After testing the first solar panel powered pump for more than a month, we recently installed the solar panels on the second pump shed. It took a little longer to get the solar controller wired up to another hand wired prototype pump motor controller, but today was the first day that that was turned on and is now up and running.

An interesting tidbit I haven’t mentioned yet is that there was a bit of a design flaw in the way I ran two pumps to the larger sand filtration area. The larger filtration area contains two large 12″ diameter pipes with small holes in them buried in gravel and covered in sand with pumps at the other end of those pipes effectively sucking water down through the sand and pumping it out in the middle of the pond. But if only one of the two pumps is on, it will simply suck water out of the other pump’s pipe rather than suck down through the sand. In effect, it is sucking water backwards through whichever of the two pumps is off.

One of those two pumps was in the first pump shed that was wired up to the first set of solar panels, but I couldn’t turn it on until the second shed was wired up.

So today, I went from having one circulation pump running (the only pump that pumps water from our smaller sand filtration area) to having 3 pumps running (one pumping from the small filtration area and two from the larger one).

Here are some photos of the solar controller in operation towards the end of the day:

You can see that it is running the load at 25.6V and 4.5A, which comes out to about 115W.

I estimated the flow rate of the pump at about 20cm per second. I haven’t had the chance yet to measure this with our jury rigged water flow rate sensor, but we have a very long handled lightweight sponge that we use to clean the solar panels, and I put it in the water so the sponge is at the outlet of the pump and it appears to want to move away from the pump outlet at about 20cm per second. Maybe I’ll get a chance to try to water flow rate sensor soon.

Since the outlet of the pump has about 0.07 m^2 of cross sectional area, multiplying this out gets us 0.2m/sec * 0.07 m^2 = 0.014 m^3/sec of flow rate from each pump. This comes out to about 122 L/sec / kW which is about 10x less than the value we measured in our initial test of the efficiency of this pump design.

I will need to double check that my estimate of the flow speed is right and the current sensor on the solar controller isn’t confused by the PWM of the motor (it didn’t say anywhere that it is “true RMS”). But even so, this result may not be too surprising. There are friction losses in the ~50m of PVC pipe. Additionally, the water level hasn’t crested the retaining wall yet so the water circulation is going through the tiny cracks between the bricks, and this could easily be slowing down the water flow there creating a larger back pressure. So hopefully this is a worst case scenario, and based on our measurement of two commercial pumps a while back, we are still about 6x more efficient than our measurement of the “pump payanak” and almost 12x more efficient than a large aquarium water pump.

As a final note, I found it interesting to calculate the turnover rate of the pond now that I have all three circulation pumps on.

First, I noticed that Google Maps satellite view has updated to the point where you can see our pond in its early stages of digging, so I used that to take some rough measurements of the oddly shaped pond and came out with approximately 1600 square meters of surface area. At present, I would estimate an average depth of less than 1.5 meters, so using 1600*1.5 = 2400 cubic meters should be a reasonable estimate of the volume of water in our pond. (Maybe a bit of an overestimate.)

Multiplying the above mentioned flow rate by 3 (for 3 pumps) gets us 0.042 m^3/sec.

2400 m^3 of water / 0.042 m^3/sec = 57143 sec to do a single turnover of the water in the pond. At first this might sound like a lot, but actually it is less than 16 hours. (Bear in mind that our pond covers almost half an acre.)

So our solar powered, always on, 3 circulation pump pond filter system should have a turnover rate of about 1.5 turnovers per day in its current configuration.

According to this CDC web page, home swimming pools should have a turnover rate of about once every 6 hours or 4 turnovers per day which is about 2.5x faster than I currently have. But then again, this same page says that the purpose of filtering isn’t to disinfect but to remove particulate matter to make the water more clear. They are using “rapid sand filtration” which doesn’t build the bacteriological layer that a slow sand filter creates in order to disinfect, and based on the size of the sand biofiltration areas, this definitely falls within the category of “slow sand filtration”.

I will try to measure this rate more accurately once the water level crests the regeneration zone retaining wall. This might reduce some back pressure resulting in reduced power for the same flow rate. I might also be able to optimize some other parameters. But in the end, I’m pretty happy with the numbers I’m already seeing. I remember a while back seeing mention of the fact that “natural swimming pools” have turnover rates of something like 1-2 days. So I am already around there. And the current draw is such that a couple of solar panels and batteries can power it 24 hours a day.

So far so good!