  Proof-of-Work (PoW) is a popular blockchain consensus algorithm that is used
in cryptocurrencies like Bitcoin in which hashing operations are repeated until
the resulting hash has certain properties. This approach uses lots of
computational power and energy for the sole purpose of securing the blockchain.
In order to not waste energy on hashing operations that do not have any other
purpose than enabling consensus between nodes and therefore securing the
blockchain, Proof-of-Useful-Work (PoUW) is an alternative approach which aims
to replace excessive usage of hash functions with tasks that bring additional
real-world benefit, e.g. supporting scientific experiments that rely on
computationally heavy simulations. This publication consists of two parts: In
the first part, important properties of conventional hash-based PoW are
described. In the second part, theoretical PoUW concepts such as Coinami,
CoinAI and the first successful PoUW cryptocurrency Primecoin are analyzed with
respects to how PoW properties can be retained while doing useful work.
