SEED Workshop 9: Energy Storage

Lesson Outline

• Explain basics of energy storage and need for it
• Common examples of energy storage
• Mechanical storage techniques
• Electro-chemical storage techniques
• Hydrogen fuel cells
• Energy storage projects worldwide for RES

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Demonstration: Miniature hydrogen fuel cell and solar-powered battery hub

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WHY STORE ENERGY?

• Sometimes inconvenient or impossible to produce energy where and when it is needed
• Variations in energy demand (especially electric) often don’t meet capacity
• Sometimes energy production methods (especially RES) inherently variable

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BASE-LOAD VS. PEAK-LOAD POWER PLANTS

• Base-load power plants run continuously over an extended period of time
• Base load is the minimum level of electricity demand required over a period of 24 hours
• e.g. fossil fuel plants, nuclear, hydroelectric, geothermal
• Peak-load power plants are energy sources which start up during periods of high demand
• Peak demand occurs for short instances during the day
• e.g. wind turbines, solar panels, diesel generators

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WHY STORE ENERGY? RENEWABLE ENERGY LIMITATIONS

• RES like solar and wind are peak-load systems
• Inherently variable electricity output
• Energy needs to be accumulated and stored for future use in order to meet demands of base-load
• Storage involves conversion of one form of energy to another - always <100% efficient

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STORAGE TECHNIQUES: MECHANICAL

Pumped hydroelectric - most economical and practical for large-scale electricity storage

• Been used extensively since 1930s
• Water pumped into reservoir during low demand, and allowed to flow into lower reservoir during high demand to produce electricity
• Reliable, low technology, low maintenance

Flywheels - accelerating a rotor to very high speed - holds energy as rotational energy

• Spin at 20,000-50,000rpm in vacuum enclosure
• Low maintenance, long lifetime, high energy density

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STORAGE TECHNIQUES: MECHANICAL

Pumped air energy storage

• Off-peak Hours: Air pumped in
• Peak Hours: Air is heated, fed into low and high pressure expanders
• Difference in pressure rotates generator

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STORAGE TECHNIQUES: ELECTROCHEMICAL

Rechargeable batteries - many different types and sizes

• Lead-acid batteries hold largest market share of energy storage products. Inexpensive, low-labor
• Lithium-ion batteries have best energy-to-mass ratio and slow self-discharge
• As of 2017, largest battery storage power plant has capacity of 129MWh
• Supercapacitors - fall between capacitor and rechargeable battery
• 10-100x more energy to mass than regular capacitors
• Delivers charge much faster and tolerate charge/discharge cycles better than batteries

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HYDROGEN FUEL CELLS

Fuel cells directly convert chemical energy into electricity via electrochemical reaction between a fuel (typically H₂) and oxygen

• Require a continuous source of fuel and oxygen to sustain chemical reaction
• Hydrogen produced via electrolysis of water
• Gas can then be oxidized to produce electric current
• Hydrogen energy density = 142 MJ/kg. Compare with gasoline = 44.5 MJ/kg

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HYDROGEN FUEL CELLS

Every fuel cell consists of anode, cathode, and electrolyte layer

• Anode: H₂ undergoes oxidation via catalysis (platinum powder)

H₂ -> 2H⁺ + 2e^-

• Cathode: Contains oxygen gas. H⁺ flows into cathode, electrons drawn into it from external circuit. Oxygen reacts with H⁺ and electrons to form H₂O (catalyzed by nickel plating)

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ENERGY STORAGE PROJECTS WORLDWIDE FOR RES

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Tesla Gigafactory

• Produces 20 GWh of battery storage per year
• Car Batteries, Power Packs, Powerwalls

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Lithium-Ion Batteries

• Powerpack
• South Australia
• 129 MWh capacity
• Enough to power 30,000 homes

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Lithium-Ion: Home Storage

• Powerwall
• Total Cost with Installation is around $10,000
• Off the Grid

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Puerto Rico

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McIntosh CAES Plant

• Only CAES plant in the US
• 110 MW Turbine
• Powers 110,000 homes for 26 hours
• Used for natural gas but easily adapted for renewables

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