Carianne Schamhart explored the technical aspects of living at sea, as part of the university program for sustainable development. Here you can read her full master’s thesis as a PDF document. Her thorough analysis leads to the conclusion that already just one wave damper yields enough energy for 100 households…

Floating Wave Dampers

Feasibility of the use of floating wave dampers to protect and sustain housing at sea

Or how to make use of the space, energy and water of the ocean.

Carianne Schamhart, August 2005, TU Delft

Summary

Population growth, urbanization and increasing prosperity. These are the main elements that increase the demand for space, water and energy, especially in large cities. To find a sustainable solution that meets these needs the sea-surface is considered. For coastal metropolises the sea offers an abundance of space, water and energy, but is as yet inaccessible. A research program has been started to enable living at sea with the ecoboat as projected solution. An ecoboat should provide housing, water and energy to its inhabitants in a sustainable manner. This thesis investigates the feasibility of the use of floating wave dampers to protect an ecoboat settlement. It also investigates the feasibility of ecoboats and dampers as a sustainable form of living. The research to find the damping properties is restricted to the effect of one damper on one ecoboat for one wave propagation direction. The investigation of the sustainability is restricted to household consumption.

Protection

The feasibility of the use of the dampers as a protection system is investigated by comparing the motions of the ecoboat in waves behind a damper with the ecoboat motions in undisturbed waves. To this purpose a basic damper is designed. The damper requirements are derived from the ecoboat. It should protect an ecoboat of 35m diameter, 7m total height, 2m submerged and located at the coastal zone of the Randstad. The longest 5% of the wave periods at that location are excluded since these require too large wave dampers. Internal stabilization is assumed to reduce the ecoboat motions for these waves. Therefore a design wave regime is selected of T2 = 5.75s and H1/3 = 4.1m and a design frequency of w0 = 0.85 rad/s. The dampers will need to reduce the wave height from these waves. ‘Heaving and pitching bodies’ is the wave energy conversion principle chosen to convert the wave motion to mechanical motion. This principle is relatively simple and therefore most reliable. It uses the motion of the structure directly to excite an internal mechanism. Therefore the damper can be designed as a float and the motions of the float can be used to calculate the power generation. The damper geometry is selected based on its damping characteristics and the practicality of its size. Large diameters yield more damping, but also need more height. The resonance frequency of the damper should be equal to the design frequency for maximum damping of the waves. This means that the total height of the damper increases with increasing diameter, since a damper with a larger diameter needs a higher position of the center of gravity to have the pitch resonance peak at the design frequency. Therefore the selected damper geometry is

The resonance peak of this damper geometry is not a perfect match with the design frequency and the performance of the will improve for a better match. An iterative design approach is required to improve this result. After the damper geometry is selected the wave regime behind the damper can be calculated. Three different wave regimes are used to illustrate different circumstances of the ecoboat: the design wave regime, the most occurring wave regime and a high load wave regime. The damping of the three motions that are considered for the symmetrical situation, is given in table 1.

Table 1: Damping of ecoboat motions It is clear that the dampers yield a strong reduction of the motions of the ecoboat. Although the surge and the heave motion in the most occurring wave regime are increased, the actual motion remains very small and can therefore be neglected. The largest motion of the ecoboat in all waves is the pitch motion. This motion is also the most uncomfortable motion for inhabitants. Therefore the damping achieved by the damper for that motion is considered most relevant and sufficient to recommend the use of the dampers for the ecoboats’ protection.

Sustainability

A sustainable solution should counteract the population growth, increasing consumption and degradation of natural resources by using far more efficient technology. This requires an improvement of (at least) a factor-10 more efficient technology compared to the current situation or a different technology that does not limit the use of resources for a future generation. The feasibility of the ecoboat and dampers as a sustainable solution to the need for space, energy and water is investigated by comparing the consumption of a land-based with an ecoboat household. The land-based reference building for this comparison is defined with four consumption statistics: average use of electricity, water, natural gas and building materials. These characteristic values are used as an indication of the total differences between sea and land-based living. This comparison should be extended in a next design phase to include all aspects of living at sea for a conclusive answer on the sustainability of the system. The ecoboat uses the electricity yield of the wave dampers. One wave damper with solar panels on top yields 446437kWh/year for the most occurring wave regime. This is sufficient for the households situated on one ecoboat, but can also supply another 100 land-based households. Fresh water for the ecoboat is made from salt water with reverse osmosis, heating is generated by electrical heat pumps and the materials are paid for with full cost pricing. This means that all environmental effects are incorporated in the prices so they can be compensated. Table 2 gives the reduction or change in consumption for one ecoboat household.

Table 2: Reduction of household consumption for the ecoboat
The overall consumption is reduced, but not with a factor-10. However the change to re newable energy, salt water and full cost pricing makes the system sustainable, since these technologies do not limit the use of resources for future generations. It is worth while to consider this solution in a next phase of design.

To download the whole thesis, click on the PDF icon at right. In this 112 page 3 MB document you will find detailed information including formulas.