An article in the LA Times suggests an idea I’ve seen frequently — use electric car batteries to meet peak power demand on the grid. After all, you have a car, and it’s plugged in, and it has a big battery, so instead of just charging it, have it send juice back to the grid when it most needs it.
The reason this is attractive is that a large part of the cost of the grid is building it to handle the peak load. Most of the capital cost is for that, and fuel costs are based on the real, variable load. Softening the peak is very valuable to the power company — to the point that power companies give rebates and credits to people who do things that will soften that peak.
This is also one of the virtues of solar. It tends to provide power during the day, which is always when the peak is. However, solar peaks at noon, while the demand peak is the hottest part of the day, which tends to be later in the afternoon. The big peak tends to be around 4-6pm when it’s hot, and people have started turning on things in their houses to get ready for dinner. On the spot markets power costs the most then.
Contrast that with the night. Because nuclear plants and some big coal plants aren’t easy to dial back, then sometimes even produce more power than is being used, and they end up discarding the power into giant resistors. That makes power at night cheap.
I’ve never seen it done, but there could even be merit in the idea of mounting fixed solar panels pointing west, so that they catch less power in the morning but do better in the later afternoon when the price of electricity is highest. I presume this doesn’t happen because net metering home owners don’t get access to the “true” spot power price which would justify this. If they are lucky they do get time-of-day metering so they sell power at a high price in the day and buy it cheap in the evening, but some don’t even get that. The harsh reality is that most grids were not built to have a lot of generation at the edges, and power companies are pushing back on net metering and grid-ties that feed back too much power. Indeed, for cost reasons here in California, people should size their solar systems to not quite meet needs, and buy the rest at the cheap “tier 1” price, rather than try to sell back.
Most solar panels are erected facing due south, tilted to the latitude which maximizes total kwh, but peaks at noon. Actually, most are mounted on a section of the roof that is closest to south. If you have to choose between SE and SW, it might be that SW is best, at least for the grid. (Sadly, a number of solar panels are mounted on the front of houses, even if that points north! People are more keen on looking good than doing good. I hope that’s rarer than I’ve been told.)
Anyway, back to the cars
There are a few issues with using the batteries in the car for the peak load.
- The peak time is unfortunately a very popular time for driving. People either want to drive in the late afternoon — it is called the rush hour for a reason — or they plan to drive soon and want their car’s battery to be full to meet their driving needs. They don’t want to find their car half-empty at 6pm because it sold power to the grid. A study of car usage patterns detailed the numbers.
- The batteries in cars are expensive. Charging and discharging the battery uses up its lifetime. We don’t know how long car batteries are going to last but a typical estimate is around 150,000 miles, or about 40,000 lifetime kwh. If it’s the 22kwh pack in the LEAF (which costs $12K or so today) that’s 27 cents/kwh lifetime. Plus the cost of the electricity that went in to be resold. The peak price ranges from 25-30 cents/kwh in the west but hits as much as 48 cents in New York. So it could be profitable in New York, but barely so. Big, heavy lead batteries are more cost effective.
There are some factors, though, which could change this:
- Battery packs will get cheaper, and their lifetimes will increase. That will drop the cost of putting a kwh into and out of a battery.
- Cars like the Tesla model S have huge batteries, far more than they actually need. This, it turns out is quite wasteful, since you buy a lot of battery and rarely use it. If you know you don’t plan a 200 mile trip, you might be tolerant that your long-range car is half-empty at 6pm, and happy to sell that excess capacity. You already paid for the capacity, after all to give you that long-trip freedom. You will still shorten the battery life, but you’ll be paid for that.
- Weather forecasts are getting quite accurate, so demand can be predicted and this managed better.
- The car can also be a backup in the event of grid power outages. There, the 35 cent/kwh price (and loss of driving ability) are minor compared to the burden of having no power in your home.
Calling all cars!
Now, as you might expect on this blog, robocars are also game changers here. The inverters and equipment to feed power back to the grid are expensive, so most people won’t have them. But if the robocars have a means to plug in, they can bring the power to where it’s needed. A power company, seeing a brownout coming, could send out an alert on the net. “Calling all cars” — if you have spare capacity, we’ll buy it at the following rate. Please drive to the nearest two-way intertie and plug in soon. While ideally some sort of automatic connection would be possible, this could even be a charging lot with human staff who plug in the cars as they arrive and unplug them when they have to leave.
Such charging lots might well exist for cars that need charges at night or other non-peak times. Due to cost, cars will strongly wish to avoid charging at peak cost times. This puts them to use then. Inductive charging also works (at a loss of about 10%) and robotic plug-in is actually quite doable — there are already robotic gasoline filling stations out there. A robocar charging lot could be dense-pack, valet style, so not take a lot of land. But it would take megawatts — but that’s OK. The robots don’t care how convenient it is, so put it next to the transformer station.