What are strings?

adam1984
adam1984 Solar Expert Posts: 72 ✭✭✭✭
Forgive me, I am relatively new to this. I have a bachelors in EE and have done very little work with solar. I keep hearing the term strings. "5 panels to a string... you will need to string them in parallel..." Can someone please enlighten me on what exactly makes a string? You can speak in technical terms as although im a beginner to solar, i do have a background with electronics. Also, this site has been great for me so thanks in advance to you all.

Comments

  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Re: What are strings?

    For the engineering mind think of this

    String = Series , like resistors or other parts

    So a string of 5 solar panels is the panels connected in series, plus to minus to have the voltage increase
  • adam1984
    adam1984 Solar Expert Posts: 72 ✭✭✭✭
    Re: What are strings?

    So it is not only referring to solar cells that make up a panel? Like there can be a string of cells in series to make up a panel or a string of panels to make up an array? And to reiterate, with solar, if I have a string of 10-10watt 1amp panels i would have 100 watts at one amp. And if i had the same 10-10watt 1amp panels in parallel i would have 10watts at 10amps? Thank you guys for not getting frustrated with beginner questions.
  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Re: What are strings?

    You have it right

    Sometimes the "build it yourself panel" posters may refer to cells within a panel as stings, but most discussions here are at the panel level, either way its all the same
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: What are strings?
    adam1984 wrote: »
    ...to reiterate, with solar, if I have a string of 10-10watt 1amp panels i would have 100 watts at one amp. And if i had the same 10-10watt 1amp panels in parallel i would have 10watts [BB: should be 100 watts] at 10amps? Thank you guys for not getting frustrated with beginner questions.

    Your confusion probably stems from forgetting:
    • Power=Voltage*Current
    So, if you have 10 watt panel at 1 amp:
    • P=V*I
    • V=P/I=10 watt / 1 amp = 10 volts
    If you put them 8 panels in parallel (just to stop using the number "10" everywhere which may be causing some typo/confusion too):
    • P=V*I=10 volts * 8 parallel connection * 1 amp = 10v*8*1a = 80 Watts
    If you put the same 8 panels in 1 series string:
    • P=V*I= 8 panels in series * 10 volts * 1 amp = 8*10v*1a = 80 watts
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: What are strings?

    adam,
    this, "10watts at 10amps" for 10 panels, more simply put is wrong as you would still have 10 panels x 10w = 100w. bb is correct by the power fornula, but in any case just know that the wattages are additive whether you add in series or parallel.

    as guppy said, strings are in reference to series.
  • adam1984
    adam1984 Solar Expert Posts: 72 ✭✭✭✭
    Re: What are strings?

    That completely clears up the confusion. All the 1's and 0's (no pun intended) with my example was the confusing part. I did get power was additive, should have read 100w either way. As guppy stated I was thinking more along the lines of panels not cells, which added to the confusion. Also, thanks for all the input. Sad that the term strings was so confusing to me as a 4th year ee student.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: What are strings?

    It is OK--We won't string you up for using it wrong--or string you along and pretend you where using it correctly, or let the string of errors propagate. ;)

    Confusing--never. :-)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • adam1984
    adam1984 Solar Expert Posts: 72 ✭✭✭✭
    Re: What are strings?

    Haha thanks bill. So can i bother you guys one more time. Last question on strings i swear. Is there a reason that there is 5 panels to a string? Is it optimizing efficiency that way? Just wondering why you couldn't have 2 strings of 10 panels or 5 strings with 4 panels. Does the question make sense?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: What are strings?

    Solar panels are just batteries that run off of sunlight... So, just like batteries, you can create any series/parallel/series-parallel configuration that makes sense to support the defined loads.

    The "big difference" between solar panels and the typical chemical battery is that chemical batteries tend to be "voltage sources" vs Solar Panels which tend to be "current sources".

    We all know how voltage sources work... the "battery" tries to keep ~12 volts and you can model a series resistor to account for losses (resistance in plates, wiring, etc.).

    Current sources are not as common in most peoples experiences... The solar panel output current up to the Vopen circuit rating of the panel. Once you get much below Vmp (Voltage maximum power)--the solar panel will output a fixed amount of current no matter the voltage at the load--and to make it more complex, the current is pretty much proportional to the amount of sunlight energy hitting the panel.

    Take a look at this PDF document for a generic data sheet for a crystalline silicon panel.

    Now--when you start connecting the panels together in series/parallel/etc.... You need to look both the source of power and the load to figure out the optimum configuration. In fact, you will see companies have created software to help installers/customers configure the panels for their needs. Panel Specs, Min/Max voltage of controller, Min/Max temperature of the installation, etc. all need to be taken into account. Here are a couple from Xantrex (links may go away soon--Xantrex has been split and swallowed--don't know the new sites yet...

    Xantrex Grid Tied Inverter Family
    Xantrex Off Grid Charge MPPT Controller

    The choices are fairly complex and do involve trade-offs... In general, the MPPT type controllers are a percent or so more efficient with lower solar panel input voltages (capacitance losses from input switching circuits).

    However, when the panels get hot (summer weather, full sun, no wind)--the Vmp voltage drops from STC specifications--so you may have to place another panel in series to keep the voltage high enough in summer.

    And in winter, freezing weather raises Vopen circuit -- so you cannot have too many panels in series or it may over voltage the MPPT controller.

    So, if you choose minimum Vpanel voltage-From the P=I*V equation, if we have low voltage, then we have to have high current--lots of copper wire to carry the current (copper=$$$$).

    Also, if you put more than two solar panel strings in parallel--you have to add series protection fuses/breakers to each string... You could have a short in one panel and the other parallel connected panels would feed the short with >Isc (current short circuit maximum rated).

    So--you may end up choosing a High Vpanel voltage and low Ipanel current (keep wiring current low (so solar panels can be mounted some distance away from battery shed/GT inverter), fewer series protection fuses/breakers needed, save money on $$$ costs). Then you look at the panel's Vmax--older panels may have a maximum rating of 70 VDC or less--and the MPPT battery charge controllers typically support up to 150 VDC maximum... (GT inverters can support 600 VDC maximum--+/- depending on brand/model/design).

    Anyway, you probably get the idea... The first time you do the configuration calculations--it is very confusing. The fifth time you do it--sort of makes sense.

    Sorry to make it so confusing--but it is.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • adam1984
    adam1984 Solar Expert Posts: 72 ✭✭✭✭
    Re: What are strings?

    Bill you should teach an alternative energy course. ALOT OF INFORMATION TO TAKE IN. But actually really clear and it makes alot of sense. When I break it down and make the array look like a small circuit containing just batteries it makes alot more sense. I understand this may not be the best way, but for now it really helps me get this. Everything you said makes complete sense. I just couldn't wrap my head around such a large scale when im used to RLC circuits, microelectronics, but looking at it as a basic circuit/ schematic really helps me formulate the equations. Ohms law still applies, P=vi still applies, which i obviously knew, but you made this much simpler than how I was looking at it.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: What are strings?

    Somebody should have put "string" in the glossary.

    Uh, that should have been me, shouldn't it? :blush:

    Niel, could we trouble you for a quick edit? :D
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: What are strings?

    I am a bit tired and may have missed it, but can you have a series/parallel array and still call it a "string" or once you move from series into parallel you now have an strings within an array?

    Tony
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: What are strings?

    String, string array, or PV string would be two or more photovoltaics connected in series.
    Any multiple panel set-up, either series or parallel, is an array.
    At the moment I can't think of an analogous term for 'string' that applies to parallel connections.

    Feel free to disagree. :D
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: What are strings?
    icarus wrote: »
    I am a bit tired and may have missed it, but can you have a series/parallel array and still call it a "string" or once you move from series into parallel you now have an strings within an array?

    Tony

    that would be paralleling multiple strings so you can't call them a string on the whole as it would be plural. it is still referenced to those that are in series only. as such the word series is not needed when talking of strings, but it is a descriptive often used as a reminder that this is for series only.

    this means you could have a string, but when you add another string in parallel to the original it becomes strings as the word string indicates continued linearity and any parallel or broken strings do not constitute that continuum. to place 2 separate strings together in series however it now becomes a single string due to the unbroken linearity.

    i'm sure you guys are quite confused:cry:, but a visualization would probably be easier in helping you understand as words do not do it justice. well, at least my words don't.:roll::p

    do you guys really think it needs to be in the glossary?:confused:
  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    Re: What are strings?
    String, string array, or PV string would be two or more photovoltaics connected in series.
    Any multiple panel set-up, either series or parallel, is an array.
    At the moment I can't think of an analogous term for 'string' that applies to parallel connections.

    Feel free to disagree. :D

    Okay, I will. :D

    How about... string is series, and array is parallel.

    Thus, say 4 PV panels - arranged as "an array of two strings".

    (The strings are two panels in series, and the two strings are paralleled into an array.)

    But then...that would preclude a single string being called an array.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: What are strings?

    An array would either be "nondenominational" (not specifically series or parallel)--or could even be specifically referring to a Series/Parallel panel setup. From math/computer science, your array can be 1-3 or more in dimensions...

    I think that this is going a bit off the "deep" end of the gene pool...:roll:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset