Energy Storage on the Electric Grid

Electric grid designers and operators increasingly look at energy storage as an essential tool in improving and maintaining grid reliability and flexibility.

Energy can be stored in a variety of ways, including:

  • Pumped Hydroelectric.  Electricity is used to pump water up to a reservoir during periods of light demand.  Water is then released from the reservoir during periods of high demand, where it flows down through a turbine to generate electricity.  Refer to the FCS blog on Intermittent Renewables and Pumped Storage Facilities for more information.
  • Compressed Air.  Electricity is used to compress air and store it, often in underground caverns.  When electricity demand is high, the pressurized air is released to generate electricity through an expansion turbine generator.
  • Thermal Energy Storage.  Electricity can be used to produce thermal energy, or thermal energy from other sources may be used, such as solar energy and geothermal energy, which can be stored until it is needed in the form of large, heavily-insulated water tanks, molten salt storage, heat storage in rock caverns, cryogenic energy storage, ice storage, and so on.  For example, solar energy can be used to melt a salt mixture to be stored in an insulated tank.  When demand is high, the molten salt may be used to create superheated steam to generate electricity in a steam turbine generator.  Refer to the FCS blog on Thermal Energy Storage Systems for more information.
  • Batteries.  Electricity is used to charge large battery systems.  This stored energy can then be used, through an inverter and transformer, to supply electrical energy back to the grid when demand is high.  These systems can use lithium-ion, lead-acid, lithium-iron, or other battery technologies.
  • Flow Batteries.  A flow battery is a rechargeable battery that operates to store electrical energy as described above.  However, it uses two different chemical components (electrolytes) stored in two sets of tanks.  These electrolytes flow through one or more electrochemical cells with an ion-selection membrane separating the two flows.  The energy storage capacity may be quite high with multiple electrolyte storage tanks in series and in parallel.  Some common electrolyte combinations include zinc/bromine, polysulfide/bromium, iron/chromium, and vanadium/vanadium (different valency states).

The reasons behind why it is beneficial or necessary to store energy vary depending on the facility; however, everyday purposes include the following:

  • Peak Shaving
  • Solar and Wind Integration
  • T&D Investment Deferral
  • Microgrid Support

Peak Shaving

The electric bill for commercial and industrial electricity customers typically includes two distinct charges for their electricity usage each billing cycle:

  • Supply Charges:  The cost of the electricity consumed, measured in kilowatt-hours (kWh).
  • Delivery Charges:  The charges associated with delivering electricity, which includes the demand delivery charge, or “demand charge.”  The Demand Charge is based on the highest period of demand for electricity during a billing period, measured in kilowatts (kW).

The demand charge is in place to account for the requirements of the utility to maintain a robust infrastructure that can deliver a sufficient, reliable electric supply to their customers.  Utilities spread the costs out to all users so no one entity pays more than their fair share.  For many commercial customers, the demand charge can account for 30% to 70% of a monthly electricity bill.

An energy storage system may be used to decrease peak demand (higher cost) by storing the energy (e.g., charging the batteries) when demand is low (lower cost) and strategically discharging this stored energy during times of peak demand.  The allows the stored energy to absorb the peaks, keeping the peaks seen by the utility at a lower level, resulting in a lower demand charge.

Solar and Wind Integration

There are two issues with solar and wind integration that impact electric grid reliability:

  • Impact on grid frequency stability
  • Impact on grid voltage stability

Many photovoltaic (PV) solar and wind generating units are incapable of participating in response to deviations in grid frequency and of producing reactive power to support voltage.  This implies reduced grid reliability with increased solar and wind.

Additionally, solar and wind are non-dispatchable (supply electricity only when the sun is shining or the wind is blowing), and therefore cannot be started on command to respond to a contingency.

The electric power grid operates based on a delicate balance between real-time electrical generation and demand.  One way to help balance fluctuations in electricity supply and demand and increase reliability is to store electricity during periods of relatively high production and low demand, then release it back to the electric power grid during periods of lower production or higher demand.

Many modern energy storage systems can discharge real power on command to support frequency stability and support grid voltage by injecting and absorbing Reactive Power on command.

For the reasons above, electricity storage could help the grid operate more reliably, reducing the likelihood of brownouts during peak demand and allow for more wind and solar resources to be used.

T&D Investment Deferral

The best way to discuss this is with an example:

Consider an existing T&D system equipment’s load-carrying capacity is 12 MW.  The daily load peaks currently go as high as 11.85 MW, and the system demand is growing at 2% per year.  This implies that the peak loads are expected to exceed the existing T&D equipment’s load-carrying capacity in the next year by about 77 kW on average and fall about 112 kWh short of meeting total daily energy needs. By year two this will grow to 318 kW and a 757 kWh shortfall.

An upgrade to the T&D capacity is in the planning stages to increase the load-carrying capacity to 16,000 MW, for a 33.3% increase.  The cost is beyond the budget of the coming year to make the necessary upgrades to the system, but legislation for a rate adjustment is forthcoming and it is expected that these upgrades will be affordable in a couple of years.

The installation of an Energy Storage System will allow the utility to flatten the load curve by discharging the energy during peak hours, supplying the estimated energy shortfalls on a daily basis to “buy some time” before the necessary T&D capacity upgrades are built.

The size of the Energy Storage System installed will be planned to be more than large enough to allow the deferment of the infrastructure charges for two years, giving the utility flexibility with its capital budget.  This is similar to Demand Charge Reduction (Peak Shaving) discussed earlier.  However, peak shaving was to lower the end user’s peak load.  The scenario described here is for a utility to flatten its entire load profile.

Microgrid Support

A Microgrid is a small network of electricity users with a local source of supply that is connected to a centralized grid but, can function independently.  Most microgrids will be connected to the grid more than they would be in island mode.  However, a microgrid will go into island mode whenever there is an outage on the main grid.  The microgrid is now running off of its own energy sources.  However, should a cloud block the sun or the wind subside, power may be lost.

No matter what type of microgrid, energy storage is vital to the success of the system.  To store energy for future use, a microgrid owner needs an energy storage system.  When the main utility cannot supply power, a microgrid takes over seamlessly if it has an energy storage system.  Should a cloud block the sun or the wind subsides, a microgrid can use its stored energy to keep the power on.  An energy storage system helps the microgrid store power to carry a military base, hospital, or university from the time the grid goes down to when it returns online.

In Conclusion

The continuing development and implementation of energy storage technologies will allow the future electric grid to become more distributed and, ideally, more reliable and efficient.  Electric grid operators will need to adapt to these changes. The ability to add more renewables such as wind and solar to the power mix is not only good for the environment but will provide much more flexibility than the traditional grid model.