Newbie! Replacing an old system: can someone check what I got so far?

Re: Newbie! Replacing an old system: can someone check what I got so far?

Welcome to the forum Bob!

Wow--What a nice cabin system. My first suggestion is conservation... It is usually better to use less electricity than to build out a larger system. The one standout for me is the Well Pump... Your system is probably fine for the well pump (check below), but if you can use a smaller/lower power pump (less starting surge, possibly more efficient--Even possibly pumping to cistern+small DC pressure pump (use the large pump to bring up a day or few days of water, small pump for sinks/shower/etc.). We can talk more about the pumping issues/options if you wish.

And your system (looking at the numbers, not worrying about the equipment at this point):

I have replaced the old batteries with 16 new 6 volt golf cart batteries (220AH=880AH total???) wired in series/parallel to make 24 volts.

Golf cart batteries can be a good idea for beginning systems. Pretty rugged and not to expensive--especially if something goes bad (hardware failure, kids leave lights/well pump on over winter, etc.).

Make sure you have them wired something along the lines of one of these configurations. Also, I am not a fan of too many parallel battery strings... One string is ideal (in my humble opinion), two to three parallel strings can be OK. With more parallel strings, you have lots of cells to check water levels, making sure the current is properly shared across strings, you should have a series fuse per string to prevent a short circuit from being feed by other battery strings. Getting an inexpensive DC Current Clamp Multi Meter (like this one from Sears) can be very helpful to make sure your battery strings are working correctly.

The battery bank is the "heart" of your system... It will define the capabilities for power supply/surge current/etc. We like to use 2 days of storage and 50% maximum discharge for off grid power systems. Usually pretty close to optimum for battery costs/life/etc...

880 AH * 24 Volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 maximum discharge = 4,488 WH per day (basically 2 days of no sun)

That is usually more than enough power for supply a reasonably efficient full time off grid home (refrigerator, lights, well pump, washing machine, TV, laptop computer, etc.).

And power from such a battery bank:

880 AH * 24 volts * 0.85 inverter eff * 1/20 hour discharge = 898 Watts average (~5 hours a night, for two nights)
880 AH * 24 volts * 0.85 inverter eff * 1/8 hour discharge = 2,244 Watts maximum continuous load (less than 4 hours for 50% discharge)
880 AH * 24 volts * 0.85 inverter eff * 1/5 hour discharge = 3,590 Watts maximum short term load
880 AH * 24 volts * 0.85 inverter eff * 1/2.5 hour discharge = 7.181 Watts maximum surge (starting well pump, etc.)

So, looking at a 4 kW AC inverter and a 1.5 HP well pump--Looks like a pretty well balanced design.

Next, look at the amount of solar array needed to support your battery bank. We recommend 5% to 13% or so of the battery bank's capacity for charging. 5% can work fine for a weekend place--10%+ for a daily use/full time off grid home occupied 9 months or more per year:

880 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,657 Watt array minimum
880 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 3,314 Watt array nominal
880 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 4,309 Watt array "cost effective" maximum

You have:

12 Kyocera 180KW 24 volt panels

I assume that is 180 Watt solar panel (12*180W=) 2,160 Watt array

With a slightly smaller array, you might have to pay a bit more attention to your loads--Especially if you have day time loads (i.e., day time loads use current that would otherwise go to recharging the batteries). Should not be an issue during weekends/summer use--But full time, you might want to look at 3.3 kWatts or more of panels.

Using PV Watts for Bakersfield California, fixed array, tilted to 35 degrees from horizontal:




Month
Solar Radiation
(kWh/m²/day)


1
3.47


2
4.64


3
5.63


4
6.44


5
6.78


6
7.31


7
7.28


8
7.30


9
6.90


10
6.19


11
4.67


12
3.21


Year
5.82




If we toss the bottom 3 months and use November as the "break even" month for generator usage:

2,160 Watt array * 0.52 end to end system eff * 4.67 Hours (long term average) of sun = 5,245 Watts*Hour = 5.2 kWH per day

That is a very good sized system.

How does that line up with your thoughts about the system and your needs?

-Bill

Re: Newbie! Replacing an old system: can someone check what I got so far?

bobertcom wrote: »

Thanks for the reply bill! I lurked on this forum for the last year figuring out my system, which looks to have paid off.

Glad that we were of help. Let us know how the system works out for you and any suggestions you may have for the next person.

You pretty much nailed it. Even down to my concerns about our booster pump.

As it stands now, we have a large water tank (8 foot tall by 10 feet wide) so I run the generator to fill it and lighten the inverter/battery load. Since its so large, I have only had to do that a few times since we bought the cabin. I am going to research the well pump more for more specs next time I'm up there.

The booster pump is another story. It gobbles up a bunch of current and is on the list to be switched out to something more efficient. However, the pressure tank is bad, so that gets priority.

Are you talking about a second booster pump that you have now that is pressurizing at (going bad) pressure tank? There are some DC pumps (intended for RVs and Marine use) that use a whole bunch less power than most AC booster pumps. They certainly do not last as long (diaphragm pumps, bushed motors), but they can last a few years between service.

Some others have taken a high end submersible, like a Grundfos pump (scary expensive) and used those as booster pumps. They work extremely well at that and are very solar power friendly (efficient, almost no surge current, ~900 watts maximum load, etc.).

Read/research about pumps. There are lots of issues with them (lift pumps with inlet screens/filters that get plugged and kill the "lift" of a pump, cavitation on pumps that draw/lift water to pump. Pumps that leak and flood the pit/basement for outside of tank below water level pumps, submersable pumps that leak internally, etc.). They can be a nightmare.

Question about PV panel angle: Is 35 degrees a must? is there any advantage to having less an angle? I ask because I will be building the PV racking off the roof next time I'm up there and am trying to plan it out...

Look at where your power needs are at... If it is for A/C in the summer, tilting back panel 15 degrees can give you more summer harvest. If you have snow, then a panel near vertical can shed snow and even pick up sun reflections from the snow field in front.

Quite a few folks have made adjustable mounts and plan on changing tilt 4+ times per year. Most end up changing till 2x a year in the end (too much work for little extra power gain).

There are 1/2 axis tracking mounts... Cost of those mounts can exceed just buying more panels. And there is the concept of "Virtual Tracking". Buying extra panels (and cheaper fixed mounts). Face 1/2 array southeast, and 1/2 array southwest. Gives you more hour of sun per day to charge your battery bank. And your battery bank really can use more hours per day of steady charging current vs a high peak current in the middle of the day (especially into winter).

Play with a couple of solar insolation programs and see what looks interesting to you. A couple "simple ones":

http://rredc.nrel.gov/solar/calculators/pvwatts/version1/
http://solarelectricityhandbook.com/solar-irradiance.html

Also, is replacing that old 12 volt wind generator worth the trouble/cost? Is there a way to use that old 12 volt wind generator on my 24 volt system if that wind generator works? (I'm pretty sure the answer is "no")

Wind turbines.... I am not the guy to ask. Personally, I think wind turbines (especially small wind turbines frequently used on homes/cabins) are hardly worth the cost and effort. If you have good winds (i.e., it is uncomfortably windy outside during much of the winter/winter nights), then maybe. If you already have a 60+ foot tower with good siting (and 30 feet minimum above obstructions/trees/buildings/etc. within ~500+ of tower), perhaps upgrading the turbine would be "interesting". The turbines tend to be the "less expensive" part of the overall wind system costs.

We have one person, Ben from Pika, that has posted here a few times. You might talk with them about their system if you are not going to go with the DIY design and build.

PIKA 1800 watt

I don't know anything about their system--Other than it looks like they have put a lot of effort into the design (and several methods of over speed control--Yea!).

Can you make your 12 volt turbine charge your 24 volt battery bank... Yep, there are a few ways. Depending on the turbine, they may be able to re-wind the stator for a higher voltage. You could charge a 12 volt battery bank and use a voltage doubler (switching power supply) to charge the 24 volt battery bank and such... Is it worth it. Uh... Don't know. Do the trees "flag" from prevailing winds in your area? Do you have hours of >10-12 MPH wind in the winter/periods of low sun? How will you service the turbine (rent lift/crane? Tilt up tower+donkey?).

-Bill