Cabin Off Grid Design Part 2 - Panels

SunJammer

My my estimations above are conservative, so I'm going to take the 4.65 kWh/day.  Here is the irradiance in hours per day for the area I'm in considering SSE facing 59 degree angle roof that they would be mounted on and this is where I got this data.  The cabin would be a seasonal summer cabin.

Jan	Feb	Mar	Apr	May	Jun
2.35	3.37	4.44	4.95	5.09	5.41
Jul	Aug	Sep	Oct	Nov	Dec
5.31	4.81	3.95	2.92	2.02	1.79

So I'm going to pick a value of 5 for the sun hours per day:

So calculating my solar array:

1.5 fudge factor * 4.65 kWh/day = 7 kWh/day

So my panel watts per day need to be 7 kWh/day divided by 5 hours of sun per day = 1.4 kW of solar power from my array minimum.  

I have found some B type panels rated to 330 watts at 12 VAC.  I'm thinking of going with 5 so that would give me 60VAC of solar panel voltage which would be pumping max 1650 watts.  

I'm thinking of using an outback 80A MPPT Charge controller.  I'm liking the idea of the outback because it seems to handle the high input voltage to keep the thickness of the solar panel wiring down which will provide versatility of where I mount the batteries.  Output of the charger controller would be a little under 70 amps @ 24VDC

I'm leaning towards a 24VDC battery bank although I realise the 48VDC bank lends itself to being connected to an inverter better.  

I've been quoted a 24VDC battery bank:  " Refurbished, 2V/cell, 24V bank, lead acid, price: $1,250+Tax, 1 year warranty. 21KWH @80% Capacity and 70% DOD = 11.8KWH." which are GNB Absolyte glass matt batteries.  They seem to be worth the risk of buying refurbished, the company comes with great reviews.  The batteries performance are awesome from every standpoint IMO.  

The inverter I'm thinking of the a 3000 watt or a 5000 watt inverter.  

I would then use a transfer switch AC panel which would feed the cabin that would allow myself to run the cabin on 1) a solar - battery powered - DC to 120VAC inverter, or 2) to run on a generator.  

I will consider having a DC generator / AC generator + charger that I will build or buy depending on final voltage. 

Would be great to hear advice here.  

  

littleharbor2

Need panel specs. 330 watts @12 volt is odd. You must use the Voc. for figuring maximum voltage input to the charge controller. Most 12 volt panels are in the 18 volt Vmp. , and 22 volts, open circuit, Voc.. 

Really think you would do better with a pure sine wave inverter. Some loads don't do well on modified (square) wave inverters.

Estragon

In addition to providing AC at more like grid specs, a decent PSW inverter in that size range may include; a quality, programmable battery charger, and a transfer switch for pass-through of generator AC to loads, and the ability to dial back charging current if needed for loads to avoid overloading the generator. Considering the cost of getting these separately, going with a cheap inverter becomes less attractive.

At >3kw, I'd highly recommend rethinking 24v. You're getting into 100s of amps DC.

I'm not sure what "refurbished" AGM batteries are. There isn't really a lot you can do to refurbish an AGM (sealed) battery. They may be older telecom batteries? If you're okay with the possibility of schlepping them in and out annually, they might not cost much when taking scrap value into account.

There are pros and cons to AGMs. One of the pros is they tend to have low self-discharge, especially if kept cool/cold. With an unattended system over the winter, this may work well in your application. Self-discharge tends to increase as batteries age though.

mcgivor

The design of an offgrid  system should start with a load calculation, which is done, the next step should be focused on batteries to support the loads, factoring in type of use, full or part time, days of autonomy, generator use, type of battery, system nominal voltage and so forth. Once a capacity and nominal voltage is established, then the PV array to charge them would be calculated, so is a really three step process, focusing on the battery alone to discuss which choice is best, will eliminate confusion. These are my thoughts and opinions on how the process should evole, for what that's worth. There are many here with years of offgrid experience willing to assist offering suggestions based on mistakes made, which will limit to some degree potential pitfalls along the way.

Estragon

To clarify a bit on the batteries, I assume you'd be buying 12 x 2v cells with a nominal capacity of 21kwh/24v= ~875ah (20 hr rate?). These batteries are apparently warranted to have a minimum of 80% capacity for one year, so you're assuming 80% of nominal capacity. The 70% DOD (Depth Of Discharge), is really 70%SOC (State Of Charge, relative to the 80% warranted capacity), taking you to about 11.8kwh remaining after one cloudy day (and ~5kw usage by loads). A second such day would take you to ~40% SOC, during which you'd fire up the generator for a couple of hours to get back to say a 60-70% SOC.

SunJammer

Yes there would be 12 cells.

Ok so I am hearing the idea of selecting the battery bank. I’d like to assume this 24 VDC battery bank because in terms of available KWH this 24 bank is the best deal I can find, and the depth of discharge that is acceptable from what the data sheets show. The batteries can handle the winter left alone for up to 6 months completely disconnected.

So does this battery bank make sense?

SunJammer

For the solar panels I’ve found a good deal on, here are the panels:

Canadian Solar 270W poly B solar panel CS6K-270P (65"x39")

Canadian Solar 300W mono B solar panel CS6K-300MS (65"x39")

Canadian Solar 335W poly B solar panel CS6U-335P (78"x39")

Data sheet on one of them:

https://www.canadiansolar.com/downloads/datasheets/en/new/2018-4-12-v5.562/Canadian_Solar-Datasheet-MaxPower_CS6U-P_v5.562_EN.pdf

Estragon

IMHO, you'd be better off going for 24x2v for a 48v system if you think you really need a 3-5 kw inverter and ~5kwh/day. Assuming 80% of 875ah capacity, thats 700ish ah @48v - about 3 days autonomy with no sun to 50% SOC. A 5kw PSW inverter might take 100a@48v, surging to maybe twice that. At 24v, surge could be more like 400adc, which is some serious current needing pretty heavy wire (and faith in the inverter internals). 48v also makes more efficient use of charge controllers. If a 2ish kw inverter would work, 24v would make more sense.

The linked-to panels appear to be pretty standard ~37Vmp. I think the Outback controller is good to 150v, so strings of 3 in series could work well at 48v, or 2 for 24v.

mike95490

SunJammer said:

So does this battery bank make sense?

Never heard of refurbished AGM batteries.  I'd trust the warranty for as far as you can carry 4 of the batteries.

""The Absolyte battery was developed by GNB Industrial Power in conjunction with Sandia National Laboratories. This was the first VRLA AGM, large capacity deep cycle battery with extended partial state-of-charge operation. It allows for deep discharge recovery. Their wide band of temperature operation, from -40°C (-40°F) to +50°C (122°F) retains more capacity in cold temperatures than traditional flooded batteries. Life expectancy in float condition is 20 years @ 25°C (77°F) with proper charging.

  Life expectancy in cycling conditions is 1200 cycles to 80% DOD with proper charging. Sealed cells with absorbed glass mat (AGM) separators eliminate the need for periodic water additions as found in flooded cells. Periodic visual inspections, voltage readings, and connection retorquing are all that is required.""

mcgivor

Because the proposed charge controller is MPPT, the less expensive 60 cell GT panels would be a viable option, one thing to remember is the output will be less than the rating so the 1650W for the 5 × 330W panels in post #1 would in reality be ~1200W, 6 would be closer, besides, having odd numbers don't work for strings. The battery capacity determines the array size, too small it will have a hard time catching up on a lost days production, not a big deal if running a generator for a 2 to 3 hours or more is not an issue. Based on the daily consumption  figure of 7Kwh, my guesstimate would be the array should be closer 3000W, my consumption is 3.5Kwh, array 1500W, 5 hours average sun, works fine but falls short after a few days of overcast, exceptionally rare here but dose happen every 2-3 years. I'm certain if my loads were doubled it would never cope. Battery capacity 450Ah  so basically my system is half the size.

Inverter wise, think carefully about when the loads are used, it seems many make the mistake of buying too large a wattage, often sacrificing quality over quality. Consider inverter chargers with a reputable name, all things considered its false economy to buy an inverter and a separate unprogramable charger, additionally they are often offered in split phase pure sine wave, allowing the use of 240V pumps etcetera, they are packed with features the cheaper offerings lack, and have a warranty (usually 5 years) that actually means something.

SunJammer

Thank you I’ll research all that today.

SunJammer

Visited the place that has the good deals on panels and market pricing on mppt charge controllers.  I’m leaning towards 6 panels, which could be pretty close to 2000 watts of panels. I am thinking of a 60 or maybe even 80 amp outback charger controller. I think the 80 would  allow for expanding to 8 or 9 solars panels if need be in the future at 24 vdc battery bank and the 60 would handle 6 panels (2 sets of 3 in series, parrlelled together).  

For the inverter.  I’m noticing idle power gets high of the larger inverters.  So I’m thinking of maybe using dc lighting and a dc fridge, with an inverter that runs the rest of everything and run the Cpap machines off of wired dc.  The. The rest of the loads I could use the larger inverter In Power save mode or turn it off altogether when it is not in use.

the other option I see is to purchase two inverters... one for the lights and fridge and one for the the rest of the AC circuits as above.  

My research has me interested in the http://www.aimscorp.net/documents/PICOGLF10w-60w%20071618.pdf AIMs pure sinewave inverter / charger units.  

Estragon

My personal preference is to run things like charge controllers conservatively, in other words not expect a 60a CC to run at 60a output for multiple hours/day for many years.

Big inverters definitely tend to have big low load losses. I run two systems (see sig line), which is one way to handle, but there are lots of others. I'm just finishing up installing 12v LED lighting to supplement the 120vac cabin lighting. Many just get more pv to offset the inverter losses, which is perfectly reasonable.

I'm not a fan of the Aims inverters for your type of application. I

SunJammer

 Can you expand on the Aims inverter aversion?  They are pure sinewave and have charged controllers that are soohisitcated as far as I can tell.

westbranch

One of the rules of thumb about inverters is that a 12V system supports a 1000 W inverter  , 24V a 2000 W inverter   and a 48V system above that. The reasons are that those are the approximate break lines for economy of cable needed to supply the larger inverter. to supply 5000W from a 48 v bank is just a bit more than 100 Amps, for a 12 volt bank you would need ~ 416 Amps and ~208 Amps for 24 V...

I don't know about you  but I handle  High amperage with great respect... BB has some very explicit video links on that topic!

Low voltage means high amps and a much higher cost to cover modest distances and higher voltage loss due to distance...

Now look at the top line inverters ans see how many high Wattage inverters they offer in the 3 categories available... not to say those products do not work but for how long will they work.... and then there is the repair-ability factor, just down the street or half way around the world?

Estragon

My aversion to Aims is really twofold.

The first, as @westbranch points out, is low voltage / high current devices have potential issues.

In your application, (a fixed, hardwired dwellling, presumably), a device apparently designed for a portable (note the AC outets on the back, and switched neutral) application may not work well. It can probably be made to work well, but my personal preference would be to get a product made for the intended purpose in the first place.

I may have missed it, but I also didn't see an ETL reference. Certainly not perfect, but ETL does give some degree of comfort (and possibly insurance cover).

SunJammer

I beleive the low voltage discussion and points are all warranted and relevant to my design.

However from an inverter manufacture selection AIMs has a full line inverters which include 48 VDC. So when we talk about AIM VS a more reputable supplier like Schneider or some equivalent - I don’t see why the battery voltage comes into the conversation. It’s more a function of the battery bank I have in mind. I do understand the risk of buying used batteries and I also hear this communities recommendation WRT wire size and amperage.

I think what is really getting me here is the UL/ CSA approval.

mcgivor

The difference between reputable makes and those lesser known is basically quality, the less expensive brands fall short in areas such as power quality, total harmonic distortion, claimed surge ratings and overall quality. Good middle of the road units, Samlex and Cotek for example, are better, which is why reputable dealers sell them. Then there are the more industrial type, Outback, SMA, Magnum, Schneider etc. which cost more but offer much more robust electronics, interfaceing, stackability and real warranties, which typically last 5 years. Naturally not everyone can afford the best, that's understandable, but you really do get what you pay for.

SunJammer

I've looked at Samlex - they have CSA / UL approved devices.  This already is a winner since I plan on feeding a hardwired cabin.  I'd say UL listed is a bare minimum for me, CSA even better if I ever have to have it inspected.  Thank you for putting me onto Samplex and Cotek.  

Right off the bat the his looks nice:  1200 Watt, 120V Pure Sine Inverter/Charger

mcgivor

Do you have a link to the model?