Development of a model for calculating global spectral solar irradiance under all-sky conditions in Thailand

Abstract

This thesis presents a model for computing global spectral solar irradiance under all-sky conditions in Thailand. The model expresses the global spectral irradiance under all- sky conditions as a multiplication of two functions, namely a function of global spectral irradiance under clear sky conditions and a cloud modification function. To create the model, global spectral solar irradiance was measured at four stations in main regions of Thailand, namely Chiang Mai (18.77˚ N, 98.97˚ E) in the northern region of Thailand, Ubon Ratchathani (15.25˚ N, 104.87˚ E) in the north-eastern region of the country, Nakhon Pathom (13.82˚ N, 100.04˚ E) in the central region of this country, and Songkhla (7.18˚ N, 100.60˚ E) in the southern region of Thailand. The spectral data from these stations were collected and then separated into two groups. The first group (January, 2017- December, 2020) was employed for modeling and the second group (January- December, 2021) for validating the model. The first function was constructed using the first group of data. Clear sky conditions were identified by using sky images obtained from a sky camera installed at each station. To build the second function, satellite-derived cloud index obtained from Himawari-8 image data at the stations was used. To validate the model of the global spectral solar irradiance under all-sky conditions, the model was used to compute the global spectral solar irradiance at the four stations for the year 2021 and the result was compared to the global spectral solar irradiance obtained from measurements at the four stations. It was found that the spectral irradiance calculated from the model agreed well with those obtained from the measurements with the discrepancy in terms of root mean square difference relative to the mean measured value (RMSD) of 9.48%. We concluded that the model performed well in calculating global spectral solar irradiance.

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