Much more to consider about urban wind power
November 25, 2008
By Greg Bird
Michael Timm’s November article entitled “A look at small-scale urban wind power” is a helpful contribution to understanding the potential of using wind power in cities and on buildings. Certainly, Michael’s interview with Mick Sagrillo, one of the world’s foremost authorities on small wind systems and wind power generally, brings a strong voice of experience to the issue.
However, some common-sense observations also need to be considered.
Wind is Not a Constant
The similarities of liquid and gaseous fluids cited in the article might be more helpfully understood if we remember that most rivers, like the wind, don’t flow in a straight line, but, rather, confined in their channel, meander from one bank to the other. One sees that in the scouring of a river bend against a bank, the mass of moving water passing close to the shore (but not at the high-friction boundary layer closest to the stationary bank) has a relatively less-turbulent force that carries both floating twigs and sub-surface eroded soil more rapidly downstream than in the eddies nearer the opposite shore.
The atmosphere is less confined than a river, so wind dynamics are less predicable, especially in our relatively flat Midwestern terrain (compared to constrictions at mountain passes, for instance). Just look at the flag on top of the Allen-Bradley clock tower shift around as winds change direction, both in the horizontal and vertical planes.
Many of us in Bay View have directly observed the force of winds that have toppled trees and damaged roofs. These winds aren’t necessarily from tornado-like forces, but rather from straight-line winds sometimes called “microbursts,” where limited winds approaching jet-stream force come down near Earth. Some have observed that gravity works on the atmosphere, especially in mountainous areas, pulling upper-level winds down the slopes. And microbursts, especially with their density multiplied by precipitation, are also partially affected by gravity.
Relative to What?
Now, most of us don’t get up on the peaks of our roofs, especially when it’s windy.
Rather, we experience the wind from our yards or sidewalks, and occasionally, the wind we might experience there feels pretty strong. Sure, turbulence because of all the obstructions on the ground messes up the continuous strength of the wind, but similar to the wind being of a better quality dozens of feet above the treetops, the quality of the wind at the peaks of our roofs is of a better quality than we experience on the ground.
Does that make the quality of the rooftop wind good enough to justify spending money for a rooftop wind turbine? Depends on the roof and where the rooftop is.
Is it at the edge of a field stretching out to the northwest, where the wind (mostly coming from the northwest) can pick up some minimally-obstructed speed and force before it meets the roof, or is it like we have in Bay View, surrounded by close-by houses and trees? Well, again, one doesn’t necessarily need to spend the money for a professional wind assessment to get an idea of what the chances are that a particular roof or site has production potential.
Is the roof on a building that’s on a hill? Much of Bay View, for instance, is in the Deer Creek basin, which is obviously not prime for wind resource power.
On the other hand, Bay View has some hills, like Sauerkraut Hill near Ellen and Bennett, with a bit of a ridge extending east toward Oklahoma and KK, at Logan and Idaho, at the rise near Dover Street School, and at other areas which present more advantageous venues, though still in the turbulence-causing obstructive matrix of mature neighborhoods.
Use of Existing Structures
Then, there is the issue of existing structures that rise above turbulence-causing treetops.
If one had to choose a few locations to monitor wind strength in Bay View, Bay View Terrace, Lincoln Court, Dover Street School, and Bay View High School would be pretty obvious. And looking out across the larger Milwaukee cityscape, schools, churches, water towers, stacks, lighting masts, utility pylons, commercial structures, etc., do stick up above the treetops in their hundreds or thousands-some even to the cited 30-foot height. And those are outside the central business district with its tower skyscrapers and tunneling streets (most of which in Milwaukee are disadvantageously situated down in the river valley).
What about the shape of existing buildings? Are those shapes optimal for allowing wind to flow strongly over them and provide wind that can help produce power? Most often, they are not. If an existing building were to be considered for wind turbines, then some engineering analysis of fluid dynamics for the building might be advantageous to see if limited and cost-effective additions to building exteriors would pay off in improved wind flows and/or whether particular placement of wind devices would help.
Evaluating Structural Integrity
The story about the upstate New Yorker having a turbine rip off his roof brings us to the issue of wind turbine technology on structures and in an urban environment.
If the roof has a projected average wind regime that could make sense for producing enough power to pay off the investment well before replacement of the turbine (profit), then the roof needs to be strong enough to support the additional maximum-plus loads caused by the turbine sticking up beyond the roof line in a strong gale (except for tornados and microbursts, the Midwest usually doesn’t get hurricane-force winds).
So, if you think your roof might be such a candidate, the next time you tear off the roof shingles for a re-roofing and expose the under-decking and roof rafters, you might want to consult with an engineer about adding structural supports that would allow safe placement of a turbine or an array of turbines, or even a wind monitoring device.
Another issue with mounting a turbine to a roof is vibration. Many horizontal-axis turbines (typical three-bladed turbines) create a pulse when each blade passes the mounting structure/tower, which translates into vibrations and noise. Turbines may be isolated with elastomer bushings to reduce vibrations, and/or a vertical axis turbine may be used that eliminates the blade/tower pulse and, sometimes, noise.
Considering Standalone Towers
What about a standalone tower? The choices are typically a latticework/truss tower like one sees being used for ham radio antennas or other communication purposes, which can be free-standing if tapered and with a substantial foundation, or guyed with wires or attached to the house frame, or a single tubular mast, which has similar variations and requirements.
Well, a standalone tower that sticks up 30 or more feet above treetops in a Bay View-type neighborhood would present some very real liability problems should it fall. Though rare, it could happen with a turbine failure severing a guy wire, or a tree branch being blown against it. And there are appearance problems that some would object to.
The example of the Urban Ecology Center presents an interesting test case. Riverside Park’s athletic field just to the north allows some descent of wind flows from the north and north-northeast to come closer to the ground, gaining some additional force before hitting the UEC pitched roof. Perhaps an array of small turbines at the peak of the UEC roof would minimally stress the UEC building structure and provide a less-than-optimal demonstration of limited wind power in an urban environment (like at Discovery World) without spending a lot for a mounting system. And the center’s observation tower directly to the west of the main building is a substantial well-anchored metal-frame mass that might well allow an attractive, customized, latticework outrigged mast (or four?) to be attached to it for stability that extends significantly higher than the observation tower and surrounds.
Thinking Outside the Box
If one begins to think “outside the box,” homeowners might think about porch rebuilding.
Many older homes have porches that are deteriorated and need replacement. Building a large quadrangular-section open metal latticework frame that could both support porch floors and roofs, and that then extends above treetops into higher-quality winds with device arrays mounted on the full-section tower or on multiple mini-towers, might, by its large size and footprint, be significantly over-engineered to provide assurances to neighbors that it wouldn’t likely end up on their houses. By serving multiple uses, such an approach may become cost effective.
Then there are non-residential opportunities for towers. Reinforced towers for communication, utility, water, lighting, advertising, etc. may present opportunities for safe mountings, as well as with new towers. And with the proposals to rebuild the Hoan Bridge, one could contemplate incorporating wind turbine mounting systems into the bridge superstructure in the form of masts for lighting, for example. Picture each massive roadbed support with an outrigged mast on either side extending well above the roadway, each with a small array of safe wind devices.
Variety of Turbines
Now, to the turbines themselves.
There are many designs for turbines, many trying to fit into an urban environment. For example, in China, which has the most installations for thermal solar on apartment blocks, wind systems of small ((approx. 8-inch)) diameter arrays of plastic turbines arranged in a net matrix that can be hung from a high-rise balcony are being manufactured. Whether they can keep a battery charged to power just an exit sign bulb depends on what kind of bulb is being used. A light emitting diode (LED) would have a better chance of being so powered, of course, but keeping a battery charged for electronics, lighting, cooking, or heating seems to be the goal in heavily populated areas which need to reduce demands on over-strained central generating and distribution systems.
Horizontal-axis systems with wooden blades are a traditional design for small systems, but such blades may splinter, causing a catastrophic failure that results in the entire turbine falling off the tower. That’s unacceptable in an urban environment. Use of modern fiber-reinforced plastic or metal materials for blades reduces these types of failures, and can be seen to the west of Discovery World. However, the visual effects of this form have consistently drawn objections.
Vertical-axis devices are inherently simpler machines because they do not require a second axis to orient blades into the wind. More complex blade designs, higher start-up wind speeds, and low rotational speeds reduce power output compared with horizontal-axis machines, but they are quieter and safer, and can present less visual objections.
Promise of Future Technology
Still, small machines are not going to drive big generators and have big generator economies of scale. Can this be overcome? Not in a residential application.
On the other hand, one might speculate about a large generator on the ground using a magnetic bearing system in a vacuum to minimize friction. If an external induction propulsion disk were coupled to the generator such that pumped air from an array of wind-driven turbines (think Hoan Bridge and the breakwater) were able to play on cups at different diameters on the disk, the large generator might be driven.
Or, current from arrays of small devices might generate hydrogen and oxygen by electrolysis, then stored and burned on demand to drive a large generator/combustion turbine set.
Is wind energy an essentially rural and offshore technology, with utility-scale multi-megawatt generators being driven by giant foil lift blades dozens of feet long? Certainly-as wind technology is now practiced. But cities are where most of the demand is. Along with underlying demands to reduce atmospheric carbon and reduce dependence on foreign fuel sources, those are tremendous incentives to figure out urban wind.
Some of the best minds in the world are working on new ways to find free-fuel ways to produce power, so expect new technologies.
An abridged version of this letter appeared in the December 2008 print edition of the Bay View Compass.
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