PV grid tied for self use

Hi

I'm wondering about the pv GridTied systems for self consumption. You know, where instead of send excess to the grid, you produce what you need to consume during the day and use that instead of grid power.

Like any GridTied system, you connect your inverter out lines to your mains panel. This of course means that now your mains panel gets energy from both the grid and your inverter. I can visualize how if you are producing 3KW but only pulling 1KW with your home loads, the excess 2KW has to go somewhere so it goes to the only other outlet which is the grid.

But if you're producing 3KW and you're consuming 3KW, using the water analogy, both hoses (grid and inverter) have the same pressure pushing into your mains panel. How come the inverter produced power get preference into the home circuit over the grid power?

Is it because the inverter hose doesn't accept power in the other direction but the grid hose does? Or is there something special about the installation that must be made in order for that to happen?

Comments

  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: PV grid tied for self use

    It doesn't happen.

    A standard GT inverter pushes current to the wiring. After that it doesn't know or 'care' where the power goes. Whether it is consumed by household loads or sent back to the grid is purely a matter of how much he loads are:

    Loads less than GTI output = difference in power sent grid.
    Loads equal to GTI output = zero power flow to or from grid.
    Loads greater than GTI output = difference in power pulled from grid.

    Limiting GTI output to match only the household loads is very difficult and complex. Off-the-shelf GTI's will not do this; don't expect them to.

    This is not the same as a hybrid GTI (with batteries) that can be set not to sell to the grid; they are more complex and have two (or three) AC connections: one for loads and one for grid (third for generator input).
  • quiquequique Posts: 252Solar Expert ✭✭
    Re: PV grid tied for self use

    Yeah, you say:

    "Loads less than GTI output = difference in power sent grid."

    What I want to know is what is the mechanism by which that GTI output gets sent to the grid. Why wouldn't the grid push it's way in?
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: PV grid tied for self use
    quique wrote: »
    Yeah, you say:

    "Loads less than GTI output = difference in power sent grid."

    What I want to know is what is the mechanism by which that GTI output gets sent to the grid. Why wouldn't the grid push it's way in?

    Two different types of power source. The grid is Voltage-based and only produces current when there is a load on it. The inverter is current-based and will always push current within the accepted Voltage and frequency parameters (qualification). So to the grid the GTI looks like a "negative load" and to the GTI the grid looks like a "positive load". Since the two waveforms are in sync they act in harmony, the same as two generating plants on the same grid do.
  • quiquequique Posts: 252Solar Expert ✭✭
    Re: PV grid tied for self use

    Wow, as always a very nice explanation!. I feel that it's almost on the tip of my brain :-). Would you happen to have an analogy to visualize that?
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: PV grid tied for self use
    quique wrote: »
    Wow, as always a very nice explanation!. I feel that it's almost on the tip of my brain :-). Would you happen to have an analogy to visualize that?

    Actually, no. I'm not good with analogies and the usual water one falls completely apart when you start synching power sources of different types. For one thing, water doesn't have any 'waveform' - unless it's in a lake or ocean.
  • BB.BB. Posts: 28,084Super Moderators, Administrators admin
    Re: PV grid tied for self use
    quique wrote: »
    But if you're producing 3KW and you're consuming 3KW, using the water analogy, both hoses (grid and inverter) have the same pressure pushing into your mains panel. How come the inverter produced power get preference into the home circuit over the grid power?

    Is it because the inverter hose doesn't accept power in the other direction but the grid hose does? Or is there something special about the installation that must be made in order for that to happen?

    As a first approximation--This is actually a good analogy (think DC power)... You have a water tank 100' up the hill. You have your faucets that can turn on and off. And you have a constant volume (fixed volume) pressure pump.

    Faucet open, it consumes 2 gpm. Faucet closed, no consumption.

    Water pumping at 3 gallons per minute. Faucet open, 2 gpm to faucet, 1 gpm to water tank on hill (notice, no check valves or anything).

    Faucet closed, all the water is pumped to the bank.

    If you are familiar with car electrical system... The battery is basically the "heart" of your power system. It tries very hard to keep voltage around 12.0 to 14.2 volt volts... You can connect a load "anywhere" in the system (behind a fuse/breaker to protect wiring against short circuit). And you can connect the alternator to a battery. And you can even connect an AC battery charger to the battery too.

    When the car starts, the battery (grid) supplies all the energy. When the car starts, the alternator starts to recharge the battery (regulator estimates how full the battery is and adjusts current from alternator as needed). Turn on the headlights, current may come from battery, alternator, or a mix of the two.

    There is no special electrical components in the AC mains system... The utility/grid looks for all the world like a giant AC battery. The loads consume power. And the GT inverter pushes power back into the system.

    The only "special" device is the utility power meter... Ideally, it runs "forward" as you consume power and can run "backwards" if you generate more power than you need. Different power companies can have electronic meters that measure/attribute power for billing in different ways. Forwards/backwards is "net metering". I have Time of Use metering--So my meter has two "registers"... One for total power used, the second for "peak power" used--They both can run backwards and forwards.

    Other utilities can bill differently (for example bill at $0.20 per kWH for power you buy... And pay you $0.05 per kWH for power you "sell" to the utility).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • quiquequique Posts: 252Solar Expert ✭✭
    Re: PV grid tied for self use

    Well I'm having issues because I installed the system, I've tried having ACs and fans and everything else on when it's producing but it's not consuming what I produce. It seems to be going out the meter.
  • BB.BB. Posts: 28,084Super Moderators, Administrators admin
    Re: PV grid tied for self use

    If both your loads and your GT solar system are on the same side of the meter--Then the loads are consuming the power produced locally by your GT inverter--There is no other way for it to work (other than the GT system flat out not working).

    If the loads are behind one meter, and the GT system is behind a second meter (common for some utilities--They pay you for the power you generate then turn around and charge you for ALL the power you consume... And at the end of the month, the balance the generation meter against the consumption meter.

    You certainly can try things to make sure your GT and your Loads are on the same main panel... Cut the main breaker and all your loads and your GT inverter should go dead too. If the GT inverter stays alive (producing power), then there is a wiring error or something else (like a dual meter system).

    Then there is the "anti theft" meter... Many (most, all?) new non-GT billing meters will only "run forwards". This is to stop people from stealing power (plug the meter in upside down for 2 weeks of the month) and to stop people that do "guerrila solar installs" from not signing up with the utility.

    For example, if your loads are 500 watts and the GT inverter is generating 2,500 Watts--The meter will show a "2,000 Watt load"--And the utility will charge you for that power you "generated" -- Just like it was a 2,000 watt load instead.

    There are other, older meters that may have a one-way clutch in them... They only "run forwards" if there are loads--And if there is excess power generated (GT solar), the meter will stop.

    Anyway--Those are the major issues I can think of... Any sound plausible for your situation?

    You can certainly watch the meter (and the direction the "wheel" or LCD blocks move. First turn off the GT inverter--Watch what happens. Then with very little loads (or turn off the branch circuits), turn on the GT inverter and see which direction the meter moves and how fast (assuming the GT inverter is generation a lot of power). That will tell you if the meter is working (roughly) correctly or not.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • quiquequique Posts: 252Solar Expert ✭✭
    Re: PV grid tied for self use

    I have a 3kw inverter which is indeed behind the only power meter in the house.

    I have been testing with the inverter working and the normal house loads during the day.

    Wondering what the best setup would be to test.

    I do have the type of meter that adds consumption with production which is why I've been careful to have more consumption than production but I don't seem to be getting it right.
  • BB.BB. Posts: 28,084Super Moderators, Administrators admin
    Re: PV grid tied for self use

    The problem is that it is not very "obvious" when you have 2 kW loads and 3 kW of GT inverter output vs 3 kW of loads and 2 kW of GT inverter--The utility meter will spin/log the same in either case.

    You either need to find a clip on whole house power monitor that actually can measure the directon of power flow (needs both current and voltage probes) or get an old mechanical bi-directional power meter and install that in series with the power meter (probably not a very good solution).

    Otherwise, you put current clamps on your major load circuits (add those up), and another current clamp on your GT inverter output--When GT output nears the sum of the load currents, then you are at risk of pumping energy to the grid.

    If you have 120/240 VAC split phase power, you need to measure both L1 and L2 current.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • quiquequique Posts: 252Solar Expert ✭✭
    Re: PV grid tied for self use

    I made a few tests, im not sure I want to bore you posting all the results. But here's the nitty gritty.

    INVERTER ON TESTS:

    #1 Produced 10KWh : Used Washer & Dryer : Meter Moved 13 : Daytime 9 : Nighttime 4
    #2 Produced 7KWh : Used Washer & Dryer : Meter Moved 12 : Daytime 8 : Nighttime 4
    #3 Produced 0KWh : Used Washer & Dryer : Meter Moved 8 : Daytime 4 : Nighttime 4

    The inverter was OFF on Test #3. It seems Im "counting" more energy with the days with the inverter on.

    My calculations are:

    Nighttime Usage = Fridge and 3 ceiling fans. Fridge is about 100W and ceiling fans are about 100W as well, so at nighttime, thats 400w in 12 hrs = 4.8KWh which is about right.
    Daytime Usage = Fridge & washer & dryer. Fridge is 150W, washer is 400W and dryer is 3000W. Fridge 150x12 + washer 400*0.5 + dryer3000*0.5 = 1800+200+1500 = 3500.

    So in a regular day, I should be consuming around 4.8 at night and 3.5 in the day. That adds up to 8.3kwh which seems to be inline with my consumption for Test #3. It also seems to be inline with my typical bill which is around 300KWh a month (8.3X30=249 ~ 300).

    TEST1
    So if thats my baseline, it would seem that on Test 1 I consumed 8.3, Meter moved 13, which would mean I produced 5 which moved out the meter and got added to my 8.3 consumption. But my inverter says I produced 10. But that would mean I produced 10, consumed 5 and sent 5 to the grid. But that doesn't make any sense because it means I consumed 8.3 + 5 (which no corresponding loads to have consumed them) plus the 5 extra ones i sent to the grid.

    TEST2
    Similarly, I consumed my 8.3, moved 12, which means 4 got added to my meter. Since I produced 7 and 4 got sent to the grid, I consumed 3 extra ones. Once again, I do not have any extra loads that would have consumed those 3 extra KWh on top of the identified 8.3.

    What Im wondering is, what could be a more accurate way of finding out what's going on? I only have the 1 meter in my house and only 1 service panel. I just checked again right now and it seems that today, the second day with my inverter off, it will have moved 8 in the past 24 hours once again. This seems to confirm my baseline consumption is 8 daily. It was only in those days that my inverter was on, that my consumption basically doubled.
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: PV grid tied for self use

    What's going on is that this is an illegal grid-tie installation. Were it legal there would be no issue with sending power back to the utility when the local loads do not consume all that is produced.
  • BB.BB. Posts: 28,084Super Moderators, Administrators admin
    Re: PV grid tied for self use

    For your existing Grid tied system... You are sort of hosed. There is no easy way to control the mix of GT+Loads. And, technically, you have an install that the utility could pull your meter and the city red tag your home if they found out/pushed the issue.

    Your only solution would be to convert to an off grid system and run your loads from the inverter+battery system. This will probably not save you much (if any money due to the costs/extra losses of an off grid/battery system).

    The only other options are to disconnect/down size your array to ~1/4 its current size or install a load that can use the extra power (such as water pumping).

    Assuming your utility will not allow a grid tie system and/or you cannot get permits.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: PV grid tied for self use

    As far as balancing loads with output goes, you are confusing Watt hours which is cumulative with Watts which is on-going. In order for you to achieve net zero at the meter the load must equal production at the moment of production. So if the inverter is putting out 3000 Watts you need a 3000 Watt load at the same time in order to avoid power being back-fed to the utility and running your meter up instead of stopping it.

    An electric hot water heat consumes this much power, except that it is thermostatically controlled and shuts down when the temperature reaches the 'OFF' set point.

    Without some rather complicated measuring system you will not be able to automate the load/production balance so that no power goes to the grid.
  • quiquequique Posts: 252Solar Expert ✭✭
    Re: PV grid tied for self use

    Yes, the loads were matched within the production hours, so thats not the issue.

    Its definitely a meter issue. Now that I powered down the inverter, the meter is barely moving about 2 kWh a day and yesterday all of a sudden it moved 9.

    I live in Honduras, the law has been passed but the power company hasn't replaced the meters yet. We are waiting on them to replace our meter, we even bought it for them, but they may be having some political issues :)

    We'll just have to wait and see
  • stephendvstephendv Posts: 1,571Solar Expert ✭✭
    Re: PV grid tied for self use

    There are commercially available devices that will match consumption with PV production. They all use a resistance heater of some sort, e.g. hot water heater and control the power being sent to it so that it matches your excess PV production. Here are some links:

    - http://revosolar.com/solar-shop/es/accesorios-/188-inhibidor-inyectar-en-la-red.html
    - http://www.immersun.co.uk/
    - http://www.coolpowerproducts.com/uk/index.html
  • quiquequique Posts: 252Solar Expert ✭✭
    Re: PV grid tied for self use

    So it basically consumes whatever excess is leftover before it goes to the meter?
  • stephendvstephendv Posts: 1,571Solar Expert ✭✭
    Re: PV grid tied for self use
    quique wrote: »
    So it basically consumes whatever excess is leftover before it goes to the meter?

    Yes exactly. The first one is from a Spanish company because the laws around self-consuming PV power didn't allow injecting into the grid (before they went bonkers with the current law). And the other two are from UK companies where there's a financial incentive to use your own PV power in normal grid tied systems.
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