View Full Version : Solar powered water pump project

06-10-2013, 11:16 AM
I have finally had a chance to test my water pump project to verify it will run well on the solar panel I have available. This pump is to circulate water from the collector to the 45 gallon heat exchanger used to preheat very cold well water. That heat warms the water on the way to the electric water heater. Our well water comes in at about 43F year around and after a run of a few good days sun leaves the exchanger at around 90F. That cuts the electric consumption to nearly zero since the water heater is set to 120F. The impact on the utility bill is significant. In winter it takes takes flue waste heat from the natural gas heater and wood burning stove.

Until now I have been circulating the exchanger water using a 1/4 hp submersible pump. I wanted to eliminate that power use and have a panel to charge a small battery bank to power LED lighting. I have an 80 watt Sharp panel and I have now verified it can power the pump I built as well as charge batteries. The pump needs to lift water about 10 feet and overcome the resistance of around 100 ft of PEX tubing at a low flow rate. More important it needs to both run for thousands of hours but also shut down when the sun isn't shining.

We have had weather that reminds me of the rain planet sequence from the movie The Illustrated Man. Finally yesterday we had good enough sunshine for me to do a little testing of the completed pump. The exchanger tank is a 45 gallon fruit juice poly drum with a hatch in the top. The pump must sit near the cold bottom of the tank with the motor extending out of the tank.

I decided to use a positive displacement lobe pump design as that will provide circulation even at low motor rpm during weak sun conditions. The batteries will be charged with an isolation diode. This results in the pump automatically stopping when the sunlight is too weak to provide heat or when it goes down. I like automatic.

The pump in action:


More images of the pump design:


Pump parts and materials. Note that I had to change the design slightly. This first iteration had the layers clamped together with SS through screws which caused the center outer housing to compress unpredictably and jammed the rotors. I had to allow for the expansion and contraction of the plastics over a range of at least 60 degrees. I changed the design so that the front and back plate are screwed to the center housing so it is not under compression. The extra holes in the inner and outer rotors are jig holes for screwing the parts down to prevent distortion during machining.
PTFE and UHMW both have extremely low dry friction and when wet have almost no friction at all. The design also follows the principal that is also used in metals: Use different materials when rubbing contact exists. This is just as important to plastics.


Unfortunately the lobe pump design is not practical to make without a CNC mill. The shape of the curves is complex involving three different curves blended to form the lobes. They must be accurate to within a few thou as the clearances determine the amount of pressure that can be produced. I did a lot of research and studied patents on these pumps. There are numerous styles but I selected the most efficient type according to what I read. I decided to design my own curves which was also an interesting exercise. It seems to work well with decent pressure. I haven't measured the available pressure but I included plenty of screws since the plastic won't take much holding torque.

Machining UHMW is an interesting exercise. UHMW is highly abrasion resistant, about 10 times more so than mild steel and even twice as good as abrasion resistant high manganese steel. The molecules are millions of atoms long which makes it extremely difficult to break them. When machining this makes it difficult to get a smooth cut. It requires a lot of attention to feed and speed in all operations. That also includes drilling. It is essential to use a new and very sharp cutter.

When drilling to prevent nasty long strings from forming on the bit you must select feed and speed that produces nice clean spirals of swarf such as you normally see from metals. Even a little too fast on the rpm and the plastic melts and pulls out of the hole to form a nasty mess.

Proper drilling rates look like this. If it doesn't then slow down and/or increase feed rate.


06-10-2013, 11:28 AM
This was designed using SketchUp.



That includes all the patterns for CNC cutting which were exported as DXF. CamBam was used to create the G-code.

06-10-2013, 03:05 PM
What a nifty pump. It looks familiar, but I can't put a name to it. Lobe pumps typically have two rotors, but maybe that's what it is. It resembles the Gerotor http://en.wikipedia.org/wiki/Gerotor but it's different.

Did you specify the profiles, or are there plans?

[edit] According to this, it is a Gerotor http://en.wikipedia.org/wiki/Gear_pump. And it's like an automotive gear pump (it's been a long time since I disassembled one of those).

06-10-2013, 04:30 PM
It's a gerotor pump and yes, there are plans. I need to tidy it up a bit but I will be posting the model on the 3D Warehouse. I will mention it when I do.

Gerotor pumps exist in a multitude of designs, mainly to evade patents I suspect. From my reading I see patents where the only difference is a very slightly different curve. What makes it a valid new invention is that the curve has a different mathematical name so it cannot be considered a derivation of a similar design that has a mathematically different curve. It doesn't matter if the two curves just happen to share a large portion that are nearly identical. All you have to do is claim that yours works better because of the entirely different curve. The patent office doesn't care if something actually works even though they are called "Utility Patents".

06-10-2013, 04:41 PM
These are the curves. What is not immediately apparent is the the inner rotor curve is not the exact inverse of the outer rotor curve. It cannot be, the number of teeth are different. The curves have formulae, the simple one is the half circle on the outermost part of the curve. That blends into a short section of an ellipse which then blends into the inner curve which is a section of an epicycloid.


If you are wondering how accurate the curves are, this shows the data points of the various curves.


06-10-2013, 04:45 PM
Could the same have been done with a gear type setup or is that not positive displacement. Engine oil pump style. Just trying to learn myself?

06-10-2013, 04:58 PM
A pair of ordinary gears will pump just fine. It is not nearly as efficient in terms of volume per revolution. It also need four bearings, this only has one. Two lobe rotor pumps require gearing to synchronize the lobes which is a lot more expensive in terms of parts count. This pump has only two moving parts in the pump itself.

06-10-2013, 06:51 PM
Evan, Another concept to move water over low lifts is to use a fan driven air compressor to inject air into concentric tubes. A trade name around here was Bowjon. Sometime there is more wind power available than solar. Our ground water temperature is around 65F and the solar water heaters worked well enough that the Government gave a tax incentive to install them.

06-10-2013, 07:24 PM
This system needs to be free of air. The collector circuit is multiple round loops of tubing mounted at about 45 degrees so if it has air in it it becomes very hard to pump around all the loops. Once it is full of water then it is like a siphon and the only power required is to overcome the resistance. Wind won't do any good since I am collecting heat more than power. I also have lots of trees which are staying up since they help block the wind. Wind in winter causes huge heat losses.