To know the solar radiation of the next fifteen minutes is one of the objectives of the working group solar energy meteorology in DLR’s Institute of Solar Research. In cooperation with the companies CSP Services GmbH and TSK Flagsol Engineering GmbH, the scientists have developed the new nowcasting system WOBAS, which is now being demonstrated under realistic conditions in the solar power plant La Africana in southern Spain.
|All sky camera in the parabolic trough plant La Africana. Source: DLR (CC-BY 3.0)|
The consideration of the expected irradiance makes it possible to operate solar power plants more efficiently, makes solar power more cost-effective and its availability more predictable. The prediction system is based on cloud cameras and can now be applied in commercial power plants.
For the assessment and prediction of the solar irradiance on the solar field, four cloud cameras are positioned at different positions in the power plant. With a 180-degree fisheye lens, each of them takes images of the clouds above the solar field and its surroundings, from which a special software derives the cloud coordinates and their motion vectors.
|All sky image used for the nowcasting system. Source: DLR (CC-BY 3.0)|
In combination with the existing ground measurements of direct irradiance, the WOBAS system calculates the current irradiance values on the solar field and its development for the next fifteen minutes in time steps of one minute. The irradiance maps have a spatial resolution of five by five meters. Radiation transmission through clouds is taken into account.
The knowledge of the irradiance for the next fifteen minutes enables for instance to control the flow of the heat transfer fluid in the power plant under variable cloud conditions in such a way, that the desired temperature level is maintained. This increases the power output of the power plant. Also the lifetime of the plant components is increased because thermal stresses due to abrupt changes of the incoming radiation can be reduced.
In the solar thermal power plant La Africana, the WOBAS system provides predictions for direct normal irradiance, but the system can also derive irradiance maps for global irradiance or irradiance on inclined surfaces that are needed for photovoltaic (PV) systems. For example, the use of the information delivered by the prediction system could avoid the abrupt interruption or increase of the feed-in of PV power.
WOBAS fulfills two important requirements for the creation of reliable forecasts: its cameras deliver high quality images of the sky with strong contrast and large amounts of data are accurately evaluated immediately after being captured.
In the future, the WOBAS system will be linked with the forecast system CSP-FoSyS, which provides meteorological forecasts as well as heat and electricity production forecasts up to 14 days. Using this combined system, the plant operator can make a sound decision regarding the start-up and the shut-down of the power plant during partly cloudy days. The forecasts are also used to improve the participation in the day-ahead and intraday market. CSP-FoSyS was developed by DLR’s German Remote Sensing Data Center (DFD) together with TSK Flagsol. Both systems are already available separately; the combination of both forecasting tools will be ready in the course of 2017.
The project WOBAS is funded by the Federal Ministry of Economics and Energy.