The rankine cycle system includes an evaporator coupled to a heat source and configured to circulate a working fluid in heat exchange relationship with a hot fluid from the heat source so as to heat the working fluid and vaporize the working fluid. An expander is coupled to the evaporator and configured to expand the vaporized working fluid from the evaporator. The exemplary expander is operable at variable speed. A condenser is coupled to the expander and configured to condense the vaporized working fluid from the expander. A pump is coupled to the condenser and configured to feed the condensed working fluid from the condenser to the evaporator.