X Международная студенческая научная конференция Студенческий научный форум - 2018

Technology

Superconducting transmission lines are an innovative and promising transmission option that can be one of the many components needed to achieve a more sustainable transmission and distribution of electric energy. The manifold advantages of SCTL like a potentially much higher efficiency and small size requirements can have a direct positive impact on the environment and would likely increase the public acceptance. The access and utilization of renewable energy sources can indirectly be facilitated by faster approval procedures. SCTL have a much lower visual impact on landscape than standard OHL and also require much less space than standard cables for high capacity transmission. They can alleviate the ROW problematic and lead to an increased public acceptance. Underground transmission lines with several GW of power could be realized using existing right-of-ways like highways, train tracks or standard cable conducts in cities. For small capacities the size advantage of SCTL is less evident but a technological advantage can still exist, especially for transmission system operators. For instance can the operating voltage be tailored by adjusting the operating current with still keeping the same small size what allows to reduce the amount of transformer equipment and free space. Superconducting transmission lines based on MgB₂ can potentially be cost competitive without taking savings from reduced ROW requirements into account. Cost of SCTL will also decrease in time due to cheaper production processes of the superconductor itself and if the demand and output is higher (both, HTS and MgB₂). Standard transmission lines have gone through this process already long time ago and cost reductions are not expected. The losses and related costs depend on the actual load factor and the capacity. In general, high capacities and high load factors work in favor of SCTL. The size of superconductors makes it possible to minimize the outside magnetic field by choosing a proper layout for the cable system design. A coaxial design of a bi-polar cable leads to zero magnetic fields if both opposing current are equal. Heat dissipation of resistive conductors deny this design option for high capacity standard HVDC cables. For long distance and high capacity transmission SCTLs can be the best choice of all transmission options. However, the global output of superconducting wires and tapes would not be enough to supply one multi-GW and 1000 km long transmission line at the moment but can once SCTLs are accepted as a mature and cost competitive technology and output is increased to meet demand. Reduced CO₂ emissions are another sustainability asset and can further increase the acceptance of SCTL.

Superconductors (SC) are materials that can conduct electric energy without losses below a certain critical temperature TC, i.e. they are non-resistive below TC. That distinguishes them from standard conductors like copper that are resistive and have power losses dissipated as heat. A cryogenic envelope is needed to keep the superconductor cooled below its critical temperature. State-of-the-art cryogenic envelopes allow less than 1 W of heat per meter length to enter the cryogenic system as heat influx from the environment. Since the second law of thermodynamics states that in a heat engine not all supplied heat can be used to do work, the mechanical power that is needed at room temperature in order to have the desired cooling power at the cryogenic temperature is much higher. The theoretically most efficient thermodynamic cycle is the Carnot process characterized by the Carnot factor, which defines the efficiency of the process and depends on both the cryogenic temperature and the higher temperature of the environment (T=300 K). The Carnot factor is 3 for liquid nitrogen (T=77 K) and 14 for liquid hydrogen (T=20 K), meaning that the cooling efficiency is 4–5 times higher if using liquid nitrogen compared to liquid hydrogen. However, in a superconducting transmission line the electric losses due to cooling can be kept small for all considered coolants, as compared to the transferred power and to the losses of standard conductors.

The critical temperature of a SC varies in a wide range and there are basically two types of superconductors, low-temperature superconductors (LTS) like niobium titanium (NbTi, T_C=9.2 K) and high-temperature superconductors (HTS) like yttrium-barium-copper-oxide (YBCO, T_C=93 K). Most LTS need to be cooled by liquid helium (T=4.2 K), while HTS can be cooled by liquid nitrogen (T=77 K) allowing for a simpler design of the cryogenic envelope and opening the door for electric grid applications. With the discovery of superconductivity below T=39 K in magnesium diboride (MgB₂) in 2001, a promising new superconductor has come on the scene, that can be cooled by either gaseous helium or liquid hydrogen, is based on raw materials that are very abundant in nature and is therefore cheaper than any other competing superconductor.

In the more than 100 years since its discovery, superconductivity has been successfully applied to a significant number of large-scale particle-physics experiments, for instance superconducting magnets, superconducting accelerator cavities and detectors used in accelerators at CERN, DESY, Brookhaven and Fermilab, as well as the fusion machine ITER. Additionally, superconductivity is today widely used in a number of commercial applications, for instance in NMR magnets, generators (wind turbines, hydro power plants, ship engines), transformers, wireless receivers in communication technology, inductive (metal) heating systems, magnetic levitation train (Maglev), fault current limiters, and superconducting magnetic energy storage (SMES).

One of the first proposed practical applications of superconductivity, envisaged for it already in 1915 by its discoverer Heike Kamerlingh-Onnes, is the transmission of electric power without losses. Apart from the lack of resistive losses, the very high current densities associated with superconductors allow for much smaller dimensions of the conductor and cable compared to the case of standard conductors.

The overall design of SCTL shares many similarities with natural gas pipelines, as far as carrying a highly pressurized medium and the need for refrigeration/compressor stations along the line. However, the dimensions are smaller (a few 10 cm compared to 140 cm diameter) and the maximum pressure is much lower (20 bar or less compared to 85 bar). There is no availability data for large-scale SCTL with cooling stations several tens of km apart because they have not been implemented so far. To give an impression of the reliability and availability of a large cryogenic system one can refer to the Large Hadron Collider (LHC) of CERN which has the longest and most complex cryogenic system in the world with a length of 27 km. The magnets operate at a temperature of 1.9 K, which is much more challenging than the cooling temperatures of 15 K or 70 K necessary for MgB₂ and HTS SCTL. 10,080 ton of liquid nitrogen and 136 ton of liquid helium are necessary to keep 36,000 ton of cold mass (magnets, equipment) at its nominal operating temperature. The system consists of about 60.000 inlets and outlets and has been running continuously from 2007 to 2013. It achieved a global availability of 94.8% for the year 2012 and an availability of 99.3% for each of the eight 3.3 km long cryogenic segments. The non-availability time was caused by the cryogenic system (3.3%), by scientists conducting experiments/users (0.4%) and by other events (1.2%) triggered by single experiment events, IT or electricity supply by utilities. Thus, the cryogenic system of SCTL considered in this paper can have a much higher availability. Not only would the setup be much simpler for cooling only a bi-polar conductor, but the operating temperatures would be much higher and operation less demanding.

Advantages of superconducting transmission lines

SCTL share the advantages of underground cables compared to overhead lines:

1. Very low visual impact on the landscape due to their underground location.

2. Generation of lower electromagnetic fields that could affect the surrounding area.

3. Smaller environmental footprint than overhead lines (except for wetlands).

4. Minimization of land use and property acquisition, leaving the value of local real estate unchanged.

5. No affection by most natural weather phenomena such as wind, fog, snow and ice.

6. No emission of noise.

In addition to these advantages buried cables have in general, SC cables comprise several other advantages compared to standard HVDC underground cables. The following points highlight the advantages of superconducting power lines compared to the most modern underground standard HVDC cables (±320 kV XLPE HVDC):

1. A size advantage (a few 10 cm width of only one needed SC cable compared to a 17 m wide trench consisting of 24 cables for 10 GW capacity for a standard HVDC ±320 kV cable installation - not including 2.5 m safety area on both sides).

2. Much smaller land use potentially as low as 10% of standard HVDC cable installations depending on the capacity, area (urban or land) and regulations.

3. Appealing option for long-distance and high-capacity electric energy transport if underground cables are required because standard conductor cables have high losses (>6%/1000 km at 100% load for ±320 kV XLPE HVDC).

4. Adjusting the nominal current to meet the desired or existing operating voltage, especially that of medium and low voltage grids. Thus eliminating transformers results in less occupied space and less components in the grid chain that are prone to technical failures.

5. Much better option for hot climates because of the vacuum-isolated cryogenic envelope that prevents heat from entering the system and therefore stabilizes the temperature of the SC conductor. The capacity of standard HVDC cables is reduced by higher soil temperatures (the resistivity of Cu and Aluminum increases with higher temperatures and so do the power losses).

6. Do not heat the surrounding soil (does not alter soil humidity).

7. Option for a hybrid transmission line, transferring not only electrical energy but also hydrogen, the fuel with the highest energy density per weight (Please note that the efficiency of the hydrogen liquefaction process is rather low and that it takes 40% of the chemical energy of hydrogen to liquefy it from 300 K to 20 K).

8. Much easier use of existing right-of-ways (ROW) to transfer GWs of power

9. Can potentially be operated in AC with much smaller losses than standard HVDC cables. No cost-intensive AC–DC converter would be needed.

10. The cryogenic system can store energy by cooling to lower operating temperatures at times of high renewable energy input.

It is apparent, that the advantages of SCTL address many public concerns. Especially the size advantage can potentially decrease the public opposition against new transmission lines. The technological advantages of SCTL are evident and TSOs can profit from installing and operating SCTLs (potentially reduced delays, technological advantages) creating a win-win situation with affected communities.

Conclusion

It is very important to inform about the progress in the area of superconducting power transmission. This is all the more necessary since on-going events in Europe as well as other regions show that it is crucial to engage the local communities in the early stages of the planning process. It is crucial to further invest in and foster the development of superconducting transmission lines in order that they become a commercially available option for the future electric grid as an alternative to standard technologies with a number of advantages.

References

Thomas; et al. (2016). "Superconducting transmission lines – Sustainable electric energy transfer with higher public acceptance?". Renewable and Sustainable Energy Reviews.

Paul Preuss (14 August 2002). "A most unusual superconductor and how it works: first-principles calculation explains the strange behavior of magnesium diboride". Research News. Lawrence Berkeley National Laboratory. Retrieved 2009-10-28.

Dirk vanDelft & Peter Kes (September 2010). "The Discovery of Superconductivity"(PDF). Physics Today. American Institute of Physics. 63 (9): 38–43. Bibcode:2010PhT....63i..38V. doi:10.1063/1.3490499.

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