Can electric vehicles allow the grid to accommodate greater levels of variable renewable energy? That’s what I’m looking to find out.
The grid of the past
The power grid has traditionally operated under the assumption that electricity demand simply happens, and that supply must be ready on command to meet it. This requires developing a system with several layers of electricity supply resources:
- “Baseload” plants of high capital cost and low operating cost run as often as possible
- Flexible “load-following” generators ramp up and down to match hourly swings in demand
- “Peaking” units, cheap to build but expensive to operate, run for relatively few hours per year
Systems have target voltages and frequencies, but there is some tolerance for short-term mismatches of supply and demand. (See “How the Electricity Grid Works” for more details.)
As a 2013 paper notes:
“The operating principle of fossil generation is ‘burn when needed,’ a principle simple enough that it could be followed without computers, digital high-speed communications, or weather forecasting—precisely the conditions when today’s electric system was created, early in the 20th century.”
In this graphic, “Nuclear,” “Coal,” and “Geothermal” are baseload plants, “Hydro” and “Gas CC” are load-following plants, and “Gas CT” represents peaking units. The other resources do not fit neatly into those traditional classifications. Source: The Western Wind and Solar Integration Study Phase 2, National Renewable Energy Laboratory, 2013.
The grid of the future
Now, though, we do have computers, digital high-speed communications, and weather forecasting. We also have flexible loads such as air conditioners, electric water heaters, and electric vehicles chargers, alongside those that basically need power on demand, such as light bulbs and computers. These flexible loads can shift the times in which they draw power by a few minutes (or even hours in some cases) with no adverse impact on the quality of energy services delivered.
We’ve also developed technologies that, although cleaner than fossil fuels and cost-competitive, cannot be dispatched on command.
We have also greatly improved battery technology, and developed other options for energy storage. Storage can make renewable electricity available whenever needed, and can help baseload resources supply varying demand. Even as costs of storage come down, it may be more economical to first use price signals and automatic controls to align flexible demand with supply as much as possible, and then use storage to help meet the remaining inflexible demands.
Diversity of loads and supply
Individual loads can be highly variable. Small appliances are turned on and off frequently (a microwave, for example, uses large amounts of power for a minute or two at a time). Air conditioners automatically cycle on and off to maintain the proper temperature, at times resulting in very large swings in power demanded.
But when you aggregate many individually variable loads, the variability diminishes considerably. This is the same effect seen with solar power, aggregated across a wide area. Isolated clouds passing over one photovoltaic (PV) array will take a few minutes to impact the production of another array a few miles away.
Source: “Implications of Wide-Area Geographic Diversity for Short-Term Variability of Solar Power,” A. Mills and R. Wiser, Lawrence Berkeley National Laboratory, 2010.
We can mitigate some of the short-term variability of PV systems just by having many of them spread out.
Time of use rates and smart charging
On the timescale of hours, variations in solar power output occur from large cloud systems and from Earth’s rotation. The second source of variation is entirely predictable—we know when the sun will rise and set. PV systems reach peak output around noon, with variations depending on the angle of the panels, the location within a time zone, and Daylight Savings Time.
Since peak power demand is generally mid-to-late afternoon or early evening, we’d want to shift some loads to run when abundant solar power is feeding in to the grid. One way to do this using PV and EVs is with a rate structure that offers low-cost electricity in the middle of the day. Electric companies in Hawaii have proposed this. Using “time of use rates” in this way has several benefits:
- Retail prices paid by customers would reflect the wholesale market, in which mid-day power is already becoming less expensive in some locations due to abundant solar electricity.
- Electric vehicles at workplaces could charge with low cost clean power.
- “Smart chargers” could adjust the charging rate if needed to accommodate variations in power supply.
- Flexible loads such as air conditioners could run at low cost during the mid-day hours to pre-cool homes and reduce the evening power draw.
- The electric grid would see a more gradual “ramp” from the mid-day load to the evening peak.
- Solar power systems selling at wholesale (such as utility-scale systems) would see increased revenues from their power sales due to greater daytime demand.
If a car is plugged in at work for nine hours, and needs only two full-power hours to charge, there is a great amount of flexibility in when the charger is used and what rate it charges at.
Shifting the charging time to align with a period of surplus solar power, through “time of use” rates, helps address solar power variation on the timescale of hours. Starting, stopping, or varying the rate of power transfer (“smart charging”) can help address shorter-term variation.
My research: how PV and EVs can best work together
There are dozens of pilot projects on “smart charging,” and hundreds of research efforts in this area on topics ranging from consumer behavior to battery lifetime impacts to communication protocols. Some companies already earn real revenue from providing ancillary services to the grid (including both one-way “smart charging” and two-way “vehicle-to-grid” arrangements).
Part of my work at UCS is identifying how these options can increase value for EV owners, for electricity grid operators, for renewable energy providers, and for the public. Right now it is possible to integrate variable energy resources into the grid at a low cost. Can flexible loads reduce this integration cost further as variable renewable generation grows? If so, how can we align incentives so that the market prioritizes lower-cost solutions for grid integration of variable renewables over more expensive ones?
Stay tuned.