|Fig. 1: Altamont Pass Wind Farm. (Source: Wikimedia Commons)|
California is planning for the future, and that future will be full of renewable energy. In 2006 California accelerated an already established Renewables Portfolio Standard requiring that 20% of electricity sales be served by renewable energy resources.  Renewable energy sources such as the wind farm seen in Fig. 1 will become even more important as these standards increase even more. In January of 2015, California Governor Jerry Brown proposed increasing the states renewable energy goal from 33% to 50% by 2030 when it was estimated that already the state was getting 25% of its power fromRPS-eligible renewable generation, placing the state two years ahead of schedule.  Solar and wind power are the best options for potentially increasing the usage of renewable energy in California.
In order to meet the requirement of the 50% Renewables Portfolio Standard (RPS), four scenarios were evaluated. The four options would all meet the 50% RPS in 2030, but would do so in various forms of energy. The first scenario is a large reliance on utility scale solar PV resources.  Next, is a smaller solar reliance, which would rely on 1-20 MW ground-mounted solar PV systems and an increase in the amount of wind power obtained.  Another scenario is the rooftop solar option which would rely on residential and commercial rooftop solar PV installations.  The final scenario is more diverse then the other three options and would consist of large, utility-scale resources, some solar thermal with energy storage and some out-of-state wind.  The four options are all possible but some are much more effective for the cost of implementing them.
One of the challenges that arises with attempting to procure 50% of California's energy from renewable sources is that affordable technology for both wind and solar power is not readily available. This issue leads to another problem referred to as over generation. Over generation occurs when must-run generation, non-dispatchable renewables, combined heat-and-power, nuclear generation, run-of-the-river hydro and thermal generation that is needed for grid stability is greater than loads plus exports. 
Luckily, there are solutions to all of these potential challenges. Energy storage for example is an issue with technology. Addressing the problem of technology is both simple and complex. The complexity exists in creating the technology, but the simplicity is that it is known what must be accomplished to solve this problem and a solution will be put in place. The most valuable integration solutions are those that can reduce solar-driven over generation during daylight hours when the system experiences low load conditions. Downward flexibility solutions, including increased exports, flexible loads, and diurnal energy storage help to mitigate this over generation. 
The future of energy not only in California, but around the world is going to depend on renewable energy. Therefore, it is beneficial that California is addressing this problem early and finding solutions before it is too late. In order for California to meet the demands of a 50% RPS at the lowest possible cost, then a portfolio with diversity of renewable resources is best.
© Michael Humphrey. 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.
 "California's Renewable Portfolio Standard: Slashing Pollution, Creating Jobs," Natural Resources Defense Council, July 2015.
 "Investigating a Higher Renewables Portfolio Standard in California," Energy and Environmental Economics, January 2014.