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Help with hydraulic circuit for wave energy device
3

Help with hydraulic circuit for wave energy device

Help with hydraulic circuit for wave energy device

(OP)
I am a mechanical engineer with a small energy consultancy.  I remember the basics of hydraulic circuits from university but I'm a bit stuck as for how to go about getting the power out of this wave energy concept I've been working on.

The basic idea is a matrix of 25 large (5m x 5m x 5m) floats.  As waves pass underneath the floats bob up and down.  My idea is that each of these floats drive a hydraulic pump (much like a hand pump) that pumps on both the upstroke and the downstroke.  The flows then combine to drive a hydraulic motor which is hopefully rotating at about 1500rpm so as to drive an induction generator.

So on to the questions...

-How can I keep the flow output from each float pump constant?  Is there a way of doing this with an accumulator?

-How can I vary the speed of the motor so as to give more torque at the same speed if there are bigger waves (ie more displacement from the pumps).  Would a variable displacement   swashplate type of motor be capable of this if there was some kind of control system?

-How would I go about combining the flows from 25 pumps?

Any help with this will be very much appreciated.  Thanks for your time!

Pete

RE: Help with hydraulic circuit for wave energy device

I don’t think you will get much help on this project for free. If you ask hydraulic sales companies to design a system it may not work as required. The complexity of what you are trying to do should be modeled in software for kinematics, stress, fluid power and control design. Some of this complexity has to do with the uneven wave movement and energy available.

Ed Danzer
www.danzcoinc.com
www.dehyds.com

RE: Help with hydraulic circuit for wave energy device

Hello...

I know of a few similar applications in the UK currently under development. They have a float attached to a hydraulic cylinder, that when driven up and down displaces oil into a hydraulic motor, which drives an electric motor to generate power...in basic terms!

The issue I see with combining flow to feed one motor is that with random energy input into the pumps, there will always be one that has less flow/pressure and therefore cannot feed into a system that is operating at a higher pressure. Feeding into an accumualtor to supplement flow would be just as random as the accumulator could empty and then wait for a wave big enough to recharge it. It seems better to have autonomous units feeding power back to a collector on the shore.

I have designed constant tension systems on marine winches that needed to compensate for random wave patterns. The only reliable way to compensate for wave power was to use a PID controller. That was to ensure that people were not thrown out of a boat, your application might not need that much control. It really depends on the amount of control you need, but I think you would need a high level of control to ensure a constant and stable flow from the pump to the motor.

Hope this helps a little

Regards

Adrian    

RE: Help with hydraulic circuit for wave energy device

You seem to have answers for the first two question.  The pumps would be combined using check valves.  There would be a check valve to let oil in and another to let oil out at the end of each cylinder or pump.  The headers can be combined to fill an accumulator.  This is kind of like a AC to DC power supply where the AC is rectified using diodes ( check valves ) and filtered or smoothed using a capacitor ( accumulator ).  And yes you can get a controller that will vary the swash plate angle.  These problems are simple to solve. Even the control is simple.

Here is the one you should be asking. How much power do you think you will get from this? You must realize that the force pushing down is going to be limited by force of gravity and the upward force is limited by how buoyant the floats are.  The weight and buoyancy of the floats is crtical so the forces will be roughly equal.  If the pistons are not double rodded you may need to change the ratio of the upward and downward forces.

RE: Help with hydraulic circuit for wave energy device

(OP)
Thanks a lot, all the answers have been very useful!  I figured out the check valve thing myself but I was glad to know I was more or less on the right track with an accumulator.  You're right about the buoyancy forces, what I did was vary the mechanical advantage for the ones that pump on the downstroke when it's acting under gravity because a hell of a lot of extra metal as weight will be required to equalize the forces, so there is simply less displacement but at the same pressure.  Hopefully different floats will be at different points in their cycles so this will be cancelled out to an extent at the motor.  As this is just a concept I'm just worrying about the rough sizing of the main components so far and I'm not worrying about modelling it.  Again thanks for your help, I'll keep checking the forums so I can hopefully give something back.

RE: Help with hydraulic circuit for wave energy device

in an array of pumps you would stagger their locations where the oceanic wavefronts would provide multiphase pump flows, that when added would provide a relatively smooth constant.

An example might be on pump float positioned on a crest, next one out on a 50% fall, next in trough, next 50% rise of next wave. This would give you a 4 phase sinsodual flows whose net flow ripple could be mitigated by the next arrangement positioned slightly out of phase with the next. A study of wave frequencies should identify ideal locations.

Your problem will be in the fact that in running a generator, you want constant speed, where your hydraulic flows will be highly variable. This would indicate a need for a variable displacement motor. Most suppliers provide electrical compensator options.

RE: Help with hydraulic circuit for wave energy device

On top of staggering the pump locations feed all the Pump Outputs to Weight Loaded Accumulators designed to receive oil at 100-200 PSI lower than the pumps minimum output pressure (Rod Side/Cap Side). You can even use a modified Regeneration Circuit to have equal volume and pressure with the correct size Rods in the Cylinder/Pumps

Out of the Weight Loaded Accumulators use a Reducing Valve set 150-200 PSI lower than the Accumulator pressure so you have a constant pressure supply to the Motors. At each Hydraulic Motor input use a Pressure Compensated Flow Control to set Motor Rotation Speed.

Monitor the Accumulator Volume to check the amount of available oil due to Wave Height change throughout the day and drive more Motor/Generators for High Waves and shut off some Generators during Low Wave action.

That would be a Low Tech solution I believe.

Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

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