A Simplified Estimating Model For In-Plane Irradiation Using Minute Horizontal Irradiation Hiroyuki Unozawa*, Kenji Otani **and Kosuke Kurokawa* *Tokyo University of Agriculture and Technology Naka-cho, Koganei, Tokyo, 305 Japan/ phone:+81-423-88-7132/fax:+81-423-85-6729 **Energy Technology Division, Electrotechnical Laboratory (ETL) Umezono, Tｓ ukuba, Ibaraki 305-8568 Japan/ phone:+81-298-54-5152/fax:+81-298-54-5829 ABSTRACT To develop a new formula for calculating irradiation onto an arbitrary orientation-inclination plane, detailed examinations have been made by using data obtained at every one minute interval with four types of pyranometer, such as horizontal, diffuse, direct normal, and tilt global. Instantaneous irradiance values sampled with 1 min. interval have been verified by clearness index, which is a parameter independent from the lapse of time. Scattered tendencies observed on direct and diffused com- ponent ratios have been studied in detail by classifying data to different classes of clearness indices. In our model, moving averages of time-series irradiance data provide the information of weather condition in order to select model parameters prepared for assumed three types of clouds con- ditions. Only global irradiance data monitored at one minute intervals are used for the input of the model. The result of this work demonstrates that an algorithm associated with Perez model can estimate the in-plane irra- diation with a RMSE of about 50W/m 2 . 1. Introduction For analyzing the performance of photovoltaic systems ac- curately, it is essential to obtain the in-plane irradiation onto PV array. However, some systems lack costly pyranometers enough to monitor the in-plane irradiation. For such systems, it is necessary to substitute conver- sional irradiance data for actually measured data. By the appearance of building integrated module(BIPV) recently, it is considered that these necessity will increase. In order to estimate in-plane irradiation, it is nec- essary to separate global irradiation into two components, such as diffuse irradiation and direct irradiation. After such separation process, in-plane irradiation is calculated by composing them. Some models for estimating the diffuse fraction of global irradiation using clearness indices, which are defined as the ratio of the global irradiation to horizontal extraterrestrial irradiation , have been already proposed by many authors and are often used. However, almost such models naturally have widespread errors in their estimation in medium irradiation conditions. Japan Weather Asso- ciation (JWA) have improved their model by adding sunshine duration time data into the model in order to enhance the accuracy of the estima- tion in the medium irradiation conditions[1]. In this study, the authors propose a simplified model for estimating the diffuse fraction, which uses time series of minutely global irradiation data instead of the sunshine duration time data. 2. Pyranometers One-minute sampled data have been obtained with the measurement facilities installed on the top of a building at Koganei Cam- pus. Four types of irradiance are obtained at every 1 minute intervals, i.e., horizontal global irradiance, horizontal diffuse irradiation, normal direct irradiance, in-plane irradiation at an incline of 35 degrees. 3. Model for Estimating Diffuse Irradiation Analysis for distribution of Diffuse Component Ratio 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Fig.1 Relationship between diffused component ratio and clearness index (January 1998) Figure 1 illustrates a relationship between diffuse compo- nent ratios and instantaneous clearness indices. In the study, instantaneous time-series data were used. A general tendency of them is classified to 3 Diffuse Component Ratio Clearness Index