Battery-Powered Autonomous Robot for Cleaning of

Battery-Powered Autonomous Robot for Cleaning of Dusty Photovoltaic Panels in Desert Zones Michele Gabrio Antonelli, Pierluigi Beomonte Zobel, Andrea De Marcellis and Elia Palange Department of Industrial and Information Engineering and Economics University of L’Aquila, Via G. Gronchi 18, 67100 L’Aquila, Italy {gabrio.antonelli,pierluigi.zobel, andrea.demarcellis,elia.palange}@univaq.it

Abstract. Storms in desert areas cause sand accumulation on the surface of photovoltaic panels so producing a decrease in the electrical conversion efficiency per day of solar farms ranging from about 0.6% up to 80%. Hence, continuous activities for cleaning the photovoltaic panels are required in total absence of water and under severe environmental conditions for the workers. In this communication we present a 12V battery-powered autonomous robot for cleaning of dusty photovoltaic panels. The cleaning strategy adopts two helical brushes placed in front of and behind the robot. According to the robot movement direction, the forward brushes remove the sand until it falls out from the photovoltaic panels. The robot has been conceived to be moved as a half-track equipped with two independent rubber belts powered by two DC motors. The electronic on-board control system is based on ARDUINO DUE platform and employs ultrasonic sensors providing the real-time effective position of the robot, the movement regulations as well its direction and speed. Keywords: Unmanned vehicle∙ Photovoltaic panel cleaning∙ Ultrasonic sensors∙ ARDUINO DUE∙ Battery-powered autonomous system

1

Introduction

Dust accumulation (resulting in soil, sand and other particles) on the surface of PhotoVoltaic (PV) panels is one of the major cause for the reduction of the solar plant conversion efficiency that must be constantly monitored/measured through suitable sensing systems [1-6]. Environmental factors (wind and dust storm, air pollution), dust type (soil and sand, clay, carbon), solar farm location area (latitude and longitude, sandy area, industrial area) [7] are at the origin of dust accumulation. To remove dust, several solutions can be employed. Rainfall is one of them. Nevertheless, in presence of water insoluble particles (for example bird droppings) or after a dry period, rain could be inefficient and is not programmable. The manual scrubbing of dust can be carried out in small extension solar plants, generally requiring water supply. Recently advanced solutions have been implemented [8] that make use of water jets, air compressors to blow away the dust, mechanical systems for the PV panel rotations allowing to fall out dust of the PV panels, guided moving systems equipped with me-

chanical brushes acting with water, self-cleaning hydrophobic coatings. In desert zones where extensive solar farms are located, after wind storms the sand accumulation on the PV panel surface causes an average decrease in the photovoltaic conversion efficiency per day from about 0.6% [9] up to 80% [8]. For these reasons, continuous cleaning activities of the PV panel surface are required. In general, the existing cleaning solutions cannot be applied for these kind of solar farms: the use of water is problematic and too expensive; air compressor systems and mechanical apparatus for the rotation of the panels are costly and require very high power consumption; the hydrophobic coating are inefficient in absence of water; human labour for the cleaning activities is difficult to schedule due to the environmental severe conditions (i.e. high temperatures), thus resulting much more demanding than other manual activities, for example in agriculture, where robotic devices are employed [10, 11]. Some commercially available cleaning devices (i.e. robots) have been developed for the desert zones. Without an autonomous navigation system [12], Ecoppia E4 [13] is a robot with two degrees of freedom: a frame moves laterally along the PV panel area and the robot itself moves up and down the panels; two rotating brushes provide for the cleaning action. Nomadd [14] is a modular fully automated guided system made of a rotating brush moving along the panels. The Solar Cleaning Robots of Miraikikai Inc. [15], instead, is an autonomous robot equipped with three wheels and two brushes for collecting and removing the accumulated sand on the panel surfaces. About the first two solutions, for an extensive solar farm the use of external guides for the lateral motion of the brushes is expensive, requires continuous maintenance and is not suitable for wavy solar farms. About the third device, the rotation of both the brushes at the same time could result in an uncontrolled sand dispersion with subsequent sand accumulation in other zones of the PV panels system. Additionally, the uphill motion of the robot requires high power consumption. This work describes the prototype of an autonomous robot developed for cleaning dusty panels in desert zones. The cleaning strategy is based on two brushes alternatively rotating: one is placed in front of and the other behind the robot. Only the brush facing along the motion direction can rotate in order to remove the accumulated sand until it has fallen out of the panel. The robot moves as a half-track: two independent rubber belts provide for the motion of the robot by means of two DC motors. The electronic on-board control system is based on ARDUINO DUE platform [6] and employs ultrasonic sensors for a real-time detection and regulation of the position, direction and speed of the robot. The paper is divided as follows: Section 2 reports the mechanical design of the robot through the definition of the technical specifications and the description of the cleaning strategy; Section 3 describes the electronic control system apparatus. Finally, the results of the experimental testing activities of the autonomous motion of the robot are reported.

2

The mechanical design

The definition of the technical specifications and the assessment of the motion of the robot for the cleaning strategy were at the basis of the functional design of the robot.

These asppects are described in the foollowing sectiions. 2.1

Teechnical speccifications

Accordinng to the requ uirements of the external environment and of a typpical solar plant, thee following tecchnical specifi fications have been defined: - the PV panels are mo ounted on a m main underlyin ng frame by aluminium a clipps; typical dimensions of the pan nels mountingg structure arre reported in n Fig.1. A fluuent linear motion of the robot neeed to avoid coollisions betw ween the retain ning clips andd the moving parts of the robot with w the excep eption of the cleaning c devicces. For this re reason, the maximum m width of thee robot has beeen chosen low wer than 530 mm; m - the totaal length of th he PV panel m matrix is geneerally equal to o about 100 m and the height is equal to abou ut 3.5 m or 3 m for the veertical and ho orizontal placeement, respectivelyy; - the PV panels p tilt ang gle respect to tthe horizontall plane is in the range 2° - 3 0°; - the rem moval of the saand must be m mechanically carried out without w the usee of water and the clleaning devicees must avoidd to scratch thee PV panels; - the robbot must be to otally autonom mous. In partticular, it must be equippeed with: a power suupply system; an independennt traction sysstem without connections w with external framees; a control sy ystem withoutt wiring with external e devicces; - the robbot must be eq quipped withh seals to avoid the contam mination of thhe moving parts withh sand; - the roboot must operatte within a tem mperature rang ge from -10 to o 70 °C.

PVi,j

PVi+1,j

clip

PVi,j+1

PVi+1,j+1

Fig. 1. Tyypical installatio on and dimensioons (in mm) of a 2x2 PV panell matrix. The paanel matrix can be reaalized with two or three rows ((panels horizon ntally placed, as shown) or withh two rows (paneels vertically placed).

2.2

Th he cleaning sttrategy

Taking innto account th he PV panel m matrix, the cleaaning strategy is based on thhe following speciffications: - the roboot is equipped d with two alteernatively worrking brushes. Only the bruush placed along thee motion direcction can operrate. Each brush has a helical profile: thee envelope

of the hellix angle prov vides moving tthe sand forw ward the motion direction annd in areas of the PV V panel matrix x that will be rremoved after by the robot; - the roboot starts to mo ove from a pit placed on onee side of the row r placed at tthe higher height. Thhen, it cleans the whole PV V panel upper row until it moves m down too the lower row. Thee sand is alwaays moved forrward and do ownward, untiil it falls out oof the PV panel mattrix. When thee robot complletes the clean ning proceduree, it returns onn the starting pit thrrough the shortest path from m the final/currrent position;; - at the end e of each ro ow of the PV V panel matrix x (see Fig.2), the robot mooves backwards diaagonally and realigns itselff respect to th he upper panels row. Durinng the row crossing, both brushess rotate in ordder to avoid any contaminaation betweenn the accumulated sand and the moving tracttion system. The sand is always a movedd towards areas thatt will be clean ned later. Referringg to Fig. 2, greey rectangles aare the dusty PV panels. Th he outside whhite rectangle is thee starting pit of o the robot. T The square with w the black circle is the rrobot with the two brushes. b The sllash and backkslash lines rep present the bru ushes helix anngle.

a)

b)

c)

d)

e)

f)

g)

h)

Fig. 2. Mootion phases off the cleaning prrocedure: a) rob bot at rest; b) clleaning of the fi first row; c) crossing to the first halff of the second rrow; d)-e) clean ning of the whole first half of tthe second row; f) crrossing to the seecond half of thhe second row; g)-h) cleaning of o the whole seecond row.

The working brush is represented by the rectangle with thick edges. The thin and dotted arrows represent the current and previously described trajectory of the robot, respectively. Finally, the big arrows represent the sand removing direction. Even though only a 2x2 panel matrix has been represented in Fig.2, the cleaning strategy can be easily extended for any PV panel matrix dimension. 2.3

The functional design

The mechanics of the robot is constituted of three systems: the traction system, the frame system and the cleaning system. The traction system is realised by using two identical modules in order to move the robot as a half-track. Each module uses a closed belt around two pulleys and an electric motor, directly mounted on one pulley, provides for the power. The dimensioning of the couple of motors was carried out according to the following specifications: mass of the robot equal to 20 kg; whole mass of the pulleys equal to 6 kg; pulleys radius equal to 50 mm; maximum path slope to be followed equal to 25°; transmission efficiency equal to 0.9; maximum speed rate equal to 0.5 m/s. The maximum assessed torque was equal to about 2 Nm; the maximum power was equal to 24 W. A couple of OSLV MR 62 dc electric motors was employed. A HTD 8M high friction rubber belt was chosen: the belt internal part in contact with the pulley is made of polyurethane and the external one is covered of Linatex to avoid the slip of the motor on the panels. The frame system was conceived as a box with the aim to: provide for the assembly of the two traction modules; collect the battery for the power supply and the electronic components of the control system. The cleaning system is made of two identical modules: one is placed in front of and the other behind the frame system. Each module is made of a brush powered by an electric motor through a belt transmission whose speed ratio is equal to 1. The brush is made of the helix placed at an angular distance equal to 120°. The length of the brush is equal to 830 mm; the external diameter of the helix is equal to 130 mm; the pitch of the helix is equal to 400 mm. A Como Drills 975D41 12V motor was adopted. A T10 polyurethane belt was employed for the transmission. To reduce the overall mass of the robot, the mechanical dimensioning considered aluminium components. 2.4

The prototype

The length and the width of the frame system mounted with the transmission systems are 644 and 510 mm, respectively. The maximum width of each cleaning system is equal to 950 mm. The overall dimensions of the robot are: length equal to 910 mm; width equal to 950 mm; height equal to 130 mm. In the lower parts of the external edges of the frame system and of the traction system a set of linear brushes was placed to avoid the contamination between the sand and the robot moving parts. Fig.3 shows the overall prototype, without the ultrasonic sensors (on the left) and in its complete shape during the testing procedure on the PV panel matrix.

Fig. 3. Thhe prototype off the robot

3

The electron nic control system dessign

The roboot overall conttrol system haas been develo oped employin ng an ARDUIINO DUE board, baased on a 32--bit ARM corre microcontrroller (i.e., Atmel SAM3X X8E ARM Cortex-M M3 CPU), mou unted on a PC CB that includ des the modules for managiing all the motors annd sensors, as shown in F Fig. 4. The AR RDUINO DU UE board has been programmedd to manage all a the robot parts as welll as the operaations to be pperformed (moving and cleaning operations). Starting from m the robot loccated at an innitial position, the ARDUINO DUE D board coontrols the ov verall timing of its actionss and motions: slow start and sttop, solar paneel cleaning, co ontrol/change of direction aand speed, return to the initial possition at the ennd of the cleaaning procedu ure. The design gn, simulation and prototyping p (ii.e., PCB layoout) of the elecctronic circuittry and system m architecture havee been perform med in OrCAD D PSpice envirronment. More in detail, the rotation speedd and directio on of the two DC D electric m motors employed foor the robot motion m are reggulated throug gh two externaal motor contrrol shields with NovvalithIC BTN8982TA by Innfineon. They y are controlleed by PWM-bbased signals and are a capable to o drive, in bi-ddirectional mo ode, DC motorrs in H-Bridgee configuration. Thhe other two DC motors eemployed for rotation of th he cleaning brrushes are activated//deactivated by b means of thhe two 250V 10A electro-m mechanical reelays modules T0100010 by Tink kerKit. In ordeer to perform m a real-time detection d of thhe current position of o the robot and a regulate/ccontrol its direection, speed and motion, ffour highresolutionn ultrasonic raange finders hhave been em mployed (MB1023, HRLV M MaxSonar EZ Seriees). These sen nsors, powereed at 5V and performing internal i speedd-of-sound temperatuure compensaation, allow foor high accuraacy proximity detection, froom 30 cm to 5 m with w 1 mm ressolution, provviding analogu ue output volttage proportioonal to the

measuredd distance. Alll the circuitryy is powered by a 12V 22 2Ah REC22-112I valveregulatedd lead acid batttery by YUA ASA. Moreoveer, a further PC CB has been ddeveloped for the suupply voltagee managemennt of the robo ot including power p controll modules, voltage reegulators/stab bilizers, batterry charge conttrol/indicator and protectionn devices, as show in i Fig. 5. In particular, p twoo 15A DC/DC C converter by y DROK (inpput 4-32V, output 1.2-32V) have been employyed as voltagee regulators to t provide sui uitable and stable suppply voltages to the electroonic circuitry and, in particcular, to all thhe motors. The samee PCB includees also a mainn power switch, an input prrotection 6x322 mm 15A 250V slow-blow ceram mic fuse as weell as a protecttion circuit for monitoring tthe charge level of the t battery thaat automaticallly turns off the t robot in th he case of higgh current consumpttions and/or lo ow battery levvel.

Fig. 4. The PCB with ARDUINO DUE and the shields s for the control of the m motors

Fig. F 5. The PCB B for the power supply manageement

4

Experimenta al verificattion of the robot r

Experimeental verificattions of the auutonomous op peration of th he robot have been performed at a the laborato ory scale by uusing a 3x3 PV V panels mattrix with a tottal area of about 12.4 m2 for tilting angles resspect to the horizontal plan ne equal to 0°°, 15° and 25°. Starting from thee initial positiion at one corrner of the matrix, m the robbot moved followingg the trajectory y illustrated inn Fig. 2, acting only the bru ush positionedd along the moving direction. d Oncce arrived at thhe matrix corrner opposite to t the initial pposition, a first compplete lap endeed and the robbot returns tow ward the startting point folllowing the PV panell matrix diago onal. At this ppoint, after few w seconds the robot departss again for a second lap and so on. We verifieed that the rob bot is capablee to perform aat least 50

consecutive complete laps corresponding to a total cleaned surface area of about 640 m2 without any deviation from the right direction, stopping perfectly at the end of each PV panel row. We performed some experiments to investigate the ability of the electronic control system to recover the robot movement for deviation angles up to 30° respect to the starting trajectory. Under these conditions, we measured that the robot returns again along the right path at a distance of 0.5 m from the position where the deviation occurred. Further experiments are in progress to verify the correctness/efficiency of the cleaning procedure by putting sand onto the PV panel matrix.

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