دانلود رایگان مقاله انگلیسی تعیین ضریب انتقال حرارت در سیستم های ذخیره سازی حرارت ناپایدار تماس مستقیم - الزویر 2018

Abstract

This study presents a novel concept for a direct contact latent heat storage, which can be used for storing thermal energy from industrial waste heat or renewable energy systems. A storage medium, phase change material (PCM), and mineral oil heat transfer fluid (HTF) are placed in direct contact. The PCM and the oil are not miscible. The direct contact leads to an improved heat transfer and faster loading and unloading periods with a high storage density. Only affordable salt hydrates or mixtures of salt hydrates and additives are used as the PCM. For the scaling of the system, the heat transfer coefficient of the complex system must be known. A cylindrical channel inside the PCM is created, and the heat transfer coefficients between the oil and solid or melting PCM is investigated. Subsequently, the transmission in a real channel is described and discussed.

Conclusion

In this study, the heat transfer coefficient of solid and melted PCM was experimentally investigated in a direct contact latent heat storage consisting of a vertical cylindrical storage tank with a salt hydrate as the PCM and a mineral oil as the HTF. The results show the effect of the flow rate on the heat transfer coefficient (for solid PCM). Furthermore, the study shows the effect of an additional liquid PCM phase on the heat transfer coefficient by melting the PCM. From these observations, different types of heat transfer phenomena can be explained. The conclusions are summarized as follows:

1. When melting the PCM, the diameter of the channel increases. Liquid PCM partially accumulates on top of the solid PCM. Another part of the PCM forms a liquid PCM layer at the solid PCM wall. The channel diameter increases until a critical diameter is reached, at which point the liquid PCM falls down into the channel, and the channel becomes filled with liquid PCM. At this time, a different type of heat transfer occurs. Oil droplets rise up through the liquid PCM.

2. While melting the PCM, the heat transfer coefficient increases with the time respective to the proportion of liquid PCM until the point at which liquid PCM falls down into the channel.

3. The heat transfer coefficient of solid PCM is nearly constant at approximately 87 W/m²K for all oil mass flows.