Designing a solar assisted bicycle without destroying the original preferred characteristics of a bicycle (such as low cost, light weight, slim width, two wheels, dimensional constraints, sturdy and robust frame, symmetrical weight distribution for ease in balancing) is more challenging than designing a solar three or four wheeler. Advances in solar and microcontroller technologies have come to the aid of the solar bicycle design. Today, solar bicycles are available that can be pedalled in a solar assisted mode in a comfortable manner with light/ leisure effort, at a speed of 21 kms/hour. With the availability of good sunshine, the bicycle would keep going on and on, with no distance limit as long as the rider is willing to pedal softly. In the solar assisted pedalling mode, the battery does not get drained and its charge can be retained for emergency or night travel. Henceforth, in this article, by solar bicycle I mean a solar assisted bicycle.
What is the importance of a solar assisted bicycle? It addresses five major issues faced by the current generation, namely, a) CO2 mitigation, b) Fossil Fuel Depletion, c) Deteriorating Health, d) Urban Congestion, and e) Oil imports.
How Can Solar Assisted Bicycles Reverse the Car Trend?
Cars are addictive. More and more Indians have become private car owners and they have already formed a habit to drive even for short distances under 8 kilometres. Such distances are ideal for bicycles. The Indian people have got a taste of the sedentary lifestyle and are finding it difficult to replace the sitting time spent driving their cars with a healthy one riding a bicycle.
It is easy to substitute one sedentary sitting time with another. For example, it is easy to replace the (sedentary sitting) time driving a car with the time sitting in trains, trams or buses. However, the public transport network, particularly that of roads, such as trams and buses are yet to be well developed in India.
It is also easy to shift from one powered vehicle to another, but difficult to shift to an unpowered bicycle after a sedentary lifestyle. In such a scenario, solar bicycles can act as an enabler offering elegant travel modes to shift systematically from a sedentary to a healthy lifestyle. This amazing vehicle supports three travel modes, namely, (a) throttle mode, (b) solar assisted pedal mode, and (c) regular (unassisted) pedal mode. With the passage of time, the rider acquires the confidence to wean himself away from the powered modes by gradually, slowly and steadily shifting from modes (a) to (b) to (c).
Solar Assisted Bicycle – Combining Solar Energy with Metabolic Energy
Solar assisted bicycles are an interesting category of vehicles that not only has its own renewable, mobile energy source – solar photovoltaic panel, but also has the capability of combining (mixing) the solar energy with human metabolic energy and consuming them together to perform useful work (personal travel)!
Solar energy is a natural source that is constantly replenished (does not run out), and clean (does not produce harmful greenhouse gas emissions). Hence, it can help mitigate climate change which is important for protecting people, wildlife and ecosystems.
Metabolic human energy is produced from the food consumed. Any kind of food intake represents carbon consumption; accordingly, the energy produced is carbon based. Metabolism takes place in small or large amounts depending on the activity performed. While simply sitting on a sofa (sedentary) would burn 75 kcal per hour, bicycling consumes about 430 kcal per hour. To be fit and healthy, one needs to be physically active and hence, though detrimental from the point of view of carbon emissions, bicycle pedalling or other aerobic exercises are considered good.
Since solar energy is represented in watts, and the metabolic human energy is represented in kilocalories. Before combining or mixing the energies, it is important to find that constant that allows us to equate these different energy types together. Before the introduction of solar assisted bicycles, never was the need so desperately felt to equate these two energy types. More significantly, before the advent of solar assisted bicycles, never was the need felt from the energy perspective, to unify the physical science (work, travel a distance) world (joules) with the nutritional (food consumed/fat burned) world (calories), kinanthropology (exercise intensity) world (METs), electrical (motor, battery) world (watt, watt-hour) and the solar (irradiance, capacity) world (watts/m2, watt peak Wp) . Let us begin by revisiting the various forms of energy.
Physical Science World: What is a Joule?
Energy is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat and light. Though ‘energy’ is at the core of physics curriculum, it is one of the most complex topics in science education since it occurs in multiple ways, such as motion, sound, light, and thermal energy.
Energy is a conserved quantity — the law of conservation of energy states that energy can be converted in form, but not created or destroyed.
Due to the variety of relevant scales and abstractness of the term energy, physics teachers experience difficulties while introducing energy. The concept of energy becomes meaningful in the context of human senses and hence students are asked to compare different sensory perceptions by using energy as a unifying concept. In our case, solar bicycles can be used as an effective energy study tool by the students to experience and understand the combination of human calories with solar watts and their substitution in different ratios and proportions, while riding to school.
Work and Energy
Work and energy are interconnected. When a force applied to an object (solar bicycle) produces a displacement (travel a distance), work is done. Energy is the capacity to do the work. Power is the work done per unit time.
SI Unit of Energy
The unit of measurement for energy in the International System of Units (SI) is the joule (J) and is the same as that for work. The name is kept in honour of James Prescott Joule, a British physicist whose works contributed to the establishment of the energy concept. In terms of other SI base units, 1 J = 1 kgᐧm2ᐧs-2 and dimension is M L2 T−2.
One joule is equal to the amount of work done when a force of one newton displaces a mass through a distance of 1 metre in the direction of that force.
One joule is also equivalent to any of the following:
- It is the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second.
- The work required to move an electric charge of one coulomb through an electrical potential difference of one volt, or one coulomb-volt (C·V). This relationship can be used to define the volt.
- The work required to produce one watt of power for one second, or one watt-second (W·s). This relationship can be used to define the watt.
J = kg·m·s-2
= N·m
= Pa·m3
= W·s
= C·V
Energy can exist in various forms – potential, kinetic, chemical, thermal, nuclear, electrical and is named accordingly. Other units of energy are kWh, BTU, calorie, eV, erg, foot-pound.
Electrical World: What is a Watt?
Power: Power is the rate (with respect to time) at which work is done. It is the amount of energy transferred or converted per unit time. In older works, power is sometimes called activity. Power is a scalar quantity.
SI Unit of Power: The SI unit of power is watt (W) equal to one joule per second. When a body does work at the rate of 1 joule per second, its power is 1 watt. In SI base units 1 watt = 1 kg⋅m2⋅s−3 and its dimension is L2MT-3.
Commercial Unit of Energy: One joule or one watt-sec is a very small unit and hence not considered appropriate by the utilities for electrical energy billing purposes. The commercial unit of energy is one kilowatt-hour (kWh). One kWh is defined as the amount of energy consumed by a device in one working hour at a constant rate of one kilowatt. The relationship between commercial and SI units of energy is:
1 kWh = 1 kW × 1 h = 1000 W × 3600 s = 3.6 × 106 Ws = 3.6 × 106 J ………………………………………………. (4)
Hence, one unit of electrical energy (1 kWh) is equal to 3.6 megajoules (MJ).
In our solar bicycle, if we use a Li-ion battery of nominal voltage 36 V and current capacity of 5.2 Ah, then it represents 36 × 5.2 = 187.2 VAh = 187.2 Wh = 0.1872 kWh = 3.6 × 0.1872 MJ = 0.67392 MJ = 673.92 kJ of energy storage.
Nutritional World: What are calories?
A calorie is a unit of energy. It is the amount of energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere. One calorie (cal) is a really small amount, so in everyday life, we use kilocalories (kcal), which is one thousand calories. Accordingly, one ‘kcal’ is the amount of energy needed to raise the temperature of one kilogram of water by one degree Celsius at one atmospheric pressure.
It is a common practice (particularly by dietitians) to name ‘food energy’ as ‘calories’ and to refer to its larger unit – ‘kilocalorie’ also as ‘calorie’ or simply as ‘cal’. But this practice is scientifically incorrect, results in confusion and should be avoided. In this article, we refer to the larger unit, namely ‘kilocalorie’, correctly as ‘kcal’.
Energy Intake
Today, the ‘Atwater indirect system’ is used instead of a calorimeter to measure calories. The calories provided by the energy-containing nutrients – fat, carbohydrate, protein and alcohol are added up to calculate the total calories. The fibre component of carbohydrate that is not digested and utilised by the body is subtracted from the total carbohydrate before calculating the calories.
The Atwater system uses the average values of 4 kcal/g for carbohydrate, 9 kcal/g for fat, 4 kcal/g for protein, and 7 kcal/g for alcohol. Thus, a label on a chocolate bar that contains 29 g of carbohydrate, 12 g of fat and 3 g of protein should read 236 kcal.
Kinanthropology World: What are ATPs and METs?
Kinanthropology is an interdisciplinary study of human movement, size, shape, composition, maturation, and gross function. This study relates to physical education, sports, recreation, rehabilitation and physiotherapy. It can help us understand growth, exercise, performance and nutrition.
Crucial to a metabolic process in the human body is the molecule Adenosine Triphosphate (ATP), considered by biologists to be the ‘energy currency’ of life. Almost every process in the body that uses energy gets it from ATP, and in the process converts it to ADP (adenosine diphosphate)+ phosphate. The energy from the oxidation of food (metabolism) is used to convert the ADP back to ATP, making the energy available for body processes.
Metabolic Energy Expenditure
One litre of oxygen (O2) can produce 5.04 kcals, 4.68 kcals, or 4.48 kcals if it burns carbohydrates, fats or proteins respectively. The amount of metabolic energy produced by the four food categories is roughly proportional to the amount of oxygen used, so that the metabolic rate can be measured by measuring the rate of oxygen consumption. The average is 4.7 kcal of energy released for each litre of oxygen consumed. On an average, an adult sitting at rest consumes about 16 litres of O2/hour. This gives a nominal basal metabolic rate of 75 kcal/hr which translates to 87 watts. With normal activities at higher metabolic rates, a person consumes on an average, 550 litres of O2 daily.
Metabolic Equivalents of Task (METs)
Metabolic Equivalents of Tasks (METs) are a simple and practical way to express the energy cost of physical activities. METs are calculated by comparing the metabolic rate during a physical activity to a reference – sitting at rest metabolic rate. One MET is defined as the amount of oxygen (O2) consumed while sitting at rest, which is equal to 3.5 millilitres of oxygen per kilogram of body weight per minute.
1 MET = 3.5 mL O2/kg/min = 210 mL O2/kg/hr
= 4.184 kJ/kg/h = 1 kcal/kg/h = 1.1622 W/kg energy expenditure……………………………………………………….. (5)
An activity with a MET value of 4 means you’re using four times the energy than you would if you were sitting still.
Based on Eq (5), if we wish to find the calories burned by a person weighing Wb = 75 kgs, sitting at rest: Calories burned/hour = METs × Wb = 1 × 75 = 75 kcal/hour.
However, in the literature, we find a common formula used by exercise trainers to be coarse approximation: Calories burned/ hour = METs × 3.5 × 60 × Wb/ 200 Calories burned/hour = 1 × 3.5 × 60 × 75/200 = 78.75 kcal/hour which has a +5% error.
In our paper, we will continue to use the more accurate formula given by (Eq 5):
Calories burned/hour = METs × Wb.
Hence one MET for a person weighing 75 kgs (sitting at rest), means she would be burning 75 kcal/h, or 1800 kcal/day or 0.657 million kcal/year. Since all activities have a MET value greater than one, it would not be incorrect to assume that a person metabolises approx. 1 million kcal/year and generates over a quarter million litres of CO2/ year.
The MET formula provides a very broad estimate, and it will not provide the exact number of calories burned during a particular activity. If you want to get an accurate number, one way is to go to a lab where machines attached to your body will determine the calories burned during the exercise from your maximum oxygen uptake (VO2 max).
To be continued…
Vithal Kamat has a Doctorate in Artificial Intelligence from the University of New Brunswick, Canada as a Commonwealth Scholar in 1996. He completed Masters in Control and Instrumentation from IIT Bombay. He is associated with the Centre for Apparent Energy Research, Anand, Gujarat.
