Abstract: Direct conversion systems for in situ conversion of geothermal energy into electricity are described in which thermoelectric generators and/or thermionic convertors directly convert the earth's thermal energy into electrical energy. The choice of a thermoelectric or thermionic system is dictated by the temperature domain of the particular geothermal formation.The basic unit of the described system includes at least two coaxially-intersecting heat pipes, extending between a high temperature geological strata and a low temperature geological strata. Thermal-electric energy conversion devices are positioned within an annulus between the intersecting heat pipes.