Problem: How do we power the economy with solar power at night?
Solution: Store the energy from the day in large batteries.
Problem: Modern lithium-ion batteries are far too expensive to store power for the grid.
Solution: Integrate the battery into the solar cell.
At least, that is what is being proposed in a recently published study by Prof Song Jin of the University of Wisconsin-Madison. For together with his research team, he has created a device that skips the step of making electricity, and instead stores the energy directly in the battery’s electrolyte.
This has been possible as the researchers used a ‘redox flow battery’. RFB’s are different because, as the online journal Science Daily explains, “Unlike lithium-ion batteries, which store energy in solid electrodes, the RFB stores chemical energy in liquid electrolyte.” By making this change, the team has been able to create a, “… single device that converts light energy into chemical energy by directly charging the liquid electrolyte. In the new device, standard silicon solar cells are mounted on the reaction chamber and energy converted by the cell immediately charges the water-based electrolyte, which is pumped out to a storage tank.”
The research team also claim that discharging the battery is very simple, with Prof Jin stating that, “We just connect a load to a different set of electrodes, pass the charged electrolyte through the device and the electricity flows out.”
Whilst the idea of attaching a redox flow battery to a solar cell is not new, indeed models are already available on the market, what is different here is, as Prof Jin explains, “We now 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. [By doing this] 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.”
The results have been published in the scientific journal Angewandte Chemie International Edition, where the researchers give detail to their discovery describing it as, “An integrated photoelectrochemical solar energy conversion and electrochemical storage device is developed by integrating regenerative silicon solar cells and 9,10-anthraquinone-2,7-disulfonic acid (AQDS)/1,2-benzoquinone-3,5-disulfonic acid (BQDS) RFBs. The device can be directly charged by solar light without external bias, and discharged like normal RFBs with an energy storage density of 1.15 Wh L−1 and a solar-to-output electricity efficiency (SOEE) of 1.7 % over many cycles. The concept exploits a previously undeveloped design connecting two major energy technologies and promises a general approach for storing solar energy electrochemically with high theoretical storage capacity and efficiency.”
Whilst the new device has yet to be scaled up to an industrial scale, it theoretically seems a very possible option. As Prof Jin states 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.” Jin continues to describe the devices many advantages, saying, “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.”
Better still, the model used in the study contained no expensive rare metals, instead the tank that makes up the liquid electrolyte is filled with organic molecules. However, the team is continuing to search for, “electrolytes with larger voltage differential, which currently limits energy storage capacity.”
This discovery is very promising for the solar energy industry, as not only does open the door to improved solar cell efficiency, but also increases solar energy’s adaptability for use on the grid. Furthermore, as solar energy becomes a more widely used form of power generation, then it is possible that this device will become the standard battery for all future solar panel designs.