In an effort to lessen the weight of renewable power sources carried by military in the field, the National Renewable Energy Laboratory (NREL) and the U.S. Office of Naval Research are exploring ways to make solar technologies lighter.
“The military has recognized the utility for our soldiers to have durable, lightweight solar cells due to the ever-growing advanced technologies they employ to accomplish their missions,” says NREL scientist Matt Reese, noting that lightweight solar power sources can enable a soldier to carry fewer batteries and extend mission times. What is more, lightweight solar power sources available in the field can reduce the dependence on supply convoys for diesel gensets — which could have direct impact on reducing casualties, making troops more agile and potentially generating electricity cheaper than diesel gensets.
Reese says that while CIGS and CdTe are thin-film photovoltaic technologies that have been commercially scaled at low cost and efficiencies (currently topping at around 17%) are steadily increasing, these modules are heavy. “This weight, however, is largely coming from the packaging,” the researcher says. “A module might have a package weight around 16.7 kg/m2. The thin-film PV layers themselves contribute about 0.03 kg/m2, where the glass is around 16 kg/m2.”
Decoupling growth process from final package
Looking at this disproportional distribution of weight between the glass and the actual photovoltaic layer, Reese and his collaborators thought of a creative solution: “We would like to decouple the growth from the final package, while introducing minimal additional cost,” he says.
Lighter solar power sources for the military
As Reese points out, it is the packaging rather than the absorber layer that dominates the weight of a thin-film photovoltaic module. He notes that moderately lightweight and/or flexible thin-film photovoltaics have already been demonstrated in virtually every PV technology. However, he says that the growth substrate typically puts limitations on efficiency. “Some technologies, due to their growth order, require a transparent substrate (e.g., CdTe), while others can be opaque (e.g., CIGS),” says Reese. “The highest efficiency CIGS and CdTe have tended to be grown at temperatures above 500 degrees Celsius.” The problem? Such high processing temperatures are not compatible with the lightest-weight package: plastic.
“Presently, the highest specific-power solar cells have been demonstrated using ‘lift-off’ approaches for III-V photovoltaics, such as GaAs,” Reese says. The technique had originally been developed so wafers — the source for the major cost in growing III-V photovoltaics — could be reused. “It has the added benefit of decoupling their growth from the final packaging choice, such that ultra-lightweight plastic packages can be used,” the scientist points out.
While III-V technologies are the most efficient, Reese says they also presently cost about two orders of magnitude more than commercially available CdTe and CIGS photovoltaics in a rigid form factor. “Thus, for man-portable applications, the present cost of III-V PV is too high,” he concludes. “Flexible a-Si modules are much lower in cost than III-V, but low efficiency ultimately limits their specific power — or power-to-weight ratio.”
Reese goes on to explain that flexible copper indium gallium selenide (CIGS) is generally grown on stainless steel to enable high temperatures and good efficiency, but that adds significant weight. Reducing growth temperatures to grow semiconductors on plastic has historically limited efficiency for CIGS and CdTe-based designs, according to the expert.
“We are developing methods to controllably delaminate polycrystalline thin-film photovoltaics — CIGS and CdTe — from typical standard glass substrates such that they can be repackaged in a manner similar to III-V,” says Reese. “The goal is to have as few changes to the growth process as possible from standard rigid modules. The methods we must use to lift off our polycrystalline films are different from single crystal III-V, however.”
Conversion efficiency of NREL’s lifted-off cells
“Production module efficiencies for CIGS and CdTe are in the 14–17% range,” says Reese, pointing out that there is typically a 2–4% difference between cells and modules. He also reminds that the research work is still in its early stages — the team is only about six months into the three-year project. “We have had preliminary successes on small-scale devices in which we have maintained circa 60% of the starting efficiency after we have delaminated the cells,” Reese reports. “We understand the cause of the losses, and we hope to push the efficiencies up higher during the project.”
The lightest weight package for a purpose
As the NREL scientist pointed out, the cells themselves are extremely lightweight, about 30 g/m2). He says some of the lightest packaged cells in industry are around 350 g/m2. “Fundamentally, if we can demonstrate this process, we think, we can use the lightest-weight package that will serve a particular purpose.”
Overcoming the weight/performance trade-off
Historically, solar cell makers had to balance growth considerations, such as temperature and chemical and dimensional stability, against a module’s final weight. “We are endeavoring to enable high performance by using standard growth on standard glass substrates and then put lifted-off films in the desired final package,” Reese says, explaining that packaging requirements are dictated by application. “For instance, the package requirements for man-portable photovoltaics in a military setting are different from that of an unmanned aerial vehicle or when integrated into a portable building structure.”
Addressing key applications for this new kind of lifted-off lightweight solar cell, Reese anticipates man-portable photovoltaics for military, disaster relief applications and consumer (e.g., outdoors) applications as initial uses for the innovative technology. Once costs can be sufficiently reduced, Reese says non-military applications could include ground transportation, building-integrated PV, floatovoltaics or even uses at the utility scale as a way of reducing installation costs.
“We have a lot more work until a true product from this process is possible,” says Reese, looking at the remainder of the three-year research project. “We want to be able to get our delamination to work over large areas uniformly every time while not have any fractures in the polycrystalline films afterwards,” he says. “Luckily, we are still early on in the project and trying to walk before we run as we understand the fundamentals.”
Written by Sandra Henderson, research editor Solar Novus Today