In this demonstration we look at an application of heat transfer: the Stirling Engine. In this fundamental thermodynamic cycle a gas is alternately heated and cooled, compressed and expanded with the cycle converting a temperature differential into useful work. While it has some advantages over other cycles, in modern times it has only been used in a few specialized applications. Interestingly, NASA has been looking into using the Stirling engine as a power source for deep space (replacing the "TG" of the RTG we've been looking at) due to its far greater efficiency. The Wikipedia article on the Stirling engine is quite informative (our demo is in the "gamma" configuration), and the NASA technical report may be found here.
A key element in a Stirling engine is the thermal regenerator: basically a porous energy storage unit which absorbs and gives up heat as gases pass through it making the engine much more efficient. Thermal regenerators are also important in chemical engineering, particularly for catalytic oxidation reactions which are only slightly exothermic. The idea is to trap the heat in a "hot zone" of catalyst pellets by reversing the flow periodically. An interesting example of the use of reverse-flow autothermal reactors for removing methane from mine ventilation exhaust gasses is given here.