Stray electric field energy harvesting technology using MEMS switch from insulated AC power lines Hoseong Kim, Dongkil Choi, Sungmin Gong and Kyungjin Park A new energy harvesting technology that extracts energy from the stray electric field around a three-wire AC power line is presented. It is observed that when 20 cm of the insulated power line is surrounded with a conductive sheet, about 20 mJ of energy is harvested in 15 min with a 1 μF storage capacitor. An autonomous harvesting circuit was designed and built adopting an MEMS switch as a low leakage, low power consumption and hysteretic switch. It is demonstrated that the harvested energy is utilised to drive a commercial Zigbee-based wireless sensor module autonomously. Since it is easy and safe to install, the proposed stray electric field harvesting technology should greatly expand the field in which wireless sensors are required, such as home automation, smart grid, building energy management and structural health monitoring, as long as power lines are available nearby.
clearly shows that the distance between two adjacent lines is increased significantly, and it is calculated that the stray electric field intensity is reduced by one-thirtieth (B, B′) and the high electric field is concentrated inside the capacitor, which means that we can use the energy stored in the capacitor. In our primitive experiments, aluminium foil wrap as the conducting sheet is used [4] and it is replaced with copper tape for easy control of the surrounded length and for tight coverage as shown in Fig. 2a. Then, the parasitic capacitances between the copper sheet and the wires can be described as shown in Fig. 2b. The equivalent circuit of the power line surrounded by the copper sheet and the measurement circuit are shown in Fig. 2c. copper sheet
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Stray electric field energy harvesting: Fig. 1a shows a cross-section of a common insulated single-phase three-wire household power line and the equipotential lines in normal usage and with a surrounding copper sheet. The nominal voltage between the hot line and the other two lines is 220 Vrms (311 V at peak) in Korea. The neutral and safety ground wires are connected together at the service entrance and are usually at earth ground potential. The equipotential lines are drawn with equal voltage difference between two adjacent lines, which means that denser equipotential lines represent stronger electric field intensity at the point. max: 311 V B′ 224.01 V/m
B 7.07 kV/m insulation
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Fig. 2 Analysis of surrounded power line a Image of insulated household power line surrounded with copper tape b Diagram showing parasitic capacitances c Equivalent circuit of measurement system
Using this circuit, it is possible to estimate and measure the harvested energy. Since the geometry of the power line is almost symmetric, the capacitances between the surrounding copper sheet and the three wires are the same: CCH = CCG = CCN. Moreover, the capacitances between the hot wire and the other two wires are the same: CHG = CHN. Since the neutral and ground wires are connected together at the service entrance, the capacitance is zero. The measured CCH is 1.65 pF per unit centimetres of the surrounded power line. CHG and CHN have no effect on the output voltage. Since the AC power source has a capacitive load only, current flows with 90° phase shift to the voltage; therefore, the electric power flows back and forth through the space around the power line according to the Poynting vector. This AC power flow is rectified using a voltage doubler consisting of diodes D1 and D2 and stored in storage capacitor Cs. Fig. 3a shows the simulated and measured storage capacitor voltage against accumulation time. These are in very good agreements. Note that the saturation voltage is as high as 204 V (about two-thirds of 311 V). When the surrounded length is 20 cm, it takes about 15 min for the 1 μF capacitor to reach the saturation voltage regardless of line current. Fig. 3b shows the available average power for resistive loads that are connected parallel with the storage capacitor. It is found that the maximum available power is a linear function of the surrounded length, but it is only of the order of μW/cm. However, in this Letter, we demonstrate that such low power can be harvested and utilised to drive a wireless sensor module when an autonomous, hysteretic, low-power consumption MEMS switch is employed.
Fig. 1 Cross-section of 220 Vrms, three-wire power line and equipotential lines
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a Equipotential lines in normal usage b Equipotential lines with conducting sheet and capacitor
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As shown in Fig. 1a, at the peak line voltage, the electric field inside of the power line is very strong and a much weaker electric field, the so-called stray electric field, exists everywhere outside of the power line. Even though the intensity of the stray electric field, for example at B, is about a 20th of the intensity at A, long-term exposure to it might be hazardous to human health. When the power line is surrounded with a conductive sheet that is connected to ground via a capacitor as shown in Fig. 1b, the conductive sheet blocks the outward flow of the stray electric field and concentrates it into the capacitor. Fig. 1b
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Introduction: All portable or wireless devices require electric power; these accelerate the development of the energy harvesting technology that extracts electric energy from ambient heat, light, vibration or electromagnetic waves [1]. The harvested energy, as a battery substitute, can be utilised to drive electronic devices, for example, wireless sensors to monitor environments, power consumption or structural health in our daily life [2]. However, the harvested power is usually in the order of microwatts, and, more seriously, varies with time and with weather conditions, which makes it difficult to realise ‘deploy and forget’ wireless sensor nodes and hampers the prevailing technology [3]. In this Letter, we report an innovative energy harvesting technology that extracts quite stable power from the stray electric field around an insulated household AC power line without removing the insulation.
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ELECTRONICS LETTERS 14th August 2014 Vol. 50 No. 17 pp. 1236–1238
concrete wall Zigbee module
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Fig. 4 Wall-mounted Zigbee-based temperature sensor and power management circuit a Image of mounted sensor module using concrete wall as ground b Circuit diagram
In this circuit, the moving electrode A of the MEMS switch is normally disconnected from electrode B, and therefore the initial gate voltage of the MOSFET is zero. The off-state MOSFET isolates the 100 μF storage capacitor Cs from the step-down converter. When the voltage is applied to the power line, the stray electric field energy begins to accumulate in Cs and the voltage of Cs increases since Cs does not have a discharge path. When the voltage increases to the on-voltage (21 V) of the MEMS switch as shown in Fig. 5a (upper trace), the electrostatic attraction force between the moving electrode A and the ground electrode C exceeds the elastic force of the springs (not shown), then the electrode A moves down to be connected with B. This results in a positive voltage on the gate, which turns on the MOSFET, and thereby the energy stored in Cs is supplied to the stepdown converter and sensor module. Then the LED on the sensor module turns on as shown in Fig. 5a (lower trace), and the sensor module senses temperature, processes data and transmits a RF signal to a remote master node. These operations consume energy, which results in a decrease of capacitor voltage. When Cs voltage decreases to off-voltage (19.8 V) of the MEMS switch as shown in Fig. 5a (upper trace), the attraction force becomes weaker than the elastic force. Then the electrode A moves up to be disconnected from B, which isolates Cs again and turns off the MOSFET. Then the stray electric field energy begins to accumulate in Cs again, and the whole sequence repeats periodically. When the ground is provided using a concrete wall as shown in Fig. 4a, the transmission period is 5 min as shown in Fig. 5a. Fig. 5b shows the temperature data received by a remote master node for 24 h when a concrete wall is used as the ground.
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Driving a Zigbee-based sensor module: To harvest sufficient energy to drive a commercial Zigbee-based wireless temperature sensor module, 20 cm of the power line is surrounded with copper tape as shown in Fig. 4a. A power management circuit with a 100 μF storage capacitor was developed adopting a voltage doubler, MEMS switch, MOSFET and a step-down converter as shown in Fig. 4b, where schematic diagram of the MEMS switch is inserted. The MEMS switch (Omron, 2SMES-01) provides the advantage of low leakage and autonomous hysteretic operation without any stable DC power supply. The MOSFET provides high current handling capability [5–8]. A step-down converter is used to reduce 21 V to 3.3 V, since the operation voltage of the sensor module is 3.3 V, whereas the on-voltage of the MEMS switch is 21 V. The sensor module has a power indicator LED that turns on when power is supplied to the sensor module. The ground is obtained by attaching a metal plate on the concrete wall with conductive adhesives after peeling off the paint as shown in Fig. 4a.
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Fig. 5 Experimental results a Storage capacitor voltage (upper trace) and light intensity of power indicator LED mounted on sensor module (lower trace) b Measured temperature for 24 h
Conclusion: In this Letter, first, it is demonstrated that, regardless of line current, stable power can be extracted from the stray electric field around an insulated AC power line by simply surrounding it with a conductive sheet. Secondly, it is demonstrated that the energy harvested from the stray electric field can drive a commercial Zigbee-based sensor module periodically. Since there is no need to remove the insulation, there is no need to shut down for sensor installation without any risk of electric shock. It is believed that the proposed stray electric field harvesting technology should greatly expand the field in which ‘deploy and forget’ wireless sensors are required, such as home automation, smart grid, building energy management and structural health monitoring for long-span bridges or super tall buildings, as long as power lines are available nearby. Acknowledgments: This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2011-0011506) and the Chung-Ang University Research Scholarship Grants in 2012. © The Institution of Engineering and Technology 2014 30 April 2014 doi: 10.1049/el.2014.1264 One or more of the Figures in this Letter are available in colour online. Hoseong Kim, Dongkil Choi, Sungmin Gong and Kyungjin Park (School of Electrical and Electronics Engineering, Chung-Ang University, Huksukdong-gil 84, Dongjakgu, Seoul 156-756, Republic of Korea) E-mail:
[email protected] Kyungjin Park: Also with the Korea Research Institute of Standards and Science, Daejeon, Korea References 1 Paradiso, J.A., and Starner, T.: ‘Energy scavenging for mobile and wireless electronics’, Pervasive Comput., 2005, 4, pp. 18–27 2 Roundy, S., Wright, P.K., and Rabaey, J.M.: ‘Energy scavenging for wireless sensor networks’ (Kluwer Academic, Norwell, MA, 2004) 3 Mathúna, C.Ó., O’Donnell, T., Martinez-Catala, R.V., Rohan, J., and O’Flynn, B.: ‘Energy scavenging for long-term deployable wireless sensor networks’, Talanta, 2008, 75, pp. 613–623 4 Chang, K., et al.: ‘Electric field energy harvesting powered wireless sensors for smart grid’, J. Electr. Eng. Technol., 2012, 7, (1), pp.75–80 5 Uno, Y., et al.: ‘Development of SPDT-structured RF MEMS switch’. Int. Conf. Transducers, Solid-State Sensors, Actuators, Microsystems, Denver, CO, USA, June 2009, pp. 541–544 6 White Paper: ‘RFMEMS switch: what you need to know’. 2013. http:// www.ttiinc.com/docs/IO/22190/2SMES-01_RFMEMSSwitch_White paper.pdf 7 Kim, H., et al.: ‘Micro-electro-mechanical system/field-effect-transistor hybrid switch for energy scavenging system’, Jpn J. Appl. Phys., 2010, 49, 06GN19 8 Park, K.J., et al.: ‘Energy scavenging system utilising MEMS switch for power management’, Electron. Lett., 2012, 48, (15), pp. 948–949
ELECTRONICS LETTERS 14th August 2014 Vol. 50 No. 17 pp. 1236–1238
