Upgrading cabin solar system

System
System Posts: 2,511 admin
in Off Grid Solar & Battery Systems #1
Hello all,

I've got a small off-grid system that I installed in our weekend cabin at 10,000 feet elevation back in April 2003. Our electricity demands have since increased and there are also a few new loads that we are considering: a ceiling fan and a on-demand propane water heater. Right now we sometimes run low on power, particularly during cloudy periods in the summer (if a week-long visit) and during 3-4 day visits in the winter. I'd like to add battery capacity so I worry about it less and so I'm seeking advice for upgrades.

The main load is a Flowlight Booster Pump (12 Volt, 10 amps) that pumps from our underground water storage tank to the pressure tank (set at about 50 psi). We don't use a lot of water since we've replaced the toilet with a low-flow unit and have kept showers to a minimum. The rest is mostly lights (120 Volt, all CFL), though we do run a vacuum (occasionally...). Our frige is propane though it can run on 120V (never tried it).

Here is the configuration, which is essentially designed to charge the batteries between visits and then run off the batteries when we are there.

3 Shell (Siemens) SM-110-12V, 110 Watt, 12 Volt, 6.3 amp panels wired in parallel with a 60' run to the controller with AWG #6 multistrand wire. The controller reads about 18-22 amps input in the middle of a sunny summer day.

ProStar PS-30M, 30 amp 12/24 volt charge controller

Prosine PS-2500/12, 2500 Watt Inverter, 12 Volt, 120 VAC Output

4 Surette S-460 6 Volt, 460 amps @ 20 Hr Rater batteries (wired in series-parallel configuration for 12 V).

The batteries, inverter and charge controller all reside in an unheated cellar below the cabin, though the battery bank is well insulated.

We've had absolutely no problems with the system over the years. Only routine maintenance has been checking and filling the batteries and adjusting the tilt on the panels 4 to 6 times per year.

Potential options on the battery side:

(1) Double the number of batteries to 8, which in this series-parallel configuration would result in 24V to the inverter. Unfortunately, I believe this inverter is configured for 12V only. So I would have to wire the second four batteries in series-parallel for 12V and then wire these two battery banks (4 batteries each) in parallel to provide 12V to the in inverter.

(2) Add 4 more batteries and buy a new 24V inverter. Any major advantages here considering the cost?

Potential options on the charging side:

(1) Purchase a MPPT controller that may result in more efficient charging.

(2) Purchase an additional solar panel. I've got room for one more on the mount.

There may be other options as well and I'm sure open to any advice. Part of the issue is that our old Paloma on-demand water heater froze up and it looks like it is unrepairable. Most of the new ones seem to require AC, though not a lot (less than 2 amps when heating water, much lower when in standby). So any thoughts about non-electric propane on-demand water heaters would also be welcome. And finally if anyone has experience with efficient ceiling fans I'd also like to hear about it.

Thanks,
Karl
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Upgrading cabin solar system
    Our electricity demands have since increased

    Absolutely no one is surprised about that! It's as inevitable as death and taxes. :p
    designed to charge the batteries between visits and then run off the batteries when we are there

    That was a mistake. It should be designed to recharge daily, if at all possible (i.e. good sun).
    Right now you've got 330 Watts of panel trying to charge 920 Amp hours of battery (unless I read something wrong, which happens). If you've been using this long, those very good and expensive Surette batteries are rapidly becoming scrap metal. This is not a problem that shows up right away; this is the evil sulphation destruction that doesn't show up until it's too late. Vis:

    330 Watts @ 80% efficiency = 264 Watts/14.2 Volts = 18.5 Amp hours. No place near the 5% minimum of 46 Amps that size battery bank would require.
    I'd like to add battery capacity

    If you add battery you must add panel. Right now you are so severely under-paneled I'd be surprised if there's any life left in those batteries. They've been chronicly under-charged for seven years. They'd probably last twenty, btw; Surette makes good batteries.

    My advice, for whatever little it's worth:

    1). Check the Specific Gravity on each battery. It should be consistent and it should be above 1.250. I very much doubt it will be either, even after a good full charge.
    2). Buy panels. If you need that much battery capacity, you need about 1600 Watts of panel. You'll also need two charge controllers, as no one will handle 92 Amps @ 12 Volts.
    3). Re-calculate all your loads and see where this massive amount of power is going to. If it's all being used as "days of capacity" instead of being replaced daily, you may not need such a huge battery bank. That would be a good thing, as you're going to have to replace it.

    Sorry for the bad news. But you're not alone; lots of people have done exactly this same thing - too much battery/too little panel.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    Re: Upgrading cabin solar system

    92a as coot suggested may be going too far as these don't require the 10% charge rate, but he is right that they are undercharging and this is destroying them through sulfation. your current rate of charge is at 2% and is not good for charging batteries. others that have opted for a slower charge usually go in the charge percentage range of 3-5% with 3% pushing things as there isn't any vigorous stirring of the electrolyte that normally occurs when given a good charge. we normally ask that people bulk charge batteries in the 5%-13% charge range, but yours is a circumstance that differs from those that use and charge daily. they do need a good vigorous bulk charge, but before we go on here have you ever checked the electrolyte levels? are the plates showing at all? if the plates are showing the batteries won't last too much longer even if you refill them with distilled water and give them a good bulk charge.
    in any case you do need more pvs. if you add more battery capacity than you presently have with all new batteries then the pvs need to be increased to match and try to get the charge rate at least in the 4%-5% range. for 920ah of batteries that is equal to 36.8a to 46a. if you had the more efficient agm batteries then i'd say ok to going possibly as low as 3%.
    you may not gain that much with the use of an mppt controller with such infrequent use, but when they do bulk charge they allow for a better efficiency in charging that may benefit you. that will be your call to upgrade the controller.
    another problem area you may have is the wire. with that distance at 18.9a of rated pv you have about a 1.14v drop for 9.5% on that voltage drop. it should be lower than 3% and some recommend 2%. adding more pv will increase this voltage drop even worse. either get very large wires like #0 or shorten the run length to accommodate the wire gauge you use. there is a calculator you can use here on the forum.
    http://forum.solar-electric.com/showthread.php?t=29
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  • System2
    System2 Posts: 6,290 admin
    #4
    Re: Upgrading cabin solar system

    Hmmm. I had no idea this system was so out of balance. When I originally purchased this (entire) system from NAWS, they recommended just two panels. I later installed the third panel.

    Electrolyte levels have always been pretty good. I don't recall ever seeing the plates exposed. It's been a few years since I measured SG, but I'll do that next time we're there.

    The only way to shorten the run from the panels to the controller/batteries/inverter is to move the later up to a closet on the main floor. Unfortunately, I will then have to compensate with a long 12V run to the water pump.

    I'll have to give this some serious thought. Thanks for the input.

    Karl
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #5
    Re: Upgrading cabin solar system

    a side benefit to some mppt controllers is that you may run the pvs in series to be converted by the controller back to 12v for the batteries. not all mppt controllers will downconvert and some may allow for 24v to 12v only. this allows a smaller current with that higher voltage and may facilitate wires of a smaller gauge. again this is quite allot to invest for your modest system and would be up to you how you may wish to handle it.
    i'm surprised naws recommended only 2 pvs for those batteries. in guessing, it may have been a salesman who was inexperienced or short lived with the company.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Upgrading cabin solar system

    A few random but associated comments. :p

    Thinking has changed over time. The "put back the Amp hours over time" attitude used to be prevalent, but has been replaced with the "charge 'em back up fast and strong" concept as that give you longer battery life. It isn't that the first method doesn't work, it's just that getting 12 years out of a set of batteries instead of six is nice when you consider how expensive they are.

    I always say "shoot for 10%" because you probably won't get it. Efficiency losses will probably eat that down to 7-8%, and loads going out while charge is coming in drops the effective charge rate as well. So shoot for 10% and you may get a realistic 5% peak charge rate.

    I did mention re-examining the loads, didn't I? (already forgot). Maybe you don't need such a big battery bank, in which case maybe you don't need 1600 Watts of panels. You could be better off with a smaller bank being well and fully charged every day plus a bit of load management to make use of the sunlight you otherwise wouldn't be 'harvesting' once the batteries are full.

    Niel's quite right (as usual) about your potential benefit from an MPPT controller. You've got quite a long run of wire there. It's a cost/benefit issue: more Voltage & MPPT or different wire? Your call.

    One problem you'll have is not being able to get matching panels for your old ones, so it may be necessary to "bite the bullet" and go with a higher Voltage array & MPPT controller.

    Specific Gravity is your friend; check it often. :p
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  • Panamapat
    Panamapat Registered Users Posts: 22 ✭✭
    #7
    Re: Upgrading cabin solar system
    BlueSky79 wrote: »
    Hello all,


    Potential options on the charging side:

    (1) Purchase a MPPT controller that may result in more efficient charging.

    (2) Purchase an additional solar panel. I've got room for one more on the mount.

    There may be other options as well and I'm sure open to any advice. Part of the issue is that our old Paloma on-demand water heater froze up and it looks like it is unrepairable. Most of the new ones seem to require AC, though not a lot (less than 2 amps when heating water, much lower when in standby). So any thoughts about non-electric propane on-demand water heaters would also be welcome. And finally if anyone has experience with efficient ceiling fans I'd also like to hear about it.

    Thanks,
    Karl

    Hi Karl;

    I've got a remote cabin as well for weekend use and a similar configuration. A couple of thoughts in response to your post:

    a) On the load side of the equation, you mentioned water heating requiring AC. I've had success with an imported propane powered unit that runs on 2 D size batteries and is quite inexpensive. I have not found the exact version of it here in the USA, but there are similar ones to it at these links:

    http://www.eztankless.com/

    Very similar to the one I'm using at our remote cabin... except the one we have is a 9 liter per minute unit.

    or these units:

    http://stores.gonetankless.com

    Mine has built in freeze protection (automatically fires up below certain temp), but its a feature I don't use as I drain the system in the winter.

    B) on the PV side, you might find that an your incremental solution adding another PV panel and an MPPT controller might just do the trick. If you do get an MPPT, you can wire the 4 panels for 24 volt and reduce losses thru the wiring from panels back to controller. Also the MPPT controller at 10k feet of elevation may work out quite well with the panels running cooler and the MPPT controller able to deliver more juice from the higher resulting panel voltage. I put in a Rogue MPT-3024 and have been thrilled with the units performance. The combination of the two might give you a 35-45% boost in solar output (4x110/3x110 * 0-10% MPPT boost)

    Lastly you might want to consider adding a batter monitor like a Bogart Trimetric. It has been very helpful in keeping track of consumption, charging and relative battery charge levels.

    Good luck to you.

    Pat

    btw: where is your cabin? ours is at near 10k elevation as well in the Gunnison National Forest of Colorado
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