There is a theory that proof-of-work (PoW) may exhaust all energy available to people. PoW converts energy into a monotonically increasing double-spend barrier for any given transaction. This is comparable to the energy expended in securing any money against counterfeit (by its own issuer or otherwise).

The purpose of any security measure is to create a cost necessary to overcome the measure, i.e. a financial barrier. Bitcoin creates its double-spend barrier by compelling the attacker to replace the branch of the targeted transaction with one of probabilistically greater work. Interestingly, such a replacement raises the barrier to subsequent attackers. The energy expended is not independently important, the erected barrier is the attacker's necessary financial burden.

The security barrier (S) of a block is the product of unit hash cost (C), hash rate (H), and period (T).

S = C * H * T

The adjustment varies hash rate to maintain a constant period for a given hash cost and security.

T = S / (C * H)

A constant period implies that hash rate is inversely proportional to cost for a given security.

H ~ S / C

As energy supply is reduced its price must increase, which reduces the amount expended for a given level of security. Therefore energy cannot be exhausted by mining and the theory is invalid.