Researchers from the Department of Chemistry at Columbia University and Stanford University have developed a new, scalable and low-cost “dip and dry” method for fabricating a highly efficient selective solar absorber (SSA). This SSA can harness and convert sunlight to heat for use in many energy-related applications from heating water and generating steam to residential heating.
The researchers determined the plasmonic-nanoparticle-coated foils created by their method perform just as well or better than the existing SSAs and have high efficiency through the day, regardless of the angle of the sun, because of the wide-angle design. They propose that the simple, inexpensive and environmentally-friendly process provides an appealing alternative to current SSA fabrication methods.
"We saw an unmet need for a facile, inexpensive, and sustainable method for fabricating high-performance SSAs," said Yuan Yang, assistant professor in the department of applied physics and applied mathematics at Columbia University. "We were pleased that our relatively simple process produced SSAs that performed on par with commercial SSAs and designs reported in other research. To our knowledge, this is the first time a plasmonic SSA has been made using such a process, and the scalability and cost of this approach bring us closer to making solar energy a practical reality for more people."
Harvesting sunlight for renewable energy is a primary objective for scientists. Solar-thermal converters can absorb light across the entire solar spectrum and convert it to heat at very high efficiencies. They offer a highly promising pathway for solar-energy harvesting. But attaining high-efficiency solar-thermal conversion at low cost is a challenge.
SSAs are ideal as a surface component of solar-thermal converters because they have contrasting optical properties for solar and thermal radiation. They are very dark black across all colors of sunlight and can absorb almost all the light and become very hot. But, unlike common black surfaces, they are metallic (i.e., non-remissive when it comes to thermal radiation (mid- to far-infrared light)). Heat is not lost as radiation and can be used for things like heating water or generating steam.
Most SSAs are made using sophisticated, energy-expensive or hazardous manufacturing processes like vacuum deposition or electroplating. This increases the environmental footprint and cost while limiting accessibility. As a basis for manufacturing SSAs, a dip-and-dry process is an attractive option. It yields SSAs that are highly efficient while bypassing the costs and environmental hazards associated with other approaches.
Researchers worked with instruments and facilities in Columbia Engineering laboratory and the Columbia Nano Initiative (CNI) to fabricate metal-based plasmonic SSAs using an inexpensive process that can tune the SSAs to suit different operating conditions. It is compatible with industrial manufacturing methods.
By dipping strips coated with a reactive metal (zinc) into a solution containing ions of a less reactive metal (copper), solar-absorbing nanoparticles of copper can be easily formed on the zinc strips by the galvanic displacement reaction.
"The beauty of the process is that it can be done very simply," said Jyotirmoy Mandal, lead author of the study and a doctoral student in Yuan Yang's group. "We only needed strips of metals, scissors—to cut the strips to size—a salt solution in a beaker, and a stopwatch to time the dipping process."
The SSA addressed another long-standing problem that is faced by solar-absorbing surfaces: the ability to absorb sunlight through the day from sunrise to sunset. In tests, the resulting SSAs showed a higher solar absorption at all angles than the existing designs (97 percent absorption when the sun is above 80 percent when near the horizon).
The team plans to test other combinations of metals besides zinc-copper and zinc-silver and explore ways to increase efficiencies. They are excited about the potential for the simple and affordable process to be utilized for solar conversion in developing countries.
A paper on this study called “Scalable, ‘Dip-and-Dry’ Fabrication of a Wide-Angle Plasmonic Selective Absorber for High-Efficiency Solar-Thermal Energy Conversion” was published in Advanced Materials.