new frontiers

island nations

less industrialized nations

marginal wind locations

distributed generation projects

repowered sites

ecologically sensitive areas

Marginal Wind Locations

A land rush to secure and develop the windiest class 5 and 6 sites using 80m tall turbines is substantially underway in the United States and Europe. In order to meet the aggressive growth goals of the US federal government, the National Renewable Energy Laboratory “NREL”, estimates that very large penetration levels must be reached. NREL has determined that Class 3 and 4 lower wind speed sites which represent more than 10 times the land area of the Class 5/6 sites must be developed to achieve the goals. As shown by NREL’s DNS model, the class 3 and 4 sites represent the majority of wind farm development over the next 25 years.

The energy available at a Class 3 or 4 site can be less than ½ that of the Class 5/6 sites at 80m tower height. Low wind speed turbines with larger rotors have been developed to improve the energy capture but this achieves only a linear energy increase with rotor area increase. Since turbine energy capture increases with the square of the wind speed, and because wind speed typically increases with hub height, taller towers can achieve substantial improvements in energy yield. Wind farms using taller towers at the Class 3/4 sites are able to improve yields sufficiently to be competitive with many of the Class 5/6 sites.

Exploiting Wind Shear with Taller Towers

Wind speed typically increases with height above ground. The amount of wind increase with height is calculated using the wind shear power law. The shear exponent typically used to estimate wind speeds at hub height is commonly considered to be .14 or greater and is estimated to exist at over 70% of all sites on an average annual basis. However, in many locales the wind shear is typically greater than .20 and can average .25 or more. The chart below shows that using the power law and various wind shears that the energy at 125m hub height can be more than 40% greater than at 65m, thus illustrating that the lower wind speed sites can achieve capacity factors nearly equal to those at high wind sites at lower hub heights.

The cost of steel tubular towers increases exponentially with height thus the net present value of the increased revenues from increased energy production over the life of the turbine is less than the incremental costs to go taller. In fact wind shears much greater than .25 are typically needed to achieve positive economics for 125m hub heights. Since less than 10% of sites have shears this high, taller tubular towers offer very little in economic gain for low wind speed site development. In contrast, the Space Frame Tower cost increases slowly with height with no substantial increase in installation costs when using the Hi-Jack System thus even very low wind shears of .1 achieve increased economics. Since over 90% of sites have at least this level of wind shear, low wind speed development opportunities on a large scale open up. For more details on the Space Frame Tower™ cost metrics click here.