California to require solar panels on all new homes

[ggunn]

That same could be said of portable engine-driven generators, but with a little bit of intelligent load shedding, it should be possible to power the fridge, furnace and a few essentials during a blackout.

Except that generators behave as voltage sources and can deliver current on demand.

 

[GoldDigger]

Except that generators behave as voltage sources and can deliver current on demand.

This is particularly a problem for starting motors. A generator with energy stored in the rotation of the generator and prime mover can deliver high peak currents and will degrade the output voltage gracefully when overloaded. Inverters without a battery as a secondary energy source do not have this peak reserve available and would require special design to avoid a complete voltage collapse on overload. Even inverter type generators may have less peak reserve capacity than their more traditional counterparts.

 

[ggunn]

Good to know and I stand corrected.

Followup: I just looked, and the secure power supply is shown for the -22 series but not on the -40 series. I thought that the -22 series was discontinued.

http://files.sma.de/dl/27676/SB30-77-US-DUS174221.pdf

 

[electrofelon]

True enough, but how would you get the inverter output to track the load?

So we have two cases, when the array (at or near MPP) is making more than the load, and when it is making less than the load. The latter case is easy, there is nothing that can be done. For the former case, I dont think that is big a deal (in theory) and of course done all the time with off grid charge controllers by PWM. IT could also be done by operating the array at different parts of the power curve, the limitation being the amount of bucking or boosting the inverter is capable of. I assuming it would take some redesign of a grid inverter to make it capable of this. It would be interesting to hack into a GTI, remove the anti islanding scheme, and reprogram it to seek stable output voltage rather than max power and see how it works.

 

[ggunn]

So we have two cases, when the array (at or near MPP) is making more than the load, and when it is making less than the load. The latter case is easy, there is nothing that can be done. For the former case, I dont think that is big a deal (in theory) and of course done all the time with off grid charge controllers by PWM. IT could also be done by operating the array at different parts of the power curve, the limitation being the amount of bucking or boosting the inverter is capable of. I assuming it would take some redesign of a grid inverter to make it capable of this. It would be interesting to hack into a GTI, remove the anti islanding scheme, and reprogram it to seek stable output voltage rather than max power and see how it works.

Maybe, but if it were that simple I'd think someone would already be doing it with a integral ATS and a protected loads output, similar to what the Sunny Island has.

 

[drcampbell]

Im still mad at them for ruining the gas can :rant:

Seems like a reasonable enough accommodation, in exchange for undoing decades of ruining the air.

True enough, but how would you get the inverter output to track the load?

Configure it for constant-ac-voltage output.

 

[electrofelon]

Maybe, but if it were that simple I'd think someone would already be doing it with a integral ATS and a protected loads output, similar to what the Sunny Island has.

I still speculate that it is as easy as changing a few lines of code, and of course adding an ATS or a separate output like the SMA's with the plug. IMO its just not practical: You would have changes in irradiance, shading, clouds all conspiring to make the consumer frustrated that is stuff keeps coming on and off. Add to that the typical limited duration power goes out and that, obviously, no matter what, 12 hours a day you are out of luck anyway.....

 

[ggunn]

Configure it for constant-ac-voltage output.

Convert it from a current source to a voltage source? Well, duh.

The devil is in the details.

 

[GoldDigger]

Maybe, but if it were that simple I'd think someone would already be doing it with a integral ATS and a protected loads output, similar to what the Sunny Island has.

The SMA Secure Power Supply design takes advantage of the fact that the array's nominal power is several times the design capacity of the inverter. It does not have to optimize operation to get the absolute maximum power. It does not have to (but may) include enough capacitance for energy storage through the peak of each half cycle.

And, of course, the Sunny Island is a battery based system. The ATS and monitoring that allow it to be used in a hybrid system rather than with just a charge controller are a nice touch.

 

[jaggedben]

Besides an ATS, I think the bigger design/engineering obstacle to turning a grid tie inverter into a dual mode is providing the neutral. Current designs use either an autotransformer or two 120V inverters. At any rate, this stuff more or less doubles the cost of an inverter. It's really not worth it if you're not doing batteries. SMA made their solution cheap by making it very low power and totally manual.

 

[winnie]

True enough, but how would you get the inverter output to track the load?

It would not be trivial, but IMHO would be do-able.

You could not simply set the inverter for constant voltage output, because you would also need to manage the array voltage so that the power output of the array would match the power consumption of the load.

This might simply 'magically' work if the array voltage is being bucked in the inverter to produce output. If you are on the part of the power curve where current drops if voltage increases (out of the part of the IV curve where the array looks like constant current) then if the load drops, the array voltage will go up and its power will go down. Sounds reasonably stable to me.

If you are on the constant current part of the power curve, then you need an inverter that can deal with the fact that the system is _unstable_; if the output power drops, then the array voltage will increase leading to more power from the array. You would either need a dump resistor, or be very very smart about quickly increasing output voltage (slightly, still in tolerance) to increase output power faster than array power is increasing, and get back to the target steady point.

Hmm. I think I know which part of the power curve I would use

-Jon

 

[electrofelon]

You could not simply set the inverter for constant voltage output, because you would also need to manage the array voltage so that the power output of the array would match the power consumption of the load.

But with PV we can just open circuit it if we dont need/want the power. GTI's already do this if array power exceeds inverter capability.

 

[jaggedben]

True enough, but how would you get the inverter output to track the load?

FWIW, this isn't so hard in a grid tied situation either. Just put a meter at the point of entry to the house (or wherever you're trying to reduce the load to zero).

SolarEdge's StorEdge inverter will do this. You can set an export limit, the limit can be zero, and it will do a decent job of matching inverter output to local load with no exports. You'll get a few watts going back and forth but nothing the utility would be likely to care about.

 

[ggunn]

The bottom line for me is that as a PV design engineer I am limited to designing systems with equipment that actually exists. If and when someone comes out with a GT inverter without batteries that will merrily keep supplying power when the grid goes away, I'll use it. Until then, conjecture is fun but that's all it is.

I'll just add that I believe that the odds of us (most of us with limited exposure to PV systems) coming up with an inverter design concept which hasn't been considered by the design engineers at all the inverter companies in the world are vanishingly small.

 

[gadfly56]

The bottom line for me is that as a PV design engineer I am limited to designing systems with equipment that actually exists. If and when someone comes out with a GT inverter without batteries that will merrily keep supplying power when the grid goes away, I'll use it. Until then, conjecture is fun but that's all it is.

I'll just add that I believe that the odds of us (most of us with limited exposure to PV systems) coming up with an inverter design concept which hasn't been considered by the design engineers at all the inverter companies in the world are vanishingly small.

What is the raw, unsubsidized by any credit, off-set, or government-largess-of-any-kind cost for a 3kW array on a south-facing, second story, simple roof in your area? Grid-tied only. Here in NJ I hear numbers of around $30,000.

 

[jaggedben]

What is the raw, unsubsidized by any credit, off-set, or government-largess-of-any-kind cost for a 3kW array on a south-facing, second story, simple roof in your area? Grid-tied only. Here in NJ I hear numbers of around $30,000.

I think you must mean a 10kW array.

For a 3kW array around here it would be about $10,500, installed. There are economies of scale.

 

[ggunn]

I think you must mean a 10kW array.

For a 3kW array around here it would be about $10,500, installed. There are economies of scale.

It's not my department but I would agree that ~$3/W will get you pretty close.