Stored Energy Options
By Kay A. Modi, JHCGA's Energy Analyst in Residence
Stability of electricity production has the projected need to balance the variability of solar and wind production of electricity with stored energy systems. Wind and solar are referred to as variable renewable energy (VRE) sources. Diverting a portion of the VRE peak electricity production that typically occurs in mid-day and then using it when consumers of electricity have their highest demand in early evening would help the bulk electric system (BES), commonly referred to as the grid. Storage of the diverted energy for future needs comes in many forms according to a 2022 MIT study and is broadly characterized as electrochemical, chemical, mechanical, or thermal. The ability to respond to peak demands from daily user fluctuations or from weather related extremes characterize the value to the BES.
This article is Part Two of a series and it addresses the stored energy options for stabilizing the BES. Part One of this series discusses why energy storage is needed.
Stored energy systems are not forecast to completely close the gap, but they do present low carbon emission options and may be considered dispatchable electricity. Dispatchable sources can quickly respond to user demands compared to other resources such as nuclear, geothermal, and hydropower. Natural gas is the most readily available dispatchable resource.
The U.S. Energy Information Administration (EIA) classifies stored energy projects based on numerous attributes, including: voltage support, storage for excess wind and solar generation, load management, system peak shaving, transmission and distribution network deferral, backup power, and energy arbitrage (where arbitrage involves effectively moving the electricity from one time period to another).
Delayed use attributes can be valuable for daily peak demands which are typically later in the day. The graph below is another version of the “duck curve” representing daily fluctuations needs and the impacts on natural gas fired power plants (ramping is indicated by the arrow) from increasing the number of VRE sources per year. The duck curve specifically applies to California sources which have a much higher percentage of solar plants. Wind is not limited to sunlight hours, but has low production periods late at night and has much more variability than solar. The graph shows the desired movement of overgeneration from VRE sources (green) in the middle of the day to later in the 24-hour day (red). Currently, some VRE sources in California are curtailed part of the time, which means that they reduce their production output due to overgeneration. The curtailing is necessary to keep other power plants from unstable low-level operations in the middle of the day. Thus, the BES has less decarbonized electricity production (which increases greenhouse gases). The graph compares the hours of the day to the amount of natural gas power plant megawatts (MW) needed.
Mitigating Overgeneration by VRE Sources
Capturing Overgeneration and Discharging During Peak Load Hours
Stored energy systems have other benefits for the BES, such as the ability to restart in blackout conditions and to provide emergency supply during power disruptions. Benefit cost analyses for stored energy are complex and utility planners are weighing the vast array of benefits. These may include reduced greenhouse gas emissions, reduced water consumption, avoidance of capital costs of new power systems, and lower rate payer bills. Another benefit of stored energy systems is reduced use of fossil fueled generators during emergencies, such as hurricanes (combined solar and battery mobile units).
The following are examples of stored energy systems. More options can be reviewed in the 2022 MIT study for each category.
The most widely discussed type of electrochemical storage is the lithium-ion (Li) battery. It has six defining characteristics: near instantaneous (dispatchable) electrical supply, very high conversion efficiency (approaching 95%) compared to mechanical, high-energy density for reasonable space allowance near production sites (wind or solar), lower capital costs and short installation period compared to others, life cycle requirements for replacement, and limited hours of storage before dissipating for Li batteries. This type of battery is available for the daily peak demand experienced by the localized BES (refer to the duck curve). The combination of a variable renewable energy (VRE) and electrochemical storage reduces the impact of power source drop, but does not solve the gap in electricity generation in early morning hours or fulfill the complete peak energy demand.
Other electrochemical battery options are based on electrodes for chemical reactions and have the ability to hold power supply in modular tanks for longer periods (approaching 8 hours). Each of the electrochemical storage systems may be sensitive to temperatures and need protection if located in cold and hot weather climates.
An attribute of small electrochemical storage with its affordability for private ownership is the ability for a company to avoid daily peak demand charges by BES managers. A company may be able to manage peak demand with a short-term energy release from onsite systems.
The production of hydrogen gas from water electrolysis and storage of the hydrogen is considered a stored energy option. This would occur using electricity during over generation periods from VRE sources. Because hydrogen is a very light gas, it needs extreme compression to store. Large volumes of hydrogen can be stored if underground salt caverns are available. Hydrogen is readily burned to make electricity and does not create greenhouse gas in the process. No conversion efficiency is available due to numerous ways hydrogen can be stored and used for conversion to power or chemical feedstocks.
The options for stored energy using hydraulics include storing water in high elevation dammed lakes. Large western utility companies are currently studying a number of options for constructing lakes in high elevations that would be conjoining with nearby existing lakes. The envisioned storage systems would have 8- to 12- hours of electricity production and near indefinite, very large holdings. The difficulty with the hydropower option is the long construction periods of building dams and associated infrastructure in remote areas. Energy Vault is currently constructing hydraulic lifting of heavy weights within a tall structure in several European locations plus Texas, Nevada and California. These types of mechanical energy conversion have efficiencies near 80 to 85%.
Thermal energy storage is more broadly known as heat storage from large power production sources such as nuclear or geothermal power plants (plus some solar plants) that create high temperatures. Some new generation nuclear power plants have planned salt vaults to store their excess heat. These types of vaults have an issue with dissipation and thus need to be used within the day of the stored energy. The efficiency of heat storage improves on the high temperatures of nuclear power compared to others. Other types include storing heated and pressurized water for later use to generate electricity.
Natural Gas to Fill the Gaps
Early morning hours, daily peak demand or seasonal issues with extreme weather episodes require responsive, reliable electrical supply and power systems. Natural gas power plants are critical to meet this demand when it is needed most. The future of carbon capture or transformation of natural gas to green fuels such as hydrogen-fired systems may be the last step to fill the gaps.
Cover picture credit: National Renewable Energy Lab