Smarts for Solar Arrays

Start-ups are vying to squeeze more energy out of solar panels, with distributed intelligence

Photo: Tigo Energy

Sunroof: Clif Bar & Co.'s rooftop solar array is smarter than average.

More than a dozen venture-capital-backed start-ups are vying to bring some smarts to solar farms in the hope of boosting photovoltaic arrays' ability to deliver carbon-free energy. The offering by Tigo Energy, based in Los Gatos, Calif., may be the simplest and most cost-effective of these distributed intelligence schemes. The three-year-old firm is exploiting wireless communications to minimize the added cost and complexity of PV arrays' energy-harvesting electronics, an approach that could quickly win over risk-averse system installers.

Tigo's strategy is getting its highest-profile test at the El Cerrito, Calif., headquarters of sports foods producer Clif Bar & Co. Solar installer Sun Light & Power, of Berkeley, Calif., is festooning Clif Bar's carport and rooftops with nearly 2000 silicon solar panels. Sun Light & Power is betting that by individually monitoring, controlling, and optimizing each panel, Tigo's system will squeeze 6 to 8 percent more energy from the 530-kilowatt array.

Such potential exists thanks to the inherent inefficiencies in the way PV arrays are designed today: The panels blindly feed their direct current to a centralized inverter. The inverter turns the array's DC into AC in sync with the power grid, but it must also maximize the DC flow coming in by controlling the entire array's impedance.

Centralized impedance control wastes energy, because a PV array's panels are rarely uniform. Variable shading from trees, buildings, or shifting clouds turns PV arrays into an electrical mosaic. The problem only gets worse with age as panels degrade at varying rates from their factory-shipped specs. The centralized inverter must pander to its array's lowest common denominator, setting an impedance that maximizes the panel's harvest of charges generated on the lowest-performing panels. Extra charge generated by overachieving panels is left to recombine and dissipate as heat.

The early market leader among the firms addressing PV's "module mismatch" problem, Enphase Energy, proposes to replace the big centralized inverter with microinverters attached to each panel. But in doing so, it may have limited its potential market. Electronics market consultancy IMS Research concluded this August that the microinverters' higher cost would preclude their use in utility-scale solar plants.

Gary Gerber, Sun Light & Power's president, says he is concerned about failure-prone components in microinverters such as liquid-filled capacitors, which add reliability risk to rooftop installations. "You put thousands of those out on roofs and they start failing, and you have a lot of work to do to repair and replace units, especially in residential installations where the panels are generally flush to a roof," says Gerber, a past president of the California Solar Energy Industries Association, which represents many installers.

Tigo's approach should be fairly fail-safe because its distributed electronic devices are comparatively simple. The modules attached to each solar panel contain a basic circuit that modifies impedance and a 2.4-gigahertz wireless interface; all processing capabilities reside in a centralized management unit. In operation, Tigo's modules transmit a power reading every 3 seconds, the central unit crunches the data to calculate the optimum impedance for each panel, and those marching orders are transmitted back to the modules.

The result is that each panel is assured to generate maximum DC power, leaving the centralized inverter to concentrate on its forte: efficiently churning out synchronous AC. Gerber says Tigo's hardware is edging up Clif Bar's PV price tag by roughly 4.5 percent, but he predicts that increased energy flow when the array starts up later this winter will more than pay back the added costs, boosting Clif Bar's overall return on investment.

Profit margins should increase as Tigo works with PV manufacturers to build its system into PV panel junction boxes at the factory. Several leading PV manufacturers have announced plans to integrate Tigo's circuitry, including Schott Solar, Solyndra, and Suntech Power. Tigo vice president Jeff Krisa says those partnerships should beget several Tigo-optimized solar projects at the 10- to 20-megawatt scale by the end of 2011. Gerber says it could become mainstream practice within three to four years.

Gerber points to one more benefit that has less to do with dollars and cents and more with organizations like Clif Bar wearing their green credentials: The data flowing into Tigo's central management units is easily output to the Internet, to be incorporated in Web sites or in-building displays that lay open an array's output in real time. "I can't overemphasize the value of the display technology," says Gerber. "That's a huge PR benefit."

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