FIELD TESTS ON CPV ISFOC PLANTS

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1 FIELD TESTS ON CPV ISFOC PLANTS F. Rubio, M. Martínez, J. Perea, D. Sánchez, P. Banda Instituto de Sistemas Fotovoltaicos de Concentración (ISFOC) C/ Juan Bravo Puertollano (Ciudad Real) Spain Tlf: , Fax: , mail: ABSTRACT In order to generate key knowledge on CPV technology, ISFOC has already installed 1,4MW of CPV and is executing 3MW of power plants incorporating seven different technologies which will be finished in The objective of these pilot plants is to assist the industries in the setting up of pilot production lines and to obtain very valuable information such as reliability, suitability and production. In collaboration with the various suppliers, ISFOC has followed in detail all the qualification tests and their results. Therefore a great body of knowledge and experience is being built up. After the completion of the plants, ISFOC has started the campaign of measurements, following its own methodology. It is based on the equations of the Shockley model and only one measurement is needed to establish the nominal power of the CPV system. Heat-sink temperature to calculate the cell temperature through the thermal resistance, DNI with a pyrheliometer and the I-V Curve are measured in this procedure. But, ISFOC will also test other rating procedures, like the ASTM 2527-E or the IEC draft for CPV modules. The results will be shown in this paper. Keywords: CPV, Concentration photovoltaic, field tests, IEC tests, standards, Power rating 1. INTRODUCTION During many years the prototypes of CPV were installed only in Universities or R&D centers [1,2]. But, in 2006, once special feed-in tariffs schemes for PV were established in some countries of Europe, especially in Spain, this technology has started the way into profitability. At this important point of time, in order to support the first steps that would allow stepping from CPV prototypes to technology industrialization, the Institute of Concentration Photovoltaic Systems (ISFOC) was created [3]. ISFOC was instrumented through an agreement between the Science and Innovation Ministry of Spain and the Government of Castilla La Mancha. The main goal of ISFOC is to support the growth and thrust the industrialization and commercialization of the most advanced CPV technologies, through the installation of various CPV pilot power plants. Since its establishment, ISFOC is executing a number of power plants incorporating various concentrator technologies Currently, there are 1.4 MW already completed from various technologies: Isofoton from Spain, Solfocus from US and Concentrix from Germany. A second phase has involved four more companies: Emcore from US, Arima Eco from Taiwan and Sol3G and Renovalia CPV from Spain with a total of 1.3MW, which will be operational in The objective of these pilot plants is to assist the industries in the setting up of pilot fabrication lines and help the development of industry standards. There will also be very valuable information obtained in the process such as reliability, suitability and production from each technology. ISFOC has become a national and world reference in CPV QUALIFICATION IEC TESTS AND RELIABILITY The first selected companies carried out, for the first time in world, the most important tests according with the draft of the new Standard IEC available on September 2006, following the requirements of the ISFOC call for tenders. The four suppliers of the second call for tenders should carry out all the tests of the standard as it was passed in December In collaboration with the various suppliers, ISFOC has followed in detail all the tests and their results. Therefore a great body of knowledge and experience is being built up. High and Low Concentrator Systems for Solar Electric Applications IV, edited by Lori E. Greene, Proc. of SPIE Vol. 7407, SPIE CCC code: X/09/$18 doi: / Proc. of SPIE Vol

2 As the standard procedures were being performed, improvement of designs with some modifications to comply the IEC tests was realised. The benefit of the standard is proven The isolation test and the Damp heat test are one of the most difficult one, as the modules need to show watertighness and/or electrically isolation. ISFOC recommendation is to carry out some pre-tests before the final certification. Currently there are a few laboratories in the world, mainly in USA, Spain and Germany, which are certified or ready to be certified for this standard. Additionally, ISFOC is working, together with some Spanish laboratories and R&D Centers in reliability studies of the systems, including cells, optics and modules. Currently, ISFOC is also working in the field degradation of the systems with the plants which are already installed. Figure 1: Prototypes of various technologies during IEC62108 testing. 3. OUTDOOR TESTING: ISFOC S PROCEDURE As no any standard for power rating was established when ISFOC has installed the first demonstration power plants, ISFOC has developed its own procedure for concentrator rating. This methodology has been described in several papers [4,5] and it is used by ISFOC for the acceptance of the CPV power plants[6]. It is based on a set of measurements in a short period of time which are translated to the standard conditions with equations based on the Shockley model. The standard conditions for concentrator characterisation are defined at 850 W/m2 and 60ºC of cell temperature. The measurement conditions are established as: - The sky around the sun should not have clouds during the measurement. - The direct radiation should be higher than 700W/m2. - The wind speed should be lower than 3.33m/s In order to determine the DC Power of the concentrator, the following variables are monitored during the measurements: - The I-V curve of the system is measured with a capacitive charge, as described before. - Direct radiation is measured by two pyrheliometers - Back plate temperature is measured with several thermal sensors on the back part of the module behind the cell. - For this method it is necessary to know the internal thermal resistance between the cell and the back plate. Applying equation (1) we can obtain the operating cell temperature from the actually measured back plate temperature. T cell = T + B R (1) h s cell backplate - Wind conditions, speed and direction, are also measured in order to determine their effect on the final rating in the future. Proc. of SPIE Vol

3 The current and voltage measured values are then translated to as-defined standard test conditions following the equations (2) and (3), deduced from the usual Shockley equation model: Boper I 2 = I1 (2) B mea Where I 2 I 1 B mea B oper Standard test conditions current Measured current Measured Direct beam radiation Standard test conditions direct beam radiation V 2 = V N ( T oper T cell ) ( I I ) ( I I ) ( I I ) 297 ln Lmea 1 Lmea 1 ( ( ) ) T oper N E g1 + E g 2 + E g 3 V OCmed 1 T cel I i Lmea 2 I Lmea 2 i I Lmea 3 Lmea 3 i (3) Where N I Lmeaj E gj V oc T oper T cell Number of cells in series connection Shortcircuit current of each junction Band gap of each junction System Open circuit voltage Standard test conditions cell temperature Measured cell temperature This method allows the calculation of DC power of the concentrator using only a few representative measurements 4. DC AND AC PLANTS ACCEPTANCE ISFOC is using its internal rating procedure for the DC Acceptance of the plant. For the first plants, ISFOC has measured all the concentrators to validate the procedure and to assure the function of all the systems. For the determination of the Nominal Power of a concentrator, first of all the ambient conditions are checked to verify if they fulfill the requirements. The performance of the system depends directly of the DNI, therefore, we select a period were the DNI is stable to obtain less dispersion. Once these periods are selected, the I - V curves are translated to the Standard conditions using equations 1, 2 and 3. Proc. of SPIE Vol

4 The Nominal Power calculations for all the concentrators of a plant are represented in Figure 2. With these results the plant can be accepted, because all the concentrators are between the +/-10% of the nominal power as it was established in the ISFOC s call for tenders. Normalized Power Figure 2. Statistical results for all the concentrators of a plant The AC rating is carried out to calculate the real performance of the plant due to modules performance in different conditions of spectrum, temperature, low DNI, or errors in the tracking function along the days. ISFOC s AC rating procedure consists of a control of the Energy produced during a period of time. The DNI data and the heat-sink temperature from several modules from different concentrators in the plant (selected in a random way) are stored. Comparing this theoretical Energy production with the real data of the energy meter of the plant, a factor (F) is defined. Finally the AC Nominal Power (P AC ) of the plant is calculated with equation (4) using the inverter efficiency (η inv ) and the factor F with the DC Nominal Power (P DC ). P AC = P η F (4) DC inv If the final AC Power is 100kW +/- 10%, the plant will be accepted. This procedure will be proposed as international technical specifications for acceptation of plants by the IEC Committee TC NEW IEC STANDARD PROPOSAL FOR POWER RATING The Working Group 7 of IEC TC82 Committee is working very actively to develop the new standards needed for the improvement of the CPV technology and market. This group is headed by Robert McConnell, nowadays working by Amonix, but previously member of NREL. One of the most important standard which should be established as soon as possible is the power rating, because is the only way to compare the behavior of the modules of different manufacturers. This subgroup of the group 7 of the TC82 of the IEC is headed by Sarah Kurtz (NREL) and has already presented to the IEC Committee a New Work Item proposal with the title Concentrator Photovoltaic (CPV) Module and Assembly Performance Testing and Energy Rating: Part 1: Performance Measurements and Power Rating - Irradiance and Temperature The purpose of this standard is to define a testing and rating procedure, which provides the CPV module power (watts) for a set of defined conditions. A second purpose is to provide a method for determining a set of characterization parameter values for the module. This standard is based on the IEC (Power rating methodology for flat-plate PV modules) [7] which provides the PV module power (watts) at maximum power operation for a set of defined conditions. As the power Proc. of SPIE Vol

5 depends of the radiation and temperature level, a table with the different levels of radiation and temperature shall be fulfilled as shown in table 1. Irrad. Air Mass Module temperature 15ºC 25ºC 50ºC 75ºC 1100 AM AM AM AM AM AM1.5 Table 1: Set of parameters for IEC Obviously these set of parameters and the procedures to carry out the measurements need to be adapted to the CPV conditions. In the standard there are two possible procedures to carry out this test; one is using a solar simulator for the indoor measurements and the second is to perform it with natural sunlight: - Indoor measurement challenges One of the best ways to fulfill the table of different set of conditions is performing these measurements indoor under controlled conditions. This measurements need to be done with a solar simulator for CPV, which need collimated light. Nowadays some solar simulators are being developed with very good results [8] and, we hope that in the near future, they could be used to characterize the CPV modules under this new standard. The challenges of these new solar simulators will be how to control the temperature, the radiation level and even the spectrum. As ISFOC has not yet a solar simulator, we have checked this procedure in outdoor conditions. - Outdoor measurements For the natural sunlight case, the standard describes a procedure to shade the modules for changing temperature conditions or to filter the radiation to modify its value, but this methodology needs to be very well developed because is not easy to perform with the size of some CPV modules and the filter of radiation could have spectrum variations, which could be more important in this type of modules than in the flat PV. To check this standard ISFOC has used this method in a whole concentrator system where is impossible to use shades or filter to change the conditions. Therefore we have taken the measurement during different days at different ambient temperatures and radiation, to try to have different set of conditions. 6. POWER RATING RESULTS The first task to adapt this proposal of standard is to translate the conditions for CPV. Now the radiation should be the direct radiation, which is used in CPV. Proc. of SPIE Vol

6 We have analyzed the data of the direct radiation in Puertollano between the months of January and April 2009 collected with the ISFOC s meteorological station, which can perform the measurement every day with a 1 minute frequency and we have obtained the values presented in figure % 100% Percentage 80% 60% 40% 20% 0% Radiation range (W/m2) Figure 3: Frequency (in %) of DNI values, for which the radiation is larger than the abscise value, in (Puertollano between January and April 2009) In Figure 3 we observed that 70% of the time we had more than 700 W/m2, and almost never we have less than 400 W/m2. Therefore we can only consider data above 700 W/m2, as it is typical in CPV standards, because is very unusual to obtain stable data below this value. We didn t measure the module temperature every day; therefore we have only the data of 15 days for two concentrators. The module temperatures were always between 45ºC and 75ºC, depending of the radiation and ambient temperature. If we are conditioned to fill the Table 1 with real data at AM1.5, we would not have enough values to fulfill all table. Therefore we have taken the data with different air mass values. Anyway any geometric value of Air Mass 1.5 doesn t assure the standard or the same spectrum value. With these measured values we can consider that the whole table 1, could be only fulfilled partially at outdoor conditions as shown in table 2. Irrad. Air Mass Module temperature 1100 AM AM AM AM AM AM1.5 15ºC 25ºC 50ºC 75ºC Table 2: In color set of parameters that could be fulfilled for outdoor conditions in CPV To have a wider set of conditions, we have created the table 3 with radiation values from 700 W/m2 to 950 W/m2. To have enough data we have defined different levels every 50 W/m2. For modules temperatures we have taken values between 45 and 70ºC, every 5ºC. Proc. of SPIE Vol

7 To fulfill all the conditions data we have carried out set of measurements during four months. All the measurement taken for this paper has been carried out at ISFOC facilities over two concentrators: A and B. These concentrators are HCPV systems, with concentrator modules on a two axis tracker. Both concentrators are defined to have a nominal power of 6100 W at 850 W/m2 and 60ºC of equivalent operating cell temperature. After four month of measurements we could fulfil only a small part of the table, because low values of temperatures with high radiation or high temperatures for low radiation did not occur DNI (W/m2) Module temperature (ºC) Table 3: Array power output in Watts for a set of values of Radiation (W/m2) and Temperature (ºC) With the available data, we have obtained only regression lines for 55ºC and 60ºC back plate module temperature. With these lines we calculate the output power at 850 W/m2 and back plate at 60ºC, obtaining P0= W For comparison reason if we take the 850 W/m2 data and 44ºC for the module back plate, which is equivalent to 60ºC at the cell (like the ISFOC standard condition), then the output power is P0= W. That means that the calculated value is 6% higher than the nominal one. 7. POWER RATING PROCEDURES COMPARISON For comparison reasons, ISFOC has analyzed other rating procedures: 7.1 Results with ISFOC s procedure We have used data of the same data to perform the calculation with ISFOC procedure, together with other measurement that we have taken before. We have chosen one set of parameters of these days with stable radiation and temperature. With only one calculation we have obtained the results shown in table 4 and 5: Concentrator A Date B (W/m2) Tplate (ºC) Tcell (ºC) Pmeas (W) Pcal (W) Diff 18/02/ % 05/05/ % 25/03/ % 06/10/ % Table 4: ISFOC s methods calculated power in different conditions for concentrator A. Concentrator B Date B (W/m2) Tplate (ºC) Tcell (ºC) Pmeas (W) Pcal (W) Diff 23/02/ % 03/04/ % 21/08/ % Table 5: ISFOC s methods calculated power in different conditions for concentrator B. We can see that the results are very near of the nominal power. The maximal error is 1.91%. The validity of this method has been tested with different concentrators, different ambient conditions and in different days of the year. Proc. of SPIE Vol

8 7.2 Bilinear Interpolation As it is very difficult to obtain a table with enough different data to do the study of interpolation or regression, we have decided to use the methodology of bilinear interpolation described by Bill Marion for flat modules in [9]. In this procedure we need only four I-V curves in different conditions: - High radiation and low temperature HRLT - High radiation and high temperature HRHT - Low radiation and low temperature LRLT - Low radiation and high temperature LRHT Between our collected data we have tried to find these conditions and it was very difficult to obtain the four curves, especially with a fixed Air Mass = 1.5. This is, because the real CPV working conditions are, in fact, very narrow. Therefore we used different air mass conditions, but always below AM 2, to fulfill the four cases. Finally we have used for the analysis the data shown in table 6. Concentrator B Day Time Air mass Rad+Temp Radiation (W/m2) Temp mod (ºC) 1 03-abr 11: HRLT ago 14: HRHT feb 11: LRLT feb 15: LRHT Table 6: Set of measurements chosen for the bilinear interpolation method The four I/V curves of concentrator B are represented in figure 4. Current (A) (1) HRLT (2) HRHT (3) LRLT (4) LRHT Voltage (V) Figure 4: I-V curves of the four set of chosen measurements. In this procedure we need to translate curve 1 and 2 to curve 5, and curves 3 and 4 to curve 6, using several equations: Current (A) Interpolated Curve 5 Interpolated Curve 6 Interpolated curve Voltage (V) Figure 5: Translated and interpolated curves 5, 6 and 7 by bilinear interpolation method. Proc. of SPIE Vol

9 The final power calculated with this method is P0= W at 850W/m2 and 60ºC of module (back plate) temperature. If we try to do the calculations with 44ºC of module temperature (to be near of the 60ºC of the cell temperature of the ISFOC standard conditions), the translation is not so proper, because all the curves are translated to a lower temperature and a higher Voc. Therefore the results are not very correct The power calculated with this methodology for the ISFOC standard conditions is P0= W, which is almost 8% smaller than the nominal one. 7.3 Regression Methodology It is based on the American Standard Test Method for Rating Electrical Performance of Concentrator Terrestrial Photovoltaic Modules and Systems under Natural Sunlight ASTM E [10]. This regression method has been used in USA for several years. In this method the determination of the performance of a CPV system consists of measuring the maximum power over a wide range of irradiance, air temperature and wind values. A multiple linear regression is used to rate the maximum power at standard concentrator reporting conditions, defined as T 0 =20ºC, v 0 =4m/s, E 0 =850W/m 2 In the procedure the variables to measure are the direct solar irradiance E, the air temperature T a, the wind speed v and the maximum power P. The calculation of the results is carried out computing the regression coefficients a 1, a 2, a 3, and a 4 by performing a multiple linear regression of P as a function of E, v and Ta using the equation (5) ( a + a E + a T + a v) P E a = To calculate the nominal power of the system, we obtain the value of P0, substituting the values of T 0, v 0, E 0, and the calculated values of a 1, a 2, a 3 and a 4 in the equation (6) (5) P ( a + a E + a T + a ) 0 = E v0 (6) To carry out the regression we have used the same data that we have measured for the interpolation methods. Therefore we will have the data of 7 days for each concentrator. Before doing the calculations, we have filtered the data for radiation higher than 700 W/m 2 and with variation lower than 0,4% every minute. The wind should not have variations higher than 5m/s every 5 minutes, which, anyway, is very unusual in Puertollano. To perform the linear regression we have used a Mathlab function and we do the regression with the data of the measured power, radiation, ambient temperature and wind. The results are clearly dependent on the amount of data used. If we use the data of only one day the results are not very representative. The calculated power for one day data is P 0 = 5518,8042W at the nominal conditions (T 0 =20ºC, v 0 =4m/s, E 0 =850W/m2) If we use the data of 3 days the calculated power is P 0 = W And if we use 5 days, the result is P 0 = W at standard conditions. If we try to use this method with the ISFOC standard conditions (850 W/m 2, 60ºC cell temperature and 3.3m/s max. wind), we need to force 10ºC ambient temperature, for closing the cell temperature to 60ºC. With this temperature and 3.3m/s wind speed we obtain: P 0 = W, which is 3% smaller than the nominal value. Proc. of SPIE Vol

10 7.4 Methods comparison resume and conclusions The resume of the array power output at standard conditions of DNI= 850 W/m 2, Cell temperature=60ºc and wind lower than 3.3 m/s for an entire concentrator system, are shown in table 7. Power in STC conditions (W) Diff Nominal Power % PV method % Bilinear interpolation % Regression method % ISFOC worst case % Table 7: Array power output of the different procedures in ISFOC standard conditions (DNI= 850 W/m 2, Cell temperature=60ºc and wind lower than 3.3 m/s) and the comparison with the nominal power. After the comparison of all the rating procedures, we need to say, first of all, that these results are not definitive, because they come from only four months of measurements in one location, with only two concentrators. Futures studies will confirm or not these first results. Secondly we need to remember that some of the studied methods were developed for modules and indoor measurements and we have used all the procedures for entire systems and outdoor measurements under real conditions. After the study of the different methodologies we can conclude: The first method based on the PV standard needs a large number of measurement days to obtain the table of data, and even so, we have not obtained all of the required ones. The procedure to fulfill the table with shading and filtering the light is not too feasible for a whole concentrator. This procedure need to be improved for module power rating in indoor and outdoor conditions. The method of bilinear interpolation needs also many different measurements in different moments of the year and a lot of physical and mathematical calculations. The regression methodology needs at least 3 or 5 days to have good results. The final coefficients don t have necessarily a physical interpretation as shown in the example. The ISFOC method has proven that with only few measurements in the given measurement conditions and with only one simple calculation, the results are repetitive and stable, even in different moments of the year. The narrow operating conditions of the CPV systems allow the good stability of corrections. 8. CONCLUSIONS ISFOC has already installed 1,4MW of different CPV technologies and has started the first measurements of the DC and AC power of the demonstration plants with our own procedure. This procedure has been validated and it is going to be proposed as international technical specifications for CPV plants acceptance. The workgroup 7 of the TC82 IEC Committee has proposed a new standards for the CPV module power rating based on the IEC (Power rating methodology for flat-plate PV modules) which provides the PV module power (watts) at maximum power operation for a set of defined conditions. A new set of conditions should be defined for the CPV modules. ISFOC has checked this new standard and other 3 methodologies for outdoor concentrator testing, showing that the ISFOC s procedure is very well adapted to the outdoor measurement. The new standard proposal need to be adapted for indoor and outdoor measurement conditions. Proc. of SPIE Vol

11 ACKNOWLEDGEMENTS The authors would like specially thanks the fruitful discussions with the Professor Gabriel Sala of the IES-UPM (Instituto de Energía Solar of the Politechnical University of Madrid) The authors also want to thanks to the ISFOC personal involved in the acceptance process: José Carlos Hernández, Oscar González and Antonio Usero for the good work made for the acceptance and for making the rating process easier. ISFOC wants to thanks all the partners of the first phase, Concentrix Solar, SolFocus and Isofotón for their collaboration. All this work has been partially financed by La Junta de Castilla la Mancha, the Spanish Ministry of Science and Innovation, the FEDER funds and the European project NACIR REFERENCES [1] E.L. Burgess, DA Pritchard. "Proc 13th Photovoltaic Specialists Conference" IEEE New York, 1121 (1978) [2] G. Sala, A. Luque, "Past Experiences and New Challenges of PV Concentratiors". In Concentrator Photovoltaics. Eds A. Luque & V. Andreev, Springer Series, (2007) [3] F. Rubio, P. Banda, J.L. Pachón and O. Hofmann. "Establishment of the Institute of Concentration Photovoltaics Systems - ISFOC". Proc 22nd EU PVSEC, Milan, (2007) [4] F. Rubio, M. Martinez, R. Coronado, J.L. Pachón, P. Banda, G. Sala, A. Luque, "Deploying CPV Power Plants - ISFOC Experiences" 33rd IEEE Photovoltaic Specialist Conference, San Diego, (2008). [5] ISFOC, "Specifications of general conditions for the call for tenders for concentration photovoltaic solar plants for the Institute of Concentration Photovoltaic Systems (ISFOC)" (2006) [6] M. Martínez, O. de la Rubia, D. Sánchez, M.L. García, R. Coronado, F. Rubio, J.L. Pachón, P. Banda. "Concentrator Photovoltaics connected to the grid and systems rating". Proc 23rd EU PVSEC, Valencia,(2008) [7] IEC, "IEC Ed.1: Photovoltaic (PV) module performance testing and energy rating - Part 1: Irradiance and temperature performance measurements and power rating".(2009) [8] C. Domínguez*, S. Askins, I. Antón, and G. Sala, "Characterization of five CPV module technologies with the Helios 3198 Solar Simulator". 34th IEEE Photovoltaic Specialist Conference, Philadelphia, (2009). [9] B. Marion et al., "Current-Voltage Curve Translation by Bilinear Interpolation", Progress in Photovoltaic, 12: (2004) [10] ASTM, "Rating Electrical Performance of Concentrator Terrestrial Photovoltaic Modules and Systems under Natural Sunlight" ASTM E 2527-(2006) Proc. of SPIE Vol