A Greenhouse type solar water heater has been built to evaluate the thermal performance of the new system and to extend the application field of thermosyphon solar heating system. The system consists of flat plate absorbers, greenhouse, water storage tanks and connection pipes. The greenhouse has dimensions of 34.6m in length and 9.2m in width. Ten cylindrical tanks of 0.5m in diameter and 1.5m in length and the flat plate absorbers with total area of 138m2 are connected in parallel. The roof and south wall of the greenhouse are made from polycarbonate which has high light transmission and low heat transmission and other walls are from Sandwiched color steel plates which are thermally insulated from the ambient environment. The water storage tanks, the flat plate absorbers and the connection pipes, which need no other insulation because of greenhouse effect, are placed in the greenhouse. The experimental data includes solar radiation, wind speed, ambient temperature, greenhouse air temperature, storage water temperature, and absorber temperature et al. To investigate new system performance, it is necessary to develop mathematical model which is different from models of traditional thermosyphon solar water heater because the new system combines natural circulation system and greenhouse technology. The transient thermal model of greenhouse type solar water heater is derived, which is based on energy balance of the flat plate absorbers, the floor, the air in the greenhouse, the roof, and the storage tank. Greenhouse air temperature and storage water temperature are predicted and mass flow rate is calculated according to the balance between the frictional pressure drop and pressure due to density differences around the thermosyphon circuit. It shows that the experimental data fit well with the estimated values according to the new mathematical models.

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