In this work, a new device for tidal energy conversion is presented. The main purpose of this research is to investigate the energy conversion of tidal energy into electrical one by building a small scale prototype using a pneumatic system representing the energy conversion device. The tidal energy conversion device consists of a concrete in cylindrical shape, a moving base that moves inside the concrete cylindrical body and two single acting pistons connected to a power turbine. The system specifications that mainly affect the amount of energy are the spring stiffness and hose diameter. It was found that there is a possibility to convert the tidal energy into electrical energy using the designed prototype. The maximum amount of electricity generated using the proposed prototype was about 5 Volts.
The renewable energies are free energy sources, such as wind, solar radiation, geothermal, and ocean energy. The energy in the seas and oceans can be classified into five main categories: wave energy, tidal energy, ocean thermal energy, ocean current energy, and salinity gradient energy [
Wave energy is indirect form of solar energy since the wind produces waves [
The power that can be extracted from the oceans on earth is around 1 TW. It corresponds to massive energy production which can compete with hydro and nuclear power production plants. Wave energy can be converted to electricity by several ways [
Blunden et al. [
The devices facilities that use tidal energy are usually of two types which are: marine barrages with turbines that use the advantage of the water level difference and marine turbines that use the marine currents kinetic energy produced by tides [
The proposed device in this work has a great advantage over other tidal devices due to the fact that the turbine is not in direct contact with ocean water (saline) since the compressed air is running the turbine blades.
In this paper, a new device is designed to convert the tidal energy to electrical energy. The objective of this paper is to present the new design with some of basic formulations in energy production.
The maximum height of the tidal level in Kuwait is 4.5 meters. In this work, a prototype tidal harvester with the total height of 1020 cm and the storage thank height of 410 cm is designed to obtain tidal power from the two pneumatic cylinders. The tank is made from steel sheet with 10 mm thickness which can withstand 400 MPa stresses. The maximum pressure acting on the base is calculated to be 45 kPa. The schematic diagram for the tidal energy harvester prototype is shown in
The concrete frame using 2 cylindrical shape woods was designed. First wood has a diameter of 330 mm which is placed into the other cylindrical shape wood with 430 mm. The concrete mixture will be placed between the 2 frames and that will give a cylindrical shape concrete with an inner diameter of 330 mm and outer diameter 43 mm. The concrete mixture is specifically used for a long life in the sea water. The concrete mixture was made of 25% sand, 10% cement, 45% small aggregate and 20% water. The main components of the device are: Housing of a turbine, Tank, Concrete body that will hold the cylinders with the tank, Valve, Cylinders, Springs, Motor, Turbine, Fan, and Pneumatic cylinders. The turbine used in this research is small wind turbine (see
Water properties and the conditions strongly affect the energy produced from tides [
Fwall is the force exerted by the wall on the base, hence we can write:
where θ is the base misalignment angle as
Using Equations (1) and (2), we obtain an Equation (3) from which the base thickness can be calculated using iteration procedure
Substituting Fw = 26745.2t N, Equation (3) can be rewritten as
Using iteration technique, the thickness t of the base is obtained as t = 0.01 m.
The maximum height of the tidal level in Kuwait is 4.5 meters. For this reason we chose the height of the device 4.5 m, to obtain maximum tidal power from the device. This height results in pressure of 45 kPa. The steel sheet with 10 mm thickness can withstand around 400 MPa, and that allows the device to be as big as it can be. Since the maximum pressure acting on the base is only 45 kPa, the diameter of the base can get up to 5.2 m maximum according to the standards of the steel sheets that come in maximum 5.2 m width.
In order to obtain the number of cylinders that are needed to obtain the tidal energy due to the tidal height, the total force acting on the base should be determined. The water volume above the base with 4.5 m can be obtained from
The total force that will act on the cylinders is:
The standard size of the pistons is the maximum diameter of air cylinder is 250 mm with length of 700 mm. In this work, the length has to be in the range of 4.3 - 4.5 m. The specifications of the air cylinder that is to help finding the number of cylinders we will use in the device. For 250 mm piston it can push up to 600 kPa, for 29.45 kN. Since the total force acting on the base is 979.15 kN, around 33 cylinders with maximum size according the tidal height are needed in the device.
The turbine used in this research is small wind turbine which is connected to a generator that produces electric while rotating. The compressed air that will come from the cylinder will be directed to the blades of the turbine in order to run the turbine. The air speed that will allow the turbine to rotate should be measured so that the number and size of the cylinders that will be used can be determined.
Air speed that comes from the compressor after leaving the cylinders is controlled by controlling the distance between the compressor and the turbine. Once the smoothest results are obtained the distance between the turbine and the compressor is marked and after that Air Speed Indicator (ASI) is used to measure the air speed of the compressor that needs to rotate the turbine. The recorded result of ASI was 36.45 m/s.
The piston plus the rod mass is 2.5 kg which is 25 N. To fit the spring inside the cylinder, it should have 90 mm diameter and 300 mm length.
The stiffness of the spring can be obtained from the following equation
where F is the force, K is the spring stiffness and x is the spring elongation.
The piston and the rod weight will move the spring in a very small distance because the target of the spring is to move when the water weight is applied. In this case when the piston and the rod weight applies the spring will move maximum 40 mm (measured) along the cylinder. The stiffness of the spring is obtained as K = 613 N/m.
The force that can move the spring from 300 mm long to 50 mm is needed to have the maximum volume of pressurized air from the piston cylinder. Using the stiffness of the spring, this force is obtained as 153 N.
In this research, 2 cylinders were needed to double the force needed, i.e., 306 N for which the volume of water can be found equal to
The chosen dimensions, diameter and height are 0.32 m and 0.4 m, respectively.
The concrete body that holds the cylinders within the tank was built, it was placed into wet conditions so that it can be saturated up to 28 - 30 days. After that the fiberglass sheet of the tank is placed with the moving base. Before placing the moving base, the rod on the piston is attached into the moving base then both placed into the device while the cylinder fits into its place in the concrete body. The base is checked by moving the piston up and down to check if it is going smoothly. The output air from the cylinder is connected from the forward position and the backward position direct to the turbine. The turbine will produce power that is measured in terms of voltmeter as shown in
Tidal energy is a type of clean energy that may contribute as one of the main energy sources in the future. It produces energy from the tidal of the sea or ocean. In this work, a device basically consists of a tank with a moving base is designed to be filled with sea water during tidal changes. As long as the water fills the tank, the moving base will be forced to move downward to compress two pneumatic cylinders. The compressed air then is directed to rotate a wind turbine connected to an electric generator.
Volume (m3) | Height (m) | Diameter (m) |
---|---|---|
0.032 | 1 | 0.2012 |
0.032 | 2 | 0.1427 |
0.032 | 3 | 0.1165 |
0.032 | 4 | 0.1009 |
0.032 | 5 | 0.0903 |
0.032 | 0.9 | 0.2128 |
0.032 | 0.8 | 0.2257 |
0.032 | 0.7 | 0.2412 |
0.032 | 0.6 | 0.2606 |
0.032 | 0.5 | 0.2855 |
0.032 | 0.4 | 0.3192 |
0.032 | 0.3 | 0.3685 |
0.032 | 0.2 | 0.4514 |
0.032 | 0.1 | 0.6383 |
Test # | Voltage (V) | Water height (m) |
---|---|---|
1 | 0 | 0.06 |
2 | 0 | 0.12 |
3 | 1.02 | 0.18 |
4 | 1.52 | 0.24 |
5 | 2.14 | 0.3 |
6 | 3.22 | 0.36 |
7 | 3.73 | 0.42 |
8 | 3.98 | 0.48 |
9 | 4.34 | 0.55 |
10 | 4.76 | 0.6 |
11 | 5.22 | 0.67 |
12 | 5.39 | 0.73 |
When the tank base reaches its dead bottom center, there will be an exit valves which allow the water to exit the tank when the tidal level decreases. The inlet water is controlled to move the base in slow speed until the tidal level starts to decrease. In this way, the device can operate during 24 hours a day by using the pressurized air inside the cylinder when the piston is going forward and when the spring is pushing the piston backward as well. Future work should be done on tidal energy convertors, which should focus on economic and social impacts of ocean energy. Focus on the area of maintenance and operation should be expressed and will help in improving the tidal energy conversion devices. More experimental work should be conducted on the proposed device in order to improve its design parameters.
El Haj Assad, M., Bani-Hani, E., Sedaghat, A., Al-Muhaiteeb, A., Khanafer, K. and Khalil, M. (2016) New Pneumatic System for Tidal Energy Conversion. Journal of Power and Energy Engineering, 4, 20-27. http://dx.doi.org/10.4236/jpee.2016.412002