Bastian Zinsser

Performance of Photovoltaics Under Actual Operating Conditions

Amongst the various renewable energy sources, photovoltaic (PV) technologies that convert sunlight directly to electricity have been gaining ground and popularity, especially in countries with high solar irradiation. Over the past years PV has shown rapid development and a wide variety of new technologies from different manufacturers have emerged. For each PV module type, manufacturers provide typical rated performance parameter information which includes, amongst others, the maximum power point (MPP) power, efficiency and temperature coefficients, all at standard test conditions (STC) of solar irradiance 1000 W/m2, air mass (AM) of 1.5 and cell temperature of 25 °C. As this combination of environmental conditions rarely occurs outdoors, manufacturer data-sheet information is not sufficient to accurately predict PV operation under different climatic conditions and outdoor PV performance monitoring and evaluations are necessary. The objective of this chapter is to provide an overview of different PV technologies ranging from crystalline silicon (c-Si) to thin-film and concentrators. Subsequently, a summary of the main outdoor evaluation performance parameters used to describe PV operation and performance is outlined. An overview of the effects of different environmental and operational factors such as solar irradiance, temperature, spectrum and degradation is also provided along with the results of previously published research efforts in this field. In the last section of the chapter, the installed PV and data acquisition infrastructure of a testing facility in Cyprus is presented and a thorough analysis of the climatic conditions and the performance of different grid-connected PV technologies that have been installed side-by-side and exposed to warm climatic conditions, typical of the Mediterranean region are given.

Outdoor efficiency of different photovoltaic systems installed in Cyprus and Germany

The energy produced by different photovoltaic (PV) systems depends to a great extent on the inverter and photovoltaic module outdoor efficiency which in turn vary according to the irradiance and temperature of the field conditions. The objective of this paper is to describe the investigations performed to evaluate the outdoor efficiencies of thirteen (13) different types of PV modules which have been exposed to real conditions in Stuttgart, Germany and Nicosia, Cyprus. The inverter and PV array efficiencies have been evaluated by using direct outdoor measurements and data analysis on the measured metereological and operational data collected by the installed sensors present at the two test sites. The measured efficiency of the PV modules has been further evaluated on a seasonal basis reflecting the obvious effects of the seasonal variations of incident irradiation, operating temperature and seasonal spectral shift on the outdoor operating efficiency. The average annual inverter efficiency measured in Stuttgart was 89.8 % while the corresponding inverter efficiency in Nicosia was 90.9 % for the same period and for identical inverters that are rated at a European Efficiency of 91.6 %.

Degradation of different photovoltaic technologies under field conditions

Over the past years a number of testing facilities have been monitoring the performance and degradation of PV systems according to the established standards of indoor and outdoor testing. The objective of this paper is to present the initial first year and longer-term rate of degradation of different PV technologies installed at the testing facility of the University of Cyprus, based on outdoor field measurements and methodologies. The first year degradation of the technologies was obtained using a data filtering technique of DC generated power at Maximum Power Point (MPP) at irradiation points of higher than 800 W/m2 and normalising the measured power to Standard Test Conditions (STC). Over the first year, mono-crystalline silicon technologies showed degradations in the range 2.12 % – 4.73 % while for multi-crystalline technologies the range was 1.47 % – 2.40 %. The amorphous silicon system demonstrated the highest first year decrease in power with an average degradation of 13.82 %. For validation purposes the first year degradation was also obtained using a second technique by evaluating outdoor measured data-sets under Air Mass (AM) 1.5 (morning and afternoon) conditions and during noon (high irradiance and temperature). In this case the evaluated results showed deviations of up to 6 % and 3 % for mono-crystalline and multi-crystalline technologies respectively whereas for thin-film this was 5 %. Finally, the longer-term degradation rates were evaluated by using the least-square fit method on average monthly data-set blocks of (i) Performance Ratio (PR), (ii) PR evaluated by filtering outage data-sets and restricting to high irradiance conditions and (iii) the Photovoltaic for Utility Systems Applications (PVUSA) rating methods, for the period June 2007 – June 2009.

Rating of annual energy yield more sensitive to reference power than module technology

At present, many different photovoltaic (PV) technologies share the market. Especially investors want to know how much energy each of the PV technologies produces. This paper discusses the measured annual energy yield EAC of twelve PV technologies under different climatic conditions in Germany and Cyprus over three years of operation. In order to compare the annual yield of different PV technologies, the EAC data are normalized to the rated power PN, to the flasher power Pflash, and to the measured field power Pfield. An error analysis is done for both, the energy measurement EAC and the nominal power PSTC. It is found that the typical uncertainty for an energy yield comparison is ±5 %. This means that a difference of 10 % in the annual energy yield between PV technologies can not be traced back to the technologies themselves. The performance analysis of all PV systems shows that the differences in the energy yield are smaller than the error bars on the reference power. Therefore it is not yet possible to decide which PV technology is the best. Moreover, despite obvious trends on the data, we can not unambiguously conclude that PV modules with a better temperature or low light behavior will ensure a higher energy yield in general, since the propagation of state-of-the-art nominal power rating errors outbalances the well recognized effects of low light and temperature dependencies.

Photovoltaic model uncertainties based on field measurements

The purpose of this paper is to compare the combined uncertainties inherent in three PV models the single-point efficiency, the single-point efficiency with temperature correction and the PVUSA. This evaluation was performed using outdoor measurement data from 12 different, 1 kWp, grid-connected PV systems, operating in Cyprus since June 2006, along with measurement data from the meteorological sensors on-site. The different models were associated with different uncertainties which affected the accuracy of the annual energy yield prediction of the different PV technologies. For the single-point efficiency model, the combined uncertainties on the annual dc energy yield of all PV systems were in the range ±7%, resulting from the name-plate efficiency at standard test conditions (ηSTC) and the global irradiation measured on the plane of array (GPOA). By applying temperature correction on the model, the combined uncertainties dropped to an average of ±5.68% over the evaluation period. Lastly, the combined uncertainties for the PVUSA model were even lower, on average ±1.59%.

Two year performance evaluation of different grid connected photovoltaic systems

The performance and energy produced by photovoltaic (PV) systems installed at any location depends to a great extent on the environmental conditions they are exposed to and their technology. This paper presents the two year evaluation of thirteen (13) different types of PV systems which have been evaluated under real conditions in Nicosia, Cyprus and Stuttgart, Germany. The fixed flat plate systems under test have shown an average annual energy yield (ac) of 1580 kWh/kWp with a solar irradiation at the plane of array (POA) of 1997 kWh/m2 for the first year of operation in Nicosia, whereas in Stuttgart the respective average energy yield (ac) was recorded to be 1194 kWh/kWp and the solar irradiation at the POA was 1460 kWh/m2. During the second year of operation, the fixed mounted average energy yield (ac) was 1609 kWh/kWp (+1.8%) and 1066 kWh/kWp (-10.7%) for the systems in Nicosia and Stuttgart respectively. The solar irradiation at the POA in Nicosia was 2050 kWh/m2, an increase of 2.6% from the first year and in Stuttgart, 1306 kWh/m2 showing a decrease of 10.5%. Accordingly the performance ratio (PR) on the ac side for the systems in Nicosia, was between 73% - 85% in the first year whereas in the second year between 70% - 84%. In Stuttgart the systems have shown higher ac PR than in Nicosia, in the range of 76.2% - 88.5% in the first year and 72.3% - 88.5% in the second year of operation.