microtidal power generation idea, suggestions wanted

Hello,
I live in a place where the tides are an average of 15 feet different from low to high, so thats 15 feet of up or down every 6 hours.
I am thinking of a fairly simple gear system to harness a bit of this to make electricity. components would be
1.A float of some kind
2. A gear rack (float pushes a straight member with 'teeth' up with the incoming tide, or pulls it back down with the outgoing tide and the weight of the float, this up and down motion creates a very slow turning iin the other part of the gear rack, maybe a gear with 1.5 inches diameter. at 1.5 inches diameter and 2 feet of linear motion by the tide in an hour, there would be about 5 revolutions in one hour of the initial gear.
3. several gears to increase the rpms. Only a few changes of 1.5 inches to 24 inches diameter pulleys would be needed to get this very low rpm up to around 500 rpms
4. an alternator and then on to the normal Renewable energy battery bank system.

so in theory it seems it would work great, producing high rpms at mid tide and slowing to a stop at low and high tides, and would be a constant source of energy regardless of the weather.
its just a question of how much buoyancy and weight would be needed in the flotation system and how strong do the gears have to be.
I would love some input about the feasability of this plan by anyone with real fabrication or engineering experience as well as being pointed in the directions of the right equations i would need to figure it all out. the speed of the tide is one fixed variable (fixed in the sense of always falling within a specific range) and then at the other end would be the alternator which would require a certain torque range and rpm range, so depending on what alternator was selected, those variables should result in specifications for the float and gear system.

it would take alot of force to turn that many gears - and even more to drive the alternator.

i saw a gunius tidal wave generator system on shark tank the tv series. remember those shake flashlights that charge when you shake them? a guy designed huge ones that float. i thought it was genius free energy- and easy - plus easy to drive. the sharks said NO to his investment proposal but its genius. may be patented

From what I've heard of tidal power, it is very hard to get the efficiency up high enough to be worth the investment. There is so much tidal force, but it's VERY difficult to capture this in terms of generating potential. Considering the amount of the Earth's cities and resulting population which are near the tidal zone, you would definitely see A LOT of interest if you could pull it off. That said, the reason you find virtually nothing on the internet about tidal power, is that virtually nobody is even attempting it. It takes innovation and investment to get moving. Sometimes all the investment that is needed is time/personal energy. Go for it, if you have the time/energy to invest and you think that you can translate that force to power, but just be aware of the rest of this post. Lot's of people have wanted to harness this potential, and... there are no examples that I have seen or heard of that are worth it to build.

The thing I like about the tides, is that it is the ultimate perpetual motion machine. If someone, like Gates, with enough cash to front the project, was involved, I'm sure that a up/down train-wheel like drive system could be effectively produced with the right gearing and drive alternator, but everything would probably have to be custom machined to get it all right.

What i am envisioning is a very micro system. similar in scale to a windmill, using, for example, this generator http://www.windbluepower.com/Permanent_Magnet_Alternator_Wind_Blue_Low_Wind_p/dc-540.htm

if you look at projects on that website, there is one where they used the pumping action of cars driving across a road and translated that tiny up and down movement through a gear system to get enough rpms.

If i calculated right, that particular dynamo (dc generator) will have a max output of 236 watts. 18 amps at 12 volts at 2000 rpm. it produces 1 amp at 12 volts at 130 rpm. i'm trying to figure out the math to see how much weight and buoyancy would be needed supply the thing with enough torque at max capacity. Here, maximum potential would be midtide with a 30 foot tide difference from low to high. Thats 30 feet in 6 hours, and using rule of 12/ths we get 1, 2, 3, 3, ,2 1; so the 3rd and 4th hour of that tide cycle there would be 3/12ths of 30 feet or 30/4= 7.5 feet.
so 7.5 feet/hour is .125 feet per minute
(for gear/pulley ratios, I'm using http://www.blocklayer.com/pulley-belt.aspx)
if that is set up with a gear rack turning a 2 inch gear, one circumference of the gear is 6.28 inches, or .523 feet, so that would give .239 rpms. if there is a 24 inch gear on the same shaft turning another 2 inch gear, the next 2 inch gear will have 2.868 rpms, now if we do it again, a 24 inch gear on that shaft turning a 2 inch gear, we are up to 34.4 rpms, now we do it again, and we get 413 rpms, and one last time, but this time the small gear we turn is attached to the generator and has a diameter of four inches and will spin at 2478 rpms as our approximate maximum speed. (it will be a little higher exactly at midtide) (the generator has no upper speed limit)
so, for the system so far we have a gear rack with a very long straight piece, then a 2 inch gear mounted on the same axle as a 24 inch gear, then another, then another,then another then the generator with 4 inch gear, so we have 4 axles plus the generator to get us up to proper rpms.

now lets go in reverse and see what it takes to get the minimum functional output of 130 rpms.
we go in reverse, that 4 inch gear will need the 24 inch one before it turning at about 22 rpms, so the 2 inch gear on the same axle will need its 24 inch driver turning at 1.8 rpms, then the 2 inch gear on that axle will need the 24 inch gear powering it turning at 0.15 rpms, which will require the initial axle to spin at .0125 rpms. if we divide 2478 by .239, we get 10,368
and if we divide 130 by.0125 we get 10,400 so that confirms the math for a potential gear scenario and rpms. now this .0125 rpms translates to about .39 feet per hour.
our smallest 'hold up' tides are around 10 feet in difference, so that gives us approximately one foot per hour during the slower part of the cycle, so we would still be operational.

now i think the torque will translate at the same ratio as the rpms, so for every foot pound of torque at the generator, we will need 10,400 at the initial gear, except that at the generator our gear is double the radius, so we divide by 2 and get a multiple of 5,200.
Now we know the generator has a max output of 236 watts at 2000 rpms. Since watts measures power, we should be able to calculate the torque, so how much torque is required at 2000 rpms to produce 236 watts of power?

using this calculator, http://planetcalc.com/1908/

236 watts at 2000 rpms requires about 1.13 Nm of torque. Now we convert Nm to foot lbs (http://www.convertunits.com/from/newton+meters/to/foot+pounds) and we get 0.83 foot lbs. so we multiply this by 5,200 and get 4,316 foot lbs of torque needed at original driver. that seems like quite a lot. now, the radius of the 2 inch gear is 1 inch, so we have 1/12th of a foot fulcrum so now we have to multiply by 12 and get over 50,000 pounds of force required, so if my math is right, the concept is a no go, as it would be really impractical to have a gear that could withstand that force and enough weight and flotation to drive it. : ( I'm not sure but it would seem however you gear it, the initial flotation to final watts ratio is going to be the same. anyone please correct me if i'm wrong, as i'm not an engineer.

Can you set up a reservoir to drive the generator hydraulically? Every week I go by a pond with a culvert connected to the bay on the other side of the railroad tracks. This creates a very powerful flow through the culvert.
I don't know what your shoreline restrictions are there. Permitting for a floating dock is pretty common but you could call it a dry dock and then use it for power when nobody is painting the bottom of their boat.

I think a flotation system would be very complicated, especially if you wanted to produce power on both the rise and fall of the tide. Using the float to compress air or water and force it through a hydraulic motor might require less engineering than using gears, but I suspect it would still be expensive and challenging to fabricate, and would probably require very large floats.

What might be cool, as Hans suggested, and if you had the ideal spot for it, would be some adaptation of the medieval tide mill approach - catching water at high tide and releasing it through a turbine at low tide.

well if my math was right above, or even nearly right, i would need some huge floats and some very strong gears which would make it not worth it. the idea was simple, have the appropriate weight and enough flotation to float twice that weight, so you get power up and down, but when that weight is 50,000 lbs it becomes very impractical and expensive, especially for only 236 watts. i'm much better off going with wind. the dam idea is good but not possible here for me due to land prices and regulations. i tried to do the math another way just calculating how much force would be required to get 236 watts of power at the very slow speed of the tide and got around 80k lbs. I'm not sure where the discrepancy comes from, probably bad math somewhere, but getting convinced its not a practical solution for me. thanks all : )

In Brittany , France there lots of old tidal mills used for grinding corn and making linen . It's very simple the tide fills a lagoon and then they let the water turn a wheel at low tide letting the water out . I always fancied doing one up to generate power . These were quite common until the industrial revolution in Europe There are even traces of them next to London
also in Brittanny they have the worlds only commercial tidal power station working on the same principal but a tad larger

David

There is even a natural lagoon like that near me (called rusty's lagoon) that fills and drains on all but the smaller tide cycles, its probably 5-10 acres area of the lagoon and the inlet/outlet is only about 16 feet wide. its on protected land, (kachemak bay state park)_ though.

Draw up a proposal for a park enhancement. A 16 foot high wheel With lights on it for enjoyment of the motion at night, If you take the gear ratio off the wheel at say14 feet that gives you a tremendous boost in speed for the generator. You could make it portable on floats like they do the fish wheels. Once people see the concept works then it may be easier to get consideration for other more permanent projects.

sounds fun but it might disturb the pristine nature of the area. we already have a huge hydro power project just up the bay at bradley lake. : )