Pushing Pedals David Haaren c.1992 David Haaren One simple way to make electricity is to push pedals. The pedals turn a dynamo that charges a battery. To some the idea might sound as practical as trying to catch their dinner with their bare hands. Others who have tried it are enjoying the benefits of indoor exercise and free electricity. My wife, Kathy, and I have been riding the system shown here for several years to supplement photovoltaic power. We are marketing the system as a kit that specifically adapts the Schwinn DX 900 exercise bike. It uses a thermax 5 Ampere DC permanent magnet generator. Power is transmitted from the flywheel to the motor using Berg sprockets and a self- lubricating steel-reinforced plastic chain. We typically charge our two 12 Volt deep-cycle lead-acid batteries in the 3 to 6 Ampere range, while reading, strumming the mandolin, or just pedaling. We find we only need to pedal in the winter. I get motivated when I see our battery voltage getting low. Our time on the bike varies, but we may average about 30 minutes a day between us when required. Thanks to all the work done with efficient appliances, even an extra 2 Amp-hours really can brighten our electrical outlook. Where Do The Watts Come From? Our bodies produce mechanical power in our muscle cells using food or stored fat as fuel. This conversion is about 20 to 30% efficient. There are also losses in our creaking joints and when one muscle contracts against another. The rest of the energy manifests itself as body heat or is used to maintain vital functions. One food Calorie contains about 4000 Watt-seconds or about 1000 Watt-seconds of mechanical energy assuming 25% efficiency of the human engine. The 150 Calories in one large banana should be good for about 42 Watt-hours of energy and about 430 BTUs of body heat. Our muscles can produce power over a wide range of forces and speeds. We can push with a force exceeding our body weight (if we pull down on handlebars or push against a recumbent seat) or we can push lightly. We can pedal slowly or spin very fast. The experts recommend a high rpm (70 to 90) when bicycling to reduce the forces and strain on the muscles and bones. Pedaling slower may be vastly more tolerable. What's the hurry? Everyone has their own preference. Here are some typical values for the human body's ability to produce Watts of mechanical power: INSERT TABLE These numbers agree with other literature. A factory worker can be expected to work at a rate of 75 Watts. World-class bicyclists can maintain over 300 Watts for races that last over an hour. Thus we can expect our pedal power to be in this range. Our muscles are easily trained and made stronger. We can expect to find it easier to pedal the more we pedal. Gearing Up It is the gearing that determines what amount of force is required at what rpm. The simplest approach is to use a single gear. That gear must be carefully chosen so that suitable forces are developed at suitable rpm. With a constant force applied to the pedals, the power produced is proportional to the square of the rpm. Increasing the crank length is a way to decrease the forces required for a given power output at a given rpm. Higher forces will feel like a higher gear. At the end of the day our legs will tell us if we have or have not found the right crank-length and gear to use. Where Do the Watts Go? We should anticipate a smaller electrical yield per banana than the mechanical Watts we produce because of the inevitable losses in converting mechanical power at the pedals into electricity. There are losses due to friction at every bearing, including the pivoting links of the bicycle chain. There is even resistance from the air. Fortunately if you use positive-drive transmission (chains or gears), these frictional losses can be kept very low. Rollers and smooth belts are generally very inefficient and should be avoided. The blocking diode introduces electrical losses because of the voltage drop across it (at half a Volt that's 4% of the power at 14 Volts). But the major source of losses for the system shown here can be summed up in two words: dynamo heat Dynamo Heat Dynamos use magnets and motion to induce voltage and current in a conductor, which is in the form of coils of wire called windings. (The phenomena can be witnessed by moving a magnet near a TV and watching the stream of electrons deflect.) The magnets can be electro- or permanent magnets. The latter are preferable because they produce a magnetic field without current. The wire generally needs to be coiled in windings to build up a useful voltage. Any length of wire has some resistance and that resistance is responsible for the unpleasant, but inevitable, dynamo heat. These winding losses can be calculated by multiplying the winding resistance in Ohms by the amperage squared. For instance, with a winding resistance of one Ohm there is one Watt of dynamo heat at one Ampere and 100 Watts of dynamo heat at 10 Amperes. Thus the losses to dynamo heat increase from 7% at one Ampere to 42% at 10 Amperes (assuming a 14 Volt charging voltage). Here's how it's figured: if you're pedaling hard enough to put one Ampere of current into the battery, the battery is getting 14 Watts (1 Amp X 14 Volts), while 1 Watt ((1 Amp)2 X 1 ohm) is wasted as heat. You produce 15 Watts, but only 14 go to the battery, while 7% (1 Watt/15 Watts) is lost. Ten Amperes into the battery is 140 Watts, with 100 Watts (10 Amp)2 X 1 ohm) or 42% (100 Watts/240 Watts) wasted as heat. This decrease in efficiency will actually be even worse because the increase in the resistance of the wire windings with temperature was not taken into account. One nice side effect of this decrease in efficiency is that it justifies not pedaling too hard. Multiply the dynamo efficiency by the mechanical Watts to calculate the electrical Watts into the battery. Dynamo Dementia Two curved ceramic permanent magnets in a steel tube form the body of the dynamo (or generator). The tube completes and thus strengthens the magnetic field in which the rotor rotates. The windings are on the rotor which is contacted with carbon- copper brushes at the commutator. The commutator 'switches' the 10 sets of windings as the rotor turns so that direct current is produced. The brushes should last over ten years in this application as they last two or three years on a windmill. The brushes do not seem to add much mechanical friction. The winding resistance of the dynamo measures over one Ohm when at rest and varies with the rotor orientation. The actual working winding resistance when the machine is in motion is not known. I have briefly measured a maximum of 15 Amps at about 15 Volts going into our batteries. I can't maintain even 10 Amps for long, and the dynamo just heats up, so I don't bother. We think the system represents a good compromise between efficiency and cost for this application. The Ultimate Hand-built Dynamo I recently purchased an amazing book from New Zealand called The Homebuilt Dynamo, by Alfred T. Forbes. Words and hundreds of photos show how to build the ideal dynamo for pedal power from scratch! The author charges a 65 Ampere-hour lead acid 12 Volt battery for about an hour a day (two in winter) at 8 Amps to provide all the fluorescent light needed for his home. The dynamo is large, the parts are not cheap, and the construction looks like a challenge. But the winding losses are low right on up to 10 Amps of charge. You can get the book for $65 from Todd-Forbes Publishing P.O. Box 3919, Auckland, New Zealand. Shipping Included It's Only Natural Pushing pedals can make the difference for a battery-based alternative energy system. (A typical residential transformer has windings that are always turning about 28 watts of electricity into heat. Pedaling into the utility grid seems futile.) Alternative energy enthusiasts invest large amounts of money in efficient appliances and photovoltaic panels and batteries and other hardware. A few weeks of cloudy weather doesn't have to spoil that investment. We want to keep our batteries charged so they will last a long time and be ready with power. When the sun is gone, we have reason to pedal. I strongly recommend this type of battery charger. Without any sun, it's often more pleasant indoors anyways, and pushing pedals can help to push both morale and battery voltage a little higher. Access Author: David Haaren, POB 6, Westminster Station, VT 05159. 802-722-4122 Kits: Pedal Systems, same address Reference Bicycling Science by Frank Rowland Whitt and David Gordon Wilson. The MIT Press. Second Edition. 1982. This book is an interesting look at the physics of bicycling and body power.