Page 310 - Handbook of Energy Engineering Calculations

must be turned away from the sun because there is too much heat for the
turbine capacity. When this occurs, almost every other row of mirrors must
be turned away. However, in the winter, when solar radiation is the weakest,
every mirror must be employed to produce the required power.
In the Mojave Desert plant, the mirrors focus the collected heat on the

HCEs—coated steel pipes mounted inside vacuum-insulated glass tubes. The
HCEs contain a synthetic-oil heat-transfer fluid, which is heated by the
focused energy to approximately 735°F (390.6°C) and pumped through a

series of conventional heat exchangers to generate superheated steam for the
turbine-generator.
In the Mojave Desert plant, several collectors are assembled into units
called solar collector assemblies (SCA); generally, each 330-ft (100.6-m) row
of collectors comprises one SCA. The SCAs are mounted on pylons and

interconnected with flexible hoses. An 80-MW field consists of 852 SCAs
arranged in 142 loops. Each SCA has its own sun sensor, drive motor, and
local controller, and is comprised of 224 collector segments, or almost 5867
2
2
ft (545 m ) of mirrored surface and 24 HCEs. From this can be inferred that
2
2
some (5867/80) = 73.3 ft (6.8 m ) per MW is required at this installation.
4. Plan for an uninterrupted power supply
To ensure uninterrupted power during peak demand periods, an auxiliary

natural-gas fired boiler is available at the Mojave Desert plant as a
supplemental source of steam. However, use of this boiler is limited to 25
percent of the time by federal regulations. This boiler serves as a backup in

the event of rain, for night production when called for, or if “clean sun” is
unavailable. According to Luz International, clean sun refers to solar
radiation untainted by smog, clouds, or rain. Figure 2 shows the firing modes
for typical summer (left) and winter (right) days. Correlation of solar
generation to peaking-power requirements is evident.