Solar grid tie battery 3

[itsmoked]

Trying to build a system that has a couple of solar panels to charge a bank of batteries.
Once the battery bank is fully charged (the normal state) the solar transitions to grid tie.
The bank is only for power failure backup.

Anyone doing this or have any experience with it? Suggested equipment?

A couple hundred watts. Output 120V.

I've combed ebay for a while but I'm not seeing it.
It's such a logical configuration it makes me think there's a different way everyone does this that I haven't recognized yet.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

Amps -90 to +90 degrees of the corresponding phase voltage are forward and amps 90 to 270 degrees of the corresponding phase voltage are reverse. How's the meter set up, forward might be into the service or it might be into the system.

 

[waross]

If the sensor shuts the inverter off at zero Volts, there will be no reverse power.
But for dependability the current is probably compared to the voltage.
The single sensor system is dependable only for 120 Volt inverters.
The two hot lines of a residential service are often unbalanced with regards to current.
Consider a few lights and a toaster are turned on. The toaster draws about 1200 Watts.
The lights happen to be on the other line.
So we have 200 Watts on one line and 1200 watts on the other line.
If the sensor is on the side with the toaster, it will allow a 240 Volt inverter to push out 2400 Watts before it starts to limit the power. That is 1200 Watts on each line.
So our total load is 1200 Watts plus 200 Watts or 1400 watts. The inverter is pushing out 2400 Watts. That is 1000 Watts going into the grid on the line without the sensor.
With the default setting on a modern meter, you will be charged for that 100 Watts as if you were consuming it.
Moral: If you are using current limit with a single sensor, stick with 120 Volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

I was wondering if someone would mention that HBPowerwall install:

Speaking of which:
Get a load of this setup!
Youtube link to a LARGE DIY Solar/Power-Wall/Grid-Tie system

That install is an excellent example of DIY and a number of the pitfalls involved in such an exercise, including exposed conductive surfaces, the likelihood that the circuit breakers aren't adequately rated to break the possible fault current and so on. As I understand it, that system is not a grid connect one, as its quite unlikely that it would be accepted by a utility for connection to their network.

In all seriousness though, if you forgo the need to 'use' the extra solar once the batteries are charged (or the ability to seamlessly transition to and from the grid), its quite possible (and a lot cheaper) to run a small solar install with an MPPT controller, batteries, and an inverter. Something like this is still likely too large for your overall needs, but I can't recall any of the smaller units off the top of my head.

Moltenmetal, the CT is only part of the picture, any grid connect inverter already monitors the voltage and can establish direction based on both of these aspects, effectively as what davidbeach said. Some units have that equipment internally, some do need a CT (often called a 'Power Measurement Unit') at the connection point to detect. Expect that most units on the market won't just accept a standard CT being connected, rather they'll all have their own specific unit to do a similar function.

EDMS Australia

 

[itsmoked]

Molt; PVWATTS is nice - thanks.

And, to agree with Freddy the current coil is connected to the inverter that knows the voltage waveform intimately since it has to correctly drive into the grid.

Bill; I'm pretty sure that inverter is 120V only 1 or 2kW so the coil only needs to see one phase.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[FreddyNurk]

moltenmetal, this is probably the device you'd need in order to manage the export capability, although there's probably a different regional variant.

Attempting to DIY may be possible if you can work out how it communicates with the inverters (and your inverter units are compatible...) but generally isn't worth the effort compared to what it costs to buy one.

EDMS Australia

 

[waross]

Two issues Keith.
1. They are offering 240 Volt inverters with only one CT.
2. These inverters are plug connected. If the unit works properly plugged into the top connection of a split receptacle, it won't limit properly plugged into the bottom half.
Just something to be aware of.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[moltenmetal]

Freddy- an excellent solution if I were to have about 10 panels over which to amortize its $480 USD cost. However, that represents more than I paid for the two Enphase microinverters I have, which themselves cost almost as much as each panel I bought...

Thanks for the explanation davidbeach: I guess I understood AC a little better than I thought. So people are assuming that the inverter is comparing the phase angle of the current waveform it is generating with that of the power entering/exiting the house to determine the direction of current flow. We agree that the device would be of no use on a 240 V split phase AC setup i.e. most North American homes, because the draw on each phase can be highly imbalanced. It would be best to use two separate 120V units if you were to go this route.

Obviously I won't be doing any of this due to cost and complexity. I'm still hoping someone has a nice simple analog circuit that would allow AC current direction detection by comparing the phase angle of the voltage and current waveforms on each phase. Regrettably, even that wouldn't be all that useful to me, as all I can do with my microinverters is connect or disconnect them to/from the grid, and they seem to take several minutes before they start generating power after they are reconnected to the grid- no doubt a safety measure. They claim per the datasheet to consume only 65 mW when dark, but last I checked, they were drawing a non-negligible amount of current when their panels were dark.

 

[cavbilly]

I saw a clip yesterday by an MIT professor, and his team that made molten salt magnesium/antimony battery for a grid-scale energy storage, but this is way out of most individual's league at the moment. For short-term 200-800 W power, I would stick with not more than a few lead acid batteries, charge them using a charge controller powered by PV panel(s), and never attach the inverter until needed. Or use a UPS properly scaled for the specific application.

 

[waross]

I have been wondering about grid tie inverters for customers who have automatic backup generators.
I hope that the no-backfeed feature is switch selectable. That would be a solution for a grid tie inverter that DOES NOT back feed into a standby generator.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Bill; This is part of the reason for this exploration, a learning opportunity.

Seems to me a grid tie inverter could always use an external input that stops the function. This would allow any kind of reverse power relay or generator ON relay to dump the output. But, maybe they look at the "grid" and decide the little generator doesn't look stable enough or is 59Hz or 60.4Hz and out of spec. None of the data sheets seems to go into any of this.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

A grid tie inverter must have some frequency agility. The grid is not 100% stable. At one time I had a background display that showed the actual grid frequency and recent history in real time in the Pacific North West. I don't remember the range of deviation, but a grid tie inverter would have to track frequency.
No problem with the inverter reducing the load on the generator. When the load on the generator is zero, and back feeding is a concern, the frequency with 3% droop will be 61.8 Hz on a good quality residential set.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Point taken.

I've seen a lot of UPSs that considered any generator to be "bad power" and would run constantly until dead taking down the "protected" load.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[FreddyNurk]

waross, itsmoked, you're right, they do track frequency, the key is being able to set the allowable frequency deviation. Some inverter units will allow for a 'generator' mode selection that has a much wider frequency range than what would be expected for grid, usually the ones with inbuilt relays. Those sorts of units generally allow for charging of the battery under low sun conditions, and often have capabilities (sometimes with additional accessories) that can start the generator, load the generator up by charging the batteries quickly, and then shutting off the set again.

As an aside, a lot of these systems are DC coupled, rather than AC coupled, thus the issue of reverse powering the generator is not the same as running it AC coupled against a standard grid connect inverter, those units will run flat out unless there's some means to tell them not to (like an export metering unit).

Most UPS units (at least within a certain size and cost range anyway) expect to only see grid level deviations, running a set in droop will often result in the UPS considering it outside acceptable frequency and not transition back to mains.

EDMS Australia

 

[Sparweb]

Keith,
I take it you are not interested in selling the power, since the revenue from your pittance of kWhr would not likely make up for the fees you pay for the privilege.

If you had a device that would do the below, would you be satisfied?
It's not on the market any more, but they can still be bought used for ~300 USD (yeah e-bay)

This excerpt if from my Xantrex SW series machine. The Xantrex DR series is smaller, cheaper, does the same stuff.
These devices have 20 other modes of operation so if this isn't suitable, maybe another is.

Something I notice in your OP is the hint of an assumption that the inverter would be involved in charging the batteries in conjunction with the solar panels.
Normally it wouldn't be, since the solar panels can charge the batteries directly, or through a charge controller (which is better).
This device, being an inverter/charger, not just an inverter, would allow you to charge the batteries from the grid, if necessary after many cloudy days, for example.


STF

 

[itsmoked]

Thanks for the info Spar!

I was actually considering building something that does this. I hadn't seen anything that did it except the guy in here with the solar refrigerator controller thingy, but then his has never actually made it to the market.

The DR series is nice.

Amazing how fast Xantrex dumps it's products. It's almost as bad as APC with their absurd number of UPSs.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[FreddyNurk]

Amazing how fast Xantrex dumps it's products. It's almost as bad as APC with their absurd number of UPSs.

That's because they're both owned by the same parent company...

EDMS Australia

 

[Sparweb]

Glad I could help.
The DR and SW inverter/chargers are not on the market any more, true, but 20 years ain't bad in the electronics world, IMHO, which is why they still aren't hard to find on e-bay. (They were introduced in the late 80's).
Devices with similar capabilities are on the market, but the "super-size me" culture has driven them well above the operating range you're looking for.

I'm a bit of a shill for their equipment because I still run a bunch of that stuff here. Just tweaked the charge controller this morning, in fact.

Like Freddy, I'm having trouble keeping up with the Schneider changes, too.

STF

 

[wroggent]

Any of you ever fear getting in trouble with your utility when using a peak shaving type system?

The service agreement with my utility requires that I apply for and use their generation rate plan if "any portion of the load is not served by the utility". So, just ensuring that I do not back feed the utility is irrelevant. I've been collecting dust on a panel I got for free a while back out of fear that the utility would (1) somehow notice the change in my use behavior (to become aware that I've done something, despite not back feeding through the smart meter), and then (2) bill/sue me for lost revenue. The generation rate plan is structured such that you will need a rather massive PV system to overcome the steep increase in energy charges compared to a standard rate plan. This utility also decided to no longer allow net metering back in 2015.

 

[EddyVl]

I've seen the post of ITSMOKED to set a PWM-charger in parallel of a grid tie inverter.
The question is whether the solar charger will start charging, since they both use the voltage across the solar panels to determine wheter they should work. Possibly the voltage will drop across the solar panels because the inverter will start working. The solar charger will be thinking the voltage is to low and wont work.

There is possibly another solution: put a SPDT relay between the solar panels and the inverters.
The relay should be connected with the NC to the solar charger, the NO to the inverter, common to the solar panels.
When the sun comes up you wait till the battery is fully charged. This is the case when the solar panels voltage is 1 V higher then the battery voltage. Then you switch to the inverter. Then you wait till the voltage drops below 5 or 6 V, and you switch back to the solar charger. It can be done with an analog opamp system. (Schmidt trigger) If you're using the backup system, and you want to return to solar instead of the invertor, you can detect the net voltage (relay, opto-coupler) or measure the discharge current of the battery. The voltage measurement may be a little tricky, cause some solar chargers are connected to the + of the battery, and switch the - side.
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1510248138/tips/Solar_pzpdak.bmp[/url]

 

[itsmoked]

wroggent; I'm not worrying about it for 500 measly watts. The inverter just plugs into an outlet. I also have a 'dumb' meter that turns backwards with reverse power. it's also single phase 120V. My house probably uses about 350W during the afternoon.

I picked up the panels today! So shortly I should be experimenting.


EddyVl; Thanks for the suggestion.

Part of this project is for learning. I'll try the paralleled charger and inverter and see what happens. They may play together or not.. we'll see! I'll feedback here so we all learn something.


Keith Cress
kcress -

http://www.flaminsystems.com