Basic Research Challenges for Solar Thermal Utilization
CURRENT STATUS
Solar thermal utilization can be categorized into low-temperature solar thermal systems, which
may not involve sunlight concentration, and high-temperature solar thermal systems, which
require sunlight concentration. Concentrated photovoltaics (CPVs), although not a solar thermal
process, crosscuts with solar thermal utilization through the use of concentrators.
High-temperature Solar Thermal Systems
High-temperature solar systems use various mirror configurations to concentrate the light and
then convert the sun's energy into high-temperature heat. The heat can be converted into
electricity through a generator, or it can be used to drive chemical reactions. A plant consists of
three parts: an optical system that collects and concentrates the light, a receiver or reactor that
converts the light to heat, and an “engine” that converts heat to electricity or “reactor” that
converts heat to chemical potential.
We will survey the principles and state of the art of the optical systems used for concentration,
discuss the engines or other components that convert the concentrated heat into electricity, and
finally evaluate the state of the art for reactors that convert the heat into chemical fuels.
Solar Concentrators
The current status of solar concentrators, including current research directions, is treated in the
Solar Thermal Technology Assessment, Appendix 1. We offer a brief survey for the convenience
of the reader.
Line Focus Systems. In line focus systems, incident sunlight is “folded” from a plane to a line.
In most cases, the optical configuration is that of a trough tracking the sun from east to west and
a target that rotates accordingly (Figure 14a). The main inherent advantage of the system is its
compatibility with large engines (i.e., steam turbines of hundreds of megawatts). The main
inherent disadvantage is the low operating temperature, limited to less than 750K by the
relatively low concentration and long tubular receiver configuration. Lower temperatures reduce
the efficiency of the heat transfer to the fluid located in the tubular receiver; this fluid provides
the thermal energy to drive electricity generation cycles. The current systems range from
350 MWe to newer small-scale 1-MWe systems. Current installed cost is approximately $3/W;
the short-term goal is to reduce this cost to $2/W.