Comparative Operating Costs:

Energy Storage

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)

Energy density storage drives the choices that can be made:

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`

Of course, you can always just refill the lake in times of low demand

More on pumped hydro

FLYWHEELS and ENERGY STORAGE

A flywheel, in essence is a mechanical battery - simply a mass rotating about an axis. Flywheels store energy mechanically in the form of kinetic energy. They take an electrical input to accelerate the rotor up to speed by using the built-in motor, and return the electrical energy by using this same motor as a generator.Flywheels are one of the most promising technologies for replacing conventional lead acid batteries as energy storage systems.

So, in other words. During times that your generating more power than you need, you can spin the fly wheel up, so to speak. When you need to recover that energy, you let the fly wheel spin down.

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. 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

Fused Silica Flywheels High tensile strength material allows it to be rotated very fast without flying apart

The model with the small yellow disc tends to stop when the crank and connecting rod are in a straight line ('dead' spots) - because sliding the brass knob exerts no turning force on the shaft. In the model with the big yellow flywheel, it is easy to keep the disc turning, once it has started, due to the effect of the flywheel. The mass and the size of the big flywheel helps resist the slowing down of the model as it is turning.

Comparative Advantages/Disadvantages of high speed flywheels

Flow Batteries:

Excitement over flow batteries derives from their attributes, which combine aspects of conventional batteries and fuel cells. They are relatively simple, efficient, scalable, durable, and can optimize either power or energy output, as desired. Flow batteries can respond in fractions of a second and can cycle rapidly and deeply at high or low power output with minimal battery degradation.

Flow batteries are scalable from a few watts and kilowatt-hours to tens or hundreds of megawatts and megawatt-hours.

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

Ultracapacitors New Technology: ultracapacitors are a new energy storage technology ideally suited for applications needing repeated bursts of power for fractions of a second to several minutes.

ultracapacitors pack up to 100 times the energy of conventional capacitors and deliver ten times the power of ordinary batteries.

An application:

Energy Storage for Prototype Military Transport Vehicle powered by Hybrid Electric Drive System

Summary