Proof of Memory Facade
It has been proposed that a proof-of-memory (PoM) can replace some fraction of the proof-of-work (PoW) energy cost with hardware, even relying on existing memory devices. As shown in Energy Waste Fallacy, a constant level of security requires a constant ongoing expenditure. Therefore such a system would require a comparable level of hardware consumption to offset any reduction in energy cost. In other words total energy consumption cannot be reduced, it can only be transferred to hardware manufacture, operation and disposal.
In December 2017 the estimated annualized cost of energy consumed in Bitcoin mining was $1,628,000,000, based on the approximations of 32.56 terawatt hours consumed at an average $.05 per kilowatt hour energy cost. Contemporaneously this cost level equates to the consumption of 32,560,000 terabyte drives at an average price of $50 per drive. Utilization of existing underutilized memory reduces the unit cost and therefore comparably increases the size requirement.
It is worth considering the economic behavior of a theoretical system in which PoM is determined by an existing (cost free) fixed pool of memory with no expiration or operational costs. As the cost of mining is zero, rewards flow at no expense in proportion to memory (assuming no pooling pressures). Any increase in average fee increases this reward for memory. Capital invested is zero and therefore rate of return is perpetually infinite. Despite unbounded incentive, the assumption of zero expansion precludes competition. But since the proof is externalized, competition cannot actually be restricted. In an actual system hardware manufacture expands perpetually for a given fee level, and this expansion accelerates with fee level increases.
Proof-of-memory is equal to proof-of-work in terms of resource consumption and there is no reason to assume a reduced energy component of that cost. The hardware acts as a proof battery, representing energy provably consumed in its manufacture. This is a facade analogous to the "zero emission" battery-powered car.