# Will Exponential Growth Get Us to Our Solar Shangri-La?

There’s so much bad news these days that it can be hard to focus on the good stuff. The global trend toward clean energy is, however, one recent trend worth celebrating. So, being a bit of an energy nerd, I thought I’d delve into some of the details.

### How’s Kurzweil’s Solar Prediction Faring?

Back in 2011, I read an article claiming that futurist, author and inventor Ray Kurzweil had predicted that solar power would be able to satisfy the entire world’s energy needs in 16 years (that is, by 2027) due to exponential growth in its usage. At the time, I was skeptical but interested. Kurzweil is well-known for his predictions of various future events based on the idea that some trends grow exponentially.

So, here it is 2022, just 5 years shy of the time when the world is supposed to be fully solar.” How are we doing? Well, that depends on your perspective.

In many respects, Kurzweil has proven to be correct. The growth in the amount of solar energy produced in the world has, in fact, been growing exponentially since the year 2000. By my reckoning, the number of terawatts produced by solar power has doubled every 2.375 years. Whereas solar produced just 1.08 terawatt hours (or TWh) of electricity in the year 2000, it produced 1,032.5 terawatt hours in 2021. That’s a remarkable run!

But even if it continues to double every two years over the next five years, solar will still produce roughly 8,000 TWh of power in 2027. Not bad. In fact, it’d be about a quarter of all the electricity humanity generated in 2021 (28,214.07 TWh). Still, we will not be powering the whole planet with solar by then.

How about if we add wind energy to the mix? Based on my own back-of-the-proverbial-envelope projections, the combination of solar and wind energy has been doubling every 3.5 years or so since 2000. Together, solar and wind produced 2,894.44 TWh in 2021. If that number doubles over the next four years (let’s say), it’ll represent 5,788.88 or so TWh in 2025. Again, pretty great, but by no means powering the whole world.

The Global Growth of Solar and Wind Energy, 2000 to 2021

If solar/wind were to continue to double every four years, however, then it would represent 23,155.52 TWh in 2033 and 46,311.04 TWh in 2037, which is far more electrical energy than humanity produced in 2021. So, Kurzweil’s forecast may well come true, though a few years later than predicted.

Even if it does, however, it won’t be all solar and wind. Hydropower, for example, won’t be going away. Nor will all the nuclear plants be shuttered. But it’s quite possible that between 2033 and 2038 the vast majority of sources of electricity will be completely renewable.

The problem, of course, is that sooner or later, solar and wind will hit the so-called S-curve. As the World Resources Institute notes, “Historically, technologies that are growing exponentially have a ‘top speed’ for growth — a maximum growth rate that is achieved, that lasts awhile and then slows down as it approaches 100% adoption. This pattern is known as an S-curve.”

S-curves are probably inevitable for these two forms of renewable energy. After all, it’s one thing to grow from 2 TWh to 4 in a couple of years. It’s quite another to grow from 20,000 TWh to 40,000 TWh in just two years. You’re essentially taking much of the previously installed solar power over a period of many years and then doubling it in just two!

Below is an example of a possible S-curve related to renewables, as shown in “Explaining the Exponential Growth of Renewable Energy.”

### When Will the S-Curve Flatten Out?

Nobody quite knows when the S-curve will flatten, slowing the growth of solar/wind power. There are too many variables that remain uncertain. But I think the most important variable is energy storage. If humanity can solve energy storage pretty quickly, then the S-curve will not flatten out for some time. If we can’t solve it, though, then it’ll flatten faster because the all-too-variable renewables will need to be use used in tandem with more reliable sources such as fossil fuels and nuclear.

#### Lightning in a Bottle

So, will today’s scientists and engineers be able to develop and implement enough robust, cheap and plentiful energy storage to allow renewables to meet their potential? After all, if we could efficiently store energy from the sun and wind so that is available when the wind’s not blowing nor the sun shining, then renewable will be reliable enough to power nearly everything.

[Note: Some experts believe that more infrastructure can serve essentially the same purpose as energy storage: that is, if you have enough renewables plugged into an expanded grid, then some energy source will always be available. For example, the wind will always be blowing somewhere. Of course, this assumes nations will invest in such infrastructure and NIMBY types allow it to be built.)

So far, we haven’t come close to perfecting energy storage, but there has been considerable progress. Since 2019, there have been a rash of articles on how the combination of solar-plus-batteries has become the least expensive way of generating electricity. Science magazine reported that “Solar plus batteries is now cheaper than fossil power.” Forbes reported “New Solar + Battery Price Crushes Fossil Fuels, Buries Nuclear.”

But, as welcome as such headlines were, the stories themselves came with a catch. That is, the lithium-ion batteries in these projects can’t hold a charge for very long and the batteries themselves wear out. In fact, the batteries cited in the Forbes article could only provide electricity for four hours past the time when the sun stops shining. That’s clearly not long enough to make this model capable of getting us to 100% renewable energy.

Nonetheless, we are about to see a ton more of these solar-or-wind-plus-lithium-ion-battery power plants (aka, hybrid projects) come online over the next few years. By the end of 2020, there were 73 solar and 16 wind projects that, altogether, provided 2.5 gigawatts of power generation and .45 gigawatts of storage, Bigthink.com reports.

By the end 2021, about 223 gigawatts of proposed hybrid solar plants were in the works, along with 19 gigawatts of hybrid wind. That’s some pretty Kurzweilian growth there.

#### Salt, Gravity, Hydrogen and Other Storage Hopefuls

Lithium-ion batteries are the state-of-the-art right now, but plenty of other energy storage tech is in the wings. There is a ton of innovation in this area right now, with news stories popping up every day. Yesterday, for example, it was Scientific American reporting on molten salt batteries.

Researchers at Pacific Northwest National Laboratory (PNNL), a Department of Energy national laboratory in Richland, Wash., are developing a battery that might solve this [long-term storage] problem. In a recent paper published in Cell Reports Physical Science, they demonstrated how freezing and thawing a molten salt solution creates a rechargeable battery that can store energy cheaply and efficiently for weeks or months at a time. Such a capability is crucial to shifting the U.S. grid away from fossil fuels that release greenhouse gases and toward renewable energy.

Then there are ideas such as using gravity to store energy, which is pretty much what hydropower already does. Now, however, they’re trying to make gravity work without water and favorable local geologies. For example, there’s the company Energy Vault, which has been celebrated in the media but is now being criticized as an unworkable idea by some analysts. It’s hard to know if anything will come of these gravity schemes.

Perhaps more promising is green hydrogen–that is, hydrogen fuel produced with renewables. The problem with using hydrogen for energy storage is that it’s not very efficient to turn renewable energy into hydrogen and then use hydrogen to power something else. S&P Global Market Intelligence reports:

The technology to convert power to hydrogen and back to power has a round-trip efficiency of 18%-46%, according to data that Flora presented from the Massachusetts Institute of Technology and scientific journal Nature Energy. In comparison, two mature long-duration technologies, pumped-storage hydropower and compressed air energy storage, boast round-trip efficiencies of 70%-85% and 42%-67%, respectively. Flow batteries, a rechargeable fuel cell technology that is less mature, have a round-trip efficiency of 60%-80%.

On the other hand, I’ve also read that engineers are working at developing processes with a much better conversion efficiency for hydrogen. Even if hydrogen does not turn out to be the most efficient way of storing energy, it could still win the energy storage game if the world produces so much cheap renewable power that a substantial level of inefficiency become acceptable.

Then there’s the really high-tech stuff that is not ready for primetime but may utterly change things when it is. For example, Swedish scientists have reportedly created “an energy system that makes it possible to capture and store solar energy for up to 18 years, releasing it as heat when needed.”

This kind of tech sounds almost magical. “This is a radically new way of generating electricity from solar energy. It means that we can use solar energy to produce electricity regardless of weather, time of day, season, or geographical location,” said research leader Kasper Moth-Poulsen, Professor at the Department of Chemistry and Chemical Engineering at Chalmers University of Technology in Gothenberg.

### Now, If We Can Just Reinvent Ourselves

A world in which renewable energy quickly becomes our primary source of electrical feels inevitable at this stage…unless we screw it up via the worst impulses of our hominid natures. We’ll need, for example, to avoid destroying our infrastructures (not to mention ourselves) via nuclear war while electing responsible politicians who are not beholden to the fossil fuel lobbies and who are better at bringing people together than driving them apart. The world’s richer states will also need to help the poorer ones make the transition to green energies as quickly as possible. And, of course, the scientists and engineers will need to keep doing what they’ve done so well so far: reinventing the way humanity generates its energy.

In short, we’ve solved most of the engineering problems associated with clean energy, and we’re quickly making headway in the one area not yet solved: the storage and stable dissemination of renewable power. We can make it to a green energy global economy if we can just figure out how to listen a little more closely to the better angels of our nature.

`Featured image from Lunkwill / derivative work: McSush - Exponential.png`