Oscillating water column improvements
Brian Kirke
Adjunct Senior Research Fellow, Sustainable Energy,
Barbara Hardy Institute, University of South Australia
http://www.unisanet.unisa.edu.au/staff/
Homepage.asp?Name=brian.kirke
And
Technical Director, SEADOV P/L (www.seadov.com )
A presentation at the
Center for Ocean Energy Research
National University of Ireland, Maynooth
http://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 energy
2. Floating – deeper water, possibility of resonance
3. Seaworthy ship hull for storm survival, easy
deployment and relocation if required
4. One way air flow: avoid inefficient bidirectional
turbine
5. Steady air flow: improve efficiency
6. A water turbine could be even more efficient
than an air turbine
7. 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
Approx 16 km
Bunbury
Wave energy attenuation in shallow
Wave height reduction in shallow water off
water off the west coast of Denmark [13]. 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/generalinterest/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
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 for
RO plant
OWCs
Optimized Aframax tanker hull, after [19].
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 full
Wing tanks empty
Small rotational inertia
Short natural roll period
Central tanks empty
Wing tanks full
large rotational inertia
Long natural roll period
Amplification 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
90 m
Maximum wind load on 3 towers = 0.18 MN
While wind turbines are operating, 1 MN in 60 m/s
Storm gust
Weight of 3 towers @ 300 tonnes = 8.8 MN
50 m (approx)
72,400 tonnes displacement
= 709 MN buoyant force,
Displacement from centre =
W2tan/(12D) = 21.1 tan
Hull C of G 2m
above surface

D=8m
15m
W = 45m
Hull weight half full of ballast = 693 MN
Improvements to OWC turbine efficiency
• Wells turbine with reversing, fluctuation
flow has low efficiency.
• High and low pressure air tanks with nonreturn 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.
Efficiency drops in
irregular wave conditions
“One way of reducing
the sensitivity of the
efficiency to flow
changes (at the expense
of higher mechanical
complexity and cost) is
to employ variable
geometry machines, as
is the case of Kaplan
water turbines”
A.F. de O. Falcao, P.A.P. Justino. OWC wave energy devices with air flow control.
Ocean Engineering 26 (1999) 1275–1295
Variable pitch air turbine:
Increased efficiency
But extra complexity.
Efficiency may be about 50% when the air is flowing, but that’s < 50% of the time
One way turbine,
steady flow
Low pressure
Air reservoir
High pressure
Air reservoir
Exhaust valve
closed
Water rising increases air
pressure, opens inlet
valve, recharges high
pressure air reservoir
I had a bright idea: a possible arrangement
for steady, one way air flow
One way turbine,
steady flow
Low pressure
Air reservoir
High pressure
Air reservoir
Exhaust valve
open
Water dropping
decreases air pressure,
opens exhaust valve, lets
air out of low pressure
air reservoir
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
One way air flow
through turbine
Low pressure
air reservoir
high pressure
air reservoir
Kelly, T., Dooley, T., Campbell, J. and Ringwood, J. (2013). Modelling and Results for
an Array of 32 Oscillating Water Columns
But perhaps we could take it one step further, eliminate the air spring and use a water turbine
Ref. Hydro Power, Mohammed Taih Gatte and Rasim Azeez Kadhim, Ministry of Sciences
and Technology, Babylon Department, Hilla, Iraq
Operating head
Low pressure
reservoir
Wave crest
High pressure
reservoir
One way turbine,
steady flow
Exhaust valve
closed
Inlet open
OWC body resonates,
moves down out of phase
With wave crest
diaphragm
Wave crest high, OWC low,
increased pressure, opens
inlet valve, recharges high
pressure reservoir
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
Operating head
Low pressure
reservoir
Exhaust valve
open
High pressure
reservoir
One way turbine,
steady flow
Inlet valve
closed
OWC body resonates,
moves up out of phase
With wave trough
Wave trough
diaphragm
Wave trough low, OWC high,
decreased pressure opens
exhaust valve, drains low
pressure reservoir
Thanks for inviting me to talk
Any questions, comments etc?