Energy storage: batteries and capacitors

How do the fundamental mechanisms of storing electrical energy in batteries and capacitors dictate the main performance differences (power density versus energy density) between them as energy storage devices?

Capacitors have low energy densities in comparison to batteries because the charge is only stored on the surfaces of the electrodes in capacitors, as opposed to the entire volume Capacitor electrodes have high electrical conductivity, so the charge can leave the capacitor much more quickly than in batteries, which rely on electrochemical processes to discharge (capacitors have high power density) Discharging & recharging in capacitors doesn't require volume or interface changes, making them more reliable long term

NiCd batteries

Cd anode, KOH electrolyte, NiO(OH) cathode Susceptible to memory effect (useful time between charges decreases over life of battery)

What are the basic general requirements for practical electrode materials in batteries?

Electrochemically active substance (what gets oxidized or reduced) Fluorinated polymeric binders Carbon black or carbon nanotubes to raise electronic conductivity --but which, having no Li themselves, dilute the charge capacity of the electrode Pores that electrolyte can penetrate

What advantages does grid-scale energy storage offer?

Just-in-time delivery system Capacity resource: peak demand support Flexibility resource: system ramping and dynamic voltage control Reliability/Resilience: temporary local power for outages

Li-ion batteries

LixC6 anode, organic liquid salt dissolved in solvent electrolyte, LiMn2O4 or LiCoO2 or LiFePO4 cathode Advantages: have almost completely replaced NiMH for rechargeable electronic devices in individual consumer products; have completely replaced NiMH for electric vehicles; have high power and energy densities Disadvantages: kinetic of the rxns slow down if lithium must diffuse long distances into and out of active electrode material; expensive; shortened lifetime

Disadvantages of batteries and capacitors

Lower specific power density [W/kg] than gasoline ICE Lower specific energy density [0.8 kWh/kg] than fuel cells and gasoline ICE

NiMH batteries

Metal hydride anode, KOH electrolyte, NiO(OH) cathode Advantages: low production costs; durable Disadvantages: most common anode is AB5 where A is a rare-earth mixture of lanthanum, cerium, neodymimum, praseodynium B is nickel, cobalt, manganese, or aluminum

Pb-acid batteries

Pb anode, sulfuric acid electrolyte, PbO cathode Advantages: Establish technology; inexpensive Disadvantages: Low energy and low power density, environmental impacts, addressed through recycling

What other electrical energy storage technologies, besides batteries and capacitors, are in common use?

Pumped hydroelectric storage Hydrogen storage Thermal storage Compressed air

How are electrochemical capacitors for grid-scale energy storage different from capacitors for use in e.g. portable electronic devices?

Require high surface area carbon for electrodes Electrolyte: sulfuric acid or acetonitrile

Ultracapacitors

Store charge via separation of electrolyte ions near plates < 10Wh/kg Voltage limited by dielectric breakdown of electrolytes Good for short bursts of power Long lifetime, low cost, reliable

Psuedocapacitors

Surfaces and interiors of electrodes can be used Electrodes store electrical energy as products of chemical reactions > 10Wh/kg Same performance issues as batteries

Why was our current electrical power grid not designed from the start to integrate energy storage?

Technical challenges: Performance, life, efficiency Economic challenges: High costs, small value streams Regulatory challenges: Lack of clear definition, framework designed for existing grid

Advantages of batteries and capacitors

Wide range of power scales Maintenance-free Exhaust-free Quiet (no moving parts) Portable power Efficient - up to 90% of the energy put into charging them Could store electricity generated during low-usage times for use during high-usage times: so generators do not have to run constantly at their peak power to meet peak demand