Could multiple ram pumps be used to fill a watertower for a pico hydroelectric system?

Question

Since a watertower stores potential energy just like a reservoir does for a hydroelectric dam, could I use that to power a pico (<5kW) hydroelectric system for my homestead?

Now, I have read that the problem is not that this model will not create hydroelectric power, it is that the energy required to refill the tank after consumption is greater than the energy generated. Consequently, most of the pico hydroelectric systems on homesteads use the actual water source as the energy for the hydroelectric generator. In other words, the topography of their land is what they use to create the pico hydroelectric system, and the manipulation of their water source is part of the design. Either they build a reservoir or they already have land that gives them an adequate head to generate power.

Additionally, I have seen the use of ram pumps for all kinds of energy transfer inventions on numerous homesteads. Specifically, the use of ram pumps to replenish a homestead water tower irrigation system was the inspiration. But, there was only one ram pump in this case (which was plenty). If the water source is good enough, why could numerous ram pumps not be used in different spots in the source for replenishment? I would guess most people have water sources for dozens of feet if not hundreds.

1. Assuming that someone has a good running water source on their property with enough flow and volume to run one or more ram pumps to the watertower for either demand determined replenishment or constant flow generation, that source would essentially be limitless and ram pumps could run all the time.

2. Even with a constant flow system, a ram pump could be installed at the outflow to recycle additional water. The outflow will simply just be sent downstream, so why not use it again?

3. If this setup is not adequate to replenish, an additional small power pump could be added for replenishment. Collect it in another tub and pump it back up as close to the tower as possible, preferably.

I understand that ram pumps vary to a great degree in efficiency. However, this USDA report states that some ram pumps could be upwards of 60% efficient.

It just seems like there would be enough with all this to create an energy surplus. Would this be something that might work?

Answer 1

Every time water flows through a ram pump it loses energy (slows down). That means there is less energy available (in the water) for the next ram pump, which will make its yield (the amount of water it can lift to a higher elevation) smaller. Eventually you reach the point where the water is moving so slowly that it doesn't contain enough energy to operate the pump at all, and the ram pump will stall.

The total amount of energy available to you is simply related to the mass of water entering your property and the vertical distance it falls before exiting your property. That number doesn't change regardless of how many ram pumps you install. Whether you build one big ram pump to extract all of the energy, or five small ram pumps, doesn't matter. In fact, because of the diminishing returns mentioned earlier, it's better (more efficient) to go with as few pumps as possible — ideally one — and make them as big as possible.

Remember, it's running water that makes ram pumps work. Anything you do that slows the water down undermines their yield. Ram pumps don't work in pools, ponds or lakes. You're not harnessing water energy in the horizontal plane — the energy is stored in the vertical plane.

PS: I don't think you appreciate what a pitifully small amount of energy can be stored in a 'water tower'.

Let's take a hypothetical — yet massive — tower that contains 100,000L of water. Let's say that you are able to magically elevate this tower 100m above your property. The total amount of potential energy stored in the water = mgh = 100000*9.8*100 = 98,000,000J. That would power a 5kW hydroelectric system for 19600 seconds (5.4 hours).

Now, that may sound useful, but given that it would probably take you — under ideal conditions — more than a week to fill that tank up with a ram pump... it's hardly practical.

98MJ is equivalent to 27.2kWh of electricity — worth about $10 in this part of the world. A 100kL tank would cost a minimum of $10k to build, which means the tank would need to be filled 1,000 times before it paid for itself. If you fill it each and every week, you would be waiting 20 years for it to pay for itself, and before you saw even a single cent of 'profit'. Depending on the material and climate, the tank would likely have started leaking by then, and be due for replacement anyway... so in the real world you'd never see a profit.

Pumping water into a manufactured container of any type makes no financial sense — at any scale — even if the energy used to perform that pumping is completely free. The only way the math works out is if you can use (something like) natural terrain as a container to eliminate 99.999% of the construction costs. That's why hydro systems primarily get built in hilly/mountainous regions.

tl;dr: Ram pumps exploit the energy embodied in running water to redirect a fraction of the flow to a higher elevation. Every pump you add removes energy from the water and suffers from diminishing returns. Eventually there is not enough energy in the water and pumping fails. The potential energy in elevated water is tiny, so you need enormous volumes and really high elevations to do anything worthwhile. Unless you are able to exploit hilly/mountainous terrain to create truly vast amounts of elevated storage for virtually free, the savings from any power produced will never cover your construction costs.