Design Help Needed: Small Shed PV System

stoneunhenged

I'm currently working on a self-sufficiency project to grow tilapia in a small shed that is located in pasture on a farm. I've grown tilapia before with the benefit of grid power, but this shed is off the grid. I have six slightly used 175w Solar World PV panels that I scrounged from this parking lot where they are being taken down for redevelopment.

Attachment not found.

The tilapia shed will be 16' x 12' with a roof sloping at 35° (optimum angle for our latitude). The system load is created by the need to drive two air pumps for the fish tanks. Each pump uses 15w/0.3A/120v. The pumps will need to operate 24 hours a day. There will also be an efficient overhead light and an outlet that I may use from time to time for things like small power tools.

I would like your recommendations on the optimum configuration to use the six panels for the intended application. Obviously, I need an inverter and batteries, but I'd like to know your specific recommendations on these as well as other components.

Thanks. I begin building the shed these weekend so I'll give you updates as the project progresses.

vtmaps

Re: Design Help Needed: Small Shed PV System

stoneunhenged wrote: »

Each pump uses 15w/0.3A/120v.

wow, those pumps have incredibly low power factor... 36 VA but only 15 watts. Fortunately, in this system it won't hurt you too badly because battery storage needs are based on watts, whereas inverter size is based on VA. In your case the peak loads are low enough that almost any inverter (perhaps MS 300) will be able to handle it.

Your pumps will draw 720 wattHours per day, and your peak loads are small enough (except perhaps, the occasional power tools) that you can use a 12 volt system and the venerable MS 300 inverter. Your need for power tools may have a great effect on your system design. Also, how many days autonomy do you need for cloudy weather? Are you willing to use a small generator (with battery charger) for prolonged cloudy weather? If so, that will save you quite a bit in your system design.

--vtMaps

mike95490

Re: Design Help Needed: Small Shed PV System

I'd consider a fish farm to be a critical load, and would plan for 3 days of foul weather, but start the genset (away from the fresh air intake) whenever it looks like 2 days.

As to load, inverters have to provide VA, and supply it via watts from the batteries, so if I use the 36VA x 2 = 72w load, x 24 hr * that gives me a
1728wh load - daily. At 12V, that's 144ah, just a bit over what 2, 220ah 6V batteries in series could supply. That would be a pretty deep cycle, so I'd say you need 6 of those batteries. For a low current load, could 3 parallel strings manage it - maybe.

I'd run 2, MS sure sine300 inverters, one for the critical pumps, and the other for other loads. and if one fails, you have a spare all set !

*
So, I'm just guessing about the VA - Watts the inverter has to have the overhead to supply the VA, but is that entire amount feed from the batteries, I'm guessing yes, because it always seems to work dis-favorably for me. My inverter records a 1,000 w load for my 1/2 hp pump. I'm pretty sure that is the PF nailing me, and those watts are really coming out of my battery.


Charging : 6, 175w panels. Not knowing the volt/amps of them, I'll just play with watts. (1050w)
a bank of 660ah @ 12V would want to charge at 60 amps if it was low. 1050 w @ 13V = 80 amps, so that looks fine there. You seldom would see 80A from the array at dawn, so I think a 60A controller would be fine. But I'd use a MPPT and wire the panels up to feed HV DC to the controller, and let it down convert to battery voltage. Something like the Midnight Classic, or other high end MPPT would do it. Factor in the cost of any needed controllers, meters, panels etc, and the price of the Classic is right there.

Cariboocoot

Re: Design Help Needed: Small Shed PV System

Your Solar World 175 Watt panels are "24 Volt" panels with a Vmp of 35.7, so there's a problem using them for a 12 Volt system; requires an MPPT charge controller.

1050 Watt array is capable of about 2kW hours AC daily (on 4 hours good sun). You've got a 72 Watt load running 24 hours or 1728 Watt hours. Not much insurance against clouds.

You could recharge a 673 Amp hour 12 Volt bank with that, but you'd need a big MPPT controller to handle the current potential (FM80 for example). That would give you (at 25% DOD) roughly 1800 Watt hours AC, so that all works.

But maybe you should be looking for more efficient pumps.

vtmaps

Re: Design Help Needed: Small Shed PV System

Cariboocoot wrote: »

You've got a 72 Watt load running 24 hours or 1728 Watt hours.

Caraboocoot, I think he has a 30 watt load running 24 hrs a day. His 30 watts is 72 VA, but as far as the battery is concerned he has a 720 wattHour per day load. Am I confused about this? --vtMaps

Cariboocoot

Re: Design Help Needed: Small Shed PV System

vtmaps wrote: »

Caraboocoot, I think he has a 30 watt load running 24 hrs a day. His 30 watts is 72 VA, but as far as the battery is concerned he has a 720 wattHour per day load. Am I confused about this? --vtMaps

Well, one of us is. Since I have a PhD in Confusion it's probably me.

At any rate, it looks okay for supplying the high-end of the power requirements with the right equipment.
But those pumps aren't great. I wonder what makes their PF so bad?

stoneunhenged

Re: Design Help Needed: Small Shed PV System

Thanks very much for your responses so far.

It is possible to buy a 24v DC air pump for this application. See:

http://www.alita.com/airpump/pi_020dc_en.php

Do you think this is an option I should consider? If so, specifically what equipment would I need to build the 24v system?

And just to give you a sense of what I'm talking about, here's one of the two tilapia tanks that will be in the shed.

Attachment not found.

Cariboocoot

Re: Design Help Needed: Small Shed PV System

Buying 24 Volt pumps would simplify a lot: no inverter & its consumption, no need for an MPPT controller to down-convert from panel to system Voltage (use a cheaper PWM type instead), no conversion losses. At about 1 Amp, each pump would use 24 Amp hours per day. That's not a lot of power to have to produce. A couple hundred Amp hours would be two days' worth.

Before you jump on it, check with the pump maker about the Voltage range, as fully charging a 24 Volt battery bank requires upwards of 30 Volts and the pumps may not like that.

Oh and what else will you be running from this? Lights? Fans? Remote video monitoring system?

stoneunhenged

Re: Design Help Needed: Small Shed PV System

Okay. So please walk me through this. Assume I go with the 24v air pumps. Which specific controller would you buy and which batteries? I'm not looking to go cheapest route. I'm looking for maximum reliability. Thanks.

BB.

Re: Design Help Needed: Small Shed PV System

Will there be somebody there at least once per day? Will there be a generator/backup power source? Will the generator have autostart? Will there be "something" to connect an alarm too (cell phone, radio transmitter, phone line?).

-Bill

stoneunhenged

Re: Design Help Needed: Small Shed PV System

There will generally be someone to check the shed each day. I could install an auto-start generator if you the system design demands it. I could install an alarm system although I would prefer to avoid the cost.

BB.

Re: Design Help Needed: Small Shed PV System

I forgot--Are you near Mobile Alabama?

Anyway, using PV Watts for Mobile AL (although, you may be much farther north at 35 degrees)... Using a fixed mounted array at 30 degrees (for Mobile), we get the "hours of sun" per day by month as:

Month Solar Radiation (kWh/m 2/day)
1 3.79
2 4.56
3 5.06
4 5.48
5 5.41
6 5.31
7 5.11
8 5.15
9 4.96
10 5.31
11 4.04
12 4.05
Year 4.85

So, the minimum sun is in January at an average of 3.79 hours per day.

Regarding the air pump--I generally like AC powered devices (tend to be cheaper/more reliable). However if the 24 VDC one is efficient and reliable--why not.

To calculate the "typical worst case efficiency" of your system:

• 0.77 panel+charger derating * 0.80 flooded cell battery eff = 0.62 end to end efficiency (DC loads)
• 0.77 panel+charger derating * 0.80 flooded cell battery eff * 0.85 inverter eff = 0.52 end to end efficiency (AC loads)

I will use the 0.52 for AC loads--But you can substitute your own numbers/configuration (put in a spread sheet and you can play around with the numbers/costs):

• 30 Watts AC load * 24 hours * 1/3.79 hours sun per day * 1/0.52 system eff = 365 watts of solar array minimum

Battery bank wise, assume 1-3 days of "no sun" and 50% maximum discharge (for longer life). Use 3 days for your system (more battery backup--will require a larger solar array to keep larger bank properly charged):

• 30 watts * 24 hours per day * 1/12 volt system * 1/0.85 inverter eff * 3 days no sun * 1/0.50 max discharge = 423 AH @ 12 volt battery bank

Note, I chose a 12 volt system because MorningStar makes a killer 12 volt 300 watt TSW inverter--You would be hard pressed to find a better match in 24 volt or 48 volt systems (you would need a larger inverter which has more losses when driving very light loads).

Next, we recommend 5% to 13% rate of charge--With 10% being a nice sized array (good charging current, quicker recharging vs 5% array). So, basing the array size on battery bank AH capacity (larger banks usually need larger arrays for proper/recommended charging current):

• 423 AH * 14.5v batt charge * 1/0.77 * 0.10 rate of charge = 797 Watt solar array

Note, the minimum array (based on my guesses) to power your loads is ~365 watts. However, if we go with a "very large" battery bank, the array should be around 2+x larger. If you can live with a smaller bank, then you can downsize the array.

You are looking at charging current of:

• 797 watt array * 0.77 solar derating * 1/14.5 volts = 42 Amps "nominal"

So, you are looking at a 45 amp minimum solar charge controller. MorningStar makes a nice MPPT 45 amp solar charge controller. And quite a few companies make 60+ amp MPPT charge controllers (Midnite, Outback, MorningStar, etc.).

For a backup genset, I think a Honda eu2000i (or Yamaha 2400i type) would be a good fit. Read this thread on picking the "optimum" charge controller (you would be better off with an "optimum" charge controller with the large battery bank--A smaller battery bank could use a less than optimum AC charger just fine):

Question about battery charger selection with EU2000 generator.

Of course, the hours of sun are average... You should only run a continuous load at 60-75% of rated output unless you plan on starting the genset every few days (cloudy weather)... And you probably would need to run the genset every 1-3 days during "bad" weather (with a Honda/Yamaha, you are probably looking at ~1 gallon of fuel (SWAG) for every day of "no sun" (heavy storm clouds).

We can go into more details--Just wanted to see your reaction to the above numbers/assumptions/guesstimates.

In the end, load power is critical, so I would recommend a kill-a-watt meter (for 120 VAC) or a DC AH/WH meter and buying one air pump to see how they really perform.

I am a fan of Battery Monitors (Victron is another good brand)--If setup correctly, will give you very good information on the state of the battery storage and at least 1-2 day warning before you over discharge the battery bank (below 50%) so you can start the generator and/or debug electrical problems. (Just log the SOC and battery bank voltage every day--And have a simple sheet that says >75% SOC/14.0 volts (sunny day) is fine, if less than 75% may need to start genset next day if no sun predicted, if 50% or less, start genset now and call you for help.

Again, this a "rule of thumb" design... If you have good reasons, we can adjust the system to better meet your needs (sort of worst case design/assumptions--so should work well as designed--but not necessarily "optimum" for your needs).

-Bill

vtmaps

Re: Design Help Needed: Small Shed PV System

stoneunhenged wrote: »

Okay. So please walk me through this. Assume I go with the 24v air pumps. Which specific controller would you buy and which batteries? I'm not looking to go cheapest route. I'm looking for maximum reliability. Thanks.

Don't worry about 24 volt DC pumps vs 115 volt AC pumps. The DC pumps mean you don't need an inverter, but if you use an inverter it will be small and reliable and will have a low no-load draw. The six panels you have will provide enough power for your needs. You need to figure out whether there are DC pumps that are satisfactory for you, then you can design the rest of the system.

stoneunhenged wrote:

There will generally be someone to check the shed each day. I could install an auto-start generator if you the system design demands it. I could install an alarm system although I would prefer to avoid the cost.

If something goes wrong, how long before the fish go belly up? Is once a day checking enough? Perhaps you should have two systems (one pump per system) for redundancy. Split your battery bank and panels in half (no extra cost). You would, however need an extra controller, and if you go for a 115 volt system you would need an extra inverter. Considering how small the systems would be, an extra controller and inverter would not be terribly expensive.

--vtMaps

stoneunhenged

Re: Design Help Needed: Small Shed PV System

I worked on the foundation of the shed this weekend, and after recalculation the roof area I think I can mount 12 panels instead of the originally contemplated six panels. So, I'm thinking I set up a 24 volt system with a Honda eu1000i or eu3000is generator as back up. (I have both generators now and would rather use the smaller one if it will work.) Any thoughts on this approach generally and any specific recommendations on the best charge controller? Again, performance is more important than low price.

Cariboocoot

Re: Design Help Needed: Small Shed PV System

Okay the first thing you do is send four of those panels to me ...

Kidding aside, you would have a 2100 Watt array. That is a lot of power potential. Over 4kW hours of AC per day.
Enough array to support 670 Amp hours of 24 Volt battery.

vtMaps suggestion of redundant systems/pumps is a good one. I would still choose strictly DC since the 24 Volt pumps appear to be more efficient.

You'll need to carefully evaluate your power needs. No sense putting in a system that's larger than necessary just because you can. By the same token, no sense in short-changing yourself on power capacity and having the fish run out of air.

BB.

Re: Design Help Needed: Small Shed PV System

So, several ways to work out the detail:

1. Measure/Define Loads, then battery bank, and finally balance of system
2. Figure out Array Size (watts/orientation), define battery bank, then balance of system
3. Define battery bank, size array, then design balance of system

I highly recommend #1, but we can do #2 and then talk about system capabilities.

So, first, roughly, where will the system be installed (do you have problems with marine layer/summer thunderstorms/etc.)? I guessed Mobile, but I think you are farther north.

At this point, your system looks like 12x175 watt panels? Are these the Vmp=35.7 volt versions?

If Vmp~35.7 volts, you could, in theory, use a PWM charge controller and save a few hundred dollars on hardware.

However, if the array will be mounted somewhat distant from the controller/battery bank, then you really should look at MPPT controllers. You are close, but a 60 amp MPPT charge controller (minimum) would be fine with that array. (For your size system, an MPPT controller is probably a better choice).

Do you have a choice on how many days of "no sun" you want to plan for (1-3 days, 2 days is "nominal" and usually a good fit for most people).

-Bill

ggunn

Re: Design Help Needed: Small Shed PV System

Cariboocoot wrote: »

No sense putting in a system that's larger than necessary just because you can. By the same token, no sense in short-changing yourself on power capacity and having the fish run out of air.

The only way to be sure of not doing the second is to (at least somewhat) do the first.

stoneunhenged

Re: Design Help Needed: Small Shed PV System

To clarify, I'm on the same latitude as Mobile, Alabama.

I'm for maximizing the electric output of the shed because I can easily see future electric needs creating demands that the shed can helpfully meet. For example, I have a PV powered well that is currently running off two panels (on a tracker) at 12vdc. By wiring it to the 24vdc system in the shed I could double its pumping ability. So, yes, I will be installing twelve 175 watt panels.

I'd prefer to plan for two days of no sun and use the generator back-up option.

Any recs on the controller for this set-up?

BB.

Re: Design Help Needed: Small Shed PV System

You have to run through the math again... So, starting with the assumption of a 10% rate of charge:

• 12 x 175 watts of panels * 0.77 system derating * 1/0.10 rate of charge * 1/29 volts charging = 557 AH @ 24 volt battery bank

The current from the array would be:

• 12 x 175 watts of panels * 0.77 system derating * 1/29 volts charging =55.8 amps

So, any good quality MPPT charge controller from 60 amps and up will work fine.

MorningStar makes a 60 amp MPPT (TS MPPT family) charge controller.

Midnite and Outback make controllers ~ 60-80 amps.

The MorningStar and Midnite both have Internet access for logs and some control (you may not need).

With this size array, a 24 to 48 volt battery bank is probably a better fit. Unfortunately, this knocks out the MorningStar 300 watt TSW 12 volt inverter--which would have be perfect for driving your air pump loads.

Air pump wise, you still have the choice of 24 volt or 120 VAC... If your primary 24x7 loads will be the air pump, you might want to spend the money and get a 24 volt unit to inspect/test and see how will it works (this appear to be linear motors--basically a little plate that vibrates left and right from the 60Hz power of an AC line--Wonder what they are doing for DC version--a small internal inverter?).

Otherwise, if you will have other substantial 120/240 VAC loads, then bite the bullet and get a good quality AC inverter. The problem is that with 24 and 48 volt inverters, these are (typically) larger units which burn more power just being on. So, your extra solar panels and battery bank will be helpful here too.

And, with AC air pumps, if you need to use a backup genset, the pumps can be powered directly from the genset instead have having the genset power a battery charger through the battery bank.

AC Off Grid Inverters... There are a lot of options. Simple units, those with "search low power modes" (inverter pulses the AC line once a second looking for >6 watts of load, then turns on). Combination AC inverter/chargers (DC input from battery bank, AC input from genset. AC from genset can power loads and recharge battery bank at same time). And even inverters with 120/240 VAC capabilities (if you have large pumps and/or long distances to run larger loads, 240 VAC will save a lot on wiring costs).

You might want to install two smaller inverters (one for each pump). Or install a smaller one to run the two pumps 24x7, and a larger one to power your other loads when needed...

You could even get one DC pump and one AC pump and see which setup works better for you.

-Bill

stoneunhenged

Re: Design Help Needed: Small Shed PV System

Yes, there will be someone there every day. There will be an autostart generator. There will not be something to connect an alarm to.

stoneunhenged

Re: Design Help Needed: Small Shed PV System

Hey, guys. Stone here. Just wanted to let you know I have not been waxing my weasel. The solar shed is going vertical. Soon I will need your help pulling together the PV system.

stoneunhenged

Re: Design Help Needed: Small Shed PV System

A little more progress. Soon, the PV panels.

BB.

Re: Design Help Needed: Small Shed PV System

I cannot tell for sure--But shear walls? Or some sort of significant cross bracing against strong winds?

-Bill

gww1

Re: Design Help Needed: Small Shed PV System

I definatly like your solar shed better then what I have started.
Attachment not found.
very nice.
gww

stoneunhenged

Re: Design Help Needed: Small Shed PV System

Actually, I've think you've done a nice job. One of the better wooden racks I've seen.

stoneunhenged

Re: Design Help Needed: Small Shed PV System

BB. wrote: »

I cannot tell for sure--But shear walls? Or some sort of significant cross bracing against strong winds?

-Bill

There's only one enclosed wall to the north. What you can't see in the pic are two things: 1) we live in the lowest average windspeed area in the entire Southeast (the peak monthly average speed is 7.5 mph) and 2) there is a large stand of tall trees immediately to the north of the wall that act as a windbreak. The lower portion of the structure is completely open on three sides and allows for the wind to pass through. Just to be sure, we installed a lot of straps in the structure.