The drop in battery prices is enabling battery integration with renewable systems in two contexts. In one, the battery serves as a short-term power reservoir to smooth over short-term fluctuations in the output of renewable power. In the other, the battery holds the power for when renewable power production stops, as solar power does at night. This works great for off-grid use, but it adds some complications in the form of additional hardware to convert voltages and current.
But there’s actually an additional option, one that merges photovoltaic and battery hardware in a single, unified device that can have extensive storage capacity. The main drawback? The devices have either been unstable or have terrible efficiency. But an international team of researchers has put together a device that’s both stable and has efficiencies competitive with those of silicon panels.
Solar flow batteries
How do you integrate photovoltaic cells and batteries? At its simplest, you make one of the electrodes that pulls power out of the photovoltaic system into the electrode of a battery. Which sounds like a major “well, duh!” But integration is nowhere near that simple. Battery electrodes, after all, have to be compatible with the chemistry of the battery—for lithium-ion batteries, for example, the electrodes end up storing the ions themselves and so have to have a structure that allows that.
So, the researchers used a completely different sort of chemistry. Flow batteries use solutions of two chemicals that can undergo charge-exchange reactions, shifting them between two chemical states. The battery basically borrows those charges in order to produce current when discharging, or it pumps charges back in to shift the chemicals to their alternate state, thus charging the battery. Flow batteries have the advantage that their total storage capacity is simply dependent upon the total volume of solution you use.
While there are many chemistries capable of working in a flow battery, the researchers started with their photovoltaic system and used that to choose the battery’s chemistry.
Even here, they didn’t exactly use off-the-shelf hardware. There was silicon involved, but it was part of a two-layer solar cell. In this setup, one photovoltaic material absorbs a set of wavelengths that aren’t absorbed by a second; the first layer, by contrast, is transparent to those wavelengths absorbed by the second. This allows a single cell to absorb a much broader range of wavelengths than would be possible otherwise, upping its overall efficiency.
For their device, the bottom layer was silicon. On top of that is a layer of perovskite photovoltaic material. Perovskites are a potential next-generation solar material, useful because they’re made from cheap ingredients and can be created simply by evaporating a solution of the perovskite. Unfortunately, these chemicals also have a propensity to decay, which has made for short lifetimes in many experimental setups. The researchers here don’t try to solve all of these problems; they simply use a perovskite-on-silicon photovoltaic setup and don’t try to run it for long enough that chemical decay is an issue.
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