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.
Previous records for a solar flow battery show the tradeoffs these devices have faced. The researchers used a measure of efficiency termed solar-to-output electricity efficiency, or SOEE. The most efficient solar flow devices had hit 14.1 percent but had short lifespans due to reactions between the battery and photovoltaic materials. More stable ones, which had lifespans exceeding 200 hours, only had SOEEs in the area of 5 to 6 percent.
The new material had an SOEE in the area of 21 percent—about the same as solar cells already on the market, and not too far off the efficiency of the photovoltaic hardware of the device on its own. And their performance was stable for over 400 charge/discharge cycles, which means for at least 500 hours. While they might eventually decay, there was no indication of that happening over the time they were tested. Both of those are very, very significant improvements.
Obviously, given that both batteries and photovoltaic cells can potentially last for decades, 500 hours shouldn’t be viewed as a definitive test—especially for a device that’s proposed to enable off-the-grid electrical production. But the demonstration that voltage matching provides such a large efficiency boost should allow researchers to identify a wider range of battery and photovoltaic chemistries that have improved efficiencies. That accomplished, researchers will then be able to search among those for stable configurations. Whether all of that is compatible with low cost and mass production will be the critical question. But, at this stage of the renewable energy revolution, having more options to explore can only be a good thing.