Tidal Power & the Magnus Effect

svetz

Works in theory! Practice? That's something else
Staff member
Moderator
There are a number of great uses for Hydroelectric (like dragging a hydro turbine behind your sailboat) when the conditions are right.

But what about tidal power?

The Magnus effect seems promising. See https://www.diysolarforum.com/index.php?threads/the-magnus-effect.26/ for discussion in air.

The density of sea water is 1000x that of air, and it's not uncommon to see currents over 0.5 mph over 18 hours a day under bridges or other restricted flow areas. Since the Magnus effect is linear to fluid flow; just multiply the numbers in the other post by 1000.



None of what's here is new, folks have been looking into this for a long long time. But AFAIK, none of these are being used for power generation anywhere; so it makes me think there's some fatal flaw with it. The US Navy did a report on it here: apps.dtic.mil/dtic/tr/fulltext/u2/a165902.pdf, I really like the rudder/keel applications.
 

aboy

New Member
There are a number of tidal powerplants:
But the best/biggest streams are not yet developed. There are enormous streams in north east of Canada and north east of Russia, but they are far from cities. The biggest tidal streams near cities is probably those around Great Britain.
 

svetz

Works in theory! Practice? That's something else
Staff member
Moderator
...The density of sea water is 1000x that of air, and it's not uncommon to see currents over 0.5 mph over 18 hours a day under bridges or other restricted flow areas. Since the Magnus effect is linear to fluid flow; just multiply the numbers in the other post by 1000.

How is it I never ran the numbers here?
F/L = ρv 2πr²ω, where r is the radius of the cylinder and ω is the cylinder rotational speed.

So, if the cylinders had a 1' radius, then for 1m cylinder length at the power is:

Code:
                        power kW, cylinder revolutions per second
flow, mph   flow m/s   0.25     0.5      1        2       3       4
   .25      .111        .06      .117     .236     .471    .705    .942
   .5       .237        .126     .252     .5      1       1.5     2
   1        .447        .237     .474    .949     1.9     2.8     3.8
   2        .894        .474     .949   1.9       3.8     5.7     7.6
   3       1.341        .711    1.4     2.8       5.7     8.5    11.4

From the table in the OP the water speed is between .5 and 1.5 for 2 hours/d, so at an average of 1 mph with a 1 revolution per second would generate 12 kWh per cylinder/d. Rougher is supposed to be better, so perhaps fouling wouldn't have a big impact. I could see a revolving line of these under bridges. Easy to adjust the power output by altering the spin rate.

Let's change the units to revolutions per minute but keep the diameter and length the same for the 1 mph case:
Code:
                        power W, cylinder revolutions per minute
flow, mph   flow m/s    1     5    10     15     20      30     60
   1        .447       16    79   158    237    316     395    949

Using the same case as above, let's increase the radius from a foot to a meter:
Code:
                        power W, cylinder revolutions per minute
flow, mph   flow m/s    1     5    10     15     20      30     60
   1        .447       76   378   757    1135   1513   1891   4540
 
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