Researchers from the Competence Centre Thermal Energy Systems & Process Engineering at the Lucerne University of Applied Sciences and Arts have conducted a series of experiments together with the EuroTube Foundation to create a compact cold accumulator for capsules to be used by Hyperloop, a vacuum train the idea of which was proposed by Elon Musk back in 2013. Ordinary water was used as a material for keeping the system cold and as a coolant: in frozen form, it accumulates cold, and in liquid form, it brings it to equipment elements that need to be cooled.
The need for this system is due to Hyperloop’s peculiarities: in the rarefied atmosphere of the tube, passive cooling is largely ineffective, and the heat from the engines, batteries and electronics can range from 6 kW to tens of megawatts. The cooling system must be lightweight, compact and able to quickly respond to changes in load.
In a CCS (compact cold storage) setup, an 850-mm-long cylinder with a 44-mm internal diameter, the scientists looked into how the shape of ice and the direction of the water flow affect the system’s performance. They tested three shapes: crushed ice 5–30 mm in size (packing density 56%, theoretical capacity 90.8 Wh), 15×30 mm cubes (47%, 81.1 Wh) and a solid block frozen in the tank (98%, 142.2 Wh). They also changed flow directions: top-down, bottom-up and horizontal. Cooling power varied from 100 W to 300 W, and the water delivery temperature was set to 15°C, 25°C or 35°C.
The results showed that flow direction was key. Feeding water top-down provided the highest capacity and the lowest outlet temperature due to natural stratification: cold water (~4°C) sinks down and ice floats up, creating a stable layer.
Crushed ice turned out to be the best in terms of response speed and stability. With a top-down flow, it reached an operating temperature of about 3°C in less than 5% of the experiment time, maintaining it even when power rose from 100 W to 300 W. The utilization factor reached 0.80–0.98, approaching the maximum. The only drawback was a smaller cold reserve compared to a solid block.
Meanwhile, the solid block had the largest capacity, although it responded to load changes more slowly: it took at least 20% more time to reach a stable temperature, and the average temperature at the outlet was higher, about 7°C at 100 W and up to 10°C at 300 W. Under high load, the block sometimes broke away from the walls and floated up, with a layer of cold water forming underneath and acting as a buffer, albeit with an increased temperature.
This is why a combination of crushed ice and a top-down flow turned out to be optimal, providing a fast response and stable cooling. In the future, the researchers plan to select an intermediate fraction of ice that would combine high packing density and a fast response, as well as to test the system on a real scale, taking into account vibrations and a real thermal profile.
Experts believe that studies like this could bring the Hyperloop project closer, as its commercial launch remains a distant possibility. Currently, the project exists mainly in the form of lab samples, pilot sites and preliminary technical and economic calculations. For instance, a demonstration center for the Hyperloop Development Program is currently under construction in the Netherlands, with a 420-m test track. In Munich, a 24-m test tube is already in operation as part of the TUM Hyperloop program, with plans to extend it to 400 m. China has been especially successful in this regard: last year, a magnetic levitation train in a 2-km closed tunnel achieved a speed record of more than 623 kmph.
