Ultracapacitor-battery hybrid energy storage
Energy storage and power management options are many for engineers, across multiple industries and applications. Batteries offer simplicity along with low cycle life, temperature sensitivities, and inefficiencies. Careful analyses of cost, power, and performance of combined battery/ultracapacitor solutions make a compelling case for hybridized energy storage.
Manufacturers evaluating hybrid battery/ultracapacitor energy storage often ask:
- Will it cost more?
- Will dual-storage systems be overly complex?
- What additional electronics will be required to balance two technologies in one component?
- What is the timeline for developing a hybrid solution?
When traditional batteries are paired with ultracapacitors optimized for higher voltage levels:
Ultracapacitors are more attractive than batteries. Price of these components has fallen 99% in the past decade, while battery costs have fallen 30%-40% in the same time period. That trend will likely continue as the price of raw materials falls and the market continues its widespread adoption of ultracapacitors.
While batteries alone are a simpler system, in application they fail to measure up in almost every way to a hybrid component.
In most applications, a primary energy source handles a continuous energy demand. At times, there are peak power demands, and engineers can either size batteries to handle peak demands or use ultracapacitors to bridge the demand. Using ultracapacitors also allows downsizing of the primary energy source.
High-power ultracapacitors provide burst power required by high current demands associated with acceleration, starting, steering, and regeneration. Pairing a capacitor with a battery improves the power density of the hybrid supply, which has the added advantage of allowing the battery to operate without seeing the large current spikes that would be present in the absence of the capacitor.
Extend battery life
A hybridized approach allows a battery to perform better and for longer periods of time when paired with an ultracapacitor. An ultracapacitor enables the battery to do what it was designed to do: provide high-energy density.
For example, an accelerating hybrid vehicle creates an enormous demand for amps (current). Putting ultracapacitors in parallel with batteries and control electronics allows ultracapacitors to provide high current, enabling the battery to become strictly an energy source, rather than an energy and power source.
Since ultracapacitors have a much lower internal resistance and much faster charge rate than batteries, they make battery-powered systems run more efficiently. Ultracapacitors make batteries last longer because they do the brunt of the work when the load is initially switched on and allow the battery to pick up load gradually, preventing high current draws from the battery. Avoiding high current drain on batteries avoids thermal, chemical, and mechanical stresses. Current spike (and internal temperature) reductions extend battery life as much as 400%, depending on the application.
A typical starter battery will degrade quickly if it is required to supply high current for any length of time. So-called deep cycle batteries are designed specifically to supply higher currents, but even such batteries, with thicker lead plates, are not immune from damage due to repeated deep cycling. A parallel configuration of a battery with an ultracapacitor can dramatically reduce the deep cycling of the battery under heavy load conditions. Doing so extends the life of the hybrid power supply and provides a more efficient supply. Hybrid design also can decrease the warranty and replacement cost of the batteries, making the system economically attractive.
Cycles and Celsius
Ultracapacitors also perform under a wider range of climate conditions than a battery, with a comfortable range of 70 ºC to -40 ºC. Batteries claim a range of 60 degC to -20 degC, but at and below zero, batteries lose most of their available energy.
Ultracapacitors also deliver greater return in cycle life. Batteries rely on a chemical reaction to dissipate stored energy and have life of hundreds to low thousands of cycles. Ultracapacitors store energy in an electrostatic field, allowing life of more than a million cycles.
Ultracapacitors offer 95%-98% efficiencies, and lead-acid batteries top out at 70%. Combined ultracapacitors and battery energy storage systems can reduce the size, weight, and number of batteries in a system. Hybridizations are more efficient and use fewer materials. They can also extend the cycle life of the battery component, which makes the whole system “greener.”
While a dual-technology system is not as simple to deal with as a hybrid one, the payoffs are too significant to ignore. In a hybrid energy storage system, both elements work together to help the other, resulting in a more efficient system with a longer, better performing lifespan.
Brendan Andrews is vice president of sales and marketing at Ioxus Inc. and also responsible for education.
At a glance
- Ultracapacitors offer 95%-98% efficiencies, and lead-acid batteries top out at 70%.
- In the last 10 years ultracapacitor prices have fallen 99%; batteries prices have fallen 30%-40%.
- In a hybrid energy storage system, reductions in battery current spikes (and internal temperature) extend battery life as much as 400%, depending on the application.