Energy storage is the key component for creating sustainable energy systems. Current technologies, such as solar photovoltaics and wind turbines, can generate energy in a sustainable and environmentally friendly manner; yet their intermittent nature still prevents them from becoming a primary energy carrier. Energy storage technologies have the potential to offset the intermittency problem of renewable energy sources by storing the generated intermittent energy and then making it accessible upon demand. In addition to energy grid applications, energy storage technologies also have the potential to transform the transportation system. Functioning energy storage devices could replace the powertrain systems of current transportation technologies from a chemical fuel-based powertrain into an electricity-based powertrain. The electric car is a prime example of how energy storage technologies can transform the transportation system into a more sustainable model. Electronic devices, which have become ubiquitous in modern society, are also heavily reliant on energy storage technologies. The breadth of products and industries which energy storage affects shows how valuable advances and breakthroughs in this field will be in the future.
Currently, the dominating energy storage device remains the battery, particularly the lithium-ion battery. Lithium-ion batteries power nearly every portable electronic device, as well as almost every electric car, including the Tesla Model S and the Chevy Volt. Batteries store energy electrochemically, where chemical reactions release electrical carriers that can be extracted into a circuit. This can be illustrated with the example of the lithium ion battery: During discharge, the energy-containing lithium ion travels from the high-energy anode material through a separator, to the low-energy cathode material. The movement of the lithium releases energy, which is extracted into an external circuit. When the battery is charged, energy is used to move the lithium ion back to the high-energy anode compound.
Schematic of Li-Ion Battery – Image from Azom.com
The charge and discharge process in batteries is a slow process and can degrade the chemical compounds inside the battery over time. As a result, batteries have a low power density and lose their ability to retain energy throughout their lifetime due to material damage.
The supercapacitor uses a different storage mechanism. In the supercapacitor, energy is stored electrostatically on the surface of the material, and does not involve chemical reactions. Given their fundamental mechanism, supercapacitors can be charged quickly, leading to a very high power density, and do not lose their storage capabilities over time. Supercapacitors can last for millions of charge / discharge cycles without losing energy storage capability. The main shortcoming of supercapacitors is their low energy density, meaning that the amount of energy supercapacitors can store per unit weight is very small, particularly when compared to batteries. Additionally, the cost of supercapacitor materials often exceeds the cost of battery materials due to the increased difficulty in creating high-performing supercapacitor materials, such as graphene. However, recent advances in creating new supercapacitor materials and improving material production methods may soon bridge the energy density gap for some commercial applications.
Energy Density & Power Density of Energy Storage Technologies – Image from Wiki Commons
The two technologies in this energy storage war, the battery and supercapacitor, remind me of the turtle and the hare from the famous fable. The battery represents the turtle as a slow and steady energy supplier for large energy demands, and the supercapacitor represents the hare that charge and discharge quickly for low energy demands. Yet, instead of viewing them as competing technologies, it may be more useful to view them as complementing technologies: Supercapacitors from Maxwell Technologies (NASDAQ: MXWL) are currently being used in Chinese hybrid buses. The supercapacitors are charged when the bus breaks and then discharged to help the bus accelerate. Yet, the supercapacitors alone cannot sustain the energy needs of the bus. Tesla’s electric cars operate completely on batteries, but the weight of the battery significantly limits the range of the car and most electric power is used during acceleration. If the two technologies are combined, an improved electric car would accelerate using supercapacitors, which would decrease the amount of batteries required, decrease the weight of the car and thereby extend the range. In this case, the turtle and the hare are not competing in a race, but working together to reach a common goal.
Hybrid Supercapacitor Bus in Shanghai – Image from MIT Technology Review