ABSTRACT
Global horizontal irradiance (GHI) is the total solar ux available from the hemispheric sky dome that is incident on a horizontal surface. By convention global irradiance implies a hemispheric, 2π steradian, eld of view on any surface. GHI is the sum of the solar radiation coming directly from the solar disk, also called the direct normal irradiance (DNI) or “beam irradiance,” which is normal to the rays from the sun, projected (Equation 5.1) onto a horizontal plane and the solar radiation coming from all other directions of the sky dome, or “sky irradiance” (Figure 5.1). The solar radiation coming from all parts of the sky dome other than directly from the sun is called diffuse horizontal irradiance (DHI). Planar surfaces of any orientation receive solar radiation directly from the sun plus diffuse radiation from the sky and reected radiation from the ground. Historically, this has been termed total solar radiation on a tilted surface. Those involved with photovoltaic systems refer to this total irradiance as plane of array (POA) irradiance. Studies of the thermal performance of buildings rely on the amount of solar irradiance on various parts of a building envelope and refer to this total irradiance as global irradiance on tilted surfaces. Traditionally, the study of nonconcentrating thermal collectors has also relied on global irradiance available to tilted at plate collectors. In this book global horizontal irradiance (GHI) will refer to the total solar irradiance on a horizontal surface. When discussing irradiance on a surface oriented at any angle other than horizontal, the irradiance will be referred to as global tilted irradiance GTI.
