Electricity, that wonderfully efficient medium for distributing power, has a behavioral quirk imparted by natural law: you can’t save it.  This simple fact dictates the way we generate electricity and also poses a problem effectively using some forms of renewable energy.  A myriad of new technologies under development may help both traditional and renewable generating sources by providing cost-effective, large-scale energy storage for the electric grid.

It is an inviolate rule that the electric grid must have enough generating capacity at any instant to meet the ever-changing demand.  Compounding this situation, the daily demand for electricity fluctuates significantly by season, by day of the week and by the time of day.  Electricity producers have searched for decades for ways to store electric energy when demand is low and release it at peak demand times to minimize the generating capacity needed to serve the peak load.

In a similar vein, wind and solar power could benefit from a large-scale energy storage solution, as their generation is also situation dependent.  Eliminating the uncertainty from wind and solar generation by storing their output would solve a major shortcoming of these renewable energy sources.

How close are we to practical, grid-scale energy storage?  Here’s a run down on the state of the technologies.

Pumped Hydro

Currently, the most widely used grid-scale energy storage scheme is “pumped hydro.”  Like a hydroelectric plant, pumped hydro uses reservoirs at different elevations instead of a river.  Water is pumped to the higher reservoir during times of low demand, then it is released through hydro turbines at times of high demand to generate electricity. 

The Electricity Storage Association (ESA) reports that 90 GW of pumped hydro is installed worldwide.  Simple and reliable, pumped hydro has its drawbacks: high capital costs, long construction times, site constraints and a massive footprint. 

Compressed Air Energy Storage (CAES)

As the name implies, CAES uses surplus electricity to compress air into large underground caverns.  Electricity is produced by burning a mixture of compressed air and fuel (oil or natural gas) in a modified gas turbine.  CAES returns the energy stored in the compressed air by eliminating the need for the gas turbine to compress its own air (which may consume up to 60% of turbine’s energy consumption). 

CAES has seen limited application since the first installation in Germany in 1978.  While CAES enjoys lower capital costs, shorter construction times and smaller footprints than pumped hydro, the technical and environmental complexities might be preventing this technology from getting traction.

Batteries Galore

Battery technologies for portable devices have made great strides in the past decade.  Unfortunately, powering a cell phone and powering the national grid are very different matters.  There are interesting battery technologies in development for grid-scale storage but none has garnered overwhelming acceptance thus far.

Lead-Acid Batteries

Lead-acid batteries, like the one in your car, have been used for years in large-scale uninterruptable power supplies (UPS) to provide emergency back up power.  Readily available, well-understood and low cost, lead-acid batteries have been used for limited grid-scale storage.  The biggest downside with these batteries is the relatively small number (<1,000) of charge and discharge cycles they can provide before replacement; however some strides are being made to extend lead-acid battery life.

Lithium-Ion Batteries

Li-ion batteries dominate the portable electronics market.  And, while they offer 2 to 4 times the number of charge/discharge cycles and pack more power than lead-acid batteries, the Li-ion battery is 4 or 6 times the cost.  Li-Iron Phosphate (Li-FePO) batteries promise to be the next generation of lithium technology.  Hybrid vehicles will be the first application – grid energy storage may follow.

The products above can be as simple as a conventional, integrated-cell battery.  The battery technologies that follow are systems requiring multiple components and control systems.

Sodium Sulfur (NaS) Battery Systems

Utilities have deployed nearly 300 MW of NaS battery systems, mostly in Japan.  Far from your typical battery, these behemoths utilize molten sodium and sulfur at nearly 600ºF.  Capable of storing large quantities of energy and operating through many cycles, the NaS battery is gaining popularity as evidenced by General Electric entering this market.  Addressing the challenges posed by very corrosive liquid metals and keeping them at high temperature make these systems expensive.  Cost and reliability should improve as more experience is gained with these systems.

Vanadium Redox Battery (VRB) Systems

The VRB system is an example of a flow battery: two electrolytes stored in external tanks are pumped past opposite sides of a membrane with an electron flow passing through the membrane.  In the reduction-oxidation (redox) flow battery, the volume of electrolyte determines energy storage capacity.  The vanadium redox process provides many desirable operating characteristics but commercial viability will depend on reducing the cost.

Zinc-Bromine Battery Systems

The Zn-Br is a hybrid flow battery system since zinc is deposited on the negative electrode during the charge cycle.  Unlike the redox flow battery, the energy storage capacity of the Zinc-Bromine hybrid is constrained by the amount of surface area available for deposition.  Zn-Br battery systems promise very competitive life cycle costs and are currently being deployed in smaller capacities on a limited basis. 

Ultracapacitors

The electric double-layer capacitor (EDLC) or ultracapacitor provides storage capabilities thousands of times higher than the traditional electrolytic capacitor.  EDLCs can store and discharge relatively huge amounts of energy much faster than batteries over hundreds of thousands of cycles.  Currently, EDLCs find ready application in combination with batteries of all types used in electric hybrid and electric vehicles (EHVs & EVs).  The EDLC buffers the battery from high in-rush current of electric motors by virtue of its very fast, high power discharge – significantly lengthening battery life.  Born of carbon nanotube technology, EDLCs will continue to improve in terms capacity, capability and cost as production quantities rise.  The unique characteristics of EDLCs make them very attractive for different grid-energy storage schemes.

Flywheels

Flywheels store energy in the form of the inertia available in a large mass rotating at speeds in the range of 16,000 RPM.  Modern flywheels employ high-strength, composite materials and spin in a vacuum to achieve higher power densities and efficiencies.  Offering very long life cycles, simplicity and high reliability, the relatively small energy capacity flywheel modules can be ganged together to provide as much storage as needed.  The downside of flywheels is their high cost compared to other emerging storage technologies, and significant cost reductions may be hard to obtain.

The good news about grid-scale energy storage may be that we can see the light at the end of the tunnel.  Technology advances and cost reductions are being made on a regular basis.  This other clean technology gives us reason to look forward to the use of more renewable energy sources, higher efficiency from conventional generating sources and better quality power from the electric grid.