  When a black hole evaporates, there arises a net energy flow from the black
hole into its outside environment due to the Hawking radiation and the energy
accretion onto black hole. Exactly speaking, due to the net energy flow, the
black hole evaporation is a nonequilibrium process. To study details of
evaporation process, nonequilibrium effects of the net energy flow should be
taken into account. In this article we simplify the situation so that the
Hawking radiation consists of non-self-interacting massless matter fields and
also the energy accretion onto the black hole consists of the same fields. Then
we find that the nonequilibrium nature of black hole evaporation is described
by a nonequilibrium state of that field, and we formulate nonequilibrium
thermodynamics of non-self-interacting massless fields. By applying it to black
hole evaporation, followings are shown: (1) Nonequilibrium effects of the
energy flow tends to accelerate the black hole evaporation, and, consequently,
a specific nonequilibrium phenomenon of semi-classical black hole evaporation
is suggested. Furthermore a suggestion about the end state of quantum size
black hole evaporation is proposed in the context of information loss paradox.
(2) Negative heat capacity of black hole is the physical essence of the
generalized second law of black hole thermodynamics, and self-entropy
production inside the matter around black hole is not necessary to ensure the
generalized second law. Furthermore a lower bound for total entropy at the end
of black hole evaporation is given. A relation of the lower bound with the
so-called covariant entropy bound conjecture is interesting but left as an open
issue.
