Wind energy technologies can be classified into two categories – macro wind turbines that are installed for large-scale energy generation such as wind farms, and micro wind turbines used for local electricity production. Micro wind turbines are suitable for application at the building scale and are called ‘building-integrated wind turbines’. The main components of a wind turbine include blades, rotor, gearbox and generator. Small wind turbines were originally designed with a horizontal axis, also known as HAWTs. To reduce the need for a high tower, and for aesthetic reasons, vertical axis wind turbines (VAWTs) become increasingly popular for integrated building applications. Furthermore, VAWTs are also quieter (resulting in less noise nuisance) than HAWTs during operation.
Wind turbines can be grid-connected or off-grid. Off-grid systems require battery storage to store surplus electricity, thereby providing a more stable electricity supply. Their application is most suitable for rural and remote areas, such as remote villages and small isolated islands, where grid power is not available. Conventionally, grid-connected systems require power converters to convert the generated DC electricity to AC electricity to be compatible with power grid and AC-electricity-based appliances. As technologies improve, modern wind turbines can also directly generate AC power.
Recent developments in building integrated wind turbine technologies involve improving reliability, improving efficiency at low wind speeds and lowering capital cost. Wind turbine blades are now designed with lightweight materials and aerodynamic principles, so that they are sensitive to small air movements. Furthermore, the use of permanent magnet generators, based on rare earth permanent magnets, results in lightweight and compact systems that allow low cut-in wind speeds. In this way, electricity can be generated with wind speeds as low as a few metres per second.
To be more attractive for integrating into buildings, micro wind turbines are also being designed to be more visually attractive, without compromising their performance. Another objective is to reduce/eliminate noise associated with blade rotation and gearbox/generator noise. This can be achieved by using low-noise blade designs, vibration isolators to reduce sound and sound absorbing materials around the gearbox and generator. Lastly, simplifying wind turbine components/systems also adds to the attractiveness of wind turbine application and reduces maintenance costs. Efforts in this area include the integration of inverters into the nacelle (rotor hub) (EWEA, 2009).
Lastly, to lower the product costs, advanced blade manufacturing methods, such as injection moulding, compression moulding and reaction injection moulding, are being applied to reduce labour and increase manufacturing quality.
In terms of applications, development of wind home systems (WHSs), based on the idea of solar home systems is a growing trend. A typical wind home system comprises a micro wind turbine, a battery, and various DC electrical appliances. Research shows that in coastal island areas with frequent windy conditions (e.g., Kutubdia and St Martin islands in Bangladesh), the application of WHSs is more cost effective compared with solar home systems (Khadem, 2006).
Micro VAWTs are often installed at locations with frequent windy conditions. Prior to installation of a wind turbine, it is important to collect wind data in the immediate vicinity of a building or installation site. Based on the wind data, a suitable type of wind turbine and suitable location can be determined to maximise the electricity generation. One important criterion is to match ambient wind conditions with a wind turbine’s cut-in wind speed, rated wind speed and cut-out wind speed.
Prior to installation of wind turbine(s), especially in a large number, on an existing building rooftop, it is important to ensure the roof structure is strong enough to hold the additional loads. These include the weight of wind turbine(s) and vibration from wind turbine operation. Vibration absorbent technology should be applied in order to prevent damage to building structure and to reduce interior noise in the building. As wind turbines are usually installed on the high point of the building, prevention measure from lighting damage should be in place. Accessibility for maintenance should also be planned for.
Research and development is the initial step for large-scale implementation of building integrated wind turbines in a region that has no precedent for wind turbine application. In particular, what is required is local wind mapping to understand wind speed, frequency, and wind directions at various heights and various settings. This data is crucial to determine the feasibility and the suitable types of wind turbines to be implemented in a particular area. If the feasibility study shows positive results, with a feasible return on investment, supporting policies and financial mechanisms should be in place to make building integrated wind turbines commercially viable for large-scale adoption by building owners, developers and related professionals and trades. Supporting policies should include but are not limited to the following:
- Reducing or removing subsidies for fossil-fuel-based electricity supply.
- Reducing or removing import tariffs on wind turbine components.
- Clearly identifying power grid expansion plans (for rural and remote areas) and communicating these plans clearly to the public. This is necessary for building developers to calculate payback period in the decision making process to invest and implement wind turbine off-grid systems including wind home systems.
- Setting up smart grid and incentivising feed-in tariff (in urbanised areas) as a platform to promote wind turbines for on-grid use.
In addition to the above incentivising policies, local building and construction authorities should regulate the installation of building integrated wind turbines in the following aspects:
- Structure safety
- Noise pollution control
- Grid connection
- Urban-scape design guidelines.
Another important factor for large-scale implementation of building integrated wind turbines is capacity building, especially in the following areas:
- Technical knowledge to compute, simulate and deploy appropriate types of wind turbines at appropriate locations to maximise their performance and aesthetic integration with buildings and urban-scape
- Installation skills and techniques for local workforce
- Maintenance procedures for building owners and facility management personnel
- Manufacture of micro wind turbines and related components. In this way, the products are locally available with low embodied carbon, and at the same time the local green economy is supported with new jobs creation and income sources.
In recent years, wind turbine technologies have enjoyed strong market growth globally. The global average annual growth rate of wind power capacity from 2003 to 2007 was close to 25% (i.e., from 40,000MW at the end of 2003 to 94,000MW at the end of 2007) (EWEA, 2009). China is reported to be the largest market for small wind turbines (REN21, 2009). As a general observation, the market penetration for wind turbines in the regions near the equator is low, due to the small range of temperature change year round – a natural phenomenon that results in lower wind speed in compared to regions further away from the equator.
For micro wind turbines, the initial markets were villages and developments on off-shore islands and remote rural areas. In these areas, the cost of installing micro wind turbines can usually be justified when compared to the high infrastructure cost to extend the power grid or building a power plant. Taking Inner Mongolia as an example, there are already about 250,000 micro wind turbines installed, and the use of WHSs is considered a norm. The manufacturing capability in Inner Mongolia is about 40,000 units annually (EWEA, 2009). Micro wind turbine grid-connected systems have also found a foothold in residential and commercial buildings in urbanised areas. The European Wind Energy Association (2009) anticipates this market sector to expand rapidly, thanks to the trend of higher energy prices and increasing demand for on-site power generation.
Wind power is a key component of renewable energy utilisation. Implementation of building integrated wind turbines contributes positively to the environment as a climate change mitigation option.
Wind turbine technologies, used in wind home systems in particular, contribute to social development by improving the quality of life to villagers in remote islands and rural areas, similar to that of solar home systems. These benefits include:
- Better environmental health and reducing fire hazards by avoiding use of kerosene for lighting
- Making information and entertainment accessible through the use of radio and television.
Building integrated wind turbines offer opportunities for local economic development, including:
- Less financial burden to households due to lower electrical costs
- Opportunities for households/building owners to sell surplus electricity back to the grid
- New skills and job opportunities for the local workforce
- Mechanism to grow the local green economy.
Financial requirements for the implementation of building integrated wind turbines include investment and maintenance costs. Investment cost covers not only the products and their installation, but also feasibility studies and system design related activities. One of the most critical activities is to analyse (for existing buildings) and predict (for new buildings during design stage) the wind conditions on and around the building to determine the feasibility and location for installation.
The cost components of wind turbines vary in a wide range, depending on the type, capacity rating, and local availability. Return on investment depends greatly on the actual wind conditions and performance onsite, and partially on the incentive level of feed-in tariff and local electricity pricing.
Figure 3: The Bahrain World Trade Centre is a good example of building-integrated wind turbines in a large-scale commercial building application. The high-rise building integrates three HAWTs, each with a 29m rotor diameter, into sky-bridges linking two 50-storey towers. Total cost for the wind turbines were reported to be around 3.5% of the total project cost. The three wind turbines generate between 1,100MWh to 1,300MWh annually, meeting about 11% to 15% of the building’s electricity demands.
EWEA (2009). Wind Energy: The Facts. European Wind Energy Association 2009. [Online]: www.windenergy-the-facts.org
Khadem, K.S. (2006). Feasibility Study of Wind Home System in Coastal Region of Bangladesh. Homer Energy. [Online]: http://homerenergy.com/webcast-downloads/WE58_FeasibilityWHS_Bangladesh.pdf
REN21 (2009). Renewable global status report: 2009 update. Renewable Energy Policy Network for the 21st Century.