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 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)

Energy density storage drives the choices that can be made:

At the turn of the century electric vehicles were common place (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

• 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

Next Lecture

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The Electronic Universe Project

e-mail: nuts@moo.uoregon.edu