Enlarge /. Existing solar + battery solutions include two separate hardware parts.
The decline in battery prices enables batteries to be integrated into renewable systems in two ways. On the one hand, the battery serves as short-term electricity storage to compensate for short-term fluctuations in the performance of renewable energies. In the other case, the battery keeps the electricity when the generation of renewable energy is stopped, as is the case with solar energy at night. This works great for off-grid use, but it leads to some complications in the form of additional hardware to convert voltages and currents.
In fact, there is an additional option that combines photovoltaic and battery hardware into a single, unified device that can have a large storage capacity. The main disadvantage? The devices were either unstable or have terrible efficiency. However, an international team of researchers has put together a device that is both stable and more efficient than silicon plates.
Solar flow batteries
How do you integrate photovoltaic cells and batteries? In the simplest case, you make one of the electrodes that draws electricity from the photovoltaic system into the electrode of a battery. That sounds like a big "well!" But integration is far from easy. After all, battery electrodes must be compatible with the chemistry of the battery – for example, in lithium-ion batteries, the electrodes store the ions themselves and must therefore have a structure that enables this.
So the researchers used a completely different kind of chemistry. Flow-through batteries use solutions made from two chemicals that can undergo charge exchange reactions and can switch between two chemical states. The battery essentially borrows these charges to generate electricity when discharged, or pumps back charges to put the chemicals in their alternate state to charge the battery. Flow-through batteries have the advantage that their total storage capacity simply depends on the total volume of the solution used.
While there are many chemicals that can work in a flow-through battery, the researchers started with their photovoltaic system and used it to select the chemistry of the battery.
Even here, they didn't necessarily use standard hardware. It was silicon, but it was part of a two-layer solar cell. In this construction, a photovoltaic material absorbs a series of wavelengths that are not absorbed by one second. In contrast, the first layer is transparent to the wavelengths absorbed by the second. This allows a single cell to absorb a much wider range of wavelengths than would otherwise be possible, which increases its overall efficiency.
For their device, the bottom layer was silicon. There is a layer of perovskite photovoltaic material on top. Perovskites are a potential next generation solar material that is useful because they are made from cheap ingredients and can be made simply by vaporizing a solution of the perovskite. Unfortunately, these chemicals also tend to disintegrate, which has led to short lifetimes in many test setups. The researchers here are not trying to solve all of these problems. They simply use a perovskite-on-silicon photovoltaic assembly and don't try to keep it running for so long that chemical decay is a problem.
Put the parts together
The key concept of the researchers was to start with this photovoltaic material and adapt the battery chemistry to its properties. Photovoltaic cells have a voltage based on the bandgap (the voltage difference between the insulating and the conducting state of their electrons) of the materials from which they are made. Batteries also have a potential, measured in volts, based on the energy difference between the two chemical states that supply them. Adjust these tensions, the researchers say, and you'll get a far more efficient system.
Using data from their photovoltaic hardware, they were able to identify the chemistry of the flow-through batteries with a potential that corresponded to the voltage. (The actual chemistry involves reactions between two different organic molecules, bis (trimethylammonio) propyl viologen and 4-trimethylammonium TEMPO. I'm sure you'd ask.) The reactions these chemicals take up between their two states are quick enough that they occur in the absence of catalysts, which simplifies the use of electrodes.
This is important because another problem with flow-through batteries is that their chemicals also react with many photovoltaic materials, which would significantly shorten the life of these devices. Therefore, the researchers covered the silicon with a thin gold layer that was both conductive and inert. Obviously, a cheaper inert metal would be preferred if it went into widespread production.
The resulting hardware can operate in one of three modes: providing power as a solar cell, using sunlight to charge as a battery, or providing power as a battery.
Previous records for a solar power battery show the compromises that these devices have been exposed to. The researchers used a measure of efficiency called solar-to-output electricity efficiency (SOEE). The most efficient solar power devices had reached 14.1 percent, but had a short lifespan due to reactions between the battery and photovoltaic materials. Only SOEEs in the range of 5 to 6 percent had more stable ones with a lifespan of more than 200 hours.
The new material had an SOEE in the 21 percent range – about as much as solar cells already on the market and not too far from the efficiency of the device's photovoltaic hardware. And their performance was stable over 400 charge / discharge cycles, ie at least 500 hours. While they could eventually expire, there was no indication that this was happening while they were being tested. Both are very, very significant improvements.
Given that both batteries and photovoltaic cells can potentially last for decades, 500 hours should not be seen as a final test – especially for a device designed to enable off-grid power generation. However, the demonstration that voltage adjustment offers such a large increase in efficiency should enable researchers to identify a wider range of battery and photovoltaic chemicals that have improved efficiency. Once this has been achieved, researchers can look for stable configurations among them. The key question will be whether all of this is compatible with low cost and mass production. However, in this phase of the renewable energy revolution, it can only be good to have more opportunities to explore.
Nature Materials, 2020. DOI: 10.1038 / s41563-020-0720-x (Via DOIs).