|Fig. 1: Solar One power plant in Mojave, California. (Source: Wikimedia Commons)|
At this point in time, nearly 23% of California's energy comes from renewable sources, namely solar and wind. With this in mind, the state seems to be wholly on track to its 2020 goal of 1/3 or 33% of energy coming from renewable sources. This major increase in renewable energy production poses its own problems amidst its benefit to the climate. Put simply, the state is producing more renewable energy that is consumed on the grid. This has to do with both overall energy output, but also timing of solar supply and time of demand. Current infrastructure has very limited capability to store this solar energy for uses outside of daylight hours when solar energy capture is at its greatest. Thus, there is a significant discrepancy of when solar energy is available on the grid and when it is needed , which is mostly in the evening hours after the sun has subsided. California is faced with the issue of how to carry over this excess of solar energy to be applied to hours of peak demand in the evening. This issue is explained through the California Independent System Operator (CAISO) Duck Curve.  With a baseload required power generation of 15,000 MW, but a net load far below 15,000 MW during peak sunlight hours, there is an overgeneration risk that could pose problems for generators and motors that are connected to the grid.  The solution to this is storage, but battery technology hasn't quite reached the point of providing a complete solution. Without it, increased capacity of utility-scale PV solar will be largely wasted during hours of overgeneration and left inaccessible during peak hours.
15,000 MW of solar energy are currently installed in California, which represents only about 6% of penetration when residential and utility scale are taken into account. Another 20,000 MW are projected to be installed in the next 5 years.  This massive increase in solar poses problems with the temporal demand discrepancy outlined above. Along with the growing market of solar photovoltaic panels is growth of the energy storage market. While there are many technologies being tested and implemented, perhaps the most popular is the lithium ion battery in industrial scale and home application.  These batteries are tested for their ability to provide solar photovoltaic firming (smoothing of output), peak electricity demand shaving, and ramp rate control. They are currently superior to alternative storage means in their ability to deliver more cycles in their lifetime than other battery systems. This lends itself to a better ability to provide ancillary services to the grid. Further, they have high charge / discharge efficiencies which is paramount for solar PV applications. 
Utility scale development of solar power comprises about 4% of California's overall energy consumption through facilities like the Solar One plant in the Mojave shown in figure 1. In order to increase this number, which poses one of the most viable routes for clean energy in California, massive investment needs to be made in both R&D, manufacturing, and installation. Currently, costs pose the larger barrier to wide-scale implementation. Research done by the National Renewable Energy Lab reports that in order to achieve a very high goal of 50% of California energy consumption from solar PV would require 19GW of storage capability assuming high grid flexibility.  There are generally 3 sets of primarily applicability criteria for these batteries. The first is low cost. Current battery applications, particularly in lithium-ion technology remains expensive, although costs have been dropping as technology improves as well as economies of scale due to higher scalability.  Second, is efficiency. With energy efficiency calculated as charged energy / discharged energy. Energy losses can occur during charge, discharge, and idle time. Minimizing these losses will help retain solar PV power and augment the effect of peak demand shaving. Third, is maintenance. These systems must be relatively self-sufficient and requiring low upkeep and continued investment, especially in off-grid systems. 
While industrial size batteries are being further developed in order to provide lasting solar integration into the grid, home and corporate installations of PV storage systems have been growing rapidly. By implementing lithium-ion storage systems with power management integration systems, homes and corporations can effectively decrease their reliance on the grid. These home based systems reduce the total demand on the duck curve. By reducing the net load on the grid due to storage energy produced by installed PV panels at the consumer site, the grid is put under less energy demand pressure at peak hours. Companies like SolarCity, Tesla, SunPower, and Solar Grid Storage have moved into this space with increasing success. It is projected that over 700 MW of commercial energy storage will be deployed by 2020.  If we look at the Tesla Powerwall 2 battery as an example, it can store 13.5 kWh. For reference, the average California residence uses about 6,500 kWh per year.  This means about half of all annual electricity consumption could be fulfilled solely by stored solar energy and not the direct PV power itself. Not to mention, the consumer can link the batteries to provide excess capacity.
With California's ambitious goal of having 33% of power consumption from renewable sources, solar is placed as a potential leader. However, with current limitations posed by the Duck Curve, PV storage systems must be developed and implemented across the state in order to facilitate the expansion of photovoltaic utility capacity. While companies mentioned in this paper have identified this as a massive market, major steps need to be taken in order to provide grid flexibility and capital requirements necessary for expansion.
© Matthew Stevens. 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.
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 M. Burnett, "Energy Storage and the California Duck Curve," Physics 240, Stanford University, Fall 2015.
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