IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 19, NO. 2, JUNE 2004 441
Energy Storage and Its Use With
Intermittent Renewable Energy
John P. Barton and David G. Infield
Abstract—A simple probabilistic method has been developed to
predict the ability of energy storage to increase the penetration of
intermittent embedded renewable generation (ERG) on weak elec-
tricity grids and to enhance the value of the electricity generated
by time-shifting delivery to the network. This paper focuses on the
connection of wind generators at locations where the level of ERG
would be limited by the voltage rise. Short-term storage, covering
less than 1 h, offers only a small increase in the amount of elec-
tricity that can be absorbed by the network. Storage over periods
of up to one day delivers greater energy benefits, but is significantly
more expensive. Different feasible electricity storage technologies
are compared for their operational suitability over different time
scales. The value of storage in relation to power rating and energy
capacity has been investigated so as to facilitate appropriate sizing.
Index Terms—Energy storage, interconnected power systems,
modeling, power distribution, voltage control, wind power
generation.
I. INTRODUCTION
E
NERGY storage systems installed within an electricity
system can be provided by a range of technologies and
can add value in a variety of ways as summarized in Table I.
Income may be derived from an energy store by charging
it when the local electricity value is low and discharging it
when the value is high. If, at some times, the grid at the point
of connection of the embedded renewable generation (ERG)
cannot absorb the entire output of the generator, then output
must be curtailed and the value of the excess is effectively zero.
If, at other times, demand is high and expensive generators are
used to meet that demand, then the price will be high. Another
source of income results from supplying ancillary services, for
example, reactive power, voltage, and frequency control and
emergency power during a power outage. This paper focuses
on the first possibility identified, in which the value of a store
depends on time variations in the local cost of electricity. In
this study, the local grid voltage has been modeled in detail,
but the impact on transmission losses has not. The costs of
a store comprise capital costs, maintenance costs, any cost
penalty associated with possible technical failure of the store,
and the cost of electricity losses (both standing losses and
energy transfer losses). In this paper, electrical losses have been
modeled by a simple, overall round-trip efficiency, calculated
at a typical time scale for the particular storage technology.
Manuscript received October 25, 2002.
The authors are with the Centre for Renewable Energy Systems Technology
at the Department of Electronic and Electrical Engineering, Loughborough Uni-
versity, Leicestershire LE11 3TU, U.K. (e-mail: j.p.barton@lboro.ac.uk).
Digital Object Identifier 10.1109/TEC.2003.822305
Wind power is the fastest-growing renewable energy source
and poses the most immediate grid connection problems; the
time variable nature of wind power is well documented. For
these reasons, this paper focuses specifically on the use of en-
ergy storage with wind power and from this point on, ‘ERG’ can
be taken to be wind-powered. The output from a wind turbine
depends on the wind speed, which varies across a wide range
of time scales. The Van Der Hoven spectrum [1] describes vari-
ations up to 1000 h. Corresponding spectra must be calculated
from local wind speed data in order to predict accurately the
character of variations in wind power at a given location, re-
quired for the design of a suitable energy store.
The loads on an electricity supply system, which are reflected
in the spot price of electricity, vary according to the time of day,
day of the week, season, and also other, unpredictable factors.
The latter, essentially stochastic, elements are minor in compar-
ison to wind power fluctuation, and have been ignored in this
study. The amount of ERG that an electricity grid can absorb
at a particular location is usually limited by voltage rise. The
voltage drop caused by local loads offsets the voltage rise of
ERG and, therefore, higher levels of ERG are permitted when
local loads are high. In general, this tends to happen when total
system electricity demand is high and when wholesale elec-
tricity prices are high. Short-term energy storage will smooth
out the effects of wind turbulence on the power output from
ERG and so facilitate a higher installed capacity, without ex-
ceeding local voltage limits. Longer-term energy storage (i.e.,
over several hours) could time-shift ERG power to a time when
the local loads are higher and, therefore, allow even more gen-
eration capacity. The absorption of reactive power reduces the
local voltage but the injection of reactive power increases the
local voltage [2]. Energy storage may be an economically fa-
vorable alternative to allowing increased local reactive power
loads (or rather, not reducing these); a strategy sometimes used
to limit the voltage rise caused by ERG. However, a study of
reactive power control is outside the scope of this paper.
II. MODELING ASSUMPTIONS
A. Storage Technologies
Table II summarizes the key properties of each storage tech-
nology. The capital costs have been discounted at an annual in-
terest rate of 5% to give annual costs. This reflects typical cur-
rent interest rates. Note that in the case of hydrogen and redox
flow cells, the energy rating of the store in kilowatt-hours can
be chosen independently of the power rating of the store in kilo-
watts, as indicated by a “Y” in the third column. For these tech-
nologies, when designing a store for a given time scale, the
0885-8969/04$20.00 ? 2004 IEEE