By Liam Critchley (chemistry and nanotechnology writer)
As the world transitions to new and renewable energy sources, the discussion of how to connect these new energy sources to the electric grid and then to consumers has taken on new urgency, and the limitations of copper are becoming central to that discussion.
Copper has been the “go to” material for electrical transmission for more than two centuries because it has relatively high conductivity and stability. However, the red metal doesn’t have very good thermal conductivity for dissipating heat during operation. Anyone who has put their hand on an electrical coil knows that it’s hot because of the energy wasted to heat from resistance. Moreover, copper mining is harmful to the environment. Plus, copper is heavy, increasingly expensive and its supply will be challenged to keep up with demand, which is expected to double to over 50,000,000 mt annually by 2035. This projected annual consumption is more than all the copper consumed in the world between 1900 and 2021.
Despite these formidable challenges, plans for renewable energy infrastructure continue to rely on ever larger amounts of copper. For example, millions of feet of copper wiring will be required to upgrade the world’s power grids and build the charging infrastructure for electric vehicles (EVs). Hundreds of thousands of tons of additional copper are needed to build transmission lines to connect wind and solar farms to the grid. The additional copper required for electric turbine and motor coils will be the subject of another article but, as an example, an offshore wind turbine contains 8 mt of copper per megawatt of generation capacity. The average electric vehicle requires two and a half times as much copper for its battery connections and electric motors as an internal combustion engine vehicle.
Source: National Renewable Energy Laboratory | The 2035 map is based on the “All Options” path from NREL’s 100% Clean Electricity by 2035 Study. Both maps show utility-scale renewable projects, but do not include distributed installations, like rooftop solar.
Copper May Not Be Able to Deliver the Potential of New and Renewable Energy
If electric transmission grids continue to suffer outages under extreme levels of heat, then power supplies will become even more strained as global temperatures continue to rise. For example, data from the Energy Information Administration shows that both the frequency and duration of power outages increased between 2013 and 2021—with the power outage time doubling from 3.5 to 7 hours. As temperatures continue to climb, we’re only going to see more and more power outages if the existing infrastructure doesn’t change.
Extreme heat is also known to make transmission lines less efficient. This is due to a combination of material properties and more energy running through the wires to support demand. When already hot line temperatures increase from electrical resistance and are combined with warmer air, the transmission lines can swell. This swelling can lead to sagging of the transmission lines, which can then ultimately lead to damage, failures and outages.
If power lines can be built with materials that are mechanically robust and thermally stable, without a drop in electrical performance, fewer transmission lines will go down—even with the continually increasing temperatures and energy demand.
This is where the discussion of graphene comes in. Graphene can replace or, at a minimum, improve copper because of graphene’s combination of robust mechanical, thermal and electrical properties.
What Is Graphene and Why Is It Such a Good Electrical Conductor?
Graphene is a two-dimensional form of crystalline carbon. It has a hexagonal arrangement of atoms, which is the same as graphite. Each carbon atom is bonded to three other carbon atoms, leaving a free electron per carbon atom. This free electron is important because it floats above the atom’s surface and enables a high charge carrier mobility and electrical conductivity.
Graphene’s excellent electronic properties also stem from the way its atomic lattice is ordered and the effect it has on the electrons. Graphene has a reciprocal lattice and the valence and conduction bands of graphene meet at points in the lattice known as the Dirac point. At the Dirac point, the two bands intersect and there is no gap between them. This causes the electron and hole charge carriers to overlap, leading to no electronic band gap (i.e., a zero-band gap conductor). If a material has no band gap, then hole charge carriers can pass into the conduction band freely as there is no electronic barrier to overcome—leading to a high electrical conductivity and charge carrier mobility.
The Dirac points in graphene also exhibit a cone-like shape (known as Dirac cones). This shape causes the electrons to behave as if they have no mass (because the electrons and holes have a linear energy/momentum relation) and are known as massless Dirac fermions. Because the hole charge carriers now have no mass, it allows them to travel at very high speeds across the layer and are also responsible for graphene’s excellent electrical conductivity and charge carrier mobility.
Comparing Copper to Graphene
Copper and graphene are two materials that have begun to compared for electrical applications. Copper has been the longstanding choice, but graphene is the ‘new kid on the block’. Copper does have excellent electrical conductivity of 57-59 MS/M, which is why it has been used for so long. On the other hand, graphene has an electrical conductivity of up to 100 MS/M. Overall, LTDF graphene can have an electrical conductivity that is up to 70% higher than copper.
But electrical properties are not just about conductivity, as the charge carrier mobility is also important. The charge carrier mobility is the velocity at which the free electrons (or holes) move throughout the material and determines how fast they move under an applied electric field. In materials with a higher charge carrier mobility, it takes less time for those charge carriers to travel through the material and/or device (if we’re talking about electronic devices). Moreover, the current of an electrical system depends on the charge carrier mobility, and materials with higher charge carrier mobilities generate higher currents. While copper has a high electrical conductivity, it only has a charge carrier mobility of around 4.5 cm2 v-1 s-1. In comparison, graphene has a charge carrier mobility of around 15,000 cm2 v-1 s-1 making it significantly better on overall electronic properties than copper. Graphene also has a very large charge carrier concentration of up to 1011 – 1012 cm2.
Another aspect is tensile strength. While local, urban or even suburban transmission lines may not require high degrees of strength, transmission lines that cover long distances or go over obstacles such as rivers or gorges, benefit from material with a high tensile strength to overcome any mechanical stresses on the lines – especially during periods of extreme heat, wind or icing. In this regard, graphene is far superior because it is a material that has one of the highest known tensile strengths and has great mechanical properties overall. Specifically, copper has a tensile strength between 200 and 400 MPa and a Young’s modulus of 120 GPa. In comparison, graphene has a tensile strength of 130,000 MPa and a Young’s modulus of 1000 GPa, highlighting the substantial difference in mechanical strength between copper and graphene.
Finally, thermal properties are key to ensuring a safe and optimal function in transmission lines. If the heat generated by the transmission line—or high heat from the surrounding environment—is not dissipated well enough, it can lead to damage and failure of the lines. This is because lower tensile strength materials can swell and sag in such hot conditions, leading to potential damaging contact with tree limbs and other obstacles on the ground. Graphene has both excellent thermal conductivity and a high thermal stability that could help to prevent line damage while improving transmission efficiency through better heat dissipation.
Compared to copper—which has a thermal conductivity of 400 W m-1 K-1 and can withstand temperatures up to around 1085 °C/1985 °F before melting—graphene can withstand temperatures above 2300 °C/4200 °F and has a thermal conductivity of around 5300 W m-1 K-1. This higher thermal conductivity is much more efficient at dissipating heat. While copper can theoretically withstand high temperatures, once the temperature of the copper increases, the resistivity of copper starts to increase, reducing its effectiveness at transmitting electric current. So, the hotter copper gets, the lower its conductivity gets—with the resistivity of copper increasing by around 0.4% per each 1 °C increase in temperature.
Overall, graphene has many property benefits that are better than copper for electrical transmission applications. While the electrical conductivity and charge carrier mobility of graphene is much higher, a high-quality graphene that contains few defects is key—because any defect sites in the graphene will scatter the electrons, slowing them and reducing the electrical conductivity across the graphene layers. Defects can also affect the symmetry of the graphene layers, change the length of the carbon bonds in those layers, or alter the electron orbitals of the carbon atoms, all of which will affect the electrical properties of graphene.
Defects also reduce the thermal conductivity, tensile strength and Young’s modulus of graphene. The combination of these three key properties is vital if graphene is to compete with copper in transmission and many other electrical applications, so there’s a need to create graphene materials that have very few defects (as no material has zero defects). This is where large, thin and nearly defect free (LTDF) graphene comes in because its larger than average flake size and lack of defects make it an ideal material for facilitating the movement of electrical currents.
Could we use Copper and Graphene Together?
While graphene has the potential to replace copper in electrical transmission and power applications, what about using it alongside copper to improve the performance of existing transmission line materials? This is also a possibility. Various studies have reported that graphene has been used to create more robust conductive composite materials with copper.
One study from Kang et al. has created graphene-copper composites that show a 450% increase in electrical current carrying capacity, a 41% higher electrical current and a 224% thermal heat dissipation increase compared to pure copper. These composites haven’t been specifically created for transmission cables – instead having been tailored towards electrical applications in general – but it shows the potential of using both materials in transmission applications.
Aside from copper, graphene is gathering interest in other composite materials that are used in transmission lines—such as aluminium conductor steel-reinforced (ACSR) cables. There’s currently a project ongoing in Korea that is looking to use graphene fibres to take ACSR cables beyond their current power transmission efficiency (as they have reached their technical limit). So far, the integration of graphene has yielded lighter cables with transmission capacities three times that of conventional aluminium power cables, with more results still to come.
Conclusion
The road to Net Zero should not be paved with vast amounts of copper.
Copper has been the standard for electrical transmission for more than 200 years. But, as the world transitions to new and renewable energy sources, copper’s many limitations require alternative materials to connect these clean power sources to energy consumers.
While copper played an important role in the first generation of electrification starting in the 1820s, it is heavy, increasingly expensive and its supply will be challenged to keep up with demand. In addition, copper mining pollutes the atmosphere and environment.
Governments and industries should seek lighter, more efficient and more plentiful materials that can be scaled to meet the demand for the second generation of electrification. With its superior electrical conductivity and ultra-light weight, LTDF graphene has emerged as an attractive option. However, work needs to be done to utilize LTDF graphene-enhanced materials as a substitute for copper in electrical transmission.
References:
https://www.eenews.net/articles/heat-wave-slams-the-grid-heres-what-to-know/
https://www.scientificamerican.com/article/increasing-power-outages-dont-hit-everyone-equally1/
https://energyeducation.ca/encyclopedia/Electrical_transmission
https://www.electricaleasy.com/2016/07/types-of-conductors-used-in-overhead-lines.html
https://www.graphene-info.com/koreas-kepco-launches-rd-project-develop-graphene-based-power-lines
https://www.engineeringtoolbox.com/resistivity-conductivity-d_418.html
https://mae.osu.edu/events/2023/03/can-graphene-based-electrical-conductors-replacecopper
https://www.bosch.com/stories/can-graphene-compete-with-copper-in-electrical-conductivity/
Myronov M., Chapter 3 – Molecular Beam Epitaxy of High Mobility Silicon, Silicon Germanium and Germanium Quantum Well Heterostructures, Molecular Beam
Epitaxy (Second Edition) From Research to Mass Production, (2018), 37-54.
Bartos M. et al, Impacts of rising air temperatures on electric transmission ampacity and peak electricity load in the United States, Environmental Research Letters, 11, (2016), 114008
Blaikie A. et al, A fast and sensitive room-temperature graphene nanomechanical bolometer, Nature Communications, 10, (2019), 4726.
Lei S. et al, Thermal-Resistance Effect of Graphene at High Temperatures in Nanoelectromechanical Temperature Sensors, Micromachines (Basel), 13(12), (2022), 2078.
Knych T. et al, New Graphene Composites for Power Engineering, Materials (Basel), 15(3), (2022), 715.
Roscher S. et al, High voltage electrochemical exfoliation of graphite for high-yield graphene production, RSC Adv. 9, (2019), 29305-29311.
Jawaid M. et al, Functionalized Graphene Nanocomposites and their Derivatives, Synthesis, Processing and Applications A volume in Micro and Nano Technologies, 2019.
