Wind Power

As wind power generation takes off in North America, the engineering issues continue to evolve. Experts from Hatch Acres explain what’s driving the business, and describe their technical approach.

The Business Case

by Rick Donnelly, P.Eng.

The origins of the modern wind power industry have been attributed to the activities of local inventors and anti-nuclear activists in Denmark in the late 1970s. This Danish technology spread quickly across Europe and crossed the Atlantic in the early 1980s, fueled by the California “wind rush.”

Since this initial push, wind power has emerged as a competitive energy source. The industry is growing dramatically around the world — an average cumulative rate of 29% in the last six years. According to the Global Wind Energy Council, last year was another record year. It saw the installation of 11,769 MW, compare with 8,207 MW the previous year. The total worldwide installed wind power capacity now stands at 59,322 MW. Canada installed 239 MW in 2005.

Currently, the five markets of Germany, Denmark, Spain, the United States and India dominate, with almost 75 per cent of the world’s wind power situated in Europe. However, the situation is changing as the interest in wind power in other areas of the world, and increasingly in North America, grows.

Costs for wind power typically remain slightly above those for more conventional forms of energy production, resulting in the need for incentives for the technology to grow. Power purchase agreement prices for the recent clean energy proposal calls issued by Ontario were reported to be in the 8.5 cent per kilowatt-hour (kWh) range. The same prices received in response to the Quebec government’s 1000-MW award in October 2004 are understood to be in the range of 6.5 cents per kWh exclusive of transmission costs, with the full evaluated price also being about 8.0 cents per kWh (see table, page 24).

In North America, the wind power industry has grown largely in response to an often mandated requirement for increased clean energy and government incentives.

Canada’s production incentive, known as the wind power production incentive, or “WPPI,” was instituted in the federal budget of 2001 for 1,000 MW over five years. That incentive has since been extended to a target of 4,000 MW. The program provides a $0.01 k/Wh incentive to developers over a 10-year period if the wind farm is commissioned before March 31, 2006 and $0.008/kWh if it is commissioned before March 31, 2007.

Wind and greenhouse gas credits

Overall, growth in the Canadian wind sector in response to the WPPI program and provincial government requests for renewable energy generation can be expected to continue at least for the next decade given the issues surrounding the Kyoto protocol and the need to reduce greenhouse gases.

Legislation is expected with respect to a greenhouse gas trading system for Canada. One question that has yet to be answered, though, is how much credit, if any, a zero-emissions facility should receive. An early proposal left wind facilities out of the program, but following strong protests from renewable energy advocates the Canadian government agreed to allow wind generators to create credits. Now the question is, how much will each Megawatt of wind power be worth when traded?

It is expected that the Canadian government will discount wind credits to some degree. While wind creates no greenhouse gases, this fact may not be recognized fully in the trading system, since no guarantee exists that the credits will actually reduce overall emissions. A wind park may add clean power to the grid, but that power could simply create a surplus and will not necessarily displace a dirty, coal-fired generator, nor reduce Canada’s total emissions.

The Canadian Wind Energy Association supports the idea of discounted credits for wind (to reflect the fact that not all wind-generated energy will displace conventional coal-fired generation), but wants to see other new energy sources, including gas-fired generation, subject to the same discounted credits.

Another issue under debate is whether or not projects that receive the WPPI can participate in the trading program. Some government officials call this double-dipping. The WPPI depends on government funds, while money for greenhouse gas credits comes from the private sector. The WPPI was designed only to cover half the gap between the cost of wind energy and conventional energy production. Revenue from credit trading will not close that gap and do away with wind energy’s premium. In fact, wind energy producers will come under pressure to put their prices up as system operators impose new interconnection and transmission costs.

Turbine Technology Advances

By Michael Morgenroth, P.Eng.

Since the fuel for wind power is abundant and free, the ratio of input over output is not that important. What is prominent in the wind power developer’s mind is the amount of capital it has invested in the site and equipment in order to produce a maximum yield in electricity production.

In this context, advances for better efficiency have been accomplished in features of the turbine such as better suited blade profiles, higher efficiency gear boxes — or the omission of a gear box altogether — and improvements in the generator and inverter technology. There have also been advances in the blade controls to reduce the hysteresis of cut-out and re-start in high winds.

The modern utility scale wind turbine design, also named the Danish concept, has emerged as a standard. It shows almost invariably as a three-bladed horizontal axis turbine with upwind rotor, actively pitched blades and actively yawed (turned into the wind) nacelle. The standard model uses induction type generators and inverter technology, and is mounted on a conical tower.

The impressive size of today’s turbines is made possible by technology improvements in design tools and material science. While it might have been unthinkable a decade ago, a single turbine of 6 MW output is a reality today, with a rotor diameter well over 100 metres.

Improvements in reliability are a result of wind power entering the mainstream of the electricity generation industry and vice-versa. The big international electricity companies such as Siemens, GE and ABB have entered the field and they are able to cross-fertilize wind energy technology with approaches from other industries. In this way wind energy is benefiting from these companies that have sizeable R & D budgets.

Significant costs and technical challenges still await innovative solutions not only in the design and manufacture of turbines, but also in erecting and maintaining them. They require the largest cranes available for any commercial hoisting operation. By reducing the component weight through the use of multiple lighter generators instead of a single heavier one, components can be changed using an onboard hoist rather than the main erection crane.

The sub-supplier base is responding to the growing market. As a result, purpose-designed and built components are now post-tensionel available. This stands in contrast to the earlier practice which was to adapt industrial components for service in a wind turbine.

The volatility of steel prices on the world market is a concern for the wind industry. One way out is to build towers out of pre-cast post tensioned concrete that overall use more material, but less steel.

Among the technical issues being solved is how to make the wind generator capable of supplying reactive power to the grid and sustaining short-term grid induced voltage drops, thus improving power quality on the grid.

Foundation Design

By Ivor A. Shaw, P.Eng.

To determine the appropriate design approach for wind turbine foundations, the engineer has to apply the applicable code constraints for the jurisdiction and for the foundation configuration, while producing the most
economic, constructable and safe solution. The pertinent codes for the current generation of wind turbine installations are being reviewed by the regulators in all Canadian provinces and the Canadian Standards Association. Hatch Acres is working with the CSA on this project.

As the overturning moment of a wind turbine is very large — around 30,000 kNm — and can apply in any wind direction, the optimum ground conditions for the foundations are dry, with rock or hard strata close to surface. Towers are 80 to 100 metres tall.

Wind turbine foundations are generally symmetrical reinforced concrete gravity bases. Most bases are octagonal and around 15 metres across. Some bases are circular, using European precedents. When weak strata exist at the ground surface, piled bases are used, again with an octagonal or circular concrete structure under the tower but with a smaller diameter of about 8 metres.

The fatigue of the steel elements also must be considered due to the continually varying loading.

Selecting The Site

By Margaret Trias, M.A.Sc.

Ideally wind farms are located where the best wind is, but unfortunately, those areas don’t always coincide with where the energy demand is. A wind farm located far from a population base often requires the construction of transmission lines, which increases the cost of the project and results in increased transmission losses. Therefore, the siting of a wind farm has to balance the wind resource and energy generation with other factors that affect the return on investment.

Wind speed will, of course, dictate the energy generation potential of a site. Typically, wind farms are best located in cleared areas and on topographic highs where turbulence effects from trees and other features are minimized. Some of the best locations in Canada are along the coastline in Newfoundland and in the rolling foothills of the rocky mountains in Alberta. Other sites are on cleared farm land, near large bodies of water, and along ridges where the turbines are located along the length of the highest points of the ridge.

Offshore wind farms situated in the ocean or, potentially in the Great Lakes, can offer significant advantages. However, offshore operations require additional interconnection costs and they have civil engineering challenges, both to construct their foundations and for operation and maintenance.

In some cases a wind farm may be incompatible with the existing environment or social values. Public relations are important if a developer wants to get the local community on board with the project. Also, municipal by-laws do not necessarily include zoning for wind power development, representing a further challenge.

The geotechnical conditions of the site are important and there are logistics and accessibility aspects to consider. Roads and bridges may have to be improved to allow the transportation in of the turbine, tower and blade parts.

Noise Effects

By Margaret Trias, M.A.Sc. and John B. Codrington, P.Eng.

Wind turbines make a “swishing” noise that may be perceived as a nuisance. In fact, the sound emissions from wind turbines come from two sources: the aerodynamic noise produced by the flow of air over the rotor blades and the mechanical noise originating from the gearbox, generators and auxiliary equipment.

The mechanical noise emissions have decreased thanks to the improved design of blade airfoils, gearbox and generators that allow more of the wind energy to be converted to rotational energy and less into acoustic noise. Turbines can include acoustic insulation and baffles to the nacelle, with low-speed cooling fans, and precision gear teeth to reduce their noise emissions. Newer, larger wind turbines have also reduced aerodynamic noise because of the lower rotor tip speeds, lower blade angles of attack and modification of the blade trailing edges.

Low frequency noise can be minimized by restricting the number of downwind wind turbines.

Because wind turbines generate some noise that propagates to surrounding areas, site-specific noise assessment studies are required. There are international standards defining how noise measurements should be made and how to estimate propagation (IEC 61400-11 and IEC 9613). Currently, however, there are no common international noise limit regulations.

Most countries or jurisdictions define noise limits based on upper bounds of exposure that may vary according to time of day and zoning (rural, urban, commercial, residential etc.). These levels may be fixed or, as in Ontario, where Ministry of Environment guidelines NPC-205/NPC-232 apply, the limits increase with increased wind speed. Wind turbines generate more noise at higher speeds.

Impact On Birds

By Trion M. Clarke, Ph.D.

Considerable attention has been focused on the impact of wind turbine generators on birds. Behavioural studies indicate that the most common response is for birds to recognize the turbines as obstacles and fly around them. However, some bird mortalities have resulted from collisions.

Earlier turbine models, particularly those with lattice towers and higher blade speeds, tended to have higher collision rates. However, in recent years, avian fatalities have averaged approximately two birds per turbine annually, according to U.S. data compiled for various wind projects (Erickson et al, 2001). These deaths are a small fraction compared with losses to predators or when compared to collisions with other structures such as communication towers, windows, automobiles, tall buildings and transmission lines.

Mortalities for bats are generally slightly higher than for birds, though still generally very low.

Measures to minimize bird collisions include:

* Using innovative designs such as tubular towers (rather than lattice towers) to discourage perching.

* Siting of turbines to avoid sensitive areas such as wetlands.

* Avoiding construction during bird breeding and nesting seasons.

* Lighting of towers to minimize impacts on nocturnal migratory birds.

* Shutting down turbines during peak nocturnal migration periods, if warranted.

C. Richard Donnelly, P.Eng. is Hatch Acres’ director for water and wind power for Atlantic and Central Canada. He is based in Niagara Falls, Ontario. Michael Morgenroth, P.Eng., Ivor Shaw P.Eng., Margaret Trias, M.A.Sc., John Codrington P.Eng. and Trion Clarke, Ph.D. are all experts in energy production with Hatch Acres.

Prince Wind Farm, Ontario

The Prince Wind Farm is located about eight kilometres northwest of Sault Ste. Marie, Ontario. Hatch Acres along with Breton, Banville & Associates of Quebec are the owner’s engineers for Brookfield Power during the engineering and construction of the two-phase project. The wind farm is being constructed engineering, procurement and construction (EPC) contracts. Once completed, the farm will have a total of 126 wind turbine generators producing 189 MW.

Phase I to be completed this fall involves the installation of 66 1.5-MW wind turbine generators on 80-metre towers, a 34.5-kV collection system between the turbines, a 34.5 kV/230 kV substation and a 230-kV overhead transmission line. The line will carry the power to a tie-in point with a transmission line owned and operated by Brookfield Power. Phase II involves 60 1.5 MW wind generating turbines to be delivered in August 2006.

The turbines are the General Electric (GE) Wind Sle model which has a three bladed, upwind, horizontal axis with rotor diameter of 77 metres. The turbine generator and nacelle will be mounted on a tubular tower, giving a rotor hub height of approximately 80 metres.