More PV-Power With Heat-Resistant Device

Schematic energy flow of standard and solar thermophotovoltaic (STPV) systems. In standard solar cells, a PV cell (left)  is directly illuminated by the sun. Whereas in STPV systems (right), an absorber placed between the sun and PV cell is heated by absorbing the broad solar spectrum. A thermal emitter connected to the absorber converts the heat into narrowband thermal radiation that illuminates the PV cell (Source: M. Chirumamilla)

Schematic energy flow of standard and solar thermophotovoltaic (STPV) systems. In standard solar cells, a PV cell (left) is directly illuminated by the sun. Whereas in STPV systems (right), an absorber placed between the sun and PV cell is heated by absorbing the broad solar spectrum. A thermal emitter connected to the absorber converts the heat into narrowband thermal radiation that illuminates the PV cell (Source: M. Chirumamilla)

The photo­voltaic cells in tradi­tional solar cells convert sunlight efficiently within a narrow range of wave­lengths determined by the material used in the PV cells. This limits their effi­ciency, as long wave­lengths of sunlight are not converted at all and the energy of short wave­length light is largely wasted. Scientists have sought to increase the efficiency of photo­voltaics by creating multi-junction solar cells, made from several different semi­conductor materials that absorb at varying wave­lengths of light. The problem is, such multi-junction cells are expen­sive to make.

Broadband solar absorption pre­viously has been achieved using metal-insu­lator-metal resonators, which consist of an insulator sand­wiched between a thick bottom and a thin top layer, each made of metals like chromium and gold. The metal components used in MIM resonators have rela­tively low melting points. Tempe­ratures that are reduced further when the materials are in very thin layers, as in the resonators, because of a pheno­menon called melting point depression, in which the melting point of a material scales down as the dimensions of the material decrease. The metals in standard MIM reso­nators melt at around 500 degrees Celsius, hindering their use­fulness in solar cells.

Now a group of researchers in Denmark have disco­vered an alter­native method to capture a broad spectrum of sunlight using a heat-resistant device made of tungsten and alumina layers that can be fabricated using inexpensive and widely available film-depo­sition techniques. “They are resistant to heat, including thermal shock, and exhibit stable physical and chemical pro­perties at high tempe­ratures,” explained Manohar Chiru­mamilla of Aalborg Univer­sity in Denmark. This allows the absorbers to maintain their structural properties at very high tempe­ratures.

In experiments, the new absor­bers were shown to operate at a tempe­rature of 800 degrees Celsius and to absorb light of wave­lengths ranging from 300 to 1750 nano­meters, that is, from ultra­violet to near-infrared wave­lengths. “MIM reso­nators absorbing in the spectral region from UV to near-infrared can be directly employed in different appli­cations, such as solar thermo­photovol­taic systems and solar thermal systems,” Chiru­mamilla said. “Other poten­tial appli­cations include in so-called tower power plants, where concen­trated solar light gene­rates steam to drive a gene­rator. This is the first step in uti­lizing the energy of the sun in a more effi­cient way than with current solar cells,” he added. “Using an emitter in contact with our absorber, the generated heat can then be used to illu­minate a solar cell which can then function more effi­ciently when it is placed directly in the sun.” (Source: OSA)

Reference: M. Chirumamilla et al.: Multilayer tungsten-alumina-based broadband light absorbers for high-temperature applications, Opt. Mat. Ex. 6, 2704 (2016); DOI: 10.1364/OME.6.002704

Link: Dept. of Physics and Nanotechnology, University of Aalborg, Aalborg, Denmark

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