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Photovoltaic System and Application Team

Study on a stable long-term energy source of photovoltaic systems 

Over View

 In the Fifth National Energy Strategy approved by the Cabinet in July 2018, it is necessary to "continue to actively promote efforts for the smooth mass introduction of renewable energy to reduce costs to a level that is competitive with other power sources and to become independent from feed-in tariffs (FITs), so that it can be sustainable as a long-term, stable main power source that plays a role in Japan's energy supply. In order for photovoltaic power generation to take root as an energy infrastructure and become the main power source, the realization of a stable long-term energy source of photovoltaic systems has become an important issue.

Research Target

 Our team is engaged in research and development of safety-related design and operation technologies for photovoltaic systems, as well as infrastructure development, in order to realize long-term stable energy supply of photovoltaic systems. We are also conducting research and development of power generation forecasting technology for supply and demand operation in order to smoothly integrate photovoltaic power generation into the power system at the time of a large amount introduction.

Research Outline

 Our team has a role to play in making photovoltaic generations an effective energy technology and a viable energy infrastructure. In addition to supporting the domestic photovoltaic industry, our research aims to contribute to the sustainable diffusion and expansion of photovoltaic systems in Japan from the perspectives of "owners and users of photovoltaic power generation systems" and "photovoltaic systems that form energy infrastructure.


(1) Initiatives for long-term reliability and safety of photovoltaic systems
 
Photovoltaic systems are generally expected to operate for more than 20 years, but the most important component, the photovoltaic module, is installed out of the user's sight and is soundless and motionless during operation. In addition, photovoltaic power generation has fluctuation affected by the weather, making it difficult to understand the degradation of power generation performance and the increase in safety risks over a long period of operation.
 In addition, during the daytime, the photostatic modules themselves continue to generate DC voltage even when the photovoltaic system is shut down, so there is a potential risk that workers will be electrocuted in the event of a fire in a building equipped with a photovoltaic system during the daytime.
Therefore, our team is examining technical and institutional measures to improve the safety of photovoltaic systems through various surveys and analyses.

(2) Approaches to photovoltaic power generation forecasting technology

 Concerning “photovoltaic system as a player of energy infrastructure”, on the other hand, the methodology of forecasting photovoltaic power generation has been discussed.
Not to mention it that, output electricity strongly depends upon weather condition. The characteristics of photovoltaic power generation systems are their instabilities. When the large amount of photovoltaic systems will be connected to a utility grid, the inevitable fluctuation of their output may have a harmful influence on the utility grid. Using energy storage devices is one of the promising solutions, but their installation should be as small as possible from an economical viewpoint. Our team is conducting research and development of technologies for understanding and predicting solar power output from both scientific and engineering approaches, such as weather observation and forecasting techniques and machine learning.

Main Research Facilities

(1) Experimental a photovoltaic array (2) Photovoltaic systems' facilities for demonstration
Figure1:photovoltaic systems’ facilities for demonstration
This is a photovoltaic array for conducting various experiments on photovoltaic systems, and it is also possible to conduct tests with a PCS connection (location: Tsukuba).
Figure2:Solar power generation facilities for demonstration
There is a total of about 800 kW of photovoltaic systems consisting of a 3 to 4 kW photovoltaic array and PCS for residential use.
Experiments in actual power generation conditions are possible (location: Tsukuba).
 

Major Achievements

1. Development of infrastructure for the safety of photovoltaic systems
As the foundation for the safety of photovoltaic systems, we have released technical information on the DC safety of photovoltaic systems that summarizes electric shock and fire risks and countermeasures, on-site maintenance and inspection procedures, near-miss incident information, and design support tools for snow loading in structural design. For more information, please refer to "Information on the Safety of Solar Power Generation".
Guide and Technical Information for DC Electricity Safety of systems (2nd Edition) in Japanese, etc.

2. Development of estimation technology for photovoltaic systems
Using meteorological satellite observation technology, we are developing a method for estimating and understanding the power output of photovoltaic systems with high temporal and spatial resolution. Figure 5 shows an example of the estimation of photovoltaic output using the geostationary meteorological satellite Himawari 8 and 9 for the Kyushu Electric Power Company area (left: estimated solar power generation by municipality, right: mapping of ground solar radiation estimated from the meteorological satellite). This study has made it possible to simply and economically estimate and understand the power output with a temporal and spatial resolution of 2.5 minute intervals for each municipality, without having to collect detailed data from power generation facilities.
 

 
Figure3:Estimating solar power output by municipality in the Kyushu Electric Power Company area using meteorological satellites (April 15, 2016, 10am) 
  
3. Development of prediction technology for solar power generation
We are conducting research and development of a method for detecting major deviations in solar power generation forecasts in advance based on weather forecasting technology.


Figure4: Japan devises index to detect when solar radiation forecasts are significantly off (29 June 2018)
 (a) Prediction error by day and (b) the large error detection index developed in this study. 

4. Development of power equalized circuits
We have developed a circuit that automatically shapes the current-voltage characteristics caused by variations in output characteristics in the shade of a building or on a curved surface. This reduces the circuit mismatch loss and improves the output performance.

Team Member

Title Name
Leader Oozeki Takashi
Senior Researcher Kato Kazuhiko
Senior Researcher Takashima Takumi
Senior Researcher Ohtake Hideaki
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