Batteries won’t do for renewable energy storage
The world needs to find a way to store renewable energy as we know the future will rely on it. Even disregarding environmental concerns, the fact is fossil fuels are finite resources. While nuclear power is still practical, renewable energy will likely make up most of the world’s supply.
But many renewable energy sources, including solar and wind, have a serious drawback: they cannot produce energy on demand. Solar farms only harvest energy when the sun is shining. Windmills only generate electricity when gusts are strong enough—about 6.7mph or faster.
To provide electricity when people need it, we must store the energy produced by renewable sources. A solar farm might produce 200MWh of power in a day, but only during the sunny hours. To meet demand for the rest of the day and night, half (or more) of the power generated will require storage for at least a few hours.
The best method for storing renewable energy is a matter of debate. This article explores the options.
Battery storage
Batteries of various types can store energy in an efficient (80-90% for a charge/discharge cycle) and compact manner. But those are the only benefits of battery storage. In every other respect, batteries are a poor choice for storing energy at this scale.
The most glaring issue with batteries, from a financial perspective, is their expense. A battery array large enough to store the energy from a solar or wind farm could cost more than the solar panels or windmills themselves. And that is not a one-time cost, as batteries have a brief service life and require eventual frequent replacement.
The situation from an environmental perspective is even worse. Most modern battery types require heavy metals and other toxic materials. Not only are elements like cadmium and lithium finite resources, just like fossil fuels, but they also pose a significant risk to the environment—both during the mining process and after use.
It is difficult to make batteries practical for storing energy at this scale, but there are alternatives. A battery stores chemical energy, which is just one form of energy. Other forms of energy, such as kinetic and thermal energy, are easier and cheaper to store in large amounts.
Hydro storage
One method for storing kinetic energy is a pumped hydro-storage system almost identical to modern hydroelectric dams. Instead of harnessing the natural flow of water down a river, this system moves water as necessary to store potential energy.
To understand how this works, imagine two lakes. One lake sits at a higher elevation than the other. When the solar or wind farm outputs more energy than its service area requires, the excess energy powers pumps that transfer water from the bottom lake to the top lake. Then, when the farm cannot meet current demand, water flows back down from the top lake to the bottom lake through a conventional hydroelectric generator.
Hydro-storage system
This system has many advantages. It can deliver greater than 80% efficiency, is cost effective, uses tried-and-true machinery, and poses little risk to the environment after the initial construction. It is also capable of storing vast amounts of electricity in the form of potential kinetic energy and does so in a safe manner.
There aren’t many disadvantages, but those that do exist are sometimes prohibitive. Such a system requires a large volume of water, which many regions lack. While it is possible to concurrently use the water for other applications, such as typical municipal use, some areas do not have enough water to store the required energy.
For a hydroelectric storage system to work, one water reservoir needs to be at a higher elevation than the other. In flat regions, this would require building tall water towers or digging deep storage reservoirs. The cost of such construction might still be less than a battery array, but it isn’t trivial.
Finally, freezing weather is also a concern. The water reservoir itself wouldn’t freeze solid—even in the coldest inhabited regions, ice rarely exceeds more than two feet in thickness. But ice flows can cause issues for the hydroelectric machinery that are expensive to solve, as we see in today’s hydroelectric dams.
Mechanical storage
It is possible to store potential kinetic energy in a mechanism, with the simplest example being a spring. When you stretch or compress a spring, you store some of the expended energy as potential kinetic energy to utilize later.
It isn’t necessary to build a series of gigantic springs to store energy, because the same concept applies to many other mechanisms. If, for example, you hoist a bag of sand off the ground with a pulley and tie the rope off, you are storing the potential energy of gravity pulling on the bag of sand.
One method of applying this on a large scale looks a lot like a cable trolley. When there is surplus power output available, an electric motor winches the cable and pulls the “trolley” (a train car laden with cheap, heavy material, like sand) up a hill. When power output can’t meet demand, the trolley rolls back down the hill. As it does, the cable spins the electric motor, which acts as a generator to produce electricity.
A similar “gravity battery tower” system proposed by Energy Vault has efficiency up to 90%, making it competitive with lithium-ion batteries and pumped hydro storage at a fraction of the cost. This has a levelized cost of storage (LCOS) below $0.05/kWh, compared to $0.30/kWh and $0.17/kWh for lithium-ion batteries and pumped hydro storage, respectively.
Gravity battery tower
Gas compression storage
A related, but distinct, method involves compressing gas. A powered pump compresses the gas, which can be plain air, into a storage tank. In many proposed systems, natural caverns act as massive storage tanks. When demand is high, gas released from the storage tank spins a turbine to generate electricity.
The cost of compressed air energy storage is very low when natural caverns are available. That and its high efficiency make it one of the most practical storage methods on this list.
Thermal storage
Steam engines drove the industrial revolution and the physics behind them could revolutionize renewable energy storage, too. There are many ways to convert heat into useful electricity, from steam engines to modern thermoelectric generators that rely on temperature differentials.
But to take advantage of thermal energy in this context, one must store heat for an extended period. It is difficult to keep heat from dissipating through convection or conduction. For that reason, most thermal storage techniques have poor efficiency.
These techniques rely on heating a material and insulating that material as much as possible. That material can be something as cheap and ubiquitous as gravel or sand, which retain heat well. While the efficiency of thermal storage is bad (one estimate puts it at 60-65%), it has great real-world potential thanks to how affordable and scalable it is.
Of the methods on this list, thermal storage is the cheapest by a wide margin.
Hydrogen storage
Hydrogen is a chemical energy source, but it offers many advantages over batteries for storing large amounts of renewable energy. A battery is costly and has a high environmental impact; hydrogen is a simple gas that is storable in a conventional tank or the same kinds of natural caverns proposed for compressed air.
Producing hydrogen is easy through electrolysis. All one needs is H20 (water) and electricity. With power from a solar or wind farm, electrolysis converts H20 into hydrogen and oxygen. The oxygen can vent to the atmosphere or be collected for other purposes. The hydrogen becomes a combustible fuel for powering generators.
When hydrogen burns, it bonds with oxygen to form water once again. This means that there is little harm to the environment and that the water used during the electrolysis process isn’t “wasted.”
Hydrogen storage is even less efficient than thermal storage at 18-46%. It is also quite expensive, even when stored in natural caverns. The major benefit of hydrogen is that it is a combustible fuel. That means it is useful for much of the machinery already in use today.
Moving forward
These energy storage methods show that the primary challenge for renewable energy—storage—is practical to overcome. As the world phases out fossil fuel, these storage techniques will become commonplace as a necessary component of the power grid.
Energy storage comparison
Some of these have yet to be proven at a large scale, but all of them rely on solid science and equipment that is readily available today.
However, one of these methods is already in widespread use today. Pumped hydro storage was first used in the late 19th century and, according to the U.S. Office of Energy Efficiency and Renewable Energy, today accounts for “93% of all utility-scale energy storage in the United States.”
The other methods may not yet be in widespread use, but each has a purpose and could fill a niche when pumped hydro storage is impractical.
