Lithium-ion batteries rule the world. They are in cell phones, cars, and laptops, among many other products we use on a regular basis. They are lightweight, have high-energy density, and are easy to recharge. The movement of positively-charged lithium ions creates free electrons, which creates a charge of current. That current flows through the device and back to the battery.
Perfectly suited for small devices with short-term use, the problem with lithium-ion batteries is one of scale. The lithium-ion battery in the Tesla Model 3, for example, costs about $190 per kilowatt hour (kWh). The wider industry and several governments are working diligently to see that cost drop to under $100 per kWh. It might one day get there as lithium-ion is going to maintain a strong presence in hardware. It remains the pack leader of the battery world.
Tesla installed the largest lithium-ion battery system at the Hornsdale Wind Farm in Australia in 2017. The 129-megawatt hour (MWh) system has enjoyed service comparable to a coal or natural gas facility. Charged by the wind farm it is paired with, Tesla’s facility can discharge 100 megawatts for 75 minutes. Since it is a chemical system rather than a mechanical system, it can supply this energy practically instantaneously, without any need to ramp up the way spinning generators of traditional fossil-fuel facilities do. Tesla’s facility’s energy sales brought in more than $9 million in the first half of 2018 while the battery itself cost approximately $66 million.
One of the most efficient and effective ways to store energy is through pumped hydro. The method is elegant: shifting water from a low elevation to a high elevation when electricity is cheap, and shifting it back when electricity is expensive, thus recouping stored energy in the process. The problem, however, is obtuse. Such a system is dependent upon an existing type of geography, and while the most suitable sites in the U.S. have already been dammed, the remaining potential sites have been blocked off from major development due to environmental concerns.
While some researchers are reimagining locations for pumped hydro, Energy Vault, a Swiss-Californian startup launched through Bill Gates’s Idealab, is trying to take the efficiency of pumped hydro and bring it to any type of topography. Their utility-scale battery uses gravity power through a system of six cranes that electrically stack heavy bricks made of recycled waste into a tower when electricity is cheap, and release those bricks again to harvest energy when electricity is more expensive.
This is not the first theory for gravity power, and some experts are critical of such a system, but the cheap and effective build that Energy Vault promises means that we should see real-world results soon enough. Energy Vault estimates that they will be able to deliver four megawatts/35 MWh of energy at a lower cost—$200 to $250 per kWh—than any competing energy storage system, and all at 90 percent efficiency.
The Energy Vault system would remain fully automated and robustly durable for decades of self-management. While pumped hydro systems would still produce more energy, Energy Vault’s system imagines itself to be easier to install, cheaper to build, and accessible to anyone, regardless of geography.
Dozens or hundreds of large mirrors, known as heliostats, collect heat from sunlight to create energy. On these solar farms, sunlight is directed towards a central thermal tower that can heat quickly and boil stored fluid, which can, in turn, be used to power a steam turbine. When the sun goes down, however, the heat stops coming, and energy needs must be filled by older utilities like hydroelectric, nuclear, or coal power.
The relatively new idea is to transfer solar heat to molten salt where it can be stored for a longer amount of time and keep solar farms producing energy well past sunset. Molten salt can be heated to 1,000 degrees Fahrenheit, and is highly effective at retaining heat. The Gila Bend facility in Arizona, which was the first U.S. solar farm to use molten salt storage, generated around 250 megawatts (Mw) in its first year of operation.
Three new recently approved facilities in Chile are expected to generate 2,600 gigawatt hours (GWh) of electricity annually. One downside, however, is water usage—thermal storage consumes more water than any other type of power plant, which is particularly a problem when most solar plants are in hot, dry climates where water is scarce.
Alphabet’s moonshot factory, X, has proposed cutting out the solar part of the equation altogether. Its Project Malta, led by Nobel-winning Stanford physics professor Dr. Robert Laughlin, envisions a grid-scale storage system that takes renewable energy and stores it as heat in molten salt and as cold in tanks of fluid similar to antifreeze, where it could be kept for days or even weeks.
The components are inexpensive. They utilize conventional technology and safe and easily harvested natural resources. The molten salt could be charged and recharged many times, for up to 40 years. While specifics are not quite understandable to the layman, the project will move into the real world soon, which might mean a great leap forward for thermal storage.
Redox flow batteries charge and discharge energy through the electrochemical reactions of reduction and oxidation. In practice, these batteries tend to be quite large, often involving shipping containers full of electrolytes (usually vanadium) that then flow into a common area to interact and react to create an electrical charge.
One of the largest flow battery sites is a 2 MW/8 MWh installation in Washington State, housed in 20 shipping containers. But that won’t be the largest for long. In 2016, the China National Energy Commission approved a massive 200 MW/800 MWh flow battery site to be connected to a wind farm in the Liaodong Peninsula.
Nant Energy is taking the idea of redox flow batteries and going smaller, wider, and cheaper. Its zinc-air battery operates in a fundamentally similar way to other flow batteries but replaces vanadium with much smaller components. When the sun is shining and electricity is cheap, photovoltaic solar panels fuel the battery’s conversion of zinc oxide to zinc and oxygen, and the zinc serves as a form of storage for energy. To discharge the energy, the battery system oxidizes the zinc again, generating electrons. NantEnergy’s battery can repeat this charging cycle thousands of times without deterioration.
Shooting for one GWh of production capacity in a full-scale facility planned to open in California in 2019, Nant Energy believes it can lower costs to $100 per kWh once they have achieved 100 MW in scale, making them far cheaper than lithium-ion storage. They have already installed 6,000 zinc-air batteries in nine countries over the last six years, resulting in 55 MWhs of capacity and 1.2 million usage cycles.
More than 100 communities and 200,000 people get 100 percent renewable-based electricity with microgrids backed by Nant Energy, and the product line has expanded to offer hybrid systems that use ultracapacitors and even lithium-ion storage to boost shorter-term power delivery. While there are plenty of alluring conceptual models in grid-scale energy storage, Nant Energy’s real-world adoption and interoperability will continue to turn heads in green tech circles.
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