Available online at www.sciencedirect.com

Mathematics and Computers in Simulation 91 (2013) 16–28

Original articles

Intelligent sun-tracking system based on multiple photodiodesensors for maximisation of photovoltaic energy production

G.M. Tina a,∗, F. Arcidiacono a, A. Gagliano b

a D.I.E.E.I. University of Catania, D.I.I. University of Catania, V. le A. Doria n. 6, 95125 Catania, Italyb D.I.I. University of Catania, D.I.I. University of Catania, V. le A. Doria n. 6, 95125 Catania, Italy

Received 31 October 2011; received in revised form 14 June 2012; accepted 23 July 2012Available online 1 September 2012

Abstract

To maximise the energy collected by a given photovoltaic system (PVS), it is important to track the position of the sun so thatthe PV panels are exposed to the maximum global radiation at any given time. Thus, it is useful to determine the direction ofthe maximum global radiation, which does not correspond with the condition of orthogonality between the plane of the array andthe beam component of the solar radiation. Thus, it is worthwhile to investigate a low-cost solar radiation sensing system thatcan determine this direction. The proposed system consists of a structure of nine photodiodes (FDs); a suitable analysis of theoutput signals calculates the azimuth and solar height angles, which are related to the direction of maximum solar irradiance. Thispaper reports the development of hardware (HW) and software (SW) for a prototype system, which is called SoliSector. Outdoorexperimental results regarding testing and calibration of the proposed system are also reported.© 2012 IMACS. Published by Elsevier B.V. All rights reserved.

Keywords: Photovoltaic system; Tracking system; Solar radiation; Sensing transducer

1. Introduction

The growing interest in “bankable” solar resource data for the justification of solar energy conversion systemfinancing has raised questions regarding the use of single- or double-axis tracking systems to increase the yearlyenergy production with an acceptable increase in capital and M&O costs.

The double-axis PV system tracks the sun by adjusting two angles: the azimuth and the inclination. In this way, thepanel can be directed along the direction of the sun. The single-axis system tracks only one of these two angles.

There are many sensors that can measure solar irradiance and its components (beam and diffuse). These sensors arebased on different technologies (such as thermopiles, photodiodes and photovoltaics). Silicon-photodiode irradiancepyranometers were developed as a low-cost alternative to thermopile-based pyranometers to provide integrated solarresource information for climatological research. Many studies have been performed to compare the performances of

pyranometers based on photodiodes and of those based on thermopiles. The main sources of inaccuracy, which areintroduced by systematic, time-of-day-dependent variations in the solar spectrum, the solar angle of incidence, andthe operating temperature, are analysed in [6,8,11]. Much progress has been made to overcome traditional problems.

∗ Corresponding author.E-mail address: Giuseppe.tina@dieei.unict.it (G.M. Tina).

0378-4754/$36.00 © 2012 IMACS. Published by Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.matcom.2012.07.020

G.M. Tina et al. / Mathematics and Computers in Simulation 91 (2013) 16–28 17

Ip

iwa

so

s

soi

ibIo

2

bac

2

app

tp

Fig. 1. Bottom layer view of the nine-photodiode array (left) and a schematic of the functional principle (right).

n [9,12], a new photodiode-based pyranometer that features characteristics similar to those of thermopile-basedyranometers is presented.

Among the different types available on the market, it is interesting to observe the system developed in [2]: it is anntegrated device (called BF3) that uses an array of photodiodes with a unique computer-generated shading pattern,hich enables simultaneous measurements of the horizontal global and diffuse irradiance and the presence of sunshine

t any time.The common available solutions for tracking systems rely on the knowledge of the geographical position of the

olar panel on the earth’s surface. With this knowledge, it is possible to determine the relative position of the sun basedn the time of day, according to the well-known equations.

On the other hand, evident improvements can be achieved in the energy collected by double-axis PV systems ifuitable position algorithms are used [5].

In this context, significant effort has been focused on the optimisation of sun-tracking systems, as documented byeveral studies (e.g. [4]) and registered international patents (e.g. [7]). The system described in [1] seeks the directionf the maximum photovoltaic production of a two-axis tracking system by scanning a certain number of positions untilt finds the one that maximises the PV production.

This paper focuses on optimising the electric energy production of PV power plants through the development of anntelligent sensor applied to sun-tracking systems, the principle of which was reported in [3]. The sensor developed,ased on photodiode technology, is innovative compared to other similar solutions that are available on the market.n fact, the proposed solution is static and autonomous in terms of the information needed to process the optimalrientation, and it is intelligent because it performs on-line monitoring of the photovoltaic energy production.

An experimental prototype was built, and field results have demonstrated good performance of this tracking system.

. System description

The idea of the “SoliSector” (from Latin: sun tracker) arose from the need to increase the efficiency of PV modulesy positioning them along the direction of maximum solar irradiance. The aim is to determine the “optimal” azimuthnd tilt angles through the elaboration of data from solar transducers, in this case, photodiodes. The SoliSector systeman be divided into two parts: hardware and software.

.1. Hardware

The HW is composed of two fundamental parts: the electronic circuits and a suitable cylindrical plastic support,s shown in Fig. 1. The entire unit is placed on a base with a level control to ensure horizontality. In this system, theosition and “status” of the sun are estimated by processing the data gathered from nine high-speed and high-sensitivity

hotodiodes.

The plastic support is a black PVC pipe, which has a very thin wall, a diameter, D, of 50 mm and a height, Hhat depends on the latitude of the site. In particular, the height of the black cylinder and the distance between thehotodiodes laying along the circumference and the base of the cylinder, d, have been chosen in such a way that, at

18 G.M. Tina et al. / Mathematics and Computers in Simulation 91 (2013) 16–28

Fig. 2. Solar height for the SoliSector at midday of both the summer and winter solstices (H = 25 mm, D = 50 mm, d = 5 mm). Site: Catania (Italy)Lat. 37◦24′N.

midday of the winter solstice, the central photodiode (FD 9) is illuminated, whereas at midday of the summer solstice,the lateral photodiode (FD 5) is also illuminated (see Fig. 2).

When the sun moves, the shading pattern changes, so the photodiodes receive a different intensity of radiation(some receive global radiation, others receive diffuse radiation). Thus, there is a correspondence with the output of thephotodiodes, as shown in Fig. 3.

The conditioning circuit adopted for each photodiode is depicted in Fig. 4. The current output for each photodiode(BPW34) is voltage-converted through the Rs resistance; subsequently, the operational amplifier LMC6062IM causesa signal amplification, which is necessary for the voltage levels of successive blocks, whereas the Rp1 resistance isused to tune the output of the nine photodiodes under controlled and uniform irradiance conditions (low and highilluminance).

This choice of pyranometer sensor element (photodiodes) required an exhaustive study of the commercially availabledevices because it constitutes one of the key elements for obtaining better performance from the pyranometer [6]. Aphotodiode was required with a response within the visible spectrum, a high output value and as much linearity aspossible. In particular, a PiN junction rather than a p-n junction is used. A PiN photodiode requires a reverse bias forits operation due to the presence of an intrinsic region. This reverse bias has several consequences:

• Reverse bias introduces a noise current, which reduces the signal to noise ratio• Reverse bias offers better performance for high bandwidth applications• Reveres bias offers better performance for high dynamic range applications

Fig. 3. Shading pattern inside the plastic support.

G.M. Tina et al. / Mathematics and Computers in Simulation 91 (2013) 16–28 19

tssttr

owu

Fs

Fig. 4. Conditioning circuit diagram for each photodiode (Vcc = 5 V).

When a photodiode is used to measure the instantaneous solar irradiance, its accuracy is lower than that of ahermopile sensor. This inaccuracy is due to errors introduced by systematic, time-of-day-dependent variations in theolar spectrum, the solar angle of incidence, and the operating temperature [6,8,11]. In particular, the relative spectralensitivity of the PiN (type BPW34) diode is centred with respect to the solar spectrum at airmass (AM) 1.5. However,he solar spectrum of the diffuse component of the solar radiation has a spectral distribution shifted mainly towardshe ultraviolet compared to the beam component. Thus, when the distance travelled through the air mass of the solaradiation, the spectrum moves towards ultraviolet wavelengths.

The sensitivity is measured by the ratio of the radiant energy (W) incident on the photodiode to the photocurrentutput in amperes, expressed as the absolute responsivity in amperes per watt. A typical responsivity curve, normalisedith respect to the maximum value, is shown in Fig. 5(a), whereas in Fig. 5(b), the spectrum of the solar irradiance

nder standard conditions is given.

ig. 5. (a) Relative spectral sensitivity of a PiN (type BPW34) diode versus wavelength of the incident light (λ) and (b) AM 1.5 standard referencepectrum.

20 G.M. Tina et al. / Mathematics and Computers in Simulation 91 (2013) 16–28

Table 1Table of correspondence among FDi output voltages and levels.

Voltage Level

0 ≤ Vi ≤ 0.1 VMAX 10.1 VMAX < Vi ≤ 0.3 VMAX 20.3 VMAX < Vi ≤ 0.5 VMAX 30.5 VMAX < Vi ≤ 0.7 VMAX 4

0.7 VMAX < Vi ≤ VMAX 5

2.2. Software

The software for the SoliSector system was implemented in a MATLAB environment. The execution of variousscripts controls the following steps:

• data extraction from the database;• conversion of voltage values into levels;• reference model construction;• conversion of reference model outputs to levels;• data elaborations and comparisons;• routine error management;• correction of angles in comparison with the reference model;• azimuthal and solar height angles related to the direction of maximum irradiance.

The output voltage of each photodiode, Vi, is converted to a level following Table 1, where VMAX is the outputvoltage of the sensor corresponding to the highest irradiance value. This value is calculated by means of a mathematicalmodel, described below, that can calculate, for any given day, the maximum radiation at the earth’s surface under clearskies.

The conversion process must consider three factors:

1. possible errors during data acquisition;2. dynamic voltage output amplitudes;3. variable irradiance throughout the year.

For the first aspect, it is important to verify that a high level, which is associated with a high voltage value, does notcorrespond to a low irradiance value. If this condition is not observed, once the data are converted, it will be impossibleto find the errors again because the conversion process erases the departure data.The second factor accounts for thefact that the dynamic output voltage of the photodiode changes over the year. If we do not take this into account,the measurements will not be correctly interpreted.In fact, it is possible that, during the level assignment, parts of theoutput dynamics could be deleted due to a wrong choice of voltage range associated with different levels.The systemresolution and its sensitivity are considered together during the assignment of levels, especially in the case of verysmall variations in the output voltage of the photodiode.The last thing to consider is the change in radiation levelsthroughout the year, allowing possible errors to be examined, as indicated above for the first factor.

Fig. 5 depicts the daily trend of the global irradiance, under cloudless skies, on a horizontal plane in Catania overthe course of a year. Solar radiation coming from the sun is attenuated by the earth’s atmosphere before reaching theground, and the atmosphere can be classified into two broad types: clear sky atmosphere and atmosphere with clouds.For the aims of the present study, the total radiation under cloudless skies was modelled; because a highly precisemodel is not necessary, the ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers),

algorithm is used [2,10] (Fig. 6).

If the output voltage ranges associated with high levels are too narrow, important dynamics for some months couldbe eliminated. Thus, a range of output voltages equal to 1 V was assigned to levels 4 and 5.

G.M. Tina et al. / Mathematics and Computers in Simulation 91 (2013) 16–28 21

Fig. 6. Daily global irradiance according to the clear sky model (left graph) and solar path (right graph), over the course of a year on a horizontalplane in Catania – Italy (lat. 37.5◦N, long. 15.1◦).

ac

f

t

Fig. 7. Level conversions on a scale for the voltage output of FD 9, V9.

The conversion process is applied for the output voltage dynamic of each photodiode during the day, from 8:00.m. to 4:00 p.m. A conversion example, relate to the FD 9, is illustrated in Fig. 7, whereas in Fig. 8 is illustrated theonversion of the photodiode voltages vector [V] into levels [S].

The ASHRAE model provides the theoretical instantaneous values of the solar irradiance under clear sky conditions

or a surface oriented in any direction.

Starting from the calculation of the sun’s path, i.e., the solar altitude (βS) and azimuth (γS), a MATLAB script findshe projection of the cylinder of the SolySector on a horizontal plane. As shown in Fig. 9, there are two circles. Then,

Fig. 8. Example of a voltage-to-states conversion scheme.

22 G.M. Tina et al. / Mathematics and Computers in Simulation 91 (2013) 16–28

Fig. 9. Graphic representation of the reference model.

Fig. 10. Example of a distance-to-level conversion scheme.

another script implements a level conversion based on the Euclidean distance between the centre of each photodiodeand the centre of the cylinder projection.

The conversion process for the model data is simpler than the conversion for the measured data. In fact, in the levelassociation, there are no other factors to consider except for the distances, and naturally, there are no acquiring errors.The distance-level conversion is shown in Fig. 10, and the relationships between distances and levels are shown inTable 2.

The last MATLAB script, IrradianceDir, receives two vectors of levels as inputs, which are related to the measure-ments [S] and the reference model [Sref]. Then, a level comparison between the model and measurements for eachphotodiode in each data acquisition is conducted. If there is a perfect equality, a vector containing the azimuth andsolar height angles is built. In particular, these angles are the same as those calculated by the reference model because

the maximum irradiation corresponds to the direction of the present astronomic sun position.

If there are differences between the levels, a correction routine searches for the solar height errors, assuming thatpossible azimuthal errors can be neglected.

Table 2Distance-level conversion.

Distance range [cm] Assigned level

[5, +∞] 1[3.5, 5] 2[2.7, 3.5] 3[2.2, 2.7] 4[0, 2.2] 5

G.M. Tina et al. / Mathematics and Computers in Simulation 91 (2013) 16–28 23

After comparisons between the measurements and the model are made, the outputs of the script are the azimuth andsolar height angles relative to the maximum irradiance direction, which does not correspond to the astronomical sunpath. In particular, these angles are calculated after several steps; in fact, the MATLAB script takes into account thestandard deviation of the measurements and the model data according to the following relationships.

Sum of the stored levels for each acquisition:

S(m) =9∑

j=1

lev(m)j (1)

S(ref ) =9∑

j=1

lev(ref )j (1′)

Arithmetic mean of the levels for each acquisition:

M(m) = 1

9

9∑j=1

lev(m)j (2)

M(ref ) = 1

9

9∑j=1

lev(ref )j (2′)

Standard deviation:

DS(m) =√√√√1

9

9∑j=1

(lev(m)j − M(m))

2(3)

Fig. 11. Software for the SoliSector system: flowchart of MATLAB routines.

24 G.M. Tina et al. / Mathematics and Computers in Simulation 91 (2013) 16–28

Fig. 12. (a) Model-SoliSector dynamics: superposition of the voltage signal and the relative levels. (b) Model-SoliSector dynamics: superpositionof the voltage levels and the reference levels.

G.M. Tina et al. / Mathematics and Computers in Simulation 91 (2013) 16–28 25

DS(ref ) =√√√√1

9

9∑j=1

(lev(ref )j − M(ref ))

2(3′)

From Eqs. (1)–(3), it is possible to calculate a correction parameter, Corr(tilt), as follows:

Corr(tilt) =(

1 − DS(m)

DS(ref )

)· βCS =

(1 − DS(m)

DS(ref )

)· (90 − tiltCS) (4)

Here, βCS represents an equivalent solar height; it is calculated as the difference between 90◦ and the tilt angle, tiltCS.This angle is the optimal inclination of a PV module under clear sky conditions. The expression for tiltCS is reportedin (5), where C is a parameter that takes into account the contribution of the diffuse irradiance to the global irradiance.

tiltCS = a tan

[cos(βS)

sin(βS) + (C/2)

](5)

Fig. 13. Sun paths for the Model-SoliSector overlapping for: (a) clear sky conditions (24th March 2011) and (b) variable conditions (30th March2011).

26 G.M. Tina et al. / Mathematics and Computers in Simulation 91 (2013) 16–28

The solar height angle correction is finally obtained using the standard deviations previously calculated accordingto relationship (6):

βPV = βCS + Corr(tilt) (6)

The output of the SoliSector is represented by a vector containing the solar azimuth angles calculated by the referencemodel and the solar height angles obtained, where necessary, with the routine correction management. These anglescorrespond to the direction of maximum solar irradiance, which can then be sent to the double-axis solar trackingsystem.

Another graphical scheme of the SoliSector system, where the flowchart of routines developed in MATLAB isspecified, is shown in Fig. 11.

3. Experimental results

The most important point of this experimental system is represented by the choice of levels. A rough assignment thatdoes not consider the variable sun paths throughout the year could partially eliminate the dynamics of the photodiodesignals. Therefore, significant effort has been devoted to correctly calibrating the conversion to levels. Fig. 12 showsa notable overlapping of the dynamics after the level conversion for the nine photodiodes.

It is worth noticing the signal noise presents in the output voltage of each FD especially when a FD is shaded, furtherthe symmetry of the FD outputs can be detected taking into account how the Solisector is positioned and the symmetricpath of the sun from sunrise to sunset. Data from both the measurements and the model give a vector of the azimuth andsolar height angles as an output. These pairs of angles are sent to the double-axis solar tracker to position it along thedirection of maximum irradiance. Fig. 13(a) and (b) shows the input (solar irradiance) and the output (equivalent sunheight) of the software associated with the SoliSector sensor. Two different meteorological conditions are considered:(a) clear sky conditions and (b) variable conditions.

Some errors affecting the optimal angle are present, which are due to

1. the sensitivity of the photodiodes and their temperature dependence2. the choice for level conversions3. attenuations due to the Plexiglas hemispheric caps.

The first issue of sensitivity, combined with a strong dependence on temperature, causes an increase in the inversecurrent in the photodiodes, which clearly affects the final result because the system is exposed to elevated temperatures(especially in the summer) for long periods of time.

The second aspect is important because, in the calibration process for the choice of levels, there are most likelyother dynamics that have not been considered.

Finally, the attenuation in the Plexiglas hemispheric cap accounts for part of the incoming radiation.

Fig. 14. Outdoor installation of the SoliSector sensor.

G.M. Tina et al. / Mathematics and Computers in Simulation 91 (2013) 16–28 27

t

w

a

Fig. 15. View of the double-axis tracker on the roof of the Idrilab DIEES laboratory (Catania – Italy).

These considerations imply that a gain factor might be inserted in the equations to calculate the solar height angle,hus taking into account the items listed above.

Figs. 14 and 15 show, respectively, the SoliSector installation for outdoor testing and the double-axis solar tracker,

hich was installed on the roof of the Idrilab laboratory of the University of Catania (Italy).Finally Fig. 16 shows how the SoliSector has been integrated with a double-axis tracker control system, where the

mbient, mechanical and electrical measurements are performed.

Fig. 16. Experimental set-up to test the SoliSector with a double-axis tracker.

[[

[

28 G.M. Tina et al. / Mathematics and Computers in Simulation 91 (2013) 16–28

4. Conclusions

To efficiently exploit solar radiation through photovoltaic conversion, the present system was developed to maximisethe energy produced by a double-axis solar tracker. In particular, the system can find the equivalent optimal azimuthand solar height angles corresponding to the direction of maximum global solar irradiance.

The SoliSector system is in fact a component of a multisensory system developed at the University of Catania,Idrilab laboratory, for complete control and monitoring of a photovoltaic system.

The SoliSector system is divided into hardware and software: the HW includes the electronics card construction andcalibration processes, and the SW includes data and error elaboration processes using various scripts in the MATLABenvironment.

In comparison with the hardware of the previous version presented in [4], some improvements have been introducedin both the operation of the system and the data resolution. The number of photodiodes has been reduced from twelveto nine. The three photodiodes used to observe the solar height were removed because, with the new sensor, the samefunctions can be done without these elements.

Moreover, the system resolution is notably increased. During the testing phase, in an exit voltage dynamics analysiswith an oscilloscope, the 5 V power supply was found to introduce some fluctuations, and as a consequence, the systemshowed a resolution of approximately 0.2 V.

It is likely that the heat from the power supply was responsible for this noise because such heat is difficult to dissipate.Therefore, by replacing the supply with a stabilised 5 V power supply, the fluctuations nearly vanished, revealing aresolution of 10−2 V with a consequently notable increase in the precision of the system.

Finally, the low cost of this prototype (approximately 90 Euros) is worth noting; in fact, when compared with thepossibility of increasing the yearly energy production of a PV panel, especially under national incentives (e.g., feed intariff incentives), this cost is negligible.

References

[1] Datasheet FoxhunterTM, http://www.sunnyenergy.it/, December 2010.[2] E.A. Farber, C.A. Morrison, Clear Day Design Values, ASHRAE, New York, 1977, ASHRAE GRP. 170.[3] J.M.G. Figueiredo, J.M.G. Sá da Costa, Intelligent sun-tracking system for efficiency maximization of photovoltaic energy production, in:

Proceedings of ICREPQ’08, Santander, Spain, 2008.[4] S. Gagliano, D. Neri, N. Pitrone, N. Savalli, G. Tina, Low-cost solar Radiation Sensing Transducer for Photovoltaic Systems, WSEAS

Transactions on Environment and Development 2–5 (2009) 120–125.[5] K. Kaufmann, Choosing your detector. Hamamatsu Corp. Available from: http://sales.hamamatsu.com/assets/applications/Combined/choosing-

detector.pdf (accessed 15.08.11).[6] D.L. King, W.E. Boyson, B.R. Hansen, W.I. Bower, Improved accuracy for low-cost solar irradiance sensors, in: Proceedings of 2nd World

Conference and Exhibition on Photovoltaic Solar Energy Conversion, Vienna, Austria, 1998.[7] M.A. Martínez, J.M. Andújar, J.M. Enrique, A new and inexpensive pyranometer for the visible spectral range, Sensors 9 (2009) 4615–4634.[8] D.R. Myers, Quantitative analysis of spectral impacts on silicon photodiode radiometers, in: Proceedings of SOLAR 2011, Raleigh, NC, USA,

2011.[9] F.R. Rubio, M.G. Ortega, F. Gordillo, M. López-Martínez, Application of new control strategy for sun tracking, Energy Conversion and

Management 48-7 (2007) 2174–2184.10] Solar energy utilization for heating and cooling, in: ASHRAE Applications Handbook, ASHRAE, New York, 1978 (Chapter 58).

11] F. Vignola, Solar cell based pyranometers: evaluation of the diffuse response, in: Proceedings of the 1999 Annual Conference American Solar

Energy Society, 260, 1999.12] J. Wood, T. Muneer, J. Kubie, Evaluation of a new photodiode sensor for measuring global and diffuse irradiance and sunshine duration, Journal

of Solar Energy Engineering 125 (2003) 43–48.