Feasibility of Harvesting Solar Energy for Self-Powered Environmental Wireless Sensor Nodes

electronics

Article Feasibility of Harvesting Solar Energy for Self-Powered Environmental Wireless Sensor Nodes

1, 1, 1 2 2 Yuyang Li †, Ehab A. Hamed †, Xincheng Zhang , Daniel Luna , Jeen-Shang Lin , Xu Liang 2 and Inhee Lee 1,*

1 Department of Electrical and Computer Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA; [email protected] (Y.L.); [email protected] (E.A.H.); [email protected] (X.Z.) 2 Department of Civil and Environmental Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA; [email protected] (D.L.); [email protected] (J.-S.L.); [email protected] (X.L.) * Correspondence: [email protected] These authors contributed equally to this work. † Received: 5 November 2020; Accepted: 30 November 2020; Published: 3 December 2020

Abstract: Energy harvesting has a vital role in building reliable Environmental Wireless Sensor Networks (EWSNs), without needing to replace a discharged battery. Solar energy is one of the main renewable energy sources that can be used to efficiently charge a battery. This paper introduces two solar energy harvesters and their power measurements at different light conditions in order to charge rechargeable AA batteries powering EWSN nodes. The first harvester is a primitive energy harvesting circuit that is built using elementary off-shelf components, while the second harvester is based on a commercial boost converter chip. To prove the effectiveness of harvesting solar energy, five EWSN nodes were distributed at a nature reserve (the Audubon Society of Western Pennsylvania, USA) and the sunlight at their locations was recorded for more than five months. For each recorded illumination, the corresponding harvested energy has been estimated and compared with the average energy consumption of the EWSN with the most power consumption. The results show that the daily harvested energy effectively compensates for the energy consumption of the EWSN nodes, and the battery charge capacity of 295 mAh can reliably support their daily dynamic energy consumption.

Keywords: batteries; energy harvesting; environmental monitoring; illumination; solar energy; sunlight; wireless sensor networks (WSNs)

1. Introduction Wireless Sensor Networks (WSNs) have extensive popularity in the field of environmental monitoring [1–5]. Recently, an Environmental Wireless Sensor Network (EWSN) has been exceptionally enhanced due to the development of the Internet of Things (IoT) [6]. EWSN nodes are deployed in open areas (e.g., forests); accordingly, they are not connected to the main power supply because of high wiring cost and difficulty. Instead, an EWSN node usually depends on a rechargeable or non-rechargeable battery to operate; while a non-rechargeable battery requires regular replacement, a rechargeable battery can be recharged by energy harvesting and thus more reliable and effective in the long-term cost [6]. A high capacity battery can be used to lengthen the period of successive replacement or recharging, but it typically increases the node size and weight [7,8]. Alternatively, an energy harvester can be utilized to scavenge the surrounding ambient energy (such as heat, electromagnetic, motion, and light energy) and convert it to electrical energy that charges a battery and thus powers an EWSN node [9,10]. Efficient conversion from ambient energy to electrical energy is crucial for a practical energy harvester. Conversion from thermal energy to electrical energy is established based on the Seebeck

Electronics 2020, 9, 2058; doi:10.3390/electronics9122058 www.mdpi.com/journal/electronics Electronics 2020, 9, x FOR PEER REVIEW 2 of 13

Efficient conversion from ambient energy to electrical energy is crucial for a practical energy Electronicsharvester.2020 Conversion, 9, 2058 from thermal energy to electrical energy is established based on the Seebeck2 of 13 effect [6,9,11–16], which depends on the temperature difference between pairs of p‐type and n‐type esemiconductorffect [6,9,11–16 ],plates. which The depends thermoelectric on the temperature conversion di hasfference low betweenefficiency pairs because of p-type it is andlimitedn-type by semiconductorCarnot cycle efficiency plates. The [9,15,17], thermoelectric and the conversion conversion has process low e ffistopsciency at becausestable heat it is when limited both by Carnot plates cyclesettle eatffi ciencythe same [9,15 temperature,17], and the [6,18–20]. conversion On process the other stops hand, at stable the heat ambient when bothRadio plates Frequency settle at (RF) the samesignals temperature (radar, Wi [‐6Fi,,18 –Bluetooth,20]. On the and other cellular hand, thenetworks ambient [9,21–23]) Radio Frequency are sources (RF) signalsof electromagnetic (radar, Wi-Fi, Bluetooth,energy that and can cellular be harvested networks and [9, 21converted–23]) are sourcesto electrical of electromagnetic energy. The widespread energy that canexistence be harvested of RF andsignals converted and their to vast electrical frequency energy. range The make widespread them appropriate existence of for RF powering signals and numerous their vast WSN frequency nodes rangein a large make area. them Nevertheless, appropriate the for harvested powering energy numerous greatly WSN varies nodes according in a large to the area. distance Nevertheless, between the harvestednode and RF energy source greatly [9,24]. varies according to the distance between the node and RF source [9,24]. The fact that EWSN nodes are deployed in a natural environment makes them subject to wind and sunlight.sunlight. TheThe kinetic kinetic energy energy of of wind wind can can be convertedbe converted to electrical to electrical energy energy using using turbines turbines or rotors, or givingrotors, highgiving harvested high harvested power [ power6,25,26 ].[6,25,26]. However, However, harvesting harvesting wind energy wind requires energy requires large wind large turbines, wind andturbines, sometimes and sometimes the wind isthe neither wind availableis neither nor available predictable nor predictable [6]. Alternatively, [6]. Alternatively, sunlight is a sunlight remarkable is a renewableremarkable source renewable of energy source that of energy can be that harvested can be e harvestedffectively [effectively6,27], using [6,27], photovoltaic using photovoltaic solar cells thatsolar convert cells that solar convert energy solar to energy electrical to energyelectrical [1 ,energy28]. Solar [1,28]. cells Solar are connectedcells are connected in series in and series parallel, and formingparallel, modulesforming modules or solar panels,or solar to panels, increase to theincrease total harvestedthe total harvested power [6 ].power In addition [6]. In toaddition solar cell to size,solar thecell solar size, energythe solar obtained energy fromobtained the sunfrom is the also sun proportional is also proportional to the illumination to the illumination level, which level, is inconstantwhich is inconstant and varies accordingand varies to time,according region, to and time, weather region, [6,29 and]. Therefore, weather a [6,29]. rechargeable Therefore, battery a isrechargeable often included battery with is a often solar energyincluded harvester with a solar to store energy the solar harvester energy to in store the chemicalthe solar formenergy [9, 30in ,31the]. Itchemical is used toform power [9,30,31]. the EWSN It is used nodes to when power the the light EWSN source nodes is unavailable. when the Figurelight source1 shows is anunavailable. overview ofFigure the solar 1 shows energy an overview harvesting of process the solar starting energy from harvesting the solar process panel andstarting ending from with the the solar EWSN panel node. and Solarending energy with the cannot EWSN be controlled,node. Solar but energy it is predictablecannot be controlled, during the but day it andis predictable seasons [6 during,32]. To the keep day a nodeand seasons battery [6,32]. from energyTo keep depletion, a node battery the average from energy energy depletion, consumption the average of an EWSN energy node consumption must not surpassof an EWSN the average node must harvested not surpass energy the from average the sun harvested [6]. energy from the sun [6].

Figure 1. SolarSolar energy energy harvesting process for the target application.application.

In this work, we propose two energy harvesters to avoid the battery replacement of the ESWN nodes. The firstfirst harvesterharvester is is a a primitive primitive energy energy harvesting harvesting circuit circuit that that is built is built using using elementary elementary off-shelf off‐ components,shelf components, while while the second the second harvester harvester is based is based on a on commercial a commercial boost boost converter converter chip. chip. They They are designedare designed using using commercially commercially available available components components so that so the that harvester the harvester design design can be rapidlycan be appliedrapidly toapplied similar to applications. similar applications. Moreover, Moreover, the effectiveness the effectiveness of the harvester of the harvester is verified is by verified using by light using intensity light measurementintensity measurement on the exemplary on the EWSN exemplary nodes EWSN [7,33–35 nodes] in a nature [7,33–35] reserve in a (Audubon nature reserve Society (Audubon of Western Pennsylvania,Society of Western USA) Pennsylvania, for 5 months. BasedUSA) onfor the 5 measuredmonths. Based light profileon the and measured harvesting light effi profileciency, and it is estimatedharvesting that efficiency, both harvesters it is estimated are able tothat compensate both harvesters for the energyare able consumption to compensate of the for EWSN the energy nodes. A battery with a maximum charge capacity of 295 milliampere hour (mAh) can reliably support the consumption of the EWSN nodes. A battery with a maximum· charge capacity of 295 dailymilliampere dynamicꞏhour energy (mAh) consumption can reliably of support the EWSN the node.daily dynamic Moreover, energy we find consumption that the primitive of the energyEWSN harvesternode. Moreover, can cost we at find least that USD the 10 primitive less than energy the second harvester harvester. can cost However, at least USD the primitive 10 less than energy the harvestersecond harvester. provides However, degraded batterythe primitive charging energy ability harvester when light provides intensity degraded becomes lessbattery than charging 1 kilolux (klx)ability due when to a light significant intensity harvesting becomes e lessfficiency than 1 drop kilolux under (klx) weak due light. to a significant harvesting efficiency dropThe under rest weak of the light. article is organized as follows: Section2 describes the primitive solar energy harvesterThe rest and of component the article selection; is organized Section as3 follows:introduces Section the utilization 2 describes of bq25570the primitive chip in solar harvesting energy solarharvester energy and and component how to determine selection; theSection values 3 introduces of its external the utilization passive components; of bq25570 chip Section in 4harvesting presents the measured light intensities at the deployed nodes and shows the generated current from the solar

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solar energy and how to determine the values of its external passive components; Section 4 presents harvestersthe measured and the light estimated intensities remaining at the deployed charge nodes in the and battery, shows then the it generated discusses current the important from the results. solar Finally,harvesters Section and5 concludes the estimated the paper.remaining charge in the battery, then it discusses the important results. Finally, Section 5 concludes the paper. 2. Primitive Solar Energy Harvester 2.The Primitive proposed Solar primitive Energy Harvester solar energy harvester is attractive by its low cost, as it mainly consists of only twoThe cheap proposed off-shelf primitive components solar energy (nMOS harvester transistor is attractive and diode), by asits shownlow cost, in as Figure it mainly2, making consists it a goodof productiononly two cheap choice off to‐shelf serve components for a large (nMOS number transistor of WSN nodes. and diode), The harvester as shown circuit in Figure is connected 2, making to a solarit a panelgood asproduction a source for choice input to power serve andfor a rechargeable large number batteries of WSN as anodes. charge The destination. harvester Thiscircuit circuit is hasconnected two tasks: to the a solar first ispanel to stop as a charging source for the input batteries power after and reaching rechargeable their batteries maximum as voltagea charge to protectdestination. the load This circuits circuit from has being two tasks: damaged, the first and is theto stop second charging is to preventthe batteries the batteriesafter reaching from their being dischargedmaximum through voltage the to protect leakage the current load circuits of the harvesting from being circuit. damaged, For and testing the second purposes, is to Keithley prevent the 2400 SourceMetersbatteries from are being used todischarged measure thethrough voltage the and leakage current current of the of batteries the harvesting and turn circuit. on/o ffForthe testing nMOS transistor.purposes, In Keithley reality, the2400 EWSN SourceMeters node measures are used battery to measure voltage the voltage and provides and current the switching of the batteries digital signaland that turn indicates on/off the the nMOS full charge transistor. of the batteries.In reality, The the nMOSEWSN transistor node measures short-circuits battery the voltage solar paneland provides the switching digital signal that indicates the full charge of the batteries. The nMOS and stops harvesting when the battery voltage (VBAT) exceeds 3.6 Voltage (V), which is the nominal operatingtransistor voltage short‐ ofcircuits three the 1.2 solar V NiMH panel AA and batteries stops harvesting connected when in series.the battery The voltage leakage (V currentBAT) exceeds of the nMOS3.6 Voltage transistor (V), has which to be is a the minimum nominal tooperating avoid unwanted voltage of lossthree from 1.2 V the NiMH harvested AA batteries energy connected through it. in series. The leakage current of the nMOS transistor has to be a minimum to avoid unwanted loss Table1 shows the measurement results of two samples of three transistor models (TN5325N3, BS270FS, from the harvested energy through it. Table 1 shows the measurement results of two samples of three and ZVN2110A) at a battery voltage of 3.6 V and illumination of 100 klx. Their leakage current does transistor models (TN5325N3, BS270FS, and ZVN2110A) at a battery voltage of 3.6 V and illumination not exceed 0.5 nanoampere (nA), which is negligible compared with the average current consumption of 100 klx. Their leakage current does not exceed 0.5 nanoampere (nA), which is negligible compared of the target EWSN node. ZVN2110A is selected for the proposed harvester design since it has the with the average current consumption of the target EWSN node. ZVN2110A is selected for the minimumproposed leakage harvester current design with since a small it has variation. the minimum leakage current with a small variation.

(a) (b)

Figure 2. Primitive solar energy harvester. (a) Circuit diagram of the harvester. (b) Testbench of Figure 2. Primitive solar energy harvester. (a) Circuit diagram of the harvester. (b) Testbench of power powermeasurements. measurements.

Table 1. Measured leakage current of nMOS transistors at 100 klx and VBAT = 3.6 V. Table 1. Measured leakage current of nMOS transistors at 100 klx and VBAT = 3.6 V. Leakage Current (pA) Leakage Current (pA) ModelModel 1st1st Sample Sample 2nd Sample2nd Sample TN5325N3TN5325N3 300 300 400 400 BS270FSBS270FS 20 20 300 300 ZVN2110AZVN2110A 30 30 30 30

The diode prevents the batteries from being discharged by the harvesting circuit at weak light conditions.The diode It prevents needs to thehave batteries a low voltage from being drop dischargedand leakage by current the harvesting since the diode circuit voltage at weak drop light conditions.decreases It the needs minimum to have light a intensity low voltage to recharge drop and the leakagebattery, and current the diode since leakage the diode current voltage reduces drop decreases the minimum light intensity to recharge the battery, and the diode leakage current reduces Electronics 2020, 9, 2058 4 of 13 batteryElectronics charging 2020, 9, x current. FOR PEER TableREVIEW2 shows the measurement results of two samples from four4 diode of 13 models (BAX16, 1N4148FS, 1N4149, and FDH333) at a battery voltage of 3.6 V. The diode voltage drops battery charging current. Table 2 shows the measurement results of two samples from four diode are measured at three diﬀerent lighting conditions such as 1 klx, 10 klx, and 100 klx, and the leakage models (BAX16, 1N4148FS, 1N4149, and FDH333) at a battery voltage of 3.6 V. The diode voltage currentsdrops are measured at at three 100 klx. different The leakage lighting currents conditions of allsuch diodes as 1 klx, are 10 negligible klx, and (100<5 nA).klx, and BAX16 the is selectedleakage for currents the proposed are measured harvester at 100 due klx. to the The lowest leakage voltage currents drop. of all diodes are negligible (<5 nA). BAX16 is selected for the proposed harvester due to the lowest voltage drop. Table 2. Measured leakage current and voltage drop of four diode models (two units for each model) at diﬀerent light conditions and V = 3.6 V. Table 2. Measured leakage currentBAT and voltage drop of four diode models (two units for each model)

at different light conditions and VBAT = 3.6 V. Voltage Drop (V) Ileakage (nA) Model 1 klxVoltage 10 klx Drop (V) 100 klx 100Ileakage klx (nA) Model BAX16 0.591 klx/0.58 0.7210/ 0.71klx 0.82100/0.81 klx 4.72/1003.78 klx BAX16 0.59/0.58 0.72/0.71 0.82/0.81 4.72/3.78 1N4148FS 0.62/0.61 0.73/0.73 0.82/0.82 3.12/3.03 1N4148FS 0.62/0.61 0.73/0.73 0.82/0.82 3.12/3.03 1N41491N4149 0.62/0.61 0.62/0.61 0.730.73/0.73/0.73 0.820.82/0.82/0.82 2.922.92/2.79/2.79 FDH333FDH333 0.68/0.68 0.68/0.68 0.790.79/0.79/0.79 0.850.85/0.85/0.85 0.030.03/0.03/0.03

FigureFigure3 shows 3 shows the the measured measured output output power power of of the the proposedproposed primitive harvester harvester with with four four solar solar panels (SM531K80L, Adafruit200, LL200‐3‐37 (Serial), and LL200‐3‐37 (Parallel)) at different light panels (SM531K80L, Adafruit200, LL200-3-37 (Serial), and LL200-3-37 (Parallel)) at diﬀerent light conditions (1 klx, 10 klx, and 100 klx). SM531K80L has the highest output power at 100 klx for ~3.6 V. conditions (1 klx, 10 klx, and 100 klx). SM531K80L has the highest output power at 100 klx for ~3.6 V. However, it produces lower output power than other panels at weaker light conditions. On the other However, it produces lower output power than other panels at weaker light conditions. On the other hand, Adafruit200 has appropriate output power around 3.6 V battery voltage for a wide range of hand, Adafruit200 has appropriate output power around 3.6 V battery voltage for a wide range of light light conditions, which makes it a good choice to feed the harvester circuit. conditions, which makes it a good choice to feed the harvester circuit.

(a) (b) (c)

FigureFigure 3. 3.Measured Measured output output power power of of the the primitiveprimitive harvester circuit circuit with with four four different diﬀerent solar solar panels panels at at diﬀdifferenterent light light conditions: conditions: (a ()a 1) 1 klx; klx; ( b(b)) 10 10 klx; klx; ( (cc)) 100100 klx.klx.

3. Solar3. Solar Energy Energy Harvester Harvester Based Based on on bq25570 bq25570 ICIC TheThe Bq25570 Bq25570 chip chip from from Texas Texas Instruments Instruments isis aa devicedevice that integrates integrates energy energy harvesting harvesting and and powerpower managing. managing. The The chip chip is is designed designed to to eefficientlyﬃciently extract low low power power in in the the range range of of microwatts microwatts to to milliwatts.milliwatts. Moreover, Moreover, bq25570 bq25570 provides provides overchargeovercharge protection for for batteries batteries and and buck buck converter converter for for outputoutput regulation. regulation. Moreover, Moreover, it it has has a a programmable programmable maximum power power point point tracking tracking (MPPT) (MPPT) module module whichwhich optimizes optimizes the the energy energy extraction extraction from from diﬀ differenterent sources sources (such (such as solaras solar panels), panels), while while the the chip chip itself consumesitself consumes only hundreds only hundreds of nano of amperesnano amperes current current [36]. [36]. According According to the to the advantages, advantages, bq25570 bq25570 has beenhas utilized been utilized as a solution as a solution to recharge to recharge the batteries the batteries of the of EWSN the EWSN nodes, nodes, which which can can be comparedbe compared with thewith primitive the primitive harvester harvester for performance for performance comparison. comparison.

Electronics 2020, 9, x FOR PEER REVIEW 5 of 13 Electronics 2020, 9, 2058 5 of 13 Figure 4 shows the energy harvester circuit utilizing bq25570. External resistors are used to determine voltages necessary for required operations for the bq25570. First, the bq25570 uses the harvestedFigure4 current shows to the charge energy CSTOR harvester until its circuit voltage utilizing exceeds bq25570.the under‐ Externalvoltage (UV) resistors threshold are which used to determineis internally voltages set to necessary~1.95 V. Then, for required the transistor operations between for VSTOR the bq25570. and VBAT First, is theturned bq25570 on, and uses the the harvestedbattery begins current to to be charge charged. CSTOR The battery until its over voltage‐voltage exceeds (OV) protection the under-voltage threshold (UV) is set threshold using ROV1 which and is internallyROV2 according set to ~1.95 to the V. Then, following the transistor equation between [36]. When VSTOR the and battery VBAT voltage is turned exceeds on, and this the value battery begins(VBAT_OV), to be charged. the connection The battery from over-voltage the input to the (OV) VSTOR protection is turned threshold off to stop is set charging using the ROV1 CSTORand and ROV2 accordingthe battery. to the following equation [36]. When the battery voltage exceeds this value (VBAT_OV), the connection from the input to the VSTOR is turned oﬀ to stop charging the CSTOR and the battery.

(a)

(b)

FigureFigure 4. 4. EnergyEnergy harvester harvester based based on bq25570. on bq25570. (a) Circuit ( diagrama) Circuit (modified diagram from (modiﬁed [36]). (b) Designed from [36 ]). (b)harvester. Designed harvester.

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! 3 ROV2 VBAT_OV = VBIAS 1 + . (1) 2 ROV1 here, VBIAS nominal value is 1.21 V. ROV1 and ROV2 are chosen to be 1 and 1.05 megaohm (MΩ), respectively, to limit the over-voltage (VBAT_OV) to 3.72 V. Similar to the over-voltage threshold, the operating range of the battery voltage has to be independently programmed between 3 and 3.6 V using the following equations [36]. ! ROK2 VBAT_OK_PROG = VBIAS 1 + . (2) ROK1 ! ROK2 + ROK3 VBAT_OK_HYST = VBIAS 1 + . (3) ROK1 here, VBAT_OK_PROG is the lower threshold for the battery, and VBAT_OK_HYST is its upper threshold. ROK1,ROK2, and ROK3 are chosen to be 2, 3, and 1 MΩ, respectively. The buck converter is designed to give 2.9 V output voltage according to the following equation [36], by selecting ROUT1 and ROUT2 to be 1 and 1.4 MΩ, respectively. However, for a fair comparison between the primitive harvester and bq25570-based harvester, the EWSN node in this work is connected directly to the battery and no current is absorbed from the buck converter. ! ROUT1 + ROUT2 VOUT = VBIAS . (4) ROUT1 Table3 shows the values of all the selected resistors and their corresponding voltages. CREF, CIN, CSTOR, CBYP, COUT, L1, and L2 are chosen to be 10 nanofarad (nF), 4.7 microfarad (µF), 4.7 µF, 10 nF, 22 µF, 22 microhenry (µH), and 10 µH, respectively [36]. The maximum power point threshold is set to 80% of the open-circuit voltage of the solar cell.

Table 3. External resistors are used to set up the bq25570 and their corresponding voltages.

Actual Resistors Corresponding Voltages

ROV1 1.00 MΩ VBAT_OV 3.72 V ROV2 1.05 MΩ

ROK1 2.00 MΩ VBAT_OK 3.03 V ROK2 3.00 MΩ VBAT_OK_HYST 3.63 V ROK3 1.00 MΩ

ROUT1 1.00 MΩ VOUT 2.90 V ROUT2 1.40 MΩ

4. Measurement Results The target EWSN nodes are developed based on IRIS [37] and MicaZ [38–40] motes [7]. The motes are connected to MDA300 data acquisition board [41] to measure temperature and humidity. Moreover, the acquisition board digitizes analog signals from three external soil moisture sensors [7], using its analog-to-digital converter. Operating on TinyOS 2.1.2, the nodes sample sensor data packets every 15 min, including measured temperature, humidity, and soil moisture data. The EWSN nodes are classified to relay nodes that improve network connectivity without operating external sensors, and regular nodes with fixed locations and external sensors [7]. Thus, there are four different nodes, namely, relay nodes based on the IRIS platform, relay nodes based on the MicaZ platform, regular nodes based on the IRIS platform with three soil moisture sensors, and regular nodes based on the MicaZ platform with three soil moisture sensors. Table4 shows the current consumption and duration for different operation of the nodes. The most power-hungry node was the regular node Electronics 2020, 9, x FOR PEER REVIEW 7 of 13 Electronics 2020, 9, 2058 7 of 13 (mAs)/day. Considering a 5% margin, 127,026 mAs/day is used to check the feasibility of the basedproposed on the energy MicaZ harvesters. platform, which has a charge consumption rate of 120,978 milliampere second · (mAs)/day. Considering a 5% margin, 127,026 mAs/day is used to check the feasibility of the proposed Table 4. Current consumption of the target nodes [7]. energy harvesters. IRIS MicaZ Table 4.CurrentCurrent consumption of the target nodesCurrent [7]. Operation Duration Duration Consumption Consumption IRIS(ms) MicaZ (ms) (mA) (mA) Current Current SensorOperation Sampling Consumption6.3 Duration341.8 (ms) Consumption6.8 Duration337.0 (ms) w/o external sensors (mA) (mA) Sensor Sampling Sensor Sampling w/o 59.16.3 341.8 341.8 91.8 6.8 337.0337.0 w/externalexternal sensors sensors Data Packet Sensor Sampling 6.559.1 1.5 341.8 91.89.2 337.02.4 Transmissionw/external sensors Data Packet Data Packet Transmission21.1 6.5 23.6 1.5 22.7 9.2 2.424.0 Reception DataIdle Packet Listening Reception 19.1 21.1 5.4 23.6 23.3 22.7 24.03.7 SleepIdle (Relay/Reg.) Listening 0.1/1.0 19.1 N/A 5.4 0.3/1.1 23.3 3.7N/A Sleep (Relay/Reg.) 0.1/1.0 N/A 0.3/1.1 N/A The EWSN notes have a supply voltage operating range between 2.7 and 3.6 V [7]. It can be provided by connecting three 1.2 V batteries in series. Considering AmazonBasics 1.2V NiMH AA The EWSN notes have a supply voltage operating range between 2.7 and 3.6 V [7]. It can be rechargeable batteries (2000 mAh), the battery voltage mainly stays at 3.6 V by energy harvesting, provided by connecting three 1.2 V batteries in series. Considering AmazonBasics 1.2V NiMH AA which will be clear at the end of this section. Figure 5 shows the harvested current and the end‐to‐ rechargeable batteries (2000 mAh), the battery voltage mainly stays at 3.6 V by energy harvesting, end efficiency of the primitive harvester and the bq25570‐based harvester at 3.6 V across light which will be clear at the end of this section. Figure5 shows the harvested current and the end-to-end intensities. efficiency of the primitive harvester and the bq25570-based harvester at 3.6 V across light intensities.

(a) (b)

FigureFigure 5. 5. MeasuredMeasured energy energy harvesters harvesters with withdifferent diﬀ illuminations:erent illuminations: (a) generated (a) current; generated (b) end current;‐to‐ (b)end end-to-end efficiency. eﬃ ciency.

ToTo judge judge the the feasibility feasibility of of the the proposed proposed energy energy harvesters, harvesters, wewe taketake the advantage of of the the outdoor outdoor multi-hopmulti‐hop EWSN EWSN by theby University the University of Pittsburgh of Pittsburgh and Indiana and University Indiana University Purdue University Purdue Indianapolis University (IUPUI)Indianapolis (https: //(IUPUI)sites.google.com (https://sites.google.com/site/aswpitt/)/site/aswpitt/) located inside the located Beechwood inside Farms the Beechwood Nature Reserve Farms of AudubonNature Reserve Society of Audubon Western Pennsylvania Society of Western in the Pennsylvania greater Pittsburgh in the areagreater [35 Pittsburgh]. The reserve area is [35]. densely The woodedreserve with is densely varying wooded stages with of mixed varying deciduous stages of forest,mixed deciduous an old conifer forest, plantation, an old conifer clear plantation, meadows, shrubland and wetland vegetation (http://www.aswp.org/pages/beechwood). As shown in Figure6, Electronics 2020, 9, 2058x FOR PEER REVIEW 8 of 13 Electronics 2020, 9, x FOR PEER REVIEW 8 of 13 clear meadows, shrubland and wetland vegetation (http://www.aswp.org/pages/beechwood). As weshown recordedclear in meadows, Figure the 6, illuminations weshrubland recorded and of the sunlightwetland illuminations vegetation on the of EWSN sunlight(http://www.aswp.org/pages/beechwood). nodes on withthe EWSN the goal nodes that with the surroundingthe goal As that thediverse surroundingshown plant in Figure community diverse 6, we plantrecorded will enablecommunity the illuminations the study will enable results of sunlight the to have study on wide the results EWSN applications. to nodeshave widewith theapplications. goal that the surrounding diverse plant community will enable the study results to have wide applications.

FigureFigure 6. 6. 55 5 EWSN EWSN nodes nodes with MX2202MX2202 light light sensors. sensors.

HOBOHOBO PendantPendant Pendant MX MX MX loggers loggers loggers (MX2202) (MX2202) (MX2202) are areare attached attached to to eachto each each node node node and and collectand collect collect light light intensitieslight intensities intensities every every 5 min for more than 5 months (11/2/2019–4/17/2020). Figure 7 plots the measured light every5 min for5 min more for than more 5 months than (115 months/2/2019–4 (11/2/2019–4/17/2020)./17/2020). Figure7 plots Figure the measured 7 plots lightthe intensitiesmeasured fromlight intensities from the MX2202 for Nodes #1 and #5 as examples. The measured light intensities are intensitiesthe MX2202 from for Nodesthe MX2202 #1 and for #5 Nodes as examples. #1 and The #5 as measured examples. light The intensities measured are light diﬀ erentintensities since theare different since the orientation of the nodes and the shade conditions by trunks, branches, and leaves differentorientation since of the the nodes orientation and the of shadethe nodes conditions and the by shade trunks, conditions branches, by and trunks, leaves branches, are diﬀerent. and leaves are different. are different.

Figure 7. Measured light intensity using MX2202 light sensors at two nodes (#1 and #5). Figure 7. Measured light intensity using MX2202 light sensors at two nodes (#1 and #5). FigureFigureFigure8a shows 8a 7.shows Measured the the calculated calculated light intensity harvested harvested using current, current,MX2202 atlight Node sensors #5 #5 with withat two the the nodes weakest weakest (#1 andlight light #5). condition condition among the five nodes, for the primitive harvester in the blue line and the bq25570‐based harvester in among the five nodes, for the primitive harvester in the blue line and the bq25570-based harvester in the the red line, based on measurement results of the harvester itself (Figure 5) and environment light red line,Figure based 8a onshows measurement the calculated results harvested of the harvester current, itself at Node (Figure #5 5with) and the environment weakest light light condition intensity intensity (Figure 7). Figure 8b,c show the predicted remaining charge of the batteries every 5 min. (Figureamong 7the). Figure five nodes,8b,c show for the the primitive predicted harvester remaining in charge the blue of the line batteries and the every bq25570 5 min.‐based The harvester remaining in The remaining charge is predicated by adding the estimated harvested charge (‘generated current’ × thecharge red isline, predicated based on by measurement adding the results estimated of the harvested harvester charge itself ((Figure‘generated 5) and current’ environment5 min) light and intensity5 min) (Figure and subtracting 7). Figure the 8b,c average show charge the predictedconsumption remaining of 0.125 mAh charge every of 5the min. batteries However, ×every it takes 5 min. subtractinginto consideration the average that chargethe battery consumption charge does ofnot 0.125 exceed mAh the maximum every 5 min. capacity However, of 2000 mAh. it takes The into The remaining charge is predicated by adding the estimated harvested charge (‘generated current’ × considerationremaining charge that theis different battery among charge the nodes does since not exceedlight intensities the maximum are different, capacity as shown of in 2000 Figure mAh. 5 min) and subtracting the average charge consumption of 0.125 mAh every 5 min. However, it takes The7 remaining with the examples charge isof diNodesfferent #1 and among #5. the nodes since light intensities are different, as shown in intoFigure consideration7 withThe important the examples that results the ofbattery from Nodes Figurescharge #1 and 7does and #5. not8 for exceed both harvesters the maximum (primitive capacity and ofbq25570 2000 ‐mAh.based The remainingharvesters)The important charge are issummarized different results from among in Table Figures the 5, showingnodes7 and 8since forthe average bothlight harvestersintensities and maximum are (primitive different, illumination and as bq25570-basedshown of the inlight, Figure 7harvesters) withthe theaverage examples are summarizedand maximumof Nodes in #1estimated Table and5 #5., showing charging the currents, average and and the maximum estimated illumination minimum remaining of the light, the averagebatteryThe important charge. and maximum All results the tabulated estimated from Figures numbers charging 7 are and currents,calculated 8 for both and for harvestersthe the five estimated deployed (primitive minimum nodes andfrom remaining bq25570 November battery‐based harvesters)charge.2019 All to the are middlesummarized tabulated of April numbers in 2020.Table are The 5, calculated showing average andthe for average themaximum five deployedand illumination maximum nodes values illumination from cannot November ofindicate the 2019 light, to directly to the most efficient harvested energy. Node #1 has the highest maximum illumination and the middleaverage of and April maximum 2020. The estimated average and charging maximum currents, illumination and the values estimated cannot minimum indicate remaining directly to maximum charging current, while Node #2 has the highest average illumination and average batterythe most charge. efficient All harvested the tabulated energy. numbers Node are #1 hascalculated the highest for the maximum five deployed illumination nodes from and November maximum 2019charging to the current, middle while of April Node 2020. #2 has The the average highest and average maximum illumination illumination and average values charging cannot indicate current; directly to the most efficient harvested energy. Node #1 has the highest maximum illumination and maximum charging current, while Node #2 has the highest average illumination and average

Electronics 2020, 9, 2058 9 of 13 Electronics 2020, 9, x FOR PEER REVIEW 9 of 13 nevertheless,charging Nodecurrent; #3 nevertheless, shows the highest Node #3 minimum shows the requiredhighest minimum charge, whichrequired makes charge, it thewhich most makes eﬃ cient nodeit in the exploiting most efficient the harvestednode in exploiting solar energy. the harvested This observation solar energy. validates This observation the idea validates that the the existence idea of lightthat over the a long existence period of oflight each over day a islong more period important of each than day itsis more maximum important or average than its intensitymaximum since or the batteryaverage cannot intensity acquire since a charge the battery over cannot its maximum acquire a capacity. charge over its maximum capacity.

(a)

(b)

(c)

Figure 8. Estimated battery charging current and remaining charge. (a) Estimated charging current at node 5 for both harvesters. (b) Estimated batteries remaining charge for the primitive energy harvester. (c) Estimated batteries remaining charge for the bq25570-based energy harvester. Electronics 2020, 9, 2058 10 of 13

Table 5. Summary of the most important readings in the measured and estimated data for both harvesters at the ﬁve nodes over more than 5 months.

Avg. Charging Max. Charging Min. Remaining Illumination (Lux) Node Current (mA) Current (mA) Battery Charge (mAh) Avg. Max Primitive Bq25570 Primitive Bq25570 Primitive Bq25570 #1 3161 88,760 2.61 2.42 49.0 41.1 1818 1793 #2 4814 85,852 3.86 3.54 47.8 40.1 1923 1921 #3 3969 75,796 3.38 3.13 43.5 36.8 1938 1937 #4 3627 77,087 3.05 2.83 44.1 37.2 1895 1890 #5 2505 59,105 2.27 2.14 36.1 31.0 1734 1705

Both harvesters are estimated to continuously power any of the five nodes at the given 2000 mAh batteries without a power outage. The difference in the remaining charge is slightly better in the primitive harvester, the batteries charge drops from 2000 to 1734 mAh before starting to recharge again, while, for the bq25570-based harvester, the batteries charge drops to 1705 mAh. It makes the battery charge not to drop below 85% of its maximum charge capacity for both harvesters. Moreover, the average charging current from all harvesters is above 2 mA, which is higher than the average current consumption of 1.5 mA by the most power-hungry node in [7]. It keeps the batteries charged and thus ensures the ability of both harvesters to continuously power the nodes. In this work, the AA batteries have been considered to replace the non-rechargeable battery used in [7]. Since the batteries support enough charge storage capability in the given illumination profile, smaller size of batteries with lower maximum charge capacity can be applied. For example, an AmazonBasics 1.2 V NiMH AAA rechargeable battery has the maximum charge capacity of 800 mAh with the same output voltage. The three batteries reduce the volume from 23,088 cubic millimeters (mm3) to 11,559 mm3 (50% lower) and the price for three batteries from USD 4.5 to USD 3.2 (29% lower) while maintaining the remaining charge higher than 63%. However, the smaller remaining charge makes the battery output voltage lower in general so that the efficiency of the primitive harvester can be reduced as shown in Figure3b,c. In addition, if environmental light intensity is unexpectedly weaker than the exemplary light profile, the node with smaller batteries can experience power outage due to the reduced energy margin stored in the batteries. Moreover, different types of batteries can be applied, such as a lithium coin battery or lithium polymer batteries, to further reduce the physical battery volume with a commercially available option. For instance, as an extreme case with negligible remaining charge capacity, an Illinois Capacitor RJD3048 3.7 V lithium coin battery (300 mAh) and a Jauch LP422339JU1s1p 3.7 V lithium polymer battery (330 mAh) only take a volume of 3393 and 4324 mm3, respectively. However, the high battery voltage cannot be applied to the nodes directly. It is required to add a voltage regulator between the battery and the node, increasing design complexity. The primitive harvester only has two components, and the total cost of the harvester circuit does not exceed USD 1 excluding the solar panel. The ZVN2110A nMOS transistor costs USD 0.68, and the BAX16 diode costs USD 0.19. On the other hand, the cost of bq25570 IC alone is USD 7.7. The cost becomes easily beyond USD 10 by considering other discrete components to build a complete bq25570-based harvester circuit. It makes the total cost at least USD 10 higher than that of the primitive harvester. However, Figure5 shows that the e fficiency of the primitive harvester significantly drops for illumination lower than 1 klx. For applications where environmental light mainly cannot be higher than 1 klx, the bq25570-based harvester can be a better choice since the charging current of the primitive harvester can be substantially reduced. Electronics 2020, 9, 2058 11 of 13

5. Conclusions In this paper, two solar energy harvesters are introduced; the first one is a primitive harvester with very low cost; the second one is based on bq25570 IC from Texas Instruments. The two harvesters are exposed to various light conditions, and the corresponding harvested current is measured. The measured values are used to estimate the generated current corresponding to given light conditions, and these estimations are applied to the illuminations recorded at five different nodes deployed at ASWP for more than five months. The estimated current generated by both harvesters at the locations of five nodes shows their feasibility to power real-world WSN nodes deployed for environmental monitoring applications. In the worst case, both harvesters can keep the batteries charged up by only using 15% of 2000 mAh. The efficiency of the primitive harvester lies below that of the bq25570-based harvester in low light conditions but overpowers it above 3 klx while keeping its reasonably low price. It makes the primitive harvester a feasible choice to power numerous EWSN nodes, without increasing the overall budget of the system.

Author Contributions: Conceptualization, Y.L., J.-S.L., X.L. and I.L.; methodology, Y.L., X.Z. and D.L.; validation, Y.L. and X.Z.; data curation, Y.L. and E.A.H.; writing—original draft preparation, E.A.H.; writing—review and editing, Y.L., E.A.H., D.L., J.-S.L., X.L. and I.L.; visualization, E.A.H.; supervision, I.L.; project administration, I.L.; All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding Acknowledgments: The authors are grateful to the Audubon Society of Western Pennsylvania (ASWP), Pittsburgh, PA, USA, for their support. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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