Energy Storage and our Unpredictable Future

It’s a fine spring day and you decide on a whim to go camping. By early afternoon you’ve reached a sheltered clearing in the woods, the sky is clear, and you relax against a tree trunk rejoicing that “The best things in life are free!” as you soak up the abundant warmth of the sun. As the sun goes down, though, the temperature drops to near freezing, you shiver through a long night, and you resolve to be better prepared the next night.

And so by the time the sun sets again you’ve invested in a good down sleeping bag, you sleep through the long night in comfort due to your own carefully retained heat, and then you greet the cold dawn by cheerfully striking a match to the pile of dry sticks you had gathered and stacked the day before.

In this little excursion you’ve coped with variable energy flows, using technologies that allowed you to store energy for use at a later time. In short, you’ve faced the problems that Graham Palmer and Joshua Floyd identify as critical challenges in all human civilizations – and especially in our own future.

Their new book Energy Storage and Civilization: A Systems Approach (Springer, February 2020) is an important contribution to biophysical economics – marvelously clear, deep and detailed where necessary, and remarkably thorough for a work of just over 150 pages.

The most widely appreciated insight of biophysical economics is the concept of Energy Return On Investment – the need for energy technologies to yield significantly more energy than the energy that must be invested in these activities. (If it takes more energy to drill an oil well than the resulting barrels of oil can produce, that project is a bust.) While in no way minimizing the importance of EROI, Palmer and Floyd lay out their book’s purpose succinctly:

“we want to argue that energy storage, as both a technological and natural phenomenon, has been much more significant to the development of human civilizations than usually understood.” (Energy Storage and Civilization, page 2)

Central to their project is the distinction between energy stocks and energy flows. Sunshine and wind energy – primary energy sources in a renewable energy future – are energy flows. Grains, butter, wood, coal, oil and natural gas are energy stocks. And storage mediates between the two:

“Energy storage deals with the relationship between stocks and flows: storing energy, whether by natural or anthropic processes, involves the accumulation of flows as stocks; exploiting stored energy involves the conversion of stocks to flows.” (page 1)

Our current industrial civilization relies on the vast quantities of energy stored in our one-time inheritance of fossil fuels. These energy stocks allow us to consume energy anywhere on earth, at any hour and in any season. If the limited supplies of readily accessible fossil fuels weren’t running out, and if their burning weren’t destabilizing the climate and threatening the entire web of life, we might think we had discovered the secret of civilizational eternal youth.

Fossil fuels are higher in energy density than any previous energy stock at our control. That energy density means we can ship and store these stocks for use across great distances and long periods. Oil is so easy to ship that it is traded worldwide and is fundamental to the entire global economy.

In particular, fossil fuel stocks can be readily converted to electrical energy flows. And electricity, which is so magnificently versatile that it too is fundamental to the global economy, cannot be stored in any significant quantity without being converted to another energy form form, and then converted back at time of use – at significant cost in energy losses and further costs for the storage technologies.

This is the crux of the problem, Palmer and Floyd explain. The vision of a renewable energy economy relies on use of solar PV and wind turbines to generate all our electricity – plus electrification of systems like transportation, which now rely directly on fossil fuel combustion engines. A major part of the book deals with two closely related questions: How much storage would we need to manage current energy demand with the highly intermittent flows of solar and wind energy? and, Are there feasible methods known today which could create those quantities of energy storage?

Beyond simple technologies like huge tanks or reservoirs of oil and gas, and stockpiles of coal, our current economy has little need for complicated means of energy storage. Batteries, while essential for niche uses in phones and computers, store only tiny amounts of electrical energy. But in Palmer and Floyd’s estimations, to maintain an economy with today’s energy consumption without fossil fuels, we will need to expand “current technologically-mediated storage capacity by three orders of magnitude”. (page 28)

What might a thousand-fold or greater expansion of storage technology look like? Palmer and Floyd provide some excellent illustrations. Pumped hydro storage is one promising candidate for managing the intermittent energy flows of solar PV or wind generators. Where suitable sites exist, surplus electricity can be used to pump water to an elevated reservoir, and then when the sun goes down or the wind calms, the water can flow down through turbines to regenerate electricity. This is a simple process, requiring two water reservoirs that are close geographically but at significantly different elevations, and is already used in some niche markets.

But for pumped hydro storage to be a primary means of managing intermittent renewable electricity production – that’s another story. By Palmer and Floyd’s calculations, to produce half of current US peak electricity demand via pumped hydro storage, the combined water flow from all the upper reservoirs would need to be far greater than the typical flow of the Mississippi River, and closer to the total flow of the Amazon River (depending on the average elevation differences between the reservoir pairs).

Building sufficient battery storage is equally daunting. Palmer and Floyd look at the challenge of converting the world’s gas- and diesel-powered passenger vehicles to battery-electric propulsion. Even after making appropriate allowance for the far greater “tank-to-wheels” efficiency of electric motors, they find that to replace the energy storage capacity now held in the vehicles’ fuel tanks, we would need battery storage equivalent to 142 TWh (TeraWatt hours). As shown in Palmer and Floyd’s illustration below, the key material requirements for that many batteries are vast, in some cases greater than the entire current world reserves. And that is to say nothing of the energy costs of acquiring the materials and building the batteries, or the even more difficult problems of electrifying heavy freight vehicles.

Barring unknown and therefore unforeseeable possible developments in storage technologies that might provide order-of-magnitude improvements, then, it is highly unrealistic to expect that we can simply replace current world energy demands from renewable energy sources. Far greater changes are likely: combinations of changes in technologies, trading practices, regulations, social practices, ways of life. The layers of interacting complexity, Palmer and Floyd argue, are beyond the capacity of computer models to predict.

Their book is a bit of a complex system, too. Although many of the ideas they present are simple and they explain them well, there are sections which go beyond “challenging” for readers who have no more than an ancient memory of high-school-level chemistry and physics. (I plead guilty.) Such readers will nevertheless be rewarded by persevering through difficult parts, because Palmer and Floyd do such a good job of tying all the strands together. The second-to-last chapter, for example, provides a lucid explanation of why the “hydrogen economy” offers real potential for replacing some of the energy storage and transport capacities of fossil fuels – while incurring very significant energy conversion penalties that would have major economic implications.

Civilizations both ancient and contemporary need practices that provide a sufficient Energy Return On Investment – but a high EROI is not sufficient cause for a technology or practice to come into wide use. Rather, we need complete socio-technical systems that provide the right combination of adequate EROI, and adequate and flexible energy storage.

Energy Storage and Civilization is a superb overview of these challenges for the waning years of fossil fuel civilization.

Photo at top by Radek Grzybowski – A stack of wood lays in front of a snowy and foggy forest, Gliwice, Poland; from Wikimedia Commons.