Helping Photovoltaic Arrays Work Anywhere

The News: Two California companies, one big (National Semiconductor), one small (Enphase Energy), launched products last year that improve the reliability and efficiency of terrestrial photovoltaic solar arrays by as much as 50 percent – great news for anyone contemplating a five-figure investment in solar panels.

Shade can be cast by many things. Image from icanhascheezburger.com.

The Space 2.0 connection: Photovoltaic cells – which convert solar energy directly into electricity – came right out of NASA research. PVs have been a dependable, primary source of power in space for more than 30 years, powering almost all Department of Defense and NASA satellites and the International Space Station, whose most distinctive feature is its wing-like solar arrays.

Shade is the enemy of terrestrial solar panels. Arranged in a series, solar panels suffer reduced output from even a small amount of shading. Shadows are cast by chimneys and other building elements, trees, leaves, dust, power lines, bird droppings and poorly thrown flying toys.  Shade that covers just 10 percent of a PV array can reduce its efficiency by half.

National Semiconductor’s SolarMagic power optimizer modules connected to individual solar panels can boost power output by 10 to 30 percent, the Sunnyvale company says. Petaluma start-up Enphase Energy sells a  micro-inverter system that uses similar technology to

Ralf Muenster, director of National Semiconductor’s Renewable Energy Systems division, told RenewableEnergyWorld.com why shade matters (or, to quote the story’s headline, “Shade Happens”):

The answer lies in the way cells are connected within solar panels and the centralized form of performance optimization, carried out by the array inverter. Most solar arrays are made from panels connected in a series of parallel “strings.” Each panel feeds a DC current into the inverter, which then converts it to AC while also optimizing the PV array’s power generation through maximum power point tracking (MPPT). In turn, each panel is comprised of cells also connected in series.

To prevent the whole string of cells failing when one cell underperforms — like Christmas tree lights, which are also strung together in series — the typical installation is equipped with “bypass diodes.” These reroute the current around the underperforming cells. The catch is that rerouting the current loses not only the potential energy from these cells, but also lowers the entire string’s voltage.

This leaves the inverter with a dilemma: optimize the voltage for the underperforming string or maximize the energy harvest from the unaffected strings. In most cases the inverter chooses the former, causing the energy harvest of the impaired string to drop to near zero.

Thanks to @AltEnergyNews for the pointer.