A potentially low-cost method of storing coastal wave energy

Posted on September 17, 2015
Posted By: Harry Valentine
Topic: Hydro
 

The installation of wind turbines at coastal locations is becoming more popular internationally, the result of wind blowing at higher velocity over expansive open areas of water than over land an delivering higher power output, the result of the velocity3 factor. If wind blows at 30-mi/hour (44-ft/sec) over water and 26-mi/hour (34-ft/sec), the wind turbine will operate at slightly higher efficiency (38% vs 36%) and deliver 2.25-times the output. However, if wind speed is too high, turbines are locked in neutral and no power is available, same if wind speed is too low.

A second form of wind energy manifests along coastal locations in the form of wind-driven waves that near or break along the coast. Parties involved in the research and development of ocean wave energy are divided between offshore wave power conversion and shore-based conversion. Some hybrid technologies involve wave-driven water pumps being secured offshore with high-pressure hoses connecting to shore-based turbines that drive electrical generators. In some installations, multiple wave-driven offshore pumps can supply water under pressure to a single shore-based turbine that along with electrical generating equipment is easily accessible to maintenance staff.

Other shore-based wave energy technologies are based on a pair of converging breakwaters that `focus' wave energy from a wide inlet to a narrow shore based outlet at higher elevation, with a coastal storage reservoir connected to the outlet. The optimal angle for the breakwaters is 90° to gain maximum amplitude or wave height from each incoming wave. Seawater would leave the coastal reservoir through a turbine placed outside of the converging breakwaters, with an operating `head' of 0.75-metres or higher to allow turbines to operate at minimum 70% efficiency. Turbines and electrical equipment are easily accessible to maintenance staff.

Off-Peak Power Generation:

However, both wind power and traditional wave power technologies have a propensity to generate electrical power during market off-peak periods, a factor that requires investment into some form of energy storage technology. Various forms of grid-scale electrochemical battery technologies are available. However, such technology would have a usable life expectancy of 5 to 10-years before replacement at substantial cost is required. While compressed air energy storage is possible at many locations internationally, it is a technology that is most productive when operating in mega-scale proportions and with companion thermal technology easily and readily available.

There are 2-forms of pumped storage capable of grid scale energy storage, one technology involving storage reservoirs located at high elevation and the over involving underground storage chambers such as abandoned mines near the coast. Coastal mountains and coastal plateaus occur at many locations internationally, with prospects for development involving pumped storage between upper and lower reservoirs. Many years ago along the coast of Cornwall UK, mines were excavated along the coast that went below sea level and into the earth under the sea. That precedent proves the possibility of underground chambers near or next to the coast.

Venturi Water Pumps:

Venturi pumps involve short sections of concentric pipes, with a small high-speed stream of water actually combining with water from an underground reservoir to pump that water to higher elevation. A series of venture pumps built into success locations along a single pipe can move water uphill over an extended elevation, such as from an underground reservoir to empty at sea level. An array of wave-driven offshore water pumps connected to the shore by high-pressure hoses could provide the energy source to activate multiple venture pumps that push water from an underground reservoir to sea level.

Alternatively, wave driven piston pumps would be quite capable of very directly pumping seawater to higher elevation, including 500-feet above sea level. Wave technology that operates 24-hours per day offshore could pump sufficient water into a coastal reservoir to provide a small community with about 6-hours of peak hour electric power when water from high elevation flows through a sea level turbine and drives an electrical generator. An underground storage system would require installation of turbine and electrical generator deep underground, a concept that may be viable if a suitable underground chamber such as an abandoned mine were available.

Shore-based Pumps:

There is also potential for shore-based wave energy to pump water to higher elevation for storage where the reservoir a few feet above sea level has limited storage capacity. Water would flow from the reservoir near sea level perhaps over a head of 2-feet or greater to ram pumps located at sea level, outside of the converging breakwaters. These units involve 2 x one-way valves and a pressurized storage chamber and can pump a small percentage of the water flowing through them to an elevation of 10 to 25-times the `head' to a storage reservoir at much higher elevation.

If the coastal reservoir can provide a `head' of 10-feet to sea level, the ram pumps could push a small percentage of water to elevations of 100 to 250-feet above sea level. The increase in `head' between reservoir and sea level would translate to higher operating efficiency for the turbine that drives the electric equipment. Whereas a turbine operating over a 5-ft head could deliver relatively low output over extended time duration (24-hours), a turbine operating from a 100-ft head would generate much greater output at higher efficiency for a short duration (4 to 6-hours).

Conclusions:

The option to combine wave energy with low cost energy storage capability could increase the attractiveness of wave energy conversion that could pump energy into storage for 18 to 20-hours per day, delivering peak output over 4 to 6-hours duration during peak demand periods. Wave-driven energy storage involves relatively basic, low-cost technology. Coastal-based technology offers the advantage of being easily accessible to maintenance staff while offshore technology would require that malfunctioning pumping units be disconnected and towed to shore for repairs. Where geography allows, wave-driven storage technology could be attractive to the distributed-generation energy market.

 
 
Authored By:
Harry Valentine holds a degree in engineering and has a backround in free-market economics. He has undertaken extensive research into the field of transportation energy over a period of 20-years and has published numerous technical articles on the subject. His economics commentaries have included several articles on issues that pertain to electric power generation. He lives in Canada and can be reached by e-mail at harryc@ontarioeast.net .
 

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