Basic Research Needs for Solar Energy Utilization

Direct Sunlight Tubular Receiver

Linear Collector

Concentrated

Sunlight

Rotational

Axis

Figure 74 Trough (line focus) system configuration

generate electricity. 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.

About 350 MW of capacity, generated by trough systems, was installed in California in the
1980s and early 1990s and has been continuously improved (Price et al. 2002). Recent
developments have focused on means to increase operating temperature and/or reduce
components and electricity cost. These include optical improvements with new concepts (Mills
et al. 2004) and new designs to reduce optical losses in trough systems (Herrmann et al. 2004).
Receiver efficiency can be enhanced by developing selective coatings capable of reaching higher
temperature and by introducing smaller and more durable glass-to-metal seals and connections
between receiver pipes (Benz and Kuckelkorn 2004). Working fluids and heat transfer means can
be improved by direct steam generation in the trough (Almanza et al. 2002) or through the use of
molten salt as working fluid. Thermal storage can be introduced by using solid materials (Laing
et al. 2004) or high-heat-capacity fluids and phase-change materials (Tamme et al. 2004).

Central Receiver Systems. Central receiver systems are fundamentally Fresnel reflector arrays
where the reflectors (heliostats) have two axes of rotation and their common focus is stationary,
at the top of a tower (see Figure 75). The two-axis tracking enables higher concentration ratios
and higher operating temperatures than those of the line focus configuration, but as the system
size increases, the optical efficiency declines and this advantage is diminished. The power
conversion unit of large central receivers (20–200 MW) is likely to be a steam Rankine turbine,
while smaller central receivers can accommodate Brayton and combined cycles operating at
higher temperatures. Increasing the temperature generally leads to higher power conversion
efficiency but lower optical efficiency; thus, system optimization is required.