There is increasing interest in the way that the design of buildings can incorporate renewable technologies including wind turbines. Up to now such machines have been regarded as an adjunct to buildings but a concept patented by Altechnica of Milton Keynes demonstrates how multiple turbines can become a feature of the design.
The system is designed to be mounted on the ridge of a roof or at the apex of a curved roof section. Rotors are incorporated in a cage-like structure which is capped with an aerofoil wind concentrator called in this case a 'solairfoil'. The flat top of the solairfoil can accommodate PVs. Where the rotors are mounted at the apex of a curved roof, the effect is to concentrate the wind in a manner similar to the Croatian cowling (see Fig. 5.7).
The advantage of this system is that it does not become an over-assertive visual feature and is perceived as an integral design element. It is also a system which can easily be fitted to existing buildings where the wind regime is appropriate. Furthermore it indicates a building which is discretely capturing the elements and working for a living. Altechnica has also illustrated how small scale rotors can be incorporated into the design of a high building. This approach has been demonstrated by Bill Dunster Architects in their SkyZed concept.
This is a multi-storey residential block consisting of four lobes which, on plan, resemble the petals of a flower, hence its colloquial name the 'flower-tower'. The curved floor plates serve to accelerate the wind by up to a factor of four at the core of the building. Here vertical-axis generators are positioned at every fourth floor. This is the only type of turbine appropriate for
this situation, being virtually silent in operation. Also at every fourth floor there are access platforms linking the accommodation lobes and serving as platforms to view the rotors. This building will be considered in more detail as a case study (see Figs 15.11 and 15.12 on pp. 155-6 and also www.zedfactory.com).
One of the most ambitious wind projects has emerged from Australia with the idea of a power station in the sky. High-altitude winds have high velocity and are constant. The idea is to install flying wind generators at 15,000 feet to harvest this energy. 'High altitude wind power represents the most concentrated flux of renewable energy found on Earth'. Depending on location, flying generators could be 90% efficient, which is well over three times that of onshore counterparts. Bryan Roberts, Professor of Engineering at the University of Technology, Sydney, has teamed up with Sky Windpower of San Diego, which has approval to conduct tests over the California desert. GPS technology will keep the turbines stable in space. The system is rated at 240 kW rotor diameters of 35 feet. The system will use existing rotor technology but also benefit from very strong but lightweight tether ropes. Even stronger strength to weight ratio materials are being developed. The device would clearly pose a hazard to aircraft. Its inventors claim that airspace restricted for these turbines would be less than that already restricted for civil aviation use. Military aircraft are another matter. If employed in mass the makers claim a cost of less than 2 cents (US) per kilowatt hour, taking account of life cycle costs.
A Canadian company, Magenn Power Inc., has also developed an airborne tethered wind rotor system (see Fig. 5.9). Horizontal-axis blades rotate around a helium-filled cylinder at heights up to 300 metres (1000 feet). Power is transmitted down the tether to a transformer on the ground which feeds into the electricity grid. The 'Magnus effect' (the effect of the aerodynamic forces on a cylinder) of the rotation provides extra lift whilst keeping the device stable within a restricted location. The company claims that it can operate at wind speeds between 1 and over 28 metres per second. It has a load factor of 40-50%, placing it well ahead of conventional wind turbines.
In addition to its efficiency and low installed cost per watt, the system has several advantages:
• clusters of machines can be sited close to high concentrations of demand, greatly reducing transmission losses and costs
• it can reach altitudes at which winds are stronger and more constant than is possible with fixed turbines
• maintenance is straightforward and easily performed
• it is portable and can easily be moved to sites with more favourable wind conditions
• it can provide rapid access to power in emergency or disaster situations where grid power is not available
• it offers a reliable source of cheap energy for remote locations in developing countries where grid power will probably never be available. It can afford near-continuity of supply in conjunction with solar and possibly micro-hydropower
• technically the system can be scaled up to very large units which can supplement an established grid in large urban areas.
A domestic scale 4 kW version was due to be marketed from early 2006. The company is confident that the system can produce electricity cheaper than any other wind system. The installed cost is estimated at £7000. See Figure 5.10.
Finally the subject of wind power cannot be left without reference to changes in distribution technology which are likely to come increasingly in evidence as offshore wind farms are developed at ever greater distances from land. It concerns transporting electricity as medium to high-voltage DC current instead of the conventional AC. As distances from the shore increase from the present 5-15 km to 50-200 km, AC cables become increasingly inefficient. At a distance of 100 km, AC cables use so much electricity generating heat that at the destination there is virtually no usable power. The principle is that the AC current generated by the turbines is converted to high-voltage DC for transportation and then back to AC in a conversion terminal onshore. By changing to DC transportation the theoretical transport capacity of existing lines could be increased by a factor of two. This switch has been made economic by the introduction of DC transformers, powerful switches and custom-developed safety and protection devices.
This DC conversion technology offers great advantages in urban situations whenever it becomes necessary to enlarge the capacity of the existing grid. Medium-voltage DC (MVDC) technology involves raising the cable voltage and this can be done with existing AC cables without having to expose the cables. Thus the curse of all urban dwellers, the digging up of roads, could be considerably curtailed.
The European Commission White Paper on Renewable Sources of Energy set a goal of 12% penetration of renewables across the EU by 2010. Within this target the goal for wind energy is 40 GW of installed power producing 8 TWh of electricity. This would save 72 million tonnes of CO2 per year. The European wind Energy Association considers this to be conservative
since current growth rates suggest that a target of 60 GW is feasible. That would mean that by 2020 wind would account for almost 10% of all EU electricity production.
Another important statistic is that the latest version of a 600 kW turbine will save between 20,000 and 36,000 tonnes of CO2 over its 20-year life. The difference is due to varying site and wind conditions. As yet the cost benefit of such a technology in terms of avoided damage to the biosphere, human health, plant damage etc., is not acknowledged in the price regime. However, the European Union Extern-E study has sought to put a price on the damage inflicted by fossil fuels compared with wind energy. The research has concluded that, for 40 GW of wind power installed by 2010, and with a total investment of €24.8 billion up to 2010, CO2 emissions could be reduced by 54 million tonnes per year in the final year. The cumulative saving would amount to 320 million tonnes of CO2, giving avoided external costs of up to €15 billion.
This is the first sign of a revolution in the way of accounting for energy. When the avoided costs of external damage are realistically factored-in to the cost of fossil fuels, the market should have no difficulty in switching to renewable energy en masse.
During 2005 there was increasing interest in mounting small turbines on the roofs of buildings. The domestic market was particularly targeted with attractive claims regarding their energy production (see Fig. 5.11).
Nick Martin of the Hockerton Housing Project,2 which has building-integrated PVs and two free-standing wind turbines, is sceptical about some of the claims being made for these micro-wind machines. He warns that the gap between the claims and the reality reflects badly on the whole industry.3 Micro-turbines should perhaps carry a pay-back time health warning. In the Hockerton Project there are four earth-sheltered houses built to superinsula-tion standards. Two 5 kW wind turbines and a PV array are enough to ensure that the homes are net zero CO2 and their energy demand is about one tenth that of an average home.
Nick Martin's concerns should be allayed by an elegant triple-helix vertical-axis quietrev-olution wind turbine ideal for urban situations produced by energy consultancy XCO2 (see Fig. 5.12). It is rated at 6kW, producing ~ 10,000 kW hours per year at average wind speeds. Free-standing, it is around 14 m high with a swept area of 3 m. Mounted above buildings, its height is 8 m. This machine promises to represent a breakthrough in aesthetically pleasing and cost-effective compact wind generators, which should be attractive to business and housing developers.
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