IEEE Electrification Magazine - September 2015 - 32

Substation

PV with
Batteries

Centralized
Generation

Foss
siiill Fu
sil
Fuel/
el/l//
N lear
Nuc

Pumped
Hydro Storage

Grid
Electric
Vehicles and
Charging Station

Wind
Power
Concentrating
Solar Power

Renewable
Generation

Photovoltaic Farm
Compressed-Air
Energy Storage

Figure 1. A scenario of different energy-storage technologies integrated with centralized grid, DG, and transportation. (Source: Alfred Hicks, NREL.)

operation and the future low-carbon economy. The development of EVs has dramatically reduced battery cost and
improved battery reliability, and these batteries may eventually serve as electric storage for power quality and
DG applications.
Typically, energy storage starts with electricity and converts it into some other form of energy that is stored and
later converted back into electricity. Direct electricity storage stores ions or electrons in a battery or capacitor. Other
storage methods convert electric energy into some form of
mechanical energy, as in pumped storage hydropower
(PSH), compressed-air energy storage (CAES), or flywheels.
Figure 1 shows some storage technologies integrated in
MES, DG, and transportation.
PSH was originally built to assist utilities in providing
electricity during peak electrical demand times. PSH often
consists of a high reservoir and a lower reservoir that are
separated vertically, with pumps pumping water during
off-peak hours and water flowing down through the turbines to generate electricity. Round-trip efficiencies that
exceeded 75% can be achieved. Technically, underground
PSH using flooded mines or caverns is possible, and the
open sea can also be used as the lower reservoir, with
possible synergy with offshore wind generation.

I E E E E l e c t r i f i cati o n M agaz ine / SEPTEMBER 2015

Conventional PSH uses constant-speed motors for water
pumping, so power consumed in the pumping mode is
constant. New variable-speed pump technology coupled
with PSH will provide not only peak demand but also
ancillary services. The introduction of adjustable-speed
technologies allows the addition of a high level of flexibility for power control, load following, and rapid response.
CAES stores energy by compressing air in an airtight
underground storage cavern. To extract the stored energy,
compressed air is drawn from the storage cavern, heated,
and then expanded through a high-pressure turbine that
captures some of the energy in the compressed air. The air
is then mixed with fuel and combusted, and the exhaust is
expanded through a low-pressure gas turbine. The CAES
gas turbine uses 40% of the gas used in conventional gas
turbines to produce the same amount of output power.
Energy-storage methods are characterized by widely
varying power scales and costs. Here, we introduce major
energy-storage development and testing efforts at NREL.

Developing and Testing Energy-Storage
Technology at NREL
NREL is working toward advancing energy-storage devices
and considers them a crucial pathway in grid integration of

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2015