Two-way charging of electric vehicles in solar-powered homes could be just the first step towards a dynamic vehicle-to-grid power system.
30 April 2014
After Hurricane Sandy pummelled the eastern coast of the United States, causing severe power outages, one of the smaller news items to come out of the 2012 storm was how some electric vehicle (EV) owners used their Nissan LEAFs as emergency backup power devices to reverse-charge their homes.
They were not exactly using their cars as approved, but it was still ingenious. And Nissan and other companies had already thought of the idea. After the Fukushima accident, Nissan brought out in Japan its LEAF to Home system to address demand for vehicle-to-home (V2H) power; it has since sold more 2 000 systems. The package, which sells for the equivalent of about USD 4 300, not only functions as an emergency backup device but also allows for an interconnection with rooftop solar panels. This means that during sunny hours when the car is parked at home, the battery is charging. At full charge, a Nissan LEAF battery can provide almost two days’ worth of electricity consumption for a typical Japanese household. Other companies are also developing and marketing similar systems.
Cars are parked 95% of the time, and given the increasing challenge of storing electricity generated by variable renewables as well as when and where to source the electricity to charge EVs, a framework linking them to solar-powered homes looks tantalising. The trick is to make sure that the vehicle is charged when the driver needs it, that the home utilises the battery for consumption when possible, and that the solar panels are being well used. Optimally, V2H increases EVs’ potential value, supports home solar generation and makes it possible to get off the grid to better fit overall supply with demand.
Powering much more than just homes
But a broader and dynamic application, vehicle-to-grid (V2G), can increase the reliability of the entire electricity system by helping the grid avoid peaks, like when people all plug in their EVs at much the same time after coming home from work, and providing extra decentralised energy storage. V2G is not a prerequisite for large-scale vehicle electrification, but the 2014 edition of the IEA flagship technology publication Energy Technology Perspectives estimates that the on-board battery storage in EVs could provide cost-effective demand response capacity, fully halving the need for capital-intensive large-scale storage technologies required for an electricity system that limits global temperature rise to 2 degrees Celsius by 2050.
Software is part of the necessary backbone for all V2H and V2G systems, but a critical physical component is the power control system (PCS) that makes electricity management bidirectional, determining when the household draws from the battery and when the car charges from the solar panels or the grid. Besides Nissan, India’s sole EV manufacturer, Mahindra Reva, recently released the e2o, which also offers bidirectional capacity.
Ford Motor and SunPower proposed another solution this year: a non-plug-in hybrid car that charges using an advanced solar canopy roof. The costs and technology have not yet come to a point where the technology is entirely viable, but it opens up a new way of considering vehicle electrification without relying on the grid. Similarly, BMW has teamed up with SolarCity (whose chairman, Elon Musk, is better known as the CEO of Tesla Motors) to offer customers of BMW’s i3 EV a home solar package solution with a 10% discount for panels.
Pilot projects leading the way for V2H
In one Nissan pilot programme in Osaka, the cost savings ranged from the equivalent of about USD 190 for cars driven 12 000 kilometres (km) a year, to USD 460 for those driven 7 000 km. The higher rate of savings for less driving came from the greater amount of time the car was parked. The end result of the Osaka programme was that solar power utilisation rose from 40% to 70% and dependence on the grid dropped from 75% to 50%.
Despite the news from Hurricane Sandy, V2H may have less impact in the United States than Japan because Americans drive more and their households use more electricity; a LEAF’s battery can power “only” one day of the average US household’s electricity consumption.
Still, a project in San Antonio, Texas, uses electric delivery trucks to play an active role in demand side management. Similarly, a project on the Hawaiian island of Maui by Hitachi and Nissan is demonstrating the potential for connecting renewable generation with EVs.
The programme relies on a smart grid but also a Direct Load Control, which connects EVs to households so they can power (in this case) electric water heaters. The demonstration project, which wraps up in March 2015, involves 40 households and is part of a range of demonstrations on the island that involve 350 EVs. It will be a while before final results are delivered from the V2H pilot programme and the rest of the suite of experiments, but already the Maui tests are showing the high potential for isolated systems such as islands to demonstrate the viability of vehicle-to-grid/home/building (V2X).
A technology with uncertain prospects
Nevertheless, the potential for V2G is currently uncertain, with a recent roadmap by the California Independent System Operator stating that despite its technical feasibility, “knowledge about the economic, environmental and grid benefits is underdeveloped, inconsistent or not validated”.
But as the IEA projection for storage capacity shows, EVs are more than cars: they are batteries that want to be used, especially as battery degradation from energy exchange with V2X is less than from driving and as the various trials for homes and other buildings further limit the effect by trying never to deplete batteries to less than half their charge.
The next application after homes could be office buildings. There, EVs can “shave the peaks” of electricity load during working hours, when people are at their desks while their cars sit outside. In this scenario, EV batteries could provide power to the building until late afternoon, when the cars draw back a charge before the workers use them to return home. In one test, Nissan estimated that 70 LEAFs cut 8.5% of workday consumption at a major office centre.
The problems of success
Load-demand is increasing across the board (in the United States, it is rising by 4% per year), putting pressure on the antiquated electricity grid to adapt and become more dynamic. The system’s integrity is also a priority as increasingly frequent harsh weather events threaten the grid.
To address these issues, especially considering the growth of variable generation from wind and solar power, the driving force behind the demonstration projects in Hawaii is not environmental targets nor job creation per se, but rather to solve the problem of successful deployment of solar power. In Hawaii, 10% of households have rooftop solar, which has begun to overload the grid. The experiments aim to find out whether or not V2H may be the solution to successful integration of variable renewables as well as a way to help maintain grid integrity.
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