  We review theoretical approaches to analyzing efficiency of steady state heat
to work conversion which is crucial in the timely problem of optimizing
efficiency of small-scale heat engines and refrigerators. A rather abstract
perspective of non-equilibrium statistical mechanics and dynamical system's
theory is taken to view at this very practical problem. Several recently
discovered general mechanisms of optimizing the figure of merit of
thermoelectric efficiency are discussed, also in connection to breaking
time-reversal symmetry of the microscopic equations of motion. Applications of
these theoretical and mathematical ideas to practically relevant models are
pointed out.
