	Introduction

Energy storage technology has great potential to improve electric power grids, to enable growth in renewable electricity generation, and to provide alternatives to oil-derived fuels in the nation's transportation sector. In the electric power system, the promise of this technology lies in its potential to increase grid efficiency and reliability—optimizing power flows and supporting variable power supplies from wind and solar generation. In transportation, vehicles powered by batteries or other electric technologies have the potential to displace vehicles burning gasoline and diesel fuel, reducing associated emissions and demand for oil. In recent years, federal policy makers have become increasingly interested in promoting energy storage technology as a key enabler of broad electric power and transportation sector objectives. Statements such as those above highlight not only the technical opportunities for energy storage in the grid and in electric transportation, but also the attention being paid to energy storage technologies at the highest levels in the federal government. Finally, there are many aspects of market structure and economic regulation that affect energy storage deployment. This report attempts to summarize the current state of knowledge regarding energy storage technologies for both electric power grid and electric vehicle applications. It is intended to serve as a reference for policymakers interested in understanding the range of technologies and applications associated with energy storage, comparing them, when possible, in a structured way to highlight key characteristics relevant to widespread use. The report also discusses how aspects of policy and market structure affect competition among both mature and emerging technologies. It also discusses non-technical barriers including environmental, material, market, and policy challenges to widespread deployment. Though important, short-term discharge services can often be provided by non-storage options such as power electronics. However, in some applications, thermal storage can be functionally equivalent to electricity storage with efficiencies exceeding 90%, which is higher than most other storage technologies. The first is to improve the technical and economic performance of the electric power grid ("the grid"). Briefly, however, a primary historical challenge of storage deployment has been the limited ability of utilities to estimate and capture the full economic value of electricity storage, especially the many dynamic benefits to the grid of fast responding storage technologies. Their ability to provide energy services is more limited by both the storage capacity of the battery and the high cost of battery cycling. For hydrogen fueled FCEVs or HEVs, entirely new infrastructure is needed for fuel production, transport and refueling. R&D will likely further improve battery technical performance and reduce costs for multiple technologies. For grid applications, challenges to hydrogen storage include capital costs and low round-trip efficiency (well under 50%) compared to other commercially available energy storage technologies. Existing PHS facilities in the United States have high availability and few forced outages. There are also a significant number of proposed plants both in the United States and internationally. Typical instantaneous roundtrip efficiencies range from 80% to 90%. Load-shifting and frequency regulation flywheels also are dispatched differently. Conclusions

As a technology, TES has several advantages over conventional electricity storage devices. Table of Acronyms

AC - alternating current
AFC - alkaline fuel cell
ARPA-E - Advanced Research Projects Agency – Energy
ARRA - American Recovery and Reinvestment Act
BSCCO - bismuth strontium calcium copper oxide (type of superconductor)
CAES - compressed air energy storage
CAISO - California Independent System Operator
CEC - California Energy Commission
CER - charging electricity ratio
CSP - concentrating solar power
CT - combustion turbine
DC - direct current
DMFC- direct methanol fuel cell
DoD - depth of discharge
DOE - Department of Energy
EC - electrochemical capacitors
EPRI - Electric Power Research Institute
ERCOT - Electric Reliability Council of Texas
EV - electric vehicle
FCEV - fuel cell electric vehicle
FERC - Federal Energy Regulatory Commission
FOA - funding opportunity announcement
GW - gigawatts
HEV - hybrid electric vehicle
HHV - higher heating value
HTF - heat-transfer fluid
ISO - Independent System Operator
ITC - investment tax credit
KOH - potassium hydroxide
kW - kilowatt
kWh - kilowatt hour
Li-Ion - lithium-ion
LTS - low temperature superconductor
MCFC - molten carbonate fuel cell
MWh - megawatt hour
NaS - sodium sulfur
NERC - North American Electric Reliability Corporation
NiMH - nickel metal hydride
NOx - nitrogen oxides
NREL - National Renewable Energy Laboratory
NYSERDA - New York State Energy Research and Development Authority
O&M - operations and maintenance
OCC - overnight construction cost
PAFC - phosphoric acid
PEM - polymer electrolyte membrane
PEMFC - proton exchange membrane fuel cells
PHEV - plug-in hybrid electric vehicles
PHS - pumped hydro storage
PSI - pounds per square inch
R&D - research and development
RD&D - research, development, and deployment
RTO - regional transmission organization
SLI - starting, lighting, and ignition
SMES - superconducting magnetic energy storage
SOFC - solid oxide fuel cell
T&D - transmission and distribution
TES - Thermal energy storage
TIO - technology improvement opportunities
TOU - time of use
TWh - terrawatt hour
UPS - uninterruptible power supply
V2G - hehicle-to-grid
VG - variable generation
VMT - vehicle mile traveled
VOC - volatile organic compound
WWSIS - Western Wind and Solar Integration Study
YBCO - yttrium barium copper oxide (type of superconductor)
Energy storage technology has great potential to improve electric power grids, to enable growth in renewable electricity generation, and to provide alternatives to oil-derived fuels in the nation's transportation sector. In the electric power system, the promise of this technology lies in its potential to increase grid efficiency and reliability—optimizing power flows and supporting variable power supplies from wind and solar generation. In transportation, vehicles powered by batteries or other electric technologies have the potential to displace vehicles burning gasoline and diesel fuel, reducing associated emissions and demand for oil.
Federal policy makers have become increasingly interested in promoting energy storage technology as a key enabler of broad electric power and transportation sector objectives. The Storage Technology for Renewable and Green Energy Act of 2011 (S. 1845), introduced on November 10, 2011, and the Federal Energy Regulatory Commission's Order 755, Frequency Regulation Compensation in the Organized Wholesale Power Markets, are just two recent initiatives intended to promote energy storage deployment in the United States. Numerous private companies and national laboratories, many with federal support, are engaged in storage research and development efforts across a very wide range of technologies and applications.
This report attempts to summarize the current state of knowledge regarding energy storage technologies for both electric power grid and electric vehicle applications. It is intended to serve as a reference for policymakers interested in understanding the range of technologies and applications associated with energy storage, comparing them, when possible, in a structured way to highlight key characteristics relevant to widespread use. While the emphasis is on technology (including key performance metrics such as cost and efficiency), this report also addresses the significant policy, market, and other non-technical factors that may impede storage adoption. It considers eight major categories of storage technology: pumped hydro, compressed air, batteries, capacitors, superconducting magnetic energy storage, flywheels, thermal storage, and hydrogen.
Energy storage technologies for electric applications have achieved various levels of technical and economic maturity in the marketplace. For grid storage, challenges include roundtrip efficiencies that range from under 30% to over 90%. Efficiency losses represent a tradeoff between the increased cost of electricity cycled through storage, and the increased value of greater dispatchability and other services to the grid. The capital cost of many grid storage technologies is also very high relative to conventional alternatives, such as gas-fired power plants, which can be constructed quickly and are perceived as a low risk investment by both regulated utilities and independent power producers. The existing market structures in the electric sector also may undervalue the many services that electricity storage can provide. For transportation storage, the current primary challenges are the limited availability and high costs of both battery-electric and hydrogen-fueled vehicles. Additional challenges are new infrastructure requirements, particularly for hydrogen, which requires new distribution and fueling infrastructure, while battery electric vehicles are limited by range and charging times, especially when compared to conventional gasoline vehicles.
Substantial research and development activities are underway in the United States and elsewhere to improve the economic and technical performance of electricity storage options. Changes to market structures and policies may also be critical components of achieving competitiveness for electricity storage devices. Removing non-technical barriers may be as important as technology improvements in increasing adoption of energy storage to improve grid and vehicle performance.