Abstract-- The PV plant at Politecnico di Milano represents the first installation under the national PV Rooftops Programme of Ministry of Environment in Italy. The role of this pilot plant has a significant importance in respect of the promotion and development of photovoltaic technology. The study and the design were made in detail not only to improve system productivity and efficiency, but also in respect of its visibility to illustrate a good example of the integration of photovoltaics into buildings from an architectural point of view.

After a five years long working life, the monitored data and parameters represent an important benchmark to analyze the PV technology application in the northern Italy. Index Terms—Photovoltaic, Performance Ratio, PV efficiency, PV payback time

I. NOMENCLATURE A nomenclature list, if needed, should precede the

Introduction.

II. TETTI FOTOVOLTAICI PROGRAMME : THE SUBSIDY GRANT SCHEME

The Italian Ministry of Environment in cooperation with regional administrations launched in 2001 the National Tetti Fotovoltaici (Photovoltaic Rooftops) Programme to support the installation of grid connected photovoltaic plants (1 to 20 kWp) integrated in buildings (roofs, façades and other elements, e. g. urban infrastructures). The access at the first demonstrative part of the Programme was restricted at the PV plants realized on public buildings, afterwards the subsidies were extended to the private buildings too, by means of regional grants.

The financial support was provided by a subsidy grant to the initial investment up to 75% of the eligible cost of the plant, including components, support structures, integration, planning and labours. Based on the market price, the acceptable cost has been fixed in the different calls directly managed by the regions. These calls should have been announced annually but because of the mismanagement and non homogeneity among the different regions, between 2001 and 2005, not more than two calls have been announced by each region.

According to the programme, annual energy production should had been less than the average electric

consumption of the building. In this way it was possible to apply the mechanism called net-metering, i.e. the possibility for the users to exchange energy with the local electricity grid. The electricity grid works as a storage for the PV plants and the surplus energy produced can be supplied to the grid at the same price of the energy consumed. More in details, the photovoltaic electricity fed into the grid had not been paid directly to the owner of the plant, but deducted from his energy bills. Consequently, the measurements had been done across a double meter that measures electricity fed to and withdrawn from the grid by the owner of the plant. Since the PV electricity production can not be greater than the electricity consumed by the user, the manager of the grid is not obliged to purchase the electricity produced from the PV plants (an important factor that could conflict with the liberalization of the national electricity market), but only allows to store for its successive utilization.

Within 2007, while the brand new feed-in tariff is showing the first achievements, the last PV Rooftop Programme’s installations are expected to be completed. The definitive data are not still available, but it is estimated that about 30 MWp PV power has been installed thanks to the Tetti Fotovoltaici programme.

III. PV PLANT DESCRIPTION The 11.25 kWp PV plant at Politecnico di Milano has

been designed by researcher and technicians of the Politecnico university, with the cooperation of CESI (Centro Elettrotecnico Sperimentale Italiano), and it was installed in December 2001 on the roof of the Faculty of Architecture.

The basic idea that aimed the whole project was the exploitation of the slightly sloped roof of the “11A” building, one of the main facility of the Faculty of Architecture, that contains didactic rooms and the library; the chosen roof showed immediately interesting characteristic for PV integration, such as the South exposition, the almost total absence of obstructions toward solar paths, and a relevant number of sheds suitable to be covered by PV modules.

On the roof there is a total number of 30 sheds, aligned on 3 rows of 10 elements. Each of them has been utilized as a support for five 75 Wp – PV modules, in order to obtain an homogeneous distribution of the PV generator and a full exploitation of the possibilities

The first installation under the Italian PV Rooftop Programme: a performance analysis

referred to five years of operation N. Aste*, F. Groppi**, and C. Del Pero* * BEST department, Politecnico di Milano, Italy

** CESI, Italy

offered by this configuration.

IV. ARCHITECTURAL DESIGN The choice of the PV modules represented

undoubtedly one of the most important steps of the project, because these components, employed as external building envelope, were evaluated considering their dimensions, fixing system, chromatic appearance, cells and frame type.

After a number of comparisons, the choice fell on the Shell Solar RSM75 module, a component whose size (1220x580 mm) was particularly suitable to meet the requirements of the project. In this way it had been possible to install 5 modules on each shed, that corresponds to a total of 150 PV modules for the whole plant.

Each PV module is constituted by a glass-tedlar laminated wafer which incorporates 36 12.5x12.5 mm multi-cristalline cells and offers an efficiency of about 10.6%. A special pyramid-shaped external surface of the glass increases the module efficiency when sunrays form a large incidence angle.

All PV modules are oriented in the same direction with a tilt of 60° and the azimuth toward South.

The PV modules have been fixed to the sheds by means of special steel supports, L-shaped (edges) and Ω-shaped (middle). The 10 cm air-gap between the external shed surfaces and PV modules allows the convective air flow and thus the natural ventilation of both the surfaces, especially in the hot season

Fig. 1. View of the Politecnico PV plant

V. SYSTEM DESIGN The PV plant at Politecnico di Milano represents a

good example of electric self-production in an urban context of high utilization.

The 150 multi-cristalline silicon modules, with 75 Wp of rated power each, form a 11.25 kWp PV array. These modules have been connected together in order to constitute 8 strings on the whole: six 20-modules strings and two 15-modules strings. Three 3kW-inverters convert the power from the six 20-modules strings and two

1.1kW-inverters convert the power from the two 15-modules strings. Although all the inverters are equipped with embedded Surge Protection Devices (varistors), the large extension of the PV array suggested a reinforcement with further and stronger class II SPD placed close to the arrays.

Each inverter contains an insulation transformer in order to electrically separate the DC section from the AC section. In this way the DC section constitutes an IT system, that is a circuit considered safe from electric shocks, even if the open-circuit voltage of the strings may reaches 436V at STC. The DV604 interface device, along with those incorporated in the inverters, represent a good protection against eventual islanding effects that may be caused by the disconnection of the electric grid. Since the power supply from the utility to the building is performed at 20 kV, this voltage value must be internally reduced by means of a 800 kVA transformer. For this reason the PV plant is connected to the 400V three-phase internal grid of the building.

The PV plant main electrical characteristics are summarized in the following table.

TABLE 1

ELECTRICAL CHARACTERISTICS OF THE PLANT Sub-array

N° of PV

modules

Peak power[Wp]

Strings Rated voltage

[V]

Open-circuit voltage

[V]

Rated current

per string [A]

Short-circuit current

per string [A]

1 40 3000 2 340 436 4,3 4,6 2 40 3000 2 340 436 4,3 4,6 3 40 3000 2 340 436 4,3 4,6 4 15 1125 1 255 327 4,3 4,6 5 15 1125 1 255 327 4,3 4,6 150 11250 8

VI. DATA ACQUISITION The PV installation is equipped with a centralized

acquisition system, through which it is possible to get all the necessary information to monitor the PV plant. The examined data are:

• Irradiation on the module plan, expressed in W/m2, measured at regular time steps of 15 minutes by a PV reference cell.

• Ambient temperature, in °C; • Alternate current output power, expressed in kW; • Daily, monthly and yearly amount of electric energy

produced by the system, in kWhel; • Photovoltaic modules temperature, in °C (obtained for

correlation with NOCT parameter and the environmental temperature).

VII. EFFECTIVE EFFICIENCIES ANALYSIS A detailed analysis of the monitored data has been

carried out in respect of the PV plant. The values of the experimental efficiencies have been calculated for a set of sample-days through the following formula.

PV

ACPV AI

P×

=η (1)

where: PAC represents the instantaneous power converted by the inverter [kW]; I is the value of the irradiance on the module surface expressed [kW/m2]; APV is the total area of the modules [m2].

The obtained values represent the instantaneously effective efficiency, measured at regular 15 minutes steps, by which the photovoltaic system converts the solar incident radiation into electricity.

VIII. THEORETICAL EVALUATION OF THE PERFORMANCE COEFFICIENT AND PV EFFICIENCY

The complete working-records database referred to the 5 years operative life of the Politecnico PV plant allowed to develop a simulation model, very useful to elaborate reliable forecasts of electrical production of similar installations. The calculation process has been tested and adapted on the real data obtained by the acquisition system of the plant, allowing to validate the theoretical hypothesis assumed in the model.

The entire method needs as an input several parameters like modules inclination ad orientation, site latitude, hour angle, zenith angle, declination, NOCT, modules area and coefficient of temperature. With these parameters it is possible to calculate the incident irradiation on modules surface and the modules real operative temperature. Subsequently, by evaluating the influence factors on modules and system efficiency it is possible to quantify the expected PV plant Performance Ratio (PR) in actual operating conditions.

The PR parameter quantifies the overall effect of losses on the rated output of a PV plant, due to inverter inefficiency, wiring, string mismatch, module temperature, incomplete use of irradiance by reflection from the module front surface, soiling or snow and component failure.

Having preventively estimated the value of this coefficient, it’s possible to compare the effective experimental efficiencies (ηPV) and those quantified through the evaluation model (ηPVt). This last parameter is calculated by the following relation.

)( pnPVt PR ηηη ∆−×= (2)

where: ηn is the modules nominal efficiency measured in STC; ∆ηp represent the predicted annual performance degradation.

IX. COMPARISON BETWEEN MEASURED AND CALCULATED PERFORMANCE

In order to validate the evaluation model, a

comparison between measured and calculated performance parameters was carried out on a basis of 40 sample days, chosen as representative of the whole year.

The daily average difference between the two values allows to establish the accuracy of the model. As an example are shown the comparison diagrams for a clear and a cloudy day.

0,001,002,003,004,005,006,007,008,009,00

10,0011,00

6.30 7.30 8.30 9.30 10.30 11.30 12.30 13.30 14.30 15.30 16.30 17.30

Hours

ηη ηη (

%)

Theoretical efficiency Real efficiency

Fig. 2. Theoretical and real efficiency (clear-sky)

0,001,002,003,004,005,006,007,008,009,00

10,0011,00

6.45 8.00 9.15 10.30 11.45 13.00 14.15 15.30 16.45 18.00Hours

ηη ηη (

%)

Theoretical efficiency Real efficiency

Fig. 3. Theoretical and real efficiency (cloudy day)

Observing the graphs it’s possible to assert that the

shape of the estimated efficiency curves follow fairly the development of the curves obtained tracing the courses of the measured instantaneous efficiencies. The correspondence among the curves turns out practically exact during the clear days, while shows a greater variability, even if contained, during the cloudy days, due to the non-homogeneous weather conditions. In any case, the average difference between the calculated and measured efficiencies, in the 40 sample days analyzed, is always less than ± 1 %, for both clear-sky and cloudy days.

As an example, in the following table are shown the system PR and efficiency values, related to any of the sample days during 2003 and 2004.

TABLE 2 DAILY MEASURED PERFORMANCE DATA

Clear sky Cloudy sky

Day PR Efficiency

[%] Day PR Efficiency

[%] 02/06/03 0,80 7,95 01/30/03 0,69 6,88 03/10/03 0,78 7,84 02/10/03 0,73 7,29 04/13/03 0,81 8,06 03/11/03 0,73 7,30 05/31/03 0,74 7,36 05/01/03 0,71 7,07 06/27/03 0,73 7,28 06/07/03 0,67 6,74 07/28/03 0,73 7,30 07/14/03 0,64 6,36 09/14/03 0,79 7,86 09/08/03 0,61 6,11 09/26/03 0,81 8,07 10/04/03 0,72 7,15 10/08/03 0,78 7,83 11/03/03 0,70 6,96 03/03/04 0,87 8,67 02/26/04 0,77 7,71 03/22/04 0,82 8,20 03/27/04 0,71 7,08 04/24/04 0,78 7,81 04/15/04 0,73 7,35 05/17/04 0,80 8,01 05/30/04 0,68 6,79 06/08/04 0,75 7,51 06/20/04 0,61 6,08 07/14/04 0,79 7,91 07/08/04 0,78 7,82 07/29/04 0,77 7,72 09/15/04 0,69 6,88 08/13/04 0,74 7,38 10/01/04 0,72 7,24 09/18/04 0,79 7,89 11/08/04 0,71 7,08

Average 0,78 7,81 Average 0,70 6,99

X. ANNUAL ENERGY PRODUCTION In order to evaluate the long term performance of the

PV plant, its energy production were at first estimated using the climatic data of the nearest weather station (reference site: Milan) and standard data on PV plants viz. photovoltaic modules characteristics, BOS etc. Subsequently, the estimated performance was compared with the measurements carried out during the 5 working years of the plant to verify the data reliability. Also on annual basis the assumptions were verified with the mean yearly measured production on average 4% higher/lower with respect to the estimated values.

The estimated electrical energy production of the PV plant is calculated by the following expression:

EPV = PPV × ηBOS × KPV × H/ISTC (3)

where: EPV is the annual electricity production by the plant [kWh/year]; PPV, is the peak power of the plant [kWp]; ηBOS is the efficiency of BOS (inverter, wiring, etc.) assumed as 0.9; KPV is the reduction coefficient due to the real operational conditions deviation from the standard test conditions (STC), considered as 0.8; H is the annual solar irradiance [kWh/m2 year]; ISTC indicates the solar irradiance in standard test conditions, equal to 1 kW/m2.

The (ηBOS × KPV) product corresponds to the above mentioned PR parameter; usually, it falls within the range

of 0.6 to 0.8. The value assumed in the present calculations is 0.74 in agreement with operational performance results collected from experimental data. The amount of solar radiation over the PV modules have been calculated using the standard methods, taking in to account the different inclinations and orientations of the PV modules.

In the following graph is represented the monthly energy production of the PV plant, measured during the 2003.

0

200

400

600

800

1000

1200

1400

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Total 2003 = 10256 kWh

Fig. 3. Monthly and annual energy production during 2003

To be more specific, in the successive economical

evaluations, it is considered that the performance of a PV plant reduces during its operation period and is progressive in time. The PV modules used in the plant, are guaranteed to maintain at least 80% of the initial nominal power after 25 years of operation. This means that the maximum loss in efficiency is expected about 0.8% annually. It should be mentioned that this value represents the upper limit, however in real conditions, it should be considerable much lower. In any case, taking into consideration also other components of the installations (BOS) and applying a reasonable safety margin, the loss in efficiency is considered as 1% annually.

The total electrical production of the Politecnico PV installation in a period of 25 years are estimated about 24,9 MWh. It should be mentioned that here the useful life of the plant is considered 25 years, however the average life of a PV installation could be considerable higher. Nevertheless, considering also the guarantee provided by the manufactures, the assumed time limit can be considered reasonably correct.

XI. ECONOMICAL ANALYSIS The investment costs related to a PV plant depend on

the technology used and on the system nominal power, while the corresponding cash flow depends on the cost of the electric power produced from the plant and therefore not purchased from the electrical net and from the maintenance costs. The realization of the Politecnico plant had a cost of approximately 87.300 € detailed in the following table.

TABLE 3 PLANT COSTS

Description Quantity Total price

Shell Solar RSM 75 PV modules 150 € 53'091.00

SMA inverter 5 € 7'798.50 Switchboards € 2'065.83

Installation and wiring € 7'230.40 Mounting system € 9'296.22

Planning and designing € 7'800.00

Plant total cost € 87'281.95"Tetti fotovoltaici" grant € 66'622.94Total paid by the Politecnico € 20'659.01 As shown in the table, the above mentioned program

grant covered about the 75% of the investment total costs necessary for the realization of the plant. Concerning the components costs and the planning e designing costs, in the calculation has been considered just the fraction related to the PV plant, excluding the data acquisition system. The charges due to this last part of the plant are detailed in the following table.

TABLE 4

DATA ACQUISITION SYSTEM COSTS

Description Quantity Total price

Data acquisition system 1 € 8’000.00

Display 1 € 6’700.00 Installation and wiring € 2’100.00 Planning and designing € 7'800.00

Data acquisition system total cost € 24'600.00 Taking into account just the total cost paid for the

plant, data acquisition system excluded, it has been possible to evaluate the investment payback time. The economic data are shown in the next table.

TABLE 5

ECONOMIC PAYBACK TIME EVALUATION

Item M. U. AmmountAverage annual electrical production kWh 10’500

Electricity average costs €/kWh 0.18 Maintenance annual costs € 0

Annual saving € 1'890

Payback time years 12 In the calculation the electric power cost has been

considered equal to 0,15€/kWh1, supposing an annual 2% medium increase of such price2. The maintenance costs have been rather considered not significant; in fact during the monitored 5 years long working period just an inverter fault has been registered, but it was related to a

1 Enel tariffs, 2005 2 Osservatorio nazionale Tariffe e Servizi: average increase of the electric power cost from 1995 to 2003, net of inflation.

product flaw covered by the manufacturer warrantee.

-30000

-20000

-10000

0

10000

20000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Year

NP

V (

€)

Fig. 4. Politecnico PV plant NPV calculation

As is shown in the previous chart, considering a 25

years long lifetime, we can conclude that the investment turns out reliable.

XII. CONCLUSIONS L’impianto del Poli costituisce un importante punto di riferimento per la progettazione e la realizzazione di impianti fotovoltaici in contesto urbano. I dati raccolti nei 5 anni di vita operativa costituiscono un utile database per comprendere nel dettaglio le possibilità produttive di un impianto fotovoltaico. Il modello di calcolo tarato su questi dati, inoltre, può essere impiegato come strumento di progetto e di valutazione preliminare utile a guidare le scelte nel campo dell’integrazione architettonica dei PV

REFERENCES [1] M. A. Quintana, D. L. King I. E. Cannon, J. R.

Woodworth, Recent advances in outdoor performance evaluation of pv systems, 23rd IEEE PVSC Conference, Louesville, KY (USA), 1993.

[2] L. A. Hecktheuer 1, A. Krenzinger, The effects on the photovoltaic system response of parameters: reflection, spectrum, voltage drop and temperature, 17th EPVSEC European Photovoltaic Energy Conference, München (D), 2001.

[3] N. Aste, “Il fotovoltaico in Architettura”, Gruppo Editoriale ESSELIBRI-SIMONE, Napoli 2002

[4] J. F. Randall, C. Droz, M. Goetz, A. Shah, J. Jacot, Comparison of 6 photovoltaic materials across 4 orders of magnitude of intensity, 17th EPVSEC European Photovoltaic Energy Conference, München (D), 2001.

[5] N. Aste, F. Groppi, “ The 12 kwp PV plant at Politecnico di Milano:a first example of sustained rooftop PV project in Italy”, 4th ISES Europe Solar Congress “EuroSun 2002”, Bologna, 23-26 giugno 2002.

[6] N. Aste, S. De Nardi, F. Groppi, “Integrazione architettonica di un impianto fotovoltaico da 11,25 kWp: ricerca, sviluppo e applicazione presso il Politecnico di Milano”, Conference and Exhibition “PV in Europe from PV Technology to Energy Solutions”, Roma, 7-11 ottobre 2002.

[7] Osservatorio nazionale Tariffe e Servizi, Statistiche energetiche, 2003

[8] D. L. King, W. E. Boyson, J. A. Kratochvil, Photovoltaic array performance model, Sandia National Laboratories, paper nr. SAND2004-3844, 2004.

[9] F. Groppi, C. Zuccaro, Impianti solari fotovoltaici a norme CEI, Editoriale Delfino, 2005.

[10] N. Aste, C. Del Pero, “Simulation model for forecast of the energy performance of PV plants”, ASME ATI Conference, Milano,14-17 maggio 2006

.