Can anybody talk about large scale PV unit

Hello!

I find many topics in this forum are about small units for home using. I want to know the application of large scale PV in US, is it popular? Maybe some technical point is different between these two kinds application.

Comments

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Can anybody talk about large scale PV unit
    rgs315 wrote: »
    Hello!

    I find many topics in this forum are about small units for home using. I want to know the application of large scale PV in US, is it popular? Maybe some technical point is different between these two kinds application.


    it's in a few large scale users of power with even a few electric companies trying it, but popular is not a word i'd use as it is a great investment many companies and individuals alike either can't or don't like to pursue. in general, it is a hard sell as it is like prepaying your electric bill for xx numbers of years and is easier to do when dealing with smaller numbers so in many cases it is a small supplemental power, unless you are one of the growing off grid pioneers that do without so much anyway that a few kwhs a day is heaven sent to them. large scale would work just as small scale can and does, but a bit more planning may be needed for large scale as a ready, fire, aim is then more costly.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Can anybody talk about large scale PV unit

    In the US, there are solar electric power plants... But I would not say they are popular (1% or less of grid generation sources?).

    Hydro Electric (dams and turbines) tend to be more popular (where you have mountains and streams/rivers) as hydroelectric power can be turned on and off in minutes... Solar (and Wind) only works when the energy source is available.

    There are several major types of solar electric plants... One uses traditional solar PV panels (like home systems). Others use some sort of process to collect heat and run boilers/steam turbines to generate power (or alternatives like Stirling Engines). Thermal based power systems may be more efficient and offer the possibility of storing heat for later use/peaking.

    All solar/wind systems in the US usually get some sort of tax breaks, power credits, and such--which makes them cost competitive with conventional fossil fule plants. Without the subsidies, many/most would probably never be built.

    One of the big problems with all of these plants is the "NIMBY"--Not In My Back Yard"--for various reasons, many times groups of people don't want anything large (fields of glass panels, Wind Towers, etc.) built near their homes or in recreational areas (for example, we could build a huge set of solar power stations in Death Valley (desert park)--but people don't want to industrialize open space. Also, when these plants are built "in the middle of no-where", they need to build transmission lines to distribute the power--again, "people" don't like that either.

    Where do you want this discussion to go? I know that China is supposed to be setting up large solar power stations--is that becoming an issue in China yet? Or, do people see them as reducing coal fired power plants and reducing pollution?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Can anybody talk about large scale PV unit

    "One of the big problems with all of these plants is the "NIMBY"--Not In My Back Yard""

    good point bill as we have even heard the excuse that pv would stop plant growth under the panels. to that end i say tear down your house as it too has done this. plants do still grow in shaded areas as pvs would shade no more than the tall trees of a forest would imho. the same can't be said of the home roof of the person making such a comment.
  • rgs315
    rgs315 Registered Users Posts: 9
    Re: Can anybody talk about large scale PV unit
    BB. wrote: »
    Where do you want this discussion to go? I know that China is supposed to be setting up large solar power stations--is that becoming an issue in China yet? Or, do people see them as reducing coal fired power plants and reducing pollution?

    -Bill

    Thanks!

    Maybe the word “popular” is not right for PV plant because the solar electric power is still very low. Just as what Bill said, PV become to be a more and more import issue in China because of environment protection. You know, US is at leading position in many area, so I want to know the large PV plant application in US. There are much difference between large unit and small one in technical issue. Just as what I asked in the thread” How often the tracker should act”, for home using, tracker is used when the land is limited, but for large scale unit, tracker is used only when the land is big. Another example, someone told me that many trackers were used in large PV unit in US about two decades before, but the result is very disappointed, most trackers broke down in two years. However I cannot confirm whether it is true.

    So I ask for some assistance to help me know more about the large PV application in US, or where I can find such information?
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Can anybody talk about large scale PV unit

    it would seem you have a predisposed idea of what a large system should be and it has biased you somewhat as it is not customary to track either large or small systems because those trackers are mechanically unreliable for their costs while reaping little. it would seem you also wish to hear a predisposed answer to suit of which you will not get from me.
  • peakbagger
    peakbagger Solar Expert Posts: 341 ✭✭✭
    Re: Can anybody talk about large scale PV unit

    In order for PV solar to be competitive on a large scale at current pricing, there has to be very substantial subsidies in place to drive the capital cost down. Someone has to pay the subsidy and most ratepayers are unwilling to pay that subsidy. At some point when there is a value to carbon emitted, the economics may line up in some areas.

    Large scale solar thermal has a lower installed cost and has some capability for thermal storage which also has a lower capital cost than current electric storage costs and therefore thats why MW scale solar thermal plants are being built in the west. Note that these plants are also subsidized quite heavilly.
  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    Re: Can anybody talk about large scale PV unit

    The world leader in grid connected PV is Germany - and that was brought about purely by the economic incentives. I think the vast majority of their PV is roof installed because of the attractive feed in tarrifs. If you read some of the posts on the german solar forums, you'll find a lot of "ordinary" people with no real environmental or green agenda - just people looking to make a good investment.

    When the bank gives you 3% interest on savings, but a PV system gives you > 10% guaranteed for 25 years, it's easy to see why PV is so popular.

    For the original poster, not quite clear what you're asking. The principles of small scale or large scale systems are the same, just the dimensions differ. More powerful inverters, remote management and monitoring is more important as is getting good discounts for volume purchasing of panels. Maximising cost effectiveness is paramount, so choosing the right panel for the given environment at the right price is essential (e.g. a-si for hot climates to maximize summer output, etc).
  • rgs315
    rgs315 Registered Users Posts: 9
    Re: Can anybody talk about large scale PV unit
    stephendv wrote: »
    For the original poster, not quite clear what you're asking.


    Thanks!
    I am sorry for my question is not very clear. In fact, I just want to look for some questions. My present project is a 20MW PV plant, including 18MW fixed arrays, 1MW single-axis tracker, 1MW dual-axis tracker. Such scale PV plant is not popular in the world, so I am afraid the biggest problem is I do not know where the problem is. Thus, I want to find more information about the similar unit. For example, we use 2MW to test if the tracker is good enough to be widely application. Although the tracker is arguable, at first I should know the practical problem for it. Also, I am familiar with the grid-connection of traditional fossil fired power and know the automatic synchronized system can do it. For PV plant, maybe the inverter can do the same work, but is there any problem? What is the key point about commissioning……..
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Can anybody talk about large scale PV unit

    Certainly home sized trackers can require maintenance/replacement every few years. Don't know about a large commercial tracker--I am sure more rugged and more expensive.

    For home systems, the cost of the trackers can be more than simply adding more solar panels. Others here like the trackers because it extends the charging period to late in the day (handy if you are running air conditioning in the afternoons/evenings).

    For your installation--You have a mix of array types, so you can see what works well for you.

    The inverters are (at least for small systems) synchronous inverters that track (slave to) the incoming AC voltage/frequency. They cannot operate without the incoming power (is a safety concern--to make sure that my "small" home array does not try and power the utility system if there is a power outage or a car takes down a power pole.

    For a large system--I am sure you and your engineers know more about them than I do... A large power station needs to control the frequency/phase of the network and cannot just be a slave to the network--A GT type inverter will shut down if the voltage or frequency is out of specifications. A "real power plant" will attempt to adjust the voltage, frequency, phase, current, etc. based on the need to keep the network regulated (and to synchronize with other power plants).

    The "problem" I have seen with any solar panel based power system is that the wiring and panels (and inverters) can fail. You may have 100,000x 200 watt solar panels (typically some number like 10-20 in series to reach the appropriate voltage, and then you probably have a bunch of these series strings tied in parallel to power the local inverter).

    When a panel or string fails, it is a very small part of the entire power output... So, do you pay to have each string monitored (remote reading voltage/current instrumentation) or do you simply look at the output of inverter 124A and see that it is generating 1% less than inverter 124B--then send out a couple guys with voltmeters and clamp on DC Amp Meters and start hunting down which string(s) has/have failed.

    Realistically, unless you have good data, it would be difficult to tell if even 10% of the strings (maybe 1% of the panels) have failed unless you are monitoring and logging. You cannot just go and look at a power meter and guess that the output is 10% low unless you have a reference (another inverter or perhaps an instrumented logging array) so you can account for temperature, wind, sun, clouds, haze, time of day, etc...

    On my system, I have two parallel strings, so if one fails, I have 50% output reduction.

    You, if your system is down 5%, that is a lot of power--and profit. So, how you "manage" the system can be the difference between profit and loss.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rgs315
    rgs315 Registered Users Posts: 9
    Re: Can anybody talk about large scale PV unit
    BB. wrote: »

    On my system, I have two parallel strings, so if one fails, I have 50% output reduction.

    You, if your system is down 5%, that is a lot of power--and profit. So, how you "manage" the system can be the difference between profit and loss.

    -Bill

    Thank you, Bill!
    I wonder which string is parallel? From where to where?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Can anybody talk about large scale PV unit

    rgs,

    I hope this helps (sorry if this is not your question):

    Think of a 3 cell flash light with 1.5 volt batteries... The light needs 4.5 volts to operate--so 3 cells in series (+ to - to + to -) increases the voltage by 3x1.5=4.5volts

    Now, say those batteries will power the flashlight for 30 minutes, but you want it to last for 120 minutes. You would take each string of 3 batteries (4.5 volts per string), and connect 4 of those strings together + to +, - to -. The voltage remains the same, 4.5 volts, but the stored energy is now 4x larger because you have 4 strings in parallel. Each string providing 1/4 the current to the light bulb. (notice I have 3 batteries per string times 4 strings, for a total of 12 batteries to power the lamp).

    Here is a web page with some series/parallel examples using batteries (pictures are better than words),

    With solar panels, it is similar. You put (in my case) 10 panels in series, each with Vmp=30 volts, so I get 300 volts DC (my inverter needs ~200-600 VDC from the solar array to operate).

    Note: Batteries are "a voltage source"--for example a 12 volt battery ties to keep 12 volts, no matter the current... From 0 amps to 100 amps--or whatever the maximum current of the battery may be.

    Solar panels are a "current source"--For example a solar panel will output 5 amps in full sun, from 0 volts (dead short) to Vmp (say 30 volts DC). Once the voltage goes about 30 volts, the current quickly falls off. (output current is pretty much proportional to the amount of sunlight hitting the panel).

    Most people are familiar with voltage sources (a battery, an AC wall outlet, etc.). However, current sources are less common in day to day life (arc welders are an example of a current source; florescent lights need a "ballast" which is also a current source--to keep the right amount of current flowing through the florescent lamp regardless of the lamp's voltage).

    Now, for my system, each panel is good for 5 amps, so I have 300v*5a=1,500 watt.

    But I want more power, so I add another series string of 10 panels in parallel with the first string. Voltage adds when connected in series. And current adds when connected in parallel... So the second string adds another 5 amps for a total of 10 amps.

    300 VDC * 10 amp = 3,000 watts

    In your case, your inverters are 10 or even 100x or larger than mine... So, instead of having 2 parallel strings, you may have 20 to 200 strings in parallel.

    Where my system has two parallel strings, if one string fails, I lose 1/2 my power--pretty obvious.

    In your system if you lose 1 string, you will lose 1/20=0.05 or 1/200=0.005 of the total output.

    For my system, it will generate around 2,000-3,000 watts in the middle of sunny day, depending on clouds, wind, temperature, how dirty the panels are, etc.

    Dropping 1/2 is pretty obvious... 1,000-1,500 watts. If I had 100 parallel strings (like your large system may have), that would be the equivalent of my system having a loss of 1/100 or 20-30 watts--I could never see that on my system unless I had a lot of lab gear and/or something measuring the sunlight available.

    Ideally, to find out if 1 of 100 strings has a problem, each string should have its own voltage/current monitoring circuit--so a computer could measure all 100 at the same time... If 99 show 300 volts and 5 amps, and 1 shows 300 volts and 0 amps--you know immediately which one has failed and can send somebody out to fix it.

    However, it costs money to install and monitor 100 strings with a computer. It might be "cheaper" to send somebody out with a "DC Current Clamp" meter to measure the current one a week or once a month in sunny weather to find any "dead" strings. This will work, but will not be as accurate.

    You may also end up with monitoring to make sure the panels and hardware meet their performance guarantee from the Manufacture or Installer... If you can show they are 5% under performing--you can get them to fix the problem or pay a performance penalty.

    I am taking a guess that I am answering your question and making sense to you. I don't know your background or exactly what your questions (your English is probably better than mine).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rgs315
    rgs315 Registered Users Posts: 9
    Re: Can anybody talk about large scale PV unit
    BB. wrote: »
    rgs,

    I hope this helps (sorry if this is not your question):

    ....

    I am taking a guess that I am answering your question and making sense to you. I don't know your background or exactly what your questions (your English is probably better than mine).

    -Bill


    Thank you, Bill, you are so kind. Thank you!
    I am familiar with the basic knowledge you said because my major is electrical engineering, anyway I thank you for your detailed explain.

    Just as you said, monitoring is a big problem, and we should find the right point of performance and cost. It will be good to monitor more panels, but the cost is also added. For my project, we used nearly 50 SATCON inverters (each is 500KW), it has not been decided about the monitoring system. It is unpractical to make the string parallel; maybe the monitor is the only way.

    Another question about the grid-connection. For example, we have 50 inverters; the grid is unwilling to let the 50 inverters to connect to grid one by one—they said that will lead big disturb to the gird. Do you know any good method?

    Sorry for my poor english because it is not my native language, but I will try my best to make me understood.

    Thank you!!
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Can anybody talk about large scale PV unit

    You are pretty much at the mercy of the power company. I would guess that the issue is not connecting at the beginning of the day or at the end. Power would ramp relatively slowly as the sun travels across the sky.

    Their issues, I would guess, are around having 20MW of power turned off (say voltage or frequency went of out of tolerance), then 5 minutes later, all 20MW turned back on within a few 1/10th's of second of each other).

    I would get the requirements from your utility company (if they even have detailed requirements written down yet) and meet with Satcon to discuss how they would adjust their monitoring and control software/hardware to conform with the utility's requirements.

    You probably will have to do some "Horse Trading" (negotiations) to see what the utility can give up on vs what Satcon can do for your installation.

    I can understand the utility's concerns. If you are a large percentage of the their "loads" -- having somebo6dy else turning off and on 2MW of electrical generation in the middle of the day can be a real problem.

    Part of what you may have to figure out is what does the utility need to manage their network. Example (complete guesses on my part):
    • No notice required if 500 kW or less is dropped/added (such as maintenance)
    • 1 hour notice for 500kW-5MW emergency service ($xxx charge)
    • 24 hour notice for >5MW emergency service ($xxxx charge)
    • 7 day notice for >1MW scheduled downtime
    • Each 500 kW of inverter will delay turn on/turn off by 5 seconds (to prevent all turning on/off during maintenance and/or restart procedures).
    • If >1MW of inverters will be brought back online (after Voltage/Frequency shutdown), must contact Utility Network Manager for permission to bring back on line between 9am and 4pm.
    • Your Plant must be able to (via telephone, Email, Pager, Internet, etc.) shutdown generation during a network emergency/transmission line failure/etc within XX Minutes.
    • You offer them reports/Read-Only Internet capability to monitor your system performance to better plan their daily load plans.
    I have not worked with anyone on large scale power--so the above are just guesses and suggestions of how to approach the issue and make the Utility confident that your system will not cause them problems and that your local management (and Satcon interface) can meet their overall requirements.

    On the other side, your company only makes money if it "sells" power. Any outages caused by your utility (realistic and pain in the behind requirements) can cause loss of revenue for your company. At 20MW, that is probably $2,000 USD per hour worth of revenue (at $0.10 per kWhr). A five minute outage required by the utility is ~$167 in loss revenue (at peak power).

    So anything that you give up to the utility has to be balanced against your company's revenue expectations/requirements too.

    Good Luck and I would look forward to hear how things work out for you.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • blackswan555
    blackswan555 Solar Expert Posts: 246 ✭✭
    Re: Can anybody talk about large scale PV unit

    you are already using Satcon ?
    Their "Solstice power conditioning unit" http://www.satcon.com/pv_inverters/index.html seems to tick all the box`s right down to individual MPPT + string monitoring , ?

    Have a good one
    Tim