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Have researchers at last found an energy storage solution for solar devices that do not have space for a bulky battery? A University of Wisconsin-Madison electrical engineer has developed a design that allows a simplex standard-size solar cell to simultaneously generate and store power, creating a self-sustaining system for small-scale applications of solar energy.

Storing the electricity harvested from the solar cells and solar panels is always critical, says Dr Hongrui Jiang, the Vilas Distinguished Achievement Professor of electrical and computer engineering at UW-Madison. Having solar cells that can perform both harvesting and storage could significantly reduce weight, size and cost.

What sets UW-Madisons invention apart from previous photovoltaic self-charging cells is the ability to provide energy continuously. How? We introduced a polyvinylidene fluoride (PVDF)/zinc oxide nanowire array onto the counter electrode of a dye-sensitised solar cell, Jiang explains. When the photovoltaic cell is converting sunlight into electrons, most electrons flow out of the device to support a power load while some are directed to the PVDF-coated zinc-oxide nanowires. The PVDF has the high dielectric constant required to serve as an energy storage solution.

The promising device could provide steady photocurrent output under sunlight illumination, while stocking away photo-generated electrical energy for a rainy day. In fact, when solar illumination stops e.g., at night the solar cell automatically continues to provide electrical output from the stored energy without any additional control.

Jiang and his students developed the technology as an offshoot of a National Institutes of Health grant to design a self-focusing contact lens that adapts to the eyes of adults suffering from presbyopia. Powering the contact lens required a multifunctional, tiny device with a design that balances energy harvesting, storage and usage. In addition to a contact lens, Jiang envisions a myriad of other applications for electronics. Another potential is in localised micro power grids, where electricity is generated and stored for local usage.

The proof-of-concept device currently performs at about 4% power conversion efficiency. We demonstrated the feasibility of integrating these two functions into one single device, the microscale devices specialist indicates. The performance has much room to be improved, though, both in the photo-to-electric conversion efficiency and the storage charge density. He says his ultimate performance goal is a conversion efficiency comparable to the best dye-sensitised solar cells and a storage capacity comparable to that of supercapacitors. It could be a long time, though, before the technology becomes commercially viable Jiang estimates 1015 years. In the meantime, he and his colleagues will conduct more fundamental research to come up with the optimised design, taking advantage of state-of-the-art nanotechnology. In addition to improving performance, his team will also work on development in large-scale manufacturing, addressing cost, robustness, etc.

The research is detailed in the paper Dye-Sensitized Solar Cell with Energy Storage Function through PVDF/ZnO Nanocomposite Counter Electrode, published in the journal Advanced Materials.

Written by Sandra Henderson, Research Editor, Solar Novus Today

Labels: energy storage,self-sustaining,solar cell,polyvinylidene fluoride,PVDF,zinc oxide,nanowire,DSSC,dye-sensitized solar cell

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