The supercapacitor could be set for a starring role as grid operators look to smooth loads. The short-duration electrochemical energy storage device — which looks much like a chubby battery — can deliver large amounts of power, without some of the complications of existing battery technologies.
Despite only playing a minor role in the energy transition so far, supercapacitors are being touted as a potentially key technology for regulating grid voltage, frequency, and harmonics, all of which are vital in avoiding blackouts. The application is one of several expected to drive a 30% compound annual market growth for supercapacitors between 2021 and 2030, according to the U.S. Department of Energy.
In 2019, Tesla bought a supercap manufacturer called Maxwell Technologies, causing an initial flurry of interest — but that purchase was ultimately more about Maxwell’s dry electrode technology manufacturing process than the supercapacitors it was making.
Today, though, vendors report growing interest in combining supercapacitors with devices called static synchronous compensators, or STATCOMs, to simulate the action of thermal power plant turbines. The large rotating mass of a turbine helps keep grid voltage and frequency in check, but turbines are being lost as coal and nuclear plants disappear from a growing number of grids.
STATCOMs can address this issue by using reactive power, which is the power flowing between an AC circuit’s phase conductors and the neutral conductor, to correct voltage and harmonics. However, their use comes at a price, since reactive power reduces the efficiency of electricity transmission.
Grid operators are trying to claw some of that efficiency back through measures such as dynamic line rating. But some argue that the better option is to cut the need for reactive power altogether, which can be done by combining STATCOMs with active power from energy storage.
Active power nudges can correct voltage and frequency by simulating the inertia provided by rotating masses, resulting in lucrative ancillary market revenues for battery plants and stimulating interest in so-called eSTATCOM plants that combine STATCOMs with storage.
But the question is which technology to use: lithium-ion batteries or supercapacitors? While widely available and low-cost lithium-ion batteries are an obvious choice for arbitrage and ancillary services, experts say the technology is ill-suited to applications where discharge times are often measured in seconds rather than minutes or hours.
“If you need 300 megawatts for two seconds, it becomes unfeasible to use a lithium-ion battery due to the high-power nature of the application,” said Lilli Hamari, sales manager at Skeleton Technologies, which makes supercapacitors in the Sachsen region of Germany.
Beating lithium-ion
There are other reasons for choosing supercapacitors over lithium-ion batteries for short-duration applications. Supercapacitors do not suffer from thermal runaway problems, are not in demand for tasks such as vehicle electrification, and don’t rely on materials such as lithium or cobalt that could face supply chain shortages.
Furthermore, because supercapacitors store energy in electric fields instead of chemical reactions, they can outgun lithium-ion many times over in terms of charging times and cycling rates. Skeleton claims its supercapacitors can go through more than a million charge-discharge cycles, compared to around 5,000 for lithium-ion.
“For mixed applications with lots of fluctuations, the supercapacitor is mechanistically the better choice,” said Hugh Sutherland, a consultant who has worked with U.S. supercap developer Carbon-Ion Energy.
Power technology vendors seem to agree. In February, Hitachi Energy announced what is thought to have been the world’s first supercapacitor-based eSTATCOM announcement, supplying two systems totaling 300 MW of power to the German state-owned transmission system operator TransnetBW.
“This pioneering technology supports the energy transition by strengthening and stabilizing power grids handling larger amounts of renewable energy,” said Hitachi Energy in a press release.
Other grid equipment makers touting eSTATCOM systems include Siemens Energy, which is an investor in Skeleton, and GE Vernova, which unveiled a STATCOM-plus-supercapacitor combo in August. With such products only recently having made it to market, it could be some time before eSTATCOM systems go mainstream. But TransnetBW’s units, which are due to be commissioned in 2026, could be the first of many.
TransnetBW is one of four German grid operators jointly publishing eSTATCOM standard ratings that they said are due to be applied “to all new projects as soon as possible.” And Hamari said other German operators such as 50hertz and Tennet are also investing in eSTATCOM systems.
Growing global demand
Germany appears to be leading the introduction of supercapacitor-based eSTATCOMs, which have yet to take off in the same way elsewhere. In the United States, for instance, grid operators CAISO and PJM told Latitude Media they had yet to see the technology being used on their networks.
However, Hamari said Skeleton was working with an unnamed U.S.-based system integrator and had seen interest from grid operators in Eastern Canada. And Skeleton’s vice president of government affairs and strategic partnerships Arnaud Castaignet said that demand from North America “is already growing fast.”
“Massive investments in grid infrastructure in that region are expected in the next five years and beyond,” he said.
Skeleton has so far been primarily focused on providing supercapacitors for transportation, but by 2030 the company expects two-thirds of its business to come from products used for grid and data center applications.
The company is expanding operations to cope with the uptick in demand. Alongside its current production capacity of 300,000 supercapacitor cells a year, or roughly 164 MW, Skeleton is also investing more than $223 million in Europe’s largest supercapacitor factory, near Leipzig in Germany.
The new plant is due to enter operation next year with an initial capacity of 4.4 million cells or 2.4 gigawatts per year. That has the potential to expand to 12 million cells or almost 6.6 GW. Such volumes would represent a major boost for a technology that has had a relatively low profile in the energy transition to date.
Today, Hitachi’s head of power and grid quality, Marco Berardi, confirmed the use of supercapacitors “is growing, especially in Europe,” due to rising levels of renewable energy on power grids.
“The attributes of supercapacitors to provide high amounts of power for short periods of time is especially applicable for stability-related services in the transient stability time domain of a few seconds or less,” he said. “Interest is skyrocketing.”
