Chemical reactions enhance efficiency of key energy storage method


Date:
January 6, 2022
Source:
Oregon State University
Summary:
Researchers have uncovered a way to improve the efficiency of a
type of grid-scale storage crucial for a global transition toward
renewable energy.



FULL STORY ========================================================================== Research by the Oregon State University College of Engineering has
uncovered a way to improve the efficiency of a type of grid-scale storage crucial for a global transition toward renewable energy.


========================================================================== Moving toward net-zero carbon emissions means dealing with the
intermittent, unpredictable nature of green power sources such as wind
and solar and also overcoming supply and demand mismatches, said OSU's
Nick AuYeung, who led the study along with Ph.D. student Fuqiong Lei.

Those challenges, AuYeung notes, necessitate energy storage through
means beyond pumped hydro plants, which feature a turbine between two
water reservoirs of different elevations, and huge lithium-ion batteries.

The computer modeling study spearheaded by AuYeung, associate professor
of chemical engineering, and Lei found that one of those additional
energy storage technologies, compressed air, could be improved via
chemical reactions.

The reversible reactions can absorb energy in the form of heat and
subsequently conserve energy that would otherwise be lost.

Findings, published in Energy Conversion and Management, are also
applicable to a related technology, liquid air energy storage, AuYeung
said.



==========================================================================
As their names suggest, the liquid and compressed air techniques harness
energy that can be accessed when needed by allowing stored air -- either pressurized or cooled to a liquid form -- to expand and pass through electricity-generating turbines.

However, both CAES, as compressed air energy storage is typically
expressed, and LAES (liquid air) score somewhat poorly in a category
known as round-trip efficiency, AuYeung explains. With either, only
about half the energy put in can be pulled out -- think of it as making
a bank deposit of $1,000 but, due to various charges, only about $500
is available for withdrawal.

"An advantage of CAES is that it allows energy to be stored at large
scales, which is a hurdle for electrochemical battery technologies,"
he said. "But a major challenge for traditional CAES is reaching high round-trip efficiency." In a conventional CAES process, electricity is
used to compress air, and the compressed air is stored below ground in a
cavern or in a pressure vessel, AuYeung said. When the air is compressed,
its temperature rises, but that heat is typically regarded as waste and
thus goes unrecovered and unused.

"To discharge the air to produce power, it's usually heated with natural
gas to increase the turbine feed's enthalpy, the total system energy,"
he said.

"Factoring in heat lost during storage, the result is that the overall
round- trip efficiency -- the ratio of turbine output work to work
consumed through compression -- is only between 40% and 50%." AuYeung and collaborators at OSU, Mississippi State University and Michigan State University came up with a storage scheme to improve that efficiency - - thermochemically recovering lost heat -- and developed a mathematical
model for its design and operation. An advantage of thermochemical
energy storage, or TCES, over other methods is a higher energy density
made possible by capturing heat in the form of chemical bonds, he said.



========================================================================== Using their model, the researchers analyzed the performance of TCES incorporated into thermal energy storage via "packed beds" -- vessels
filled with some kind of solid packing medium, where energy reaches the
solid by means of a heat transfer fluid such as air. The filler material
is typically alumina, ceramic or crushed rock.

Packed beds are classified as "sensible" storage because energy is
harnessed by virtue of the filler material changing temperature.

"We looked at TCES with packed beds filled with rocks and barium oxides," AuYeung said. "Our results showed a similar round-trip efficiency
between beds with TCES and beds without because of the relatively low
heat capacity and heat of reaction for the barium oxides. We got to 60% round-trip efficiency for both systems with a 20-hour storage time after charge. Other means of thermal storage cannot store the heat for long
periods of time since they cool down." Importantly, he noted, with TCES material placed atop the packed beds, there was a more stable turbine
air inlet temperature -- higher for longer -- which is a key to optimal
power generation and thus desirable to utilities. In addition, AuYeung
said the model shows that with future advanced materials, round-trip
efficiency and storage time could improve as well.

"To better illustrate the potential of the concept, we came up with
a hypothetical material with the same heat capacity as rocks but a thermochemical storage capacity three times that of barium oxides, and
we looked at that hypothetical material in our model," he said. "Results
showed that a potential round-trip efficiency improvement of more than
5% can be obtained, as well as longer storage durations. Also, 45% less
filler volume would be needed to achieve storage capacity similar to rock-filled beds." AuYeung said the barium chemistry the initial model
was based on was the most obvious that came to mind for the researchers'
but has a downside because its heat capacity is fairly low.

"There are non-oxygen chemistries such as hydrates and carbonates that
have the hypothetical properties -- high heat capacity, high heat of
reaction -- we looked at, but right now we haven't identified one for
a redox material that operates on oxygen swing," he said. "A next step
perhaps for us, or for others with more materials expertise, would be
to try to discover new materials." Oregon State University supported
this research through the OSU Advantage, which assists work related to entrepreneurship, intellectual property and technology transfer.

The collaboration included David Korba, Like Li and Wei Huang of
Mississippi State University, who were instrumental in building the mathematical model, and Kelvin Randhir of Michigan State University,
who helped in the conceptual development.

========================================================================== Story Source: Materials provided by Oregon_State_University. Original
written by Steve Lundeberg. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Fuqiong Lei, David Korba, Wei Huang, Kelvin Randhir, Like Li, Nick
AuYeung. Thermochemical heat recuperation for compressed air energy
storage. Energy Conversion and Management, 2021; 250: 114889 DOI:
10.1016/j.enconman.2021.114889 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220106143711.htm

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