16-04-2013, 02:12 PM
GENERATING ELECTRICITY USING OCEAN WAVES
GENERATING ELECTRICITY.pdf (Size: 1.2 MB / Downloads: 134)
EXECUTIVE SUMMARY
In 2007 the Hong Kong Electric Company (HEC) initiated a green energy fund on
promotion and application of renewable energy. A sum of HK$119,000 was allocated
from this fund to this project.
The final result of this project is the installation of a prototype demonstration unit at
the Word Wide Fund for Nature (WWF) Education Centre at Hoi Ha Wan. It is
expected that thousands of secondary school students will view this equipment. The
concept is to promote renewable sources of energy.
Part of this project also involved the design and construction of equipment that could
measure and log wave conditions and tide levels at Hoi Ha Wan.
Prototypes 1 and 2 were respectively produced between October 2006 and March
2007. Prototype 1 being officially inspected by HEC in February 2007.
The final unit was installed at the City University Laboratory at the centre pending the
completion of the renovation works at the centre. The unit will then be relocated in
the main auditorium at a suitable time in the future.
The electricity generated by this unit will be used to power the DataBuoy Project
currently being installed in the Hoi Ha Wan Marine Park.
[b]BACKGROUND[/b]
The worldwide concerns regarding the depletion of fossil fuels have been a hot topic for
many years. This, the increasing demand for energy by developing and expanding economies
and the recognized impacts of the emission from fossil fuels on the environment, particularly
Global Warming, has led to a rethink of methods of useful energy production. Currently only
a small percentage of the power being used by nations is from renewable sources. It is
becoming a well understood fact that this has to be changed.
Renewable sources of energy are also becoming more attractive, especially as the prices of
traditional sources of energy become more expensive. More effort is being placed on
developing these technologies and to tap into more natural renewable energy sources
especially for electrical power generation.
The most common useful technologies are based upon the natural energy sources of water,
wind, solar and heat, the latter usually being geothermal energy sources. Each energy source
has its own advantages and disadvantages. For example water based commercial generating
systems are known as hydro-electric and involve the damming of suitable rivers. This in itself
can cause a large amount of natural damage defeating the ecological benefits of using them.
Wind based systems generating electricity are reliable and can produce a significant amount of
power, however wind farm RADAR invisibility and the fact that the large rotating blades can
kill birds and bats causes concern. They can also be noisy. One fact that few people realize is
that wind farms have to be shut down during extremely windy weather conditions. Solar
based systems have to be large and can only operate during the hours of daylight. The key
issues for geothermal is the availability, safety (usually installed in areas with volcanic activity)
and access in areas where power is needed.
A short note about the DataBuoy Project
One of the key design factors was to design a power generating system that could provide
power for a marine area monitoring project. The DataBuoy Project is a bold initiative by the
City University and the Oceanway Corporation Limited to install a real time 3-Dimensional
data logging system into the Hoi Ha Wan Marine Park. The first phase, a single buoy, has
been running for over one year. The second phase expands this by adding two more buoys,
making it the first 3-D system installed in Hong Kong. In the second phase the system will
also be connected to the internet allowing the Authority, students and other interested people
to access the data. This will allow information about salinity, light intensity and temperature
to become available for reference and research.
It is very appropriate that the in-field equipment be as stand alone as possible and it should
run on renewable energy sources. Apart form the conservation message this illustrates, the
wave energy generator of this project also provide opportunities to investigate the feasibility
of using waves to power future equipment installed in remote places as the DataBuoy project
expands further.
THE DEVELOPMENTAL STAGES
Ocean Wave Theory
The idea to somehow extract useful electrical power from the energy of ocean waves has been
a challenge for many years. There have been a myriad of designs and contraptions made,
based upon different technologies. All have operated with different degree of success and
failure.
Marine waves can be categorized into two main types; deep water waves and shallow water
waves. The difference is determined by two main characteristics, the mean depth of seabed
where the wave is currently travelling and the wave length of the wave. A deep water wave
represents wave in water which the depth exceeds six times its wave length and shallow water
wave when wave is in water depth of less than six times of the length of wave.
This can present an interesting situation, in that a deep water wave will become a shallow
water wave as the water depth decreases, and a shallow water wave can become a deep water
wave as the water depth increases. It is also possible for the nature of a wave set to change as
the wavelength of that set changes. This means that at relatively shallow water depths
(10~15m) it is possible for deep water waves to become shallow water waves and vice versa
as the wavelength of the waves changes.
Measuring Waves and Tides.
There are three methods considered suitable for measuring tides and waves in local waters;
ultrasonic, capacitive sensor and pressure sensor. All methods involve the relatively quick
determination of water height at fixed intervals in time. Ocean wave frequencies are normally
in the order of 1~2 Hertz, so any height determination needs to operate at a minimum
frequency of ~10Hz in order to give a reasonable resolution. Accepted current technology
includes water pressure measurement, ultrasonic distance measurement and changes in
capacitance of a sensor as water height rises and falls. All of these technologies are easily
operated at read frequencies of 10Hz, so it was decided to construct three units based upon
each of these technologies. In order to remove external influences, the heights were
measured inside vertical tubes placed alongside the pillars that support the laboratory building.
The setup was designed so that at least one third (1.5m) of the tube remained above the
surface of the water during the high tide mark.
The Concept of Capacitive Sensor Wave Height Measurement
The basic structure of a capacitor; two parallel conductors with a dielectric in-between them,
lends itself to accurate liquid height measurement. A totally sealed encapsulated parallel
conductor twin-lead wire is placed into the liquid. The capacitance of this wire is measured
at one end with the other end left open. The surrounding liquid forms the dielectric. As the
liquid rises and falls the capacitance of the cable changes correspondingly. The relationship
between capacitance and the height of the liquid is hyperbolic so can be a bit difficult.
This method of liquid height measurement is surprisingly accurate provided the surrounding
conditions are very stable. There is, however, a lot of care required when measuring the
capacitance especially if there is any distance between the sensor and the actual measuring
electronics. It is strongly recommended that the capacitive measuring circuit is connected to
the measured end of the sensor. A novel way to improve the accuracy is to loop the
conductor set into the liquid. This construction actually doubles the effect of the liquid and
solves the issue of trying to seal the open end of the cable underwater.
This unit was tested in the laboratory and deemed to be two unpredictable for use as a wave
height measuring system for the centre. The issues seemed to be that changes in water
salinity also changed the capacitance. The sea water at Hoi Ha Wan has salinity that varies
from 5ppm to 35ppm. Some form of compensation would be required if the measurement
was to be accurate. Another consideration was the proximity of other metals. Unfortunately
the pillar that the unit was to be installed on still has the remains of the mild-steel caisson
jacket on it. It is obvious that as this rusts away the influence of this metal object, on the
sensor, will change over time.
Methods of Wave Energy Collection
The first obstacle is to determine a suitable method to collect the physical wave energy in a
suitable form that can be eventually converted to electrical energy. The simplest method is to
use a “Water Piston”. This is basically an open tube placed vertically in the sea. The top end
is connected to a series of air hoses; the lower end is placed no deeper than 2m down below
the surface of the sea. As waves passes the tube, their kinetic energy makes the water inside
the tube rise and fall in sympathy with them. The net result is that the air inside the tube will
undergo compression as the wave crest passes and will undergo decompression as the trough
of the wave passes. Figure 4 shows the process.