Thermal machines based on surface energy of wetting: Thermodynamic analysis



This work proposes an original thermodynamic-energetic analysis of the feasibility and ideal performance of thermal machines based on the wetting phenomenon proposed by V. A. Eroshenko. The extension or contraction of a liquid film is taken as a “tutorial” example to introduce the basic thermodynamic relations of this 2-D transformation. It implies both mechanical and thermal effects, and this coupling allows conversion of heat to work (thermal engine) or conversely to pump heat (refrigeration/heat pump effect). A similar approach is then developed for the interface between a liquid and a highly microporous solid, having a large internal surface area. The thermodynamic behavior of this interface involves as state variables the surface tension of the liquid, the contact angle, and their dependence on temperature. Depending on the relative magnitude and sign of these quantities, and, therefore, on the working couple and the temperature range, a variety of machine cycles are feasible, or excluded, and a method is proposed for a comprehensive inventory. Order-of-magnitude calculations of the energy densities are presented based on the existing experimental data for several systems involving water as the fluid. The tentative conclusions are that the energy densities are very small on a mass basis compared to conventional systems based on vaporization, but the contrary is true on a volume basis because the phase transformation (extension of the surface) occurs in a condensed state. There may, therefore, be some niches for thermal machines of this type, but they remain to be identified and validated.