Energy Storage

Energy Storage Needs

A grid with a renewable energy focus will need a substantial amount of energy storage to account for the daily sun cycle and variable weather.

Daily – Accounting for energy demand peaks, nighttime for solar.

Variable – low wind periods, cloudy periods for solar.

Seasonal – Energy generation and demand will also vary seasonably.  Seasonal storage is expensive (building a storage device for 1 cycle per year).  Currently it costs about 10 cents to consume one kwh and it costs over $100 to store one kwh so the seasonal economics aren’t great.

Pumped Hydro Storage

Pumped hydro storage is the most widespread energy storage in the world.  Water is pumped from a lower reservoir or river to an upper reservoir in a time of energy surplus and then run through a generator in reverse during an energy demand.  While cheap and efficient it needs a good site and has a long timeframe to plan, design and build.

Pumped Hydro $165 per kwh, Return on electricity 80% [1]

Kruonis pumped hydro storage plant, kaunas Conty, Lithuania. Water is pumped up to the higher reservoir in times of energy surplus and run through a turbine in reverse to meet extra demand

Batteries

The recent rise in electrical vehicles as well as more emphasis on electric utility energy balancing has greatly increased the demand, improved the technology, decreased the need for environmentally sensitive ingredients like cobalt, and driven down the cost of battery technology.  Modern batteries degrade much more slowly per cycle and more slowly over time than in older technology.

Lithium Ion $469 per kwh, 86% Return on Electricity [1]

Compressed Air Storage

Compressed air storage at a large scale involves compressing air in an underground cavern and then releasing the pressure at a later date to convert back into electricity.  Compressed air storage has shown to be even cheaper than pumped hydro, however it is less efficient, meaning you have to have more energy generation to store the same amount of energy.

Compressed air $105 per kwh, Return on Electricity 52% [1]

Hydrogen

Hydrogen is a much less efficient method to store energy than the battery as it incurs energy losses during electrolysis, compression, storage, and converting back to electricity.  The energy recovered can be about 30% of the energy put in [2]. However, it does have advantages.  It is much more energy dense. it is easier to move around, and takes up less space to store energy.

Hydrogen is the simplest element with 1 proton and 1 electron, the process to make it is simple in applying energy to water to create hydrogen and oxygen.  Finally, when hydrogen is used for energy it combines with the oxygen in the air to go back to water, an environmentally friendly process.

The cheapest way to store hydrogen is in a salt cavern.  By pumping water into the cavern and washing out the salt a void is created to store the hydrogen.  Underground hydrogen storage may be a storage medium of the future.

Thermal Storage

Thermal storage is often the most efficient way for a building or group of buildings to store heat or cold and use it later for space heating/cooling.  Thermal energy storage can be cycled each day all the way to seasonal storage if needed.  A simple example is the sun or fire heating water or rocks and then using that heat later.  A cooling example is using ice to cool things.

In sensible heat storage (SHS) a fluid is put through a heat storage medium surrounded by insulation.  The storage medium is either heated or cooled, and then that heated or cooled medium is used later.  Materials such as water, salt, rocks, or concrete can be used to store heat.

You can also store energy through a phase change.  In latent heat storage (LHS) the storage material goes through a phase change instead of changing temperature.  Various polymers, gels, and salts are among the materials used in LHS.

Other Technologies

Other storage technologies not listed here are flywheels, flow batteries and molten salt.  At present, pumped hydro and lithium-ion batteries are likely to continue to be the grid storage mediums of choice.  Pumped hydro is a low cost, high efficiency option but needs a good location and takes time to plan, site, and build. Lithium-ion batteries are more expensive but they provide an efficient return on electricity and are easily deployable.

References

  1. K. Mongrid, V. Foledar, V. Viswanathan, V. Koritarov, P. Balducci, B. Hadjerioua, J. Alam. “Energy Storage Technology and Cost Characterization Report” HydroWires. U.S. Department of Energy. July 2019. Tables 4.3 and 4.4
  2. Volkswagaon. “Hydrogen or Battery? A Clear Case Until Further Notice” https://www.volkswagenag.com/en/news/stories/2019/08/hydrogen-or-battery–that-is-the-question.html#. Accessed March 11, 2021