Substitute diesel with humans, then the power station would need to pay the humans weekly 0.77 the price of one lt of diesel:
The ruler of a hypothetical small island faces two very serious problems. The inhabitants of the country are unemployed and at the same time, the island’s power station, using diesel generators, has to import diesel at a price of £1.2 per litre. Thus, the ruler comes up with a plan. He wants the population to work as a labour force in order to produce electricity through high efficiency hand crank dynamos and stop importing diesel altogether, killing two birds with one stone. Is this a plan worth doing?
First, let’s look at the energy needs of a human being. A very fit human being has the ability to convert 20-30% of the human metabolic energy available as kinetic energy (this is an average approximation of course). Thus, from all the calories that a human consumes an average of 25% can be used in order to power the dynamos. The rest of the calories are mostly spent in order to retain homeostasis (our bodies’ ability to keep the same internal conditions), repair tissues and provide nourishment to the brain. By consuming more calories we can provide more kinetic energy, but our muscles stop being so efficient leading to muscle fatigue, metabolites building up in the body, mechanical stress in our connective tissue etc. Humans also have energy stores in the form of fatty tissue and muscle glycogen, but the assumption is that the humans trying out for the job only consume the amount of calories they need in order to remain the same without gaining or losing weight.
A 38 year old male with a height of 1.80m and a weight of 83 kg that has a very active lifestyle, needs to consume roughly 3000 food calories (kcal) per day (Calorie Calculator, 2017). The kinetic energy that this person would be able provide would be 0.25*3000=700 kcal. Some of the best hand crank dynamos can boast efficiencies of 78% with high quality bicycle hub dynamos having efficiencies of 68% (Electric Pedals, 2017). Assuming that the ruler of the island country only uses the best dynamos on the market, then the above human would be able to produce 0.78*700= 546 kcal worth of electricity.
This human needs to be fed to be able to provide the above figure, though. On average, looking at a macronutrient perspective, humans need a ratio of 30% fat, 20% protein and 50% carbohydrates to their diet (World Health Organization, 2017). Assuming a milk and oat ratio of 20/4.5, the above macronutrient targets for a 3000 kcal can be reached with 1234 kcal coming from milk (corresponding to 2l of milk) and 1777 kcal coming from oats (corresponding to 450g of oats). Thus, from a macronutrient perspective, the above combination can satisfy the above person’s caloric needs. But this food comes at a cost, since oats cost £1 per kg and milk costs £1 per 2.27lt (MySupermarket, 2017). Therefore, the daily total cost of food for the human is £0.88 (milk) and £0.45 (oats) giving a total of £1.33- in order to provide 546 kcal worth of electricity don’t forget.
Since the food calorie (kcal) isn’t a unit used widely in the energy sector a conversion to kWh is needed. 546 kcal is equal to 0.63 kWh. For a quick recap, a (very fit) human needs £1.33 worth of daily food in order to provide 0.63 kWh of electricity per day, or £2.02/kWh.
This human needs to be able to substitute a diesel generator. An average diesel generator has an efficiency of 40% (Independent Energy, 2017). Diesel has a calorific content (energy content of a fuel) of 43.4 MJ/kg (Engineering Toolbox, 2017) or 36.45 MJ/lt (based on a diesel density of 0.84 kg/lt). By converting the MJ to kWh, in order to have a direct comparison, we find that the calorific content of diesel is 10.13 kWh/lt. Since the average diesel generator has an efficiency of 40%, then 1l of diesel can provide 4.05 kWh worth of electricity. And since this litre of fuel costs £1.2, then we can easily find that electricity from diesel fuel costs £0.27/kWh.
So a recap of the brief analysis shows that:
Human electricity cost: £2.02/kWh
Diesel electricity cost: £0.27/kWh
The comparison is heavily in the favour of the fuels. But there is also another side to this story. If the ruler of the hypothetical country were to use humans as labour in order to produce electricity, then a workweek from a worker would be able to provide 3.15 kWh of electricity (0.63 kWh per human daily multiplied by 5). That amount of energy is 0.77 times of what is found in one litre of diesel.
Assuming that the power station simply substitutes diesel with humans, then the power station would need to pay the humans weekly 0.77 the price of one lt of diesel. Thus a workweek for a human would pay £0.92. In the meantime the human must have consumed £6.65 worth of food. It should also be noted that the milk and oat combination was used because only it is cheap and provides enough calories with and adequate macronutrient ratio. The micronutrient breakdown (vitamins and minerals) is far from ideal, leading to all sorts of possible dietary deficiencies.
As a conclusion, think about this. Originally humans were using manual labour and then we started to use beasts of burden – oxen, horses etc. Now we use machines. We don’t use machines because we are lazy, we use them because they are a much more efficient approach. It makes sense from an economic point of view. Also, we should factor in that the human used for this example is a very fit, healthy human, something that unfortunately isn’t the average.
From an environmental point of view though? That’s a completely different approach. Greenhouse gas emissions from humans and from food production are (mostly) parts of the natural carbon cycle, whereas greenhouse gas emissions from diesel fuel are not, thus, although cheaper, diesel electricity production less sustainable. Could human electricity production be used as a means to minimise both greenhouse gas emissions and unemployment? The ruler of the hypothetical country might think so. Do you?
George Loumakis, Lecturer in all things concerning Energy, Glasgow Caledonian University, George.firstname.lastname@example.org.
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Electric Pedals 2017. Electric Pedals. [Accessed 20 November 2017]. Available from: https://www.electricpedals.com/hand-crank-generator/.
Engineering Toolbox 2017. Fuels – Higher Calorific Values. [Accessed 20 November 2017]. Available from: https://www.engineeringtoolbox.com/fuels-higher-calorific-values-d_169.html.
Independent Energy 2017. Efficiency Facts. [Accessed 20 November 2017]. Available from: http://www.independentenergyllc.com/Efficiency.html.
MySupermarket 2017. mySupermarket – Compare & Save 30% on your groceries. [Accessed 20 November 2017]. Available from: http://www.mysupermarket.co.uk/.
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