Power Generation from Renewable Energy Sources

Fall 2013Instructor: Xiaodong Chu

Email ： chuxd@sdu.edu.cnOffice Tel.: 81696127

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Electrochemically Stored Energy

I. Batteries: Invented in 1800 by Alexandro Volta

"... In this manner I continue coupling a plate of silver (Ag) with one of zinc (Zn), and always in the same order, that is to say, the silver below and the zinc above it, or vice versa, according as I have begun, and interpose between each of those couples a moistened disk. I continue to form, of several of this stories, a column as high as possible without any danger of its falling".

V

Cathode (-)

Anode (+)

Paper with Electrolyte, H20Zn

Ag

Vbatt = V1 + V2 + V3 +...+ Vn

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Electrochemically Stored Energy

I. Types of Batteries

Non Rechargeable:- Carbon Zinc- Cadmium Zinc- Zinc Chloride - Manganese Dioxide (Alkaline)- Magnesium – Air Fuel Cell- Zinc Air- Zinc-Bromine- Lithium Manganese- Silver Oxide - Mercuric Oxide

Rechargeable:- Nickel-Cadmium- Nickel-Metal Hydride- Nickel-Zinc- Lead Acid

- Lithium Ion

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Electrochemically Stored Energy

Rechargeable Batteries Comparison

1000

1.75 - 5.25

Which type of battery would be the most suitable for a low power lighting installation?

1000+

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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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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