Heat Transfer

Thermal technologies and thus also solar thermal always involve heat transfer within a body or between bodies. In some cases the heat transfer has to be enhanced in order to achieve a highly efficient optical conversion of solar radiation into heat, in other cases a minimal heat loss over the system boundaries to the environment is the target. In particular, the optical conversion of short wave solar irradiance (electromagnetic waves) to heat occurs in solids. Within the solid body the heat is released mostly volumetrically in thin layers or layer stacks causing not only temperature rise but also differential thermal elongation. This heat has to be transported via passive means towards a heat sink for utilization. At the same time, however, a body that is warmer than its surroundings loses this heat in the form of thermal radiation and convection, and as the temperature rises, this proportion increases rapidly. This conflicting feature emphasizes the excellent importance of understanding passive heat transfer mechanisms in solar thermal applications. In this context we consider as passive heat transfer processes such as heat conduction and heat radiation, which are not associated with a medium transport. Convection, however, which enhances heat transport through molecular heat conduction in fluid media by a movement of fluid volumes, will be treated in the Chap. 5 . The driving mechanisms of passive heat transport are scalar quantities such as pressure and temperature, which characterize the thermodynamic state in a continuum, as well as waves, which propagate in a medium. The propagation of the waves in a solid or fluid medium, in turn, depends on its molecular thermo-physical properties and in cases of solids even its lattice structure. For this reason, any temperature difference leads unavoidably to a heat transfer changing the thermal properties of a solar thermal system, but also represents a key variable in component design. An important technical parameter here is, among others, the maximum permissible wall temperature. After an introductory description of the distinction between heat transfer and transport processes, the steady and transient heat conduction in gases, fluids and solids is covered. Successively, radiative heat transport in different configurations as well as methods to evaluate to transferred quantities are elaborated. Finally, as a result of a heat transport by conduction and radiation across interfaces mixed situations may occur, in which density differences in the adjacent fluid/gas are induced yielding to natural buoyant convective fluid motion increasing the heat transfer. Some selected cases of combined heat transfer problems are covered in the last section.