Encapsulated Pumped Storage, Part 1: Introduction

Series Introduction

Solar and wind energy are inherently intermittent. The world has co-evolved with energy sources (such as fossil fuel, nuclear power, and hydroelectricity) that are not intermittent in the same way that solar and wind are; power on demand is what makes today’s global civilization possible. We can, and must, do everything possible to adapt our energy usage to the sources we can safely use, instead of the sources we’ve always preferred—and we must also reduce our overall energy usage as fast as possible.

But those changes won’t eliminate the need for reliable power, so most experts agree that we need energy storage, including grid-scale, long-duration (multi-hour) storage of electricity. Some believe that batteries alone will solve this problem, but that will only be possible if they become much cheaper than they are now, and projections of future prices are always uncertain. So we would be wise to exploit other storage options too, if they seem competitive with batteries in terms of cost and other characteristics.

Most grid electricity storage capacity currently is, and has always been, pumped hydroelectric storage. It lends itself to very large scale, uses proven technology and non-exotic materials, and is relatively efficient. We would like to have more of it, and in fact many countries are building more of it, all around the world, as we speak. But pumped storage has always relied on a combination of special topography (elevation change) and natural watercourses. Unexploited sites where pumped storage can be built, at reasonable cost, and with acceptably low damage to ecosystems, are so few that it’s widely regarded as a built-out technology that can’t expand much beyond its current niche.

I propose a new form of pumped hydroelectric storage that can be deployed quickly to hundreds of sites around the world, providing an aggregate amount of storage many times greater than currently exists, with low environmental impact and at a cost that is cheaper than any commercially available battery at similar scale. For reasons that will soon be clear, I call this “encapsulated pumped storage” (EPS).


This is an evolution, not a revolution. Pumped storage works fine already. Turbines, pumps, generators, pipes, and valves are all mature technologies that don’t need to be reinvented. We just need to combine these elements in a new way that gets around the limitations mentioned above. The key strategies that make this possible are:

Completely eliminate interaction with natural watercourses of any kind. EPS, like all pumped storage, relies on water, but the water is fully contained and the same water is reused for the the multi-decade life of the facility. The facility can be quite far from any natural watercourse, and also far from well-watered, productive ecosystems. (“Closed-loop” pumped storage is not a new idea and is already being done, but fully containing the water is a step further.)

Do not build reservoirs. Instead, take advantage of modern materials that combine low cost, high strength, and durability to contain the water in multiple, mass-produced, flexible containment units, or modules. This eliminates evaporation, makes repair and expansion simple, and has other important benefits. The water essentially sits “on” the terrain rather than “in” it.

Exploit very large elevation differences. The energy stored by a pumped hydro system is proportional to the elevation difference between the upper and lower storage facilities. (In the industry, this elevation difference is referred to as “hydraulic head,” or simply “head.”) Doubling the hydraulic head doubles the working pressure and the energy storage potential, but at much less than double the system cost. Most existing pumped storage systems operate at relatively low head, due to the difficulty of finding sites with large, flat areas where reservoirs can be built, especially at the higher of the two elevations. This means that to scale up their storage capacity, they have to scale up the reservoir volumes—both at the top and at the bottom. Larger water volume increases the cost much more unfavorably than higher head does.

Allow water to be stored at more than two elevations. A traditional reservoir has a single surface elevation at any moment. All of the water at that surface is at exactly the same gravitational potential. But terrain isn’t generally flat, especially in mountains. To build a big reservoir in a mountain range might entail massive excavation and other earthworks, and construction of dams (which, if made of concrete, have a very large carbon footprint). But individual water-containing modules do not have to be sited at a single, common elevation. They can follow the terrain to an extent, as long as each module sits on its own leveled pad.

However, we need the modules to be inexpensive, so that the overall cost per gallon of water stored is lower than it would be for reservoirs. And inexpensive modules cannot be built to withstand large pressures. The solution is to use remotely controlled valves to isolate the pressure within any module to always stay within its modest limits. The valves and interconnecting pipes can be made to higher (though still moderate) pressure specifications.

Those are the biggest differences between existing pumped storage and what I propose. They lead to a number of important consequences—such as the ability to scale a facility up over time from a small initial size up to very large storage capacity, construction times that are drastically shortened, cost savings due to standardizing components, low and reversible environmental impact, ability to use sites that have little to no competing economic value, and more. All of these will need to be examined in order to convincingly show that the bold claims made for this system can be achieved.

Further articles in this series will include:

Part 2. Historical Context

Part 3. No Monolithic Reservoirs

Part 4. Some Storage Basics

Part 5. An Interesting Scenario

Part 6. Powerhouse Components

Part 7. Layout And Plumbing

Part 8. Pressure Regimes

Part 9. Transient Valve Operation

Part 10. A First Look At (Dollar) Costs

Part 11. Bags

Part 12. Water

Part 13. A First Look At Benefits

Part 14. Modeling The Benefits

Part 15. Modeling The Benefits With Storage Added

Part 16. Lessons From Modeling

Part 17. Large-Scale Potential

Part 18. Summary, So Far

and more.

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