Stanford researchers working on rooftop PV array that also cools buildings efficiently

Stanford solar professor

Professor Shanhui Fan and postdoctoral scholar Wei Li atop the Packard Electrical Engineering building with the apparatus that is proving the efficacy of a double-layered solar panel. The top layer uses the standard semiconductor materials that go into energy-harvesting solar cells; the novel materials on the bottom layer perform the cooling task. (Image credit: L.A. Cicero)

Stanford electrical engineer Shanhui Fan wants to revolutionize energy-producing rooftop arrays.

“We’ve built the first device that one day could make energy and save energy, in the same place and at the same time, by controlling two very different properties of light,” said Fan, senior author of an article appearing Nov. 8 in Joule.

Today, such arrays do one thing – they turn sunlight into electricity. But Fan’s lab has built a device that could have a dual purpose – generating electricity and cooling buildings.

The sun-facing layer of the device is nothing new. It’s made of the same semiconductor materials that have long adorned rooftops to convert visible light into electricity. The novelty lies in the device’s bottom layer, which is based on materials that can beam heat away from the roof and into space through a process known as radiative cooling.

In radiative cooling, objects – including our own bodies – shed heat by radiating infrared light. That’s the invisible light night-vision goggles detect. Normally this form of cooling doesn’t work well for something like a building because Earth’s atmosphere acts like a thick blanket and traps the majority of the heat near the building rather allowing it to escape, ultimately into the vast coldness of space.

Check out the 2018 Solar Builder Projects of the Year!

Holes in the blanket

Fan’s cooling technology takes advantage of the fact that this thick atmospheric blanket essentially has holes in it that allow a particular wavelength of infrared light to pass directly into space. In previous work, Fan had developed materials that can convert heat radiating off a building into the particular infrared wavelength that can pass directly through the atmosphere. These materials release heat into space and could save energy that would have been needed to air-condition a building’s interior. That same material is what Fan placed under the standard solar layer in his new device.

Zhen Chen, who led the experiments as a postdoctoral scholar in Fan’s lab, said the researchers built a prototype about the diameter of a pie plate and mounted their device on the rooftop of a Stanford building. Then they compared the temperature of the ambient air on the rooftop with the temperatures of the top and bottom layers of the device. The top layer device was hotter than the rooftop air, which made sense because it was absorbing sunlight. But, as the researchers hoped, the bottom layer of the device was significant cooler than the air on the rooftop.

“This shows that heat radiated up from the bottom, through the top layer and into space,” said Chen, who is now a professor at the Southeast University of China.

What they weren’t able to test is whether the device also produced electricity. The upper layer in this experiment lacked the metal foil, normally found in solar cells, that would have blocked the infrared light from escaping. The team is now designing solar cells that work without metal liners to couple with the radiative cooling layer.

“We think we can build a practical device that does both things,” Fan said.

Shanhui Fan is the director of the Edward L. Ginzton Laboratory, a professor of electrical engineering, a senior fellow at the Precourt Institute for Energy and a professor, by courtesy, of applied physics. Postdoctoral scholars Wei Li of Stanford and Linxiao Zhu of the University of Michigan, Ann Arbor, also co-authored the paper.

The research was supported by the Stanford University Global Climate and Energy Project, the National Science Foundation and the National Natural Science Foundation of China.

— Solar Builder magazine

JA Solar sets new wattage record for 60-cell mono-Si PV module

JA Solar

JA Solar Holdings Co. says the output power of its 60-cell PV modules assembled by mono-Si PERC cells has exceeded now 325 W. The actual output power clocked in at 326.67 W as measured and certified by TUV SUD. Buy that measure, this sets a new world record for 60-cell mono-Si PV modules.

PERC technology has increasingly become the mainstream approach for high-performance PV products in the past few years. According to SEMI PVITR, PERC products will account for 40% of total product shipments by 2020. The advance of PERC technology as well as the wide adoption of the technology has significantly impacted on the solar powered energy generation in terms the reduction of levelized cost of energy and grid parity. Currently, the average power output of the 60-cell PV modules assembled with mono-Si PERC cells JA Solar offers is 300 W.

PV Pointers: Silicon heterojunction solar cell technology moves beyond the lab

As one of the largest PV product manufacturers in the world, JA Solar has been instrumental in developing advanced PV technologies through invention and innovation. Some history:

  • In 2010, the company filed an invention application for its industrial PERC cell structure and the method of production, the patent was granted by the Patent and Trade Mark Office of Chinese National Bureau of Intellectual Properties in 2012.
  • In 2013, JA Solar was the first company to break the 20% conversion-efficiency barrier for the industrial version of PERC solar cells using screen-printing metallization process, and started mass production of PERC-based cells and modules in the following year. JA Solar also holds the intellectual property rights for bifacial PERC cells with a Chinese patent granted by the National Bureau of Intellectual Properties of China in 2016.
  • In 2016, JA Solar provided 422MW PV modules with 40% of them being mono-Si PERC for the first 1GW demonstration phase of this “Front Runner” initiative as the largest module supplier in the project.

“Setting a new world record of over 325W output power from a 60-cell mono-Si PV module is remarkable achievement enabled by PERC technology,” said Dr. Wei Shan, Chief Technology Officer of JA Solar. “It is also a testament of the unrivaled efforts at JA Solar that focus on developing high-efficiency, cost-effective PV products meet the ever increased demand for clean energy through technological innovation and continuous performance improvement, as well as the commitment and the tradition of JA Solar to provide our customers high-performance PV products with high quality and reliability.”

— Solar Builder magazine

Trina Solar says it hit a new mono-crystalline PERC cell efficiency record

Feels like its been awhile since a module manufacturer touted an efficiency record. The wait is over: Trina Solar Limited announced that its State Key Laboratory of PV Science and Technology of China set a new world conversion efficiency record of 22.61 percent for a high-efficiency p-type mono-crystalline silicon (c-Si) solar cell.

How it happened

Trinasolar

The record-breaking solar cell was fabricated on a large-sized boron-doped Cz-Si substrate with a low-cost industrial process of advanced PERC (Passivated Emitter and Rear Cell) technology that integrates back surface passivation, front surface advanced passivation and anti-LID (Light Induced Degradation) technologies. The 243.23 cm2 solar cell reached a total-area efficiency of 22.61%. This result has been independently confirmed by the Fraunhofer ISE CalLab in Germany.

Trina’s record-breaking history

Trina Solar achieved a world conversion efficiency record of 21.40% for a large-area PERC mono-crystalline p-type solar cell in 2014, and the Company subsequently beat this with a 22.13% efficiency record in 2015. In July 2016, Trina Solar announced that its production lines were able to produce the same type of PERC solar cells in large volume with an average efficiency of 21.12%, which is only 1 percentage point less than the record efficiency that was achieved in 2015.

Trina Solar announces that it has broken its previous efficiency record by about a half percentage point, reaching the highest efficiency level to date for a PERC cell fabricated with a low-cost industrial process on a large-area p-type mono-crystalline substrate.

“We want to demonstrate all the possibilities of PERC technology on an industrial scale, and to approach as close as possible to the 25% efficiency level that was achieved by solar researchers at The University of New South Wales in the laboratory more than 17 years ago,” says Pierre Verlinden, VP and Chief Scientist of Trina Solar.

— Solar Builder magazine

Researchers exceed the theoretical limit of silicon solar cells (hit 30.2 percent)

silicon solar cell efficiency

Record-breaking cell not pictured.

So, get this: Researchers at the Fraunhofer Institute for Solar Energy Systems ISE together with the Austrian company EV Group (EVG) say they have successfully manufactured a silicon-based multi-junction solar cell with two contacts and an efficiency exceeding the theoretical limit of silicon solar cells.

Tell me more!

For this achievement, the researchers used a “direct wafer bonding” process to transfer a few micrometers of III-V semiconductor material to silicon, a well-known process in the microelectronics industry. After plasma activation, the subcell surfaces are bonded together in vacuum by applying pressure. The atoms on the surface of the III-V subcell form bonds with the silicon atoms, creating a monolithic device.

The efficiency achieved by the researchers presents a first-time result for this type of fully integrated silicon-based multi-junction solar cell. The complexity of its inner structure is not evident from its outer appearance: the cell has a simple front and rear contact just as a conventional silicon solar cell and therefore can be integrated into photovoltaic modules in the same manner.

Blowing past the limits

A conversion efficiency of 30.2 percent for the III-V / Si multi-junction solar cell of 4 cm² was measured at Fraunhofer ISE’s calibration laboratory. In comparison, the highest efficiency measured to date for a pure silicon solar cell is 26.3 percent, and the theoretical efficiency limit is 29.4 percent.

“We are working on methods to surpass the theoretical limits of silicon solar cells,” says Dr. Frank Dimroth, department head at Fraunhofer ISE. “It is our long-standing experience with silicon and III-V technologies that has enabled us to reach this milestone today.”

RELATED: Panasonic breaks module conversion efficiency record by full percentage point 

More intense science talk

The III-V / Si multi-junction solar cell consists of a sequence of subcells stacked on top of each other. So-called “tunnel diodes” internally connect the three subcells made of gallium-indium-phosphide (GaInP), gallium-arsenide (GaAs) and silicon (Si), which span the absorption range of the sun’s spectrum. The GaInP top cell absorbs radiation between 300 and 670 nm. The middle GaAs subcell absorbs radiation between 500 and 890 nm and the bottom Si subcell between 650 and 1180 nm, respectively. The III-V layers are first epitaxially deposited on a GaAs substrate and then bonded to a silicon solar cell structure.

Subsequently the GaAs substrate is removed, and a front and rear contact as well as an antireflection coating are applied.

“Key to the success was to find a manufacturing process for silicon solar cells that produces a smooth and highly doped surface which is suitable for wafer bonding as well as accounts for the different needs of silicon and the applied III-V semiconductors,” explains Dr. Jan Benick, team leader at Fraunhofer ISE. “In developing the process, we relied on our decades of research experience in the development of highest efficiency silicon solar cells.”

RELATED: How new solar module technology lifts efficiency, limits price 

“The III-V / Si multi-junction solar cell is an impressive demonstration of the possibilities of our ComBond cluster for resistance-free bonding of different semiconductors without the use of adhesives,” says Markus Wimplinger, Corporate Technology Development and IP Director at EV Group. “Since 2012, we have been working closely with Fraunhofer ISE on this development and today are proud of our team’s excellent achievements.” The direct wafer-bonding process is already used in the microelectronics industry to manufacture computer chips.

On the way to the industrial manufacturing of III-V / Si multi-junction solar cells, the costs of the III-V epitaxy and the connecting technology with silicon must be reduced. There are still great challenges to overcome in this area, which the Fraunhofer ISE researchers intend to solve through future investigations. Fraunhofer ISE’s new Center for High Efficiency Solar Cells, presently being constructed in Freiburg, will provide them with the perfect setting for developing next-generation III-V and silicon solar cell technologies. The ultimate objective is to make high efficiency solar PV modules with efficiencies of over 30 percent possible in the future.

— Solar Builder magazine

Panasonic breaks module conversion efficiency record by full percentage point

Panasonic Corp. is the latest module manufacturer to break the efficiency record. The company reports it has achieved a photovoltaic module conversion efficiency* of 23.8 percent** (aperture area***: 11,562 cm2) at research level, a major increase over the previous world record for crystalline silicon-based PV modules.

Panasonic module efficiency record

The previous record for the conversion efficiency of a crystalline silicon-based photovoltaic module was 22.8 percent****. Panasonic has broken the record for the world highest conversion efficiency by a full percentage point. Panasonic had announced a world’s highest conversion efficiency of 25.6 percent***** in its silicon heterojunction cells in April 2014. With this achievement, Panasonic holds the world records of conversion efficiency for both crystalline silicon-based solar cells and modules.

Panasonic developed a unique silicon heterojunction structure (Technology for junction formation required for solar cells that covers the silicon base surface with an amorphous silicon layer. This technology has the key feature of superior passivation to compensate for the many flaws around the silicon base surface area) composed of crystalline silicon substrate and amorphous silicon layers, and has continuously improved its photovoltaic module HIT using silicon heterojunction since the start of commercial production. This new record was achieved by further development of Panasonic’s proprietary heterojunction technology for high-efficiency solar cells and modules adopting a back-contact solar cell structure (Technology for eliminating the shadow loss on the front side electrode with the electrodes on the back of the solar cell, which allows the more efficient utilization of sunlight).

Going forward, Panasonic will continue to pursue technology development of its photovoltaic module HIT, aimed at realizing higher efficiency, higher reliability and lower costs, and will work towards mass production.

If you are new to this whole Panasonic HIT module world, definitely check out the video below for more information.

* According to research by Panasonic as of Feb. 18, 2016, for crystalline silicon photovoltaic modules.
** Result of evaluations at the National Institute of Advanced Industrial Science and Technology (of Japan).
*** The module area is the aperture area opened by the masks (11,562 cm2).
**** SunPower November, 2015. Judged from the “Solar cell efficiency tables (version 47)” [Prog. Photovolt: Res. Appl. 2016; 24:3-11]

— Solar Builder magazine