World’s most efficient thermoelectric material developed
"The new material, which is based on the common semiconductor telluride, is environmentally stable and is expected to convert from 15 to 20 percent of waste heat to electricity."

And for those ready to try this now:

http://thermonamic.en.alibaba.com/pr..._Recovery.html
Type: Thermoelectric Generator

Hot Gas: Temperature > 673 K

Output Power: 350 W

Open circuit voltage: 48 VDC

Matched Load Output Voltage: 24 VDC

Matched Load Output Current: 15 A

Internal Resistance: 1.6 Ohm

Dimension Size: 595 mm * 140 mm * 265 mm

Weight: 17.8 Kg

For those wanting to start out smaller:

http://www.ebay.com/itm/Thermoelectr...-/251817363025
Dimensions: hot side - 62 x 62 mm; cold side - 62x62 mm

Produces 13 W matched load.

Temperature differential: Tc = 50 Celsius; Th = 280 Celsius.

U-oc - 3.4 V

R-in - 2 Ohm

R-load - 2 Ohm

U-load - 1.33 V

Pretty expensive technology though, considering you'd need about 10 of the smaller units above to produce vehicle voltage. So you're looking at an initial investment of about $800 to produce a measly 13 watts maximum.