How Can Solar Energy Be Stored for When the Sun Isn’t Shining?

By Megan Crimmins

Source: Igor Savin

Many buildings that are powered by solar use it directly during the day when the sun is shining without there being any necessity for storing the power. In this case, any excess power that isn’t being used will be sent to the grid. When it’s dark or cloudy out, buildings that rely on solar and don’t have any storage for it will use other forms of energy. However, as the conversation around clean energy has evolved, there is a growing interest in how to store solar power so that it can be used when the sun isn’t shining, and the answer may be quite obvious: batteries.

There are numerous types of batteries that can be used for solar power storage such as lead-acid batteries, lithium-ion batteries, nickel-cadmium batteries, and flow batteries. While multiple options exist, lithium-ion batteries seem to be the most frequently used and up and coming of these options. These are the same type of batteries that are found in cell phones, computers, and other types of high tech. To understand how lithium-ion batteries work for storing solar, it is necessary to first understand how lithium-ion batteries work in general.

For example, think of a phone or computer. These devices have rechargeable lithium-ion batteries in them. Lithium-ion batteries contain three parts: a positive electrode called a cathode, a negative electrode called an anode, and a chemical layer between them called an electrolyte. This chemical layer allows electrical charges to flow between the anode and cathode.

So, how does the battery gain charge to begin with? Well, when someone plugs their device in, that introduces electrons to the battery on the anode side. These electrons then attract lithium ions from the cathode side to the anode, charging the battery. When they unplug their phone, these lithium ions at the anode lose this source of electrons and start to break with the electrons they had been attracted to. These lost electrons flow through a circuit towards the cathode, and this circuit provides the energy used to run your phone or computer.

The process for charging a battery through solar power is pretty similar, though some key additional elements have to be taken into consideration. In this case, we are discussing photovoltaic solar which creates electricity through the use of solar panels. Solar panels consist of layers of electrically charged silicon, and the silicon atoms are bound together by electrons. The sun produces tiny particles of light called photons, and when a photon hits a solar panel, it can knock an electron free. This electron—and the hole it leaves—move to different sides of the panel because of the aforementioned electric charge. Then, these loose electrons flow through a circuit and generate an electrical current.

Animation by Isabel Plower

With solar power, the electrons that would be introduced through something like plugging your phone into an outlet are now introduced directly from the solar panels. To store that electricity, rather than sending it directly to a battery, it must pass through a regulator first. The job of this regulator is to ensure that the energy flow stops once the battery is fully charged so that it doesn’t get overcharged and burnout.

Animation by Isabel Plower

Once the power has gone through the regulator and been stored in the battery, there is one more step before this electricity can be used. Solar typically produces electricity in the form of a direct current (DC), but much of the electricity that we use is in the form of alternating currents (AC). This makes it necessary to have another piece after the battery called an inverter. The job of the inverter is to turn this DC current into AC current.

Animation by Isabel Plower

From here, buildings can use this battery storage of solar power on cloudy days or after the sun has set!

There is no “perfect” energy source—even solar has its challenges—which is why experts typically advocate not one source, but a portfolio of sources that together, could comprise a low- or zero-carbon future. That said, this battery storage method could make dark skies or bad weather a non-issue for solar power. Though solar faces other hurdles, battery storage opens up the potential for solar to take on an even larger share of our electric grid in the future.