Brian KirkeAdjunct Senior Research Fellow, Sustainable Energy, Barbara Hardy Institute, University of South Australia

http://www.unisanet.unisa.edu.au/staff/Homepage.asp?Name=brian.kirke

AndTechnical Director, SEADOV P/L (www.seadov.com )

Oscillating water column improvements

A presentation at the Center for Ocean Energy ResearchNational University of Ireland, Maynoothhttp://www.eeng.nuim.ie/coer/ 10 April 2014

Greetings from a town 61 km south of Dublin! No, not Wicklow, not Arklow..But Adelaide, South Australia, where we love ocean energy

Dublin town hall, South Australia

Dublin is a small town on the Adelaide Plains in South Australia, 61 kilometres (38 mi) north of the state capital, Adelaide. .... At the 2006 census, Dublin had a population of 241.

http://en.wikipedia.org/wiki/Dublin,_South_Australia

For sale: Lot 282 port wakefield road Dublin SA 5501

A better alternative than the shoreline oscillating water column (OWC)

1. Offshore where there is more energy2. Floating – deeper water, possibility of resonance3. Seaworthy ship hull for storm survival, easy

deployment and relocation if required4. One way air flow: avoid inefficient bidirectional

turbine5. Steady air flow: improve efficiency 6. A water turbine could be even more efficient than an

air turbine7. A diaphragm could isolate the working fluid from the

ocean and minimise corrosion.

Advantages of an offshore platform for wind and wave energy conversion

• More wind and wave energy available• No visual or noise pollution

Perth

Bunbury

Approx 16 km

Wave energy attenuation in shallow water off the west coast of Denmark [13].

Wave height reduction in shallow water off Perth, Western Australia [13].

Advantages of a floating offshore platform

• floating hulls experience considerably lower impact loads than fixed structures in extreme wave conditions.

• significant because much of the cost of offshore devices is incurred by the need to survive extreme storms

• with the increasing frequency of extreme events related to climate change, this is becoming increasingly important

Advantages of an offshore platform for reverse osmosis seawater desalination- No expensive feedwater and reject brine

pipelines, just a fresh water pipe to shore.- No expensive coastal real estate needed- Unlimited feedwater:

- can use a low recovery rate - so lower osmotic pressure and less energy

needed per kL of fresh water produced - less dilution of reject brine needed

Advantages of a ship hull- Easily fitted out in port, deployed, accessed for maintenance or

relocated if required- Seaworthy, design evolved over centuries, able to survive the worst

storms- Already divided into tanks, easy to convert some wing tanks to

oscillating water column (OWC) wave energy converters- Natural periods of roll and heave can be adjusted to match

dominant wave period by pumping to and from wing and central tanks so vessel motion resonates with waves and increases amplitude and power output of OWCs

- Stable enough to hold large wind turbines (even after some tanks converted to OWCs).

- Wind turbines can be erected in port where large cranes are available: no need for expensive barges offshore

- Plenty of space for reverse osmosis (RO) desalination plant housed in hull

- Can be relocated as required: natural disasters, el Niño (drought in Australia)/la Niña (drought in America) events

Wave power unit collapses at site off Scottish coast news item, 09/04/1995

http://www.ogj.com/articles/print/volume-93/issue-36/in-this-issue/general-interest/wave-power-unit-collapses-at-site-off-scottish-coast.html

• Alternative energy took a step backward last week when the world's first commercial wave power generator collapsed.

• The mishap occurred only weeks after the unit was installed inshore on the northern coast of Scotland.

• Damage to two of nine ballast tanks drew the blame for collapse of the Ocean Swell Powered Renewable Energy (Osprey) structure. The damage was discovered when Osprey arrived on site, and engineers struggled to repair it when a storm broke.

• Applied Research & Technology Ltd., Inverness, built the 2 million ($3.2 million) unit. It was designed to produce 2,000 kW of electricity from waves, to be fed into the national power grid, and to have capacity for retrofitting of a 1,500 kW wind turbine generator.

• The 850 metric ton structure was described by an Applied Research spokeswoman as being like a large artificial cave, two thirds below water and one third above.

19 years later, the same problem:

Wave energy unit damaged while under tow (it was designed to sit on the bottom, not to be seaworthy)

“A $7 million wave energy unit has run into trouble .... the unit, which is being towed by a tug boat, has suffered serious damage to the airbags supporting the 3,000-tonne structure, ....” (www.abc.net.au/.../wave-energy-unit-damaged-while-under-tow/529648... Mar 4, 2014)

A floating offshore wind turbine, showing large, high drag support structure which would be difficult to tow.

Photo credit: fukushima-forward.jp

A tanker is already divided into tanks. Easy to convert some wing tanks to oscillating water column (OWC) wave energy converters

Optimized Aframax tanker hull, after [19].

OWCs

OWCs in wing tanks both sides, open at bottom only, 10 m wide to maximise point absorber effect. Diaphragm across openings if using water turbine with treated working fluid.

Space forRO plant

Natural periods of roll and heave can be adjusted to match dominant wave period by pumping to and from wing and central tanks so vessel motion resonates with waves and

increases amplitude and power output of OWCs.

Central tanks fullWing tanks emptySmall rotational inertiaShort natural roll period

Central tanks emptyWing tanks fulllarge rotational inertiaLong natural roll period

Ampl

ifica

tion

ratio

Ratio of wave frequency f to body natural frequency fn

When natural frequency is close to wave frequency we get resonance, a big increase in amplitude and more energy capture

Ocean swells are usually made up of components of different frequency, caused by storms in different areas, but most of the

energy is usually at one dominant frequency or period

Dominant frequency 0.1 Hz, i.e. Period = 10 sec. And this dominant period generally changes

only gradually over periods of hours or days, as shown by the 5 day record below, so there is plenty of time to pump water between tanks to adjust the natural period of roll of the hull.

An Aframax tanker hull is stable enough to hold large wind turbines (even after conversion to OWCs).

- Turbines can be erected in port where large cranes are available: no need for expensive barges offshore

- And there is a synergy between wind and wave: a big gust causes hull to heel, increases OWC amplitude, and a big wave causes hull to heel, increases wind turbine movement and power output if vertical axis wind turbines used.

Maximum downstream drag on 3 wind turbines = 3 MN

Weight of 3 wind turbines @ 250 tonnes = 7.3 MN

Weight of 3 towers @ 300 tonnes = 8.8 MN

90 m

50 m (approx)

15m

W = 45m

Hull C of G 2m above surface

72,400 tonnes displacement = 709 MN buoyant force,Displacement from centre = W2tan/(12D) = 21.1 tan

D = 8 m

Hull weight half full of ballast = 693 MN

Maximum wind load on 3 towers = 0.18 MNWhile wind turbines are operating, 1 MN in 60 m/s Storm gust

Improvements to OWC turbine efficiency

• Wells turbine with reversing, fluctuation flow has low efficiency.

• High and low pressure air tanks with non-return valves can provide steady flow and improved efficiency.

• Kaplan water turbines with much higher efficiency over a wide range of flow can be used in place of air turbines.

A.F. de O. Falcao, P.A.P. Justino. OWC wave energy devices with air flow control. Ocean Engineering 26 (1999) 1275–1295

“One way of reducingthe sensitivity of the efficiency to flow changes (at the expense of higher mechanicalcomplexity and cost) is to employ variable geometry machines, as is the case of Kaplan water turbines”

Efficiency drops in irregular wave conditions

Variable pitch air turbine:Increased efficiencyBut extra complexity.

Efficiency may be about 50% when the air is flowing, but that’s < 50% of the time

One way turbine, steady flow

High pressure Air reservoir

Low pressure Air reservoir

Water rising increases air pressure, opens inlet valve, recharges high pressure air reservoir

Exhaust valve closed

I had a bright idea: a possible arrangement for steady, one way air flow

One way turbine, steady flow

High pressure Air reservoir

Low pressure Air reservoir

Water dropping decreases air pressure, opens exhaust valve, lets air out of low pressure air reservoir

Exhaust valve open

Kelly, T., Dooley, T., Campbell, J. and Ringwood, J. (2013). Modelling and Results for an Array of 32 Oscillating Water Columns

Low pressure air reservoir

high pressure air reservoir

One way air flow through turbine

But then I read a paper by Tom Kelly et al. So I wrote to Tom, and got a friendly reply, so here I am

Ref. Hydro Power, Mohammed Taih Gatte and Rasim Azeez Kadhim, Ministry of Sciences and Technology, Babylon Department, Hilla, Iraq

But perhaps we could take it one step further, eliminate the air spring and use a water turbine

One way turbine, steady flow

High pressure reservoirLow pressure

reservoir

Wave crest high, OWC low, increased pressure, opens inlet valve, recharges high pressure reservoir

Exhaust valve closed

Wave crestOperating head

OWC body resonates, moves down out of phase With wave crest

diaphragm

Concept for OWC with (i) one way flow, (ii) high and low pressure reservoirs for steady flow,(iii) efficient water turbine (e.g. Kaplan 90% over wide flow range), and (iv) diaphragm to separate working fluid (treated to prevent marine growth)from ocean water

Inlet open

One way turbine, steady flow

High pressure reservoirLow pressure

reservoir

Wave trough low, OWC high, decreased pressure opens exhaust valve, drains low pressure reservoir

Exhaust valve open

Wave trough

Operating head

OWC body resonates, moves up out of phase With wave trough

diaphragm

Inlet valveclosed

Thanks for inviting me to talk

Any questions, comments etc?