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TIDAL CURRENT POWER
WHAT IS TIDAL CURRENT POWER?
Tidal current power generation is one type of renewable energy, which is a form of hydropower that converts the energy of tides into electricity. Tides are created by gravitational impact of sun and moon on the ocean. There are two kinds of tides, high tides and low tides, and the height difference between these two tides create tidal current, or tidal stream, that are used to generate electricity.
HOW DOES IT WORK?
There are two main ways to generate electricity using Tides. They are what we call Tidal range generation, or Tidal stream (current) generation.
Tidal range generation basically uses the height difference between low tides and hight tides. We make dam or barrage to let the tide flow in and to capture them. Then, the water will enter sluice gate, which is connected to generator, and then generate electricity. Those sluice gates are being opened during high tides to capture the water, but during low tides, it is closed so that the water stored in dam won't go out, and so that there will be difference between water level. And the minimum required water heigh difference is 16 feet. However, in Cordelia, we can't build a dam, since the city will be floating on the ocean, so we use Tidal stream power.
Tidal stream is the flow, or current of water itself created by ebbing and flooding. In Tidal stream power generation, we use this water current itself to generate electricity, without capturing it into some kind of resorvoir.
This tidal stream power generation is really similar to wind power generation, since they both use the flow of something (kinetic energy) to generate electricity. The only difference is that tidal stream power generation uses water current, and wind power generation uses the flow of air. So in Cordelia, there are tidal turbines attached on the bottom of the city (underwater) using the poles. Then the tidal current created by ebbing and flooding (which are caused by gravitational force of sun, moon to earth) of ocean will turn the turbine.
And this turbine is the one that is connected with generator, so as turbine turns, the generator will generate the electricity. Then the electricity will be transmitted to the city via the electrical cable that is inside the pole connecting turbines and the city.
Tidal Turbines are construced in the ocean about 60~120 feet deep, with tidal currents of velocity above 5~6 miles per hour, which is the perfect and enough circumstance to obtain maximum energy.
Why tidal current power?
Then why do we use this Tidal current power generation? Well basically, there are four reasons for that.
1. Cordelia is a city that 's floating on the ocean. So to get advantage of Cordelia's surrounding environment, we are using tidal current power. Then somebody might ask why don't we use hydropower instead. But the reason we don't use it is that for hydropower, we need to build a huge dam, which is impossible for our city, and it costs too much. But for Tidal current power, we only need several tidal turbines. So it is really cheap, but we can get a lot of energy from it.
2. Water is 832 times denser than the air. So tidal current contains much more energy density than those of air current, which means, with a rotor with same size, we can generate much more electricity in water rather than outside of it.
3. Tidal current power is fully renewable energy source, using the tides created by gravitational force of sun and moon on earth, which means that it doesn't harm the environment, and it is almost infinite (If you want to know more about it's impact on environment, click ). Also, it doesn't emit any pollutants or Co2, and some people say it might harm marine ecosystem, but it is currently being said that the impact on marine ecosystem or fisheries will be extremely small.
4. Tidal current power is completely predictable energy source, since all tides are developed in regular cycle. So we are able to build tidal turbines already knowing how fast tidal current will the turbines be exposed to. And it is independent against weather and other hindrances. So it can generate regular amount of electricity throught the years.
Some people might think Tidal Current Power won't be able to provide enough energy for the entire city. Then let's see how much potential energy is stored in Tidal Current power generation.
The power in tidal stream can be shown as:
HOW MUCH ENERGY IS STORED IN TIDAL CURRENT?
where P states the density of water (ex: 150kg/m3), A states Cross Section, t states time, and v states for velocity, as shown in the picture beside. As shown in the chart, the flow speed of water gets faster and faster, as it goes from just wet places (Amazon), to river (Thames) and then to some real ocean (near shore), as well as the power density. And Cordelia will be a city that is floating in the middle of the Mediterranean Sea, hich means it is so suitable for Tidal Current Power.
HOW MUCH ELECTRICITY IS GENERATED FROM TIDAL CURRENT POWER?
In Cordelia, we have 10 1 MWh tidal turbines - which means 1 tidal turbine can generate 1 megawatt per hour - attached under the main part of the sea with metal poles. However, the energy efficiency will be 35%. It means only 35% of the entire energy will be generated into electricity. On the main city of Cordelia, which is called downtown, 10, 000 people will be living there. And on the other 8 sections of Cordelia, which are used for tourisms, 2, 000 people will be allowed in each of them (For more info, click ). However, 2 of the sections are economical resources section, so only 6 sections will be allowed from travelers.
So in total, there will be 10, 000 + (2, 000 times 6) = 22, 000 people in Cordelia. And each people will be using 1 ~ 2 KW, or 1 ~ 2 KJ/s per day (1KW = 1Kilojoules per second). So in maximum, all the people in Cordelia will be using 44, 000 KJ/s per day, which is equal to 44 MW, or 44 MJ/s (1 MW = 1 MJ/s) per day. Also, including some other electricity being used in other places, such as theater, museum, malls, restaurants, or swimming pools, and the electricity that will be used to move Cordelia on the ocean, the total electricity used per day will be about 65 MJ/s.
If 1 tidal turbine can generate 1 MJ/s per hour, 10 turbines will be able to generate 10 MJ/s per hour. Considering energy efficiency of turbines (35%), 10 turbines can generate 3.5 MJ/s per hour. Then in 24 hours, they'll be able to generate 84 MJ/s in total per day (in maximum capacity). So it is enough electricity, considering we can't generate 84 MJ/s every day.
And also, the city will be floating around the ocean. When we are moving around (moving toward the tidal current), the power of tidal current will be stronger, by the affect of kinetic energy created by the movement of city, which means those tidal turbines can generate more electricity then what they actually can.
So Tidal current power will be able to generate enough electricity for the entire city. However, in case that Tidal current power can't generate enough electricity for the entire city, we also use another energy source, which is Solar power
(For more info, click ). Also, if we generate too much electricity from Tidal current power, that can lead to a big problem. Some people might think it will be better if we have more and more electricity. However, if there are electricity left, without being used, the frequency of the electricity rises. Basically, the electricity is generated by the rotation of generator, and the rotating time of the generator per second makes the frequency. So if a generator spins 60 times per second, the frequency of the electricity will be 60Hz. In Cordelia, the frequency of electricity will be 60Hz. So if the electricity is left without being used, those electricity cause generator to rotate faster and faster. If frequency changes more than 0.3Hz, the generator will go wrong, and if it goes worse, there will be a blackout in the entire city.
So to prevent that, there will be several huge accumulators connected with the electrical cables (which are inside the pole connecting city and turbines). Accumulator is a type of battery cell, which is used to store electricity. If there are overplus electricity generated, those electricity will be sent to accumulators. And those electricity will be used when the city is out of electricity, or when all the energy sources don't work.
Tidal turbines that we use in Cordelia has efficiency of 35% and can generate 1 MJ/s per hour. And this type of tidal turbine costs 8, 337, 710 Dirham per one. So in total, 10 tidal turbines cost 80, 337, 710 Dirham. This is the capital cost for tidal turbines.
However, including maintenance costs and generation costs, one tidal turbine needs about 1 Dirham to generate 3.6 MJ/s. And considering efficiency, 10 tidal turbines generate 84 MJ/s in one day. So in total, they need about 23 Dirham per day for maintenance and generation.
This menas we basically need 80, 337, 710 Dirham for capital cost, and 8, 395 Dirham per year for maintenance and generation cost.
HOW MUCH DOES IT COST?
