Ultracapacitors in light rail in Arizona?
After we published the blog about ultracapacitors in light rail and regenerative braking, Bonnie Baker from TI asked:
The capacitive array is on the roof of this vehicle. I am wondering what the effects of temperature changes on this stored energy. I know that Oregon temperatures are from moderate to lower temperatures, but in other parts of the country, such as Arizona, the temperatures can easily range up above 100 degrees. Would this vehicle be appropriate there?
We spoke to Maxwell’s application engineer, Chuck Cook, and he told us:
Maxwell works closely with designers to develop a robust design in any type of environment and they will provide the proper charts and graphs to the customer for this effort. In this case the ultracaps should be moved from the top of the vehicle to somewhere lower and out of direct sunlight.
The general rule of thumb is that for every 10oC reduction in temperature the life estimate can be doubled. The first graph below is the rule applied for a Maxwell K2 3000F Ultracapacitor. The specified life at 2.7V at 65 oC is 1500 hours. As you can see from the graph, the life will increase as the average operating temperature is decreased.
The second graph is from Cooper Bussman’s website, and shows a similar trend. As you can see, voltage also has an impact and reducing the operating voltage will increase life. Temperature, however, is the more dominant factor.