Electrical Question - Grid Tie Power Inverters for Home Use

[mearvk]

Electrical Question -- Grid Tie Power Inverters for Home Use

Since we are being careful let us assume we have a doll house that is powered by only a 12v source. Inside the doll house, Julie, the doll living there, gets it in her head to construct a 1.2v solar system on the roof of her doll house.

Luckily she realizes that she has an old 1.2v to 12v, DC to AC inverter in her doll garage. Julie doesn't want to mess with batteries and the cost of having to replace them every couple years, especially since there are so few doll battery dealers in her area.

Julie wonders as she's watching TV one evening if she can't simply run the inverter into the doll house wall socket by using a male-to-male extension cord and let the house use what it will, saving her from the dreaded high energy bills of a post-2012 doll world.

Is Julie's thinking correct at least in theory? If not, why not?

 

[voko]

If the house system is DC, then, yes, she could plug in the inverter - but she will have to mind the polarity!

If the system as AC, then things are trickier, because "minding polarity" then becomes having to maintain the frequency and the phase of the inverter's output very closely to the other source.

In either case, assuming both systems running in parallel, the "battery" source will still be used even if the solar system could supply all the power required. She will need a switch to optimize that.

 

[Sylvia Else]

Julie might be disappointed as to the savings. Assuming reasonably constant daylight during the day, the solar panels will produce a reasonably constant output, but the dollhouses demand will vary considerably. If the solar panels are small, then it's possible their entire output will always be consumed, but that also means that most of the dollhouses demand will be met from its 12v source. Conversely, if the solar panels are large enough to meet the higher demand peaks of the dollhouse, then for much of the time their output will be unused, and the investment wasted.

That aside, the big person who manages the 12v source may, having disconnected it for maintenance work, be under the mistaken impression that the wires to the dollhouse are no longer powered, and if the big person is susceptible to 12v shocks, they could die as a result.

 

[mearvk]

@voko: House is 12v AC @ 60Hz. There is no battery source, only the 12v power from the doll power company.

@Sylvia Else: Wouldn't whatever power going into the doll house be used before power coming in from doll house power company? If this is true the amount of power being consumed from doll power company will always be used after the solar energy, which during periods of daylight is ideal. The size of the panels isn't the primary issue here.

Evidently they sell grid-tie power inverters for this purpose. People power them with 12 or 24 volt power and they output 120v synchronized at 60Hz. The problem is that they cost about twice as much as normal inverters and they do basically the same thing. The difficultly of synchronizing two phases in terms of electronics probably would add another $20 to the price but no way would it justify 100% markup. Does anyone know a way around this for Julie's sake?

 

[the_emi_guy]

Might burn down doll house.

Several problems with this.

Most obvious is that the AC from the solar panel will not be in phase with mains. If they are 180 degrees out of phase you have a short circuit.

Even if you phase lock inverter to mains, you are bypassing circuit breaker protection that prevents wires in walls from overheating in the event of short circuit in appliance.

I have no personal experience with using solar panels, but I have to believe that there is lots on info on the Web about how to do this correctly.

 

[mearvk]

Grid-tie inverters would take care of the phase issue. Wouldn't the 12v doll house line tie into the doll house's internal breaker box? Might it be the case that the fuse issue would exist only on the branch that got its power before the breaker box. If you tied it in the doll house breaker panel directly it would avoid this issue, of course this would require an doll electrician.

 

[jim hardy]

Grid tie inverters also disconnect the inverter when incoming utility power goes away.

That is done for safety of utility workers who might be working to restore power. You don't want them electrocuted by your inverter while he's fixing the wire outside your house, or a few blocks away where a tree has taken down the wire going to your house.
That's a big part of the extra expense.

I worked for an electric utility in Florida. We not infrequently lost linemen during hurricane restoration work.
People don't realize the transformer behind their house is bi-directional, so it will step their 120 volt gas generator or battery powered inverter up to several thousand volts and send it wherever the wire goes. That's a hazard for linemen and electrocutions are becoming more frequent.

So if you're contemplating this for real, do it right.

 

[berkeman]

Grid tie inverters also disconnect the inverter when incoming utility power goes away.

That is done for safety of utility workers who might be working to restore power. You don't want them electrocuted by your inverter while he's fixing the wire outside your house, or a few blocks away where a tree has taken down the wire going to your house.
That's a big part of the extra expense.

I worked for an electric utility in Florida. We not infrequently lost linemen during hurricane restoration work.
People don't realize the transformer behind their house is bi-directional, so it will step their 120 volt gas generator or battery powered inverter up to several thousand volts and send it wherever the wire goes. That's a hazard for linemen and electrocutions are becoming more frequent.

So if you're contemplating this for real, do it right.

Good post, Jim.

@mearvk -- Do *not* try to do this on your own. Fire, electrocution, liability issues, etc. are all involved in getting these systems right.

 

[mearvk]

Thought we were talking about a doll house. Good thoughts all around.

 

[russ_watters]

Thought we were talking about a doll house.

I doubt anyone here is that naive.

 

[mearvk]

Seems to me that having to change 120v to 12v for purposes of not getting this thread locked might be considered a naive safety measure. Mind you I'm not saying that's what I've done here. I have a doll house and Julie is the doll that lives there and she's interested in running the whole deal on solar power; it's just that her wi-fi is out right now.

 

[berkeman]

Seems to me that having to change 120v to 12v for purposes of not getting this thread locked might be considered a naive safety measure. Mind you I'm not saying that's what I've done here. I have a doll house and Julie is the doll that lives there and she's interested in running the whole deal on solar power; it's just that her wi-fi is out right now.

We've allowed discussion of Grid Tie Power Inverters in the past, as long as the safety issues are discussed, and nobody goes off and naively tries to jury-rig up their own system. The power company will take an understandably dim view if that happens.

 

[mearvk]

Seems to me a relatively inexpensive diode type device on the exterior 240v would take care of the electrical company's worries with respect to outages. I do understand their concern over a potentially hazardous situation however.

 

[berkeman]

Seems to me a relatively inexpensive diode type device on the exterior 240v would take care of the electrical company's worries with respect to outages. I do understand their concern over a potentially hazardous situation however.

If you stick with the standard 50/60Hz AC power, diodes won't work. You need a full disconnect (like a relay).

 

[mearvk]

You can see that this guy has some tools that plug into the wall sockets which show probably amperage and wattage.

http://www.youtube.com/watch?v=UlYsHufsDl4&t=15s

What are these things called?

 

[the_emi_guy]

Grid-tie inverters would take care of the phase issue. Wouldn't the 12v doll house line tie into the doll house's internal breaker box? Might it be the case that the fuse issue would exist only on the branch that got its power before the breaker box. If you tied it in the doll house breaker panel directly it would avoid this issue, of course this would require an doll electrician.

Any electrical wiring that is sealed up in your walls is required to be protected with circuit breakers between source of power and loads. If you use a male-male extension cord to feed your inverter into the house wiring you may not have this protection, unless the inverter itself is current limited or includes circuit breakers. If an appliance on that branch develops a short and your inverter can deliver 200A through your extension cord, then you may ignite your doll house.

 

[Windadct]

The benefit of the GTI is that your excess energy is sold to the utility - this combined with the full time functionality is worth the 2x - as for the reason for the cost it is much more difficult to control the inverter - as it has to effectively "follow" the utilities connection - and there are a lot more regulations ( = hardware) needed to meet the code.
Spring for the GTI.

 

[sophiecentaur]

Julie might be interested in the actual savings she can expect to make with her proposed system. They are marginal unless things are 'just right'. The only real clincher in any decision about PVs at the is the lunatic rates that the Government is prepared to give people who are lucky enough to qualify for Feed In Tariff. Meanwhile, all the rest of us are subsidising the government's cosmetic bit of green energy supply.
Doing it yourself is nothing like as attractive, financially - not to mention the safety aspect. Julie ought to shop around for an approved system and she wouldn't even need to do the work herself. The FIT rates are not as good as they were but still attractive, I believe.

 

[Windadct]

I think a lot of the people putting solar are not just paying to save - but get added values out of - environmental impact, being more independent - etc. I would also like to point out - that here in PA after Sandy and 2 other outages in the last 2 years- I would also include the "value" of having an on line standby generator. -- I would include batteries, they should last more then a "few" years if taken care of. With about 5KW-7 KW of solar - yes a decent amount, I would be able to selectively ( not all at once) run pretty much all of my appliances - except the AC - I do not Know if I could get it to start - although the Solar panels + batteries may have enough overload capacity to get it running.
Just adding a few panels to save a few $ - is actually the lowest "value" proposition.

 

[jim hardy]

For use in blackouts a gas generator is sure cheaper to own and operate. Better yet, a propane one, its carburetor won't gum up from old gas.
I had one big enough to run a tiny window airconditioner. After Hurricane George it let us sleep in comfort.

A friend installled a few KW solar system around 2000. His batteries lasted about five years and at ten he had to replace the panels. My opinion of them is they're just a fashion statement.

old jim

 

[sophiecentaur]

My opinion of them is they're just a fashion statement.

old jim

Not if your government is daft enough to subsidise you when you do FIT. But it must depend on where you live - both for the Sun consideration and the infrastructure situation.

 

[Sylvia Else]

I think a lot of the people putting solar are not just paying to save - but get added values out of - environmental impact, being more independent - etc.

As far as the environmental impact is concerned, the effect of solar panels is far from clear.

If one assumes a typical grid generating mix of coal or nuclear, combined cycle natural gas, single cycle natural gas, which covers the base load, medium load, and peak load, with each type being chosen based on the balance between capital costs (paid regardless of whether the plant is running) and fuel costs (paid only when the plant is running), then one has to look at the impact of including solar panels which displace the highest marginal cost plant that's running at the time. At times, that will be the combined cycle plant, thus reducing its return on investment.

Future investment decisions may then be to build single cycle plant instead of combined cycle plant. But single cycle plant is less fuel efficient, which is to say it produces more CO2 per unit of energy generated. Whether or not one personally believes that matters is beside the point - reducing CO2 is what people who install solar panels believe is their environmental contribution. Yet in the longer term, this may not actually be the outcome.

Sylvia.

 

[Sylvia Else]

With about 5KW-7 KW of solar - yes a decent amount, I would be able to selectively ( not all at once) run pretty much all of my appliances - except the AC - I do not Know if I could get it to start - although the Solar panels + batteries may have enough overload capacity to get it running.

I obviously cannot speak for your specific air conditioner, but they don't all require large starting currents. Indeed, when I did some specific tests on a Daikin model that I have, I found that its startup requirements were very benign. It is one of the "inverter" type, which I think are pretty much the norm now.

Sylia.

 

[dlgoff]

A friend installled a few KW solar system around 2000. His batteries lasted about five years and at ten he had to replace the panels. My opinion of them is they're just a fashion statement.

Personally, I'd rather have a small grid tied nuclear powered generator sitting in my backyard. And I'd buy a professional Automatic Transfer Switch.

 

[the_emi_guy]

Personally, I'd rather have a small grid tied nuclear powered generator sitting in my backyard. And I'd buy a professional Automatic Transfer Switch.

http://www.nuscalepower.com/

 

[dlgoff]

http://www.nuscalepower.com/

:!) Thanks guy. :!)

 

[wirenut]

A few years back (6 or7) we had a major snowstorm in october (NE USA) and at the time almost all electricians were getting power restored to homes (some were out for 7+ days), and the utility company went down the street. If you had a portable generator or non registered permanent one they cut the power lines to the house @ the utility pole for the safety of all the linemen (local and out of state volunteers). A break before make approved transfer switch is required for all generator hook ups, whether automatic or manual. The NEC now has a section dealing with solar panel installations, and the AHJ and local power co. also have regs for the installation and tie in of on site power generation. Best to check with all of them to reduce hazard and liability.