Solar Field Outlet Temperature

Hi,

I'm simulating a CSP Trough Empirical IPP. The default value of Solar Field Outlet Temperature is 391°C. When I decrease it to 350°C, I obtain an higher net annual energy. Why?

Thanks for your comments.
Matt

Dear Matt,

As you observed, for the default empirical trough system with IPP financing, changing only the design field outlet temperature from 391 degrees C to 350 degrees C increases the net annual energy. A 10% change in the design temperature results in a 1% change in annual energy:



Design Field Outlet Temp (deg C)Net Annual Energy (MWh)


391370,955


350375,034



When you change the design solar field outlet temperature on the Solar Field page from the default 391 degrees C to 350 degrees C, and the field layout mode is Option 1 Solar Multiple, SAM adjusts the field aperture area to account for the decrease in thermal losses under design conditions.

For example, for the default system with a solar multiple of 2, the design HCE thermal losses and field aperture area depend on the design field outlet temperature as follows:



Design Outlet Temp (deg C)Design HCE Losses (W/m2)Field Area (m2)


39136.1877,580


35030.1867,233



You can see the design HCE thermal losses and solar field area on the Solar Field page under Solar Multiple (Design Point). As you change the field design temperatures, the thermal loss and field area values change too.

The lower design field outlet temperature reduces HCE losses, which increases the system's annual energy. However, because SAM also adjusts the field area to compensate for the reduction in losses (at least under design conditions), the increase in annual energy is less than you might expect.

If you run the model using Option 2 Solar Field Area, you can fix the area to isolate the effect of the design field outlet temperature on annual energy. For example, here are the results with the field area fixed at 877,580 m2 (equivalent to a Solar Multiple of 2 at 391 degrees C):



Design Field Outlet Temp (deg C)Net Annual Energy (MWh)


391370,955


350378,307



This time, the same decrease in design field outlet temperature increased the annual energy by 2% instead of 1%.

Best regards,
Paul.