Gravitational Potential Enery and Tidal Power Generation

1. Apr 1, 2009

stathi

After walking along the Thames every day on the way to work, I noticed that the water lever difference is between low and high tide is about 7-8m. Thoughts soon turned to how to harness this for power generation.

The idea I had is to have a large weight buoyed so that it would rise with the tide (for example a large container with two compartments; the top compartment filled with water, and the bottom (larger?) compartment filled with air or a vacuum). The weight lifts with the incoming tide, and on full tide it is secured into place effectively storing the gravitational potential energy gained from the lifting tide. The weight would be attached to some sort of gear system so that when it is released electricity is generated.

This seems simple enough.. but would it work??? I have many questions, hopefully one or two can be answered:

1) Is it possible to generate electricity from a falling weight (via gears or whatever)? If attached to such a system, how fast would the weight fall? How efficient would it be?

2) How many of these weights would be needed to generate a significant amount of energy? One big weight? An array of smaller weights?

3) Where would the displaced water go? (The bottom container would be fully submerged.)

4) Has this been thought of before (probably!), and if so, what's wrong with the idea for no one to have implemented it?

Thanks,
Stathi

2. Apr 1, 2009

Staff: Mentor

This idea has been considered in many forms.

Hydroelectric plants use falling water to turn tubrines, which turn generators. One needs a time varying magetic field to generate an force or potential to drive a current. Hydroplants use rotating generators which produce AC at 50 Hz or 60 Hz depending on the country.

For a falling mass m, the amount of potential energy = mgh, where h is the distance from reference to maximum height through which the mass falls. In free fall, the mass would accelerate at g, or slightly less if there is resistance. If one is attempting to extract energy from the mass while it falls, then there would be an opposing resistance, and if the opposing resistance = mg, then the mass would fall at constant velocity.

Tidal generation simply makes use of the change in elevation of the water due to natural tidal forces and the flow of water. There really wouldn't be a benefit to supplementing tidal flow or a reservoir with weights.

In a closed system, one needs upper and lower storage reservoirs. In an open system, one simply needs pools or ponds. In the case of the Thames, it's one large reservoir, in addition to being an estuary. The Thames, of course, is connected to an even larger reservoir which is the English Channel, and the world's oceans.

3. Apr 1, 2009

Danger

Welcome to PF, Stathi.
Most tidal generating systems involve using the up and down motion of floats to compress air which is then transmitted to auxiliary equipment. Hydraulics could also be used.

4. Apr 3, 2009

Dr.D

This has been studied specifically for the Thames, back in the 1950s or 1960s as I recall.