Wind power has long challenged the known rules of engineering. Yet for years the industry was too small to drive the design modifications to off-the-shelf components that the sector’s pioneers knew were required. They had no choice but to use standard parts designed for other purposes. Today, however, the wind sector’s double-digit growth has made it a force for change. Little-used generator concepts are being given a new lease of life, the demand for longer and lighter blades is driving advances in composite materials, and even the humble wind turbine tower is challenging engineers to make it stronger and taller without making it too big to transport. Meantime, measuring wind velocity and direction gets increasingly sophisticated in the quest for ultimate efficiency when harvesting energy from thin air.
What it all means is that component suppliers from the broad engineering sector are now investing in product optimisation to meet demands that never existed before, such as specialised parts for wind turbine drive trains. The wind industry’s calls for slow-speed generators and generators that are efficient at part load have the power-engineering industry exploring concepts, which until now received scant attention.
Connection of a wind turbine’s slow-turning rotor directly to a slow-speed generator is a component-saving option compared with taking the more obvious route and connecting a conventional high-speed generator to the rotor through a gearbox. These days, production of slow-speed generators at a cost that makes sense for wind power generation is a whole new discipline for electrical engineers. As for the little-explored niche of permanent-magnet generators (PMGs), they have come into fashion like never before. The greater efficiency of PMGs when operating at less than their rated capacity gives them a particular advantage when employed in wind turbines, which by their nature operate at partial load for most of the time: winds do not often blow at the exact speed that a wind turbine’s rated capacity is designed for.
By driving the evolution of power engineering in new directions, the wind industry can build bigger turbines. Technical size limits to economic wind power production have evaporated under the onslaught of human engineering ingenuity. Nowhere is this truer than for wind turbine rotors, where innovations by the composites industry have helped push scientific progress in leaps and bounds. A decade or so ago, blades as long as 60 metres, the size used on today’s 5MW turbines, were considered both a technical and economic impossibility. Conventional wisdom held that their weight, even if they could be built to survive, would drive up costs to outstrip the value of the electricity produced. A doubling of blade length, the old maxim ruled, would lead to an eightfold increase in weight. Practice has proved those theories wrong: material advances and better understanding of aerodynamic loads mean the addition of weight to add strength has been greatly reduced. The sky is now the limit, it seems, with 120-metre blade designs on more than one drawing board and diameters of 250 metres for offshore wind turbines under discussion.
More recently the presumption has been that physical limitations on transporting wind turbine towers would lead to size caps. Narrow roads and tunnels preclude land transportation of wide-based towers that can support more tower-head weight. But new combinations of materials and new tower shapes are leading to towers with restricted base diameters that are just as strong as the stiff steel towers used to date (page 26). For really big turbines, pouring concrete towers on site is another solution to what at one time was seen as an intractable transport problem.
Another significant recent change is the much wider geographic footprint of the knowledge base involved in designing and building wind turbines. Companies with engineering backgrounds as varied as China and North Korea, Germany and Finland, and Britain and the United States, cannot fail to take innovation further and faster still. In the quest for the perfect wind turbine, no end is in sight. The extra cost of building offshore is a primary driver for making turbines larger to gain economies of scale, while the plethora of emerging markets is calling for turbines suitable for an expanding range of challenging terrains and wind strengths. What the market demands is not a single technology solution or size, but a range of products for a wide variety of purposes. No wonder the diversity in type and size of wind turbines is getting bigger, not smaller, as the industry matures.