Optimal design of a solar water heating system for multiple purposes in low energy buildings

This paper presents a systematic methodology for optimizing the key design parameters of a solar water heating (SWH) system with multiple heating purposes in low energy buildings. The methodology is achieved through two steps. In the first step, a simplified energy model of the SWH system is developed and used to estimate system dynamic characteristics and annual energy performance, which is implemented in a spreadsheet. The behavior of the system with different collector areas and storage volumes can be determined realistically through matching multiple heating loads with the solar heat gains. In the second step, the law of diminishing marginal utility is employed to optimize the sizes of the system in order to maximize the life cycle net energy saving. The law describes that the marginal energy saving of the system decreases with the increase of the system size. Therefore, the optimum values of the system size can be determined in an easy-to-understand way, i.e.: the optimization objective (the maximal net energy saving of a SWH system) is achieved when the marginal operating energy saving equals to the marginal embodied energy. A case study on the application of the proposed method in a low-energy building is presented as well.