Solar energy is the way of the future — powering our homes, businesses and even our cars. This sustainable, renewable energy source is being studied extensively at the University of Wisconsin-Madison, and in May of 2022 the Wisconsin Alumni Research Foundation (WARF) awarded $100,000 in grant funding to four research groups through the Accelerator Electrification Challenge Grant.
One of the labs selected for this award, the Song Jin research group, is working on a new solar powered battery that could change how we power our world.
This project is led by Professor Song Jin alongside Assistant Professor Dawei Feng and graduate student Ethan Auleciems. The project is focused on high-performance solar flow batteries for public and commercial use. These batteries are a step above the traditional ones we use in our everyday lives.
The batteries used in your cars, phones and other gadgets use reduction-oxidation (redox) reactions to produce electricity. During a redox reaction, molecules dissolved in the electrolyte solution gain and lose electrons. During oxidation, neutral (non-charged) molecules lose electrons. This reaction creates a positively charged cation and free electrons. In contrast, reduction happens when a cation gains electrons, changing back into a neutral molecule.
This chemical reaction occurs at the current collectors — the positive and negative sides of the battery. Neutral molecules go to one current collector while the electrons go to the other. The positive and negative sides cannot be in contact with each other or else the battery will stop producing electricity. A barrier in the battery allows electrons and neutral molecules to move through the solution but keeps the positive and negative sides separate. The movement of these molecules allows us to power our houses, cars, buildings and more.
Flow batteries, however, take a different approach.
Auleciems said his lab is “going away from [traditional batteries].”
“We are trying to separate the component parts of the battery into a flow battery,” he continued.
Flow batteries are unique because they separate the electrolyte from the collectors. Rather than store the electrolyte with the collectors, flow batteries store it in separate storage tanks.
“This allows us to separate the storage and chemistry of it,” Auleciems explained. “Normally, if you want to increase the capacity of a battery, you have to buy a whole new battery, a bigger battery. With a flow battery, to increase the capacity you just need to buy a bigger tank.”
This separation makes flow batteries more flexible and cost efficient than traditional batteries. As energy needs increase, the electrolyte tank can be expanded for little cost.
“Going into a solar flow battery, you are basically adding a solar cell as a third electrode,” Auleciems explained.
This “third electrode” can serve as a donor of electrodes or a donor of neutral molecules, depending on the type of solar panel utilized.
A solar cell battery has three modes. During the first mode, the solar cell battery can be used similarly to a normal solar panel, converting sun into electricity. The second mode is used for storage, where sun is converted into a charged electrolyte, but not turned into electricity. The third mode can be used during the night, or days where there is very little sun, where the stored and charged electrolyte is then converted into electricity.
Regular solar panels store their electricity in lithium ion batteries or other chargeable batteries. Auleciems noted that these types of solar panels can be good for smaller applications, but have a limited ability to expand because of the high cost.
“To power the Memorial Union, you don’t want [traditional batteries],” he explained. “For large applications, flow batteries are very attractive, but the research isn’t there for energy density.”
Energy density and energy efficiency are problems Auleciems and the lab are working on. Energy density is the amount of energy that can be stored in a system, while energy efficiency is focused on limiting the amount of energy loss as molecules move through the system. Auleciems’ project is focused on a solar flow battery with two solar cells instead of one. This doubles the voltage of the battery, in turn doubling the energy density of the system.
Energy efficiency has been a problem in flow batteries since they were first introduced in the 1980s. Recent progress from the lab group has increased the efficiency from 1.4 to 20%, an exponential increase in energy efficiency.
“The end goal is to get [energy efficiency] as high as possible, but the thing is, at what cost?” Auleciems stated.
Generally, solar panels that are more expensive have a higher efficiency, but Auleciems’ goal is to make this technology as accessible and affordable as possible.
“The holy grail would be to have a purely silicone based solar cell, which is something I am working on right now,” Aluciems explained.
Silicone has the advantage of being cheap, easily accessible and very well studied. Pairing these solar cells with a similarly matched electrolyte could push the energy efficiency even higher while keeping costs reasonable.
Auleciems’ vision with this project is to provide small communities with reliable year round power, especially in remote areas of the world.
He explained communities in the southern, mountainous regions of Brazil, for example, need electricity but are unlikely to be connected to the main grid because of their location. Solar cell batteries are a solution that can grow and shrink with the size of the community and the energy needed, all while being sustainable and relatively inexpensive.