The thermal conversion process of solar energy is based on well-known phenomena of
heat transfer (Kreith 1976). In all thermal conversion processes, solar radiation is
absorbed at the surface of a receiver, which contains oris in contact with flow passages
through which a working fluid passes. As the receiver heats up, heat is transferred to the
working fluid which may be air, water, oil, or a molten salt. The upper temperature that
can be achieved in solar thermal conversion depends on the insolation, the degree to
which the sunlight is concentrated, and the measures taken to reduce heat losses from
the working fluid. Since the temperature level of the working fluid can be controlled by
the velocity at which it is circulated, it is possible to match solar energy to the load ·
requirements, not only according to the amount but also according to the tempera.ture
level, Le., the quality of the energy required. In this manner, it is possible to design conversion
systems that are optimized according to both the first and the second laws of
thermodynamics.
2
The collection and conversion of the solar radiation to thermal energy depends on the
collector design and the relative amounts of direct beam and diffuse radiation absorbed
by the collector (Kreider and Kreith 1981). As indicated in the following discussion of
solar thermal collectors, the collectors used for higher temperature applications can collect
only the direct radiation from the sun. Figure 2 shows the annual average daily
direct normal solar radiation for the contiguous United States, Alaska, and Hawaii;
values range from under 2.78 kW/hr/m2 (10 MJ/m2) to over 7.22 kW/hr/m2 (26 MJ/m2)
(Solar Energy Research Institute 1981). Peak direct solar radiation at noon during a clear
day averages about I kW/m2• Generally speaking, the southwestern and western regions
of the country receive direct normal solar radiation levels sufficiently high for most high
temperature solar thermal conversion applications.
High temperature heat is needed by industry for process heat and by utilities for electricity.
In 1980, the last year for which statistics are available, industry and utilities
accounted for approximately 73 percent of the 76.3 quads of energy consumed in the
United States (Energy Information Administration 1980). The industrial process heat portion
alone was 20.6 quads (17 percent). Figure 3 displays a recent analysis by the Solar
Energy Research Institute (Krawiec et al 1981) of the distribution of industrial process
heat requirements by process temperature. It can be seen that 48.9 percent of the process
heat total falls below 500° F (260° C) and 34.0 percent is above 10aaoF (538°C).