Negative AC net monthly output for High-X CPV model

Thank you for sending the file.
There are many factors that are causing the output to be negative during the low resource months. I don't have enough information to help you design your system, but hopefully I can help you see where to look so you can do it yourself.
You can explore the hourly results using the Time Series graph (on the Results page) to help understand what the model is doing.
The solar resource does not look great for a CSP system: The annual DNI is only 1,080 kWh/m2 (that's compared to 2,636 kWh/2 for the default trough case in Arizona). From October through March, the hourly DNI rarely exceeds 600 W/m2. Those are the same months that you see the negative net AC output. By contrast, for the Arizona location the hourly DNI is fairly constant throughout the year.
In the first few days of January, the receiver thermal losses are quite high, ranging from an average of 100 to 400 W/m2 (that's compared to a range of 50 to 175 W/m2 for the default case with Therminol VP-1 in Arizona).
It appears that the Schott PTR70 receiver uses the default emissitivty and optical properties. According to this SolarPaces article from Schott Solar , those values need to be modified to model that receiver with a salt HTF. You can edit the values in the absorber emittance table and other parameters on the Receivers (HCE) page.
I hope that helps you sort things out. Please let me know if you have any other questions.
Best regards,

Paul.