With the ClusterCTRL Stack, one can harness the power of up to 5 Raspberry Pi boards simultaneously. The ClusterCTRL Stack can control up to two fans to keep the clustered Raspberry Pi SBCs cool too.
https://www.notebookcheck.net/Raspberry-Pi-Transform-the-single-board-computer-into-a-powerful-cluster-system-with-the-ClusterCTRL-Stack.484927.0.html