Offshore Wind Overview
- Energy Source: Wind
- Energy Type Converted: Kinetic
- U.S. Theoretical Potential: 7200 TWh (varies with wind speed)
- Power Density: 2-10 W/m2
- Device Types: Horizontal axis, vertical axis, floating
- LCOE: 12 – 20 cents/kWh
Although not a “true” form of marine renewable energy since it doesn’t directly harvest energy from the ocean, offshore wind faces similar challenges. Unlike the other technologies, offshore wind is technologically mature and other countries have seen explosive growth in its development. Soon the U.S. will too.
I’ll assume that you have a basic understanding of what wind is and how it is created. If words like “westerlies” and “Coriolis effect” mean nothing to you, then check out this page, which goes into more detail about how wind is created.
Offshore Wind Introduction
The best offshore wind sites around the U.S. are clustered along the east and west coasts. When looking at the heat map below it is unsurprising that the first operating offshore wind farm in the U.S. is in the northeast near Block Island, RI.
Wind does have seasonality, but this varies with location. For example, in the San Francisco Bay Area the windy season is from March through October. In the Outer Banks off the coast of North Carolina the wind is best from September through April.
Talk to any windsurfer or kite surfer and they’ll tell you that wind is finicky. Weather forecasts can give you an idea of the conditions, but on any given day the wind speed and direction can change dramatically. A wind turbine obviously depends on wind speed for generating power, so if conditions don’t shape-up as forecasted, this can make it difficult to integrate it with the rest of the grid, let alone plan ahead. In other words: wind is plagued by the problem of intermittency.
Wind speed and direction not only change with location and season, but also with altitude. Typically the wind becomes less turbulent and faster the higher one goes. This is for the simple reason that there are less obstructions higher in the atmosphere. Large scale wind turbines typically shoot for heights of around 100 meters to take advantage of this cleaner and less turbulent wind.
Looking at the heat map for wind potential in the U.S. it may seem like there are plenty of places to develop new wind farms (a collection of wind turbines). But finding a good spot to build is not only limited by wind conditions, but also by social pressures. The chief complaints about offshore wind farms is that they are noisy, destroy beachfront views property values, kill sea birds, and use-up too much space. All these factors have led developers to look further offshore where there are less concerned parties, and cleaner wind (and less birds).
To understand how a turbine actually generates power, it’s important to understand lift. The blades on a turbine have a unique shape and curvature modeled around an airfoil. If you’ve ever been sailing or been on an airplane you have first-hand experience of how an airfoil works.
As air particles flow across an airfoil a low-pressure area develops on one side resulting in a force that acts on the foil in a direction that is perpendicular to the wind. This is lift. If the turbine is oriented correctly the lift on the blades will cause the rotor they are attached to to spin. There is another force produced by wind flowing over a blade, but which acts in a direction parallel to the wind, that we call drag. Wind turbines can take advantage of either of these forces, but most blade designs try to maximize lift and minimize drag. For a more in-depth discussion on lift, see this article.
The Wind Turbine
A wind turbine is nothing new. They are essentially a modernized version of Don Quixote’s windmills. Its job is to extract the kinetic energy of wind and use it to drive a shaft connected to a generator to produce electricity.
Wind turbines are characterized by their axis of rotation relative to the ground (or water): either horizontal or vertical. Horizontal axis wind turbines (HAWT) resemble an airplane propeller and are situated such that they face upwind or downwind. The HAWT design is the most common on land and the only design currently used offshore.
Vertical axis wind turbines (VAWT) have not seen the same levels of adoption as HAWTs. However, their generator is more easily accessible on the ground and the design doesn’t care what direction the wind is coming from. The Darrieus and Savonius designs are two common types of VAWTs that use lift and drag, respectively, as their driving forces. There are a few offshore prototypes in the works, but none that are operating on the grid.
A wind turbine’s ability to generate power is dependent on the wind speed. Its power output is given by the equation:
P = power
ρ = air density
A = swept area of the rotor
v = wind velocity
The important thing to notice about this equation, and why I present it, is that power is proportional to the cube of the wind velocity. So even a small increase in wind speed leads to huge improvements in power, or vice versa. Power is also proportional to swept area of the blades, hence the tendency to build larger turbines.
One might assume that more wind is always a good thing. Wrong. Turbines are designed for an optimum range of wind speeds, but beyond that they actually cut out and stop producing power. So just as too little wind can be a bad thing, so can too much.
Offshore Wind Systems
Offshore wind turbines are nearly identical to their shore-based counterparts, minus a few differences. Offshore wind turbines are generally larger than onshore systems. As we saw in the power equation, a larger swept area equates to more power. This is also because of economies of scale (bigger is actually sometimes cheaper) and the cleaner offshore wind that improves performance.
Offshore wind have more elaborate foundations. When it comes to supporting the turbine there are three possibilities: fixed, floating, and well, floating.
The most common is using a bottom mounted system. In these systems the supporting tower is either driven into the seafloor (monopile); attached to a large, heavy base that uses gravity to keep it upright; or a multi-legged foundation where each leg is drilled into the seafloor. All of these fixed foundation types are limited to water depths of no more than 60 meters.
Beyond 60 meters deep a turbine must rely on a floating platform. Floating offshore wind is a relatively new option that calls on a proven technology used in the oil and gas industry for drilling platforms. In this system, a semi-submerged platform is partially sunk and then tethered to the seafloor. This partially submerged structure is what supports the turbine and the nacelle.
There are a variety of submerged platform shapes and designs. The submerged tension-legged platform (TLP) uses taught mooring cables to connect the platform to the sea floor. A spar buoy uses a ballasted tank to provide the necessary weight and stability. And lastly the tri-float design uses three interconnected pontoons to keep the turbine afloat.
These designs are not as common as the fixed foundations, but they are well tested by the oil and gas industry. Principle Power has successfully demonstrated its WindFloat design and it’s likely we’ll see more of them connected to the grid soon.
The last type of floating offshore wind system isn’t floating on the ocean, but rather in the air. These systems are still technologically immature, but there is potential for them in the offshore industry. The Makani Energy Kite is a wind turbine that glides 800 to 2000 ft above the ground and generates power via several small turbines. Altaeros has a different design that uses a helium filled tunnel with a turbine in middle. Both systems are still in pilot testing phases, but keep an eye out for them in the not-too-distant future.
- “Onshore and Offshore Wind Energy: An Introduction” 2012; Lynn, Paul A.