According to a study done by USAID in early 2018 Kenya’s total installed capacity stands at 2,336 MW with geothermal at 28%, hydropower at 36%, thermal at 31% and other the renewable sources such as wind and solar at 5%. Fossil fuel (thermal) power generation releases carbon-dioxide into the atmosphere and it is harmful to the environment as the layer of greenhouse gas is getting thicker, which is in turn making the Earth warmer hence causing climatic change around the world. Fossil fuel consumption in Kenya still stands at 31% and thus there is an increased call to get reduce reliance on fossil fuels while promoting renewable sources of energy. Solar and wind technologies on the other hand are still not reliable to meet the world’s needs since they are unpredictable.
The ocean still has an enormous potential and there is a lot of ever growing interest around the world in the utilisation of wave energy and marine currents to generate of electricity. Marine currents are predictable whereas wave energy is inherently less predictable as it is a consequence of wind energy. The conversion of these resources into sustainable electrical power will offer vast opportunities to countries with such resources, Kenya being one of them.
Tidal energy is an example of a renewable energy source and it is produced as a result of the gravitational fields of both the sun and the moon, these together with the earth’s rotation around its axis usually lead to both high and low tides and energy harnessed from this is the source of power generation. In other terms as the Earth, its Moon and the Sun rotate around each other in space, the gravitational movement of the moon and the sun with respect to the earth, causes millions of gallons of water to flow around the Earth’s oceans creating periodic shifts in these moving bodies of water. These vertical shifts of water are called “tides”.
Little is being done to invest in tapping the ocean energy especially in Kenya as clearly shown by the chart on Kenya’s 2031 targets on energy. Investment in marine energy is still seen as a high risk due to the initial capital investment and the complexity in the technology. Tidal energy however offers an exciting potential for its exploitation although it is not as established as solar and wind power, it is estimated that about 3800 terawatt hours per year of tidal power can be taken from the world seas if fully harnessed.
Tidal power has gained worldwide popularity as a future source of renewable energy because: it’s clean, doesn’t spoil the landscape and its totally powerful and predictable in terms of time and magnitude hence a very reliable source of energy. It is also less likely to face opposition from residents unlike wind turbines that are set up on land.
As per the ocean energy Europe 2017 neglecting Africa and Europe the estimated global potential of tidal energy is uniformly agreed to be more than 120 GW. An average of 1.9GW can be extracted from tides and tidal power across the tidal cycle that happens every fortnight. Imagine the vast amount of energy to be produced if these two continents were to be included.
There are two methods that are used in the extraction of tidal energy: tidal range and tidal stream. The first one exploits the rise and fall of sea levels using barrages and energy is extracted from the potential head of the water. This method is similar to a hydropower generation. The second harnesses local tidal currents by extracting the kinetic energy of the moving water by means of tidal current energy converters such as tidal turbines and it is similar to the wind turbines. Underwater turbines are an example of the tidal stream.
Generating energy from tidal current turbines are more environmentally friendly as compared to tidal barrages generation. Tidal turbines work by using the ebb and flooding of the tides from high to low which happens twice a day this moves huge quantities of water in the ocean hence creating the currents. The turbines consist of sets of blades and the force of the water will turn the blades and makes the shaft of submerged rotors to rotate and it is this rotational energy which is converted into electricity by a generator. Tidal turbines are normally fixed to the seabed and are connected to the grid via an armored power export cable and are typically controlled via a standard SCADA system
Water is also 832 times denser than air and it exerts more force hence energy from tidal currents can be captured and later converted into a usable form without the need for big devices. Ocean currents also have more kinetic energy than 220 miles per hour wind, these currents have a higher energy density hence smaller devices are required to harness tidal energy than wind energy. Tidal current turbines are also smaller in size as they do not require large blade to capture as much energy as opposed to wind power.
There are three types of tidal current energy converters which are the horizontal axis tidal current turbine which are similar to the wind turbines, vertical axis tidal current turbine and other types of non-turbine devices.
In horizontal axis tidal current turbines the axis of rotation is normally parallel to the direction of the current flow and it is the most common type worldwide because of its effectiveness. Vertical axis tidal current turbines have their axis of rotation perpendicular to the current flow whereas other non-turbine devices consist of a range of diversified design concepts.
There are three different methods used in installation of turbines including: seabed installation, floating devices and attachment to a rigid structure such as monopole. Sea bed installation is the most common method where the turbine and its whole structure is towed and lowered and a ballasted supporting structure fitted to the seabed. This type of installation is friendly to shipping and other marine activities and is less susceptible to overturning drag forces. Maintenance is however hectic as it involves using other vessel like cranes in detaching and lifting the turbine above sea level which is time consuming.
Floating turbines on the other hand are installed to exploit the kinetic energy on the top third of the current flow and its installation and maintenance is less complex as it can be towed to the site as compared to the sea bed installation. This type of installation may however cause a lot of disruption and result to danger to the marine and also shipping activities.
The last method of turbine installation involves attaching the turbines to a standing structure such as a monopole with the best example as the seagen turbine where the arm holding the twin rotor turbines is attached to a monopole. The construction involves civil work, the turbine can however be raised above the sea level for maintenance and repairs.
When deploying tidal turbines; technical efficiency and components of the turbines, site considerations, funding among other factors need to be considered. According to Betz’s theory no turbine can capture more than 59.3% of kinetic energy in wind and this is the same law that governors the horizontal axis tidal current turbines. The power co-efficient of these horizontal axis tidal current turbines have to be in a range of 0.39 to 0.45. The efficiency of these turbines also has to be taken into consideration.
The tidal current velocity in reality will vary with the water depth and the velocity profile defined by power laws hence performance of these turbines depends on its location in water. The top third of the water column contains the highest velocity and hence it’s the most preferred location for deployment of turbines. Slight specification changes in the gearbox of the horizontal axis tidal turbines will affects the rotational speed causing considerable changes in the output energy over a full spring seven day cycle.
When constructing the underwater turbines for tidal energy the position of individual turbines have to be assessed to know which arrangements in the water would capture the most power from the tidal currents both when the water is floods out and ebbs in.
Technology advancement has made this source of energy to be less harmful to the water animals, leave less silt deposition and also require less resources for construction, operation and maintenance. Tidal current devices have to be made in a way that they will appear to be more robust because of the hydro-dynamic forces they will undergo
When doing a sea deployment the site where the tidal farm is to be placed has to be looked into by assessing the site resources, distance from the shore, the installation scheme and other environmental factors. Tidal current submerged turbines operate optimally with a tidal velocity of 2.5m/s and at a depth of 25 to 50m hence water depth and tidal velocity range need to be taken into account. In order to obtain the correct velocity and kinetic energy flux of a potential site along the coastal line data has to be collected, with spatial and also temporal variation of these parameters being quantified through direct measurements and surveys. If there is no secondary data on this, a month’s worth data will suffice to assess the energy potential of a location as tidal energy is predictable. When doing these measurements future expansion plans also need to be taken into account to ensure the site is optimized.
The tidal energy devices also need to be connected to an area that is close to the electrical grid. Taking into account that sea bed cables are pretty expensive choosing a site where one would need a shorter cable to land is more desirable economically and it should be considered. Reduction of development costs can only be done once environmental factors such as energy flux are determined. Costs of expansion and integration of the plant will lead to additional costs when connecting to the grid. There should be a dynamic model available for each device to assess the behavior of the plant and also evaluate its impact on the grid stability.
Since installation the tidal plants may have an impact on the surrounding, the environmental factors need to be put into consideration such as how it may affect the marine life. Audio devices could be installed to scare away the marine life and hence have a minimal impact but the noise should be minimal to avoid unnecessary disturbances to marine life especially during the construction phase. There should be proper procedures in order to avoid accidental discharge of oil. Metallic copper cladding is suggested for anti-fouling and biofouling of blades should be monitored regularly to maximize the efficiency.
Permanent turbines that are immovable are encouraged so as to avoid huge weights which can hinder the flow of water. Permanent turbines can be set on monopoles which are placed very deeply in the sea bed a technology used in wind turbines.
The choice of tidal power equipment has to be put into consideration by putting into mind the design, size, capacity and also the site where it is going to be installed in terms of depth. The need for full withdrawal of the turbines to have the site restored back to its original state has to be taken into account too.
Problems likely to affect the set-up of systems to tap the tidal energy include: high initial costs, human skills, level of technology of systems, the reliable annual energy density and political instability.
There is a very high initial cost required to build the structures as it would cost about 1.2 billion dollars to build and run a 1085MW plant. The total breakdown of costs in USD on average based per kW to set up the structure as per Mrs. Lucy Onundo are as below:
Power conversion system=1,428
Structural steel elements=517
Subsea cable cost= 130
Turbine installation= 1,741
Subsea cable installation=1,636
Onshore electric grid interconnection=241
The initial investment cost is still high and there are costs that will still be incurred during operation maintenance and they are:
Power conversion system=894
Structural steel elements=506
Subsea cable cost= 20
Turbine installation= 593
Subsea cable installation=313
Onshore electric grid interconnection=50
These are average costs and there are other costs such as insurance costs that have not yet been taken into account which are likely to increase the costs. There are also other risks involved in setting up such a system which would include danger to world life but this is non-lethal due to the slow movement of the turbines which makes it hard for them to cause harm to the sea life. The technology of harnessing tidal energy is also not fully developed and hence there will be challenges that arise in terms of site selection in order to choose a site which is reliable and has the most currents to tap this energy.
As per a research conducted by Mrs. Lucy Onundo she clearly states the sites chosen indicated no potential for load base generation but here were still locations that had high potentials for tidal stream energy such as Watamu that had speeds greater than and there were propitious future potentials for the Kenyan coastline and hence integration of this new energy technology would require a thorough understanding of the resources. More research needs to be done on all areas around the coast tidal energy if it is to be used in future as a method of energy production as there is a lot of potential that lies there-in. The areas that exhibit high currents should also be made more public so that future researchers would compare their research with what has been documented previously.
Although the cost per kWh to produce tidal energy and also build tidal systems is still high, if tidal energy projects are increased over time this is conceived to reduce significantly the cost of electricity generation, in comparison with competing energy technologies and bring a benefit to the society due to its high reliability nature.
More research also needs to be done on the coastline since although all tides can produce power so far only a few locations have been looked into details and of the locations that have been examined there only few locations where tidal power can be harnessed. Recent focus has been only on proving that the technology works and on also lowering the costs associated with the technology. If successfully deployed tidal energy will bring about energy security, greatly reduce reliance on fossil fuels, improve access to education and healthcare among other benefits and the potential for this source of energy still lies in Africa.