POWER December 2021 - 22

HYBRID POWER
storage, such as with ceramic bricks,
rocks, concrete, and packed beds; liquid
(or molten) salts; and underground thermal
energy storage. Latent heat storage
involves phase-change materials (PCMs),
which essentially enable change to a material's
phase (typically from a solid to a
liquid) to store thermal energy. A prominent
example is ice-thermal storage.
Thermochemical heat storage, which
has a higher energy density than sensible
and latent heat storage, involves two
technology families: reversible reactionbased
storage and sorption-based energy
storage. " Thermochemical systems
without sorption are based on a reversible
reaction of two separate chemical substances
where a high amount of energy
is generated as a result of an exothermic
synthesis reaction. In a sorption process,
heat is stored by breaking the binding
force between the sorbent and the sorbate
in terms of chemical potential, " IRENA
explains. Examples include chemical
looping, salt hydration, and adsorption
systems. Finally, mechanical TES systems
involve TES systems coupled with
mechanical energy storage technologies,
such as compressed air energy storage
and liquid air energy storage.
Promising TES Technologies for
the Power Sector
POWER's analysis suggests a broad
range of TES technologies are currently
under development or already in use for
power plant and grid integration. Here
are just a few prominent examples.
Liquid Salts. The most widely
commercially applied
TES technology
involves using molten salts at hightemperature
concentrated solar power
(CSP) stations. At the end of 2019, the
estimated worldwide power generation
capacity from CSP molten salt systems
was 21 GWh (60 GWhth, with an average
duration of seven hours). However,
molten salt hybrid configurations are also
being explored at solar PV and wind configurations,
as part of an integration with
natural gas combustion, and even to improve
the efficiency of existing coal and
advanced nuclear plants.
German entities RWE and RWTH
Aachen University, for example, in 2019
kicked off work to integrate a molten salt
system heated (to 600C) with surplus renewable
power to create steam, which
is then fed into a turbine at an existing
coal-fired plant in the Rhenish lignite
area. TerraPower's Natrium, which is set
to demonstrate a 345-MWe sodium fast
reactor in Wyoming under the U.S. De22
2.
Siemens Gamesa Renewable Energy's (SGRE's) electric thermal energy storage (ETES)
system in Hamburg, Germany, began operating in June 2019. The system contains about
1,000 tonnes of volcanic rock as an energy storage medium. It is fed with electrical energy
converted into hot air by means of a resistance heater and a blower that heats the rock to
750C. When demand peaks, ETES uses a steam turbine for the re-electrification of the stored
energy. " The ETES pilot plant can thus store up to 130 MWh of thermal energy for a week, "
SGRE said. Courtesy: SGRE
partment of Energy's (DOE's) Advanced
Reactor Demonstration Program within
seven years, will notably use a nitratesalt
molten salt system that its developers
claim has the potential to " boost the
system's output to 500 MWe of power
for more than five and a half hours when
needed. " The system derives its technology
from a system of similar scale
that is employed at the 280-MW Solana
CSP plant in Arizona. Nitrate-salt storage
system designs are also proposed for
fluoride-salt-cooled high-temperature reactors
with solid fuel and liquid salt coolants,
and molten salt reactors with fuel
dissolved in the salt.
Heat Transfer Oils. Another innovative
medium derived from the CSP sector-specifically
from parabolic trough
CSP plants-involves using heat transfer
oils such as Eastman's Therminol-66.
One example is a 16.6-MW CSP project
that forms part of the Brønderslev hybrid
solar-biomass plant in Denmark. Therminol-66
(along with ethylene glycol and
alumina beads) are slated for testing at
Idaho National Laboratory's experimental
Thermal Energy Distribution System,
a project that in December 2020 began
evaluating the interoperability of nuclear
reactors, energy storage, and ancillary
processes in a real-world setting.
Crushed Rock Heat Storage. TES
systems that use crushed rock are gaining
prominence throughout the power
space mainly for their low-cost ability to
provide large-scale heat storage. Since
its 2019 launch of a 30-MW/130-MWh
Electric Thermal Energy Storage (ETES)
pilot (with a 5.4-MW resistive heater)
in Hamburg (Figure 2), for example,
www.powermag.com
Siemens Gamesa Renewable Energy
(SGRE) says it has racked up interest
in the system that has a temperature
range of 180C to 750C. SGRE says its
technology, which essentially draws
power from the grid to heat volcanic
stones, can be converted back into power
using a 1.4-MW steam turbine generator
and produce power for up to 24
hours. The approach could give thermal
plants a second life, it says.
This June, the New York Power Authority
and the Electric Power Research
Institute (EPRI) launched a project
to explore Israeli firm Brenmiller Energy's
high-temperature crushed rock
TES system (Figure 3) in a range of
fossil generation assets. A Brenmiller
4-MW/23-MWh system was also installed
at an Enel combined cycle gas
turbine plant in Italy, between the gas
turbine and steam turbine. " The bGen is
charged with residual low-value steam
and discharges superheated steam at
peak tariff hours to allow energy shifting,
faster ramp-up and other revenue
streams, " said Brenmiller.
On the nuclear front, Westinghouse
is exploring a system for new-build pressurized
water reactors where steam is
used to heat oil that in turn transfers it
to concrete in prefabricated boxes. The
solution uses " thin plates with narrow
gaps " to create " huge surface area relative
to volume and minimizes oil fraction. "
In South Korea, researchers have
designed a nuclear heat storage and
recovery system, interfaced with the
APR1400 reactor plant. The system comprises
a packed bed of Hornfels rock,
with heat supplied by Therminol-66 oil.
POWER | December 2021

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POWER December 2021

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