Because
of their practical energy storage, solar power towers have
two features that are particularly desirable for utilities:
flexible capacity factors and a high degree of dispatchability.
Power
towers can be designed with annual capacity factors up to
60 percent, and as high as 80 percent in summer when the days
are longer. 
This means a power tower can operate at capacity for up to
60 percent of the year without using fossil fuel as a back-up,
thus being able to deliver power during most peak demands.
Without energy storage, the annual capacity factor of any
solar technology is generally limited to about 25 percent.
A solar power tower's high capacity factors are achieved by
building the solar portion of the plant with extra heliostats
so that during daylight, sufficient energy is collected to
power the turbine, while extra energy can be put into the
thermal storage system. At night or during extended cloudy
periods, the turbine is powered with stored thermal energy.
The dispatchability of a solar power tower - its ability to
deliver electricity on demand - is illustrated above, where
three different parameters are plotted against time of day:
the intensity of sunlight (insolation), the amount of energy
stored in the hot-salt tank, and the
output power from the turbine generator. In this example,
sunrise on a winter's day is around 7 a.m., and the intensity
of sunlight rises quickly to reach its maximum at noon and
drops off at sunset around 5 p.m.
The
solar plant begins collecting energy shortly after sunrise
and stores it in the hot-salt tank - the level of energy in
storage increases during daylight hours. The turbine is brought
on-line not at sunrise, but when the power is needed, in this
example at 11 a.m. The output power of the plant is constant
throughout the day, even though there are fluctuations in
the intensity of sunlight. After sunset, the turbine continues
to operate on energy from the storage tank; note the level
of energy in storage declines after sunset. The turbine operates
continuously until 9 p.m. using the thermal energy in storage.
In the summer when the days are longer, the turbine would
be able to operate a larger fraction of each day.
In
designing a power tower, the size of the turbine, the fraction
of the day it is in operation, and the period when it is operated
are completely flexible. The plant's efficient thermal storage
system provides dispatchability, and by adjusting the size
of the solar field and the size of the storage tanks, the
capacity factor can be tailored to meet the specific needs
of a utility.
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