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diy solar

Hi from Houston, Texas

alfred2307

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Apr 22, 2020
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Interested in off grid solar to power RV park wastewater treatment system. Just eight 1HP pumps and four 120 watts aerators.
 
Welcome!!


Interested in off grid solar to power RV park wastewater treatment system. Just eight 1HP pumps and four 120 watts aerators.
"Just" 8 1HP pumps is a lot!!

1 HP = 745.7W

So your system needs to be able to drive a minimum of:

8 * 745.7W + 4 * 120W = 6445.6W. However, with things like the motor power factor and start ups surge lets round that up to 7KW. That is not outlandish, but it is certainly not small. It is bigger than most of the DIYrs on this forum attempt.

A critical aspect that you did not mention is how long everything has to run every day.

For a worst case, I will assume the system will run 24hr/day.

You never want to run your inverter at 100% so the inverter(s) on the system will need to be something in the 10K or more range. That is getting beyond almost all home systems.

Each day it will use 7KW * 24Hr = 168KWhrs That is huge. If you got 100% output out of 400W panels and you are in an area with insolation of 5, you would need 168KWh/(400W/panel * 4hr) = 84 panels to power the system. However, you won't get 100% output and you need to have power to run through cloudy days so you need more like 125-150 panels.... probably more.

You will need storage as well. Let's assume you need storage for 2 days. That means you need 2 * 168KWhrs = 336KWh of storage.
The current sweetspot on cells are in the 280Ah range. Those are 280Ah * 3.2V = 896Whr. Therefore you would need 375 of the 280Ah cells.
 
RickP, you are right. There is a piece of info missing (minor??). The pumps are main and spare in four sumps. Normal operating time, one pump for less than 20 minutes total per day for each of the four sump pump installations. But the system has to be able to run six pumps simultaneously as water usage from people are not constant. There may be times when two pumps in a sump will run at the same time for two minutes to handle the surges.

Three of the four aerators will be running 100% of the time.

So while the batteries and inverter needs to be able to provide the AMPS for brief periods, the actual AMP-Hour averaged over a day will not be very high. That's what I am hoping. I understand that the power supply is supposed to be able handle the instantaneous load of motor starting.

AC
 
RickP, you are right. There is a piece of info missing (minor??). The pumps are main and spare in four sumps. Normal operating time, one pump for less than 20 minutes total per day for each of the four sump pump installations. But the system has to be able to run six pumps simultaneously as water usage from people are not constant. There may be times when two pumps in a sump will run at the same time for two minutes to handle the surges.

Three of the four aerators will be running 100% of the time.

So while the batteries and inverter needs to be able to provide the AMPS for brief periods, the actual AMP-Hour averaged over a day will not be very high. That's what I am hoping. I understand that the power supply is supposed to be able handle the instantaneous load of motor starting.

AC
Yup.... That makes a huge difference. It is still going to be a big system... just not insanely big

With 6 pumps running at the same time:

6 * 745.7W + 3 * 120W = 4834.2W

I would recommend going with at least 6000 W of inverter. Perhaps 2 stacked 3000W so you have a bit of redundancy.

For storage and generation we have:
(4 * 745.7W * .5h) + (3 * 120W * 24h) = 10131.4 Wh That is still a lot.. The Aerators are by far the biggest part of the energy consumption. If there is anything that can be done to improve their efficiency it would be good.

Since you need to both charge after a cloudy day as well as run the equipment, I would recommend nothing less than 15KW daily production from your solar panels. Even then it will take 2 days to recharge if the batteries are completely drained.

For two days storage, you need 20.2Kwh. That works out to 23 of the 280 Ah cells. You could go with a 24V 3P8S system (24 Cells) but I would be tempted to go with a 48V 2P16S (32 Cells) system.


BTW: I assume the power numbers you are giving me are the nameplate numbers. If so then they are probably higher than what you will actually see. If you can actually measure the power draw of one or all of the aerators it would be able to make a more precise estimate of the usage.

Also, all of your loads are inductive motors. This means your power factor is probably going to be pretty bad. Depending on the motors in the pumps and airators, it could be as poor as .6. That means for each watt generated by the inverter .6 is used and .4 is wasted. With a power factor of .6, the storage size, inverter size, and solar production numbers above would need to be multiplied by 1.67. The good news is that if you can install properly sized run-time capacitors on the motors, you can get the power factor back up to .9 or even .95. The other good news is that some equipment comes with the power factor correction built in. (Most of the time power-factor is not considered in designing for home solar/storage. However, in you case it could be a big deal so you definitively want to consider it in the design of the system.)
 
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