POWER July 2017 - 52

ENERGY STORAGE
4. Lighting up the night. Using sensible heat storage, the Crescent Dunes project is
able to produce energy long after the sun has set, helping power the lights of Las Vegas. Courtesy:
SolarReserve
requires less material to do the same job.
Where the molten salt system used at Crescent
Dunes requires two large tanks, one for
the " cold salt " and one for the heated salt,
the TerraFore system requires only one tank
and less material.
Within the tank, three different salts
would be stacked. At the top are salts with
the highest melting point and at the bottom
are salts with the lowest melting point,
with a layer of salts with a mid-range melting
point in the middle. The system uses a
thermal transfer fluid, which flows from the
solar receiver to the top of the tank where it
then flows through the stacked bed of capsules,
heating them in turn as the fluid slowly
cools. Once it reaches the bottom of the
tank, the thermal transfer fluid is pumped
back to the solar receiver.
That cycle continues until the stored energy
is needed, at which time the heat transfer
fluid is pumped from the tank into the
power block where steam is generated to
drive a turbine.
hours a day, though typically it only runs
12-14 hours a day. Generally, the plant is
producing electricity for NV Energy from
around 10 a.m. to 10 p.m. (Figure 4) simply
because it is not needed for more than that.
" In some markets, like say Chile or other
markets, they really want a project like
this to run 24 hours a day. The U.S. has a
pretty diverse mix of power; there's still a
fair amount of coal-burning power projects
out there and a lot of nuclear, and those facilities
have to run 24 hours a day, " Smith
noted.
However, as the nation's energy mix continues
to change, SolarReserve predicts that
Crescent Dunes could be operating for longer
stretches. " Longer-term, a lot of [coal
and nuclear] will be phased out, and as the
U.S. markets change over the long-term, we
may see that we could be operating more 24
hours a day. But for right now, it's more the
peak demand periods 8, 10, 12, 14 hours a
day of generation and typically well into the
evening hours, " Smith said.
Implications for the Future
While sensible heat transfer currently dominates
the TES field, there is a lot of research
and development activity in PCMs and
52
TCS. One such project, an encapsulated
PCM developed by TerraFore Technologies,
has received significant support from
the Department of Energy and may be on
the verge of commercialization.
As noted, PCMs hold an advantage over
sensible heat storage mediums due to their
increased energy density. The best way to
take advantage of PCMs, Mathur said, is to
encapsulate them. However, doing so is no
easy task.
" The big issue of encapsulating is how do
you encapsulate a solid which expands almost
20%? " he said. " That means you have to have
a capsule, think about a ball partially filled
with salt, how do you make that? You cannot
open it and fill part of it and seal it because
sealing becomes an issue. "
The answer, according to Mathur, is polymers.
" I thought about it, dreamed about it,
so I said why can't we put some kind of a
polymer on top of my capsule, on the salt?
Then put my shell material, and then heat it,
and then the polymer will disappear, leaving
a void behind, " he explained. " So that
gives it expansion volume. It is a chemical
way of creating a void inside the shell. "
TerraFore's system improves on the
traditional sensible heat system in that it
www.powermag.com
Finding a Good Match
Mathur doesn't see his PCM process being
adopted at large-scale CSP projects like
Crescent Dunes in the very near future, but
he hopes that at some point it might make
its way into that sector. For now, he believes
the best place for his technology to enter the
market is in distributed CSP.
Working with a company in Sweden,
Mathur believes he may have found a good
match. " They have developed a dish Stirling
system, which can operate at about 800-
900C, but it works exactly like a PV-when
the sun shines it generates electricity. So
they cannot be competitive with PV because
PV costs are so low. Now, if they can put
storage with it, they become competitive
with a dispatchable standpoint, " he said.
For the time being, SolarReserve is happy
with the technology it has developed.
The company currently has a number of
projects under development around the
world, all based on the same technology
used at Crescent Dunes. " The core technology,
which is our receiver technology,
does not really need to be modified very
much when we go from location to location, "
Smith explained.
However, that doesn't mean the company's
leadership isn't keeping an open mind.
" We're keeping our eye on other options out
there on thermal storage and other materials
that we can utilize, " Smith said, " but right
now we really don't see anything that's on
the horizon near-term that can be done on a
commercial scale. " ■
-Abby L. Harvey is a
POWER reporter.
POWER | July 2017

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POWER July 2017

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