on those nutrients, including naturally occurring strains of electrogenic bacteria.

When fed with wastewater and illuminated in a solar simulator, the PEC-MFC device showed continuous production of hydrogen gas at an average rate of 0.05 m3/day, according to LLNL researcher and coauthor Fang Qian. At the same time, the turbid black wastewater became clearer. The soluble chemical oxygen demand--a measure of the amount of organic compounds in water, widely used as a water quality test--declined by 67 percent over 48 hours.

The researchers also noted that hydrogen generation declined over time as the bacteria used up the organic matter in the wastewater. Replenishment of the wastewater in each feeding cycle led to complete restoration of electric current generation and hydrogen gas production.

Qian said the researchers are optimistic about the commercial potential for their invention. Currently they are planning to scale up the small laboratory device to make a larger 40-liter prototype continuously fed with municipal wastewater. If results from the 40-liter prototype are promising, they will test the device on site at the wastewater treatment plant.

"The MFC will be integrated with the existing pipelines of the plant for continuous wastewater feeding, and the PEC will be set up outdoors to receive natural solar illumination," Qian said.

"Fortunately, the Golden State is blessed with abundant sunlight that can be used for the field test," Li added.

Qian and Hanyu Wang, a graduate student in Li's lab at UC Santa Cruz, are co-first authors of the ACS Nano paper. The other coauthors include UCSC graduate student Gongming Wang; LLNL researcher Yongqin Jiao; and Zhen He of Virginia Polytechnic Institute & State University. This research was supported by the National Science Foundation and Department of Energy.  

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Invisible World Discovered: Planet Alternately Runs Late and Early in Its Orbit, Tugged by Second Hidden World

Invisible World Discovered: Planet Alternately Runs Late and Early in Its Orbit, Tugged by Second Hidden World

  7:20:00 PM    Science Lover

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Usually, running five minutes late is a bad thing since you might lose your dinner reservation or miss out on tickets to the latest show. But when a planet runs five minutes late, astronomers get excited because it suggests that another world is nearby.

The "invisible" world Kepler-19c, seen in the foreground of this artist's conception, was discovered solely through its gravitational influence on the companion world Kepler-19b - the dot crossing the star's face. Kepler-19b is slightly more than twice the diameter of Earth, and is probably a "mini-Neptune." Nothing is known about Kepler-19c, other than that it exists. (Credit: David A. Aguilar (CfA))

NASA's Kepler spacecraft has spotted a planet that alternately runs late and early in its orbit because a second, "invisible" world is tugging on it. This is the first definite detection of a previously unknown planet using this method. No other technique could have found the unseen companion.

"This invisible planet makes itself known by its influence on the planet we can see," said astronomer Sarah Ballard of the Harvard-Smithsonian Center for Astrophysics (CfA). Ballard is lead author on the study, which has been accepted for publication in The Astrophysical Journal.

"It's like having someone play a prank on you by ringing your doorbell and running away. You know someone was there, even if you don't see them when you get outside," she added.

Both the seen and unseen worlds orbit the Sun-like star Kepler-19, which is located 650 light-years from Earth in the constellation Lyra. The 12th-magnitude star is well placed for viewing by backyard telescopes on September evenings.

Kepler locates planets by looking for a star that dims slightly as a planet transits the star, passing across the star's face from our point of view. Transits give one crucial piece of information -- the planet's physical size. The greater the dip in light, the larger the planet relative to its star. However, the planet and star must line up exactly for us to see a transit.



The first planet, Kepler-19b, transits its star every 9 days and 7 hours. It orbits the star at a distance of 8.4 million miles, where it is heated to a temperature of about 900 degrees Fahrenheit. Kepler-19b has a diameter of 18,000 miles, making it slightly more than twice the size of Earth. It may resemble a "mini-Neptune," however its mass and composition remain unknown.

If Kepler-19b were alone, each transit would follow the next like clockwork. Instead, the transits come up to five minutes early or five minutes late. Such transit timing variations show that another world's gravity is pulling on Kepler-19b, alternately speeding it up or slowing it down.

Historically, the planet Neptune was discovered similarly. Astronomers tracking Uranus noticed that its orbit didn't match predictions. They realized that a more distant planet might be nudging Uranus and calculated the expected location of the unseen world. Telescopes soon observed Neptune near its predicted position.

"This method holds great promise for finding planets that can't be found otherwise," stated Harvard astronomer and co-author David Charbonneau.

So far, astronomers don't know anything about the invisible world Kepler-19c, other than that it exists. It weighs too little to gravitationally tug the star enough for them to measure its mass. And Kepler hasn't detected it transiting the star, suggesting that its orbit is tilted relative to Kepler-19b.

"Kepler-19c has multiple personalities consistent with our data. For instance, it could be a rocky planet on a circular 5-day orbit, or a gas-giant planet on an oblong 100-day orbit," said co-author Daniel Fabrycky of the University of California, Santa Cruz (UCSC).

The Kepler spacecraft will continue to monitor Kepler-19 throughout its mission. Those additional data will help nail down the orbit of Kepler-19c. Future ground-based instruments like HARPS-North will attempt to measure the mass of Kepler-19c. Only then will we have a clue to the nature of this invisible world.

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