In this paper, the power transfer efficiencies (PTEs) of magnetic resonance (MR) wireless power transmission (WPT) and radio frequency (RF) WPT are compared as a function of the distances between resonators (or antennas). The PTE of the C-loaded loop resonators during MR WPT was theoretically calculated and simulated at 6.78MHz, showing good agreement. The PTE of the patch antennas, whose area is the same as the C-loaded loop resonator during MR WPT, was theoretically calculated using the Friis equation and the equation by N. Shinohara and simulated at 5.8 GHz. The three results from the Friis equation, the equation by N. Shinohara, and from a full wave simulation are in strong agreement. The PTEs, when using the same size resonators and antennas are compared by considering the distance between the receiver and transmitter. The compared results show that the MR WPT PTE is higher than that of the RF WPT PTE when the distance (r) between the resonators (or antennas) is shorter. However, the RF WPT PTE is much higher than that of the MR WPT PTE when the distance (r) between the resonators (or antennas) is longer since the RF WPT PTE is proportional to r^{-2} while the MR WPT PTE is proportional to r^{-6}.
Wireless power transmission (WPT) the transmission of energy across free space without the use of wires has received considerable attention because this technology could be applied for the next IT products such as wearable displays and internet of things (IoT) devices. Magnetic resonance (MR) WPT makes use of the strong magnetic coupling in the near-field region. It consists of two resonators, one for the transmitter and the other for the receiver, which operate at the same resonant frequency.
MR WPT systems can transfer energy over a longer distance than magnetic induction (MI) WPT systems.
However, the power transfer efficiency (PTE) of MR WPT dramatically decrease with increasing distance. Thus, it still has limitations when in longer distance power transfer is required.
Radio-frequency (RF) WPT has become an alternative solution for longer distance power transfer. Thus, the RF WPT is actively being developed [1-3]. Even though RF WPT is capable of transmitting power over longer distances, there has been no quantitative comparison between the PTEs of MR WPT and RF WPT to date. In this paper, the PTEs of MR WPT and RF WPT are compared as a function of the distances between the resonators (or antennas) when the areas of the antennas and the resonators are the same. It is expected that this comparative study will provide guidelines on selecting a more efficient WPT method depending on the applications.
As shown in Fig. 1, the MR WPT model is considered as an equivalent circuit comprising of a 2-resonators systems, which means that the primary loop is a transmitter and the secondary loop is a receiver.
The MR WPT PTE (
where
The maximum PTE can be achieved by substituting Eq. (2) with Eq. (1). Then, the PTE is a function of
where
where
On the other hand, the RF WPT PTE can be obtained by the Friis equation, as follows [7]:
where
Eq. (7) can thus be used to estimate the PTE in the nearfield region of RF WPT.
To compare the MR WPT and RF WPT PTEs, the PTEs for the same size of antennas and resonators of 38 mm × 36 mm × 1.6 mm are shown in Fig. 2 since the PTE is strongly dependent on the size of the antenna (or resonator). Fig. 2(a) shows the configuration of the resonators for MR WPT. The resonator consists of a loop and a capacitor (11.3 μF) in series. Fig. 2(b) shows the configuration of the patch antennas for RF WPT.
Fig. 3 compares the calculated and simulated PTEs for both MR WPT and RF WPT as a function of distance and at frequencies of 6.78 MHz and 5.8 GHz, respectively. The calculated MR WPT PTE result is given by Eqs. (1)–(4) when the diameter of the circle loop (2
As shown in the calculated and simulated results in Fig. 3, the MR WPT method is better than the RF WPT method for a short distance of 14–15 cm. However, the RF WPT method has better PTE over longer distances, since the MR WPT PTE is proportional to
This paper presents comparisons of MR and RF WPT PTEs as a function of distance. The PTEs were estimated both through theory and simulation. The theory predicts that the RF WPT PTE is proportional to