Introducing
a Biomimetic Solution for Renewable Wave Energy
Benjamin Gatti
Keywords:
Wave energy generator renewable
Abstract
World energy demand could be satisfied on a renewable and thermally-neutral basis if just 5% of oceanic wave energy were captured. Wave energy is proximate to current demand, it compliments solar energy both seasonally and geographically, and it improves the concentration and predictability of the wind resource with less impact than upright wind devices. Modern attempts to utilize wave energy, starting in the 1970’s with Professor Salter’s “Edinburgh Duck,” illustrate the primary challenges: 1. Survivability at sea, and 2. The huge mass required to resonate at ~ 0.15 Hertz.
A
novel device is introduced which addresses these issues. It is a self-healing,
compliant membrane composed of isolated pumping chambers which operate in a
manner similar to the human circulatory system. The device attenuates the wave
energy by spanning the wave profile with a semi-rigid planer membrane. The
action of the wave is resisted in a wide band of frequencies with adjustable
force.
If we accept that mass is a meaningful estimate of cost, initial numerical simulations show that such a device could be competitive with gas turbines in seastates as low as 5 KW per Meter.
Introduction
In 1882, Edison turned the lights on in
Renewable energy, to compete on cost, will most likely have
to compete on mass. True, some mass costs more than others; however by some
estimates, shipping is more than half the cost of wave energy, and so it is
difficult to discount the primacy of specific power for determining competitive
advantage. The recent EPRI study of wave devices[iii]
indicates that current wave devices have a specific power much closer to
The Author has invented an inflatable device which improves the specific power by an order of magnitude. The WaveBlanket uses thin polymer film filled with compressed air as both its dynamic and structural components. Such devices are inherently lightweight and have the unique characteristic which can only be defined as infinite elasticity – a feature which is critical to surviving the unpredictable forces of the sea. The Blanket rests on the surface of the ocean, it is pushed upward by the waves which is obvious; what is less obvious is the equally strong downward force, conveyed by vacuum, against a surface membrane which was discovered while testing another device.
Initial numeric analysis suggests the WaveBlanket could compete with a nuclear reactor in a half-meter seastate and with a diesel engine in a full meter seastate. Such seastates are common near large populations, while the exotic seastates targeted by heavier devices are scarce.
The
WaveBlanket represents a unity of material density and impedance to the energy
concentration of the reliable wave resource. Assuming a 1 meter wave, the
pressure on the interface is ~ 10 kPa while the pressure in a typical steam
turbine is ~ 2000 kPa, thus wave energy is more than two orders of magnitude
less concentrated than a steam or gas turbine; to be mass competitive
therefore, a wave device ought to have ~1/200th the material density of a gas
turbine. In that sense, using concrete to harness the waves is a bit like
trying to catch a butterfly in a steel trap.
Details of the Design
The WaveBlanket is a compliant membrane comprised of two or more layers of isolated pneumatic chambers communicated through check valves and a manifold with one or more turbines. The chambers are compressed and relaxed as the plane is bent by the perturbation of the ocean surface. All moving parts are sealed from the elements in a closed system. Like the human heart, the working fluid is pushed into relaxed chambers by the diastolic pressure, raised to the systolic pressure by the bending moments of the wave, where it is forced through a check valve and the turbine, which reduces the pressure again to the diastolic level. The use of two turbines of unequal ratings provides three efficient power levels for tuning the device to the seastate. Reservoirs can be included to smooth the power output. The use of a compressible fluid helps avoid rupture during impact stresses at the cost of some adiabatic entropy.
Experimental Protocols
The operation of the WaveBlanket has been evaluated using numerical simulation. The device is represented as a series of trapezoids which change shape to conform to the surface. The sea is represented by a triple sine wave and the volume of each chamber is calculated at a resolution of .three per second. Work was calculated using Boyle’s law for an ideal gas in this formula:
The equilibrium volume at which systolic (high) pressure is reached is the term (p1*v1/p2) where the diastolic (low) pressure (p1) times the initial volume (v1) is divided by the systolic pressure (p2). The difference between the equilibrium volume and the final volume (v2) multiplied by the systolic pressure (p2) yields the work in Pressure * Volume which can simply be reduced to Force * Distance.
Some
100,000 simulations were run with random values for the size of the device, as
well as internal pressures and seastate. The optimal work and actual work were
logged in a database along with the test parameters and the best results for
each seastate were returned by a query.
The Mass is based on the performance ratio of a Mylar-like film used by the company Aerotube LLC. This film has a weight of ~2 oz. per square foot inflated. Because higher pressures can be accommodated by increasing the thickness, the mass is determined by the size multiplied by the internal pressure.
To evaluate the global potential for such a device, NASA’s ubiquitous Earth at Night image was aligned with a gradient map of the average wave energy published by the World Energy Council. These were processed using spatial filters to identify the relative sphere of opportunity. Additional weight was given to isolated areas due to their limited access to alternatives. The relative potential is indicated by the color scheme shown in the legend. Current consumption in range of the ocean is shown it white with more distant regions masked in red.
Representation of Relative Opportunities for Moderate
Seastate Wave Reactors
Results
The
query of best results by wave suggest a potential price curve which is competitive
even in very low seastates. While the simulation is reasonably accurate with
respect to the size of the device and the power output, estimates of price are
subjective at best this early in development. However, if we consider that an
inflatable mattress includes similar materials as well as the turbine motor,
assembly, shipping and transaction costs in a mature market for about $23 per
square meter of inflated space, then these projected costs based on that number
are not wholly unreasonable. Operation costs are not yet estimated and assumed
to be similar to other wave devices. The most cost-effective profile is some 6
meters in height with a pressure of 5-6 PSI and a turbine pressure differential
of ~0.03 PSI. The width of the blanket simulated was 50 meters.
Discussion
Wave energy is a singularly concentrated and predictable source of energy which is within 300 miles of much of the world’s concentrated energy consumers. As other’s have noted, wave energy continued to receive less attention than is warranted by its potential.[iv]
The UK Department of Trade and Industry (DTI) lists three goals for the next phase of Wave energy development:[v]
- reducing capital cost
- increasing efficiency
- improving reliability
The initial analysis strongly supports the conclusion that thin-film compliant mechanisms can likely meet these objectives and supply renewable energy at market rates from the moderate wave climates which are ubiquitous near populations.
Reducing capital cost
Capital cost is the product of size and complexity. The WaveBlanket has lo mass and is relatively simple. Compliant mechanisms, by their nature, involve parts that flex rather than pivot, and the WaveBlanket, in that sense has but a single moving part – the turbine rotor. The flexing members include check valves, and the chamber matrix, which is both operational and structural. Even in moderate seastates, the WaveBlanket has a power per mass ratio that competes with fossil fuel and nuclear power plants.
Increasing power capture efficiency
I would interpret “capture efficiency” to mean the amount of energy captured per unit mass. It could be taken other ways, but based on the body of published costs analysis for such machines, specific power is the single strongest indicator for competitive advantage. At some future point, competition for the wave resources could justify investments in diminishing rates of return but such is neither the case now nor likely to be in the future.
Instead, the relationship of the resource is such that it is
diminished in proximity to demand, both in terms of space and time, such that
the wave are lowest in the summer, when demands are highest, and they are
generally lower near heavy populations. As a consequence, we might do better to
look at the resource as it is when and where we need, than to focus on the two
months of the year when the unpopulated tip of
This observation can be expressed theoretically, that if material cost can be reduced in relation to the reduction of the wave resources, then the Specific Power and therefore the cost of energy can be normalized. Indeed, if the energy yield of gasoline were as random as the sea, the internal combustion engine would need to tolerate rare stresses some 100 times larger than average stress they would also be very expensive.
The solution in either case is not to harness the rare high energy, but to bleed it off; the WaveBlanket does this very well. Because its structural members are fully elastic, they can bend under extreme moments of force, vent their internal pressure, and be re-inflated once the stress is removed. This strategy of yielding to rare energy states allows the design to be optimized for targeted wave conditions – near demand – during demand. Using this approach, the plant is rated only for the power it can capture during the peak demand season, costs for the balance of plant are fully utilized, and the project can justify capacity payments in addition to renewable energy credits because it offsets the need to build additional peak shaving plants.
Improving reliability
Wave reactors have a history of being destroyed by storms waves. Exposure to saltwater is also a reported concern. The WaveBlanket addresses both of these concerns; first, it is infinitely elastic, it could even be described as a self-healing mechanism because 100% of its at-risk components can be re-inflated following a structural failure; secondly, its interfacing surface is a polymer which has little or no reaction with sea salts with the one moving part exposed only to the working fluid which is recycled in a closed system. Additionally, the entire interfacing component is a soft-good which is already amortized over a shorter period (5-7 years)[1], so its complete loss to a hurricane would not place the financial viability of the plant at risk.
This said, mechanical availability is only a part of supplying reliable electricity, as the reliability of the prime energy must be considered as well. While wave energy is more reliable than oil in the long run, the highest energy seastates are less reliable in the short run. Moderate seastates however can be very reliable. Consequently, the reliability of the design depends on optimizing for moderate seastates with a minimal cost increase. The WaveBlanket can be optimized for reliability because its survival strategy does not involve resisting all possible wave conditions, but only those in the target range.
To overcome the issue of intermittency, the Wave Blanket can be implemented from ships – perhaps some of the 400 single hull tankers currently scheduled for early retirement – using dual driven generators in which LNG fired turbines provide power during lulls in the wave energy. This would ensure a capacity payment for most of the plant and the turbine can be incorporated into a hydrogen storage scheme as that technology becomes available.
Another new solution to intermittency is the frequency excursion triggered switches[vi] being conducted at the Pacific Northwest National Laboratory is particularly relevant to the reliability of intermittent sources. A cursory investigation of storage devices reveals that the opportunities for stabilizing the grid by rescheduling residential loads may be more cost effective than supply-side alternatives. I would propose a frequency-triggered device integrated into a circuit breaker – combined with arc and ground fault interrupters, which could be installed in 10 minutes on hot water heaters, refrigerators, air conditioners, pool pumps etc… to effect Demand Response and the latest safety measures for a cost of ~ $30.
Summary
Wave energy is the final energy state of a nuclear reaction which started hours or days earlier in the Sun. This sun’s energy is converted first to wind and then to waves. Ocean waves are a free and efficient method of transporting energy over great distances, and thus the great power of the sun is literally delivered to our doorstep in the form of ocean waves. Populated coastal regions tend to have moderate waves in the 15-30 kw/m range. Thin-film structures provide a sealed mechanism which resists reaction with the seawater while providing high strength to weight and infinite elasticity can be used to harness this energy for the environmentally conscientious benefit of humankind.
Accordingly, the peoples of the world should adopt policies which provide safe and clean energies a fair and level playing field. A level playing field means providing risk-mitigation equal in both impact and longevity to the Price Anderson Act. A level playing field means spending 300 billion dollars in research for renewable energy. A level playing field means leading the armies of our best and brightest young people in the fight to conquer national dependency on the imperialism of foreign oil.
The current
Acknowledgements
I would like to thank Manfred Steibli of Aerotube LLC who contributed technical specifications and the results of pneumatic and fatigue tests with the interlocked polymer used by his company.
Many thanks to Jamie Taylor of the