Shade Happens

In fact, whether caused by a neighbor's trees or their own chimneys, homeowners may be losing between 20% and 40% of the potential output of their solar installations because of shade. Just 10% shading of a solar array can lead to a 50% decline in efficiency and even, on occasion, total system shutdown. The chart, below, shows field test results revealing how large, disproportionate power losses can be caused by a tiny amount of shading.

 Shade and Power: The Multiplier Effect

Why does shading cause such disproportionate power loss? 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.

The shading that causes this loss of efficiency can come in many forms. Depending on the object causing the shading, it may only be seasonal, or for a few hours each day, resulting in apparently mysterious fluctuations in the power. The loss of energy caused by partial shading of solar modules is difficult to predict because it depends on several variables including: internal module-cell interconnections; module orientation; how modules are connected within an array and the configuration of the inverter.

Solutions: Optimal Module Output

Finding a solution to this problem will make solar power more reliable for homeowners, and will further strengthen its economic rationale. Installers also have an interest in resolving this issue, since many installers carefully design rooftop installations to avoid structural shading. By reducing system power losses caused by shading, installers can potentially increase the available area for solar arrays, allowing them to upsell larger installations with more panels.

Installers already have one relatively effective strategy for avoiding structural partial shading - carrying out an accurate study of the proposed photovoltaic (PV) site prior to installation. Different shading analysis software programs, with three-dimensional simulation capabilities, are available to analyze shading dynamics. However, on its own this approach can often be inadequate as it can result in a corresponding loss of PV surface area and awkward rooftop designs with panels not centered, or large swaths of the installation site left bare.

While these simulation solutions can lead to a lower average generation of electricity, a more complete solution could simultaneously optimize array size while maximizing the output of each module at any given moment in time. Researchers and engineers at National Semiconductor have been developing an electronic solution to this problem. By identifying and harvesting the maximum power potential of each panel individually, power optimizer technology will be able to recapture up to 50% of the energy lost due to partial shading.

Such a solution will not only increase the output of current rooftop installations, but it will also allow installers to design systems that maximize precious roof space while helping customers achieve quicker returns on their investments. Along with the much anticipated renewable energy-friendly policies of the new administration, the introduction of power optimizer technology to will mark a major step towards lowering the cost of solar energy.

Ralf Muenster is director of the Renewable Energy Segment at National Semiconductor Corp. In this role, he is responsible for leveraging National's strength in energy-efficient technologies to develop strategies and products for efficient energy generation in the renewable energy market. In 2008, Muenster was instrumental in the successful launch of National's SolarMagic technology, created to maximize the output of solar panel installations when affected by shade, debris or panel-to-panel mismatch.

Prior to joining National in 2007, Muenster held various roles in the semiconductor industry, most recently serving as marketing and applications director for power products at Micrel Inc. He also held a management position in the automotive business segment at Advanced Micro Devices (AMD). Muenster is credited with founding a successful computer start-up company in Germany and was a scientist at the University of California at Berkeley. He holds a masters degree in physics from the Technical University in Munich.