Wind Power

 

Offshore Wind Farm, Vattenfall

What is wind energy?

Wind energy is the kinetic energy of moving air masses in the atmosphere. Wind exists due to the uneven heating and cooling effect on the Earth, creating pressure differences (gradients) that cause air to move. Simply put: the sun heats the Earth unevenly, generating pressure gradients that create kinetic energy, which can then be harnessed to produce work.

How is wind energy harvested?

Depending on the technology available and application, wind can be captured in different ways:

• Electricity generation using wind turbines (most common application)
• Horizontal-axis wind turbine (HAWT)
• Vertical-axis wind turbine (VAWT)
  
  
• Mechanical wind energy (traditional use)
• Windmills
• Water pumping windmills
• Ventilation systems
• Small-scale or micro wind systems
• Decentralized systems
• Remote locations

For the purpose of electricity generation, wind farms can be built either onshore, or offshore. 

Onshore Wind Farms

Installing wind turbines on land is cheaper and easier to build and maintain. However, wind speeds on land exhibit less stable and more turbulent profiles over time because of complex terrain, trees and buildings, taking into account the surface roughness. Rougher surfaces on land increase friction with the airflow, which slows the wind down near the ground, causing turbulence and Eddies, hence producing detrimental effects on the overall electricity production.

Wind Park in Lund, Sweden

Els Escambrons Wind Park, Catalonia-Spain

Offshore Wind Farms

At offshore farms, on the other hand, turbines are located at sea, usually several kilometers off the coast. It is at these locations that wind speeds are higher, steadier and are more predictable. Furthermore, turbines with larger rotor diameters are widely used since there are no land constraints; however, a major tradeoff is that installation, as well as operation and maintenance (O&M) is significantly more expensive due to the harsh marine environment. Among other challenges, offshore projects demand not only higher upfront capital investments (CAPEX), but also specialized installation protocols and safety standards. 

Offshore Wind Farm in Sweden - Wikipedia

My experience visiting Lillgrund Offshore Wind Farm

Lillgrund was a pioneering offshore project in the Baltic Sea that not only opened possibilities for renewable expansion in Sweden, but also provided lessons that shaped the industry today. It is located in the Øresund Strait, approximately 10 km off  Sweden's southern coast and just south of the Øresund Bridge between Malmö and Copenhagen. I had the opportunity to visit the site as part of a study trip organized by my professor, Jörgen Svensson, who contributed to the development of Denmark's offshore wind farms.

Lillgrund Wind Farm

Being at the heart of such an astounding renewable energy project has been a dream of mine for over a decade. There are so many details that you don't realize until you are just a few meters from the turbines, not even after reading books for a while. Some highlights of my field trip are:

• Turbine spacing: An absolute imperative. Low spacing between rows causes a compounded reduction in power output due to the wake effect (insufficient wind speeds for turbines downstream of the main row due to the unrecovered wind speed profile)
• Tower height: Although they look quite imposing and massive -which they are- on paper, being right in front of them made me realize they actually look much more slender than their rated power suggests
• Sea state & access constraints: You can immediately tell how complex an offshore project is by looking for a while and asking yourself: how many turbines do they transport per trip ? Installation ? Turbine erection ? Marine operations ? Corrosion control ?

These aspects put the critical stages of the project into perspective, and the challenges associated with not only installation and commissioning, but also maintenance logistics. Suddenly, it all makes sense. 

Offshore Electrical Substation

Lillgrund: Technical Aspects

• Lillgrund's encompasses 48 Siemens wind turbines, each producing 2.3 MW of output power, therefore an installed capacity of 110 MW
• It has an annual electricity production of 330 GWh 
• The rotor has a diameter of 93 m
• Towers are 115 m tall!
• The average wind speed is 8.5 m/s
• It has been in operation since 2007
• An upfront investment of 200 MEUR!

Single-line diagram for the Lillgrund wind farm, Jörgen Svensson - Vindkraftsystem

Although the average electricity consumption considered for a Swedish house is  somewhat conservative (around 16,000 kWh/year), it is estimated that the wind farm's annual production is sufficient to supply electricity for more than 60,000 houses in Sweden! (Energy Monitor, 2013). Nevertheless, it is absolutely impressive to see how a project of this size can positively impact our modern lifestyle while helping us transition to a less polluting energy infrastructure.

REFERENCES

1. Power Plants: Lillgrund - Vattenfall
2. Lillgrund Wind Farm - Wikipedia
3. Jörgen Svensson: Vindkraftsystem - Lecturer at LTH, Lund University
4. Energy Monitor
5. Wind Energy Explained- J.Manwell