Irradiance Data (DNI) for CSP Project


Im looking for DNI data to use for a simulation of csp plants. I found this document 18490-radiation-quantities-ecmwf-model-and-mars.pdf but dont understand how to convert total sky direct solar radiation at surface to DNI. I`ve tried converting the data using the cos of the average zenith angle for the given hour, however the result doesnt seem to make sense as the values massively increase during evening hours (as it makes sense given that the zenith angle approaches 90°). Am I missing something or is there another way to get the DNI data from the ERA5 hourly data on single level?

Thanks in advance