Yet another of my papers now put into the public domain. Here is a suggestion for a thermoelectric converter that could operate at room temperature.
Enjoy!
And here is another paper, this time using a ferroelectric cathode that emits electrons on receipt of mechanical impulses (like the piezo gas ignitor does). Since there is a useful source of impulses in the exhaust from car engines, a converter could be designed to convert waste heat into useful electrical power. Beyond the reach of garage tinkerers but maybe could be considered as a feasibility project by a University or a car manufacturer.