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founded in 1954 and is now incorporated as a non-pro t educational and scienti c institution. With participation encompassing 100 countries, ISES® serves as a centre for information on research and development in solar energy utilisation. Through its publications and its sponsorship of technical conferences, the Society provides a world forum for the active consideration of solar energy.

This study aims to improve students' critical thinking skills through the application of learning materials with the PBL-SETS method. They were developed with the 4D model. This research is pre-experimental used one group pretest and posttest design. Data collection methods used validation, observation, and test. The data analysis technique used is the paired sample T-test. The learning materials have a very valid category and it can be used to improve critical thinking skills. The average critical thinking post-test score is 84 with an N-Gain of 0.7 which categorize as medium, while the overall student completeness reached 87%. The results of data analysis show that t-count = -21,967 and t-table = 1,693 (at df 32 and the significance level of 0.05). Research shows that the use of PBL with the SETS method affects increasing critical thinking skills. The responses of students towards learning by 88% (very good). Based on the results of this study it can be concluded that learning materials of the PBL model with the SETS method can improve the students’ critical thinking skill on fluid dynamic topic. PBL with SETS learning is expected to be able to make students more actively in the process of problem solving directly so that students are easier to develop critical thinking skill. From the indicator of critical thinking skills, students can more easily understand the concept and develop their critical thinking skill.

A solar air heater (SAH) is a simple heater using solar radiation that is useful for drying or space heating. Unfortunately, heat transfer from the absorber plate to the air inside the solar air heater is low. Some researchers reported that obstacles are able to improve the heat transfer in a flat plate solar air collector and others found that a v-corrugated absorber plate gives better heat transfer than a flat plate. Yet, no work of combining these two findings is found.This paper describes the result of experimental study on a SAH with v-corrugated absorber plate and obstacles bent vertically started from 80oto 0owith interval 10oon its bottom plate. Experiments were conducted indoor at five different Reynolds numbers (1447 Re 7237) and three different radiation intensities (430, 573, and 716 W/m2).It is found that the obstacles improve SAH performance. Both the air temperature rise and efficiency increase with inserting obstacles bent at any angle vertically. Unfortunately, the air pressure drop is increasing, too. Obstacles bent vertically at smaller angle (means more straight) give higher air temperature rise and efficiency. However, the optimum angle is found 30o. The air temperature rise and efficiency will be 5.3% lower when the obstacles bent 30oinstead of 0o, but the pressure drop will be 17.2% lower.