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Review

A Review of Technology Readiness Levels for Superconducting Electric Machinery

by
Bárbara Maria Oliveira Santos
1,*,
Fernando Jorge Monteiro Dias
1,
Frederic Trillaud
2,
Guilherme Gonçalves Sotelo
3 and
Rubens de Andrade Junior
4
1
Electrical Engineering Department, Universidade do Estado do Rio de Janeiro, Rio de Janeiro 20550-900, Brazil
2
Instituto de Ingeniería, Universidad Nacional Autónoma de México, Mexico City 04510, Mexico
3
Electrical Engineering Department, Universidade Federal Fluminense, Rio de Janeiro 24210-240, Brazil
4
Department of Electrical Engineering, Universidade Federal do Rio de Janeiro, Rio de Janeiro 21941-909, Brazil
*
Author to whom correspondence should be addressed.
Energies 2023, 16(16), 5955; https://doi.org/10.3390/en16165955
Submission received: 14 July 2023 / Revised: 31 July 2023 / Accepted: 4 August 2023 / Published: 12 August 2023
(This article belongs to the Special Issue Advanced Manufacturing and Materials for Next Generation Electrical Machines)
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Review Reports Versions Notes

Abstract

:
Superconducting electric machines (SEMs) have the potential to be commercially available in the coming years. This commercialization depends on the availability of high-temperature superconductors (HTS) produced on a large scale. HTSs have high current densities and low losses, making them the leading technology choice for future light and compact high-power-density superconducting rotating machines, with a particular niche for high torque at low frequency. The advantages of SEM in its fully superconducting design or hybrid configuration (conventional stator, superconducting rotor) inherit from the characteristics of the superconductor material. So, they can show greater efficiency at a higher power density and lighter frame than their conventional counterparts for an equivalent power rating. Applications like electric aircraft, naval propulsion, and wind turbines, among others, are likely to use SEMs if the rated power has to be increased beyond what is technically available with conventional technology. In this context, this paper reviews SEMs and their applications. However, it also aims to highlight the main the literature projects with a minimal Technology Readiness Level (TRL) larger than three. Due to the diversity of the superconductors’ characteristics and the variety of machines, the modes of operation of SEMs can be quite distinct from conventional machines. Taking into account such diversity, SEMs are presented and sorted out by their operational principles and the choice of superconducting material. Finally, the future perspectives of SEM are discussed.

1. Introduction

The use of superconductors in electric machinery has been investigated since the 1960s. Its roots lie in the advantages of the superconducting materials. They have zero resistivity in DC and negligible resistivity in AC; they exhibit perfect diamagnetism in specific conditions; and they can trap large magnetic fields. All these aspects can improve machine performance by enhancing the prospective power rating or increasing the power-to-weight ratio, making them lighter and/or smaller than conventional machines for the same power rating.
Up to this day, many Superconducting Electric Machinery (SEM) projects have been proposed and published. Many of these projects involve academia, governments, companies, and even the military. Some have yielded patents. There is a profound interest in probing the capabilities of such machines in the context of energy sustainability. SEMs have been considered one of the key technologies that enable the electrification of aircraft, for instance [1,2]. However, none of these projects has generated commercial products. Having been discovered at the beginning of the 20th century, superconducting materials had a boom in their development between the 1960s and the 2000s, when materials with characteristics that were adequate for large-scale power applications were discovered. These characteristics involve temperature, current, and magnetic field limits. Superconductors usually operate under 100 K, give or take. This implies that the devices relying on superconductors need very specific cooling systems to operate. In addition to their cost, these cooling systems reduce the overall efficiency of the device. There is also a need to understand and model their electromagnetic properties correctly. Electromagnetic modeling, simulations, and experimental techniques are very active research fields. There is a lot of ground to cover regarding project development for superconducting devices, how much? is the question for the SEM.
One way to address this question is to apply standardized technology assessment to SEMs. The Technology Readiness Level assessment, developed by the National Aeronautics and Space Administration (NASA) in the USA, is a good approach for this task [3]. This is a standard process to evaluate whether a given technology is ready to be applied to spacecraft. The readiness stages are divided into nine TRLs. TRLs 1 to 3 focus on concepts and proofs of concept. TRLs 4 to 6 focus on prototype development and evaluation. TRLs 7 to 9 focus on validation in the target environment and the completion and maturity of the system. The steps defined for TRL assessment can also apply to any technologies for any use if appropriate adaptations and modifications to the assessments are made. This is the contribution of this paper. Here, we discuss the TRL achieved by various SEM projects while also detailing the adaptations needed to apply TRL assessment to SEMs. Our endeavor is to evaluate proposals that have surpassed TRL 2, which means they are TRL 3 or above. So, any works focusing on simulations, topology optimizations, etc., are not considered here. We thoroughly analyze projects with TRLs equal to or greater than 5. We selected works that were published after 2017, up to the time that TRL of older projects had been already assessed.
This paper is divided as follows: Section 2 presents an overview of the superconductivity phenomenon, the superconducting materials applied to machines, and the types of SEMs reviewed in this paper. Then, Section 3 presents the TRL assessment process, its application to SEMs, and a review of the TRL levels of current projects. The prospects of the technological advancements in SEMs are discussed in Section 4. A conclusion is made in Section 5.

2. Superconductivity and Superconducting Electric Machinery

In this section, a summary of the basic elements of the superconductivity phenomenon and its applicability to electric machinery is provided. First, the physics behind superconductivity is described. Then, the types of superconducting materials applied to electric machines and the types of machines are discussed.

2.1. Superconductivity

The superconductors are classified into two types: I and II. Type I superconductors exhibit perfect diamagnetism and zero DC resistivity under critical values of temperature ( T c ) and magnetic field intensity ( H c ). Type II superconductors exhibit the same behavior under T c and the first critical magnetic field intensity ( H c 1 ); however, they experience a mixed state between H c 1 and a second critical magnetic field intensity ( H c 2 ). In this mixed state, a part of the magnetic field penetrates into the bulk of the superconductor, allowing large quantities of current to flow. The superconductor presents a nonlinear resistivity, the intensity of which depends on the current density, the magnetic field intensity, and the temperature. This resistivity increases exponentially as the current density approaches and passes the critical current density ( J c ), marking a limit on the capability of the superconductor to transmit current without losses. Because of the mixed state, type II superconductors are the only superconductors that are technologically applicable to large-scale applications.
Besides the ability to conduct large currents, another interesting characteristic of the mixed state is the trapping of magnetic flux when the superconducting piece is field-cooled (FC), i.e., when it is cooled in the presence of magnetic fields. In this case, after removing the external magnetic field, magnetic flux is trapped in the superconductor, giving it a behavior similar to those of permanent magnets. Fields up to 17.7 T [4] have been trapped in superconductors. Both behaviors can be used in electric machinery design, as explained in Section 2.3.
The mixed state resistivity ( ρ s c ) is usually modeled with a power law, as follows:
ρ s c = E c J c | J J c | n − 1
with E c being the critical electric field, ranging from 0.1 to 10 μ V/cm, and n being the power law index. Figure 1 shows an example of the normalized E-J characteristic of stabilized type II superconductors from which resistivity can be inferred around the J c . Up to J c , the electric field is almost zero. As the current density approaches J c , it increases sharply. J c s are within the range of 10 8 A/m 2 for superconducting bulks and 10 10 A/m 2 for superconducting tapes. This means that the superconductor can typically carry greater current densities than common conductors. This allows the reduction in device sizes and weight. The specific types of superconductors applied to electric machinery are discussed in the next section.

2.2. Superconducting Materials in SEMs

The development of the first rotating electrical machines can be traced back to the mid-1960 s with the development of hybrids synchronous machines [9] and DC homopolar machines [10]. Such machines had a NbTi-wound rotor. The rotor was cooled by using liquid helium at 4 K, and the conventional stator was operated at room temperature [11]. NbTi had just been discovered in 1962 [12]. This material was quickly made into stabilized wire to be used in applications. It was the main technology for building superconducting power devices using helium cooled system until the 1990s, still benefiting from the development of low AC loss conductors [13].
Besides NbTi, Nb 3 Sn was developed. Some preliminary work showed potential for applicability [14,15]. However, despite its better superconducting characteristics, it lacked the maniability of NbTi, being a brittle material. Its better performance in field at a larger current margin than its counterpart NbTi did not outweigh the hassle of its handling using the wind-and-react (W&R) or the react-and-wind (R&W) methods to fabricate superconducting devices [16].
With the discoveries of HTS in the mid-1980s [17], new possibilities emerged to use superconductors at temperatures greater than 4 K. A new range of temperatures could be exploited from 20 K to more than 77.3 K, with the latter being the temperature of liquid nitrogen at atmospheric pressure. Thus, newly manufactured commercial HTSs have shown a clear advantage over LTSs by avoiding the complexity of dealing with liquid helium. The first HTS, referred to as first generation (1G) or BSCCO, demonstrated the first real potential for large-scale manufacturing and practicability for power applications. BSCCO is found in two compositions: Bi 2 Sr 2 CaCu 2 O 8 (BSCCO-2212) and (Bi,Pb) Sr 2 Ca 2 Cu 3 O 10 (BSCCO-2223) [18]. Bi2212 composition has never found a commercial niche [19], leaving the market to the most popular Bi2223, which is commercialized nowadays by Sumitumo Electric Industries Ltd. Bi2223 is sold as tape with a production length in the order of kilometers with stable and consistent superconducting properties [20]. It was considered in several machine demonstration projects in the late 1990s. Its glory has fallen away in recent years [10] for YBCO, and BSCCO has never led to any actual commercial products from that point on.
In the 2000s, YBCO, referred to as second generation (2G), was not competitive yet but it was already gaining a lot of traction from HTS manufacturers driven by the prospect of a lower production cost than BSCCO technology, despite its low current density at the time. After a continuous effort and investment, it is nowadays the main material used in the design of SEM [21]. YBCO has now been replaced by REBCO, RE for Rare Earth, as other elements from the lanthanide family can substitute the yttrium (Y) in the fabrication of the ceramic material. Such elements may be Gadolinium (Gd) or Europium (Eu), among others [22]. The choice of the element depends on the manufacturer.
In 2001, a known compound (synthesized in the 1950s), MgB 2 , was found as a superconductor with a critical temperature in the order of 39 K [23]. Its manufacturing is cheaper than any HTS due to the low cost of the materials. However, there are still some issues to overcome, such as the fabrication of reliable electrical joints and some practical issues similar to those associated with Nb 3 Sn. Indeed, techniques such as W&R and R&W should be employed to make coils since MgB 2 requires heat treatment to create the superconducting phase [24]. Conceptual designs and preliminary experimental works have been carried out thus far, but no demonstration projects have been conducted yet [25,26].
For LTS and MgB 2 , the machine design considers impregnated coils directly or indirectly cooled by gas or liquid helium, and cryocooler in specific cases (Nb 3 Sn, for instance). For HTS, the design is either using coils wound with tapes or wires for BSCCO or bulks and tapes for REBCO [27].
Table 1 provides some statistics on the superconducting materials considered and some time frames used for demonstration projects in SEM since the 1960s. Today, LTSs have been superseded by HTSs. REBCO is the main superconducting material for SEMs, taking over the place held since the late 1990s by BSCCO. There is very little use for MgB 2 in machine design, at least until now [21].

2.3. Types of SEMs According to Their Operational Principles and Materials

There are several types of conventional electrical machines produced commercially on the market. In the case of superconducting electric machines, there is no commercialization yet. So, the references for the existing machines are prototypes described in the literature targeting specific applications. The classifications and naming of these superconducting machinery have varied throughout the years. According to the form of superconductors used for construction, the SEM can be categorized as follows [21]: wire- and tape-based design, HTS bulk design, and HTS stacked tape design. The same reference [21] has helped in assessing many machine proposals and their classification in an orderly manner. Here, we present a summary of the most relevant types of machines, explaining their principle of operation.
With regard to speed and torque, SEMs are subjected to the same classification of conventional machines: either, they are synchronous or they are asynchronous. Usually, synchronous machines are the ones where the superconducting field winding is directly fed with the DC current. This machine makes use of the zero resistivity characteristic of the superconducting mixed state. Synchronous SEM can also be designed with superconducting coils and permanent magnets. On the other hand, asynchronous SEMs are the ones that essentially operate in an asynchronous manner, even though they may momentarily operate as a synchronous machine. Superconducting induction machines, for example, have torque while operating synchronously [29]. The same may be said about hysteresis machines and trapped-flux machines. In these cases, they make use of both low resistivity and magnetic flux trapping to operate. Low resistivity helps to keep losses low during asynchronous operation and magnetic-flux trapping allows these machines to operate synchronously. Because the field is trapped, there is almost a constant magnetization available to interact with the armature field producing torque. If the machine loses synchronism, it continues to operate, albeit with AC losses.
SEMs may also be sorted between radial magnetic flux and axial magnetic flux machines, just like conventional ones. Both have been researched, while radial flux machines are more common, axial-flux machines are of interest for high torque applications.
One specific classification analysis for SEMs is focused on the localization of the superconductor in the machine. If the superconductor is located either in the armature winding or in the field winding, the SEMs are considered partially superconducting or sometimes hybrid machines. This means that, for these machines, the superconductor interacts with the windings (field or armature) of common conductors or with permanent magnets. However, if all the windings are superconducting, the machine is considered fully superconducting.
In the present case, as the stators are similar for both a superconducting and a conventional machine, we will present only the rotors with the superconducting material. The most common SEM’s rotors found in the literature are summarized in Table 2. One common kind of SEM utilizes trapped magnetic flux in HTS bulks or stacks of 2G tape [29,30,31,32,33,34,35,36]. It is named a trapped-flux machine and operates as a synchronous or hysteresis machine. In synchronous motors, multiple phases AC currents applied in the stator produce a rotating magnetic field. This rotating magnetic field attracts the rotor field, which is produced by the trapped field in the superconductor, leading to the production of a torque. If the load torque is under a maximum limit given by the properties of the superconductor, the rotor follows the rotating magnetic field without slipping, both with the same speed. If the load torque is increased over this limit, the flux pinned in the superconductor becomes smaller than the Lorentz’s force, making the HTS enter a hysteresis cycle. Typical rotors applied in this kind of SEM are presented in the first five lines of Table 2. The literature also presents trapped-flux SEMs based on radial flux operation mode (the first three lines of Table 2) and axial-flux SEMs (the fourth and fifth lines of Table 2).
Synchronous generators operate based on the induced voltage in the stator, which is produced by a DC magnetic field source in the rotor. In an SEM, the magnetic field can come from the trapped flux in the superconductor (bulks or stacks of 2G tapes) or a direct current applied to an HTS coil, as presented in lines six and seven of Table 2. The rotor can use ferromagnetic material and present a core design similar to traditional machines [37]. This type of machine takes advantage of the high superconductor transport current capacity, producing elevated magnetic flux densities with fewer turns in the coil than conventional conductors. Moreover, it is possible to produce a field coil without using a ferromagnetic material, which could result in a lighter machine. One SEM project highlight is the EcoSwing [47], the rotor HTS coil of which is illustrated in the eighth line of Table 2. The field coils are wound with an HTS-coated conductor or REBCO to produce the DC magnetic field. The EcoSwing is the world’s first demonstration wind turbine using superconducting material, presenting a nominal power of 3.6 MW.
An induction/synchronous motor was proposed in [43,44,45,46] using BSCCO HTS. The rotor of such SEM is presented in the last line of Table 2. This kind of motor may trap flux in superconductor bars and operate as a synchronous machine. If the load torque is increased over a certain limit, the trapped flux in the superconductor changes and the rotor speed becomes slower than the synchronous one.
The literature also presents SEMs with superconducting materials in the stator. As aforementioned, they may be fully superconducting machines or partial superconducting machines. In the first line of Table 3 (see [35]), a BSCCO coil is constructed. The second line of Table 3 corresponds to a superconducting coil made of 2G tapes [48]. Both machines are synchronous and operate based on the attraction force between the magnetic flux produced by an applied current in the superconducting coil and the magnetic field of the permanent magnets.
SEMs with radial flux stator have also been introduced in the literature. A superconducting synchronous machine with permanent magnets was proposed in [49]. The illustration of the designed stator is presented in the third line of Table 3. The superconducting stator coils used 1G tapes assembled in a double-wound pancake configuration, whereas the rotor houses rare earth permanent magnets.
A 50 kW fully HTS induction/synchronous motor was presented in [50]. The three-phase four poles HTS toroidal stator of this machine is illustrated in the fourth line of Table 3. It used a so-called ring-winding configuration and was laminated with silicon steel. The BSCCO tapes were wound to produce flux in the azimuthal direction in the stator core. Every pole has two coils supplied with currents in opposite directions to produce a resultant radial flux. The rotor of this machine is a squirrel cage with BSSCO bars, similar to the one illustrated in the last line of Table 2. A superconducting stator similar to the previous one was proposed in [51] and is given in the last line of Table 3. This SEM included a copper coil in every stator pole to increase the magnetic flux to the rotor direction, similar to a Halbach array [52]. There are also the homopolar SEM stators discussed extensively in [21]. This kind of machine is not considered in this manuscript, and we refer the reader to [21].

3. Review of TRLs of Current Projects

This section introduces a review of the Technology Readiness Levels of current superconducting machine projects. First, a discussion about TRLs and their application to superconducting machines is laid forward. Then, the full review is presented with a focus on projects with TRL ≥ 3, starting from 2017.

3.1. Technology Readiness Levels

Technology Readiness Levels are a useful tool to assess the maturity of a technology. NASA proposed it in the 1980s to evaluate the states of technologies to be used in the Aerospace industry. Nowadays, it is part of NASA’s Systems Engineering Technology Assessment process. Systems engineering is a multi-disciplinary methodology to develop, manage, operate, and retire a system [53]. A system can be defined in many ways, but overall, it can be thought of as a set of devices, personnel, and software that, working together, meets a certain need [53]. Technology evaluation or assessment is crucial to the systems engineering approach because it provides all the information necessary for system design and operation.
TRL assessment involves the entire technological development process. The evaluation starts with basic research and goes on to prototype development, system development, and launch operation [53]. There are nine Technology Readiness Levels, as presented in Table 4. The first three levels are focused on basic research. TRL 1 is achieved when the basic principles have been observed and published. TRL 2 comprises the definition of the technology and the applications proposed for it. Furthermore, the third TRL, TRL 3, focuses on the proof of concept conducted via analysis and experiments. In TRL 3, the critical function of the device is demonstrated.
From TRL 4 to TRL 6, technology development is focused on product validation. Validation, in this context, is linked to stakeholders’ expectations. A product has been validated if it meets the stakeholders’ expectations to operate appropriately in the intended environment. The stakeholders (all people, companies, laboratories, governments, etc., involved in the project) and their expectations of the technology must be clearly stated. TRL 4 is achieved when a technology has been validated in a laboratory environment. TRL 5 focuses on validation in a relevant environment, emulating operating conditions. TRL 6 is achieved when a prototype is fully demonstrated in a relevant environment, close to the one expected during actual operation without all the systems.
TRLs 7 and 8 are part of the system development. TRL 7 is achieved when the system prototype is demonstrated in the target environment. Demonstration, in this case, means a full demonstration and assessment of the technology in all possible modes of operation. TRL 8 is achieved when the system is completed and fully validated. TRL 9 is achieved when the technology is in full use.
The NASA Engineering Systems Handbook [53] recommends that to assess the TRL of a given system/subsystem correctly, it is necessary to ensure that all the terminology used in TRL assessment is clearly defined for this particular subsystem; for instance, the definitions of the basic principles, what validation means, and what type of measurements, simulations, and modeling should be carried out to establish a certain TRL level.
This paper aims to address this problem by suggesting a step-by-step approach, alongside the terminology, that could be applied to the assessment of Superconducting Electric Machinery. This ensures that, even with a few judgment calls that may happen to all TRL assessments, the SEMs are evaluated with the most straightforward, clearly stated criteria. This approach is likely to generate a more objective than subjective evaluation of SEMs, contributing to their future developments and applications.

3.2. Application of TRLs to Superconducting Electric Machinery

TRL 1 focuses on the definitions and investigations of the basic principles of the technology. For electric machinery, the basic principles are related to the correct energy conversion from electric energy to mechanical energy or vice versa. This means that the device must develop force or torque if electric energy is provided to it or generates electric energy given some external force or torque. This, of course, applies to SEMs. Here, it is proposed that the basic principles expected from a superconducting machine are validated by using numerical and/or analytical results and/or experiments demonstrating the energy conversion as intended. Simulations, analytical, or experimental proofs of basic torque/force–speed relations and/or torque/force–input current/voltage relations are necessary to prove the operation as a motor. To prove the operation as a generator, current/voltage–force/torque curves from simulations and/or experiments are needed. As the superconductor is highly dependent on the temperature, some information about the operating conditions, such as fixed temperature, adiabatic system, etc., should be stated to be considered in the model. Analysis of expected regions of operation regarding temperature, magnetic field, and current density should be addressed, as they define the state of the material (superconductor or normal resistivity). This allows the move up the TRL ladder.
TRL 2 is the definition of the technological concept and the application. In this TRL, more technical details should be included and/or discussed, as it is at this stage that the end application is defined. So, in this TRL, it is important to clearly state the purpose (for instance, aircraft propulsion, aircraft control, vessel propulsion, wind power, etc.) and to address how the SEM will be built for it. Hence, a more specific design is needed, such as the designated number of magnetic poles and expected speed (synchronous, asynchronous, both, speed ranges, etc.), expected force/torque, and current/voltage requirements. Furthermore, some information on the cryogenic system is welcome, such as the definition of expected temperature range and the type of cryogenic system that may best suit this application.
In this work, the discussion concerns Superconducting Electric Machinery projects that have achieved TRLs equal to or greater than 3. This means that they should have been tested via simulations and experiments in a laboratory environment. This excludes topology and design proposals and optimizations conducted solely with simulations. To achieve TRL 3, a project needs to have demonstrated the SEM’s critical functions. This includes experimental proof that the machine has the electromechanical characteristics defined in TRL 2. Consequently, force/torque versus speed and force/torque versus current/voltage measurements are expected, along with temperature information. For example, for a synchronous machine to be operated at 77 K, experimental proof should demonstrate that the machine can achieve and hold synchronous speed at its operating temperature.
According to [53], TRLs 4 and 5 depend on “component and/or breadboard” validation. In the present case, a “breadboard” prototype is a subscale prototype that has been tested in a laboratory environment. A full-scale prototype needs to be tested in a proper environment that simulates the expected conditions of the basic operation of the machine. This definition is important to distinguish TRLs 4 and 5. Here, it is proposed that the validation steps for TRLs equal to or greater than 4 should include a full assessment of the machine. This includes not only the measurements required for TRL 3 but also thermal measurements or, at least, some description of the thermal conditions and electromechanical measurements (torque, speed, voltage, and current) with and without load, giving a full analysis of all possible operation points of the machine. All these aspects should be validated, i.e., compared to the stakeholders’ expectations: to the intended design, to application requirements, etc.
Furthermore, for TRLs 5 and 6, a “relevant environment” is needed. As mentioned previously, a relevant environment is the one that mimics or emulates the actual environment, also called target environment by the Systems Engineering Handbook [53]. As it depends on the proposed application, it might vary between projects. For example, for a machine whose purpose is to be used in aircraft propulsion, one may look for experiments conducted in flight demonstrators, where the machine is submitted to conditions similar to those encountered during flights. For machines applied to wind turbines, one should look for experiments conducted with the presence of the turbines and so forth. The experiments should consist of the same type as the ones conducted in TRL 4, or more thorough. For TRL 6, one expects the full system to be validated, including the machine itself, the control systems, the cryogenic system, and any other system identified as needed for machine operation during the previous stages of development.
TRL 7 also depends directly on the application, so one looks for machines fully tested on-site. TRLs 8 and 9 mean a fully completed system, and for TRL 9, proof of continued use is required. A fully completed system for an electric machine means that the machine can be fully controlled and is fully stable and its operation is reliable. Another important aspect to consider is the possibility of change in the proposed application/use environment. According to NASA [53], if a technology is rated at TRLs greater than 5, but its environment has changed, the TRL goes back to TRL 5. This rule is also applied in the present case. Table 5 summarizes the TRL assessment for SEMs.

3.3. Review of the TRLs of SEM Projects

Reference [10] has made a thoughtful analysis of SEM projects, including TRL assessment, for projects up to 2017. So, the analysis presented in this work extends from 2017 to the present day.
In our overall evaluation, one project demonstrated enough details to be classified as TRL 7: a synchronous generator for wind power applications developed by the EU 2020 EcoSwing project. According to the project website [47], it is the “world’s first demonstration of a superconducting low-cost, lightweight drivetrain on a modern 3.6 MW wind turbine”. The project included nine institutions from academia and industry. They developed a full-scale generator installed in a wind turbine [40,41]. The HTS machine is composed of superconducting field windings placed in the rotor and a conventional armature winding in the stator. A rotor back iron is used as a flux concentrator. The machine was first tested on the ground, where HTS winding excitation, short-circuit, stator-heat run, no-load, and partial power production tests were performed. A quench happened in one of the HTS coils, which was replaced with ease. Then, the prototype was installed in the wind turbine and tested. The full test was conducted in five steps: rotor cool-down, first excitation of the field winding, first power production, second excitation of the field winding, and second power production. Excitation and power production were divided into two steps to reduce risks. During the power production stages, three short-circuit events happened. According to [41], the HTS technology had excellent performance, even during the short-circuit events. With more than 650 h connected to the grid, the HTS generator provided more than 600 MWh to the Danish electric grid, including the first time in history that an HTS generator delivered electric power to a grid [41]. The project successfully places the HTS generator for wind power applications in a TRL range between 6 and 7, according to [41], which is in agreement with our evaluation. It is a very important project to the field, as it provided a fully tested prototype, developed by academia and industry.
Another advanced project was the one developed by Yanamoto et al. [42], which has been attributed a TRL 5. This project has been led by The Tokyo University of Marine Science and Technology as well as the Kawasaki Heavy Industries, with the Ministry of Economy, Trade and Industry from Japan since 2007. In [42], the authors described the load test program applied to a 3 MW HTS motor. These tests were conducted in the HTS motor test facility, which has the capability to reproduce real-life load scenarios of sea vessel propulsion. The partially superconducting machine is a radial-flux synchronous motor with a DI-BSCCO winding and an air-core rotor shaft. They ran constant load (performance and 100 h endurance) and variable load tests. According to [42], the motor demonstrated reliable operation during the tests. In the present case, the machine has been thoroughly tested in an environment built specifically to mimic real-life operation, being eligible to be considered a “relevant” environment in the TRL classification procedure.
Table 6 summarizes all the papers and projects that were reviewed and their TRL. All projects were considered to have achieved TRL 1, as the basic principles of all types of SEMs presented here had already been tested. TRL 2 is very application-specific, and it is common to find projects that would fulfill TRL 2 based on a basic, non-application-specific design defined by the stakeholders. They were also attributed TRL 2 for this evaluation, but it is recommended that future projects include more application-specific design in their research, as we argue later in the text. Most of the evaluated projects amounted to TRLs 3 and 4, meaning that the overall SEM technology has been experimentally proven as proof of concept, and most topologies were investigated with a prototype tested in a laboratory environment.
Most prototypes were classified as synchronous machines, working as either generators, motors, or both. In this case, as explained in Section 2.3, the field winding is superconducting and directly fed with DC current. Both partially and fully superconducting machines have been investigated. For all machines, but especially for fully superconducting machines, the targeted scenario is to dramatically reduce or eliminate the use of ferromagnetic materials, meaning that the machine operates with air cores in the rotor and/or the stator. One of the main concerns for this type of machine is related to the AC losses and, therefore, heat generation.
This research has found that the number of projects of trapped-flux radial machines, induction machines, and linear machines is lower than the number of projects dealing with synchronous machines. For trapped-flux machines, there is no current directly supplied to the field winding, rather it is induced. Most prototypes are partially superconducting machines with some type of ferromagnetic core. This is important because, as defined in Section 2.3, those cores help the magnetization process. The main concern in this case is to correctly assess all modes of operation of these machines. In operation, the machines can switch between synchronous and asynchronous modes, or they can be used in one of the two modes. This has been conducted for the investigated prototypes [29,31,32,33], with special attention to the synchronous mode of operation, as this is the one with the lowest AC losses.
The linear machine project reviewed here is a transverse flux linear motor [68]. It uses a hybrid secondary coil with Aluminum and short-circuited superconducting HTS tapes, relying on induced current in the stacks of HTS tapes.
With regard to magnetic flux, both radial-flux and axial-flux rotating machines have reached up to TRL 4. Projects with axial-flux machines tend to be less common. However, many different axial-flux machine configurations are investigated, for example, prototypes with ferromagnetic cores in [35] and without ferromagnetic cores [48] have been proposed, or under different operations, such as motor [61] and generator [35], as well. Material and shape diversities are also observed in the prototypes: 1G HTS coils [35], bulks, and NbTi coils [62], for instance.
As for applications, wind power applications are more advanced and constitute the majority of the projects, with the largest TRL, such as the EcoSwing project at TRL 7. Aircraft propulsion applications come second in the number of projects, followed by wave energy, transportation applications (MagLev, etc), marine propulsion, and, finally, power system stability.
Additionally, it is interesting to sort out the SEM patents that have been submitted within the same period of time (2017–2022). Our research has found 12 different patents that were submitted and/or published as patents in the US, China, Japan, South Korea, Europe, Germany, and the international patent offices. Four companies appear as patent holders: General Electric (GE), American Superconductor (AMSC), Siemens, and Rolls-Royce, as listed in Table 7. Some of these projects clearly state the machine application, mostly wind power. Furthermore, one can observe the particular focus on partially superconducting machines with the superconductors used in the field winding. All projects were submitted as patents to more than one patent office. Here, only one patent ID is used.

4. Future Perspectives of the Technological Development of SEMs

Finally, it is interesting to look ahead and ask what the perspectives for SEMs are. A good outline for future trends of SEMs is found in [21], where the authors point out some trends in the SEM research and development field. The first one is toward fully superconducting machines, which can be potentially more efficient than partially superconducting machines. The main challenge, in this case, are the AC losses in the armature winding. Another interesting trend is the attempt to reduce system complexity by avoiding having superconductors in the moving part of the machine.
An additional trend concerns radial- and axial-flux machinery, while radial-flux machines are more common, as we have noted, the axial-flux topology may be much more interesting for applications that require high torque density. Thus, it is expected that axial-flux machines may be predominant in the SEM research landscape in the near future.
In this work, a TRL evaluation framework for SEMs is proposed. This framework may also be applied as a guide to the research and development of SEMs. For example, research could start with the goal of achieving TRL 3. First, the project can prove via simulations/theoretical work or experiments the basic electromechanical energy conversion principle of the machine, therefore moving to TRL 1. TRL 2 is established with the design of a machine that fulfills the application requirements. TRL 3 is achieved as this design is applied to a small-scale prototype. The prototype ought to be experimentally investigated so that the machine’s critical functions are demonstrated. These steps help to ensure that even a small-scale prototype has been tested with the end-goal application in mind, increasing the quality of the research and development process.
The move from TRLs 3 to 4 may be very fast, given that they may be reached with the same small-scale prototype and experiments in a laboratory environment; while TRL 3 is achieved if the critical functions are proven, TRL 4 is achieved only after a thorough analysis of all possible states of operation through experiments, including electromagnetic, mechanical, and thermal analyses for all the machine parts. The move to TRL 5 can be challenging because this requires experiments in a relevant environment. This may mean adding more resources in order to build/adapt laboratories and/or experiment sites.

5. Conclusions

A review of the Technology Readiness Levels (TRLs) achieved by Superconducting Electric Machinery (SEM) projects published since 2017 was carried out. Firstly, an overview of superconductors and their applications to electric machinery were presented. Then, the TRL assessment system and its adaptation to the evaluation of superconducting machines, as well as the review of the levels achieved by current projects were covered.
Most projects fall into the TRL 3–4 range, meaning that prototypes have been tested in a laboratory environment. Two projects have surpassed this range. The first project, led by the Tokyo University of Marine Science and Technology of a synchronous superconducting motor, achieved TRL 5, having the prototype tested in a special facility that reproduced real-life load conditions. The second project, the EU 2020 EcoSwing, dealing with a synchronous generator, falls into the TRL 6–7 range. For this project, the full-scale prototype was thoroughly tested and even generated power for the Danish electric grid. These two projects show that SEMs have the potential to be the drivers for technological enhancements in power systems onshore and offshore.
Finally, a short commentary about the future prospects of this technology was provided to the reader. SEMs may have a bright future ahead of them. Collaborations between academia, governments, and industry seem to be one of the key drivers to improve TRLs targeting specific applications. These applications lead to the development of new topologies made into prototypes to be ultimately tested in situ.

Author Contributions

Conceptualization, G.G.S.; methodology, B.M.O.S., F.J.M.D., F.T., G.G.S. and R.d.A.J.; investigation, B.M.O.S. and F.T.; writing—original draft preparation, B.M.O.S., F.J.M.D., F.T., G.G.S. and R.d.A.J.; writing—review and editing, B.M.O.S., F.J.M.D., F.T., G.G.S. and R.d.A.J.; funding acquisition, G.G.S. and R.d.A.J. All authors have read and agreed to the published version of the manuscript.

Funding

This work has been financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, CAPES, Brazil—code 001; by the Conselho Nacional de Desenvolvimento Científico e Tecnológico, CNPq, Brazil; by the Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro, FAPERJ, Rio de Janeiro, Brazil; and by the Instituto Nacional de Ciência e Tecnologia em Energia Elétrica, INERGE/CNPq, Brazil.

Data Availability Statement

As this is a Review, no new data has been created. Data sharing not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Energies 16 05955 g001
Figure 1. Normalized power law for different superconducting materials with their typical n values. The electric field is E; the current density is J. Four types of materials are represented: YBCO, with n value from [5], BSCCO, with n value from [6], MgB 2 , with n value from [7], and NbTi, with n value from [8].
Figure 1. Normalized power law for different superconducting materials with their typical n values. The electric field is E; the current density is J. Four types of materials are represented: YBCO, with n value from [5], BSCCO, with n value from [6], MgB 2 , with n value from [7], and NbTi, with n value from [8].
Energies 16 05955 g001
Table 1. Statistics on superconducting materials used in SEMs to date [9,10,21,28]. The total number of cited materials is 137.
Table 1. Statistics on superconducting materials used in SEMs to date [9,10,21,28]. The total number of cited materials is 137.
MaterialConsidered in Design and Manufacture over All the 137 Referenced Projects (%)Year Span
NbTi331960s–1990s
Nb 3 Sn21980s–2010s
MgB 2 42010s–2020s
BSCCO231990s–2020s
REBCO382010s–2020s
Table 2. Typical superconducting rotors found in the literature applied in SEM.
Table 2. Typical superconducting rotors found in the literature applied in SEM.
Superconducting Rotor ConfigurationsMachine TypePower RangeReference
Energies 16 05955 i001Trapped FluxNA[30,31,32]
Energies 16 05955 i002Trapped Flux500 W[29,33]
Energies 16 05955 i003Synchronous Motor540 W[34]
Energies 16 05955 i004Axial-Field Synchronous500 W[35]
Energies 16 05955 i005Axial-Field Synchronous3 kW[36]
Energies 16 05955 i006Synchronous1 kW to 200 kW[37]
Energies 16 05955 i007Synchronous300 kVAr[38]
Energies 16 05955 i008Synchronous50 kW to 3 MW[39,40,41,42]
Energies 16 05955 i009Induction/Synchronous550 W to 50 kW[43,44,45,46]
Table 3. Typical superconducting stators found in the literature applied in SEM.
Table 3. Typical superconducting stators found in the literature applied in SEM.
Superconducting Stators ConfigurationsMachine TypePower RangeReference
Energies 16 05955 i010Axial Flux2 kW[35]
Energies 16 05955 i011Axial FluxNA[48]
Energies 16 05955 i012Synchronous2.5 to 10 MW[49]
Energies 16 05955 i013Synchronous20 kW to 50 kW[46,50]
Energies 16 05955 i014Synchronous2 kW[51]
Table 4. Technology Readiness Levels (TRLs) as defined by NASA in [53]. For the purposes of this work, the words “demonstrations” and “validations” may be used interchangeably.
Table 4. Technology Readiness Levels (TRLs) as defined by NASA in [53]. For the purposes of this work, the words “demonstrations” and “validations” may be used interchangeably.
TRLNASA’s Definition
1“basic principles observed and reported”
2“technology concept and/or application formulated”
3“analytical and experimental critical function and/or characteristic proof-of-concept”
4“component and/or breadboard validation in laboratory environment”
5“component and/or breadboard validation in relevant environment”
6“system model or prototype” validation “in relevant environment”
7“system model or prototype” validation “in target environment”
8“system completed through test and demonstration”
9“actual system “flight proven” through successful mission operations”
Table 5. TRL assessment for SEMs presented step by step. The bold font highlights the achieved TRL. Previous TRLs are in gray. All of the previous TRLs must be completed before the current TRL is achieved.
Table 5. TRL assessment for SEMs presented step by step. The bold font highlights the achieved TRL. Previous TRLs are in gray. All of the previous TRLs must be completed before the current TRL is achieved.
TRLQuestions
1Have the basic principles (electromechanical conversion of energy) been established and proven?
1,2Have the application and the specific design (electromagnetic and thermal) been defined?
1 to3Has the critical function (electromechanical characteristics and temperature) been measured with a basic prototype in a laboratory?
1 to4Has a small scale or basic prototype been fully tested (all possible operation points, thermal measurements) in a laboratory?
1 to5Has a small-scale or basic prototype been fully tested (all possible operation points and thermal measurements) in a relevant (emulating the target) site?
1 to6Has a prototype to scale with all its subsystems been fully tested (all possible operation points and thermal measurements) in a relevant (emulating the target) site?
1 to7Has a prototype to scale with all its subsystems been fully tested (all possible operation points and thermal measurements) in the target site?
1 to8Is there a completed system (electromechanical, cryogenic, or others) fully functioning?
1 to9Is there a completed system (electromechanical, cryogenic, or others) that has been fully functioning repeatedly without failure?
Table 6. Superconducting Electric Machine projects and their attributed TRLs.
Table 6. Superconducting Electric Machine projects and their attributed TRLs.
Country(ies)Superconducting?Machine TypeApplicationTRLReference
The Netherlands; Denmark; Germany; FrancePartiallySynchronousWind power7[40,41]
JapanPartiallySynchronousMarine propulsion5[42]
USA; South KoreaPartiallySynchronousAircraft3[54]
ChinaPartiallyFlux-switchingWind power3[55]
South KoreaPartiallySynchronousWind power/aircraft4[39,56]
ChinaFullyDirect-drive power converterWave energy3[57]
JapanFullySynchronousAircraft3[58]
RussiaFullySynchronous?4[59,60]
RussiaPartiallyTrapped-field; brushlessCryogenic electrical systems (ground and space)3[36]
JapanFullySynchronous; asynchronousTransportation systems4[45,50]
ChinaPartiallySynchronousPower system stability3[38]
FrancePartiallyAxial-flux-4[61]
FrancePartiallyAxial-flux-3[35]
Algeria; RomaniaPartiallyAxial-fluxAircraft3[62]
Japan; UKPartiallyInductionAircraft4[63]
ChinaPartially-Wind power3[64]
ChinaPartially-Wave energy3[65]
ItalyPartiallyAxial-flux-4[48]
ChinaPartiallyHomopolarAircraft3[66,67]
ChinaPartiallyTransverse flux linear motorMagnetic levitation4[68]
RussiaPartiallySynchronousTransportation4[37]
UK; GermanyPartiallyTrapped-flux; synchronous-3[31,32]
ChinaPartiallyDoubly fed induction-4[51]
BrazilPartiallyTrapped-fluxAircraft3[29,33]
South KoreaPartiallySynchronousWind power4[69]
JapanFullyInduction; synchronous-4[46]
Table 7. List of patents of Superconducting Electric Machinery made available since 2017.
Table 7. List of patents of Superconducting Electric Machinery made available since 2017.
Patent IDTitleHolder
US11261847B2Wind turbine having superconducting generator and method of operating the sameGE
US20220302815A1Field coil support structure and modular field coil design in a superconducting machineGE
US20210211036A1Partial cryogenic shielding assembly in a superconducting generator and methods of assembling the sameGE
US8436499B2Electrical machine with superconducting armature coils and other componentsGE
US20220014072A1Superconducting generator driven by a wind turbineGE
US10601299B2High-temperature superconductor generator with increased rotational inertiaAMSC
US20210408888A1Rotor with superconducting winding for continuous-current-mode operationSiemens
US20210375541A1Electrical machine and method for fabrication of a coil of an electrical machineSiemens
US20210344256A1Rotor and machine having superconducting permanent magnetsSiemens/Rolls-Royce
DE102018205170A1Maschinenkomponente sowie elektrische Maschine mit supraleitendem SpulenelementSiemens/Rolls-Royce
DE102016205216A1Elektrische Maschine mit supraleitfähigem Permanentmagneten und Verfahren zum Magnetisieren des PermanentmagnetenSiemens/Rolls-Royce
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Santos, B.M.O.; Dias, F.J.M.; Trillaud, F.; Sotelo, G.G.; de Andrade Junior, R. A Review of Technology Readiness Levels for Superconducting Electric Machinery. Energies 2023, 16, 5955. https://doi.org/10.3390/en16165955

AMA Style

Santos BMO, Dias FJM, Trillaud F, Sotelo GG, de Andrade Junior R. A Review of Technology Readiness Levels for Superconducting Electric Machinery. Energies. 2023; 16(16):5955. https://doi.org/10.3390/en16165955

Chicago/Turabian Style

Santos, Bárbara Maria Oliveira, Fernando Jorge Monteiro Dias, Frederic Trillaud, Guilherme Gonçalves Sotelo, and Rubens de Andrade Junior. 2023. "A Review of Technology Readiness Levels for Superconducting Electric Machinery" Energies 16, no. 16: 5955. https://doi.org/10.3390/en16165955

APA Style

Santos, B. M. O., Dias, F. J. M., Trillaud, F., Sotelo, G. G., & de Andrade Junior, R. (2023). A Review of Technology Readiness Levels for Superconducting Electric Machinery. Energies, 16(16), 5955. https://doi.org/10.3390/en16165955

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