The designs of cars, airplanes and houses are determined by cheap energy. When our own bodies supply the energy, the forms change. The bicycle, canoe and man powered Gossamer Condor are interesting examples of design when energy is expensive.
Let us also build man powered air conditioners. It should be interesting, for Nature offers no examples; she only cools passively. Twenty-five years after the invention of the Condor, we have not graduated from cars and highways to commute silently like so many giant moths, but a man cooler might have great consequences.
Until recently man could not fly by his own power. He could fall, he could glide, he could rise on thermals, but until Paul McCready and the Gossamer Condor he could not fly, as birds do, by his own efforts. It wasn't for lack of trying. Old pictures show winged men leaping, racing and crashing, vainly trying to get airborne. A large prize for flying went unclaimed from
1959 until McCready's success in 1977.We only cool ourselves passively. Passive cooling is giving heat to someplace cooler; by convection, if the air is cooler; by radiation, if cool is in sight; or, mysteriously and around corners, water will accept heat at temperatures far below that of the air if there is no traffic jam of humidity. Evaporation is why panting and sweating works so well to cool us animals. Water vapor finally condenses in a cold cloud or on the earth as dew.
The pilot sits on a bicycle seat pedaling a propeller to drive the 96' wing span, 70 lb. airplane. The cooler pedals a compressor on a similar seat in an insulated cabin that is covered inside and out with finned heat exchangers. Most air conditioners need 1 unit of work to pump out three units of heat. We wish the COP was 10 not 3. Then anyone would have a chance to pump out as much heat as he created on the treadmill, for most of us are at least
10% efficient.Pedaling or cranking the compressor on an air conditioner lifts the heat to however high a temperature necessary to flow into surrounding air. This is real cooling, no relying on Nature's tricks. Sweating is no more real cooling than gliding is flying. Passive flying and cooling work only as long as one's weight or heat is headed for someplace lower.
The challenge to fly is different than to cool. Both depend on simple ratios - power-to-weight for flying and power-to-heat for cooling. While the athlete can affect his power-to-weight by trying hard, a burst of effort guarantees nothing for his power-to-heat ratio. More effort brings more heat, too. A flyer might fail on the cooler.
Washing out on the cooler is a familiar failure - like all those whose body does not obey the mind, the person who can't stop blushing, or who can't sleep, or worse, suffers an episode of embarrassing impotence. Imagine the desperate sinking pilot who discovers he becomes heavier by pumping harder.
The problem can be simply stated. To cool, the COP of the air conditioner multiplied by the efficiency of the man powering it must be greater than one.
The ASHRAE Handbook states people have efficiencies of 10% to 20%, and sales literature for air conditioners talk of COPs as high as 4.4. With a COP of
4.4 even a 20% efficient man could not crank or pump a standard air conditioner and cool. He does 10,000 ft. lbs. of work, adds 50,000 ft lbs. of heat, but pumps out only 44,000 ft. lbs. of heat. The temperature in the cubicle would rise higher and higher, yet the numbers are close enough to make one think with more investment in the air conditioner and training for the man things would workStandard conditions should be air temperature above body temperature, 98 F.
The matching of man and machine will be interesting. The man can't get the job done by just being big and strong, or trying hard. He must be thermally potent, able to produce lots of power with little heat.
The machine is the passive partner and must be beautifully made with an efficient compressor and large heat exchangers. Like all joint efforts, there will be recriminations if things don't work.
Building a man powered man cooler is less dramatic, but finally more interesting than flying. Nature flies, but she leaves all her cooling to passive evaporation, conduction and radiation. Where she must have a high metabolism at high ambient temperatures, as with birds, she ups the body temperature and continues to cool passively. Doubtless, we will come to reflect on this if we build man coolers.
Perhaps the first man coolers will use elastic bands, liquid pistons or other Stirling engine components, not Freon refrigerants. They may be as different from conventional air conditioners as the Condor is from a jet airplane. After man coolers have been made to suit even the least efficient of us, someone will accidentally discover the real reward for our efforts. This invention made to cool man wants to run backwards, especially if its radiator is struck by sun. Although some would doubtless try to pass laws against it, God's thermodynamic laws would bless man coolers fed cheap, low temperature heat to power households.
The man cooler, dizzy, turning backwards, but forever obedient, could yield lots of work on a diet of 160 F heat discharged at 80 F.
No one could build these engines as engines; they will be too heavy, too large and too strange looking. Ridicule would defeat their designer. They can appear accidentally as I have explained.
Steve Baer September 2004