|
|
| Fig. 1: Schematic of resistances of superconducting (red curve) and non-superconducting (blue curve) materials as a function of temperature; the superconductor's resistance drops abruptly to zero below the critical temperature Tc. (Image source: M. Hurley) |
In a conductor, electrons move in response to an electric field, forming a current. This relationship is captured quantitatively by Ohm's law, V=IR, where V is the applied voltage, I the current, and R the resistance of the conducting material. This resistance arises due to scattering of the electrons off impurities, thermally excited atomic motion, and other electrons. As these collisions happen, the electron imparts energy to the surrounding environment in the form of heat, a process called resistive heating (or Joule heating). The corresponding dissipated energy per unit time is given by
A class of compounds known as superconductors display zero resistance. [1] In these materials, currents are unimpeded and no electrical energy is dissipated. Such properties have made superconductors an object of much interest in engineering and basic sciences.
Superconductors display zero resistance only below a particular temperature, known as the critical temperature (see Fig. 1). This value varies with material, but to date has found its maximum (at standard pressure) around -135°C for the compound Hg0.8Tl0.2Ba2Ca2Cu3O8. [2] With such low critical temperatures, existing superconductors require intensive refrigeration when used for application, often rendering them impractical. The advent of a "room-temperature superconductor," which would maintain its zero-resistance state up to or beyond ~25°C, is frequently heralded as a transformative advance in reducing the world's energy demands. We interrogate this notion here.
Energy consumption is frequently divided into five energy use sectors: electric power, residential, commercial, industrial, and transportation. We consider each of these in turn, focusing on usage in the United States. [3]
The electric power sector concerns the production and transmission of electricity. [3] The mechanism of electricity production depends on the energy source; fossil fuels, which currently provide roughly 2/3 of the overall energy expended towards electric power, are burned as input to a heat engine (for instance, a steam turbine), which converts thermal energy into mechanical energy (e.g., the rotation of an output shaft); this mechanical energy is then converted into electrical energy via a generator. For natural gas, the efficiency of this process is around 45%; for coal it is only 35%. However, these numbers are limited almost entirely by the fuel combustion and heat engine, which are constrained by the laws of thermodynamics. Generators do lose some energy to resistive heating, but are very efficient: those used in grid-scale electricity production have efficiencies over 98%. [4]
Once generated, electricity must be transported, via the electrical grid, to consumers. A room-temperature superconductor would be able to eliminate resistive losses during transmission. However, the amount of transmission loss is only 5% that of the net electrical energy produced and comprises just 2% of the present overall energy expenditure on electricity. [3] The electrical grid primarily uses alternating current transmission; for direct current transmission, the losses can be even lower, around half as much per unit distance. [5] Transmission lines are efficient because they carry a relatively low current at high voltage, thus transporting large amounts of energy while limiting the Joule heating I2R. Using a very good conductor, such as copper, keeps the resistance R low.
The residential and commercial sectors combined made up ~20% of the 2023 U.S. energy budget. [3] A bit less than half of these sectors' energy use was in the form of electricity, and this quantity accounts for 74% of all electricity usage in the U.S. In the residential sector, primary electricity sinks include air conditioning, space heating, water heating, refrigeration, and lighting. [6] Commercial usage is similar, with ventilation, lighting, cooling, refrigeration, and computing comprising the top-end categories.
It is difficult to ascertain exactly how much of this energy is wasted through resistive losses; however, we can attempt some rough estimates, taking a residential air conditioner as an example: Suppose an AC unit runs on 3000 W of power with a standard voltage of 120 V, thus drawing 3000 W/120 V = 25 A of current. The resistance of the unit's wiring, likely made of copper, can be calculated as the "resistivity" (1.7 × 10-8 Ωm for copper) times its length divided by cross sectional area. [7] Wire of size American Wire Gauge 12 has a diameter of 0.0808 in (0.205 cm), so that a length of 100 feet (30 meters) will have a resistance of
| R = | 1.7×10-8 Ωm
× 30 m π (0.00205 m/2)2 |
= | 0.15 Ω. |
The power dissipated by the wire through Joule heating is then
This value is roughly 3% of the power used by the AC unit.
The industrial sector accounted for 28% of U.S. total energy usage in 2023 and 25% of electricity consumption. [3] Only a small part (~13%) of industrial energy consumption is electrical; the vast majority comes directly from natural gas or petroleum that are primarily used either as feedstocks (raw materials) or as fuel for process heating and boiling. In manufacturing—the largest subset of industry in the U.S., responsible for roughly three-quarters of its energy demands—process heating is an essential component of a huge number of operations, such as the curing of chemicals, melting of scrap metal, waste incineration, and food production. [6] However, it is difficult to improve heating efficiency; electric boilers can have efficiencies with upwards of 98%. [8]
Nearly half of electricity consumption in industry is distributed amongst process heating, process cooling and refrigeration, electro-chemical processes, and facility support (including HVAC and lighting). [6] The other half is dedicated to machine drive, such as motors, pumps, and fans. Motors use an electromagnet to exert force on a current-carrying conductor, producing mechanical energy, and therefore have losses associated with resistive heating. However, while exact values vary with machine, larger motors can achieve efficiencies in excess of 95%. [4,9] If we allotted a 10% reduction in motor energy usage thanks to superconductivity, the result would be savings of less than 1% of all manufacturing energy input and < 0.2% of the entire U.S. energy budget.
The last and largest energy sector is transportation, which lay claim to 30% of U.S. energy consumption in 2023. [3] Currently, less than 1% of this quantity is in the form of electricity, due to the dominance of gasoline-powered vehicles. Furthermore, electric vehicles tend to have very efficient motors, at 90%-95% or larger, with low resistive losses. [10]
Finally, magnetically levitating ("maglev") railway systems are an actualized application of superconductivity today. Such systems eliminate the rolling resistance of the trains' wheels on the track, though for high-speed transport energy losses tend to be dominated by air resistance. [11] Even if one could surmount the massive practical challenges in applying similar technology to cars, the rolling resistance of a car is only about 5% of the vehicle's total energy demand. [12]
Here we have tried to gain some sense of the amount of energy wasted by ordinary metallic conductors via resistive heating, through rough estimates and upper bounds. Obtaining an exact number is difficult. It is more difficult still to predict all future technologies that could arise from the remarkable properties of a room-temperature superconductor, or the ways in which such a phenomenon may impact energy usage through indirect means: for instance, maybe superconducting magnetic energy storage could accelerate the transition to a decarbonized grid.
Even so, the above estimates of superconductor energy savings are small. Most applications of electricity are quite efficient and leave little room for improvement; we saw that transmission losses in transporting electric power are around 5% and most end-use losses even less than that amount. Moreover, we could in principle shrink such losses further: double the diameter of a wire, and its resistance falls by a factor of four, reducing the wasted power by 75%. The limiting factor here is not physics, but cost; the excess material is not worth the marginal gain in energy. Conventional metals are simply excellent conductors—ten million times better than seawater—and the discovery of a room-temperature superconductor would likely be less transformative than was the discovery of copper.
© Matthew Hurley. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. Tinkham, Introduction to Superconductivity, (Dover Publications, 2004).
[2] G. F. Sun et al., "Tc Enhancement of HgBa2Ca2Cu3O8 by Tl Substitution," Phys. Lett. A 192, 122 (1994).
[3] "Monthly Energy Review, November 2025," U.S. Energy Information Administration, DOE/EIA-0035(2025/11), November 2025.
[4] B.C. Mecrow and A. G. Jack, "Efficiency Trends in Electric Machines and Drives," Energy Policy 36, 4336 (2008).
[5] M. Ardelean and P. Minnebo, "HVDC Submarine Power Cables in the World," European Commission, 2015.
[6] L. Schwartz, et al., "Electricity End Uses, Energy Efficiency, and Distributed Energy Resources Baseline," Lawrence Berkeley National Laboratory, LBNL-1006983, January 2017.
[7] N. W. Ashcroft and N. D. Mermin, Solid State Physics (Cengage Learning, 1976).
[8] "Uniform Test Method For the Measurement of Standby Loss of Electric Storage Water Heaters and Storage-Type Instantaneous Water Heaters," U.S. Code of Federal Regulations, 10 C.F.R. 431(G) App. B (2024).
[9] "Energy Conservation Standards and Effective Dates," U.S. Code of Federal Regulations, 10 C.F.R. 431.25 (2024).
[10] G Pellegrino, A. Vagati, and. Gugiemi, " "Performance Comparison Between Surface-Mounted and Interior PM Motor Drives for Electric Vehicle Application," IEEE 5764534, IEEE Trans. Ind. Electron. 59, 803 (2012).
[11] E. Fritz et al., "Energy Consumption of Track-Based High-Speed Transportation Systems: Maglev Technologies in Comparison With Wheel-Rail Systems," Transp. Syst. Technol. 4, 134 (2018).
[12] T. Markel, et al., "Advisor: A Systems Analysis Tool for Advanced Vehicle Modeling," J. Power Sources 110, 255 (2002).