The solar resource is the average quantity of solar energy that a specific site (or a large area) receives during a specified period, such as a year. The larger the solar resource, the more energy will be produced by any solar system. The difficulty that all solar system designers face is to obtain reliable and accurate solar resource data for a specific site under scrutiny, or a specific area in the world. Furthermore, the resource is not constant from day to day, and also varies seasonally and from one year to the next. Finally, diferent types of solar collectors use different components of the resource. For intance, fixed flat-plate collectors use global radiation (the sum of direct radiation from the sun and of diffuse radiation from the sky and ground reflections), whereas concentrating collectors use only direct radiation. The resources corresponding to a fixed tilted collector, a vertical wall, or a tracking collector are all different, and are not the same as the radiation incident on a horizontal surface. All these geometrical characteristics complicate the evaluation of the solar resource potentially usable by a specific collector. Solar resource measurements are scarce in the world, so that, in most cases, one must rely on modeled data. No model is perfect (and no measurement either!), so that one also has to evaluate the uncertainty in solar resource data. All this delicate process is conveniently described as "Solar Resource Assessment". Some sources of data are indicated in the Links section of this website.
For instance, the map below, which shows the annual daily-average direct normal irradiance (or "DNI") for the world, is redrawn (for clarity) from the information provided by NASA's POWER-SSE methodology.
Sources of modeled or measured data have increased in recent years. Some are free to use, but most are proprietary and commercial in nature. The fact is that they all use different models and different input data, or different measurement methods, thus generating significantly different results at any given site. These differences are sometimes so large that the economic feasibility of a project may become either exaggerated or highly improbable. This creates confusion, and may slow down the development of solar energy projects, due to excessive uncertainty.
To prevent such problems, we provide expert assistance to project designers, by carefully selecting for them the best possible estimates of the solar resource at a specific site and for a specific type of solar system.