Optimum fixed orientations and benefits of tracking for capturing solar radiation in the continental United States

Optimum fixed orientations and benefits of tracking for capturing solar radiation in the continental United States Matthew Lave and Jan Kleissl Department of Mechanical and Aerospace Engineering University of California, San Diego Abstract Optimum tilt and azimuth angles for solar panels were calculated for a grid of 0.1⁰ by 0.1⁰ National Solar Radiation Data Base (NSRDB-SUNY) cells covering the continental United States. The average global irradiation incident on a panel at this optimum orientation over one year was also calculated, and was compared to the solar radiation received by a flat horizontal panel and a 2-axis tracking panel. Optimum tilt and azimuth angles varied by up to 10⁰ from the rule of thumb of latitude tilt and due south azimuth, especially in coastal areas, Florida, Texas, New Mexico, and Colorado. Compared to global horizontal irradiation, irradiation at optimum fixed tilt increased with increasing latitude and by 10% to 25% per year. Irradiation incident on a 2-axis tracking panel in one year was 25% to 45% higher than irradiation received by a panel at optimum fixed orientation. The highest increases in tracking irradiation were seen in the southwestern states, where irradiation was already large, leading to annual irradiation of over 3.4 MWh m -2 . 1. Introduction Solar photovoltaic (PV) systems are quickly gaining popularity in the United States (U.S.), thanks to incentive programs and enhanced interest in environmental sustainability and energy independence. As more PV systems are installed across the U.S., it becomes increasingly important to maximize their power output. Aside from increasing a panel’s solar conversion efficiency, power output can be increased by considering the solar geometry as well as the seasonal and daily variation of atmospheric transmissivity at a particular site. Specifically, it is important to know what the optimum tilt and azimuth angles are at which to mount a fixed tilt panel on a flat roof or on the ground such that it receives maximum irradiation. In addition, knowing the increase in solar radiation incident on a two-axis tracking panel will allow analysis of the economics of tracking PV systems, which are more expensive to install and maintain. Since the power production of a PV panel is close to linearly proportional to the amount of solar radiation (photons) reaching the panel surface, incident irradiation is an excellent proxy for power output. To maximize absorption of solar radiation in clear skies, the normal to the plane of the PV panel should be pointing towards the sun such that the solar direct beam is perpendicular to the panel surface. While a fixed tilt panel can only be normal to the incident sunlight once a day, a two-axis tracking panel improves over a fixed tilt panel by following the sun through the sky such that the plane of array normal is always parallel to the incident sunlight. However, when the majority of global irradiance is diffuse, horizontal alignment often provides the maximum global irradiance [1]. Some previous studies used modeled extraterrestrial radiation incident on the top of the atmosphere to find equations for optimum tilt over a large area [2,3]. This method accounts for the deterministic (celestial) variables which affect solar radiation, but it does not consider the stochastic (clouds and other weather) variables which also affect the optimum angles. Using an extraterrestrial