�Analysis of a PV System with Single-axis Tracking Energy Production and Performances

R. Faranda*, M. Gualdoni*, S. Leva*, M. Monaco**, A. Timidei** * Politecnico di Milano – Dipartimento di Energia, (Italy)

** Convert Italia S.p.A., (Italy)

Abstract--This paper presents some results of a performance analysis of a PV-plant located in Tuscania (Latium – Italy) with a single axis tracking based on performance ratio and transposition factor indices. The data are collected by in site monitoring system with reference to the period from 01/06/2009 to 31/05/2010.

Index Terms-- Performance Ratio, Transposition Factor, PV System, Monitoring, Tracking, Energy.

I. INTRODUCTION

The photovoltaic (PV) system performance analysis is becoming more and more important with the aim to evaluate the quality of a photovoltaic system during operation. Furthermore a great deal of interest has developed in the use of sun-tracking mountings for normal flat-plate PV-systems. Such systems deliver more energy for the same nominal power but its performances and the increment of delivered energy versus the higher cost of PV-tracking system have to be carefully evaluated.

The Performance Ratio (PR) index - indicating the net amount of daily/monthly/annual energy output from the entire PV system compared with the theoretical input from the PV array - is extremely important for fixed PV plant and in case of tracking systems also, but it cannot qualify in a significant way the efficiency of the tracking systems and neither estimate the energy behavior of a tracking systems under analysis [1], [2].

The Transposition Factor (TF) is another important index in particular for tracking PV systems. It represents the ratio between the in-plane irradiation and the horizontal irradiation. This factor gives important information about the amount of captured irradiation by the PV panels in comparison of fixed panel thanks to the tracking but it does not represent the real increase of energy produced by the PV System.

There are a lot of paper that compare the performances of the fixed PV systems based on measured data [3]-[8],but the performance of tracking systems are estimated mainly basing on theoretical analysis or on the simulated average daily profiles of irradiation and temperature [10]-[13].

This paper presents some results of the analysis of these indexes with reference to a PV-plant located in Tuscania (Latiun – Italy) with crystalline silicon module and single axis tracking system by in site monitoring system with the aim to characterize the PV tracking

system and make some comparisons in terms of solar energy received by the PV-modules thanks to the tracking system. All the indexes will be evaluated basing on measured data and will be compared with the theoretical ones.

In particular, the PV plant under investigation and the main features of the one-axis horizontal tracking system will be described. The results obtained from the data measured in the PV plant, with reference to the period from 01/06/2009 to 31/05/2010, in terms of day average trend for the irradiation measured on the horizontal plane and on the PV-plane, the PR and the TF are reported. Furthermore, in order to evaluate the ideal advantageous of the tracking system, the input energy obtained with the Photovoltaic Geographic Information System (PVGIS) for the PV plant in Tuscania are then here reported and the actual and simulated data are compared. Furthermore, to qualify a tracking system the energy consumption is also analyzed and evaluated.

II. PERFORMANCE RATIO AND TRANSPOSITION FACTOR

The Performance Ratio (PR) is an index of the performance of a PV array defined by CEI EN 61724 [1]and IEA PVPS Task II [2].

PR is a dimensionless quantity which assesses the net daily/monthly/annual output energy from a PV array with respect to the theoretical system input energy. It does not quantify the amount of energy produced: in fact low PR systems situated in good irradiation site can produce more energy than high PR arrays but located in a lower irradiation site. It represents the total efficiency of all the devices necessary to the system operations in real working condition. In simple terms, the higher is the PR the more solar energy is converted to electrical energy.

From an analytical point of view the PR can be expressed as follow:

,,

0 0

1f I refuse use

r r I

Y GE EPR

Y P G P Y�

�,

r

�� � � � ��

(1)

Where Yf is the Final Yield - ratio between the generate energy Euse,� in the considered time interval �r measured after the inverter (before the connection to the grid) and the nominal power peak P0 of the PV array under investigation - and Yr is the Reference Yield - ratio between the solar irradiation in the considered time interval over the plane of the module and the

solar irradiance

r dayG� �� I

,I refG under Standard Test Condition

(STC) [3], [7]: in-plane irradiance of 1000W/m2

(perpendicular to the module plane), radiation spectrum

978-1-4244-8927-5X/11/$26.00 ©2011 IEEE 130

equivalent to the solar spectrum at air mass 1.5 and a module temperature of 25°C.

As regards the PV system with solar tracking, another important index in the Transposition Factor (TF). TF is a dimensionless quantity given by the ratio between the incident solar energy over a differently oriented and inclined plane (the reference yield Yr) and the solar energy incident on the horizontal plane (the ground reference yield Yrg):

r rgTF Y Y� (2)

It describes the higher amount of energy which is possible to receive with a modules orientation and inclination different than respect to the horizontal position.

III. PV SYSTEM DESCRIPTION

The PV plant under analysis is a system developed by Convert Italia S.p.A. located in Tuscania (location: 42°25'9" North, 11°52'13" East, elevation: 181 m a.s.l.,), in the center of Italy. It is composed by many subsystems connected to different inverters as shown in Fig. 1.

In particular the design of the PV plant account for 9 different PV arrays each equipped with 4560 poli-crystalline silicon PV-module, 12 inverters and 115

single-axis solar tracking system in order to maximize the quantity of irradiation received by the system and hence the energy produced.

The first 8 PV arrays has a power peak value of 50 kW each one while the last array is a 400 kW PV power array. The total amount of peak power installed in the power plant is therefore of 800 kW.

In order to evaluate the performance of the PV plant and the real advantageous derived from the utilization of a tracking system, many sensors and devices have been installed on the system to monitoring the quantities of interest.

To perform PR and TF calculation at least a measure of the energy produced after the inverter, of the irradiance on the ground plane and of the irradiance on the modules plane are necessary, so at least an energy counter (for each PV array) and two piranometers (sensors of irradiance) had to been installed in the PV plant.

Other measurements not strictly necessary to evaluate the PR and TF have been registered by the control system, as the external air temperature, the wind velocity, the module temperature, the quantity of rain fall in the considered period and the pollution amount in the air.

IV. TRACKING SYSTEM DESCRIPTION

Fig. 1. PV System in Tuscania (Latium – Italy)

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A solar tracker is a device that orients various PVpanels toward the sun. Trackers are used to minimize the angle of incidence between the incoming light and the PV panel. This increases the amount of energy produced by a fixed amount of installed power generating capacity, in particular the energy gathered from the direct component of the incoming light.

In standard PV systems, the energy produced by the direct light beam drops off with the cosine of the angle between the light and the PV panel. Thus trackers that have accuracies of ±5° can deliver greater than 99.6% of the energy obtainable by all the direct light beam. Hence in these typical cases high accuracy tracking are not typically used.

PV trackers can be grouped into classes by the number and orientation of the tracker’s axes. There are several common implementations of single axis trackers [10]-[12]. These include horizontal single axis trackers, vertical single axis trackers, and tilted single axis trackers. Horizontal tracking systems have the faces of the modules oriented parallel to the axis of rotation.

Compared to a fixed system, a single axis tracker increases the annual output energy by approximately 30%, and a dual axis tracker an additional 6% (see Fig. 2) [10]. Appropriate spacing between the PV panels can maximize the ratio between the energy production and the cost, this being dependent upon local terrain and shading conditions and the time-of-day value of the energy produced.

The tracking system developed by Convert Italia S.p.A. is a horizontal single axis tracker with the axis parallel respect to the ground, typically used for large distributed generation projects and utility scale projects (see Fig. 3). As a module tracks, it sweeps a cylinder that is rotationally symmetric around the axis of rotation. In these, a long horizontal tube is supported on bearings mounted upon pylons or frames and the axis of the tube is on a North-South line. Panels are mounted upon the tube, that rotates on its axis to track the motion of the sun through the day.

The range of permitted rotation is ±60° allowing an

ideal increment of production energy of about 20% considering a ground cover ratio of the plant of 0.32 [14].

Thanks to the linear actuator which is supplied by 230 Vca the centralized control system at the trackers edge moves the tracking system (tracking error ±1.87° which correspond to a power lose of 0.05%) in accordance with an established algorithm (Fig. 4).

The trackers algorithm is an astronomical watch with backtracking that enables to take the modules without shading perpendicular to their axis of rotation. As a matter of fact, one aspect of tracking systems is the problem of shadowing. In the morning and evening, the modules cast long shadows that may cover other modules, reducing the power output. For this reason it may sometimes be a better strategy not to let the modules track the sun all the way from sunrise to sunset, but to let them revert to a more south-facing position to avoid shadowing [14].

The motor driving each tracker develops a force of 6000 N under normal condition and of 8000 N in case of emergency, with a velocity of 1 mm/s, while the power absorbed is 340 W for a total waste of energy of about 20 kWh/year plus 19 kWh/year for its local control. Also 1400 kWh/year losses which is feed by 24 Vdc due to the central control system of the complete PV system has to be taken into account to evaluate the real advantages of a PV tracking system.

Fig. 3. Horizontal single axis tracker developed by Convert

Fig. 2. Irradiance on the ground plane and irradiance on the modules plane in a PV plant with one-axis solar tracking

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Fig. 4. Some particulars of the single axis tracking

V. MONITORING RESULTS

A. Irradiation evaluated with the software PV-GIS

In order to validate the irradiation measured on horizontal plane and on the modules plane of the PV system located in Tuscania, these values are here compared with the one obtained with the software PVGIS of the Joint Research Centre [10]. This software have been also used by other Authors to simulate the tracking systems [10]-[12]. The methods used by PVGIS to estimate PV system output have been described in a huge number of papers. The basis for the European part of PVGIS is a dataset with 10 years of data from several ground stations in Europe measuring global horizontal radiation and in some cases diffuse radiation. The station data were collected and processed as a part of the European Solar Radiation Atlas and published as monthly averages of daily irradiation sums. Spatial interpolation methods were used to estimate the irradiation at geographical locations between the sites where ground station data is available.

Hence a spatial interpolation and a time average have been necessary in order to obtain iso-irradiation maps which represent the entire European region. Since the data collected are referred to the solar irradiation on horizontal plane, mathematic algorithms have been developed to obtain the irradiation on planes differently

inclined from the data available. The average of the solar irradiance at any time during the day for a typical day in each month can be reconstruct for an arbitrarily placed plane and also for a sun-tracking system.

The average daily solar irradiation Hd e the average monthly irradiation Hm provided by PVGIS [10] as regards a PV plant situated in Tuscania are reported in Table I. Table I(a) considers a PV system installed on a plane parallel to the ground. In this case the irradiation is the same of that available on a horizontal plane at latitude of the site under study. Table I(b) instead considers a PV system with the same characteristics of the previous one but equipped with a one-axial tracking system with similar features to that realized by Convert Italia S.p.A., which allows the modules installed in direction North-South to follow the sun movement from East to West.

Finally the solar irradiation established by PVGIS with reference to a modules inclination of 30°, which corresponds to the optimal inclination for a PV plant located in Tuscania, is reported in Table I(c).

It can be seen that the irradiation really obtained with a tilt angle of 30° (1690 kWh/m2), is greater respect to the irradiation obtained with the modules installed on the horizontal plane: this consideration shows that for the latitudes under study is advantageous to incline the modules with respect to the ground.

Another useful reference in order to evaluate the tracking performance is reported in [5]. In particular, a system similar to that realized by Convert Italia S.p.A. allows an energy production in Tuscania of about 15-20% greater respect to the production energy of a fixed PV system with the modules inclined to the yearly optimum angle.

B. Irradiation measured and Transposition Factor

The irradiations actually measured on a horizontal plane and on the PV-plane in the PV plant located in Tuscania are reported in Table II.

Fig. 5 shows the average daily irradiance on horizontal plane and on the modules plane in Tuscania, referred to

TABLE I MEAN DAILY Hd AND MEAN MONTHLY Hm IRRADIATION ON (a) HORIZONTAL PLANE, (b) ON MODULES PLANE, AND (c) ON A PLANE INCLINED BY

THE YEARLY OPTIMUM ANGLE (30°) IN TUSCANIA

(a) (b) (c)

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the solar time. The yearly irradiation on the horizontal plane is of

1528 kWh/m2; it differs of only 4% from the irradiation of 1470 kWh/m2 provided by the software PVGIS.

The irradiation calculated by PVGIS considering a PV plant with one-axis tracking from East to West and with module parallel to the ground installed toward the North-Sud direction is of 1990 kWh/m2. The same irradiation measured in the site of Tuscania is of 1975 kWh/m2, showing a narrow margin of solely 0.74%.

The TF values calculated from the data measured in the PV system situated in Tuscania are reported in the following. In particular Fig. 6 and Fig. 7 show the daily and monthly TF values respectively for the PV system, with reference to the period from June 2009 to May 2010.

TABLE II MONTHLY IRRADIATION ACTUALLY MEASURED ON PV – AND ON

HORIZONTAL – PLANE

Month Hm (kWh/m2) Hmg (kWh/m2)

June 2009 252.570 196.350 July 2009 303.986 228.687

August 2009 261.857 196.540 September 2009 173.580 133.350

October 2009 146.661 108.469 November 2009 75.060 56.640 December 2009 57.164 45.012

January 2010 66.619 53.847 February 2010 79.716 64.120 March 2010 145.855 115.165 April-2010 205.050 160.980 May 2010 207.080 169.570

Total 1975.20 1529.73

Fig. 5. Average daily Irradiance on horizontal plane Grg and on the modules Gr plane in Tuscania

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Fig. 6. Daily Transposition Factor from June 2009 to May 2010

Fig. 7. Monthly Transposition Factor from June 2009 to May 2010

The annual TF related to the years from June 2009 to May 2010 is 1.29. This value, considering the geometrical configuration of the PV array situated in Tuscania (the ground cover ratio is 0.4), allows affirming that the tracking system is extremely effective and increment a lot the delivered energy by the PV system.

The comparison between the irradiation on a plane inclined by 30° and that effectively received by the PV array with one-axis tracking system situated in Tuscania is useful in order to evaluate the increment of input energy in PV system thanks to the tracking in relation to an optimal modules inclination. In fact, the TF evaluates the increment of energy respect to a system with modules on horizontal plane, but the real advantage due to installation of a tracking system is established only by comparing the energy received with the energy available on a system with modules inclined to the optimal angle in relation to the latitude of the site.

This comparison leads to the conclusion that the one-axis tracking allows to increment the input irradiation of the PV array by 17% than respect to a fixed-mounted system with modules inclined by an optimal angle of 30°, for the site under investigation.

This value is completely in accordance with the results reported in [10], [11] and [13].

C. Performance Ratio

In practice, the energy output of a PV system depends on several factors, such as module temperature, surface reflectivity (that takes into account the difference respect to STC), wiring losses and inverter-transformer efficiency (the Balance Of the System – BOS - losses/efficiency).

All this factors may be summarized in term of system PR. In the following the PR values calculated from the data

measured on the PV system with reference to the 9 different PV arrays are reported. In particular, As regards the year under investigation from June 2009 to May 2010 the monthly PR are also traced in a diagram of Fig. 8 and Table III highlights the minimum and maximum monthly PR for each month and for each PV array.

The evaluation of the performance ratio with reference to a PV system located in Tuscania, on duty since two years ago, leads to the following considerations: � as regards the measured annual PR, it is permanently

greater than 81% for the arrays from 1 to 8; � the PR of the array 9 (the biggest array) is the worst

although it is just about 80%. This can be due to the distance from the inverter to the energy meter which is larger with respect to the other distances from the inverters to the meters;

� however the PV system under study is a system with good features both from the point of view of the material used and of the planning type [3]-[8].

Fig. 8. Monthly Performance Ratio from June 2009 to May 2010

TABLE III MINIMUM AND MAXIMUM MONTHLY PR FOR EACH ARRAY AND

YEARLY PR: THE MAXIMUM IN BOLD AND THE MINIMUM UNDERLINED

Minimum Monthly PR

Maximum Monthly PR

PR June 2009 - May 2010

Array 1 0.7739 0.8538 0.8155 Array 2 0.7816 0.8540 0.8191 Array 3 0.7740 0.8646 0.8280 Array 4 0.7891 0.8638 0.8268 Array 5 0.7494 0.8688 0.8106 Array 6 0.7610 0.8648 0.8146 Array 7 0.7445 0.8640 0.8165 Array 8 0.7833 0.8683 0.8362 Array 9 0.7435 0.8467 0.7953

In the computation of the monthly and yearly PR the output energy on the October 20th and May 12nd for all the array are not considered because the data are not measured for a fault. The same problem has been recorded on the April 18th-19th for the Array 5 and on the November 18th for the field 8.

D. Output Energy analysis

Among the different factors affect the performance of the PV system some effects depend on the type of mounting: PV module conversion efficiency due to high temperatures, reflectivity of the module surface at

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shallows angle, and conversion efficiency of BOS. Regarding the effects of temperature some Authors

[10] shown that it will take approximately one percentage point extra off the energy output for two-axis systems compared to fixed systems at the optimum angle, in the South Europe. In Central Europe the effect is about 0.5 percentage point while in Scandinavia the value is almost zero. The effect of reflectivity goes in the opposite direction. The losses for two-axis tracking systems are in the range of 1.3-1.7% while the fixed angle systems suffer losses of 2.5-3.5%, generally rising towards North. Thus reflectivity shows values 1.0-1.5 percentage points lower for two-axis tracking than for fixed modules.

The combined effects (of temperature and reflectivity) on the comparison between the fixed mounting and the two-axis tracking will very nearly cancel in Southern Europe while in Central Europe the net effect is about one percentage point in favor of two-axis tracking systems and in the North the net effect will be an added advantage of two-axis tracking of about 1.5% [10]. For the single axis system we can considers that the combined effects of temperature and reflectivity for the center of Italy is nearly to 0.5%.

With reference to the BOS, the increasing of the cable losses due to higher currents flowing in the single-axis tracking system than in the fixed system are compensated by the increasing of the efficiency of inverters. As a matter of fact, the conversion efficiency of inverters depends on the power level, and tracking system have a higher probability of high irradiance and consequently the working power level is in average higher respect to fixed systems.

The total energy (motor and control system) used to drive each single-axis tracking for a total waste of energy of about 55 kWh/year for 7 kWp of modules. It means 7 kWh/year/kWp of the 1596 kWh/year/kWp average value of the energy output, equal to 0.46% of it and 0.60% of the estimated energy produced by PV fixed modules inclined at the yearly optimum angle. These values decrease to 0.40% and to 0.50% respectively if the trackers is used at its full potentiality mounting 8.28 kWp PV modules.

Overall, the TF also represents the increment in the PV system energy.

VI. CONCLUSIONS

The transposition factor, with reference to the 12 month considered and the geometrical configuration implemented in Tuscania (ground cover ratio about 0.4), is 1.291 and this value means that the one-axial tracking system is extremely effective.

Furthermore, an assessment of the mayor input energy to the system with respect to a system with modules inclined to an optimum angle is established by comparison of the measured irradiation on the modules plane and that estimate with PVGIS and a tilt angle of 30°. The results show the greater irradiation received by the PV system with tracking, of about 17% bigger respect

the ones received by the system with modules fixed at an optimal angle.

Taking in to account that the different factors affect the performance of the PV system depending on the type of mounting compensate one to another, the TF also represents the increment in the PV system energy.

The evaluation of the performance ratio with reference to a PV system located in Tuscania, on duty since two years ago, give as a result an annual PR of about 81% which leads to the conclusion that the system is a good system, both from the point of view of the material used and of the planning type.

The year here analyzed is the second with regards to the life of the PV array. This is an important consideration since the performance of a PV system decreases, in terms of PR, of about 1% for year.

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