  Several studies have combined heat and gravitational energy exchanges to
create novel heat engines. A common theoretical framework is developed here to
describe thermogravitational cycles which have the same efficiencies as the
Carnot, Rankine or Brayton cycles. Considering a working fluid, enclosed in a
balloon, inside a column filled with a transporting fluid, the cycle is
composed of four steps. Starting from the top of the column, the balloon goes
down by gravity, receives heat from a hot source at the bottom, rises and
delivers heat to a cold source at the top. Unlike classic power cycles which
need external work to operate the compressor, thermogravitational cycles can
operate as "pure power cycle" where no external work is provided to drive the
cycle. To illustrate this concept, the prototype of a thermogravitational
electrical generator is presented. It uses a hot source of low temperature
(average temperature near 57{\deg}C) and relies on the gravitational energy
exchanges of an organic fluorinated fluid inside a balloon attached to a
magnetic marble producing an electromotive force of 50 mV peak to peak by the
use of a linear alternator. This heat engine is well suited to be operated
using renewable energy sources such as geothermal gradients or focused
sunbeams.

General Physics