[Note that this article is a transcript of the video embedded above.]

On the hill above Granada, Spain, sits the Alhambra: a medieval palace and fortress complex of the historic Islamic world. Built and modified over centuries, the Alhambra is now a UNESCO World Heritage site and stands as one of the best-preserved palaces in the world.

Every city needs a reliable source of water, and that stood as a challenge for the Alhambra, perched high above the nearby rivers. Medieval engineers used a lot of creative solutions to divert natural sources of water and distribute it to the cisterns, baths, and fountains within the complex. Another YouTube channel, Primal Space, has an excellent video on all the ingenious ways they managed water, and one of the details in that video really caught my imagination.

Alcazaba is the stone fortress on the western tip of the Alhambra that sits higher than most of the palace city. Apparently, throughout the Renaissance (and maybe even starting in the medieval period), the fortress was supplied by water using a pump that had no moving parts. In 1764, a priest observed the device. He couldn’t understand how it worked, but he did his best to describe it anyway. More than a century later, a Spanish engineering professor, Cáceres, took it upon himself to try and recreate the device using the priest’s description. By that time, remnants of the device were gone. Historians estimate it existed until the end of the eighteenth century, when a higher canal replaced it. Even so, the professor got it to work, presenting his results at a 1911 scientific congress in Granada.

Was it the actual pump design the priest described? We’ll never know for sure, but it seemed likely to that professor, and more recent historians have found it plausible. And that’s pretty fascinating to me. A pump with no moving parts, able to lift water above its source, quietly serving a hillside fortress centuries ago. It is clever, effective, and, all these years later, mostly unknown today. You can’t pick one up off the shelf at your local hardware store, at least not yet. So I decided to take after Professor Cáceres and try to build one myself. I’m Grady, and this is Practical Engineering.

There’s something really magical about taking advantage of flowing water to accomplish work. I don’t know exactly what it is. Seeing a natural force, like the flow of a river, interacting with human ingenuity to do something important - it’s really cool to me. And it’s especially cool when it’s purely mechanical. Don’t get me wrong; I love electronics, circuits, and sensors. But doing a job with water alone - you have to admit that there’s something special about it. I’ve covered a few devices like this before. I built a trompe, which is basically a water-powered compressor. I also built a ram pump, which is a water-powered pump that uses check valves to harness kinetic energy, converting it to pressure. But I have to admit that Primal Space’s video is the first time I had ever heard of what seems to be mostly referred to nowadays as a pulser pump. And there really isn’t much information out there about them, despite the fact that they’ve been around for centuries. The idea isn’t really that complicated, but the details are a little tricky, so I decided I would try to come up with a design that boils it down as simply as possible. And you know we have to break out the acrylic.

I just had to tap the holes… then glue everything together. Now, let’s turn on the water so we can see this in action.

Step one is this basin up top. Rather than connecting directly to the hose, I wanted a free surface of water at the top, just so it’s clear, from an energy perspective, that this is the starting point. This tank provides a simple, consistent, and obvious input for the pulser pump. It’s the equivalent of the end of a canal in an ancient palace, and the goal is to raise the water above this level.

From the basin, the water falls down this vertical pipe. But if you look carefully, you can see it’s not just water. The water flows into this tee fitting that acts like a vent, allowing the stream to kind of swirl around and draw in air. There are quite a few ways to intentionally mix air and water. The historical description of the pump at the Alhambra was pretty unclear when it comes to this part. The priest didn’t provide much detail about how the air was entrained in the downward flow. Professor Cáceres tried two methods and had the most success using a whirlpool to draw water and air downward. I don’t know if this is exactly what he tried, but it is dead simple, and it worked surprisingly well. You can see the water in the pipe is full of bubbles, and it’s moving fast enough to carry them into the next tank.

The goal in this area is to separate all the air from the water. You can see the bubble float upward while most of the water continues onward. The sloped top helps trap the bubbles, so the flow exiting on the right is just water.

So far, this is basically just a trompe. I mentioned I built one of these before in my backyard and made a video about it. It looks a little different from this one, but the concept is basically the same. Entrain bubbles of air in a stream of water, carry them downward, and then separate them out - now under pressure - so the air can be used for things like smelting, powering tools, or in my case, blowing some dry grass around. It was just a scale demonstration.

Trompes aren’t used much these days. It’s easier to buy a compressor than to build a piece of infrastructure. But it’s still a cool idea, and their use is being explored to aerate remote pools of mine waste to speed up the bacterial reactions that can help clean up contamination. There are probably quite a few edge cases where a source of pressurized air is more valuable than a source of moving water, and a trompe basically lets you make that trade with no moving parts or electricity.

The sloped top helps trap the bubbles, so the flow exiting on the right is just water. You can see in my model, there’s a riser on the right, just like with the trompe demo. The purpose of this is to create enough pressure to encourage the bubbles upward. You can imagine if there was no back pressure on the system and I just let the water out at the bottom of the separator, eventually it would just fill up with air. That’s not what we want. So the water has to flow up the riser and then out through this hose, keeping the bubbles under pressure so that they’ll flow out of this tube: the discharge line for the pump.

I tried all this in my garage first, but kept spraying the ceiling, so I eventually decided to do this outside. My discharge line runs up above the inlet tank. As bubbles move into the separator, they float upward and out of this pipe. But, because the pipe is pretty narrow, water gets kind of trapped between the bubbles. This is a little finnicky, but basically, the buoyancy of the air mixed with the water occasionally creates enough lift for the water to make it all the way to the top. And now you can see why they call this a pulser pump. You don’t get a very continuous flow. But look at that! The water is actually going a lot higher than where it started in the upper tank. We are moving water uphill with no moving parts.

Actually this part of the pump is a pretty ubiquitous design. It’s usually called an air lift pump. Basically, pump air bubbles to the bottom of a pipe, and let them carry water upward. These are often used in dirty situations where you don’t want sand, grit, or plant matter clogging up the impeller of a more traditional pump. They’re not very efficient, but useful in certain situations like wastewater plants and dredges. And, this is also how coffee percolators work. The steam bubbles carry the liquid water to the top where they can percolate downward through the grounds.

I’m recirculating the water in this demo just using a bucket and pump below the table, and that drives home a couple of key points here. For one, all the water running through the pump does not actually get pumped. In fact, in my little demo here, I didn’t actually measure it, but I’d guess the discharge flow rate is somewhere less than five percent of the total flow rate through the pump. You need a lot of water to move just a little bit upward. So for two, this is not a free energy device in the same way a hydropower turbine isn’t producing free energy. In a practical sense, the pulser pump is extracting energy from the flowing water to push water bubbles downward, temporarily storing the energy. Then it’s extracted again to push some of that water back up.

So it really is that simple. A pulser pump is basically a combination of two steps: a trompe to supply the bubbles, and an air lift pump that uses those bubbles to carry water upward. But in some ways, it’s not simple at all. Two phase flow, where air and water move together, is pretty complex. If you thought fluid dynamics was tricky with one fluid, just try using two! You can tell just by looking at my demo that there’s not a lot of stability here. At the down tube, sometimes you get a regular stream of small bubbles, and occasionally you get one big one. At the discharge, sometimes you get regular pulses; sometimes you get big bursts. Every step of the process is just so …gurgly. There are a lot of knobs to turn here, and they all affect the system in different ways.

Let’s say you have a fixed flow rate, and a fixed amount of height between your inlet and outlet. You still have to select the diameter of your down pipe, which will affect the fluid velocity, and so how much air you can draw in. There are probably many different ways to mix the water and air that are more or less efficient, depending on the configuration. And there’s the diameter of the discharge line. A bigger pipe can move more water, but too big and the bubbles don’t crowd up enough to carry water with them. There is quite a bit of engineering guidance out there for air lift pumps, since they’re pretty widely used. Not so much for trompes, although I did find an interesting paper in the Journal of Applied Thermal Engineering. The author called them “hydraulic air compressors” and that’s actually one of the tricky parts to finding more information on devices like this. Since they’re pretty obscure, there’s not much consistency in terminology. The most I could find on pulser pumps was a few old YouTube videos and college projects. And this recent paper on the hydraulic techniques for water supply at the Alhambra doesn’t even venture a name for the device used there.

So this is still kind of just trial-and-error engineering. I’m sure I could spend hours trying different configurations and improving this demonstration. If you’re a grad student looking for a thesis idea, I think pulser pumps would make a pretty interesting project, because I can see some applications here. In fact, I’m not the only one.

Hydraulic ram pumps are pretty popular around the internet and in rural areas that have abundant water but no electricity. They were well known by the time Professor Cáceres did his experiment in 1911. In his paper, he said about the pulser pump:

“This arrangement will always have, over the hydraulic ram, the advantage of eliminating valves entirely, since it contains no moving solid parts. Doing away with the ram strokes seems to remove any source of fatigue in the pipes and, of course, the very annoying noise that makes the ram inapplicable near living quarters.”

I can’t help but think back to him in his lab, seeing the water spurt out from the top of the discharge line for the first time. You can tell his excitement in the paper:

“Beyond its historical appeal, the idea has real value for modern engineering. In cases where efficiency is not critical, reviving it could solve practical problems, using a layout so simple that it is remarkable it has not become common knowledge after several centuries.”

I wonder if he would be a little disappointed that the idea never really did catch on, despite its novelty. But I still think it’s pretty cool. And maybe someone will see my demo working and try it for themselves, carrying the ancient idea forward for new applications. Thank you for watching, and let me know what you think!