Timeline for Is there a fundamental efficiency limit for generators?
Current License: CC BY-SA 4.0
12 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Aug 31, 2019 at 10:04 | comment | added | JanKanis | The chat discussion on the Betz limit appears to be gone. I'd just like to add that the Betz limit also assumes a turbine in a free flow that can also flow around the turbine, such as a wind turbine. For a water turbine in a dam that is typically not the case, which is another reason Betz's limit does not apply there. | |
| Dec 16, 2018 at 11:04 | vote | accept | ckrk | ||
| Dec 11, 2018 at 20:10 | comment | added | Thorondor | Let us continue this discussion in chat. | |
| Dec 11, 2018 at 20:06 | comment | added | Tygo Huurnink | you can generate more energy from it by placing another turbine behind it, but at the moment kinetic energy is extracted from the air, the air cannot mix with the other air / expand. If you were to prohibid expansion/interaction inbetween the 2 turbines, the limit will still be 16/27 from the original air flow. | |
| Dec 11, 2018 at 20:02 | comment | added | Tygo Huurnink | an Archimedean screw converts rotational energy into gravitational energy. The fact that the limit for Betz law is 16/27 is because the flow cannot be stationary for a nonzero starting velocity. Such limitations do not apply to Archimedean screws :) | |
| Dec 11, 2018 at 19:57 | comment | added | Thorondor | @TygoHuurnink Also, what do you think happens to wind after it passes through a wind turbine? Only a little of the remaining energy gets converted into heat, so most of it must stay in the air flow. Why do you think that a second turbine can't use that energy to generate more electricity? | |
| Dec 11, 2018 at 19:51 | comment | added | Thorondor | @TygoHuurnink To consider a simpler example instead, it's very well known that a screw turbine (i.e. an Archimedean screw used as a turbine) can have an efficiency above the Betz limit. For example, here's a 3D printed one with efficiency of about 80%. | |
| Dec 11, 2018 at 19:35 | comment | added | Tygo Huurnink | I think they mean 80% with respect to Betz's limit. A heterogeneous flow of air will only make it less efficient, as far as I understand it. | |
| Dec 11, 2018 at 19:21 | comment | added | Thorondor | @ckrk Betz's law applies only to the very specific situation of turbines extracting energy from fluid flow using a single rotor made of thin aerodynamic surfaces. To see this, note that in the proof it is assumed that the fluid flows through the turbine at a single constant velocity v. Designs that gradually extract energy using multiple layers of blades, such as this one, can do better than the Betz limit. | |
| Dec 11, 2018 at 14:30 | comment | added | ckrk | Thank you, Thorondor and @Tygo Huurnink for your answers! Both answers are interesting but seem somewhat contrary for me. Betz's limit would seem to be a reasonable limit, in the sense i would have imagined. However, the example Thorondor gives seems to clearly violate Betz's limit. From my understanding Betz's law should apply to all machines that use newtonian fluids as a working medium. Thorondor, would you care to remark why Betz limit seems to be inapplicable in the examples you give? Are they relying on some kind of non-newtonian working medium? | |
| Dec 10, 2018 at 23:41 | history | edited | Thorondor | CC BY-SA 4.0 |
deleted 11 characters in body
|
| Dec 10, 2018 at 23:34 | history | answered | Thorondor | CC BY-SA 4.0 |