Reducing Energy Consumption, while Protecting the Environment - Energy and Climate: How to Achieve a Successful Energy Transition - page 61

expensive. Sodium-sulphur batteries are also being investigated for
large capacity storage applications. These batteries operate at a high
temperature (350 C). They use liquid sulphur and sodium electrodes,
separated by a solid electrolyte. The storage capacity of such batteries is
around 100Wh/kg [72].
Liquid phase electrochemical storage systems, developed recently, look
promising. They include a reversible battery, equippedwith two compart-
ments separated by an ion exchange membrane.
Each compartment is connected to an electrolyte reserve. Each electro-
lyte is pumped through a circulation circuit and, while one of the
electrolytes is oxidised at the contact of the membrane, the other one is
reduced [73]. The system is reversible and is thus able to store energy. One
of the most attractive electrolyte couples is based on the use of vanadium
ions and uses the couples V 2 þ /V 3 þ and V 4 þ /V 5 þ . Three devices operat-
ing according to this concept, with a storage capacity ranging between
500 kWh and 2000 kWh, have been installed.
Electrochemical storage systems cannot be used for storing the amounts
of energy needed to ensure a seasonal adjustment between supply and
demand. Presently, only gravity hydraulic storage can be used on such a
scale.
Underground storage (saline cavity or aquifer) represents another
potential way of achieving a large storage capacity (compressed gas,
heat, hydrogen) and might be more widely used in the future. Under-
ground compressed air storage has already been implemented. It is thus
possible to store 12 kWh/m 3 of air at 100 bar. At Huntdorf, in Germany,
air compressed at 70 bar is stored in two caverns representing a volume of
310 000m 3 . In the future, large quantities of hydrogen might be stored in
underground saline aquifer systems, similar to those which are used for
storing natural gas. It is also feasible to store hot water underground at
comparatively shallow depths.
In the residential area, the use of thermal storage materials helps to
recover heat during the hottest hours of the day and to release this extra
heat during the coldest hours. Different materials can be used for such a
purpose and in this area too, innovative technologies are being investi-
gated, including the use of phase transition materials. Such materials can
store excess heat during the day and release this heat during the night.
These heat storage systems are especially useful for buildings supported by
a light infrastructure.
Energy storage is certainly a key technology for the energy transition
period, and it is essential to make it more cost effective during the years to
come.