Pressure drop field

I am trying to study the parasitic consumption of a CSP parabolic trough plant due to the main HTF pumps. I am simulating with SAM (version 2011.12.2) a typical 50MW no TES plant of current construction in Spain. After running the simulation I check the calculated hourly outputs for field pumping and the values SAM has calculated seems quite low compare to real values from a 50MW plant. Obviously, these values vary along the year, but lets focus just in once case so it is easier to analyze the results. For the design conditions (875W/m2) SAM calculates around 5kg/s per loop for a 112 loops plant. For that case, the field pumping SAM yields is around 0.3MWh. However my experience tells me that for that 5kg/s the field pumping should be quite close to 1MWh.

Field pumping is calculated based on the flow rate and the pressure drop so since I am confident about the flow rate figures SAM is calculating I have check the pressure drop field and I have noticed it is quite low, around 3.7 bar for the 5kg/s per loop. I have perform several studies about pressure drops in a parabolic trough plant and my experience tells me 3.7 bar is only the pressure drop in the loop not in the entire solar field. In theory pressure drop field should be around 10 bars (3-4 bar in the loop and 6-7 bar in the piping)leading to the 1MWh I mentioned before.

I looked at SAM's help and manuals and it clearly says pressure drop field includes pressure losses in both loops and headers. I would like to get some light about this. Could it be possible that currently SAM is wrongly considering only the pressure drop in the loop as the entire solar field?

This is quite important since if I am right about this field pumping power consumption, SAM would be underestimating the parasitic consumption leading to higher net enery yields.

The pressure drops through the loop and headers depend on several of the inputs the solar field page. Would you mind attaching a zsam file to this discussion thread that gives the results you mention? That will help us explain the results.

Best regards,
Paul.

Dear Paul,

Please find attached a sample zsam file for a 50MW that can serve to clarify my question. Please, mind that the purpose of the file is just to analyze the field pumping consumption and therefore the economic and financial inputs are the default ones. The other technical inputs are set to represent a typical 50MW parabolic trough plant with no TES.

To make things easier I have used the Dagget location that SAM has in its database. It is already copied in the project avoiding any trouble regarding the TMY. Please not that the design DNI I have set of 875W/m2 might not be very representative of Dagget but that is not the point of discussion here.

After I run the simulation I use the results options of SAM to plot and analyze the following three variables: “single loop mass flow rate”, “solar field pressure drop” and “field pumping power”. Since those variables vary along the year let’s focus in the nominal operating conditions (plant at 100% load) that are achieved during most of the hours of the year. The 2nd of July can be serve as reference to make sure we are looking at the same moment in the plots. For that time SAM calculates the following:

1) single loop mass flow rate around 4.9kg/s
2) solar field pressure drop of 3.8bar
3) field pumping power of around 300kW (I understand this is electric power and not hydraulic power, so it already includes the 0.85 default pump efficiency defined in the solar field input section)

However based on my experience and calculation I get the following figures:

1) single loop mass flow rate around 5kg/s which is in line with the 4.9kg/s calculated by SAM
2) solar field pressure drop of around 9 bar (3 in the solar field loop and 6 in the piping). SAM calculated 3.8bar
3) field pumping power of around 1100kW (around 900kW hydraulic power which leads to 1100kW electric power with an overall efficiency of 82%). SAM calculates 300kW

I would like somebody to explain the reason of those big discrepancies in points 2 and 3.

Besides, I want to add an important note regarding pumping consumption. Appart from the 9 bar I have calculated as pressure drop in loops+piping there are other pressure drops that should be considered, mainly in the steam generator which usually add another 4bar for a 50MW plant. Therefore the total pressure drop in the system in around 13bar and this is what the pump need to provide and the figure I use to calculate pumping power. Im not sure about how SAM is dealing with this. Is SAM neglecting this steam generator pressure drop when calculating the pumping consumption? This is my second question that add to the main one about the discrepancies of my figures and SAM results.

I would like to get your feedback on this. Does anyone have thought about this too??

Thanks

Dear Paul and others,

After checking the zsam sample i attached, does anyone have any comments regarding the comments I posted about pressure drop calculation? I believe this is an important issue that I would like to clarify.

Best