|Fig. 1: Denmark wind power, 9 Jan 09. The inset shows the power used for EVs. |
Denmark has more wind power per capita than any other country in the world.  Its wind power capacity is 3,166 MW, supplying around 16% to 19% of the country's electricity. [1,2] One issue with using wind energy in the power grid is that fluctuations in wind strength lead to large, unpredictable variations in power generation, while the difficulty of storing electricity creates an inelastic moment-to-moment demand. Currently, Denmark handles the variability of its wind power through connections to the Swedish and Norwegian power grids, which are particularly elastic because of the large number of hydro plants in both countries that can be turned on an off easily.  A new method to balance this variation in load could benefit Denmark economically, allow for further expansion of Denmark's wind power, and create a model for expanding wind power elsewhere.
One idea is to store surplus energy in electric vehicle (EV) batteries while they are plugged into the grid. The idea is that if EVs are plugged in for longer than their charge time - overnight for instance - then they could use "smart charging" to charge only when there is a surplus on the grid, thereby smoothing out variation in supply. The EDISON consortium, a group that includes two of Denmark's largest energy companies as well as Siemens and IBM, was organized in 2009 to study this possibility. The Palo Alto startup Better Place has plans to introduce between 20,000 and 100,000 EVs to Denmark by the year 2020, with grid electricity storage in mind. [3,4]
|Fig. 2: Total wind production in Denmark over the course of the week 2 - 9 Jan 09. |
There are many complications in analyzing this scenario, but a naïve look at the numbers indicates that while Denmark's wind power may help EVs, it is unlikely that the introduction of EVs will help stabilize wind power. Consider, for example, the total wattage of the Danish EV fleet in the scenario proposed by Better Place. The average Dane drives 40 km per day, and Better Place's battery uses 24 kWh per 160 km. [5,6] Assuming the high estimate of 100,000 EVs by 2020, this means that a total of 600 Mwh will be used in transportation every day. Thus, there will be an extra load on the grid of 600 Mwh per day from charging EVs. But there are constraints to how this load can be distributed. It is generally assumed that batteries will be charged mainly at night. [7,8] Also, household charging rates would be at most 16A at 220 V, which means that over all EVs no more than 350 MW can be consumed at any time. [5,8] Fig. 1 illustrates roughly the relative orders of magnitude involved on a windy night. Thus, using the actual driving habits instead of the total capacity of the batteries indicates that, at least by the year 2020, there will not be enough EVs to make an appreciable difference in the effectiveness of wind power.
How would this picture change if Denmark's entire fleet of 2.1 million passenger cars were replaced by EVs?  Then they would use about 125 mwh per day, with a maximum charging rate of 8,400 MW. Certainly, this is large enough to have an effect on wind power. But there is still the problem that cars must be charged at night, and that they will need to be fully charged by the morning. The reason this is a problem is that on a national scale, variations in wind power generation occur on the scale of days, not on the scale of hours. (Figs. 2 and 3). There are periods of nearly a week with less than 1,000 MW of wind power. Cars that are plugged in only overnight cannot temper variation over multiple days.
|Fig. 3: Total elecricity consumption (top) and total wind production (bottom) in Denmark over the course of a month (May 2009). |
Of course, since the cars are charging at night when the load is smaller, the total load on the electric grid will not necessarily increase. In fact, the grid could supply an extra 26,000 mwh at night on weekdays without having to increase its capacity. (see Fig. 4). However, this would not necessarily benefit wind power. A study of EVs in Germany suggests that non-renewable power plants would bear the increased load at night since the prioritization of renewable sources means that currently it is non-renewable sources that have the largest day-night fluctuation. 
|Fig. 4: Total electricity consumption In Denmark in the winter and in the summer. |
Thus, on the basis of this analysis, we find that Better Place's plan to introduce EVs to Denmark would not have a large effect on the load variation on the Danish power grid cause by wind power. But this analysis leaves out several possibilities. For one, the battery storage will be distributed across the country, so it is possible that EVs could have a local effect on wind energy variation that gets lost in the national average. Also, it has been proposed that EV batteries could actually feed power back into the grid in addition to using smart charging.  In this case the actual driving habits would be less important the total storage capacity of the batteries. The possibility of a symbiosis between the two green dreams of electric vehicles and wind power is still an interesting possibility, and it is one that is currently being actively explored.
© Peter Smillie. 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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