Thermocouple efficiency

Has anybody calculated the generating efficiency of a simple metal thermocouple? I have built a few bismuth telluride generators and have found them to be a bit fragile, as any operating temperatures above, say, 150 deg C melts the solder and destroys the chip. However I have found that if I make a number of metal thermocouples in series, they easily generate a good voltage and can be used up to 1000 deg C +.