VGB POWERTECH 10 (2019)

Heat storage systems
A journey through 100 years VGB | VGB
VGB
POWERTECH
PowerTech 1/2
1/2
l 2013
(2013)
electric
storage
– electric
field energy
– magnetic
field energy
– potential
energy
– kinetic
energy
Energy storage system
mechanial
storage
chemical
storage
thermal
storage
– fuels – heat storage
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3 4 6 7
5
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K 1 K
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1 engine/generator
2 compressor
3 cooler
4,6 valve
5 underground cavern
7 combustion chamber
8 gas turbine
K coupling
Fig. 1. Energy storage systems.
Fig. 3. Compressed-air storage power plant.
merous applications in electricity networks
(in isolated networks, as backup solutions
for power outage conditions, etc.), “E-mobility”
and “vehicle-to-grid” concepts are
being met with great interest. In summary
and based on the data available on output
(MW), capacity (MWh), energy efficiency
(self-discharge rates, energy loss, etc.) and
long-term experience with this type of application,
however, it must be stated that
accumulator systems are obviously not
very commonly used in power plants and
power supply systems for storing energy on
a large scale.
Mechanical energy storage [7]
Pumped-storage power plants (F i g u r e 2)
are typical examples of mechanical energy
storage systems.
Usually, this concept uses the difference in
altitude between an upper reservoir and a
lower reservoir in order to generate power
in water turbines in combination with
generators. This principle is utilised often
in periods of high power demand (peak
load demand). For this purpose, the upper
reservoir is filled by powerful pumps during
times when sufficient power supply is
available and inexpensive. Besides the criteria
relevant to the necessary difference in
altitude between the upper and the lower
reservoir (normally in mountain areas),
especially the local requirements regarding
water resources, environment and nature
conservation, as well as the possible grid
connection need to be taken into account.
Numerous large-scale plants of this type
valve
pump
coupling
upper reservoir
M
G
lower reservoir
valve
turbine
coupling
Fig. 2. Pumped storage power plant.
h
have been installed all over the world and
are delivering outputs of ⪢100 MW and capacities
of >1,000 MWh. Positive technical
operating experience has been gathered
and documented for more than 100 years.
The projects currently discussed in Germany
and Europe are primarily affected
by the issues of environmental protection
and nature conservation [8]. In addition,
the economical and regulatory framework
for investors and plant owners are
presently not positive and/or long-range
planning is difficult [9 to 12]. However,
with respect to their output (MW), capacity
(MWh), energy efficiency (plant efficiency,
energy losses) and proven scope of
application, pumped-storage power plants
can be viewed as the storage system most
frequently used in the world that is capable
of storing large amounts of energy in
connection with major power plants and
power supply systems.
Compressed-air storage power plants
(F i g u r e 3) are another example of mechanical
energy storage [13, 14].
In this case, an underground cavern is used
for storing compressed air. In periods of
high power demand, the air can be used for
operating a gas turbine. With the help of
compressors, the cavern is filled (charged)
with compressed air during times when a
sufficient amount of power is available and
inexpensive. Only a small number of largescale
plants exist in the world that have outputs
of > 100 MW. Positive technical operating
experience has been documented for
several decades though. The cost-benefit
source: dena
ratio of such plants is very much affected
by the use of natural gas. Research & development
projects are currently focusing on
the improvement of energy efficiency, e.g.
by utilising the thermal energy produced in
the process. In summary, it must be stated
that compressed-air reservoir power plants
are not very commonly used as large-scale
energy storage systems.
Chemical energy storage
Basically, fuels (such as natural gas, biomass,
oil, coal, hydrogen, etc.) can serve as
media for chemical energy storage. In the
context on hand, however, primarily systems,
which are suitable for short-term or,
at maximum, seasonal energy storage, will
be taken into consideration. For instance,
an idea which is being widely discussed
nowadays is the “power-to-gas” concept
(Figure 4).
Numerous R&D projects are underway,
focusing on the development and testing
of such plants and systems [15, 16]. Many
complex tasks and objectives need to be
taken into account for this approach, e.g.
concerning the specific issues of process
engineering, system integration, energy
efficiency (plant efficiency, energy losses,
etc.), environmental protection and health
& safety, as well as the operational longterm
behaviour of the associated systems
and components – and the expected costbenefit
ratio, of course. So far, the outputs
of the plants presently installed are < 10
MW (per plant or module). Long-term data
still needs to be gathered and documented.
Thermal energy storage
There are different types of thermal energy
storage concepts which differ in terms of
– substances/heat transfer medium used
(e.g. water, salts, sand, concrete),
– pressure and temperature,
– technical procedure/systems (e.g. with
or without phase change),
– integration into power systems (e.g.
power plants, CHP plants, heat networks),
and
– task objectives (e.g. short-term, seasonal,
long-range storage).
On a global scale, a wide variety of types
and applications of thermal energy storage
systems have been implemented. As
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