PVWatts V1 Input Parameters

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MDuarte
PVWatts V1 Input Parameters

Hello once again,

Following up on my last question, I´ve been tinkering with the VBA code in order to try and run a PVWatts v1 simulation.
However there are certain input variables on which I have some doubts, and I believe you could help shed some light on what they are and/or their uses in the simulation.

Before that however, just a quick side question: in the SSC SDKtool, on the require column, * stands for required, ? stands for user defined but not required, and ?="value" means not required and defaulted to "value"? Just to make sure I´m understanding it right from the start.

The input variables for PVWatts v1 I have questions about are:

fd - diffuse fraction: is this an input for the Perez model irradiation calculation? If so, given that I´m using my own POA values, can I omit it?

w_stow - wind stow speed: I haven´t seen any mention of this in any of the manuals, is this a wind speed above which the panels are meant to be stowed and nonoperational? Omitting it means that no such stowage shall occur, independently of wind speed?

concen & fhconv - concentration ration & convective heat transfer factor: from the names I think this might be inputs for the thermal model calculation with the Fuentes model, is this right? If so should I leave them in their default value of 1?

ar_glass - enable anti-reflective glass coating (Beta): from what I gathered, PVWatts V1 only applied an AOI correction to direct beam incidence for angles of above 50º, and only in PVwatts V5 was a physical model of transmittance incorporated to improve that AOI correction and model AR coating adequately. However, this variable in PVwattsV1 leads me to believe there is a (beta?) version of this physical model already in PVWatts v1, is this correct? If so, what are the differences towards the final model used in PVWatts V5?
If I choose to use it, by setting this variable to 1, will it run at all? Considering it applies AOI correction to direct beam irradiation but I will be using my own POA values, there will be no array of Ib (beam Irradiation) for it to perform the AOI on. On a related note, even if I don´t use the glass coating correction, instead opting for the standard correction just accounting for angle of incidence, as it also applies to Ib, will it calculate adequately if I only supply user_poa array?

u0 & u1 - thermal model coefficient: what are these coefficients? they have no standard default value and I do not see them mentioned in any of the PVWatts manuals.

Thank you Paul, once again, for any and all help. I know this question is quite extensive, if you find you would rather reply through e-mail I can send this directly to your support address at: sam.support@nrel.gov

Thanks and kindest regards,
Miguel Duarte

Paul Gilman

Dear Miguel,

First of all, as you know, the pvwattsv1 module in the SDK is a legacy version. The current version of PVWatts in both SAM and the PVWatts website use pvwattsv5. I know you are aware of this, but I wanted to mention that for other readers.

Section 3.5 of the SSC Guide describes the Required flags. You are correct about the * and ? values. For a variable with a flag of ?="value", if you do not assign a value, the model will use the indicated default value, so I would consider it a required value from the model's perspective, but you do not need to assign it a value.

The variables you are asking about are experimental and intended to make it possible to use the pvwattsv1 module for concentrating photovoltaic collectors. Those inputs were never exposed in the SAM and online PVWatts models.

The fd input is the diffuse fraction. The model uses it to calculate the POA irradiance as follows poa = ibeam + fd * ( iskydiff + ignddiff ). You can use it to make adjustments to the diffuse portion of the POA irradiance. For example, for a concentrating collector you could set fd = 0 to remove diffuse irradiance from the POA irradiance.

The w_stow input is not used by the model. It looks like it was intended to force the array's output to zero when the wind speed was greater than the stow speed, but that is not implemented in the model. Some trackers move the modules into a "stow" position when it the wind is strong to avoid damage to the array and tracking equipment.

The concen, U0, U1, and fhconv inputs are used to calculate the cell temperature pvt as follows:

If you assign values to U0 and U1: pvt = wf.tdry + poa*concen/(U0+U1*wspd_corr), where wspd_corr is the wind speed from the weather file set to zero if it is a negative value.

If U0 and U1 are not defined, then the cell temperature is calculated using the Fuentes model as described in Chapter 8 of the PVWatts Version 1 Technical Reference with the following inputs to the cell temperature function: poa*concen, wspd_corr, wf.tdry, fhconv.

Chapter 7 of the technical reference describes how pvwattsv1 models the module cover given the POA irradiance, whether it is calculated from the beam and diffuse irradiance or provided by the user. If you set ar_glass = 1, the model uses the following set of b coefficients for the Sandia model polynomial equation instead of the ones shown in the manual (for a Sunpower SPR-E20-327 module):

b0 = 1.0002
b1 = -0.000213
b2 = 3.63416e-005
b3 = -2.175e-006
b4 = 5.2796e-008
b5 = -4.4351e-010

Best regards,
Paul.

MDuarte

Hello once again Paul,

Thank you VERY much for this thorough explanation, this will go a long way to clear up some of the remaining issues I have with my current work.

All the best and kindest regards,
Miguel D.

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