Would this off-grid system work?

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Chrispy
Chrispy Registered Users Posts: 2
Hi,
I’m new to solar and off-grid in general but my plan is to build a system for a small cottage that is powerful enough to power two hot plates 2000W, small oven 2000W, coffeemaker 1000W, fridge, TV and some lights. Everything does not have able to run at once.
If thinking of getting these ETFE solar panels:
s.click.aliexpress.com/e/b5IAZX8M
4 of them, they are 12V 100W so I'm thinking of connecting 2 and 2 in serial to get 24V. Is that a good idea?
Then I will get a MPPT controller for 24V 40A
s.click.aliexpress.com/e/cPYcXSVi
Then for the power bank I will buy 3 Lifepo4 batteries with each of them being 24V 100Ah that I will connect in paralell. Maximum discharge is 1C. So maximum output would be 300A x 24V = 7200W
Finally I want to connect a pure sine wave 
inverter rated for 3000W with 6000W peak.
s.click.aliexpress.com/e/6GFVGXA
Do you think this system would work? Am I missing something? Suggestions? How many watt hours can I get expect to get from the solar panels? 
No one will live or stay over night in the cottage and it will only be used during summer time. 

Comments

  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
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    In many locations, ~70-75% of STC rated output is reasonable to expect, so roughly 300w for 4 x 100w panels.  With marine layer haze etc, somewhat less, high altitude more, but that's about average.  I personally wouldn't use flexible panels unless the application really needs the flex.

    Watt-hours per day depends on your location.  You could check an insolation calculator like pvwatts.nrel.gov

    Using 5hrs full sun equivalent, for example, you might expect about 300 x 5 = 1500 watt-hours per day on average.  That might run the fridge, but wouldn't be enough to offset inverter losses, other loads, or for catch-up battery charging after a rainy day.  Just having the inverter on might use 25w or more, which is 600 watt-hours before powering any loads.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Welcome to the forum Chrispy,

    You can do a lot with off grid solar power... But it does cost a lot to run mechanical (i.e., water pumping, A/C and heat pump, etc.) and electrical heating... In the USA where grid power is something like $0.10 to $0.40 per kWH, off grid runs around $1.00 to $2.00+ kWH (can be less, can be more)... And the farther from the equator, the larger your arrays need to be, and the worse your winter/non-sunny day harvest.

    I will use 3,300 WH per day (3.3 kWH per day or ~100 kWH per month). This is enough to power an off grid cabin (full size refrigerator, LED lighting, solar friendly water pump/well pump, clothes washer, LED TV, laptop computer, cell phone/tablet charger... With lots of conservation (I call it a "near normal" electrical existence).

    I would suggest around a 1,200-1,500 Watt AC inverter for such a system--In reality, your peak power is limited by battery bank design/capacity in AH and working voltage (12/24/48 volts)... It is really easy to put a 3,000 Watt @ 12 volt AIMS inverter on some deep cycle flooded cell lead acid batteries (FLA)--But if you are looking for a "reliable" system over time and usage--It is not the best way to approach the problem.

    So--I like to do a few paper designs first--And see if they meet your needs (paper/computer bytes are cheap). Once you have a good paper design, then you can select the hardware that will meet those needs.

    For paper designs, scaling is easy... If you need 50% more kWH per day, simply multiply the results by 1.5x and check some simple rules of thumbs...

    First, the "heart" of your system is the battery bank. You look at your power needs (conservation is almost always cheaper than "building a bigger system" for "inefficient loads"). For example, we start at 3,300 WH per day, 2 days of battery storage (kind of optimum from our experience for FLA batteries) and 50% maximum discharge for longer battery life. So, sizing the battery bank will look like:
    • 3,300 WH per day * 1/0.85 AC inverter eff * 1/24 volt battery bank * 2 days storage * 1/0.50 max discharge = 647 AH @ 24 volt battery bank
    I cheated here a bit because I knew the answer for the minimum battery bank voltage. In general, we have 12/24/48 volt to choose from. And I highly suggest that you keep the battery bank below ~800 AH for various reasons... Keeps the wiring AWG smaller and uses fewer solar charge controllers.

    Next, we need to make two solar array calculations. One is based on Battery AH and voltage calculations (larger battery banks need more solar charging current) and the second based on your loads an hours of day of sun (your location, clear of trees and buildings, local weather, etc...---Any shading and clouds greatly reduce or even your kill solar harvest).

    First suggest 5% to 13%-20% rate of charge... 5% minimum (weekend/sunny weather usage) and 10%+ for full time off grid use.
    • 647 AH * 29 volts charging * 1/0.77 panel+controller * 0.05 rate of charge = 1,218 Watt array minimum
    • 647 AH * 29 volts charging * 1/0.77 panel+controller * 0.10 rate of charge = 2,437 Watt array nominal
    • 647 AH * 29 volts charging * 1/0.77 panel+controller * 0.13 rate of charge = 3,168 Watt array "typical cost effective maximum"
    Then there is sizing based on your daily energy usage, location of system, and seasonal usage... Let's say you are around Stockholm Sweden, fixed array:
    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Stockholm
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 31° angle:
    (For best year-round performance)

    JanFebMarAprMayJun
    0.93
     
    1.96
     
    3.42
     
    4.56
     
    5.19
     
    5.03
     
    JulAugSepOctNovDec
    4.97
     
    4.47
     
    3.60
     
    2.16
     
    1.26
     
    0.71
     

    And this is a pretty "dark" location (it appears to have "poor weather", farther north, etc.)... Nominally, I would suggest that you would not use much power in winter, and/or genset... So toss the bottom three months gives us 1.96 hours of average February sun:
    • 3,300 WH per day * 1/0.52 off grid system eff * 1/1.96 hours of Feb sun = 3,238 Watt array February "break even"
    So--For that region, just a typical (medium sized) cabin solar power system would require ~3,238 Watt array and a generator (for winter usage). During summer you have more sun and "excess energy"... But your base loads should be closer to 50% to 65% of your predicted loads (conserve during poor weather, run washing machine and water pumping during sunny weather).

    Roughly, the suggested AC inverter would be 250 to 500 Watts per 100 AH of 24 volt FLA battery capacity.
    • 250 Watt * 647 AH / 100 AH (@ 24 volts) = 1,618 Watt AC inverter "nominal suggested"
    • 500 Watt * 647 AH / 100 AH (@ 24 volts) = 3,235 Watt AC inverter "maximum suggested"
    This is all based on FLA battery type... A ~600 AH battery bank can be made out of 4x 6 volt @ ~200 AH batteries in series (for 24 volts) and 3x parallel strings for (3*200AH=) 600 AH battery bank... In the US, "golf cart" batteries are relatively cheap and rugged. And probably most people "murder" their first battery bank.

    You asked about Li Ion (typically LiFePO4 for off grid power usage). It is a possibility, but you need to measure/estimate/tell us more power your energy needs. Using a Kill-a-Watt type meter to measure "real" loads and your planned usage (get a hotplate and cook a day's worth of meals on it):

    https://www.amazon.co.uk/s?k=kill+a+watt+meter

    Do all of this for your planned cabin appliances... Using a 2,000 Watt hot plate for 20 minutes is 2,000w*1/3 hour = 667 WH per meal

    Of course, there are thermostats and variable heat hot plates... You should probably look at induction cookers instead (more efficient):

    https://www.amazon.co.uk/s?k=induction+cooker

    And very popular with Asians are Thermos type slow cookers (heat/boil your stew/soup/etc. and let sit for 4 hours in Thermos to "cook"):

    https://www.amazon.co.uk/s?k=vacuum+thermos+cooker

    This is where I remind you that "extreme" conservation (especially in less than optimum solar conditions) is usually a good idea. And using propane/alcohol stoves is cheaper long term.

    Batteries--There are a lot of options out there... FLA, AGM, some other chemistries, Li Ion, etc. For example, I think you are looking at 12 volt @ 100 AH LiFePO4 batteries (lithium Iron Phosphate--Relatively stable and "safe" for Li Ion chemistry). For your battery bank, you would need 2x 12 volt in series (for 24 volt) and 3x parallel strings for (3*100AH=) 300 AH battery bank. Or a total of 2*3=6 batteries total.

    LiFePO4 batteries are getting better in pricing, and their performance is much better than FLA... However, they are expensive and many people use a BMS (Battery Monitor System) to ensure that they don't damage the LiFePO4 batteries (over discharging, over charging, you pretty much "kill" a cell/battery). So you have BMS questions and figure out if the BMS will work with your solar charger, battery choices, and AC inverter...

    Other issues include getting a larger/cheaper AC inverter... A  3,000 Watt AIMS per sine wave AC inverter is not my first recommendation--But the issue with large PSW inverters is they take just a lot energy when turned on:

    https://www.aimscorp.net/3000-Watt-Pure-Sine-Inverter-Charger-24-Volt.html
    • Idle Consumption: 46.5 Watts
    • Power Saver Mode Idle Consumption: 18.9 Watts
    For a "medium" to small AC power system, 47 Watts is a lot if you plan on running 24x7:
    • 46.5 watts * 24 hours per day = 1,116 Watt*Hours per day (AC inverter "on" and no loads)
    That is literally 1/3rd of my suggested energy usage for an off grid "near normal" electrical existence... Or you would need to make the solar array and battery bank 1,116 WH larger (1.33x) just to "waste" power for the inverter.

    I will stop here for the moment--With a warning that you not purchase any solar power equipment until you have done a few back of the envelope (above) and paper system designs (picking equipment, confirming specifications, local suppliers, etc.).

    Your thoughts/questions?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
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    Chrispy said:
    Hi,
    I’m new to solar and off-grid in general but my plan is to build a system for a small cottage that is powerful enough to power two hot plates 2000W, small oven 2000W, coffeemaker 1000W, fridge, TV and some lights.
    .......
    To be able to run this, is not going to be easy, cheap or small.

    To improve your situation, consider a Induction Hotplate.  These require you to use a pan with a steel or iron insert, which is heated with a magnetic field.  It's much more efficient than a glow coil hotplate or even a microwave oven.

    When any single load passes 1,000 watts, a 12V system is not suitable, and you should scale up to a 24 or even 48V system.
    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 ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • PNW_Steve
    PNW_Steve Registered Users Posts: 79 ✭✭
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    Have you considered propane for cooking and heating? 
  • Chrispy
    Chrispy Registered Users Posts: 2
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    Thanks you all your suggestions especially BB for taking the time to go through almost all aspects. 

    So I understand I need to scale up quite a bit. If I use 10 100W panels instead of just 4, for my location which is much closer to Copenhagen than Stockholm I should according to pvwatts.nrel.gov be able get 4kW per day during the summer months. Even if it's less in the real world, say 3kW that would be almost enough to charge half the battery which I think will be fine for this application as the cottage isn't going to be used daily.

    I've been offered what I think is a good deal for Lifepo4 batteries, a battery bank of 300AH 24v for around 1500 USD and since they can be discharge to much lower levels than other types of batteries. 
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Another thing to look at is 100 Watt Vmp~18 volt panels vs using 200+ Watt Vmp~30 volts (or higher) panels.

    In general, 200+ Watt panels are 1/2 the price (or less) than of 100 Watt panels. Look at the price per Watt.

    If you use Vmp~30 volt panels (which are less expensive), you have to use an MPPT type charge controller, which is more expensive.

    Overall, it is usually less expensive to use an MPPT controller+the less expensive large format panels. Plus the cost of wiring from the array to the charge controller is less.

    Downside is that >175 Watt panels, you need two people to move them safely (large/heavy). Also, shipping is more expensive for large panels (generally must ship by truck on pallets).

    Shop around and figure out what panels/equipment is available to you at the best price. Ordering everything from China has its downsides too... Support and warranty repairs are more difficult. And it may take a month or more to receive your product. Any many of the low cost hardware is of questionable quality (and reviews are rare/not always real).

    If you have solar installations in your area, you should check with the people around you about their experiences.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
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    BB. said:
    Another thing to look at is 100 Watt Vmp~18 volt panels vs using 200+ Watt Vmp~30 volts (or higher) panels.

    In general, 200+ Watt panels are 1/2 the price (or less) than of 100 Watt panels. Look at the price per Watt.

    If you use Vmp~30 volt panels (which are less expensive), you have to use an MPPT type charge controller, which is more expensive.

    Overall, it is usually less expensive to use an MPPT controller+the less expensive large format panels. Plus the cost of wiring from the array to the charge controller is less.

    Downside is that >175 Watt panels, you need two people to move them safely (large/heavy). Also, shipping is more expensive for large panels (generally must ship by truck on pallets).

    Shop around and figure out what panels/equipment is available to you at the best price. Ordering everything from China has its downsides too... Support and warranty repairs are more difficult. And it may take a month or more to receive your product. Any many of the low cost hardware is of questionable quality (and reviews are rare/not always real).

    If you have solar installations in your area, you should check with the people around you about their experiences.

    -Bill
    Bill, what you say is correct with regards to lead acid, with Lifepo4 it would be possible to use GT panels with a PWM controller because the maximum charging voltage required is 28.8V, often lower to extend cycle life expectancy. However in this application, being in a cold climate the benifits would favor an MPPT controller.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • rollandelliott
    rollandelliott Solar Expert Posts: 834 ✭✭
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    Have you considered a solar oven/cooker? they are basically a bunch of mirrors that concentrate the heat of sun on a pot to cook with. that combined with a small propane backup would reduce system cost a lot. 
  • NANOcontrol
    NANOcontrol Registered Users Posts: 263 ✭✭✭
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    Personally I think most people under panel. They are the cheapest part of you system and give you the most benefit if you have the space. Inverters are a personal choice.  I've never seen a reason to have more than a MSB. I do have a 3KW XANTREX under my bed I've never gotten around to installing. Grid tie panels are the way to go. I generate all my power needs even on a rainy day. Remember you have to generate enough power to recover the batteries too.  I have all the comforts of home and almost nothing for a battery. I heat water with excess PV. You would be amazed at how much energy the average solar system wastes.