Published on February 4th, 2019 | by Dr. Maximilian Holland
February 4th, 2019 by Dr. Maximilian HollandÂ
Ultracapacitors store electrical energy, like batteries, but rather than electro-chemically (batteries), they store the energy electro-statically. Thereâ€™s also a notable difference in balance between energy density and power density. Lithium-ion batteries have energy density typically in the 150â€“250 Wh/kg range, and power density in the 250â€“350 W/kg range. Maxwellâ€™s current commercial ultracapacitors, such as the DuraBlue range pictured above, have much lower energy density of 8â€“10 Wh/kg (around 5% that of lithium-ion), yet much higher power density of 12â€“14 kW/kgÂ (around 45Ă— that of lithium-ion).
In the context of EV applications, this means that a 50 kg array of ultracapacitors could potentially input or output 650 kW of burst power (although, at 0.18 kWh, this would last just a second or so). Lower power levels would obviously be sustained for proportionately longer. For context, thatâ€™s about twice the power that the Tesla Model 3 Performanceâ€™s 480 kg battery pack is currently tuned to provide (331 kW).
If the economics made sense, a modest ultracapacitor array could work alongside the battery pack as a cache of energy, to reduce the load on (and/or work in parallel with) the main battery during short bursts of hard acceleration or strong regenerative braking. Since ultracapacitors can perform reliably over hundreds of thousands of cycles, this could also reduce the cycling load on the lithium-ion pack, and potentially allow it to have a chemistry that prioritizes energy density over power density. The round-trip energy efficiency of Maxwellâ€™s ultracapacitors is in the 80% efficiency range, which is pretty decent (lithium-ion is 80 to 90%).Â In a mid-2018 conversation with the San Diego Business Journal, Maxwell reported having already sold 6.1 million ultracapacitors to automakers.
With their extremely fast response, high power density, and high cycle durability, ultracapacitors also have applications in fast-response stationary storage applications and grid load balancing (read more about Maxwellâ€™s case studies of these).
Another interesting technology that Maxwell has developed is its dry battery electrode manufacturing process. Maxwell believes it has potential to lower traditional battery manufacturing costs:
â€śWe believe that our patent-protected, proprietary manufacturing process, which has been utilized through many years of ultracapacitor production, can be applied to the manufacturing of battery electrode without the use of solvents to produce a highly reliable electrode material with uniform characteristics resulting in enhanced product performance, long-term durability,Â and lower manufacturing cost.â€ť (Maxwell Annual Report, 2017)
Maxwell undertook proof of concept pilot testing between 2016 and 2017 with an automotive OEM and tier 1 supplier, which the company believes â€śhas demonstrated the significant performance and cost advantages of our dry electrode manufacturing process compared with wet electrode manufacturing, while providing the required consistency and reproducibility in manufacturing a pilot-scale dry electrode roll.â€ť (Maxwell Annual Report, 2017)
You can see Maxwellâ€™s other claims for the technology in the above presentation slide, from the Needham investor conference in mid January this year. Maxwellâ€™s 2017 annual report claims that, â€śThe dry electrode can be further applied toÂ advanced battery chemistries, offering well over 300 Wh/kg at the cell level.â€ť Itâ€™s not clear whether these energy densities are enabled by their technology, or are simply compatible with the technology â€” the above slide appears to suggest the technology has a direct bearing on energy density. The claims of 2Ă— durability improvement and 10â€“20% cost reduction will also no doubt have interested Tesla.
The ultracapacitor technologies and/or the dry battery electrode technologies could have been the attraction for Tesla. Given Teslaâ€™s deep investment in lithium-ion battery production, for both EVs and stationary storage, the potential cost savings and performance benefits from the dry battery electrode process is clearly interesting.
The ultracapacitors also have potential for both stationary and mobile applications. The response speed, power density, and robustness would certainly make sense in heavy-duty grid applications, likely as a fast and powerful energy cache used alongside lithium-ion storage. Although itâ€™s not so clear that thereâ€™s a significant need or benefit for ultracapacitors in Teslaâ€™s passenger EV applications (beyond potentially enabling a different balance of lithium-ion cell characteristics, as mentioned above), the Tesla Semiâ€™s heavy-duty use case may make more sense for employing an ultracapacitor cache. There even may be a case for using them in the Roadster for burst power.
It will be very interesting to see how Tesla leverages Maxwell Technologies. Readers will no doubt have many ideas about how Tesla will benefit from this acquisition. Please do jump in and share them in the comments.Â