Im new, have a few questions about designing a new system that need answers please.

Re: Im new, have a few questions about designing a new system that need answers please.

Welcome to the forum Cliff.

Yep--It does come down to loads. That is the reason for the off grid power system--If the system does not power your loads, then it is of little use.

And an over designed/expensive system costs more to maintain (batteries still age and fail) and, if in the middle of no-where, then theft is a real concern too.

Your loads--They are problematic. High efficiency lighting is a great start--Look at LED... CFL's tend to take a while to start/warm up in cold climates (as you probably know better than I here in California).

The 1 hp pump is the problem. It takes a lot of energy to start/run -- But for a cabin, it does not run long and therefore you need a large battery bank+inverter to run the pump, but the bank may be way over-sized for your needs otherwise (a 100 watts of power to run lights, charge cell phones, run a laptop, etc. a few hours per day).

My first point to address would be the water pump. Options include getting a new pump that runs from AC (or DC) that is more efficient/smaller. Most people do not really need a 1 HP pump running hours a day unless a very deep well, irrigation systems etc. But these "solar friendly pumps" are not cheap either.

An alternative would be to use a simple/cheap genset to pump water to a cistern, then use a 12 or 24 VDC pump to pressurize the cabin plumbing (aka like an RV pump). Can run cold water and even a small RV water heater, or tankless water heater. Does not take much of a battery bank to run this kind of pump.

Alternatives--The almost mythical/magical LiFePO4 battery (getting where I can touch type LiFePO4 pretty well now ) could be in interesting solution. Lots of current to run the well pump + big inverter (perhaps even a big/cheap MSW inverter). But still smallish AH capacity (enough to run small loads) and light enough that you can haul home (if theft is an issue--LiFePO4 batteries are not cheap). The only other major draw back for the LiFePO4 (got to keep in practice typing that), is that they don't work well under freezing--So you have to keep it "warm" in the cabin, or at least a well insulated box.

So--What would a 4x 6 volt @ 215 AH battery bank system look like....

First, the "average power" would be 1-2 days of storage (backup genset) and 50% maximum discharge (for long life):

• 24 volt * 215 AH * 1/2 days of storage * 0.50 max discharge * 0.85 AC inverter eff = 1,097 WH per day average

That is a ~1kWH per day system for a cabin--You should have plenty of power for your daily needs--Assuming the water pump runs for 10's of minutes per day (no irrigation, etc.).

Then, there are two calculations need to keep the batteries happy. First is 5% to 13% rate of charge for your battery bank. For a weekend cabin, you can probably get away with 5% rate of charge--But 10% to 13% rate of charge is nice, especially now that solar panel pricing is historically cheap.

• 215 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 405 Watt array minimum
• 215 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 810 Watt array nominal
• 215 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,053 Watt array cost effective maximum

Next, you need to size the array based on the amount of power you need and amount of sun you will be receiving. Using PV Watts for a fixed array tilted to 45 degrees, in Houghton MI:

Month    Solar Radiation (kWh/m 2/day)
1      2.56     
2      3.70     
3      4.68     
4      5.72     
5      5.25     
6      5.56     
7      5.58     
8      5.38     
9      4.48     
10      3.20     
11      2.09     
12      2.11     
Year      4.19

For winter, mounting panels vertical with room for snow to slide off (90 degrees):

Month    Solar Radiation (kWh/m 2/day)
1      2.56     
2      3.70      
10      3.20     
11      2.09     
12      2.11     
Year      4.19

If you assume 1,097 WH per day average loads, and 4.0 hours minimum sun (mid Feb to mid October):

• 1,097 WH * 1/0.52 system end to end eff * 1/4.0 hours of sun = 527 Watt array minimum

If you wanted that amount of power in deep winter with 2.0 hours of sun per day:

• 1,097 WH * 1/0.52 system end to end eff * 1/2.0 hours of sun = 1,055 Watt array minimum

So, somewhere between a 527 Watt and a 1,055 Watt array would make sense for your system. You would always start with the smaller array and add more panels later if needed.

And what would a 24 volt @ 215 AH battery bank power?

• 24 volts * 215 AH * 0.85 inverter eff * C/20 Hour rate = 219 Watt "average load" 5 hours per night
• 24 volts * 215 AH * 0.85 inverter eff * C/8 Hour rate = 548 Watt maximum continuous rated load
• 24 volts * 215 AH * 0.85 inverter eff * C/5 Hour rate = 887 Watt short term maximum load (hour or so)
• 24 volts * 215 AH * 0.85 inverter eff * C/2.5 Hour rate = 1,754 Watt surge load (seconds to minutes starting well pump, etc.)

So--It appears it would be possible to run your well pump from this bank--But it is close. In cold weather (sub 50F battery bank temperature), you might have problems plumping with the 1 HP pump.

Your thoughts?

-Bill

Re: Im new, have a few questions about designing a new system that need answers please.

The more complex the system, the more wild and wonderful ways the system can fail.

Off grid solar is difficult to leave it running unattended. You have the choice of automating health status via Internet (i.e., simply report state of charge, battery voltage, etc.) or automate recovery (panels fail, poor sun, etc.) and have an AGS on a genset.

Then, if the genset fails to start... What then, a second genset + AGS? And generators are known to catch fire, crank until the starting battery dies, etc.

I am certainly on the side the keep is simple. I.e., it is cheaper to bring food back/toss a bit of excess food than to create a power system that can keep the food in the freezer cold for months at a time. And save the battery bank from getting over discharge in poor weather/during a genset failure.

Simple monitoring may be a good option if the place is close enough to visit to address problems or there is a neighbor near by to prevent damaging your battery bank (i.e., the price of a killing the battery bank vs the cost of wasted food).

Not right or wrong answers on off grid/backup power--Power usage is a highly personal set of choices... Those solutions have to make sense to you.

-Bill

Re: Im new, have a few questions about designing a new system that need answers please.

What size array/system (I gave you a range of solar panels and a rough amount of power it would supply based on one string of batteries--You were asking about 2 strings too).

Will you have a backup genset? How much energy use per day winter vs summer, etc... 1/2 HP well pump? etc...

Basically, you will want to do these as paper designs first and run out the pricing and capabilities vs your needs.

In general, for a cabin that is used seasonally, a few days at a time--You probably would want to keep the system small (1 string battery bank), 1/2 HP or less water pump, size the array to run your power needs in the summer, and use a small genset to bump the batteries with a couple of hours of charging ever day or two in winter... (something like a Honda eu2000i or similar ~2kWatt Yamaha inverter-generator--Quiet, light weight (take come for security), and pretty fuel efficient--A gallon of fuel would last you several days).

Again--What makes most sense to you (minimum solar array, genset run time+fuel; or lots of solar panels and not very much run time regarding winter use). Will you tilt the array vertical in winter to help them shed snow?

-Bill