Nathan Schroeder, a mechanical engineer at Sandia National Labs, arranges landscaping gravel in a thermal energy-storage unit. (Image: Craig Fritz, Sandia National Laboratories)

I first came across the term “energy storage” when I started working in high-power electronics. A focus of the company was pulsers, in which energy was stored in a bank of capacitors and discharged into a load with a high-power electronic switch. Often, it was to trigger microwave radar tubes, like klystrons. But one of the most “entertaining” applications was for a cable fault locator.

New York City has miles of underground power cables, many of them really old and prone to failure. The electric utility has to send people underground to locate the faults and repair them, so it’s helpful to be sure about the fault’s location before descending into a manhole. One technique was to hit the cable with a high-voltage pulse to cause a short circuit at a weak point and listen for the underground bang.

When I was ready demonstrate the system, I was careful to limit the surge current because an energy-storage capacitor could explode. But the engineer was not satisfied — he wanted to send the pulse into a dead short. So, I backed away and triggered it remotely, causing a bang that made me almost jump out of my skin. I’ll never forget his words: “That’s what I was looking for.”

I tell this story because I love telling stories, but I also have a point. There is a great distance between storing energy to make a bang and storing energy to, say, use excess daytime solar energy at night. But they both are energy-storage technologies. There are other means of storing energy as well: thermal, hydro, and mechanical, for example. And there is a whole range of sizes from utility scale to chips for smart phones.

Energy storage has become a “hot” topic in recent years because the burning of hydrocarbons is the major cause of increasing global average temperatures. Unfortunately, many of the non-polluting sources of electrical energy, solar and wind, are intermittent in their outputs — sometimes exceeding demand and, at other times, producing less than is needed. So, an obvious solution is to store some of the energy when the source is abundant and use it later, say at night, when there is no sun.

The other major energy-storage market is electric vehicles (EVs), which are beginning to replace hydrocarbon-burning internal combustion engines.

Most of the press releases about energy storage that show up on my computer screen have to do with making a better battery — increasing the energy density; reducing the charging time; increasing the life; increasing the current surge capacity; and extremely important, improving their safety — especially the fire hazards of Li-ion.

But every once in a while there are completely different approaches — for example, using heated gravel or abandoned oil and gas wells. I tend to be drawn to these less-common technologies — after a while my mind goes into a fog after reading one more article about changing the chemistry of the electrolyte or anode of a Li-ion battery. I know these things are important, but it’s hard for me to distinguish the claims for one chemistry vs. another.

I think we need a more holistic approach — we should start with the general category of energy storage and analyze the pros and cons of different technologies to solve different problems. A good mindset for doing that is to think of what energy storage means.

I like the following definition: Energy storage is a method for storing energy produced at one time to be used later.

So, for example, if you want to heat a single-family home sustainably, you could use a photovoltaic panel to generate electricity to run a heater. Then you would have to decide how to keep the heat on overnight — and batteries might be the best way to do that. On the other hand, the storage medium could be water in an electric heater. Or perhaps the sun could directly heat the water, which would then be stored in an insulated tank for future use. That would eliminate generating electricity as an in-between step. Or how about a geothermal heat pump , which doesn’t depend on sunlight because it uses the earth as the storage medium.

Energy can be stored chemically in batteries; or thermally; or mechanically, with flywheels; or with pumped hydro; or electrically in supercapacitors (designed for storage over time rather than an instantaneous bang).

My theme is that deciding on an energy-storage technology should be analyzed for a particular use case, based on variables such as location, cost of installation and operation, size scale, and the particular needs of the application. For example, batteries probably make most sense for EVs, but even for them, there might also be a role for hydrogen fuel cells or even flywheels.

To sum it up, I think we should start with a high-level view of an energy-storage application and then drill down to the specifics, rather than the other way around.