Indoor WiFi Energy Harvester with Multiple Antenna for Low-power ...

Indoor WiFi Energy Harvester with Multiple Antenna for Low-power Wireless Applications Ermeey Abd. Kadir, Aiguo Patrick Hu, Morteza Biglari-Abhari

Kean C Aw Department of Mechanical Engineering The University of Auckland, New Zealand

Department of Electrical and Computer Engineering The University of Auckland, New Zealand

planar antenna was adopted to make a small and attachable radio frequency harvesting system. Microstrip antenna has been chosen at early stages [2]. Planar-inverted-f-antenna (PIFA) is also capable to harvest ambient RF energy and increase the frequency bandwidth of the harvesting system [3]. Researcher applied co-planar waveguide antenna as the energy harvester when a single layer antenna is needed [4]. As proof of concept, antenna such as Reflector [5], Horn [6], Helical [7], Yagi [8] and Planar Yagi-Uda [9] antenna were also used as the RF energy harvester. From the literature findings, outdoor and indoor RF sources were used as the energy sources for the radio frequencies harvesting system. It could be from commercial or free radio transmission. The targeted frequency for the radio frequency harvesting system spans from Mega Hertz (MHz) up to Giga Hertz (GHz). Three categories of frequencies are workable for the radio frequencies harvesting system. The three categories are single frequencies less than 1 GHz, single frequencies more than 1 GHz, and multiple frequencies. The effectiveness of the radio frequency harvesting system relies on efficient energy conversion circuit and control. The previous converters circuit used both off-theshelf discrete electronic components and custom-made integrated circuits. Although the converter circuit used different types of electronic components the arrangement of the converter circuit still based on a basic rectifier circuit. A single Schottky diode is used to rectify the harvested energy [2]. Rectifying harvested energy within the range -20 dBm (0.00001W) to 20 dBm (0.1W) at the harvester antenna front has been conducted by using two Schottky diodes; typical circuit arrangements are Voltage-doubler [10], Dickson Voltage-doubler [11] and Villard voltage-doubler [12]. Rectifying and at the same time increasing the output level from the energy harvester, Schottky multiple-stages circuit [13] and Cockcroft-Walton circuit [14] were used as the converter circuit. A custom-made integrated circuit rectifier circuit was used when the harvested energy values is in microwatt or nanowatt [15]. Energy storage is another essential part of a radio frequency harvesting system. This is because the received power from an energy harvesting antenna is normally too low to drive a wireless sensor directly. It is used to store the energy over time, which able to supply higher power

Abstract – This research proposed a WiFi energy harvester for low-power wireless applications. The proposed system harvests energy using three antennas to cover three ISM (Industrial, Scientific, and Medical) channels with central frequencies at 2.412 GHz, 2.439 GHz, and 2.462 GHz. For each channel, a coplanar waveguide antenna is designed to harvest energy from indoor WiFi transmitters. FR4 substrate with relative permittivity of 4.3 and loss tangent of 0.025 is used to form the antennas. The output from each harvester antenna is then connected to a seven-stage multiplier circuit. The multiplier circuit is to rectify and boost the harvested energy to a higher voltage level and then stored temporarily in a super capacitor. A dc-dc boost-charger circuit with battery management is used to increase the output voltage level to 2 V. An experiment with the proposed system has been conducted using transmitted energy from available WiFi transmitters. The power density at the harvesting antenna front is between -80 dBm and -50 dBm. The proposed harvester system takes about 6 to 7 hours to charge up the first stage super capacitor up to the minimal threshold voltage (0.45V). This minimal threshold will start the boost-charger circuit charging the secondary storage device. This research demonstrates that the proposed system can supply energy for low-power wireless sensors that operate with an input power less than 1 mW. Index Terms—Energy Harvesting, super capacitor, WiFi, wireless sensor.

I. INTRODUCTION Energy harvesting is the process of capturing energy from the ambient atmosphere and converts it into useful electrical energy. Frequently, the term energy harvesting is applied when a solution is needed to provide a supply source to wireless applications. A variety of energy sources exists for energy harvesting such as electromagnetic energy, solar energy, thermal energy, wind energy, kinetic energy etc. In order to prove that energy harvesting has the potential to replace the function of a battery in wireless application, the study in the fields related to energy harvesting has continued to be highly engaging. Radio frequency (RF) energy harvesting system is one of the attractive studies in this field. It is more attractive when it is used as an energy supply for wireless sensors that be installed in hazardous and unreachable area. Different types of antenna are used to collect ambient RF energy. To gather energy from variable transmission angles, Omni-directional antenna [1] is one of the candidates. A

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required to drive low power wireless sennsors. A battery supplies slow and steady energy for large energy e demands [16] and a super capacitor practically chargge and discharge quickly for low energy demands [17]. Recent RF energy harvesting systems usiing off-the-shelf components have successfully harvested transmitted RF energy, especially when the power densityy at the selected area is more than -50 dBm. When an enerrgy harvester is needed to operate with a power density loweer than -50 dBm, researchers opt to CMOS technologyy. Using this technology, the circuit design of the energyy harvester will become more complex and increases the fabrrication cost. A RF energy harvesting system relying on a single RF frequency and source are often not reliabble. Especially, when the power density from the RF sourrce is extremely low or absent. To develop an energy harvester with sensitivity less than -50 dBm, the harvesting system must be equipped with an energy harvester that can harvest ambient RF energy at multiple frequencies. This papper proposes an indoor WiFi energy harvester for low power p wireless applications. The proposed system is develloped using offthe-shelf discrete components and harvvest transmitted energy from indoor WiFi transmission system using multiple antennas.

III. PROPOSED DESIGN D APPROACH The typical existing reseaarch approach for developing a radio frequency energy harvvester is summarized by a flow chart in Fig. 2 (a). In order to t develop the proposed system, new methodology is used as highlighted in Fig. 2 (b).

START

START

MEASURE AMBIENT RF ENERGY

MEASURE AMBIENT RF ENERGY

DESIGN HARVESTER ANTENNA

DESIGN HARVESTER ANTENNA

DESIGN CONVERTER CIRCUIT

DESIGN CONVERTER CIRCUIT

NO

NO IMPEDANCE MATCHED

IMPEDANCE MATCHED YES

YES

II. THE PROPOSED SYSTEM

DESIGN ENERGY STORAGE CIRCUIT

The proposed energy harvester system is illustrated as in Fig. 1. Transmitted energy from available WiFi W is harvested by the antennas. Each antenna is fabricaated to harvest energy from available channels used in 2.45 2 GHz WiFi system. The harvested energy is convertedd by the multistage circuit. Passive balancing technique ussing resistor (Rn) is used to balance the voltage between super capacitor (SCn). The harvested energy stored in the suuper capacitor is the input for to the boost-charger and batteery management unit (BCBMU). The function of the boost-charger is to increase the output voltage from the firrst stage super capacitor. The battery management unit is used to manage the input and the output voltage that stored at a the first stage and secondary storage (SS) device, respectivvely.

DESIGN ENERGY STORAGE CIRCUIT

ATTACHED WIRELESS APPLICATION

ATTACHED WIRELESS APPLICATION

SYSTEM EVALUATION NO END

APPLICATION FUNCTION PARAMATERS ACHIEVE YES SYSTEM EVALUATION

END

WiFi channel 1 Multistage circuit 1

R1

Vref r

WiFi channel 2 Multistage circuit 2

(a)

SC1

R2

SC2

Rn

SCn

Fig. 2. Research methodology foor developing a RF energy harvesting system. (a) typical methodoology (b) proposed methodology

Vin BCB MU

A. Ambient WiFi Power Denssity Measurement With the advances in computer engineering, power density from WiFi transmittter at a selected area can be measured by using free-to-usse network monitoring software. Since, this research concenntrates on harvesting available WiFi transmitted energy, Netspot software is used to measure the WiFi power density rather than using a spectrum analyser. Measurem ment on the WiFi power density is performed inside the Radio R System and Microwave laboratory at Electrical and Computer Engineering

SS

WiFi channel n Multistage circuit n

(b)

Fig. 1. Proposed radio frequency energy harvesster system

527

Department, The University of Auckland, and a the average power density measured is between -80 dB Bm to -50 dBm. Most detectable WiFi channels are channel 1, 1 6 and 11. \

B. Converter circuit design A typical WiFi access point transmits onlyy 100 mW [18]. The power density from the available WiFi transmitters is less than -50 dBm (0.00001 mW). Energy harvesting from WiFi access point requires high efficient circuitry, c due to the low-power transmission level. To arcchive this goal, Schottky multiple-stages circuit is used. Nine zero-bias lowbarrier Schottky diodes from Skyworks, SMS7621 with high-saturation current (IS = 4 x 10-8 A) are used as the rectifying diode in the converter circuit. The converter circuit arrangement is shown in Fig. 3. Computer Simulation Technology (CST) Design softw ware was used to simulate the optimal number of stages for f the voltage multiplier circuit. Seven-stages of voltage-doubler arrangement increased the output voltage froom 0.5 mV to 30 mV.

Open-circuit stub (a)

(b)

(c) Fig. 4. Proposed harvester antenna (a) Harvester antenna arrangement (b) antenna directivity and (c) return loss (S11) simulation result for 2.412 GHz anntenna

E. Energy storage circuit The energy storage unit foor the proposed harvester system is a combination of an ultraa-low power boost-charger with battery management as show wn in Fig. 5. The boost-charger with battery management uniit is using BQ25505 from Texas Instrument. A boost-chargerr circuit is more suitable than a buck or buck-boost circuits for RF harvesting system. The energy required to achieve thhe expected input voltage for the boost-charger circuit to start charging the super capacitor is

Fig. 3. Multistage converter circuit

C. WiFi energy harvester antenna Three harvester antennas are used to harvvest transmitted energy from the available WiFi transmitters. Each antenna operates at a single centre frequency, thatt is 2.412 GHz (channel 1), 2.439 GHz (channel 6), annd 2.462 GHz (channel 11). These antennas are designed based on a coplanar waveguide arrangement due to its design simplicity and simple impedance matching capability. The dimension of the harvester antenna is 28.5 mm x 288 mm. Fig. 4(a) shows one of the proposed harvester antennnas operated at 2.412 GHz. The antenna gain for the propposed harvester antenna is 3.6 dBi. One of the proposed anntenna direction pattern is shown in Fig. 4 (b).

0.45

-

0.33

= 0.00468C Joule

(1)

Average current measured at the RF energy harvester end is about 3 µA. The expectedd output voltages at the energy harvester end are between 0.33 0 V to 0.45 V. That give an average voltage for about [(0.45+0.33)/2]) = 0.39 V. In addition, assuming time needded to harvest energy from 0.33 V to 0.45 V is 21,600 secondd (6 hours). Hence;

D. Matching circuit A single open-circuit stub is used to matchh the impedance between the harvester antenna and the harveester circuit. The position of the single stub element is shown in Fig. 4 (a). By adjusting the dimension of the open-circuuit stub and the output port is fixed at 50 Ω, return loss S11 of the proposed harvester antenna at 2.412 GHz is about -38 dB as shown in Fig. 4(c).

0.00468

0.39

3

2 21600,

0.54

(2)

Therefore, three 0.22 F, F 2.5 V super capacitor are connected in series connecttion, and a 0.47 F, 5 V super capacitor is connected across the series capacitor connection, resulting in a caapacity of 0.543 F, and output voltage is 5 V.

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capacitor, and secondary super capacitor to about 1 V and 3 V, respectively to speed up the testing process. NI ELVIS II is used to logging the voltage values for the input and secondary storage voltage. The measurement results for the input and secondary storage voltage values are shown in Fig. 7.

The targeted voltage at the secondary storage super capacitor is between 1.5 V to 2.0 V. The energy required to be stored at the secondary storage is 2.0

1.5 = 0.875C Joule

-

(3)

From measurement held with the boost charger circuit, the average current between 1.5 V to 2.0 V is 25 µA, and the average voltage is about [(2+1.5)/2] = 1.75 V. In addition, the expected lifetime is 21,600 second. So 0.875

1.75

25

2160, C = 0.945 F

Harvester Antenna

Converter Circuit

(4)

Therefore, a 1 F, 5 V super capacitor is connected at the output of the boost charger circuit. 1 stage super capacitor Vin

BQ25505EVM-218

V_Sec_stor BCBMU

Vref

Secondary storage

Fig. 6. Indoor WiFi Energy Harvester with Multiple Antennas

First stage Storage

3 2.5

Fig. 5. Energy storage circuit

2 Voltage

F. Charging process The process of charging the secondary storage device starts when the boost-charger circuit extracts power from the first stage super capacitor storage. The boost-charger circuit is started when Vin is approximately 450 mV, and once started, it can continue to harvest energy down to Vin = Vin – 0.25 mV or Vref. Vref is a value set to the microcontroller at the battery management circuit. This value is the minimum voltage when the boost charger circuit stops extracting power from the first stage super capacitor. The boost-charger circuit will stop charging the secondary storage device when Vin is less than 330 mV. When the boost-charger circuit stop extracting power, the energy harvested from the ambient WiFi energy is stored in the first stage super capacitor until it reaches 450 mV thresholds and will recharge the secondary storage device again. This process runs continuously until the secondary storage voltage value is more than 2.0 V. When the voltage of the secondary storage devices is between 2.0 V to 4.2 V, the BQ25505 will be in the sleep condition.

1.5 1

Vin (V) V_Sec_Stor(V)

0.5 0 0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 Time Fig. 7. Voltage measurement for input voltage, Vin for the boost charger circuit and voltage reading at secondary storage device, V_Sec_storage

Table 1 summarizes the comparison of the proposed system with other existing RF energy harvesting system. The power density used in [17] is 3.2 times higher than this research. However, both systems produce a similar maximum output voltage. Even though the power density used in this research is 75% lower than [18], the proposed system efficiency is 9% higher. The energy harvester in [19] is fabricated on a low loss substrate, and its maximum output voltage is 0.2 V smaller than this research. The energy harvester in [20] is developed using CMOS and it is operated with a similar power density which is less than -50 dBm. The maximum output voltage different between both system is 1 V.

IV. PRACTICAL RESULTS The proposed system indoor WiFi energy harvesting system is built as shown in Fig. 6. The proposed system takes a significant long time (approximately 2 days) for the first stage super capacitor to be charged up to 450 mV using transmitted energy from WiFi transmitters. For that reason, an initial charge is applied to both first stage super

529

[8]

TABLE 1. RF HARVESTING SYSTEM COMPARISON References Frequency Sensitivity (dBm) Maximum output voltage System efficiency (%) Fabrication

This Work 3 WiFi Channels

[17]

[18]

[19]

[20]

2.45 GHz

2.45 GHz

2.45 GHz

1.9 GHz

< -50

-15.6

-12.47

-30

-87

2

2

3.44

1.8

1

18.6

2.7

16.91

NA

NA

FR4

FR4

FR4

Rogers RO6010

CMOS

[9]

[10]

V. CONCLUSION This research has demonstrated that harvesting indoor WiFi energy using a harvester with multiple antennas is achievable. Adopting the proposed methodology and the system design arrangement, the proposed harvesting system is able to maintain 2 Vdc at the secondary storage device. This stored energy can be used to drive a wireless sensor that needs less than 1 mW of power to operate. Further works and optimizations are required to extend the proposed system functionality as an energy source for low power application. Integration with a low-power wireless sensor and its operating period will be investigated.

[11]

[12]

[13]

REFERENCES [1]

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