Electrical elevators (also known as traction lifts) consume energy and generate energy. When the elevator is going down with a higher load than the counterweight, or up with a lower load than the counterweight, the electrical motor is actually acting as a generator. Of course, due to the overall mechanical and electrical efficiency of the complete system, the generated energy is always lower than the consumed one. Traction elevators’ energy consumption and generation is complicated to determine. Mainly because these are highly dependent on number and duration of trips and the particular load in each case, as well as the actual mechanical efficiency of the system.
There are mainly three alternatives to handle the generated energy of an elevator:
- The most common one would be to waste it (as heat) in a braking resistor. Resistors are activated by the drive and avoid that the excess energy would eventually destroy the drive due to internal overvoltage.
- The second one would be to return it back to the grid. This is accomplished by something known as regenerative drive. A normal unidirectional drive needs to be replaced with a bidirectional (regenerative) one that injects back to the AC grid the excess of energy. This energy needs to be used by any other system in the building at the time that is being generated. In this type of drive, the elevator not only consumes the same as before, it actually consumes more because of the higher standby consumption of the regenerative drives. It is true, energy is not really wasted, and eventually it is used in the building or returned to the grid. Regenerative drives require special filters to grant that the injected energy complies with standards.
- The solution proposed and described in this article STORES the braking energy in supercapacitors (described in detail in the next section) and the elevator itself makes use of this energy, thus making the elevator to consume less. The bidirectional energy storage process is carried out in DC (direct current) thus avoiding additional filtering. This type of solution is depicted in figure 1. It does not require any changes in an existing elevator that already has a drive and it does neither require interaction with the elevator control.
WHAT ARE SUPERCAPACITORS? WHY USE THEM?
Supercapacitors are an energy storage technology that is often compared to batteries, but these two technologies have quite different features. The benefits of supercapacitors are well-established: long lifetime of about 15 years, more than 1,000,000 charge-discharge cycles, the ability to charge and discharge in seconds, high power density, and high reliability, lowering maintenance and replacement costs significantly.
A Kinetic Energy Recovery Systems is in many ways an ideal application for supercapacitors; fast charging and discharging, and the high number of charging cycles are what supercapacitors excel in.
Skeleton Technologies’ supercapacitors are based on the company’s patented raw material, curved graphene, which provides their supercapacitors with higher power density compared to competing products on the market, which in turn leads to a smaller footprint due to less supercapacitors needed to perform the operation.
HOW DOES THE KERS OPERATE?
Every VVVF drive rectifies the three-phase main grid and transforms the 50Hz AC grid into DC (about 560Vdc) to then again transform this DC voltage into an AC one that is variable in voltage and in frequency in order to manage properly the synchronous motor. As it has been mentioned before, supercapacitors are low-voltage storage elements, so an intermediate element is needed to transform voltages in the range of 600V to lower voltage values. If we need to store and use the energy, this element needs to be bidirectional and is known as DC/DC converter. So, in essence, the KERS by Epic power is a piece of power electronics that transforms energy in DC voltage to energy in low DC voltage in and out the supercapacitors.
To make it functional and practical for the application, the converter needs to be:
- Very efficient. As the energy goes back and forth, any losses in the process happen twice. Electrical efficiencies of more than 98% are required otherwise ROI decreases substantially.
- Easy to connect. With plug-and-play functionality. It automatically detects when the motor is generating and stores the energy before it goes to the braking resistor and returns the energy before the drive takes it from the network. If supercapacitors are already full, the energy will still go to the braking resistor, and when supercapacitor energy is depleted, energy will be fully absorbed from the grid. The system also needs to automatically (and smoothly) pre-charge the supercaps when rebooted.
- Compact. The module includes supercapacitors and power electronics all-in-one for best simplicity.
- Multibrand. No adjustments need to be made regardless of the VVVF or elevator manufacturer.
- Flexible and sizable. Not all elevators are the same in terms of power consumed and generated and in terms of the energy needed for an average trip. Therefore, a basic system is designed to transfer a certain power in kW and store certain energy in kJ but it can easily be parallelized to double the power or the energy. Parallelization in DC is very simple and only requires a connection with no need to additional cabling or communications.
The KERS is the result of the combination of Skeleton’s high-power supercapacitor cells and the intelligent power electronics applied to the elevator market from Epic power.
ABOUT EPIC POWER
Epic power designs and produces bidirectional DC/DC converters. Since 2012, they have been engaged in providing specific solutions for the elevator market, integrating energy storage, supercapacitors and batteries as well as solar panels in industrial electronic systems.
ABOUT SKELETON TECHNOLOGIES
Skeleton Technologies is the global leader in graphene-based ultracapacitors and energy-storage systems. We deliver high power, high energy, reliable and long-life storage solutions across the industry. Through the use of patented ‘curved graphene’, we have achieved global breakthroughs in ultracapacitor performance and successfully commercialized our ultracapacitors, in trucks, buses, and grid applications.
Since our foundation in 2009, we have raised over 51 M EUR to support manufacturing scale-up in Germany and in Estonia and grown our headcount from 4 to 115 people.
Our ultracapacitors deliver twice the energy density and 4 times the power density offered by other manufacturers. Our current customer base ranges from leading Tier One automotive firms and industrial equipment OEMs to truck fleet operators and aerospace prime contractors.
 ISO 25745 – Part 2 describes well how to make energy calculations depending on the class (use) of the elevator.
Dr. Pilar Molina Gaudo, CEO, Epic Power
Ants Vill, Vice President of Product, Skeleton Technologies