Constant weather inputs but non-steady state gross output

8 posts / 0 new
Last post
williampaulbell
Constant weather inputs but non-steady state gross output

Hello Paul

Could you help with the following query please regarding constant weather inputs but non-steady state gross power output?

Table 1 below shows the input for SAM that is kept constant for every hour. Table 2 shows a sample of a daily cycle from SAM's output that is the Gross Power Output.

I assume that SAM recognises the constant inputs and adjusts the DNI to follow an idealised daily cycle based on the time, latitude, longitude and elevation of the site.

I also assume that SAM recognises non-constant input of DNI and does not adjust the DNI for some idealised daily cycle.

Could you advise if my assumptions are correct?

Regards

William Bell

Table 1: SAM input - constant weather variables
DNI (W/m2): 767
Dry temperature (°C): 26.1
Relative humidity (%): 55
Pressure (hPa): 989.5

Table 2: SAM output - daily cycle but expecting steady state
Gross cycle power output (MWh), hourly
0
0
0
0
0
0
0
1.90196
23.0868
32.7346
35.2751
35.2808
11.3107
8.00759
7.24606
6.26285
0
0
0
0
0
0
0

Paul Gilman

Dear William,

Does Table 1 show values that you used to create an hourly weather file with constant values?

It's hard for me to tell what the results in Table 2 mean without seeing the SAM file (.zsam) and weather file. Would you mind attaching those to your original post (Edit the post and then attach the files)?

Best regards,
Paul.

williampaulbell

Hello Paul

Thank you for you reply.

I have up loaded the input and output files for SAM to the original support request.

Regards

William

Paul Gilman

Dear William,

Thank you for sending the files.

The linear Fresnel and other solar models determine the solar radiation incident on the solar field (or array) based on the solar radiation data in the weather file and solar angles that the model calculates from the hour of year, latitude, longitude, and time zone. For your artificial data with constant radiation values, the solar energy incident on the field varies with the day as the solar incidence angle changes throughout the day. That's why you see the system's electrical output varying with time of day in spite of the constant solar radiation data.

Best regards,
Paul.

williampaulbell

Hello

Thank you for the clarification regarding the artificial data that I was using for a sensitivity analysis.

So, SAM shapes the DNI data according to the the hour of year, latitude, longitude, and time zone. This works well for constant input to shape the data into something more realistic.

However, we have DNI data based on readings from the site of the proposed LFR. These DNI readings are a fair indication of the DNI incident on the proposed plant and don't require modification. Could you let me know if SAM would also shape our DNI readings measured taken at the proposed site?

Regards

William Bell

Paul Gilman

Dear William,

SAM does not have an option to use incident irradiance data as input. It always converts either horizontal or direct normal irradiance data to incident, depending on the type of system you are modeling. The linear Fresnel model assumes that the field uses single-axis tracking, so the hourly incident irradiance is not the same as the direct normal irradiance.

Best regards,
Paul.

williampaulbell

Hello Paul

Thank you for the clarification.

Regards

William

Paul Gilman

Hi William,

Here's a little more on your question based on input from one of the NREL CSP engineers.

There is a difference between the weather file input DNI and the receiver incident flux. The former is a value that you can set in the weather file, and the latter is a value that SAM calculates and is dependent on solar position, tracking, and optical properties. You can't change the solar position values because those are calculated by the model. That means that you could not force the receiver to run at night simply by assigning daytime DNI values to nighttime hours, for example, because SAM assumes the sun is down during nighttime hours, regardless of the DNI values.

SAM's physical trough model performs some checks on the weather file DNI values to ensure that those inputs make sense relative to the solar position and global solar constant. The linear Fresnel model does not perform those checks, so it will run simulations even when the DNI values are unreasonable. For example, we ran the linear Fresnel model with the TMY3 file for Blythe, CA, but multiplied the DNI values by 10 to create obviously unreasonable data. The linear Fresnel model ran without errors.

In general, the current version SAM is designed to run with "standard" weather files that have realistic values and does not perform systematic test on the weather file data to check for potential errors in the data.

Best regards,
Paul.

Theme by Danetsoft and Danang Probo Sayekti inspired by Maksimer