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. Energy is to power as distance is to speed.
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, at the same rate.
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 three 4-hour 187.5 MW bursts, during which the pack would have to 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 what will charge the battery, 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 first 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 first figure is at zero).
The first figure shows the Moss Landing Battery output at its theoretical (actually, its claimed) maximum, over a theoretical 24-hour day. Note that there are three 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; the third from six p.m. to nine p.m. This obviously rules out solar as the recharge power source for those rechargings. Solar-based recharging is really possible only in the mid-day recharge period. And that would require hundreds of thousands 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 for solar panels to recharge a single Moss Landing battery pack for one of the rechargings shown in the first figure. 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.
Let me repeat: the power (green curve) in the second figure performs one of the rechargings that must occur in the “trough” (zero output) portions of the first figure. Note that this single recharging occupies the entire day. Most important, remember from the first 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 (forget about the source of that 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 desirable, the idea is obviously to supply that power from solar panels. As mentioned, hundreds of thousands of panels would be needed just for a single recharging. They’d need millions of square meters of panels, plus seven more battery packs, to make solar-charged batteries anything close to a replacement for the old Moss Landing gas-fired generator.
Unlike the hundreds of thousands of panels in the second figure, which to repeat are equipped with sun tracking technology and are of the theoretically highest efficiency, many if not most of the panels to charge the Moss Landing battery pack would actually be mounted on the roofs of high-priced urban and suburban homes. This would drive down their collective capacity factor, and thereby drive up the per-kilowatt-hour price they would have to fetch in order to make the proposition worthwhile for the owners of the rooftops.
This would be an expensive proposition both because of difficulty of installation and the value of the installation platform.
Would all this drive up the per-kWh cost of that solar electricity, and hence grid electricity, just as it did in Ontario? It most certainly would.
Add in the cost of battery storage, and you’re looking at ruinously expensive electricity. My guess is, long before California gets anywhere close to actually outputting battery power at anything close to the level of the old Moss Landing gas-fired steam unit, the state will have abandoned this patently idiotic idea and jumped onto whichever other harebrained green energy scheme Silicon Valley has fallen in love with.1
But in the mean time, what will this spike in electricity prices do to the price of menial labour in California?
In spite of the obviously dubious economic case underpinning the battery project, the California Public Utilities Commission okayed PG&E’s proposal for it.
The Ontario media criticizes the new Ontario PC government for pulling out of the cap and trade system with California (the Western Climate Initiative), 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.
- The official proponents of the Moss Landing battery scheme already couch their words carefully. None actually says that batteries will replace the old unit’s output. As the lead proponent, PG&E, told a local newspaper, “[w]e believe that battery energy storage will be even more significant in enhancing overall grid reliability, integrating renewables, and helping customers save energy and money.” i.e., the battery’s actual purpose is to provide ancillary services, not supply the grid with renewable energy. But the latter claim, on saving energy and money, pretty much undoes the care with which PG&E avoids making a false promise about the battery replacing the gas-fired generator.