CSP systems - condenser calculations

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emiliano casati
CSP systems - condenser calculations

Good morning Forum!
I've been following the detailed description presented in the 2011 "technical manual for the SAM physical trough model" (section 3.3, from page 63 on).
I'm trying to replicate SAM's outputs and I'm facing problems with the dry-condenser model, both for on- and off-design calculations (attached the model I'm simulating as well as the script I'm using)

summarizing:
-"SAM incrementally reduces the cooling system capacity according to the 'cooling system part load levels'", and "parasitic fan loads are also reduced in proportion with the partial operation" (page 67).

-air mass flow can be adjusted passing from 100% to 50% depending on the thermal load to be rejected (formula 3.14 page 69).

ON-DESIGN OPERATION
the air mass flow is = to the design value and the "cooling system capacity" is not altered. In this case, applying formulas 3.26 and 3.27 (assuming DT_out constant) I obtain a value off by 7% (average of absolute values along the hours whereby condenser is working in nominal conditions). Also the fan power is off by +29%. both are shifts (approximately constant).

Trying to fix this point, I found out that probably SAM calculates the thermal power to be rejected (q_rej_des, formula 3.12) considering the NET design power (W_des) AND the gross cycle conversion efficiency ETA_cycle_des. This affects both the calculated p_cond and fan power, and in my opinion leads to wrong results. Assuming this ambiguity in my calcs the "offset" in these w/r to SAM's output is consistently reduced (2 % in T_cond and 15% in fan power).

Another point is that there are several hours whereby, even though the cycle delivers net electrical power, the fan power is = 0 !!

OFF-DESIGN OPERATION
From the manual I cannot really understand how it's managed...what's the "cooling system capacity" being reduced with steps of amplitude 1/N_hr_pl (page 67) ?? is the condenser area? how is the fan consumption adapted?

SUMMARIZING QUESTIONS
1) is it correct to consider DT_out constant during on- and off- design calculations (as it is explicitly said regarding the wet cooled condenser)?
2)how is the thermal load to be rejected calculated?
3) how it's possible to have net electric power production without fan consumption?
4)how is the off-design managed in more details?
5)might be possible to have access to the fortran code of the condenser?

Thanks in advance

Emiliano Casati

emiliano casati

...still believe it contains several interesting points...

Paul Gilman

Dear Emiliano,

Sorry for the slow response. It took some research for us to answer your questions. I've repeated portions of your questions in bold below with an answer immediately below each question:

-"SAM incrementally reduces the cooling system capacity according to the 'cooling system part load levels'", and "parasitic fan loads are also reduced in proportion with the partial operation" (page 67).

-air mass flow can be adjusted passing from 100% to 50% depending on the thermal load to be rejected (formula 3.14 page 69).

This is true only if there are 2 part load levels. For example, if there are 4 part load levels, then the fans speeds can be 100%, 75%, 50%, and 25% of design.

ON-DESIGN OPERATION

the air mass flow is = to the design value and the "cooling system capacity" is not altered. In this case, applying formulas 3.26 and 3.27 (assuming DT_out constant) I obtain a value off by 7% (average of absolute values along the hours whereby condenser is working in nominal conditions). Also the fan power is off by +29%. both are shifts (approximately constant).

Trying to fix this point, I found out that probably SAM calculates the thermal power to be rejected (q_rej_des, formula 3.12) considering the NET design power (W_des) AND the gross cycle conversion efficiency ETA_cycle_des. This affects both the calculated p_cond and fan power, and in my opinion leads to wrong results. Assuming this ambiguity in my calcs the "offset" in these w/r to SAM's output is consistently reduced (2 % in T_cond and 15% in fan power).

In Equation 3.12, W_dot_des represents the net design power output of the power cycle. This value is equal to the turbine output less the power required by the feedwater/condenser pumps. From an energy balance on the cycle it can be shown that the thermal power input is equal to the sum of the net power output and heat rejected.

Another point is that there are several hours whereby, even though the cycle delivers net electrical power, the fan power is = 0 !!

We identified the bug in the code causing this problem. The bug was a programming issue – not a problem with the basis of the equations.

SUMMARIZING QUESTIONS

1) is it correct to consider DT_out constant during on- and off- design calculations (as it is explicitly said regarding the wet cooled condenser)?

Yes, dT_out is constant. ITD is calculated based on the heat rejection load and mass flow rate of the air. See Equation 3.27.

2)how is the thermal load to be rejected calculated?

The regression modeled (detailed in Section 3.2) calculates the power output and heat input, with some assumed condenser pressure. This information constrains the thermodynamic power cycle and allows the heat rejection to be calculated. The cooling model then uses the heat rejection to calculate the condenser pressure. Therefore the regression model and the cooling model are solved iteratively using successive substitution.

3) how it's possible to have net electric power production without fan consumption?

See above

4) how is the off-design managed in more details?

5) might be possible to have access to the fortran code of the condenser?

The cooling models are contained here in the TRNSYS source code folder, which by default for SAM 2013.1.15 in Windows is SAM\2013.1.15\exelib\trnsys\source\sam_mw_pt_HRmod.f90

Best regards,
Paul.

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