Indoor Light Energy Harvesting System for Energy-aware Wireless Sensor Node

Transcription

1 Available online at Energy Procedia 16 (01) International Conference on Future Energy, Environment, and Materials Indoor Light Energy Harvesting System for Energy-aware Wireless Sensor Node Hua Yu a*, Qiuqin Yue b a College of Optoelectronic Engineering, Chongqing University a The Key Laboratory for Optoelectronic Technology & Systems, Ministry of Education of China, Chongqing,400044, China b Department of Electromechanic Engineering, Chongqing College of Electronic Engineering, Chongqing,400044, China Abstract This paper proposes a novel energy harvesting and power management circuit that includes maximum power point tracking (MPPT) circuit, energy storage circuit, energy instantaneous discharging circuit, and DC-DC boost converter. It harvests energy from indoor faint light by imposing the harvester to work close to its maximum power point and supply power for sensor node. The measured result shows that the prototype circuit can harvest energy from indoor light condition with input power of 7.74µW and successfully drive a smart wireless temperature and humidity sensor node with power consumption of 105mW per operating cycle Published Published by Elsevier by Elsevier B.. Selection Ltd. Selection and/or peer-review and/or peer-review under responsibility under responsibility of International of Materials [name Science organizer] Society. Open access under CC BY-NC-ND license. Keywords: Energy Harvesting; Power Management Circuit; Maximum Power Point Tracking; Wireless Sensor Node 1. Introduction One key bottleneck is the limited battery lifetime for most wireless sensor networks. The frequent maintenance efforts associated with battery replacement significantly increase the system operational cost. Energy harvesting technology presents a promising solution to battery-less wireless sensor networks. Solar energy harvesting is a comparatively fledged technology for wireless sensor networks used for outdoor applications. However, for indoor applications, it is not suitable because the efficiency of photovoltaic cell is very low under low indoor light luminous intensity. With low light intensity, the energy harvested may not be enough for powering for the wireless sensor node. Thus, the special power management circuit should cater for the large difference between the scavenged energy and the power dissipation of wireless sensor node load [1-]. Lately, more and more research has been devoted towards to the energy harvesting which powers for wireless sensors networks [3-6]. However, there is little information available in above literatures about the energy harvesting system that can operate at an input * Hua Yu. Tel.: ; fax: address: yuhua@cqu.edu.cn Published by Elsevier B.. Selection and/or peer-review under responsibility of International Materials Science Society. Open access under CC BY-NC-ND license. doi: /j.egypro

2 108 Hua Yu and Qiuqin Yue / Energy Procedia 16 (01) power under dozens of μ W. When the P cell size scales down to a few cm, the harvested power drastically drops into the range of μ W. This severely power constrained region presents new design challenges for power management circuit. More research is needed in order to develop a micro-scale indoor light energy harvesting system which can work well under input power of dozens of μw and drive wireless sensor node with load of dozen of mw per operating cycle. This paper presents a novel micro-scale indoor light energy harvesting system that includes photovoltaic cell, maximum power point tracking (MPPT), energy storage, energy instantaneous discharging circuit, and DC-DC boost converter. This presented power management system can operate well for powering wireless sensors while input energy is as low as dozens of μ W. The paper provides an overall block diagram of the system circuit with descriptions of each block. Finally, the measured results, discussion and conclusion are presented.. Proposed micro-scale indoor light energy harvesting system Fig. 1 shows the block schematic of the proposed power management system. The basic design idea is to store the power generated by photovoltaic cells in super-capacitor (SC) and deliver it when it is enough to supply the load for an established amount of time. The proposed power management system consists of light energy transducer, MPPT circuit, energy storage circuit, energy instantaneous discharging circuit and DC-DC boost converter. The different parts of the circuit system are analyzed with detailed information in the following parts. Fig.1. the block schematic of the proposed power management system.1. MPPT circuit MPPT refers to drawing power from energy harvesting source at a levels that maximizes the power output. For DC sources such as P cells, the maximum power point is a voltage-current combination that maximizes the power output under a given light condition and temperature. We propose a MPPT circuit based on fractional open circuit voltage method which adopts a hysteresis voltage comparator and regulates the photovoltaic cell voltage to be a fixed fraction of its open circuit voltage [7-8]. This MPPT circuit consists of the MPPT control unit circuit and the MOSFET switch 1. A hysteresis voltage comparator 1 is used as a control unit. It generates control signal to turn on/off the MOSFET switch by comparing the reference MPP and the P cell operational voltage. The measured output wave is shown in Fig.. By adjusting the hysteresis, the threshold voltage of the comparator can be changed, thus, the sensitivity of the MPP tracking can be adjusted. Super capacitor can be charged with the maximum power of the P cell by the use of the MPPT circuit.

3 Hua Yu and Qiuqin Yue / Energy Procedia 16 (01) Fig.. Output curve of MPPT circuit.. Energy storage element In order to drive low power wireless sensor nodes successfully, energy harvested from P cells which is normally of the order of 1 μ W, must first be buffered in energy storage element. The super-capacitor has a much higher number of charging/discharging cycles than battery during a relatively long lifetime and they can deliver or accumulate important peaks of power because of their low equivalent series resistance (ESR). What is more important is that super capacitors do not require a special complex charge circuit as long as their current and voltage does not exceed the nominal value. So a proper size of super capacitor (SC) calculated by equation can be used in this power management system as a storage element [3,9]..3. Energy instantaneous discharge circuit Energy instantaneous discharge circuit is composed of a hysteresis voltage comparator and switch. The ultra low power hysteresis voltage comparator monitors super-capacitor s voltage and controls supercapacitor s charging and discharging. The hysteresis in a comparator creates two trip points: one for super-capacitor charge voltage ( CHA ) and the other for the super-capacitor discharge voltage ( DISCHA ). The voltage difference between the trip points is the hysteresis voltage ( HB ). Hysteresis voltage can be decided with three resistors R 4, R5 and R 6 using positive feedback (Fig.1), which can decide how much energy from the super-capacitor s discharging. Use the following procedure to calculate three resistor values respectively: 1) Select R6 = min I For example, when using the MAX9064 (Internal I R 6 = 1μA. Choose a standard value for 6 ) Choose the hysteresis band required ( HB ). 3) Calculate R4 according to the following equation: REF CC REF R 6, (1) R6 I R6 R. REF 0. = ) and CC = 4. 5, and if we choose R 4 = R6( HB / CC ) ()

4 1030 Hua Yu and Qiuqin Yue / Energy Procedia 16 (01) ) Choose THR > REF ( R4 R6 )/ R4 (3) CHA + This is the threshold voltage at which the comparator switches its output from low to high as supercapacitor voltage rises above the trip point. 5) Calculate R5 as follows R5 = REF CHA R R4 R6 High threshold and low threshold are as follows respectively: (4) CHA = REF R R4 R5 R6 (5) R 4 CC DISCHA = CHA R6 (6).4. DC-DC boost converter In order to improve efficiency of DC-DC step up converter, the two measures in the circuit are applied. One is to reduce inductor power loss and the other is set to enable start-up signal for DC-DC converter. The inductance depends on the maximum current, which must be safely sustained to prevent components rupture [10]. In order to minimize the inductor power dissipation, small inductors have to be considered, since their parasitic resistance is approximately proportional to their value. Inductor with inductance 10µH is used in the proposed circuit. If the input voltage for DC-DC converter (i.e. the super-capacitor voltage) is lower than a defined start _ up, the DC-DC consumes unnecessary power because it is not able to boost the output voltage. Thus, to overcome this drawback, we introduce a supervisor that continuously checks the super-capacitor voltage and enables the DC-DC output stage only when it can be successfully started up. In conclusion, a complete shutdown with output disconnection suppresses any additional power consumption, reducing the charging time of the SC and boosting the overall efficiency. In the output stage, a special DC-DC converter circuit is chosen because of its high efficiency and synchronous step-up DC/DC conversion with output disconnect. It offers a compact, high efficiency alternative to super capacitor applications. The DC-DC boost converter offers a stable output voltage of Wireless sensor node load The wireless sensor node with temperature and humidity sensor is adopted. The power consumption analysis is based on the experimental results of the wireless humidity sensor node, as illustrated in Fig.8. The operating cycle time is about 60 ms. The normal communication distance of the sensor node is 60m~130m at a frequency of 915 MHz and a data-transmitting rate of 50 kbps [10]. The power of the transmitting data is 85 mw at a transmitting time of ms.the current and the power of sensing data are less than 6 ma and 18 mw at a sensing time interval of 60 ms, as shown in Fig.3 respectively. The power consumption in the transmitting state is the largest.

5 Hua Yu and Qiuqin Yue / Energy Procedia 16 (01) P/mW P/mW t/ms t/ms Fig. 3. The measured power dissipation curve Fig. 4. The photo of the proposed power management circuit 3. Experimental results and discussion A photo of power management circuit for the proposed indoor micro-scale light energy harvesting system is shown in Fig. 4. The proposed energy harvesting system can successfully drive the wireless humidity sensor load when the humidity sensor node transmits signal. The great improvement of the proposed converter is its maximum power point tracking circuit and super-capacitor instantaneous discharging circuit. Output voltage of super capacitor drops from 0.69 to 0.65 while the voltage of the wireless humidity sensor node is transmitting the data. In the proposed circuit, the measured high threshold value is 0.69, and low threshold value is Therefore, the energy of super-capacitor discharging is as follows: ( ) = 40. mj 1 E sc 1 1 C THR C THF = 1.5 F = (11) The energy consumed by the wireless humidity sensor node while working one time is as follows: E load PT = P1 T1 + PT = 0 mw 618 ms + 85 mw ms = mj = (1) So the proposed energy harvesting system can successfully drive the wireless sensor node. 4. Conclusion Energy harvesting technology shows great potential as a promising approach to powering for wireless sensor nodes. However, we will be faced with many unexpected challenges and problems during the design of power management circuit for energy harvesting system under very low energy input conditions. In this paper we present a complete design flow and some design considerations. A power management circuit prototype is designed and tested with a wireless temperature and humidity sensor node. The measured results show that the proposed system can successfully drive the sensor node, which indicates the feasibility of micro-scale indoor light energy harvesting for wireless sensor network applications under extremely low light energy input environments. Acknowledgments This work is funded by the National Natural Science Foundation of China (Nos ), the Natural Science Foundation Project of CQ CSTC (Nos. 009BB034), isiting Scholar Foundation of Key Lab for Optoelectronic Technology & Systems in Chongqing University and the Fundamental Research Funds for the Central Universities (Nos. CDJZR ).

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