CSP power cycle modeling

Hi, I am a student using SAM for the first time to analyze dry cooling techniques for CSP. It doesn't seem like there are many outputs directly relating to air-cooled systems, so I am focusing on the power cycle model. Do you have any advice for investigating cooling methods more in depth?

In terms of the power cycle, I am trying to match values with a Rankine cycle analysis in Engineering Equation Solver (EES), but I'm running into some issues and have some questions that I hope you can help me with.

- Firstly, dis the Rankine cycle model on page 54 of the technical manual used to calculate SAM outputs?

- If so, is this system optimized for evaporative cooling?

- Do the number of turbine stages change based on the system you're trying to model? Or is there a way to manipulate that?

- When I input the design parameters outlined on page 55 of the technical manual in to EES, I find a discrepancy between mass flow rate and rated mechanical turbine output. When I constrain the mechanical work to be the 10 MW given value, I get a mass flow rate that is two orders of magnitude smaller. I didn't account for parasitic losses, but I don't think I should be seeing such a large error. Are these numbers found directly from this model or calculated in a different way?

Is there a better way I can confirm the values of each state in the Rankine cycle than what I am doing now?


Thanks for the help

Hello,

In SAM 2015.1.30, the outputs relating to the heat rejection system include:
* Condenser pressure (Pa)
* Condenser fraction of operating bays
* Parasitic power condenser operation (MWe)
* Cycle cooling water mass flow rate - makeup (kg/hr)
* Cycle efficiency (gross)

The cycle shown in the reference manual is used to determine the off-design performance scaling of the power cycle. SAM's model uses input that you provide to specify:
* design-point power cycle gross efficiency,
* inlet and outlet HTF temperature from the steam generator,
* design gross power output,
* design point ambient temperature, and
* design point condenser temperature rise (ITD at design point for air-cooling, Ref. condenser dT + Approach temperature for evap-coolint).

With these inputs, the cycle performance -- including mass flow rates -- is fully constrained for design point operation. Note that we don't actually model a particular turbine configuration, feedwater heating sequence, etc. -- rather we use the design-point data that you provide to make an inference about how the cycle operates under the specified conditions and then scale performance with non-dimensional curves.

The EES power cycle model is used to generate the curves that represent scaling in power production and heat absorption by the cycle (i.e. cycle efficiency) as a function of 3 variables: HTF mass flow rate, HTF inlet temperature, and condenser temperature. As these 3 variables deviate from their design point value, the curves correct cycle performance accordingly. These curves are non-dimensionalized, so they operate on the design point configuration without requiring specific knowledge of the design point settings.

If you reproduce the EES cycle model, you will likely notice that the efficiency of that cycle is substantially lower than the design-point value that we assume as the default value in SAM. The default of 0.3774 comes from the expected cycle efficiency of current trough systems, not from the EES scaling model.

Regarding the mass flow discrepancy, I would recommend that you replicate the cycles published by Lippke (see -- Lippke, F. W. (1995). Simulation of the part-load behavior of a 30MW SEGS plant. Sandia National Laboratory. www.osti.gov/scitech/biblio/95571 ). His report provides detailed information regarding thermodynamic states and equipment sizing that you might find helpful in resolving your questions.

Finally, regarding your inquiry on investigating cooling methods in more depth, it is difficult to provide good guidance without knowing which issues or questions you specifically have in mind. A few points to consider: (1) Three cooling technologies are available in SAM - dry, evaporative, and hybrid. Each option is described in the technical manual. These three models capture a large share of proposed cooling methods. (2) You can manipulate the sizing of the cooling system by adjusting the design point temperature rises in the Cooling System settings. (3) The performance of each system depends on the climate in which it operates. Hopefully this helps.

Best,
Mike Wagner
NREL

Dear Mike

I trust you are well.

I would like to ask your advice on something.

I have modeled my own parabolic trough power plant and I find that my cycle thermal efficiency is mostly lower than the default value in SAM (0.356). A frequency plot shows that my model achieves ~0.34 majority of the time. 

To improve the fit between my results and SAM's results, should I rather change the SAM default cycle thermal efficiency to the 0.34 of my model? The 0.356 seems a bit too high for my model. 

Any advice on how to account for this parameter will be greatly appreciated.

Kind regards
Louw

Hi Louw,

You might try that approach and then compare the "PC efficiency: gross" output variable in SAM's time series results to your results.

Best regards,
Paul.