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solar power after the storm  

 

GeoffMichelle
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October 21, 2008 4:22 am  

i have a question about how it works with photovoltaic power panels for electric that are tied into the grid with a netmeter. if the grid is out, can you still use the system during the day, or do you need to have some battery capacity. what happens if you try to pull too much power from your panels and there is no grid to make up difference. just curious how it works. i know there are some posters that will have answers.
geoff


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Cory
 Cory
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October 21, 2008 12:55 pm  

i wrote a huge explanation, then i hit the wrong button and erased it all, heres my copy and paste version!! HAHA

Solar Electric...the short version

A solar electric system is comprised of 4or 5 main components, solar panels that generate electricity from exposure to the sun, a charge controller to manage the output of the panels, an Inverter which converts DC (Direct Current) from the charge controller to AC (Alternating Current) for your home and finally batteries to store the electricity you generate. A system does not need all these components to provide for your electrical needs.
Systems may be configured in three primary ways.

Grid Tie Grid Tied systems connect the inverter AC output directly to your home electrical system.
When the sun is out and you are using power you use the solar output first followed by power from the grid.
When the sun is not out you use power from the grid exclusively.
When the sun is out and you are not using power, your solar electricity is sent out to the grid, turning your electric meter backwards.
It is smart to size your grid tied system carefully to produce enough power to run your meter to zero, but not feed excess power to the grid that you will not be compensated for.

Non Grid Tied Non Grid Tied systems do not have a connection that sends power out to the grid when you are producing more than you are using.
Non Grid tied systems almost always have a battery storage system to store power when you are not consuming, and release power when you are consuming and there is not enough sun.
In general non grid tied systems also include a generator to charge batteries when the sun fails to appear for days.
Non grid tied systems can be part of a house system that is powered from the grid. For instance you might pull computers and lighting off your existing electrical panel and run them directly off the solar system. When the power goes out you maintain lighting and computers while the rest of the house systems are down

Grid Tie with battery Backup Grid tie with battery backup is the most complex and flexible solar electrical system.
You can use grid power to charge batteries when there is no sun for days.
You can run entirely off solar panels and batteries, drawing from the grid only as a last resort.
You can maintain full operation even after the grid has gone down.

Your access to grid power and your power consumption will help determine which of these systems works best for you.

Photovoltaic panels also known as PV are made in the bulk of cases, from silicon based semi-conductor materials.

When sunlight hits the semi-conductor layer of the panel electrons travel across the junction of two different materials creating a positive flow of electricity. There are many different types of panels manufactured today. The three most common are crystalline, poly crystalline and amorphous. Each technology has it benefits and negatives. Often location, available space and quantity of sun will determine which technology is best for your needs. Photovoltaic panels can be roof or pole mounted depending on your space and aesthetic requirements.

Charge Controllers are the gate keepers between the solar panels and the rest of the PV system.

Since output from a set of solar panels can vary wildly throughout any given day, the charge controller is put in place to regulate the flow of electricity between the panels and the system.
If for instance you had a non grid tied battery backup system and your batteries were fully charged from a good full morning of sun you do not want to continue to force more power into the batteries running the risk of damaging them. So the charge controller monitors the panel’s output and the battery’s charge condition to determine whether to turn off the panels or let more current flow.
A charge controller can also provide a great deal of information about the performance of your system.

Batteries provide a place to store power you are creating with solar panels, that you are not consuming.

It’s great to be able to create solar electricity but you don’t want any of it to go to waste. Batteries for solar usage are generally deep cycle batteries. Deep cycle batteries are designed to release power in small quantities over an extended period of time. In general deep cycle batteries can be drawn down to 20% of their capacity and be fully recharged thousands of times with no harm done. It is recommended that 50% is a safer number and battery banks should be sized for that target. Your car battery is not a deep cycle battery. It is designed to provide huge output in a short time and then quickly be recharged so it’s performance does not fit a solar application. There are two common types of batteries used in solar storage systems. Lead Acid, or “wet cell” batteries and gel-cells. Wet cell batteries are the most common in solar systems because of cost and longevity. A well maintained wet cell battery could provide 10 years or more of good performance with only minor monthly care. Gel-cells on the other hand require no maintenance but the cost more and have a shorter life.

An inverter is a device that converts DC, (Direct Current) from your batteries and or charge controller, into AC, (alternating current) for your home or business electrical system. The inverter outputs clean stable power from your renewable sources after converting the power from DC to AC. The power is sent into your electrical system to be used or sent out to the grid.

If you are installing a grid tied system the final two components are the automatic disconnect and a new electric meter. The disconnect switch shuts down the power going to the grid in the event of a grid power failure. This insures that a lineman is not injured by power sent out to the grid from your solar system. The new meter will be installed by your power company. This new meter will actually spin backwards when you are producing more power than you are consuming. This meter allows you to sell power back to your provider at the same price that you purchase power.

SO...if you have a grid tied system with no batteris, and the grid goes down...you will NOT have power

...If you have a grid tied system with batteries, and the grid goes down...you WILL have power

Hope this helps!

Cory K


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goalusvi
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October 21, 2008 1:13 pm  

Thank you, Cory for that explanation. Some of the discussion on this board about solar power have been super complex and your explanation really helped me understand the basics of using solar. Thanks!!!!!


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GeoffMichelle
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October 21, 2008 1:22 pm  

hi cory,
i was hoping you would answer. i'm still a little confused. if you have a grid tied system, and grid is out, (such as after OMAR), or during any of the short term power outages. during daytime the panels are still making electricity. can't you somehow use this electricity? or do the controllers shut everything down.
can you install some type of load shedding devices that would take off load, so as to balance the panels output, or just use resistive loads that woudn't be harmed by low voltage?
batteries are very expensive, so i was just hoping that a PV system could have some use in a power failure.
geoff


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Cory
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October 21, 2008 2:08 pm  

Geoff...

Many variables to cosider and i dont know your needs but...

This question is very common because people dont want to spend the $$ on batteries, but for a relatively small increase in cost you will get a more flexible system.

Yes its possible to do using a voltage regulator/limiter, for some DC appliances such as a fridge or a water pump. However if you are going to spend $25k on a system, would you rather get an AC appliance and no more than $1000 in batteres/CC to run critical loads or would you rather buy a DC fridge and other system components...

Long story short, yes its possible, but you are better off just getting a few batteries. 5-15 years of life depending on battery quality, DOD and cycles.

The power may be out for a month, but not the sun, if you "critically need" your applainces to run, batteries are a better solution than DC regulators. You shouldnt need more than 2-3 days of power...if the sun doesnt come out.

There are DC fridges & pumps that you can run with a single PV panel and a small battery if its that critical, but if you have already spent alot of money on a system to power the house it desnt make sense to go the DC regulator/limiter route.


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GeoffMichelle
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October 21, 2008 2:25 pm  

thanks for the answers cory. i guess i thought batteries were expensive. i spent almost $1000 for four 6 volt AGM batteries for my boat, and i don't think that would be nearly enough for a house. is there some particular battery voltage that you recommend for the backup. i'm used to 12 volt or 24 volt battery banks for boats. but i have seen 120 volt banks for more storage, without driving up the amperage so much.


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Jim Dandy
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October 21, 2008 2:28 pm  

Grid tied inverters have a built in safety feature which is the equivalent of a transfer switch. The purpose is to prevent back feeding the grid during a power outage and electrocuting a lineman working on the grid or people that are driving over downed power lines after a hurricane has knocked them down and assume that the power is turned off.

You can buy more expensive and more complicated systems that will allow you to use your solar power as a stand by source during an outage but the more features you add the more the equipment costs.

It would not be unusual to have 10 -12 deep cycle batteries to store power so you can run your home during non sun hours. At $200 - $300 a battery this is an expensive option. You also need the space to store this many batteries.

The cost and storage of this many batteries is why most people go with a grid tied inverter and use a generator when the sun isn't shinning or the grid isn't available.

Jim


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Cory
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October 21, 2008 3:04 pm  

thanks for the answers cory. i guess i thought batteries were expensive. i spent almost $1000 for four 6 volt AGM batteries for my boat, and i don't think that would be nearly enough for a house. is there some particular battery voltage that you recommend for the backup. i'm used to 12 volt or 24 volt battery banks for boats. but i have seen 120 volt banks for more storage, without driving up the amperage so much.

It depends on what you are trying to run on battery power and for how long....

Batteries are expensive....higher quality batteries last longer and are more cost effective in the long run.

Are you powering a whole house or just a fridge or pump? and for how long...

Grid tied inverters have a built in safety feature which is the equivalent of a transfer switch. The purpose is to prevent back feeding the grid during a power outage and electrocuting a lineman working on the grid or people that are driving over downed power lines after a hurricane has knocked them down and assume that the power is turned off.

The cost and storage of this many batteries is why most people go with a grid tied inverter and use a generator when the sun isn't shinning or the grid isn't available.

Jim

Exactly!


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EngRMP
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October 21, 2008 4:08 pm  

I think we might be missing one aspect of the question: if we already have a grid-tied system and the grid goes down, then can we use the system to get SOME relief from lack of power?

Here is what I think the problem would be (let's look at some scenarios):
- if the Sun is out at full strength, the system is supplying perfectly usable power:
--- if the system produced 2 kW of power (just for example) and you only were drawing 2 kW (equivalent of 20 100W light bulbs) you'd be OK
--- if you turned something else on (so you asked for more power than the system can supply) I believe that the inverter would essentially shut off (like popping a fuse - in fact it might pop the fuse/breaker at the inverter output)
--- OK, so that's not so bad... a little experimenting and you figure out how to live within the available power

- now, let's say that you have full Sun and you've figured out what you want running, and everything is fine. Now a cloud passes overhead and blocks the Sun:
--- the solar panels instantly drop their power output, so you are now probably drawing more power than is available and I believe that the inverter will shut off (and I'd think it would have a safety feature that it quickly shuts off instead of letting the voltage "sag" too much).
--- so, in this case the power is quickly dropping off with every cloud that passes. I guess that might be OK for refrigerators, and most other devices; but, you'd want to think this one through a bit to make sure you're not causing damage to yourself or equipment.

Other cases include early/late day Sun and overcast days where the system is supplying less than full rated power. You'd have to "learn" how much power is available through experimentation (unless the system has some display that can tell you) and what devices you can have on at each point. As an engineer I would love this puzzle, but I can tell you that it would drive my wife crazy.

Cory, have I got this right... is this the way the system will act?


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Cory
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October 21, 2008 11:45 pm  

I think we might be missing one aspect of the question: if we already have a grid-tied system and the grid goes down, then can we use the system to get SOME relief from lack of power?

Not unless you have batteries, not unless its a grid tied with Battery backup is my best answer.

There is a way to run some DC appliances directly, but if you have a grid tied PV system designed for your house this really doesnt make sense. If one wanted to have a panel hooked up directly to a pump for example, to have it run during the day for a water fountain or something. If one wanted to have a single panel, small battery and charge controller to run a DC freezer..this is possible.

Put simply...A grid tied system is NOT powering your house, it is working in conjunction with your hook up to WAPA. If you produce more than you consume, the meter in effect spins backwards. A grid tied system DOES NOT power specific appliances, it is a system designed to produce power as efficiently as possible to lower the amount of power you need to buy from wapa. It has nothing to do with powering specific breakers in your main electric panel, this is where battery back up comes in. When the power for wapa goes off, the batteries automatically begin to power specific breakers(critical load) only. SO...even though you have panels on the roof, unless you have batteries in your grid tied system you dont have power.

If you have EMERGENCY BATTERY BACK-UP with a grid tied system, THEN you can use the power you produce.

Heres a straight forward explanation and diagram of grid tied and grid tied with battery backup

http://www.aessolarenergy.com/grid_tied_with_battery_backup.htm


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EngRMP
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October 22, 2008 3:18 pm  

Maybe I'm missing something, but according to the grid tied (w/out battery) diagram:

How it works:

1) Sunlight hits the solar module, which is attached to your roof with the mounting racks.
2) The solar (or photovoltaic) cells inside the module then convert the light into electricity (DC Electricity)
3) This electricity travels through wires to the inverter, which takes the electricity from the solar module (DC electricity) and converts it into the electricity your home needs to run your appliances, lighting, etc. (AC electricity)
4) This AC electricity then travels to your standard AC service panel (breaker box), and is supplied to your new utility net meter (supplied by XCel) - which then feeds the electricity both to your home and to the electrical utility grid.

Unless the dead grid stops the inverter from feeding the electrical panel???

So, based on that description (item 4) I believe that the PV system would supply power to your home if the grid died. You're still subject to the caveats in my previous post (full Sun, clouds, overload, etc).


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GeoffMichelle
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October 22, 2008 4:08 pm  

maybe you need a different type of inverter to handle non-grid connected situations. i imagine that the inverter for grid tied system has to reference frequency of the grid in order to synchronize phase.
when you have shaded solar cells, does the voltage decrease with shade, or does the amp output just decrease? if the voltage decreases, then the charge controller or inverter would have to accept variable input DC voltages in order to yield a constant output voltage.
thank you cory and engrmp for your input on this question.
geoff


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Jim Dandy
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October 22, 2008 4:45 pm  

A grid tied inverter in the standard and simplest form only delivers power when the grid is hot. If you want to use solar as standby power when the grid is not available you need a more complicated and therefore more expensive system.

The PV panels are still making electricity is just isn't being used. The inverter shuts itself doen to prevent back feeding the grid.

Jim


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EngRMP
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October 22, 2008 4:55 pm  

Oh, interesting Jim... thanks... I thought the meter would have the disconnect switch, so that you could power the house somewhat during these times. I wonder why that design decision was made. Hmmm, I'm guessing there is a (safe) way to bypass that cutoff, get power to the house, and keep the power from getting back on the grid... not for the average homeowner though - legalities would be HUGE.


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GeoffMichelle
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October 22, 2008 5:28 pm  

i guess best solution is to have the batteries. i suppose if you are just trying get over the normal power outages, and don't put AC on it, you can get by with a fairly small battery bank. then if power goes out for a long period, you could still monitor battery charge from the solar and get some power usage from the panels, albeit somewhat limited. but if you are used to living on boats, then you probably have alot of experience in being careful with power usage.
if you don't want to be so concerned about power usage, then an engine driven gen-set would be the solution, which could be set up for direct charging of the batteries, and wouldn't have to be run all that often.
thanks everyone.
geoff


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Linda J
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October 22, 2008 9:47 pm  

This posting is EXACTLY the reason I will stick with WAPA until I can't afford power!! All I want to do is walk in the door and flip a switch.


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Jim Dandy
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October 23, 2008 1:41 am  

While deep cycle batteries can tolerate being 80% discharged it is strongly recommended that you limit your discharge to 50%.

Therefore you will need many more batteries than you think to run a house.

For instance we still have no WAPA on STX where I live. I run my generator until 10 PM. I then use two small 12V batteries and an inverter to run just a small floor fan during the night. Depending on how well I have gotten the batteries charged I can run my fan for just 4 - 6 hours. Then my inverter shuts down to save my batteries while I wake up in a pool of sweat.

With better batteries I probably could do better, but this example just demonstrates how inefficient it is to use batteries and inverters to run 110V appliances.

Jim


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GeoffMichelle
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October 23, 2008 2:20 am  

say you have (2) 100 amp-hour batteries, so you can only pull 100 out of them to 50% discharge, you only have 1200 watt-hours avail, so you would have to use 300 watts for your fan. that seems high, that would be about 3/8 horsepower. so your small batteries must be less than 100 amp-hour rating. maybe you can find a fan that doesn't use so much power, or get some small dc fans like you'd use on a boat and go direct to your batteries.


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SunOrSki
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October 23, 2008 2:56 am  

I'm with you Linda J. I love the power of a switch!

I am pretty ignorant to power; although, rising rates have made me more cognisant of what I am consuming. I am not on STX now so I really do not even know if my house has reliable WAPA power now or not, that would make a difference. If I had been on the island over the last week without WAPA power I would probably be more decisive.

I am very interested in this subject but the pace is too fast for me to totally comprehend. I need more time than the contributors to absorb the concepts. I am thinking about adding solar to my house but first I need to decide if I will own long enough to reap the rewards of any payback. I am not convinced adding solar instantly adds value to the property as I do not think I would necessarily pay more for such an equipped home.


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EngRMP
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October 23, 2008 7:42 pm  

Ooops, I spaced out on the diagram... I see that it says that the disconnect is in the inverter. And, I think the reason for this is that the inverter requires 115VAC to drive it's internal circuitry... so, there's a catch 22 if you try to use the output of the inverter to drive the internal circuitry. I still think it'd be feasible to get usable power without batteries and without the grid, but only for emergency use (which would be worthwhile)... but that's just this geeky engineer thinking; and there is probably some practical reason (that I'm missing) why you don't want to have this capability.


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Jim Dandy
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October 23, 2008 8:21 pm  

They are batteries designed to jump start an automobile or boat engine. One battery is rated 300Ah. the other is rated 400Ah.

Jim


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GeoffMichelle
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October 24, 2008 2:02 pm  

jim, the 400 amp jump start battery has very little amp-hour capacity. i looked up the info in the instruction book. it says it can power an 80 watt load for 1.5 hours. that works out to just over 9 amp-hours. so not a good comparison to a real battery bank, (even a modest one would be a couple of hundred amp-hour capacity.
geoff


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