Ocean Current and Wave Energy Harvesting
MEEM Group for Renewable Energy
As shown further in this text, every square meter of ocean-current flowing at 1 m/s (3.6 km/h) contains 500 W of power. Currents twice as fast have been reported in HK waters and ten times as fast elsewhere. Wave motion can additionally speed up this flow periodically, doubling it for open-sea waves that are 1.5 m high and 40 m apart [Ref], thus quadrupling the available power to 2000 W per square meter (from the flow component alone). When waves of about 1m height pound on the beach, they dissipate an average of 10kW per metre of beach. This is the power of a small car at full throttle, every five metres. [Douglas L Inman in Oceanography, the last frontier, 1974].
Relatively simple machines have been built to capture at least some of this energy. Objectionable on environmental grounds, most generation schemes for static hydraulic head call for damming of estuaries or building of elevated reservoirs that are refilled by waves. However, opting for the open seas poses technical challenges. On the one hand, devices must be capable of harvesting relatively calm seas with wave heights of one meter or so. On the other hand, they must also survive storm conditions with wave heights of 10 or 15 m. The difficulty of anchoring and the salty environment are additional problems to overcome.
Our initial designs explore a number of concepts
- Pairs of long turbines with alternately funnelled flow in reciprocal directions;
- Air compression by pumping action of waves in water-columns equipped with rectification valves;
- Water mills with hydro-sails (figure PDF - 221KB)
- Oscillating Wave mechanism (figure PDF - 221KB)
These mechanical systems would be supplemented with sensing and controlled survival mechanisms in storm conditions. For example, the funnels would be reoriented to neutral position when experiencing excessive loading, and wave impacts would be neutralised to the largest extent possible through opposite directionality of wave motion along the wave to minimise the demand for anchoring.
Our subsequent efforts would include harvesting thermal gradients between warm surface water and cold deep water.
"The potential marine energy resource…is huge" [Climate Change 2007; Mitigation of Climate Change, Working Group III Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change]. These plentiful renewable resources are all around Hong Kong.
Illustrative Calculation of the Ocean Current/Wave Flow Energy
Wind turbines require minimum wind speeds of over 13 km/h to operate. By analogy, because seawater has nearly 900 times higher density than air, the equivalent performance in water is expected at 30 times slower sea-current. This implies that sea-currents as slow as 12 cm/s could be harvested.
"Hydraulic head" h for ocean current with the flow speed of v=1 m/s is composed of velocity head alone (no gravity term): 
. This is minute relative to tens of meters in elevation difference provided by typical hydroelectric dams. To capture an appreciative amount of power P from small hydraulic head h, large volume of fluid flow i has to be captured: 
(ρ is water density, g gravity). This necessitates that large area of flow A be covered since i=vA. Hence, preliminary conceptual designs in this proposal feature large and numerous hydrofoil surfaces. For reference, the volume flow per square meter section of current flowing at 1m/s is i=vA=1 m3/s and the power is then 
. While not all of these 500W per square meter of flow could be captured, hydraulic machines are renowned for their high efficiency, which is in the 95 percent range.
Last modified on 7 April, 2009