April 25, 2016 Categories: California Clean Energy Climate Change Low-Carbon Fuel

Ducking solar integration challenges with workplace charging

It would have been hard to miss the hype around the unveiling of the Tesla Model 3. Not that we wanted to miss it – finally, consumers will have the opportunity to choose a long range electric vehicle (encased in a sleek shell) at a price competitive with traditional gasoline-fueled vehicles.

It is about time. While the Clean Power Plan and the markets for clean energy technology are reshaping and cleaning up the electricity sector, the transportation sector still accounts for one third of total United States emissions of heat-trapping pollution. The scientific urgency of reducing carbon pollution from all sectors of our economy has only increased. Now, the same disruption that has upturned the electricity sector is coming to the personal vehicle market.

But the rise of electric vehicles like the Tesla, the popular Nissan LEAF, and the pending 2017 Chevrolet Bolt doesn’t just mean cleaner cars. The mass adoption of electric vehicles can do double-duty in modernizing our electric grid for seriously high levels of renewable energy with grid-enabled workplace charging.

Good problems to have

A foundational goal for our clean energy transformation is to derive most of our electricity (for powering as much of our economy as possible, including our cars) from renewable sources like wind and solar. The state of California is already on this path, consistently providing over 25 percent of state energy needs from renewables today and aiming for 50 percent renewable generation by 2030.

With higher levels of variable resources, their generation does not always directly coincide with electricity consumption patterns. At some times there can be a big mismatch, such as when we all come home from work and turn on our lights and appliances just as the sun sets and before wind generation really kicks into high gear later in the evening. This dynamic can create a challenge known as the “duck curve,” where it may be necessary to forgo using some of the available solar energy generated at peak hours or to quickly ramp up other electricity sources as the sun goes down. (See figure below).

Screen Shot 2016-04-22 at 11.55.00 AM(source: CAISO, 2016 https://www.caiso.com/Documents/FlexibleResourcesHelpRenewables_FastFacts.pdf)

When the sun is shining in the middle of the day,  solar panels can generate more electricity than we can immediately use. See the image below of California’s grid on March 27, 2016—excess renewables, that cannot be immediately used, are the area in red. We can go a long way towards smoothing things out with a more advanced grid that can help us shift demand for electricity (using smart thermostats and water-heaters, for example). But there is also an important role to be played by electric battery storage. If we can save some of that solar “over-generation” from midday and use it to bridge the gap between when demand decreases and when wind ramps up at night, then we can get even larger portions of our electricity from these renewable sources.

Solar_-Grid_Desktop-1440x832(source: KQED Science 4/4/16 http://ww2.kqed.org/science/2016/04/04/what-will-california-do-with-too-much-solar/ )

Workplace charging will flatten the duck, smooth the curves

Smart workplace charging of electric vehicles could help smooth out the demand and generation curves at an extremely low marginal cost. Consider the possibilities posed by the 12.5 million cars in California that sit in a parking garage, lot, or space during the peak hours of solar in the middle of each workday.[1]  Smart, grid-enabled charging stations could dynamically price electricity to encourage consumption during the sunniest hours of solar generation—during the middle of the day, when most people are at work—rather than in the evening when people return home and the sun is setting.

Assuming electric vehicle efficiency of 30 kWh per 100 miles, which the 2016 Nissan LEAF currently achieves and is also projected for the Chevrolet Bolt, a car that commutes 15 miles on a full charge would have at least 4.5 kWh of battery space empty upon arriving at the office. Over four hours, a car plugged into a 120 volt (Level 1, 1.2 kW) charger might take in 4.8 kWh of electricity. Perfect.

The potential additional storage capacity of 12.5 million cars charging at workplaces over the course of one year is 11,250 GWh. To put these numbers in context, for a California grid with 108,259 GWh total variable generation, of which 24 percent of total generation is from solar, 32,478 GWh of variable generation are projected to be “curtailed” (i.e. turned off) annually due to grid balancing requirements, assuming that no integration strategies are deployed.[2]  The 11,250 GWh of vehicle energy needs can be dispatched strategically to eat up about one-third of this generation that might otherwise go to waste. Workplace charging of all passenger vehicles used for commuting would be a convenient component of a comprehensive plan for integrating greater amounts of renewables into the grid.

Just as important, vehicle charging is immensely affordable. The installation costs for a Level 1 charging station are easily as low as $100, thus providing storage during those four peak solar hours for about $21/kWh. Meanwhile, the cheapest battery storage option on the market for most consumers is Tesla’s Powerwall, which comes to about $469/kWh. Using well-timed vehicle charging as energy storage makes plenty of financial sense and offers great potential to move us towards a clean energy economy.


[1] Source U.S. Census, 2009-2013 5-Year American Community Survey Commuting Flows, “Table 1. County to County Commuting Flows for the United States and Puerto Rico: 2009-2013,” accessed 4/6/16. Method: sum, “Drive Alone” and one third commuters reporting “Carpool.” http://www.census.gov/hhes/commuting/

[2] Denholm et. al, National Renewable Energy Laboratory, “Overgeneration from Solar Energy in California: A Field Guide to the Duck Chart,” 11/2015. http://www.nrel.gov/docs/fy16osti/65023.pdf. Drawing from pp. 11 & 22.

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