Takumi Takashima

Maximum output control of photovoltaic (PV) array

A photovoltaic (PV) system output depends on environmental parameters such as the solar insolation and the PV module temperature. If it is possible to predict the maximum power point under the outdoor environment and to operate at that point, the PV system can generate the maximum output every time. In this paper, a maximum power point control method that maximizes the output of a PV array is proposed. This method determines the maximum output operation point from the I-V characteristics introducing empirically the effects of the solar insolation and the module temperature. The authors derived two main parameters from this analysis; one is the power gain G, and another is the environmental operation parameter X. At the operation point determined by this method, G becomes larger than that of under the same environmental conditions. G becomes large with the increase of X, and the large X mainly means low solar insolation. The characteristics of PV module which will supply more power especially at large X should satisfy the following points; the fill factor of the module should be lower and the short circuit current of the module should be larger than those of arrays currently available in the market.

New proposal for photovoltaic-thermal solar energy utilization method

Abstract One of the most effective methods of utilizing solar energy is to use the sunlight and solar thermal energy such as a photovoltaic-thermal panel (PV/T panel) simultaneously. From such a viewpoint, systems using various kinds of PV panels were constructed in the world. In these panels, solar cells are set up at an absorber collecting solar thermal energy. Therefore, temperature of solar cell increases up to the prescribed temperature of thermal energy use, although it is lower than the cell temperature when using only solar cell panel. For maintaining cell conversion efficiency at the standard conditions, it is necessary to keep the cell at lower temperature. In this paper, electric and thermal energy obtained from a PV/T panel is evaluated in terms of energy. Based on this evaluation, the method of not to decrease cell conversion efficiency with collecting solar thermal energy was proposed.

Fault detection by signal response in PV module strings

A fault detection and location method in PV module strings was experimentally studied. Even if the conventional diagnosis methods such as I–V measurement or Voc measurement could find the fault existence, those methods could not locate the fault position in the string. In this paper, the time domain reflectometry (TDR), which applies the voltage signal into the string and observes the signal response waveforms, was applied to the PV strings containing faults to detect the fault and locate its position. The results showed the disconnections and the degradations with series resistance increase between modules were detected with the response signal voltage rise and the signal rise timing shift. The signal voltage rise meant the impedance change in the string and the shift meant the disconnection / degradation positions, respectively.

Experimental Studies of Failure Detection Methods in PV Module Strings

The diagnosis and monitoring of PV systems are important to minimize the outage period and maximize the lifetime output. The objective of this research is to develop the failure detection technologies for PV systems which can be integrated into the power conditioners. There are some failure detection methods such as thermal methods, visual methods, and electrical methods. Between these methods, the electrical methods seem to be the most appropriate to integrate into the power conditioners. To determine the position of the failed module in PV array, the time domain reflectometry (TDR) is the most promising in electrical failure detection methods. In TDR method, the inputted signal into PV cell/module/string and the reflected signal from the circuit are compared. The signal delay and the change of waveform shape are translated into the failure position in the line and the type of failure. In our fundamental experiments under the dark conditions, the line length of several cells string and the line length of a module in which many cells are connected in series could be detected by TDR. These results show the possibility that TDR can find the electrical property changes of PV modules and can be used for failure detection method. Based on these fundamental experiments, the detailed diagnosis methods should be studied in more depth for their realization into power conditioners or test equipment

A study on a thermally regenerative fuel cell utilizing low-temperature thermal energy

Abstract We have proposed a thermally regenerative fuel cell operated by low-temperature thermal energy such as solar thermal energy and low-temperature waste heat. It consists of the chemical reaction of 2-propanol dehydrogenation at the negative electrode and the acetone hydrogenation at the positive electrode by using the principle of a fuel cell. As the first step of this research, we investigated the acetone hydrogenation. Activity of ruthenium and platinum composite catalyst (3 wt.%) supported on a carbon plate for the reaction was much higher than that of ruthenium catalyst or platinum catalyst at 90°C. Activity of ruthenium and platinum composite catalyst was much higher when it was supported on carbon felt or cloth than a plate. We adopted ruthenium and platinum composite catalyst supported on carbon cloth as electrodes of the cell and examined its characteristics. First, we used molecular hydrogen instead of 2-propanol as a proton source. Under this condition, the open-circuit voltage and the short-circuit current were 104.6 mV and 8.98 mA, respectively. As loading of the catalyst became higher, the short-circuit current also became larger. The short-circuit currents were 11.5 and 26.7 mA when loading of the catalyst was 5 and 30 wt.%, respectively. Then we used 2-propanol as a proton source. We investigated effects of 2-propanol concentration, catalyst loading and reaction temperature on the cell efficiency. When 2-propanol was diluted with water and supplied to the negative electrode, it was shown that 2-propanol concentration of 50–70 vol% was the best for cell efficiency. The cell efficiency was improved with increasing catalyst loading. As for reaction temperature, 80°C was better to improve the efficiency.

Photovoltaic power production forecasts with support vector regression: A study on the forecast horizon

The objective of this study is to verify how different forecast horizons affect the accuracy of a method to forecast photovoltaic power production using support vector regression and numerically predicted weather variables. One year of power production forecasts were done for a 1 MW photovoltaic power plant in Kitakyushu, Japan. Two forecast horizons were evaluated, up to 2 hours ahead and up to 25 hours ahead. The results showed a variation of the accuracy of the forecasts according to the forecast horizon. The root mean square error for 1 year of up-to-2-hours-ahead forecasts was 0.104 MWh; whereas for the up-to-25-hours-ahead it was 0.118 MWh. The mean absolute error was 0.065 MWh for the up-to-2-hours-ahead forecasts and 0.076 MWh for the up-to-25-hours-ahead forecasts. In percentage terms, the root mean square error and the mean absolute error increased 13% and 17%, respectively, with the increase of the forecast horizon.

On the Use of Maximum Likelihood and Input Data Similarity to Obtain Prediction Intervals for Forecasts of Photovoltaic Power Generation

The objective of this study is to propose a method to calculate prediction intervals for oneday-ahead hourly forecasts of photovoltaic power generation and to evaluate its performance. One year of data of two systems, representing contrasting examples of forecast’ accuracy, were used. The method is based on the maximum likelihood estimation, the similarity between the input data of future and past forecasts of photovoltaic power, and on an assumption about the distribution of the error of the forecasts. Two assumptions for the forecast error distribution were evaluated, a Laplacian and a Gaussian distribution assumption. The results show that the proposed method models well the photovoltaic power forecast error when the Laplacian distribution is used. For both systems and intervals calculated with 4 confidence levels, the intervals contained the true photovoltaic power generation in the amount near to the expected one.

Case studies of large-scale PV systems distributed around desert area of the world

Abstract To show the huge potential of PV systems, the authors have been studying the feasibility of large-scale PV plants. If a PV module cost is assumed to be 100 ¥ W , it gives the electricity at a cost of 7.70−13.12 ¥ kWh for a 100 MW plant size located at 6 desert sites around the world, considering the site irradiation,local labor cost, etc. for each site. In spite of the fixed, flat plate, the cost can reach a fairly low level. The station will be composed of 20 sub-units × 10 units of 500 kW optimum size sub-units.

An evaluation method for smoothing effect on photovoltaic systems dispersed in a large area

PV is expected as a major carbon-free primary energy source which is inevitable to proceed to a carbon-free economy. However, as a variable power source, the large PVs penetration into the power system may cause various kinds of power system operation issues and/or expansion needs. In order to technically-and economically-feasible solution for these issues and needs, it is quite important to quantitatively analyze and evaluate the variation of PV generation. This paper will present the smoothing effect of the total PV generation output in a broad area by means of the maximum output fluctuation as fluctuation index.

Performance and Reliability of 1 MW Photovoltaic Power Facilities in AIST

The AIST 1 MW PV facilities consist of a total of 211 inverters of the 4-kW type designed principally for residential uses, and tens of inverters of the 10-kW unit designed principally for industrial uses. These inverters are connected to low-voltage (200 V) distribution line in AIST. There are 13 kinds of PV modules from 5 different technologies and tens kind of PV inverter from several manufactures. Benchmarking these PV components reveals the differences of the performance and reliability among other PV components under actual conditions. During the first two years, there were several failures in the system components, mainly caused by improper installation works. But the PV facilities showed proper performance ratio (around 70%) and perfect availability in total. The mean time of failure was 7.9 times. More than 64% of individual PV systems had no failures. The mean time to repair was only 3.3 days