Question

Need help on part D & E

Problem 2: Pedal Power

Many villages in the developing world lack access to electricity. One proposed solution is the ‘Free Electric’ bicycle, which is a stationary bike hooked up to a generator and battery. Pedaling the bike powers the generator, which creates electricity that is stored in the battery. The inventor claims that 1 hour on the bike can produce a whole day of electricity. Let’s put it to the test!

Assume the following:  The human body is 25% efficient in converting food to mechanical kinetic energy.  An adult can maintain a mechanical power output of 125 W sustained over an hour of biking.

. For every 100 J of mechanical energy delivered to the bicycle pedals, 60 J of energy is ultimately stored as usable potential energy in the batteries Fully charging a tablet takes 36 Wh of energy, fully charging a cell phone takes 10 Wh of energy, and small LED lightbulbs consume 7 W of power .The energy content of rice is 200 Calories (kcal in SI units) per cup of cooked rice . Plants are 1% efficient in turning sunlight into chemical potential energy (a) Energy production: How much mechanical energy (in kWh) will a person generate in one hour of biking? (2 pts) (b) Energy available for household use: How much electricity (in kWh) is stored in the batteries from this one hour of biking? (4 pts) (c) Household electricity: If a household charges 100% of a cell phone and 25% of a tablet every day, how much electricity (in kWh) is left over for lightbulbs? If they use lightbulbs for 4 hours a night, how many lightbulbs would this power? (6 pts) (d) Food consumption: How many cups of cooked rice will it take to provide this daily electricity? Give your answer to 2 decimal places. (6 pts) (e) Efficiency: Now let's look at the whole conversion pathway. We'll start with the "simple" version: sunlight -> rice -> pedaling battery. This is summarized in the diagram below with boxes for each stage of energy and arrows for each conversion. On your answer sheet, draw this same diagram and label it with your answers to parts (i) and (ii) Sunlight (radiative KE) Rice (chemical PE) Pedaling (mechanical KE) Battery (chemical PE) Metabolisnm & muscles Bicycle + generator synthesis Efficient Efficient Efficient (i) What is the efficiency of each conversion? Write your answers in the blank space below each arrow. (3 pts) (ii) What is the efficiency of the entire process from sunlight to battery? (3 pts) (ii) Write the amount "200 kcal" in the box labeled "rice." This is the amount of energy in each cup of rice. In order to get this rice energy, how much sunlight energy (in kcal) is needed? And how much pedaling energy and battery energy are produced (also in kcal)? Write your answers to each of these amounts in the boxes. (6 pts)

Answer 1

From tye question and the solved part we know that:

• Energy produced per hour pedaling is 0.125 KWh
• Energy per cup of rice is 200calories with conversion efficiency of 25% into mechanical energy.

So to solve part (d) we just need to find it as shown in the image below.

For part (e) we need to multiply all the efficiency percentage, from sunlight to battery, inorder to calculate the net efficiency of the whole process.

d From the quest e Know Mechanial enegy fra hn com the bi 0 125 Kwh ne几 Con) s 6.232 Kwh Nins , efficieny H mech energy con verction from cooKed tice f, vt fCe refuc ree. xy = 2.155 cups ..2.,55 cup'어 60 iled Rice Peda ling Belery Re 200 Kc50 ?5, 30 eff ) 시et effi ciency become - 25x 60-0, 0015 10-15 % Nth