The Real World Costs Of Backing Up Weather-Dependent Electricity Generation With Battery Storage

August 8, 2023Francis Menton  @ Manhattan Contrarian 

A recurring question at this blog has been, how do the world’s politicians plan to provide reliable electricity without fossil fuels? Country after country, and state after state, have announced grand plans for what they call “Net Zero” electricity generation, universally accompanied by schemes for massive build-outs of wind and solar generation facilities. But what is the strategy for the calm nights, or for the sometimes long periods at the coldest times of the winter when both wind and sun produce near zero electricity for days or even weeks on end?

When pressed, the answer given is generally “batteries” or “storage.” That answer might appear plausible before you start to think about it quantitatively. To introduce some quantitative thinking into the situation, last December I had a Report published by the Global Warming Policy Foundation titled “The Energy Storage Conundrum.” That Report discussed several calculations of how much energy storage would be required to get various jurisdictions through a year with only wind and/or solar generation and only batteries for back-up, with fossil fuels excluded from the mix. The number are truly breathtaking: for California and Germany, approximately 25,000 GWh of storage to make it through a year; for the continental U.S., approximately 233,000 GWh of storage to make it through a year. At a wildly optimistic assumption of $100/kWh for storage, this would price out at $2.5 trillion for California or Germany, $23.3 trillion for the U.S. — equal or greater than the entire GDP of the jurisdiction. At more realistic assumptions of $300 - 500/kWh for battery storage, you would be looking at 3 to 5 times GDP for one round of batteries, which would then need replacement every few years.

But even these numbers wildly understate the real world costs of storage that would be needed. Here’s why: the calculations that I presented were based on actually data for particular years, and what storage would have been needed to make it through that year. For example, here is the chart from my Report of the annual charge and discharge cycle for a collection of batteries that would have been sufficient to get California through the year 2017 on a wind/solar system, fossil fuels eliminated, without running out of electricity:

As you can see, the calculation assumes that California would run its batteries right down to zero in March with the expectation that they would then begin to recharge.

But if you are planning a system that must have 99.9% reliability, you can’t just look at one year and assume you can run your storage down to zero. You need to consider the worst-case year. This is particularly true in the case of an electricity system consisting only of wind and solar generation plus batteries. If the batteries run down to zero, then what? It is not at all obvious how to restart. You might need to dedicate the generation exclusively to charging the batteries for weeks or even a month or more before you can have confidence that you can restart without immediately crashing again.

So, in the real world, how would you run such a system prudently?

There actually exists a closely analogous type of system from which we can make inferences of what kind of margins are necessary to assure reliability. That analogous type of system is the system for water supply. The supply of water from a reservoir system, like generation of electricity from wind and sun, is dependent on unpredictable weather. What kind of margins for storage are necessary to assure reliability?

The New York City water supply system makes lots of data available to investigate this question. Here are some key data points:

• New York City consumes about 1 billion gallons of water a day from its reservoir system. (Although the population has grown somewhat over the past couple of decades, that figure has remained quite stable, and actually decreased by a little, largely due to universal metering and increasing prices.)

• The New York City reservoir system has a capacity of approximately 550 billion gallons — which is about 1.5 years of consumption, or 18 months’ worth.

• Rainfall, on average, is a generous 4 inches per month, year-round. However, there can be droughts, which can continue for months on end.

The New York City reservoirs have a usual annual cycle. Usage exceeds replenishment in the summer and fall, and then the reservoirs refill in the spring with run-off from melting winter snows. In a typical year, the reservoir level never falls below 70% of capacity. However, there are periodic drought years, when reservoir levels can get much lower.

Here is a chart from New York City on historical droughts going back to the 1960s. There were droughts in 1963-65, 1980-82, 1985, 1991, 1995, and 2002. The lowest level reported for the reservoirs in this chart occurred on January 19, 1981, when the level reached 33% (which would represent approximately 6 months of usage). A drought “Emergency” was declared at that point. Another “Emergency” was declared in April-July 1985, with the reservoir level ranging between 55% and 62% (10-11 months of average usage), and again in April 2002 with the level at 57.5% (10 months average usage).

I would contend that this represents government over-reacting as usual and trying to scare the people into compliance. All of these drought conditions resolved themselves when rains came and refilled the reservoirs long before they emptied out.

But the point remains: Nobody is going to let the reservoirs get anywhere close to zero before declaring an emergency. After all, there is no further back-up when the reservoirs are empty. At that point, there is no more water until some rain shows up. And so we consider it a drought emergency when the remaining storage is somewhere in the range of 6 to 10 months of water.

Now apply that to a prospective wind/solar/battery electricity system with fossil fuel back-up eliminated. Are we really going to run such a system in accordance with the models in my Report, where we allow the batteries to drain right down to zero every spring? What if the wind and sun don’t cooperate for the next month (or two, or three)? Won’t we insist on having at least a month’s worth of spare storage at the normal low point of the year, just in case we have a worst-case situation?

In that case, I suggest that the number presented in my Report for the cost of batteries to back up a fossil fuel-free system are low by at least a factor of two, and probably more.