By Scott Kriner, Green Metal Consulting
Metal Roofing is the ideal platform for rooftop photovoltaics (PV). The useful service life of typical PV units is often around 30 years before the output is below 80%. The useful service life of unpainted Galvalume® coated sheet standing seam roofing has been found to exceed 60 years based on a study by the Metal Construction Association and the ZAC organization. Service life of this type of roofing system is extended beyond 60 years in certain climate regions in the US.
With specially designed clips and fasteners, PV can be attached to metal roofing without penetrating the roof membrane. This eliminates sources of leaks and maintains the roof integrity. Lower labor costs are associated with the installation of PV on metal roofing, as compared to the traditional penetrating fasteners used with PV installed on non-metal roofing systems. The clips and fasteners provide excellent wind uplift and the maintenance costs are reduced as well.
The Solar Energy Industries Association (SEIA) reports that in 2013 new PV installations brought 4,751 MW of capacity online. Solar energy was the second-largest source of new electricity-generating capacity in the US last year, surpassed only by natural gas. SEIA predicts that in 2014 around 6,000 MW of PV will come online. That would represent a 26% increase over the 2013 installations. Despite the continuing drop in price for PV and the record installation levels, SEIA indicates that solar energy only provides 0.2% of the country’s total energy.
The use of PV on metal and membrane roofs was the subject of a research project recently completed by DOE’s Oak Ridge National Laboratory. The work showed that a crystalline Si modular PV unit mounted to a Galvalume (unpainted or painted) standing seam roof created some benefits to the integrated rooftop application. Having the PV unit mounted above the flat pan surfaces of the standing seam metal roof allows for air to flow in the channel between the PV and the roof surface. That air flow helps to minimize solar heat gain into the building space below. The air flow also helps to reduce the surface temperature of the PV which optimizes the efficiency of the PV itself.
The configuration of the mounting of PV on standing seam metal roofing also creates shading which can help to lower the solar heat gain as well. Field measurements have shown that a 33% reduction in surface temperature in the shaded area can occur, compared to areas of the sun-baked, boldly-exposed roof surface.
The metal roofing industry has known for years that metal roofing is the ideal platform for mounting PV for all reasons cited above. At maximum efficiency, Si semiconductors used for PV applications convert less than 30% of the energy they receive from the sun into electricity but despite the benefits provided by metal roofing, and especially standing seam metal roofing, there is no way to change the fact that the PV units must face the sun to perform their conversion of sunlight into electricity. And that exposure to sun tends to increase the surface temperature of the PV which can reduce the life and conversion efficiency of the PV units. For every 1°C increase in temperature, the efficiency of PV declines by about 0.5%. Solar cells also deteriorate or age more when their temperatures increase.
Scientists recently disclosed some promising results in developing solar cells that can be kept cool. The technique involves adding a specially patterned layer of silica glass to the surface of ordinary solar cells. A team of researchers led by Dr. Shanjui Fan, an Electrical Engineering professor at Stanford University in California found a way to allow solar cells to keep themselves cool.
Under routine operating conditions, PV units can reach temperatures of 130° F or more. Dr. Fan’s team found that by embedding small pyramid and cone-shaped structures (a few microns thick in any direction) on a thin layer of silica glass, unwanted heat (as IR energy) can be redirected away from the PV surface and directed to the night sky. By controlling the dimensions of the pyramids and micro-cones the layer could be fine-tuned to redirect only the unwanted infrared wavelengths away from the solar cell and into outer space. This is similar to how prepainted metal roofing with high thermal emittance values can re-emit IR energy from the surface of the roof to the night sky, thereby lowering the surface temperature.
The scientists used some basic principles to allow visible light to pass through the added silica layer unimpeded. Silica is also transparent to visible light but it can bend and refract light of specific wavelengths.
These devices are now being fabricated and performing experimental tests on the design. The next step is to demonstrate the self-cooling of these types of solar cells in an outdoor environment.
Galvalume® is an internationally recognized trademark of BIEC International, Inc., and some of its licensed producers.