Wearable electromagnetic energy-harvesting textiles based on human walking

Abstract

The need for self-generating energy for outdoor environments is growing. To cope with emergencies in outdoor environments, sustainable, environment-independent energy-harvesting methods based on natural human movements are necessary. In this study, we developed a wearable energy-harvesting textile with a structure capable of generating the maximum energy from the swinging motion that occurs during walking. Two types of conductive yarn were manufactured and, when used as coils, are as durable as wire coils and have excellent flexibility and wearability, allowing easy integration into outdoor garments. Design variables related to the effect of energy production were investigated and the wearable energy-harvesting textile was evaluated by the average current generated. The conductive yarn coils can be connected in a serial circuit method and were evaluated to generate energy at a greater efficiency than wire coils. The average current increased as the number of turns of coil, the magnetic field, and the coil swing speed increased. The average currents along the distance between the magnet and the coil varied with magnet forces inversely proportional to distance.

Keywords

wearable energy-harvesting textiles;swing motion;electromagnetic transduction;conductive yarn coil

Wearable energy-harvesting technology allows the wearer to naturally produce and collect energy in outdoor environments. Energy harvesting through habitual and repetitive human body movements can be useful during outdoor emergencies when electricity is urgently needed. Outdoor communication disruption and GPS malfunction often occur due to the battery discharge of portable devices, which can lead to emergencies. Therefore, outdoor activists benefit from self-generating energy to prevent such situations of distress.¹

The most fundamental human movement is walking. Walking includes the movement and vibration of the whole body, ground contact with the soles of the feet, and the swinging movement of the arms and legs. Many studies have used these movement sources as means of harvesting energy: body movement and vibration during walking and other activities,^2–7 pressure between the soles of the feet and the ground,^8–12 and the swing of the arms and legs.^13–18 Power-generation methods include triboelectric,^19,20 piezoelectric,^{8–10,13,14,16–19} and electromagnetic^{2–7,11–13,15,20} transduction. Previous studies have designed harvester structures that increase energy-generation efficiency. Triboelectric generation is suitable for outdoor environments where material friction is frequent; however, humidity and friction damage lead to performance degradation.²¹ Piezoelectric generation requires a process of converting mechanical strain into an electric current or voltage.^18,19 To minimize the influence of environmental factors and friction, many electromagnetic studies have been carried out. Research related to the development of electromagnetic harvesters has used the irregular movement of the human body. In these studies, the magnets were designed to move in one direction within the coil device to maximize energy generation.²

Therefore, an energy harvester for outdoor activities must have a structure in which the human walking movement is naturally connected to energy generation. The purpose of this study is to design wearable energy harvesting textiles (WEHT) that use the regular and bidirectional motion of the arms and legs to maximize energy generation. The WEHT is unobtrusively well integrated into outdoor garments. The concept of the integrated energy-harvesting system of this study is displayed in Figure 1 and includes the WEHT based on the swing of the arms and legs during walking. Flat-type magnets and coils were used for the structure, which generates the maximum energy in the swing motion. Two types of conductive yarn were developed for the coils. Conductive yarns are more flexible and wearable than metal wire and have durability. To optimize the performance of the WEHT, many combinations of design variables were tested for energy harvesting efficacy, including conductive yarn types, the number of turns of the coil, circuit connection methods, magnet arrangement, the speed of the swing, and the distance between magnet and coil.

Figure 1.

Concept for integrated energy harvesting system design. This design intends to use wearable energy harvesting textiles for harvesting energy.

Materials and methods

Design of the WEHT

The WEHT is designed to generate energy through the arm-swing motion. In the WEHT, the magnet is on the side of the torso and the coil is on the inside of the upper arm. The WEHT applies Fleming’s right-hand rule (Figure 2). This rule indicates the direction of the current induced when a conductor attached to a circuit moves within a magnetic field. Electromagnetic force (E) is defined by equation (1), where B is the magnetic field, l is the length of the conductor, v is the velocity of the conductor, and θ is the angle between the current direction and the magnetic field direction.²³

E = Blv sin θ

(1)

Figure 2.

Fleming’s right-hand rule.

The WEHT in this study is designed using equation (1) to generate the maximum energy through the swing motion. The current direction is perpendicularly positioned relative to the movement of the coil and the magnet field. When the rectangular flat-type coil moves very close to the surface of the flat magnet θ is 90°, so the generated current in the coil is at its maximum. Figure 3(a) shows the direction of the current induced in a coil relative to the coil-moving direction near one flat magnet with the N pole and S pole arranged back and forth. Figure 3(b) shows the current induced in a coil relative to the coil-moving direction near two flat magnets arranged side by side. The current direction in the coil changes according to the coil-moving direction and magnet-field direction. Positive (+) and negative (−) voltage waveforms occur during the process. Negative (−) voltage waveforms generated through the WEHT can also be used after positive rectification through bridge diodes. The energy generated in the WEHT was expressed as the average current for the pulse period as shown in Figure 4.

Figure 3.

Current direction and waveform of the wearable energy harvesting textiles according to the moving direction of the flat coil; (a) one magnet arrangement and (b) two magnet arrangement.

Figure 4.

Expression of average current from wearable energy-harvesting textiles.

Conductive yarn coil

For the WEHT, conductive yarns are used to fabricate the wearable flat coils. Two types of conductive yarn were developed. Figure 5 shows the manufacturing process and the structure of the conductive yarns. The manufacturing method was developed in our previous research.^24,25 In the first step of the manufacturing process, a polyester filament (70 denier) was covered with Polyurethane(PU)-coated copper filament (Ø_Cu: 0.060 mm, Ø_PU-Cu 0.070 mm) in the S-direction with 150 TM. PU-copper wrapped polyester yarn was additionally twisted in the S-direction with 700 TM (conductive single yarn). The subsequent steps are to fabricate conductive plied yarns using multiple strands of conductive single yarns. For the conductive plied yarn of type 1, two strands of conductive single-yarn were wrapped with polyester filament (20 denier) with 100 TM and then additionally twisted with 500 TM. For the type 2 yarn, three strands of conductive single-yarn were wrapped by polyester filament (20 denier) with 100 TM and then additionally twisted with 450 TM. The characteristics of conductive yarns are listed in Table 1.

Figure 5.

The structure of the conductive yarns: (a) manufacturing process of the conductive composite yarns and (b) side views of the developed conductive yarns (type 1 and type 2).

Table 1.

Characteristics of conductive yarn

Yarn type	Composition (wt%) Cu: Polyester	Linear density (denier)	Thickness (mm)	Resistance (Ω/cm)	Load at yield (N)	Max. load (N)	Strain (%)
Type 1	74.3: 25.7	652	0.214	0.0329	3.255	11.994	18.808
Type 2	75.2: 24.8	980	0.272	0.0221	4.201	17.340	19.370

Conductive yarn was wound on rectangular flat-type plastic bobbins with dimensions of 20 mm width, 60 mm height, and 3 mm height, and 3 mm depth as shown in Figure 6. As shown in Figure 3, the bobbin should be sized to effectively generate a current without cancelling out the generated current. The bobbin is longer and wider than the magnet. Type 1 yarn and type 2 yarn were wound on bobbins with 50, 100, 150, and 200 turns and samples were created with both parallel and serial circuits as shown in Figure 7.

Figure 6.

Geometry of bobbin and an example of fabricated conductive yarn coil (Y2_200T).

Figure 7.

Circuit connection method of the conductive yarn coils; (a) parallel and serial connection of the type 1 yarn coil and (b) parallel and serial connection of type 2 yarn coil.

Evaluation methods

To activate the WEHT, an instrument was created to perform the swing motion. The instrument can be turned to control the speed of the swing and the distance between the magnet and the coil. As shown in Figure 8, the linear motion of the rail connected to the rolling motion device is transformed into the swinging motion of the mechanical arm. The length of the mechanical arm is 20 cm, which is the distance between the center of the mechanical arm and the center of the coil. This length was chosen to be equal to the length of the upper arm of a standard adult male. A magnet (NdFeB, width: 20 mm, height: 40 mm, depth: 10 mm, Gauss range 4000 ∼ 4300, weight: 120 g) is fixed at a position facing the attached coil.

Figure 8.

The setup of swing instrument for performance evaluation of wearable energy harvesting textiles.

To evaluate the WEHT’s ability to harvest energy, the effects on energy harvesting of conductive yarn types, the number of turns of coil, circuit connection methods, magnet arrangement, the distance between magnet and coil, and the speed of swing were considered for the experiment. As shown in Table 2, 32 modules were prepared prior to the experimentation. For the experiments, the speed of swing was set at 1.0 Hz for slow walking and 1.4 Hz for fast walking or running.^26,27 The distance between the magnet and the coil was set to be gradually increased from 1 mm to 15 mm. All evaluations were carried out four times, and the mean value was taken for each experiment. All of the WEHT’s power generation was measured using Tektronix TDS2012C (100 MHz, 2-Ch, 2 GS/s Digital Storage Oscilloscope).

Table 2.

ID classification of the wearable energy harvesting textiles

ID	Yarn type	Coil turns	Circuit connection	Magnet arrangement
Y1_50T(P)_M1	Y1	50	P	M1
Y1_50T(P)_M2			P	M2
Y1_50T(S)_M1			S	M1
Y1_50T(S)_M2			S	M2
Y1_100T(P)_M1		100	P	M1
Y1_100T(P)_M2			P	M2
Y1_100T(S)_M1			S	M1
Y1_100T(S)_M2			S	M2
Y1_150T(P)_M1		150	P	M1
Y1_150T(P)_M2			P	M2
Y1_150T(S)_M1			S	M1
Y1_150T(S)_M2			S	M2
Y1_200T(P)_M1		200	P	M1
Y1_200T(P)_M2			P	M2
Y1_200T(S)_M1			S	M1
Y1_200T(S)_M2			S	M2
Y2_50T(P)_M1	Y2	50	P	M1
Y2_50T(P)_M2			P	M2
Y2_50T(S)_M1			S	M1
Y2_50T(S)_M2			S	M2
Y2_100T(P)_M1		100	P	M1
Y2_100T(P)_M2			P	M2
Y2_100T(S)_M1			S	M1
Y2_100T(S)_M2			S	M2
Y2_150T(P)_M1		150	P	M1
Y2_150T(P)_M2			P	M2
Y2_150T(S)_M1			S	M1
Y2_150T(S)_M2			S	M2
Y2_200T(P)_M1		200	P	M1
Y2_200T(P)_M2			P	M2
Y2_200T(S)_M1			S	M1
Y2_200T(S)_M2			S	M2

M1: one magnet; M2: two magnets; P: parallel; S: serial; Y1: type 1; Y2: type 2.

Results

Figure 9 presents the generated waveforms on the WEHT. This result is consistent with the waveforms predicted in Figure 3. The peak current of the WEHT was higher in the two-magnet arrangement and serial circuit connection conditions than the other arrangements.

Figure 9.

Power generation waveforms depending on yarn type (type 1 (a) and type 2 (b)), the coil circuit connection method and magnet arrangement.

Figure 10 shows the average current generated by the type 1 coil (Y1), type 2 coil (Y2), a conductive single yarn coil (S) and PU-coated copper wire (Ø_Cu: 0.2 mm, Ø_PU-Cu: 0.235 mm) coil (W) at a two magnet and 1.4 Hz swing frequency. As the number of coil turns increases, the average current increases linearly. In general, the average current was about 95% in the type 1 coil and about 90% in the type 2 coil, and about 84% in a conductive single yarn coil compared with the wire coil (W). The reason for the lower power-generation efficiency of the conductive yarn coils (Y1, Y2, S) than the wire coil (W) is that the PU-coated copper filaments in the conductive yarns are wrapped around the polyester filament yarn as shown in Figure 5. The type 1 and 2 coils showed higher average current than a single yarn coil. Because the additional twist direction (Z-direction) for making the conductive plied yarn is opposite to the wrapping direction (S direction) of the PU-coated copper filament for making a conductive single yarn, the PU-coated copper filaments in the type 1 and type 2 are more parallel to the axial direction of the yarn than the PU-coated copper filament of the conductive single yarn. The type 1 coil was more efficient than the type 2 coil because type 1 was thinner than type 2 and copper filaments (Ø_Cu: 0.060 mm, Ø_PU-Cu 0.070 mm) in the conductive yarn were more parallel to the axial direction of the conductive yarn. The conductive single yarn produced in step 1 is very likely to be damaged or broken during textile processing. A plied yarn type such as type 1 yarn and type 2 yarn dramatically improves yarn strength, durability, and textile processability.

Figure 10.

Average current generated by type 1 coil (Y1), type 2 coil (Y2), a conductive single yarn coil (S) and PU-coated copper wire coil (W) at two magnet and 1.4 Hz swing frequency.

The type 1 and type 2 coil generated much more current in a serial circuit connection than in the parallel circuit connection (Figure 10, Figure 11). The average current of Y1_200(S)_M2, 1.4 Hz, and Y2_200(S)_M2, 1.4 Hz was 317.31 μA and 399.42 μA. This result was 2.17 times higher for type 1 coil and 2.74 times higher for type 2 coil compared to W_200_M2 (Figure 10). Additionally, the average current of Y1_200(P)_M2, 1.4 Hz and Y1_200(S)_M2, 1.4 Hz was 139.52 μA, and 317.31 μA. The average current level in the serial circuit connection was 2.27 times higher than the parallel circuit connection of Y1_200. In type 2 coil, the average current of Y2_200(P)_M2, 1.4 Hz and Y2_200(S)_M2, 1.4 Hz was 135.82 μA and 399.42 μA. The average current level in the serial circuit connection condition was 2.94 times higher than the parallel circuit connection of Y2_200 (Figure 11).

Figure 11.

Average current according by turns of coils; (a) type 1 coil (Y1), 1.0 Hz, (b) type 1 coil (Y1), 1.4 Hz, (c) type 2 coil (Y2), 1.0 Hz, (d) type 2 coil (Y2), 1.4 Hz.

Figure 11 also shows the generated current increases linearly when the turns of the coil is increased by 50, 100, 150, and 200 turns. In type 1 coil, the average current of Y1_50(S)_M2, Y1_100(S)_M2, Y1_150(S)_M2, and Y1_200(S)_M2 were 85.61 μA, 164.99 μA, 221.01 μA, and 317.31 μA. In type 2 coil, the average current of Y2_50(S)_M2, Y2_100(S)_M2, Y2_150(S)_M2, and Y2_200(S)_M2 were 123.28 μA, 206.75 μA, 295.65 μA, and 399.42 μA.

The effect of the magnet arrangement can also be seen in Figure 11. It shows that the generated current increases with a two flat-magnet arrangement rather than one flat-magnet arrangement. In the type 1 coil, the average current of Y1_200T(S)_M1, 1.4 Hz and Y1_200T(S)_M2, 1.4 Hz was 197.46 μA, and 317.31 μA. In the type 1 coil, the average current level in the two-magnet arrangement was 1.61 times higher than that in the one-magnet arrangement. In the type 2 coil, the average current of Y2_200T(S)_M1, 1.4 Hz, and Y2_200T(S)_M2, 1.4 Hz were 256.44 μA and 399.42 μA. In the type 2 coil, the average current level in the two magnet arrangement was 1.56 times higher than that in the one magnet arrangement.

Figure 12 shows that the generated current increases linearly as the frequency of the swing increases. In the type 1 coil, when the frequency gradually increased, 1.0 Hz, 1.1 Hz, 1.2 Hz, 1.3 Hz, 1.4 Hz, the average current of Y1_200T(S)_M2 also increased linearly, 226.04 μA, 246.02 μA, 260.52 μA, 285.71 μA, and 317.31 μA. In the type 2 coil, the average current of Y2_200T(S)_M2 also increased linearly: 293.10 μA, 316.57 μA, 339.67 μA, 368.71 μA, and 399.42 μA.

Figure 12.

Average current according to frequency of the swing speed; (a) type 1 coil, (b) type 2 coil.

The distance between the magnet and the coil was set to be gradually increased from 1 mm to 15 mm. The average current is shown in Figure 13, according to the magnetic force inversely proportional to the square of the distance between the magnet and the coil.

Figure 13.

Average current according to the distance between the magnet and the coil.

Conclusions

The WEHT was successfully developed to generate energy from the human swing motion that occurs during outdoor activities. WEHT is a method of electromagnetic energy harvesting using the kinetic energy of the human swing motion. The structure of the WEHT was designed so that it maximizes energy harvesting during the swinging motion that occurs during walking. Because the WEHT uses a magnet and magnetic energy is not limited by environmental factors such as temperature and humidity, the WEHT can harvest a certain amount of energy whenever a swing is completed, regardless of where it is used and the weather conditions.

The performance of the WEHT was evaluated in terms of the conductive yarn types, number of turns of the coils, circuit connection method, magnet arrangement, swing speed, and the distance between the magnet and coil. The results show the energy-harvesting method using the WEHT follows the rule of electromagnetism described in the design stage. The results also show that conductive yarn coils are more effective in generating energy than wire coils, and more energy is generated when a conductive yarn coil is used in a serial circuit connection than in a parallel connection. Conductive yarn coils can be connected in a series and are more effective in energy harvesting. This connection method generated 117% more energy in type 1 and 174% more in type 2 coil than a wire coil. As the number of coil turns increased and the magnetic force and swing frequency speed increased, the amount of energy generation increased linearly. The conductive yarn coils generated more energy when it was wound with a greater number of turns, and it was more effective when the magnetic force and swing speed were high. The average generated current was inversely proportional to the square of the distance between the magnet and the coil.

The WEHT is a sustainable energy harvester because it can continuously obtain energy in an outdoor environment whenever the wearer walks or runs. The WEHT module is cheap and easy to manufacture and can be easily attached to a jacket. The module can also be easily detached and the jacket can be washed. Further research on WEHT will be conducted for wearable garments. Reliability in an outdoor environment needs to be considered for future research.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by the Ministry of Science and ICT, Korea, under the Information Technology Research Center support program (IITP-2018-2015-0-00390) supervised by the Institute for Information & communications Technology Promotion.

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