「夜間放射冷却」による建物の冷房に関する基礎研究(第1報) : 空気式夜間放射冷却器の試作・実験及び解析

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The phenomenon, that the surface temperature of the ground and the air near by goes down far below the outdoor air temperature, as the result of radiation exchange between ground and atmosphere is well known as "nocturnal radiative cooling". The purpose of this study is to reveal the performances of nocturnal radiative coolers and thermal storage systems for this natural space cooling system through experiment and theoretical analyses. This paper as the first report of the study describes the general cooling theory of the nocturnal radiative coolers, similar to the solar air collectors, and the results of the outdoor cooling experiments using three different test-made nocturnal radiative coolers (denote "radiative cooler" hereafter). Three radiative coolers, tested are of "go through" type, through which the induced outdoor air flows to the room or to the gravel coolness storage tank. The first radiative cooler is a "fore-side cooled" type, in which the induced air, cooling medium, is cooled down on the surface of rdiative plate, plate, passing through between the plate and cover film, which is transparent to the long-wave radiation. The second one is a "rear-side cooled" type, in which the air is cooled on the rear side of radiative plate, exposed to the outdoor air without cover film, and the last one is also a "rear-side cooled" type, but which has transparent film to prevent convective heat gain. The theory and caluculation method of cooling and dehumidificating processes for those radiative coolers were made at first and applied to access the optimum structures and flow rate for test modules. Three test-made radiative coolers were experimented to evaluate the cooling performances simultaneously on the roof of our laboratory building of Shonan Campus of Tokai University in summer of 1984. Through experiments of 19 nights (Aug.12-Sept.9. except unavailable data by typhoon and air flow change), it was found that the average flux of atmospheric radiation was abont 330kcal/m^2h, and the average available temperature drops were 2.8deg℃ (No.3), 2.5deg℃ (No.1) and 2.2deg℃ (No.2) at the air flow rate of 18m^3/hm^2. But the average total cooling power, including latent heat (condensation) reached 238.5kcal/m^2 day for No.l, 189.7kcal/m^2 day (No.3), 172.5kcal/m^2 day (No.2) respectively. After confirming the coincidence of the measured temperature, cooling power data and calculated data using above mentioned theory, general cooling performance of the radiative coolers were evaluated under wide range variety of weather conditions, which we could not encounter in our experiments. As the result of the calculation, followings were pointed out. The available temperature drop and total cooling power decreased significantly as the atmospheric radiation increases (i.e., effective radiation exchange decreases) for all three coolers, but did not decrease so much even if the outdoor air veocity increases, for covered coolers No.l, No.3. Also the total cooling power was significantly improved, as the absolute humidity and/or temperature get higher. In addition, it was found that the coefficient of performance (COP) of these radiative coolers, which needed only power input for circulating fans, could be expected to be about 1.5 times (No.2), 3 times (No.3) larger than that for electric air conditioners. In conclusion, through this experimental study, useful knowledge to apply these system in practice was attained.
東海大学の論文

「夜間放射冷却」による建物の冷房に関する基礎研究(第1報) : 空気式夜間放射冷却器の試作・実験及び解析

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