# Experimental Study on Heat Transfer Performance of Vacuum Tube Heat Collector with Thermal Storage
## 1 Introduction  
The vacuum tube collector is effective in low temperature solar energy applications. However, due to natural conditions such as weather and other random factors, the available solar energy is extremely unstable. In order to overcome the disadvantages as noted above, it is necessary to put forward a suitable solution regarding the energy storage conditions such that the radiant energy from the sun is able to be utilized in the evening or on rainy days. When considering the filling material used to elevate the heat storage capacity, the author has chosen paraffin as the phase change material (PCM) which possesses many advantages. For example, paraffin excludes the super cooling phenomenon; chemical reactions and corrosion are less likely to occur in paraffin; the price of paraffin is low and so on. However, paraffin has a significant disadvantage--its thermal conductivity is low. Therefore, great attention has been paid to the search for an effective method to enhance the heat transfer of paraffin. In recent years, various methods have been proposed to enhance the heat transfer in the latent heat thermal store system. Due to the development of nanotechnology, many researchers have tried to prepare high-performance composite PCMs by adding nanoparticles. [PERSON] et al. [1] analyzed the high heat release rate of nanoparticle-enhanced PCMs by the CFD method. [PERSON] et al. [2] studied the thermal conductivity of the mixture of pure paraffin wax and carbon nanofibers. The utilization of heat pipe and thermosiphons has also been given attention. [PERSON] et al. [3] studied the effects of heat pipe spacing, fin length and numbers and the influence of natural convection on the thermal response of the thermal energy storage unit. [PERSON] et al. [4] used a novel heat pipe-metal foil approach to enhance the heat transfer progress of a PCM (99% pure n-octadecane with a melting temperature of 28 \({}^{\circ}\)C). Besides, the application of finned tubes with different configurations hasbeen studied by various researchers [5; 6; 7; 8; 9; 10]. [PERSON] et al. [5] have studied the heat transfer effect of adding metal fins in the energy storage system filled with paraffin. They placed the heat storage equipment with fins between two surfaces with constant temperature to form the heat transfer model. By measuring and photoing the temperature of the melting zone, they found that the presence of fins could provide additional heat conduction paths and enhance the natural convection effect in the melting transition zone. [PERSON] et al. [10] analyzed how the circular annulus enhanced the heat transfer in the heat storage system. They did parameter analysis on two cases in one of which the entire casting ring was arranged with rectangular fins and in the other the inner surface of the circular was arranged with round fins. From their experiment, they obtained the formula for calculating the volume fraction of the PCM at any time. Other heat transfer enhancement techniques have been reported [11; 12; 13; 14; 15]. For instance, foam metals are attracting growing attention. [PERSON] et al. [11] studied the heat transfer characteristics of a composite phase change material composed of paraffin and metal foam. In their study, the composite PCM was fabricated by using copper foam to enhance the thermal conductivity of paraffin. [PERSON] et al. [12] designed an experiment to investigate the dynamic thermal behavior of a shell-and-tube latent heat thermal storage unit. Their experimental results indicated that the influence of heat transfer fluid (HTF) temperature on the charging process is more significant than that of the HTF flow rate. [PERSON] et al. [13; 14] also studied the charging and discharging process of a shell-tube latent heat storage system. They found that the performance of the latent thermal energy storage system was affected notably by the types of PCMs, HTF temperatures and flow rates. [PERSON] and [PERSON] [15] investigated the effect of metal foams and PCM on the heat transfer performance of a heat sink.