Tag Archives: engineering

Human Power: That Other Renewable

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Bike-powered peanut sheller and blender

Of all the renewable energy sources available today, one of them is constantly overlooked by modern society. It’s a shame, because this renewable energy is easy to harness, uses little space, and is complementary to wind and solar energy. I am speaking about that other, forgotten renewable: human power.

Using machines like a hand-crank, treadle, or pedal, human labor can be harnessed as mechanical or electrical work. The most common human-powered machine is the bicycle: mechanical work from a pedal is used to turn a wheel, which propels the rider forward. Bicycles, however, are not the only possible human-powered machines. With some clever engineering, human power has been harnessed to crank washing machines, plow fields, and saw wood. A bicycle can even generate electricity if equipped with a generator, voltage regulator, and battery. It can then power light bulbs, flashlights, laptops, and vacuum cleaners.

Hand-cranked and solar flashlight and radio


Hand-cranked red pepper processor

Unlike other renewable energy sources, human power requires active labor. Modern society, with its distaste for exercise in general, rejected human-powered machines for this very reason. That’s a shame, because human-power provides a nice complement to solar technology. Pedal-power can provide a handy back-up to photovoltaic panels on cloudy days. What’s more, pedal power can create short bursts of electricity, in contrast to the steady-stream of low power provided by solar panels.

An illustration involving an LCD monitor can provide perspective. A typical monitor requires around 100W of power to operate. After cloud cover and the earth’s tilt are considered, a photovoltaic panel might produce a power of around 25W/m^2 on average (1). So to power the monitor, we would need 4m^2 of solar panels. It only takes a single stationary bicycle, however, to generate 100W. Space is only needed for the bicycle itself and a few electronics, so the whole system can be contained in around one square meter. A fit cyclist, moreover, can produce even higher rates of sustained power — up to 200W in athletes. As a result, a well-trained cyclist can produce twice the energy of a photovoltaic panel in one-fourth of the space.

Pedal-power is not unreasonably expensive. A stationary unicycle can be built for under $250, and accompanying electronics can be purchased for around $400 (2). The combined total is $650, roughly the cost of similar solar panel installations.

The real cost savings, however, are for appliances that require only mechanical power. When there is no need to purchase expensive electronics, pedal power is clearly cheaper, since these machines can be built using only donated bicycles, spare hardware, and elbow grease. One NGO based in Guatemala, Maya Pedal, has taken discarded bikes and retrofitted them to make useful tools for local farmers. Old bicycles have been used to blend soap, pump water, grind flour, shell peanuts, and thresh grain. Not only has this removed the drudgery of agricultural work, it has also increased the income of local families. These projects promote development without burning extra gasoline or coal, all while recycling old garbage.

The Western world could learn a lesson. We chronically suffer from energy shortages, and we have no lack of people needing exercise. In the United States, more than one in four Americans are obese, and six in ten overweight. Cheap energy has allowed us to live sedentary lifestyles, which shorten our lifespans and waste trillions of dollars on unnecessary healthcare. If couch potatoes were forced to pedal for their television time, the rates of Western diseases — heart attacks, strokes, diabetes, and cancers — would rapidly plummet.

This is much better than going to the gym. Not only does gym membership cost thousands of dollars, but workout machines like treadmills actually waste additional energy to power. The average treadmill consumes 1500W of power — enough power to run 20 laptops. When people drive to the gym, moreover, they further add to greenhouse gas emissions. With human-power, they could instead burn their extra fat for productive purposes. Those calories might as well be used to wash clothes, blend smoothies, and generate electricity. Why not combat global warming while getting in shape?

Pedal-powered washing machine

Whether human power can truly make a difference depends on the efficiency of the exercise machine and the power demanded by your household. The average person can produce around 35-60W of power using a hand-crank, and 100W-120W using pedal power. Cell phones, flashlights, and watches can all be powered by hand-crank, while computers and televisions can be powered by pedals.

This sounds promising — that is, until you consider our monstrous demand for power. A medium-sized, window air conditioner uses around 1000W of power. To supply the energy for just that one AC unit, it would take a team of ten cyclists pedaling at full speed for the entire day. Once you add in laptops, televisions, clothes driers, washing machines, heaters, and light bulbs, human power becomes woefully inadequate. It would take a legion of cyclists to support the typical American home.

Storing generated electricity is a problem as well. Most pedal generators use lead-acid batteries, which store energy for later use. Devices can then be plugged into the battery rather than directly to the exercise machine. This helps avoid the awkward situation of having to simultaneously pedal while using your laptop. But as Low Tech Magazine points out, lead-acid batteries require massive amounts of energy to manufacture. Sulfuric acid can also cause severe burns, and lead can cause birth defects and brain disorders. Even pedal-powered electricity, then, isn’t perfectly green.

This limitation can be largely overcome by simply transmitting work mechanically rather than electrically. One clever hobbyist retrofitted his bicycle to spin washing machines using only pulleys and belts. The Human Powered Home, a compendium of do-it-yourself pedal-powered machines, provides plans for mechanically connect your bicycle to a grain mill, sewing machine, and tool sharpener. With a little ingenuity, the mechanical applications of pedal power are endless.

Pedal-powered jig saw

Despite its flaws, human-powered electricity can still contribute to sustainable living. Every renewable technology has its limitations, and a human-powered generator is no exception. They may not be perfectly green, but neither are solar panels. When used properly, the benefits of renewable, off-grid electricity can outweigh the harm caused by pedal-power electronics.

Generating your own electricity can allow you to live off the land, which dramatically reduces your carbon emissions. One difficulty with living on rural, undeveloped land is the lack of grid electricity. Pedal power, along with photovoltaic panels, can provide electricity without an expensive connection to the utility company. One Laptop Per Child, for instance, has taken advantage of human power to design off-grid laptops. Students in remote villages often lack access to electricity, but one minute on a hand-crank can provide enough energy for ten minutes of laptop use.

Yet the most profound impact of human power is not the generated electricity itself, but rather the conservation ethic it instills. Producing electricity is hard work. When we hook up an appliance to a power outlet, we are blind as to how much energy we are truly wasting. But if we had to pedal forty-five minutes for each hour of television we watched, we would be more conscious about our electricity usage. We would never have to be reminded to turn off our lights or to sleep our computers, and few would dream of using an air conditioner. Ultimately, it’s conservation — in addition to our feet — that will provide us with the power to lower our carbon footprint.

Do-it-yourself bicycle-power plans are the most affordable and have the lowest environmental impact. There are also some commercially-available attachments. They are expensive, however, and may actually waste more energy than they produce. I encourage you to build your own instead.

  1. Sustainability: Energy: Without the Hot Air generously estimates that a solar panel in Britain produces around 22W/m^2 on average. Low-Tech Magazine estimates a power capacity of 100-150W for average cyclists and up to 300W for athletes.
  2. The Pedal Powered Prime Mover is one unicycle designed especially for pedal power. It costs around $100-$250. The exact electronics will vary depending on your needs.
  3. Photo credits: Alan Levine, CC BY. Engineering for Change, CC BY. AIDG, CC BY-NC-SA. AIDG, CC BY-NC-SA. Donkeycart, CC BY-NC. Bruce Turner, CC BY.

The Jevons Paradox

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Energy-efficiency has become the talk of the town. Scientists, marketers, journalists, and politicians alike are showering praises on the new technologies that promise to revolutionize our planet. From zero-emission electric cars, to smart electric grids, to green laptops, high-tech sustainable solutions seem to promise the world a brighter future (1). It’s a positive message at heart: to solve the world’s energy problems, all we need is better engineering. And with many prototypes near completion, who wouldn’t be excited?

The economists aren’t, for one. These contrarians are quick to point out that most attempts towards energy-efficient technology have proved utterly futile. History has repeatedly shown that energy-efficiency rarely leads to net energy reduction. In fact, quite frequently, efficiency improvements makes things worse by actually encouraging a net waste in energy. This counter-intuitive effect is known as the Jevons Paradox.

This energy-efficiency paradox was first described in the mid-1800s by a British economist named William Stanley Jevons. During this era, coal was the fuel that powered industrialization in Britain. Britain was blessed with this valuable resource: geologists estimated that it had around 90 billion tons of natural coal reserves (2). This ample supply of cheap energy provided the power for the nation’s vast array of steam engines. These engines, in turn, powered the manufacturing industries that made the British Empire wealthy.

Over time, Britain’s economy became increasingly dependent on coal. Since 1770, the amount of coal being consumed each year was growing exponentially. Assuming continued exponential growth, England would exhaust its vast coal reserves in the next 100 years — not good at all for the powerful British Empire. Engineers, therefore, were racing to produce machines with better energy-efficiency. If only efficiency increased, they believed, we could reduce the demand for coal. Jevons, however, knew better.

In 1865, Jevons published The Coal Question, which investigated the relationship between efficiency and total energy use. His results were absolutely startling: energy-efficiency was worse than useless — it was positively harmful. Historical records showed that the more efficient steam engines became, the more coal Britain ultimately consumed. Better technology within the 18th century had actually caused coal consumption to grow exponentially.

This paradox is best illustrated by example. Suppose the average car gets 25 miles to a gallon of gasoline, with each gallon costing $4. Using hybrid electric technology, engineers could create an improved car that gets 50 miles using a single gallon. As a result of this breakthrough, the improved car could produce the same amount of work using half the amount of gasoline.

The economics, however, look different from the consumer’s point of view. To the consumer, this improvement in efficiency has effectively halved the price of transportation. Whereas it used to cost $4 to travel 25 miles, now it only costs $2. Now that transportation is much cheaper, it’s possible to drive more than ever before. Our driver can now afford to do more than just commute to work; he can take cross-country road trips every month, if he so pleases.

Suppose our driver originally burned 10 gallons of gasoline each week. With new technology, he can save half the fuel, or 5 gallons of gasoline, each week. Unfortunately, our driver decides to take extra road trips, and drives an extra 150 miles each week. As a result, he ends up burning 8 gallons instead of 5. That’s 3 gallons more than what he could potentially have saved, had he kept his driving habits constant. We say that the rebound effect is 60%, since that is the percentage of potential savings that was forfeited (3).

At this point, it still looks like energy-efficiency could be of some use. After all, a 60% rebound effect still implies that 40% of the potential savings were retained. Isn’t it better to have our driver conserve 2 gallons of gas, rather than no gas at all? But here’s where it gets peculiarly disturbing: the rebound effect frequently exceeds 100%. Returning to our analogy, a rebound effect greater than 100% would means our driver is now burning more than 10 gallons of gas each week. If our sustainable car suffered from 120% rebound (3), that means 11 gallons of gas are burned instead of 10. This backfiring is the paradox that Jevons observed with steam engine technology.

What exactly happens to all of the potential savings?

For one, as machines become more efficient, engineers tend to add more features and provide better performance. Hybrid-electric cars might accelerate faster, become roomier and heavier, and include more electronics. Those extra creature comforts squander all the potential savings in fuel technology.

Drivers might also suffer from greenwashing. Let’s suppose our driver is environmentally-conscious. When he used to drive his old clunker, he’d feel guilty about wasting gasoline. But when he drives a zero-emission hydrogen car, he does so with clear conscience. After all, most people don’t stop to realize that hydrogen fuel requires energy to produce, which currently still uses coal to produce (4). Besides, he’s already spent thousands of dollars going green; what’s a little extra driving here and there?

But perhaps most importantly, average consumers simply don’t care about conservation. Many passengers take the bus each day, not because they care about the environment, but because gasoline is expensive. By increasing the fuel efficiency of cars, the effective price of driving decreases. This provides the average passenger with extra encouragement to drive instead of taking public transit.

Even non-driving activities contribute to the rebound effect. If our driver spends less money on gasoline because of fuel efficiency, he now has more disposable income. He might choose to use that extra cash for a cruise to the Bahamas or a plane ticket to Europe, activities which both waste tons of gasoline.

Altogether, these effects usually ensure that the rebound effect is greater than 100%.

The Jevons Paradox can be quite disheartening, especially after you realize how often it occurs.

  • Architects, for example, are now wasting more energy than ever before using energy-efficient LED lighting. They do this by plastering buildings with LED lights to create gigantic lighting displays (5). The buildings are lit all night, 7 days a week. Such lighting was previously too expensive using traditional lightbulbs, but energy-efficient LEDs now make it cost-effective.

  • As another example, refrigeration may result in increased energy usage. The benefit of refrigeration is that, by preventing food spoilage, consumers can save electricity the electricity used in food production. However, refrigeration inadvertently encourages us to buy too much food. Today, the average American throws away 40% of the food he purchases — you could hardly call that saving electricity (6).

  • Energy-efficient heating and cooling may have resulted in increased electricity demand. The problem is that energy-efficient air conditioners and heaters encourage people to leave these systems on longer. Lower costs might even encourage the average home-buyer to buy a bigger house. Any potential savings are thus wasted cooling and heating extra space (7).

The Jevons Paradox makes it clear that technology by itself can’t solve our present energy crisis. If new innovations aren’t accompanied by a cultural shift towards conservation, they are likely to waste more energy than ever before.

The trouble with sustainable living is that it requires a total lifestyle change. Life apart from consumerism can be difficult to imagine, so we resist. It’s much easier to just keep searching for the next big thing. We want to be environmentally-friendly, but we don’t want to give up our cheap energy, shopping sprees, fast cars, quick profits, and junk food. So we’ll be sure to see plenty of paradoxes for years to come.

  1. These technologies are an improvement, but they’re still not sustainable. Zero-emissions cars aren’t really zero-emissions; they still require electricity, which is produced by burning coal. Smart grids will save some energy, but we will just waste the savings by powering more gadgets. Lastly, even the greenest laptop will produce some e-waste, since electronics aren’t biodegradable.
  2. From Wikipedia’s summary of The Coal Question.
  3. If he had the potential to save 5 gallons, but only saved 2, then he wasted 3 out of 5 gallons, which gives 3÷5 = 60% rebound effect. An 120% rebound effect is equivalent to 120% = 6÷5. In other words, he had the potential to save 5 gallons, but actually wasted 6 more.
  4. Physics Professor David McKay writes that the Hydrogen 7, the hydrogen-powered car made by BMW, requires 254 kWh per 100 km – 220% more energy than an average European car. In other words, hydrogen cars are worse than conventional cars.
  5. Low-Tech Magazine explores LEDs and energy-efficiency paradox.
  6. From an abstract in The New Yorker.
  7. One writer argues that efficient heating and cooling has led to a rise in McMansions.
  8. Photo by nugefishes, CC BY; our own picture.