The world needs more and more electricity - and power lines. However, there is less and less space for new lines. New types of superconductors can bridge bottlenecks.
When electricity flows through an ordinary cable, it has to overcome the electrical resistance of the cable. This eats up some of the energy. Waste heat and electric fields are also generated as undesirable by-products. All of this can be avoided with superconductivity. At temperatures close to absolute zero (-273 degrees Celsius), the metallic superconductors discovered in 1911 no longer have any resistance and the current can flow without loss. However, cooling at such low temperatures requires a lot of energy.
Superconductivity saves space and transformers
Superconductors made from special ceramic materials offer this property even at a higher temperature. This is close to the boiling point of liquid nitrogen (-196 degrees Celsius). These are referred to as "high-temperature" superconductors. The Nobel Prize in Physics was awarded in 1987 for their discovery. Cables made from these materials can be used to solve a whole range of problems when conducting electricity.
For example, the considerable transformation losses that are added to the transmission losses: high voltage is required for conventional overhead lines in order to transmit the energy with the lowest possible current flow. Electricity from wind and solar parks is therefore first transformed up when it is fed into the grid and then transformed down again for the consumer.
If you want to use solar power for the electrolytic production of green hydrogen, for example, you lose some of the original energy during the transformation. With a superconductor connection, on the other hand, the current flow is irrelevant; the current flows without transformation and loss-free from the solar park to the electrolyser, regardless of whether the voltage is high or low.
Planning without distance rules
In other places, superconductivity can make new power lines possible in the first place. Conventional high-voltage lines require a lot of space, both above and below ground. In densely populated regions planning quickly comes up against the applicable distance rules. New lines cannot be laid everywhere, even underground, due to their waste heat and the unavoidable electrical fields. Thanks to the new technology, electricity suppliers can, therefore, plan sections with superconductors at critical points in order to obtain a permit at all.
The crux of the matter with superconductors is cooling. It should not consume more energy than is saved through loss-free transmission. The current-carrying capacity of high-temperature superconductors increases as the temperature falls. Messer has, therefore, developed a cooling system that provides the liquid cooling nitrogen at a temperature below its normal boiling point (-196 degrees Celsius). It can be lowered to close to the nitrogen freezing point (-210 degrees Celsius). The first system of this type was put into operation in 2014.
The principle of hypothermia
In this existing system, the superconducting cables are located in vacuum-insulated pipes (cryostats) through which the cryogenic liquid gas flows. The vacuum insulation can shield the cold from the warmer environment very effectively, but not completely. Continuous aftercooling is, therefore, necessary. A circulation pump feeds liquid nitrogen into the cryostat for this purpose. The gas is referred to as "supercooled" because its temperature of -206 degrees Celsius is well below the boiling point.
At the other end of the cable section the slightly warmer liquid nitrogen is diverted and returns to the pump via a return line. The nitrogen then flows through a heat exchanger built into the subcooler, where the absorbed heat is dissipated.
In the subcooler liquid nitrogen from the tank is used to generate cold. It is allowed to evaporate at negative pressure. This produces an operating temperature of -209 degrees Celsius, 13 degrees below the normal pressure boiling point of the nitrogen. The cooling system must be designed in such a way that it can compensate for the heat generated by the cable cryostat and return line as well as the heat generated by the pumping process. Intermediate cooling stations are also required for longer cable sections. These do not require storage tanks as they can be operated with liquid nitrogen from the circulation system. Nevertheless, the cost of installing them is considerable.
New system minimizes losses
With the new cooling system developed by Messer the combined energy losses can be reduced by up to 50 percent. It works without a return line and circulation pump, and intermediate cooling stations are not required. This also significantly reduces investment costs. The core of the cooling concept is an actively cooled cooling shield around the cable cryostat.
Liquid nitrogen is extracted from the storage tank through an expansion valve, which evaporates in the subcooler under vacuum conditions, reaching a temperature of up to -209 degrees Celsius. At the same time, liquid nitrogen is fed from the tank through the heat exchanger in the subcooler. This cools down to around -206 degrees Celsius. A pump is not required; the pressure required for the flow is generated in the tank.
Superconductors up to 100 kilometers long
The supercooled liquid nitrogen flowing out of the subcooler now flows through the inner tube of the cable cryostat, keeping the superconducting current carrier cold. The additional cooling shield ensures that the heat input in the direction of the superconductor is reduced to a tenth of that of a simple cryostat. The mass flow of liquid nitrogen required for the flow is, therefore, also reduced by a factor of 10. The flow pressure loss is even reduced by a factor of 100.
At the other end of the cryostat the nitrogen flow from the superconductor is fed into the cooling shield. There, the liquid gas vaporizes and generates the cold that compensates for the cryostat heat input. Phase separators (degassers) discharge the nitrogen vaporized in the cooling shield into the environment, thus reducing the flow pressure loss in the shield. With this technology cable sections of up to 100 kilometers in length can be implemented energy-efficiently, cost-effectively and with a very high level of operational reliability.
For more than 125 years, Messer, the today’s world's largest privately owned company for industrial gases, medical gases, specialty gases, and gases for electronics, committed to its guiding principles of safety, focus on customers and employees, responsibility for our society, sustainability, trust, and respect. Messer's Gases for Life and patented gas applications are essential for environmental protection, climate protection, decarbonization, and innovation.
