A Comparative Study of Solar Heat Transfer on Roof-Top Water Storage Tank Orientations in Saudi Arabia

The effect of tank orientation on heat transfer of a typical roof-top water storage tank is examined in this paper. The theoretical study is based on scale analysis of the governing equation that is used to evaluate the comfort time of both vertical and horizontal water storage tanks. The scale analysis is validated numerically by solving the resulting energy equation using an explicit numerical technique based on two programs written in the MATLAB software for each tank orientation. Different water tank capacities are examined for both horizontal and vertical orientations. Data for commercially available water storage tanks are used in the calculations. Three parameters play an important role in the evaluation process of the comfort time ratio. These are (1) the tank capacity, (2) the tank characteristic length, and (3) the tank surface area. It is shown that the heat transfer coefficient associated with water inside the tank is much higher than the outside air heat transfer coefficient around the body of the tank. The simplicity of the proposed method will allow the user in quick decision making as to which water storage tank orientation would provide longer comfort time. For example, in a vertical tank with a capacity of 1000 L, the water will heat up slowly and from 11:00 AM onwards, it will approximately need 3.7 h to reach the temperature of the environment. On the other hand, in a horizontal tank of the same capacity, the water will heat up much quickly and it will take only about 0.9 h for water to reach the surrounding environment temperature. Eventually, this work will contribute to less water evaporation from these tanks, thus saving water that is essential to water starved desert climates such as in the Kingdom of Saudi Arabia.