The bigger the system...

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rabbit_39
rabbit_39 Solar Expert Posts: 43 ✭
in Solar Beginners Corner #1
The more complicated it becomes. :-D But I'm trying to get a process in place so I can learn better. And by summarizing the thought processes, it makes me understand better. So here's one I'm trying to simulate, at least on paper.

System:
radio transceiver, 12VDC/1500W max, 50 watt standby
24 hour usage, but power is dependent on user, average energy used per day is 22kwh/day

My initial thoughts:
12V system would be nice, but kwh/day is too large. Will require too many amps for charging current.

Now that I know the load, I need to see how large of a battery bank is needed to support that daily load:
inverter loss = 9%; wiring loss = 2%; battery discharge losses 10%
So from the battery to the load I have 81% efficiency. Which means I need 27.5kwh from the batteries. At 50% DOD, that's 55kwh and at 2 days autonomy it's about 110kwh. If I use a 2500Ah/2V batteries at 48V it would just about cover it.

Now that I know the batteries, I can get a min/max solar panel range for charging purposes.
2500Ah * 58V charging * 0.05 charge rate * 1/0.77 derating = 9415 Watts
2500Ah * 58V charging * 0.10 charge rate * 1/0.77 derating = 18830 Watts
2500Ah * 58V charging * 0.2 charge rate * 1/0.77 derating = 37660 Watts

Here's where I'm getting to be unsure.
If I pick 21 kwp panels, into 3 arrays, each array into its own charger, I can get enough current to charge at around the 10% mark (Midnite Solar 150, 80A curve).
And of course the battery bank will go to an inverter (2000W nominal for a 1500W load, no induction loads).

The panels are 290Wp/44.20Voc/36.81Vmp/8.15A Imp). Each of the 3 arrays will consists of 24 of these panels. Each string will be 2 panels in series x 12 strings in parallel. Looking here http://www.midnitesolar.com/pdfs/fullPage12Sept10a.pdf , if my input voltage is around 80V, I can get around 80+ amperes.

Are there anything else I'm missing?

Thank you,
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #2
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    Re: The bigger the system...

    i think i'm missing something here. why would you need a radio with 1500w going 24hr? with dc power requirements like this it's outrageous in my opinion to do this with anything but a generator. now i could see maybe a 10hr backup or something similar until it becomes a decent hour, but why would you need to transmit for that much time? don't you sleep? if this is backup for ham radio contesting then you need to rethink priorities. even emergency setups don't need 1500w and if they do it may be on occasion. could you please elaborate so i can better understand why it is you need this?
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #3
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    Re: The bigger the system...

    As Niel suggested, you're looking at enormous consumption for a solar system. You had better have some very deep pockets packed with riches for this one :roll:
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #4
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    Re: The bigger the system...
    rabbit_39 wrote: »
    The more complicated it becomes. :-D But I'm trying to get a process in place so I can learn better. And by summarizing the thought processes, it makes me understand better. So here's one I'm trying to simulate, at least on paper.

    System:
    radio transceiver, 12VDC/1500W max, 50 watt standby
    24 hour usage, but power is dependent on user, average energy used per day is 22kwh/day

    My initial thoughts:
    12V system would be nice, but kwh/day is too large. Will require too many amps for charging current.

    Now that I know the load, I need to see how large of a battery bank is needed to support that daily load:
    inverter loss = 9%; wiring loss = 2%; battery discharge losses 10%
    So from the battery to the load I have 81% efficiency. Which means I need 27.5kwh from the batteries. At 50% DOD, that's 55kwh and at 2 days autonomy it's about 110kwh. If I use a 2500Ah/2V batteries at 48V it would just about cover it.

    Now that I know the batteries, I can get a min/max solar panel range for charging purposes.
    2500Ah * 58V charging * 0.05 charge rate * 1/0.77 derating = 9415 Watts
    2500Ah * 58V charging * 0.10 charge rate * 1/0.77 derating = 18830 Watts
    2500Ah * 58V charging * 0.2 charge rate * 1/0.77 derating = 37660 Watts

    Here's where I'm getting to be unsure.
    If I pick 21 kwp panels, into 3 arrays, each array into its own charger, I can get enough current to charge at around the 10% mark (Midnite Solar 150, 80A curve).
    And of course the battery bank will go to an inverter (2000W nominal for a 1500W load, no induction loads).

    The panels are 290Wp/44.20Voc/36.81Vmp/8.15A Imp). Each of the 3 arrays will consists of 24 of these panels. Each string will be 2 panels in series x 12 strings in parallel. Looking here http://www.midnitesolar.com/pdfs/fullPage12Sept10a.pdf , if my input voltage is around 80V, I can get around 80+ amperes.

    Are there anything else I'm missing?

    Thank you,

    Is the grid available to power this thing? If it is, then there is no off grid solution for you that is going to be anywhere near as economically viable as powering it from the grid. The power from an off grid system could easily be 10X or more as expensive as grid power.
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    #5
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    Re: The bigger the system...

    Let me pile on, please,

    Agree that 1500 watt output seems excessive, especially from a 12 volt power supply. 12 volt systems at this power level are inherently inefficient. Far too much current going in to get too much power out.

    Several of us assume that this system is for Ham Radio, which may be correct. In that case, choosing a good location for the station, and working on antennas is far more productive than going for BIG power.

    Perhaps this is a Remote-Base for several users, but still, Location, location .. AND Antenna, antenna, ... and so on.

    At my hammie off-grid location, 200 watt transceiver is almost always perfectly adequate. Generally, when a band is open, and one can hear well (due to a very remote location), big power is not required, and when a certain band is not open, no reasonable amount of power will compensate, at least for sky-wave propagation.

    Just MHO, Good Luck. Would be a fin project, tho. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #6
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    Re: The bigger the system...

    my further "piling on here", (i prefer adding to) if that's a good phrase to use, i am assuming this to have the grid and he wants backup. the generator is the best option. even if he has a commercial radio station at 1500w then why would he need to receive some of that time is the way i'm thinking? in any case what i said is valid as he expressed the desire to run things for several days. a solar setup by itself to carry all of the loads for days would be prohibitively large even if only to partially backup this kind of load for 10hrs. i might add that the radio may require up to 25a at 12v and would be best handled with either a converter from higher battery voltages to 12v or a 125vac power supply noting that the bulk of this will be inverted from batteries and you would want a higher voltage battery bank for certain. i know the inverter is needed because 1500w amplifiers are the rf output wattage and the input wattages seen by some amps are easilly in the 2.5-4kw area with some requiring dedicated 15a 240vac lines to power them.

    conservation is key here as you should only transmit 1500w when it's absolutely necessary and that is part of hamming too to use the least amount of power necessary for communication.

    sorry for appearing to jump on you as nothing bad was meant in my hurry to correct a possible ready, fire, aim situation.
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  • rabbit_39
    rabbit_39 Solar Expert Posts: 43 ✭
    #7
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    Re: The bigger the system...

    Actually, this is for a BTS, cellular radio. 1500W is max and while it's plugged in 24 hours, the usage is not 24 hours. Hence the average energy usage of 22kwh/day

    Grid is not available, and fuel for generator, WHEN available is about $9-$10/gallon. The location is in the remote area of Indonesia.

    There is a deep pocket for this but not infinite. Which is why I was trying to see if there's a solution for this that is renewable. Also, I'm trying to learn the calculation processes so I can be almost as good as you guys. :-D

    as to the 12V system, I ditched that (bolded the 48V battery bank size in my calculation) even though I would have to get an inverter for it. I knew even before starting out that 12V would be too small. I was just saying it would be nice if I could do it with 12V since the load will run straight from the battery bank. Incidentally, would a 48VDC to 12VDC transformer be more efficient than an inverter?

    Thank you,
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
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    Re: The bigger the system...

    There's no such thing as a "DC transformer". You either have to hunt for a DC to DC Voltage converter capable of dropping 48 Volts to 12 and handle 150-200 Amps (good luck with that) or build your own or run an inverter and a 200 Amp 12 VDC power supply (another tricky find/build) or stick to your original 12 Volt plan.

    It can be done on 12 Volts, but as the others have indicated it's not easy. The current involve to power 1500 Watts @ 12 Volts is very high: roughly 1500 Watts / 10.5 Volts (minimum system Voltage before failure) and that's nearly 150 Amps peak.

    To come up with that 22 kW hours per day you'll need 2000 Amp hours usable, or a 4000 Amp hour battery bank. In one go this is difficult to recharge: 400 Amps all at once is dangerous. I'd suggest dividing it up into at least two if not four smaller, independent banks of 1000 Amp hours each. Whether you switch from one bank to the next automatically or manually would depend on circumstances. Automatic would require some clever invention, manual would require careful monitoring.

    If you break it up into four, each of those is going to need about 2 kW of panel to recharge, and two charge controllers each (because of the need to be near 100 Amps in charge current).

    Does that help any?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #9
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    Re: The bigger the system...
    rabbit_39 wrote: »
    Incidentally, would a 48VDC to 12VDC transformer be more efficient than an inverter?
    Transformers only work for AC--not for DC (alternating vs direct current).

    Yes, you can get down converters (usually buck mode DC to DC switching power supplies) to take the 48 VDC and drop it to 12 VDC...

    Back a few decades ago, Vicor was one of the high end, high frequency, small isolated power supplies that we sometimes put into Telecom equipment.

    In the end, you will need to look at the entire power chain to figure out what will work best... Many electronics have to convert from 120/240 VAC to their internal power supply needs--various voltages and power levels).

    For example, in our equipment, we simply had an option of 120/240 VAC input supplies or -48 VDC telecom battery input--Not really any difference in efficiency.

    One of the nice things about feeding 120/240 VAC is that the equipment is all nicely isolated from each other--Ground loops (audio humm, melted signalling cables, etc.) are almost non-existent with AC power...

    With DC power systems--there are two major types... One type is "Isolated"--basically the -48 VDC input system power supply has an internal inverter and AC transformer to the DC side, which then makes local power for the electronics. There is no "common" power from AC input to DC output (just common chassis/frame/rack grounds).

    The second are non-isolated DC power systems... The "Ground" connection (the positive terminal) is carried through much of the equipment--and it is possible to get circulating ground currents through various signaling and antenna ground connections.

    You should have a systems engineer available (somewhere) that you can ask what he would prefer:
    • 120 or 230-240 VAC
    • 48 / 24 / 12 VDC (and positive or negative grounding)
    Solar power systems are designed for Negative Grounding. And true Telecom is designed for positive grounding...

    Building a positive grounded solar pv system (out of equipment that assumes negative grounded DC) may be a bit confusing.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • rabbit_39
    rabbit_39 Solar Expert Posts: 43 ✭
    #10
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    Re: The bigger the system...
    Cariboocoot wrote: »
    There's no such thing as a "DC transformer". You either have to hunt for a DC to DC Voltage converter capable of dropping 48 Volts to 12 and handle 150-200 Amps (good luck with that) or build your own or run an inverter and a 200 Amp 12 VDC power supply (another tricky find/build) or stick to your original 12 Volt plan.

    For that much hassle, I'll just go with the original inverter plan. The equipment is designed for 12VDC input, but also has 230VDC/50hz input built in. Thanks :-D
    Cariboocoot wrote: »
    It can be done on 12 Volts, but as the others have indicated it's not easy. The current involve to power 1500 Watts @ 12 Volts is very high: roughly 1500 Watts / 10.5 Volts (minimum system Voltage before failure) and that's nearly 150 Amps peak.

    Hadn't thought about it going all the way down to 10.5 volts before failure. I thought 12V was impossible solely based on how many amps it would take just to recharge 12V banks.
    Cariboocoot wrote: »
    To come up with that 22 kW hours per day you'll need 2000 Amp hours usable, or a 4000 Amp hour battery bank. In one go this is difficult to recharge: 400 Amps all at once is dangerous. I'd suggest dividing it up into at least two if not four smaller, independent banks of 1000 Amp hours each. Whether you switch from one bank to the next automatically or manually would depend on circumstances. Automatic would require some clever invention, manual would require careful monitoring.

    So here's the way I figured how much battery capacity I need from 22kwh/day:
    - total losses to charge from battery (charger losses, wiring, battery eff.) = 18%
    - storage needed just cover usage=22kwh/0.82=27kwh/day
    - 50% DoD and 1.5 day autonomy = 81kWh battery bank or 81kwh/48V=1688Ah
    - closest battery capacity = 2000Ah

    Is there something I'm missing that your calculation requires a 4000Ah?
    Cariboocoot wrote: »
    If you break it up into four, each of those is going to need about 2 kW of panel to recharge, and two charge controllers each (because of the need to be near 100 Amps in charge current).

    Does that help any?

    Yes it helps a lot.

    So if I split it into 4 battery banks, can't I charge them independently? but parallel the output?

    4 x 1000Ah/48VDC bank
    So 1000Ah x 58VDC charging voltage x 0.77 derating x 10% rate = 4500Wp?
    4500Wp * 0.77 derating /58VDC = 60A

    Am I missing something here?

    Thanks for the comments and help!
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  • rabbit_39
    rabbit_39 Solar Expert Posts: 43 ✭
    #11
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    Re: The bigger the system...
    BB. wrote: »

    You should have a systems engineer available (somewhere) that you can ask what he would prefer:
    • 120 or 230-240 VAC
    • 48 / 24 / 12 VDC (and positive or negative grounding)
    Solar power systems are designed for Negative Grounding. And true Telecom is designed for positive grounding...

    -Bill

    Thanks, I just got an earful from the telecom engineers about those same exact things :-D I was going to show them this forum...but I think I'll wait a while.

    It's an inverter solution for me. :-D
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #12
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    Re: The bigger the system...

    My 4000 Amp hour battery bank was based on your 22 kW hour per day estimate:
    22000 / 12 Volt average = 1833 Amp hours used. @ 50% DOD that's a 3666 Amp hour battery - pretty close to 4000.

    But if a 230 VAC 50 Hz inverter is a possibility, go for it! :D Here's one possible inverter for you, the Outback FX2348ET: http://www.solar-electric.com/fx2348et.html

    On 48 Volts you can run 916 Amp hours. Maybe a bank of these 2 Volt cells from Crown: http://www.solar-electric.com/repoba2vo10a.html

    You will still need a hefty amount of panels to recharge them. Around 8 kW worth, maybe more as high temps cause quite a drop in panel power. Split in two, to feed a couple of FM60 or FM80 charge controllers which would integrate nicely with the OB inverter.

    I mention Outback equipment here because I believe it is the best choice for this particular application, all things considered. It may be "out of date technology" but it's extremely reliable even under adverse conditions.
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