[RE-wrenches] Percent of DC capacity that is inverted to AC kWh?

[Christopher Warfel]

Hi all, Thank you for the feedback.  I realize there are a lot of 
unknowns.  I am attempting to show regulators and the utility that a cap 
based upon solely ac ratings of the inverters may be easy to 
administrate, but it greatly overestimates revenue erosion and any rate 
design that would capture the revenue needed to meet fixed costs.  I am 
going to have to aggregate a "unit" that reflects fairly standard and 
common installations including the information below, and see how the 
utility and regulators react.  The utility was just taken over by the 
Town and there is a lot of plant work that needs to be done. They think 
solar is taking away from that even though the Charter is all about the 
environment.......   We get into line rebuilding, distributed generation 
advantages, but we are only about 5MW peak, so they don't have the 
engineers/economists to easily do these calcs.

Chris

On 8/28/2019 10:26 AM, Dan Lepinski, P.E. wrote:
> Hello Chris,
>
> Thank you for the example.  It helped .. but not completely.
>
> See if this is what you have in mind:
>
>          PV -> Inverter ---V
>                   ^        V
>                   V       ATS  -> Grid
>                Battery    Loads
>
> PV = 2 kW STC rating.
> Inverter has an internal MPPT charge controller.
> Automatic Transfer Switch "ATS" disconnects the system in the event of a utility failure.
> Battery provides storage in the event of a utility failure.  Chemistry not identified.
>
> Presuming the above is correct:
>
> There isn't a clear-cut answer to your question due to the number of uncontrolled variables involved.
>
> However, some variables can be generally addressed:
>
> 1. Battery chemistry.  Lead-acid?  Li-Ion?
> 2. Site location (for average temperature, annual insolation, etc.)
> 3. PV mechanical considerations: Azimuth?  Tilt?  Mounting method?
>
>
> It appears you're seeking an estimated value for total annual energy production.  Subsequently, I'll proceed on that basis:
>
> For very generalized analysis, actual overall PV output is 85-88% of their label ratings.  Much of the decrease is due to cells operating above STC temperature of 25C.  Haze, pollution, weather, dirt on the PV, and other factors are also considerations.  Use 86%.
>
> There are two key aspects to charge controllers in this discussion: 1. How well do they track the actual maximum power point?  2. Conversion efficiency?  For your purposes, estimate an overall efficiency factor of 95%.
>
> Whether battery efficiency plays into this or not depends on how often the system operates on battery power only, depth of discharge, age of the batteries, and battery chemistry .. for openers.  If lead-acid, there's another aspect called the "Peukert's Law" (or the "Peukert formula") that has to be factored in.  Peukert says the higher the rate you withdraw stored energy from a L-A battery, the less you get.  It's akin to saying, the faster you drive, the worse your miles per gallon.
>
> Inverter efficiency varies depending on the point of their curve on which it's operating.  The "sweet spot" tends to be mid-range.  For a 3.6 kW, and only 2 kW of PV, you'll never exceed the inverter's mid-range.  For a typical grid-interactive inverter today, use 97%.
>
> Batteries have a round-trip efficiency of approximately 90% for new lead-acid, and slightly better than 90% for Li-Ion.  As L-A batteries age, efficiency decreases.  Near end of life, L-A batteries are in the 80-85% range.  This also varies with the method of charging.  L-A also require periodic conditioning charges (also known as an "equalizing" charge).  Li-Ion are better in that respect, losing approximately 0.5% per year, and don't require conditioning.  However, efficiency in both topologies depends on how often the battery is discharged, how deeply it's discharged, and for L-A, how long the battery sits in a less than 100% state of charge condition before being fully recharged.  Heat also takes its toll on L-A batteries in life and efficiency.  When used in repeated discharge/recharge cycles, for L-A, use 88%.  For Li-Ion, use 92%.
>
> Battery float current also varies with chemistry.  L-A batteries consume nominally 0.3 to 0.5% of the rated amp-hour value.  Thus for a 48 volt, 300 amp-hour battery .. the float current would be 1 to 1.5 amps.  This value will vary with battery temperature.  It's higher in L-A batteries than Li-Ion.  Compromise and use 1.25% for L-A, and 1% for Li-Ion.
>
> Wiring loss in a well-designed system is recommended to be 2.5% for best practices.  This is inclusive of all BOS hardware, connectors, and so forth.  Use 2.5%.
>
> Finally, on reading your reply below .. I infer from the information the site gets 4.5 hours of "good" sunlight per day.
>
> I'm unable to proceed beyond this point due to lack of information related to the PV azimuth, tilt, mounting method, and other variables.  Hopefully you can plug those in .. and with the information above .. derive a better estimate of annual AC energy from the system.
>
>
> Regards,
>
>
> Dan
>
> P.S.  You may not get admonished by Michael W for inadvertently including my identifying information in your re-post to the Wrenches.  I'm not an installer, and no longer with any manufacturer.  For that reason, Mr. Welch has deemed me "unqualified" to post to the Wrench BBS, and persona non grata where the the BBS is concerned.  One of your colleagues on the board posted one of my comments a year or so back .. and was contacted by Michael for doing so.  Michael told him if he ever posted information to the BBS that originated from me, he too would be blocked from making any future posts.
>
> You're welcome to post anything I send to you.  I would ask for your own sake, and continued participation on the Wrench board, that in re-posting information .. you omit anything that would identify me as the source.  I have a somewhat unique style of writing, so it's also suggested you change the ".." and other characteristics to resemble what you'd type.  Plagiarize away.  I'm OK with it.
>
>
>
>
> Dan Lepinski, P.E.
> Professional Solar Consultant & Electrical Design Engineer
> Serving the Solar Industry Since 1972
> 817.884.6081 (cell)
>
>
> * 2010 State of Texas Renewable Energy Industries Association Honoree
>    and Award Recipient for Meritorious Achievement in Renewable Energy.
>   
> * 2016 Memnosyne Institute Sustainable Leadership Award
>   
> * 2017 United States Department of Energy National BCAP Project - 2017 & Ongoing...
>    (One of only two engineers selected nationwide for the program.)
>
> * 2019 State of Texas Energy Advisory Council Member
>
>
> Professional Profile: https://www.linkedin.com/in/dan-lepinski-p-e-5339bb22/
>
> "Scientists investigate that which already is; engineers create that which has never been."
> ~ Albert Einstein
>
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>
> On 8/28/19 8:31 AM, Christopher Warfel wrote:
>> Hi Dan, Sorry for the confusion.
>>
>> Maybe an example would help.  Take a 2000 w dc multimode system that is
>> coupled with a 3600 w ac rated multimode inverter.  What percentage of
>> the dc capacity is inverted into ac capacity.  So, roughly 2000 x .9
>> (wiring and similar losses from array to battery) x .95 for an assumed
>> 5% usage to charge/float the batteries x .98 for dc to ac inverter
>> efficiency makes me think that 2000 x .9 x .95 x .98 * (4.5 x 365)
>> yields = 2,752 kWh annually. The utility is assuming roughly 3600 x .98
>> x (4.5 x 365) {if they are assuming anything at all} = 5,368 kWh revenue
>> lost. The regulatory assumption overestimates the capacity factor by @49%.
>>
>> I'll post to the WRENCHES too to help clarify hopefully.   Chris
>>
>> On 8/28/2019 9:10 AM, Dan Lepinski, P.E. wrote:
>>> OFF-LINE REPLY
>>>
>>> Hello Chris,
>>>
>>> The wording of your question is confusing.  You ask about the percentage of DC capacity vs AC power output, then add batteries into the mix.
>>>
>>> What system topology is the focus of your query?
>>>
>>> I'll be glad to do what I can to assist.  Just need clear direction on the configuration.
>>>
>>>
>>> Regards,
>>>
>>>
>>> Dan
>>>