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  • Subject area(s): Engineering
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  • Published on: 7th September 2019
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The following report regards an innovative way of producing renewable energy. The environment taken into consideration is the City University gym where different types of weights’ machines are offered, as well as stationary bicycles and elliptical machines. All calculations have been computed with regard to term dates, as it is the period with the highest usage of the equipment. The aim is to use the lifting of the weights in order to produce electricity through the use of a dynamo and to then use the electrical power generated to charge a battery which provides electricity to the LED lights in the gym.

1.0 Introduction

For 1.4 billion of the planet’s residents there is no electricity and millions of others do not even have intermittent electricity. 1 On the other hand, the population of developed countries strongly relies on electricity in order to power lights and heat water at home. [6] However, fuels will shortly run out due to the high demand. As a result, renewable energies are becoming the future source of our electricity supply.

Renewable energy can be defined as an energy resource that is naturally generated and it can be regenerated over a short time. Such energy is usually produced by the sun, as for example photoelectric or from some other natural mechanisms such as biomass. [15] The scheme investigated instead of using energy produced by the sun, takes advantage of the movements of the human body. The aim when using human power, is to produce energy without increasing the effort put in the movement by the body. According to a research carried out in the past year in the United Kingdom, 1 in 8 people have a gym membership. [2] This means that in 65,138 million people in the United Kingdom, 8,142 million people go to the gym. [4] Going to the gym on a usual basis consist in using the machines offered such as treadmills, steady bicycles and weight training equipment. If all these machines were to produce electricity, the emission gases in the atmosphere would be reduced.

The purpose of the scheme investigated is to use the lifting of the weights’ machines in a gym, in order to produce energy and use it to reduce the gym’s bills as well as the greenhouse gases in the atmosphere. This is doable, as dynamos and batteries already allow us to produce and store DC current in batteries, as long as some mechanical work is done in order to turn the dynamo. However, being this a new scheme, there are no previous examples that can prove the efficiency of the system. Indeed, there is no guarantee that a gym will produce enough electricity to power all the lights or that in the long term the scheme’s building expenses will provide an actual profit to the gym.

Furthermore, with common renewable energies, generators produce electricity on a large scale, meaning that they can power cities. For instance, an onshore wind turbine (2.5-3MW capacity) is able to produce up to 6 million kWh in a year time, which is enough energy to provide electricity to 1500 European households. [8] The scheme investigated instead, will not be able to produce that much energy. Perhaps, as stated before, it will not produce enough electricity to power the lights of the entire City Gym. However, whether or not it is going to be efficient, if more gyms around the country or even the world were to install such system, the greenhouse gases in the atmosphere would be reduced. In addition, from a behavioral point of view, it might increase the intensity of workouts as the more a person works out, the more electricity would be produced and some people might even find it motivational.

The electricity produced in a day will depend on how many people will go to the gym in a day, as well as the type of workout that they will do. The calculations shown in section 4.0, are based on the average number of people that attend the gym on a weekday on a term. Furthermore, it is assumed that the gym is opened from 6am to 23pm and that most of the people attending the gym will use weights’ machines. In addition, an average was taken for the number of sets done as well as for the kilograms lifted. All of these averages, are further explained in section 4.0 along with how they have been computed and how they have been used in the calculation of the total power output of the City Gym.

2.0 Previous scheme

Human power has been used before to produce renewable energy. However, at the moment, there are only three main renewable schemes in the sports’ environment. The first one, known also as one of the most advanced schemes and still in growth is the E formula, while the second and third schemes are both related to green fitness equipment. The second scheme, also known as a stationary bike, connects the back wheel of a stationary bike to an electric motor with a belt. The motor being powered by the turning of the wheel is used as a generator, then transfers the electricity produced through a diode to a battery which charges up. The following images show the structure of the bike.


The third one, instead, is an elliptical which produces electricity as a person runs on the machine. The mechanism is exactly the same as the bikes, except for the fact that here the circular movement is given by a person running on the pedals.

Such equipment, also known as Eco Fitness equipment, were first used in a gym in 2008 in America and made it possible for the first green micro gym to use 85% less electricity than a usual gym and reduce its carbon footprint to 1/10. [13]

At City University, the gym offers 16 elliptical machines, 12 stationary bikes and 20 addition spinning bikes in a separate studio. On average, a professional cyclist has a power output of 380W for an hour ride [3] while an amateur will float between 100W-200W, depending on the level (gear) he is cycling at and the rate of rotation. On the other hand, an elliptical’s average power output is 120W for an amateur. [11]

Being aware of such values, allows to calculate the total energy produced by the bikes and elliptical machines in the City Gym, and the total can be added to the energy produced by the lifting of the weights in order to then find out the energy produced by every machine if they were all changed to Eco ones.

3.0 The engineering behind the scheme

The scheme investigated, includes both mechanics and electronics. The main idea is to use the lifting of the weights in set up machines, in order to power a dynamo and produce electricity. The image below shows the structural mechanism used.

Looking firstly at the weights machines, since we want to convert the energy normally used to lift those weights into battery charge, it needs to be pointed out that the weights on the machine will need to be changed accordingly with the dynamo’s resistance. The ideal dynamo for such system, would be an electronically controlled one that provides mechanical resistance. This would mean that depending on how much the person wants to lift, the dynamo’s resistance will change and simulate the requested weight. If such system existed, no actual weights would be required. However, for the purpose of the report, the weights will be kept into place. The resistance of the dynamo is directly proportional to the electricity produced, indeed, the bigger the dynamo, the greater the resistance, the higher the amount of electricity produced. If the person lifting the weights was to do many reps very quickly, more energy would be produced rather than a person lifting weights very slowly. As on a usual basis, when lifting weights reps are done slowly in order to avoid any injuries, for the system investigated, a bigger dynamo is required. This will allow to produce electricity while lifting at a normal pace. Supposing for instance, that the dynamo will have a resistance equal to 5kg, the weights will need to be reduced by 5kg each but will keep the same values written on them. In addition, it has to be stated that the lifting force applied by the athlete, unless a dedicated dynamo control is created, will be likely to change depending on the velocity of each push.

Moving onto the mechanics behind the scheme, a cable that is going to move along with the weights is required to power the dynamo. It is known that who is using the machine can adjust the kilograms by moving a metal peg, also known as the pin, in the drilled hole in the center of the chosen weight, as it is shown in the below picture.

Fig 5 A drawing of the weights’ machine

The best option to connect the weights to a cable is to have a polyester rope tied to the pin. This will then pass through a small pulley in the pavement which will change its vertical direction to horizontal. This way, the rope will be pulled by the lifted weight that the person selected without complicating the given system. Furthermore, polyester was picked as the most suitable material for the rope, as an 8mm has a working load limit of 930kg and it is lighter than a nylon rope, which means that not extra weight is added to the lifted weights. [7] As the weights will be raised, the rope will be pulled and it will get unrolled from a spinning reel underneath the pavement. The reel will make use of a spring in order to roll back up the rope after it has been pulled. Such mechanism, is the same one used in small motor boats where in order to switch on the engine a rope has to be pulled fast. More in detail, when the weights are pulled up, the rope unrolls from the reel extending the spring which does not reach its elastic limit and as a result, when the weights are brought back to their initial position, the rope is pulled back by the reel back rolling activated by the spring.

Fig 6 A drawing to show how the scheme will be set

In addition, the reel will also be connected to a shaft, subsequently connected to a free wheel. The shaft, will move with the weights and make the free wheel turn. A free wheel is a mechanism that can turn just in one direction.  An example is given below.

Fig 7 A drawing of a free wheel

This energy will be produced only when the weights are pulled up and not on their way back to the initial position. This is due to the fact that if no free wheel was included, the spring wouldn’t be strong enough to spin back the dynamo and it would generate an alternated current This current would not be able to be stored and its alternation frequency would be dependent on the movement of the weights which would change with every person and would not be compatible with usual current. As a result, in order to avoid any energy waste, the system explained will be doubled. This would consist of the same exact system explained, but it will have one dynamo for the lifting of the weights and one for them on the way back to initial position. This is shown below.

Fig 8 A drawing of the scheme investigated doubled

As it can be noticed from the picture, the rope has a reel and a spring at each end which both make a freewheel turn. Having two different reels and free wheels will allow us to make the coil inside the dynamo turn always in the same direction and as a result produce DC.

Looking singularly at each component, the free wheel as previously stated, will make use of a dynamo generator to produce electricity. A dynamo generates electricity by inducing a current by spinning a coil of wire (usually copper) through a magnetic field. As the wire moves, an electric current is generated. [16] The electricity produced is a direct current which can then be transferred to a battery where it can be stored. The reason why a battery is used is because, it will get charged when the gym is used and either use the energy stored or get charged by common electricity when it goes below a certain charge level. The battery will be charged with dc current by the dynamo and will have a rectifier inside the charger from the normal electricity source in order to convert the ac provided to dc. Moreover, the battery will be set to use the green electricity mainly, but if this will not be enough to keep the gym illuminated, then an automatic switch will charge the battery with common electricity. When fully charged, or when enough people will be using the gym, the switch will disconnect the charger and allow the dynamo to charge the battery.

The graph shown below represent when the battery charges, as it is dependant on the tension of the rope connected to the weights. Indeed, electricity will not be produced through an entire push, but only during a certain time of the rep.  In the graph below, from O to A, B to C and D to E is the entire rep, so the weight being pulled up when the gradient in positive and being released back down to initial position when the gradient is negative. The intervals between these values, represent the time interval between one rep and the other, so when the weight returns to its initial position to then be pulled up again. Such interval is probably about a second, but it needs to be shown as it will be a longer interval for the battery.

The section of the graph where energy is actually produced and charges the battery is from the line x up. This is because only over a certain tension level the battery will be able to receive charge from the dynamo and the tension generated by the dynamo will go with the speed it will turn at. On the other hand, with the charger, electricity will be constantly available, indeed as it is shown in the graph below, the line representing the charging is a straight line at a certain tension.

The aim of the battery will be to provide power to the lights in the gym. These would be self adjusting LED lights, which depending on the amount of sunshine entering the gym, they provide more or less illumination. LED are the cleanest bulbs on the market as they require less power to illuminate and have very long lives with very little maintenance. LED lights are further explained in the section 4.0 along with their efficiency.

4.0 Calculations

The energy produced by the scheme investigated, will depend on several variables. In order to obtain a value as accurate and precise as possible, a few averages need to be taken. Among these there is the average weight lifted, as well as the length of the push. These are further explained in this section with regards to the calculation in which they are used.

As previously stated, an amateur cyclist working out in the gym will have a power output floating between 100W and 200W. [11] Taking into consideration, the spinning classes offered by City Gym, everyday there is a fast paced 40 minutes’ class. Assuming such workout has a power output of 200W and all the twenty bicycles are used, the energy produced can be computed.

In addition to this, taking into consideration that on average, around 50 people will use the steady bicycles on a weekday and assuming that they will follow the workout suggested by the machine which consists of a 30 minutes’ cycle with an average output of 100W, 180kJ will be produced per person.

As a result, multiplying such energy output times the number of people producing it, will give the total output of the steady bicycles in the gym, which equals to 9MJ.

Moreover, an average elliptical machine, used for half an hour produces around 120W. Taking into account that the city gym offers 20 elliptical machines and that around 80 people are going to use it in a day for about half an hour, the total energy produced is going to be equal to

Moving onto the actual scheme calculations, as previously stated, several averages had to be taken in order to end up with a single result. Firstly, the length of the push was measured. This is the difference in length between when the arm/leg is bended (before pushing) and when the arm/leg is stretched (after pushing). The length of the push for both arm and legs varies between 0.3m and 0.5m, mainly depending on the body size of the person working out. As a result, it was averaged to 0.4m for both. Another variable that needed to be taken into account is the weight lifted. This was set to 30kg for the arm machines and to 50kg for the leg machines. Both values were approximated on the majority of people that attend the gym. However, it needs to be taken into consideration that there might be people lifting double the average and other lifting half of it. A s a consequence, the final values obtained are not very accurate.

The first step is to compute the energy produced is to find out the force that each push produces. This can be done using Newton’s second law.

The second step, is to find the work done. The work done is defined as the force times the distance, as long as the push is in a straight line and not at an angle. As a result, for this computation it is assumed that the exercises are carried out properly, which means that the push is always perfectly aligned with the movement.

These two values represent the work done by every single push, so when multiplied by two they will equal the total energy produced.

At this point, considering the fact that most workouts consist of 3 sets of 10 repetitions each, we can state that each person will do a total of 30 pushes per machine. As a result, if we multiply the energy produced by one push times the number of pushes we obtain the total amount of energy produced per machine’s workout.

In addition, assuming that an average person uses 6 machines in an entire workout, the total energy produced per person can be calculated.

Knowing the energy produced by each person, having an average of 200 people attending the gym per day and assuming that 150 will do an arms workout and 50 a legs workout, as arms are the most trained muscle of the human body on average and there are more arms machines than legs ones, we can compute how much energy the proposed scheme would produce in a day.

In conclusion, adding up all the energies computed we get:

Considering the result itself, it can be said that the energy produced is a great amount. However, such value needs to be compared to the electricity consumption of the gym. As the scheme would be used to power the lights in the gym, I am going to investigate the electricity consumption just with regard to the illumination. At the moment, the lights installed are neon lights. The cleanest and most energy saving bulbs as previously stated are LEDs. Indeed, it would be suggestible to install self adjusting LED lights in the gym in order to reduce the power consumption.

The illumination of a room is calculated in lumens. This is an SI unit of luminous flux which is the amount of light emitted per second in a given area. [14] Generally, a 100-watt incandescent bulb gives around 1600 lumens while a 75W LED bulb gives around 1100 lumens. Indeed, an average good quality LED bulb provides at least 60 lumens per watt, but in certain cases, it can actually reach 90-100 lumens. The maximum lumens delivered per watt at the moment are 140-160. [5] For a good illumination, in a place such a gym, it is suggestible to have around 300 lumens per meter squared. The city gym is very big, it has several studios and offices which are not always in use when the gym is open. As a consequence, the illumination output was calculated considering only the gym it self as the lights always stay on. An industrial laboratory requires around 500lm which can be provided by LED lights with 5.2W/m2. A place such as a gym will require only 300lm/mq2, which can be provided by 3.12W/m2. As the area investigated is around 200m2, a total of 640W will be required.

These converted in energy (Ws) equal to 39.168MJ.

5.0 Conclusion

To sum up the scheme investigated, it has been found that 39.168MW are required to light up the gym, and the schemes produce 41MJ. However, the efficiency of the dynamos has not been taken into consideration. Modern dynamos are usual around an 80%, which if true for the ones used 32.8J would still be produced. As previously stated, if an electrically controlled dynamo was to be designed, it would revolutionize the scheme. No more actual weights would be needed and much more energy would be produced. In addition, the faster someone worked out, the more the energy output. As a result, depending on the type of workout an athlete would carry out, a certain energy would be created.

(Usain Bolt = runner = heavy weights = slow reps =low energy)

(Denis Gargaud Chanut = canoeist = small weights =fast reps =high energy)

Looking at the scheme investigated, despite a great amount energy would be produced, it has to be taken into consideration the fact that several variables affect the values calculated. These have to do with the amount of people that attend the gym as well as the type of workout they carry out. The values used where personally recorded values or values given by professionals who work at the City Gym. As a result, other people might do life more or less. In addition, the amount people attending the gym was with regard to a university term period, as it is the time of the year with the highest attendance. Indeed, in summer, a much smaller number of people might workout and as a consequence less energy would be produced.

Despite all the variable that need to be taken into consideration, and the fact that the results obtained are based on averages, the final outcome can be said to be success as a renewable energy is being produced through a very common routine.

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