NOVELTY - The compressor 16 has a first constant volume container (5) which are provided with a first and second heat exchanger (133,134) are coupled in series and provides heat to working fluid. A container connection valve (205,207) controls flow of working fluid between the first and a second container (6). A controller (116) controls valve and heats working fluid for inducing natural circulation from the first container to the second container due to difference in pressure, working fluid density and fluid level.    USE - Thermal compressor for compressing working fluid such as water, atmospheric air, refrigerants such as R134a, R1234yf, R407c, Rll, R12, R13, R14, R21, R22, R23, R32, R41, RI13, R114, R115, R116, R123, R124, R125, R141b, R142b, R143a, R152a, Organic Rankine cycle fluids such as R245fa, R141b, R236fa, R218, R227ea, R236ea, R245ca, R365mfc, RC318, ammonia, propane, carbon dioxide through use of thermal energy form thermal solar collector, turbo machine, geothermal installation, steam generating apparatus, photovoltaic energy saving installation, ambient heat of environment, internal combustion engine, nuclear reactor, burner, electrical coil, heat concentrated by mirror, concentration power plant, stirling engine, Fracking natural gas fire, thermoelectric material, and heat generated from cooling machine (all claimed).    ADVANTAGE - The compression of working fluid within the containers using thermal energy allows conversion of potential energy without additional installations and thus, energy loss is prevented. The absence of fan and moving components nullify mechanical losses and eliminates need for scheduled maintenance. Thus, efficiency and reliability of the system is increased and construction of thermal compressor is made simpler with less cost. The addition of additional heating containers results in requirement of less energy for achieving desired pressure.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a method of thermally compressing a working fluid.    DESCRIPTION OF DRAWING(S) - The drawing shows a circuit diagram explaining operation of a thermal compressor connected to a thermal circuit and a work producing system that generates electricity.    First container (5)    Second container (6)    Controller (116)    First heat exchanger (133)    Second heat exchanger (134)    Connection valves (205,207)