Solar Energy Perspectives - IEA

Solar Energy Perspectives: Solar electricity
Figure 3.7 Principle of pumped-hydro storage, showing discharge (left) and charge (right)
Upper
reservoir
Upper
reservoir
Water
Conducting tube
G
Lower
reservoir
Water
Conducting tube
p
Lower
reservoir
Pump turbine
Pump turbine
Source: Inage, 2009.
Key point
Pumped-hydro plants represent the bulk of electricity storage capacities today.
Recently, thanks to the emergence of variable-speed pumps, some plants are being used more
efficiently and frequently, switching from pumping to electricity-generating modes several times
a day, thus better fitting the requirement of absorbing variable renewables. The introduction of
variable speed and power electronics also allows these plants to deliver more ancillary services
to electric grids on various time-scales, e.g. frequency and voltage regulation, and reactive power.
Many other options exist for electricity storage, suitable for different needs on
different timescales. 1 They include flow batteries or “redox flow cells”, capacitors, dry
batteries of various types (lead-acid, lithium-ion, sodium-sulphur, metal-air, zinc-bromine),
flywheels, superconducting magnetic energy storage, compressed-air storage and others
(Inage, 2009). They may all have a role for ancillary services at various levels of electric grids,
especially the sodium-sulphur batteries for load management in isolated or end-point grids.
But only compressed-air energy storage systems (CAES), with two plants currently in
operation worldwide, represent a large-scale alternative to pumped-hydro plants.
CAES consists in using electricity to compress air to store it in large cavities, then letting the
compressed air flow through a turbine expander to generate electricity when needed
(Figure 3.8). Investment costs are in the same range as for pumped-hydro plants, but the
round-trip efficiency of CAES at 60% at best is significantly lower. Indeed, current CAES
plants lose heat during the compression of air, and need an external source of heat during its
decompression to run a turbine expander, so they burn natural gas.
Advanced adiabatic compressed-air energy storage (AA-CAES) could eliminate or at least reduce
the need for burning natural gas, and reach a round-trip efficiency in storing electricity of about
70%. The concept consists in storing separately the heat resulting from the compression of the air,
and the air itself, then using the stored heat to warm the air being decompressed before it is sent
1. In areas where wind power and CSP co-exist or are linked together, such as Spain, one additional possibility to avoid curtailment
of wind power could be to use the thermal storage capacities of CSP plants, which are partly unused in winter. Heating molten salts
or another medium with electricity, then generating electricity from that heat, would not be very efficient (30% to 40% efficiency
depending on the steam cycle). But it could prove better economics than dumping excess electricity, as the scheme would only
require the additional investment of electric heaters in the thermal storage tanks.
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