I’ve seen a lot of gizmos that generate electricity from residential water pressure. Basically a hose is connected to a turbine which turns the shaft of a generator.
Every one of these I’ve found uses a turbine of some sort.
From a mechanics viewpoint wouldn’t a piston-driven setup be more efficient? I.E. be able to generate more electricity from the same water source?
I realize a turbine is simpler - fewer parts, lower cost, and more relaible - but if I were looking to maximize power, wouldn’t a piston mechanism be better?
Hyrdo electric dams use turbines - but that’s large scale - is the small scale different?
pistons need huge explosions to move and need much power to start. a turbine starts with a finger. turbines can be cascaded for more output when appropriate.
A turbine is the most efficient way of extracting power from a flow. I am not even aware of any way to do this with a piston. Can you explain what you are thinking of a little more explicitly?
A piston mechanism would make a terrible hydro-generator. A piston is a reciprocating device, meaning it constantly moves, comes to a complete stop, then moves in the opposite direction etc. Besides, the reciprocating energy of a piston is always redirected thru connecting rods to convert it to circular motion, whereas a turbine is inherently that already (circular, and smoothly continuous not reciprocating).
On a turbine the water flow traveling at a velocity hits the blade causing the turbine to turn. Using the head (pressure) to creat the velocity.
On a piston engine water at high pressure would enter the cylinder pushing the piston down energy gained. Near botten dead center an exhaust valve would have to open. The pistion moving upwards would then push the water out of the cylinder energy used.
Rotating power from fluid flow and pistons is done all the time. Hydraulic motors used in an enormous amount of machinery are pistons, as are many hydraulic pumps - being pretty symmetric.
Probably the main issue is one of volume versus pressure. Hydraulic systems are high pressure low volume. Turbines are high volume and relatively low pressure. (As a very broad brush approximation.)
This would be inefficient if we were relying on friction to bring the piston to a stop at each end of its travel, but we don’t. The piston and the crankshaft/flywheel exchange kinetic energy with each other, and that exchange need not be inefficient at all. (there is some friction loss associated with the piston sliding in its bore, but this is unrelated to the fact that the piston reciprocates.)
As Francis notes, piston-based hydraulic motors are ubiquitous. They can also be pretty efficient. There are three fundamental piston-based hydraulic motor types in existence, and all of them tend to exhibit efficiencies over 90%.
Turbine efficiencies vary depending on type, size, and in some cases RPM. Pelton wheels are common for very small-scale hydroelectric power generation, as they perform well in situations with modest source pressure and high flow rates. This book suggests that when the RPM is suitably matched with the water jet velocity, the efficiency of a small-scale Pelton turbine can be around 85%.
So to answer Uncle Fred’s question, yes, at small scales, piston-based hydraulic motors can have efficiencies that are competitive with turbines. But as you’ve correctly surmised, a turbine is far simpler to design and build. Piston motors require numerous parts, close tolerances, and a careful choice of surface finishes. Moreover, a motor using hydraulic fluid is well-lubricated; a motor using water has a more difficult situation, in that not only is water a less-than-ideal lubricant, it may also promote corrosion of metal parts. Light surface corrosion isn’t a huge problem on a turbine, but it is usually disastrous for the longevity of sliding joints (e.g. piston-cylinder seals).
I think MachineElf has largely answered my question, but to respond to Absolute;
Picture a steam engine of any sort, with one or more pistons. I was thinking along the lines of the same thing but with water pressure the impelling force instead of (much greater) steam pressure.
I was trying to remember the name of those, all I could think of was spinning cup thing. To add only slightly to what is said, a turbine uses a simpler cycle. With a piston engine of any kind, some of the energy produced has to be used to clear a filled cylinder. Some energy, in this case stored in a flywheel, is needed to push the water out a cylinder after the water flow has filled it. It’s operating just like a steam engine except using water instead of steam.