  In this paper we have investigated the limits imposed by thermodynamics on a
dark energy fluid. We have obtained the heat capacities and the
compressibilities for a dark energy fluid. The thermal and mechanical
stabilities require these quantities to be positive. We have shown that dark
energy fluids must satisfy the stability conditions and that such requirement
put difficulties in the cosmic fluid models with negative constant EoS
parameters. We have also shown that the observational constraints imposed by SN
Ia, BAO and $H(z)$ data on a general dark energy fluid with a time-dependent
EoS parameter are in conflict with the constraints imposed by thermodynamics.
This result indicates that dark energy fluid models are unphysical.
