Water pump power requirement /sizing

[Pooh_Bear]

When our well was drilled almost 10 years ago it had a flow rate of 3gpm. We have 2 houses on that well and a total of 6 people + weekenders.
We are also 18 inches behind on rain for the year. [img]/forums/images/icons/frown.gif[/img]

Pooh Bear

[stumpfield]

Thanks for all the info. 2gpm should be plenty of water for us. We have 5000gal water storage capacity located at 20ft above the well then gravity feed down to the house approx. 100ft below. 5000gal could last a month if we use it conservatively. So, let see if I got this right...

Well is 230ft deep. 6" diameter
Pump is set at 220ft.
Water lever is at 80'
Well's flow rate is 2gpm
5000 gal of storage tank capacity 20' above the well.

Total head = 100ft.

So, I need to size based on 100ft head. Add 20% water level uncertainty... I 'm dealing with 120' total head. According to the performance curve of the pump, I need 150w of power. If everything works like it should, I should get a continous 2gpm flow into the storage tank whenever the sun is up.

If the solar panel only produce 2/3 the power (100w) or the water level drops another 100', I'm still pumping .5gpm.

I'm going to start with 150w and see how it goes or should I go with more? When I buy a solar panel that says 150w, does it produce 150w in full sun? or it goes from 0w in morning then peak out at 150w then drops down to 0w in the evening????

[jimbrown]

It only produces full power in full sun. If it is not full sun it produces less depending on the type of panel. The only way you can get full power all day is to use a tracking mount that follows the sun across the sky. And then there must be no clouds. You are not going to get it to pump at 25 watts and .5 gallons a minute. Because it won't start. Again. Unless you get the special 66 volt panels the voltage will be low and make starting even more difficult. If you go with 12 volt panels I would suggest at least three for 36 volts better yet 4 and run 48 volts. 4 40 watt panels will give you 160 watts at 48 volts. Which should work but in my opinion is still too low. I you really want to go low on power I would start with 3 60 watt panels for 180 watts at 36 volts and then if you need more power add another 60 watt panel for 240 watts at 48 volts. I say again you can't have too many watts (unless you go ridiculous).

You are going to need some type of cut off switch if you do not have one. Something like this works well.
http://store.solar-electric.com/water-floatswitch.html

[Pat]

Panel output will partially depend on your latitude, the season, (altitude and other air clarity issues) and ambient temperature. Also it matters if you have a tracker, either single or dual axis and if no tracker will you manually adjust the tilt angle a few times a year to track seasonal changes or just set to a compromise like 15 degrees off your latitude and take what you get?

The solar flux can be measured in energy per area (Watts/sqft) Insolation (not insulation) is the amount of solar power available. With no tracker you might get 5 hrs (of equivalent sun) that is, from the predawn glow to the end of evening twilight all the solar power you collect is as if you had full sun for 5-6 hrs (approx.)

In summer with longer days and otherwise good conditions you might net an equivalent of 6 hours or more depending on latitude etc. Panels are typically rated for their output with a standard sun at some specified temperature. This will not happen all day from sunup to sundown but will be about 5-6 hours or whatever you circumstances dictate.

To do the math you need to decide not just how many Watts you need to pump at a rate of 2GPH (or whatever) but how many Watt-hours you need to deliver to the pump to pump the volume of water you need to average at or above your needs. The volume is the critical factor.

For example, if you pump 2 GPH for 5 hours that is just 10 gal, not much if you have flush toilets and like to shower. You may want to size your panels to allow you to pump considerably more that 2 GPH. A higher pumping rate will not necessarily be a problem as you will be pumping the stored water in the well bore (as others mentioned) and that storage will be getting replenished overnight when you are not pumping.

One good thing is that you have no battery storage in your solar electric setup. They are not helpful in your system where your supply pressure is from a gravity head. DO NOT ADD BATTS. They waste energy and will result in less water pumped.

I suspect that you are going to have to size the panels to pump considerably more that 2 GPH or you will have to ration water rather severely. Just for a rough approximation lets assume pump output vs insolation at a given head is linear. If the panels are sized to pump 2GPH then you get 10-12 gal per day. In actuality, probably less since pump efficiency is not entirely linear over a varying electrical input. I did my cruising on a sailboat under rather stringent water use restrictions (on longer outings) and washed dishes and showered with sea water (fresh for a brief rinse only) to extend cruising range. You don't have the option of using sea water.

Note: sea water desalination via RO was available but was too expensive back then for my budget and had too high of input power requirements.

Some panels are much better on overcast days than others, preserving a higher fraction of the "clear day" output. If you have significant periods of overcast you might want to lean toward the panels that do well with longer wavelengths which penetrate overcast better.

Ever notice how on some days although there is significant overcast you can still feel the sun rather well? The right panels will have a good output on those days and worse.

Pat

[GaryQWA]

Poohbear you are forgetting total dynamic head, that includes the pressure loses in the plumbing with the water moving. The pressure losses vary with the pressure so we use the average pressure. I.E. 30/50 is 40 psi.

To the original poster, I don't give this much chance to work well long term unless you only need a small volume of water moved into the cistern between like 11 AM and 3 PM. That's because you aren't storing any energy in the present design. Now a bank of solar recharged batteries large enough to power the pump at its max open discharge flow rate with a 100-120' of TDH for a number of hours would work but... you'd need charge and possibly other controllers etc.

Here's a link to where I'm thinking I'll be buying a solar system from for my motor home that we live in full time. I still work a lot from 'home', so I have two computers running 12-16 hours a day and running a generator that long is ridiculous, expensive and noisy. So we are sitting in campgrounds more lately than we'd like, saving the money we'd spend on running the generator. By spring we should be in OR having them install the system.

There are other sites but check out the Education section here.
http://www.amsolar.com/

[jimbrown]

I must politely dissagree. Batteries are totally unecessary and an extra expense. My system has no batteries and will pump 800 to 1000 gallons on most days. I average about 300 to 500 per day. With the size of storage that he has he should not have any problem unless it snows all day for over a week, or does something dumb like I do sometimes and leave a hose or horse tank on all night. I do not have a tracking mount and mine pumps from about 0900 to 1630 everyday at 2 gallons a min or better.

[GaryQWA]

No problem, you should know much more about it than I do.

Your 800-1000 gallons over 7.5 hours comes in at 1.7 to 2.2 gpm. The 300-500 is .666 to 1.1 gpm.

[jimbrown]

The 1.7 -2.2 is about what my pump does. The .666- 1.1 is kinda meaningless as it won't pump that slow. It pumps into an 1800 gallon storage tank and simply shuts off when full untill it draws down about 200 gallons. So it normally pumps about 2 gal a min which is what it is rated for. That is kinda what I have been trying to explain to Stumpfield that that once the amperage drops below a certain point the pump will stop and not start. You can't pump .5 gallons a min all day long. You have to pump at rated amounts and then store it.

[Pat]

Thanks to Jim Brown, I need to modify an earlier post to get right with reality. If you put batteries in the system and charge those when the solar electric output is insufficient to run the pump, then yo can use the stored energy to run the pump when it might not otherwise run. I think spending the battery money on more solar panels would probably be a better investment but batteries could increase your total pumped water quantity if used to store energy when the pump isn't pumping for use when the pump would not otherwise be pumping. Theoretically true but probably not cost effective compared to more panels.

Thanks for the insightful post, Jim.

Pat

[jimbrown]

Pat it really depends on what type of system that you have. If you pump into a storage tank then batteries are not needed or desired. if you pump into a pressure tank you will almost always need batteries. I actually have both. But they are seperate systems. The well is all panels and pumps into a storage tank. A seperate system of panels and batteries then uses different pumps (2 12 volt ones) to pump into the pressure tanks. That way my pressure tanks can refill at night. The few people that I know that tried pumping directly into pressure tanks gave up and resorted to a system similar to mine. I can actually graviety feed most of my stuff without the pressure tanks But i do have a two story house so it will not feed the upstairs.