At what size do turbines stop becoming efficient?

[Kidphysics]

You never see them on small scales. Why not use small turbines with magnets on them to start a current?

 

[russ_watters]

It probably has more to do with output than efficiency.

 

[Kidphysics]

Can you elaborate on this?

 

[Jobrag]

One of the criteria that affects turbine efficiency is blade tip clearance; proportionately it is higher in a small turbine then in a big turbine limiting overall efficiency.

 

[Enthalpy]

What do you call small? Dentists have turbines in D=10mm tools.

In such a size, objects are manufactured easily with <100µm tolerance. Bigger wheels need clearance because of centrifugal force that strains parts, made worse by creeping if hot.

 

[SteamKing]

You never see them because they are small and easy to hide, like the APUs on commercial jets.

 

[Jobrag]

What do you call small? Dentists have turbines in D=10mm tools.

In such a size, objects are manufactured easily with <100µm tolerance. Bigger wheels need clearance because of centrifugal force that strains parts, made worse by creeping if hot.

Yes but I'll bet that a dentists drill has never been sold on the basis that it is more efficient then a competitors. Same with APUs efficiencey will be low down the list of desirable criteria when speccing one.

 

[tasp77]

IIRC, the neon coolant pump on the Hubble space Telescope is a very small turbine. Is that the one that spins at 400,000 RPM ?

 

[Phrak]

You never see them on small scales. Why not use small turbines with magnets on them to start a current?

Look up "The Economy of Scales".

It means it cost's less to make a few big things than to make a lot of smaller things that do the same job.

It doesn't always work. It was a rule of thumb wisely applied during the industrial revolution. It works for turbines producing power.

It costs less to make and operate 10 turbines that produce 10 megawatts apiece than 100 turbines that produce 1 megawatt apiece.

In particular, the economy of scales rule, when translated to mean "bigger is better" is broken with integrated circuit scaling. Smaller tansistors mean less wasted power, higher speed and more transistors on a chip. Smaller profiles are better in integrated circuit fabrication.