Heat pipe
The system of directed (ordered in one direction) heat flow is named as
heat pipe.
Since velocity of the vapor in one direction and velocity of the liquid
in other direction is different, the moment of the system is not equal
to zero. The compensation of this "free energy" is heat exchange with environmental
or with special heating and cooling system. The heat inflow and heat outflow
are classified as unordered energy and by this way the result sum vector
outside of the heat pipe is equal to zero.
Such sort drive for spacecraft was proposed by author in 1996,
report "Gravitation concept".