Green Propulsion from tidal currents

One of my favorite places to fish is a rip just south of Chatham Light on Cape Cod. The bottom there rises from fifty feet to just fifteen, and the prevailing southerly current rushes over the ridge at about 10 knots. Properly equipped, a boat anchored at such a spot could employ water-driven generators to recharge batteries that are later used to power electric motors that propel the boat.

With a bit of ingenuity, a single device could perform both functions, using the propeller to drive a generator winding, and then using the winding to drive the propeller.

My initial questions in the evaluation of this concept all relate to energy conversions and balance. Of course there are energy losses at each conversion (mechanical to electrical, electrical to chemical, chemical to electrical, and electrical back to mechanical.) Obviously, if the boat uses only it's drive propeller for recharging, then it will have to spend more (time x current speed) at anchor than (time x drive speed) cruising off the batteries. So there's a need for additional charging units that are deployed at anchor. The additional charging units could be deployed on cables off the stern, or mounted on davits that swing them over the side. What does the energy balance look like from a practical point of view?

Batteries are a big issue--space, weight, heat dissipation, service access, and, oh yes, cost. Since this boat would theoretically never require that the operator purchase fuel, I suppose you could figure the net present value of 10 years of projected fuel purchases that are offset by the batteries, but that will still be a big nut to crack. On the other hand, battery technology, especially for automobiles, seems to be improving, so maybe the cost factor will come down.

What about the dual-service propellers? Can you design a propeller that functions efficiently as both driver and driven? Should the propeller be rotated 180 degrees between the two functions?

How does scale affect performance? Will this work best for a 25 foot fishing boat, a 50 foot coastal cruiser, or maybe even something bigger?

How will early adopters find the best places to anchor and recharge? Maybe a website: Swiftwateranchorages.org!

If this is a harebrained scheme, please let me know before I invest ten years of my life on it. Thanks!

 

A variable pitch VP propeller would be the ideal for that.

 

Few places have 10 knot currents to harvest. If a boat was always anchored in a strong tideway, a few days charging might give enough battery for an afternoon's use. Anchoring a boat in a violent tidal race is not considered the best seamanship. Floating tide mills were once common in rivers and strong tideways. A catamaran arrangement held an undershot water wheel which ground grains to flour.
Also land based ones are still around.
http://en.wikipedia.org/wiki/Tide_mill
http://www.vicnewey.co.uk/mills/

 

The basic problem is that extracting power from a free stream and putting power back into it are completely different problems and no device will do both with any kind of efficiency (which has a separate definition for each function). An efficient prop has low slip, and an efficient turbine has a high blockage. Considering the weight penalty of batteries you need an efficient prop to minimise battery requirements. These make lousy turbines, so a separate device is needed. It is a fairly expensive way to make power even over the long haul, but it could be done with a ten knot current.


If you consider the power input as the free stream flow rate of seawater times its kinematic energy through an area equal to the turbine's swept area and you consider the power out as the prop thrust times boat speed, you'd be doing well to get 10% round trip efficiency.

turbine eff........... 25% ..... 0.25 (35% is upper limit in a lab)
transfer losses....... 5% .... x .95
gen eff .............. 85% .... x . 85
charger eff ......... 90% .... x .9
batts round trip ... 70% .... x .7
motor controller ... 90% .... x .9
motor eff ............ 85% .... x .85
shafting losses ...... 5% .... x .95
prop eff .............. 80% .... x .8
.................................. = 7.4% efficiency

The drag from the turbine would be several times the strength of the prop thrust if one hour charging were to get you one hour operation.

 

Phil--
So if my math is correct, I need 2 turbines, each with a diameter that is roughly 3 times the diameter of the propeller, just to achieve an energy balance that allows 1 hour of cruising at 10 knots for each hour spent anchored and charging in a 10 knot current.

Efficiencies might be improved by use of azopod type devices, where the winding is on the same shaft as the propeller or turbine blade--of course such a device would have to be able to handle all speeds without reduction gears, etc.

Weights of batteries would to some extent be offset by losing that diesel engine, shaft, fuel, tanks, cooling system, etc.

So the anchoring system would have to be robust to withstand the drag force on the turbines...

Thanks!
Stu

 

Sawdust,

You overestimate the weight savings from taking diesel equipment out, and dreadfully underestimate the weight of the batteries that you will need to replace it.

As a quick rule of thumb, it takes 100lbs of deep cycle batteries to replace 1lb of diesel fuel in terms of usable power, or about 715lbs of batteries per gallon of diesel you take off the boat. So if you are currently using 5 gallons of fuel per trip you will need something on the order of 3500lbs of batteries to provide the same amount of power.

This is why electric boats all go very very slow, with very high l/s ratios. It is the only way to reduce power usage enough to make them 'work' in any conditions.

 

Stumble--
715 pounds per gallon is a non-starter; rather than lead-acid deep cycle batteries, how about the lithium batteries used in hybrid cars?
Stu

 

How about use of fabric for underwater 'windmill'.

Since you need a big area to capture low-density energy of the slow tide, and, if it is Big Thingy that doesn't need to be super-strong(like a normal prop) it would be nice to be able to collapse, fold and store it.

I'm thinking of something similar to fabric windmill(but ratios shifted for water density and speed) on the end of a pole, mounted either in-line or at right angle, with an inner-shaft that would be connected to the generator top-sides(out of the water).

The WaterMill part would be made of sticks, fabric and cordage, and would be cheap and easily repairable/discard-able for when a submerged log or fishing net comes along and wrecks it. For the speed it would be rotating I don't think it would need to be precision balanced and could be repaired with a wide range of sticks, fabric and cordage.

Think "big and cheap" because we don't care if it only gathers 10% of the tidal energy passing through it. We only care about how much energy it gathers for how much it costs, and how much space it takes up when all folded up for storage.

Collapse down for storage similar to an umbrella.

I'm thinking of something about 8' diameter, but that could be rigged to with cords to be as little as 3' diameter for use in shallower waters(where tides flow fast).

 

While experimenting with this setup, I'd bring along a kicker to get home. I do a lot of sailing in that area and sure know you wouldn't want to be caught in those currents without any way to get home.