vgbe energy journal 10 (2022) - International Journal for Generation and Storage of Electricity and Heat

New storage technologies in the energy market
Energy nexus
Electricity
Additional forms of Energy:
- Magnetical
- Radiation (e.g. light)
- Kinetical (e.g. gravity)
- Nuclear
Power plant
Electr. heater
Electrolysis
Fuel cell
Dynamo
Combustion
The good news is, that energy can be converted
back and forth between the different
energy states although (sometimes significant)
energy losses occur during each conversion
(F i g u r e 5 ). Therefore, energy
conversion with all its features is one of the
key factors in designing and evaluating energy
storage systems.
To put this into perspective, converting natural
gas with CCPPs into electric energy has
an efficiency of 50 - 65 %. But also roundtrip
efficiencies of storage can be almost anything
between 10 % and 90 % depending on
the energy conversion(s) used during charging
and discharging. Since we cannot afford
to waste energy, it must be stored and made
available when needed in the most efficient
manner. And with good energy storage designs,
we can get not just a good cycle efficiency,
but also a control range from maximum
power delivery (maximum capable
load = 100 % power delivery) to a maximum
charging power (minimum capable
load= -100 % of charging). So that means if
we simplify here, that a 100 MW energy storage
has a control range of 100-(-100) =
200 % of the nominal load (differentiation
and details later in this text). Here, the market
of energy storage systems begins to
spread. It becomes more and more complicated
to rate the different storage concepts
Heat
Gasification
Chemical
Gas turbine
Friction
Electric motor
Muscles
Mechanic
NOTE:
All written functions are
examples. There could be more
possible ways to transfer energy.
Fig. 5. The Energy Nexus exemplary overview of energy conversion possibilities. Conversion losses
appear as heat. They are not shown in this diagram.
Discharge
duration
Weeks
Days
Hours
Energy storage systems provide a great variety
of solutions for various use cases
Hydrogen storage
Thermal
Thermo-/
Mechanical
Minutes
Batteries
Seconds
Miliseconds
Rotating Grid
Stabilizers
10 kW 1 MW 100 MW 1,000 MW
Power
with the different advantages and disadvantages.
So before looking at the parameters more in
detail, we also must put the storage capacity
and the duration of storage into a perspective:
An often-seen representation of energy storage
is the diagram shown in F i g u r e 6 , in
which different types of storage are rated by
Power output [MW] on the X-Axis and the
discharge duration [time range] on the Y-
Axis. Not considered here are the previously
mentioned constraints like cycle efficiency
or controllability. Nevertheless, it helps to
visualize the operational range.
Clustering this representation, a little deeper
into the time dependency of the solutions,
we must segregate into short term storages,
mainly for primary frequency control for
grid stabilization, long term storages to carry
the energy into the winter season and the
intermediate or mid-term storages to ensure
secondary or tertiary frequency control to
stabilize the grid and to “buy” enough time
to activate other energy sources.
Short-term storages: Mostly used as shortterm
storages are batteries (e.g. Li-Ion) and
super-capacitors in wide ranges from local
home storages for prosumers to industrial
applications and grid stabilizing devices,
Long-term storage with green hydrogen enables the
efficient coupling of all sectors of the economy.
Thermal energy storage improves a plant's efficiency, it's
operational flexibility and provides high-quality process steam.
Compressed air energy storage (CAES) stores energy when
demand is low and reuses it when demand is high.
lt also enables long-term storage.
Batteries support fast and zero-carbon energy supply,
enable shifting to renewable energy and avoid curtailment.
Rotating Grid Stabilizers enable the grid to
handle fluctuating renewable infeed.
Fig. 6. Different storages rated on power output vs. duration of storage.
while hydro storages and flywheels currently
only apply for larger scales. In addition,
there are large consumers that store energy
for their processes like cooling houses or the
steel production but without feeding the energy
back into the electric grid.
Mid-term storages cover the range between
short term and long-term storage and therefore
also pumped hydro, pressure storages
(e.g. Compressed Air Energy Storage), large
consumers and thermal storages (stones,
concrete, molten salt, etc.) can be considered
here. If there is one type of storage that
fits best into this category, it probably would
be redox-flow batteries, since they are
well scalable to these needs and they have a
relatively high efficiency, since they do not
require a thermal conversion or large machinery.
Long-term storages: The range of long-term
storages is wide and the most challenging,
since especially heat is required in winter
times. Sector coupling must be considered
with green H 2 (Hydrogen with green electricity
via electrolysis) / green CH 4 (Methane
– catalytic reactions between green H 2
and CO 2 ) and green NH 3 (Ammonia – classic
NH 3 synthesis with green H 2 ). Green methane
and in certain ranges also hydrogen can
be added to national gas storages to replace
natural gas and NH 3 can be stored and transported
in tanks. The drawback of Power to
X and Power to Gas currently is the cycle
efficiency, the benefit though is an easy conversion
back to electricity via gas turbines,
combustion engines or fuel cells for energy
purpose, but also and especially for mobility
applications on land, sea or in the air.
Although metal fuels (e.g., Aluminum/
Aluminum Oxide) are emerging technologies,
although at the time of writing this article
without known commercial applications.
Which parameters are used to rate energy
storage:
In the following, the different features of the
mainly available types of energy storage
are clustered and rated based on control capability,
environmental and financial aspects.
Control capability:
––
Basic Offset / working point range (Minimum
charging/discharging load):
The basic offset is the working point
around which control energy can be provided.
To offer positive control energy,
turbines must be synchronized and deliver
a certain minimum load to avoid backenergizing
or overheating. Therefore, several
types of thermal storage with an attached
steam cycle have a dead-band
between minimum load of the steam process
and a minimum charging load. To
deliver control energy, the thermal storage
must avoid this dead-band, since they
cannot participate in load control during
this time.
vgbe energy journal 10 · 2022 | 31