# Experimental Study on Heat Transfer Performance of Vacuum Tube Heat Collector with Thermal Storage
## 2 The Construction of the Experimental Platform  
### Development of Energy Storage Vacuum Tube Collector  
The U-type copper tubes with 10 mm internal diameter and 12 mm external diameter are used here as the main heat exchanger components. The cylindrical aluminum fins are uniformly distributed around the tubes radially. The fin thickness is 4 mm and the space between fins is 40 mm. The U-type tube and aluminum fins are connected by welding. The pressure leak detection is done by imposing 1 MPa oxygen on the U tube. The processing diagram of U copper tube and fin is shown in Figure 1.  
Since the potential use of PCMs in various thermal storage applications is quite limited by their cost, it is very important to choose the appropriate PCM heat storage material in the vacuum tube. According to the existing research, fully refined No. 58 paraffin is the outstanding thermal storage material. The paraffin entity is shown in Figure 2 and the thermo-physical properties of paraffin are listed in Table 1.  
The amount of paraffin to be inserted and the packaging method are of vital importance. As there is only 5 mm between the outer edge of the tube's radial fin and the inner diameter of the vacuum glass tube, it is difficult to ensure that the paraffin can be instilled to the bottom of the vacuum tube. It is also difficult to ensure that the paraffin can completely occupy the space between the fins and the vacuum tubes. In light of this problem, an unconventional method has been proposed to ensure the filling amount of paraffin wax. In this experiment, the paraffin particles are first heated and melted into liquid. Then, the paraffin is placed in a metal container for cooling and solidification. After this step, the wax is injected into the gap between the fins when it has not cured completely, as is shown in Figure 3. In the next step, the U-tube filled with paraffin is placed into the glass vacuum tube and the energy storage vacuum tube collector is placed outdoors to absorb solar radiation until the paraffin melts and takes up the space between the inner surface of the U tube and the glass vacuum tube. Finally, the liquid paraffin is added into the vacuum tube until the glass tube is closed. Before filling the tube with paraffin, the mass of the single vacuum glass tube measured is 8.80 kg, and the mass of  
\begin{table}
\begin{tabular}{c c c} \hline
**No.** & **Physical Properties** & **Unit** & **Value** \\ \hline
1 & Specific heat C & J/(kg\(\cdot\)K) & 3350 (solid)/2700 (liquid) \\
2 & Thermal conductivity \(k\) & W/(m\(\cdot\)K) & 0.35 (solid)/0.15 (liquid) \\
3 & Density \(\rho\) & kg/m\({}^{3}\) & 920 (solid)/795 (liquid) \\
4 & Latent heat \(H\) & kJ/kg & 158.2 \\
5 & Phase-transition temperature & \({}^{\circ}\)C & 55–61 \\ \hline \end{tabular}
\end{table}
Table 1: The thermo-physical properties of paraffin.  
Figure 1: U type tube and fin profile.  
Figure 2: Fully refined No. 58 paraffin granules.  
the U tube with metal fins is 4.80 kg. After filling the tube with paraffin, the whole mass of the vacuum tube is 22.6 kg. Thus, the mass of paraffin actually inserted is 9 kg.  
Another point should be stressed: the open end of the vacuum tube needs to be sealed and insulated. In this experiment, the Expandable Polyethylene (EPE) pearl cotton is used as package material. The EPE pearl cotton is made up of a large number of independent bubbles produced by physical foaming of low density polyethylene. Due to the makeup of EPE pearl cotton, it has the following characteristics: moisture proof, heat preservation, good plasticity, strong flexibility, chemical resistance and flame retardant. Finally, the rubber materials are placed outside the EPE pearl cotton for insulation.