Additional variable names request

[steve-ransome]

I'm trying to get my code for the Loss Factors and Mechanistic Performance Models working using this naming convention before the workshop on 5th Aug https://pvpmc.sandia.gov/7865-2/

There are a few parameters I found missing that may be useful to all and there will be a few that are maybe MLFM specific.
I've made a list with suggested variable names, maybe they can be added, or let me know whether they already exist, or if they should just be calculations in my code..

Useful for all

_norm (suffix for normalised measurements)
	
date_time	(suggest format YYYY-MM-DDTHH:MM:SS) or maybe pandas datetime?
wind_speed	(ms^-1)
wind_direction	(degrees)
poa_global_cSi	(unfiltered ref cell)
poa_global_cSi_kg3	(filtered reference cell)
gni	(global normal irradiance) (kW/m2)
relative_humidity	(%)
air_mass	(number)
xbi	(extraterrestrial beam irradiance) or xni?? (kW/m2)
xhi	(extraterrestrial horizontal  irradiance) (kW/m2)
kTh	(clearness index horizontal = ghi/xhi) (%)
kTn	(clearness index normal gni/xbi) (%)

beam_fraction	= (1- dhi/ghi) (%)
direct_fraction	= (dhi/ghi) (%)

Used by MLFM
rsc = -1/(dV/dI)|V=0 (Ohms)
roc = -1/(dV/dI)|V=Voc (Ohms)
i_half_vmp = current at Vmp/2 (A)
v_half_imp =voltage at Imp/2 (V)

Any help welcome!

[cwhanse]

We should add extraterrestrial irradiance and clearness index terms. 'xni' is too cryptic, but 'extraterrestiral_normal' is perhaps too long. Maybe 'extra_irrad' as the base, with '_normal', '_hor' as suffixes.

What is extraterrestrial 'beam' irradiance? Does beam refer to the plane of measurement?

'clearness_index' is understood to be GHI / extraterrestrial on the same plane, I'd prefer spelt out over 'kT' or any of the various symbols I've seen.

xbi (extraterrestrial beam irradiance) or xni?? (kW/m2)
xhi (extraterrestrial horizontal irradiance) (kW/m2)
kTh (clearness index horizontal = ghi/xhi) (%)
kTn (clearness index normal gni/xbi) (%)

[steve-ransome]

Sorry xbi or xni mean "normal to the sun" so around 1366 ± a few percent depending on the earth-sun distance from the orbit ellipticity.

[toddkarin]

Some of these parameters are already there: relative_humidity, wind_speed, wind_direction, airmass.
I just added date_time.

Small question: should we change airmass to air_mass? This would be more consistent with the rest of the variables.

I think we do not want to add a specific suffix for a reference cell measurement.

It would be good to have alignment with the choice of names for ground irradiance values. We currently have:

• poa_diffuse, poa_direct, poa_global, poa_ground_diffuse, poa_sky_diffuse, ghi, dhi

where poa stands for "plane-of-array irradiance."

I don't like using extra because that has another meaning. What about using xt or ext?

• ghi_xt, xti_direct, poa_global_xt, etc.

I could also get behind xbi and xhi.

We chose direct over beam in the past, but I think beam is a little more descriptive.

As with all these small decisions, nothing's perfect, but it's way better to just pick something than not finish this scheme.

[cwhanse]

I'm not fond of 'extra' either. I'm not strongly opposed to 'xni' or similar but haven't seen those terms used anywhere else. The common notation is H_extra or I_(some subscript) which doesn't translate to any particular words.

I also think the only extraterrestrial irradiance quantity in common use is extraterrestrial normal irradiance, i.e., outside the atmosphere on a plane normal to the the earth-sun line. Other terms may not be worth adding.

I would have no idea what 'ghi_xt' or 'poa_global_xt' meant - outside the atmosphere (xt) but measured on a what plane, exactly? 'global' doesn't add anything here, since irradiance from sources other than the sun aren't being considered.

[steve-ransome]

@toddkarin Thanks, where were the names listed? I looked on https://duramat.github.io/pv-terms/ and didn't see them

[toddkarin]

@steve-ransome I just rebuilt the docs, they should be there now.

@cwhanse I'm happy to add xbi and xhi if that's the only thing used. Do people never use a plane-of-array projection of extra-terrestrial irradiance?

[steve-ransome]

@toddkarin Thanks it has changed, wind is there now

[cwhanse]

I have only seen extraterrestrial irradiance calculated for planes normal to the sun, or horizontal to earth surface i.e. normal * cos(zenith). As far as 'xbi', since the middle character identifies the plane of measurement, I would use 'xni' so that the 'n' means the same thing as in 'dni'.

[toddkarin]

Okay, so I will add xni and xhi. Better to have an extra variable even if it is not typically used.

Are there any objections to adding these suggested values for xTh and xTn?

I'd suggest changing rsc to resistance_sc and including. Any objections?

Note that pvlib uses FD to mean the fraction of diffuse irradiance used by the module, so we should be a little careful here. I'm not sure how commonly used beam_fraction is but we can include this as well if it is a useful term.

[steve-ransome]

I'd prefer if possible to have rsc and roc as they are more tied in with isc and voc than resistance_shunt and resistance_series.

Also I use extraterrestrial irradiance on the poa as it is useful for some arrays with limited sensors as it gives a reasonable value of clearness index for a few hours around noon/

[toddkarin]

Okay, do we prefer poa_xti,xpoa or something else?

[steve-ransome]

Can we have a fill_factor?

How does interval work, is it _minute, _hour, day? How about _5min or _3hour (are they valid).

Several of the deprecated lines are wrong e.g. "imp [A]: Current at the maximum power point. [Deprecated/alternates: v_mp]"

Are the orders of the inverters going to be changed ? e.g. max_inv_dc_current should have current to the left.

Thanks

[wholmgren]

I've never cared for "extraterrestrial". The atmospheric sciences literature refers to top of atmosphere radiation, abbreviated TOA. toani? toa_ni? top_of_atmosphere_ni?

[toddkarin]

@steve-ransome Just added fill factor and fixed the deprecated lines.

For inverter inverter parameters, we should decide which style to use. E.g.

• min_inv_operating_temperature vs. temperature_inv_operating_min
• max_inv_dc_voltage vs. voltage_inv_dc_max

any preferences? The second style is more consistent with the rest of pv-terms, but I think it is also less easily understandable.

@wholmgren Yes extraterrestrial sounds like little green men. What about dni_toa, poa_toa, ghi_toa?

[wholmgren]

yes much better