Numerical Study on Heat Transfer Performance of Vacuum Tube Solar Collector Integrated with Metal Foams

Vacuum tube solar collector for solar collection is one effective way to reduce heat loss. However, the low conductivity of pure water leads to poor heat transfer performance in vacuum tube. In order to enhance the heat-transfer in tube, a vacuum tube solar collector using water as medium and metal foams as filler is presented. The vacuum tube consists of outer glass tube, inner metal tube and metal foam filler. A two-temperature model is established due to different heat transfer properties between water and metal foams. The fluid is considered as Newtonian fluids and under Boussinesq approximation. The flow is laminar, following Darcy law and the viscous dissipation of fluid is ignored. A numerical model is proposed to describe the heating process of vacuum tube with metal foam filler. The heating process of vacuum tube with and without metal foam filler during the daytime is simulated. The temperature distribution of two kinds of vacuum tube collector is studied. The temperature vs. time and the heat transfer resistance in the tube are obtained. The results show that the temperature in tube with metal foams is higher and has higher speed of rise. The metal foams decrease the heat transfer resistance in tube. It is found that the metal foams play a great role in heat transfer enhancement for the vacuum tube solar collector. Compared with the traditional vacuum tube solar collector, the proposed vacuum tube solar collector has better thermal performance and greater potential in solar building integration.