Questions regarding pvwattsfunc in SSC

Hi Steve,

I don't have MATLAB, so could not test your code directly. However, when I replicated it as an LK script in the SSCdev tool that comes in the SAM SDK package, I think I get different results than you describe:

POA before simulation = 1000
Tcell before simulation = 25

POA after simulation = 965.85
Tcell after simulation = 47.2424

Here is the LK script I used for the test -- you can copy and paste it into the SSCdev Script Editor to run it yourself:

Do you get a different result?

[BEGIN SCRIPT]

clear();

var('year', 2012);
var('month', 6);
var('day', 21);
var('hour', 12);
var('minute', 0);
var('lat', 39.742);
var('lon', -105.18);
var('tz', -6);

var('beam', 970);
var('diffuse', 86);
var('tamb', 25.4);
var('wspd', 3.6);
var('snow', 0);
var('time_step', 1/60);
var('system_size', 100);
var('derate', 0.95);
var('track_mode', 0);
var('azimuth', 180);
var('tilt', 40);
var('tcell', 25);
var('poa', 1000);

outln("before poa="+var('poa'));
outln("before tcell="+var('tcell'));

run('pvwattsfunc');

outln("after poa="+var('poa'));
outln("after tcell="+var('tcell'));
outln("after dc="+var('dc'));
outln("after ac="+var('ac'));

[END SCRIPT]

Thanks,
Paul.

Hi Paul,

Ok, thanks for taking the time to replicate the script in LK.

I think your results are aligned with mine. Here's the equivalent MATLAB output:
dc: 88642.835938 (W)
ac: 81783.929688 (W)
tcell: 47.242397 (deg C)
poa: 965.850037 (W/m2)

To see the phenomenon I'm referring to, modify the initial values of 'tcell' and 'poa' (seperately) and examine the results. For example, here are three runs I tried and the results (with some annotation):

[Script]
...
var('tcell', 50);

Hi Steve,

As you know, the pvwattsfunc module calculates the cell temperature based on the tcell input value along with the the incident solar radiation (or plane-of-array radiation, POA), wind speed, and ambient temperature. The POA value has the largest impact on cell temperature. The cooling effect of the wind is also significant.

When you change only the tcell input value, depending on the values of the other parameters, they may dominate over the effect of the difference in the tcell input value, so you won't see a change in the tcell output value.

In your example, if you change the wind speed to zero, and run the module over a range of tcell input values while holding the other parameters constant, you will see a small difference in the tcell output value:

wspd = 0:
before tcell, after tcell
25, 62.2293
30, 62.0742
35, 62.0781
40, 62.0821
45, 62.0861
50, 62.0901

wspd = 3.6:
before tcell, after tcell
25, 47.2424
30, 47.1745
35, 47.1745
40, 47.1745
45, 47.1745
50, 47.1745

Best regards,
Paul.