Light reflection model boosts solar farm efficiency
The researchers used thermal radiation readings on panels to help better understand ways to improve low-concentration photovoltaic systems. Their findings – that solar panels are not getting as much light as they could be – is published in the Institute of Electrical and Electronics Engineers (IEEE) Journal of Photovoltaics.
"We’re looking at this from a systems perspective," says Joshua Pearce, an associate professor of materials science and engineering and electrical and computing engineering at Michigan Technological University. According to Pearce, the research focused on the system rather than individual panels mostly because the current set up for ground-mounted solar panel arrays is "wasting space."
The iconic flat-faced solar panels installed in large-scale utility solar farms are spaced apart to prevent shading. As the sun shines on a photovoltaic system, sending electricity into the grid, a fair amount of that potential energy is lost as the light hits the ground between rows of panels.
The solution is simple, claims Pearce: Fill the space with a reflector to bounce sunlight back onto the panels. At present, however, reflectors – or planar concentrators – are not widely used.
According to Pearce, panels are usually warranted for 20 to 30 years, but the warranty only guarantees under certain circumstances. "If you’re putting more sunlight on the panel with a reflector, you will have greater temperature swings and non-uniform illumination, but simple optics makes wrong predictions on the effect."
Because of the uncertainty with potential hot spots, using reflectors currently voids warranties for solar farm operators. Pearce and his co-authors found a way to predict the effects using bi-directional reflectance function, or BDRF.
BDRF is often used in movies and video games to create more life-like computer generated imagery (CGI) characters and scenes. BDRF equations describe how light bounces off irregular surfaces and predicts how the light will scatter, creating indirect brightening and shadows.
For their solar panel work, Pearce’s team created a BDRF model that could predict how much sunlight would bounce off a reflector and where it would shine on the array.
"Real surfaces do not necessarily behave like perfect mirrors, even if they look like it," says Pearce. "So we applied [BDRF] models to these materials, which scatter the light instead."
By showing how the reflectors scatter light, the researchers started to take the risk out of using reflectors with solar panels. But even better, the reflectors greatly increase solar system output.
"The mathematics behind this is complicated," Pearce says. So the researchers wanted to "validate the predictive model, so the solar industry could start using our equations to design better solar farms."
The research team took their model to the field and ran an experiment on Canada’s Open Solar Outdoors Testing Field in Kingston, Ontario. The results shined much more light on the problem than predicted by others.
With standard panels, not tilted at the optimum angle for the latitude, the increase in efficiency reached 45%. Even with a panel optimally tilted, the efficiency increased by 18% and simulations show it could be pushed to 30% with better reflectors.
"We expend a lot of blood, sweat and tears to make solar panels as efficient as possible," says Pearce. "We work so hard to get a fraction of a percent increase on the module level; double digit returns on the systems level was relatively easy.
Such a large increase in efficiency at the system level could thenchange how solar panels are installed, and with the economic payback, it could even mean major retrofits for existing solar farms.
"The main goal here was to hand the solar farm developers the data needed on a silver platter, which they can then use to modify their farms and crank up their output and revenue by about a third," says Pearce.