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.

7 pages, 1 figure, 1 table. This an updated version of the article originally named "Is this the end of dark energy?"