작동유체가 양방향성 태양열 열다이오드의 열성능에 미치는 영향 평가

Abstract

An excessive use of fossil fuel leads to the limitation of coal deposits and carbon dioxide accumulation that accelerates the global warming, so the international environment regulation becomes more strict to control the greenhouse gas emission. Many researches are being made on alternative energy development to cut down fossil fuel and to decrease carbon dioxide. During the last decade, there have been active tries to utilize the solar energy that is unlimited and clean . The application of solar energy to heating and cooling of the building has much improved the economical efficiency and function with the development of high-technology materials, and it is not rare to find the recently built houses and commercial buildings equipped with solar energy systems. In particular, the advanced countries such as USA and Japan attempt the remarkable reduction of energy consumption in heating and cooling of buildings. For this, they are searching for the more effective application of various alternative energies including the solar energy. In addition, they are trying to realize the distinct zero energy conception by applying the new techniques and materials to the existing buildings. In developing the new techniques of solar energy application to heating and cooling system, the economic problems hire to be addressed, The typical problems encountered in applying the solar energy are insufficient design concept for HVAC(heating, ventilation, and air conditioning) system and low reliability, Further, the economical efficiency of the solar energy is still low to compete with the oil, and there are many limits on the realization of the actual system in a building, e.g. spatial alignments and exterior appearances. The purpose of this study is to find the improved method to increase the heat transfer efficiency of the solar energy system that are to be installed in houses and commercial buildings. For this, a series of experiments using the bi-directional thermodiode system (Smart Module System) were made. Six kinds of working fluids were used to investigate their effects on the thermal performance of a bi-directional solar thermodiode. Two kinds of thermodiodes were studied. The first one is the mono-directional thermodiode that allows heat flow in the desired direction but blocks in the opposite direction. The second one is the bi-directional thermodiode of which the direction of heat flow, surface absorptivity and heat capacity of the module can be adjusted for maximum energy efficiency. This hi-directional can be used both for the summer cooling and winter heating of buildings and shelters. Usually, the thermodiodes are simple beam shape , but in this study, they were redesigned _as two L-shaped loops mounted between a collector plate and a Storage tank. Rotable joints between the horizontal and inclined segments of the loop enable easy alteration of geat tranfer direction. The loops and tank were filled with a working fluid for effective heat transfer when the solar thermodiode was forwarded biased. The solar thermodiode was heated by a radiant heater that consisted of 20 halogen lamps that generates a heat flux of about 1000W/m² on the collector surface. The working fluids used in the study were water, acetone, ethylalcohol. In addition, three kinds of silicon oil with different viscosity were studied. And three mixtures of water and ethylalcohol of different volume ratio were used. Finally, the nano fluids were also studied. Working fluids were tested with thermal conductivity values ranging from 0.1 to 0.56 W/m-℃, thermal expansion coefficient values ranging from 1.8 × 10^(-4) to 1.3 × 10^(-3) K^(-1), and kinematic viscosity values ranging from 0.65 × 10^(-6) to 100 × 10^(-6) ㎡/s. Through the study, it was found that the circulation point(CP) at the onset of fluid flow is very important. for a given working fluid, the heat transfer and heated stability of the system depends strongly on the circulation point of the fluid stream. Also, the circulation point are different each other for different working fluids. In summary, the circulation point is the function of working fluid, geometrical characteristics of the thermodi_odes and flow conditions such as flow velocity and flow rate. The earlier occurrence of circulation point at a lower temperature leads the stable distribution of the temperature. With this respect, the working fluid of water-ethylalcohol mixture whose volume ratio is 2 to 8 were found to yield the most stable heat transfer.