|
|
| Fig. 1: Grid energy storage. (Source: Wikimedia Commons) |
The California net-zero carbon economy has been highly dependent on the rapid growth of solar and wind electricity, as well as electrification of transportation and heating. However, the increasing reliance on weather-dependent renewables can raise grid reliability challenges which mandates careful resource planning. California solar and wind power generation during the summer is 40-60% higher than winter, with frequently extended periods with limited solar generation. The high penetration of solar power in the California grid resulted in oversupply during the middle of the day. [1] The mismatch of supply and demand resulted in increased energy curtailment during extreme-supply periods and increased consumption of carbon-intensive resources during extreme-demand. Fig. 1 shows an illustration of these concepts.
With the high penetration of solar resources, it is common to have excess generation during off-peak hours, resulting in negative pricing and generation curtailment. Negative real-time hourly wholesale prices occurred in about 4% of all hours and wholesale market nodes across the United States. [2] Negative pricing hours almost exclusively occur around noon. Also, the highest occurrence rates of negative pricing hours are during the Spring season, when solar and hydroelectric generators are most prolific. Meanwhile, the lowest frequency of negative prices is observed during the Summer season due to the decrease in power supply (i.e. warmer ambient temperatures lead to lower power plant efficiencies) and the increase in demand due to the use of air conditioning during the daytime. Additionally, the intermittency of solar and wind often results in abrupt shortages in power generation, causing spikes in electricity prices. Solar resources are characterized with a diminishing effective load-carrying capability (ELCC). [3] Additional investments in solar alone would only shift the peak to later hours during the day.
While intermittent resources cannot alone drive a reliable net-zero portfolio, they can be more reliable and effective when integrated with energy storage technologies. Energy storage systems can be broadly divided into (1) mechanical storage (e.g., compressed air, pumped hydro, flywheel), (2) electric systems (e.g., supercapacitors, superconducting magnetism) (3) electrochemical systems (e.g., Lithium-ion, flow batteries), and (4) hydrogen storage. We are interested in comparing pumped hydro and Lithium-ion storage in their ability to satisfy short (i.e., span of minutes/hours) and/or long (i.e., span of weeks/months) duration storage. With its relatively low energy cost of 10-20 $/kwh and long discharge time of 1-24 hours, pumped hydro is suitable for long-duration storage. Meanwhile, Lithium-ion batteries are more suitable for short-duration storage despite high energy cost of 350-700 $/kwh as they allow for short discharge times in the range of minutes to a few hours with specific energy of 160-200 Wh/kg. [4]
In conclusion, there are many energy storage technologies that can be more or less mature. They come with different specifications which need to be carefully considered when evaluated for integration with intermittent renewable energy systems.
© Alaa Alahmed. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] F. Sioshansi, Variable Generation, Flexible Demand (Academic Press, 2020), pp. 3-24.
[2] D. Sodano et al., "The Symbiotic Relationship of Solar Power and Energy Storage in Providing Capacity Value," Renewable Energy 177, 823 (2021).
[3] J. A. Dowling et al., "Role of Long-Duration Energy Storage in Variable Renewable Electricity Systems," Joule 4, 1907 (2020).
[4] R. Amirante et al. "Overview on Recent Developments in Energy Storage: Mechanical, Electrochemical and Hydrogen Technologies," Energy Convers. Manag. 132 372 (2017).
