System sizing recommendation?

[Wallymtn]

Hi Everyone,

I currently have a 24 vdc system. My batteries have weiled there death call. I am looking to upgrade my system to manage an energy use from between 3800-5400 w AC. I am looking at using either a Magnum energy MS 4024 PAE or MS 4448 PAE because I have certain electronics and motors that I want to protect by using a pure sinewave inverter. My solar irradiation is roughly 5 (I have both a tiltable fixed rack and a tracker array (now done by hand because motor burnt out) (from the charts from the for the worst time of the year (December/Jan) because I live in the Taos area of Northern NM in the mountains, but do get permanent snow coverage during these months. I am wonder what would be the best system to invest in going forward? The only DC appliance I have is my Sunfrost frig only 24 vdc and figure I could convert 48 to 24 vdc to accommodate the higher voltage system. Any thoughts would be greatly appreciated.

thanks and have a wonderful day,
Walter

[Mike90250]

I would only look at 48V systems. Your PV needs to be above 70V to charge batteries on a hot day.

Do you have any 240VAC loads ?

What is the condition of your backup generator ? is it 120 or 240V ?

Many of the larger 48V inverters, have a generator input and transfer switch built in, and can charge batteries from the genset.

Does the wattage of your current array, keep up with your current usage ?

[Wallymtn]

Hi Mike,

So hopefully this will finally get through to you in response to your entry. I have written 2 but the first one seems to have gone into the etherworld and my second I had internet issues and then it didn't save so have had to repeat it.

First I want to thank you for sharing your knowledge. 2nd, I want to apologize if I ask questions that seem obvious in terms of answers. To answer your questions

1. I have no 240 volt loads but thought that if that changed then having an inverter that could do both in the future if needed.
2. I have a military generator MEP 002a that is in great shape. It is a 5kw diesel generator. It produces three possibilities single phase 120, 120/240 single phase, and 3 phase 240.
3. I would have to say that prior to a year ago that my solar array did manage to meet my electrical needs for the most part. I did have to run my generator occasionally but actually had no functioning generator for 1 year and only had a 3 or 4 times where I drew that power down to a point where the system shutdown (mostly when watering outside main garden).

My array totals is 1350w. I have 18 functioning astropower 7105s (75w--4.4 max amps at 17 volts per panel). I use to have 20, but in moving them down from an upper roof one of the panel's glass became shattered, so I had to take 2 panels out of action so that there wouldn't be any draw down in terms of odd match of production. I have 2 arrays. I is 10 panels (17 1/2 years old) and the other is 8 (12-13 years old). My energy usage analysis showed 170w DC and 3630 of AC. My inverters energy use is not factored in. It has a 8w search mode but I could not find a rating when it's in use. I have left it in the on position all the time. In my calculation I figured it at 600w per day. With that added in my total usage stands as stated. I may realize some energy savings as I change my lights over from CFLs to LEDs. My increase would be along the line of grow lights for my winter indoor garden. Currently I just go with my winter sun in my sun space but am thinking about trying to increase my food production, but have not made a final decision. If I did add this in then it would add in about 1000w total. My energy usage does not factor in any efficiency losses. My current battery bank system is a 900AH 24v (16 Trojan T-105). They have last about 11 or 12 years (I am not exactly sure because they were changed out while I was living in another state, so just don't clearly remember exactly when they were brought online.

The question I have is why do you think that I need to have a 48v system and is the 70v on a hot day in relation to having a 48v system or because of heating/cooling issue? If it's the latter, I don't have any heating cooling since I live at roughly 8000 feet and never get that hot and for heating I have a passive design with a wood stove backup that manages superbly. In doing my numbers I thought to increase my battery bank to 1350 AH by using 24 T-105REs in a 4x6 setup. As I stated above I got a minimum of 10 years out of the last set of batteries and possible as much as 12 years. This is why I am not sure why I need to increase to a 48v system, unless it's fire danger related which I read about in another post by Sunking on this website. I did read some information about the loss experienced by batteries stacked in parallel and how by taking from opposite corners can reduce the imbalance of draw from the first to last battery, which is the way my bank has been connected. I have a 60amp charge controller from Xantrex/Trace C-series. I realize that if I had a 48v system then I would have lower amperage and better transmission efficiency, but would then have to get a converter for my 24v fridge (Sunfrost). If I was to go to a 48v system I would effectively lose another 2 panels and would have to reconfigure the wiring on the tracker (non-operable except by hand because motor burnt out twice and got tired of replacing it) which has 10 panels. My current tiltable rack can only hold 10 panels. The fixed is faced due south. The solar insolation for the area has been stated anywhere from a low of 2.81 (due south) to max low of 5.36 (due south with a tiltable ability). I live just north of Sante Fe, NM in the mountains around the Taos area. Today my sun arose at 6:55 and set at 4:05. We do have a mix of clouds throughout the winter and do get significant cloud and snow, but also plenty of sun as well.

I am not wed to any particular product or system but thought to try to use as much of what I already have so as to minimize my outlay. I don't have a fixed budget it but am trying to keep it under $6000. I know about the federal tax credit, which I plan to use so that would help offset my cost and am looking to do a lot of the work myself in terms of installation. I helped install the original system and have done some simple wiring updates, like when I moved the panels to a different location. I am looking to have a battery system that provides 4 days of energy consumption. This is where with efficiency factors I came up with the 1350 AH battery bank idea, especially since I have a great backup generator.

Again thank you for whatever support and suggestions that you provide. Curious if this is your full-time occupation or just a love of yours. I just wonder because I am a nurse and it's the same kind of situation because as your experience increases you become the teacher/mentor for the next generation and just love to do because I got the same support when I was new.

Walter

[Mike90250]

Wow, lots of stuff to cover there.

48v vs 24v
This voltage simplifies the battery bank, making feasible a single string. With care, 2 strings of batteries can be paralleled, but it's not "wire & forget". Other choice is expensive 4V or 2V, 800ah cells to make a single string @ 24V.
Stored & useable watt hours remain the same for a 1,000ah 24v or 500ah 48v Wire losses are not an important factor, but the proper wire gauge to keep them low, is.

So your existing 24v gear is what prevents you from making the switch?

48V banks can require up to 65V to equalize, which needs an array that can deliver at least 68V when heated up in the summer (hot PV panels produce lower voltages)

You have in the past, had success with your 24V system, even with 4 parallel banks, so you have what it takes to keep them happy.

if you add a larger 24v bank, you NEED to have a larger PV array, you need to charge the batteries at a high enough rate, to get them bubbling well, for a couple hours to stir the electrolyte up. Otherwise, STRATIFICATION starts up, and your electrolyte separates into different layers of density. The rule of thumb is 8-15% of battery amps is needed for proper charging, so a 100ah battery needs between 8-15 amps of charge for several hours. 800ah of battery needs 64 - 120 amps. That takes some pretty fat wire, which leads to 48V banks instead.

That's a gem of a generator you have! If you don't have any 240V loads, a 120V system is much more "standard" and I'd not look for 240v gear at all. (some folks forget about the 240v well pump down in the hole for 30 years, or that "funny plug", on aunt Maratha's iron lung).

Drawing down you batteries to the point the inverter shuts off, is really hard on them, and each time you do it, takes months off of their lifetime. Do you have a way to charge batteries off the generator, or does it only power house loads while batteries are flat?

[Wallymtn]

First, I want to thank you for you for the sharing of your knowledge and time on this subject.

First question about present gear limiting choice. No, it doesn't limit my decision I just am trying to be as efficient as possible in using what is present where possible. Unfortunately, I have been unable to find another Astropower 7105 since they are no longer made. I thought I could try to find a panel made now that has similar matching specs (just thought of that actually). I do see the value of suggesting a 48v system, but just wanted to understand more the logic/knowledge behind it. Like I said with the fridge, I can purchase a dc to dc converter to drop the voltage down so that I don't have to replace the fridge which works excellently.

So looking at what you said about a 48v system the array needing to deliver a minimum of 65v and 68v to be able to deliver this then if I was to wire 4 of my current panels in series would produce 68v (17v per panel) if there aren't other factors that I am unaware of. Would that be cutting it to close in terms of voltage production since it doesn't factor in loss due to transmission which is minimal since I currently have a 2/0 cable to carry the power in? (per a voltage drop calculator is says the loss would be 0.17 so ending voltage would be 67.83v 48). The temperature here gets to a max of 95F, so not so bad as some places and it's a very strong sun since high elevation and often little cloud cover/low moisture.

The Generator.
Yes, I actually have come to the point where I would recommend them to anyone. They are used versions sold as military surplus. You can either by them at auctions that are held throughout the USA (cheapest way if one is close by or you can go through ebay/amazon to pick one up (if you want more information about them I be more than willing to pass it along). I spent just under $1000 and was able to pick it up in the back of my small pickup. It weighs 960 pounds. There are lots of different sizes. To answer your question about charging/house loads. The generator is wired into the inverter and is able to do both maintain house loads and charge with whatever power is left over (and since I actually have minimal load usage at any one time unless the pump has kicked or I am running my table or skill saw I there is plenty left to charge the batteries effectively.

So I am leaning toward the 48v system the more we talk. Any recommendations on battery manufactures and size/type/style of the batteries as compared to cost?

Have a great day!
Walter

[Mike90250]

......

So I am leaning toward the 48v system the more we talk. Any recommendations on battery manufactures and size/type/style of the batteries as compared to cost?

Have a great day!
Walter

Batteries. Who has what brands near by? Do you want to make 3 trips with your truck to haul buckets of acid from the dealer to your place, or do they have delivery service ?

Most systems initially murder the first bank of batteries via some user error, or configuration mistake. So, maybe cheap, 6v 200ah golf cart batteries are a good starter set. Practice on them.

[paulcheung]

If you can afford them and get them, The Rolls 5000 series and the Trojan Industrial line batteries are the two good brand batteries for off grid users.

[Wallymtn]

I can get a local delivery through a company called USAbatterysales.com that I can meet the driver in a town only 45 minutes from my house. Great chat with the sales rep. They stock/carry the Trojans. Very informed and knowledgeable about the different batteries and what the true different between the different Lead acid batteries. I didn't broach another type of batteries because the cost is too high for me to be able to afford.

[inetdog]

I can get a local delivery through a company called USAbatterysales.com that I can meet the driver in a town only 45 minutes from my house. Great chat with the sales rep. They stock/carry the Trojans. Very informed and knowledgeable about the different batteries and what the true different between the different Lead acid batteries. I didn't broach another type of batteries because the cost is too high for me to be able to afford.

Good that you have found someone knowledgeable to talk to.
Just keep in mind that there are several types of deep discharge batteries, even from the same manufacturer.
If you can get them, the Trojan models with RE (for renewable energy) in the name will have heavier plates and be more suitable for long life with deep cycling every day. Hopefully their cost will not be too much more for you than other models.

[Sunking]

Careful, Trojan makes 3 lines of batteries. They make Golf Cart batteries like T-105 and those are a 2 to 3 year battery as the whole line-up of USA batteries. Trojan also make Deep Cycles batteries like the T-105RE, and it is a 3 to 5 year battery. Trojan also makes an Industrial line which is a 5 to 7 year battery. The difference in price is not much. If you are smart, you look at Kwh cost for the life of the battery.

Example the T-105 wil cost you around $1 per Kwh over the life of the product.
T-105RE drops to around 80-cents per Kwh.
Industrial line drops to around 60 to 65 cents per Kwh

Sure you can go cheap initially with the T-105, but it is the most expensive choice you can make.