Sihun Kim, Undergraduate Student, Elmore Family School of Electrical and Computer Engineering, Purdue University
Dionysios Aliprantis, Professor, Elmore Family School of Electrical and Computer Engineering, Purdue University
April 2023
Researchers in academia and industry are developing dynamic (in-motion) wireless power transfer technology (DWPT) to directly "charge" moving vehicles through the road. At Purdue University, this research effort is supported by the Indiana Department of Transportation, and by the Advancing Sustainability through Powered Infrastructure for Roadway Electrification (ASPIRE) Engineering Research Center, which is funded by the National Science Foundation.
Why is this technology important?
According to the Environmental Protection Agency, transportation is the leading contributor of greenhouse gas emissions in the United States at 27% in 2020. [1]
Automotive manufacturers are racing to deploy electric vehicles to reduce greenhouse gas emissions at the tailpipes. However, a critical barrier to the adoption of electric vehicles is range anxiety: the fear that drivers will not have enough charge in their batteries to arrive at their destination. DWPT technology aims to dispel that fear by directly "charging" the vehicles as they move.
This technical blog will explore the DWPT standards ensuring public safety as there are electromagnetic fields associated with the technology. Complying to these standards are critical for commercialization.
Figure 1. Dynamic Wireless Power Transfer. Adapted from "ASPIRE Electrified Transportation Systems Course"
DWPT utilizes the physical phenomenon of electromagnetic induction to transmit energy between a transmitter beneath the road and a receiver underneath the vehicle. As the vehicle is driving over the road, the corresponding transmitter pad underneath the road will energize, powering the vehicle. The inductive DWPT system developed at Purdue University will operate at approximately 200kW and 85kHz to power heavy-duty trucks for long distance freight transport. The electromagnetic waves associated with the DWPT are non-ionizing and do not propagate to far distances like antenna radio waves. Some of these terms are further explained in the next section.
Review of Fundamental Concepts
Power: Rate at which energy is transferred. The unit of power is the watt (W). Example: A laptop charger may range from 40W to 150W.
Electromagnetic waves: According to [2], electricity can be static, like the electric charges that makes your hair stand. Magnetism can be static as well, like a magnet on your refrigerator. However, a dynamic, or changing, magnetic field will induce a dynamic electric field and vice-versa. These changing fields form electromagnetic waves and carry energy.
The main parameter of the magnetic field is the magnetic flux density. The unit of magnetic flux density is the tesla (T). As magnetic flux density increases, the energy stored in the field increases. Example: MRI scanners range from 1.5T to 3T, and people experience approximately up to 50µT, or 50 millionths of a tesla from the Earth's magnetic field on the surface of the Earth.
The unit of an electric field is volt/meter (V/m). As electric field increases, the energy stored in the field increases. Example: A typical electric field value near a toaster is 80V/m [3].
Waves have a frequency, which is the rate at which they oscillate. The unit of frequency is cycles per second, or Hertz (Hz). Example: A microwave operates at 2.4GHz, or 2.4 billion Hz.
Waves can be characterized by their root-mean-square (rms) value. The rms value is computed by taking the square root of the average of the square of the waveform. For a pure sine wave, the rms value is the peak value divided by the square root of 2.
Moreover, human and animal bodies can act as conductors and perturb low frequency electric fields. For magnetic fields, because the permeability of the body is similar to that of air, human and animal bodies do not significantly perturb the field [6].
According to [4], electromagnetic waves, or radiation, can either be ionizing or non-ionizing.
Ionizing radiation: Ionizing radiation is a high-energy form of radiation that can displace electrons from atoms and molecules through which it passes. What this means is that ionizing radiation can alter the cells in our body and can lead to cancer. Ionizing radiation ranges from X-rays to gamma rays and according to [5], ionizing radiation waves have a frequency starting at approximately 10^21Hz. Example: X-rays, which can penetrate our body and create images of our bones.
Non-ionizing radiation: Non-ionizing radiation lacks sufficient energy to remove electrons from atoms or molecules due to their lower frequency. Non-ionizing radiation waves range from radiowaves to Ultraviolet radiation, which has a frequency up to approximately 10^21Hz [5]. This poses less of a threat than their ionizing radiation counterparts. Example: microwaves, which use microwave radiation to heat food.
Metric System Units:
Name | Number | Prefix | Symbol |
billion | 1 000 000 000 | giga | G |
million | 1 000 000 | mega | M |
thousand | 1000 | kilo | k |
thousandth | 0. 001 | milli | m |
millionth | 0. 000 001 | micro | µ |
For example, 1kHz is 1000 Hz and 27µT is 27 millionth of a tesla.
International Reference Levels on Electromagnetic Non-Ionizing Radiation
Because of the potential safety concerns these charging systems pose regarding electromagnetic fields (EMF), numerous organizations have established regulations in order to ensure public safety, such as the Federal Communications Commission of the United States. These regulations are based off published scientific guidelines and standards. Here, we take a dive into the standards published by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) [6] and the Society of Automotive Engineers (SAE) [7]. Currently, the SAE standard applies to stationary wireless power transfer. DWPT standards are under development, but will expand on the stationary wireless power transfer standards and are expected to include similar limits on the electromagnetic fields. Complying to these standards are critical for any commercial DWPT system.
According to ICNIRP standards [6], these regulations have two relevant quantities: reference levels and basic restrictions.
Basic restrictions are defined as: Maximum level of electric fields a person can experience internally without stimulation effect or heating [6]. Stimulation effect is when electrically sensitive cells, like nerve cells, become stimulated, resulting in tingling. The heating is the same effect a microwave has on heating its food, as a microwave emits non-ionizing radiation. Meeting these basic restrictions ensure that these health complications will not occur. Basic restrictions are shown in Table 1.
Table 1. ICNIRP 2010 Basic Restrictions for General Public Exposure
Exposure Characteristic | Frequency Range (kHz) | Internal Electric Field rms (V/m) |
All tissues of head and body including central nervous system head tissue | 85 | 11.475 |
Reference levels are defined as: Maximum exposure of electromagnetic fields to ensure safety. Reference levels are derived from mathematical models of basic restrictions from published data [6].
In essence, reference levels correspond to the external fields that induce a particular electromagnetic field inside the human body. These induced electromagnetic fields correspond to basic restrictions. The reason why reference levels are established is because "the internal electric field strength is difficult to assess. Therefore, for practical exposure assessment purposes, reference levels of exposure are provided. Most reference levels are derived from relevant basic restrictions using measurement and/or computational techniques..."[6]. Reference levels are shown in Table 2.
Therefore, it is clear that reference levels are sufficient as [6] states that "Compliance with the reference level will ensure compliance with the relevant basic restriction."
Table 2. Reference levels for exposure to time-varying electric and magnetic fields (unperturbed rms values)
Frequency (Hz) | Magnetic Flux Density (µT, rms) | People Group |
50 | 200 | General Public |
3k—10M | 27 | General Public |
79k—90k | 15 | People with cardiac implantable electronic devices (CIED) |
According to [6], the magnetic field reference level for the general public is 27µT for 3kHz—10MHz. However, Table 2 also shows that the reference level at 85kHz for those with CIEDs is 15µT [7]. CIEDs include pacemakers. Therefore, the ultimate reference level that DWPT needs to meet for this frequency is 15µT.
The reference levels mentioned in Table 2 are applicable in situations when a person is beside, above, and inside a vehicle, but not under [7]. The CIED reference level was established such that the maximum magnetically induced voltage does not interfere with the operation of the CIED [7].
Further Discussion on Reference Levels and Basic Restrictions
A main source that has informed the ICNIRP standard in terms of safe electromagnetic field level is [8], which is a scientific paper published in 2005. This study produced a 3D female model from a test subject via MRI. From this model, the induced electric field basic restriction was computed via an algorithm known as the scalar potential finite differences method. The results are shown in Table 3.
Table 3. Calculated external magnetic flux density from basic restrictions [8]
Frequency (Hz) | Calculated external magnetic flux density (µT) |
50 | 412 |
1k | 20.6 |
10k | 22.0 |
100k | 25.4 |
1M | 28.6 |
10M | 32.8 |
The values in Table 3 include a reduction factor of 5 for additional safety precautions. The reduction factor, according to [6], has the following description: "Reduction of the effect threshold to compensate for various sources of uncertainty in the guideline setting process. Some examples of sources of uncertainty about exposure-effect threshold levels include the extrapolation of animal data to effects on humans, differences in the physiological reserves of different people with corresponding differences in tolerance, and statistical uncertainties (confidence limits) in the dose response function. In ICNIRP’s view, uncertainty in measurements used to implement the guidelines is a problem more appropriate to the functions of organizations responsible for the development of compliance methods."
The five black data points in Figure 2 correspond to last five values in Table 3. The red data point of 27µT is the ICNIRP reference level for the frequency range 3kHz—10MHz given in Table 2. It is plotted at the frequency 85kHz since that is the operating frequency of the DWPT system. This red data point is plotted to reveal its relative position to Table 3 values. The derivation of the red data point reference level of 27µT given in the ICNIRP standard from the Table 3 values is unclear.
Figure 2. Calculated magnetic flux density results from Table 3 and general public reference level from Table 2 vs frequency.
However, [6] does give an example for deriving the reference level from the basic restriction for the frequency at 50Hz. At 50Hz, the factor used to convert the electric field basic restriction to the magnetic flux density reference level for central nervous effects is 33V/m per T. ICNIRP does not expand on where this conversion factor came from. Additionally, a reduction factor of 3 was applied to these calculated values to allow for radiation exposure uncertainty.
According to [6], the central nervous system tissue of the head's basic restriction is (0.0004)*(frequency in Hz). The central nervous system restriction is utilized as it is most strict. Inserting f = 50Hz, the computation for this example is as follows:
The calculated reference level quantity of 200µT matches with the value provided in Table 2 for 50Hz.
Comparison to Other Standards
IEEE C95.1 standard establishes a general public reference level of 205µT at 85kHz [9]. This source does not address people with CIEDs. This reference level is approximately 7.5 times greater than the 27µT general public reference level established by ICNIRP.
Conclusion
Purdue University researchers are developing a DWPT system to meet energy demands of electric vehicles and to further accelerate the adoption of electric vehicles. In order to ensure real-world feasibility, the electromagnetic fields associated with the DWPT must meet reference levels established by various organizations. These reference levels are derived from basic restrictions by algorithms using published data from [8]. In setting reference levels, reduction factors were included to account for any uncertainties in the process and provide an extra level of safety. Non-ionizing electromagnetic waves at levels below these reference levels are safe.
Further Resources
The effects of the electromagnetic field (EMF) radiation and safety concerns are elaborated in more detail in this article by a ASPIRE industry partner, WiTricity: Analysing the safety of wireless electric vehicle charging (https://www.innovationnewsnetwork.com/analysing-safety-wireless-electric-vehicle-charging/20502/)
ASPIRE website: https://aspire.usu.edu/
Purdue/INDOT press release: https://www.in.gov/indot/current-programs/innovative-programs/wireless-electric-vehicle-charging-solution-for-highway-infrastructure/
Citations
[1] Sources of Greenhouse Gas Emissions. (2022, August 5). Retrieved March 26, 2023, from https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emissions
[2] National Aeronautics and Space Administration, Science Mission Directorate. (2010). Anatomy of an Electromagnetic Wave. Retrieved March 26, 2023, from http://science.nasa.gov/ems/02_anatomy
[3] Radiation: Electromagnetic fields. (2016, August 4). Retrieved March 26, 2023, from https://www.who.int/news-room/questions-and-answers/item/radiation-electromagnetic-fields
[4] Radiation studies: Ionizing radiation. (2021, June 29). Retrieved March 26, 2023, from https://www.cdc.gov/nceh/radiation/ionizing_radiation.html#:~:text=Non%2Dionizing%20radiation%20is%20a,%2C%20water%2C%20and%20living%20tissue.
[5] Radiation. Occupational Safety and Health Administration. (n.d.). Retrieved April 9, 2023, from https://www.osha.gov/radiation
[6] International Commission on Non-Ionizing Radiation Protection (ICNIRP) (2010). Guidelines for limiting exposure to time-varying electric and magnetic fields (1 Hz to 100 kHz). Health physics, 99(6), 818–836. https://doi.org/10.1097/HP.0b013e3181f06c86
[7] J2954_202010: Wireless Power Transfer for light-duty plug-in/electric vehicles and alignment methodology. (2020, October 20). Retrieved from https://www.sae.org/standards/content/j2954_202010/
[8] Dimbylow P. (2005). Development of the female voxel phantom, NAOMI, and its application to calculations of induced current densities and electric fields from applied low frequency magnetic and electric fields. Physics in medicine and biology, 50(6), 1047–1070. https://doi.org/10.1088/0031-9155/50/6/002
[9] "IEEE Standard for Safety Levels with Respect to Human Exposure to Electric, Magnetic, and Electromagnetic Fields, 0 Hz to 300 GHz," in IEEE Std C95.1-2019 (Revision of IEEE Std C95.1-2005/ Incorporates IEEE Std C95.1-2019/Cor 1-2019) , vol., no., pp.1-312, 4 Oct. 2019, doi: 10.1109/IEEESTD.2019.8859679.