Solving the “Christmas light” problem so solar panels can handle shade

Just 10 percent shade cover can drop electricity production by 50 percent. A new U-M-led project aims to change that.

A solar panel illuminated at night. Enlarge
Solar panels on North Campus. Photo: Joseph, Xu, Michigan Engineering

Shade is a big problem for solar panels: just 10 percent shade cover can create a 50 percent drop in electricity production. A new project led by researchers at the University of Michigan could ensure that power produced by a solar panel reflects the average over all the cells. The project is one of 40 the Department of Energy announced today through its SunShot Initiative.

“Shading is a fundamental issue with photovoltaic systems. Globally, more than 7.8 million kilowatt-hours of electricity are estimated to be lost annually due to photovoltaic shading. This lost energy is enough to power the city of San Francisco for an entire year,” said Al-Thaddeus Avestruz, an assistant professor of electrical engineering and computer science and project leader.

The reason why a little shade causes large power losses in a solar panel is that the cells are connected in a series like Christmas lights. The string of solar cells can only maintain the electrical current produced by the shaded solar cells, which isn’t much.

A variety of debris can come between solar cells and the sun, including dirt, bird droppings, fallen leaves and snow – and rooftop installations aren’t exactly easy to clean. Worse, because the shaded cells resist the current produced elsewhere, they can heat up and damage themselves, crack the solar panel’s surface or even start fires, says Avestruz.

To stop this weakest link phenomenon, he and his colleagues have proposed a way to allow the cells in full sun help out those in the shade. They did this by adding extra circuitry that allows the cells to trade charge – if the charges stored in each cell are equal, then the current produced by each cell will also be equal.

“The charge on each cell is stored like a tank of water,” said Avestruz. “We propose connecting the tanks with pipes and valves so that when we open the valve between two tanks, they equalize.”

The valves take the form of integrated circuits, which Avestruz and his team aim to produce and build into a solar panel over the next four years. In addition to overcoming the Christmas light effect seen in current solar panels, the circuits will streamline power processing by using the energy storage available at the cell level, reducing the size of the solar system and ultimately the cost of the electricity.

The $800,000 project is called “Architectures for Photovoltaic (PV) Cell-Level Power Balancing Using Diffusion Charge Redistribution (DCR).” Grant number: DE-EE-0007549.

Creating novel interdisciplinary solutions for global sustainability is a top priority for the U-M College of Engineering’s transformational campaign currently underway. Find out more about supporting sustainability efforts in the Victors for Michigan campaign.