and cools in a storage container.
The compressed air cools further
as it expands to drive a turbine,
and fuel (typically natural gas)
is burned to heat the air back to
the turbine inlet temperature.
If there is no compression heat
recovery or heat recuperation
from the turbine exhaust, the net
energy efficiency of CAES can be
40 to 50 percent [ 5]. To save fuel
and improve efficiency, TES may
be incorporated in an adiabatic
CAES system. In an adiabatic
CAES configuration (that is, one
in which no heat is gained or lost
by the system), heat from the
compressed air is transferred to a
thermal storage device and then
reused to preheat the turbine inlet
air, thereby minimizing or even
eliminating the need for natural
gas heating. In this configuration,
adiabatic CAES may achieve an
energy-usage efficiency of 50 to 70 percent.
As shown in figure 3, TES can have a ther-mal-to-thermal conversion efficiency of 95 to
CAES [ 4]. A further step
to boost the electricity
generation efficiency is
to apply “exergy uphold”
— a unique method that
may be possible for CSP
plants with TES. This
entails charging thermal storage at a high
temperature level, for
instance above 400°C
(752°F), and keeping
the low-grade heat of
below 400°C in CSP
generation to cushion
the low-end thermal
energy. In this operating mode, TES for CSP
FPO
Figure 3. Comparison of roundtrip efficiency for energy storage methods.
solartoday.org SOLAR TODA Y May 2012 25
thermal energy residing in the
CSP TES system.
Selecting an appropriate ener-
gy storage technology is a multifac-
eted process, involving econom-
ics, power requirements, storage
capacity, reliability, lifetime and
efficiency. A proper choice will
add value to the electric grid
operation and enable renewable
energy to provide baseload power.
Low-cost, high-performance TES
is currently being developed under
the U.S. Department of Energy
(DOE) CSP Program. Last year,
DOE launched the SunShot Ini-
tiative, which aims at lowering
solar power generation cost to be
on par with conventional power
plants (approximately 6 cents per
kilowatt-hour by 2020). The real-
ization of the DOE SunShot goal
can enable broad deployment of
CSP power generation via high
electric conversion efficiency and high-perfor-
mance TES that can provide additional use for
bulk energy storage. Unlike the geological require-
ments of CAES and PHS, TES is not restricted by
site, making it possible to co-locate with photo-
voltaic (PV) or wind farms to alleviate transmis-
sion load with firming renewable power. Under
this scenario, TES as a part of a CSP plant can
serve broader renewable energy storage at virtu-
ally no additional cost, as
the TES cost is paid for
upfront by the CSP plant.
TES can also be integrat-
ed into the grid for elec-
tricity storage. Figure 4
(page 26), shows a sce-
nario for mixed renew-
able and conventional
power production with
TES electricity storage.
Under this grid struc-
ture, off-peak wind, PV
and conventional electric
generation can be stored
in the CSP TES facility
by electric heating, and
the stored thermal ener-
gy then converts back to
electricity to serve the
peak demand. ➣
Figure 2. Storage power rating and energy capacity comparison
for various storage types.
keeps discharging the temperature higher than
the cold storage temperature. By superposing
electric heat on top of the CSP TES level, the
stored energy from electricity maintains its high
exergy level (i.e., its capacity to produce electric power). High-grade energy has the ability
to convert back into electricity more effectively,
and achieves high electricity-to-electricity efficiency through augmenting the prior low-end
