Tag Archives: photovoltaics

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.

Harness the Sun, Cheaply

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The photovoltaic panel isn’t the only hot technology under the sun. The solar cooker, a fuel-free alternative to the traditional stove, is another invention that can harness solar energy. While not as sophisticated as its photovoltaic cousin, the humble solar cooker is an important tool in sustainable development. It heats food by concentrating light onto a black pot using aluminum foil. Unlike a photovoltaic cell, a solar cooker requires neither rare earth metals nor integrated circuits. Its elegant simplicity makes it incredibly cheap to build. In fact, hand-made solar cookers are often made of ordinary junk lying around the house. But don’t let its modest, low-cost practicality fool you: solar cookers can reduce carbon emissions, protect forests, and enrich the poor.

To understand the value of the solar cooker, it’s important to grasp the drawbacks of a conventional stove. Every stove, whether it burns natural gas or firewood, requires fuel. Even an electric stove burns fuel, since electricity from the power grid comes from coal power plants. When fuel is burned, it generates carbon dioxide gas, which aggravates global warming. What’s worse, fuel often doesn’t burn cleanly. Incomplete combustion leads to smoke, which is made of particles that can irritate the lungs and eventually cause disease. The effects are worst for poor people cooking over fires with poor ventilation.

The price of the charcoal in the center is roughly the same price as any of the groups of food around it.

Overuse of cooking fuel leads to scarcity. Burning too much coal and natural gas depletes limited reserves, while burning too much firewood leads to deforestation. Fuel is also expensive, with the heaviest burden falling on the world’s ultra-poor. As much as half of a family’s income may be wasted on firewood alone. That income could have otherwise been invested in nutritious food, clean water, immunizations, and education. The world’s poor don’t have much money to burn.

The solar cooker provides an attractive solution. It runs entirely on clean, renewable solar energy. It produces no carbon emissions, requires no fuel, and saves one hundred percent of operating costs. Unlike with modern stoves, solar cookers can be built with common household items like cardboard, aluminum foil, glue, and a pair of scissors. A do-it-yourself version can be assembled in under an hour.

Solar cookers are based on simple principles. Light rays from the sun are not strong enough by themselves to cook food. Rays collected from a broad region, however, can be focused onto a small pot using reflective metal sheets to increase the delivered power. Pigments on a black pot absorb these light rays effectively to convert them into heat. On a hot day, a solar cooker can reach temperatures up to 165°C (330°F) — hot enough to boil, cook, and bake food.

The CooKit

The two essential components of a solar cooker are the pot and the reflective sheet metal. The ideal pot is black, since dark pigments work best for converting light rays into heat. White pots, or pots with shiny metal surfaces, do not work well because they reflect incoming light. The second component, the reflective sheet metal, is often aluminum foil because foil is cheap and widely available. The foil is glued onto a scaffold (often cardboard, sometimes an umbrella) so that it can help concentrate light rays onto the pot. Variations in design center around the arrangement of aluminum foil and the type of insulation the solar cooker uses.

Two styles of solar cookers are very popular: a box-style solar oven and a panel-style cooker. In a solar oven, the pot is insulated by durable oven walls lined with a reflective aluminum interior. Any heat-resistant scrap material, like bricks or fodder, can be used for insulation. By trapping in heat, a solar oven can maintain a uniform temperature and continue cooking for a few hours after sunset. A panel-style cooker, on the other hand, resembles a traditional stove. It only uses a thin plastic bag for insulation, but it has a large reflective surface that can capture plenty of light. Design plans for both styles are available free online. You can either buy one professionally made, or build one from scratch at no cost. Anyone up for a fun weekend project?

A box-style cooker (left) and a panel-style solar cooker (right)

One drawback of the solar cooker is that it heats more slowly than a conventional stove. With a solar cooker, meals take longer to cook and require advance preparation. The CooKit model, for example, requires double the heating time and has a maximum temperature of 120°C. This minor nuisance, however, actually provides unintended benefits. A steady, low-heat simplifies cooking since food cannot burn at 120°C. Once set up, cooking a meal requires no extra stirring or monitoring. There’s also less chance to start a fire.

Alas, there is another caveat: a solar cooker will not work in the dark. Extra insulation can help on overcast days, but don’t expect it to work during a blizzard. Because of this limitation, a solar cooker cannot fully substitute for a conventional stove. On sunny days, however, solar cookers are still the most cost-effective, light-impact cooking technology available.

Solar cookers have much to offer to the world’s poor. Solar Cookers International, the non-profit organization behind the CooKit, has distributed over 30,000 solar cookers in Africa. To help Darfuri women, it donated 10,000 CooKits to a refugee camp in Chad. Not only did these cookers help save fuel expenses, they also allowed women to prepare food without the need to venture outside camp, which reduced their risk of assault while attempting to gather firewood.

An umbrella solar cooker

Solar cookers, however, have been largely ignored in the West. It’s a pity — they are a must-have for the ultralight Greenimalist. They can be built from leftover scrap (cardboard and aluminum) and they use free sunlight as fuel. Panel-style cookers also fold well for easy traveling. What’s more, solar cookers work off-grid, making them useful in campgrounds and homesteads.

One final note: solar cooker operators should wear sunglasses to protect themselves from UV damage. All those concentrating beams aren’t so good for your eyes when you stare at them directly. Then again, wearing a stylish pair of shades might not be such a bad thing, especially if corrugated cardboard and aluminum foil isn’t your idea of green chic.

  1. Photo credits: Tom Sponheim, public domain. Tom Sponheim, CC BY-SA. Tom Sponheim, public domain. Xuaxo, CC BY-SA. rangorang, CC BY-NC-ND. Meganhelms, CC BY-SA.