Superconductors and applications in electrical energy

The current social and economic dilemma regarding the storage and usage of electric power must be addressed as the demand for power continues to increase. There are many current forms of energy storage, but new methods need to be researched and developed. As the world moves toward more renewable, clean energy and becomes more aware of the effect on the environment, there will need to be methods in place to accept that change and the continual increase in demand. These growing demands require a versatile energy generation and storage system that is adaptable enough to meet any change in supply or demand.

Current storage systems, like lead-acid batteries, nickel-cadmium batteries, lithium-ion batteries, and pumped hydroelectric storage all meet small parts of this growing demand, but none of them can meet it with the versatility of superconducting magnetic energy storage systems. Superconductors have a huge part in the future and will revolutionize the electrical energy storage industry. Introduction Superconductors make up a large part of the small niche of truly phenomenal materials that are part of the quickly expanding realm of material science.

A superconductor is typically a compound of inner crystal-structure and metal alloy whose resistive properties vanish below a certain temperature. This can be particularly interesting in the electrical world because it presents the possibility of eliminating any thermal losses, which can be a huge drawback in energy storage devices, like batteries. Over the past few years, lithium-ion (Li-ion) batteries have been quickly supplanting other forms of energy storage like Nickel-Cadmium (NiCd) and typical lead-acid batteries from their position at the top of the reliability podium in industry.

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As can be seen below in Figure 1, Li-ion batteries have longer discharge times at rated power than either NiCd or lead-acid, and out-performs NiCd in power ratings of effective module sizes[1]. Problem However, one of the primary concerns about these batteries are the thermal losses, as was mentioned earlier. Li-ion batteries are known to overheat and incite problems in devices that can range anywhere from simply overheating to thermal expansion to a veritable explosion of fire and potentially toxic chemicals. Superconductors may be able to fix that problem. It was stated earlier that superconductors have the ability to completely eradicate thermal losses within their sphere of influence. This is done through a process called quantum locking. Background Quantum locking, also known as flux pinning, is a phenomenon that allows super-cooled superconductors to “lock” in space in a magnetic field (as seen in Figure 2). These effects can only be seen in type-II superconductors. High-temperature superconductors are all characterized as type-II and are typically made of metal alloys and oxide ceramics. In a thin enough superconducting material, quantized packets of magnetic flux lines can penetrate the material in groups that are known as flux tubes. At low enough temperatures (around -321˚F or -196˚C, the temperature of liquid nitrogen for high temperature superconductors, or around -452˚F or -270˚C, the temperature of liquid helium for low-temperature superconductors), these flux tubes become pinned in one spot on the material and allow the material to float above the magnetic field generator. This is not to be confused with a similar phenomenon called the Meissner-Ochsenfeld effect, which is the property of superconductors to completely eradicate all magnetic flux lines from the material, instead of simply locking them in place. This process is achieved through the use of an extremely thin piece of inert material, typically a crystal sapphire wafer, that is coated in a type-II superconductor like yttrium barium copper oxide (YBaCuO). The material is so thin that the flux tubes can bore through tiny imperfections in the material, allowing the superconductor to lock the flux lines in three dimensions instead of bending the flux lines around it. Note that type-I superconductors do not achieve the same effect. Type-I superconductors are made up of pure materials like aluminum and lead. Overview This paper will begin with a brief overview of the history of superconductivity and its importance in the electrical industry. Then, current electrical energy storage (EES) techniques will be analyzed for a baseline of what to expect out of future applications of superconductive energy storage. History of Superconductors The history of superconductors dates all the way back to the dawn of the 20th century[3]. In July of 1908, Heike Kamerlingh Onnes, a Dutch physicist, discovered a way to liquify helium. This discovery opened up a whole new field in low-temperature physics. Until then, the lowest temperature that was previously capable of being achieved was 14K. This discovery preceded the new field of superconductivity by only three short years. Kamerlingh Onnes began by testing several metals’ resistance at low temperatures. He expected to find zero resistance at liquid helium temperatures, which is precisely what he documented in his journal in May of 1911: “Mercury practically zero.” The Advent of a Science Onnes strove to determine the truth of what would really happen to the resistivity of a metal as the temperature approached absolute zero. The two theories of the day were: the electron scattering amplitude would decrease rapidly enough to converge at zero with temperature, resulting in zero resistivity at zero degrees Kelvin; and  that the electron mobility would decrease with temperature, resulting in the resistance of a metal decreasing with temperature until it bottomed out at some value before climbing back up to effectively infinite resistance at near-absolute zero temperatures. This passion proved to be infectious as other scientists and acquaintances to Onnes became involved in the low-temperature field of superconductivity. In March of 1910, a new lowest recorded temperature was observed. Kamerlingh Onnes began his experiment by transferring liquid helium to a cryostat that had more space for testing than the liquefier. Through an array of pumps, he transferred the liquid helium from the bubbler to the cryostat in order to condense and pressurize the liquid and measure the temperature achieved. The temperature recorded was about 1.1K, the lowest yet achieved. Establishing a Science The next major breakthrough came when Onnes decided to enlarge the liquefier to be able to hold a platinum resistor and make the first low-temperature measurement of a metal. The experiment in December of 1910 was conducted with the assistance of Cornelius Dorsman for measurements and Gilles Holst to operate the galvanometer, which was equipped with a Wheatstone bridge for extremely sensitive current measurements. This experiment delivered the killing blow to the theory that as temperature decreased, the resistivity of a metal fell to a minimum value before rocketing to infinity. It turned out that the platinum wire’s resistance dropped to a steady value below 4.25K, which was presumed to be purely a function of the purity of the sample. Kamerlingh Onnes determined that if the sample were pure enough, the resistance would drop to zero as its temperature approached absolute zero. A similar experiment was later done with a new, more accurate setup on a sample of mercury. As can be seen in Figure 4, the resistance of mercury jumped from approximately 10-5Ω to about 0.115Ω in a temperature change from 4.20˚K to about 4.23˚K. This was further proof that the resistivity of metals decreases with decreasing temperature based on the purity of the material until it is approximately zero when approaching zero degrees kelvin. Permanent Currents Once again, Kamerlingh devised a new experiment to test current decay in a coil of wire placed in a superconductive atmosphere. He made a device to test this and was blown away with the results: “During an hour, the current  was observed not to decrease perceptibly… A coil cooled in liquid helium and provided with current at Leiden might, if kept immersed in liquid helium, be conveyed to a considerable distance and there be used to demonstrate the permanent-magnetic actions of a superconductor carrying a current…” -Kamerlingh Onnes This quote by Kamerlingh Onnes portrays how groundbreaking this discovery truly was. Through these discoveries, Kamerlingh found a perfect conductor of current with next to zero losses. Current Energy Storage Techniques Electrical energy storage systems are one of the biggest obstacles to the renewable energy field. As countries begin to place more demand on renewable energy sources, reliable high-power storage becomes an increasingly important factor in power management plans. This is caused by the inherent intermittency of renewables like solar, wind, and wave energy. Photovoltaic arrays are limited at night and on cloudy days, wind turbines are limited on calm days, and kinetic motion generators are limited by the tide. The only way to combat this rising challenge is to ensure that energy storage devices also advance and are able to provide adaptable power to combat the rising demand in the technology era. Figure 5 shows a comparison of various EES type available, sorted by efficiency and lifetime in number of charge and discharge cycles. Batteries are typically high-energy density and low-power density storage devices, while common capacitors are typically low-energy density and high-power density storage devices. A comparison of different types of storage can be found in Figure 6. It can be seen that supercapacitors bridge the gap between high-power density capacitors and high-energy density batteries. Lead-Acid Batteries Lead-acid batteries have been a prevalent energy storage system for quite a while. It is still more viable for larger load applications than either NiCd or Li-ion battery arrays. Based on Figure 1, a lead-acid battery array can have a system power rating of over 10MW, while a Li-ion battery array can only reach just over 1MW. However, lead-acid batteries do have a lower discharge time at that rated power than either the NiCd or the Li-ion batteries. They are also very heavy, can come in bulky packages, and contain toxic heavy metals. Nickel-Cadmium Batteries Nickel-cadmium batteries are one of the leading technologies in the high-power density battery storage applications. However, NiCd is still heavy, overly large, and made of toxic materials. These batteries also have a lower discharge time at the rated power as a typical lead-acid battery would. Lithium-Ion Batteries Lithium-ion batteries have one of the highest energy densities out of these battery choices. The versatility of LI-ion batteries is another appeal for industry, as they are not so heavy or bulky as lead-acid or NiCd batteries. This means that these batteries can be scaled for nearly any application below approximately 1MW. One of the biggest setbacks to wider use of Li-ion batteries is the cost of construction, packing, and overcharge protection circuitry. The contact points require low-resistance pads to increase the charge and discharge characteristics. Pumped Hydroelectric Storage Another promising storage application comes in the form of pumped hydroelectric energy storage (PHES). PHES systems store energy in the form of water. While a generation plant is generating excess power, like in a time of low demand, it will activate pumps to use that extra power to move water uphill into a reservoir where it can be stored until there is high demand and extra power is needed. When this happens, the upper reservoir is opened up to allow the water to flow back down through a turbine and into the lower reservoir. An illustration of this process, using an array of wind turbines, can be found in Figure 7. PHES systems are currently one of the most realistic and feasible options for industrial power storage applications. They provide the most commercially important way to store energy for an adaptable grid in the large-scale industry. Unfortunately, it does require a large body of water nearby and enough geographic space to make effective use of this technique. It also is not easily scaled for smaller applications. Superconductive Energy Storage Applications Superconducting magnetic energy storage (SMES) devices store energy in the magnetic field of a DC current in a superconductor, which results in nearly zero energy loss. Current can be injected or extracted quickly from these systems, and they have been adapted for high-power applications due to their responsiveness. The biggest advantages achieved by SMES systems is storage efficiency, storage time, and durability. With an ideal cooling system, as much as 98% system efficiency can be achieved. Energy can be stored in these systems for a very long time, and the system can cycle nearly indefinitely. Overview An SMES system is comprised of four essential components: the superconducting coil, the cooling system, the power conditioning system, and the power control system. The superconducting coil is kept insulated and isolated from the external environment. The cooling system works to keep the superconducting coil below the critical temperature, while the power conditioning system regulates electricity exchange between the grid and the SMES system via an array of electrical elements. The final vital component constantly monitors the system and adjusts the cryogenic system accordingly. Future Prospects (SMES) SMES systems are used in industry for voltage regulation, frequency regulation, power quality improvement, and stability control. Since most renewable energy generation systems like solar and wind farms have variable power output, coupling them with SMES systems is a great way to offset some of the variability. Current SMES systems are in development, and some are even in use. For example, in the USA alone, there are over 30 systems with around 50MW capacities. The largest roadblock for SMES devices is the economics of building storage devices out of low-temperature superconducting materials. In the future, SMES systems could be used for high-efficiency bulk power storage, but more research is needed into storage devices based on higher-temperature superconducting materials. Research should also be done to find ways to increase current and magnetic field intensity for these materials and to find ways to produce them more cheaply. Although they are very efficient, the main sources for losses are the mechanical components and the power conditioning and regulation of the system. Conclusions As the demand for electrical power continues to increase, the issue regarding the storage and usage of power must be addressed. Since the world is moving toward more renewable energy systems, there is an increased need for effective, high-power, responsive storage applications. Current storage systems cannot keep up with the increasing demand for versatility, but they also cannot be discarded at this time. Superconducting magnetic energy storage may, in the future, fulfill that need, but there must be much research to address the remaining problems of finding higher-temperature materials that are not as economically demanding. They may be highly versatile and efficient, but a grid based on SMES systems is still not realistic for many years.