Gross electric power output of CSP plant

Dear Paul,

After running various simulations of CSP plants I noticed that in empirical trough model gross electric output is limited by the input parameters: "design gross output" and "max turbine over design operation", as it should be. However, when I use physical trough or molten salt power tower models, the gross electric output exceeds the defined limit. Could you please explain the reason for this? I am using SAM 2013.1.15.

Thank you for your help and time.

Best Regards,

Tahir

Dear Tahir,

The empirical trough model uses a polynomial equation (with coefficients that you specify on the Power Block page) to represent the performance of the power cycle as a function of the design point capacity (See "Power Block Simulation Calculations" in the Power Block topic of the Help system), so the gross electric output can never exceed the design point.

The physical trough model uses a more physical representation of the power block by calculating hourly HTF mass flow rates and temperatures, as described in the physical trough reference manual, which you can download from the Performance Models page. The physical model adjusts performance parameters (e.g., increasing or decreasing HTF mass flow rate, defocusing collectors) to meet the design point parameters that you specify, but it may not always be physically possible to do so for every hour of the year. Note that SAM makes this adjustment accounting for parasitic loads (particularly the solar field HTF pumping power), so that the actual net output should rarely exceed the net design net plant capacity (see Sections 5.3 and 5.4 in the reference manual). Also note that the "Turb. out. fraction" (turbine output fraction) factor that you specify on the Thermal Storage page can also cause the power cycle to operate over its design point at particular times of the day and year.

Also, for the physical trough model, the "Maximum turbine over design design" factor applies to the heat input to the power cycle, not to its electrical output. That means that there may be hours when the electrical output exceeds the design point even when the thermal input does not. For example, for a power cycle with an evaporative condenser type (wet cooling) and a cooling system design ambient temperature of 20 degrees Celsius, for an hour when the wet bulb temperature is below 20 degrees Celsius and the thermal input is at the design point, the cycle's efficiency will be above the design efficiency, and the electrical output will be greater than the design point.

If your system's output in some hours is higher than you want, you may want to adjust the solar multiple to reduce the size of the solar field. Of course, that may reduce the system's total annual output, which may not be desirable from an economic perspective.

Best regards,
Paul.