In this video, we'll be taking a look at assessing ocean energy. Specifically, we'll be looking at wave energy, assessing aquatic current energy, these pictures of the Mediterranean Sea currents , and tidal energy. Let's get started. Let's begin by assessing wave energy. To assess wave energy, we need to know the wave height, the distance between the waves or the wavelength, we need to know the velocity of the waves, and we also can derive the period of the waves. It's how frequently waves occur, dividing by the wavelength, divided by the velocity. We're going to calculate wave power for deep water. In shallow water, the calculations change, but most wave energy is in water deep enough that these calculations work. Seawater density is a little denser than pure water at 1,025 kilograms per meter cubed. Gravity remains at 9.81 meters per second squared, and the wave period, we're going to take to be five seconds so every five seconds a wave occurs. We're going to say that wave height is three meters, it's pretty rough out there, and the power then is measured in watts per meter of wave crest. For every wave crest is measuring along the top of the wave and so we're going to get watts per meter of wave crest. Here's the formula below the picture. It's kind of complicated. P equals density times gravitational constant squared by 32 Pi times the wave period t times the wave height h squared. Now, the characters in parentheses are all constants so we can work those out as 981 times t times h squared. When we run the numbers, plug in the numbers for this example, we find that each meter of wave crest will deliver, or has embodied, 44.15 kilowatts per meter. That's a lot of energy when you think about the frequency of waves arriving and the length of those waves. This map shows wave energy around the world. We can see that sites with the greatest wave energy are off the southern coast of South America, going in the southern coast of Australia and New Zealand, and then off the western coast of Northern Europe. Now, in most of these sites, except for Northern Europe, there are very few people and so wave energy there isn't very useful. It is useful off the coast of Northern Europe. Again, that's one of the reasons that there's so much interest in wave energy in Northern Europe. Now let's turn to the power of flowing water or currents. The types of aquatic currents include steady ocean currents, tidal currents, and rivers. Here's an example of current power at sea. Here we have a submersed floating turbine. It's anchored to the seafloor. Let's consider a one cubic meter of water that's flowing past that turbine. When we do the calculations, we can calculate the mass flow rate as the density of seawater per second. The seawater density, again we've looked at, is 1,025 kilograms per meter cubed. Let's suppose the current velocity is two meters per second. Then we can calculate the power as 0.5 times the mass flow rate times v squared, the velocity squared. Those of you that are familiar with physics, this is simply the kinetic energy of that current flow. When we run the numbers, the power is equal to 2.05 kilowatts per meter squared. Now, understand that water density is 800 times air density so even though the water is moving much more slowly than air currents, it packs a lot more power because of the much greater density of water than air. Here's a map of global ocean currents. You can see that current speeds vary widely around the world, but it's not very useful for practical use as far as finding locations where the current is flowing rapidly. More locally, channels between islands often have fast currents around headlands, entrances to tidal lakes, estuaries, and, of course, rivers, are all places to look for fast currents where energy can be extracted. Here are some potential tidal turbine sites around the world. You can see that they're rather limited and many of them are not near any population centers. Nonetheless, there's a lot of interest in tidal turbines and other current turbines because they generate so much power with relatively small footprints. Now, let's talk about the power of dammed tidal flows with tidal barrages. We've talked about tidal barrages in the previous video, and let's now do some calculations. Again, a tidal barrage works by allowing tidal water to flow into a tidal basin as shown in the top picture. Then it's dammed there as the tide goes out. We then, in the bottom picture, force the water to flow through a turbine in order to get back out to the open sea. That's how a tidal barrage works. The tidal range is the height of the tide, let's say five meters. The tidal basin area, that's how much water is captured behind the dam, is say nine kilometers squared. Seawater density is what we've known already, 1,025 kilograms per meter cubed. Gravity hasn't changed, 9.81 meters per second squared. We put it all together and we get that the energy out is equal to 0.5 times the density times gravity times the tidal basin area times the head height squared divided by 3.6. When we do the arithmetic, we find that the energy is 314 megawatt-hours per tide or about 25 megawatts annual average in this tidal barrage. Potential tidal barrage sites are not too many, they're pretty rare. You can see the red dots here. These are places where they have the suitable estuary that would make for a good tidal barrage. One of the problems with tidal barrages of course is ecological, is that they essentially dam up estuaries where there's lots of life and a lot of commerce that goes on in estuaries so tidal barrages probably are not going to be a big part of our energy future. In summary, thinking about ocean energy, we've talked about wave energy and how to measure it, ocean current energy and how to measure it, and finally tidal energy and how to measure it.
