Wind turbines work hard in winter winds
October 25, 2017
Winter is high season for Canadian wind energy production. As many farmers and fishermen take a much-needed rest and begin to repair equipment and prepare for the next season, wind farms are harvesting as much power as they can.
While everyone settles in to their winter activities I wanted to give some insight into what wind turbines are doing during the short days ahead. Wind farm operators have gone to great lengths to conduct routine and unplanned maintenance throughout the slower summer months. The oil has been changed, bearings greased, blades repaired and major failures overhauled all in preparation for September. There are a few factors that boost the production of WTGs (wind turbine generators) as the season changes from summer to fall and on into winter. These are highlighted in this simplified equation for a wind turbine’s power potential.
The power available for a wind turbine = ½ x air density x swept area of the blades x wind velocity3*
So, the major players are air density, how big the diameter of the rotor is and how fast the wind is blowing. Since wind farms do not change the size of the blades on wind turbines from season to season I’ll focus on the other two, air density and wind velocity.
Air density goes up as temperature goes down. Those cold air molecules get packed closer together as they lose energy. It’s not really noticeable in everyday life but sailors, kite boarders, windsurfers and pilots will take this into account when they hook up their gear and put it to work. At -1ᵒC and standard pressure and humidity the density of air is about 1.3 kg/m3. At 25ᵒC air density is around 1.2 kg/m3. Based on a typical 100-meter diameter rotor that’s going to cause about an 8 per cent increase in our power potential calculation above.
If nothing else changed this density effect translates into thousands of additional MWhs of clean electricity produced in Canada each winter.
Air density clearly has the potential to contribute to the increase in performance we’re talking about but looking back at our equation, it is also clear that the cube of the wind velocity wins out for significance. Even a relatively small change in velocity can make a big difference in production. When fall and winter come around so do the high winds. Now this change is much more obvious than a change in air density. Your pile of leaves gets blown onto your neighbor’s lawn, the snow drifts around your car and maybe that fence board finally lets go. All this is due to the environmental changes that come along with winter. One significant contributor is more dramatic changes in temperature between one region and another. Solar power advocates are absolutely right when they boast that wind power is actually second hand solar power. As the sun heats a region the air pressure changes forcing the atmosphere to rebalance with neighboring areas. During a Canadian winter, temperature extremes are greater between Canada and the tropics as the Northern Hemisphere tilts away from the sun causing more rapid air transfer.
Another contributor is the lack of boundary layer resistance. By this I mean the grass, trees, crops and other vegetation that cause the wind to slow down near the ground, have shed their leaves or are covered by snow.
Most modern wind turbines in Canada have a tower height of 80 meters. For the sake of our argument we’ll take data from a public database at a 10-meter height. In my home town of Windsor, Ontario the average wind speed at the airport in August is 3.6 m/s. In January its 5.7 m/s. Taking the output for these two wind speeds we see a power increase of almost 60 per cent! Now let me reign this in for a minute. This is a very simplistic calculation and does not take into account physical limits on power capture, cut-in and cut-out wind speeds, wind shear and a host of other complications that exclude most of us from designing wind turbines in our spare time. It does however prove the point that winter is peak season for wind farmers. It also allows us to give additional acknowledgment to our friends in the solar power industry. With wind peaking in the winter and solar production at its height in the summer these complimentary, clean generation sources can share the credit for harnessing the sun’s abundant energy year-round.
*Manwell J.F., McGowan J.G, Rogers A.L. “Wind Energy Explained: Theory, Design and Application” John Wiley & Sons Ltd, West Sussex, England, 2002