Effects of Structures in the Urban Environment on Solar Irradiance Availability

Solar radiation is becoming an ever more significant resource to be used by renewable energy technologies, such as rooftop photovoltaic arrays or solar thermal collectors, allowing power production in urban environments without any pollutant emission. The study of solar radiation in urban environments is also crucial for understanding the its impacts on transport of heat and contaminants in the urban boundary layer.

The present study aims to quantify the role of urban morphologic city elements on the availability of solar irradiance in urban environments by developing a general parameterization that can be applied in any city with minimal inputs at a very small computational cost. We hypothesize that latitude, ratio between rooftops areas and the total plan area of the city (plan area fraction), average building height and standard deviation of buildings heights govern solar availability of an entire city. Shadowing effects due to surrounding buildings may considerably reduce the amount of radiation reaching building rooftops in a city area.

As a first step, two different types of city models are utilized to develop a parameterization. The first type is a "European-type" city, where buildings are characterized by a homogeneous height across the city domain, therefore the standard deviation of buildings heights is low. The second type is an "North American-type" city, where skyscrapers typically characterize the downtown and lower buildings, such as one or two floors detached houses, characterize the outskirt. The standard deviations of buildings heights for this type of cities are typically high.

To mitigate the computational expense of running entire city detailed radiation calculations, the building-resolving radiation transfer model "QES Radiant", presented by Overby et al. in 2015, was employed. Thanks to graphics processing units, it uses accelerated ray tracing techniques to simulate balances of solar radiations for millions of patches over surfaces equivalent to whole cities domains. Uniquely, this allows this study to not only include incident direct shortwave solar radiation on rooftops but also potential reflected shortwave radiation, emitted longwave radiation from buildings and the sky, and reflected longwave radiation.

REFERENCE: Overby, Matthew, et al. "A rapid and scalable radiation transfer model for complex urban domains." Urban Climate 15 (2016): 25-44.