  Dirac structures are geometric objects that generalize Poisson structures and
presymplectic structures on manifolds. They naturally appear in the formulation
of constrained mechanical systems and play an essential role in structuring a
dynamical system through the energy flow between its subsystems and elements.
In this paper, we show that the evolution equations for open thermodynamic
systems, i.e., systems exchanging heat and matter with the exterior, admit an
intrinsic formulation in terms of Dirac structures. We focus on simple systems,
in which the thermodynamic state is described by a single entropy variable. A
main difficulty compared to the case of closed systems lies in the explicit
time dependence of the constraint associated to the entropy production. We
overcome this issue by working with the geometric setting of time-dependent
nonholonomic mechanics. We define three type of Dirac dynamical systems for the
nonequilibrium thermodynamics of open systems, based either on the generalized
energy, the Lagrangian, or the Hamiltonian. The variational formulations
associated to the Dirac systems formulations are also presented.
