Thermal energy storage stores excess solar heat to
extend the operating hours for concentrating solar
power. Is it also the answer for increasing the grid
value of electricity from wind and photovoltaics?
by ZHIWEN MA, GREG C. GLATZMAIER
and CHuCK Ku TSCHER
SOLAR TODAY®
MAY 2012
VOL. 26, NO. 3
Copyright © 2012 by the American Solar Energy Society Inc. All rights reserved.
Electricity from the grid is usually produced
on demand or as needed, without storage ability. However, energy storage is already used in
transportation, mobile power and distributed
generation (DG). DG can be connected to a
grid, or just provide power to a stand-alone load.
This article presents energy storage technologies
in utility-scale power backup and for renewable
grid integration.
As renewable electricity generation technologies are deployed on an increasingly large
scale, storage will be required to abate variability.
Several electricity storage methods are available,
and many others are under development [ 3].
Of these methods, thermal energy can drive
mechanical, chemical and electric processes for
power generation, and is also easy to store and
retrieve.
For variable generation like wind and solar
power, electric storage provides a reliable and
uninterruptible power supply, reduces the need
for spinning reserve (online reserve capacity syn-
chronized with the grid and available within a few
minutes) and helps meet peak power demands
from the grid. In short-term electric storage,
discharging electricity from storage can smooth
grid fluctuations and improve power quality.
Table 1 lists three types of energy storage appli-
cations — power quality, distributed generation
and bulk energy storage — and specifies their
power and storage capacity ranges. Utility-scale
power is of the same scale as bulk energy stor-
age [ 3], defined by a power rating of 10 to 1,000
megawatts (MW) and a storage capacity of 10
to 8,000 megawatt-hours (MWh). Bulk energy
storage has several benefits:
•;Addresses;variability;and;defers;transmis-
sion congestion,
•;Firms;up;and;enhances;the;value;of;renew-
able energy generation,
•;Reduces;the;amount;of;central;power;gen-
eration capacity needed for peak and baseload
demand, and
•;Reduces;time-of-use;energy;costs.
Table 2 (page 24), compares different storage methods with their applicable scales: batteries (lead-acid, lithium-ion and sodium sulfur),
reversible fuel cells, superconducting magnetic
energy storage (SMES), flywheels, compressed-air energy storage (CAES) and pumped hydro
storage (PHS). Energy storage typically converts
electricity to another form of energy that is easily stored for later generation. The most direct
methods of electricity storage store chemical or
electrical potential in a battery or capacitor, or,
in the case of SMES, transform electricity into
an electromagnetic field. Other energy storage
methods convert electric energy into forms of
mechanical energy, as in flywheels, PHS and
CAES. Although the major electric storage technologies focus on the above methods, this article
adds thermal energy storage as a means to provide electric energy storage [ 4].
The storage capacity for each method
depends on the amount of the storage medium
and its cost. Batteries, flywheels and SMES
On the other hand,
utility-scale storage
may directly compete with the cost of conventional spinning-reserve power generation and
is cost-sensitive. The storage media for CAES
and PHS are air and water, respectively, which
are essentially free. Therefore, CAES and PHS
Zhiwen Ma, Ph.d. ( zhiwen.ma@nrel.gov), is a senior
engineer in the Concentrating solar Power group at
the National renewable energy Laboratory (NreL),
and previously worked on aircraft engines for ge avia-
tion. He now works on CsP system analysis, advanced
power cycles and thermal energy storage.
Greg Glatzmaier, Ph.d. ( greg.glatzmaier@nrel.gov),
joined NreL’s solar thermal Program in 1987, where he
demonstrated a new concentrating solar technology
and, with Coors Ceramics, developed a high-temper-
ature materials synthesis process. In 1997 he launched
a private r&d company to develop spacecraft fluid
compressor designs and feedback controls for Nasa
and the u.s. air Force. Back at NreL since 2007, glatz-
maier now manages advanced heat-transfer and
thermal-storage work.
chuck Kutscher, Ph.d. ( chuck.kutscher@nrel.gov),
joined NreL in 1978 and is a principal engineer and
group manager of the thermal systems group, lead-
ing r&d for parabolic troughs. He is chair of the World
renewable energy Forum 2012, served as chair of the
american solar energy society for 2000–2001 and was
general chair of its sOLar 2006 conference. Kutscher
edited the ases report, Tackling Climate Change in
the U.S., and writes a column on climate change for
SOLAR TODAY.
TABlE 1. ElECTrIC ENErGy SToraGE TyPES
application discharge Stored Energy Typical applications
Power rating Capacity
Power Quality 0.1− 2 MW 0.000028−0.016 M Wh End-use power quality and reliability
Distributed 0.1− 2 MW 0.05− 8 MWh Peak shaving, transmission deferral
Generation
Bulk Energy 10−1,000 MW 10− 8,000 MWh
Storage
load leveling, load reserve,
enable renewable energy generation
solartoday.org SOLAR TODA Y May 2012 23
