Energy Storage:

Why is Energy Storage Important:?

• Stored energy is what we use now --> Fossil Fuels

• Its 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

  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

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)

Energy density drives choices:

At the turn of the century electric vehicles were common place. Higher energy density of gasoline quickly dominated the way vehicles were propelled.

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 100-meters --> 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 build-up 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:

• On most McDonalds Toys
• 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*I*w*w

• I = moment of inertia --> ability of an object to resist changes in its rotational velocity
• w = rotational velocity (rpm)
• I = k *M*R*R (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 energy-to-mass ratio the flywheel needs to spin at the maximum possible speed

Rapidly rotating objects are subject to centrifugal forces that can rip them apart.

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

Example:

Consider a solid disc flywheel of radius 50 cm and mass 140 kg.

• If it spins at 32,000 RPS it has stored energy equivalent to 10kg of gasoline (2.5 gallons)
• Energy density is 0.2 KWH/kg --> therefore can't be made of steel but need carbon fiber or fused silica

Next Lecture

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The Electronic Universe Project

e-mail: nuts@moo.uoregon.edu