  In contrast to the classical concept of a Carnot engine that alternates
contact between heat baths of different temperatures, naturally occurring
processes usually harvest energy from anisotropy, being exposed simultaneously
to chemical and thermal fluctuations of different intensities. In these cases,
the enabling mechanism responsible for transduction of energy is typically the
presence of a non-equilibrium steady state (NESS). A suitable stochastic model
for such a phenomenon is the Brownian gyrator -- a two-degree of freedom
stochastically driven system that exchanges energy and heat with the
environment. In the context of such a model we present, from a stochastic
control perspective, a geometric view of the energy harvesting mechanism that
entails a forced periodic trajectory of the system state on the thermodynamic
manifold. Dissipation and work output are expressed accordingly as path
integrals of a controlled process, and fundamental limitations on power and
efficiency are expressed in geometric terms via a relationship to an
isoperimetric problem. The theory is presented for high-order systems far from
equilibrium and beyond the linear response regime.
