Sun hours

[J.P.M.]

Here is the post I was trying to use for guidance. Bruce

Bruce:

Thank you. I found it.

A question if I might: How many STC watts did it take to produce that 157 kWh ?

[bcroe]

Bruce:

Thank you. I found it.

A question if I might: How many STC watts did it take to produce that 157 kWh ?

Let me try to cover that. The inverters have an AC output rating of 15KW, at an efficiency of 96%
they can take in 15.6KW DC. My odd collection of 7 year old panels (when new) had a Standard
Test Condition rating 2.244 times that, though I have doubts some the no-names really made it.
Bruce

[J.P.M.]

Let me try to cover that. The inverters have an AC output rating of 15KW, at an efficiency of 96%
they can take in 15.6KW DC. My odd collection of 7 year old panels (when new) had a Standard
Test Condition rating 2.244 times that, though I have doubts some the no-names really made it.
Bruce

Thank you.
Another question/2 please: Are you inverter limited by the POCO or other reasons to a max. inverter size/capacity ? If so, what is it ?

Respectfully,

[bcroe]

J. P. M.,
There was quite a list of constraints trying to get this all together. First was timing. I had a decent handle on energy,
especially electrical. The long reduction of KWHwaste at the new property was concluding, and the heating levels
were established. The gas company was about to serve my country block, and I was quite familiar with their billing
escalations entirely outstripping my attempts to conserve. The solar technology was becoming competitive with
other energy, along with regulations and rebates, and I had space.

So spring 2013 I contracted to build a solar system in line with HVAC and electric consumption here. The largest
15KW option seemed about right if the house insulation was improved some. This was already larger than the base
consumer level, and as I recall approaching the largest the PoCo could accommodate with the existing 200A service.
It was also the largest I could handle using the existing buried 600 foot loop of 4 gauge AC wire. This wire was
marginally efficient with over 3% loss, and the 9V boost over the already high line voltage was on the edge. The
PoCo contract specified inverters down to the serial numbers.

Very soon discovered, was that the perpetual clouds of varying density, would severely reduce energy collection. I
often got to watch all my equipment sitting there doing very little. Increasing the system peak rating was out of the
question because of all the above constraints. And a bigger plant would have had me
watching EVEN MORE equipment doing very little.

Anyway, I did not need a bigger plant, I NEEDED to keep the existing plant busy. I just needed more panels to
increase output under clouds. Panels were only a quarter of the system cost. Pointing more panels S would exceed
the inverter rating, but phasing them E and W would solve that problem while also extending useful hours when the
sun did shine. If this had not worked, I would have needed to electronically phase in as many panels as the inverters
could handle, a dynamic and dangerous method with clouds constantly moving. That was not necessary.

The E-W array was very cheap and efficient, a mount could handle twice as many panels, and I believe the wind
loading was not increased. A south only mount would require about twice the support structure.

Later array modifications had minor energy improvements, they were mostly a traded off of convenience (snow
handling) vs first cost. The house insulation upgrade is still largely in the future, but I found other ways to compensate.
With the additional costs per KWH of these approaches, it could be concluded that this system is too expensive. My
own estimates are that ANY solar system attempting to operate under these clouds is not very much more effective,
and that that is just part of the cost of avoiding all the problems of living near the oceans. Bruce Roe

[J.P.M.]

J. P. M.,
There was quite a list of constraints trying to get this all together. First was timing. I had a decent handle on energy,
especially electrical. The long reduction of KWHwaste at the new property was concluding, and the heating levels
were established. The gas company was about to serve my country block, and I was quite familiar with their billing
escalations entirely outstripping my attempts to conserve. The solar technology was becoming competitive with
other energy, along with regulations and rebates, and I had space.

So spring 2013 I contracted to build a solar system in line with HVAC and electric consumption here. The largest
15KW option seemed about right if the house insulation was improved some. This was already larger than the base
consumer level, and as I recall approaching the largest the PoCo could accommodate with the existing 200A service.
It was also the largest I could handle using the existing buried 600 foot loop of 4 gauge AC wire. This wire was
marginally efficient with over 3% loss, and the 9V boost over the already high line voltage was on the edge. The
PoCo contract specified inverters down to the serial numbers.

Very soon discovered, was that the perpetual clouds of varying density, would severely reduce energy collection. I
often got to watch all my equipment sitting there doing very little. Increasing the system peak rating was out of the
question because of all the above constraints. And a bigger plant would have had me
watching EVEN MORE equipment doing very little.

Anyway, I did not need a bigger plant, I NEEDED to keep the existing plant busy. I just needed more panels to
increase output under clouds. Panels were only a quarter of the system cost. Pointing more panels S would exceed
the inverter rating, but phasing them E and W would solve that problem while also extending useful hours when the
sun did shine. If this had not worked, I would have needed to electronically phase in as many panels as the inverters
could handle, a dynamic and dangerous method with clouds constantly moving. That was not necessary.

The E-W array was very cheap and efficient, a mount could handle twice as many panels, and I believe the wind
loading was not increased. A south only mount would require about twice the support structure.

Later array modifications had minor energy improvements, they were mostly a traded off of convenience (snow
handling) vs first cost. The house insulation upgrade is still largely in the future, but I found other ways to compensate.
With the additional costs per KWH of these approaches, it could be concluded that this system is too expensive. My
own estimates are that ANY solar system attempting to operate under these clouds is not very much more effective,
and that that is just part of the cost of avoiding all the problems of living near the oceans. Bruce Roe

Bruce:

Thank you for the information. I would have done it differently, but you would have probably done my stuff differently as well. Either and any way, it doesn't matter much now.

Respectfully,

[DanS26]

Bruce....since the nameplate capacity for our two Fronius 7.5 inverters is the same it is interesting to make comparisons.

One measure I have used is Capacity Factor of the installed plant. The actual calc is explained here.....

https://en.wikipedia.org/wiki/Capacity_factor

I have calculated my plant at ~20% capacity and using data you posted on PVOutput.org your plant runs at ~22% capacity. Your greater plant capacity is most likely due to DC/AC ratio and panel orientation.

[J.P.M.]

Bruce....since the nameplate capacity for our two Fronius 7.5 inverters is the same it is interesting to make comparisons.

One measure I have used is Capacity Factor of the installed plant. The actual calc is explained here.....

https://en.wikipedia.org/wiki/Capacity_factor

I have calculated my plant at ~20% capacity and using data you posted on PVOutput.org your plant runs at ~22% capacity. Your greater plant capacity is most likely due to DC/AC ratio and panel orientation.

On capacity factor, for plants, I'm not a fan of using such factors as have historically been used for nuke, hydro or fossil fuel fired power producing plants as I don't think the somewhat unique factors of fuel availability and siting are accounted for in a meaningful or useful way by the capacity factor.

That said, or written, I believe the capacity factor for PV is usually and commonly understood to be based on D.C or the nameplate STC rating of the power producing equipment, that is, in front of the inverter, with any factors including orientation or inverter size reductions and other things being reflected in the (lower) capacity factor.

In Bruce's case he's got 35 kW of panels. Any losses such as those from inverters or less than optimum orientations, etc. be they necessary, or unavoidable, or as as mandated by design or preference by the owner have the effect of lowering the capacity factor.

Using that 35 kW number as the facility (D.C.) capacity, the denominator of the equation for capacity factor for PV plants referenced in your attachment becomes :

(365 days)*(24 hours/day)*(35 kW) = 306,600 kWh.

Now, I don't recall what Bruce's annual production is, but if I use your number of Bruce's capacity factor of ~ 22%, and know you are using 15 kW of inverter capacity instead of the 35 kW of STC rating, I get a capacity factor for Bruce's system of : 0.22 *(15/35) = 0.094.

For some reference only, my long term (6 1/2 years anyway) capacity factor using averaged annual system output of 9,068 kWh/yr.:

(9,068 kWh/yr)/[(365 days)*24 hours/day)*(5.232 kW)] = (9,068 kWh/yr.)/(45,832) = 0.1979.

That includes an ~ 3.5 % output reduction for shading and an inverter that clips at 5,035 W. The 5.232 kW system size is based on 327 STC W/panel *16 panels.

I could have split the array into 3 orientations and in so doing reduced the annual output to ~, say, 8,000 kWh/yr. (making it somewhat analogous to what Bruce has), or with a 206 deg. az. and 31 deg. tilt for a single array have increased the annual output to ~ 9,200 kWh/yr. If I could have cut the tree down that's at a 220 azimuth to my array, I'd have boosted array output by ~ 3.5 %/yr. If I had done any of those things, the capacity factor would have been affected in direct proportion to the changes in output.

By the definition the capacity factor of a PV system is based on the DC (or the STC) system size, not the inverter rating. As I understand it, inverter capacity can only effect the numerator of the capacity factor equation, and only then to the extent that the inverter capacity is < the STC rating of the what is feeding that inverter.

[Ampster]

..........
One measure I have used is Capacity Factor of the installed plant.
I have calculated my plant at ~20% capacity and using data you posted on PVOutput.org your plant runs at ~22% capacity. Your greater plant capacity is most likely due to DC/AC ratio and panel orientation.

Yes, I like to use those kind of metrics also. Capacity factors above 20% are not as usual because the sun only shines part of the time. Bruce has also used a ratio of annual production in kWhrs to inverter capacity in kW. That is a metric used by solar farms as one measure of cost effectiveness. You are correct that higher DC to AC ratios and orientation influence that. Whether one uses STC capacity or inverter capacity is not important as far as comparing one system to another, because they may have been designed with different goals in mind.

The use of a particular ratio or calculation is more a matter of whether one is useful for the person doing the analysis. No doubt different metrics provide insight into different elements of a system. It may even depend on the goals of the system design which may be financial, resource management, risk management or some combination of those.

[J.P.M.]

Whether one uses STC capacity or inverter capacity is not important as far as comparing one system to another, because they may have been designed with different goals in mind.

The use of a particular ratio or calculation is more a matter of whether one is useful for the person doing the analysis. No doubt different metrics provide insight into different elements of a system. It may even depend on the goals of the system design which may be financial, resource management, risk management or some combination of those.

I'm not sure the term "Capacity Factor" as it's defined and used (or misused) is a good metric for PV system design or system comparison. Historically, it was conceived for use in/for power generating facilities where the fuel source is more reliable and predictable. I believe it's use can a source of more confusion than help for PV system design and analysis - the case of systems with high DC/AC ratios being an example.

But if it is used, as a start I'd suggest it's important to state whether or not inverter capacity or system nameplate capacity is used in the denominator of the calculation.

As an alternate or modification to the metric "Capacity Factor" as the term might be applied to PV systems, I'd suggest changing the capacity factor by dividing it by a PV system's DC/AC ratio and using the more commonly used "nameplate" or STC system capacity in the denominator. Some confusion or slight of hand, or just plain deception might be avoided. Besides, what's the problem with being clear about stuff ?

Better yet, a more meaningful (but certainly not the only) metric - when discussing either energy or economic efficiency with respect to P systems anyway - is net annual plant output per installed STC kW.

(Bruce: If you're reading this, apologies about writing about you in the third person) This isn't an indictment of what Bruce has done and isn't meant as such at all. If I was near him, I might have volunteered my services to help dig holes or trench ditches, or wire something, etc., and considered the labor as the tuition of learning something. Being somewhat of a fellow eccentric, I get what he's done. More than a few of my neighbors see me fooling around with the stuff on my roof and scratch their heads. Besides, NOMB how someone allocates their resources, including time.

An example of why using inverter size for calculating capacity factor rather than system nameplate rating doesn't reveal much, if any information and may even be misleading:

Bruce has a 35 STC kW PV system. On 05/15/2019 he stated the system's latest output is ~ 28,500 kWh/yr., or ~ 28,500/35 ~ = 815 or so kWh/yr. per installed STC kW.

Without too many particulars about system design, a 20 STC kW ground mounted array facing south at a 42 deg. tilt in zip 61084 and powering 15 kW of inverter capacity will produce something like 28,600 kWh/yr. according to PVWatts. (That figure is after ~ 2,900 kWh/yr. of clipping caused by undersizing the (15 kW) inverters on a 22 STC kW system - see next paragraph).

Now let's bump that STC size by 10 % just in case Bruce's annual reported figure is for a very good year and leave the system output at 28,500 kWh/yr. to be on what's probably the very conservative side of things.

Using the 15 kW of inverter capacity with a DC/AC ratio of 22/15 = 1.467for each system, the capacity factor is essentially the same for each system: 28,500 kWh/yr/[((365days/yr.)*24hrs./day)*(15 kW inverter capacity) = 0.217.

However, which one do you think would cost less to acquire, install and operate in most any similar situation and climate ? A 35 STC kW ground mounted system with multiple orientations or a 22 STC kW ground mounted single orientation system ?

Put another way, which system do you think will have a higher probability of producing a lower LCOE - one that produces 28,500 kWh/yr/35 STC kW = 815 kWh/yr per STC kW, or one that produces 28,500 kWh/yr./22 STC kW = 1,295 kWh/yr. per STC kW ?

Bottom line, in vernacular terms, which system of the two will have the better chance of producing more long term bang for the buck ?

And, with respect to Capacity Factor, what does the above show about the usefulness of Capacity Factors without some considerations of how the system size is defined ?

For many folks with PV systems, like Joe/Jane 6 pack, the big reason they have PV at all is probably, if not solely, or at least primarily for the purpose of lowering their cost of providing electricity to their residence. I'd suggest such folks don't care as much about capacity factors which as shown here, may not reveal as much, or much of anything about as the cost of the system as they care about how much a system produces in the way of revenue or bill offset in $$.

Aside from the problems I have with definitions used, or assumed, or undefined when using capacity factor with respect to PV systems, it seems to me that Capacity Factor is pretty useless without a lot more definition, particularly with respect to exactly how "maximum capacity" is defined.

Take what you want of the above. Scrap the rest.

[Ampster]

............ Capacity Factor is pretty useless without a lot more definition, particularly with respect to exactly how "maximum capacity" is defined.

Once a system is built the only metric that really matters is kWhs produced over a year.

[bcroe]

Enjoying all the activity and comments here. A couple of my comments.

My opinion is, to compare 2 designs, they both should be simulated the same way, such as by PVwatts.
Comparing a theoretical system simulation to a different actual systems performance is not apples to
apples. Real systems have variations both externally (weather) and internally (shading, wire loss, panel
mismatching or aging, etc) not accounted for in a simulation. My shading issues are not eliminated,
but improved a bit annually.

Inverters here are recommended to over panel up through 115% total, not 147%, of the AC rating. A more
subtle design feature here has been to observe that limit, at least approximately. It will not be precise till
I get the clipping meter functioning, I do not anticipate any panel throttling needed ahead of the inverter(s).

Ground mount array supports to my requirements are more expensive than the panels they hold. The
incremental cost of converting to a 2 sided mount is small, so the mount cost for a single orientation array
approaches being double per KW. Shared return wires help. Bruce Roe

[DanS26]

Take a look at this GSES Technical Paper.......Oversizing PV Arrays...

https://www.gses.com.au/wp-content/u...Oversizing.pdf

On page 2 it says......"Basic modelling indicates that the maximum benefit of oversizing is realised with an oversizing ratio of 150%".

I sized my system based on this paper.

[DanS26]

Furthermore I built my Shoulder array "East-West orientation" based on information from this Fronius white paper.........

https://www.energymatters.com.au/ima...olar-paper.pdf

I have been very happy with the results.

[DanS26]

In addition since I'm running the system at its maximum with hours of clipping on very hot days I am very concerned with temps in the inverters and all electrical connections. I am particularly concerned about the molded case breakers since they can run very hot for extended periods. This advice from Schneider Electric provides guidance from the UL489 standard........

https://www.se.com/us/en/faqs/FA173839/

I monitor the heat at appropriate places throughout my system. I've replaced a few breakers that ran hot just as a prophylactic. A little prevention goes a long way especially if you're running on the high end. I also use silver conductive grease on all connections and terminations where appropriate and always used a torque screwdriver or wrench as appropriate. Never had a failure....knock on wood.

[J.P.M.]

Enjoying all the activity and comments here. A couple of my comments.

My opinion is, to compare 2 designs, they both should be simulated the same way, such as by PVwatts.
Comparing a theoretical system simulation to a different actual systems performance is not apples to
apples. Real systems have variations both externally (weather) and internally (shading, wire loss, panel
mismatching or aging, etc) not accounted for in a simulation. My shading issues are not eliminated,
but improved a bit annually.

Inverters here are recommended to over panel up through 115% total, not 147%, of the AC rating. A more
subtle design feature here has been to observe that limit, at least approximately. It will not be precise till
I get the clipping meter functioning, I do not anticipate any panel throttling needed ahead of the inverter(s).

Ground mount array supports to my requirements are more expensive than the panels they hold. The
incremental cost of converting to a 2 sided mount is small, so the mount cost for a single orientation array
approaches being double per KW. Shared return wires help. Bruce Roe

Thank you for your response.

I wholeheartedly and very strongly agree with your 1st opinion with respect to modeling. Before my last post I was wishing I had the particulars about your arrays/system so I could do some modeling comparisons.

To that end, before I go any further into discussing the rest of your above post, if you post all the relevant input PVWatts requires including inverter AC size pertinent to your arrays, I'll run the PVWatts analysis and compare it to a run of a south facing array at 42 deg. tilt and sized so that PVWatts produces the same output with the same inverters and total inverter size for both systems. Please include the azimuths and also seasonal tilt angles and the approx. dates of tilt changes. I'll keep the single orientation array that's compared to yours at 42 deg. tilt.

I'll even go you one better. Give me module maker, model/size and approx. date when they entered service, and inverter model(s) for SAM. I'll use the same data for both systems for the SAM input.

To the extent we agree to trust models' output, we all might learn something.