Researchers at the Michigan Technological University (MTU) in the US, in collaboration with a team from Queen’s University in Canada, have developed a method for properly modeling non-tracking planar concentrators based on a bi-directional reflectance function (BDRF) of non-ideal surfaces. The model could enable engineers to evaluate and optimize any type of reflector in low-concentration photovoltaic (LCPV) systems.
“The solar industry knows that non-tracking planar concentrators — or booster reflectors — are a low-cost method for increasing the performance of traditional solar photovoltaic systems,” says Joshua Pearce, associate professor of materials science and engineering and electrical and computer engineering at MTU. “Unfortunately, module manufacturers discontinue the warranty on such low-concentration PV, due in part to incorrect and antiquated optical models.”
Focusing on the system rather than individual panels, Pearse’s team now provides method for properly modeling this type of system using a bi-directional reflectance function (BDRF) of non-ideal surfaces, rather than traditional geometric optics. “We tested it in an outdoor system for a year and observed that reflectors improved the energy yield by 45% for a traditional flat glass module and by 40% for a prismatic glass crystalline silicon module, compared with control modules at the same orientation,” the researcher reports, adding that while they tested their BDRF method on crystal silicon modules, it should be effective for all photovoltaic materials.
“The BDRF model allows PV system designers to evaluate and optimize any type of reflector in low-concentration PV systems,” Pearse says. Today’s ground-mounted, flat-faced solar panels installed at solar farms are spaced apart to prevent shading, taking up large areas of land and wasting potential energy as sunlight hits the ground between panels, unharvested. Pearce proposes filling that space with reflectors to bounce sunlight back onto the panels. However, such planar concentrators, are not widely used.
One problem is that using reflectors typically voids manufacturer warranties for solar farmers, largely due to wrong predictions about the reflectors’ effects — e.g., greater temperature swings and non-uniform illumination. These real surfaces do not behave like perfect mirrors, though traditional modeling approaches threat them as if they would.
Instead, Pearce’s BDRF method predicts those effects more realistically by describing how sunlight bounces off the reflectors’ irregular surfaces, and predicting how much the light will scatter and where it will hit the array.
“Our results indicate that even optimized solar farms could increase their output by double digits,” says the head of the Pearse Research Group. “If adopted on a large scale, it could significantly increase PV penetration while further driving down the cost of sustainable solar electricity, far below the real costs of dangerous and polluting electricity sources.” The expert predicts that at the system level an increase in efficiency of this magnitude could not only profoundly change the way solar panels are installed in the future but also mean major retrofits at existing farms, motivated by the substantial return on the investment.
Pearson and his colleagues are now working with large-scale PV farms to demonstrate the technology at the tens-of-megawatts scale. At the same time, they are looking to partner with module manufacturers willing to maintain their warranty for LCPV systems to capture the market.
The paper titled “Photovoltaic system performance enhancement with non-tracking planar concentrators: Experimental results and BDRF based modelling" is published in the Institute of Electrical and Electronics Engineers (IEEE) Journal of Photovoltaics.
Written by Sandra Henderson, Research Editor, Solar Novus Today