Page 36 - Design of Solar Thermal Power Plants

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION 29

thermal storage system, the design point is typically defined by consid-
ering the output capacity of the collector field, which is equivalent to the
thermal power of the steam turbine generator under full load operation.
Examples are as follows:

A. Design point of some CSP plant
Time: midday of spring equinox
Solar irradiance and environmental conditions: solar direct normal
irradiance ¼ the mean solar direct normal irradiance for multiple
years at the locality on the spring equinox; ambient air
temperature ¼ mean ambient air temperature for 30 years at the
locality
B. Design point of some CSP plant
Time: midday of spring equinox
Solar irradiance and environmental conditions: solar
2
irradiance ¼ 1000 W/m ; ambient air temperature ¼ mean ambient
air temperature for 30 years at the locality on the spring equinox
Differences between A and B are as follows:

1. When solar irradiance exceeds the set value of the design point, the
thermal output of the collector field in plan A can be transmitted to
thermal storage, which means that in cases of partial thermal output
of the collector field being transmitted to thermal storage, there will
be no impacts on full-load operation of the steam turbine. For plan B,
solar irradiance has already been defined as the maximum value
possible on the earth’s surface, which does not exist.
2. When solar irradiance is less than the set value of the design point,
the steam turbine cannot operate under full load. Thanks to the
design of plan B, collector field output can never directly drive the
steam turbine to operate under full load; instead, full-load
functioning of the steam turbine relies solely on the operation of the
thermal storage system.

Based on the above, plan A is more optimized than plan “B.” However,
with plan “B,” the collector field’s energy output will never be “exces-
sive,” yet such an outcome is possible with plan A. For areas with an
extremely nonuniform seasonal distribution of solar irradiance, the
annual mean solar irradiance is low but the transient solar irradiance is
high, and this might result in collector field output that exceeds the
requested level of the steam turbine and thermal storage under certain
weather conditions. At that moment, part of the concentration field will be
closed, resulting in wasted investment; for example, in Hainan Province
in China. For arid and semiarid areas in northwestern China, the daily
mean solar direct normal irradiance is comparatively even, for which plan
“A” is suitable; for Hainan, plan “B” is more appropriate.