Rejected energy is not thrown away energy. It is a result of the fact that our technologies do not have an efficiency degree of 100%.
Bitcoin Mining uses 120 TWh/year[1] which is way to much imo. The problem is that bitcoin uses the Hashcash proof of work system[2].
But there are mechanisms like proof of stake which can solve this problem.[3] Its about time to convert all blockchains to energy-saving mechanisms..