The figure illustrates a solar thermal energy system for boiling water or another liquid. The system can be used for a variety of purposes that can include drying aqueous hazardous waste, distilling pure solvent from spent solvent, purifying water by distillation, or generating steam. The principal innovative feature of this system is an absorber/evaporator unit, which is designed to absorb radiant solar energy with an effectiveness close to that of an ideal (in the black-body sense) absorber. The design is such that unlike in some other systems, it is not necessary to coat the solar-irradiated surface with a high-solar-absorptivity ("solar black") material to obtain the desired black-body characteristic. The design is also simpler than that of other absorber/evaporator units.
Though the absorber/evaporator unit described here is cylindrical, it could just as well have a spherical, conical, or other convenient shape. The absorber/evaporator unit includes an inner cylinder and a slightly wider outer cylinder, both made of stainless-steel sheet. The inner cylinder acts as the solar-energy absorber for heating the liquid, which is pumped into the narrow annulus between the inner and outer cylinders. The narrow-annulus feature is an important element of the design in that a liquid can be boiled most effectively when confined in a thin layer. The vapor leaves through vent tubes at the top, then flows through vent hoses to the condenser. The vent tubes, the vent hoses, and the outer surface of absorber/evaporator unit (except at the opening) are covered with insulating material.
Concentrated solar radiation enters the absorber/evaporator unit through a solar-glass-covered opening at its lower end. The absorber/evaporator unit is positioned and oriented to face a paraboloidal solar-radiation concentrator; the cylindrical axis of the unit is aligned with the optical axis of the concentrator and the unit is positioned along this axis with the center of the opening at the focal point of the concentrator. Of course, the paraboloidal concentrator must be mounted on an azimuth/elevation tracking mechanism that keeps the concentrator aimed toward the Sun.
This optical arrangement ensures that most or all of the concentrated solar radiation enters the cavity enclosed by the inner cylinder, yet the opening is small enough to provide a relatively small solid angle for the escape of thermal radiation from the cavity. Once solar radiation enters the cavity, some is absorbed immediately, some is absorbed after one or more reflection(s), some is reradiated, and some of the reflected and reradiated energy escapes through the opening. The opening is small enough that the solid angle for the escape of radiation is relatively small; consequently, the incident radiation undergoes multiple cycles of reflection, absorption, and reradiation that convert the radiation field in the cavity into one that approximates a black-body radiation field, even in the absence of a high-absorptivity coating on the absorber surface.
With its thin-wall-shell design and without for a special high-absorptivity coating, the absorber/evaporator unit can be constructed relatively easily. The unit can also be disassembled relatively easily for cleaning and repairs.
This Solar Heating System for boiling a liquid features an improved absorber/evaporator unit that exploits geometry to achieve an absorption characteristic approximating that of a black body. The seemingly peculiar arrangement of vent tubes can be understood, upon closer examination, to prevent spillage of liquid when the unit is tilted toward the east or west.
This work was done by Feng-Nan Lin, William I. Moore, and Philip D. Stroda of Kennedy Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Physical Sciences category, or circle no. 160 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).KSC-11895