What primary ancillary services are needed to maintain reliable operations for modern power generation and distribution?

Grid Scale Energy Storage - The ideal technologies to balance the grid and provide generation contingency

22 September 2017

Grid Scale Energy Storage – Applications and Technologies


The growing penetration of intermittent renewable energy sources has forced increased cycling of fossil fuel plants to provide additional ancillary services and meet energy demands.  In 2013, National Renewable Energy Laboratory (NREL) in conjunction with Intertek Asset Integrity Management (AIM) reported that the increased integration of wind and solar energy raised cycling costs by $122M per year, with the cycling cost of an average fossil plant nearly tripling from $0.47/MWh to $1.28/MWh.  Power producers are looking to energy storage to curb rising cycling costs incurred by increased maintenance, fuel, forced outages, etc.

The benefits of energy storage extend beyond energy balancing.  Storage improves grid stability and security by offering a wide range of ancillary services needed for modern power generation and distribution.  According to the US Federal Energy Regulatory Commission, ancillary services are "those necessary to support the transmission of electric power from seller to purchaser given the obligations of control areas and transmitting utilities within those control areas to maintain reliable operations of the interconnected transmission system."  The primary ancillary services provided with energy storage are regulation and reserves.

Regulation services are used to balance electricity supply and demand on short-notice.   Rather than starting and stopping thermal units, energy storage can be used to balance the grid in these short-time frames by charging during times of electricity overproduction and discharging when electricity supply isn't meeting demand.

  • Load following – regulation on an hour-to-hour basis to meet rising energy demand in the mornings and decreasing demand in the evenings
  • Peaking – regulate power generation to meet peak demands
  • Ramping – regulate energy output within seconds to minutes during periods of significant power generation variability (e.g. intermittent wind, passing of clouds over solar panels)
  • Frequency regulation – regulation on a second-to-second basis to momentarily regulate electricity supply up or down to coincide with demand

Reserve services generate additional electricity used to compensate capacity loss due to the unexpected unavailability of large power sources.  Energy storage is an ideal technology to provide generation contingency because it can be deployed immediately and can provide power separately from the grid.

  • Spinning reserve – Online generation that is synchronized to the grid and can immediately respond to generation and transmission outages within 10 minutes
  • Non-spinning reserve – generation, that may be offline, capable of reaching full output within 10 minutes, but does not need to respond immediately
  • Backup supply – backup generation for spinning and non-spinning reserves available within 1 hour


There are a number of energy storage technologies available that can be used to provide regulation and reserve ancillary services, but their best suited applications will be dictated by storage capacity and response time.

Batteries store energy electrochemically.  Electricity drives the chemical reactions used to chemically store and provide energy.  Their properties (i.e. capacity, life, energy, and power) are dependent on the battery chemistry as well as size.  Batteries have the advantage of versatility; their fast response times make them well suited for regulation applications, and their ability to operate when disconnected from the grid make them an excellent energy reserve storage solution.

Compressed Air
Excess energy generated from renewable sources is used to compress air, which is then stored in underground caverns and pressure vessels.  To produce electricity, air is expanded to turn a turbine and generate power.  The capacity of a compressed air storage system is dependent on air pressure and the volume of the reservoir.  While compressed air systems work well as energy reserves, they are not well suited for grid regulation.  Compressed air storage facilities typically operate in daily cycles so as to prevent damage to their compression and generation components from excessive cycling.

Flywheels store energy as rotational energy.  Excess energy increases the angular velocity of the flywheel during "charging" and velocity is decreased upon "discharging."  The capacity of a flywheel storage system typically ranges from 3kWh to 133 kWh and is dependent on the shape of the rotor as well as the strength and density of the materials used.  Flywheel systems are more suited for short-term reserve applications, and the fast response time of these systems make them an excellent option for regulation services.

Pumped Hydro
Pumped hydro storage operates similarly to compressed air storage.  Excess energy from over generation is used to pump water into a water tower or some sort of elevated reservoir.  The stored potential energy is converted into electricity when water flows down from the reservoir to spin a turbine.  Like compressed air systems, pumped hydro storage is well-suited for energy reserves, but is not a feasible option for grid regulation.

To learn more visit www.intertek.com/aim or contact us today.

Dr. Taylor Kelly earned her Ph.D. in Materials Science and Engineering from the University of Houston, where she developed an expertise in lithium ion batteries and energy storage. Her Ph.D. thesis investigated the mechano-electrochemical coupling behavior of stretchable lithium ion batteries. Dr. Kelly has not only brought to Intertek her energy industry expertise in the areas of electrochemical testing, mechanical testing, and heat transfer, but she also provides consulting and technology assessment services to energy storage manufacturers, developers, and consumers.