Designing a system for a recreational building

Re: Designing a system for a recreational building

ferret wrote: »

Thank you for your reply!
The building and everything in it will be newly constructed/purchased, so there isn't a way to log power usage. I understand how to do the math on usage. I know the fridge will be the biggest draw. Minimum daily pull will be around 1000-1300 watts (mainly because of the fridge), with maximum pull being around 4000 on days we are using the building for a few hours of entertainment.

How much money do you want to spend--A crass question--But it does really help us to focus where to best put your limited resources. $5,000-$10,000 for such a system (as described) would not be out of the question. I can bury a lot of wire and or buy a $1,000 Honda eu2000i and run it on ~1-2 gallons of fuel per day when the power is needed.

I probably should have clarified more about the location. We have a 20+ acre property. It really isn't feasible to run wires from the house to the location on the pond where this building will be.

It is OK--We do listen, but most of us here are terribly "cheap" and don't like to waste other people's money either.

We want the ability to grow the system if our needs increase, so we would rather go big first on items like the inverter than have to get a bigger one in another year.

For many different reasons, it is actually quite difficult/expensive to "grow" an off grid solar power system. It is like buying a small car, adding a larger gas tank and engine, then bigger brakes, a trailer hitch, etc. and turning it eventually into a semi-tractor/trailer.

You can "adjust" a system size (add to it) by about a factor of 2x -- But beyond that, many times you have to rip out much of the existing equipment/wiring and start over.

Right now, my energy use expectations are something like this on a typical week:
Monday-Friday: Max 1500/day (fridge, landscape lighting)
Saturday & Sunday - Max 4000/day (any combo of: fridge, landscape lighting, tv/dish, fan, speakers, interior lighting)

That is pretty much the power needed to run a full off grid (very energy efficient) home (using propane, etc. for heating/cooking/etc.).

I see right there that 4 250w 24v mono panels won't meet my daily needs... which are basically for the fridge. The good news is that this building will see more summer seasonal use, so I don't have to account as much for winter daylight loss. Are there any efficient refrigerator models out there that would help to drop my daily needs? Again, the fridge/freezer is really more for drinks and snacks. Technically we could turn it off during the week if needed...

Refrigerators are typically the largest single load most people will drop on a "smaller" off grid power system. And they are highly variable in energy usage. In summer, running in a 90F environment, they use much more power. Making ice, adding more food/drink, opening the door, etc. all adds to the loads. Letting stuff sit around "warm", then turn on for the weekend--Refrigerator uses ~2x its rated power for the next 24 hours to cool everything down.

Engineering design is a "cruel" profession. All these "nice things" you want are scuttled by the engineer's derating everything for reliable/safe operation in good times and bad (sunny/dark/hot/cold/kids playing around system/etc.).

Batteries are just about as cruel... The demand respect and proper charging/discharging. You under charge, they die. You over charge, they die. You forget to add water for a couple months, they die. You use well water to fill batteries, the die. You have lose/dirty connections, they die. A battery "dies" for randome reasons, and it kills nearby/connected batteries.

Get the theme?

And off grid system will require a certain amount of daily/weekly/monthly maintenance. The battery banks are pretty unforgiving. A genset--You change the oil every 50-100 hours (for small genset, no oil filter), and provide fresh fuel (and/or add fuel stabilizer) and away you go.

Just to give you a "nominal" system design... You could possible get away with a 1/2 size array and battery bank (a bit more maintenance/monitoring, a bit less capable and less money up front--Your choice). A quick design based on "nominal" design (at least give you the information to price out and size installation).

• 4,000 WH per day * 1/0.85 inverter eff * 1/24 volt battery bank * 2 days storage * 1/0.50 max battery discharge = 784 AH @ 24 volt battery bank

Just to give you an idea, that is ~24 x 6 volt @ 220 AH "golf cart" batteries (not that I would tell you to use GC batteries--This is a pretty good size battery bank). You are right on the cusp of needing a 48 volt battery bank. I.e., if you want a larger system in the future, you should pick 48 volts right now so you can keep the AC inverter, DC backup charger, get the correct solar charge controllers, etc.

Then sizing the array based on the capacity of the battery bank 5% to 13% rate of charge--10% rate of charge makes for a pretty reliable/easier to manage system (big battery banks need big solar arrays even if the loads are light):

• 784 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,952 Watt array nominal

And we need to double check against the amount of sun you get for your area... Using PV Watts for Athens Georgia, fixed array, tilted to 34 degrees from horizontal:

Month    Solar Radiation (kWh/m 2/day)
1      3.69     
2      4.77     
3      5.40     
4      5.82     
5      5.70     
6      5.69     
7      5.69     
8      5.61     
9      5.31     
10      5.51     
11      4.34     
12      3.80     
Year      5.11

Looks like 4 hours of sun will cover you for most of the year (looks like summers are hazy around Athens?).

• 4,000 WH per day * 1/0.52 overall system eff * 1/4 hours of sun per day = 1,923 Watt array

So a 1,923 to 2,952 Watt array would be "justified" for this system.

For the charge controller--If you choose a MPPT type controller (more expensive, but recommended for larger systems):

• 2,952 Watts array * 0.77 panel+controller derating * 1/29 volts charging = 78 Amps

That would "fit" one ~80 amp MPPT charge controller (~$600).

Need an AC inverter--Probably a minimum of 2,000 Watt. Too large of AC inverter can waste power. Besides the refrigerator (which needs around a 1,200-1,500 watt minimum AC inverter for a modern Energy Star full size refrigerator), you will need to add up the power for the lights/stereo/etc. A 4,000 Watt inverter would be on the large size for this system (and battery bank).

An off grid system should have a means of backup power for charging during bad weather--A genset would be in the range of:

• 784 AH * 0.13 rate of charge * 29 volts charging * 1/0.80 charger eff * 1/0.67 charger Power factor * 1/0.80 genset derating = 6,893 VA rated genset

That is not a small generator and will need significant amounts of fuel to run it (call it ~1-2 gallons of gasoline per hour).

All the above is not trying to scare you off--But showing what a true "nominal design" off grid power system would look like. I could see cutting the battery bank/generator by 1/2 -- And you could go with the smaller array. Batteries will last ~3-7 years typically (if used or not).

We try to make the system support your needs. If a system does not (misses by "this much")--Then it fails. Of course, your system is (in general) not a critical use system--But if it dies at 9pm on a beautiful summer evening--What good was it?

-Bill