Energy Storage
Energy Storage I
But first to finish up with electric vehicles:
Credit: Michael Goodman
KEY COMPONENTS of an electric vehicle are energy storage cells, a power
controller and motors. Transmission of energy in electrical form eliminates the
need for a mechanical drivetrain. Regenerative braking (inset) uses the
motor as a generator, feeding energy back to the storage system each time the
brakes are used.

The Key of course is marketing
people have to buy the product
California Mandate:
 1998: 2% of all vehicles offered for sale must be zero
emission vehicles (meaning electric cars)
 2003: 10% must be zero emission
 As of Fall 1995 the Mandate has been
rolled back
Some Prototypes:
 Ford Ecostar twopassenger electric minivan used by
PostOffice and UPS sodiumsulfur batteries
 Chrysler TEVan nickeliron batteries
 $500,000 given to Yosemite to replace diesel buses with
electric buses
 $500,000 given to General Motors to loan 50 vehicles to
1000 people nationwide for test drive results
Some Internet Resources:
Energy Storage
Why is Energy Storage Important:?
 Stored energy is what we use now Fossil Fuels
 It's what is required to make low duty cycle alternative
energy sources viable especially solar. Need to store the
excess energy when the collector system is being irradiated
 Energy storage is also important for power leveling for
the power companies Generating stations operate more efficiently
if they run at constant output level want to shove unused
energy to a storage system and recover it later at times of peak
demand.
 Energy storage must consider both the amount of energy that can be
stored (energy density of the material) and the efficiency at which it
can be recovered. Some materials have high energy storage capacity but
low rate of recovery.
Energy Density of Some Materials (KHW/kg)
Gasoline  14
Lead Acid Batteries  0.04
Hydrostorage  0.3 (per cubic meter)
Flywheel, Steel  0.05
Flywheel, Carbon Fiber  0.2
Flywheel, Fused Silica  0.9
Hydrogen  38
Compress Air  2 (per cubic meter)
More on Advanced Battery Technology
Energy density storage drives the choices that can be made:
At the turn of the century electric vehicles were commonplace
(using basically leadacid batteries). Since gasoline has
much higher energy density it quickly dominated the way
vehicles were propelled.
In fact, gasoline has one of the highest energy density storage
capacities known. This makes it very difficult to duplicate
the convenience that gasoline has traditionally provided (e.g.
350 kg of batteries is equivalent to 1 kg of gasoline !).
Types of Energy Storage Systems
Pumped Hydroelectric Energy Storage:
Simple in concept use excess energy to pump water uphill
pump from lower reservoir (natural or artifical) to upper
reservoir.
Energy recovery depends on total volume of water and its height
above the turbine
 need at least 100meters
this is a stringent limit
on locations
 artificial lower reserviors can made via excavation
can achieve higher energy density due to large vertical distance
(up to 1000 feet!)
 facility does not impact free flowing stream
 sediment buildup at dam base is minimized
 Hydropower is 80% efficient (uphill or downhill). So to
pump uphill and the get energy downhill, efficiency is 0.8x0.8 =
64%
Cost Issues:
Suppose a company has a coal fired plant which operates at 36%
efficiency and uses excess power to pump water uphill. The overall
efficiency of recovering that to deliver to the consumer is
0.36 x 0.64 = 0.23 (23%)
 So stored energy is more expensive
what's the incentive?
 Need to balance this cost against the costs of building a power
planet with capacity to meet some theoretical maximum demand but the
rest of the time doesn't operate at this level
Real Life Facility in Michigan
 Use Lake Michigan as Lower Reservoir
 Upper reservoir is 75 meters higher
 Peak capacity is 2000 MW (!)
 Stored energy is 15 million KWH`
FLYWHEELS and ENERGY STORAGE
a wheel winds up through some system of gears and then delivers
rotational energy until friction dissipates it
stored energy = sum of kinetic energy of individual mass elements that comprise the
flywheel
Kinetic Energy = 1/2*Iw^{2}
I = moment of inertia ability of an object to resist changes in its
rotational velocity
w = rotational velocity (rpm)
I = kMR^{2} (M=mass; R=Radius); k = intertial constant (depends
on shape)
Inertial constants for different shapes:
Wheel loaded at rim (bicycle tire): k =1
solid disk of uniform thickness; k = 1/2
solid sphere; k = 2/5
spherical shell; k = 2/3
thin rectangular rod; k = 1/2
To optimize the energytomass ratio the flywheel needs to spin at
the maximum possible speed. This is because kinetic energy
only increases linerarly with Mass but goes as the square of
the rotation speed.
Rapidly rotating objects are subject to centrifugal forces that can
rip them apart. Centrifugal force for a rotating object goes as:
MRw^{2}
Thus, while dense material can store more energy it is also subject
to higher centrifugal force and thus fails at lower rotation speeds
than low density material.
Tensile Strength is More important than density of material.
Long rundown times are also required frictionless bearings and
a vacuum to minimize air resistance can result in rundown times of
6 months steady supply of energy
Flywheels are about 80% efficient (like hydro)
Flywheels do take up much less land than pumped hydro systems
Some Network Resources Related to Flywheels
Example Calculation:
Consider a solid disc flywheel of radius 50 cm and mass 140 kg. How
fast would it have to spin to have a store the equivalent amount
of energy that is stored in just 10 kg of gasoline when burned in
an internal combustion engine:
 10 kg of gasoline = 140 KWH
 Engine has 15% efficiency 21 KWH of useable energy
 Flywheel has a conversion efficiency of 80%
 Flywheel must therefore store 21/.8 = 26.25 KWH
 Kinetic Energy goes as 1/2*Iw^{2}. For flywheels
I =1/2MR^{2}.
 A revolution means an object moves 2 pi radians (360 degrees)
 So
Stored Energy = 1/2*Iw^{2} =
1/2*1/2MR^{2}* (2 pi* w)^{2}
= pi^{2}MR^{2}* w(RPS)^{2}
 If we measure w in revolutions per second then the
stored energy of a flywheel is approximately 10MR^{2} x w(RPS)
^{2}
 For M=140 kg and R=50cm this yields a required w of 500 RPS or
30,000 RPM
 The required energy storage is 26 KWH/140 Kg = .18 KWH/kg
which excees the energy storage density of steel  hence such
a flywheel requires construction out of carbon fiber.
Compressed Air:
Has high energy storage capacity compared to the alternatives.
About 10 times higher per cubic meter than water.
One example (in Germany) to date:
 Storage reservoir is underground cavity in a natural
salt deposit
 The storage volume is 300,000 cubic meters
 Sheer weight of the salt deposit is able to pressure
confine the air reservoir
 Air is compressed to 70 atm (1000 lbs per square inch)
 Compression is done by electrically driven air compressors
 System delivers 300 Megawatts for 2 hours by using the
compressed air to drive a turbine
 Difficult to measure the efficiency of this system. Two
major contribution to the inefficiency:
 Energy required to cool the air as it is being put
into storage this is a critical requirement (see below)
 Energy required (usually from fuel) to expand the cool
air taken from storage as it entires the turbine.
 Desireable design feature would be recycle the waste heat
from the compression stage and use it to reheat the air during
expansion stage
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