PowerWall technical discussion

[lolachampcar]

I should probably read up on the Power Optimizer thingy but my gut tells me they simply maximize DC harvest from each panel. By definition, these must be paralleled into a DC-AC inverter as no two panels (with optimizer) will produce the exact same current thus they can not be put in series. The Power Optimizer likely produces a reasonably tightly regulated DC voltage. These did not appeal to me as it meant relatively high current low voltage DC runs coming off the roof.

As for using them with a PowerWall, it would be terribly inefficient to take high current 35-60 Vdc and boosting it to the levels necessary to charge the battery. It would also be an expensive and heavy DC-DC converter.

 

[schueppert]

As the CEO of Generac pointed out, the economics of PW for backup are not great compared to NG generators. But there are many people, myself included, who can't install a permanent generator for space reasons. And portable solutions are, as you say, crazy noisy dirty. So, yeah, I'm definitely interested in PW for backup but it will depend on the inverter solution. Ideally, I'd like something that plays nicely with PW, solar and a portable generator and does smart load shedding. That may be a lot to ask for in a first generation product.

 

[mspohr]

I should probably read up on the Power Optimizer thingy but my gut tells me they simply maximize DC harvest from each panel. By definition, these must be paralleled into a DC-AC inverter as no two panels (with optimizer) will produce the exact same current thus they can not be put in series. The Power Optimizer likely produces a reasonably tightly regulated DC voltage. These did not appeal to me as it meant relatively high current low voltage DC runs coming off the roof.

As for using them with a PowerWall, it would be terribly inefficient to take high current 35-60 Vdc and boosting it to the levels necessary to charge the battery. It would also be an expensive and heavy DC-DC converter.

It appears that the SolarEdge Power Optimizer is a panel level DC-DC converter which adjusts for individual panel shading, etc. by boosting the voltage from shaded panels. This prevents a single shaded panel from blocking current from the unshaded panels in the string.
The panels are still arranged in a string with an DC-AC invertor running off the entire string.
I'm still not clear on how this works since a shaded panel could have its voltage increased but this would reduce the current in the panel (and the entire string since they are in series).

 

[schueppert]

Actually the Power Optimizers are connected in series. They use clever electronics so the current through the optimizers is the same but the current through the panels is different. Net result is you have high voltage DC coming off the roof so the losses are very low.

Don't know where you got the idea that DC-DC converters are expensive and heavy. Modern switched mode power supplies are quite a bit cheaper, lighter, smaller and more efficient than transformers at the sorts of powers we are talking about.

 

[Ampster]

Just to add to the notion that DC to DC converter are not expensive I am using inexpensive Meanwell switched power supplies as DC to DC converters. They are designed to run off AC but seem to work using DC. In my applications I don't care about efficiency but in the Powerwall application they are reported to get over 90% roundtrip efficiency.

 

[lolachampcar]

schueppert,
Logic tells me there are two ways the DC-DC optimizer scheme can work if they do indeed yield higher voltage DC.
(1) each optimizer's output is high voltage DC with each panel/optimizer contributing whatever current it can at that boosted voltage or
(2) there is magic in the optimizer that lets everyone of them know "the" current which every optimizer pushes but at different voltages depending upon harvest (i.e.. from better than rated all the way down to zero for fully shaded panels - basically, a pass through). This second option is almost exactly what the PV series panels do so it is unlikely that option 2 is what is going on.

I say magic because I am not smart enough to design that circuit.....

 

[schueppert]

There a technical note here that explains it a bit: http://www.solaredge.com/files/pdfs/se_application_fixed_string_voltage.pdf

I think it's more or less your second "magic" option.

 

[lolachampcar]

yep, magic beyond my design capabilities

 

[mspohr]

There a technical note here that explains it a bit: http://www.solaredge.com/files/pdfs/se_application_fixed_string_voltage.pdf

I think it's more or less your second "magic" option.

That's a good technical note but it doesn't say how it performs the magic of telling the different Optimizers what voltage to output. There must be some communication along the string to get everyone to coordinate their output voltage to come up with 350v for the string with different amounts of shading of different modules.
I agree, this is magic.

 

[schueppert]

Yes, it requires communications between the optimizers and the central inverter. They use powerline communication (PLC) for this; I believe Enphase does too.

I hadn't heard of SolarEdge before the Tesla announcement, but I definitely think they were a great choice of partner for Tesla. Both companies seem to share a vision of a high voltage DC bus to which you can attach solar, batteries etc. I will be really interested to see what the integrated SolarEdge/Powerwall package looks like.

 

[Cosmacelf]

With regard to using the Powerwall as a backup power source, the one thing missing from Solaredge's inverter is the ability run in hybrid mode. It's current inverter, like most solar inverters, shuts down when grid power is lost. A hybrid inverter is connected to two AC load panels, the main panel with the grid feed and a critical load panel. When power is lost on the grid, the inverter must trip a relay or equivalent to isolate itself from the grid, and then send inverted power to the critical load panel. Presumably that is what Solaredge's is building for a fall release?

 

[nwdiver]

My mother installed a SolarEdge system at her house last year; I've got a 10kW SolarEdge inverter going up on a rental once I find the time... I visited their booth at Inter-Solar last summer. Remarkable company... they've combined the advantages of a string Inverter with the Advantages of micro-inverters. They went public in March SEDG... I might have missed that boat :crying:

 

[schueppert]

Presumably. There are several other product features that may not be essential but would certainly be nice to have. My ideal inverter for backup power would support any combination of Powerwall, solar and a portable generator as power sources during grid failure and would seamlessly connect these to the critical loads in the home with smart load shedding if required. If the available power is greater than the demand and the battery is not fully charged then I would like the excess power to be used to charge Powerwall. This may be a lot to ask for a 1st generation product.

 

[mspohr]

My mother installed a SolarEdge system at her house last year; I've got a 10kW SolarEdge inverter going up on a rental once I find the time... I visited their booth at Inter-Solar last summer. Remarkable company... they've combined the advantages of a string Inverter with the Advantages of micro-inverters. They went public in March SEDG... I might have missed that boat :crying:

I looked at SolarEdge and Enphase for my solar system and ended up going with Enphase primarily because my installer was familiar with them and not with SolarEdge.
I know the SolarEdge panel convertors are cheaper than Enphase but you still have to have a string inverter with SolarEdge so the total cost of the system works out about the same.
With SolarEdge, you still have min/max limits on the string size but with Enphase you don't have strings so you can easily have any number of panels.
I was wondering what advantages SolarEdge has over Enphase? Is it really more advantageous to have a panel level DC-DC convertor plus a string inverter? It would seem to me to just add complexity and more points of failure.
I realize that there may be some advantages to charging a battery directly with DC but that has only come up as a use case recently and it seems this will require additional/different hardware.

 

[mitch672]

another important function the optimizers perform is rapid DC shutdown.
There are new UL requirements in this area, for firefigthers etc, so that the high voltage DC can be shutoff quickly.
They do this communicating over the DC power link, each optimizer can be set to output just 1V DC.
I believe the new product solaredge is designing will have an additional AC output, for the critical loads that you want to run in a grid failure scenario.

 

[nwdiver]

I was wondering what advantages SolarEdge has over Enphase? Is it really more advantageous to have a panel level DC-DC convertor plus a string inverter? It would seem to me to just add complexity and more points of failure.

SolarEdge is generally slightly cheaper than enphase... enphase is ~$0.64/w vs $0.22 (DC optimizer) + $0.35/w (3.3kW) to $0.18/w (10kW) (String Inverter) = ~$0.44/w.

Benefits to Optimizers vs Micros;
- Optimizers scale better... ~$0.40/w for 10kW residential to <$0.30/w for 20kW commercial.
- Cramming MPPT + inverter is a lot for a roof-top module. Solar edge takes a 'division of labor' approach.
- Local plant monitoring. You need a Gateway for micros... you can just look at the string inverter for optimizers.

another important function the optimizers perform is rapid DC shutdown.

Micro-Inverters also comply with this.

 

[Cosmacelf]

I looked at SolarEdge and Enphase for my solar system and ended up going with Enphase primarily because my installer was familiar with them and not with SolarEdge.
I know the SolarEdge panel convertors are cheaper than Enphase but you still have to have a string inverter with SolarEdge so the total cost of the system works out about the same.
With SolarEdge, you still have min/max limits on the string size but with Enphase you don't have strings so you can easily have any number of panels.
I was wondering what advantages SolarEdge has over Enphase? Is it really more advantageous to have a panel level DC-DC convertor plus a string inverter? It would seem to me to just add complexity and more points of failure.
I realize that there may be some advantages to charging a battery directly with DC but that has only come up as a use case recently and it seems this will require additional/different hardware.

When you look at failure rates, you have to look deeper than just the number of failure points. The problem with Enphase is that their micro-inverters need to do DC-AC conversion which requires much more complex electronics than what SolarEdge's panel optimisers do. If look at traditional inverter manufacturers, you'll find the most they can warranty their inverters for is about 12 years. Along comes Enphase and says their roof mounted micro-inverters, which operate in a much harsher environment, can work for 25 years. Hmmm. I've done some research and Enphase does have a reputation for micro-inverter failures, and their warranty doesn't include the expensive labor required to swap out a dead micro-inverter. Meanwhile I have yet to see a pattern of complaints against SolarEdge (please correct me if I'm wrong). Also, this whitepaper of theirs is very interesting: http://www.solaredge.com/files/pdfs/solaredge-reliability-methodology.pdf

 

[mspohr]

When you look at failure rates, you have to look deeper than just the number of failure points. The problem with Enphase is that their micro-inverters need to do DC-AC conversion which requires much more complex electronics than what SolarEdge's panel optimisers do. If look at traditional inverter manufacturers, you'll find the most they can warranty their inverters for is about 12 years. Along comes Enphase and says their roof mounted micro-inverters, which operate in a much harsher environment, can work for 25 years. Hmmm. I've done some research and Enphase does have a reputation for micro-inverter failures, and their warranty doesn't include the expensive labor required to swap out a dead micro-inverter. Meanwhile I have yet to see a pattern of complaints against SolarEdge (please correct me if I'm wrong). Also, this whitepaper of theirs is very interesting: http://www.solaredge.com/files/pdfs/solaredge-reliability-methodology.pdf

Not sure SolarEdge DC-DC convertors are inherently less complex since they work internally by converting DC-AC-DC whereas the Enphase just do DC-AC. I expect SolarEdge have some advantage in that they can use higher frequency AC during the conversion which lets them use cheaper parts.
Enphase has a reliability white paper also: http://enphase.com/wp-uploads/enphase.com/2011/03/Enphase_WhitePaper_Reliability_of_Enphase_Micro-inverters.pdf

As for "reputation" and "pattern of complaints", I will just quote Lebowski "Well, that's, just, like, your opinion, man."
Yeah, well thats just your opinion man. (Big Lebowski) - YouTube

 

[nwdiver]

Not sure SolarEdge DC-DC convertors are inherently less complex since they work internally by converting DC-AC-DC whereas the Enphase just do DC-AC.

In the strictest sense, yes, I suppose it would be DC-AC-DC but the kind of AC a micro-inverter must produce is an order of magnitude more complicated. ~60Hz, 240vac, synced to the grid.

The Boost/Buck circuit used by optimizers are relatively simple... as is reflected in their cost. $60 for a 300w optimizer vs $150 for a 250w micro-inverter.

At the Inter-solar conference there was talk of integrating MPPT into the module. Each PV module is usually 3 strings of cells, each string can be bypassed by a diode. Instead of a diode at some point a Boost/Buck circuit could be used allowing each cell string to operate at it's max power point.

 

[Cosmacelf]

Enphase has a reliability white paper also: http://enphase.com/wp-uploads/enpha...er_Reliability_of_Enphase_Micro-inverters.pdf

So, have you read both whitepapers? Skimmed them at least? Enphase's is very, very unimpressive compared to SolarEdge's.

As for "reputation" and "pattern of complaints", I will just quote Lebowski "Well, that's, just, like, your opinion, man."

I did ASK in my post if anyone could find any complaints about SolarEdge reliability since I couldn't find ANY. If you can show me some complaints, I'm all ears.