General question about solar PV grid tied systems

Discussion in 'General Electronics Chat' started by jimmychacha, Dec 21, 2010.

  1. jimmychacha

    Thread Starter New Member

    Dec 1, 2010
    10
    1
    Hey all, I'm an electronics student and solar energy enthusiast with a general question about grid tied systems that no one seems to have an answer to. When a solar arrray is fed into an inverter, then the AC output is fed into my breaker box, now there are two power sources fed into my box- the utility power and the pv generated power. My question is how can I be sure the loads in my home are using up all the pv power instead of the utility power? Is there an aspect of physics that governs how this usage occurs. Thanks again to all who contribute.
     
  2. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    The grid tie inverter sources power into the grid connection (ie. current flows into the AC wiring). If the current is not consummed in the house, then ipso facto (also a bit of Kirchoff) it flows out into the grid.
     
  3. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    ...and how would the power company know to give you credit when it was due??
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,795
    950
    Ah yes! That requires a meter that can record negative use. A two way meter they are called.

    And not every power utility will pay you for putting power on the grid.

    The inverter output HAS to be a slightly higher voltage as measured against the AC mains or it cannot 'feed' any power into the house wiring. If every device in the house is satisfied by the inverters output and it still has ability to put out more, then that slightly higher voltage will appear at the main breaker panel and at the meter, which are connected to the grid through a transformer. The extra power will be absorbed by a neighbor in reality, but it is technically the 'grid' which let it get there.
     
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    It is true that power companies must buy surplus power. However, they are not required to provide the equipment to link it back to them (phase matching inverter, two way meter, panel, etc). In most cases, there will not be enough of a surplus to sell back to the power company to ever cover the equipment cost. Exceptions to this are large wind power installations, or combinations of solar and wind power, or small hydro power if you have the water rights to a fast flowing stream.
     
  6. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    That sounds reasonable. The 60W solar array I have is not likely to cover all needs in the home and a combination of grid tied and charged batteries would keep most of the collected energy in my home. One battery with a 750W inverter could power my front loading washer without too much effort. I haven't checked, but I suspect the refrigerator would be simple enough also. The cloths drier, in the other hand would be an issue. Perhaps untying some circuits from the grid that only supply lighting would be possible as well.

    Q: Are there devices that switch to battery charging when the grid tied devices are not using all the power supplied?

    I hope I am not hijacking the OP's thread. Let me know, jimmychacha. I was assuming these question would also be helpful to you as well.
     
  7. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,795
    950
    You are asking about what is known as a 'Charge controller'. The fancy expensive ones are a combination of inverter, battery charger, and grid power monitor, all in one package.

    Check out(google them) Outback PV equipment. It is what I use to charge my batteries. A 2500 watt solar array with 25.25 amps @ about 62 volts on cloudy days, which feeds the charge controller and battery bank. I only have one true sine wave inverter, with 1500 watt output @ 120VAC, but it runs everything (If I'm careful of what is switched on) but the central air unit and the microwave. Oven and range are gas. :)

    I really need to get a grid tied unit as well, so I can cut the electric bill by 30-35%. Right now it just serves as a power outage back up and also runs our bedroom window AC unit.
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    If your local electric provider does provide the equipment, or even half of it, then it is often a good deal.

    I mentioned it above as some do not find out what the electric company provides or will not provide when they set up a few panels and expect their bill to go away. This will work in areas where it is sunny mostly year around, but not so well in the northern climes, essentially, north of Texas/Arizona/etc. Though in the corridor from TX to North Dakota, there is a great deal of potential wind power to be found. It isn't as unobtrusive as PV solar, but it is a good ROI in those areas.
     
  9. 'Ol Sparky

    New Member

    Dec 22, 2010
    3
    0
    Why would you want a grid-tie system anyway? Why not keep the power for yourself like me?
     
    Last edited: Dec 22, 2010
  10. jimmychacha

    Thread Starter New Member

    Dec 1, 2010
    10
    1
    iONIc, no problem taking the discussion in another direction, I enjoy reading others opinions. However I think my original question was a bit misunderstood. My scenario does not generate excess power to be bought by my utility, I am asking for an explanation that proves my PV power is consumed first, before I pay my utility for any further power needs. If I have a constant 100 amp load in my house, a constant 50 amp supply from PV, and an 200 amp supply from my utility, I assume that my utility bill will be for a constant 50 amps. I would like an explanation that guarantees that my 50 amp PV supply is used up first, before I need to purchase the other 50 amps from my utility?
     
  11. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,795
    950
    You would have to set up a current output monitor, and record the output of your PV array.

    That constant 50 amp output will only be available 4 to 5 hours a day, while the 100 amp load from the house is 24/7.

    Don't expect the bill to be cut in half, if the PV output is not constant through out the entire day and night. You have the 5 hour / 24 hour percentage to calculate as well.
     
  12. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    If you had a separate circuit box supplied only by inverted solar power then you would know that the power company is not powering a certain room or two or three. A battery bank with inverted power to this new circuit box could power some rooms for some time. There are several rooms in my home that power nothing but a few florescent lights and could all be run from a single battery inverted to 110VAC. Depending on your appliances you could also remove a refrigerator and washer from the grid and add to the "off-grid" box.
     
  13. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    jimmychacha

    "I am asking for an explanation that proves my PV power is consumed first, before I pay my utility for any further power needs. If I have a constant 100 amp load in my house, a constant 50 amp supply from PV, and an 200 amp supply from my utility, I assume that my utility bill will be for a constant 50 amps. I would like an explanation that guarantees that my 50 amp PV supply is used up first, before I need to purchase the other 50 amps from my utility?"

    The rigorous answer is Kirchoff's current law - there is no better guarantee. If you doubt that law then you can tong meter the currents for personal confidence. You may already have some metering - you will only need two current (or power)measurements to deduce the third.

    Ciao, Tim
     
  14. mwade002

    New Member

    Jan 27, 2011
    3
    0
    ...forget how much energy the house is using, with net metering, the inverter is designed to handle the array, not the load as in off grid systems. All the power at the array is pumped out, anything you need above that you get off the grid.
     
  15. jaclement

    Active Member

    Apr 15, 2009
    34
    0
    The most important thing for a grid tied system is that the inverter does not supply power to the grid if the grid goes down, so that it is safe to work on the grid in the case of a power failure. This means you lose power too.The big advantage of grid tied systems is the you don't have storage batteries that are heavy,expensive and have a limited lifetime.Grid tied systems can be a good financial investment, but the agreements with power companies will change radically. We are currently getting 13 cents per KWH that the system generates regardless of who uses it (REC meter). The power goes into our box on the custumer side of the main power meter,so the meter runs backward if we generate more power than we use. This is a contract with the power company for 12 years. We get a check from PNM each month for about $ 35 to $80. The power company has since reduced the payout to new solar systems on a sliding scale depending on how much installed solar capacity has signed up. The big boys get 16 cents per KWH for 15 years ( 1000 KW systems, I know of 3 in Albuquerque now). The power company wants to charge an 8 cents per KWH connection charge,which would kill the whole thing. We paid $26k for our system with a total tax credit of 40% on that 26K. We break even in 10 years. I works out to about 5% reurn on investment
     
  16. mwade002

    New Member

    Jan 27, 2011
    3
    0
    That's pretty good, paid for power you produce, and you get to use that power you are paid for.
     
  17. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,175
    397
    Power Co explination: tep.com/green
     
Loading...