Checking my numbers for an off-grid aeroponics system

hueyp13

I am working on an aeroponics system for sweet potato seedlings. I need a solar power system to run two .75kw 1 hp centrifugal pumps. The water pumping is done 15 minutes in every 30 minutes during day time and 15minutes in every 75 minutes during night time. Night time is considered to start at 6pm and end at 6am. There is an automatic timer that controls the start and stoppage of pumping. My questions are as follows: We are currently using a standby power generator to supply power when there is power cut. We hope to be able to use solar power to assure continuous supply of power when there is power cut. Two pumps are functioning simultaneously. Some questions we have are:
1. Can solar power system be used to replace the stand by generator?
2. In case there is power cut for a period of 24 hour or longer, can solar system provide enough power?
3. Can solar power alone be used for such a system at a site where there is no other power source? In other words can solar power system be used for such a seed production system for 24 hours per day for 4 months continuously?

This is what I have calculated up to this point:
This system will be deployed in Malawi (although I'm considering building a prototype in the Dallas area maybe for teaching/training purposes). At this stage I want to learn the calculations before I burn a hole in my wallet. The global irradiation for Malawi is approximately 2200 kWh/m2 and the solar electricity is 1550 kWh/kW peak

http://www.soda-is.com/img/map_af_glob_opta_150dpi.png

I believe I will be using a 48V battery bank. Recognizing the example in the link provided only had one 1 hp pump I thought I would need twice the number of battery banks. However, most of the literature I have read has only discussed 12, 24 and 48V banks. (***I read an earlier post that did an excellent job in explaining the difference between 12, 24, and 48 volt battery bank) So, I am going with the 48 V

http://www.offthegridnews.com/2012/03/07/building-a-battery-bank-for-your-off-grid-energy-needs/



Based on the example above, my project would be three times if I used the same number of backup days (4 backup days). Then I would have 15,000Wh times 4 is 60,000 watt-hours. And using the 50% limit on depth of discharge, would take me to 120,000 watt-hours.

The lowest average temperature is 7° C (45° F ). http://www.weather-and-climate.com/average-monthly-Rainfall-Temperature-Sunshine-in-Malawi. So, 120,000 Wh times 1.25 is 150,000 Wh. So, depending on if I divide by a 48 v battery pack my amp-hours is either 3125 amp-hours for 48v.

I need 3125 amp-hours of battery capacity to carry you through 4 days.

Next is sizing the solar array to charge the batteries. I need a system that generates at least the ~15kWh/day that your pumps will use. The closest detailed solar data I can find is for Harare, Zimbabwe so I can't model the Malawi sun exactly. If a 5kW solar array it should generate about 16 kWh per day during the lowest sun months. It would take a while to recharge the batteries after a 4 day use, so you might bump up to 6kW to give you extra capacity. A 6kW system is only about (25) 235W panels. My spidey sense says that's an awful lot of solar panels for two .75kw water pumps! Where did I go wrong?

So, I conclude that I will need

25 235W panels to support a 6kW solar array.
3125 amp-hours of battery capacity @ 48 volts will satisfy the requirement.
minimum MPPT Charge controller size which is Panel Wattage / Battery voltage = 6000 watts / 48 volts = 122 amps

Please note if there is a more straightforward and logical approach to this exercise, I'm all ears!!!

Thanks in advance.

Bernie

Cariboocoot

Re: Checking my numbers for an off-grid aeroponics system

Welcome to the forum Bernie.

I'm having a "fun" day here so this will be brief.

First up, evaluate those loads carefully; pumps tend to draw more than they say, especially on start-up.
Second, calculate the Watt hours carefully.

Otherwise:

1. Can solar power system be used to replace the stand by generator?

Yes. But solar back-up power is very expensive vs. generator power.

2. In case there is power cut for a period of 24 hour or longer, can solar system provide enough power?

Yes. Lots of us run whole operations of many sizes entirely off-grid. See answer #1: it's expensive.

3. Can solar power alone be used for such a system at a site where there is no other power source? In other words can solar power system be used for such a seed production system for 24 hours per day for 4 months continuously?

Yes. Did I mention it's expensive?

As in: why solar? No grid? Look into the cost of getting grid.

stephendv

Re: Checking my numbers for an off-grid aeroponics system

Hi Bernie,

Firstly you need to get a handle on your daily kWh consumption. Roughly: 1.5kW in pumping for 12 x 15 minutes during the day = 4.5kWh. And 1.5kW for 9 x 15 minutes during the night = 3.3kWh, for a total of 7.8kWh per day, is that right?

Losses from the battery to the loads are approx 10% in the inverter, so you need 8.6kWh from the batteries.
And losses from the solar to the batteries are approx:
- 20% for battery charging
- 5% in charge controller and cabling

So you'd need about 10.7kWh per day from the solar. The 1550 kWh/kW peak figure is that average over the year? What you really need to know is what is it during the worst time of the year and plan around that. Regarding battery sizing you'd want to keep it to max 50% depth of discharge, but how much battery you decide to install is more of a $ question. E.g. if you have the grid available most of the time and power cuts don't typically last more than 1 day, then installing a battery to handle the odd 2 day power cut is not economical. Similarly, if you have a reliable generator then you could size the battery for 1-3 days of backup and run the generator when needed. Depends on the climate there and how long the overcast periods last.

For 2 days of backup power it would be 8.6kWh * 2 / 0.5 = 34.4kWh battery at 48V = 716Ah.

Photowhit

Re: Checking my numbers for an off-grid aeroponics system

I don't think your math is bad, the points Stephendv made will correct it, and basically expensive which 'coot made somewhere in his post...

So worrking on loads is your option, aeroponics/hydroponics can have some easier ways of working with loads, moving water the 'head' or lift is often the major load, so keeping things as flat as possible will drastically reduce your loads. If your recycling or nutritian baths, you may be able to use hydrolics to lift a heavy bath a couple feet, then lock and then drop a couple feet. You might be able to help with the loads by doing some sort of teeter totter, lifting one side of a steel tank to run the liquid out and letting it drop to recover.

I'm not familiar with which energy source is most effiecient, if you can reduce the need/head it will reduce your use. If your in an academic situation, you might suggest this problem to an engineering class for some fresh ideas. I take it this is a very large production, home units often run on 5 gal buckets hand lifted to 30 gallons, for a sizable operation. I think the strawberry U-Pick only used 30 gallons and it was a pretty large garden/area.

hueyp13

Re: Checking my numbers for an off-grid aeroponics system

Thanks guys.

Stephen: 7.8kWh per day is right. The 1550 kWh/kW peak figure is that average over the year. The low temperature for the year or worst case is 7C or 45F so that is where the 1.25 factor came in. Was that incorrect? I see your point in using a 2 day backup rather than a four day, since it cuts in about half the number of panels required. You all are right the price stings. By by estimation..
2.8kW Yingli PV( $354 times 12), 5kW SMA Sunny Island 5048( $4,454.00), 900Ah 48V Forklift battery(reconditioned$2,200), 5kW Petteroid Indian diesel generator(somewhere between $1,000 and $2,000)

Hopefully the reply will go thru... I lost my work before.

hueyp13

Re: Checking my numbers for an off-grid aeroponics system

Photowit,

the system is about a 200 gallon/800 liter tank. It creates more of a mist environment. The pumps will be connected to tubing with small holes to keep the roots of the seedlings wet with the mineralized water. Nonetheless, I will consider your your points. Basically, let gravity be my friend.