CR4® - The Engineer's Place for News and Discussion®

 Previous in Forum: Fracking Next in Forum: Another Solar Power Use For Vehicles
Active Contributor

Join Date: Feb 2008
Location: Walsall, England
Posts: 15

# Wave Energy Converter

06/17/2014 8:15 PM

I think the most efficient way to capture and store the energy in waves is by first converting it to stored potential energy and then using this potential energy to generate electricity.

This can be achieved by using reservoirs that are either fixed to piles in shallow water or floating in deep water.

Water will enter the reservoir through one-way openings in its sides and exits through turbines to generate electricity. Water will enter if the pressure from the incoming wave is greater than the water pressure inside the reservoir. This will result in a head of water above sea-level in the reservoir that can be used to generate electricity using low head turbines.

The reservoirs are able to survive the large forces of the big waves because most of the energy passes unimpeded through the openings in the sides.

The bigger the reservoirs the greater the electricity output and energy storage and the less wastage of energy from overflowing.

The reservoirs would also be effective breakwaters.

Interested in this topic? By joining CR4 you can "subscribe" to

Guru

Join Date: Oct 2008
Posts: 40058
#1

### Re: Wave energy converter

06/17/2014 8:55 PM

Numerous approaches have been tried already.

Guru

Join Date: Apr 2010
Posts: 5812
#2

### Re: Wave energy converter

06/17/2014 8:58 PM

Most of the schemes I have seen for capturing wave energy utilize the up and down motion of floats riding on top of the waves. With the method that you propose you would have to use up the hydraulic head in your reservoir before the next wave arrived for you to capture any more water. I'm thinking that that would tend to limit the efficiency. You should build a model and see how well it works.

Active Contributor

Join Date: Feb 2008
Location: Walsall, England
Posts: 15
#4

### Re: Wave energy converter

06/18/2014 6:40 PM

Water moves from high pressure to low pressure zones. If the pressure inside the reservoir is lower than the pressure exerted by the incoming wave, water will enter. There is no need to use up the head after each wave.

The maximum potential head would be reached when the pressures inside and outside are equal.

Guru

Join Date: May 2007
Location: NYC metropolitan area.
Posts: 3009
#3

### Re: Wave Energy Converter

06/18/2014 12:24 PM

You're late to the party, the device you describe is called a hydraulic ram, i'ts been around for centuries. You'll also find modern incarnations of it being used exactly as you describe when you Google on the term.

__________________
Curious minds want to know, engineering minds get answers....
Active Contributor

Join Date: Feb 2008
Location: Walsall, England
Posts: 15
#5

### Re: Wave Energy Converter

06/18/2014 7:04 PM

I agree they are both based on the principle of converting kinetic energy to potential energy.

The reservoirs block the forward motion of waves and convert the energy to a head of water

Guru

Join Date: Dec 2010
Location: North West England
Posts: 928
#6

### Re: Wave Energy Converter

06/19/2014 4:12 AM

The maximum head you can build in the reservoir is the height of the waves. This is insufficient to drive a turbine.

Floating reservoirs will sit lower in the water as the level inside rises. There will be no head. Think in terms of a boat with holes in the side. IT SINKS!

Oceans and seas generally have tides (Baltic and Med. still have tides but with a much lower range) Close to shore, in many cases the tidal range will be greater than the wave height. So half of the day your reservoir on legs could sit clear of the water or submerged. Make the walls high enough to overcome this problem and you have a tidal storage facility. You no longer need the waves. Better still just build a barrage blocking a river (See; Rance Tidal Barrage Northern France, and proposed River Severn Barrage UK)

Once a reservoir on legs is full, which will happen quickly in high seas, non-of the energy is absorbed by the storage mechanism. The reservoir has to be built to absorb everything that the ocean/sea can throw at it. A very expensive and risky proposition.

Conclusion: Go back to the drawing board, this idea has so many holes that it will sink without trace. (pun intended)

Active Contributor

Join Date: Nov 2006
Location: Cape Canaveral
Posts: 24
#7

### Re: Wave Energy Converter

06/19/2014 7:21 AM

I am especially fond of this WEC http://www.bomborawavepower.com.au/technical

Clearly the most viable WEC concepts harness the cyclic energy of wave pressures to generate a pumping action and the Bombora concept seems to have the lowest environmental impact of those I've seen. Side benefits could be shore line stabilization and though manipulation of bottom contours, improved surfing.

__________________
I'm only here, because I'm not all there
Guru

Join Date: Dec 2010
Location: North West England
Posts: 928
#9

### Re: Wave Energy Converter

06/19/2014 9:16 AM

It seems counter intuitive to be increasing wave height to improve surfing while at the same time stabilizing the shoreline. Is it not wave action a major contributor to making the shoreline unstable in the first place?

Active Contributor

Join Date: Feb 2008
Location: Walsall, England
Posts: 15
#8

### Re: Wave Energy Converter

06/19/2014 7:56 AM

The maximum head will be higher than the height of the waves because the kinetic energy of the wave is converted to potential energy.

You can see this when waves crash into a rock or sea wall. The height of the spray produced is much higher than the height of the wave.

Guru

Join Date: Dec 2010
Location: North West England
Posts: 928
#10

### Re: Wave Energy Converter

06/19/2014 9:35 AM

When a wave encounters a sea wall it tries to compress into zero volume. As water is nominally incompressible it has nowhere to go but up. The higher the wave the higher the spray. The speed that a wave moves towards the sea wall is a function of the wind speed, and the wind speed determines the wave height. That faster waves throw up water to a greater height is an illusion, those waves are just higher because there is more wind. That is a very different situation from passing the water through a valve with no compression involved. The water level in your reservoir cannot be more than a millimeter or so above the height of the waves without defying the laws of physics.

Active Contributor

Join Date: Feb 2008
Location: Walsall, England
Posts: 15
#12

### Re: Wave Energy Converter

06/19/2014 3:17 PM

The wave compresses the water inside the reservoir and forces it up.The water in the reservoir blocks the forward momentum of the wave and so is compressed

The speed of waves are determined by their wavelength and water depth and not by wind speed.

Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 13882
#11

### Re: Wave Energy Converter

06/19/2014 9:41 AM

Since the kinetic energy of the wave will force it to "climb" a tangential surface that curves upward, perhaps you could simply optimize the shape of this leading edge surface to "pump" the water up and over the top, then you don't even need the one-way valves. Maximum head is the top of the lip of the reservoir-any part left under water. Why not put the thing on pilings that keep it just at sea level for low tide, but with the leading edge deep enough to capture most of the wave kinetic energy. That should not be hard to figure out with the right models. Allow storage to take place all the time, but only run the generator at low tide to maximize head. If you don't have significant tides, then whenever.

__________________
If it ain't broke, don't fix it. Just build a better one.
Guru

Join Date: May 2007
Location: NYC metropolitan area.
Posts: 3009
#13

### Re: Wave Energy Converter

06/19/2014 5:38 PM

Wave motion as being used here is really a form of wind energy capture; if there is no onshore wind of sufficient velocity tangential to the shoreline then there are no decent waves except for those caused by tidal motion (boat wakes don't count!), and their periods are much longer.

Those of us who are boaters (where's netmaker when you need him) know that on a calm day it's "smooth sailing", that as the windspeed picks up there's "light chop", and gale force winds mean "return to port". Waves tend to move in the direction of the wind, that's why the windward side of an island or barrier reef takes a pounding while the leeward side is spared.

A common misconception is that the water has a large forward motion loaded with kinetic energy, the "waves crashing into the seawall with nowhere to go" is used as an example of that energy. On a windless day the water level rises and falls on a 12 hr 25.2 min period, much different than the rise and fall on a windy day. It's the wind's energy that was transferred to the water that is being dissipated.

As the attached college-level article shows, the water at sea level undergoes an oscillatory circular motion that motion that decreases as the depth increase until it is simply going back and forth near the bottom. Any scuba diver in a reef has seen the plants moving back and forth with the wave motion, once divers are below a certain depth waves just pass over them, their position oscillating backward and forward (not up and down) unless they are where waves are breaking overhead.

All of the above is based on open water; once boundary conditions (seawalls, beaches, etc.) come into play the dynamics get a bit murky, but the kinetic energy in a 10 point wave is still a function of the wind's energy. No wind, no waves, no surfing, no energy to be harvested.

__________________
Curious minds want to know, engineering minds get answers....
Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 13882
#14

### Re: Wave Energy Converter

06/19/2014 6:00 PM

Good point, so why bother with sophisticated capture of water from the wave? Why not just use wind power where available to pump up water and release it later, much easier to implement?

The only reason to do this other project would be if you need to build a new break-water coming into a harbor. Then you could build something very large indeed, and even with relatively low head, the amount of MWh storage would be impressive.

__________________
If it ain't broke, don't fix it. Just build a better one.
Active Contributor

Join Date: Feb 2008
Location: Walsall, England
Posts: 15
#15

### Re: Wave Energy Converter

06/19/2014 7:19 PM

Wave energy is more concentrated than wind energy because water is denser than air.

Wave energy is more available than wind energy because waves are produced by local and distant weather conditions.

wave energy can be stored as I describe. You cannot store the wind as compressed air

If you control the waves you are left with a flat surface which would be ideal to build floating cities.

Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 13882
#16

### Re: Wave Energy Converter

06/20/2014 10:22 AM

Some of what you stated I can agree with, however, one can (and perhaps should) store wind energy as either compressed air, liquid air, or compressed hydraulic fluid (see hydraulic accumulator, hydraulic pump, hydraulic motor). If the hydraulic system were utilized, water could be a suitable working fluid, but certain oils have a more preferred viscosity profile. Also, only having a hydraulic pump at the top of the wind tower at the rotor allows two things: (1) rotor does not synchronize with grid, so tip speed can be increased to Mach 0.9 if so desired in the upper range of speed, and (2) reduction of mass at the top of the tower makes the entire structure less susceptible to torsional vibration in high winds (IMHO).

As to your wave capture system, you should use a concave hyperbolic ramp to help lift the most water (or maybe parabolic concave), and also have situated above this ramp cantilevered out over the normal water level, and inverted curved surface to channel in the water. If you build this high enough above water level, you do not need one-way valves (that can fail), and you will still produce enough lift to gather significant energy. Think of this as your two hands cupped over each other with a little separation. You should experiment with this design in a wave tank to see if it is a good model, but I suspect it will work every time.

__________________
If it ain't broke, don't fix it. Just build a better one.
Guru

Join Date: Jun 2008
Posts: 839
#17

### Re: Wave Energy Converter

06/21/2014 12:11 PM

Local wind has very little to do with wave energy---Long period swells, created by storms, thousands of miles away, over long periods of time, with large fetches, contain the most energy, and can be utilized when there is no local wind. A long period swell, 16- 18 seconds or much higher, say 22 seconds contains a tremendous more amount of energy. even if the wave is only 2 feet high, compared to a 2 foot high local wind swell. Read Willard Bascom on this. The longest period "swells" are the tides, moving high to low and back, twice a day, approx 6 hour periods. Tidal bores contain a lot of energy. Another water energy proposal that has be used some, and studied , is Ocean Current power, dropping undersea turbines into areas of constant current flow, like the Gulf Stream. Problems are corrosion, maintenance, and transmission line loss. 40 years ago, A colleague came up with a design that looked like a giant spider, with articulating arms, floats on the ends, and mechanical gearing at the hub. Theory being that any swell , from any direction, would lift the arm, turn a gear that created energy within the hub. Problem was conversion and transmission. Anyway, some other ideas here….

Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 13882
#18

### Re: Wave Energy Converter

06/21/2014 12:18 PM

There is also this: WimP (whatever input to mechanical power) Not sure if this has been scaled up as yet, but is seems feasible to do so, since it is strictly a mechanical device that takes any motion in x-y-z, and converts it to rotation of the output shaft.

To get back to your discussion of swells, the energy contained in such can be utilized to convert at least some of this energy to a higher potential (at less than unity power integrated, of course), but utilizing some form of ramping to capture water at a higher raised level than what the swell itself would be.

__________________
If it ain't broke, don't fix it. Just build a better one.
Guru

Join Date: Jun 2008
Posts: 839
#19

### Re: Wave Energy Converter

06/21/2014 1:13 PM

Good points. The first mechanical conversion system would be exactly what would be needed for the "Spider", an almost Gyroscopic hub design, articulating in all directions, but keeping the conversion point stable, to prevent further energy loss. With wave energy, I have even seen systems, that after being used hydraulically, being used to compress air, something like a blowhole effect. I wish more money was diverted into some of these technologies (Although maybe it is--I have no idea), as local power sources, to complement other sources, like Solar, where applicable..

Active Contributor

Join Date: Feb 2008
Location: Walsall, England
Posts: 15
#20

### Re: Wave Energy Converter

06/21/2014 6:32 PM

Ramps are used in overtopping devices such as the Wave Dragon

The problem with overtopping devices are:

1. Energy loss from friction as the wave climbs the ramp.
2. The deeper the reservoir at the top of the ramp the greater the energy loss
3. The shallower the reservoir the less energy storage.
4. They cannot be fixed in position because of tides
5. They cannot effectively store energy
6. Waves that are too small cannot climb the ramp and waves that are too big will swamp the reservoir.
7. The structure itself has to absorb much of the wave energy and so it is likely to be pushed about.

By using one-way openings instead of ramps for water to enter reservoirs all these problems are eliminated

Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 13882
#21

### Re: Wave Energy Converter

06/24/2014 9:43 AM

OK, I'll bite. What happens when the water rushes into the one-way opening? Does the momentum not still thrash the reservoir about? Won't large waves over-power the relatively small opening? What happens when the storage is full? Won't waves simply hit this "barrier" and shoot straight up?

I think you should incorporate the one-way valves and still use the ramps with slit openings to capture as much water as possible. You will never achieve high or even moderate head heights with any of these concepts, so the generator will have to be powered by specific low-head turbines.

__________________
If it ain't broke, don't fix it. Just build a better one.
2
Anonymous Poster #1
#22

### Re: Wave Energy Converter

06/25/2014 3:54 PM

GEORGB.... you gotta stop smoking those funny weeds that you find along the edge of the highway....

I did a senior engineering project in college 42 years ago. The observations and conclusions I made then are still valid today

Wave power will always be more expensive and undesireable than other renewable power sources because:

1) This power source is very diffuse, massive collection devices it will ruin the view, water access ways, ship lanes etc. that are close to costal cities.

2) The ocean corrodes everything.... yes everything....maintenance is a nightmare. Corrosion resistant alloys etc. are very expensive.

3) Every couple of years a hurricane or storm will demolish your expensive plant. No investor wants to take such a risk

Interested in this topic? By joining CR4 you can "subscribe" to