Irradiance maps from a shadow camera on a mountain range.

Spatially and temporally highly resolved direct normal irradiance (DNI) maps of the solar field can be used to significantly improve the operation of concentrating solar power (CSP) plants. Further improvements are expected if DNI nowcasts are also used for the plant operation. Currently, the most widely used method to create spatially resolved DNI nowcasts is based on all sky imagers. All sky imagers take photos of the whole sky above the CSP plant and detect clouds to derive the DNI map. Such all sky imager systems are promising, but for the calculation of DNI maps the cloud height and shape must be determined. Depending on the conditions, these processing steps can be error prone. Shadow cameras avoid these complex tasks. Mounted on an elevated position, they take photos of the ground in which cloud shadows can be detected. The RGB values of the photos and one measurement station in the camera's field of view are used to derive the DNI for each pixel of the photo. So far, a shadow camera system was mounted on a solar tower, covering an area of about 4 km². However, mirror surfaces and ground shaded by structures seen in the images cannot be evaluated with the method. Together with the low spatial extension, this so far excluded the shadow camera approach from the application in utility scale CSP plants. To overcome these limitations, a shadow camera is mounted on a mountain range. The evaluation method is adapted to the large distances between the camera and the ground and the results are compared to ground based DNI measurements. Considering that a simple surveillance camera is used the results are satisfying. The method is promising for tower plants and other solar power plants located close to mountain ranges or high buildings and can also help to provide highly resolved forecasts of global irradiance for areas with distributed solar energy systems. [ABSTRACT FROM AUTHOR]