Energy Storage

Energy Storage I

But first to finish up with electric vehicles:

The Key of course is marketing people have to buy the product

California Mandate:

Some Internet Resources:

  • EV Fact Sheet
  • Fuel Cells for EVs
  • Advanced Battery Technology
  • Components of Government Program

    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)

    Energy density storage drives the choices that can be made:

    At the turn of the century electric vehicles were commonplace (using basically lead-acid 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

    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%)

    Real Life Facility in Michigan


  • 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

  • I = moment of inertia ability of an object to resist changes in its rotational velocity
  • w = rotational velocity (rpm)
  • I = kMR2 (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. 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:


    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:

    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:

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