Saving energy the solar way

The power unit is a rectangular slab about the size of a movie theatre screen. It’s mounted on a thick steel post, and equipped with a tracking mechanism that continuously points it at the sun.

The slab is made of over 1,00,000 small lenses and an equal number of even smaller solar cells, each the size of the tip of a ballpoint pen. This contraption is part of one of the most efficient solar power devices ever made.

Semprius, a startup based in Durham, North Carolina, claims that the next generation of this power unit will make solar power the cheapest option for utilities installing new power plants. With fields of over 1,000 of those devices, utilities would produce electricity at less than 5 cents per kilowatt-hour. That is even cheaper than today’s least expensive option: a new natural gas plant. Yet for all the promise of the technology, Semprius is in a tough financial spot. For its technology to be cost-effective, Semprius must scale up the production of its solar cells significantly. Right now it can make enough solar units to produce six megawatts of power per year, but it needs to raise that to at least 200 megawatts.

High efficiency, low cost Semprius is not actually asking for that much money. Unlike many earlier solar startups that gambled on developing entirely new manufacturing equipment, Semprius mostly uses inexpensive, off-the-shelf equipment, some of it from the LED industry. It could grow simply by using excess capacity at existing LED manufacturing facilities. The company wouldn’t be possible without one key bit of new manufacturing technology, but that, too, is remarkably simple.

At Semprius’s pilot factory in Henderson, North Carolina, that technology can be found inside two glass-enclosed devices, each not much bigger than an office copier. At the end of a robot arm, and kept deliberately out of view, is a rubber stamp embossed with a pattern. This stamp is what makes Semprius’s high-efficiency, low-cost solar power possible.

The stamp, developed in Rogers’s lab, allows Semprius to improve upon a type of solar power called concentrated photovoltaics, which has been around for decades. The idea is that you can increase the amount of energy any solar cell gathers by putting lenses over the cell to focus light into it. Existing versions of the technology might use a lens with an area of about 400 square centimeters and focus it on a one-centimeter solar cell, for a concentration ratio of 400. Semprius’s stamp makes it possible to make arrays of solar cells that are far smaller and thinner than those that had been used in concentrating photovoltaics.

For the concentrating technology to work, the solar cells need to be picked up and arranged in an array so they can be paired with an array of lenses — and that’s where the rubber stamp comes in. It can pick up and transfer thousands of the tiny solar cells at once without breaking them, completely changing the economics of using small solar cells. Small cells have many advantages; because they require little material, they can be made of expensive types of semiconductors that are far more efficient than silicon.

What’s more, they dissipate heat well and can operate under very concentrated sunlight. You’d need far less material and space to generate the same amount of power as a typical solar cell. Those advantages, and some clever lens designs, allowed Semprius to break a solar power efficiency record in 2012.

Another advance, made this year, may allow the company to go even further. Semprius demonstrated another advantage of the rubber stamps — their ability to quickly and very accurately stack cells made of different semiconductors on top of each other. Researchers have wanted to do that for some time, because it would allow them to match semiconductor materials to each part of the solar spectrum. Some wavelengths of light would be absorbed by one material, and the rest would pass to the semiconductors below, and so on.

Physically stacking cells wasn’t practical with conventional manufacturing equipment. Semprius’s rubber stamp and extremely thin layers of semiconductor make it relatively easy to align the cells and electrically connect them. Next year, Semprius can break the current record, then quickly break the new one as it optimises the technology. If the company can survive until next year, that is.

Siemens acquired its stake in Semprius in June 2011. The plan had been for Siemens and Semprius to work together, with Semprius producing its concentrated photovoltaic devices and Siemens drawing on its expertise in building solar power plants. But 15 months after Siemens invested in Semprius, everything fell apart.
Dozens of promising solar startups failed and the projected market for concentrated photovoltaics shrank, convincing Siemens to get out of the business, ending the partnership and marking the beginning of hard times for Semprius.

Semprius’s solar devices are best suited for use in solar power plants. Having the backing of Siemens would have helped convince utilities to take a chance on novel technology. Without such a partner, that job is much harder.

So, Semprius continues its search for a new investor to scale up its technology. It has leads in sunny places, where its systems work best, like Saudi Arabia and parts of Mexico. At least one potential investor in China is interested.
NYT

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