  Glass is an under-cooled liquid that very slowly relaxes towards the
equilibrium crystalline state. Its energy balance is ill understood, since it
is widely believed that the glassy state cannot be described thermodynamically.
However, the classical paradoxes involving the Ehrenfest relations and
Prigogine-Defay ratio can be explained when the `effective' or `fictive'
temperature of the slow modes is taken as additional system parameter. Having
straigtened out the proper picture, it is interesting to reconsider glass from
a thermodynamic viewpoint. A shard of glass, kept at fixed temperature and
volume, relaxes towards lower energy. Heat is released, inducing apparent
violations of all basic thermodynamic laws. The most interesting application is
to use glassy or amorphous systems as a source of energy, by extracting the
configurational energy in a process of melting followed by crystallization.
