Impact Of Shading On Cooling And Heating Load

This paper explores the implications of shading the roof of a residence on the cooling and heating loads. This is done by simulating a thermal model of a hypothetical 2800 sq. ft house which takes into consideration direct and diffuse solar irradiation on its roof and walls. The simulation is performed for a home in the southwest portion of the United States, specifically, Phoenix. The simulation is performed twice: a) with half of the roof shaded with PV modules mounted on a rack, and b) with no PV modules. Even though the specific device used here to provide the shading is PV arrays, any other means would produce similar results. The dynamic thermal model of the house is implemented in TK-Solver. The simulation goes through every hour to determine the required cooling and heating load. It takes into consideration direct and diffuse solar irradiance. It uses the Sol-Air temperature approach to determine wall and roof temperatures. This approach modifies the outside skin temperature depending on direct and diffuse irradiation. Weather data for Phoenix, for the year 1990, is used in the simulation. The simulation results are used to perform an economic analysis, specifically, the impact on the payback period. The yearly cooling costs are reduced by $126, while the heating cost increases by $25. Thus the shading produced by the PV arrays has a net effect of reducing the yearly electric bill by approximately 11%. A major motivation for this study is to expose students to the tools and methodologies of modeling dynamic physical systems, especially in the energy area. Introduction In the southwest of the United States, cooling load accounts for a large portion of the total utility bill. Similarly PV arrays continue to play an increasing role in green or environment-friendly houses. This is the result of genuine concern for the environment, reduced pay-off periods caused by decreasing PV array prices and attractive government incentives. PV is particularly attractive in the southwest because of an overabundance of sunlight. Even though PV contribution is unquestioned, in real practice it has not achieved the popularity that some had expected. The main reason for this is cost, and the resulting long payoff period. Even after generous state and federal incentives, the payoff period remains around15 years. Shading has an obvious impact on Page 13695.2 Proceedings of the 2008 American Society for Engineering Education Annual Conference & Exposition Copyright @ 2008, American Society for Engineering Education the cooling load. Its effect has been investigated in many cases. For example Lam investigates the effect of surrounding buildings on producing shade [1]. This paper is also used as a teaching vehicle in the senior capstone class, in which the students are working on a design project that includes heat gain/losses in a small greenhouse, the use of PV modules to charge a battery and also drive a small, centrifugal pump. Thermal model It is important to use a model that has the ability to handle the heat storage properties of the building. For instance, if the night is particularly cold, the building model should reflect the coldness of the night and yield a building that is initially cold on the following day, one that will demand less cooling. The model schematic for the house used in this analysis is shown in Figure 1. It describes a thermal mass at temperature Ti surrounding by an insulating material, the entire house is then surrounded by an outside temperature To. Notice that the house would experiences no solar radiation on its south-facing roof if was covered with PV modules.