Linear Fresnel Molten Salt Storage Dispatch

I am simulating a linear fresnel molten salt with storage (same HTF and TES fluid) and the turbine dispatch is not that one that I would like to get. At  the end of the day, the power output decreases with the deacreasing of the solar resource and, only after a significant decrease of the power, the energy from TES is used to dispatch the turbine (Fig 1). I am using a uniform dispatch, with storage dispatch fractions equal to zero, and a unitary turbine fraction .
 I simulated a molten salt Tower plant, with similar parameters, and I get the results as expected, that means, the stored energy is used just in time when the solar resource starts deacreasing (Fig 2). I would like to know if it is possible to get a similar behaviour for the linear fresnel molten salt plant. 
Best Regards,
Rafaela Reinaldo

Please, find the power output time serie graph at the attached file

Hi Rafaela,

The molten salt linear Fresnel (MSLF) and molten salt power tower (MSPT) models use a different controller algorithm to dispatch energy from the thermal energy storage system (TES). The MSPT model has a more recent controller so works better than the MSLF controller. We are gradually updating the CSP models to all use the new controller, but have not yet had the resources to update the MSLF model. Starting with the next version of SAM to be released later this Fall 2019, the physical trough model will use the new controller.

Unfortunately, I do not have an explanation for why the TES output in the MSLF model drops at sundown and then increases. I have asked the CSP engineers to help and will follow up when I hear from them.

Best regards,
Paul.

Dear Paul,
Thank you for the reply.
I hope the new controller can give a better plant behaviour.
Best Regards,
Rafaela

Here is some additional information from the CSP engineering team:

This is caused by the solar field and TES supplying HTF to the power cycle in parallel. At the end of the day, the field temperature slowly degrades until it hits some threshold at which the field turns “off” and begins recirculating the HTF. When it sends colder-than-design HTF (at a low mass flow rate) to the cycle, that stream mixes with hotter HTF from the TES. The combination is still somewhat colder than typical operation, and as such, cycle output decrease (see image below). In contrast, we model the tower as a steady state receiver that is either delivering HTF at (roughly) the design hot temperature or is off, so that model won’t show this problem.

Best regards,
Paul.

Dear Paul,

Thank you for the explanation.

I supposed that it was possible to dispatch the storaded energy before the complete decreascing of the field thermal power produced by setting storage dispatch with solar equal to zero (The fraction of the TES maximum storage capacity indicating the minimum level of charge that the storage system can discharge to while the solar field is producing power. A value of zero will always dispatch the TES in any hour assigned to the given dispatch period; a value of one will never dispatch the TES. Used to calculate the storage dispatch levels).

The figure bellow shows that the hot tank volume starts decreasing just after the field power becames null, that means that the storaded energy is not used before. That would be interesting if we could set a minimum cycle electrical power output level to start using the storaded energy.

Hope that a future version of SAM considers this point, as well a series solar field/TES  configuration.

Best Regards,
Rafaela