Putting renewables on the grid to green your electric car is complicated

My car may be charged from these windmills, or is it?

In my Tesla review, I tossed out a line that raised some questions. The power utility here in Silicon Valley provides power only from renewable sources, in my case 80% wind and 20% solar. Does this mean that my electric car driving is zero emissions, and if so, how, since when I charge at night there is no solar and may or may not be enough wind.

So I set off to figure out what happens when I (and thousands of others) plug in cars to charge, and it turns out the answer is far from simple. So I wrote a two part series.

Part one tries to figure out what happens when you add renewables to the grid, either by putting solar panels on your house, or buying your energy from a renewable company. In the long term, it's good, but in the short term, sometimes it will cause some emissions to go up because it shifts the grid from efficient slow-response plants to higher emission fast-response plants.

Part one: Putting renewables on the grid to green your car is really complicated


Nice job on this article; I’m looking forward to part 2!

In our own little “microgrid” the goal is to use as much solar power as is available and minimize grid imports. My grid connection is my “baseline.” Over the 11 months that my system has been active, I’ve generated 34% of my power, nearly 5 MWh, and that includes an 18% increase in household consumption due to the electric car. Also, 3 of the 11 months were before the Powerwall was installed, which left a lot of power on the table (my utility does not do net men). So, what we do is run the washer/dryer on sunny days, we charge the car from 10 to 4 (at a reduced rate to stretch out demand). The Powerwall allows us to combine instantaneous solar power with up to 5 kW sustained power from the battery for high draw use cases like the water heater. On the few times when the Powerwall is about to fill up, we use the HVACs (which we can control remotely) to either overcool or overheat the house, and then let it fall to equilibrium.

Very well written article. I understand that the political reality is making the case for geothermal + high capacity (inter)national grid a moot point, but it would not hurt to be mentioned more. Especially the fact that the bigger the grid the less it is vulnerable to very short variations in demand/production. Geothermal is so cheap that even 6% of losses in the grid could be tolerated to get it to the big consumers.

One thing I'd add is that there is usually some power demand that can be shifted, either a little bit or a lot. Some electric companies (the few I know of, at least) allow you to install a device that turns off certain devices during those brief demand spikes. The device is smart, and recognizes actual spikes, not just time-of-day estimated usages.

Electric cars will benefit from this a lot. Already power companies are finding a spike in demand at a certain time of the day (e.g. midnight) when their prices go down. This spike lasts a relatively short period of time (most cars don't take very long to fully charge most nights), and then demand is low the rest of the night. A smarter way of having the chargers talk to the power companies to smooth that demand over the entire night would be preferable, and probably will be necessary if and when electric cars become more widespread.

It might also make sense for them to charge less for using a steady low (not necessarily trickle-low) amount of power and spreading it over the entire night. I'm not sure that would be better for them compared to just letting houses take turns with high power, but it would probably be easier to implement. I'm not sure if the software currently lets you just input the percent charge you want and the number of hours you want it in, but that would be an easy thing to do - in the mean time, it's easy to do the math yourself.

One big question will be whether or not the governments let them do this. Power rates are highly regulated. Billing strategies that make sense for the power companies may or may not be legal (or may only be legal after spending a lot of money on lobbying, to the legislatures or directly to the voters in the case of referendums; I'm reminded of all the money they have to spend, often unsuccessfully, to avoid inefficient billing strategies like net metering).

I'm curious how that'll work. Some rich homeowners install a bunch of solar panels and sell their power at retail prices to the grid during the day. Then they all charge their Teslas at 17 kilowatts from midnight until 12:30 AM at rock bottom prices. That's gotta be costing the electric companies a lot already, and it's only going to get worse.

This is a big thing in power management, and yes, the electric cars should participate and probably would be the easiest thing to make participate. One problem with a discount for charging slowly is that it's hard to tell that's what really happened.

I would think it would be easy (technically, not necessarily politically) to exclude from super-off-peak discounts any short spikes in usage (e.g. more than x% above average for less than y hours). The meters that can handle time-of-usage billing already collect enough data to do this don't they?

Yes, this wouldn't be specific to electric cars. But any residential customer who uses 17 extra kilowatts for 30-60 minutes every night at the same time is causing a disproportionate amount of stress on the grid.

Every kw of load is causing the same stress to the grid no matter what it comes from. However you can do tricks to discourage some loads over and above just pricing them higher. Rewards can be easier (even cheaper power at 4am.) In the past, the idea of regular fluctuation of prices did not fit in the world of energy pricing. Today, with computers handling the loads, it could.

One type of energy that you didn’t mention in your article on renewable energy is thermal storage. The four biggest residential thermal loads, space heating, air conditioning, water heating and refrigerators can all be done with electricity and combined with low-cost thermal storage. This can be done with timers or aggregated under smart grid control and can be coordinated with intermittent renewable energy sources such as solar and wind. Tank electric water heaters (resistance or heat pumps) already have potentially controllable thermal storage. I have operated my heat pump water heater under timer control for the past 17 years and have time-of-use rates. I run my water heater from midnight to 6 AM.

Yes, thermal storage is great, and we should have more of it in the future. But we don't, and I am investigating what happens with all the renewable we are putting on the grid now. One of the best things is for air conditioners to chill some water in the night and morning and use it to cool in the afternoon. The prices should make them want to do that anyway, but it's funny how the difference between capital costs and operating costs screws us.

Brad, excellent article. Peak Power has solutions and expertise which may be valuable for your Part 2 article. Please reach out if you would like to interview some of our team members. Thank you, Mark

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