Stationary Solar Thermal Collectors

Solar thermal collectors are the core components of solar thermal energy systems, converting the solar radiation into heat, which is transported to a demand location by active or passive means. Solar thermal collectors can use the whole solar spectrum, not only a specific spectral part. But converted solar radiation cannot be fully used as inevitable thermal losses from the hot absorbers to the colder environment cannot be completely avoided. Therefore, the design of solar collectors must compromise—depending on the specific application—between maximizing the optical gains and the minimization of thermal losses. Highly transparent covers and selective absorbers with small thermal emissivities are used to achieve this target. Concentration by mirror optics and consecutive reduction of receiver area is a powerful tool to obtain an even better energy balance also for very high temperatures. Stationary collector types without or with small concentration are described in their main characteristics and the internal processes described, as well as the concentrating collector types which need one- or two-axis tracking devices to be able to collect the direct solar radiation from the sun over the day. In a last part collector testing methods are described and discussed, as well as the stagnation of collectors at which the highest absorber temperatures are reached without any net energy output, as thermal losses and solar gains just cancel out.