Dual-Axis Tracked vs. Fixed PV: Energetic Response of One-Year Testing Period in Romania

Bogdan BURDUHOS, Dorin DIACONESCU, Sorin MORARU, Milian BADEA, Costin GRIGORESCU

Transilvania University of Brasov E-mail: bogdan.burduhos, dvdiaconescu, smoraru}@unitbv.ro, milianbadea@yahoo.com, costin.grigorescu@gmail.com

Abstract-The purpose of the current paper is to present and analyze the energetic results obtained from a stand-alone photovoltaic system, consisting of two differently orientated PV modules, over a period of one year of functioning. One of the modules is being pseudo-equatorially oriented along two axes based on an annual orientation program which will be described in the paper. The second PV module is installed on a fixed and optimally tilted frame orientated towards South. The paper also describes the electrical and the monitoring system used for measuring the parameters of the electrical energy produced. In the end of the paper a comparative analysis is made regarding the energetic response of the two tested photovoltaic systems in the geographical and meteorological conditions of the Braşov, Romania location during three most relevant day types (sunny, normal and cloudy days). Two main conclusions can be stated: firstly, non-deterministic orientation programs need to be developed, able to adapt themselves to the instantaneous ratio between direct and diffuse solar radiation; and secondly, the resistance of the consumer has the main role in transforming the whole incident solar radiation into electricity or only a part of it resulting in the need of using control systems capable of adapting the consumer resistance to the maximal power of the PV module.

I. INTRODUCTION

Photovoltaic modules represent a good solution for obtaining clean electrical energy from the sun. Unfortunately their conversion efficiency is quite small [3], and the life-cycle energetic balance is not always positive, depending on the operation conditions [11]. In order to improve these parameters mainly two optimizations are possible: on one hand the improvement of the conversion materials which are related to increase of production cost and on the other hand the increase of the received solar radiation using mechanical solar orientation systems [7, 17].

The second option is widely approached in literature with mechanisms allowing both single-axis and dual-axis orientation [2, 4, 13]. Literature also widely describes the effect of orientation on the electrical energy gain achieved using these systems [1, 6].

In contrast with previous literature results, where mainly the energetic results of grid-connected photovoltaic systems are presented [9], in this paper stand-alone (off-grid) systems are addressed with an active surface of 1m2. One year long comparative energetic results and their analysis are targeted, indicating the necessity of correlating the dual-axis orientation program to the on-site conditions.

The paper is structured in four chapters: firstly describing positioning aspects of the two PV systems (dual-axis tracked and fixed tilted) and secondly the components and functioning of the monitoring system are presented. Further the experimental results are presented, explained and discussed, and lastly the main conclusions and improvements of the paper are presented.

II. DESCRIPTION OF THE DUAL-AXIS TRACKING PROGRAM AND OF THE TILTED FIXED SYSTEM

The position of the sun on heavens vault can be described using several pairs of angles depending on the angular system used. One of them is the pseudo-equatorial angular system which uses the angle β as the diurnal sun angle and γ as the elevation angle for describing the suns position. Similar angles are used to describe the movement of the pseudo-equatorial tracking system in order to distinguish them from the sunray angles (β and γ), the angles of the tracking system are marked with an asterisk (β* and γ*) and approximate the variation of β and γ in steps [5, 8]. The reason why this angular system was chosen during the tested period is because for small tracking systems, with surfaces < 5 m2 the needed mechanism is simple, robust and the least expensive.

Further, the step-wise tracking program of the tested tracking system is explained in detail. This program can be fully described by the numerical values of the angles achieved by the tracking mechanism and by the values of the time when these angles are achieved. In Table 1 all these values are presented for the annual tracking program used during the one-year long testing period. From Table 1 it is obvious that the annual program was divided in 12 seasonal programs (corresponding to the step variation of the annual declination angle from Fig. 2a), each of them having a fixed elevation angle (γ*) and the same diurnal variation (β*) throughout he season.

This chapter has the role to describe the two photovoltaic tracking systems used during the period in which the tests were made. The first system is a dual-axis one (Fig. 1a) while the second one is fixed and tilted (Fig. 1b).

The annual tracking program presented in Table 1 was determined based on numerical simulations and is considered to be optimal both from the point of view of the received solar radiation in the Braşov-Romania area [14, 15, 16] and also from the point of view of the needed electrical energy for moving the system.

979978-1-4673-1653-8/12/$31.00 '2012 IEEE

a)

b)

Fig. 1. Images of the photovoltaic tracking systems installed on the roof of building E of the Transilvania University of Braşov: a) dual-axis pseudo-equatorial system; b) fixed tilted system

TABLE I OPTIMAL ANNUAL ORIENTATION PROGRAM OF A YEAR

Interval / number β* / γ*

steps γ* steps

γ* (winter time) local

hours

β* steps in the morning

β* morning (winter

time) local hours

N= 49-70 N= 272-293 6 / 1 steps

53.25º NO elevation movement

62º; 40º; 19º; 0º

9:06; 10:24; 11:48

N= 71-89 N= 253-271 8 / 1 steps

45.65º NO elevation movement

64º; 46º; 32º; 16º; 0º

8:43; 9:43; 10:43; 11:49

N= 90-108 N= 235-252 8 / 2 steps

-15º; 38.25º;

-15º 7:01; 17:37 64º; 47º; 32º;

16º; 0º 8:31; 9:37;

10:37; 11:43

N= 109-126 N= 217…234

10 / 2 steps

-15º; 31.65º;

-15º 7:33; 16:57 64º; 52º; 40º;

28º; 12º; 0º

8:09; 8:57; 9:51; 10:51;

11:51 N= 127-144 N= 199-216 10 / 4 steps

-15º; 5.85º; 26.65º;

5.85º; -15º

7:14; 8:50; 15:38; 17:14

64º; 52º; 38º; 26º; 12º; 0º

8:02; 8:56; 9:56; 10:56;

11:50

N= 145-198 10 steps

-15º; 4.05º; 23.15º;

4.05º; -15º

7:31; 8:55; 15:43; 17:07

64º; 48º; 36º; 24º; 12º; 0º

8:07; 9:13; 10:07;

11:01; 11:55N= 294-48

6 steps 62.65º NO elevation movement

-58º; 36º; 18º; 0º

9:14; 10:32; 11:50

in the afternoon data are symmetrical to solar noon

In Fig. 2, as an example, there are presented the variations of the sun angles and of the tracker angles during the beginning and end days of the spring interval.

The second photovoltaic system is the fixed and tilted system needed for making relevant comparative analyses. It is used as a reference system and is permanently oriented towards South (β*=0º) at a fixed angle γ*=33º from the horizontal plane. The fixed tilt angle was determined based on numerical simulations in the meteorological-geographical conditions of Braşov-Romania and was considered to be the angle for which the incident global solar radiation on the photovoltaic module is maximal.

Fig. 2a. The division of the year in intervals by approximating of the annual variation of the declination angle (δ) with a 12-steps-line (δ*)

Fig. 2b. Variations in degrees (°) of the sun angles (β, γ) and of the tracker angles (β*, γ*) during the day interval N= 127…144

0100200300400500600700800900

10001100

4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 12.5 13.5 14.5 15.5 16.5 17.5 18.5 19.5

Time [h]

Sola

r Rad

iatio

n [W

/m2]

dir.avail. dir.rec.dualAxis dir.rec.fixeddif.avail. dif.rec.dualAxis dif.rec.fixed

Fig. 2c. Variations of the available and received solar radiation using a dual-axis tracked and a fixed tilted photovoltaic module, during 28.08.2011

980

III. DESCRIPTION OF THE USED MONITORING SYSTEM

The main parameters needed for comparative analysis which will be made in the next chapters of the paper are, firstly, the electrical parameters of the produced photovoltaic energy (module voltage Upv and produced current Ipv) and secondly meteorological parameters regarding the ambient conditions in which the photovoltaic systems have functioned (solar radiation, air temperature).

Before describing the solution which has been chosen for monitoring the electrical parameters of the produced photovoltaic energy (Fig. 3) it needs to be mentioned that a constant resistance was used for consuming the energy, represented by two parallel light bulbs having together a power of 100W. Another option would have been to use a charge controller for charging individual batteries. This possibility has been tested at the beginning of the study and lead to problems when comparing the produced energies due to the different charge statuses of the batteries.

Another option would also have been the use of a DC/AC inverter as [9, 12], but it is not the aim of this paper because stand-alone, individual, offline (not grid connected) PV systems were addressed.

The chosen monitoring solution contains, beside the tracked PV module and the consumer also a small electronic regulation board, a data acquisition board of type National Instruments 6218 and an acquisition software made in Labview.

The electronic regulation board has the role the diminish the produced voltage levels to the interval (–10 V, …, +10 V)

so that these levels can be read by the acquisition board. It also has the role to transform the produced electrical current into an appropriate level of voltage since the acquisition board is only able to read voltages.

The acquisition board reads the two voltages and sends them further to the acquisition software which runs on a dedicated computer and which converts the measured data into the real correspondent parameters once every 10 seconds.

On the other side, for monitoring the ambient parameters a local weather station was used, Delta-T production, capable of recording the total and diffuse solar radiation together with the air temperature on a 10-minute interval basis.

IV. COMPARATIVE ANALYSIS

In this chapter the energetic response of the two photovoltaic tracking systems is analyzed from the point of view of the produced electrical energy and from the point of view of the relative energy increase due to tracking. The analysis is made considering both daily data (Fig. 4) and monthly data (Fig. 6).

Fig. 4 presents the variation of the daily produced electrical energy using the previously described tracking systems, during the year 2011. From this diagram it becomes obvious that the dual-axis system produces far more electrical energy than the fixed system. Another important observation which can be made when analyzing this diagram is that the influence of orientation on the produced electrical energy is more visible during the summer interval when the available solar radiation is higher than during the winter (see also Fig. 6).

Fig. 3. Simplified scheme used for monitoring the electrical energy produced by the orientated photovoltaic modules

981

Using the numerical results which are at the basis of this diagram the mean daily energy can be calculated for both tracked (308.23 Wh/m2) and fixed (173.78 Wh/m2) photovoltaic modules. Furthermore the annual produced electrical energy can be estimated (112.5 kWh/m2 for the tracked respectively 63.43 kWh/m2 for the fixed system). For these values the annual relative energetic increase represents 77.4% and is calculated using the formula from (1).

This high increase, compared to literature known values (30% - 50%) [1], can be explained by the fixed 100W electrical consumer used, which badly diminishes the photovoltaic produced electrical energy during periods with low solar radiation availability. The effect occurs mainly during mornings and evenings when the fixed system hardly receives any solar radiation, producing close to 0W; while the dual-axis tracked produces much more electrical energy (for example, see the morning period in Fig. 8a where the fixed system starts producing energy only ~2h behind the dual-axis

tracked system.

[ ]%100.

..... ⋅−

=fixedEnerg

fixedEnergergdualAxisEnIncrEnergrel (1)

In Fig. 5 the relative energetic increase is described throughout the year 2011. Its variation is quite normal, in the range of ~ 100%. There are only a few points on the diagram where the graphical results are at first sight abnormal: firstly there are days when the relative energetic increase is negative, meaning that the tracked PV system has produced less energy than the fixed one, and secondly there are several days when the relative energetic increase is far higher than the 100%. Both these situations will be treated separately in the next chapter of the paper.

In Fig. 7 the following interesting aspect can be observed: it is related to the fact that although the maximal electrical energy is produced during summer the relative energetic increase is at its peak during winter months.

Fig. 4. Variation of the electrical energy produced by the dual-axis and by the fixed photovoltaic systems during year 2011

Fig. 5. Variation of the daily relative energetic increase during year 2011

982

Fig. 6. Variation of the monthly produced electrical energy during year 2011

Fig. 7. Variation of the monthly relative energetic increase during year 2011

V. DISCUSSIONS

In this chapter the most three representative days of the analyzed interval are considered and for each of them the interesting aspects are stressed out from the point of view of the produced electrical power, of the available solar radiation and of the ambient temperature. These days are firstly sunny days, with prevailing direct solar radiation and secondly cloudy days when only diffuse radiation is available. For both types, days during the warm and cold season were analyzed. Thirdly normal days were considered with both direct and diffuse solar radiation.

a) Sunny days This type of day is a day in which the direct solar radiation

is preponderant; analyzing the diagrams from Fig. 8 and 9 the following observations can be made: • during both summer and winter days the moments when

the steps of the orientation program are made can be very easy identified on the blue lines of the power diagrams, indicating that the used tracking system has achieved its role and has insignificant loses when moving from one position to the next one;

• during summer days the relative energetic increase is lower than during winter days (87.33% compared to 177.62%), especially at noon time; this effect has two explanations: on one side it occurs due to the fact that the

a)

b)

Fig 8. Variation during 28.08.2011 (day during summer season) of the: a) electrical power produced by the dual-axis tracked and fixed PV module; b) global, diffuse solar radiation and ambient temperature.

a)

b)

Fig. 9. Variation during 13.12.2011 (day during winter season) of the: a) electrical power produced by the dual-axis tracked and fixed PV module; b) global, diffuse solar radiation and ambient temperature.

983

ambient and consequently the PV module temperatures are lower during cold days, with a benefic effect on the photovoltaic conversion efficiency and on the other side it occurs because during the summer season the elevation angle of the fixed system is closer to the optimal value of the considered days (γ*summer=~24°; γ*winter=~68° compared to γ*fixedPV=33°)

b) Cloudy days

During cloudy days (days in which the diffuse radiation is preponderant) there is no difference between the hot and the cold season; other observation types can be made regarding the days with abnormal relative energetic increase, previously mentioned (see Fig. 10 and Fig. 11).

The first observation addresses days with no direct solar radiation (only diffuse) like the one presented in the diagrams of Fig. 10. During these days the relative energetic increase is in most cases negative (-16.07% for the day in Fig. 10), meaning that the dual-axis tracked module has received less solar radiation and in consequence has produced less electrical energy than the fixed one. The explanation for this is the fact that the diffuse radiation is according to the isotropic model maximal in the horizontal plane [10]. In our case, during the day 01.03.2011 (Fig. 10), the tilted fixed system was closer to the horizontal plane (33°) than the dual-axis tracked one, which functioned in that interval at 53.25° from the horizontal plane.

The second observation is regarding the days similar to the one presented in the diagram from Fig. 11, days having direct solar radiation only during the extreme hours (during early morning or late evening) and having cloudy conditions during noon time. During these days the relative energetic increase of the dual-axis PV module can reach even values of ~ 2000% (757.69% for the day in Fig. 11) compared to the fixed module. This occurs due to the fact that orientation systems are mainly conceived for maximizing the direct solar radiation during the whole day, and to the fact that during extreme hours the fixed system received very little amount of direct solar radiation.

Another observation, common to all types of cloudy days is the lack of direct solar radiation which leads to a very small amount of produced electrical energy for both considered PV modules. Also, although during cloudy days there are days with great relative energetic increase, the absolute difference between the two produced electrical energies is quite small.

The two previous observations lead to the conclusion that during cloudy days the orientation of photovoltaic modules can not be justified, as a consequence tracked PV modules need rather to be positioned as close as possible to the horizontal plane (parallel to the ground) during cloudy days, in order to receive the maximal amount of global solar radiation.

Despite this fact dual-axis tracking systems are very useful also during cloudy days, but the orientation programs would need to adapt themselves to the solar radiation conditions, with the purpose of maximizing the direct but also the diffuse

a)

b)

Fig 10. Variation during 01.03.2011 (day with negative relative energetic increase) of the: a) electrical power produced by the dual-axis tracked and fixed PV module; b) global, diffuse solar radiation and ambient temperature.

a)

b)

Fig 11. Variation during 08.10.2011 (day with very high relative energetic increase) of the: a) electrical power produced by the dual-axis tracked and fixed PV module; b) global, diffuse solar radiation and ambient temperature.

984

radiation. This implies an improvement of the dual-axis tracker orientation program in order to adapt the movements to the available solar radiation, more specifically, to the ratio between direct and diffuse radiation. This improvement will be addressed in further papers, being in progress in the frame of a postdoctoral research program.

c) Normal (common) days

During normal days, with both clouds and clear sky (see diagrams in Fig. 12), no special observations can be made regarding the effect of tracking. In these days, depending on the ratio between diffuse and direct solar radiation the previously described observations may apply. The mean relative energetic increase is approximate 70%.

a)

b)

Fig 12. Variation during 09.03.2011 (day with normal-common solar radiation) of the: a) electrical power produced by the dual-axis tracked and fixed PV module; b) global, diffuse solar radiation and ambient temperature (mean relative energetic increase = 77.65%).

In order to identify the reason for the unusually high annual

relative electrical energetic increase (77.4%) the following analysis was made considering the sunny day 28.08.2011. For this day the electrical energy increase is 87.33% (see Fig. 8a) but the received solar radiation increase is only 31.5% (see Fig. 2c) and is in the normal increase range (30-50%) of tracked PV systems.

This difference between the two values shows that the two used photovoltaic panels have no identical working. The explanation resides in the use of the constant 100W electrical consumer which diminished the electrical energy production

when only small amounts of solar radiation are available (mornings and evenings especially).

This explanation is backed up by the simplified qualitative diagram in Fig. 13 which presents the variation of photovoltaic module I-V curves under different irradiation situations.

The blue line in Fig. 13 represents the I-V characteristic of a module which receives little solar radiation (in our case the fixed one: G1*) while the red line correspond to a module with more incident solar radiation (in our case the dual-axis tracked one: G2*). The two lines are intersected by the green line representing the characteristic of the fixed consumer used (R). The intersection points (Q1 and Q2) indicate the electrical power (I1xU1 and I2xU2) which can be taken from the modules using the considered resistance (R); in accordance with Fig. 13, the two PV systems (fixed and dual-axis) use the whole received solar radiation (G1* and G2* respectively) in the conversion process, only if Q2 goes into P2max and Q1 goes into P1max respectively. From here the difference between the solar increase and the electrical energy increase results; because the ratios between produced and maximal producible energy are not the same under different irradiation conditions (2). This means that the dual-axis system transforms more solar radiation into electrical power compared to the maximal values than the fixed system.

max1

11

max2

22

PIU

PIU ⋅

>⋅ (2)

The fixed PV system can achieve the same ratio (as the dual-axis PV system with resistance R) with other resistance R’>R (Fig. 13) for which:

max1

11

max2

22 'P

IUP

IU ⋅=

⋅ (3)

The presented analysis stresses out that in order to have more concluding comparative results an electronic device of MPPT type [12] is needed, which should adapt the consumer resistance to the instantaneous characteristic of the PV module so that the maximal electrical energy is produced.

The diagram model from Fig. 13 is valid in those seasons when the elevation of the tracked system is very different

Fig 13. Simplified scheme with the IV characteristics variations of a photovoltaic module under two solar irradiation conditions and of two possible consumers with different resistances.

985

from the elevation of the fixed system and in all the seasons during the morning and evening periods.

During noon time from each day of seasons in which the elevations of the two systems are the same, similar energetic results are obtained.

VI. CONCLUSIONS

The current paper presents and analyses experimental results obtained during one year by testing two photovoltaic tracking systems in the geographical and meteorological conditions of Braşov, Romania. The main conclusions which can be stated are: • During day 28.08.2011 the incident solar radiation

increase (31.5%) is situated in normal limits (30-50%). Unlike it, the electrical energy increase is much higher (87.33%).

• At annual level similar ratios are obtained with a mean daily produced energy for the tracked PV module of 308.23 Wh/m2 compared to 173.78 Wh/m2 for the tilted fixed PV, meaning an electrical energy gain of ~77.4%.

• The annual increases would have same values only if the condition from (3) was verified or if the whole incident solar radiation would be converted into electrical power (functioning at the maximum power point). Because a fixed electrical consumer was used none of the two conditions is fulfilled and therefore the difference between the increases.

• The more solar radiation is incident on the photovoltaic module the closer the module functions to the point with maximal power;

• There are days when the influence of orientation is insignificant compared to fixed systems; it is the case of days with low direct radiation energy (cloudy days) during which the tilted fixed system performs better than the oriented one, when the orientation can not be justified. This aspect was expected because the current orientation program was developed with the role to maximize only the direct solar radiation. These situations impose an improvement of the annual orientation program so that it would allow the maximization of the global solar radiation.

Thus a already known requirement is identified, namely that the maximization of the solar radiation is only efficient if an appropriate resistance control system is used that assures the functioning of the PV module in the maximal power point; only in this way the whole received solar radiation is used in the conversion process.

Also from the previous conclusions another demand becomes obvious: because the use of deterministic orientation programs, based on statistical meteorological data lead to unsatisfying results during cloudy periods, the development of non-deterministic programs becomes necessary. These programs need to consider the instantaneous rations between direct and diffuse available solar radiation. This aspect will be addressed in further papers, being already in progress in the frame of a postdoctoral research program.

ACKNOWLEDGMENT

This paper is supported by the Sectoral Operational Programme Human Resources Development (SOP HRD), Post Doctoral School, financed from the European Social Fund and by the Romanian Government under the contract number POSDRU/89/1.5/S/59323 and Doctoral School contract numbers POSDRU/107/1.5/S/76945, POSDRU/88/1.5/S/59321.

REFERENCES [1] S. Abdallah, “The Effect of Using Sun Tracking Systems on the

Voltage–Current Characteristics and Power Generation of Flat Plate Photovoltaics”, Energy Conversion and Management, 45, 2004, pg. 1671–1679

[2] M. Abu-Khader, O. Badran, S. Abdallah, “Evaluating Multi-Axes Sun Tracking System at Different Modes of Operation in Jordan”, Renewable and Sustainable Energy Reviews. 12(3), 2008, pg. 864-873.

[3] B. Allen, K. Douglas, C. Honsberg, “Very High Efficiency Solar Cell Modules”, Progress in PhotoVoltaics, 17(1), 2009, pg. 75-83.

[4] F.P. Baumgartner, A. Büchel, R. Bartholet, “Cable-Based Solar Wings Tracking System: Two-Axis System And Progress Of One-Axis System”, 25th European Photovoltaic Solar Energy Conference and Exhibition, 6-10 September 2010, Valencia, Spain, pg. 4487-4490.

[5] B.G. Burduhos, Optimization of Pseudo-Equatorial Tracking Mechanisms Used for Increasing the Conversion Efficiency of Individual Photovoltaic Modules, PhD. Thesis, Braşov, 2009.

[6] B.G. Burduhos, D. Diaconescu, I. Vişa I., A. Duţă, “Electrical Response of an Optimized Oriented Photovoltaic System”, OPTIM 2010 – 12th International Conference on Optimization of Electrical and Electronic Equipment, Braşov, Romania, 20-22 Mai 2010, pg. 1138-1145.

[7] B.G. Burduhos, C. Toma, M. Neagoe, M.D. Moldovan, “Pseudo-Equatorial Tracking Optimization for Small Photovoltaic Platforms from Toronto/Canada”, Environmental Engineering and Management Journal, 10(8), 2011, pg. 1059-1068.

[8] D. Diaconescu, I. Vişa, B. Burduhos, V. Popa, “On the Sun-Earth Angles used in the Solar Trackers’ Design. Part 2: Simulations”, Annals of the Oradea University, Fascicle of Management and Technological Engineering, Vol. VI (XVI), ISSN:1583-0691, 2007, pg. 842-849.

[9] N. Helwa, A. Bahgat, A. El Shafee, E. El Shennawy, “Maximum Collectable Solar Energy by Different Solar Tracking Systems”, Energy Sources, 22, 2000, pg 23-34.

[10] N. Kelly, T. Gibson, “Improved Photovoltaic Energy Output for Cloudy Conditions with a Solar Tracking System”, Solar Energy, 83, 2009, pg. 2092–2102.

[11] R. Laleman, J. Albrecht, J. Dewulf, “Life Cycle Analysis to estimate the environmental impact of residential photovoltaic systems in regions with a low solar irradiation” Renewable & Sustainable Energy Reviews, 15(1), 2011, pg. 267-281

[12] D. Lalili, A. Mellit, N. Lourci, B. Medjahed, E.M. Berkouk, “Input output feedback linearization control and variable step size MPPT algorithm of a grid-connected photovoltaic inverter”, Renewable Energy, 36(12), 2011, pg. 3282-3291

[13] M.D. Moldovan, I. Vişa, B.G. Burduhos, “Energetic Autonomy for a Solar House”, Environmental Engineering and Management Journal, 10(9),2011, pg. 1283-1290.

[14] M. Meliss, Regenerative Energiequellen – Praktikum, Berlin Heidelberg, Springer, 1997.

[15] M.M. Vătăşescu, D.V. Diaconescu, A. Duţă, B.G. Burduhos, “Atmospheric Pollution Evaluation Based on Turbidity Factor Analysis”, Environmental Engineering and Management Journal, 10(2), 2011, pg. 251-256.

[16] I. Vişa, D. Diaconescu, V. Popa, B. Burduhos, “Quantitative Estimation of the Solar Radiation Loss in the Braşov Area”, Environmental Engineering and Management Journal, 8(4), 2009, pg. 843-847.

[17] I. Vişa, D.V. Diaconescu, R. Săulescu, M.M. Vătăşescu, B.G. Burduhos: A New Linkage with Linear Actuator for Tracking PV Systems With Large Angular Stroke, Chinese Journal of Mechanical Engineering, 24(5), 2011, pg. 744-751.

986