Integration Of Solar Cells And Flow Batteries Could Mean Better Off-Grid Storage

This experimental solar-battery combination is still too small and not robust enough for practical applications, but it has already overcome some major obstacles to cheap storage of renewable power at the household level. Wenjie Li/UW-Madison

Direct integration of two of the most exciting clean power technologies could make renewable energy available for many of the billion people without access to an electricity grid.

Perovskite cells are widely seen as the future of solar energy, with the potential to be much cheaper and somewhat more efficient than the existing silicon versions. If the last obstacles can be overcome, they will usher in an era of astonishingly cheap and environmentally friendly daytime electricity. Flow batteries may represent the key to storing that power energy for night use, but their path to widespread adoption has been slower and more obstacle-strewn.

People looking to produce and store their own electricity currently use separate solar systems and batteries, requiring several components to integrate them together. As the price of panels and batteries fall, the so-called “balance of system” items like state of charge monitors and maximum powerpoint trackers become a bigger share of the cost. Sometimes, the price of these components is the difference between being able to afford electricity and relying on expensive, polluting diesel generators or kerosene lamps.

University of Wisconsin PhD student Wenjie Li is seeking to change that by integrating solar cells directly with aqueous organic redox flow batteries (AORFBs), reducing the price by cutting out the intermediary items. Initially, Li tried III-V tandem solar cells, which capture 26 percent of sunlight, close to the maximum efficiency available. However, much of the energy these cells produced was wasted because their output voltage was different from the 1.5-2 Volt ideal input voltage for Li's batteries – the problem that the "balance of system" components normally address. As a whole, the system was just 14.1 percent efficient, although even that was a record

Li is now collaborating with Professor Anita Ho-Baillie of the University of Sydney. Ho-Baillie told IFLScience to prevent wastage “you either need to tune the solar cells' voltage to match the batteries or the batteries to match the cells.” One of perovskite's great advantages over silicon cells is greater voltage flexibility.

Ho-Baillie created silicon-perovskite tandem cells with voltages designed to match Li's needs, with stable conductors to protect the cells' bottom layer from damage from the battery electrolyte. Solar cells work best at lower temperatures, and insertion directly into the passing electrolytes helps cool them.

In Nature Materials, Li and Ho-Baillie announce this system can collect and store sunlight with 20 percent efficiency. Ho-Baillie's cells are 22 percent efficient when bypassing the battery and sending power to be used directly, so only 10 percent is lost in storage, similar to conventional battery systems.

Moreover, by using AORFBs, Li avoids the expensive raw materials used by most existing flow batteries and the acidic electrolytes that could damage the solar cells or prove a problem in a leak.

Ho-Baillie told IFLScience there would be no technical obstacle to scaling up these integrated batteries to supply entire cities, but added that “if it is a really huge system, the cost of items like the charge monitors may not be as significant,” lessening the benefits compared to keeping production and storage separate.

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