I LOVE this app and have it running on several old devices. Their total computational power may not be massive, but every bit helps. Android devices are some of the most efficient devices in terms of compute-per-watt and will blow any laptop or desktop out of the water on this metric.
Researchers use BOINC to distribute massive computational workloads like searching for genes related to cancer (World Community Grid) or finding pulsars (Einstein@home). There are over a dozen BOINC projects so you can pick and choose what to contribute to. You get fun stats about your contributions and some projects will even credit you in their scientific papers.
BOINC live in F-Droid because it's not allowed in the regular play store. This is because it downloads binaries and runs them, which is a big play store no-no. Be careful to remove your battery and/or use the in-app settings to keep temps low to prevent battery swell. I wouldn't run this on my daily phone, but on spare devices it's great! Some of them I have run for years. Removing your case can make a big difference with temps.
Relevant lemmy community: https://sopuli.xyz/c/boinc
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