In this work we explore the thermodynamic aspects of dark energy in different scenarios : as a perfect fluid with constant and variable equation of state parameter ; and as dissipative fluid described by a barotropic equation of state with bulk viscosity in the framework of the Eckart theory and the full Israel-Stewart theory .
We explore cosmological solutions for a flat , homogeneous and isotropic universe .
When modeled as a perfect fluid with a dynamical equation of state , p = w ( a ) \rho , the dark energy has an energy density , temperature and entropy well defined and an interesting result is that there is no entropy production even though been dynamical .
For dissipative dark energy , in the Eckart theory two cases are studied : \xi = const . and \xi = ( \beta / \sqrt { 3 } ) \rho ^ { 1 / 2 } ; it is found that the entropy grows exponentially for the first case and as a power-law for the second .
In the Israel-Stewart theory we consider a \xi = \xi _ { 0 } \rho ^ { 1 / 2 } and a relaxation time \tau = \xi / \rho ; an analytical big rip solution is obtained with a power-law entropy .
In all cases is obtained a power-law relation between temperature and energy density .
In order to maintain the second law of thermodynamics theoretical constraints for the equation of state are found in the different dark energy models studied .
A barotropic dark fluid with w < -1 is thermodynamically difficult to support , but the overall effect of bulk viscosity in certain cases allows a phantom regime without thermodynamic anomalies .