Moss Landing unit 7 was a California steam cycle electricity generator that ran on natural gas. Its nameplate capacity was 739 megawatts. That meant that its operator could have run it at or near capacity, i.e. at 739 MW, for days at a time if it so wished. Though it was an uncommonly efficient steam cycle generator, its owner, Dynegy, stopped producing power from it at the end of 2016 because of inability to profit in the California electricity system. The site owner, the regulated utility PG&E, recently received utility commission approval to install a battery pack on the site, with a capacity rating of 730 MWh.
At first glance, you could be forgiven for thinking there is some similarity in the capacity of the old steam plant (739 MW) and the capacity of the new battery pack (730 MWh).
But there is a significant difference. Note the “h” in the capacity unit for the battery: it stands for “hours.” That is to say, the battery pack is rated in megawatt-hours, not megawatts. That is to say, the battery capacity is given in energy units, not power units. Power is to energy as speed is to distance.
So will the new 730 MWh battery pack at Moss Landing be able to deliver anything close to 730 MW, like the old steam-cycle gas plant was? No. Like all batteries, it will be constrained by its C-rate, i.e. its current discharge rate; or, when expressed in terms of power, its E-rate. From the report of one Tesla enthusiast, the Moss Landing battery pack would discharge electrical power at the rate of 182.5 MW, for four hours. Assuming this is true, the battery pack would then need four hours to recharge. So over say a 24-hour period, it could theoretically provide three 4-hour blocks of 182.5 MW, for a total of 2,190 MWh. And there would be three 4-hour gaps in between those four-hour 187.5 MW bursts, during which the pack would be recharged, also at a rate of 182.5 MW. See the figure.
In light of this, is it not misleading to say, as Tesla cheerleaders do, that this battery pack will replace the gas plant with clean energy? Leaving aside that we haven’t even gotten around to how the battery will be charged, it is plainly false to say that a 730 MWh battery can replace a 739 MW steam plant.
The old steam cycle gas plant running at capacity (the dotted line in the upper figure) could have provided 17,520 MWh in the same 24 hours—8 times as much energy. But most important, it would have been able to output 730 MW continuously over those 24 hours; that’s why the dotted line is flat.
To get 730 MW for 24 hours straight, from batteries of the type Tesla is building at Moss Landing, how many battery packs would it take? Nominally, at least eight (don’t forget the four-hour recharge periods—i.e. those periods in which the green line in the upper figure is at zero).
The figure shows the Moss Landing Battery output at its theoretical (actually, its claimed) maximum, over a theoretical 24-hour day. Note that there are rechargings from midnight at the beginning of the day till 3 a.m., then 7 a.m. to 11 a.m, and then 3 p.m. to 7 p.m. The first recharge period occurs in the middle of the night, which obviously rules out solar as the recharge power source. Solar-based recharging is possible in the two daytime recharge periods, with many millions of square meters of solar PV panels installed somewhere, at somebody’s expense.
To show what I mean, see the figure below. It gives what would be needed to recharge a single Moss Landing battery pack. I put the panels at 32° North (the same latitude as San Diego, i.e. in the southern part of the state), and assumed perfect sun on the first day of summer.
The power (green curve) in the figure performs one of the chargings in the valley portion of the first figure. Note it occupies the entire day. Remember from the upper figure that there are three such chargings in the theoretical 24 hour day.
But solar-based recharging would be feasible only if there were a surplus of solar power, or rather a surplus of power during those periods—otherwise why would anybody in their right mind pay money to store electricity at a time that they need electricity.
In those periods where there is a surplus of power that makes storage theoretically desireable, the idea is obviously to supply that power from solar panels. As mentioned, many millions of panels would be needed. Unlike the hundreds of thousands of panels in the lower figure, many if not most of the panels to charge Moss Landing would actually be mounted on the roofs of high-priced urban and suburban homes, an expensive proposition both because of difficulty of installation and the value of the installation platform. You could be forgiven for thinking this will drive up the per-kWh cost of that solar power, and hence grid power, as it did in Ontario.
Add in the cost of battery storage, and you’re looking at extremely expensive electricity. My guess is, long before California gets anywhere close to actually outputting electricity from batteries anywhere close to the level of the old Moss Landing gas-fired steam unit, the state will have abandoned this patently idiotic idea.
But in the mean time, what will this spike in electricity prices do to the price of menial labour in California?
The California Public Utilities Commission okayed PG&E’s proposal for the Moss Landing battery project.
The Ontario media criticizes the new Ontario PC government for pulling out of the cap and trade system with California, the CO2 permit auction proceeds of which might have been funnelled into pipe dreams like the Moss Landing battery project.
To put it mildly, it’s a good thing we pulled out.