A deep dive into electromagnetic fields from parallel wireless power systems—and what science says about staying safe.
How Safe Are Wireless Power Systems? Exploring the Science of EMF Exposure
Wireless Power Transfer (WPT) is transforming how we charge devices, from electric vehicles (EVs) to household appliances, by sending energy through magnetic fields without cables. As these systems scale up—think multiple EV chargers in a parking lot—questions arise about electromagnetic fields (EMFs) and their impact on human health. A 2017 study by Feng Wen and Xueliang Huang from Southeast University digs into this, examining what happens when two WPT systems operate side-by-side. They analyzed magnetic field leakage, induced electric fields in the body, and energy absorption (SAR), comparing the results to safety standards from the International Committee on Non-Ionizing Radiation Protection (ICNIRP). Let’s unpack their findings and what they mean.
The Experiment: Modeling Two WPT Systems
The researchers simulated two WPT systems, like EV chargers parked next to each other. Each system has a transmitter coil (Tx) sending power and a receiver coil (Rx) catching it. They tested two configurations:
- In-phase: The magnetic fields oscillate in sync, peaking and dipping together.
- Anti-phase: The fields oscillate oppositely—one peaks as the other dips. Using computer models, they calculated EMF effects on a detailed human figure (male, 30 years old, 180 cm, 70 kg, with organs like lungs and liver included). They focused on three metrics: magnetic field leakage escaping the coils, electric fields induced in the body, and SAR (energy absorption in tissues), all checked against ICNIRP guidelines.
Magnetic Field Leakage: Mapping the Spread
Magnetic fields don’t stay confined to the coils—they leak out, measurable in amps per meter (A/m). The study gauged this 1 meter above ground, a typical height for human exposure.
- In-Phase Results: When the systems operate in sync, the fields reinforce each other at the edges, reaching 0.75 A/m at 3.3 kW power. Between the coils, though, the waves partially cancel, reducing the field strength and making the middle safer.
- Anti-Phase Results: When out of sync, the fields clash in the center, peaking at 0.75 A/m—slightly above ICNIRP’s 1998 limit of 0.73 A/m, though well below the 2010 limit of 21 A/m. The edges, meanwhile, see weaker fields.
- Shielding Effects: Adding aluminum plates above the coils changed things. Large plates (e.g., 80 cm wide, exceeding the 25 cm coils) absorbed or redirected the field, lowering leakage everywhere—highly effective. Smaller plates (25–35 cm) had the opposite effect, sometimes amplifying the field, especially in anti-phase. This happens because small plates disrupt the magnetic coupling between coils, redirecting energy outward instead of blocking it.
Electric Fields in the Body: Where the Energy Lands
Magnetic fields hitting a person generate internal electric fields, measured in volts per meter (V/m), which could affect nerves if too strong. The study tested five positions (A–E) around the systems, adjusting the human model’s orientation in 30-degree steps.
- Peak Strength: The highest field, 21.0 V/m, occurred at position A (between the systems) during anti-phase, with the body facing the coils. This is well below ICNIRP’s 135 V/m limit, which prevents nerve overstimulation.
- Distribution Patterns: Typically, the field concentrates in the trunk, groin, and genitals—areas rich in conductive tissue. An exception popped up in-phase at position A: the field shifted to the legs. This occurs because the synchronized magnetic waves align differently, pushing the energy downward. Legs are less critical than the trunk (fewer vital organs), so this is a safer scenario.
- Position and Angle Impact: At the midpoint (A), anti-phase fields varied more with orientation, while in-phase stayed consistent—easier to predict and manage. At position B (off to the side), the trunk took the brunt, but anti-phase was milder overall. Positions C–E (farther out) showed weak fields across the board.
SAR: Measuring Energy Absorption
SAR, or Specific Absorption Rate, tracks how much EMF energy tissues absorb, measured in watts per kilogram (W/kg). Too much could heat cells, so ICNIRP sets limits: 0.08 W/kg for the whole body, 2 W/kg for head and trunk.
- Findings: At 3.3 kW, SAR stayed tiny—e.g., 0.119 W/kg max in the trunk, far under limits. Lungs absorbed the most (up to 4.5 × 10⁻⁴ W/kg in anti-phase) due to their low density (394 kg/m³), making them spongier targets. The liver, denser and smaller, barely registered (2.2 × 10⁻⁸ W/kg).
- Power Thresholds: The study calculated 55.5 kW as the max power to keep SAR within ICNIRP’s strict rules. For the conservative 0.73 A/m magnetic field limit, it’s 3.1 kW. Since this frequency (1 MHz) doesn’t cause shocks or burns, SAR and electric field limits take priority—55.5 kW prevails as the practical ceiling.
- Trends: Anti-phase at the midpoint spiked SAR higher than in-phase, but all values were safe. It hinges on position and system sync.
What This Means for Safety
- Strategic Syncing: In-phase is safer between chargers—good for tight spaces—while anti-phase works better at the edges, like near car doors.
- Shielding Smarts: Large plates block fields well; small ones can backfire—size matters because of how fields interact with metal.
- Body Position: How you stand or face the system shifts exposure—designers need to account for that variability.
- Power Play: Up to 55.5 kW is fine for health, even if it tops the old magnetic limit, since SAR and electric fields are the real safety yardsticks.
Wrapping Up
Wen and Huang’s study shows parallel WPT systems can coexist safely with the right tweaks—sync them properly, shield them effectively, and cap the power sensibly. As wireless charging expands to EVs and beyond, this research ensures it’s practical without putting us at risk. Next time you’re near a wireless charger, rest easy: the science has your back.
Published February 21, 2025, based on research by Feng Wen and Xueliang Huang, Southeast University.
