Why the Grid Needs Stored Energy
By Kay A. Modi, JHCGA's Energy Analyst in Residence
Stability of the electricity system has the projected need to counterbalance the variability of solar and wind production, both now and for their future expansions. Currently, these variable renewable energy (VRE) sources operate about 40 % of the time and are typically balanced with natural gas fired power plants. Sun and wind have seasonal and daily plunges that make them highly variable and leave gaps in electricity production to be filled by others. Diverting a portion of the VRE peak electricity production periods and using it when they stop production helps the stability of the bulk electric system (BES), commonly referred to as the grid. Overall, the grid is being decarbonized with more wind and solar projects to meet climate change objectives. However, grid operators need stored energy to make it reasonable to operate the entire BES throughout the 24-hour day. Consumers want low-cost electricity and less fossil fuel-based power plants, so innovative stored energy projects and strategies are needed.
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, and thermal. The ability to respond to peak electricity demands or from weather related extremes characterize their value to the BES. The storage options have various management strategies to stabilize the BES and are expected to get more complicated as more solar and wind power sources are built.
Additionally, it is important to understand that stored energy systems are only one aspect of “filling the gaps” remaining from VRE power sources. The current compensating power production is from fossil fuel-fired power systems, such as natural gas-fired power plants. Fossil fuel plants of coal and natural gas are baseload (primary) power plants to the BES and operate more efficiently and economically when held at steady state operations. The following graph shows that coal-fired power plants are also fluctuating production in response to VRE power sources. This is an unusual operation from coal plants due to high natural gas prices in March 2022. The graph represents the immense challenges of maintaining the BES’s voltage stability and responding to operational demands for electricity during customer’s peak demand periods in extreme cold. The fall and rise of the green-wind and orange-solar lines illustrate daily fluctuations during a very cold period. The corresponding response by brown-coal and blue-natural gas lines illustrate the required responsiveness for counterbalance and daily electricity demands by consumers. In other words, stored energy systems would divert VRE power production by “shaving off” their highest production periods to reduce the swings in the overall power plants daily cycling. It takes time to understand the busy graph below. Before VRE sources were added to the power mix, the traditional power plants simply responded to daily changes in electricity usage and their lines of this graph would be fairly smooth (flat with gentle rise and fall).
Rocky Mountain Area Power Production During Peak Demands in Winter
Fluctuating Fossil Fuels Responding to Demand During Extreme Cold and Wind/Solar Variability
Each power source is also presented individually below:
Stored energy systems are not forecast to completely close the gap, but they do present low carbon emission options and stabilization of the BES by diverting peaks of the variable power supply (sun and wind) for later use. Also, it is essential for solar and wind to be properly located to maximize their production utilization. Both approach 40 % utilization (operating time) for newly installed systems in their premium locations. Futuristic nuclear power plants may have some aspects of this responsiveness but prefer stable operating conditions. Interestingly, France’s nuclear power plants have this capability now and routinely respond by ramping to VRE output cycles.
A stored energy system that can store during times when VRE output is abundant and wholesale prices are relatively low and then produce electricity at times when VRE output is scarce and wholesale prices are relatively high is most effective. This delayed use aspect can be valuable for daily peak demands which is typically between 4pm to 7pm. People are coming home from work and using appliances. This greatly impacts electricity usage. Unfortunately, it does not match when the sun shines and the wind blows. The graph below is referred to as the “duck curve” of daily fluctuations based on increasing VRE sources per year. It shows the challenges of “reducing then ramping” by non-VRE power systems (e.g., natural gas) on a daily basis in California. California has a high percentage of solar power which is forecasting the difficulties ahead as more VRE sources are added to the grid. Stored energy systems can be brought online to reduce the steep curves as more VRE projects are added to the power mix. Currently, some VRE systems in California are curtailed, which means that they reduce their production output. The non-VRE power systems, however, cannot be turned down too low for effective operation. This is “not wasting” electricity that is produced. It is simply the same type of operational control that natural gas systems do in response to low electricity demands. Going back in time is helpful to understand the changes from added VRE sources to the California section of the BES. The 2012 curve does not have the steepness from the daily power needs change but the increasing amount of wind and solar added over the years has significant impacts on the curves. Currently, natural gas power plants are required to counterbalance the hourly changes.
Winter Day Power Demand Fluctuation and VRE Projections
From California Independent System Operators, Duck Curve for Natural Gas Power
Some stored energy projects may have the ability to provide their power during the early morning periods between 6 and 9am when the sun does not shine and wind does not blow.
Natural Gas to Fill the Gaps
Early morning hours, daily peak demand or seasonal issues with extreme weather episodes require responsive, reliable power systems and electrical supply. 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.
Part Two of this article covers the options for stored energy and how they can be applied to stabilize the BES as more sun and solar facilities are added to produce electricity to meet climate change objectives.
Cover picture credit: National Renewable Energy Lab