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Hello,

I've been working on specifications for a small-scale, off-grid solar AP system. Below I've listed my assumptions and calculations so far--very basic. Does this set-up seem reasonable to you?

Assumptions & Requirements

I'm leaning toward a 12v system.

The pump will pull 16 AC watts = 24 DC watts required. The pump will run 12 hours/day.

I'm assuming 2.5 hours of peak sunlight per day to cover the worst case scenario for my area (yearly avg. is 4.5).

I would like to build in enough capacity to cover a four-day period without power generation.

This will be a small IBC system with a couple of growbeds.

Calculations

Power Used by the Pump
24 DC watts/hour at 12 hours per day = 288 watts/day or about 375 watts assuming energy loss per a couple of online calculators.

Power Generated by the Panels
100-watt panels should generate at least 250 watts of power/day under the worst conditions (2.5 hours of peak sunlight/day).

***

So, if this is right, I would need two 100-watt panels to power the proposed system year-round. Correct? I'm hoping I'm wrong, as I would like to power this system with a single 100-watt panel.

I would be most appreciative for any feedback, corrections, or suggestions you could provide.

Many thanks,
Varmint

I think you may find you will want 2 x 100w panels for reliability

But you could try it on just one to start off with, then simply wire in another panel in parralel down the track if required.

Is the 2.5 hours of peak sunlight a day a summer/winter/or average figure for your area?

Can you supply us some info on your proposed battery bank including amp hours etc as that would help

Do you have access to the grid as well? If so, my personal preference (cheaper and more efficient) is powering off the solar panel during the day then using a relay to switchover to mains when insufficient voltage from P.V. This saves costly, lead filled and limited life/consumable batteries

I have been looking into this as well. I am going with 2 100 watt panels, a 30 ah charge controller, and a 125 ah battery to start with.

You are a bit out in your PV calculations. I assume you are saying 24W DC due to inverter inefficiencies running a 16W AC motor? Depending on the inverter, it may be worse than that, and pump power supply and battery recharging inefficiencies need to be allowed for, so lets say allow a safety margin of 1.5X energy required.

24W = 2A X 12hours X 1.5 = 36AH /day @ 12V required (0.43kWh). To cover 4 days of minimal sun you would need 144AH of usable battery- lets say you use a gel battery and take it down to ~50% depth of discharge so as not to kill it too quickly, then you will need a 12V ~300AH battery



Averages can hide the reality of a week of heavy overcast. Whether or not you get that in winter at your location I dont know, but you need to allow for worst case if you dont want to kill your battery, assuming you dont use mains power to charge the battery when needed.


Panels generally deliver about 80% of rated power (ratings measured at: 1.5 atmosphere absorption, 25C cell temp and 1000W/m^2 radiation), so your 100W panel will deliver 80W at best, when the sun is face on, less at other times. You will see brief peaks above this due to cloud edge effect sometimes, but not consistently.
In winter if your average is 2.5 kWh/m^2/day, then a 100W panel might be expected to deliver:
2.5h * 80W = 200Wh/day, which is less than half your requirements, so even 2 panels will struggle to keep up in typical conditions! Just remember that to recover from a deep discharge you need to meet your energy requirements plus recharge your battery. So, if you have 4 days with no sun, in order to recharge your battery in say 2 days, you need 3X your minimum required energy production - your pump requirement plus recharging in half the time it took to discharge. Charging a Lead-acid battery is not all that efficient, particularly as you approach full charge, so a LiFePO4 battery may be worth considering. You can safely discharge them to a greater depth, and they are a lot more efficient when recharging- about 95% vs Pb-acid at 85-90% at best.

Additional note to above, since edit has timed out.

The above PV calculations apply to a panel with Vmp near 15V and a PWM controller. If you have Vmp near 18V, which is more typical of most "12V" panels, then the above outputs will only be achieved with an MPPT charge controller. A less expensive PWM controller used with a Vmp 18V panel will deliver even less output, as it forces the panel to operate at battery voltage, rather than its maximum power point.

You have the best handle on Solar, Gunnagulla. Not much of it makes sense to me but its very valuable for the forum.

Thanks Charlie, its my job to know about solar stuff

Not really part of my job as it is for myself, I'm in the middle of designing (a few tweaks left to do on the drives) and building (welding almost finished) a 2kW solar tracker ATM, and have just finished welding and painting the PV panel mount for my AP system alternative power supply.

There will come a time when I want to use solar for my home... I will be talking to you first thats for sure.

Helpful post Gunagulla... nailed it mate

What is your job Gunagulla?

Surprising as it may seem, currently it is forum moderator, but not this forum.

You get paid for that?

Which forum?

Yep, only a part time casual position though, at Energy Matters.

that's pretty cool

Gunagulla and jono81:

Thank you for your detailed feedback. It is so helpful to receive guidance in plain language. I appreciate everyone else's comments, too.

I've spent some time thinking about your suggestions, my assumptions, and the reality of what I can really do with this system. Here's what I came up with:

1. The system has to be off-grid unless I want to pull a wire across a field and down a hill. I'm not willing to do that.

2. I'm in the US in North Carolina, and our winters can be rough. As I'm not going to put a solar system in that will come anywhere near powering a heater, there is no reason for me to plan for the worst case scenario of peak sunlight. I looked at an NRC publication showing peak hours by month for my part of the State, and during the months in which I'll be growing, the lowest peak sunlight rating is 3.6. I will plan based on 3.0 hours, just to add for a little more padding.

3. I was off on my forecasted energy usage. I will only need to run the pump for 6 hours a day, but it will need to be a bigger pump that can turn the water over quicker.

So I think my calculations would look something more like this now:

33 watts (AC) for the pump = 2.75 amps x 6 hours x 1.5 = 24.75 Ah/day.

4 days with minimal sun will require: 100 Ah of power, so I would need at least a battery in the neighborhood of 300 Ah.

For power generation minimum: 3 hours x 80 watts = 240 wh/day.

It has been a long week and I'm worn out, but the more I look at it, the more the ultra-low energy air lift systems, such as the one described in Jimbo's thread below, make sense. I would still like to try it the "old way," first, though.

viewtopic.php?f=18&t=11703&start=15

I will keep tinkering with this design, and I would be grateful for any additional feedback you may have. Thanks again for your help.

Varmint