PDA

View Full Version : OT Solar planning help needed please!



flylo
12-19-2012, 11:37 PM
Thinking of going solar as I can buy new US made panels for 80 cents a watt ready to install & 1/2 that if I assemble. I have an electric 50 gal water heater I'd change to LP. My average elect use is 1000kw-1200kw per month. Can anyone give me an idea how many 260watt panels I'll need. We have a very high south facing roof thats seems steep enough to work well. I'll light the shop with solar & put in a 3 phase 15kw generator. :confused:Thanks!

lakeside53
12-19-2012, 11:49 PM
Tell us more... assuming you are going to charge batteries, what's the peak power that you want from the battery or panel array? Averages don't mean squat unless you store that all away somewhere and use it for the peaks. And you generally use AC not DC, so honking inverter needed (sized for the peaks).

If no batteries, just dump the power into your hot water tank or heating system!

tdmidget
12-20-2012, 01:33 AM
"My average elect use is 1000kw-1200kw per month."

WHAT? Are you talking about kilowatt hours?
At 1000 KWH per month you would need 1,000,000/30 days/24hours= 1.388 kwh per hour. But since you have about 8 hours per day of daylight you need 412.66 kw production for those hours in brilliant sunlight. Oh wait you're in Michigan. Better 4 hours on a good day. so you need 32.05 panels for a 30 day month. At .80 per watt you need to ante up $33012.80 to start. That is until the next hailstorm which in Michigan is what, 2,3 years? So $10,000 or so per year in panels while that generator eats your ass alive for the 3 phase. Spit out that Koolaid .

remember, TANSTAAFL
with fond memories of Robert HeinLein

2ManyHobbies
12-20-2012, 02:08 AM
Please share more info, the closest I've found is just under $2/watt.

Your optimal roof angle is based off of your latitude and you can use your current roof angle to estimate average annual efficiency due to the angle. Next factor in panel efficiency, panel area, and your area's solar irradiance number.

If average solar irradiance at your location is 3kWh/day per square meter then your roof angle is ideal, you are looking at 70-74% efficiency (or 2.1kWh) times whatever the panel efficiency might be. Panel efficiencies run 7-20% depending on type. At 7% on the ideal roof angle, you'd get about 147Wh/day per square meter. You'd need about 272 square meters (2,928 square feet) to manage 40kWh/day on a 1200kWh/month cycle. The numbers change with irradiance, panel efficiency, and angle. There can be reasonable benefit to different summer/winter angles, but that can bring its own challenges. If irradiance is 4kWh per square meter per day, your panels are 20% efficient, and you use a summer/winter angle, then 67 square meters (721 square feet) should get you 40kWh/day.

My numbers are just rough and don't account for gaps, borders, or spacing either.

H8Allegheny
12-20-2012, 08:25 AM
There are multiple online calculators to help with this. Try this map-based one:

http://www.nrel.gov/rredc/pvwatts/grid.html

In general, grid-tied systems are the best as you don't need to maintain a huge bank of batteries. Of course, it helps immensely if your state has net-metering so that your PV array can spin your meter backwards! IF you do have net metering, also consider getting TOUR (time of use rates), i.e., peak/off-peak pricing - this really helps defray the cost of installing a PV system as most peak-rate times coincide with daylight hours.

Brian
Taxachusetts

Lew Hartswick
12-20-2012, 09:28 AM
Thinking of going solar:confused:Thanks!
My advice is "Stop thinking so much". :-) In any location but the "sunny southwest" , fergettit for any but incidental (like pre-heating water) use.
...lew...

J Tiers
12-20-2012, 09:44 AM
"My average elect use is 1000kw-1200kw per month."

WHAT? Are you talking about kilowatt hours?
At 1000 KWH per month you would need 1,000,000/30 days/24hours= 1.388 kwh per hour. But since you have about 8 hours per day of daylight you need 412.66 kw production for those hours in brilliant sunlight. Oh wait you're in Michigan. Better 4 hours on a good day. so you need 32.05 panels for a 30 day month. At .80 per watt you need to ante up $33012.80 to start. That is until the next hailstorm which in Michigan is what, 2,3 years? So $10,000 or so per year in panels while that generator eats your ass alive for the 3 phase. Spit out that Koolaid .

remember, TANSTAAFL
with fond memories of Robert HeinLein

A little OVERDOSE of "realism"............. that isn't entirely realistic.

Solar beats heck out of wind power...... in small systems

in reverse order....

1) 3 phase..... Outback brand inverters will chain to make 3 phase.... problem solved, no generator. Maybe other brands will also.

2) I have had panels up for about 20 years. No hailstorm has damaged them yet, and we have had a number of them, with golf ball or larger hail. What destroyed the original set was a tree on the north side of them falling and squashing them, a couple years ago.

3) If you get the right type of panels, like my original set, they produce well (half power) on an overcast but not dark day... so there is more input than you think. Old panels were single crystal.... a new multicrystalline panel I have produces well in sun but drops to nothing every time a cloud of gnats flies over it... can't tolerate ANY less than full sun... you gets what you pays for.

4) Yes, absolutely you have to be realistic about the total sunshine input. Some places are simply not candidates, but it also is not true that Michigan is impossible.... You will very substantially improve your numbers by adjusting angle seasonally, and also by sun tracking. Also "improves" your costs......if larger is better...

5) net metering and grid tie.... THIS has been a bunch of BS...... Who wants a setup that MUST BE OFF if the grid goes down? Up to recently, all grid tie systems HAD TO shut down and produce nothing if the grid was down.... STUPID.... you have no power, so you WANT to use solar, but the anti-islanding rules meant that a UL recognized inverter COULD NOT OPERATE without the grid, so when power is out, it sits like a brick, doing nothing but costing you money.

This is starting to change. Some systems are being allowed that can be used with a cutover switch similar to a generator.

Abner
12-20-2012, 10:32 AM
Solar is great if you don't use much electricity. Machine tools? With lead acid batteries? Better take a pot of strong coffee and read up on battery life and discharge rates, talked me out of it, completely.
We all should look at energy use. But buck for buck you will likely get your greatest return on conservation. Read up on German Passive houses.

This is the best I have found with full documentation of results of various projects.
http://www.nofossil.org/
more
http://www.builditsolar.com/Projects/Projects.htm
http://www.passivehouse.us/passiveHouse/PHIUSHome.html
hot air solar panel research by an engineer
http://www.youtube.com/watch?v=gBLQ2ZUeP7w

"Put on a sweater" - Former president Jimmy Carter who had a degree in engineering, and installed solar panels on the white house. A white house solar panel is now in a Chinese Museum. They were removed by former president Ronald Reagan who was a Hollywood actor.

Sounds like JT has real life experience, I don't, I would ask him about batteries.

Rosco-P
12-20-2012, 11:13 AM
Start reading Home Power magazine to get a dose of "real world" solar systems. A lot of the articles feature people living totally off grid, but they don't always live a "power rich" lifestyle. If you are installing this system yourself, it still has to meet all national, state and local electric code. As said above, don't expect full output from your panels, except maybe at noon on an absolutely clear summer day. The life of the panels is very finite, even the newer panel degrade with time. In 20 years the output of todays panels will have degraded to the point where their are nearly useless. Panels have dropped in price where watts per dollar seems cheap, but the cabling, control and monitoring equipment, inverters and suitable batteries are still the major cost of the installation.

1-800miner
12-20-2012, 11:48 AM
Grid tie or stand alone system? Two different animals. Rebates for one nothing for the other.

I live at the southern end of the Sierras,I get more sun than you so take me with a grain of salt.I use more energy for cooling than heating.
I self installed a 7kw grid tie twelve years ago. ( thirty two 180 watt panels) Best thing I ever did. The bill went from 200/250$ a month to 400/500 a year.
I pay yearly because output is really seasonal.

What I learned:
Buy as big of an inverter as you can afford,it's the heart of the system and the biggest single expense.Then later on you can add more panels if the system is too small. Max out a small inverter and you have to buy a bigger one if you want to expand.

I went by the book to determine the tilt of the panels. ( average between summer and winter). I should have made them flatter.Summer is the real production time.Winter is spotty.
Then again, a friend in Montana tips his more to the vertical in the winter to catch reflected light from the snow.

My inverter has remote computer connection terminals.I wish I had run computer wiring in the conduit when I laid it. Sure would be nice to click a button and monitor them.

If there are trees to the south of the panels,sooner or later they will get tall enough to shade the panel.
Lucky for me,I have an understanding neighbor that lets me trim his trees.

Solar trackers? I dunno,I think I would spend that money on some more panels. The radiation isn't enough until around 9/10 o'clock. Early morning and evening its traveling through too much atmosphere.

Seasonal tilt?Absolutely .Too bad I didn't think about that when I built mine.

How much wind do you have? The smaller eggbeaters have really improved.

p.s. I love my tankless water heater,I can take a shower for days.

flylo
12-20-2012, 12:24 PM
I have 2 new 12,000 watt inverters plus 3 used 2000 watts, I've always gotten 10 years + on lead acid batteries but I take good care of them.
2ManyHobbies I PMed you the info on the supplier. I'll do my homework & decide, Thanks All!

mayfieldtm
12-20-2012, 04:38 PM
I wanted to do a Grid-Tie system, however, my Electric Company here in Portland Oregon, will not approve any system that is not installed by
qualified "Good Buddy" Contractors and Electricians that use qualified equipment.
I know the Safety concerns with back-feeding power during an outage, but, the cutout circuits are not all that complicated.
I becomes so expensive, that any dollar incentives available are only a drop in the bucket and the cost is prohibitive.
They also reserve the rite to come on your property, unannounced, for regular inspections.
I understand that individuals can do the installs with the proper paperwork and approvals.
I have not been able to find any one person that was able to get through the Red Tape.
Tom M.

joe51
12-20-2012, 04:59 PM
Go to the Florida Solar Energy Center's website. It's located at Kennedy Space Center and run by the US government and the State or Florida and the University of Central Florida. They test EVERYTHING! You can get a lot of advice and factual information there instead of the manufacturer's and retailer's BullSh1T. If you're buying this through a retailer that's gong to install it for you, you'll probably find that everything that they tell is wrong or else a flat out lie! Most are absolutely clueless about what they're doing.

FWIW Florida had a rebate program up until a few years ago and the retailers were selling systems that were entitled to get the rebate but the state was two years behind in paying (the rebates had to be reauthorized every year) however the retailers SWORE that the buyers would get their rebates. NOT!

The lazy, sorry, stupid retailers here were installing the collectors flat on people's roofs no matter which way they faced and no matter which way they pointed. Solar collectors WILL NOT work that way.

My advice, go take a solar energy class at a nearby college. I did and after I knew something about SE I found a company that had a GOOD system and I bought it and it's been up for over 25 years now with very few problems.

Don't trust anything a retailer tells you! Go verify it on one of the reputable websites. Florida runs one, so does California, and I think both Texas and Arizona.

joe51
12-20-2012, 05:13 PM
I wanted to do a Grid-Tie system, however, my Electric Company here in Portland Oregon, will not approve any system that is not installed by
qualified "Good Buddy" Contractors and Electricians that use qualified equipment.
I know the Safety concerns with back-feeding power during an outage, but, the cutout circuits are not all that complicated.
I becomes so expensive, that any dollar incentives available are only a drop in the bucket and the cost is prohibitive.
They also reserve the rite to come on your property, unannounced, for regular inspections.
I understand that individuals can do the installs with the proper paperwork and approvals.
I have not been able to find any one person that was able to get through the Red Tape.
Tom M.

Oh, oh that sounds like the same arrangement that they had here in Florida! I called EVERY registered installer and talked to them and NONE of them knew anything about electricity or about solar energy systems. They were just throwing systems on people's roofs with no regard to where they pointed or if they were shaded or anything else. I TRIED to get a couple of them to install a system on a free standing frame that I could properly orientate but none of them would do anything other that a roof mount. (I also didn't want the eventual roof leaks!) I took the contractor's test and aced it with 100% score but there's not even a way to get the contractor's license any more. It seems that there was a good old boy network of power companies, retailers and State of Florida office that set up this system and only allowed in their buddies and then closed it to everyone else, including licensed electricians and contractors.

yeah it sounds like the same SCAM is going on where you live. IIRC they promise a 40% federal rebate and a certain amount ($2.40?) per KWH rebate from the state. If that's it, STAY AWAY from it! If you really want solar power do your homework and learn what you need and then buy and install it properly. The crap these folks are selling will work POORLY at best and won't last long. Go look at the specs for the inverters and panels that they're using. They're made in China and only good for ten years. (Some do have German names like Siemans but they're still MADE in China!)

The Artful Bodger
12-20-2012, 08:40 PM
Flippin 'eck, "everything is made in China" these days and even the sun shines on China before it shines of the US, about 13 hours.

J Tiers
12-20-2012, 08:46 PM
Solar is great if you don't use much electricity. Machine tools? With lead acid batteries? Better take a pot of strong coffee and read up on battery life and discharge rates, talked me out of it, completely.

Not necessarily so...... depends entirely on the machine tools, and usage..... and I do NOT mean Sherline only shops. 10 HP motors? Maybe not so good. 1HP? sure..... usually they are not running all day for people who would be reading here...... lots of setup, then shorter "cutting time".............

I run an air compressor off my inverter.

Here's an interview... seems pretty realistic

http://www.motherearthnews.com/renewable-energy/vermont-solar-zmaz94fmzraw.aspx

jkilroy
12-20-2012, 10:45 PM
I do tons of solar in my business, and with your location it is going to be hard to make the numbers work unless you have huge rebates. You don't have to worry about hail. I have installed hundreds of panels and they have been thought serious southern thunderstorms and I have never seen hail break one yet. Your problem is location, you are in one of the worst locations in the 48 states. But don't let that discourage you, the truth is very little of North America is worth a crap. You would need to get close to 45 degrees mounting angle to optimize collection.

jnissen
12-20-2012, 11:48 PM
As others have said do your homework using the online calculators. I pulled the trigger on a system here in Texas since the rebates were fairly decent. Add on the federal tax rebate as well and it made sense for me. Does it really pay for itself. Yes and no. The electric company here just changed how they do the billing so I am now getting at least decent rates for every kWh produced. Problem is they raised all the rates across the board so my actually electric rate has only gone down a little. The rate I am being paid for the energy I produced is much higher so the pay back period has been reduced from about 9 years to less than 5 for me.

I strongly suggest micro inverters if your doing a grid tie system. If you have any partial shade the micros are easily the best decision. Enphase makes a great product and for a grid tie system this is easy to manage and gives wonderful feedback. You can identify down to the individual panel how the system is performing.

A.K. Boomer
12-21-2012, 09:57 AM
I do tons of solar in my business, and with your location it is going to be hard to make the numbers work unless you have huge rebates. You don't have to worry about hail. I have installed hundreds of panels and they have been thought serious southern thunderstorms and I have never seen hail break one yet. Your problem is location, you are in one of the worst locations in the 48 states. But don't let that discourage you, the truth is very little of North America is worth a crap. You would need to get close to 45 degrees mounting angle to optimize collection.


Very true, im from Mich originally and can attest it is very poor for average sunshine, If I lived out there and lived by one of the great lakes I think Id look into wind generation before solar,

You can supercharge the panels by placing them at the correct angle but up against a white background (like siding) and such, you can pull out all the stops - but you still have to have the sun to begin with.. and generally mich. don't.

bborr01
12-21-2012, 10:01 AM
Eric,

You could use the solar to charge your home built scooter too. Have you started construction on it yet?

Brian

1-800miner
12-21-2012, 01:00 PM
I ram into the same bureaucracy crap.I was the first owner/builder to install my own system.
The county building inspector was clueless.
He just signed off anything for the contractors.But gave me the deer in the headlight look. He couldn't answer any of my questions and wouldn't be bothered to find the answers.
Same crap with the public utilities commission. One year later and the guy had not even looked at my rebate application.

On both issues I called my state representative and told her if she wanted my vote it was time to earn it. She earned it!
After that, the inspector and the p.u.c. were quite friendly and courteous.

dp
12-21-2012, 02:54 PM
The Earth receives about 1200 watts per meter squared. That includes energy in all wavelengths and many of those wavelengths are not useful. That is also the perfect square meter located in the perfect location at the perfect time of day. The rest of us have lesser square meters to work with. Today in Seattle (hell, this month in Seattle) it is raining. Very little solar. Days are short this time of year, too.

So to work out how much energy you need to capture you have to define your supportable load on a 24/7/365 basis. Collector, storage, and inverter efficiencies have to be included in the collector size. Storage capacity has to be sufficient to ride out months like we're having where solar is not much of a contributor. You only get out part of what you put in, so plan are lots of square meters.

Converting solar to electricity and then back to heat is probably a bad idea unless the area to be heated is isolated or difficult to get to with anything other than wires. Multimode solar collectors that create both heat and electricity are possible and may prove to be more effective. It all depends on your energy load.

bborr01
12-22-2012, 03:26 PM
The Earth receives about 1200 watts per meter squared. That includes energy in all wavelengths and many of those wavelengths are not useful. That is also the perfect square meter located in the perfect location at the perfect time of day. The rest of us have lesser square meters to work with. Today in Seattle (hell, this month in Seattle) it is raining. Very little solar. Days are short this time of year, too.

So to work out how much energy you need to capture you have to define your supportable load on a 24/7/365 basis. Collector, storage, and inverter efficiencies have to be included in the collector size. Storage capacity has to be sufficient to ride out months like we're having where solar is not much of a contributor. You only get out part of what you put in, so plan are lots of square meters.

Converting solar to electricity and then back to heat is probably a bad idea unless the area to be heated is isolated or difficult to get to with anything other than wires. Multimode solar collectors that create both heat and electricity are possible and may prove to be more effective. It all depends on your energy load.

Dennis,

I have never heard of the multimode collectors but they sound like they would have the advantage of cooling the cells and therefore increasing the efficiency of the cells.

Brian

wierdscience
12-22-2012, 06:12 PM
If the 50 gallon water heater is electric,then my advice is kick it to the curb and install a tankless gas fired heater and forget the solar idea.
IF you could separate the lights off to a separate panel,that could be powered by not too big a system which you could do yourself free of red tape and conartists.

jcaldwell
12-29-2012, 07:26 PM
Hi, flylo,

I'll put in my 2 cents.

I finished a solar intall on my house about 2 months ago. In New Mexico, I have more solar than many places, so I thought it would make sense. I got 34 240w panels at $.98 per watt. They were Schott panels after the Schott factory in Albuquerque closed down. I never once considered getting a big DC to AC inverter. I went from the start with Enphase m215 microinverters. They are a good match to the panels, since the panel rating is under ideal conditions of light, incidence angle and temperature--none of which I will ever see. A big advantage of the microinverters is that they convert each individual panel to AC. So, if a panel goes out, for whatever reason, the supply of electricity is not interrupted. That is not the case with some DC inverters. Installation was fairly easy, since my roof only has a 15 degree pitch. The hardest part was making mounts that would fit under my spanish-style concrete tiles, and allow the rails to be connected. Wiring was simple, though I had to use conduit to satisfy the city inspector that there was equipment ground, in addition to the regular system ground wiring from the inverters. The power company has a program to buy back excess generated energy at a nominal rate, though there probably won't be any excess while my son is in the house :-)

Over the last year, my monthly usage has ranged from about 1800Kwh in the summer to 1000-1200Kwh in the winter. My system generated about 800Kwh during the last billing month. You probably figured out that the max output of the panels is around 8300w, but I'll never see that much. Not only is the max limited by the factors I mentioned before, but the microinverters will cut off a little over 215w. So, the most I can expect would be around 7300w. I know I'll never get that much, though, because of some shading and my installation. The best I have done this month is 35kwh per day. That is pretty much the norm, unless it's cloudy. The worst, when we actually had some cloudy weather, was around 10.5Kwh. My peak power is generally around 6Kw on a sunny day. As I mentioned, that is probably due to shading and less than perfect incidence. Also, 2/3 of my array faces SE, 1/3 faces SW, because of the orientation of my house. I can give you these numbers because the Enphase system has an internet link that transmits system performance data every 5 minutes from my house to the company. I can go to their website at any time and see system and individual panel instantaneous power or energy, and power or energy for any given time period. So far, the system has generated close to 2Mwh of energy. The company monitoring capability is good, because I can see if any panel is not operating up to snuff and figure out why.

When you start to really look at a system, the first thing you need to consider is space available. I had grandiose plans of putting 40 panels on my roof, but realized, after actually scaling things out, that I had to really plan and squeeze just to get 34. (If you go with the Enphase m215 microinverters, there is also a restriction of no more than 17 panels on a single string, due to current carrying capability of the Enphase wiring.) The panels are really pretty large.

If at all possible, install the system yourself. Yes, it will be a pain in the butt. But a friend had a system installed with 16 235w panels and Enphase m195 inverters. It cost him nearly $25K. I have a system twice as big, with higher power inverters, installed myself for about $18-19000, before the tax credit.

JC

jcaldwell
12-29-2012, 07:31 PM
OBTW. I forgot to mention that my system is grid tie. And, as mentioned before, should the power go off on the grid, my system will also power down. Mostly that is for the safety of people working on the grid. However, I think, though I have not tried it, that you could work around that by shutting off the main circuit breaker tying the house to the grid and feeding AC into the breaker box, below the main breaker, via a generator. If the microinverters have power, they work. I don't know that I would advise doing such a thing, but if things were really bad for a long time.....

JC

J Tiers
12-29-2012, 10:03 PM
OBTW. I forgot to mention that my system is grid tie. And, as mentioned before, should the power go off on the grid, my system will also power down.

That just seems insane..... not you, but the fact that systems had to be that way for a long time. That isn't required for safety.

Now the use of a cutover switch is being accepted, similar to generators. But so far it has to be part of the actual inverter. An outback inverter I have worked with has an internal cutover, and will satisfy the "cease to energize" rule by disconnecting from the grid, but still supplies power to the connected loads.

Were you able to get the tax credit doing the work yourself? Many of those programs would only credit if whatever was being installed was put in by "qualified" contractors, you had to have their paperwork to turn in.

GKman
12-30-2012, 09:26 AM
A couple of things came to mind:
I wouldn't put much thought into the problem of the solar system also being shut down during rare power outages. They usually coincide with weather that wouldn't produce any solar anyway and a 5kw $450 gasoline generator doesn't amount to much in a $20,000 decision.

I don't think the necessity of a grid tie to operate is just a regulatory one. I think it's easier and cheaper to build and inverter that feeds a 240 V 60 cycle system than could be restive or inductive with a demand anywhere from a few milliamps to several kilowatts.

J Tiers
12-30-2012, 11:43 AM
A couple of things came to mind:
I wouldn't put much thought into the problem of the solar system also being shut down during rare power outages. They usually coincide with weather that wouldn't produce any solar anyway and a 5kw $450 gasoline generator doesn't amount to much in a $20,000 decision.

Batteries, batteries, batteries.

Then also, a scenario that I have seen repeatedly: One day of nasty weather takes down power in areas all over town. It takes a week to restore power, perhaps more. meanwhile the clouds have blown out of the area, and it is sunny again, but there you are with $20k of unusable investment in power production that is totally worthless to you until power is restored.

We live in a large metropolitan area, and yet several times power has been out in our area for a week or more. And it is getting worse, not better, as the power companies fire their "extra" crews and slice maintenance to the bone as a cost cutting measure.

Maybe you like to hear a generator hammering away for a week, sucking expensive gasoline that you cannot replace locally due to power outages, but I don't. Even less so if I had $20k of solar that was forced to sit there idle and worthless while I had a freezing house (some of the week long outages have been in winter....)




I don't think the necessity of a grid tie to operate is just a regulatory one. I think it's easier and cheaper to build and inverter that feeds a 240 V 60 cycle system than could be restive or inductive with a demand anywhere from a few milliamps to several kilowatts.

The inverter does not "care" about the power output, so long as it is not more than the inverter can produce.

It IS a regulatory one. The way the regulations are written makes a physical disconnection very hard. And also they virtually mandate that there is no way the inverter can operate to supply power without a grid power "signal" on its output. You essentially need a second separate inverter if you want local power with no grid.

Two main standards, UL1741 and IEEE1547, have extremely tight requirements on grid tie inverters. In some cases, an inverter is required to "cease to energize" the power mains within 6 line cycles (0.1 second) after the voltage or frequency goes out of specification. The specification is a few tenths of a Hz for frequency, and a few percent for voltage, and if these are exceeded, you MUST "cease to energize".

The rules specify that testing must do it's best to fool the inverter, with resonant loads, etc, and it still must shut down within the required time.

Theoretically, one could use a relay/contactor to satisfy the "cease to energize" requirement. That is what the power company does, but they also give themselves more time to operate. It is somewhat difficult to use relays or contactors which themselves have an inherent time delay* to operate, because that inherent time delay, which may be several line cycles, cuts into the detection time, making the requirement hard to satisfy.

There is NO requirement that your inverter must actually physically disconnect, just that it can't supply power. Therefore is quite simple for an inverter to shut down the output in microseconds by cutting the drive to the IGBTs.

Since the line crew won't be on the spot in 6 line cycles, AND the power company usually gives themselves more time to open the line, a small increase in the allowed time to "cease to energize" would make it far simpler to satisfy the rules while still being quite safe for workers.

A contactor would be more safe, most likely, as a contactor does a far more reliable "open circuit" than an IGBT which merely has no drive. The contactor actually opens the circuit, and puts an air gap of several millimeters in series. The IGBT is still connected, relying on only a few tenths of a mm of semiconductor junction to block the voltage. It can still fail shorted and potentially put hundreds of volts of DC on the mains.

If your grid tie is many hundreds of kW, then it is different... you are required to satisfy "ride through" requirements, meaning that within much wider limits, you must NOT shut down, but must CONTINUE TO POWER the grid for a longer but still limited time. Few (if any) will have that requirement for a home grid tie system.

* the time delay of a relay is the time for the magnetic field to "decay", plus the added time for the contact arm to accelerate from a stop and move from one position to another due to spring force.

bborr01
12-30-2012, 12:01 PM
[QUOTE=J Tiers;819555]Batteries, batteries, batteries.

Jerry,

If you lose your power regularly for a week at a time, batteries MIGHT be a good choice. But batteries are quite expensive and take regular care and maintenance. You are also likely to have to use a generator to keep up unless you have a very large battery bank. Using old car batteries won't work. You need at a minimum t105 style batteries or better yet a bank of l16 batteries. Lots of cash outlay and time spent maintaining them. You can buy a large inverter generator that is quite efficient for far less than a large battery bank. You also are not likely to need to run it 24 hours a day.

So, if I were to put PV collectors on my home where I am grid tied I would skip the batteries and just use the grid for my storage and use a generator when the power goes out.

My 2 cents worth.

Brian

J Tiers
12-30-2012, 12:26 PM
Suit yourself....

I am NOT grid tied, and I do have a bank of T-105 in the system, which is a backup, and not a full-power system. A rather large bank still costs comparable to a fairly small generator.... in bulk they run about $115 per each 220 AH battery. for a grand that's 8 batteries, which gives quite a decent backup. Of course it gets re-charged every day by the sun, even if only partly.

I cannot run out of gasoline, as I don't use any..... it's nice for us, and for the neighbors, I just toss an extension cord over the fence so they can keep the 'fridge cold, etc.

For the furnace, I need only a few watts, radiators and thermo-syphon operation mean I need only power the gas valve/thermostat.

Batteries are far cheaper than a natural gas powered genset, and work even if the gas cuts off