Wave Energy Driven Resonant Sea Water Pump

A wave driven sea-water pump which operates by resonance is described. Oscillations in the resonant and exhaust ducts perform similar to two mass-spring systems coupled by a third spring acting for the compression chamber. Performance of the pump is optimized by means of a variable volume air compression chamber (patents pending) which tunes the system to the incoming wave frequency. Wave tank experiments with an instrumented, 1:20 scale model of the pump are described. Performance was studied under various wave and tuning conditions and compared to a numerical model which was found to describe the system accurately. Succesful sea trials at an energetic coastline provide evidence of the system ́s viability under demanding conditions. INTRODUCTION Many sites in the ocean are vulnerable to the accumulation of substances as a result of a weak flushing rate by comparison to the speed at which a particular substance is added or generated locally. Such sites include coastal lagoons, harbours and bays. Exceeding a certain degree of accumulation, some of these substances constitute a nuisance or can be dangerous even though they might otherwise be inocuous or could be safely digested by the sea. In some cases, these dangers can be averted by diminishing the local residence time so that the pollutant is promptly dispersed before it is allowed to accumulate. Ways of diminishing the residence time at a site include pumping clean water into the area or increasing communication with the ocean by clearing obstacles. Each particular option has advantages and drawbacks which must be evaluated in a cost-benefit exercise. M. Merino, F.Ruiz and M.A.Alatorre proposed using the energy of waves for flushing out coastal water bodies. They developed a sea-water pump which amplifies waves by convergence and rectifies the crests above the mean sea level, driving a gravity flow towards the receiving body of water (Ruiz et al., 1995). In this paper, an alternative wave energy driven sea-water pump is presented as an option for diminishing the residence time at stagnant coastal locations. The pump design is based on the work of Carey & Meratla (1977) and includes improvements which enhance preformance under varying wave climate conditions. The pump may be used to flush out polluted coastal lagoons or harbours, or to inject oxygen rich sea water. The absence of moving parts allows the passage of larvae undamaged through the pump which permits using the system to recover isolated coastal lagoons as nursery grounds. Controlled flow through the pump allows for environmental impact management. DESCRIPTION AND THEORY Figure 1. Schematic diagram of the wave driven sea-water pump which operates by resonance. A schematic diagram of the sea water pump can be seen in Figure 1. The wave induced pressure signal at the mouth of the resonant duct drives an oscillating flow which spills water into the compression chamber and through the exhaust duct to the receiving body of water with each passing wave. Air in the chamber behaves like a spring against which water in the resonant and exhaust ducts oscillates. Maximum efficiency is attained at resonance when the system natural frequency of oscillation coincides with the frequency of the driving waves (see Lighthill, 1979). A resonant condition can be maintained for different wave frequencies by means of a variable volume compression chamber which adjusts the hardness of the air spring Czitrom(Patent pending). Water surface displacement in the resonant and exhaust ducts can be described using the two coupled non-linear second order differential equations 1a and 1b. ( ( ) ) && & & & & & X L T Cos X X K X X f X X 1 1 1 1 1 2 1 1 1 1 1 1 1 2 2 + + + + + + ε θ + − − − ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ − − ⎡