My recent research on the cost-drivers of wave energy (see cost-revenue conundrum) identified 3 desirable characteristics for structural efficiency:
- A primary interface combining heave with either surge or pitch.
- Gearing up from slow to fast motions as early in the power transfer process as possible.
- Minimising the ratios of mean to maximum loads, strokes and power.
To minimise this ratio for loads, we need to eliminate hard end-stop loads, and have a storm-protecting configuration where the structure is a poor load-attractor. To minimise this ratio for power, there should be smoothing, rectification, and load avoidance/shedding as early in the power transfer process as possible.
There are several types of wave energy converter that might fulfil these requirements. Here is one of them:
- An oscillating water column (OWC) with a sloped water column; so that an upward heave motion also results in (i.e. is 'coupled to') a beachward surge motion.
- An external profile that also slopes beachward, as this will reduce storm loads.
- By tapering the channel containing the water column, gearing is a built-in feature of the primary interface, resulting in a higher amplitude oscillation and hence less volume.
- If the channel guiding the water column is open at the top, this will avoid hard end-stop loads. This requires power to be captured from the water column rather than an air chamber.
- The need for smoothing suggests temporary storage prior to electricity generation. This can also be used for rectification and further gearing.
If this sounds suspiciously close to an 'Overtopping' device, this is because Overtoppers are a member of the family of sloped OWCs that capture energy from the water column, and store it in reservoirs prior to electricity generation. This family has not received the attention it deserves because it lends itself to a very simple form of power capture: overtopping a fixed-height entrance to a storage reservoir. Unfortunately this simplicity comes at the cost of inefficient power capture.
Below is a link to a report that investigates the nature of these inefficiencies, suggests solutions to each, and identifies devices that incorporate some of these solutions. The report suggests a concept called the 'Ocean Percolator' that combines all these design requirements for structural efficiency and capture efficiency.
Link to report: