I have been writing a lot recently about global warming and energy infrastructure. This is partly because there is a lot of news coming out of COP27, but also because both here and on the SGU there has been some lively and informative discussion on the issue. Also, this is a very complex issue and as people raise new points it sends me down different rabbit holes of information. I am trying to develop the most complete and objective picture I can of the situation.

The goal, of course, is to rapidly decarbonize the energy infrastructure of the world. We not only need to do this, we need to do it quickly and cost-effectively. Further, we need a plan for the next 30 years, and essentially we don’t have any second chances left. If we want to stay as far below 2.0 C temperature rise as possible, and even shoot for that rapidly fading hope of keeping below 1.5 C, then we have one shot. This means that if we have to course correct after 20 years, this may still improve the situation but will likely be too late to meet our climate goals.

I find that the most compelling arguments from experts to be those who advocate essentially doing everything. We should pick the low-hanging fruit, do all the win-wins, but also hedge our bets. If anything we want to overshoot.

One contentious issue has been whether or not it is feasible and advisable to plan on a 100% renewable energy infrastructure. The conversation gets complicated by some technical terms, so let me define them here.

“Renewable” includes wind, solar, hydroelectric, geothermal, tidal and biomass. Low carbon includes all these plus nuclear, which is not technically renewable because it burns fuel. “Intermittent” sources are wind and solar, because we cannot control when the sun shines or when the wind blows. One of the biggest communication errors I encounter is confusing “renewable” with “intermittent”, so I want to keep these terms clear.

My goal here is simply to understand what the experts are saying and what the evidence is. I otherwise have no dog in this hunt, as they say. My understanding has been that while wind and solar are the cheapest new energy capacity, they become increasingly expensive and problematic as their share of electricity capacity increases. But again, my position is frequently misrepresented, so let me clarify.

There is no controversy over the fact that above 30% or so of wind and solar penetration the cost of new capacity goes up. This is because intermittent sources require overcapacity and grid storage to work at higher percentages. This is a geometric progression, with the inflection point being somewhere around 30%. This is not a limit or ceiling, it is an inflection point. The curve really starts to turn up at around 60%.

Some experts are concerned (and I share their concern) that if the plan for the next 30 years is all wind and solar we may run into practical upper limits with no plan B. These limits include suitable locations for wind and solar, and increasing capital investments needed, availability of raw material, as well as the need for updating the grid (to make it smarter and more robust). Obviously we should be updating the grid as we go, but if this lags behind the need that can be a problem.

The other big variable is grid storage. With sufficient grid storage, well distributed, we can have 100% wind and solar. We don’t currently have such grid storage, so again we are betting on a future capacity we don’t currently have. Grid storage also needs to be able to shift energy production to demand over hours, days, and even seasons (for solar).

Again, the question is, what is the probability that a 100% renewable scheme (which would need to include about 80% wind and solar) will succeed, meaning that we have sufficient grid updates and grid storage? And if it does not succeed, what is our plan B? The default plan B is to burn fossil fuel, and that will cost us our climate goals. This is why I have found compelling the arguments of experts who say we need to maintain, at least, our nuclear capacity, and perhaps even expand it a bit, to reduce the probability that we will need to burn fossil fuels to make up for any shortfall in a 100% renewable scheme. This further means we need to develop nuclear capacity now, because it will be too late in 20 years.

This is still, I think, the most reasonable position. However, I have recently modified my understanding of how likely it is that we will be able to develop sufficient grid storage because of recent analyses of a specific grid storage strategy – closed loop pumped hydro.

Pumped hydro has always been one of the best grid storage options, and today is responsible for 99% of grid storage power. The idea is to pump water from a lower reservoir to an upper reservoir when the grid has excess energy, and then to flow the water from the upper to the lower through turbines to generate electricity when needed. This has a round trip energy efficiency of about 80%, which is pretty good as grid storage goes. Only lithium-ion batteries are better, at around 92%.

The problem with pumped hydro has been that the number of suitable locations is limited, and river-based hydroelectric plants tend to be disruptive to local ecosystems, so expanding them vastly is not a good idea.

Attention, however, has shifted to closed-loop pumped hydro. These installations are not connected to a river, and cannot be used as a source of hydroelectric power. They solely serve the function of grid storage. They can be built wherever there are two reservoirs of water that are close enough and have a sufficient difference in altitude (called the head). If, for example, the reservoirs have a head of 300 meters and are 1km apart, that is a good location for a closed loop system. There are other geological details of importance. The two reservoirs are connected by tunnels or pipes, so it matters how much rock will need to be drilled.

A recent analysis of the global potential for closed-loop pumped hydro finds 616,000 potential sites around the world. They also conclude that we only need 0.1% of these sites to develop sufficient pumped hydro grid storage to support wind and solar energy. This means we can pick the best 0.1% of sites to develop. They also argue that prior analyses of the potential of pumped hydro did not include the potential for closed loop, non-river systems.

An analysis of potential sites in the US includes an interesting discussion. First, this also finds great untapped potential, 35 terawatt-hours of capacity. But they also point out that such calculations are extremely sensitive to technical assumptions about the suitability and cost of developing specific sites.

I hope that these analyses are generally correct, and that closed-loop pumped hydro is the grid storage solution we have been looking for. That will make reaching our climate goals much easier and more likely. I do have a generic concern stemming from my experience that initial enthusiasm for new ideas and technologies is not always realized. We’ll see what happens when the idea gets more attention and other experts pick over the details. We also need to see what will happen when candidate sites are developed – how long will it actually take and actually cost?

In the US most current pumped hydro storage was built over 50 years ago, with the last facility built 30 years ago. So we are strangely in the same position that we are with nuclear power. Also, the regulatory process is very slow, taking a minimum of 5 years and often much longer (there are both state and federal approvals to navigate). Since 2018 there is now a 2 year expedited approval process, but sites don’t always quality or pass and still need the longer process. The inflation reduction act provided general tax support for grid storage projects, but otherwise did not do anything specifically for pumped hydro. And the bottom line is that there currently is no plan to build lots of closed-loop pumped hydro in the US.

But reading these studies has made me more optimistic overall. Hopefully we will see some new closed-loop pumped hydro installations by the end of this decade, providing further information to guide even more future development. If this process continues to look good, then we need to put some serious resources into developing installations, and need to streamline the regulatory process and consider ways to make financing more easily available. This is exactly the kind of investment that can make the difference in meeting our climate change goals.