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Wave Energy Converter


Cost of energy is a critical factor to the success of wave energy converters, in order for wave energy converters to compete with other forms of renewable and fossil-fuelled power generation.
The key factors affecting the cost of energy of marine renewable devices include performance, capital costs, operating & maintenance costs and risks, e.g. due to offshore environment.
Changes to the design of wave energy converters can affect both performance and costs simultaneously.


Performance

Essentially, the energy converted into electricity by a wave energy converter is a function of the resource the converter is placed in, the converter's prime mover, and the converter's power take-off system. This is a dynamic system and changes to one aspect can have a significant affect on another.

Key factors influencing the performance of WEC converters

Because of the many ways that wave energy converters can be configured, their performance characteristics vary widely. To understand performance characteristics in detail, it is necessary to look closely at specific designs. However, it is possible to make general observations about the performance characteristics of wave energy converters and identify requirements for high performance that are common to many design variants.
Some of these requirements also apply to other types of generation plant, and therefore may already be familiar, such as:

  1. efficiency,
  2. availability and
  3. variation of power with resource conditions.

Efficiency

Assessments of efficiency open up a large number of questions. These include the theoretical maximum energy that the converter could be expected to capture, the intermediate efficiencies of its prime mover and individual power take-off system components, and the certainty to which the resource's energy content itself can be described. Furthermore, overall efficiency may be influenced by certain control and operating regimes that relate to other aspects of the design, including survivability.

Operational up time

The proportion of time that the converter is ready to generate, whether or not the wave conditions are suitable for generation, directly influences the quantity of energy generated over time.

Variation of power with resource conditions

During operation, conditions of the resource vary continuously over time. For wave energy converters, three parameters are relevant:

  1. wave height,
  2. wave period and
  3. wave direction

We can reduce the description of wave power capture to a three-dimensional problem:

  1. power,
  2. wave height, and
  3. wave period.

An ideal wave energy converter would capture all the power in the waves that it interacts with. But this is not possible in practice; there are certain conditions in which converters cannot operate and consequently no power is generated.

Performance, capital cost and O&M costs can be summarized as follows

  1. Along with capital costs, operating and maintenance costs and risks, the performance of a marine energy converter is key to its cost of energy.
  2. Performance is determined by the wave energy or tidal stream energy resource, the converter's prime mover and power take-off system .
  3. Efficiency, operational availability, matching to resource conditions and sizing are important factors.
  4. For wave energy converters, one need consider a power surface on three axes: power, wave height and wave period.
  5. Cut-in, cut-out and rated conditions may apply to either wave converters. They are based on the designer's judgments about the ranges of resource conditions it is economic to generate in.
  6. Assessments of long-term energy capture require historic resource information in addition to either the power curve or power surface. By combining the two, one can further understand and optimise converter performance.

Capital and Operating & Maintenance costs

The capital cost of a wave energy converter is made-up of several parts, which generally can be divided into

  1. station-keeping, the moorings or foundations, e.g. a monopile,
  2. structural, i.e. the parts that hold the device together, the steel shell of a floating wave energy device,
  3. energy conversion components and sub-assemblies, i.e. the parts of the power train or power take-off (PTO) system, such as hydraulic pistons, hydraulic motors, gearboxes, frequency converters and electrical generators, and
  4. project costs, i.e those items, excluding the device itself, which allow it to operate at the intended location. They include further hardware such as subsea cables and the processes of transportation, installation and commissioning.

For wave energy converters, there is some overlap between structural and energy conversion components because the structure’s geometry and size has a significant bearing on the device’s ability to absorb power. For large installations station-keeping might be considered under the project costs category.

The operating & maintenance costs include planned and unplanned maintenance, license to be stationed and generate electricity at the location, insurance, and ongoing monitoring activities.