PV Contenders Battle it Out in Italian Mountains

Issue 3 and Volume 14.

Italy’s Bolzano Dolomiti airport, in the Dolomite mountain range, hosts the country’s largest PV experimental field. Through an installation of 24 technologies, the project is allowing the performance of various commercially available PV technologies to be assessed.

As PV installations under the 2007 Secondo Conto Energia ministerial decree of the Italian feed-in tariff scheme exceed 140,000, for a combined total of 2.7 GW, and new incentives encourage renewable energy developments through the 2010 Terzo Conto Energia decree, the opportunity to assess the real performance of available PV technologies have never been greater.

Charged with supplying the energy needs and reducing the CO2 contribution of Italy’s Bolzano Dolomiti airport (ABD), the field is the largest Italian PV experimental installation as well as one of the first large ground-mounted systems on Italian airport terrain.

It was developed with co-financing from the European Regional Development Fund (ERDF) 2007-2013 of the European Union. The plant is owned by ABD, while EURAC’s Institute for Renewable Energy is the scientific body responsible for studying the performance and the degradation of the different PV modules throughout the entire lifetime of the installation.

The value of such a plant is both scientific and commercial. Its purpose is to study the behavior of PV modules, in both the short- and long-term, with scientific accuracy and in a specific geographical context.

The results feed scientific publications and, at the same time, support different sections of the PV sector, such as developers, producers, installers, financing institutions, as well as decision-makers and customers, helping them to identify the most suitable technology for their applications.

ABD PV Field Features

The concept for the field, located in northeast Italy, was first mooted in 2008. In August 2010 it was connected to Italy’s medium voltage network. The energy its produces can power approximately 220 households and is expected to reduce ABD’s CO2 emissions of about 377 tonnes/year, or some 65% of current emissions.

Falling under the 2007 ministerial decree of 2007, the electricity produced by the ABD PV system has been approved under the Italian incentive scheme and receives a feed-in tariff of €0.346/kWh. Overall investment in the project has reached €2.6 million to date.

The installation has a peak power of 724 kWp and it is divided into two main parts: a 662 kWp commercial installation, comprising 8538 CdTe First Solar 277 modules with a nominal peak power of 77.5 Wp each, and a 62 kWp experimental installation, consisting of 24 different types of modules, divided into groups ranging 1-2 kW each.

While most of the modules have a 30° tilted position and are mounted on FS Schletter system supports (which are piled about 2 metres into the ground without any concrete footings), some are also installed on tracking systems.

There are two types of tracker unit installed. The DEGER Top tracker 40NT, is a single axis active tracker, has an inclination of 30° and the possibility to rotate in an east-west direction with a maximum angle of ±45° (considering the south as home position).

The DEGER tracker 7000NT, a dual-axis active tracker, has the possibility to adjust elevation and rotation. The elevation angle is 15°-90°, while the east-west rotation can be active for a 360° sweep, with adjustable limit switches.

An additional flat roof-like structure, created to test a group of Solyndra modules, completes the overview of available supports. This horizontal surface is covered with a white waterproof roofing membrane Alkorbright PVC-p from Renolit.

Given the shape of the CIGS tubular cells, the portion of energy production as related to the reflectivity from below the modules can vary from 0-39.3%, according to the position of the sun.

The whole PV field is connected to 105 SMA inverters of different sizes. Among them, 30 are located in the experimental section, including those linked to the two trackers.

The first row of the CdTe-based commercial part of ABD PV field (Source: EURAC/O. Seehauser)

The plant has a surface area of 18,860 m² (205×92 metres) covered with 20 cm of white gravel, a material with a reflectivity of around 25-30%. The field orientation is 8.5° west from south.

The ABD field was designed to assess the performance of the majority of the PV technologies currently available on the market and for use specifically in an Alpine valley. In addition, the two tracking supports allow comparison between fixed and moveable options.

The purpose is to evaluate and quantify differences in terms of energy performance among the different PV technologies and solutions, so to identify the most economically convenient to install in a specific environment, being the South Tyrol region or another comparable location.

Nevertheless, the results obtained from the monitoring activity underway at ABD also aim to define the energy rating of various PV modules under specific irradiance levels and spectral characteristics of sunlight.

For this purpose, an on-site meteological station has been installed to detect weather data and solar radiation in all its components (global, direct, diffuse, albedo) to support the characterisation of the photovoltaic modules.

It includes instruments, such as the three pyranometers, a pyrheliometer and an albedometer, as well as ambient temperature and wind sensors. A sun photometer operating in the AERONET network (coordinated by NASA) is next to be installed to detect aerosols, ozone and vapors in the atmosphere to better characterise the solar spectrum. PV reference cells (crystalline-Silicon and KG5-filtered) are mounted in-plane with the modules, as well as on the two trackers.

To maintain the quality of the monitoring instruments and assure the quality of data, regular checks and maintenance, as well as calibrations, are foreseen.

In addition to the monitoring instrumentation, the performance and the degradation of the modules under test at the ABD installation will be evaluated indoor using a pulsed solar simulator, requiring the maintenance of at least an internal reference module for each type installed on the field.

Mutual Support Research-Industry

In the year since it began operating the ABD field has attracted significant interest from all over Europe, ranging from PV module producers to installers and from scientists to the general public.

EURAC’s Institute for Renewable Energy participates in the Enertour programme, organised by TIS Innovation Park, to guide groups of interested people through relevant and innovative renewable energy applications in the South Tyrol region. The ABD PV field is among the installations of major interest.

Those participating in Enertour are mainly players from the PV sector looking for information about module technologies, as well as technical options for installation.

In the past few years — and in light of Italy’s booming PV market — there were a significant number of approaches made to the PV sector as players attempted to capitalise on the large market opportunities potentially available.

With 24 different types of modules, ranging from frameless solutions to the more standard module with an aluminum frame, and from transparent to opaque ones the variety of mounting clips and supports are also widely covered.

In additon, as the modules are tested in real working conditions, their performance is often far from the normal STC operating conditions frequently used in laboratories.

A working approach was adhered to for module acquisition by following the installer’s order process, meaning that even the PV production industry was itself brought under scrutiny.

The ABD field is thus a cross-roads in which electricity production becomes useful for research and performance evaluation. In such a ‘playground’ science and industry can meet, discuss and support one other.

A First Performance Comparative Evaluation

Data including electric variables, module temperature for each group, and ambient information is collected each day. An example evaluation of the energy performance of four PV technologies mounted on 30°-tilted supports — crystalline silicon cells (c-Si), copper indium selenide (CIS), copper indium gallium selenide (CIGS) and cadmium telluride (CdTe) — is shown below.

Final yield (Yf in kWh/kWp) for some technologies installed at ABD PV field evaluated from monitored data on a six month period.

The performance assessment of various PV technologies is based on indicators such as the performance ratio (PR) and the final yield (Yf) in kWh/kWp.

Following the indications of the international standard IEC 61724, the performance of a PV system can be expressed by the performance ratio which shows the overall effect of losses (due to module temperature, irradiance usage, low components efficiency, faults) on the power output of the plant.

The PR is calculated according to the formula PR=Yf/Yr, where Yf = E/Pο and Yr = H/G, and respectively E is the energy produced by the PV system [kWh], Pο is the installed peak power [kWp], H is the in-plane insolation [kWh/metre²], and G is the irradiance at STC [1 kW/metre²].

The PR value allows evaluating modules behaviour on different time scales (instantaneous, daily, monthly, yearly). A very interesting comparison is the irradiance dependence of the module’s behaviour. This dependence can be shown plotting instantaneous values of performance ratio with respect to irradiance values.

The PRs are calculated on a 15 minute-basis for every month and not just for selected sunny days.

Performing no filtering process shows the situation as it is in reality, and not just the ‘ideal’ case. In fact, filtering for only clear sky days, would reduce the spread in the calculated PR, but would also give the impression of a very insensitive system.

Random variations depend on measuring instruments, the items tested, the test/calculation procedure and the test environment.

The conducted analysis shows a better performance of c-Si and CdTe modules in comparison with CIS and CIGS. CdTe technologies have already been assessed for good performance in the Alpine region, due to the increased level of diffuse irradiance, and this comparison confirms their very good performance at low irradiance values.

The performance ratio values are shown without any adjustment, just as calculated from monitored data, thus showing the PV system and the monitoring system as they actually work.

A possible validation of the performance ratio can be performed by extrapolating power to STC conditions during stable and high irradiance data sets using the temperature coefficient data for power for each PV technology. This procedure gives the possibility of determining a correction coefficient and reducing system uncertainty.

Only The Beginning

As the ABD field is still in its first year of activity, many evaluations of the various PV technologies are set to be undertaken in the coming months. Stabilisation processes will be highlighted in data collected for winter and sping, with positive and negative trends possible in the different technologies.

The influence of temperature on the modules’ performance will be also discussed, as well as the effect of the different components of the irradiance will be shown in detail.

A recent comparative analysis conducted in collaboration with RSE-GSE from Milan, has already shown variable performance tendencies for the same modules in three different Italian locations. This means that it is already effectively determined that the best outcomes arise where the behavior of a PV technology is studied in close connection with the environment that is going to host it.

Alessandra Colli, Lorenzo Fanni and Wolfram Sparber are part of EURAC’s Institute for Renewable Energy. The Authors would like to thank the European Regional Development Fund (ERDF) for co-financing this project and the Fondazione Cassa di Risparmio from South Tyrol for the support of the research activities.

Acknoledgements are due to the management of the Airport of Bolzano Dolomiti (ABD), Thaler Engineering Studio, Leitner Solar, and the whole CoPES Consortium.