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| Fig. 1: Renewable power output for August 15, 2019 in the areas under the jurisdiction of the California Independent System Operator. [10] All sources adhere to the Renewable Portfolio Standards (RPS) listed in Table 2. [2] |
In 2018, California passed a state law dubbed "The 100 Percent Clean Energy Act" (Senate Bill 100). [1] This law mandates that "that eligible renewable energy resources and zero-carbon resources supply 100% of all retail sales of electricity to California end-use customers and 100% of electricity procured to serve all state agencies by December 31, 2045". [1] The bill also revises previous legislation that calls for unambiguous renewable energy goals. These goals are tabulated in Table 1.
"Eligible renewable energy sources" are defined by the California Energy Commission (CEC) and are found in Table 2. [2] "Zero carbon resources" do not generate any carbon dioxide when it is being used, while "net-zero carbon resources" may generate carbon dioxide, but other processes associated with the resource remove the same amount of carbon dioxide from the atmosphere. [3] An example of a zero-carbon, but non-renewable resource is electricity generated by nuclear power. An example of a net-zero carbon resource can be using a fossil energy resource combined with carbon capture and storage technology if the capture and storage are sufficiently effective (currently it is not).
One of California's key mandates has been the Renewables Portfolio Standard (RPS). The RPS requires any merchant of energy in California to increase its procurement of eligible renewable energy resources (defined in Table 2) to 33% of retail sales by 2020 and 60% of retail sales by 2030. The CEC calls these merchants load-serving entities (LSEs) and are defined as "a term to refer to retail sellers, POUs (public owned utilities), and all other entities serving retail sales of electricity in California that are obligated to participate in Californias RPS". [2] The RPS 2019 annual report shows that of all the retail sales of energy in California in 2019, 37% came from eligible renewable energy resources. [4] An average of 80% of this 37 percent comes from solar and wind. [4]
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| Table 1: Major upcoming milestones in enewable energy retail sale requirements for sellers and local publicly owned electric utilities in California. [2] (* The bill states that "eligible renewable energy resources and zero-carbon resources" supply 100% of retail electricity sales.) |
California's main strategy for building a renewable grid has been to massively scale up and incentivize solar generation. In January 2007, California launched a multi-entity $3.3 billion (USD) ratepayer-funded program called "Go Solar California". [5] The goal of this program (and its sub-programs) is to install 3,000 megawatts of new solar generation over 10 years to lay the foundation of a self-sustaining solar economy. This program can be understood as a subsidy to bring the price of solar generation down by stimulating an increase in solar energy supply.
A 2020 program assessment of one of Go Solar California's sub-programs (California Solar Initiative) shows that by the end of 2019 approximately 9,607 megawatts of solar capacity were installed. [5] As a result, California has exceeded its target of 3,000 megawatts by 220%. This massive integration of solar generation has pushed the operation of California's electrical grid into a new regime, where there is often too much (renewable) energy and not enough demand for it. In other words, there is an oversupply. Oversupply of electrical energy is managed through curtailments. [6,7] Curtailments are the intentional reduction of an energy resource so that the supply better matches the demand. This solution seems counterintuitive because it reduces output from renewable energy sources. Other solutions are being developed, like improving energy storage capability or improving demand response. [6,7] For more information on the oversupply issue and the feasibility of large energy storage facilities, see Burnett. [8]
In addition to oversupply, a related issue with solar generation is that it is intermittent. Generally, energy demand peaks during the late afternoon/early evening. [9] Essentially, as people return home from work (assuming a 9:00 am-5:00 pm schedule), the demand on the grid increases just as solar generation is ramping down. To highlight this, let us consider an example. On August 15, 2019, according to the California Independent System Operator, the peak instantaneous demand was 44,301 megawatts at 5:50 pm. [9] Fig. 1 shows that this corresponds to when solar PV generation is ramping down with an instantaneous supply of approximately 7500 megawatts. At midnight the demand is still around 30,000 megawatts, while all renewables approach negligible amounts of power output. [10]
The usual discussion surrounding solar generation focuses on either oversupply or intermittency, as I have done above. [9,10] However, A recent study by de Chalendar and Benson shows a 100% renewable grid can be misinterpreted as producing lower emissions than it actually does. [11] They highlight the importance of timing when doing greenhouse emission calculations.
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| Table 2: Summary of eligible renewable resources as defined in Table 1 of the 9th edition of the RPS Eligibility Guidebook. [2] A retail seller or a publicly owned utility may produce or procure one or more of these resources to satisfy the RPS requirements. | ||||||||||||||||||
In California, current and popular methods of estimating greenhouse gas emissions use yearly averages. [11] This means if you are an entity that can consume and produce power, all the calculations are being done using end-of-year data. So, using this yearly-average system, anyone who claims to be 100% renewable may not cover all of their power use with renewables. This is because they can purchase or generate enough energy to match 100% of the energy consumed at the end of the year. So that leaves room for solar dominated entities to sell excess solar power in the afternoon and purchase more carbon-intensive energy during the night. At the end of the year, if we consider only the energy generation, it will look like the entity is adhering to the RPS. However, in reality, the net carbon emission is non-zero - a non-intuitive result considering the entity has the label of being "100% renewable". A yearly system is only suitable when fluctuations in renewable power generation is on the same time-scale. This means that either (1) the generation has little fluctuation throughout the day or (2) there is effective storage of the energy.
In California, where solar power is the leading source of renewable energy, a 1-megawatt constant load, using yearly averages could overstate the emissions reductions associated with solar power by more than 50% when compared to hourly averaging by 2025. [11] If we do the same comparison, but using only wind-power, the emissions reductions are actually underestimated by 4%, when compared to hourly averaging. [11] As a result, purchasing more solar energy in a grid that already has a large amount of solar generation does not result in zero emissions.
So, as the fraction of solar energy in the grid increases, institutions that have targets to procure 100% renewable energy should use hourly accounting to more accurately estimate the carbon emissions reductions. The reason why hourly accounting paints a more accurate picture is because the energy output changes on the same time scale (see Fig. 1).
California's legislation surrounding its 100% renewable energy goal by 2045 is interdisciplinary. A thorough discussion involves economics, social science, political science, chemistry, geology, and physics. In the end, however, what matters is to bring emissions zero. While California has reached its goals thus far by leveraging solar generation, there are many subtleties to consider and address. The pursuit of renewable energy generation needs to be in lock-step with greenhouse (carbon) emissions decrease. Otherwise, the problem is being pushed further into the future. As de Chalendar and Benson recommend, investment in wind generation will help with providing consistent energy throughout the day - ultimately resulting in an overall climate benefit.
© Kevin Multani. 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] "SB 100 California Renewables Portfolio Standard Program: Emissions of Greenhouse Gases," California Statutes of 2018, Ch. 312, 10 Sep 18.
[2] C. Crume and L. Green, "Renewables Portfolio Standard Eligibility, 9th Ed.," California Energy Commssion, CEC-300-2016-006-ED9-CMF-REV, January 2017.
[3] K. Levin et al., "Design and Communicating Net-Zero Targets," World Resources Institute, July 2020.
[4] M. Albright, C. Cox, and A. Singh, "Renewables Portfolio Standard Annual Report: November 2019," California Public Utilities Comission, November 2019.
[5] "2020 California Solar Initiative, Annual Program Assessment," California Public Utilities Commission, June 2020.
[6] "Impacts of Renewable Energy on Grid Operations," California Independent System Operator, May 2017.
[7] "What the Duck Curve Tells us about Managing a Green Grid," California Independent System Operator, 2016.
[8] M. Burnett, "Energy Storage and the California 'Duck Curve'," Physics 240, Stanford University, Fall 2015.
[9] "CAISO Peak Load History from 1998 Through 2019," California Independent System Operator, 31 Dec 19.
[10] "CAISO Renewables Watch for Operating Day Thursday, August 15, 2019," California Independent System Operator, 15 Aug 19.
[11] J. A. de Chalendar and S. M. Benson, "Why 100% Renewable Energy Is Not Enough," Joule 3, 1389 (2019).