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28th European Photovoltaic Solar Energy Conference and Exhibition; 661-663, Under concentration conditions, it is important to manage the operating temperature of a concentrator photovoltaic (CPV) module, because a high-density solar energy enters into the solar cell. We estimated the operating temperature of the CPV module using structural mechanics simulation. In the case of the non-uniform light condition, inhomogeneous temperature distribution was generated in the solar cell. Moreover, the highest thermal stress at the center of the finger electrode was observed. In the uniform light condition, the maximum temperature and thermal stress were decreased. The developed model can be applied to the design optimization of CPV module which achieves low operating temperature and low thermal stress.

The low-temperature ultrasonic dispersion in the transverse (C11 - C12)/2 mode of PrMg3 with the non-Kramers-doublet ground state has been investigated in temperatures down to 20 mK. The dispersion is caused by a thermally activated motions characterized by an Arrhenius-type relaxation time of τ = τ0 exp(E/kBT) with low activation energy, E = 500 mK, and slow attempt time, τ0 = 8.5 x 10-10) s. The coupling of the electric quadrupoles of the non-Kramers-doublet ground state to transverse lattice vibrations leads to a vibronic ground state with dissipation. The vibronic state in PrMg3 releases the entropy of kB ln 2 with lowering temperature across the activation energy. A Kondo-like singlet state due to the binding of the non-Kramers doublet to the lattice vibrations appears at low temperatures far below the activation energy.

27th European Photovoltaic Solar Energy Conference and Exhibition; 134-136, This study investigated the influence of dew condensation on the annual generated energy of a concentrator PV (CPV) system by using field test data from June 2004 to May 2005 collected in Toyohashi. The occurrence of dew condensation was identified using the daily curves for the module efficiency and fill factor (FF). Then, the daily generated energy of the system was estimated by correcting the measured values of the module efficiency to the ideal value and using measurement values of direct solar irradiance for each day. Because the output characteristics of a CPV system using three-junction cell are strongly influenced by seasonal variations in the spectrum of direct solar irradiance, the ideal daily efficiency curves were made for each month from data measured for day with clear skies. In the results, dew condensation was observed on 46 days throughout the year, and the annual energy generated by the CPV system was estimated to increase by 1.21%. Thus, the average value of the annual module efficiency increased by 0.25 points.

26th European Photovoltaic Solar Energy Conference and Exhibition; 159-162, Failure of III-V multi-junction solar cells has been acknowledged by experienced CPV system engineers. The frequency and significance in the real field, however, were not reported. For this purpose, 30 kW system consist of 5,250 III-V solar cells and 42 strings was intensively monitored for about three years. Current sensors were applied in all DC strings and observed the change of contribution of each string to detect when failure occurs and how significant to the loss of system output power. The modules that were found containing many dead cells were removed and intensively studied. The damaged cells are found concentrating in a single production lot of receivers and also in a single sorting level, namely, highest Isc bin. This fact suggests there are certainly “weak cells” and “robust cells”. The monitoring to 42 strings reveals that it looks the cells are degraded or dead simultaneously. Another distinct fact is the dead cells are found electrically closed position, rather than physically closed position. These facts imply that some external electrical stress is the direct reason although the robustness of the cell varies. The fact that the internal resistance of CPV array circuit is low means that the Q-value of resonance circuit consist of capacitance from junctions and inductance from interconnects is high, which induces high reverse voltage to the junction due to high frequency resonance by electrical cirtuits. In order to assess the robustness to the electrical stress, a new test is developed applying high voltage spark between the ground and one of the interconnects. Although the test does not apply forward bias current to the cell junctions, the spark kills one or several cells along the interconnects as is seen in the field. The mortality was significantly improved by protecting cells against surge voltage. The cause of field failure of III-V concentrator cells was understood. The infant mortality and field failure of III-V concentrator cells was expected significantly improved by proper protection of the cells. It may be concluded it is important to protect cells from outside electrical stress, rather than expecting III-V cells to be robust and defect free.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 114-117, The new model on the shading loss by multi-tracking CPV system considering spectrum mismatching loss as a function of sun height, shading of obstacles and range of tracking, seasonal change of sunshine duration and mismatching loss among modules. The observation and analysis of shading loss were done by comparison of string currents. The 30 kW CPV multi-tracker system consists of 42 strings. If specific string currents drop more than 50 % of the standard string currents, it will be regarded as being shaded. The shaded string current data sampled by every 1 second were integrated an entire day with weight of total power output. The measured and calculated shading loss was compared (see figure 1). Good agreement was obtained. Depending on local conditions including sunshine duration among seasons, latitude, shape of lands and etc, the annual shading loss of CPV multi-tracker system of 0.18 of space factor was 5 %.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 106-109, III-V concentrator photovoltaic systems attain high efficiency through the use of series connected multi-junction solar cells. As these solar cells absorb over distinct bands over the solar spectrum, they have a more complex response to real illumination conditions than conventional silicon solar cells. Estimates for annual energy yield made assuming fixed reference spectra can vary by up to 15% depending on the assumptions made. Using a detailed computer simulation, the behaviour of a 20-cell multi-junction concentrator system were simulated in 5 minute intervals over an entire year, accounting for changes in direct normal irradiance, humidity, temperature and aerosol optical density. The simulation was compared with concentrator system monitoring data taken over the same period and excellent agreement (within 2%) in the annual energy yield was obtained.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 5271-5275, This paper discusses optical impact to the cultivation area with comparison of flat-plate PV and tracking PV – typically CPV. It is reasonable to think of utilizing PV-uncovered area for agriculture. This paper discusses the space factor of the PV field by comparison of measured and calculated results as well as possibility of utilizing the land underneath PV array by agriculture. The farmland equipped with high-pole CPV has about 2 times more cultivated land, and the minimum illumination level of the cultivated land is around 2 times, the total sunlight energy to the farming land is almost 3 times. The PV output per lost cultivated land is about 2 times.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 782-785, A diesel generator system that is independent of the electric power system supplies electric power to a customer on an isolated island. In this study, the reduction effect of fuel consumption of the diesel generator is investigated by introducing a stand-alone concentrator PV(CPV)/diesel/battery system on an isolated island. In this study, the optimum operation schedule of the generator is determined by dynamic programming (DP) using data on the electric load and CPV output, and then, the annual fuel consumption is calculated. The results show that the annual fuel consumption increases on increasing the battery capacity, because of the charge/discharge energy loss. On the other hand, the CPV system effectively reduces the annual fuel consumption when its capacity is less than 20%.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 882-886, After the 5 years of the field experience in HCPV in Japan, it was found that a new kind of CPV trackers would be required to the application to Japanese market, where the sun is always covered by thin clouds, soil is soft and wind velocity is high. The new HCPV tracker is based on open-loop control equipped with software correction of installation and alignment errors. The correction is based on a liner model. A 2.4 kW prototype was constructed and tested. Based on the successful demonstration of that 2.4 kW system, a 30 kW CPV power plant consist of 6 enlarged trackers is designed. The CPV power plant is scheduled to be finished by December 2008.