Lothsahn
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Re: Dumbest excuses people have given for NOT installing PV

Tue Apr 02, 2019 12:23 pm

goldbrick wrote:
SageBrush wrote:Reliability aside, I would avoid micro-inverters due to the high i^2 losses. I personally also find the parallel wiring scheme
to more difficult to work with and my impression from a few projects is that a lot more cabling is used.
I'd think micro-inverters would require smaller cables and generate less heat in the cables (i^2*R losses). The power through the wires = V*I and with micro-inverters the voltage is much higher so the current is much less. That is the reason long distance power lines are such high voltage; it reduces the transmission losses.
Aren't microinverters outputting 240v AC while strings are typically around 400V DC?
goldbrick wrote: I'll probably sketch out the design with both configurations before I decide. I am a bit worried about the reliability factor (more active parts means more chances of failure to me...) but the reliability is supposedly good and they are covered by a warranty. And if one or two fails, I'd still be generating power from the remainder of the panels.
FYI, if you go with String inverters, you're still probably going to want DC optimizers on your roof (due to shading issues and safety issues*). This means that you'll still have an electronic component on your roof under each panel, whether it's a micro inverter or optimizer. Also, some micro inverters can service two panels, resulting in half as many components.

For ground-mounted, unshaded systems facing the same direction, string inverters have the lowest TCO and lowest # of components. However, string inverters have a shorter warranty than micro inverters, as they're handling significantly higher amounts of power.

* Each power optimizer is equipped with the unique SafeDC™ feature which is designed to automatically reduce modules' DC voltage to a safe level whenever the inverter or grid power is shut down (unless connected to a StorEdge™ Inverter that is operating in backup mode).
https://www.solaredge.com/us/products/power-optimizer
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goldbrick
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Re: Dumbest excuses people have given for NOT installing PV

Tue Apr 02, 2019 2:16 pm

Lothsahn wrote:Aren't microinverters outputting 240v AC while strings are typically around 400V DC?
oops! I guess my ignorance is showing again. :? I do believe you are right. This must mean the 'string' is wired in series which raises some interesting questions (for me at least....) about how the frames, etc of the panels are grounded/insulated. I guess I better go back and school myself some more on this before I decide on the best approach.

Lothsahn
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Re: Dumbest excuses people have given for NOT installing PV

Tue Apr 02, 2019 3:33 pm

goldbrick wrote:
Lothsahn wrote:Aren't microinverters outputting 240v AC while strings are typically around 400V DC?
oops! I guess my ignorance is showing again. :? I do believe you are right. This must mean the 'string' is wired in series which raises some interesting questions (for me at least....) about how the frames, etc of the panels are grounded/insulated. I guess I better go back and school myself some more on this before I decide on the best approach.
String is wired in series. The number of panels per string is defined by the inverter that you are using--you can't have too few or too many. This is also why a single panel or DC optimizer failure can take out the entire string's production.
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RegGuheert
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Re: Dumbest excuses people have given for NOT installing PV

Wed Apr 03, 2019 5:21 am

Lothsahn wrote:
RegGuheert wrote:Simply put, DC is much more dangerous than AC, both in terms of a shock hazard and as a fire hazard. SolarEdge has done a lot to make it safe in their system, but the regulations had already been put in place due to issue with prior technologies.
Safety: Can you explain why this is? If you do a quick google search DC vs AC safety, you'll get a bunch of answers saying DC is safer.
https://www.brighthubengineering.com/po ... omparison/
https://www.electronicproducts.com/Powe ... ltage.aspx
I should be more clear: DC PV systems are much more dangerous than AC PV systems. There are many reasons for this, some of which you have already mentioned. First, let's start with what is the same:
- In both types of systems, the AC voltages are the same: 240 VAC split-phase here in North America, which means that the highest voltage ever seen is 120 VAC (about 170 Vpeak) away from ground.

That is where the similarity ends:
- In microinverter-based systems, the highest DC voltage outside any electron module is the voltage from one PV module, which is no more than 50 VDC in the worst case. In string inverter, the maximum DC voltage can be as high as 1000 VDC in some countries (with the capability to sustain as much as 15 or more amps.
- High voltages are on the roof whenever the sun shines.
- At night, high voltages are even present just from lights, such as those used by firefighters.

By contrast, in a microiverter-based system, the AC voltage on the roof is provided by the power grid. If you want to remove it, you simply need to throw a switch to turn off grid power to the array. Since the microinverters are DC-powered, they CANNOT produce AC at night and they are engineered so that they DO NOT produce AC when the grid is switched off. The provision of a switch allows firefighters and anyone working on or around the array to be safe on the roof. Not so with string inverters, which will have high voltages present regardless of whether a switch is turned off or not. (I have even been shocked by a DC system hours after sunset. Fortunately, that was only by four panels in series!)

But, as I said previously, SolarEdge has engineered most of these risks out of their system by adding the "Power Optimizers" which break up the DC string when the main inverter is switched off. They also limit the DC voltage to about 400 VDC when operating and include arc fault detection and circuit interruption.
Lothsahn wrote:Fire Hazard: I would expect heat to be current*resistance... roughly proportional to the power flowing through them, given equivalent sized cables. Why is DC solar more likely to catch fire, or be a hazard during a fire?
This topic has been discussed previously in this thread.

Basically, it is MUCH more difficult to extinguish an arc in a DC system than in an equivalent AC system. This is because the AC voltage and current go through zero 120 times per second which the DC current is continuous. A microinverter-based PV system limits the voltage of the DC component, thus limiting the arc capability in the system. Enphase has done a demonstration to show the difference in the DC arc risk in the two types of systems:



Click here for the link to the YouTube video in case you do not see it immediately above.

If you search for news of fires caused by photovoltaics, you will find that those fires were inevitably started by string inverters. I do not know of a single fire caused by a microinverter.

And you are correct in a later post you made: Now that SolarEdge has created central inverters with ridiculously-high efficiencies and tackled the shading problem, DC systems should be more efficient.
RegGuheert
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Lothsahn
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Re: Dumbest excuses people have given for NOT installing PV

Wed Apr 03, 2019 6:13 am

RegGuheert:

Thank you so much for your informative post. This is the first and only information I was able to locate on why DC PV systems are more dangerous and exactly what I was looking for!

Do you agree that a DC Optimizer system has roughly the same safety as a Microinverter, or are there still added risks? It sounds like the major added risk is that, when operating, the output voltage is significantly higher (400V vs 240V/50V)

Also, if the micro inverters are providing 240V AC, how is the single panel voltage only 50V? Doesn't the microinverter have to output 240V AC along the trunk line to the breaker box? Or are you saying that the voltage inside each panel (and the wiring to the microinverter) is 50V?
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Oilpan4
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Re: Dumbest excuses people have given for NOT installing PV

Wed Apr 03, 2019 8:01 am

The main reasons that are legitimate are if the utility is anything like my last electricity provider excel-energy.
Not only did they evoke every regulation, approval and inspection process known you had to pay an additional $30 per month for a cogen meter. The only solar that excel wants on their lines is from assets they own and control.
So that pretty much kills it.

For my solar panel power system each panel or pair of panels is going to be ran all the way back to a junction box so I can reconfigure or modify the setup on the ground.
I have almost unlimited emt and wire so not worried about running some extra wire or larger conduit. Probably add a disconnect that breaks the string in 3 places that way each isolated string only has a max of about 150vdc when the switch is opened.

I have the green light from the coop to install up to 67kw on my property as is.
(No hoa or any bs like that)
But it doesn't make sense for me to install more than around 10kw.
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RegGuheert
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Re: Dumbest excuses people have given for NOT installing PV

Wed Apr 03, 2019 4:46 pm

Lothsahn wrote:Thank you so much for your informative post. This is the first and only information I was able to locate on why DC PV systems are more dangerous and exactly what I was looking for!
You are welcome!
Lothsahn wrote:Do you agree that a DC Optimizer system has roughly the same safety as a Microinverter, or are there still added risks?
I recommend SolarEdge equipment without reservations, including safety.
Lothsahn wrote:It sounds like the major added risk is that, when operating, the output voltage is significantly higher (400V vs 240V/50V)
While that is true, according to this link, both 400 VDC and 240 VAC (which only gets 120 VAC from ground) are capable of causing ventricular fibrillation (thresholds given are 200 VDC and 40 VAC).
Lothsahn wrote:Also, if the micro inverters are providing 240V AC, how is the single panel voltage only 50V? Doesn't the microinverter have to output 240V AC along the trunk line to the breaker box? Or are you saying that the voltage inside each panel (and the wiring to the microinverter) is 50V?
Here is what I actually wrote above:
RegGuheert wrote:In microinverter-based systems, the highest DC voltage outside any electron(ic) module is the voltage from one PV module, which is no more than 50 VDC in the worst case.
I think it is clear that I was discussing DC voltages, since that is what I wrote.
RegGuheert
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lorenfb
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Re: Dumbest excuses people have given for NOT installing PV

Sun Apr 07, 2019 9:27 pm

Re-title: Reasons for not installing PV

Having a gable roof where the ridge line is in the east-west plane, and the southern section of the gable roof has skylights is problematic
for a PV install. Besides, when the roof load is near max with heavy composition shingles (350 lbs/square), a PV install will not be approved
by the city's planning department.
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Oilpan4
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Re: Dumbest excuses people have given for NOT installing PV

Mon Apr 08, 2019 5:41 am

I went to the home and garden show here this weekend.
2 solar installers were there. Last year there were 0 at the show.
They both said the typical system they install is around 6kw. Both want approximately $15,000.
Yeah forget that.
One company does micro inverters on each panel the other does optimizers and a big central inverter. Both use panels in the 250 to 265 watt range. One uses the cool expensive proprietary aluminum racks the other uses unistrut or unirack.

I'm looking at more like $5,000 to $6,000 for 6kw.
Used 330w panels at least, preferably used 435w panels, freight ship them to where I work and just about cut freight shipping in half, unistrut racking on a metal roof, already have all the wire and conduit, get a used big string inverter with multiple inputs. I will try my echo and Crossman solar panel optimizers first. Then I can add traditional ones later on if it appears necessary.
Last edited by Oilpan4 on Mon Apr 08, 2019 8:22 am, edited 1 time in total.
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SageBrush
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Re: Dumbest excuses people have given for NOT installing PV

Mon Apr 08, 2019 6:40 am

^^ I'm under the impression that the optimizers have to talk with the central inverter. That is certainly the value added by the SolarEdge systems.
If generally correct, your mix&match scheme is not going to work.

On the other hand, you may be able to do without optimizers if you do not have shading issues. I say 'may' since Colorado is now requiring panel level shut-down electronics to be code compliant. I do not know if NM has the same.
They both said the typical system they install is around 6kw. Both want approximately $15,000.
Yeah forget that.
If the quote is before tax credit then ~ $10k for 6 kW is an outstanding turn-key price. The system is paid off in ~ eight years.
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