Selection between single-phase and two-phase evacuated-tube solar water heaters in different climate zones of China

• Evacuated-tube solar water heaters are evaluated.
• Numerical models of single-phase and two-phase systems are developed.
• Energy and cost performance for applications in China are compared.
• Single-phase system is found more economical.
• Two-phase system is better in environmental performance.

Evacuated-tube (ET) solar water heaters are popular in China. Both single-phase and two-phase thermosyphon ET collector systems are available in the market. However, the system energy and economic performance has not been evaluated systematically. In this paper, the numerical evaluation of these two types of solar water heaters in different climate zones of China was reported on the aspects of system energy performance and economical return. The single-phase system is found having better cost payback period in all these cities, and can be a suitable choice except for locations with extreme cold weather. The two-phase design is an attractive and sustainable alternative since its thermal efficiency is higher under the same weather condition and this is important for supporting zero-carbon building design. On the other hand, it can be readily used to enhance the outside appearance of a building. Hence when considering climate adaptability and aesthetic compatibility, the potential of the two-phase system is higher.