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Solar grid tie battery 3

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itsmoked

itsmoked

Electrical
Feb 18, 2005
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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 -
 
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Are you expecting a lot of power failures?
Normal grid supply is pretty dependable with a long time between outages, but I know that there are exceptions.
Have you considered charging the batteries slowly with something like a wall wart?
Your project has at least three design challenges;
1. Transitioning from a grid tie inverter to UPS mode on power failure.
2. Using the batteries only when in UPS mode.
3. Charging the batteries.
The design challenge of charging the batteries may possibly be solved by the use of a small independent charger and charging the batteries over a fairly long time period.
If you start with charged batteries, you don't need much current to maintain the batteries.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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I think you're overparsing the problem. I think the typical grid tie prioritizes direct loads over the grid, i.e., if you have a demand prior to the grid, that gets satisfied before power goes to the grid, so the standard grid tie system will essentially do what you ask for without any additional design requirements.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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A standard grid tie inverter knows nothing of loads, local or remote. All it does is pump Watts out the point of connection at a voltage that enables the flow. Watt, Ohm, and Kirchhoff see to it that "local" load is fed from the "local" source and that any left over capacity feeds "remote" load.

Probably change the focus from inverter plus batteries to batteries plus PV and see what the battery energy storage folks can do for you. I think you'll do better building it around the storage system than trying to build it around the PV.
 
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Yes there are solar inverter's that are designed for that called grid tie-battery backup. It also requires specialize switches. inverters are very specialized pieces of equipment. first they must be able to frequency syn to your inverter to the utility system. the inverter must convert dc to ac at the required voltages and cycles and on and on . All these problems have been solved by the mfg of the inverters but they are all very different. So you need to pick one that does what you need. Check with your solar store, vendor or on line mfg hot lines. Goggle YouTube too. Also the switching arrangement is complicated-it requires transfer switches. Putting the wrong inverter or switching incorrectly can cause the switches or inverter to explode. Remember utility voltages are very dangerous if you are not trained for handling electricity-you may need to get a solar electrician to help you out. Saving a few bucks to do it yourself is not worth injury or death. After saying the that the key to your problem is to select the correct inverter and switches and connecting them together properly. If you are qualified to handle electricity The gird tie- battery backup inverter have connection diagrams to configure the switches and wiring to do what you want.
 
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Thanks everyone for your sage advice.

Bill, you make an excellent point about usage from the battery bank (probably) being very infrequent so charging it in any form is not particularly important.

A grid tie inverter, by definition, only feeds into a bigger active supply so they don't typically become standard inverters and if they did you'd have the dead grid connection problem anyway.

As advidana mentions a transfer switch is needed to safely switch from grid tie generation to inverter service.


advidana I'll look for a "grid tie battery backup" and see what I come up with. Thanks.

Keith Cress
kcress -
 
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An off the shelf solution may be two devices:
1. A grid tie inverter without batteries.
2. A ups with the battery charging dialed way down.
Now the problem becomes finding or designing a unit where a single inverter serves both functions.
You will need at least one contactor to provide galvanic isolation from the grid and a synchronizing circuit to reconnect "on the fly" to the grid when power is restored.
You have two problems:
1. A Grid Tie Inverter. By itself not a problem.
2. A UPS. Old mature technology and not a problem.
It is not always possible to solve two issues with one solution.
The challenge now is to find a solution such that neither the Grid Tie Inverter nor the UPS compromise the action of the other device.
Please share your findings with us. (Prices also if possible.)

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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You might want to find out whether your utility, presumably, PG&E, has installed, or can install, a net meter ( pg. 29). It's supposed to keep track of power flows, and that's where the adjudication of which way power is flowing is done on my installation.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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Most of the electronic meters that are in common use now may be configured to record both positive and negative power flow.
This makes it easy to set and bill different tariffs for positive and negative power flow.
The old mechanical meters indicated net power. They ran backwards on reversed power flow.
Some utilities did not credit consumers for the VARHrs produced by a leading power factor during light loading. In these instances a ratchet could be installed to prevent the meter from running backwards.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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There are definitely off the shelf solar / battery / grid-tie /islanded units that can do all of that on the market.

However, they're much larger than the size you're talking about. Some of the units end up being 3 or 4 different pieces of equipment, some are 'all in one' units. The Nedap PowerRouter is one device that can handle the entire lot (at 48V DC for the batteries, rather than 120V), I don't have a link unfortunately.

The Outback Power Radian Inverter is one that requires a separate interface for the solar to battery stage, but handles the connection to grid seamlessly (it has internal relays, and separate connections for the powered load and the grid connection).

All of them start at about 1.2kVA though, which is a bit bigger than what you're looking for.

EDMS Australia
 
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A couple of hundred Watts doesn't seem worth the time to even think about grid tie ins.
How about just a UPS?
Disable the battery charger and charge the batteries direct from the solar cells.
I suspect that there is a good reason why not. Can't wait to find out why. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Freddy you're right, those Grid tie / battery / island units are crazy expensive. They seem to start about $2.7k.

Bill; The price of solar is so low now.. Way below $1/watt for panels that even 500W has short ROI. Then only $49 for a 300W micro-grid-tie inverter. Combined about $1/watt. This pays back quickly by its shaving the higher tier power off call it 30¢/kWHr power. That's fast payback with the addition of some short term backup power.

Here's what I'm thinking of trying: Put a PWM charge controller for battery charging in parallel with the grid tie inverter across the panels. The MPPT function of the grid-tie will just throttle back thinking (maybe) "clouds" while the PWM charge controller is topping up the battery bank. As the bank goes to trickle the MPPT G.Tie will crank back up to maximum output.

Keith Cress
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itsmoked: here's the route I went. I put two 250W panels on opposite sides of the N-S ridge on my garage, each with a grid-tied microinverter. With panels oriented E-W, I spread my generation out over the longest period of each day, and the microinverters make sure that part shade (one panel in shade, the other in sun) doesn't matter. That tiny system does behind the meter "peak shedding". I have a lot of mature trees, so not enough sun-exposed roof for a system large enough to pay back the permits and other fees associated with a full-on micro- FIT installation, so this is the most I can do.

I could fit and use a few more panels, but because I am not be allowed to back-feed any electricity into the grid, I would need a means to measure my demand and supply and shut off panels/inverters to get the two to match my demand but NEVER backfeed. I haven't found the clever and inexpensive way to do that- yet- but hope one day someone will figure that out for me!

The grid-tied inverters I have now are safe to linesmen working on the lines in a power failure because they must have grid power to sync to, and of course also useless to me during power outages.

My UPS needs are limited to my sump pump and, should a long power failure occur in winter, my boiler and its circulator. For those, I have a separate 150 W 12V panel with a cheap PWM charge controller and a couple deep cycle 12V floodies. The main AC sump pump is backed up by a 12V bilge pump connected to the floodies directly, so no inverter needed. If I need to back up the boiler/circulator, I have a standalone inverter capable of driving them, but I'd have to run an extension cord and I'd become the "transfer switch". In a long-term emergency, I have 18.5 kWh stored in my EV battery which I could use, if I really wanted it badly enough.

For fun, I also use the 12V system to operate a garden fountain pump, some garden LED lighting, and a little pump which uses water from the sump to automatically water my wife's hanging baskets- saves getting someone to come by to water them when we're on vacation.



 
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itsmoked said:
Bill; The price of solar is so low now
Click to expand...
Keith, can you post some links to your suppliers?
I have a customer who has been bugging me for 10 years or more about a solar installation.
My answer has been, unless you can get the government to pay, forget it.
It looks like that has changed and it's time to get to work on a system for him.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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For areas where export to the grid is not feasible.
Search Ebay for
"grid tie inverter with limiter"
Link
Gaia Defender said:
*** If the Solar Panel is producing less power and the GTI cannot deliver all the power needed by the load, then powe will be supplemented by the Grid.

*** Power Meters (Except smart meter) are not aware of the direction of Power flow. The user then pays for the power exported and delivered by the Grid.
Do Not Worry! No Problem .

* Now ,we have the limiter type inverter , if you do not want to sell electric power.
Click to expand...
What is not mentioned is that these systems monitor only one line of the 120/240 Volt service.
For the proper functioning of the power limiter, the inverter must be used on 120 Volts, and must be connected to the side of the line that the monitor sensor is connected to.
The third generation may address this limitation, we hope.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Cat: Thanks for the links though they're way expensive and generally too small.

Thanks Molt! Understood.
Here's the inverter Bill was alluding to. It uses a clamp-on sensor to prevent back-feeding.
No Net Backfeed Grid tie Inverter

Your case would sort of be like what I'm trying to do. You could warehouse your afternoon power in a DIY power wall and then feed it back during night hours staying net zero.

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

Bill;
Micro Inverter

Common Micro Inverter

Micro Inverter with lots of informational feedback

Typical sub dollar/watt panel offerings follow.
A typical solar install costs a fortune, like a new car $20K+ which really does demand a subsidy to make sense. And, virtually all subsidies demand installation by "a company" which in my opinion renders the subsidy a null-event. However, if you can do your own installs, these days, you can shop for and buy sub dollar/watt panels and come in well below the cost of null-subsidized installs for about half the time to full payback.

Typical Inexpensive Used Panel find - Pickup Yourself limitation

Free Shipping Panels #1

Free Shipping Panels #2

Free Shipping Panels #3

Free Shipping Panels #4 W/self mounting

A you must Pick-up panel, local to me, I'll likely get

I've seen flexible and adhesive backed panels for a little more than $1/watt.

Lets say I use two of the 260W panels. (total $300) And a 1kW MPPT grid-tie. ($120)

Total hardware cost = $300 + $120 = $420

Call it 5 hours a day solar.
260W x 2 x 5hr/day = 2.6kWHr/day
30day/month x 2.6kWHr/day = 78kWHr/month

I pay 0.29/kWhr in tier 2.
0.29/kWhr x 78kWHr/month = $22.62/month

$420/$22.62/month = 18.5 months... To me that's a no-brainer investment.
Isn't that something like a return of 64% on the money a year?



Keith Cress
kcress -
 
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I'm puzzled how you can use a simple AC current transformer on the power feed lines to the house to differentiate between 1A of forward current being fed to the house from the lines outside, and 1A of reverse current flowing from the inverter BACK into the grid- both would show up as 1A of AC current to that current transformer, wouldn't they? How is the direction of the AC current flow actually detected, and what scheme could I use to do so, given that I'm not going to throw out my microinverters so I'd have to do this detection myself if I wanted to add more panels? Is this done with some kind of phase shift measurement? That's the part I just don't get- maybe I'm being thick, but it's likely I just don't understand AC well enough. The fact that the inverter web page you linked to is full of spelling errors doesn't inspire confidence in their ability to explain how this thing actually works, either...I'd love it if one of ye who are so learned in all things electrical can explain this one to me in language a chemical engineer can understand!

As to the calcs, I know I can only make about 1000 kWh/yr per kW of installed panel capacity given my latitude and panel orientation, and even so my little system has a meaningful payback period because it is so simple and so low in capital- and because rates here are weighted heavily for time of use, with noon to 5pm being on-peak in summer.

If I had more roof area in sun I'd go off grid entirely during peak periods, drawing from the grid only to top up a battery off-peak and maybe to run the clothes dryer off peak too. I'd end up using a battery from a crashed Chevy Volt likely- at the very modest current demands of running a house, those batteries would likely last forever if you have a decent BMS on them to keep them within a safe range of SOC. But I'm not cutting down trees to feed sun to solar panels- the trees are worth more to me than the dollars I'm feeding the local utility, and the local grid here is already pretty green- the trees are doing more environmental good than I would be by dropping my power consumption really.



 
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