Solar and wind are playing a greater role in power generation, leading to hundreds of gigawatts worth of new renewable energy generation. However, without effective energy storage techniques, natural gas and coal are needed for times when weather conditions aren’t favourable. Large-scale storage is becoming instrumental if society is to shift away from a world dependent on fossil fuels, and will define the next decade.
Though lithium’s improved manufacturing techniques and economies of scale caused costs to fall by 85% since 2010, it doesn’t mean it’s necessarily economical for other grid applications. It will remain too expensive for most grid-scale applications, it can’t store more than a few hours’ worths of energy, it poses a fire risk, and its ability to hold a charge fades over time.
Overall, the energy storage market is predicted to attract $620 million dollars in investments by 2040. Billions of dollars are being spent to find alternatives to lithium-ion. To address this, entrepreneurs are experimenting with a variety of different solutions in order to achieve long-duration energy storage, as well as reduce costs by 10 to 20 times.
Sodium-sulphur batteries are another alternative to lithium and have already seen significant use at scale in sites around the world. The sodium sulphur battery is a high-temperature battery, and it operates at 300°C and utilizes a solid electrolyte, making it unique among the common secondary cells. One electrode is molten sodium and the other molten sulphur, the reaction between these two is the basis for the cell reaction. In sodium-sulfur batteries, the electrolyte is in solid-state but both electrodes are in molten states. Sodium-sulphur batteries have a longer lifespan than their lithium-ion counterparts, of around 15 years compared to the two or three years expected from lithium batteries. Sodium and sulphur are abundant and inexpensive materials, which mitigates one of the main problems with lithium batteries. In February 2019, Abu Dhabi installed the world’s largest storage battery, which makes use of sodium-sulphur battery cells.
Zinc’s abundant supply, fundamental stability, and low cost make it an attractive alternative to lithium, but efforts to make it commercially viable at scale have been rare. Zinc, which is stable in air and compatible with aqueous electrolytes, presents a low-cost and potentially safer option for rechargeable batteries than lithium and sodium, which typically use flammable organic electrolytes. In many rechargeable zinc-based batteries, a thick zinc foil serves as the anode and source of zinc ions. However, the extra zinc hides inefficiencies in the battery’s charging and discharging which will increase the battery’s cost. NantEnergy’s zinc-air battery system replaces a second electrode with one that “breathes air”, using oxygen from the atmosphere to extract power from zinc.
One of the main alternatives being explored is a flow battery. Unlike lithium-ion, flow batteries store liquid electrolytes in external tanks, which means the energy from the electrolyte and the actual source of power generation are decoupled. With lithium-ion tech, the electrolyte is stored within the battery itself. Electrolyte chemistries vary, but across the board, these aqueous systems don’t pose a fire risk and most don’t face the same issues with capacity fade. Once the manufacturing scaled up, the actual price can be competitive with lithium-ion. Primus’s modular Energy Pod, for example, provides 25 kilowatts of power, enough to power five to seven homes for five hours during times of peak energy demand, and between 12 to 15 hours during off-peak hours. While most systems use multiple Energy Pods to further boost capacity, the company says what sets it apart is its simplified system: instead of two tanks, which every other flow battery has, Primus only has one. They were able to separate the electrochemical species by taking advantage of the density differences between the zinc-bromine and the bromine itself, and the more aqueous portion of that electrolyte.
Also operating in this space is ESS Inc, an Oregon-based manufacturer of iron flow batteries, founded in 2011. Its systems are larger than Primus Power’s; Batteries are in a shipping container and they can provide anywhere from 100 kilowatts of power for four hours to 33 kilowatts for 12 hours, using an electrolyte made of iron, salt, and water. So far, ESS has at least six of its systems, called Energy Warehouses, operating in the field and plans to install 20 more. It’s also in the process of developing its Energy Center, which is aimed at utility-scale applications in the 100 megawatts plus range. That will be 1000 times more power than a single Energy Warehouse.
Nevertheless, flow battery companies like Primus and ESS Inc still aren’t really designed to store energy for long periods of time (days/weeks). Many of the flow battery technologies still suffer from the same fundamental materials cost challenges that make them incapable of getting to tens or hundreds of hours of energy storage capacity.
Currently, about 96% of the world’s energy storage comes from Pumped Hydro technology; excess energy on the grid is used to pump water uphill to a high-elevation reservoir. Once there’s energy demand, the water is released, driving a turbine as it flows into a reservoir below. This requires land, disrupts the environment, and can only function in very specific geographies. Energy Vault, a gravity-based storage company founded in 2017, was inspired by the concept. Instead of moving water, Energy Vault uses cranes and wires to move 35-ton bricks up and down, depending on energy needs, in a process automated with machine vision software. The company has a system tower crane utilizing excess solar or wind to drive motors and generators that lift and stack the bricks in a very specific sequence. When power is needed from the grid, that same system will lower the bricks and discharge the electricity. This system is sized for utility-scale operation, and the company says a standard installation could include 20 towers, providing a total of 350-megawatt hours of storage capacity, enough to power around 40.000 homes for 24 hours.
Thermal storage, on the other hand, has the potential to store energy for longer than flow batteries with a smaller footprint than gravity-based systems. Excess electricity on the grid is used to heat up cheap carbon blocks, which are insulated inside a container. When needed, that heat is converted back into electricity using a heat engine, which would be a steam or gas turbine. Berkeley, California-based Antora Energy, founded in 2017, has developed a novel type of heat engine called a Thermophotovoltaic heat engine (TVP), believed to be solving a need that is currently and will continue to be unmet by lithium-ion batteries, and that will enable the next wave of integration of renewables on the grid. TVP is a solar cell, but instead of capturing sunlight and converting that to electricity, it cell captures light radiated from the hot storage medium and converts that to electricity.
There are other creative solutions. For example, Toronto-based Hydrostor is converting surplus electricity into compressed air, and U.K. and U.S.-based Highview Power is pursuing a cryogenic storage plant, using excess energy to cool down air to the point where it liquefies.
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