TECHNICAL PAPER #22
UNDERSTANDING ENERGY
STORAGE METHODS
By
Clyde S. Brooks
Technical Reviewers
Paul L. Hauck
LeGrand Merriman
Lester H. Smith, Jr.
Published By
VITA
1600 Wilson Boulevard, Suite 500
Arlington, Virgnia 22209 USA
Tel: 703/276-1800 . Fax:
703/243-1865
Internet: pr-info[at]vita.org
Understanding Energy Storage Methods
ISBN: 0-86619-222-0
[C]1985, Volunteers in Technical Assistance
PREFACE
This paper in one of a series published by Volunteers in
Technical
Assistance to provide an introduction to specific state-of-the-art
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their
situations.
They are not intended to provide construction or
implementation
details. People are urged to contact VITA or a similar
organization
for further information and technological assistance if they
find
that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and
illustrated
almost entirely by VITA Volunteer technical experts on a
purely
voluntary basis. Some 500 volunteers were involved in the
production
of the first 100 titles issued, contributing approximately
5,000 hours of their time. VITA staff included Maria Giannuzzi
as editor Julie Berman handling typesetting and layout, and
Margaret Crouch as project manager.
The author of this paper, Clyde S. Brooks, has been a VITA
Volunteer
for many years. He
holds a B.S. in chemistry and has done
graduate work at Duke University and Carnegie-Mellon
University.
Currently, Brooks performs independent research
consultancies in
applied physical chemistry. His experience includes coal
chemical
processing, chemical stimulation of oil recovery, and energy
conversion processes. The reviewers of this paper are also
VITA
Volunteers. Paul J.
Hauck has been a mechanical engineer for
Westinghouse for the past 20 years. He designs piping
systems and
pressure vessels and operates and maintains pumps, motors,
heat
exchangers, valves, etc. LeGrand Merriman is an electrical
engineer
who worked for Westinghouse for 31 years. His duties
included
directing the installation, start-up and servicing of
electrical equipment.
Lester H. Smith, Jr., an electrical engineer,
is a founding partner of an electrical consulting firm
responsible for various medical, institutional, commercial,
and
residential projects in the United States.
VITA is a private, nonprofit organization that supports
people
working on technical problems in developing countries. VITA
offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to
their
situations. VITA maintains an international Inquiry Service,
a
specialized documentation center, and a computerized roster
of
volunteer technical consultants; manages long-term field
projects;
and publishes a variety of technical manuals and papers.
ENERGY STORAGE METHODS
By
VITA Volunteer Clyde S. Brooks
I. INTRODUCTION
Energy storage capability is essential if the maximum
economic
advantage is to be gained from small power plants. Unless
the
power plant is operated at full load on a continual basis,
there
will be periods when there is a lower load demand upon the
plant.
As a result of this lower demand, excess energy will be
generated
by the plant. The use of an energy storage system will allow
for
the recapture of this surplus energy and its later use
during
periods of high demand.
This paper presents a critical review of the technical
features,
state of development, and economics of various energy
storage
systems and their compatibility with small power
plants. The
small power plants examined here have generation capacities
within
a range of 1 to 50 kilowatts (kW) and consist of systems
such
as windmills and small-scale hydropower.
Energy storage systems potentially compatible with small
power
plants include batteries, flywheels, pumped water, and
compressed
air.(*) In selecting
an energy storage system for small power
plants in developing countries, the most important factors
to
consider are storage capacity required; capital costs;
operating
costs; nature of storage/generation duty cycles; system
complexity
in terms of how easily the system can be built, operated,
and
maintained; hardware availability; form of energy
recoverable
from storage; conversion efficiency; and the country's
current
state of technical development in related fields.
In this examination of energy storage systems, emphasis will
be
placed on the overall technical features of the systems and
their
comparative performance and efficiency. The characteristics
of
the various energy storage technologies are considered below
individually and then compared with each other. Based on
this
comparison, recommendations as to the most promising storage
systems for use in combination with small-scale hydropower
and
wind energy generators are made. It should be noted that the
discussion of economic factors (e.g., operating costs) is
based
on data obtained for the most part from large power plants
in
highly industrialized countries such as the United States.
----------------------
(*) Other more advanced energy storage technologies are
beyond the
scope of this paper.
One word of caution: It is beyond the scope of this paper to
provide a detailed
engineering or economic analysis of energy
storage systems. A feasibility study will have to be
performed
for any given site.
Nevertheless, this paper will aid in the
selection of promising energy storage system that merit more
detailed study.
II. SYSTEM ALTERNATIVE
Several energy storage systems will be examined in this
section:
batteries, compressed air, pumped water, and flywheels.
BATTERIES
Batteries are commonly used to store the electricity
generated by
wind machines and small-scale hydropower plants. A typical
system
couples the drive shaft of the power source to a direct
current
(DC) generator. The rotating shaft produces mechanical
energy,
which is converted to electricity by the generator. Excess
electricity
can then be stored in banks of batteries.
Before choosing any generating and storage system, you must
determine how much power you will need. Tables 1 through 3
show
average annual power usage for electric home heating and
appliances
in the range of 5,000-8,000 kilowatt-hours per year
(kWh/yr). A small wind power system of 5 kW, such as one
currently
marketed by an American company, is estimated by the
manufacturer
to provide about 1,0000 kWh/yr under average wind
conditions.
Such a system would be more than adequate to meet the
energy requirements of an individual household in a highly
industrialized
country such as the United States. (No attempt is made
here to specify the wind conditions essential for the
economic
operation of windmills. But it is fairly well established
that if
the wind velocity does not achieve or exceed 12 miles per
hour
for most of the year, the siting of even a small wind
machine
would be economically impractical.) Based on this estimate,
even
a household with many appliances could generate sufficient
excess
power to justify the cost of battery storage.
In order to determine the cost of a combination generation
and
battery storage system, the capacity and number of wind or
hydropower
generators would have to be established, as well as an
appropriate bank of storage batteries.
Proper design of battery storage capacity must be based on
anticipated
excess power for storage and recommended battery charge
and discharge rates.
Table 1. Average
Annual Energy Requirements of 110 Volt Electrical Appliances
Average Power Estimated
Required per Annual
Energy
Appliance
Consumption
(Watts)
(kwh)
* Food Preparation
Blender
385
15
Broiler
1,436
100
Carving Knife
92
8
Coffee Maker
894
106
Deep Fryer
1,448
83
Dishwasher
1,201
383
Egg Cooker
516
14
Frying Pan
1,196
185
Hot Plate
1,257
90
Mixer
127
13
Oven
(microwave) 1,450
190
Range
with oven
12,200
1,175
self-cleaning
oven 12,200
1,205
Roaster
1,333
205
Sandwich Grill
1,161
33
Toaster
1,146
39
Trash
Compactor 400
50
Waffle Iron
1,116
22
Waste Disposer
445
30
* Food Preservation
Freezer (15 cu
ft) 341
1,195
Freezer (2 cu ft
frostless)
440
1,761
Refrigerator (12 cu
ft) 241
728
Refrigerator (12 cu
ft
frostless)
321
1,217
Refrigerator/freezer
(14 cu ft)
326
1,137
(14 cu ft
frostless) 615
1,829
Low Energy Model
1973, 21 cu ft
frostless
starting
2,480
running
320
1,200
* Health & Beauty
Germicidal
lamp 20
141
Hair Dryer
381
14
Heat Lamp
(infrared) 250
13
Shaver
14
18
Sun Lamp
279
16
Tooth Brush
7
0.5
Vibrator
40
2
* Home Entertainment
Radio
71
86
Radio/Record
Player 109
109
Television
black & white
tube type 160
350
solid state
55
120
color
tube type
300
660
solid state
200
440
* Housewares
Clock
2
17
Floor Polisher
305
15
Sewing Machine
75
11
Vacuum Cleaner
630
46
* Lights
75 Watt bulbs (8
each) 600
864
* Laundry
Clothes Dryer
4,856
993
Iron (hand)
1,008
144
Washing Machine
(automatic)
512
103
Washing Machine
(non-automatic)
286 75
Water Heater
2,475
4,219
(quick
recovery) 4,474
4,811
* Comfort Conditioning
Air Cleaner
50
216
Air Conditioner
(room) 1,565
1,889
Bed Covering
177
147
Dehumidifier
257
377
Fan (attic)
370
281
Fan
(circulating) 83
43
Fan (rollaway)
171
138
Fan (window)
200
170
Heater
(portable) 1,322
178
Heating Pad
65
10
Humidifier
177
163
* Tools
1/4"
drill 250
2
Sabre Saw
325
1
Skill Saw
1,000
5
Typewriter
40
7
Water Pump (1/3
HP) 420
150
3" Sander,
Belt 770
10
* Electric Home Heating [a]
Measured Living
Area
1,000 Sq. Ft.
17,000
16,300
1,500 Sq. Ft.
21,500
20,800
2,000 Sq. Ft.
26,000
25,500
Sources: Electric Energy Association, 90 Park Avenue, New
York, New York; Henry
Clews,
"Electric Power from the Wind," Business Week, March
24, 1973.
Note: The estimated annual kilowatt-hour consumption of the
electric appliances
listed in this table are based on normal usage. When using
these figures for
projections, such factors as the size of the specific
appliance, the
geographical area of use, and individual usage should be
taken into
consideration. Please note that the wattages are not
additive since all units
are normally not in operation at the same time.
[a] Based on figures published by local utilities for
electrically heated homes.
Table 2. Typical Home Power Usage
Average Power
Daily Energy
Required per
Consumption
Type of Appliance
Appliance (Watts)
(kWh) [a]
Refrigerator:
14 cu. ft.
frostless 615
5.00
1/2 HP oil burner
400
3.21
Lights (100-watt bulb)
100 x number of lights
5.60
TV color tube
300
1.80
Coffee maker
900
0.60
Toaster
1,146
0.40
Frying pan
1,196
0.60
Clocks (3)
2
0.14
Hot plate
1,257
0.42
Vacuum cleaner
630
0.63
Dishwasher
1,201
0.80
Clothes washer
512
0.25
Clothes dryer
4,856
2.41
Total
21.86
Source: Grumman Aerospace Corporation, Living with Wind
Power
(Bethpage, New York, 1975), p. 4.
[a] 21.86 x 30 = 655.80 kWh per month; 655.80 x 12 = 7,869
kWh
per year.
Table 3. Planned Home Usage
Average Power
Daily Energy
Required per
Consumption
Type of Appliance
Appliance (Watts)
(kWh) [a]
Refrigerator: 21 cu. ft.
frostless Philco
Ford 320
2.56
1/2 HP oil burner
400
3.21
Lights (40-watt bulb)
40 x number of lights
2.24
TV color solid state
200
1.20
Coffee maker
900
0.60
Toaster
1,146
0.40
Frying pan
1,196
0.60
Clocks (3)
2
0.14
Hot plate
1,257
0.42
Vacuum cleaner
630
0.63
Dishwasher
1,201
0.80
Clothes washer
512
0.25
Clothes dryer
4,856
2.41
Total
15.46
Source: Grumman
Aerospace Corporation, Living with Wind Power
(Bethpage, New York, 1975), p. 4.
[a] 15.46 x 30 = 463.80 kWh per month; 463.80 x 12 = 5,565.5
kWh
per year.
Specific questions that must be considered in designing such
a
system are:
1. The types of
electrical loads to be served by the system.
Whether
direct current (DC) power only is required or
whether
inverters must be included to complete the conversion
of stored DC
electricity to alternating current
(AC). If the
loads to be served are largely incandescent
lighting and
heating, the output of the battery system
may remain
direct current since incandescent lamps and
most heat
producing equipment (space heaters, toasters,
irons)
operate successfully on DC or AC. If the loads are
motors (pump
drives, fans) of 1/2 horsepower and larger
or are
communication equipment (radio and television
transmitters), inverters will be required as a part of
the storage
system.
2. Whether a
multiple power generation and multiple user
system is
required. In most applications, a single prime
mover
(windmill, turbine) will be required. However, if
multiple
generators are employed, additional equipment
must be added
to the system to enable paralleling of
electrical
output. Multiple battery installations accompany
multiple
generators as a general rule. For most
applications,
a single prime mover, generator, and battery
bank will be
preferred due to the simplicity of
installation, operation, and maintenance.
Where extended
systems to
serve more loads are desired, an increase in
capacity of
the single system is the preferred approach.
3. Whether
commercial hardware with established performance
characteristics is available. While it
is possible to
assemble and
fabricate a system from unrelated components,
the chances
for successful operation will be enhanced
by using
factory-assembled systems that have been
designed to
match one another. A compromise in development
of the system
would be to purchase and match groups
of commercial
equipment. For example, a prime mover and
generator
could be purchased and matched to a battery
bank,
charger, and inverter.
4. Energy source
characteristics, by day and by season. If
wind is the
source of energy, its availability must be
determined,
on average, for each day of each season. Its
velocity must
also be estimated. If water is the source,
the same
determinations must be made. Whether the energy
source is
wind or water, these determinations must be
made in
advance of designing the storage system. For
example,
winds usually vary in velocity throughout the
day; during
periods of low or no wind, the battery system
must be
capable of making up the electrical energy the
generator
cannot produce during those periods. Similarly,
knowing the
length and time of occurrence of strong wind
velocity will
enable a designer to estimate how large a
battery bank
can be recharged.
5. Electrical
load demand characteristics, by day and by
season.
The daily, weekly, and seasonal characteristics
of the
electrical load demand must be determined in
advance of
design of the system. To make electrical
energy
available at the moment it is needed requires an
accurate
estimate of how much is needed at what hours of
which days during the year. For example,
if water is to
be pumped for
irrigation, it will likely be a continuous
load
throughout certain seasons. Lighting loads will
appear only
in the early morning, evenings, and early
hours of the night, but these loads will
appear every day
of the year
even though the number of hours will vary
each day. If
space heating will be provided, it will
likely appear
as a load on the system only during a
specific
season.
The costs of a given system will have to be estimated, based
on
discussions with specific hardware suppliers regarding:
*
performance specifications for the system;
*
capital costs;
*
shipping costs;
*
power consumption and efficiency of
operation;
*
labor commitment required for system
operation; and
*
anticipated life of hardware components.
Having stated these requirements for initial system design
and
pricing, it is clear that an experienced electrical engineer
should be selected to plan and oversee system installation.
Once
a system has been assembled, semi-skilled laborers could
become
operators, but there should be supervision by someone
sufficiently
trained in the component hardware to conduct all necessary
routine maintenance.
No attempt is made here to specify hardware, which must be
done
by the electrical engineer selected for system design, in
collaboration
with specific hardware suppliers.
There are many types of storage batteries. Many of these, in
various stages of development, have performance
characteristics
superior to the lead-acid battery. However, in terms of
overall
demonstrated performance, cost, useful life, and commercial
availability, the lead-acid battery is the most conservative
and
economical choice (see Table 4). Industrial lead-acid
batteries
with power ratings to 225 ampere-hours and regeneration life
cycles to about 1,800 are available commercially.
Table
4. Comparison of Today's Storage
Batteries
Battery
Density By: [b]
Cost [a] Weight
Volume
Life[c]
Battery Type
(Dollars/kWh) (Wh/kg)
(kWh/cu.meter)
(Cycles)
Silver-Zinc
900 120
310.8
100/300
Nickel-cadmium
600 40
127.1
300/2,000
Nickel-iron
400 33
49.4
3,000
Load-acid:
50 22
91.8
1,500/2,000
Source: D.L.
Douglas, "Batteries for Energy Storage," Symposium
on Energy
Storage, 168th National Meeting, American Chemical
Society,
Preprint Fuel Division, Vol. 19, no. 4
(Washington,
D.C.: ACS, 1974), pp. 135-154.
[al Cost to the
user.
[b] Battery
capacity is inversely related to rate of discharge.
The values
shown are for the 6-hour rate.
[c] Cycle life
depends on a number of factors, including depth
of discharge,
rate of charge and discharge, temperature, and
amount of
overcharge. Range shown is from most severe to
modest duty.
COMPRESSED AIR
The drive shafts of wind power systems or small-scale
hydropower
plants can be linked to conventional gas compressors and
used to
store air at pressures on the order of 600 pounds square
inch
(psi). The compressed air can be depressurized subsequently
through conventional turbines to generate electricity, or it
can
be linked through gearing for use of the stored energy to
power
any mechanical machinery driven by a rotating shaft or drive
belt. Efficiencies of 75 percent can be attained for
utilization
of the stored energy.
The compressed gas can either be air or fuel gases (e.g.,
natural
gas or hydrogen).
However, for purposes of this paper, the discussion
will relate to compressed air only.
The economics of storage will be most favorable if existing
underground storage capacity such as depleted oil fields,
coal
mines, or aquifers can be used.
Underground storage of natural
gas is a widely used and economical technology.
If underground
storage containers are used, costs are minimized, but a
certain
amount of unrecoverable residual gas loss (20 percent or
more)
will have to be accepted as a penalty.
High pressure gas can also
be stored in steel containers.
However, if new containers must be
purchased, the capital costs for a large power plant may be
greatly increased.
For small plants, steel tanks are a practical
alternative.
PUMPED WATER
Pumped water, stored above ground or underground, can also
be
used as an energy storage device in combination with either
small-scale hydro or wind energy generators.
Pumped water as an
aid in peak leveling for electric hydropower generation has
been
used in the United States since the early 1930s.
The options for
energy retrieval are quite similar to compressed air with
perhaps
5-15 percent' less overall efficiency than that obtained
from
compressed air.
Underground storage in various types of depleted
mines or aquifers offers some cost advantages over surface
storage,
since the costs of reservoir construction can greatly
increase
the total cost of power plant construction.
Pumped water storage in a special reservoir can be provided
during high river flow periods.
During spring thaws or rainy
seasons the river flow may be able to develop more power
than the
electrical system can consume.
The stored water may then be
released for power generation during future peak load
periods or
dry seasons.
Extensive areas of land must be flooded to provide
sufficient storage or pondage for a hydroplant.
Losses due to
evaporation, irrigation, and infiltration into the soil are
difficult
to estimate and may vary from time to time.
When evaporation
rates are high, a shallow pond with a large surface area is
disadvantageous.
The available data on costs for pumped water storage systems
are
derived entirely from megawatt size power plants.
For small power
plants, applicable cost data will have to be calculated for
any
given site considered.
FLYWHEELS
The flywheel is a device that permits storage of energy in
the
form of a rotating wheel.
Mechanical energy such as that from the
rotating shaft of a wind energy or hydropower system can be
converted to the kinetic energy of a low-friction flywheel
for
storage. Surplus
energy from a wind or hydropower system stored
in the rotating flywheel can be subsequently recovered as
rotating
shaft mechanical energy or possibly converted to electrical
energy via a generator to satisfy peak demands.
The energy stored in the flywheel is given by the formula
W = 1/2 [Iw.sup.2] where "W" is the stored energy,
"I" is the moment of
inertia of the flywheel, and "w" is the angular
velocity in radians
per second of the flywheel.
One of the attractive features
of the flywheel is its adaptability to a wide range of
energy
requirements for small power plants in the 1-50 kW
range. The
mass of the flywheel and its angular velocity can be varied
to
obtain this range of storage capacities.
Efficiencies are potentially
high and energy densities of 66 watts/kilogram can be
attained
for power peaking rotation speeds of 1,800 to 3,600
revolutions
per minute (rpm) by gearing to the rotating shaft of
small power generators, whether wind or hydro.
Successful performance requires careful design and
high-strength
materials. Steel has
been used for years, but modern composites,
such as metal alloys, glass fiber, and polymer/carbon fiber,
provide
the strength required for coherence during extended duty
cycles to prevent catastrophic failure of the flywheel at
high
rotation speeds.
Actually, wood and bamboo are low-cost, high-strength
flywheel materials that are economically competitive
with the synthetic composite materials cited above.
The flywheel is quite competitive with alternative energy
storage
systems for small power plants in terms of efficiency,
storage
energy density, and cost.
Small flywheels that provide 30-1,000
watt-hours (Wh) of energy storage for around $50-100/kW
have been developed (see Figure 1).
ues1x11.gif (600x600)
Flywheels are small, but are high technology devices
requiring
sophisticated engineering know-how on the part of those who
will
select the hardware and design the match to the wind or
hydropower
installation. Once
installed, semi-skilled operators can
maintain these installations under the supervision of an
engineer.
III. COMPARISIONS
AND RECOMMENDATIONS
Tables 5 and 6 give comparisons of the energy densities,
conversion
uest50.gif (600x600)
efficiencies, state of technical development, cost data, and
potential applications of the various types of energy
storage
systems. These
comparisons, however, were based on data obtained
from large power plants, and therefore must be adjusted for
small
power plants.
The essential criteria for selecting an energy storage
system
are: (1) the
technology should provide high conversion efficiency;
(2) commercial
hardware should be currently available; and
(3) costs should be
favorable compared to alternative options.
Based on the above criteria, the energy storage systems most
likely to be both technically feasible and economical are:
1.
Conversion to electricity via generators and
storage in
lead-acid
batteries.
2.
Storage as mechanical energy in a flywheel
with recovery
as
mechanical energy.
3.
Compressed air storage, combined with a
turbogenerator
for recovery
of stored energy as electricity or as mechanical
energy.
4.
Pumped water combined with a turbogenerator
for recovery
of stored
energy as electricity or as mechanical energy.
BIBLIOGRAPHY/SUGGESTED READING
LIST
Abelson, P.H., ed. Energy:
Use, Conservation and Supply.
Special
Science
Compendium. Washington, D.C.:
American Association
for the
Advancement of Science, 1974.
Adams, J.T. Electricity and Electrical Appliances
Handbook. New
York, New
York: Arco Publishing Co., 1976.
Ayer, Franklin A.
Symposium on Environment and Energy Conservation.
EPA
600/2-76/212:PB-271 680. Washington,
D.C.: U.S.
Environmental
Protection Agency, 1975.
Berkowitz, J.B. and Silverman, H.P.
"Energy Storage."
Proceedings
of Symposium,
October 6, 1975. P.O. Box 2071,
Princeton, New
Jersey
08540: New Technology Subcommittee and
Electrothermics
and Metallurgy
Divisions, Electrochemical Society,
1976.
Bockris, J.O. Energy
Options. New York, New York:
John Wiley &
Sons, 1980.
Brookhaven National Laboratory.
Proceedings of the ERDA Contractors'
Review Meeting
on Chemical Energy Storage and Hydrogen
Energy Systems.
CONF-761134.
Upton, New York:
Brookhaven
National
Laboratory, 1976.
Chubb, T.A.
"Analysis of Gas Dissociation Solar Thermal Power
System." Solar Energy
17. New York, New York:
Pergamon
Press, 1975, pp.
129-136.
Cohen, R.L. and Wernick, J.H. "Hydrogen Storage
Materials: Properties
and
Possibilities." Science 214, 1981,
pp. 1081-1095.
deWinter, F. and Cox, M., eds.
"Mechanical Energy Storage System
for a 10 kWe
Solar Power Pack." Sun--Mankind's
Future Source
of Energy.
New York, New York:
Pergamon Press, 1978.
Douglas, D.L.
"Batteries for Energy Storage."
Symposium on Energy
Storage.
168th National Meeting, American Chemical
Society,
Division of Fuel
Chemistry. Preprints Vol. 19, No. 4,
135-154.
Washington, D.C.:
American Chemical Society, 1974.
Duffie, J.A. and Beckman, W.A.
Solar Energy Thermal Processes.
New York, New
York: John Wiley & Sons, 1974.
Fickett, A.P.
"Fuel-Cell Power Plants" Scientific American
293(6), 1978,
pp. 70-76.
Gross, S., ed. Battery Design and Optimization.
Proceedings of
Symposium. Vol.
79. P.O. Box 2071, Princeton, New Jersey
08540:
Battery Division, Electrochemical Society,
1979.
Grumman Aerospace Corporation, Living With Wind Power.
Bethpage,
New York:
Grumman Aerospace Corporation, 1975.
Harboe, Henrik. The
Use of Compressed Air for Energy Storage.
168th National
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