Post by John Mulrow, PhD Candidate in Civil Engineering at University of Illinois Chicago
INTRODUCTION
In the last Human v Machine blog we determined that humans stand little chance of matching the performance of heavy machinery the likes of an industrial compost screener. But how about for lower power tasks you might find machines doing at home? Student participants of the Women in Engineering Summer Program (WIESP) helped me investigate this question during a workshop on energy, held at UIC. The students performed a variety of human-powered tasks and gathered data on their power output, energy expenditure, and production efficiency. Their results are then compared to the everyday home equivalents, powered by municipal electricity.
METHODS
Students were given three tasks to perform:
Grind coffee beans
Blend smoothies
Generate light
The tools available to them were all human-powered. No appliances were plugged into a wall socket during this exercise!
Mortar and pestle
This is a hand-made wooden mortar and pestle. The handheld part (the pestle) is used to pound, smoosh, and grind the beans against the bottom and sides of the goblet-shaped vessel (the mortar).
Rotary grinder
This little hand-powered grinder is used for grinding coffee beans. A hand crank turns the center burr of the grinding mechanism and the funnel shape keeps whole beans moving down into the grinder.
Bicycle-powered blender
This piece of equipment – called the “Fender Blender” – was provided by Climate Cycle, a local non-profit that engages youth in sustainability issues. The Fender Blender is a stationary bike with a front wheel that is driven by the pedals. The forward spin of the wheel is converted to rotary motion of the blender blade by way of a simple bearing assembly. One biker can easily blend up a pitcher’s worth of frozen fruit and yogurt with this tool.
Bicycle-powered light bulb array
The “Energy Bike” was provided by SCARCE, a non-profit that conducts environmental education and runs a textbook/school supplies reuse depot for Chicago-area teachers. The same mechanism that turns the blade on the Fender Blender is used here, but instead turns a dynamo which causes a current to flow through the array of light bulbs on the display. The bulbs can be switched on or off to demonstrate the change in biking effort required to power different types of bulbs. The difference in incandescent, compact fluorescent, and LED bulbs is obvious from the below photos!
RESULTS
Human-powered technology comparison
The table below presents the average estimated energy inputs, power generated, and energy efficiency calculated for 2-3 trials of each task. The energy inputs are estimated using an assumed value of 3.3 nutritional Calories burned per minute during moderate physical activity. This is equivalent to 0.004 kilowatt-hours per minute. This value was revised upward for the more difficult tasks of lighting CFL and incandescent bulbs.
The energy efficiency value represents how much product or service is yielded per unit of energy put into the task. In the case of smoothies and coffee-grinding, the product is a physical quantity: cups of finished smoothie or ground coffee. In the case of lighting, the product is a duration of sustained lighting. A biker who keeps four light bulbs lit for a full minute thus provides 4 light-minutes. Again, think of efficiency as measuring the energy input per service provided. This metric is represented by column A.
You could also flip the equation to get service provided per unit energy, given in column B. Think of the energy efficiency metric we are all used to using for transportation: miles per gallon. In this case, miles (ie. distance traveled) are the “service” provided, and gallons (of gasoline) are a stand-in for energy use.
The data show that for grinding things (like coffee beans or smoothie ingredients), bike-powered grinding is clearly the most efficient on a per kcal basis (remember, 1 kcal = 1 nutritional Calorie). The bike blender yielded 0.71 cups of finished smoothie per nutritional Calorie expended, while the hand-driven methods of grinding coffee beans yielded less than 1/10 cup for each technology. Of course, coffee beans are more compact than smoothies, but it’s not surprising that leg power beats hand power in efficiency.
The data also show the drastic difference in energy efficiency among light bulb technologies. It takes less than 1 nutritional Calorie for a bike-rider to generate a minute of LED light, compared to 2.5 Calories for CFL light and 4.1 Calories for Incandescent light.
Human versus Machine
The next table presents a comparison of energy efficiency calculated for each human-powered process and that for a standard electrical appliance that performs the same task.
The data show that for short-burst grinding/blending applications, humans (with the assistance of a rotary tool) perform at about the same efficiency as electric appliances. The Fender Blender actually outperformed the Vitamix in this experiment, yielding 0.7 cups of smoothie per nutritional Calorie versus 0.6 cups per nutritional Calorie (equivalent) for the Vitamix.
In the case of light generation, the Energy Bike must first convert the mechanical energy of biking into electric energy, which is then sent to the light bulbs. This conversion step is what drops the efficiency of using human energy to directly power light bulbs.
This comparison naturally leads us to another set of questions:
How would we factor in the energy used to grow the food which ultimately provided the calories or Human energy expenditure?
How would we factor in the energy used to extract, process, and convert the fuel sources used to generate home electricity?
Once these items were factored in, how might the comparison change?
DISCUSSION
It’s worth asking what any of this has to do with circular economies and sustainability. Comparing the human body’s capacity to do work with that of our equipment and appliances highlights two important concepts in this realm. First is the reliance on high-power commercial energy for almost every aspect of modern life. Humans, on average, expend about 1 kilowatt-hour of energy per day simply digesting, thinking and moving body parts. But add up all the energy that goes into the rest of daily life and it is orders of magnitude larger. If you’re a US resident, your home electricity use is likely to be around 30 kWh per day. And if you move anywhere at a speed greater than 20 mph you’re relying on an engine that converts fuel to motility at a rate of at least 30 kWh per hour. Experiencing what it feel like to physically produce the work that appliances normally do is just one way of contextualizing our modern dependence on power and speed.
This leads us to a consideration of an all-important circular economy concept which is local resource availability. For every work process we rely on throughout our day, some material (such as gasoline) or other resource (such as solar rays) are expended in driving that process. If you’re not doing the work yourself, something else is. And it should be obvious that a massive swath of space and resources, much larger than ourselves, is required to meet our daily energy requirements. Just imagine, to meet your daily home electricity requirement of 3 kWh you would need a crew of about 18 bikers on dynamo-equipped stationary bikes, biking for 8 hours each. That’s more stationary bikers than would fill your apartment or your driveway and more than you could afford to pay! Imagine drawing on only local renewable resources to meet such an energy requirement. These Human v Machine experiments help characterize the challenge (and great importance) of cultivating local circular economies.
Original data, collected and recorded by WIESP participants. July 2018