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235W Solar Panels are too high voltage for GTI, Suggestions?

Discussion in 'Renewable Energy' started by Jules_Theone, May 6, 2012.

  1. Jules_Theone

    Jules_Theone Member

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    I've recently moved house, finally got round to tinkering with the solar panels and grid tie inverters today, haven't got all the kit yet but started putting things together to test.

    I have 2x 235W panels, 1x 40W panel and 1x 80W panel, 2x 350W (Chinese) GTIs. Will buy isolation switches, electric meter, circuit breakers etc to do it properly later.

    My problem is that the GTIs are designed for 14V to 28V which worked perfectly for the 80W and 40W panels but I measured the open circuit voltage from the 235W panels and they were putting out around 34V which means the GTIs don't start. It sort of works if I cover part of the panel so the voltage comes down then the GTI loads the panel and it continues to work when I uncover it. I tried connecting the GTI to 2/3 of the panel but that would mean it would not use the whole panel. Also tried putting 6 diodes in series with the panel to bring the voltage down but with little current flowing (40mA) the voltage doesn't actually come down much.

    I had some ideas about putting a small load on the panels to bring the voltage down to 28V but haven't tried it yet and think it would probably be a waste of power, alternatively, I thought to add a voltage clamp sort of thing like a FET and zener diode to trim the voltage and so would stop conducting when the GTI is loading the panel (bringing the voltage down) which seems better than a resistive load. It's gone dark now so I'll experiment again tomorrow, hopefully it'll be sunny... One last thought I had was that it seemed the problem was that the GTI refused to draw (much) current because the voltage was too high so if I can get the GTI to load the panel(s), the OC voltage would drop and it would work. I connected a 235W and 80W panel in parallel and it seemed to work, but that might have been that I connected the 80W THEN the 235W.

    Are any of the above ideas any good or does anyone have better ideas.

    Thanks,

    Jules.
     
  2. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    What does your GTI pull the 235w panels voltage down to when its running?

    If it was me I would just use a zener and a fair sized NPN transistor or similar setup to make a simple voltage limiting shunt of sorts.

    I would suspect that to pull the panels OC voltage down from 34 to 28 may not take all that much actual current.
     
    Last edited: May 6, 2012
  3. alec_t

    alec_t Well-Known Member Most Helpful Member

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    As the panels have different ouput voltages (34V v <28V) presumably you have some way of preventing reverse current flow through the lower voltage panel?
     
  4. dave

    Dave New Member

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  5. Jules_Theone

    Jules_Theone Member

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    I think the voltage comes down to 17 - 18V, when I was measuring current (like 5Amps in bright sunshine) it didn't seem very high for such a large panel but my DMM had a flat a battery which I later found affected it's accuracy. I have some high power resistors I was going to try loading the panel with tomorrow to see how much it takes to bring the voltage down. Obviously that isn't the ideal long term solution, just to see how big the transistor needs to be. Typically the panels came from left over from a 340kW solar array and so not exactly made for home use. Ideally I want to add panels as I go along, I'm guessing there isn't an issue with connecting different power rated panels in parallel as each has a diode in series with them. The only thing I can think of is the MPP for diff size panels is different.

    Another question I wanted to ask was i know I can connect pretty much as many GTIs onto the same mains supply but is there any issues with connecting them on the DC side too, 235+235+80+40W panels connected in parallel to 350+350W GTIs with DC inputs and AC outputs joined together?

    Alec_t, all the panels have blocking diodes in series so no issues with reverse current etc.

    Thanks,

    Jules.
     
    Last edited: May 6, 2012
  6. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    At 34 volts OC I would take a guess that they may be rated to produce around 9 - 10 amps at 24 VDC. If your GTI needs to see 28 volts or less the panel may only need a amp or two of load to keep it at 28 VDC so from there you may only need to dissipate around 60 watts of power which is not much for a common TO-220 or larger sized transistor to handle.

    The thing is if your GTI's are pulling the panels down below 28 volts when running the voltage limiter wont be getting used at all.
    Also if the system is hooked up all the time I would assume that every morning as the sun comes up the GTI would be coming on automatically while the panel voltage was well below 28 volts keeping the voltage down anyway.

    Thats my take on this so far.

    How about som pics for us? :D
     
  7. Jules_Theone

    Jules_Theone Member

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    I agree, the voltage limiter would hardly ever be used. I can imagine if the inverter restarts or the mains goes down it would release the DC to rise and get stuck again. I did some more experimenting today, while it wasn't raining and found that with just the 80W panel connected it drew 5-6Amps. With the 235W connected after, the current only came up to 7-8Amps. I think that the 80W panel isn't really working very hard once the 235W is connected. I haven't got enough DMMs yet to measure every parameter and annoyingly my meters can only measure up to 10Amps. I haven't found the box of high power resistors yet so made a current shunt with some copper wire to be able to measure higher currents than 10A. Seems to work, see pics. I'm going to build a triangle mounting to make the panels face into the sun better. And make up a board with all the GTIs, switches, elec meter etc on.

    It might be better to have the larger panels separate from the smaller ones.

    I've got a pretty new electric meter (Landis Gyr E110 Type 5235A) which appears to be the basic model which isn't capable of measuring power flow in both directions. It has a pulsing red LED which blinks for every watt-hour I use and goes to solid red when I try to feed back onto the grid - I think that means it won't run backwards when I produce more power than I use. Annoyingly it doesn't have tenths of kWh so I can't easily see it running backwards. I've bought an electric meter to put on the same board as the GTIs. I think if I switch everything off and measure 1kWh from the solar panels with my bought elec meter and compare it with if the E110 meter registers a decrease. I also plan to have a PIC circuit to log DC power / AC power generated from the array.

    Jules.
     

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    Last edited: May 7, 2012
  8. debe

    debe Active Member

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    With electric supply meters be carefull. Here in Australia a rotating disc type meter will run & register in reverse with a Grid tie inverter. But a plain Digital type will continue to register & will charge you for energy you export. With a grid tie system you need an Import/Export meter.
     
  9. strantor

    strantor Active Member

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    Maybe you could setup a little comparator with a 28V reference and a time delay; if the panel output is >28V, a mosfet shunts the panel with some low resistance load for a bit, to give the inverter time enough to come on.
     
  10. Jules_Theone

    Jules_Theone Member

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    For those interested in a picture... Attached is the board I made up for the solar grid tie at my parents house before I moved out. The array was only 145W of various panels but it could charge lead acid batteries or connect to the grid (manually switched). It had a standalone inverter for when there's a power outage, but typically it never happened.

    Yesterday I phoned up the feed-in tariff people to ask about connecting to the grid, getting the meter changed or what equipment I needed (not really the right people to ask about technicals) and got some general information. It seems like a lot of bureaucracy if the system is less than 1kW (initially) and if I wanted to add to the system later I would have to get it re-certified. I want the freedom to experiment with the system and that means that I probably can't get paid the feed-in tariff.

    Also the guy suggested that effectively if I don't use the power I generate locally in the day and it feeds the grid, that it will disappear, thats not how I understand it - I can generate say 10kWh in the day when it's sunny but only use 5kWh locally, meaning that 5kWh will go onto the grid and power someone else's house. Then when it gets dark and I stop generating, I continue to use 5kWh overnight, overall I have used no electricity from the grid. I believe with my current setup (with the digital consumer meter) the meter won't run backwards and if I don't use it locally I will lose it.

    Apparently I can get paid 9p for every kWh I generate if my house is rated E-G on a EPC certificate but 21p per kWh if my house is A-D, basically if my house is energy efficient I can get paid more.
     

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  11. debe

    debe Active Member

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    This is the import / export meter the power company installed its an ltron & they are made in England. They come in mode1 import only meter doesnt register during rev energy flow. Mode2 Uni-direction & records the sum of the energy flow, inport & export kwh in one register. Mode3 Bi direction, the meter records import & reverse energy flow in seperate registers, labeled import (3) export (9).
     
  12. Jules_Theone

    Jules_Theone Member

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    I've figured out I can get the higher rate payment if I go through getting the setup certified.

    Over the weekend I had a phone call from the electricity company I'm switching to to discuss what I wanted to do RE: grid-tie and feed-in tariffs. It all has to be completely certified - solar panels, GTIs, meters, isolation switches, installed by a MCS certified installer (not me). This seems overly expensive for in my opinion a simple install I can do myself.

    I haven't had the opportunity to test the digital consumption meter to see if it runs backwards (unlikely), reads only consumption or reads as consumption in either direction (bad) as my housemates have been there using appliances, perhaps try tomorrow.

    I've done some rough calculations for price of systems:

    The equipment I've bought so far cost £695 for 590W of capacity,

    £695/590Wp = £1.18/Watt

    Whereas the energy saving trust website calculates:

    1kWp system fully installed cost approx. £4400
    £4400/1000W = £4.40/Watt

    2kWp
    £7200/2000W = £3.60/Watt

    3kWp
    £10000/3000W = £3.33/Watt

    4kWp
    £12800/4000W = £3.20/Watt

    Which seems like 3 times the price, correct me if I'm wrong.
     
  13. mr.s.p

    mr.s.p New Member

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    Type approved, professionally fitted equipment costs a lot in rip-off Britain. DIY is what our dad's did, no room for that any more in the nanny state...

    Does your chinese grid-tie kick in when the sun rises? The output of the panel must reach 12v before it gets to 28v and if the inverter starts up somewhere between the two, it will never reach too much voltage.
     
    Last edited: May 14, 2012
  14. debe

    debe Active Member

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    Is Australia the only country that subsidises household Solar GTI systems? My 1.5Kw system was $1799 fitted after the subsidy. You can get cheaper if you live in a large city, as im in the country & 250Ks from nearest city they charge travel expense.
     
  15. mr.s.p

    mr.s.p New Member

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  16. debe

    debe Active Member

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    I would expect your low voltage inverter & panel to load the panel voltage up as the sun rises & hold the voltage to an acceptable level. Here is what my system does & its a chinese GTI Aerosharp. The solar panels are in a series string of 8 of & give 369V DC open circuit. As the sun comes up the voltage slowly rises to about 270V DC & stays there, & the mains voltage is about 258V AC, if the mains goes over 260V AC which it does sometimes the Inverter disconects its self & registers Grid fault.
     
  17. debe

    debe Active Member

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    Some interesting reading there. Here the feedin tarrif was redused drasticly last year, this also saw a large reduction in the cost of instilations eg 1.5Kw system went from around $5000 to around $1800. When this happened Solar took of & there was a back log of instilations. On around June 30th here the Govt instilation subsidy is to be reduced, will be interesting to see where the prices go then.
     
  18. Jules_Theone

    Jules_Theone Member

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    I was thinking of installing the equipment myself and getting a MCS certified company to check it and sign it off but they probably wouldn't want to do that. I understand that these Chinese GTIs are probably lower quality than a SunnyBoy etc inverter and the solar panels are just various types and sizes of panels (I'll note down the exact makes / models) but if it feeds onto the grid ok what's the problem? Ideally I don't want the system to be 'closed' to me - I want to be able to connect extra panels on later or try connecting exercise equipment to the grid-tie (not much power generated but it's more fun than useful). I can see that a system open to tinkering is also open to fraud like wiring things before the meter etc.
     
  19. mr.s.p

    mr.s.p New Member

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    In the genuine, type approved, professionally installed system, is it possible to use some of your generated power before it gets to the export meter? Or do you have to export it all to the grid and then draw any power you need from the grid?
     
    Last edited: May 16, 2012
  20. debe

    debe Active Member

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    If you are using apliances during the day & the system is generating power, then the solar system will try & match it. It will only export exess. This is a total for the 30days of my 1.5Kw system. Imported 131.8Kw, Exported 110.4Kw. Total solar production 168Kw.
     
  21. Jules_Theone

    Jules_Theone Member

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    I believe the way it will work is if I'm generating at a moment in time say 1kW and consuming 500W it would effectively have 500W flowing onto the grid. Over a day I consume 10kWh and I also generate 10kWh but because I'm not consuming at the exact rate that I'm generating, I have to buy some of it from the grid at 14p/kWh and sell the excess at 3.2p/kWh. If I had a consumption meter that could run backwards I could buy and sell at the same rate of 14p/kWh. The electric company said they will pay me 21p/kWh I generate, 3.2p/kWh I export to the grid (they assume I will export 50% of what I generate. I'm trying to figure out what would be the best to do and what size system to get. The best setup for me would be to have a consumption meter that runs in both directions (consumption is positive and generation is negative), a generation meter on the solar panels to measure how much I generate. This would mean that for every kWh I generate I would get 21p+3.2p and it would offset my consumption no matter when I'm generating or consuming. Whereas if the two meters can only read in one direction, I'm losing out if I don't generate and consume at the same time of day. Thinking about it, I guess that's what they want - at peak times when I'm consuming, everyone else is also consuming but I'm not generating so its not neccessarily helping the electric company. What I need to do is store the electric to be used when I'm consuming.

    For instance:

    If I have solar panels, they will generate all the electric in the daytime when I'm at work and not consuming much. I will consume the most electricity in the evening / at night when the solar panels are not generating.


    debe: according to my calcs -

    Imported: 131.8kWh
    Exported: 110.4kWh
    Generated: 168kWh

    Consumed = Imported + (Generated - Exported)

    = 131.8 + (168 - 110.4)

    = 189.4kWh

    I'm assuming that both meters only run in one direction and you aren't consuming at the same time and rate as generating, so if I had those results:

    Price per unit = 14p/kWh
    Export price = 3.2p/kWh
    Generation price = 21p/kWh

    Price of importing = 131.8 x 0.14
    = £18.45

    Price of exporting = 110.4 x 0.032
    = £3.53

    Price of Generation = 168 x 0.21
    = £35.28

    Total = 18.45 - 3.53 - 35.28
    = £-20.36 (I've made money)

    But I'm not going to have an export meter to measure actual export so:

    Price of exporting = (168/2) x 0.032
    = 84kWh x 0.032
    = £2.69


    New total = 18.45 - 2.69 - 35.28
    = £-19.52 (not much difference)


    Whereas if the consumption meter could run backwards:

    Import or Export amount = Generation - Consumption
    = 168 - 189.4
    = -21.4kWh (imported from the grid)

    Price of importing = 21.4 x 0.14
    = £3.00

    Price of exporting = (168/2) x 0.032
    = £2.69

    Price of Generation = 168 x 0.21
    = £35.28

    Total = 3.00 - 2.69 - 35.28
    = £-34.97 (extra £15 made compared to single direction meters)
    I believe this would be the case if generation and consumption was completely matched.


    Now if I generated half the previous but consumed the same amount:

    Consumed: 189.4kWh
    Imported: 105.4kWh
    Generated: 84kWh

    Price of importing = 105.4 x 0.14
    = £14.76

    Price of exporting = (84/2) x 0.032
    = £1.34

    Price of generation = 84 x 0.21
    = £17.64

    Total = 14.76 - 1.34 - 17.64
    = £-4.22 (still made a small amount of money from the system)


    Now if I generated twice the previous but consumed the same amount:

    Consumed: 189.4kWh
    Generated: 336kWh

    Import or Export amount = Generation - Consumption
    = 336 - 189.4
    = 146.6kWh (exported to the grid)

    Price of importing = -146.6 x 0.14
    = £-20.52 (Not importing overall)

    Price of exporting = (336/2) x 0.032
    = £5.38

    Price of generation = 336 x 0.21
    = £70.56

    Total = -20.52 - 5.38 - 70.56
    = £-96.46 (made a larger amount of money, made the meter run backwards more than forwards that month so might be a problem billing etc.)

    I think this last scenario is the problem and why the consumption meter must not run backwards.

    Sorry for the length of this post, I'm just thinking out loud. Can someone check my math and or logic.

    Thanks,

    Jules
     
    Last edited: May 17, 2012

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