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                          TECHNICAL PAPER #22
                         UNDERSTANDING ENERGY
                            STORAGE METHODS
                            Clyde S. Brooks
                          Technical Reviewers
                             Paul L. Hauck
                           LeGrand Merriman
                         Lester H. Smith, Jr.
                             Published By
                   1600 Wilson Boulevard, Suite 500
                     Arlington, Virgnia 22209 USA
                Tel:  703/276-1800 . Fax: 703/243-1865
                     Internet: pr-info[at]
                 Understanding Energy Storage Methods
                          ISBN: 0-86619-222-0
              [C]1985, Volunteers in Technical Assistance
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
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.
Several energy storage systems will be examined in this section:
batteries, compressed air, pumped water, and flywheels.
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
   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
   (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
   black & white tube type       160                       350
    solid state                    55                       120
   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]
  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.
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
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
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.
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.
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
     4.   Pumped water combined with a turbogenerator for recovery
         of stored energy as electricity or as mechanical energy.
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
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Ayer, Franklin A.  Symposium on Environment and Energy Conservation.
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     Environmental Protection Agency, 1975.
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     Jersey 08540:  New Technology Subcommittee and Electrothermics
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Brookhaven National Laboratory.   Proceedings of the ERDA Contractors'
     Review Meeting on Chemical Energy Storage and Hydrogen
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Chubb, T.A.  "Analysis of Gas Dissociation Solar Thermal Power
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     Storage.   168th National Meeting, American Chemical Society,
     Division of Fuel Chemistry.  Preprints Vol. 19, No. 4,
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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 Meeting, American Chemical Society, Division
     of Fuel Chemistry.  Preprints Vol. 19, No. 4, 155-161.  Washington,
     D.C.:   American Chemical Society, 1974.
Jensen, J.  Energy Storage.  London, England and Boston, Massachusetts:
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