1 ENERGY STORAGE Part II Dr. Dave Irvine-Halliday ENEL 581.

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ENERGY STORAGEPart II

Dr. Dave Irvine-Halliday

ENEL 581

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Chemistry of Lead Acid Batteries

When the battery is discharged:

• Lead (-) combines with the sulfuric acid to create lead sulfate (PbSO4),

Pb + SO4 PbSO4 + 2e-

• Lead oxide (+) combines with hydrogen and sulfuric acid to create lead sulfate and water (H2O).

PbO2 + SO4 + 4H + 2e- PbSO4 + 2H2O• lead sulfate builds up on the electrodes, and

the water builds up in the sulfuric acid solution.

When the battery is charged:

• The process reverses; lead sulfate combining with water to build up lead and lead oxide on the electrodes.

Lead Acid Batteries Consist of:

Lead (Pb) electrode (-) Lead oxide (PbO2) electrode (+) Water and sulfuric acid (H2SO4) electrolyte. PbSO4 + 2e- Pb + H2SO4

PbSO4 + 2H2O PbO2 + H2SO4 + 2e-

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Sealed Lead Acid (SLA) Batteries

• Instead of water and sulfuric acid the SLAs have the acid in form of a gel

• The battery is valve regulated to prevent the build up of gases which are produced during charging.

• Maintenance free

• Safer against leakage

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Using SLA batteries in a Solid State Lighting System

• Deep discharging will shorten the battery life time

• Safe limit - do not discharge the battery more than 20% of its full capacity

• Keep the battery charged all the time

• Never short circuit the battery terminals

• Let the users be aware about the proper handling of the battery

• Operating Temperature Limits(-30º C to 65º C)

• Heat can kill the battery • Cold slows down chemical

reactions inside in the battery

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Discharge Pattern of SLA batteries in a Solid State Lighting System

Example:A 12V 7.2 Ah battery can store

*E = Voltage (13.2v) x Capacity (7.2 Ah)

E = 95 Wh or 342 KJ

A luxeon lamp takes 110 mA when battery voltage is at 13.2 V

Then

Pconsum = 13.2 V x 110 mA = 1.4 W

Assuming 75 % power transfer efficiency from battery to lamp)

Tdisch = Capacity / consumed current x 0.75 = [7.2 Ah / 110 mA] x 0.75 = 49.09 h

or Tdisch = Energy / Power consumption

= [95 Wh /1.4W] x 0.75 = 50.89 h

Capacity of a battery (C) is measured in Ampere-hours (Ah)

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Discharge Pattern of SLA batteries in a Solid State Lighting System

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Electrochemically Stored Energy

Net reaction:2H2 + O2 2H2O + Electricity + Heat

Chemistry of a Fuel Cell

II. Fuel Cells: Convert chemical energy into electric energy

Chemical Process:

1.- Platinum Catalyst (electrode) Separates Hydrogen gas into electrons- and Ions+.

2.- Hydrogen Ions+ pass through membrane only.

3.- With help of the Platinum catalyst Hydrogen Ions- combine with electrons and oxygen to form water.

Proton Exchange Membrane

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Electrochemically Stored Energy

Net reaction:2H2O + 4H+ + 4e- 2H2 + O2

Electrolizer

II. Reversible Fuel Cells / Electrolizer:

Chemical Process:

1.- Platinum Catalyst Separates Water into Oxygen and Hydrogen electrons and Ions+.

2.- Hydrogen Ions+ pass through membrane only.

3.- With help of the Platinum catalyst, Hydrogen molecules are formed when hydrogen Ions- and electrons are combined.

Proton Exchange Membrane

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Fuel CellsUsually named according to their electrolyte and categorized according to their operation temperature.

High temperature fuel cells(600° to 1000° C): Solid Oxide Fuel Cell (SOFC) Molten Carbonate Fuel Cell (MCFC)

Low temperature fuel cells (< 200°C): Polymer Electrolyte Membrane Fuel Cell (PEMFC) Direct Methanol Fuel Cell (DMFC) Phosphoric Acid Fuel Cell (PAFC) Alkaline Fuel Cell (AFC)

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Fuel Cells

Advantages:

Environmentally Friendly (When Hydrogen obtained using RE) High energy density Quiet operation compact size scalable

Disadvantages:

Requires Refill of Hydrogen Low Efficiency (55% - 25%) Cost ($3/W - $4/W)

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Magnesium - AirFuel Cell Powering Three LUTW WLED (1 W) Lamps (Feb2006)

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Electric Energy Storage

I. Capacitor: is an electrical device which serves to store up electricity or electrical energy.

Q = CV

Q = charge (Coulombs)V = voltage (Volts)

C = 0.0885 x10-12 K · A / d

C = Capacity (farads)K = dielectric constantA = area of one plate (square centimeters)d = distance between plates (centimeters)

Stored energy: E = ½ C · V2

d

A

e.g. 1000μF at 35 volts will store 0.6125 Joules (enough to power 1 W WLED lamp for ~ 0.5 seconds, assuming 90% power transfer efficiency and 1.2 W of lamp consumption)

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Electric Energy StorageII. Ultracapacitors or Supercapacitors: Similar to a normal capacitor, a supercapacitor or ultracapacitor stores energy electrostatically by polarizing an electrolytic solution. Highly porous carbon-based electrodes increases the area to be charged as compared to flat plates.

Capacitance: 2500 - 5000 FaradsVoltage: 2.5 VCharging/Discharging Efficiency: 90%Charging/Discharging Cycles: 500 000Stored Energy:

E = ½ C · V2

E = 7.81KJ to 15.62 KJ

Enough to power a 1W WLED lamp for ~ 1.6 to 3.2 Hours(assuming 90% energy transfer efficiency and 1.2 W lamp consumption)

Negative electrode

Positive electrode

Ion-donor electrolyte

Ultracapacitor cross section view when is being charged

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Lead Acid Batteries Ultracapacitors

1000 Charging Cycles 100K – 500K Charging Cycles (Years?)

Lifetime 10 years Deteriorates 80% in 10 years

*Require discharge controllers *Not require charge controllers

*Toxic compounds (H2SO4, Pb) *No toxic compounds

Slow charge and discharge Safe fast charge and discharge

High energy density Low energy density

Low power density High power density

*Cost – US $0.11/ Wh (Initial) *Cost US$ 12.8 / Wh (Initial)

Efficiency 75% to 80% Efficiency 95%

Lead Acid Batteries vs Ultracapacitors

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Supercapacitor powers a 1 W Luxeon WLED for more than 1 hour 25 Feb. 2002

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Show IEEE Spectrum article:

http://www.spectrum.ieee.org/nov07/5636

“The Charge of the Ultracapacitors”

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• More Power. More Energy. More Ideas.

The new HC family of products includes compact, cost-effective, 25-, 50- 150-farad cells, all rated at 2.7 volts.

Key features and benefits include:

• Reliable performance for 500,000 or more charge/discharge cycles • Zero maintenance over estimated 10-year operating lifetime • Broad operational temperature range (-40 to +65C) • High power and energy density in low-volume, lightweight package • Two-pin radial design for easy mounting • Resistant to reverse polarity • Scalable to higher voltages via multi-cell configurations• Today more than ever, system designers recognize that ultracapacitors enhance energy

efficiency and functionality and provide 'life of the application' durability for virtually any electronic device or system. The new HC product family responds to growing demand by delivering Maxwell's industry-leading technology in new form factors that are suitable for a broader range of electronic applications.

Typical applications benefiting from ultracapacitor cells in the 25-to-150-farad range include:

• Robotics and factory automation • Uninterruptible power supply (UPS) systems for industrial and telecommunications installations • Renewable energy systems, including solar and wind energy generation systems • Cordless power tools • Consumer electronics• Visit our website, www.maxwell.com for more information on this and other exciting new

developments from Maxwell Technologies.

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Electrochemical Storage Devices Comparison

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Superconductive Magnetic Energy Storage (SMES)

In SMES, Energy is stored in the magnetic field produced by a current passing through a superconductive coil immersed in liquid helium vessel.

Rapid response for either charge/discharge It is claimed that SMES are 97-98% efficient. Commercial SMES systems are able to store up to about 6 MJ.

Superconductive no resistive losses 0.1% of stored energy is used for the cooling system, needed to mantain superconductivity in the coil (~ -200°C).

L = Coil Inductance (H)I = Current (A)

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Superconductive Magnetic Energy Storage (SMES)

Advantages:

SMES systems are environmentally friendly Capable of releasing megawatts of power within a small period of time Recharges within minutes Can repeat the charge and discharge sequence thousands of times

Disadvantages:

Complex expensive parts & maintenance Big size Cost

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References:

I. Mechanically Stored Energy Flywheels – http://www.eere.energy.gov/femp/pdfs/fta_flywheel.pdfhttp://www.toolbase.org/tertiaryT.asp?DocumentID=2081&CategoryID=966Air Compression - http://www.ridgeenergystorage.com/caes_overview.htm

II. Electrochemically Stored Energy Batteries - http://www.batterystuff.com/tutorial_battery.htmlhttp://www1.jaycar.com.au/images_uploaded/recharge.pdf

Fuel Cells – http://www.nrel.gov/hydrogen/ http://www.energy.ca.gov/distgen/equipment/fuel_cells/fuel_cells.html

III. Electric Energy Storage Devices Capacitors - http://www.europhysicsnews.com/full/23/article2/article2.htmlUltracapacitors - http://www.epcos.com/inf/20/35/ds/technology.pdfSuperconductive Magnetic - http://www.mcs.vuw.ac.nz/events/EMW/downloadable-pdfs/emw_1_2004_tuleasca.pdf