What's the best way to charge millions of electric vehicles at once?

September 16, 2015 by Lisa Zyga feature
(a) In a distribution network, EVs randomly choose a charging node and plug in to the node until fully charged. (b) shows the circuit corresponding to the subtree rooted at node j. Credit: Carvalho, et al. ©2015 IOP Publishing Ltd

(Phys.org)—About 350,000 plug-in electric vehicles (EVs) have been sold in the US from 2008—when they first entered the market—to mid-2015. Although EVs still represent a small fraction of the country's 250 million total vehicles, the continual increase in sales suggests that EVs will become even more popular over the next few decades. This raises the question of how millions of EVs may be charged at once on a grid that was not originally intended to supply such large amounts of power.

The main problem, as researchers Rui Carvalho and coauthors from the UK and Slovakia explain in a recent paper published in the New Journal of Physics, is congestion—not road traffic congestion, but charging traffic congestion. In their paper, they show that when the number of EVs being plugged into the reaches a critical point, the system undergoes a phase transition from a "freeflow" state (where all vehicles can be fully charged within the expected time period, say 4 hours) to a congested state. In the congested state, some vehicles have to wait for increasingly long times to fully charge, resulting in queues of vehicles rapidly building up that will then face even longer charging times.

"With high penetration of electric vehicles, charging at home will increase the stress on the 'last mile' of local distribution networks," Carvalho, a researcher at Durham University in the UK, told Phys.org. "The conventional solution would be to lay copper under the road, so as to increase network capacity. The cost of upgrading the last mile of the network, however, would be prohibitive, and we present an alternative, much cheaper approach that could be implemented with minimal hardware requirements: a software layer and controllers at the point of charge."

As the researchers explain, the congestion problem can be avoided, at least to an extent, by managing how the power is allocated throughout the charging network. A good management strategy can increase the critical number of vehicles that pushes the system over the threshold into its congested state, thereby allowing more vehicles to be charged in their normal charge time.

Distributing charge quickly and fairly

In their paper, the researchers compared two charging strategies ("max-flow" and "proportional fairness") with the aim to guide network designers in deciding which algorithms to implement in the real world. Both algorithms investigated here rely on recent advances that combine tools from optimization and critical phenomena. As vehicles randomly plug in to the network, the network must continually solve the congestion control problem and allocate each an instantaneous power using the algorithm. The researchers compared the outcomes of both algorithms using simulations that are only possible due to techniques developed since 2012.

As the researchers explained, a good algorithm will have two features: it charges more vehicles at once, and it does so fairly, meaning all vehicles' charging times are roughly equal. As an example of unfairness, the "max-flow" algorithm charges vehicles closer to the main power source faster than those further away, which the researchers expect will not be socially acceptable. Fairness can be quantified by the Gini coefficient, which is traditionally used to measure income inequality. For comparison, the researchers note that Sweden has a Gini of 0.26, the US has a Gini of 0.41, and the Seychelles has the highest Gini of 0.66. The researchers explain that these values might provide a useful benchmark for identifying socially acceptable values for EV charging algorithms.

"The proportional fairness algorithm reaches a maximum Gini of 0.45, which is comparable with the level of inequality in the US society, and thus may be judged sociable acceptable," they write. "The max-flow algorithm, however, reaches a Gini of 0.91, which measures a level of inequality considerably higher than present in any contemporary society."

Fairness beats greed

The proportional fairness algorithm not only scores better on fairness, but it also allows more vehicles to be charged compared to the max-flow algorithm before reaching the critical threshold. The researchers say that they were surprised by the superiority of the proportional fairness algorithm, since the max-flow algorithm maximizes the total instantaneous power, which would seem to lead to a maximization of the number of charged vehicles, but this is not the case. The researchers explain that the downfall of the max-flow algorithm is its "greediness"—its focus on total instantaneous power makes it sub-optimal compared to proportional fairness, which instead focuses on a fair allocation of instantaneous power, and as a result achieves a higher optimum.

"Intuitively, network designers and operators might be led to the conclusion that there will be a price to pay for being fair to users on these local distribution networks, and disregard fair allocations," Carvalho said. "We show that it is possible to 'have your cake and eat it' in the free-flow state: network operators can charge a higher number of vehicles with fair algorithms than with the greedy max-flow, before the system is congested."

Overall, the analysis shows that the most obvious choice isn't always the best, and that careful examination of different algorithms before they're implemented could provide large cost and time savings down the road.

"We see EV charging as a theoretical problem of allocation of scarce (network) resources to a population of heterogeneous and mobile agents," Carvalho said. "From this point of view, we think our paper only scratches the surface of this potentially rich field (for example, our agents are homogeneous). Some questions that come to mind: What would be the effect of heterogeneous agents? How to implement such a congestion control mechanism with a pricing scheme on the links? How to prevent users from cheating on the network?

"Consider the last question. One simple way of cheating would be for an EV to pretend to be not one single vehicle, but many cloned vehicles. Such a 'super car' would then be able to get a much higher allocation than the rest, because the network would see it as many vehicles. This is the mechanism explored in peer-to-peer networks: users get very high bandwidth by breaking down a file to be transferred into file segments, and then sending these segments in parallel over several network connections. How can we design network controllers to prevent this?"

Explore further: New online mechanism for electric vehicle charging

More information: Rui Carvalho, et al. "Critical behavior in charging of electric vehicles." New Journal of Physics. DOI: 10.1088/1367-2630/17/9/095001

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Eikka
4 / 5 (16) Sep 16, 2015
Kinda have to say "told you so".

There's also an earlier article that quantifies some of the figures involved:

http://phys.org/n...tml#nRlv
The rising number of electric vehicles on the road is putting grid operators under pressure. Low voltage networks for domestic consumers are not designed for the kind of loads that are generated by recharging electric vehicles at home. "A vehicle draws up to 22 kilowatts (KW) of power. So if you have multiple vehicles plugged in at the same time, then current grids quickly reach their limits,"


The proposed solution here is basically energy rationing, which means there's a big brother who has the power to monitor and dish out energy, similiar to the fuel shortages in past years when people with odd or even license plates could refuel on alternate days...
Eikka
4.6 / 5 (9) Sep 16, 2015
This is the mechanism explored in peer-to-peer networks: users get very high bandwidth by breaking down a file to be transferred into file segments, and then sending these segments in parallel over several network connections.


The segmentation is how the network works in the first place. That's not cheating. Every user gets high bandwidth because they're getting bits and pieces off of everyone, rather than everyone trying to get all of them from any one member. The actual cheating comes when users pretend to be more users to capture a larger portion of the limited number of connections between peers.

Perhaps rather than being coordinated centrally by a "big brother", the cars too could form a P2P network where nearby vehicles who already got theirs could share portions of energy with ones who are still waiting, easing the load on the substations.
rgw
1.5 / 5 (8) Sep 16, 2015
Solution is simple; build hundreds of new coal fired plants.
david_king
3 / 5 (4) Sep 16, 2015
Having many electric cars also represent a huge buffering storage capacity for the grid overall which might easily smooth over shortages during peak demand with a higher proportion of renewable energy in the mix. Cars are generally used for commuting and could potentially be left plugged into the grid when they aren't being driven. The car owners could potentially stipulate what the minimum charge they need at the times they commute and would pay a premium for the privilege of having a fully charged vehicle at all times.
We can choose to see this as a disaster in the making or an opportunity to evolve a smarter, fairer grid that's a whole lot more resilient.
denglish
3.2 / 5 (9) Sep 16, 2015
Don't worry. Our current capable energy sources will be vilified out of existence, and the alternatives (unless we pull our head out and go nuke) won't even be able to power our cities, much less our homes.
MR166
2.5 / 5 (8) Sep 16, 2015
Utopia keeps getting better and better doesn't it. How come progressive solutions always wind up with more and more personal limitations? Save the world, buy a car that you cannot charge without getting permission from some mindless bot.
ab3a
4.7 / 5 (14) Sep 16, 2015
Speaking as one who is involved in standards committees, this is precisely what is pushing development of the smart grid. Actually, that's a misnomer. It should really be called the smart load because generation facilities have been quite "smart" for a long time.

Electric vehicles scare the crap out of most electric utilities because they know that they have no chance of charging these things up in the evening after commuting home but before everyone goes out for a night on the town.

Some look at these cars as distributed energy storage devices. However, one would have to configure the car's charging system to know that you won't need it to go anywhere for the evening.

The car could send demand needs to the grid managers via existing SCADA systems. But the problem of cyber security remains.

This is a huge, messy problem. There are no easy answers. We need better energy storage technologies.
MR166
2.3 / 5 (3) Sep 16, 2015
To think that an electric vehicle can supply power to the grid is just plain silly. That implies that the vehicle has excess capacity to begin with. With electric cars fighting customer range anxiety this makes no sense at all.
MR166
1.8 / 5 (5) Sep 16, 2015
I would love to know the percent of electric car owners that also own a gas powered car as a backup vehicle. That is the only way EVs make any sense at all.
ab3a
4.8 / 5 (16) Sep 16, 2015
To think that an electric vehicle can supply power to the grid is just plain silly. That implies that the vehicle has excess capacity to begin with. With electric cars fighting customer range anxiety this makes no sense at all.


And yet, this is exactly the sort of thing that people are discussing in smart grid design discussions. The hope is that storage can be relatively light weight and power dense. The hope is that during peak demand, the car can help shave load off of the grid so that generation plants can run closer to the edge. Cars with less storage would get more power allocated to them. Cars with more storage would supplement the home.

Of course, you'd have to authorize this, and you'd need to ensure that the car isn't lying to you. In a perfect cooperative society, this scheme could work. The problem is that we're not dealing with a perfectly cooperative society.

Don't shoot, I'm just the messenger...
ECOnservative
5 / 5 (1) Sep 16, 2015
Use a pseudo-random backoff algorithm that's aware of the history or each car and what time it's usually operated the next day. Give each car a time slot to even out the load. Using cars as storage seems fraught with issues.
Egleton
not rated yet Sep 16, 2015
Professor Hagelstein is addressing the IEEE just north of Boston. No entrance fee.
https://meetings..../m/35303
antigoracle
3 / 5 (2) Sep 16, 2015
Here's an idea. Power for the rest of your home is diverted to charging your car. Your charging station monitors how much power you are using and regulates itself based on a preset limit.
Bloodyorphan
4 / 5 (3) Sep 16, 2015
Every car needs a 22kVA (Or appropriate for the car wattage requirements) home based storage unit that charges at 3kVA during the day.

When you look at the voltage rise issues with current small scale home generation, using them to supplement supply is no easy task either and probably not worth the trouble.

Many issues though, who wants a 22kVA transformer in their house let alone two or more of them.
RM07
4 / 5 (4) Sep 16, 2015
A 10kW Tesla Powerwall is selling for about $3K. They're sold out for all next year at that price. 10kW is 1/8 of the capacity of a Tesla, which is the only car with such a capacity today.

By the time this is a real problem, we won't need a "fairness algorithm", there will be technological solutions already in place.
Egleton
5 / 5 (1) Sep 17, 2015
Sold out you say? Would that be an indication on market acceptance? Possibly.
And here in Australia the car manufacturers have gone out of business competing with China.
So executives sit around their big polished round table, agonizing. "What to do. What to do!"
Like monkeys caught with the peanut trap. Let go of the peanut, monkey.
They were trying to sell zoom, zoom, zoom. That was their advertisement, I kid you not.
(How much do these executives get paid? $100 per annum would way too much.)
retrosurf
4.3 / 5 (6) Sep 17, 2015
> How come progressive solutions always wind up with more and more personal limitations?

I can't answer for everyone, but in your case I think it's because you've got a liberal boogy-man in your closet that you blame for most of the ills of the world, instead of seeing the economic and engineering realities that are at play here.

Do you think it's a socialist plot that the power lines that run to your house have real, limited capacity that cannot be exceeded? Do you imagine that attempts to protect that vital infrastructure from overload are just encroachments on your personal liberty to consume, without concern for the other consumers that share that network's capacity?

Maybe you believe what someone else told you. Maybe you're smart enough to figure it out yourself, but you like the poisoned kool-aid better. Maybe you're a paid shill, but you said "utopia", so I think you just like kool-aid.
retrosurf
3.7 / 5 (3) Sep 17, 2015
Every car needs a 22kVA (Or appropriate for the car wattage requirements) home based storage unit that charges at 3kVA during the day...


You are confusing power and energy here. I think you mean 22 KWh energy storage charging at 3 KVA. That would be charging for 7-ish hours (time) at 3 kilowatts (power) for a total of 22 kilowatt-hours (energy).

Bloodyorphan
2 / 5 (1) Sep 17, 2015
No I wasn't RS, and after looking up the specs for a Tesla (58 miles of travel for 1 hour charging at 80 amps on 240V) you may have to increase the total power storage depending on mileage requirements and local supply voltages.
Eikka
4.2 / 5 (10) Sep 17, 2015
Do you think it's a socialist plot that the power lines that run to your house have real, limited capacity that cannot be exceeded?


That's hardly the point, now is it?

The power lines can be upgraded at a cost, which in a free market situation would be borne by the electric car owners who have to go to the utilities to say "I want to charge my electric car, upgrade my lines" and then the utility would send them a bill for it. Then the grid would be upgraded gradually to meet the demand.

But instead, and because there's this -political- rush and push for electric cars rather than having people voluntarily pick them up on their own merits, the proposed solution is to give up more of peoples' privacy and practical freedoms and diffuse the cost to everyone, to make it seem more palatable.

I'm all for socialist policies, just not when they're used to make things worse for people.
Eikka
4.6 / 5 (9) Sep 17, 2015
Do you imagine that attempts to protect that vital infrastructure from overload are just encroachments on your personal liberty to consume, without concern for the other consumers that share that network's capacity?


There's more than one way to skin a cat.

The point here is that electric cars are very very expensive directly and indirectly, and hence generally unaffordable to society in any great numbers. By schemes such as big brother watching and rationing energy, you trade some of the value of that expense into other forms, such as from having to pay for your charging infrastructure in money to paying it with a loss of practicality, freedom and privacy.

The scheme here is like getting gasoline for your car only if you agree to install a GPS tracker and settling to leave your house on pre-registered hours, or risk a penalty fee.

There's no way that could be abused - right?
ricegf2015
3.4 / 5 (5) Sep 17, 2015
I would love to know the percent of electric car owners that also own a gas powered car as a backup vehicle. That is the only way EVs make any sense at all.


For current EVs other than Tesla Model S, that's true - though the longer range affordable vehicles announced for 2017 such as the Tesla Model 3 and Chevy Bolt, coupled with the build out of DC Fast Chargers along Interstates, will likely eliminate gasoline entirely for many drivers.

We own a Leaf and a gasoline car, but the vast majority of miles are driven using the Leaf. We also have a 100% wind power contract for our Texas ranch house. I think this makes great sense for us and for the environment. Do you agree?
Joker23
2 / 5 (4) Sep 17, 2015
Just another reason NOT to buy electric.......wait until they figure out how to tax the electricity that electric cars use. Only problem, with the share my wealth with you crowd, is that we, who still drive regular cars will end up paying for someone else's use tax as well as the outrageous gasoline tax we already pay.
MR166
2 / 5 (4) Sep 17, 2015
"We also have a 100% wind power contract for our Texas ranch house"

So when the wind is not blowing you have no power right. That is what 100% means to me.
antialias_physorg
4 / 5 (4) Sep 17, 2015
I would love to know the percent of electric car owners that also own a gas powered car as a backup vehicle. That is the only way EVs make any sense at all.

Whether it makes sense is highly dependent where you live. If everything is close together and there is good rail infrastructure (as in some european countries) then you may not need secondary car - even if your EV has low range. In places like the US (with basically no locally easily accessible rail or bus infrastructure) EVs make no sense unless they have Tesla S type range.

Just another reason NOT to buy electric.......wait until they figure out how to tax the electricity that electric cars use.

Wait. What? So your argument is:
"Yay capitalism, because I'd rather pay the tax on gasoline than NOT pay a tax on electricity for fear that someone might add one"
ricegf2015
3 / 5 (4) Sep 17, 2015
"We also have a 100% wind power contract for our Texas ranch house"

So when the wind is not blowing you have no power right. That is what 100% means to me.


It's not what 100% means in the power industry - for example, see powertochoose dot org.

The grid isn't designed to steer specific electrons from the wind generator to my EV charger. Rather, I pay the owner of the wind farm for the kWh I use, and they put the same kWh onto the grid as managed by the grid operator (in our case, Oncor).

The net effect is to provide a profit incentive to the wind farm owner, which seems to be working quite well - 14% of Texas generating capacity is now wind, the highest in the country, and 100% wind power contracts (as defined by the power industry) are competitively priced with mixed source.
antialias_physorg
4.2 / 5 (5) Sep 17, 2015
"We also have a 100% wind power contract for our Texas ranch house"

These contracts mean: For every kWh someone uses their power provider must buy/manufacture 1kWh from renewables.

All power plants (renewable and otherwise) are connected to the grid.
(It's weird to have to spell this out to you but you obviously don't know this, given your comment above)
Which one of those drives the electrons that light your porchlight can't be determined at any one time.

MR166
1 / 5 (4) Sep 17, 2015
Lets face it these contracts are just a way to give the purchasers a warm and fuzzy feeling. It deludes them into thinking that they are not using fossil fuels for the majority of their electric needs.
ricegf2015
3.7 / 5 (6) Sep 17, 2015
(It's weird to have to spell this out to you but you obviously don't know this, given your comment above)


It's weird that you would attempt to poorly summarize the very clear explanation I gave with references in the comment directly preceding yours! :-D

Again, the net effect we're seeking here is one of incentives. By paying the wind farm investors for the power I use, more investment in wind power results, and the more environmentally friendly grid-charged EVs prove to be. Google "well to wheels study" to learn more about how this works.

And it does work quite well. Don't knock it just because some people (not to call any names) seem to naively think that wind turbines are directly wired to each house junction box!
ricegf2015
3.7 / 5 (6) Sep 17, 2015
Lets face it these contracts are just a way to give the purchasers a warm and fuzzy feeling. It deludes them into thinking that they are not using fossil fuels for the majority of their electric needs.


You fail to understand how Renewable Energy Credits work to expand renewable power capacity. Check the national trends.
gkam
1.3 / 5 (13) Sep 17, 2015
We are shooting at ghosts which do not yet exist. Because we have identified a potential problem with doing something a certain way does not mean we are stopped.

I think your attitudes may be a reason YOU are not doing it where you live.
antialias_physorg
4.3 / 5 (6) Sep 17, 2015
Lets face it these contracts are just a way to give the purchasers a warm and fuzzy feeling. It deludes them into thinking that they are not using fossil fuels for the majority of their electric needs.

People need power. If you go to a supplier then they will have to produce (or at least buy) that power for you. If you obligate them to do so with renewables at least you're not adding to the problem (you're not solving the problem caused by those that don't care - but that's a sociological/political issue).

It's certainly not perfect but it's a hell of a lot better than all those others who are just freeloading off of your willingness to save their sorry behinds.
gkam
1.3 / 5 (13) Sep 17, 2015
Who says all cars will be charged at night? They can be charged at home or at work during the day, and spread the load.

But it maybe the cost of storage which will obviate the need for strict control over loads.
HocusLocus
5 / 5 (5) Sep 18, 2015
"What's the best way to charge millions of electric vehicles at once?"

Nuclear power of course.

Oops! Sorry, I see now it was a trick question. The 'actual answer': come up with a algorithmic network theory that promises to 'manage' resources based on an (arbitrary, incorrect) Malthusian shortage of energy and a (tragic) unwillingness to just build out energy sources (except wind turbines, several million ~3MW lollipops for environmentalists to suck on) and infrastructure. Like we used to do.

People who want electric cars and fear nuclear energy do not want electric cars after all.
I'm holding out the the next generation be smart enough to realize that they're fucked because their parents disliked nuclear energy.
Also hoping they'll go easy on us after the problem is fixed.
Do you realize... we have WASTED 40 years??
If Oak Ridge had not abandoned Thorium research we'd have 100% nuclear grid by now.
With surplus to charge those millions of cars.
Lex Talonis
1 / 5 (3) Sep 18, 2015
Well with have RETARDS like Elron Musk, putting out power gouging super cars, and the idiots that buy that shit.

Then we have people who have things like electric power assisted bicycles, and can keep them charged up and running from a 200W solar panel for ever, and ride to and fro - and maybe include a solar panel charger at the destination for the longer commutes.

The retard Merkins and their monster V8 gas guzzlers, used for driving 2 blocks down the street and back, who's brains are programmed for stupidity from birth, to be the advertising agencies puppets...

gkam
1 / 5 (14) Sep 18, 2015
"People who want electric cars and fear nuclear energy do not want electric cars after all."
---------------------------------

I do not fear nuclear energy, I fear the goobers who want to use it. They have proven over and over they are incapable of controlling it.
gkam
1 / 5 (12) Sep 18, 2015
HocusLocus can sign up to buy power right now from the Vogtle Reactors in construction in Georgia, or the ones Excelon needs subsidies for, or the ones First Energy requires massive help with to keep it online.

Shoot, he can buy entire nuclear powerplants from them right now!!

Here is your chance, hocus, go for it!
gkam
1 / 5 (12) Sep 18, 2015
http://www.taxpay...ctors-34

Goldman Sachs and Zacks Investment Research have rated Southern Company as a "sell." Golden Sachs cited "accelerating capital spending on Vogtle nuclear project and ongoing litigation with the plant's contractors" as well as the Kemper coal gasification plant and a GA Power rate case that has since resulted in a disappointing ruling for the company. Zacks pointed to weak share earnings, increased expenses, and high risks associated with the construction of reactors 3 and 4 at Plant Vogtle. With the likelihood of additional delays and cost overruns, Zacks states "the project cost could easily end up around $20 billion."
Bloodyorphan
2.3 / 5 (3) Sep 18, 2015
It's not only generation capacity, it's amps on cables.
If every house has two 22kVA chargers then the total amps for every house on the grid goes up by a factor of 540 %

That means every low voltage distribution cable is now running at two or three times it's actual capacity.

The fact is low voltage distribution grids were not designed for those kinds of load conditions.
chapprg1
5 / 5 (1) Sep 19, 2015
'Fail Soft' is still a failure. Or as we say in engineering terms, 'You can't get blood out of a turnip'.
docile
Sep 19, 2015
This comment has been removed by a moderator.
docile
Sep 19, 2015
This comment has been removed by a moderator.
bluehigh
5 / 5 (2) Sep 19, 2015
.. where nearby vehicles who already got theirs could share portions of energy with ones who are still waiting ...


You don't know much about human nature, right?
gkam
1.3 / 5 (13) Sep 19, 2015
"You don't know much about human nature, right?"
--------------------------------

Not all of us are selfish conservatives. Many of us believe we are all in this together, and can work together in systems for all of us, not descend into dog-eat-dog.
Edenlegaia
5 / 5 (2) Sep 19, 2015
The best way to charge so many cars is to have a lot of energy and a good way to distribute it anywhere it's needed. Lot of energy? Either shitloads of renewables, or nuclear plants. Both will be advanced enough to supply the energy those cars will need when common citizens will be able to buy electric cars without selling their organs. We'll just have to wait.
gkam
1.3 / 5 (12) Sep 19, 2015
I disagree. We do not need central plants requiring secure and very large transmission and distribution systems, but distributed generation locally, such as SOFC powerplants in neighborhoods. The value of distributed and diversified sources is the reliability, efficacy, and efficiency of them. Not to mention security, since they can be cut into small self-sustaining cells.
gkam
1.4 / 5 (11) Sep 20, 2015
I see four snipers gave me a one rating above. Do any of them have something to discuss, or is it purely personal and offensive?

Let's discuss the benefits of distributed generation, shall we?
OdinsAcolyte
1 / 5 (2) Sep 28, 2015
Go fuel cells. Traditional electric cars are inefficient. A WASTE of energy.
OdinsAcolyte
1 / 5 (1) Sep 28, 2015
Go fuel cells. Traditional electric cars are inefficient. A WASTE of energy.

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