Dear Mwende,
1. You can see the thermal output of the fossil backup system by looking at the "Fossil thermal power produced (MWt)" hourly variable in the results. In your file, for an LHV efficiency value of 0.65, the total annual fossil energy value is 129,366 MWht/year compared to 93,431 MWht/year for a LHV value of 0.9.
2. You can see the total value of the hourly fossil MWt values on the Statistics tab of the Results page. I don't understand the second part of your question -- the empirical trough model does not report TOD period numbers in the results. I see that you are using the fossil fill fraction inputs on the Thermal Storage input page to control how much fossil backup is used at different times.
3. Yes, the HTF properties are one factor to consider in choosing the design inlet and outlet temperatures. You want to avoid exceeding the fluid's maximum and minimum operating temperatures, which are typically provided by the HTF manufacturer.
Best regards,
Paul.