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In the United States, major portions of the annual electrical and primary energy are consumed for buildings. To help reduce the energy consumption of non-renewable energy sources, this study investigates a new technology for harvesting solar energy using a boiling-condensing cycle with water in a solar collector. The fluid circulation is under natural forced convection. A solar collector is made of a black lacquer copper with 2 meters in length is used. The design of the system is presented to simulate heat transfer rate in a cold climate an average daily solar irradiation of 4.5kWh/m2/day, e.g. for Omaha Nebraska. The tube surface temperature is calculated based on specified ambient temperature and solar radiation, i.e. 302.6K. A constant tube surface temperature is considered in this simulation-based study. The solar collector will not freeze since this system uses the lower pressure inside the tube. The correlations for the heat transfer coefficients in non-boiling and boiling regions are presented. This study analyzes the heat transfer development of a single phase flow and two-phase flow boiling process in different regions such as subcooled flow region, saturated flow boiling region and vapor region in various pressures. This research investigates how to maximize heat transfer in a single vertical tube. The proper subcooled flow region in the overall tube length and also a specific pressure are optimized and estimated to calculate the maximum total heat transfer rate in a solar collector. It is concluded that the maximum heat transfer rate will be obtained when the vapor region is minimized and the subcooled and saturation region are maximized in overall tube length.