  In research articles and patents several methods have been proposed for the
extraction of zero-point energy from the vacuum. None of the proposals have
been reliably demonstrated, yet they remain largely unchallenged. In this paper
the underlying thermodynamics principles of equilibrium, detailed balance, and
conservation laws are presented for zero-point energy extraction. The proposed
methods are separated into three classes: nonlinear processing of the
zero-point field, mechanical extraction using Casimir cavities, and the pumping
of atoms through Casimir cavities. The first two approaches are shown to
violate thermodynamics principles, and therefore appear not to be feasible, no
matter how innovative their execution. The third approach, based upon
stochastic electrodynamics, does not appear to violate these principles, but
may face other obstacles. Initial experimental results are tantalizing but,
given the lower than expected power output, inconclusive.
