A Beowulf Cluster is basically a cluster of multiple identical computers running together for more processing power. One "mastermind" computer will assign tasks for the other computers (workers) to do. example: one computer can handle logins, one computer can handle world generation, one computer can handle player movement, one computer can handle mob ai, etc. By splitting the tasks into jobs for multiple computers instead of making one computer do all, there may be a potential increase in performance. Many supercomputers are beowulf clusters I was thinking that maybe this can be done with around 5-10 raspberry pis to create a cheap (in terms of electricity) but 'powerful' server to run pocketmine
A PocketMine server runs on one machine. I'm pretty sure that you can't have one part of the server on one machine and another part handeled run another. This is because a PocketMine server itself is a single task. You can however have a bunch of PocketMine servers running accross different machines (or one machine) to create a network. So, no this isn't possible (unless someones come up with some hard way to) plus, PocketMine doesn't take up too much system resources anyway.
excuses dosent meant this shouldnt be tried AFAIK PMMP have no such support but you may try running a clustered PHP compiler, and see if how it reacts or that even exist that is