Power/Performance Bits: Nov. 21

Solar: greener greenhouses; butterfly thin films; quantum dot record.


Greener greenhouses
Researchers at the University of California, Santa Cruz are testing greenhouses capable of generating some of their own energy, without hampering plant growth. Greenhouses use electricity to control temperature and power fans, lights, and other monitoring systems.

Electricity-generating solar greenhouses utilize Wavelength-Selective Photovoltaic Systems (WSPVs), a novel technology that generates electricity more efficiently and at less cost than traditional photovoltaic systems. These greenhouses are outfitted with transparent roof panels embedded with a bright magenta luminescent dye that absorbs light and transfers energy to narrow photovoltaic strips, where electricity is produced. WSPVs absorb some of the blue and green wavelengths of light but let the rest through, allowing the plants to grow.

Cost per panel of WSPV technology is 65 cents per watt–about 40 percent less than the per-watt cost of traditional silicon-based photovoltaic cells.

The team monitored photosynthesis and fruit production across 20 varieties of tomatoes, cucumbers, lemons, limes, peppers, strawberries, and basil grown in the magenta glasshouses.

Plants grown in this magenta greenhouse fared as well or better than plants grown in conventional greenhouses. (Source: Nick Gonzales/UC Santa Cruz)

“Eighty percent of the plants weren’t affected, while 20 percent actually grew better under the magenta windows,” said Michael Loik, professor of environmental studies at UC Santa Cruz. Tomatoes and cucumbers are among the top greenhouse-produced crops worldwide, he said.

In additional experiments, small water savings were associated with tomato photosynthesis inside the magenta glasshouses. “Plants required 5 percent less water to grow the same amount as in more conventional glasshouses,” he said.

Reducing the energy consumed by greenhouses has become a priority as the global use of greenhouses for food production has increased six-fold over the past 20 years to more than 9 million acres today–roughly twice the size of New Jersey, according to Loik. “It’s big and getting bigger,” he said. “Canada relies heavily on greenhouses for vegetable production, and their use is growing in China, too.” Plastic greenhouses are becoming popular for small-scale commercial farming, as well as for household food production, he added.

“If greenhouses generate electricity on site, that reduces the need for an outside source, which helps lower greenhouse gas emissions even more,” said Loik. “We’re moving toward self-sustaining greenhouses.”

Butterfly wing solar cells
Scientists at the Karlsruhe Institute of Technology (KIT) enhanced thin-film solar cell light absorption up to 200% by mimicking nanoholes present in the wings of certain butterflies. While thin-film photovoltaics are a cheaper alternative to conventional crystalline silicon solar cells, absorption rates are lower, so they are primarily used in systems requiring little power.

“The butterfly studied by us is very dark black. This signifies that it perfectly absorbs sunlight for optimum heat management. Even more fascinating than its appearance are the mechanisms that help reaching the high absorption. The optimization potential when transferring these structures to photovoltaics (PV) systems was found to be much higher than expected,” says Dr. Hendrik Hölscher of KIT’s Institute of Microstructure Technology.

Nanostructures of the wing of Pachliopta aristolochiae can be transferred to solar cells and enhance their absorption rates by up to 200 percent. (Source: Radwanul H. Siddique, KIT/Caltech)

The team reproduced the butterfly’s nanostructures in the silicon absorbing layer of a thin-film solar cell. Subsequent analysis of light absorption yielded promising results: Compared to a smooth surface, the absorption rate of perpendicular incident light increases by 97% and rises continuously until it reaches 207% at an angle of incidence of 50 degrees. “This is particularly interesting under European conditions. Frequently, we have diffuse light that hardly falls on solar cells at a vertical angle,” said Hölscher.

Prior to transferring the nanostructures to solar cells, the researchers determined the diameter and arrangement of the nanoholes on the wing of the butterfly by means of scanning electron microscopy. After simulating the rates of light absorption for various hole patterns, they introduced disorderly positioned holes in a thin-film PV absorber, with diameters varying from 133 to 343 nanometers.

The efficiency of a complete photovoltaic system may not reach the same improvement, warned Guillaume Gomard of KIT’s Institute of Microstructure Technology. “Also other components play a role. Hence, the 200% are to be considered a theoretical limit for efficiency enhancement.”

Still, the researchers believe any type of thin-film PV technology can be improved with such nanostructures.

Record quantum dot solar efficiency
Researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) and the University of Washington established a new world efficiency record for quantum dot solar cells, at 13.4%.

Because of their astonishingly small size (typically 3-20 nanometers in dimension), colloidal quantum dots possess fascinating optical properties. Lead sulfide quantum dot solar cells emerged in 2010 with an efficiency of 2.9%. Since then, lead sulfide has improved, reaching a record of 12% set last year by the University of Toronto. The improvement from the initial efficiency to the previous record came from better understanding of the connectivity between individual quantum dots, better overall device structures and reducing defects in quantum dots.

The latest development in quantum dot solar cells comes from a completely different quantum dot material. The new quantum dot leader is cesium lead triiodide (CsPbI3), and is within the recently emerging family of halide perovskite materials. In quantum dot form, CsPbI3 produces an exceptionally large voltage (about 1.2 volts) at open circuit.

“This voltage, coupled with the material’s bandgap, makes them an ideal candidate for the top layer in a multijunction solar cell,” said Joseph Luther, a senior scientist and project leader in the Chemical Materials and Nanoscience team at NREL. The top cell must be highly efficient but transparent at longer wavelengths to allow that portion of sunlight to reach lower layers. Tandem cells can deliver a higher efficiency than conventional silicon solar panels.

The multijunction approach is often used for space applications where high efficiency is more critical than the cost to make a solar module. The quantum dot perovskite materials could be paired with cheap thin-film perovskite materials to achieve similar high efficiency as demonstrated for space solar cells, but built at lower costs than silicon technology, potentially making them a viable technology for both terrestrial and space applications.

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