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Using cars for backup grid energy probably doesn't work

Pumping water into a reservoir is great for storage

In the world of electric cars, some people talk about an idea called "vehicle to grid" or V2G. Renewable energy's biggest challenge is storage -- wind and solar only come at certain times of the day, but we need electricity all day. The V2G hope is to use all the batteries in electric cars as a means of grid storage.

Initial V2G plans just involve electric cars that are sitting plugged into charging stations. They would fill up, as they always do, but mostly during the night when power demand is lowest. They would also charge up, if plugged in, after working the morning commute.

Then, if the grid had a shortage of power, it could send the message out, and ask cars to send power back out of their batteries back into the grid to make up the shortfall. This could help handle the top peak loads, for which there is simply not enough generation capacity available, or it could also reduce the need to fire up "peaker" plants that burn fossil fuels.

Storage for the grid is challenging, and many are working on using grid-based batteries as they get cheaper. Other options include pumping water back into reservoirs (one of the top choices) or fancier approaches like compressing gas in underground chambers. V2G suggests using the big fleet of roving batteries.

Robocars add another dimension to this. Unlike regular cars, if there is a major power outage and they are not in use, they can drive themselves to a connection point to sell their energy. All the idle cars can get in the game if there are enough connection points in the right places. Energy can be bought at night for 6 cents/kwh and sold at high peaks for 30 cents/kwh or more.

So many people have mistakenly identified this as a possible win.

Car batteries vs. stationary batteries

A number of factors work against this idea:

First, car batteries must be as small and light as practical. All weight must be carried around. This is why they are all Lithium batteries today. Grid batteries don't care about weight or size or safety in a collision. They care only about cost, and in particular the cost per lifetime kwh. Lifetime kwh is the number of kwh you can put into and get out of the battery before it has to go out of service.

It turns out that even though weight is unimportant for grid batteries, lithium batteries are still the most cost effective technology thanks to their falling cost. In cars, cooling in a compact space is a big issue. (Even worse for cars that are not moving to gain air cooling.) Some other technologies should be better for large facilities, such as flow batteries (for predicted peaks,) zinc batteries and advanced lead-acid. There are also some interesting technologies in the lab which promise much lower cost.

In cars, we care not just about weight and size, but also range. Range is precious, especially in human driven cars. Having paid to put it there, we are very reluctant to give it up.

Batteries are, for now, a consumable resource

The clearest problem for V2G is that all batteries today are a consumable resource. They have a lifetime, because they degrade slightly with each kwh that goes in and out. Their capacity reduces until it gets too low to be acceptable. In a regular car, the effect is immediate, because people crave range and feel their car is much less valuable with lower range. In a robotaxi, the lowered range reduces the amount of time it can operate in a day. With a grid battery, it barely matters until capacity is so low that the real estate to house it is no longer justified.

Degradation is not completely linear. The faster you put energy in and out, the more degradation there is. If you discharge a battery all the way to the bottom, or charge it all the way to the top, lifetime is shortened. Car driving, particularly with jackrabbit starts and Level 3 supercharging, causes more degradation than grid use, so the real answer is complex but the main point remains true.

If your car provides power to the grid it uses up some of its lifetime. Unless the battery has "surplus lifetime" because it sits in a car that is very rarely driven, V2G is hard to justify.

Do some simple math. Imagine a car with a battery good for 160,000 miles. This means it will roughly store and provide about 40,000 kwh in its life. Typical human driven cars will go about 10,000 miles/year (2,500 kwh) while taxis will go 60,000 miles per year (15,000 kwh.)

Say the car is providing power to the grid at 6kw for 2 hours/day for 208 days -- I have chosen that because that's also 2,500 kwh per year which makes the calculation obvious. Now the battery is wearing out twice as fast. If the one battery would last the life of the car, now you need to buy two batteries.

Instead of buying two batteries, it makes more sense just to buy the one battery, and put the money of the 2nd battery into a grid battery -- or more likely, a share of one. Because of the lack of need to worry about weight, volume and crashes, you can get more for your money in a grid battery. Yes, two batteries requires more interest on capital cost, but that should be offset by the lower cost of grid batteries and the cost of swapping.

(There is one factor here in the other direction which is harder to predict. The future battery might be significantly cheaper than it is today. That has been the pattern so far, though most expect a floor to appear based on raw materials cost, but it could also do well. If you're sure of this, using up your battery faster at a profit could make sense.)

A battery in a car is not yet a resource that is just sitting there unused, wasted if you don't take advantage of it. It's a consumable resource, and it does not make sense to burn it up on grid energy when it's designed for something very different.

Some day, we hope to have battery designs that don't degrade with use. When we do, then a car battery just sitting there can be a wasted resource waiting to be exploited. We'll want all our batteries to be of this new design. This does not alter the other issues discussed.

By the way, a great use for depleted car batteries is transfer out of the car to grid storage. A half capacity battery is a big waste in a car, but no problem in a grid facility, until it gets so bad it's cheaper to recycle it.

In the taxi, which is using up far more battery capacity per day, there is even less incentive to use up "spare" capacity on the grid. For a taxi fleet operator, building a grid storage unit is something more practical than it is for an individual. As battery lifetimes increase, once they exceed the life of the car, there would be "spare" capacity to sell off at a profit. For taxis, robotaxis in particular, the battery will not last as long as the rest of the powertrain, and so using it for V2G means replacing it sooner.

If that's not enough, though, it gets worse.

Time of selling the power

The next problem is that the peak demand on the grid tends to run from 3:30pm to 8pm. That's the combination of the built-up heat of the day needing air conditioning, and the homes coming online while the offices are still running. After 8pm it drops quickly until about 7am. The only time V2G is likely to be wanted is in that peak zone, especially the top period from 5pm to 6:30pm.

Maybe you've noticed the problem. This is the peak of rush hour. This is the time when the cars are all out on the road. And the time before it is the time where people want their cars to be fully charged because rush hour is coming. It's the last time you want to drain your car to help the grid.

Yes, there will be some cars that are able to sell electricity during the peak:

  • Vehicles that are offline for maintenance
  • Long range cars which are sure they will not need their range that day
  • Privately owned cars which know they won't be driven in that rush hour
  • Cars that have gotten home from the rush hour drive and have extra power left for the later section of the peak.

Of course to do so, they must be plugged in to a charger which has the necessary inverters to convert their power to grid power (220vac in the USA.) I haven't even talked about the cost of all that special equipment at the charge/discharge stations.

A decision to sell the energy in a car's battery is a decision to decrease its range for that day. It has to be rare, otherwise you put a wastefully large battery in the car.

Super peak

It could make a lot more sense to use V2G during the "super peaks" -- those few hot days when the grid runs out. Building capacity to use just a few days a year is expensive, be it peaker plants or grid batteries.

Here the problem is that it's also expensive to put a grid-quality inverter at a lot of charging stations to only use a few days a year. Here's where robocars are a win. You don't need to build a lot of fancy inverters. Instead you build a few garages with the right grid intertie equipment (possibly using the DC direct from the compatible cars) and then you dispatch the robocars there on demand. It does mean longer wait times at rush hour, but if the price offered for the electricity is high enough, it could be worth it.

Time of charging the battery

One last issue -- if the rest weren't enough -- is a future one. Our dream is of a mostly solar grid, with so much excess solar that we need to store that extra energy for use in the peak and at night. While cars don't drive all day, they day is when they do most of their work -- and that's the only time excess solar will be available.

Cars instead prefer to charge at night, when the power comes from baseload -- nuclear, coal, geothermal and some wind. Hydro also runs at night as needed, but since hydro and natural gas can be turned up and down on demand, they usually are best for the loads of the day.

Update: Super-long lifecycle batteries

My friend Carl points out that battery systems are getting better, and eventually will have extremely large numbers of recharge cycles possible before end-of-life. This would change the equation enough to make V2G practical, but not completely. In that world, there is "spare" capacity in an existing battery because it will not lose much lifetime by feeding the grid, and so cheap night energy can be sold at a profit at 5pm. But only, as noted above, if the car is not planned for use in that evening rush.

In today's world of human cars, a surprising number of them are not in the evening rush. With enough profit incentive, people might give up their range on days when they are pretty sure they don't need it.

The bad news is that in the robotaxi world, there aren't many "spare" cars. While I don't think everybody would convert by giving up car ownership and using only robotaxis, for those who did, the fleet will be in full utilization driving at the rush hours. I say full utilization because even though you only need enough cars for the annual peak, and the daily peak is a bit less, the size of your "idle but on call" fleet is what governs how quickly you can serve a passenger -- their wait time. Short wait times, which come from having every car ready to serve, are a big competitive factor once a city gets competition.

So there's no such thing as an idle robotaxi which is ready to give up its energy, unless you have a majorly overprovisioned fleet. However, as the peak goes on and traffic starts going into decline, a fleet can safely take cars out of service and let them sell their extra energy. This would probably happen around 6:30pm or so, but that's just a guess.

On the plus side, out in the country, there will be more owned cars and no robotaxis, and these cars, once it is known they are not driving any more than evening, could sell their energy.

So why the excitement in V2G?

So what factors keep people thinking about V2G? The arguments I made about cost are based on a more mature world of grid batteries. Today, car batteries are the subject of volume production and high research, which is making them cheaper than other batteries. Factory capacity is in place and it's a (high end) consumer technology, while industrial technologies can end up costing more even though their raw cost is cheaper. This has driven the cost of Lithium batteries down so that they now beat lead-acid in lifecycle cost. (Lead acid, due to its maturity, is recycled extremely well compared to other technology.)

There may be some attraction for those who want to use an electric car as storage not for the grid, but their house. People have a strong (mostly irrational) desire to be free from the grid. If they have solar panels on their houses, they delude themselves into thinking they are running their car on solar energy.

A storage battery that disappears during the day is not a very good solution. It's unlikely to be there at the peak times when your power needs are high (air conditioning) and energy is expensive. I mean, it could help, but it's a sucky solution.

Perhaps of more interest is the ability to use the car as emergency backup power for a home when the grid is down. That's more reasonable, though it requires inverters and a complex transfer switch to protect the grid. If you do this, you will be shortening the lifetime of the car battery, but this time it's well worth it.

The real efficient solution is not yet practical. That's neighbourhood grid storage. Blocks of houses could pool the money they are not spending on replacing their car batteries faster to install a neighbourhood grid battery that would offer both cheaper energy during the expensive part of the day, and emergency backup power. By sharing the cost of inverters and installation, it would be the win -- if the power companies would facilitate it.

Comments

Ever part of this analysis is totally broken.
Its sad when good electronics engineers assume they know power engineering when they have never studied it. Not knowing what you don't know.
Need to pay close attention to the revenue from selling energy in to the grid. Energy is not watts. Power meters are well designed, and there are opportunities to take it to the wholesale marketplace.
You can't evaluate this by making wild ass guesses about relative efficiency of parts of the system. You have to respect the cash register. How it works today at retail, commercial, and wholesale levels. All are exploitable to great advantage today.
The cost of battery degradation. They can last forever when cycled respectfully.
The insane stated collective goal of all-solar grid, as opposed to "clean" negative emissions, toxic pollution free energy. Which is obviously going to be renewables plus 2018 redesigned nuclear.
The cost of "equipment" to implement VTG... is pretty much software... pretty cheap stuff to manufacture.
Careful attention to how power bills work show very large revenues to car owners, and very large savings to utility operators.
I plan to write a paper on this when a demonstration is complete.

Of course energy is not watts. I would never knowingly say it was. I made only one reference to watts in the article and it is correct. (Noting that a car in a home charger typically can output about 6kw as that's the rating of the 30a x 220v circuit it's typically on.)

I am aware of the prices. You do have to respect the cash register. I am saying that, even though there might be night power to be bought for 6 cents which can be sold for 30 cents in the day, the fact that this is profitable is not the issue. It's going to cost you money (in terms of battery lifetime) to put a kwh in and to get it out. How much money depends on the life cycle numbers for your battery and the cost of your battery. I am saying that a battery designed to sit in a building is going to be more cost effective at that, than a battery designed to fit in a car.

So you can either use the battery in the car and take the take the 24 cents in delta but pay 10 cents of battery life in the car, or take the 24 cents and pay less in battery life in the battery (or other tech) designed for grid storage.

Now, you're trying to say that there is no such thing as battery degradation. That the batteries in cars can last forever. That's the only claim in your comment that refutes what I have written. Everything else I see in the literature right now suggests a much more finite number of cycles in lithium batteries before they reach a point of lowered utility for a car. If that's wrong, then my analysis is partly wrong. It is now reasonable to use a car's battery because it's sitting there unused. The other factors -- about grid peak happening in evening rush hour -- are still true. I've added an update section on that due to your prompting.

You are saying that typical cars today can generate 220vac pure sine wave power to feed back to the grid with just a bit of software?

I don't doubt there can be revenues to car owners. Or savings for utilities. That's not part of my argument. The argument is that there are more cost effective ways to give revenues to car owners. Utility savings are the same.

And as for the future grid -- I agree that nuclear should play a big role. Unfortunately that is politically challenging, if you look at what's happened in Germany and some other places. Right now, in sunny places, solar is the cheapest new type of power plant to install, and since it has so many other good attributes (close to perfect except for the pesky fact it only provides power when the time and weather are right) it seems like what you would want for your ideal grid of the future, which is, in spite of conservation, a much bigger one globally than we have today.

The degradation issue may be less extreme than you suggest, particularly for cars with larger batteries.

Tesla Model S 85 has probably the most longevity data for a battery of this size. Per Pluginamerica's dataset, the trend is approximately 1% degradation per 11000 miles. Assuming linear degradation, at 300000 miles the vehicle will have degraded from 265 to approximately 200 miles of range. Community forum surveys show even lower degradation, approximately 1% per 15000 miles.

At 15000 miles of annual driving, these long range batteries will have significantly fewer than 100 annual cycles. IF the useful lifespan of the battery significantly exceeds the useful lifespan of the vehicle, then V2G has minimal marginal cost.

Additionally, the simpler option may be to simply to deploy networked EV supply equipment that can be shut off upon demand, provided that a target charging contract can be met (ex: vehicle negotiates to reach 80% charge by 4 pm or 6 am). For a carefully managed grid, avoided demand can be roughly equivalent to supplied power, and can help shape grid demand curves based upon short interruptions in renewable supplies.

While it is good news to get degradation down, as long as it exists, it has a cost and you decide to you want to spend it on grid arbitrage or driving. In particular, the less it is, the more life is left in the battery when the car is junked, and so that battery can be put into full-time grid operation, where degradation is not an issue.

I know, really obvious, but neighbourhood grid storage would be a lot more practical when designing a large multi story apartment block.
Also in places where developments are more like private suburbs, multiple apartment blocks shopping malls and other infrastructure all in one.
Possibly these types of developments are more common in third world countries where infrastructure is less reliable.
In this case battery storage + solar might make more sense.

Overall though WRT energy storage, I'm not yet convinced that distributed generation and storage (household solar and batteries) is more efficient than monolithic generation and storage (e.g massive solar/wind + hydro storage).
Getting 'off the grid' seems like an unobtainable dream because of the need for energy security 100% of the time, 99% is completely inadequate for most people.
The times when local generation + storage fails is likely to be over a widespread area due to widespread weather conditions, so at these times the carrying capacity of the grid needs to be 100% what iit is today.
The cost of the grid seems to be (almost) constant regardless of local generation.

Going off the grid is a false and non-green dream, unless your generation and demand can be kept in exact harmony, which they can't -- especially if you have solar or wind. Storage is good for surges and those sources and backup. But it's always better to find somebody else who can use your surplus power, and to make use of other people's surplus, than it is to store it. Thus a grid, though it doesn't have to be a large grid. Just large enough so that power is not wasted.

Agree you don't go off the grid.
Not sure about the size of the grid, you will still run out of power in winter months therefore the grid capacity doesn't change at all.
Perhaps that's not what ypu mean?

Have lower demand because there is no air conditioning, but of course less sun. Now, we haven't talked about heating, which today is mostly fossil fuel. A world of electric heat is another step up -- it's common in places like Quebec that are crawling with hydro. On the plus side, if it's heat you want rather than electricity, you can store heat fairly easily from the day to the night.

Going off grid may make little sense from an engineering POV, however changing electricity generation and pressure on prices could still make it economically logical. Although I accept using EV vehicle batteries for this purpose makes far less sense.

In my own area a mild climate (little heating or cooling), a cheap spot price for electricity during the day (overbuild of solar) with higher prices in the evenings are making PV solar self consumption with fixed batteries more competitive with grid prices. My government is trying to claw back overgenerous PV solar installation subsidies by allowing utilities to charge much higher 'daily connection charges'. This last point is now very close to creating a tipping point for the economics to favor off grid electricity. No grid connection then no 'daily connection charge'. If battery prices fall much further then grid defections may start to become a real issue and put pressure on the entire pricing system.

The main reason to keep on the grid is not just money. It's that going off grid is not green. When you go off grid, you have batteries, and you like to keep them fairly full. Once they are approaching full, if your solar panels are enjoying the sun and you have nowhere to put the power, you just throw it away. You end up throwing a lot of your power away. When you could feed it to the grid, and offset fossil fuel plants. Not the green thing to do.

Agree it is not the best green solution. If it becomes more economic to do so then it’s a sign of the pricing system failing to keep up with technology.

Sorry I did not have time to explain Apparent.com again.

Your analysis considers only one sort of arbitrage a battery can help you perform.
Time of day.
Suffers from a drastically oversimplified view of energy marketplace.

There are several independent dimensions of value creation using electric storage that you aren't thinking about.

I'm going to list several. But as a Singularity University guy i'm going to have to ask you to do some multiplication. Most of the value creation opportunities I'm listing are orthogonal and leverage each other. One set of slightly smarter hardware and control can leverage most of these ideas at the same time, and the value multiplies.

The simplest is retail demand charges- a PG&E customer is punished for their highest 15 minutes of the month for the whole month. Make sure that peak is shaved and in many cases you can save half your bill. Modest storage and maybe some load controls can do it. Just pay attention to the power meter.

Next simplest... smart meters and even dumb ones actually do charge for reactive power-- if you read the "About Your Meter" section on PG&E website you will find out that when they say KiloWatts on your bill they actually mean KVA which accounts for the reactive component.

A big error is your repeated assertion that the grid is a sine wave and the PV inverter makes a "perfect" sine wave. You are 129 years out of date. Voltage might be a sine... but not watts or current. You are assuming the whole grid is in one small town and all the loads are incandescent light bulbs as Nikola Tesla created it. That was the original design but we have come a long way.

A mutual friend of ours has a farm on Skyline. I measured the power waveform using a Fluke power quality meter. It was a perfect sawtooth. A sine wave would be better... but power factor correction gear costs money and it's a bit rural, so those inductive power lines aren't balanced by the pole capacitors, and he has lots of inductive motors on, and all those switching power supplies feeding in harmonics. It's about a 70% power factor a lot of the time.

Often on commercial bills, the real and reactive components (real and imaginary parts of the complex number we use to represent AC) are split out, and the reactive imaginary part is 5x more expensive. That's the utility asking you not to demand that, because it costs them 10x. The grid is built for 90% efficient transmission of sine waves, and everything that's not a sine wave (reactive power) is moved with 10% efficiency. Sadly you can't lose energy-- so that 90% of the reactive energy, that doesn't make it to the customer, is converted to heat in the worst possible place and time, limiting the transmission capacity, and leading to blackouts and brownouts.

But hey, your power meter is measuring reactive power and works backwards, and it's worth 5x, so you would be dumb not to make it when you can... and the solar/storage industry is dumb, they make a "perfect" sine wave. So they could make $1 bills or $5 bills at the same cost and they choose $1 bills. But Apparent.com makes $6 because... real and reactive are just ways of describing the shape of the waveform. You make the ideal waveform, you get both, no sacrifice, and "energy capacity" and revenue increase. (Now most EE's will call foul here.. because they say power supplies sacrifice real power to make reactive... which is true of 50 year old RC power supply designs, but if you have a $0.01 micro-controller and know calculus, you don't have to make the wrong thing and re-work it.)

But it's worse. If the power factor is bad, your equipment may function with more heat and less work. If the grid power factor is 80% and you are trying to sell them a "perfect" sine wave, you will fail to get the last 20% out. Impedance mismatch, at the worse possible time... they need your energy but you failed to deliver it in a usable form. Silent curtailment. But if your inverter systems impedance match, you can get out 120% or more. Helping force the grid to be a sine wave. And the commercial power meter is already set up to do it-- no novel business arrangement required.
Note that a storage unit's ability to help convert real power to reactive only requires 1/60th of a second storage- so it's the power rating, not the energy rating, that matters. Even the NiHM battery in a Prius hybrid could be in the money.

So all that stuff I mentioned above can be done with no reference to the outside world, no info feed, and no special business deals. Measure the load waveform at your site, produce that exactly, and you can stop your power meter. Then deliver the most lucrative waveform with excess power available.

I can get it for you wholesale
Now let's think about what we can do if the system does have access to wholesale market data.

There's no reason not to do business, wholesale, if it's a win win win win opportunity between utilizes, regulators, the planet, and the distributed generation (solar storage) owner. Regulators have done their job with the "Smart inverters working group" and making sure at the PUC/ ISO / FERC level that smart distributed generation assets can be aggregated and operated as virtual power plants with software defined "inertia" balancing the grid.

OK, understand your market. Wholesale.

Get the CAISO phone app "ISO TODAY" and check the renewables map. Click on the prices on a warm day. You will see day ahead and 15 minute markets wildly fluctuating.
Get on CAISO and look at the 11 different products that are traded constantly to produce a reliable grid.
Note that natural gas plants often make less than half of their revenue supplying a "sine wave real power" product. It's a minor product for them. Oddly it's the only product the solar farms have even tried to produce. So ideally they would be making half as much money as the gas plants-- but it's worth. Turns out on a sunny day the value of solar goes negative and they get curtailed... The AC grid is a system that moves a resource described by a 2 dimensional number. If you attempt to sell only one dimension, that is always wrong, and at non-trivial scale (over 20% penetration) constitutes vandalism.

But Apparent.com 's PVUSA historic DOE site is never curtailed, because they are supporting the grid, instead of vandalizing it, and the grid needs reactive support, for frequency control, voltage control.. all those things grid operators do. Making sure power flows where it's needed, and not in circles around the grid.

No one denies that power factor correction equipment can make money-- prudcts exist, and industrial sites are often required to buy it. You might note capacitor banks at ski lifts for example. The economics suffer when you buy a lot of hardware and all those capacitors get replaced every 6 years.
But if you can get that for free... You already bought the hardware for another purpose, and you just add software.
You can get the AC optimization and wholesale marketing features. Apparent.com also does it without any short lifespan parts.

So these business models and hardware products are all working today. The industry doesn't know it for the most part. And the academic power grid analysts generally ignore these details. But the grid operators know about it. And the regulators have done a good job on the roadmap to a true distributed grid.

We do need a lot of control and "inertia" to avoid blackouts as solar / wind grow above 20%. This smart tech leveraging storage can get us to 50% renewables I think, without substantial storage.

I still think getting the last 20% of fossil fuel shut down will require some form of nuclear redesigned in 2018. Fortunately distributed storage improves the efficiency of transmission and leaves more money on the table for all suppliers and consumers. Storage helps nuclear, solar, and wind displace fossil fuel. Storage also helps coal deliver energy to customers more profitably-- so it's not an automatic win for the climate.

The Vehicle to Grid opportunity is interesting because we can get the storage we need cheaper than free. Only one million EV"s in California would have the same power capacity as all of California's generators. If those EV's can earn money when they are parked, that offsets the cost of ownership. How much is the subject of our current analysis.

I feel that we should change California's Pavley Bill that regulates EV batteries, requring a 10 year lifespan. Fear of landfill waste and toxics drove the idea.
That's unfortunate because it means a conservative old battery that can be insured to last 10 years is put in new cars. ANd 5 years down the road, you want the latest battery, not the one you bought earlier. And there's an aftermarket. And the batteries aren't scary toxic NiCd or PbNa anymore.
So a new VTG law and deal means we can have a win win win... Relax the 10 year rule.
The state gets grid services NOW. The driver gets a car with the best battery available NOW. The driver get's a source of revenue from the car in the garage, without sending it to Tesla Robo Carshare Pool. The car manufacturer gets off the hook for the long battery warranty, saving money NOW. The climate does better. People have nicer transportation. Fewer blackouts. It's all an improvement.

That's the dream!
Thanks for listening, if you made it this far!

As you say, many utilities and solar people are unaware of these elements, so the math on it takes some time. When I talk about the sine wave inverter, that's the one you need to power your house or sell power back to the grid today. A fully dynamic inverter able to generate complex waveforms to match the reactive loads on the grid is a fancier device, so what does it take to get these into cars or into charging stations (connection points?) What will they cost? Of course, if you can sell the power for a high price, their price is justified.

I am still unsure about the car battery being there to offset your peak load, because again, the car is not there during your peak load, probably. That short burst load is interesting though, if you can predict it, because while the robotaxi fleet needs to be "on call" for the evening rush from 3 to 7 (which is when many of these peaks occur) cars can wait connected to power to provide short bursts. They just can't sit there feeding the grid for all of the rush hour. It is more useful to be using their energy to provide transportation at rush hour than to use it supplementing the grid at grid peak.

The next bit of math is to figure out how many cars you need. How much battery capacity is needed to counter all the expensive spikes and reactive loads?

One way robocars can help is they can bring the battery to you. And they can travel to special connection points, where they could make a DC connection. This might allow several cars to come to that point, if they can match their voltages, and then all get put into the grid using a single fancy circuit to generate the desired waveforms.

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