(Continued from Part 1. This part concludes the article.)
The #1303 model of Dankoff Slow pumps has the highest ‘gpm’ (gallon per minute) rating of the less expensive 1300 series. The #1303-24 pumps the most water for the money, and to a height that it will pump that is adequate for most situations. The advantage of a 24 volt system over a 12 volt pump, is that most surface water sources would likely be in a shaded environment, so the panels would need to be located some distance from the water source to obtain the needed amount of sunshine. We should place the panels where the sun shines most to get the needed solar power output to pump the *maximum* amount of water for your individual requirement.
If we need 225 gallons per day, we only need sunshine for the time needed to pump that 225 gallon goal. More sun is not necessarily needed. For example, if the system as installed produces 1 mpg with full sun, and 225 gallons are needed, then 4 hours of full sun is required to produce 225 gallons. On cloudy days it will produce less. However, the garden will require less water on cloudy days. I use 20, or 55 gallon drums, and 40 gallon troughs to store water for times I would like to water the garden using the gravity fed method through a valve, or siphon effect, but slowly saturating, or flooding the garden could also be done. If enough water is held in reserve for cloudy days, one could just let the pump run and saturate the ground, but we would then be putting excessive time on the pump. It’s best not to expect the maximum output as solar conditions are usually less than optimal, so give your system a real world test, and see what it really does. We should estimate that the actual output will be less, therefore we should have built in, a wide margin of error just to make sure we can easily meet our greatest estimated requirement.
The pump as you may install it, could meet my estimate of 1 gpm, or close to it by using only two 100 watt panels. Under the best conditions, and the given 204 watts listed on the chart, the pump could potentially pump 2.5 gpm x 60 minutes x 6 hours = 900 gallons per day. My 1308-12 usually puts out only half of it’s hypothetical maximum when wired directly to the panels, so let’s assume the proposed 1303-24 will only produce 1/2 or 1.25 gpm x 60 minutes x 6 hours = 450 gallons per day to height of 100 feet above the water source. That is enough water for one acre of the thirstiest crop during hot and dry weather.
I will now further de-rate the pumps output, because the proposed site is somewhat shaded in the morning and afternoon, and receives only 3 hours of full sun. 1.25 gpm x 60 minutes x 3 hours = 225 gallons per day. That would be enough water for a 1/2 acre garden. We can justify de-rating the pump’s output another way to hedge our estimate. If using only two 12 volt, 100 watt panels wired in series to create 24 volts, the maximum amperage that can be made is only 5.5 amps at 24 vdc. The pump needs 8 amps to pump at it’s maximum rating of 2.5 gpm to a height of 100 feet. This is why we are assuming the pump will not pump at the full rated 2.5 gpm, but only potentially half (1.25 gpm), and because solar conditions are usually less than ideal, I would want assume a worst case scenario, that it produces only one fourth of it’s rating, or 0.75gpm and see if that low amount would be adequate. 0.75gpm x 60 x 6 hours = 270 gallons per day. That is still enough water for a one half acre garden. I have read that 1/4 acre garden could feed a family of 4, but then I have never tried that. If true, then I would want to double that, in case crop loss is excessive.
If 225 gallons does not produce enough water for your needs, add another set of two 12vdc 100 watt panels for a potential total of 11 amps at 24 vdc, and it will produce twice the water, nearer it’s full rated output of 2.5 gpm in less than full sun, or ideal conditions. It may then produce say a solid 2.0gpm, but begin pumping earlier in the morning and continuing into the evening, producing more than 900 gallons per day. Again, the maximum potential output with 6 hours of full sun is 900 gallons per day at 100 feet above the water source. Adding 2 extra 100 watt panels will cause the pump to run much longer throughout the day, albeit at a reduce rate, yet produce significantly more. This is difficult to estimate, yet that is what these pumps can do. I believe these estimates to be realistic, if not conservative. However, as all regular SurvivalBlog readers know: Your Mileage May Vary (YMMV). In these rough estimates, we have given ourselves a generous margin of error, so we will be less likely to be disappointed.
Consider DC Cable Voiltage Drop
IMHO, the 1303-24 (24vdc) model pump is the most cost-effective solution for tight-budgeted preppers who have access to surface water. Look before you leap, and do your own assessment of your property and water needs. If the water source must be more than 800 feet from the location of the solar (photovoltaic) system, the 1303-48 (48 volt) would be necessary. The maximum wire run using 6 AWG (well pump wire), and the 24 volt pump, to run at full capacity, or at 8 amps, would be 800 feet. Using the much less expensive, and common 12 AWG romex wire for our tight budget installation, the maximum run would be 200 feet. Hopefully a 200 foot run will put the panels in full sun. If not, then increase the size of wire–to reduced line loss.
Using common 2-wire romex in 10 AWG wire, would increase the maximum run to 350 feet, should the pump be pulling it’s maximum amperage of 8 amps at 24 volts. Perhaps 10 AWG would work? Less wire, in terms of number of feet, and less wire in terms of wire size, is less money needed. Use a free on-line ‘voltage drop calculator’ during the design phase to determine the gauge, and length of wire that can be used that does not reduce the voltage at the end of the line more than 5%. This means that the correct size, and length of wire used, will deliver the full potential power output in amps at it’s highest potential voltage it creates when receiving full sun light.
The #1303-24 pump can use no more than 8 amps at 24 volts, so plug those numbers into the voltage drop calculator. Because these pumps can be run directly from the panels, we can use the unregulated voltage output of the panel in the calculator. The actual unregulated voltage of a nominal 12 volt panel is about 18 volts, and 36 volts for a nominal ’24’ volt panel. Use those higher voltage numbers for the voltage drop calculation along with the amperages given, namely 8 amps that is the maximum draw for the 1303, 24 volt pump. If the voltage drop is 5% or less, it would be considered to be an efficient system for standard a ‘solar system’ that charges batteries. Because this is a ‘slow pump’, being electrically efficient is not as relevant. However we’ll use the standard 5% voltage loss limit as a maximum acceptable. This pump is designed to operate on a voltages much lower than it’s rating without damaging the motor, but we are after a production of 225 gallons per day, or more, so we’ll do our best to deliver as much power as we can.
What this pump requires more than voltage, is amperage. So let’s supply it with enough watts to make it operate, and not sweat the voltage quite so much. If you live where is it often cloudy, then adding another set of two 100-watts panels might be necessary, or use more water storage, to compensate. 100 panels at 24 volts will not harm the pump. Add as many 24vdc pairs of panels as you need. The pump will pump earlier in the morning to later into the evening, and better on cloudy days, greatly extending the time the pump will run, and provides potentially hundreds of extra gallons of water.
One source recommends that if running directly off panels, in other words, without using a linear current booster that costs $700 bucks, to compensate for the absence, use at least 40% more wattage than is suggested by the manufacturer’s pump output chart. We may, or may not need the full output, so let’s go with only one set of two 100 watt panels, and see what happens. Add more latter if needed. Looking at the chart, 40% of 200 watts, would be 80 watts, but because we need 24 volts, and because it is less expensive to buy two 100 watt panels than one 24 volt panel, we can supply it with 100% more power, instead of 40% more power, or 400 watts instead of 200 watts, or 11 amps instead of 5.5 amps. Of course, shop around.
In my experience, because the shipping costs are included, it makes the 100 watt panels from Renogy.com, or windynation.com, the best deals. And consider that if the pump will only be used 5 or 6 months out the year, that those panels can be used to power other essentials, such as radios, or other during the darker winter months when power is hardest to come by. In my experience with radio, 200 watts during the winter in Montana is my bare minimum. I would like more.
Pump Assures a Key Resource
This is a time-tested pump, invented in 1983. The wearing parts are said to last 10 years, given a filtered input. It is efficient, and reliable. Time tested. Mine is set up to be portable, and will likely out-last me. If possible, spare parts are recommended, and should be the motor brushes, and a spare pump head. A portable rig can service several small gardens, and would be one of the most important item to go with me should I have to leave.
We happen to have surface water everywhere around here in NW Montana. It does, however, require a 10 micron filter. Get a lifetime supply, one for each season, up to 20. Do not use a 5 micron filter, it does not allow enough water to flow and can ‘starve’ a pump that needs water for lubrication and cooling. This recommendation comes not from the literature provided by the stores, but from the boys in the back, the technicians that build and repair the pumps. I called and asked. If you must run a 5 micro filter, then use two filters that separately feed into the intake. With a clean water source such as ours, a filter should last several seasons, but I would hedge my bets, as it might be used in relatively dirty water such as pond or cistern.
Bleach can be used to ‘clean’ a filter of algae at the beginning of the growing season. Be sure to also get the low water shut off device, or better, the pump temperature sensor that protects the pump head that is made of brass, or best yet, both, and insure your ability to grow food. I have the temperature sensor shut off. My 1308-12 will provide more than I need, but my needs are low, and it is using an LCB that will eliminated when I can buy more panels.
Currently, my tested slow pump in not in use at this time to preserve it for when I will truly need it. I consider it to be one of the most precious of tools, right up there with garden seeds. It can be taken wherever I go, and irrigate whatever land I might find when I get there, should I be forced to leave. Irrigating by buckets would greatly limit food production.
Here are links to the best system component prices that I could find. (Of course, shop around.):
- 10 spare Dankoff water filter cartridges (10 year supply), Fire Mountain Solar
- Dankoff DRS2500 Dry Run Switch, Fire Mountain Solar
- 1/2 inch foot valve, brass, Amazon.
- 250 foot roll of 2 wire romex, 12 AWG, Amazon
- Rain Bird T70-500S Drip Irrigation 1/2″ (.700″ OD) Blank Distribution Tubing, 500′, Black
- Dankoff 11003 Float Switch Kit, 10 amps, Fire Mountain Solar:
- 1/2 inch brass check valve, Amazon
- Dankoff filter kit, Fire Mountain Solar
- #1303-24 pump, Fire Mountain Solar
- PV panels from windynation.com