Electric Vehicles for Energy Storage to Stabilize Utility Grid

by Gordy Slack

The state’s hundreds of thousands of electric vehicles could be efficient and portable sources of energy storage if they can be  woven into the grid, says Joel Kubby, Associate Professor of Engineering at UC Santa Cruz. He and his colleagues are working toward that goal using a 25kw, three-wheeled electric vehicle (EV), which Kubby calls “a big battery pack on wheels.” With help from a 2011 CITRIS seed grant, they are installing an EV recharge station and developing a smart-battery management interface system to integrate the car into their nanogrid testbed.

Electric vehicle batteries can be used as storage devices.

Their timing could not be better. Earlier this year, Governor Jerry Brown signed an executive order that established the goal of 1.5 million zero-emission cars on the state’s roads in by 2025 and for all cars to be zero emission by 2050.  This mandate should prove a valuable impetus in fueling the growth of electric vehicles and in research using their batteries as storage devices.

The research can have a big effect, but the application is on a small scale. “A ‘microgrid’ is a campus-scale network of energy supply inputs and users,” says Kubby.  “The smaller ‘nanogrid’ is a system just large enough to run a single home.”  Both micro- and nano-scale grids must have some storage capacity and be equipped to manage the fluctuation of energy sources, prices, and demands throughout the day. 

The EV and its smart-battery management system could serve both functions for the nanogrid, says Kubby.  Three years ago, Kubby bought a three-wheeled EV called a Triac. He chose it primarily because of its large, 25-kilowatt-hour battery. “It is like a three-wheeled golf cart that goes 80 miles per hour,” he says.
The Triac will be charged by renewable energy sources, plugging into stations that are fed by wind turbines and solar collectors.  It is an efficient way to convert wind and sun into transportation, says Zachary Graham, a Ph.D. student in Electrical Engineering at UCSC. It is  also a good place to store power, both when it comes free from the renewables  and when it is charged relatively inexpensively, like at midnight when there is low demand on the utility grid.

The smart-battery management interface will track the EV’s whereabouts, its battery’s age, capacity, and level of charge, as well as environmental conditions that might influence that charge. It will consider all these, and the routines of its user, when determining how to allocate the resource in the EV’s batteries.

“Since EVs already have these big battery packs, and most people drive their cars for less than an hour each day, we might as well put those batteries to work storing and providing energy,” says Kubby.

Graham just returned from Denmark, where he was working with an EV that can reverse the flow of the electricity, enabling EV owners to sell it back to the grid. Employing similar technologies, electric vehicles in California could also present an opportunity for their owners to participate in the energy market, charging up their EV batteries when electricity is abundant and cheap, and using it or selling it back to utilities, when demand and prices are high.

The extra storage capacity could help utility companies by providing an emergency backup source of energy during peak use. Instead of firing up expensive and polluting standby generators, or resorting to brownouts, utilities could buy energy back from individuals, or from collectives composed of EV-owners.  The result would be a more stable and efficient system that is also less expensive and less polluting.

For the plan to work, a few things need to be worked out first. The energy market is complex, keeping full-time brokers busy buying low and selling high. A software management application should be able to track all the variables though, and come up with a plan that gets the EV driver where she’s going, conserves electricity, buys it cheaply only when necessary, and enhances stability of the whole utility grid.

Modeling applications will allow users to track an EV’s battery levels, the environmental conditions that affect its charge, as well as energy prices.

Kubby’s smart-battery application will start with HOMER (Hybrid Optimization Model for Electric Renewables), a real-time home-energy modeling program created by the National Renewable Energy Lab, a division of the U.S. Department of Energy. HOMER Energy was incorporated in 2009 to commercialize HOMER.  Kubby is working with Peter Lilienthal, Ph.D., Chief Executive Officer at HOMER Energy, to modify it. The resulting program will track an EV’s whereabouts and monitor its battery levels, the environmental conditions that can influence the rate of charge, as well as energy prices and the energy needs and expectation of the driver.

Kubby’s testbed currently includes solar and wind sources in two different sites that are networked, accessible, and manipulable through a web interface. At the Advanced Studies Laboratories on the NASA Ames campus, Kubby plugs into an array of adjustable photo voltaic (PV) collectors and a small wind turbine. There are several more PVs and a larger turbine on the Santa Cruz Pier, another site that is part of Kubby’s testbed. A wave-energy generator may soon be added to the pier site.

Some of that electricity is helping to power the Pier’s businesses, but the testbed’s primary function at this point is to allow students and researchers to ground-test their theories about how best to manage a functional grid this size. “People do a lot of modeling,” Kubby says, “but not a lot of validating.” 

A battery pack like the one in the Triac can only be charged and depleted about a thousand times before its performance begins to nosedive, says Graham. Since batteries typically make up about one third of an EV’s total purchase price, calculating their lost value over time and use is going to be an important component of any EV-inclusive nanogrid’s economic model. “It will have to be accounted for before anyone would lend out their batteries to a utility company,” Kubby says. Further complicating things, wear and tear on a battery depends not only on the number of times it is charged and depleted, but also on the rate at which that charge or depletion occurs. Faster discharge leads to shorter battery life.

To extend battery life and to help stabilize the grid, says Kubby, a smart-battery manager might choose to charge batteries from the grid more slowly, and hurry only when necessary to accomplish high priority tasks, like getting a driver home from work. The important thing, though, is that the cost of the battery’s loss is accounted for in a way agreed on by both the owner and the utility, says Graham. Kubby’s smart-battery management program could facilitate that kind of calculation and communication.

The $75K CITRIS grant helped to send Graham to Denmark’s Technical University’s Center for Electro Technology program for six months. The Danes are trying to use market incentives to shift EV users’ recharge patterns. “Right now,” says Graham, “there is a peak in energy use in the evening right after work when people get home and plug their cars in. It would be much better for the grid if users would wait and plug their cars in at night, when the country is usually overproducing electricity from wind.”

However the EV industry’s battery system is eventually set up here in California, taking advantage of the grid-enhancing opportunity EV’s present will require some way to track and evaluate the condition of a vehicle’s batteries and to match those to the opportunities in the energy market and the needs of the driver.