DC or AC power transmission over long distances from solar and windmill?

crystaleye
crystaleye Registered Users Posts: 5 ✭
in General Solar Power Topics #1
Dear all, I will have solar panels and windmill turbines mounted on a mountain top about 1000m high and above in an asiatic humid tropical region.

Because of water storage issues, I would need to locate my house a bit of a distance away from the mountain top.. about 50-100m is my bet..

my question to you would be if i place my batteries 50m-100m away from my house, and i wanna transmit the power from the batteries to my house, is it more efficient to use DC or AC to transmit the power over long distances? thanks..
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #2
    Re: DC or AC power transmission over long distances from solar and windmill?

    welcome to the forum.
    basically that would depend on which one may have the higher voltage to overcome v drop losses. now are you using ac power appliances and lights in your home? if so then the inverter, batteries, and cc located 50-100mtrs away from your home might be the better choice. this is because the higher dc voltages needed to overcome the v drop losses would present an efficiency loss when the mppt cc converts back down to the battery bank voltage. i am assuming you are off grid.
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  • hfrik
    hfrik Registered Users Posts: 15 ✭
    #3
    Re: DC or AC power transmission over long distances from solar and windmill?

    Well, 50-100m is not a long distance regarding power transmission, and the decision AC/DC ( here we talk about >1000km ...)
    So the question in your case is - which voltage do you have in AC (I guess for use in your house), what voltage do you have from your PV and what voltage do you have from your wind generatoe?
    And what currents are running there?
    usually it would be useful to let the higher voltage run the longer distance. For the position of the batteries, also maintenance and theft could be a topic.
    Can you give more information?
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #4
    Re: DC or AC power transmission over long distances from solar and windmill?
    crystaleye wrote: »
    is it more efficient to use DC or AC to transmit the power over long distances? thanks..

    If the AC and DC voltages are the same, DC is slightly more efficient (less capacitive loss). However, these losses in a small system such as yours are not important. Much more important are the practical maters of switching and fusing, in which AC is the better choice.

    In most off-grid systems the AC is higher voltage than the DC and is therefore the logical choice for transmitting over distance. One more thing... if you put your inverter on the mountain top, you will probably want to have a monitoring and control device back at the house. The cables for that monitoring/control device need to be in separate conduit... do not run them with the power transmission cable.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: DC or AC power transmission over long distances from solar and windmill?
    hfrik wrote: »
    Well, 50-100m is not a long distance regarding power transmission,

    It is quite a long distance for the low Voltages usually associated with off-grid power systems. For those who are metrically challenged: 162 to 325 feet. Try that with any standard PV array Vmp and see how big the wire size has to be to carry any significant amount of current.

    Niel's advice of converting to 230 VAC (location: Asia = power standard 230 VAC @ 50 Hz most likely) at the charging site and running that to the use location is best.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #6
    Re: DC or AC power transmission over long distances from solar and windmill?

    You may even need/want to invest in a pair of 480V transformers, use 480VAC for the long run, one at mountain top inverters, and one at house for step-down. Figure 5% loss in each transformer.

    And for sure, look into power monitoring scheme, the new Conext ComBox looks to be a winner for one brand.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #7
    Re: DC or AC power transmission over long distances from solar and windmill?
    vtmaps wrote: »
    If the AC and DC voltages are the same, DC is slightly more efficient (less capacitive loss). However, these losses in a small system such as yours are not important. Much more important are the practical maters of switching and fusing, in which AC is the better choice.

    In most off-grid systems the AC is higher voltage than the DC and is therefore the logical choice for transmitting over distance. One more thing... if you put your inverter on the mountain top, you will probably want to have a monitoring and control device back at the house. The cables for that monitoring/control device need to be in separate conduit... do not run them with the power transmission cable.

    --vtMaps

    in theory yes, but you forgot the down conversion losses presented by the cc in bringing the higher voltage dc down to a lower voltage dc for the battery bank. at least if physically near to each other one can have the pv voltages closer to the battery bank voltage to improve the conversion efficiency.
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #8
    Re: DC or AC power transmission over long distances from solar and windmill?
    mike95490 wrote: »
    You may even need/want to invest in a pair of 480V transformers, use 480VAC for the long run, one at mountain top inverters, and one at house for step-down. Figure 5% loss in each transformer.

    I think that's overkill for the distances he mentioned. I looked into transformers for a 300m 230V cable with a 2kW well pump on it; just putting thicker wires was FAR more economical than 2 transformers.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #9
    Re: DC or AC power transmission over long distances from solar and windmill?
    stephendv wrote: »
    I think that's overkill for the distances he mentioned. I looked into transformers for a 300m 230V cable with a 2kW well pump on it; just putting thicker wires was FAR more economical than 2 transformers.

    more efficient too as transformers would present a loss.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #10
    Re: DC or AC power transmission over long distances from solar and windmill?

    More efficient is whatever has the higher voltage.

    If your AC is 230V, then AC is probably more efficient.

    For DC to be more efficent, it has to be more than 230V, say 350V. For that you need at least 12 big panels wired in series and special controller to handle the voltage.

    There are other considerations besides efficiency, such as convenience, accesibilty, climate for batteries, noise, smell etc.
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  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    #11
    Re: DC or AC power transmission over long distances from solar and windmill?
    NorthGuy wrote: »
    ...
    There are other considerations besides efficiency, such as convenience, accesibilty, climate for batteries, noise, smell etc.

    Smell? As in sulphuric acid from the batteries, or something else?
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #12
    Re: DC or AC power transmission over long distances from solar and windmill?
    techntrek wrote: »
    Smell? As in sulphuric acid from the batteries, or something else?

    That's exactly what I meant! :cool:
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #13
    Re: DC or AC power transmission over long distances from solar and windmill?

    i was thinking he use ac, but not to put transformers in as you would be better off putting in thicker wire than buying transformers. do you really think the transformers will be cheaper than thicker wire? i don't. thicker wire offers more efficiency and the transformers would lose efficiency. that translates to a loss for transformers.

    for instance at 230vac with a 225ft run (~75mtrs)(450ft total) with #12 at 10a the v drop % is nearly 4%. put in #10 at a somewhat modest cost and the v drop % drops to just under 2.5%. the transformers would cost more and cause a loss.

    now i will add that if say the voltage is 115vac and in keeping with the same wattage this would be 20a. the v drop % would be high for that same run near 10%. you would need to go to #4 to achieve the <2.5% v drop. still a cost yes, but cheaper than 2 costly transformers to up voltages and back down again and lower resistances in wires allow more of the power to flow ratherthan face losses due to transformer losses.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #14
    Re: DC or AC power transmission over long distances from solar and windmill?
    niel wrote: »
    i was thinking he use ac, but not to put transformers in as you would be better off putting in thicker wire than buying transformers. do you really think the transformers will be cheaper than thicker wire? i don't. thicker wire offers more efficiency and the transformers would lose efficiency. that translates to a loss for transformers.

    I agree about transformers too.

    Unless OP has a big array, plain 230V AC run is the most efficient choice.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #15
    Re: DC or AC power transmission over long distances from solar and windmill?
    stephendv wrote: »
    just putting thicker wires was FAR more economical than 2 transformers.

    Agree. And on the subject of thicker wires... the distance is long enough to consider using aluminum cable. It is complicated to calculate the cost savings, if any. Aluminum is cheaper, but costs money to terminate properly. Aluminum is thicker than equivalent copper, harder to pull through conduit, and may require a larger size conduit. But with 100 meters distance, the aluminum may be viable... depends upon the voltage he intends to transmit.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #16
    Re: DC or AC power transmission over long distances from solar and windmill?
    vtmaps wrote: »
    Agree. And on the subject of thicker wires... the distance is long enough to consider using aluminum cable. It is complicated to calculate the cost savings, if any. Aluminum is cheaper, but costs money to terminate properly. Aluminum is thicker than equivalent copper, harder to pull through conduit, and may require a larger size conduit. But with 100 meters distance, the aluminum may be viable... depends upon the voltage he intends to transmit.

    My cable length to the house is only 100 feet. It is aluminum with copper neutral. At these sizes, aluminum is usually cheaper even though you need a thicker wire to achive the same result.
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