Energy Storage

Why is Energy Storage Important:?


  • 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 (KWH/kg) 

       Hydrogen ------------------------- 38
 Gasoline  ------------------------ 14
 Lead Acid Batteries -------------- 0.04
 Lithium Ion Batteries ------------ 0.15
 Flywheel, Steel ------------------ 0.05
 Flywheel, Carbon Fiber ----------- 0.2
 Flywheel, Fused Silica ----------- 0.9


Energy density storage drives the choices that can be made and is essentially a tradeoff between stored power density and stored energy density.

Power = energy x time of usage so systems with large power densities but small energy densities means that they discharge their power relatively quickly. Systems with large stored energy densities generally mean systems that discharge power at relatively slow rates.

Only gasoline and hydrogen have both high power and high energy storage capacity. Batteries have good power storage but they tend to discharge pretty fast which compromises their energy storage.






Industry goal is to reach at least 400 watt hours per kg on a production scale - Envia Systems But as of Dec 2013 Envia Systems is no more and this is a major setback.



New form factor - Lithium Polymer (LiPO) - has emerged and these can have energy densities as high as 250 watt hours per kg.





Figure of Merit for "practical EVs":

A driving range of 300 miles requires about 400 KWH of storage energy (e.g. 10 gallons of gas).

At energy density of 150 watt hrs per kg it would require 2666 kg of batteries to store 400 KHW of energy - this is more than the weight of the vehicle. This is the basic problem with current battery technology and current vehicle design.

Now let's suppose one has a lightweight vehicle that could get 60 mpg (like the Insight does). At 60 mpg one would use about 5 gallons to go 300 miles and that's equivalent to about 200 KWHs of storage. With a battery material that could get 500 watt hours per kg, then you would need a mere 400 kg to cover this range. Those technologies would make pure electric vehicles feasible. We are not there yet (or even close).

Who killed the electric vehicle Poor energy density storage of batteries!

More on Batteries Good overview of different types; advantages and dis-advantages and the like. Read this resource in detail.



Types of Energy Storage Systems





Pumped Hydroelectric Energy Storage:



Simple in concept: use excess energy to pump water uphill or back behind a dam

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 30-meters this is a stringent limit on locations
  • artificial lower reserviors can be 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; 2000 MW drains in 7.5 hours



















China now has the largest engineered facility in Asia:

Specifications:

  • Two storage reservoirs, 1 km apart
  • Elevation is 590m
  • Storage volume is 8 million cubic meters. This equates to 4 GWHs (gigawatt hours)
  • Reservoir drainage: Two 7 meter diameter pipe branches into 3 3.2-m diameter pipes
  • Each 3.2 m diameter pipe is connected to a 306 MW turbine.
  • Total capcity is therefore 1.8 GW meaning the system can drain for about 2.5 hours to get 4 GWHS.
  • Total project cost estimated to be 1.1 billion dollars so that's less than 1.1/1.8 dollars per watt (about 60 cents per watt).

Also the world's first seawater pumped storage facilty recently came on line. Height is 600 m above sea level; total capacity is 600 MW.



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.



Example of Flywheel/Piston arrangement:
Inertia of the Flywheel helps keep the system going.





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.



Flywheels are about 80% efficient (like hydro)

Flywheels do take up much less land than pumped hydro systems

Fused Silica Flywheels are possible: High tensile strength material allows it to be rotated very fast (100,000 rpm) 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.

Beacon Power - the Leader in Flywheel Technology

Frequency Regulation



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.



The Turlock Project

1 MW Iron-Chromium (ICB) Flow Battery



The concept of using large flow batteries at Wind Farms, which you would think would be a no-brainer, has finally started to catch on. Duh!

Alas the expansion of this project from a 2.5 MW wind farm to a 600 MW wind farm, however, has now been scrapped but some wind+storage facilities do exist



Compressed Air: In principle:





In practice only 2 facilities in the world were ever built.

  • 290 MW in Germany late 1970s
  • 110 MW in Alabama 1991
The Iowa Stored Energy Park project has been cancelled. This was to build a wind farm on top of an evacuated aquifer to compressed air storage. A good idea - apparently good ideas' don't pan out ...

Summary: