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From Helios to Our House PUBLIC ACCESS

Looking at Fan Blades Aerodynamically Leads to Higher-efficiency Cooling.

[+] Author Notes

Associate Editor

Mechanical Engineering 123(08), 42-46 (Aug 01, 2001) (5 pages) doi:10.1115/1.2001-AUG-1

A study commissioned by Florida Power and Light Co. of Juno Beach reports that energy savings in Florida from ceiling fans came only with the raising of air conditioner thermostats. Making fans with high-efficiency motors offered one way of achieving that goal and aerodynamic blades offer another. Infrared thermogram of a typical ceiling fan shows heat generated by the motor. Temperature scale at bottom reads from coolest to hottest. With simulations having helped the decision on the choice of design, the engineer team set out to discover a building method for making the prototype blades. As work proceeded on the blades, Florida Solar Energy Center, in Cocoa, also considered what sort of control system would eventually operate a production version. Even after undergoing many redesigns from the original wooden-blade prototype development team tested back in 1997, the two production models offer substantial improvements in energy efficiency.

A casual comment started Danny Parker down the trail of invention. A principal research scientist at the Florida Solar Energy Center in Cocoa, Parker said that a conversation with his father-in-law began his inquiry into ceiling fans. In the five years since, Parker and his colleagues developed a more efficient fan that today Home Depot sells nationwide.

Ceiling fans that were available five years ago exhibited a strangely prevailing feature.

Flat blades were the rule. As Parker and his father-in-law talked on the porch, the older man, a retired pilot, recalled fan blades looking very much the same as they did when he was a kid. They hadn’t changed in the ensuing years. None had turned aerodynamic.

At about the same time, a study commissioned by Florida Power and Light Co. of Juno Beach reported that energy savings in Florida from ceiling fans came only with the raising of air conditioner thermostats. When air conditioner thermostats were not set higher, or when fans ran full time, total electrical load in a house could actually increase, the study found. The motors turning the blades also produced heat that added to air conditioning burdens.

Sections of the fan blade airfoil, generated by computer, plot the increasing twist of the blade as it moves from the tip to the root.

Researchers in the study projected as much as a 25 percent energy savings if air conditioned houses could raise their thermostats several degrees to take advantage of the increased sensible cooling brought on by air moving over skin.

Infrared thermogram of a typical ceiling fan shows heat generated by the motor. Temperature scale at bottom reads from coolest (at left) to hottest.

Grahic Jump LocationInfrared thermogram of a typical ceiling fan shows heat generated by the motor. Temperature scale at bottom reads from coolest (at left) to hottest.

Using 78°F as a baseline thermostat setting, researchers determined where a setpoint had to be in order for ceiling fans to reduce energy use. It wasn’t until thermostat settings rose to 79° that additional electricity consumed and heat added by fan motors were overcome by less air conditioning use.

To Parker, improving ceiling fan efficiency could spell significant savings for the energy consumed by Florida homes during the long cooling season. Making fans with high-efficiency motors offered one way of achieving that goal. Aerodynamic blades offered another, Parker suspected. By switching to improved fan blades, Parker hoped that a less expensive, lower-power motor could stand in for the big motors found on fans moving high volumes of air.

A UL standard for ceiling fans limited blade speed, so the blade design had to work at rotational rates between 50 and 200 rpm. That presented an immediate hurdle, Parker said. Propellers ordinarily spin much faster, so finding an aerodynamicist conversant with low-speed applications proved tough.

Partway through construction, prototype blade maker Jeff Sonne holds the first of four airfoils he would build by hand. He cut balsa sections of the airfoil from the computer-generated plot.

Grahic Jump LocationPartway through construction, prototype blade maker Jeff Sonne holds the first of four airfoils he would build by hand. He cut balsa sections of the airfoil from the computer-generated plot.

Eventually, Parker remembered the Gossamer Albatross, the human-powered aircraft that Bryan Allen peddled across the English Channel in 1979. The propeller on that craft turned slower than those on conventional prop airplanes.

In tracking down the Gossamer Albatross and its predecessor, the Gossamer Condor, Parker came upon Aerovironment Inc., the company founded by human-powered aircraft designer Paul McCready. At the firm’s headquarters in Monrovia, Calif., Parker contacted Bart Hibbs, a critical mission engineer and, later, Guan Su, an aeromechanical engineer, who agreed to help him build a better blade.

Parker bought a number of fans and, in examining them, discovered that the best ones moved air at velocities near 2 meters a second. Parker asked the Aerovironment team to match that flow rate in the new blade configuration, while limiting the fan speed to 200 rpm so it could meet UL requirements.

A specialist in full laminar flow airfoils, whose work propels NASA’s new Helios solar aircraft, Hibbs fed Parker’s constraints to a program he had written for low-speed props. Hibbs said he used the same procedures with which he designed the Helios propellers (a modified Atkins method) to design the fan blades. But when the program prompted him for the fan propeller’s forward speed, he entered a very small number rather than zero to avoid sending the program into convulsions.

A flat paddle’s two major flaws are a nearly even chord length over its entire span and a fixed pitch angle, Su explained. Because the tip of a paddle moves through air faster than its root does, airflow over a conventional fan blade is lowest near the hub and highest at the tip. Smoothing this distribution would make an entire blade surface contribute equally to moving air.

Hibbs initially chose a public-domain airfoil, designated GM15, for the fan blade shape. This airfoil originated at the wind tunnel at the University of Illinois.

But the profile was too thin for the fan, so Hibbs and Su worked up eight fan designs using three other airfoils. Eventually, a twisted, tapered airfoil emerged that promised twice the efficiency of standard flat-blade fans for moving air. Their predictions showed that the new blade could meet performance requirements with only 8 watts of electrical power at the fan shaft.

With simulations having helped the decision on the choice of design, Parker and his associates set out to discover a building method for making the prototype blades. According to Jeff Sonne, a research engineer at FSEC, the first attempts at prototypes—laying fiberglass over wooden cores—failed. So he began the laborious task of building four blades by hand with a combination of metal spars, wooden struts and profiles, and balsa skins.

The first sheets of balsa begin covering the skeletal spar. Manual building methods produced airfoils that came quite close to the original design.

Grahic Jump LocationThe first sheets of balsa begin covering the skeletal spar. Manual building methods produced airfoils that came quite close to the original design.

To make each blade, Sonne cut a dozen or so profiles from balsa sheet, then glued them to a thin metal tube every 2 inches, tilting them in accordance with the outlines plotted by the software. He used an instant glue to set them rapidly, and followed that with an epoxy coating at each joint to strengthen the bond. To stiffen the developing airfoil, Sonne glued small balsa blocks between adjacent profiles. Then, using several balsa sheets 1/64-inch thick, he covered the blades.

A testament to the importance of the prototype blades, Sonne said he spent some 140 hours building the quartet. The skills he acquired building model boats in his youth he applied to assembling the quiver of foils.

By spring 1997, Parker and his associates had rigged a test lab at FSEC. Instrumentation there, which included a hot-wire anemometer, a digital watt-hour meter, and an infrared tachometer, was used to compare the flow, power, and speed of the handmade airfoils with the performance of flat blades on three popular purchased fans.

The researchers checked airflow for all the fans, starting at their axial centerlines and working out through a dozen points at 6-inch intervals. Diameters did not exceed 52 inches, so the last six measurements in the series revealed how little beyond the sweep of any fan the cone of moving air extended, regardless of whether its blades were flats or foils.

Two fans, of which one was the experimental unit, hit the same peak airflow. But the experimental fan matched the best off-the-shelf model while consuming about half the energy. In a side-by-side comparison, where researchers mounted aero-blades and paddles on identical motors, the foils more than doubled the flow rate of stock blades for the same expenditure of energy.

As work proceeded on the blades, FSEC also considered what sort of control system would eventually operate a production version. Many users in Florida ran their fans all the time, surveys showed, even while no one was in a room to feel the effects. Many users either had the impression that moving air around actually served to cool it, or they simply left a fan on all day because it turned out to be the least bothersome way of operating it.

Among the control options considered were a presence sensing circuit, a thermostatic input, and a timer. One prototype used a compound Fresnel lens to monitor movement in a room panoramically through an infrared motion detector. FSEC shelved this particular innovation in the initial production versions, but it did incorporate both thermostat circuits and timers to enable the fan to turn and stop in response to temperature or according to preset durations. The complexity of operational decisions compared with a basic three-speed fan and light kit precluded simple wall switch controls. Instead, the production models take their orders from remote keypads.

The search for manufacturers started in 1997, Parker said, only to be set back by difficulties in fabricating the aero-blades. Along the way, one manufacturer built many stereolithographic rapid prototypes, working closely with Su to refine the design. But finding a way to make the blades in production-size quantities proved frustratingly elusive, Parker said. Eventually, FSEC and Aerovironment began licensing talks with another manufacturer, King of Fans Inc. of Fort Lauderdale. King of Fans, the maker of Hampton Bay ceiling fans—a brand that Home Depot sells exclusively—knew about injection molding from the outdoor fans it made with plastic blades in place of wood.

The handbuilt blades proved concepts; then rapid prototyping took over. Guan Su finally worked on the preproduction prototype last summer, following many refinements to the original design.

Grahic Jump LocationThe handbuilt blades proved concepts; then rapid prototyping took over. Guan Su finally worked on the preproduction prototype last summer, following many refinements to the original design.

After lining up the airfoil profiles and affixing them to the aluminum tube, Sonne added blocks between successive profiles to bump up rigidity.

Grahic Jump LocationAfter lining up the airfoil profiles and affixing them to the aluminum tube, Sonne added blocks between successive profiles to bump up rigidity.

According to King of Fans’ product development manager, Charles Bucher, a combination of market demand and UL requirements forced a slight detuning of the fully aerodynamic blades. On just about any residential ceiling fan sold today the blades turn both ways, he explained. Customers demand it, possibly a result of industry claims that ceiling fans promote energy efficiency in summer and winter. Fan sellers say that reversing a fan’s normal summertime downdraft pushes warm air off the ceiling, redirecting it into inhabited space during the heating season.

Whether that’s true or not—and Parker takes issue with the claim—the market acted in a way that sometimes surprises engineers. It told the manufacturer what would sell, and that wasn’t necessarily a design based solely on technical superiority.

Reversing an aerodynamic blade presents the knifelike tail, which ordinarily trails, as a leading edge, Bucher said—creating an obvious hazard out of any fan mounted within arm’s reach. UL requirements said no leading edge could be thinner than 1/8 inch.

Making the fan irreversible in the name of efficiency might seem the best solution. Except that a fan without a reverse switch probably wouldn’t sell, Bucher said.

Thus, the dialogue continued between manufacturing engineers at King of Fans and aeromechanical engineers at Aerovironment. Guan Su and Bart Hibbs tinkered with the profiles, blunting the trailing edge. Under pressure to move into the manufacturing stage, with the airfoil design having languished for so long in development, it softened until a suitable shape arrived.

 Solar Flier Climbs Higher

Engineers and Scientists from NASA and Aerovironment of Monrovia, Calif., were gathering at press time at a missile range in Hawaii to attempt the launch of a solar-powered, unmanned airplane, Helios, to a record altitude of 100,000 feet. In 1998, Pathfinder Plus, Helios’s predecessor, set the current high flight record for an airplane of 80,000 feet.

NASA has several goals for the Helios project. By flying at this extreme altitude, the agency hopes to get a sense of the flying conditions on Mars. The Helios is expected to carry a 100-lb. payload and thus demonstrate an ability to carry loads up to 600 lbs. to 70,000 feet, offering an alternative to meteorological and atmospheric monitoring by satellite. Another goal of the project is to demonstrate the feasibility of long-term flight, one in which the time spent aloft is counted in months.

Helios’s maker, Aerovironment, has already formed a subsidiary to commercialize the technology. Skytower Inc. sees the Helios and similar solar-electric aircraft as an ideal way of providing telecommunications service to residents of Earth. Compared with satellites, a Helios-style craft could offer many times more broadband capacity with lower launch-loss risk. And atmospheric satellites could compete with land-based stations through lower cost, faster deployment, and easier relocation.

Grahic Jump Location

If the high-altitude flights go well, NASA will begin fitting the craft with fuel cells to enable flights of longer duration. The fuel cells will use oxygen and hydrogen generated during the day to keep the airplane flying through the night. The test of this part of the project is anticipated for 2003, when plans call for a 96-hour flight at 50,000 feet.

Meanwhile, manufacturing decisions and aesthetic judgments started firming up the rest of the design. Though the new blades needed less power to move the equivalent air volume of fans using flat paddles, seller and manufacturer agreed that the production fans would use the same motor found in other models. This would lessen the possibility of warranty claims arising out of a smaller motor whose use had been limited to laboratory prototypes.

King of Fans smoothed out some of the rough edges in the prototypes as well. The fan—intended to be functional—would have to blend seamlessly into a variety of interior decors. The importance of pleasing design could not be emphasized enough, especially for aerodynamicists who see beauty mainly in the aerodynamic perfection that underlies mere surface shape.

In the end, two models would emerge, each available in two finishes. Both would use five Gossamer Wind blades for a total span of 54 inches. Apart from two very distinctive shapes—one is a sleek, modern fixture whose motor housing goes unadorned, while the other shows off a more traditional profile—it is their lighting that actually distinguishes the models from one another.

Parker realized early that few ceiling fans left the showroom without light kits. A fan, centrally mounted, offers a logical place to put a light. But more often than not, the lights use incandescent or halogen bulbs that can double a fan unit’s energy use. They also add to a home’s heat load. Of the two Gossamer Wing fans, one bucks convention by offering fast-starting fluorescent bulbs. Like the fan itself, the lights are controlled remotely and include a dimmer and a timer.

Simulation of energy lost (top) to turbulence (bottom) for flat and aerodynamic profiles. Images depict tip conditions 26 inches from the center. Flat blades pitch to 12 degrees; aero-blades pitch to 6.

Grahic Jump LocationSimulation of energy lost (top) to turbulence (bottom) for flat and aerodynamic profiles. Images depict tip conditions 26 inches from the center. Flat blades pitch to 12 degrees; aero-blades pitch to 6.

Even after undergoing many redesigns from the original wooden-blade prototype Parker tested back in 1997, the two production models offer substantial improvements in energy efficiency. According to Home Depot literature, the aerodynamic blades move up to 40 percent more air than standard blades. The fans look good, too. Home Depot started selling them this year and sales have been better than anyone expected.

The Windward II ceiling fan uses a fluorescent bulb to further increase energy savings. A second model uses incandescent light fixtures.

Grahic Jump LocationThe Windward II ceiling fan uses a fluorescent bulb to further increase energy savings. A second model uses incandescent light fixtures.

Home Depot actually sells a third fan that uses the Gossamer Wind blades. Intended for industrial use, it avoids some of the design restraints that slimmed down the performance gain on the residential models. It needs no reversing switch. It has no limit on speed because it must be mounted at least 10 feet from the floor. Function played a bigger part than aesthetics did in its design. Although its three blades do not taper to sharp trailing edges either, owing to manufacturing limitations, they more closely approach an aerodynamically preferred shape than the others do.

Using an infrared tachometer, Parker checks prototype’s speed during efficiency testing. Fans were evaluated for airflow and power consumption, too.

Grahic Jump LocationUsing an infrared tachometer, Parker checks prototype’s speed during efficiency testing. Fans were evaluated for airflow and power consumption, too.

In explaining the brisk sales of the new fans, Parker did not discount forces beyond anyone’s control that held up product introduction until this year—a year in which energy consumers are doing everything they can to lower electricity use. If the fans had been introduced a couple of years ago instead, when energy wasn’t on everyone’s mind, sales volume might well have been smaller.

With the fans moving quickly off store shelves, Su at Aerovironment has returned to Bart Hibbs’ computer programs, trying to figure out how to squeeze another 10 to 20 percent out of the fan’s energy use in time for next year’s production run. Specifically, he and Hibbs are looking at a design where the top surface retains the airfoil as the bottom surface flattens, Su said. The two are also “playing some tricks” to thicken the trailing edge while still gaining the efficiency that a knife edge can bring, he said.

This is another idea that was long in its realization: man-powered flight over the English Channel in 1979.

Grahic Jump LocationThis is another idea that was long in its realization: man-powered flight over the English Channel in 1979.

Other possible enhancements for next year’s models include bigger blade spans and motion-sensing controls. A longer blade could further the reach of moving air. Also, a longer blade could mean better use of energy by the big motors now driving the current models.

Parker’s journey reinforces the notion that it takes more than just a good idea to realize a successful venture. Pitfalls and compromise await. Reflecting on the trek so far, however, Parker said having a product out there that really works feels very good.

Copyright © 2001 by ASME
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