Issues and Challenges

Energy Storage

One of the greatest technical and commercial obstacles for renewable energy is energy storage. Whether a renewable energy source is available or strongest only at certain times of day - like solar and wind - or available 24 hours a day - like wave energy or run-of-the-river hydro - making that electricity accessible when it is needed most is a challenge that must be overcome.

Energy storage falls into four main categories:

  • Mechanical: compressed air, flywheel, pumped storage hydroelectric
  • Electrochemical process: batteries and capacitors
  • Thermal process: molten salt, solar pond
  • Chemical Process: hydrogen 

Excess energy is typically available during the late night and early morning hours, though solar energy may be in excess during the middle of the day. Net energy metering allows small commercial and residential customers to export that excess to the grid. But larger renewable energies face curtailment if the electricity they produce exceeds the demand on the grid after the minimum capacity of other generators has been reached.

Stored energy can be used in three ways. One is load leveling, holding energy produced when demand is low and using it when demand is high. Compressed air energy storage (CAES) and pumped storage hydro work well for this. Another is ramping. Energy from renewable sources like wind typically rises and drops off faster than typical firm-energy generators can ramp down or up to meet the change. Stored electricity can be used to fill in the difference. CAES, PSH and flow-type batteries work well for ramping. Renewable electricity output can fluctuate minute to minute causing problems in voltage regulation. Batteries, super capacitors, flywheels and Superconducting Magnetic Energy Storage (SMES). A new device, developed and patented by Hawaiian Electric engineers, called the ESA or electronic shock absorber is being tested to help control the frequency variability of wind farms that is especially difficult to deal with on small, remote grids with no inter-connections to other grids, like those in the Hawaiian Islands.

Electric storage systems on a large or utility scale are at different stages of technical and commercial development.

CommercialPre-commercial PrototypeDemonstration StageDevelopmental Stage
  • Pumped Hydro
  • Compressed Air
  • Lead-Acid Battery
  • Ni-Cad Battery
  • Sodium-Sulfur
  • Battery
  • Flywheel
  • Zinc-Bromine Battery
  • Flywheel
  • Vanadium Redox Battery
  • Electrochemical capacitor
  • NiMH Battery
  • Lithium-Ion Battery
  • Electrochemical capacitor

Different types of energy storage have different characteristics:

 Pumped Storage HydroelectricCompressed Air StorageBatteriesCapacitors
Energy Storage Capacity 22,000 MWh 2,400 MWh 50 - 250 MWh 0.5 kWh
Duration of Discharge ~ 12 hours 4 - 24 hours 1 - 8 hours 1 - 30 sec
Power Level Up to 4 GW 50-300 MW 50 kW - 50 MW 200 kW
Response Time 0.5 - 15 min 2 - 12 min 4 ms 4 ms
Cycle Efficiency 0.87 0.85 0.65 - 0.90 0.95
Lifetime 30 years 30 years 5 -15 years 106 cycles or Up to 20 yrs

GW = gigawatt = one billion watts; MW = megawatt = one million watts; kW = kilowatt = 1,000 watts;
MWh = megawatt hour; kWh = kilowatthour;ms = millisecond = 0.001 second
ms = millisecond = 0.001 second


Two promising technologies for utility scale electricity storage are sodium-sulfur batteries and pumped storage hydro.

The sodium-sulfur battery has highly efficient charge/discharge, appears to have a long cycle life and is made from inexpensive, non-toxic materials. It operates at high temperatures of 300 to 350 °C and contains highly corrosive sodium polysulfides which, according to U.S. Department of Energy, have proven safe under extreme conditions.

While such advanced batteries can increase electricity reliability for vulnerable industries such as semiconductor and pharmaceutical plants they tend to be much too expensive for large-scale applications.

When it comes to battery storage for wind farms, there are only two utility scale projects in the world, in Japan and Ireland, each using a different technology. Using Japan's technology, a similar battery system on a 30 MW wind farm would add $30 million to $60 million to the cost.

A Sodium Sulfide battery system of 34 MW has been installed at a 51 MW wind farm project in Japan developed by JWD (Japan Wind Development Co.). It was scheduled to be in operation during early 2008. This would be the first or one of the first large-scale wind-battery integrated projects.

9.6 MW x 6 hr Sodium Sulfide battery

This is a photo of a 9.6 MW x 6 hr Sodium Sulfide battery in Japan. Estimated installed cost of this battery is about $3 million per MW. The manufacturer recommends installing batteries equal to one-third to two-thirds of the total wind farm capacity. So for a 100 MW wind farm, 33 MW to 66 MW of battery storage would be needed at an extra cost of $100 million to $200 million.

In short, the wind may be free, but the equipment needed to turn it into useful energy available when customers need it is still in development and would be quite costly for our customers.

Also, the sodium sulfur battery has other challenges. At present there is a single supplier, which means there is no competition on pricing and getting parts and service may be a problem if that supplier goes out of business or cannot handle the demand. The unit must be kept hot and the site could also be classified as a "hazardous waste facility."

Pumped Storage Hydroelectric (PSH)

Pumped Storage Hydroelectric (PSH) is a proven form of energy storage for electric utilities. There are over 150 plants with 22,000 MW capacity in United States and 78,000 MW of PSH installed worldwide. No PSH plant interconnected directly with a wind farm has been constructed to date.

Pumped storage hydroelectricity is a method of storing and producing electricity to supply high peak demands by pumping water to a reservoir at a higher elevation during off-peak periods and producing electricity using flowing water during on-peak periods. Several natural geological features are needed, including adequate close land areas divided by adequate elevation. There must also be an adequate water supply, though in some cases the lower reservoir is the ocean and sea water is used.

Studies have explored the possibilities of PSH on at Koko Crater and Kaau Crater; on Hawaii Island at Puu Waawaa and Puu Anahulu in North Kona, Puu Enuhe in Kau and at Kaupulehu/Kukio. On Maui, sites for PSH have been considered at Maalaea, Honokowai in the Kaanapali area, Kohama near Lahaina and upcountry at Ulupalakua

PSH Challenges include siting, permitting, water availability, cost and the long lead time for development. 


The ESA stores electricity in ultra-capacitors and was developed at Hawaiian Electric to integrate fluctuating wind power into the electric grid by mitigating short duration frequency and voltage deviations. An original demonstration unit was installed for testing at Lalamilo wind farm on Hawaii Island in 2006 and operated successfully until it was damaged by October 2006 earthquake.

Hawaiian Electric continues to explore, monitor and study energy storage solutions in conjunction with the U.S. Department of Energy and General Electric, a leader in the renewable energy field.