A recent Environmental Building News story, "The Folly of Building Integrated Wind," (May 2009) revealed that attaching spinning things that are supposed to generate electricity to buildings is not a very good idea. In critiquing building-integrated spinning things, however, it is important not to paint with too broad a brush. The Tibetan tradition of Vajrayana Buddhism gave rise to a building-integrated spinning thing that is energy efficient, relatively low in cost, and non-polluting: the prayer wheel. The hand-held prayer wheel is more familiar in the west than the building-integrated kind, and has the obvious advantage of portability. But prayer wheels mounted on or in walls or railings (not to mention freestanding models) have the advantage of vastly exceeding the prayer-holding capacity of handheld wheels. A 2"x4" cylinder has a volume of 50 cu. in. Increasing the dimension threefold, to 6"x12" yields a volume of almost 340 cu. in, almost seven times as large. Say a hand-held wheel holds ten copies of a mantra. Each spin of the wheel releases into the universe ten repetitions. At two spins per second, in five seconds the yield is a hundred mantra repetitions. Let us say that a typical wall-mounted installation has 20 wheels, easily spun while walking by the wall in five seconds, and that each wheel has a volume seven times that of a typical hand-held wheel and holds 70 copies of a mantra. Because the input of human energy applied to a wall-mounted prayer wheel is ordinarily only momentary, the quality of its bearings has a large effect on the number of rotations it will undergo in a given use. Lubrication and other maintenance also play a role. It is probably safe to assume five revolutions. So, in our example, there are 20 wheels, each holding 70 copies of a mantra, revolving five times each, which yields 7,000 repetitions of the mantra each time a person passes by the installation and activates the wheels. With more precise machining than is usually found in places such as Tibet and Nepal, the number of revolutions could be increased, and larger wheels than are typical can hold many more mantras; either of these adjustments could push the number of mantra repetitions to 15,000 or more. Even if located in a low-traffic area where daily only 10 people pass by and spin the prayer wheels, this yields 150,000 mantra repetitions. In busy place, the yield could be in the millions. Hand-held prayer wheels are ordinarily operated by a given individual for longer periods of time than building-integrated models, and, sticking with our assumptions, generate about 6,300 repetitions per hour, if spun continuously. If spun for eight hours--impossible for many people, and probably the most that could be feasibly expected of anyone--this yields a little over 50,000 repetitions. The comparison to architectural prayer wheels is not impressive. Of course first costs must be considered. Given the same quality of materials and workmanship, a hand-held prayer wheel is certainly cheaper than a set of larger wheels set on axles within the niche of a wall, especially when the wall itself is factored in. But building-integrated prayer wheels are a shared community resource, while the cost of a hand-held model is borne by an individual or at most a household. Here again, the larger, more capital-intensive, higher capacity wheels seem to win the day.

This electric prayer wheel may have prodigious output, but at a cost.

While it makes sense to maximize the productivity of prayer wheels, generally speaking, this can be taken too far. Electric prayer wheels are increasingly popular, but they take an efficient technology with no carbon footprint and shift it into the realm of energy-consuming machinery that is only as environmentally sound as the source of its power. A better route is running prayer wheels on hydropower or wind power, which is easily done by incorporating the prayer wheel into a waterwheel or turbine which already provides the rotary motion needed. Building-integrated prayer wheels both promote and benefit from pedestrian activity. Their mantra-generating capacity is vastly enhanced by any increase in the number of users, while the opportunity for meritorious action while walking can draw motorists out of their cars, improving their level of fitness as well as keeping prayer wheels in action. At the same time, because they take some attention and effort to activate, they can function as a pedestrian traffic-calming technology, slowing bustling walkers to a more moderate pace and decreasing the likelihood and severity of accidents.