  A significant observational effort has been directed to unveil the nature of
the so-called dark energy. However, given the large number of theoretical
possibilities, it is possible that such a task cannot be performed on the basis
only of the observational data. In this article we discuss some thermodynamic
properties of this energy component by assuming that its constituents are
massless quanta with a general time-dependent equation-of-state parameter
$\omega(z)=\omega_0 + \omega_a f(z)$, where $\omega_0$ and $\omega_a$ are
constants and $f(z)$ may assume different forms. We show that very restrictive
bounds can be placed on the $w_0$ - $w_a$ space when current observational data
are combined with the thermodynamic constraints derived.
