Technology
The Slow Mill is a Wave Energy Converter consisting of a floater with blades variably connected to an anchor on the seabed. Waves push the floater up and the blades away from the anchor. This way, not only the up and down movement but also the back and forth movement of the waves is utilised. The blades go as deep as 3-4 m to extract wave power from below the surface as well. When the wave recedes, it takes the Slow Mill back to its starting position. The entire unit follows the orbital or sometimes elliptical wave path but, taking the inner bend, moves a bit slower than the wave, hence its name: Slow Mill.
Energy extraction
One of the major challenges of converting wave energy into electricity is that wave motions are irregular and hard to follow. Some strategies adapt the resonant frequency in order to align the work stroke with the wave swell. The Slow Mill is rather light and follows the waves easily, harnessing both the heave (vertical) and surge (horizontal) motions. It's designed to also capture sway, pitch and roll motions but those are minor contributions thus the Slow Mill is called a "Heave and Surge Absorber". Its blades are designed to move more or less along the orbital wave path, even when waves hit at various angles and frequencies. It doesn't need to complete a return stroke to make a new work stroke as it just makes smaller or bigger circular motions, avoiding losses due to acceleration, deceleration and out-of-phase movement.
Practical Development
The anchor is relatively large as forces during storms can become quite strong and the floater has to be pulled under the higher wave crests. But installation is rather simple as the design allows a small tug boat to quickly bring the anchor to the assigned location where it can be installed. The floater is lighter and could be installed right after or at the same time to minimise work at sea. Yearly maintenance is done by filling the anchor with pressed air and bringing it to sea level to clean, inspect and possibly repair the unit. The anchor will be made of marine quality concrete that boosts biodiversity and carbon storage to counter ocean acidification.
Design
The floater can be entered via an air tight lock as inside the climate is controlled. The floater is kept under a constant over pressure of ca. 0,1 bar to prevent salt water or vapours to enter. This way the hydraulic and electric systems are protected and can be maintained in a controlled environment. If any leak might occur, it'll be instantly noticed by a pressure drop but water still can't enter due to the over pressure. The leaking air will be supplanted by a compressor until repairs are finished.
The prototype is 20m long, 8m deep and has a 2,5 m diameter floater. The 70 t/MW machine is optimised for North Sea conditions with resonance blades that reach till about 3m under the surface to capture the lion share of the North Sea wave energy.
Anchor
The anchor is made from marine grade concrete and can contain air or water. For transport it is filled with air and gets towed to its destination. There it's filled with seawater and sunk to its final location on the seabed. For maintenance it's filled with pressed air until it floats up to the surface, allowing underwater parts and connections to be inspected and maintained. The anchor proactively enhances marine biodiversity as its structure is suitable for early food chain organisms to attach and develop.
Installation
The anchor is relatively large as forces during storms can become quite strong and the floater has to be pulled under the higher wave crests. But installation is rather simple as the design allows a small tug boat to quickly bring the anchor to the assigned location where it can be installed. The floater is lighter and could be installed right after or at the same time to minimise work at sea. Yearly maintenance is done by filling the anchor with pressed air and bringing it to sea level to clean, inspect and possibly repair the unit. The anchor is made of marine quality concrete that boosts biodiversity, ecology and carbon storage to counter ocean acidification.
Innovation
The Slow Mill Effect
An open flow of water, be it a current or wave, will choose the path of lowest resistance. This makes it hard to move along in the same direction as the flow and extract a good deal of energy out of it. The Slow Mill has a unique system of blades that changes the direction of flow temporarily to optimise the angle of attack, enabling more energy to be extracted from the current. As shown in the lab tests at TUD, high resistance to open flow and high efficiency can be observed in the models. "The Slow Mill Effect" is defined as; enhancing energy transfer from an open flow to a barrier that moves in the same direction by temporarily deflecting the flow to create an optimum angle of attack.
Survivability
Hyper Resonance
The Slow Mill "sees" a wave and moves toward it in order to capture the full movement. Then it goes up, following the wave path, moves along the wave direction and comes down when the wave passes. The model is tuned to make the same motion as the wave but move a bit slower and take the inside bend, aiming to maintain a minimum phase gap to get optimum energy extraction. We call this "hyper resonance", defined as; the smallest possible phase gap between a wave and a floating object extracting energy from it.
"The effect of the wave period on the Slow Mill at regular and irregular waves is small"
Corrosion and Abrasion
Seawater is known for its corrosive and abrasive characteristics. That's why the Slow Mill is largely made from fibreglass composite materials that don't corrode. Fatigue of composite materials is minimal and abrasion can be handled by using wear resistant top layers. A few steel components will be necessary but can be sufficiently protected by modern day preservation technology.
