Cutting Out the Middleman in Solar Energy

A solar charged liquid battery, cutting out the middleman. (Image courtesy of David Tenenbaum/UW-Madison.)

Solar power generation is in a constant state of improvement, as photovoltaic cells become more efficient and new applications of the technology find their way into the mainstream. Despite these improvements, limited battery life and storage capacity are still keeping solar power in the dark.

While Lithium-Ion (Li-ion) is the current high capacity battery-of-choice for many applications, from cellphones to Tesla’s Powerwall, the technology is prohibitively expensive to use on a large scale—like a city’s power grid.

Researchers from University of Wisconsin-Madison, along with colleagues from the King Abdullah University of Science and Technology in Saudi Arabia, have published a report on a device that converts light energy into chemical energy to be stored in a liquid or redox flow battery (RFB).

Song Jin, a professor of chemistry at the University of Wisconsin-Madison who co-authored the report, noted that "The RFB is relatively cheap and you can build a device with as much storage as you need, which is why it is the most promising approach for grid-level electricity storage.”

Solar cells paired with liquid batteries have been on the market previously, but this new design will charge the battery directly, rather than requiring an intermediate step where electricity is generated and transferred to the battery.

“We have one device that harvests sunlight to liberate electrical charges and directly changes the oxidation-reduction state of the electrolyte on the surface of the cells,” explained graduate student and first author on the report, Wenjie Li.

"We are using a single device to convert solar energy and charge a battery. It's essentially a solar battery, and we can size the RFB storage tank to store all the energy generated by the solar cells."

Let there be light! (Image courtesy of David Tenenbaum/UW-Madison.)

Combining solar cells and battery provides multiple advantages, as Jin explained. "The solar cells directly charge the electrolyte, and so we're doing two things at once, which makes for simplicity, cost reduction and potentially higher efficiency."

RFB’s are particularly efficient as well, as adjusting the flow of the battery’s electrolyte mixture will adjust the output and storage potential. As this device will both produce and store electricity, cost and efficiency will determine the success of the research.

Having proven the concept, Jin and Li are already working on improvements to their battery. One would be to match the solar cell's voltage to the chemistry of the electrolyte, minimizing losses as energy is converted and stored.

The liquid electrolyte used in the current study contains organic molecules but no expensive rare metals, which raise costs in many batteries. Jin and Li are also searching for electrolytes with larger voltage differential, which currently limits energy storage capacity.

Interested in other developments in liquid batteries? Check out this electronics story.