  Energy conversion in nanosized devices is studied in the framework of
state-space models. We use a network representation of the underlying master
equation to describe the dynamics by a graph. Particular segments of this
network represent input and output processes that provide a way to introduce a
coupling to several heat reservoirs and particle reservoirs. In addition, the
network representation scheme allows one to decompose the stationary dynamics
as cycles. The cycle analysis is a convenient tool for analyse models of
machine operations, which are characterized by different nanoscale energy
conversion processes. By introducing the cycle affinity, we are able to
calculate the zero-current limit. The zero-current limit can be mapped to the
zero-affinity limit in a network representation scheme. For example, for
systems with competing external driving forces the open-circuit voltage can be
determined by setting the cycle affinity zero. This framework is used to derive
open-circuit voltage with respect to microscopic material energetics and
different coupling to particle and temperature reservoirs.
