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

New storage technologies in the energy market
In simple words: Energy must be available
and reliable just in time, when it is needed.
The production and consumption must always
be kept in balance: ∑ produced energy
= ∑ consumed energy
This works fine as long as dispatchable and
completely controllable conventional power
sources are in their control range. The cost
of electricity is well predictable, but the situation
changes, when we reach the limits of
their control range which stretches from the
minimum capable load to the maximum capable
load. This is the time, when we run
out of control margins and as we know, price
is a function of demand compared to the offer
in the market. Both, the minimum capable
load and the maximum capable load are
not fixed values. They vary over time, since
the production of renewable energy is taken
into account, as well as the types of power
plants and their amount that are in operation
at that very time. Looking at the situation
in Germany, conventional power plants
are not allowed to dump load by opening the
turbine bypasses. So, when they reach minimum
load, they must decide if they shut
down or stay online. Shutting down the
plants must be well considered, since restarts
cost time and fuel. This also leads to
the need of feeding storage partially from
conventional sources to avoid shut downs,
since a re-start means financial losses and an
increased CO 2 -production.
This relation can be seen in the energy price
development: In F i g u r e 1 the energy production
and the energy market prices at the
German spot-market are displayed for an
example time frame in 2020. Frequently we
can see peaks in the red line (energy market
price in €) whenever we have not enough
controllable energy sources (positive peaks)
and when we produce more energy than we
can consume (negative peaks).
So, does already energy arbitrage make
sense? Solely financially analyzed, the potential
financial turnover of storages can be
calculated based on the price differences
between the highs and the lows of the energy
prices multiplied with the stored energy
under consideration of the cycle efficiency,
taxes and fees.
∑ Revenue = (selling price-buying price-grid
fees,taxes,etc.) × stored energy ×
cycle efficiency
Capacity in GW
80
70
60
50
40
30
20
10
0
-10
-20
-50
01.01.2020 11.02.2020 24.03.2020 05.05.2020 15.06.2020 27.07.2020 07.09.2020 18.10.2020
Date in MEZ
Fig. 1. Energy prices (red line) in Germany for January 2020 till October 2020 [1].
Currently this calculation of arbitrage trading
barely justifies the construction and operation
of storages, since the events of extreme
cost differences are still too rare. But
this changes, when we look more detailed
into the technology and the physical constraints
of our national grids, where the situation
indicates a much more dramatic scenario.
Grids still require black start capabilities,
capacity reserves and ancillary services.
So, technology-wise this indicates that we
are facing the limits of our national grids,
caused by the imbalance of controllable devices
to “disturbances”. Now besides consumers,
we also got producers or even “Prosumers”
(artificial combination of producer
and consumer) which in control theory act
like disturbances to the grid and the remaining
controllable energy sources need to provide
more “control energy”. Control energy
here means the difference of maximum capable
load to minimum capable load (Delta
load in % or MW) under consideration of
load ramps (%/minute or MW/min). And a
certain control energy reserve is simply required
to maintain a stable power grid.
But there also is another perspective which
shows analogies to hybrid cars: Hybrid power
plants. Storages can optimize the operational
behavior of conventional power plants
or also act as a buffer to operate around optimal
working points. This is illustrated in
F i g u r e 2 based on a combined conventional
power plant and a battery energy storage
system, called SIESTARTTM.
F i g u r e 2 shows two different scenarios.
Light blue showing the “standard operation
line” of a gas turbine (GT). Power increase
can happen “slowly” only because power increase
at a GT is mainly driven by the turbine
inlet temperature. “Slowly” is of course
a relative term, as modern gas turbines from
Siemens Energy can handle load gradients
up to 50 MW/min at F-class engines and up
to 75 MW/min at HL-class. Fast temperature
changes mean stress for the material and
have an impact to the lifetime of the parts.
The huge advantage of batteries (in combination
with a capacitor) are the fast reaction
and accelerating times of power output –
they react within milliseconds. The purple
line shows the operation line of the hybrid
200
175
150
125
100
solution, where the benefits are combined.
The battery makes the system faster to react
to the grid, it helps if a black start of the engine
is necessary (battery instead of a diesel
engine) and additionally the batteries can
be used for short-term peak power bridging.
Furthermore, the batteries can be used as an
additional energy buffer, which allows the
gas turbine to also be operated in island
mode and thus makes it fit for use in microgrids.
Additionally, the battery helps the gas
turbine to be operated at optimal load behavior
with minimized CO 2 -footprint by
temporarily consuming the surplus energy
(that is not needed by the grid) or providing
the missing energy. The main advantage of
the GT is the uninterruptible and dispatchable
power supply over hours, weeks or even
years. Another hybrid solution could be a
thermal storage behind e.g., a gas turbine
where thermal energy will be stored from
the flue gas and could be released later,
when needed.
Back to the grid constraints: Previously we
discussed the limitations of conventional
power plants. But even when all classical energy
sources would have a 100 % control
range and an infinite load ramp capability,
we will one day reach a natural limit. Latest,
when renewable energy production exceeds
the energy consumption and/or when the
transmission systems are at their limits,
large consumers must contribute to the grid
stability. This may work in a certain range
with demand side management, e.g., with
controllable consumers in the steel production
or cooling houses and partially even
at home, when for example car chargers
are being controlled by the grid. But still
we may face imbalances that cannot be cov-
Peaking
Power Power Flex operation line (with SIESTAR TM )
Standard operation line
Frequency response PFR + SFR
Fast start
Black start
Fast ramp-up and
ramp-down support
Minimum environmental
load
Islanding, off-grid
Fig. 2. SIESTARTTM optimized performance opens new opportunities for conv. power plants.
Time
75
50
25
0
-25
Prices in Euro/MWh, Euro/tCO2
vgbe energy journal 10 · 2022 | 29