POWER December 2014 - 33

TOP PLANTS
1. Energy storage. Molten salt in a dozen large storage tanks is used to store thermal energy
produced by the concentrating solar collectors. The hot molten salt can be use to produce
electricity during periods when the sun doesn't shine. Courtesy: Abengoa Solar
Club's position on water use and Solana:
" It is on private land, not public land, and is
currently agricultural land. Depending on
how you calculate it [the power plant] will
use 75 to 85 percent less water than the current
agricultural use. It is still a fair amount
of water but it is much less than it takes to
grow alfalfa. "
Storing Solar Energy
The energy storage system uses molten salts
to store thermal energy that can be recovered
later in the day. Molten salt is used because it
is liquid at atmospheric pressure, its operating
temperatures and pressures are compatible
with conventional steam turbines, it is
nonflammable and non-toxic, and the price is
right. Molten salt is also commonly used in
the chemical and metals industries, so there
is an extensive operating history with these
fluids in non-solar applications. Generally,
molten salt is a mixture of 60% sodium nitrate
and 40% potassium nitrate, which melts
at 430F and is stored as a liquid at a minimum
of about 550F.
peak electricity demands during the summer
evenings and early night time hours, "
according to an Abengoa Solar statement.
This is particularly helpful during the hot
Arizona summer months, when air conditioners
run 24/7.
The financial and operating flexibility
benefits are apparent: Solana is capable of
operating with a 38% average annual capacity
factor against a typical 25% produced by
a utility-scale photovoltaic plant. Abengoa
estimates that Solana will produce about 944
GWh of highly desirable dispatchable generation
each year. According to the U.S. Energy
Information Administration, for the first
seven months of 2014, the plant produced
374,181 MWh, which is equivalent to a net
capacity factor of 29.5%, a very respectable
achievement. That statistic will climb once
the remaining high production summer and
fall months are accounted for.
The advantages in energy production that
accrue to a CSP plant with energy storage
must be balanced against a significant increase
in plant complexity. The plant's 3,200
mirrored parabolic-trough collectors, each
consisting of 28 curved mirrors, efficiently
December 2014 | POWER
reflect and focus solar energy on a thermal
" pipe " located at the focal point of the parabolic
trough as the assembly tracks the sun's
position in the sky. Therminol VP-1, the heattransfer
fluid, is heated up to about 735F by
the sun and circulated through a web of heat
collection pipes.
A heat exchanger transfers energy from
the Therminol to a secondary water loop. The
water, much like in a conventional boiler,
flashes into vapor at 1,450 psi and is then directed
to the plant's two 140-MW steam turbines.
The steam expands through the steam
turbine and is condensed in a condenser that
is cooled by a wet cooling tower. In sum, the
power-generation cycle is comparable to that
used in a conventional fossil-fueled thermal
power plant, with the exception that the Therminol
loop replaces the fuel-burning boiler as
the heat source.
Hybrid CSP-steam plants are often
criticized for excessive water usage because
their consumption is on par with a
like-sized conventional steam plant. Sandy
Bahr, a spokesperson for the Sierra Club
at the time the project was requesting
state permits in late 2008, stated the Sierra
www.powermag.com
Molten salt is stored in 12 insulated
tanks measuring 45 feet tall, 140 feet in diameter,
and holding 125,000 metric tons of
salt (Figure 1). During the day, some of the
heated Therminol is used to heat the molten
salt up to about 730F in six " hot " tanks.
During off-peak hours, molten salt is taken
from a " hot " tank, transfers its thermal
energy into steam production, and is then
stored in a " cold " tank. During the following
day, the molten salt is reheated and the
heat recovery process repeats. A " hot " insulated
storage tank can store heated molten
salt for up to a week. The entire energy
storage and retrieval process operates at
about 70% efficiency.
The advantage for APS with this arrangement
is that it decouples the solar collection
system from the energy storage system. APS
can effectively store electricity produced during
off-peak hours when wholesale prices are
lower and dispatch it during the evening onpeak
hours or even during the day to cover
adverse weather conditions.
In early October 2013, the plant " passed
commercial operation tests, " according to
Abengoa. The tests consisted of running
the plant's generator at full power while
also ramping up the thermal storage system.
Next, after allowing the solar part of
the plant to stop once the sun was down,
the plant continued to produce electricity
using only thermal energy storage for
another six hours. Those tests were very
successful, and the plant was placed into
commercial service. ■
-Dr. Robert Peltier, PE is POWER's
consulting editor.
33
http://www.powermag.com

POWER December 2014

Table of Contents for the Digital Edition of POWER December 2014

Contents
POWER December 2014 - Cover1
POWER December 2014 - Cover2
POWER December 2014 - Contents
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