Absorption and Conversion of Sensible Heat and Solar Radiation into Mechanical Work Using a Self-Pumping Capillary Thermal Engine with Passive Cooling Potential

This paper proposes a self-pumping capillary thermal engine that absorbs solar radiation and sensible heat with a spectrally selective absorber and converts part of it into mechanical work through the evaporation and condensation of a low-boiling organic fluid, while a capillary wick returns the liquid to the evaporator without a mechanical pump. The performance is analysed for the climate of Khartoum using hourly NASA POWER data for 2025 and fluid properties from CoolProp. For an evaporator at 75 °C and a condenser at 45 °C the Carnot limit is about 8.6%, and the estimated mechanical power is 1.5-2.8 W per 0.1 m2 at solar noon. The annual mean daily output is 7.9-14.5 Wh per 0.1 m2 (79-145 Wh m-2 per day), with a stable output throughout the year. A comparison of R1233zd(E), n-pentane and ethanol shows similar power but very different maximum wick pore radii (0.05, 0.11 and 0.52 µm), which emerges as the governing design constraint. A second operating mode with an evaporator near 50-55 °C is described for cooling photovoltaic panels and machines, and the thermodynamic limits of heat rejection and routes toward higher efficiency are discussed. The results are numerical and no experiments are reported.