A new graphene-based solar heating film with over 90% sunlight absorption capacity has been developed by a team of researchers at the Swinburne University of Technology in Melbourne, Australia. While simultaneously eliminating most IR thermal emission loss—the first time such a feat has been reported.
The resultant solar heating metamaterial can efficiently heat up rapidly to 181 degrees F (83 degrees C) in an open environment with minimal heat loss.
The film can be used for thermal energy harvesting and storage such as- thermoelectricity generation, and seawater desalination.
Reducing thermal emission loss which is also known as blackbody radiation and simultaneously absorbing solar light is critical for an efficient solar thermal absorber but is extremely challenging to achieve, says Baohua Jia, founding director of CTAM (Center for Transitional Atomaterials).
“That’s because, depending on the absorbed heat and properties of the absorber, the emission temperature differs, which leads to significant differences in its wavelength,” she explains. “But we’ve developed a three-dimensional structured graphene metamaterial (SGM) that is highly absorbent and selectively filters out blackbody radiation.”
The 3D SGM is composed of a 30-nanometer-thick film of alternating graphene and dielectric layers deposited on a trench-like nanostructure that does double duty as a copper substrate to enhance absorption. More importantly, the substrate is patterned in a matrix arrangement to enable flexible tunability of wavelength-selective absorption.
The newly developed graphene film is designed to absorb light between 0.28- to 2.5-micrometer wavelengths. And the copper substrate is structured so that it can act as a selective bandpass filter that suppresses the normal emission of internally generated blackbody energy. Hence the heat generated is completely blocked and results in an increase in the metamaterial’s temperature as shown in the figure. Hence, the SGM can rapidly heat up to 83 degrees C.
“In our previous work, we demonstrated a 90 nm graphene heat-absorbing material,” says Baohua. Though it could heat up to 160 degrees C, “the structure was more complicated, [comprising] four layers: a substrate, a silver layer, a layer of silicon oxide, and a graphene layer. But our new two-layer structure is simpler and doesn’t require vacuum deposition. Moreover, the method of fabrication is scalable at low cost.”
In this newly developed solar heater graphene film, the thickness of graphene film is reduced to one third in comparison to the previous one, as a result the absorbed heat can more efficiently be transferred to the other mediums such as water for different applications. Additionally, the film is hydrophobic, which fosters self-cleaning, while the graphene layer effectively protects the copper layer from corrosion, helping to extend the metamaterial’s lifetime.
“The metal substrate’s structural parameters are the main factors governing overall absorption performance of the SGM, rather than its intrinsic features. Therefore different metals can be used according to application needs or cost,” says Keng-Te Lin, lead author of a paper on the metamaterial recently published in Nature Communications.
Aluminum foil can also be used to replace copper without compromising the performance to reduce manufacturing cost.
Researchers have fabricated a prototype using standard laser nanofabrication, self-assembly graphene oxide coating, and photo-induced reduction.
“We used the prototype film to produce clean water and achieved an impressive solar-to-vapor efficiency of 96.2 percent,” says Keng-Te. “This is very competitive for clean water generation using a renewable energy source.”
He adds that the metamaterial can also be used for energy harvesting and conversion applications, steam generation, wastewater cleaning, seawater desalination, and thermoelectricity generation.
What is a graphene film?
Graphene is an allotrope of carbon in the form of a single layer of atoms in a two-dimensional hexagonal lattice in which one atom forms each vertex. It is the basic structural element of other allotropes, including graphite, charcoal, carbon nanotubes and fullerenes.
Graphene has a special set of properties that set it apart from other allotropes of carbon. In relation to its thickness, it is about 100 times stronger than the strongest steel. Yet its density is dramatically lower than any steel, with a surface density of 0.763 mg per square meter. It conducts heat and electricity very efficiently and is nearly transparent. Therefore it has a vast application in electronic and photonic devices also used for solar panels.
The graphene film is a one atom thick sheet or film of carbon atoms. It is strong yet low weight and it conducts heat and electricity. In fact, graphene is the most thermally conductive material known to man.