A high-vacuum solar collector system with flow boiling in tube has high collector efficiency of solar energy. In this paper flow boiling heat transfer coefficient in tube was measured by changing mass flow rate (30–80 kg/m2s), heat flux (5–30 kW/m2) and inclined angle of collector plate. Inside diameter of tube is 4.4 mm, and saturation temperature is 100°C. Flow boiling heat transfer coefficient is about 8000 W/m2K and decreases at low flow rate. Effect of the inclined angle of collector plate is small. Experimental results of boiling heat transfer coefficients are similar to Sani’s correlation equation. The collector efficiency of vacuum solar collector systems with flow boiling in tube is analyzed and the efficiency is 69% at a standard calculation condition. There is 50°C temperature difference in the collector plate. Effects of the mass flow rate and the vacuum pressure on the efficiency are large. The efficiency decreases at high saturation temperature and at low solar radiation.

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