The topic of generating electricity with thermocouples comes up in the alternator-less threads.
So here's a link sent to me by a metrompg.com visitor that outlines quite an interesting test done with a professionally built thermocouple unit that uses both exhaust gas and coolant heat. It was fitted to a GM pickup and tested on a dynamometer:
http://mithril.camp.clarkson.edu/~helenbrk/Research/JoAE_AETEG_test.pdf