As the world moves away from fossil fuels, technicians are working with many types of sustainable power. By now you have certainly seen wind and solar power in action, and you've heard of hydropower that harnesses river currents. But you might not have known that we can use the tides to generate electricity. So how does tidal energy work?\n\n\n\nTidal power is a lot like hydropower. It uses the flow of water to turn generators and produce electricity. But tidal energy uses water that flows in and out with the tides. Under the right conditions, tides can provide reliable, renewable power.\n\n\n\nTides: The Moon and Water\n\n\n\nWhen we use tidal power, the ultimate source of our electricity is the moon's gravity. As the moon orbits the Earth, its gravity pulls on the water. The water level is higher on the side of the Earth that faces the moon, and also a bit higher on the side precisely opposite from the moon. It is lower everywhere else. \n\n\n\nAs the Earth turns every day that water flows through ocean currents. When the moon is directly overhead the level of ocean water will be at its highest. When the moon is close to the horizon, the water level will be at its lowest. When the moon is shining on the other side of the planet (where you won't be able to see it), the tides will be high again.\n\n\n\nHow high and how low the water will go is affected by local geography and weather.\u00a0 Tides don't affect small lakes, and typical ocean shores will see the water rise by around ten feet.\u00a0 But there are places where geography can channel and concentrate tidal flows.\u00a0 The Bay of Fundy in Nova Scotia, Canada, frequently sees tides that raise the water level by fifty feet.\u00a0 (Here's a time-lapse video.)\n\n\n\nThe tides roll in and out twice a day. The timing shifts slightly day-to-day as the moon goes around the Earth, but the tides are very predictable and reliable.\n\n\n\nUsing Tides for Power\n\n\n\nSo how do you use tides to make power? The basic idea is a lot like hydroelectric power, or wind for that matter: there is something like an underwater windmill that spins as water flows past. That is called a turbine. The turbine cranks a generator, which makes power.\n\n\n\nWhile all tidal power plants will use this basic principle, there are several different setups, all designed to make the most of what the moon and the water provide us.\n\n\n\nTidal Streams\n\n\n\nThe first option is simply to set the turbine up in a tidal stream or estuary. This narrow river-like flow of water will move back and forth with the tides. The tidal stream concentrates and intensifies the tidal water flow, making these an excellent location for turbines.\n\n\n\nYou may also see what is called a "tidal fence." This would mean a string of turbines that rotate horizontally at the water's surface like a turnstile rather than vertically like a windmill. But it is still the water's motion that drives the system.\n\n\n\nTidal Barrages\n\n\n\nA tidal barrage will look a lot like a long, low dam. The idea is that high tides will pull water up to the barrage. Here's how it works: as the tide rises, the barrage operator will open valves that let water into the barrage. The water will turn turbines on its way to a reservoir on the other side. The water will build up in a reservoir until it reaches its peak.\n\n\n\nWith the valves closed, all the water will be held up behind the barrage. Then as low tide approaches, the operator reopens the valves, and water flows out of the reservoir and through turbines on its way back out. The same water flows through the barrage in and out, at high and low tide, and is used to power generators both times.\n\n\n\nTidal Lagoons\n\n\n\nA tidal lagoon can be natural or artificial. An artificial tidal lagoon will look a lot like a tidal barrage, but unlike a barrage, a lagoon is submerged at high tide. This means less power will be generated, especially at high tide. But it also means the system will not disturb the environment as much. Fish will have the ability to swim in and out of the lagoon during high tide.\n\n\n\nTidal lagoons are still in the experimental stage. There are none currently functioning, although a couple are under construction.\n\n\n\nWhere Can We Build Tidal Power Plants?\n\n\n\nTidal power plants must be placed on ocean coasts. Although larger lakes -- like the Great Lakes system in the United States and Canada -- do have tides, the tidal water flows are minimal. The difference between high and low tides on the Great Lakes averages less than three inches, too small to be useful for generating electricity.\n\n\n\nIdeally, the plant should be located on a channel or bay that faces east or west. A fjord may also work well. The best location is one where local geography funnels tidal waters into a relatively small area -- like the Bay of Fundy.\n\n\n\nThe idea is to pick a place where the difference between high and low tides will be very large, creating opportunities for engineers to "trap" water in lagoons or behind barrages, or creating tidal streams with strong flows. All of this means a more powerful flow of water to turn turbines and crank up generators.\n\n\n\nWhat Are the Advantages of Tidal Power?\n\n\n\n1. Tidal Power is Sustainable\n\n\n\nTidal power is very much like solar power: we are using the forces of the cosmos itself. We do not have to worry about running out of tidal power, at least not any time soon. As long as there is a moon in the sky and water in the ocean, there will be tides that we can direct through turbines.\n\n\n\n2. Tidal Power is Green\n\n\n\nUsing tidal power does not release any carbon dioxide or methane into the atmosphere. It does nothing to aggravate global warming or climate change. It is also fairly non-toxic. A well-maintained tidal power system should produce practically no waste products like coal ash or spent fuel.\n\n\n\n3. Water is More Powerful than Air\n\n\n\nEven compared to other emission-free power sources, tidal power offers some important advantages. Among them is that water is heavier than air. This means that turbines powered by tidal water can turn heavier crankshafts than windmills. Tidal power can generate more electricity this way.\n\n\n\n4. Tidal Power Is More Consistent\n\n\n\nReliability is a significant edge that tidal power has over other renewable sources. Solar and wind power are vulnerable to the whims of weather. A windless day can render a windmill farm useless, and solar panels generate much less electricity on rainy or overcast days. Solar power cannot generate any power at all at night.\n\n\n\nTidal power is much more predictable and consistent. Local weather might affect water levels somewhat, but as long as you have a spot with strong tides the weather won't matter very much. The tides roll in and out as the Earth turns and the moon orbits. Practically every day will have two high tides and two low tides, so there will be four opportunities to generate power.\n\n\n\nAnd with smart design of tidal reservoirs, we can hold water in reserve and release it over several hours, turning the generators the whole time. A tidal power plant may run much more steadily throughout the day and night.\n\n\n\nWhat are the Disadvantages of Tidal Power?\n\n\n\n1. Limited Geographically\n\n\n\nWith current technology, there are only a handful of places where tidal power is practical. In the United States, potential tidal power sites have been identified along the northeast Atlantic Coast from Maine to New York, and in Alaska.\n\n\n\n2. Maintenance\n\n\n\nMaintenance for a tidal power plant will be a challenge. Unlike a solar farm, a tidal power plant will likely have many moving parts that can wear down or break -- and will need to be lubricated too. At a minimum there will be the turbines that the water turns, the shafts that connect the turbines to generators, and the generators themselves. Depending on the design there will also be valves that control the water flow.\n\n\n\nA windmill farm will also have moving parts, but tidal power will have the additional complication that much of the machinery will operate in or underneath the water, and the water will not be particularly clean. That means silt and grit will find their way into the machinery.\n\n\n\nAnd to do inspections, routine maintenance, or repairs, either you will need to hire underwater specialists with SCUBA gear or drain all the water from the area. Either of these can make a minor repair into a more complicated undertaking.\n\n\n\n3. Tidal Power Can Affect Local Ecology\n\n\n\nTo use tidal power means manipulating large amounts of water, running it through valves and turbines, holding it in reservoirs, and then releasing it. All of which can affect marine wildlife. Fish in particular can be injured by the moving machinery or trapped in reservoirs, unable to reach spawning grounds or swim back to food sources.\n\n\n\nReservoirs also change local geography, as plots of dry land are inundated. Plantlife can be destroyed by an excess of water or denied water that is redirected into tidal power facilities.\n\n\n\nNone of this is necessarily catastrophic, and there are ways to reduce the damage. Engineers have developed slow-moving turbines that minimize harm to marine life. Reservoirs and lagoons can be laid out to drain as completely as possible, ensuring that fish get back out to open water.\n\n\n\nBut designers will need to keep these risks in mind as they lay out tidal power facilities. And the tradeoffs to protect marine life may mean tidal power plants that are more expensive or less powerful than they might have been otherwise.\n\n\n\nConclusion\n\n\n\nTidal power has its limitations. There are only a handful of locations in North America where tides are strong enough to be used for electricity. But in the right location, tidal power has a lot to offer. It gives off practically nothing in terms of waste or greenhouse gases. It requires no fuel -- just water and the moon's gravity.\n\n\n\nTechnicians have developed several ways to convert tides into electrical power, But all share one basic principle: turbines that use water flow to turn generators.\n\n\n\nUnlike solar and wind power, tides are much more reliable. Unlike windmills, which are useless when the air is calm, or solar, which is weakened in rainy or cloudy weather, the tides are predictable and reliable. With the creation of reservoirs, it is even possible to hold water in reserve. Tidal power will frequently be useful at night, when solar farms will be in the dark.\n\n\n\nTidal power isn't the answer by itself, but it is one more tool we can use to create the power we need while protecting the environment.