Solar ETC Type Water Heaters – An Analysis Based on CFD Packages

Objectives: A comparative numerical analysis is conducted, to emphasize the capability of an open source CFD package (OpenFOAM) over the commercial CFD package of ANSYS. Methods/Analysis: Buoyancy driven flows occurring within the vacuum tubes of gravity-assisted solar water heaters are analyzed. A scaled model of the ETC type solar water heater has been chosen for computation. The geometry and meshing of the model was done by using the 'Design Modeler' and 'ICEM-CFD' packages available in ANSYS. The same configuration was also modeled in OpenFOAM to account for 3-D, transient, incompressible, laminar fluid flow analyses analogous to the available models in FLUENT. Findings: Boussinesq approximation is considered to be valid. Governing equations were discretized based on Finite Volume Method (FVM). The significance of the inlet velocity and varying incident solar radiation on the nature of flow and performance of the collector is examined. Mass flow rates have been varied from 0.0002 to 0.03 kg/s while the solar insolation considered was in the range of 300 to 1000 W/m². For a uniform heat flux, with decrease in velocity of the fluid entering the storage tank, temperature of the water obtained at the outlet was found to be higher. This is due to the prolonged interaction of the fluid with the tube walls, which facilitates a higher heat gain from the tube surface to the fluid. Similarly, as the heat flux increases, the magnitude of the tube surface temperature increases which results into a higher outlet temperature. The performance of ETC type solar water heaters is said to be more efficient at lower flow rates and the same is true at higher incident solar radiations. Applications/Improvements: Comparison of the results between OpenFOAM and FLUENT shows the wide reliability and urges the use of open source software like OpenFOAM for more complicated CFD modeling and computational studies.