Basic Research Needs for Solar Energy Utilization

Heliostat

Field

Receiver & Power

Conversion Unit

Heliostats

Solar Tower Reflected Light Beams

Structure

Figure 75 Central receiver (solar power tower) system configuration

Several power conversion methods have been developed for solar central receivers. A 10-MW
system using molten salt as heat transfer fluid and storage medium, combined with a steam
Rankine turbine at up to about 850K was demonstrated in the DOE Solar II project. Other
methods are (a) steam generation and superheating in the receiver, (b) heating of atmospheric air
to about 950K in the receiver and then using it to superheat steam, and (c) heating of compressed
air in the receiver to over 1,100K and using it in a solar/fuel hybrid gas-turbine.

On-axis Tracking Systems. On-axis systems, such as the parabolic dish concentrators depicted
in Figure 76, provide the highest optical efficiency of all the concentrating solar systems. Their
main drawback is the concentrator size, which is limited by practical structural considerations;
dishes with reflective area of 15–400 m^2 have been built and tested. Much of the development of
on-axis systems involved the use of solarized Stirling engines for power conversion. Recent
progress in small Brayton engine development provides the option of using a dish/Brayton
system as an alternative to the dish/Stirling system.

Recent advancements related to central receivers and on-axis tracking systems focus on
improving performance and reducing the cost of the concentrator (heliostat or dish) by increasing
the reflector size and working with low-cost structures, better optics, and high-accuracy tracking.
Optical efficiency has been increased by means of nonimaging secondary concentration
(Welford and Winston 1989; Ries et al. 1997a), improved tracking methods, and better optical
system design software. New “directly irradiated” or “volumetric” receivers have been found