Electrochemical energy conversion by-passes second law of thermodynamics

New spacecraft will explore the outer regions of the solar system where sunlight is too weak for generating the required electric power. Europa, the orbiter of Jupiter´s moon, will require 210 watts of power for its six years of post-launch lifetime. Low-power radioisotope-heated thermoelectric generators have powered 26 spacecraft on deep-space missions, but their efficiency is only around 6%. Developing a larger radioisotope general-purpose heat-source (GPHS) for the new missions is not feasible in today´s social environment. One candidate for converting heat to electric power from the existing radioisotope heater is the alkali-metal-electrochemical (AMTEC) converter. With its heat source it would weigh 18 kg and deliver 210-watts with an efficiency of 21.6%. Lifetimes in test are now over 60000 hours. The alternative energy converter is the free-piston Stirling engine in which the pistons are precisely supported with flexing diaphragms so that pistons do not have any moving contact with cylinder surfaces. Maintenance-free operating lifetimes of 100000 hours have been demonstrated. A 210-watt Stirling-cycle converter and its GPHS would weigh 16.1 kg and have an efficiency of 26%. An interesting new approach could be a combined-cycle power source in which the Stirling engine is topped by an AMTEC converter