All-weather solar cell design generates electricity come rain or shine

A team led by Qunwei Tang is developing solar cells that generate electricity not only from the sun, but also raindrops. 

Numerous research efforts have already demonstrated the potential for graphene to improve the efficiency of solar cells. Now a team of researchers in China has leveraged the remarkable properties of the wonder material to develop a new all-weather solar cell design that is able to generate electricity when current solar cells can't – when it's raining.

Amongst graphene's numerous impressive properties is its conductivity, which allows electrons to flow freely across its surface. When placed in an aqueous solution, this gives the material the ability to bind a pair of positively charged ions with a pair of its negatively charged electrons in what is known as a Lewis acid-base reaction. This property is exploited to remove lead ions and organic dyes from solutions and inspired a team led by Qunwei Tang to develop solar cells that generate electricity not only from the sun, but also raindrops.

Rather than being pure water, raindrops contain various salts that split into positively and negatively charged ions. When the water comes into contact with graphene, the positive ions bind with electrons on the graphene surface. At this point of contact between the water and the graphene, a double-layer of electrons and positively-charged ions forms, creating a pseudocapacitor. The difference in potential between the two layers is large enough to generate a voltage and current.

A new solar panel design leverages the properties of graphene to generate electricity 

from raindrops

Laboratory tests using simulated rainwater proved out this theory. A prototype dye-sensitized solar cell to which a thin film of graphene was applied, was tested with slightly salty water simulating rain. It managed to generate hundreds of microvolts and achieve a solar to electricity conversion efficiency of 6.5 percent. The researchers believe that with further refinement, the process could become efficient enough to produce all-weather solar cells that can generate electricity, whatever the weather.

3D solar towers offer up to 20 times more power output than traditional flat solar panels

Two small-scale versions of three-dimensional photovoltaic arrays that were tested by MIT researchers (Photo: Allegra Boverman) . 

While we’ve looked at the development of solar cell technologies that employ nanoscale 3D structures to trap light and increase the amount of solar energy absorbed, MIT researchers have now used 3D on the macro scale to achieve power output that is up to 20 times greater than traditional fixed flat solar panels with the same base area. The approach developed by the researchers involves extending the solar cells upwards in a three-dimensional tower or cube configuration to enable them to better capture the sun's rays when it is lower on the horizon.

Solar panels placed flat on a rooftop are most effective at harnessing solar energy when the sun is close to directly overhead, but quickly lose their efficiency as the angle of the sun’s rays hitting the panel increases – during the mornings, evenings, in the cooler months and in locations far from the equator. It is exactly in these situations that the researcher’s vertical solar modules provided the biggest boosts in power output.

After exploring a variety of possible 3D configurations using a computer algorithm and testing them under a range of latitudes, seasons and weather with specially developed analytic software, the team built three different individual 3D modules and tested them on the MIT lab building roof for several weeks. The results showed a boost in power output ranging from double to more than 20 times that of fixed flat solar panels with the same base area.

By going vertical and collecting more sunlight when the sun is closer to the horizon, the team’s 3D modules were able to generate a more uniform output over time. This uniformity extended over the course of each day, the seasons of a year, and even when accounting for blockage from clouds and shadows.

The researchers say this increase in uniformity could overcome one of the biggest hurdles facing solar energy – predictability of electricity supply that currently makes it difficult to integrate solar power sources into the grid.

They add that this uniformity, as well as the much higher energy output for a given area, would help offset the increased cost of the 3D modules, which are higher per the amount of energy generated when compared to conventional flat solar panels.

While the team’s computer modeling showed complex shapes – such as a cube with each face dimpled inward – would offer a 10 to 15 percent improvement in power output when compared to a simpler cube, these would be difficult to manufacture. In their rooftop tests, the team studied both simpler cube modules as well as more complex accordion-like shapes that could be shipped flat for unfolding on site.

This accordion-like tower was the tallest structure the team tested and such a design could be installed in a parking lot to provide a charging station for electric vehicles, according to Jeffrey Grossman, the senior author of the study and the Carl Richard Soderberg Career Development Associate Professor of Power Engineering at MIT.

Grossman and his colleagues believe that with the fall in the cost of solar cells in recent years - to the point where they have become less expensive than their supporting structures and the outlay for the land upon which they are placed - makes it an ideal time to examine the benefits of different solar cell configurations.

“Even 10 years ago, this idea wouldn’t have been economically justified because the modules cost so much,” Grossman says. But now, he adds, “the cost for silicon cells is a fraction of the total cost, a trend that will continue downward in the near future.”

Buoyed by the success of the tests on the individual 3D modules, the team now plans to study a collection of solar towers that will enable them to examine the effects that one tower’s shadow will have on another as the sun moves across the sky over the course of a day.

While the team believes its 3D solar cells could offer big advantages in flat-rooftop installations or urban environments where space is limited, they say they could also be used in larger-scale applications, such as solar farms, once a configuration that minimizes the shading effects between towers has been developed.

Lilium Electric jet

         Can't face the drive to the airport? Why not bypass the whole circus and jump in your two-seat, vertical take-off and landing (VTOL) all electric engine jet aircraft? That's the vision for the Lilium Jet, an aircraft currently being developed in Germany under the auspices of the European Space Agency's business incubation centre that boats fly-by-wire joystick controls, retractable landing gear, gull-wing doors, and a claimed to speed of 400Km/h (250mph). The creators claim that this personal e-jet that this personal e-jet could be made available to the public as early as 2018.

          Combining the vertical take-off capabilities of helicopters and the crusing abilities of fixed wing aircraft, the Lilium Jet aims to be significantly quieter than other VTOL vehicle such as helicopters, thanks to its 320 KW (435 hp) rechargeable battery powered ducted fan engines (arranged in a not-too-dissimilar from to that adorning Darpa's X-plane prototype ).

          Designed for recreational flying use during daylight hours, the Lilium Jet should be classed as a Light Sport Aircraft in Europe, with a pilot's license requiring just a minimum of 20 hours training.

          Lilium has an ambitious game plan, with its first manned experimental flight of a full-sized prototype slated for 2017. After that, the company expects to launch a fully-airworthy for flight certification by 2018, in preparation for full-scale production. There's no exact detail on a projected price, but according to Lilium it should be " far less " than similar sized aircraft and with typically lower running costs.