Jason Lee, Global product manager, supercapacitors, Eaton
Warehouse automation systems include robots, high speed shuttles and pallet movers used extensively in warehouses, storage facilities, and manufacturing plants to pick, transport, and place materials at various locations within a building. Efficient use of power throughout these systems ensure logistics companies provide efficient and rapid service.
Using highly efficient supercapacitors can help reduce operational costs through regenerative systems for material handling systems. Expanding e-commerce and shorter delivery times are needed to remain competitive and has increased the need to develop distributed supply chains. This is driving construction of industrial warehouses with automated storage and retrieval systems (ASRS) using automated guided vehicles (AGVs), pallet or carton shuttles and other electrically powered equipment. In addition to their high efficiency, supercapacitors have long life times – typically more than 10 years and/or 1 million charge/discharge cycles – and can recharge very quickly.
Secondary batteries, like Lithium ion, are typically considered for these applications but they pose several challenges for reliability and efficiency in this area. While they have a high energy density, they have a short operating lifespan – a maximum of 1 – 3 years and/or a less than 5000 full-cycle lifespan with constant usage. Most importantly, they operate at relatively slow charging rates, and suffer degradation with wide variations in temperature.
Outdoor systems can present challenging high temperature operating conditions for many electrochemical energy storage solutions while indoor freezer warehouses can render many battery technologies ineffective for shuttles. Supercapacitors offer operating temperatures from -40 °C up to +65 °C that can overcome these challenging conditions to provide optimal performance in virtually all material handling environments. They deliver reliable and efficient power for lifting and acceleration performed by material handling machines in several industries.
What are supercapacitors?
Supercapacitors are high reliability, high power, ultra-high capacitance energy storage devices utilizing electric double layer capacitor (EDLC) construction. Asupercapacitor module has a distinct number of individual supercapacitor cells that are assembled in series or parallel within a single suite resulting in higher voltage or increased capacitance. The modules offer a high-power density solution with millions of charge/ discharge cycles. Supercapacitors are maintenance-free with design lifetimes up to 20 years.
Discharge characteristics – design considerations
Supercapacitor modules are intended as energy storage with a sloping DC voltage curve in either constant current or constant power discharges. Example constant power and constant current discharge curves can be seen in Figure 1a and Figure 1b respectively. When determining required capacitance, and resulting ESR, for an application, it is important to consider both the resistive and capacitive discharge components. In high current, rapid pulse applications, the resistive component from module ESR can be the most critical. In low current, longer duration applications, the capacitive discharge component is the most critical. The formula for the voltage drop, Vdrop, during a discharge at I current for t seconds is:
Vdrop = I * (R + t/C)
To minimize voltage drop in a rapid pulse application, use a supercapacitor module with lower resistance (ESR value). To minimize voltage drop in a low current application, use a supercapacitor with large capacitance (C value).
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Figure 1b. Example voltage and current discharge curves for 500 A discharge from one module with 56 V float voltage
The EDLC and low ESR of the cells allow for ultra-fast charging – typically under 60 seconds – making them particularly beneficial in machines that perform regular and repeated operations.
OEMs can achieve customizable sizing for a wide range of applications with low-profile, configurable nature of modules. Rapid recharge and high-power density can help shrink footprints and provide longer lifetimes with reduced maintenance costs.
Series and parallel configurations
Individual supercapacitor modules have voltage ratings described earlier. As many applications require higher voltages, supercapacitor modules can be configured in series, a module strings, to increase the working voltage. Similarly, supercapacitor modules can be configured in parallel to meet application needs for current or power over the desired timeframe. To assist in determining the number of modules in series and those subsequent strings in parallel, Eaton has developed a calculator tool containing all supercapacitors. This calculator provides high level high level electrical characteristics (at data sheet end-of-life conditions) and matching these characteristics to various supercapacitor configurations / alternatives. The requested information will require the following information:
• Discharge type (constant power or constant current)
• Power or current required (based on above selection)
• Discharge time
• Maximum system voltage
• Operating voltage
• Minimum system voltage
Figure 2 is an example output from the supercapacitor calculator that illustrates the discharge curve in a constant power application using the XLM-62R1137A-R with the Figure 3 user inputs.
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Figure 2. XLM-62R1137A-R calculator output using Figure 3 inputs
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Figure 3. Calculator user inputs example
Powering pallet movers
Recognizing these benefits, one of the largest global material handling OEMs has integrated Eaton’s compact modules. The standard system uses three modules in series to incorporate into a 48 V distribution system. Depending on the individual system and project, additional strings of modules are paralleled to meet heavier weights or longer distances than standard systems, providing easy scalability. Eaton’s supercapacitors provide them with a simple ‘plug-and-play’ installation with internal wiring, minimal maintenance, and no external battery management system required.
Supercapacitors are powerful, reliable, and ultra-high capacitance energy storage devices. Its advanced technology allows supercapacitors to offer a wide variety of benefits to modern material handling and warehouse automation systems. With their fast discharge and recharge ability, long lifespan, maintenance free operation, and wide operating temperatures, supercapacitors provide industrial automation operators and logistics companies the ability to deliver efficient and rapid service.