Finding the sweet spot for hybridization

This year, Maxwell Technologies announced the deployment of two new ultracapacitor energy storage demonstration projects. In January, we announced the installation of a 1,000 kW/33 kWh (2.0 min) system on a wind farm in northern China with China Guodian Corporation. Then in March, we announced the installation of a 277 kW/8 kWh (1.7 min) system in a photovoltaic-linked sub-station in North Carolina with Duke Energy. In both of these demonstrations, the ultracapacitors were installed along with batteries to produce a hybrid energy storage system aimed at smoothing the power fluctuations of the nearby renewable power source. With these installations, we intend to demonstrate the lifecycle cost savings of hybrid systems over conventional battery-only systems.

Currently, neither system has been operational long enough to make any claims about demonstrated lifecycle costs. However, the hybridization approach and areas of expected cost savings for both systems can be understood by comparing them to published data, such as “Electrical energy storage systems: A comparative life cycle cost analysis” by Behnam Zakeri and Sanna Syri of Aalto University. The authors reviewed over 200 articles with published cost information on energy storage projects. They then fit that cost data into a lifecycle cost model for more than a dozen energy storage technologies, including pumped hydro, batteries and flywheels.

Using the same methodology, the cost structure of the ultracapacitor portion of these demonstration projects was added onto this data set, to illustrate some insights that drove the chosen technologies and hybridization approaches. The chart below shows the lifecycle cost of selected grid-tied energy storage technologies versus project-rated discharge time using the Zakeri model.

Click here for larger image

As shown, for storage systems that are only cycled once a day, battery-type energy storage, such as lithium-ion batteries (LiB) and lead-acid batteries (PbA), provide the most cost-effective solutions for discharge times greater than 1,000 seconds (about 15 minutes). Flywheels (FW) fall in the middle, and ultracapacitors are best for discharge times less than 80 seconds (about 1.5 minutes). This helps explain the technologies chosen in the Guodian and Duke projects, both of which pair a multi-hour-rated battery energy storage system with an ultracapacitor system with a few-minute rating. In this way, each energy storage system is sized to minimize lifecycle cost and the controller is tasked with diverting short discharge power requests to the ultracapacitors and longer ones to the batteries.

Even more interesting results come when we analyze lifecycle cost for a system cycling 100 times a day (or about once every 15 minutes). Looking at these results in the chart, we see that while the lifecycle costs of battery systems greatly increase with cycling, the ultracapacitor system shows virtually no increase at all. This is due to the nature of ultracapacitors storing energy electrostatically, eliminating the chances of worn-out mechanisms directly associated with charge transfer. The net result is that ultracapacitors end up having a lower lifecycle cost than batteries for discharges of all durations, if cycled more than 100 times a day.

This result drives the final hybridization strategy for the demonstration projects for which the controller is tasked with diverting all high frequency discharge demands to the ultracapacitor system to prevent wearing out the batteries and keeping lifecycle costs low. Of course, these are just projections based on backward-looking datasets. The truly interesting results will come from the demonstration projects themselves as they continue to hone their hybridization control strategies and collect real operational data and lifecycle costs. Until then, I hope this has been directionally informative in regards to the technology selection and hybridization approaches of our recent projects.

1 comment on “Finding the sweet spot for hybridization

  1. ChristopherJames
    October 10, 2018

    While it's good for us to explore the technology and circuitry for hybridization, I can't help but feel it's a bit redundant. How long will we be in this state of limbo between 2 energy sources until we start to look at conversion entirely to one or the other in order to be more efficient? And when that happens, will the equilibrium change as we become more efficient in using one more energy source over the other?

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