View Full Version : Solar DC water pump sizing and pv requirement. Please Help!!
January 16th, 2008, 5:13 PST
Hi everyone. I am wondering if someone can help me with choosing an appropriate pump, the pv panels required for the pump and the wire required.
I have spent days searching the net and am now so confused!
Here are my requirements:
My water source is a constant (although small) river in a shaded valley, which has a high limestone content ( I'm told that this can cause problems to pumps and it needs to be filtered prior to pumping? Apparently this can be done via some kind of filter tubing placed at the inlet stage?).
I want to pump 900 liters per day.
The water source is located at an altitude of 600m and is to pump to 720m.
The pipe run is about 420m.
The latitude is N 42
I will have a holding tank of 25,000 liters.
As the river is shaded I will need to place the panels near to the water deposit which I calculate as a wire run of about 350 meters.
I have included all the information that I think is necessary, but if more is needed then please ask.
January 16th, 2008, 6:38 PST
The water source is located at an altitude of 600m and is to pump to 720m.
so you have a lift of 120m ?
Do you have a zip code, or large city near you ? - many of the solar calculators take into account average cloud cover.
January 16th, 2008, 9:23 PST
I would guess that Paulno4 is probably in the Madrid region of Spain (possibly a couple hundred KM north of Madrid)...
Grundfos (Spain) (http://grundfos.com/web/homees.nsf) seems to have some pretty nice pump systems that work well with solar.
Your best bet, if you can swing it, is to have a solar system that does not use batteries. It only pumps when the sun is out and can be powered by a fuel powered generator for when you need pumping but no sun is available.
Another issue that I foresee is the problem of your 350 meter run of electrical wiring. You will want a system that can support high voltage (~100-200 volts) from the solar panels to the pump. You don't want a 24 VDC system as the wire required to run that system would be too expensive.
I am not a pump expert here--but take a look through the Grundfos site. At least on the US website, they have some pretty good catalogs with instructions for sizing and designing various pumping solutions.
January 16th, 2008, 13:32 PST
Well Bill you are absolutely correct with the location. I am just at the start of the Pyrenees in Aragon.
I have looked at the Grunfos site and have a disk from them on sizings, but it has not really clarified matters for me.
You have confirmed what I have thought about the wire size. I was kind of hoping that I could run on lower voltage. The reality is that I have to get water to my site for building works, so will have to find a solution soon.
I will in time install a full PV system for the house and this is being designed now, and probably at that stage I could use the excess load to run a AC pump from the site?
What I have no idea about is the cost of running the wire run for the 350 meters, as I really would prefer to just use a dc pump without batteries if possible.
But failing that, does anyone know what could be the appropriate pumping system at this stage?
January 16th, 2008, 17:08 PST
From what I have read, the Grundfos solutions look really neat... They have a pump line that can take either AC or DC voltages... And on DC voltages, they are MPPT (Maximum Power Point Tracking).
To transmit power "long distances", you want the voltage high and the current low. If you had a pure AC solution, you can simply put a step up transformer on one end and a step down transformer on the other for the final run to the pump. If you have solar/inverters/batteries at your place, and run a aluminum cable set (larger diameter than copper, but probably cheaper to purchase) to the pump site--it is pretty straight forward solution. Check with a local electrical contractor with experience with aluminum (in the US, typically used from the pole to the home for utility drops) that knows how to (and has the proper tools and parts) to design and terminate Aluminum to copper wiring--should not be a big issue (other than costs).
If you run a DC system (solar and/or battery), you can't use transformers to step up and down the voltages for a long run like this (can be done electronically--but I have not seen any off-the-shelf equipment designed for this...
So, the easier way is to (hopefully) choose a DC pump that can take high DC voltage and wire up the solar panels to support the high voltage--Since the SQFlex series supports 30-300 VDC, if you wire your panels for 300 VDC open voltage (cold weather/full sun/open circuit to prevent damage to your pump electronics), your system will run at least ~175 VDC (hot weather, full load, maximum power point)... The only way you will do better is if you do the full inverter/AC Voltage solution (lots more money to implement).
If, in the end, you are going to have a full off grid system at your home/building site (inverter/batteries/backup generator), it might be just worth looking into implementing the AC voltage solution with step up /step down transformers...
Of course, this all depends on where the water, sun, and home site are relative to each other. Minimizing wire runs is important. Maximizing use of funds is too (building a small solar array just for pumping, no batteries, no inverters, power to pump water only) vs the "central power station" idea (full off grid system where you have 1 system with generator, panels, batteries, ability to provide pumping power via generator for bad weather/periods of high water needs, sharing of solar panels between home and pump load requirements, etc.).
In the end, solar works best (and even water pumps), cost wise, when it is used 12 months out of the year. If, for example, you will only be building when you have vacation (say summer time), you might be better off using a generator, AC pump, and water wagon for the period that you are building (couple years), then install the final off-grid system. You could even build a small system first (for lighting, after hours radio/tv/etc.) and get some experience with off-grid solar so that you have a better idea of what you will want for your "final" system.
Sounds like a neat project--and there are probably several ways of meeting your needs.
Sorry about all the hand waving, but it is kind of confusing to dump straight into the numbers without knowing more about the problem.
To give you some rough ideas how to cost the solutions against each other...
Take all of your off-grid, no battery costs (solar panels, pump, long distance wiring) and divide it by its expected lifetime (say 20 years)... That will give you a rough cost per year for your solution.
And do the same thing with solar/inverters/batteries/replacement batteries ~5-10 year life, generator+maintenance+replacements, est. fuel costs, etc.) and again divide by 20 years and check those costs...
In the US, those numbers have always worked out to something like $0.10 to $0.30 per kWhr for utility power, ~$0.25 per kWhr for solar Grid Tie power, and ~$1.00+ per kWhr for off-grid solar+battery+generator power...
Your numbers may be higher (higher material and fuel costs), but it gives you an idea that adding inverters+batteries to a solar PV system makes the costs ~4x more per kWhr... So, if you can do without the batteries/inverters for your pump system, you are probably ahead.
January 17th, 2008, 1:52 PST
Many thanks for a well thought out and detailed reply.
You have answered many of my doubts and I think I am now on the way to finding the solution.
I agree that it would be a good idea to "test try" a smaller solution for the house/building project, but in reality I don't think this is feasible.
The calculations for costs is helpful. I would prefer to be grid tied but the cost of bringing the grid to us is prohibitive, plus I like the view clear of any pylons.
I also need to find the specifications for the pump quickly as I need to put in an application to the river authority for extraction and this can take some time.
Once again, I greatly appreciate your help.
January 17th, 2008, 8:15 PST
You can also do the same sort of calculations with the costs of running utility power to your site... Price per kWhr + cost-of-pylons/[20 years)*est-kWhrperyear-usage]. You can also factor in the increase in value of a property with utility power (if that is the case) and if there are eventually other homes in the area if there is a rebate back to you if they also connect to the original feed you paid for (still may be too expensive though).
I understand your desires for wanting to avoid the electrical lines--but sometimes practicality does get in the equation too... In the end, the costs are dependent on how much power you will be using and having a solid estimate of your peak power, daily, and seasonal usage, will greatly help you in sizing your system and estimating your costs. In the above pricing models, solar PV costs go up with usage, but utility power + cost to run lines actually (to a degree) go down with increased usage (as you spread more kWhrs over the fixed installation costs).
I am guessing that you are building in conservation measures (insulation, solar heating/hot water, appliances, lighting, etc.) to your home. In the end, those will be monies well spent in reducing your overall energy usage.
Regarding the one issue I have read about with Solar Powered water pumps (http://www.wind-sun.com/ForumVB/showthread.php?t=1000) (this board does not have a lot of traffic on water pumping) was that a Grundfos system that constantly cycled between pumping water at full flow, then dropping out of MPPT and pumping hardly any water at all. The poster and his electrician looked at everything they could and found the only way to stop the problem (at least during debugging) was to put a ~100 Watt filament light bulb as a load on the solar panel input. For some reason, that stop the MTTP hunting problems.
January 17th, 2008, 12:41 PST
These manuals might be useful references for your project:
Hoe this helps,
Jim / crewzer
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