Middle Eastern countries have plenty of two things – sun and desert sand. As part of its wide ranging and innovative research activities the Abu Dhabi-based Masdar Institute is investigating how to exploit both in the energy sector, with positive results so far.
Improving PV efficiency
While the costs of solar PV are dropping, further cost reductions along with improvements in efficiency will be the two biggest factors in enhancing the attractiveness of this technology to consumers.
Masdar researchers’ approach is addressing both of these issues, with the development of a novel, low-cost optical device that separates and concentrates the sunlight. In a traditional silicon-based photovoltaic (PV) cell, only some of the sun’s wavelengths from the visible light spectrum are absorbed and converted into electricity. Much of the sunlight at the shorter blue and green wavelengths is inefficiently absorbed – and none from the infrared region is absorbed – resulting in conversion efficiencies between 15% and 18% in commercial modules.
With the new optical device, which is made out of polycarbonate – allowing for economical mass-scale production – the visible and infrared light is dispersed and concentrated into “slices” for absorption by spectrally-matched solar cells, significantly increasing conversion efficiencies up to a theoretical 38%.
“The spectral splitting concentrator we have designed for this promising solar energy capture technology is a combination between a prism, which separates the different colours or wavelengths of sunlight, and a lens, which concentrates the light that can be used to help harness a greater amount of energy from the sun,” explains Masdar Institute PhD student Carlo Maragliano, who developed the device with Dr Matteo Chiesa, Associate Professor of Mechanical and Materials Science Engineering, Masdar Institute.
Concentrating photovoltaics (CPV) isn’t a new technology but generally these are expensive, requiring layering of multi-junction solar cells and precise tracking of the sun to maintain focus of sunlight on the cells throughout the day. The solar cell pairing technique opens the potential for efficient solar energy harvesting at a fraction of the price of most commercially available solar technologies.
Desert sand for storage
In another project, Masdar Institute researchers have demonstrated that desert sand from the UAE could be used in concentrated solar power (CSP) facilities to store thermal energy up to 800-1,000°C. The research project , ‘Sandstock’, has been seeking to develop a sustainable and low-cost gravity-fed solar receiver and storage system, using sand particles as the heat collector, heat transfer and thermal energy storage media.
“The availability of this material in desert environments such as the UAE allows for significant cost reductions in novel CSP plants, which may use it both as thermal energy storage material and solar absorber,” says Dr Nicolas Calvet, who is leading the study. “The success of the Sandstock project reflects that usability and practical benefits of the UAE desert sand.”
Replacing the typical heat storage materials used in thermal energy storage systems - synthetic oil and molten salts - with inexpensive sand can increase plant efficiency due to the increased working temperature of the storage material and therefore reduce costs.
The sand chemical composition was analyzed with X-ray fluorescence and X-ray diffraction techniques, revealing the dominance of quartz and carbonate materials. The sand’s radiant energy reflectiveness was also measured before and after a thermal cycle, as it may be possible to use the desert sand not only as a thermal energy storage material but also as a direct solar absorber under concentrated solar flux.
In parallel to sand characterization, a laboratory scale prototype was tested with a small solar furnace at the laboratory of PROMES CNRS 1MW solar furnace in Odeillo, France. In the next step of the project an improved prototype at the pre-commercial scale will be tested at the Masdar Institute Solar Platform (MISP), potentially in collaboration with an industrial partner.
The MISP, which includes a 100kW beam-down CSP facility, was inaugurated last November to spur the development of next-generation CSP and thermal energy storage technologies that are more affordable and efficient. It joins the Masdar PV testing facility, which was launched in January 2015, as part of the Masdar Solar Hub as the first independent solar testing and R&D facility in the UAE.