Brad Templeton, Forbes, 2021
The renewable power grid of the future needs energy storage and lots of it. Many have been attracted to an idea called “Vehicle to Grid” (V2G) as a possible solution. The idea is that the giant fleets of electric cars that are coming have tons of spare battery capacity. With the right gear, if they are plugged in when the grid needs power, they can quickly provide that power and get a good price for it. No need to build massive grid batteries — just use the batteries already being deployed.
Now a new partnership between Ridezum and AutoGrid wants to make this happen not with cars, but with a coming fleet of electric school buses. The theory is the electric school bus needs a big battery (it’s a bus) but only drives for an hour or two in the morning, and again in the afternoon. Otherwise it sits around — plugged in — and can help the grid. Can it? The answer is “maybe.”
The challenge that faces V2G is that the grid needs help very specifically from 4pm to 9pm, and most of all around 7pm on hot summer evenings. That’s not only the time of the most electrical demand, as people return home, offices and stores are still running, and air conditioning load is very high. It’s also the time when all the solar panels have shut down. The worst thing happens — the grid needs the most power, and it can’t get it from photovoltaic solar. We’re going to build a lot of solar, because it’s now the cheapest power you can build, and we’ll build enough to handle that load from 4pm until the sun starts fading. We’ll even wastefully point the panels west just so they can do the most at that time of day, throwing away their power in the morning. We may also build solar thermal plans, which cost way more than solar panels, but keep churning into the night.
Everything else has to come from other sources, most notably today natural gas “peaker” plants which are only used at this time. It’s expensive and emits CO2. We also use as much wind, nuclear and hydro as we can at this time. If it can make power then, it does, because the price paid is at its highest. The rest people hope will come from storage. Storage includes some hydro plants and many other new technologies, but one key method is batteries.
The V2G advocates say the batteries in the cars can do a lot here. But there are some problems:
- A large fraction of cars want to be on the road from 3pm to 7pm, what we all know as “rush hour.” Before they commute, they don’t want to sell their power, and after they commute, they have less to sell. On the other hand, batteries are big enough now that there is battery to spare — if you’re not driving.
- The car’s battery wears out the more times you put power in and take it out. If you sell power to the grid, you use of some of the life of your battery. That can be OK because the grid is willing to pay quite a lot for that, making it a net win, but in many cars the life of the battery is the life of the car, and a reduced capacity that’s lost 20% of its capacity is depreciated by a lot more than 20%.
- The car needs to have a 2-way power connection (which some do, but many, including Teslas, don’t.)
- In addition, the car or the EVSE it plugs into needs a grid quality sine-wave inverter plus electrical code transfer switches that shut it off (or disconnect the home, if doing V2Home) if the grid goes off. These are not inexpensive things.
The new project proposes that electric school buses could be a V2G answer. Unlike most private cars, school buses work on a regular schedule, especially in the school year. They drive from 7am to 9am and sit idle except for the odd school trip until working from 3pm to 5pm. They probably only go around 30 miles in each shift.
For the first shift, they will charge in the night, when power is cheap. Then after bringing in the children they return to the depot and can charge with early morning power, which is relatively cheap, and can be easily supplied by solar. In fact using solar for this is ideal.
The afternoon is the problem. The bus can’t feed the grid while bring children home. And having done so, it is less than ideal to try to charge it back from 5pm to 6pm when power as it its most expensive to sell it back from 6pm to 9pm at the same price. For V2G to work, you need to take in power when it’s cheap and plentiful, and sell it when it’s scarce and expensive.
The answer may lie in the fact that school districts are buying buses with much more range than they need to do a single shift. It’s not clear why they are doing this. In theory, you can size the battery to exactly the worst shift you expect to get. (Because it’s optimal to use the 20-80% range of a battery, that would mean to do 30 miles you might get a 50 mile range bus.) You might also get more range because longer range is longer battery life, as you distribute wear of the whole pack — but this is only modestly useful because it adds extra weight which costs range.
Some of the buses in the fleet will need to take longer journeys, like special school trips and need more range. But only some of them. In spite of this, Ridezum reports that many school districts are buying buses with 120 miles of range even though the shift is just 30 miles.
This explains the opportunity for V2G. The bus will leave fully charged for the afternoon shift and still have all its extra energy available to sell. Grid surges rarely happen by surprise — the weather forecast tells you they will happen. So the bus will refill to 100% in the morning (normally you avoid going all the way to 100%) and sell power at 5pm when it gets back. The morning power might cost 10 cents/kwh and sell for 30 cents to 60 cents in a heavy surge, and that makes economic sense. It saves the power grid from having those peaker plants.
Another useful attribute is that most school buses are not used much in the summer. They don’t do any shifts bring kids to and from school, though they may be hired out for other functions. Those that aren’t can devote all their battery to V2G, buying power at the cheapest time and selling it at a profit and helping the grid.
While Ridezum does not yet support it, the real profit comes if you are able to sell highly specialized power waveforms to the grid, instead of the basic sine wave. One of the big problems on the grid is that certain loads, notably big motors, put the current AC wave out of phase with the voltage wave. This is called the power factor, and it can gum up the grid if there isn’t something else to put things back into phase. Battery storage can do this job well, and it’s worth a lot.
On top of all this, the buses can also supply power to the depot where they park, if that depot has high demands (perhaps it’s adjacent to a school or board office.) In this case, the bus becomes a bit like a Tesla Powerwall battery, able to arbitrage the price of power (store it when cheap, release it when prices are high) and even provide backup during a power outage. You might even park buses at schools just to get this function. This also helps the grid naturally without selling it any power and can be more lucrative.
The Solar Future
The cost of solar panels is now so low that they are the cheapest type of power plant to build. Nobody is going to build any more coal or gas plants when you can build solar for less — except for the problem that solar only delivers when the sun shines. That’s why people want storage. We’ll build enough solar to meet all our needs (above our nuclear and hydro) and more, and that will produce much more than we need in the mornings. The hydro will shut off but still, we want to store that morning solar. We’ll put it into cars for driving, and may as well for V2G later.
V2G will probably be only used on the peak days — those hot summer evenings when the air conditioners are going full blast. That’s when these buses might sell their power. You don’t want to plan to sell your power every day — in that case the batteries should just go into grid storage, and shouldn’t be car batteries at all. But on the days when the choice is car batteries or a gas peaker plant, the batteries could make sense. (On the really bad days, that old gas peaker plant may run. Sure, it emits CO2 but it’s not a big deal to use it a few days a year. The problem is the cost of maintaining it so it can be used so rarely.)
The bigger application may simply be charging cars in the morning rather than at night. As EVs take over the road, they will become one of the biggest users of electricity. That solar surplus in the morning, caused by the massive solar build for the afternoon, is the perfect time to charge them. Right now the perfect time is at night, when demand is lowest. If it flips to morning, it means cars must be plugged in somewhere from 9am to 2pm to receive that power. At night, cars are at home and they plug in there. In the morning, many of the cars are not at home, and we’ll need plugs in employee parking lots, garages and even some curbs. Simply controlling when the cars charge to match grid supply and prices is a powerful enough idea that some people call it V1G — a simpler version of V2G, but not involving anything but charging cars at the right time.