Mathematical model development and optimal design of the horizontal all-glass evacuated tube solar collectors integrated with bottom mirror reflectors for solar energy harvesting

As one of the inexhaustible energy sources, solar energy as a means to provide space heating has been a public interest for decades. Many stand-alone solar thermal technologies have come into practice to replace the out-of-date systems. However, conventional solar thermal systems present two drawbacks: (1) unsteady solar sources can lead to insufficient heating in the winter, and (2) the solar collectors can become overheated in the summer. Therefore, this study proposes a conceptual design of an integrated solar harvesting unit that consists of the horizontal all-glass evacuated tube solar collectors and bottom mirror reflectors to overcome the above drawbacks to the largest extent possible. To accomplish this, a generic mathematical model of this design unit was developed, followed by the model validation process and optimal design analysis. For cities in the severe cold and cold climate zones of northern China, the bottom mirror reflectors can be regarded as solar energy collection boosters during the heating season, which can contribute solar energy ranging from 40% to 80% of the total collected solar energy depending on the inclined angles of the solar collectors and reflectors. In the summer, using such integrated unit with the solar collectors tilted at an obtuse angle, the absorbed solar radiation can be reduced by 20%, which is beneficial to overheating prevention.