Inverter oversizing for cloudy winter weather?

In the last month, we've only had a single day of sun, so we're exploring options for increasing our cloudy weather solar generation potential.

Our current configuration is two SMA TL-3000 inverters, each with a single string of 12x 224 watt panels. We're seeing 200 watts out of each inverter during typical cloudy weather.

It appears that quite a few people in Australia have taken the approach of attaching more panels to an inverter to increase their winter yield. If we understand correctly, they take advantage of the fact that modern inverters will only draw up to the rated DC power, and thus as long as voltages are not exceeded, having more DC amps (as long as it is under the rated amount for the inverter) is acceptable.

So, for example, using the SMA Sunny Web Design tool, one can configure 24 panels (2x 12 panel strings) for a single TL-3000 inverter, resulting in the following:
Parameter		Inverter	Input A		Input B
Max. DC power		3.20 kW		2.64 kWp	2.64 kWp		
Min. DC voltage		125 V		290 V		290 V		
Typical PV voltage			314 V		314 V		
Max. DC voltage (PV)	600 V		505 V		505 V		
Max. DC current (A/B)	15.0/15.0 A	7.5 A		7.5 A

(Note that the TL-3000 has two independent inputs, each with their own MPPT tracker.)

This comes up as "Conditionally compatible", and SMA has said in other threads that as long as you're not exceeding the electrical requirements of the inverter (and you have sufficient cooling for the longer period of time the inverter will be capped at max capacity during sunny times), you're good.

Any thoughts on this approach? Just adding more panels to the existing inverters seems to be the least expensive way to go, and would allow us to potentially double our system's power generation levels during these cloudy months.
House: 2x SMA SI 6048 w 24x 2V DEKA Unigy II; 2x SMA SB 3000TL-US w 24x Sharp ND-H235Q2
Cabin: 1x Magnum MS4024 w 24x 2V DEKA Unigy II; 1x Morningstar TS-MPPT-60 w 6x Sharp ND-H235Q2; 1x 200 Watt Harris microhydro
Intertie: 1x SMA WB 3800; 1x Lambda GEN-600 DC Supply; 2x PSL pQube

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Inverter oversizing for cloudy winter weather?

    My only suggestion is to look at the costs... You are seeing less than 10% of the array's output during cloudy weather--And that is not unusual.

    If your 200 watts (per inverter) were doubled to 400 watts, that would mean doubling your array size--Does that make economic sense vs just running a single Honda eu2000i on a 1/2 gallon of gas per day (400 watt output for 5 hours a day, ~9+ hours per gallon of fuel at 400 watt output).

    That would be the equivalent of adding 24x224watt=5,376 watts of panels for 400 watts of cloudy weather output==Call it $10,000 worth of panels.

    A $1,000 for a Honda eu2000i, leaves $9,000.00

    $9,000 / $8 per gallon for island fuel = 1,125 gallons of fuel

    1/2 gallon per day * 1,125 gallons of fuel * 1/365 days per year = 6.2 years of fuel

    And if you double the size of the array--I would start thinking about two more GT inverters--And could you make use of that amount of additional power during sunny weather.

    Poor sun/weather + solar just does not do much...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • AceNZAceNZ Solar Expert Posts: 104 ✭✭✭✭
    Re: Inverter oversizing for cloudy winter weather?
    This comes up as "Conditionally compatible", and SMA has said in other threads that as long as you're not exceeding the electrical requirements of the inverter (and you have sufficient cooling for the longer period of time the inverter will be capped at max capacity during sunny times), you're good.

    The SMA 3000TL-US is rated at 3.2 kW maximum DC input. Having panels that can generate 5.2 kW in sunny weather would appear to "exceed the electrical requirements of the inverter." If the inverter could just draw whatever power it needed from the input, then why would they need to specify a maximum input?

    The over-capacity type thing that I'm aware of in Australia involves having panels up to the full maximum allowable DC input, knowing that clipping in the summer will result in some power loss, while output in winter or on cloudy days will be higher. For the 3000TL-US, that would mean 3.2 kW of panels, which would result in 3.0 kW max output in summer.
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Inverter oversizing for cloudy winter weather?

    In general, MPPT based controllers will simply draw as much power as they can obtain, up to their rated input/output current/power. And they should safely clip output power if there is more than enough input array power.

    So, the hard limits are max not-to-exceed input voltage, Vmp-array MPPT tracking voltage range, min/max starting/running voltages.

    The "softer" rating is Imp/Isc of the array. You probably have a maximum input current rating--But, I would normally adjust that by using breakers (or fuses) in the input cabling (at the array) to prevent too much "uncontrolled" current from, possibly, causing a fire/meltdown in the input wiring/cabinet.

    I.e., if 30 ADC maximum input current, then 30 A * 1.25 NEC derating = 37.5 amp minimum rated branch wiring and ~40 amp maximum fuses/breakers.

    There is always the issue of running any electronics at 100% of rated power for 6+ hours per day. Some vendors have better efficiency cooling than others...

    You can simply add more GT inverters so you have more power during sunny days (if you can use it)--There is the extra losses too--Your inverter may take 30-50 watts just to "turn on" from the array (as an example). Adding those losses to an expected 200 watt output (if you double the array), is another 25% loss if you add new inverters.

    Nothing is free. :cry:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    Re: Inverter oversizing for cloudy winter weather?

    Australia, as here, enjoys reasonable output year round, and the more-PV approach works well. For isntance here at 39degrees south we get 2.5 hrs winter, and 5.0 hours summer. We designed for 2.5 and conequently dont run genertors, and have large power surpluses in the summer.

    If this ratio were more unfavourable, then the array doubling to get 2 x200W approach wouldnt necessarily work, and why gensets appear to be considered an essential part of systems in more temperate climes.

    I recall you have some hydro, but i can equally imagine how hydro systems if too cold would stop in the winter. Thats a pity becasue hydro so nicely complements solar here, where the streams dont ever freeze.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • David and LauraDavid and Laura Solar Expert Posts: 139
    Re: Inverter oversizing for cloudy winter weather?

    Re #2 - We're trying to go fossil fuel free, though the economics are making things difficult (as you pointed out). We figure that we can purchase and mount another 24 panels for less than $7K, but as you point out, that's a lot of $ for an extra 2 KWhr per day. We had figured that such a system would have a 5 year payback point when compared to diesel, though a propane-based system would be even more cost effective.

    As for additional GT inverters, we don't really need more solar when the sun is shining (as we only have 1000 AH of battery capacity to charge), and that would increase the price a fair bit.

    Re #3 - See BB.'s comment in #5.

    Re #4 - As long as we stay below the rated DC current for the array (which is 15 amps per input), we should be good. With 12 panels per input, we should be below 7.5 amps. We'd certainly add a 10 amp DC fuse to the string.

    Due to our array shading, even with the extra panels, it wouldn't extend the period of time that the inverter would be running at full power. But just in case, SMA does offer an auxiliary cooling fan option for the 3000TL, which we could look at getting.

    Re #5 - We haven't had enough rain this winter for the hydro to start working. Otherwise, that's a nice 200 watts 24/7 (effectively 4.8 KWhr per day) that we make good use of. It really does show the value of hydro — we'd have to add at least $20K worth of solar equipment to equal what a little waterwheel puts out during cloudy weather.
    House: 2x SMA SI 6048 w 24x 2V DEKA Unigy II; 2x SMA SB 3000TL-US w 24x Sharp ND-H235Q2
    Cabin: 1x Magnum MS4024 w 24x 2V DEKA Unigy II; 1x Morningstar TS-MPPT-60 w 6x Sharp ND-H235Q2; 1x 200 Watt Harris microhydro
    Intertie: 1x SMA WB 3800; 1x Lambda GEN-600 DC Supply; 2x PSL pQube
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Inverter oversizing for cloudy winter weather?

    Dave, what opportunity loads can you use, up to the max output, if you over panel? divert to residence? etc..
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • SolInvictusSolInvictus Solar Expert Posts: 138
    Re: Inverter oversizing for cloudy winter weather?

    When overbuilding a PV array for cloudy weather, an option is to point some of the PV panels to favor morning sunlight and some others to favor evening sunlight. You get the same power output on cloudy days while on sunny days the power is spread out through out the day instead of peaking around noon. This makes the power from the PV panels on sunny days available when you are more likely to use it thus reducing the load on the batteries.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Inverter oversizing for cloudy winter weather?

    solarinvictus,
    doing that doesn't make that much sense as the span from sunrise to sunset is far tighter in winter and the farther north you go it is even tighter yet. the south facing pvs are the best option. now during the summer months your idea would fly better as the sun is rising and setting over a much greater span. the further north the greater that span until one reaches the arctic circle where it never sets in summer. during the winter it is perpetual night above the arctic circle and towards spring you will see the sun span right around the southern direction and this will grow until it reaches its max for the warmer months.

    david and laura,
    i am assuming you have some ability to repoint the pvs lower to the horizon in the winter if i recall rightly for better elevation aiming and i assume you have done so already. with outputs this low i have to agree with bill that you would be throwing good money after bad to gain another 200w in winter while you can't use it all in summer. i can relate to you about the long time periods of no sunshine in the winter months as i've seen it go for a months time too. not much one can do about the fickleness of mother nature and is why many do recommend a generator for backup.
  • David and LauraDavid and Laura Solar Expert Posts: 139
    Re: Inverter oversizing for cloudy winter weather?

    Re #7 - The SMA equipment is able to regulate the amount of power taken from the solar array based on the AC line frequency. Thus, when our batteries are full and there are no loads, even under full sun, the solar inverter will pass no amps through from the DC to the AC side. The fact that the SMA Sunny Boy's can do this is part of what gives us confidence in having more amps on the DC side than will be passed through to the AC side.

    Re #8 - This is a very common configuration for the Sunny Boys that we have. They support two strings, and often are set up with a south-east facing array and a south-west facing array, so that you can extend the time when you are generating peak power. Unfortunately, this only works well when you have long days... For us, during the winter, the sun just skims the horizon for a few hours.

    Re #9 - We anticipated that our greatest challenge would be during the winter, so we mounted the solar panels at a much larger angle than usual (> 45 degrees). This also helps with shedding snow.

    Interestingly, when setting up solar for cloudy days, the best angle is 0 degrees, specifically, to place the panels flat on the ground. That lets them collect the most cloud-scattered photons.

    Given the weather data we've collected over the last three years, it seems that having 3 - 4 months of cloud with little sun will be the norm, rather than the exception.
    House: 2x SMA SI 6048 w 24x 2V DEKA Unigy II; 2x SMA SB 3000TL-US w 24x Sharp ND-H235Q2
    Cabin: 1x Magnum MS4024 w 24x 2V DEKA Unigy II; 1x Morningstar TS-MPPT-60 w 6x Sharp ND-H235Q2; 1x 200 Watt Harris microhydro
    Intertie: 1x SMA WB 3800; 1x Lambda GEN-600 DC Supply; 2x PSL pQube
  • David and LauraDavid and Laura Solar Expert Posts: 139
    Re: Inverter oversizing for cloudy winter weather?

    So, after quite a few discussions, here is what we are looking at doing:

    1. We're going to start to accumulate the parts for another array (12 panels). Assuming that this is to be optimized for cloudy weather, we have more options with respect to where we will can place the panels, so we can also try to capture some afternoon sun in the summer. This is a smaller investment, and will also help reduce the depth of our battery cycling during the summer.

    2. We're going to take a closer look at our existing waterwheel, and see if improving the headworks will help it run for a longer time. We're also going to explore replacing the waterwheel with a newer unit — perhaps we can get another 100 watts or so out of it when there is enough water.

    3. We're going to explore putting in a small propane system as a power source of "last resort". Having worked with these at remote telecommunication sites, these seem to be more reliable for unattended operation, intermittent duty, and require less maintenance. Plus, you don't have to worry about the freshness of stored liquid fuel, and they are somewhat less polluting.

    Any thoughts on the above?

    It's too bad that there aren't any good packaged biomass/small-scale steam-based generators out there designed for small off-grid applications. We certainly have lots and lots of wood...
    House: 2x SMA SI 6048 w 24x 2V DEKA Unigy II; 2x SMA SB 3000TL-US w 24x Sharp ND-H235Q2
    Cabin: 1x Magnum MS4024 w 24x 2V DEKA Unigy II; 1x Morningstar TS-MPPT-60 w 6x Sharp ND-H235Q2; 1x 200 Watt Harris microhydro
    Intertie: 1x SMA WB 3800; 1x Lambda GEN-600 DC Supply; 2x PSL pQube
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Inverter oversizing for cloudy winter weather?
    It's too bad that there aren't any good packaged biomass/small-scale steam-based generators out there designed for small off-grid applications. We certainly have lots and lots of wood...

    There are "Wood Gas" (wood gasification) systems... I am not a big fan of them (lots of carbon monoxide in the gas, explosion hazards)--But there are people out there making wood gas reactors and running generators from them... Certainly not a good candidate for full automation--But a possible alternative fuel source for attended operation.

    Wood
    gas generator - Wikipedia


    We had one poster here that said to not use the FEMA plans for wood gasification (dangerous?).

    If you have the time and can afford to experiment--Might be worth a look.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Inverter oversizing for cloudy winter weather?

    I don't remember if the container is in the shade but there is a fair bit of flat space there to use if it fits your plan...
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • David and LauraDavid and Laura Solar Expert Posts: 139
    Re: Inverter oversizing for cloudy winter weather?

    Re #12 - We aren't considering a biogas system. It's too far outside our comfort zone.

    There seems to be a lot of DYI activity in this area, and a fair number of startup companies, but nothing packaged and reliable. The most mature technology out there seems to be thermo-electric generators, which can be attached to your wood stove, but they don't generate all that much current (you need a pretty good one to get 100 watts continuously), and their reliability is also questionable.

    There are a few folks making complete steam-based systems, for example: http://woodsteamgenset.com/html/gensets.html

    But, that's getting a little off-topic... Let me see if I can wrap up this thread in the next post...

    Re #13 - Lots of space up near our array — It's just a matter of determining the most economical approach before spending any dollars.
    House: 2x SMA SI 6048 w 24x 2V DEKA Unigy II; 2x SMA SB 3000TL-US w 24x Sharp ND-H235Q2
    Cabin: 1x Magnum MS4024 w 24x 2V DEKA Unigy II; 1x Morningstar TS-MPPT-60 w 6x Sharp ND-H235Q2; 1x 200 Watt Harris microhydro
    Intertie: 1x SMA WB 3800; 1x Lambda GEN-600 DC Supply; 2x PSL pQube
  • David and LauraDavid and Laura Solar Expert Posts: 139
    Re: Inverter oversizing for cloudy winter weather?

    So, to try to wrap up this thread:

    1. It is possible to generate electrical power from solar irradiation during periods of cloudy weather. Expect 5 - 10% of your array's rated full-sun performance.

    2. For some inverter types (check with your inverter manufacturer to confirm), it is possible to connect more solar panels than the rated AC output. During times when there is more DC power than AC power that can be produced, the inverter throttles back the amount of power being drawn from the array.

    3. Oversizing arrays for an existing inverter (especially the Sunny Boy we have, where only one of the two inputs is being used) is less expensive, since additional inverters are not required. Thus, the cost is only that of the panels, mounting and DC wiring.

    4. The cost per KW of solar generation when cloudy is easily 10x that of the cost of solar generation in ideal sun conditions. This is _VERY_EXPENSIVE_. If one assumes four months of cloud (which is the only time when the power is needed), a 5 KW array of panels, $0.80/watt panels (say $1 total), and 5 hours of light per day, that works out to $5,000 capital cost. Such a system will generate 400 watts, so that works out to 120 * 400 * 5, or 240 kilowatt hours per year. Over twenty years, that's approximately $1.04 per KWhr.

    Expensive, but not too bad compared to what solar used to cost...


    Thus, to conclude, while there are some applications where oversizing the number of panels for a particular inverter makes sense (south-west array plus a south-west array to extend out the peak power generation time, for example), cloudy weather generation is still very expensive.
    House: 2x SMA SI 6048 w 24x 2V DEKA Unigy II; 2x SMA SB 3000TL-US w 24x Sharp ND-H235Q2
    Cabin: 1x Magnum MS4024 w 24x 2V DEKA Unigy II; 1x Morningstar TS-MPPT-60 w 6x Sharp ND-H235Q2; 1x 200 Watt Harris microhydro
    Intertie: 1x SMA WB 3800; 1x Lambda GEN-600 DC Supply; 2x PSL pQube
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Inverter oversizing for cloudy winter weather?

    david and laura,
    that will still be insufficient even with the doubling of the pvs. do you plan anything else or just make due with the limited output expanded system for 4 months of the year?
  • CDN_VTCDN_VT Solar Expert Posts: 492 ✭✭✭
    Re: Inverter oversizing for cloudy winter weather?

    Interesting info on the Steam works !!
    Being a techie , looking at that trailer contraption (right out of still backwoods) is very high maintenance to break. square boiler with 90*joints & straight pipes is a no-no in the pressure vessel world. Most ships I've worked on (Air to Underwater)always use stress relieving bends to flexing extrusions .. that looks dangerous!!!!
    Plus they should use a condensing exhaust to re-use the condensed water..

    VT
  • David and LauraDavid and Laura Solar Expert Posts: 139
    Re: Inverter oversizing for cloudy winter weather?

    Re #16 - If we doubled our array size to 10 KW, we would be seeing 4 KWhr per day during cloudy weather, which is enough to keep our base 150 watt (x24 hour) communication equipment running, along with the 1 KWhr per day we typically use when we're power limited.

    Re #17 - We have a healthy respect for high pressure steam (while I don't have my steam ticket, it's something that's been on the list), so we'd only ever go for something that is certified. That contraption would certainly would require a full once-over after it had been transported to where it was going to be operated, and I'd much rather the operator's controls be behind some sort of shield. Still - converting an armful of wood into 7 KWhrs of power is sure tempting.
    House: 2x SMA SI 6048 w 24x 2V DEKA Unigy II; 2x SMA SB 3000TL-US w 24x Sharp ND-H235Q2
    Cabin: 1x Magnum MS4024 w 24x 2V DEKA Unigy II; 1x Morningstar TS-MPPT-60 w 6x Sharp ND-H235Q2; 1x 200 Watt Harris microhydro
    Intertie: 1x SMA WB 3800; 1x Lambda GEN-600 DC Supply; 2x PSL pQube
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Inverter oversizing for cloudy winter weather?

    very good as i was believing it not to be enough, but being it is then go for it.
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Inverter oversizing for cloudy winter weather?

    Steam--According to their FAQ, it is roughly 40 lbs of soft wood per 1 kWH of power:

    http://woodsteamgenset.com/html/faq.html

    Getting pretty significant...

    I was not going to mention it--But if you are looking to pressurize something anyway--What about Wood Alcohol production... There may even be some larger installations in your area (lumber waste, etc.).

    At least the end result (some form of Alcohol) is much easier to store and use when needed. Canada is not probably anything like the US ATF (Alcohol, Tobacco, and Firearm) folks here regarding alcohol production (we still have remnants of prohibition hanging around our necks).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • northernernortherner Solar Expert Posts: 492 ✭✭✭✭✭✭
    Re: Inverter oversizing for cloudy winter weather?
    Re #16 - If we doubled our array size to 10 KW, we would be seeing 4 KWhr per day during cloudy weather, which is enough to keep our base 150 watt (x24 hour) communication equipment running, along with the 1 KWhr per day we typically use when we're power limited.

    I take it you don't receive much heavy cloud cover where you are located? I find that with my 2.8kw solar array, I've had one day as low as 0.2kwh, although 0.4 kwh is more typical of a heavy cloud filled day. And I live in sunny Alberta.:D I would consider using a propane powered generator to make up any shortfalls. You can easily convert any generator to run on tri fuel, although I would consider converting a Honda or Yamaha gasoline generator. I have access to natural gas to run my generator for backup, which is very inexpensive and relatively clean to run.
  • David and LauraDavid and Laura Solar Expert Posts: 139
    Re: Inverter oversizing for cloudy winter weather?

    Re #20 - Hmmm. We took that figure from another location, but I'd be inclined to believe the larger number.

    Re #21 - Most of the time we have high cloud layers or fog/mist, so it is still pretty light. But on other days (such as yesterday, when it was raining pretty hard and much darker), our existing array only generated 1 KWhr (so with an expanded array size, that would have been 2 KWhr).

    Propane is certainly the way we are leaning, due to the fuel storage benefits.
    House: 2x SMA SI 6048 w 24x 2V DEKA Unigy II; 2x SMA SB 3000TL-US w 24x Sharp ND-H235Q2
    Cabin: 1x Magnum MS4024 w 24x 2V DEKA Unigy II; 1x Morningstar TS-MPPT-60 w 6x Sharp ND-H235Q2; 1x 200 Watt Harris microhydro
    Intertie: 1x SMA WB 3800; 1x Lambda GEN-600 DC Supply; 2x PSL pQube
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