CSP Parabolic Trough Physical - Power Cycle Queries

Dear Paul:

A few queries on the power cycle behavior I could observe running the default configuration on SAM 2021.12.2:
1. Start-up time of 0.5 h seems to be not considered in a simulation with 1h time step. Start-up power is maintained for the whole time steps using more energy for the start-up process than required as per user inputs.
2. Although user sets min/max ST load on the GUI, these limits seem to apply to the PC inlet thermal power as per current implementation.
3. Start-up power seems to be factored in by reducing the PC efficieny (not deducted from the PC inlet thermal power); therefore, the PC operates below the min load as per user inputs, this happens both in terms of inlet thermal power and ST gross power.
4. It seems that the PC operates below the min load in moments where start-up is not taking place, this happens both in terms of inlet thermal power and ST gross power.
5. Could the change in material/HTF internal energy from SF to TES and PC be reported as output? As it is done for the SF piping.
6. Abrupt drop in "PC HTF Inlet Temperature" in last time step being the PC operative from TES only. It does not happen always but it happens frequently. Why?
7. PC does not go to stand-by in any time step. Conditions under which the PC goes to stand-by?

I would be thankful if you could assist me with them. I do have my analysis ready to be shared in case of help. Thanks.

Javier Pastoriza.

Hi Javier,

Question 1: Start-up time of 0.5 h seems to be not considered in a simulation with 1h time step. Start-up power is maintained for the whole time steps using more energy for the start-up process than required as per user inputs.

The "Fraction of thermal power needed for startup" value is poorly labeled. A better label would probably be “fraction of an hour at design point thermal required for startup energy”. The equation is:

     Energy_startup = q_dot_des * 1 (hr) * fraction_input (-)

So, in the default case:

     Energy_startup = 311.80 * 0.2 (hr) = 62.36 (MWt-hr)

The 0.5 hr is the minimum startup time. In the first time step, where TES is available, the controller sets the thermal power
to achieve startup in the minimum time:

     q_dot_su = 62.36 / 0.5 hr = 104.72 MWt

However, q_dot_startup, like most rate outputs, is reported as weighted value over the full hourly time step. q_dot_startup in the second time step (from 0.5 hr to 1 hr) is = 0, therefore:

     q_dot_startup_timestep = 0.5*(104.72 + 0) = 62.36 MWt

Since this term is supposed to represent the average q_dot_startup over the entire time step, it makes sense that it matches the startup energy in this case.

Question 2:  Although user sets min/max ST load on the GUI, these limits seem to apply to the PC inlet thermal power as per current implementation.

Yes, the controller uses the “turbine fraction” to adjust heat input rather than power output.

Question 3: Start-up power seems to be factored in by reducing the PC efficiency (not deducted from the PC inlet thermal power); therefore, the PC operates below the min load as per user inputs, this happens both in terms of inlet thermal power and ST gross power.

This is incorrect. Startup power requirements are taken from the inlet thermal power for the needed fraction of the time step, and this can be seen with the startup thermal energy output not having the same value over the 1 hr time step. After startup, the power cycle may operate for the remainder of the time step.

Question 4. It seems that the PC operates below the min load in moments where start-up is not taking place, this happens both in terms of inlet thermal power and ST gross power.

We were not able to reproduce these results. Please attach a .sam file that demonstrates this so we can investigate.

Best regards,
Paul.