Some members of the Stirling Engine Society have been playing about with various designs of Thermal Lag Engine. I'm hoping that there will be some examples at the midlands exhibition later this month, to have a tinker with. (I do know that one of the people who has been building these devices will be there.)
There have been various theories suggested for how these odd little engines work - some slightly more feasible than others. But there still seem to be a number of unknowns. I suspect it is high time that thermal plasmonics was thrown into the mix - just to confuse matters still further
If you google around, you can find examples of some thermal lag engines online - along with some videos. The challenge (as always) is to improve the efficiency of the current devices - so that they might find more of a use than just being cute.
I also see that our friend Allan Organ is now getting in on the act - with his latest (unaffordable) textbook.