About harvesting energy from high tempreture

The technology to cleanly and quietly turn heat into electricity without the use of a turbine or generator has existed for nearly a century. The trouble is, it has never been efficient enough for widespread practical use. 

I studied about Thermoelectric effect. There could be two methods two produce electricity from thermal energy.

First, by using stirling cycle power and converting thermal energy to mechanical energy(it is produced by difference in temperature and expanding and contracting. As a result a piston moves up and down and it is then converted to rotating energy) and then converting rotating energy to electrical power.

Second, by using Seebeck effect.(for more info see this article in wikipedia) The effect is that a voltage, the thermoelectric EMF, is created in the presence of a temperature difference between two different metals or semiconductors. This causes a continuous current in the conductors if they form a complete loop. The voltage created is of the order of several microvolts per kelvin difference. One such combination, copper-constantan, has a Seebeck coefficient of 41 microvolts per kelvin at room temperature. This little electricity can then be reproduced by DC/DC converters and finally be something about 3 or 4 volts. But the problem is that the ECT100(a thermal energy scavenging tech which I’ll introduce later) works with input voltage of at least several mv.