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Transient Analysis and Performance Prediction of Nocturnal Radiative Cooling of a Building in Owerri, Nigeria

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A study aimed at a Transient analysis and performance prediction of passive cooling of a building using long wave nocturnal radiation in Owerri, Nigeria is presented. The system modeled consists of the room of a building with a radiator panel attached to its roof, water storage tank located inside the room, pump to circulate water through the radiator panel at night and through a heat exchanger in the room during the day. The mathematical model is based on the thermal radiation properties of the local atmosphere, the heat exchange equations of the radiator panel with the sky during the night and the equations incorporating the relevant heat transfers within the space to be cooled during the day. The resulting equations were transformed into explicit finite difference forms for easy implementation on a personal computer in MATLAB language. This numerical model permits the evaluation of the rate of heat removal from the water storage tank through the radiator panel surface area, Q_(wt,out), temperature depression between the ambient and room temperatures (T_(amb)-T_(rm)) and total heat gained by water in the storage tank from the space to be cooled through the action of the convector during the day, Q_(wt,in). The resulting rate of heat removal from the radiator gave a value of 57.6 W/m^2, temperature depression was predicted to within 1-1.5℃ and the rate of heat gain by the storage water was 60 W/m2. A sensitivity analysis of the system parameters to ±25% of the base case input values was carried out and the results given as a percentage variation of the above system performance parameters showed consistency to the base case results. An optimal scheme for the modeled 3.0×3.0×2.5 m^3 room showed a radiator area of 18.2 m^2, a convector area of 28.62 m^2 and a tank volume of 1.57 m^3.These results show that passive nocturnal cooling technique is a promising solution to the cooling needs for preservation of food and other agricultural produce. It is also useful in small office space cooling. Thus the model developed is undoubtedly a useful design tool for the development of passive cooling systems that can reduce considerably the huge cooling cost requirements of mechanical air conditioning systems.

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