This chapter discusses some energy-conversion technologies where heat is directly converted into electricity or is a previously created product by energy conversions. It also analyzes conversion of solar radiation into electricity, Seebeck technology applied in thermos-couples, Schonbein/Grove technology with oxidation control, Rankine cycle technology and Brayton cycle technology. Rankine technology realizes energy transfer between the heat generator and the thermal engine using smaller amounts of fluid mass. Technologies using internal ionization and barrier semiconductor layers resulted from the first method, including photovoltaic cells which operate using radiation of hot bodies, as well as thermoelectric converters exploiting the Seebeck effect for direct conversion of heat into electricity. To clarify the mechanisms of internal ionization of bodies one must remember the principle of operation of Einstein atomic oscillators in a crystal lattices, considering the zone theory of atom and molecule structure. Solar internal ionization is a phenomenon where two different energy forms are obtained electrical and thermal.