Tackling global electricity shortage through human power: Technical opportunities from direct or indirect utilizations of the pervasive and green human energy

Abstract

With the energy and environmental problems becoming increasingly serious, human power, as a pervasive, renewable, mobile and environment friendly energy, draws more and more attention over the world. In this paper, the most basic features of human power are presented. The currently available human power harvesting theories and devices are briefly reviewed and compared. Further, direct or indirect utilization of human power in daily life, especially transportation and home appliances, such as human-powered car, watercraft, aircraft, washing machine and television etc. are summarized. Considering that the total energy from an individual is rather limited, as previously focused by most of the former works, it is conceived in this paper that an important future for large scale use of human powers lies in the efficient conversion, collection and storage of such energy from discrete people and then use it later on as desired. With the huge amount of energy gathered, the application category of human power would be significantly expended. Starting from this point, three technical ways towards efficiently utilizing human power are sketched, which are termed as human-powered grid (HPG), human-powered charger (HPC) and human-powered storage (HPS), among which, HPG is capable of collecting the electric power produced by each individual at different regions and thus can supply unique and flexible power to the customers covered in the area, without relying on the conventional electricity grid. The HPC can then charge various kinds of electrical devices instantly by a human driven generator which converts human power into electricity. Finally, the HPS can store electricity in time for later use. In this way, even for the devices requiring electricity that is strong enough, the collected human power can also serve as its reliable energy source. Meanwhile, utilization of human power becomes rather convenient and timely which guarantees its practical value. It is expected that with further research and increasing applications, human power could partially relieve the current global electricity shortage and environmental issues via its pervasive contribution.

Graphical abstract

Keywords

human energy harvesting / human-powered transportation / human-powered home appliances / human-powered grid (HPG) / human-powered charger (HPC) / human-powered storage (HPS) / biofuel

Introduction

In this day and age, with modern society running at a high speed, the shortage of fossil energy and environmental problems are becoming increasingly serious. In the summer of 2011, several provinces and big cities in China suffered “electricity shortage” one after another, which has never happened before. According to the prediction, if the insufficient electricity-coal situation continues, the gap between energy supply and actual demand would expand to 30-40 billion Watts with the coming of the electricity consumption peak in China [1,2]. In fact, similar situation has already appeared in Japan due to reduction of nuclear power which leads to 10-15 billion watts of electricity shortage [3]. And more countries keep encountering the same trouble. “Electricity shortage” would seriously restrain the development of industry and lower the quality of people’s life. A direct result everyone will have to face each day is that many daily activities such as surfing the Internet, making a phone call, watching TV, or even listening to music etc. will be hard to maintain, if not completely impossible. The complementary solutions such as restricting electricity use and raising electricity price will not solve the problem of “electricity shortage” completely.

In order to tackle such tough issues, exploiting a renewable, sustainable and environment friendly energy is of special significance. Although tremendous efforts have been made, the existing “green” energy modalities such as wind or solar power are generally restricted by weather or geographical location. As an alternative which can be exploited immediately whenever needed, the human power, as a kind of environment friendly, pervasively and permanently renewable energy and completely independent on weather or geographical location, provides a rather promising option for partially tackling the energy crisis. In fact, human power, existing as a huge electricity source, is unfortunately almost ignored by modern society. According to measurement, over 127 W of electricity can easily be generated by kinetic energy from an adult [4]. In fact, harvesting human power has been attempted over the past few years although its application is rather limited. Researchers have proposed various human power generators such as: shoe mounted magnetic generator [5], shoe mounted piezoelectrics generator [6], knee mounted magnetic generator [7], backpack mounted generator [8], liquid metal magnetohydrodynamics generator [9], boot mounted dielectric elastomer generator [10], tube magnetic generator [11], generator based on reverse electrowetting [12] and etc. Almost all of these generators gain electricity by harvesting the negative work of human body. The power levels of such generators can be well-suited for powering the wearable and implantable devices, such as cell phone, LED, MP4, computer, drug pumps and so on. The bicycle like power generator and fitness-generator, which transform positive work of human body into electricity, could significantly expand the application range of human power. With the running power spanning from milliwatt to watts magnitude [13], the potential devices drivable by human power in fact cover many mobile electronics, transportation, even home appliances and so on. Apart from using human power to drive electrical devices, utilization of human power to satisfy many daily life needs such as transportation is also attractive. In this paper, human-powered land vehicle, watercraft and aircraft are discussed respectively. Furthermore, human-powered home appliances such as washing machine, TV and air conditioner are also evaluated in the analysis. These typical appliances of human power can be depicted in Fig. 1.

In addition, if the 127 W kinetic energy which is recovered from an adult is converted into electricity, the total power produced by all the adults over the world in an hour each day for one year could be 205.471 billion kilowatthours in 2007 [4,14], which is nearly 34.7 times as much as the electric energy production of a nuclear power reactor in 2007 [15]. In fact, exploitation of only a very small ratio from that will be significant enough. However, collection of such dispersing energy poses a great challenge. To tackle the challenge, three potential ways, which is termed as human-powered grid (HPG), human-powered charger (HPC) and human-powered storage (HPS) to efficiently utilize the human power from a group of people are proposed in this paper. Their integration could clearly provide a perfect solution to the many tough issues closely related to people’s daily life and significantly expand the utilization of human power. These three ways can not only partially ease the energy shortage, relieve the greenhouse effect, save human lives in disasters and cultivate people’s health exercise habit in daily life, but also shed light on unconventional innovations in power grid and even lead to new job opportunities in the near future.

Power from human being

Characteristics of human power

Human power, one of the long neglected renewable and clean energy, has been put on the agenda in current serious situation when conflicts between energy supply and demand become increasingly severe [7,16-18]. Although human power is scattering, low in magnitude and unstable, it is huge in the total volume. In fact, all the human energy is produced from food. Through digestion food is decomposed into different kinds of organics which is then transformed into energy by respiration in cell. As the “molecular unit of currency” of energy transfer in cell, adenosine triphosphate (ATP) is continuously recycled in human body. Although averagely containing only 250 g of ATP, human body turns over its own weight in ATP each day [19-21]. ATP is the direct energy source for human body metabolism. The main forms of human energy are the kinetic movement of body parts such as hand, arm, leg, ankle, knee-joint and so on; the variation of body temperature; and the chemical reaction in the body. Among them, human kinetic energy and thermal energy are currently two types of mostly captured and utilized energy. The harvesting principles of human kinetic energy mainly include electromagnetic effect, piezoelectric effect and dielectric effect. The electrical power which is produced by human motion depends on electricity generation mechanism, location of the generator and feature of human motion. Human thermal energy is the one which is lost into environment through body skin and breath. Based on thermoelectric effect of certain semiconductor, such thermal energy can be collected by thermoelectric materials. The basic features of human power are summarized as follows:

1) Human power is scattered, thus the energy density is low.

2) Human power is huge for a group of people, which can enlarge its potential applications.

3) As a kind of renewable energy, human power is environment friendly and independent on climate and geography condition.

4) Application of human power could help maintain people’s health.

Kinetic energy from human body

Based on the characteristics of natural movement, including a range of relative motions between different body segments, walking is an alternative way for harvesting the human power. During walking, the potential energy sources are heel strikes, leg motions, center of mass motion, elbow joint and shoulder joint motion during arm swings [22]. In such activities, the work produced by linear and rotary motion can be expressed as

(1)W_L={\displaystyle {\int }_{\mathrm{0}}^{\mathrm{s}}F\mathrm{d}s},

(2)W_R={\displaystyle {\int }_{\mathrm{0}}^{\mathrm{\theta }}\tau \mathrm{d}{\theta }_{\tau }},

where W_L is the work produced by linear motion, F is the force, s is the linear displacement under the force, W_R is the work produced by rotary motion, θ is the angular displacement, and τ is the torque. During leg motion, all the rotary motions are produced by ankle, knee and hip. From the data listed in Table 1, it can be seen that the ankle produces the most positive work among these three types of joint, while the knee produces the most negative work during walking at a normal speed. Therefore, the ankle appears as a very suitable location to capture energy when consuming metabolism energy is available. However, the knee is the optimal choice for the maximum efficiency.

As the most utilized human power, human kinetic energy is produced by human muscles. During body motion, the muscles perform both negative and positive mechanical work. When they perform negative work, they generate the motion and consume the metabolic energy. However, when they perform positive work, they absorb the energy and retard or stop the motion [23]. The kinetic energy of human body is the energy converted by the muscles from metabolism energy which includes the power from the movement of body parts, joint rotation, vertical displacement of mass centers, enforcement of body weight, elastic deformation of tissues and so on. The utilization of human kinetic energy mainly includes using human kinetic energy to drive machine directly and converting human kinetic energy into electricity power to drive the devices. In addition, the direct utilization of human kinetic energy mainly appears in the field of transportation and domestic appliances such as human-powered car, watercraft, aircraft and washing machine. When based on the piezoelectric effect and the electromagnetic induction couple, human power generator is mainly employed to power portable electronic devices.

Human magnetoelectric generator

Based on Faraday’s electromagnetic induction law, human powered magnetoelectric generators could transform human kinetic energy into electricity. By using gear, spring, or flywheel mechanism, almost all traditional human powered magnetoelectric devices are solid rotary generators. The output induced voltage of the magnetoelectric generator is mathematically expressed by Lenz’s law

(3)E=n\Delta \Phi /\Delta t,

where E is the induced voltage in coil of wire, Ф is the magnetic flux through the coil, n is the number of turns of the coil, and \Delta \Phi /\Delta t is the changing rate of the magnetic flux. The changing rate of the magnetic flux is gained by changing the relative position between the magnet and the coil, which means that either the magnet or the coil moves. Under this principle the output voltage of human magnetoelectric generator depends on the turns of the coil and the changing rate of the magnetic flux. Because of the limited space and weight, the turns of the coil are restricted. Therefore, the changing rate of the magnetic flux is the main influencing factor. So in order to improve the output power, raising the relative velocity between the magnet and the coil is the most direct method. In fact, human magnetoelectric generator is just the device that utilizes human kinetic energy to drive the magnet and the coil to move relatively. In addition, under practical operating conditions, the relative movement between the magnet and the coil is a rotary and linear motion.

The shoe mounted with magnetic generator outside is a type of generator for harvesting human kinetic energy during walking based on electromagnetic effect [24]. 250 mW of electricity can be produced during a standard walk with this kind of generator. The sneaker with twin motor-generators and step-up gears can produce 60 mW of electricity. In addition, Rome et al. [8] have developed a kind of suspended-load backpack which can change the vertical movement of the loads which is carried into electricity. 7.4 watts of electricity will be generated by this new generator when carrying a load of 20 to 38 kilograms during normal walking. In 2008, researchers developed a knee-mounted electricity generator by using the negative power during walking. That generator, analogous to the braking in a hybrid car, can produce an average of 5 watts of electricity by one device on each leg [7].

Although the above-mentioned human magnetoelectric generators can produce considerable, long-term electrical power, its continuing work is hardly gained. Hence, as the harvester for daily activities, inertial power generators for human motion are studied by more and more researchers. Ideally, inertial power generator can be described as a spring-mass system which mainly contains a proof mass, a spring and a damper. The simple model of an inertial power generator is shown in Fig. 2.

The differential equation of motion can be written as

(4)m\stackrel{··}{z}(t)=-c\stackrel{·}{z}(t)-kz(t)-m\stackrel{··}{y}(t),

where m is the value of the proof mass, z is the relative movement between the proof mass and the generator, c is the damping coefficient of the damper, k is the stiffness coefficient of the spring, and y is the movement of generator. In addition, the damping coefficient c is depended on the parasitic losses c_p and electrical energy extracted by the transduction mechanism c_e. For the magnetoelectric generator, the electromagnetic transduction c_e can be expressed as

(5)c_e={\displaystyle \frac{{(nLB)}^{\mathrm{2}}}{R_{load}+R_{coil}+j\omega L_{coil}}},

where n is the number of the turns of the generator coil, L is the length of the coil assuming that the coil is square, B is the flux density, R_{load} is the load resistance, R_{coil} is the resistance of the coil, and L_{coil} is the coil inductance [25].

In 2010, Tremont Electric proposed the nPower® PEG (personal energy generator), a backup battery charger for portable electronics via harvesting the human energy while walking, running or biking. As a typical inertial power harvester, the PEG contains a tubular housing, an electromagnetically active mass, an inductive coil and springs to connect the mass to the opposite side of the housing. When moving with the PEG which can be placed in a backpack, a briefcase, a pocket and so on, the electromagnetically active mass will move up-and-down under the actuation of the first and second spring. Then electricity power could be induced in the inductive coil. Furthermore, with a weight of 0.31 kg and a diameter of 25 cm × 2.5 cm, the PEG outputs a direct current of 200 mA and 5.0 V which is transferred by a high speed USB 2.0 cable [26,27]. The structure and principle of the generator are revealed in Fig. 3.

In fact, a wearable or implantable microbattery, which has analogous structure to the nPower® PEG, was proposed earlier to provide electricity power to medical devices in 2006 [28]. Driven by human body’s kinetic energy, this microbattery is mainly comprised of a shell, an inductive coil, an inductive coil holder, a permanent magnet, a permanent magnet holder and a spring, which is demonstrated in Fig. 4(a). In 2009, the performance of this battery was tested by Wei et al. [29]. In vitro, the experimental result indicates that a maximum voltage of 4.297 V can be obtained when 4014 turns of copper coil with a diameter of 0.1 mm were used and the vibrator with a vibration frequency of 3.3 Hz was available. The experimental prototype of this microbattery is revealed in Fig. 4(b). Furthermore, the output performances of the existing human magnetoelectric generators are presented in Table 2. Comparing the data in Table 2, it can be observed that the human powered rotary generator performs better than the linear generator.

Piezoelectric devices

The piezoelectric effect was discovered by the brothers Jacques Curie and Pierre Curie in 1880. They found that when piezoelectric materials were compressed, twisted or pulled along a direction, positive and negative electrical charges appeared on the two driving surfaces respectively and the value of electric charge was proportional to the stress. The main piezoelectric materials include tourmaline, tourmaline, topaz, quartz, Rochelle salt and cane sugar. However, in early period, the output of electricity produced by piezoelectric materials was too low to drive wearable electronical devices. With the improving performance, piezoelectric materials are gradually employed to harvest human kinetic energy. Currently, lead zironate titanate (PZT) and polyvinylidene fluoride (PVDF) are two kinds of common industrial piezoelectric materials which are widely used. The model of piezoelectric generator can also be described as a spring-mass system as shown in Fig. 2, and can be expressed by Eq. (4), but the piezoelectric damping coefficient c_e is written as

(6)c_e={\displaystyle \frac{\mathrm{2}m{\omega }_n{}^{\mathrm{2}}{k}^{\mathrm{2}}}{\mathrm{2}\sqrt{{\omega }_n{}^{\mathrm{2}}+\mathrm{1}/{(R_{load}{C}_{load})}^{\mathrm{2}}}}},

where k is the electromechanical coupling factor of the piezoelectric material, R_{load} is the resistance of the load, and C_{load} is the load capacitance [30]. The schematic diagram of a piezoelectric generator is illustrated in Fig. 5.

Based on the piezoelectric effect, the energy generated in the process of walking can be captured and changed into electricity via piezoelectric material transduction. Kymissis et al. [5] have tested PVDF and PZT, two kinds of piezoelectric materials which can harvest the parasitic power in shoes. The result demonstrates that when walking with the same foot hitting the ground at a frequency of 1 Hz, the voltage peak of the PVDF was roughly 60 V, whereas that of the PZT was approximately 150 V.

Using piezoelectric materials to collect human power is the central issue of research not only in the macroscale but also in the nanoscale. Wang and his colleagues have utilized zinc oxide nanowire (NW) to covert nanoscale mechanical energy into electricity for powering nanodevice with an efficiency of 17% to 30% [31-34].

The efficiency of piezoelectric devices depends on the performance of the piezoelectric materials and the mounted-location. The properties of piezoelectric materials are nonlinear which vary with the age, the working temperature and the applied stress of the materials. Currently, application of piezoelectric materials in harvesting human power mainly focuses on the parasitical energy in shoes. So, once the performance of piezoelectric materials is improved, its application in collecting human power could be enlarged. It is worth noting that sphygmus, breathing and vibration of clothes during human body motion are also possible energy sources for piezoelectric materials.

Human kinetic energy generator based on liquid metal

As a working medium, room temperature liquid metals have recently been proposed to harvest human kinetic energy for generating electricity. Jia et al. [9] first introduced the idea to harvest human kinetic energy in 2008. The working principle of liquid metal magnetohydrodynamic electricity generator (LMME) is that the generator is driven by human kinetic energy through the liquid metal flowing across the magnetic field to produce electricity. When the geometric dimension of the LMME is fixed, the open circuit voltage increases linearly with the flow velocity of the liquid metal, which can be written as

(7)V={\displaystyle \underset{-z_{\mathrm{0}}}{\overset{z_{\mathrm{0}}}{\int }}\left(K_d{B}_{\mathrm{0}}U-{\displaystyle \frac{K_f\rho g}{\sigma B_{\mathrm{0}}}}\right)}dz=\mathrm{2}{z}_{\mathrm{0}}\left(K_d{B}_{\mathrm{0}}U-{\displaystyle \frac{K_{\mathrm{f}}\rho g}{\sigma B_{\mathrm{0}}}}\right),

where z₀ is the distance between the two electrodes of the LMME; k_d is the damping coefficient; B₀ is magnetic displacement; U, ρ, and σis the flow velocity, the density, and the electrical conductivity of the liquid metal; K_f is friction coefficient; and g is the gravitational acceleration, respectively. The structures of the LMME in vitro and vivo are plotted in Fig. 6 respectively. With the liquid metal of 5.68 g Ga62In25Sn13, the LMME can output 1.4 V/3.61 μW of power in vitro experimental setup, which would be the potential power provider for future wearable micro/nano devices.

Additionally, in August 2011, Krupenkin and Taylor developed a shoe generator which can produce 10 watts of power per footstep when the shoe contains 1000 droplets [12,35]. Running the electrowetting process backwards, they used human power to change the physical form of liquid drops, and charges were then generated between dielectric-coated plates by these drops. The liquid drop is room temperature liquid metal which could be mercury or galinstan and a gallium-based alloy. The structure of the footwear-embedded microfluidic energy harvester is exhibited in Fig. 7.

Heat energy from human body

In 1821, thermoelectric effect was discovered by German physicist Thomas Johann Seebeck which was that difference in temperature between the joint of two dissimilar materials (semiconductor or metal) induced voltage over it. Now, thermoelectric generator (TEG) is used to convert this temperature gap into electricity. As a kind of heat engine, the greatest efficiency of TEG is Carnot efficiency which can be defined as

(8){\eta }_C={\displaystyle \frac{T_{\mathrm{h}}-T_{\mathrm{c}}}{T_{\mathrm{h}}}},

where T_h is the temperature of the heat side while T_c is that of the cold side of the TEG. The efficiency of a TEG is dependent on its Carnot efficiency and the figure of merit for the thermoelectric module, which is typically described as

(9)\eta ={\eta }_C{\displaystyle \frac{\sqrt{\mathrm{1}+ZT}-\mathrm{1}}{\sqrt{\mathrm{1}+ZT}+T_{\mathrm{c}}/T_h}}={\displaystyle \frac{T_h-T_{\mathrm{c}}}{T_h}}{\displaystyle \frac{\sqrt{\mathrm{1}+ZT}-\mathrm{1}}{\sqrt{\mathrm{1}+ZT}+T_{\mathrm{c}}/T_h}},

where ZT is the figure of merit for the thermoelectric module. The temperature difference between human body and the ambient can also be used to produce electricity based on thermoelectric effect. If the heat energy dissipated from human body is captured and perfectly changed into power, the maximum power will be 2.4 to 4.8 W [4]. Although the electricity generated by thermoelectric modules under the temperature differences between human body and the ambient is rather limited, this method is significant for harvesting human energy. The Seiko thermic wristwatch [24] is a good case in point for such type of devices, which is shown in Fig. 8 [36]. Currently, the average value of ZT is 1 which limits the output of the thermoelectrical generator. Therefore, the challenge of thermoelectrical device is to improve the performance of the material.

Human-powered transportation

Transportations scavenging fossil fuel bring not only the burden of energy but also environmental problem. In this case, human-powered transportation attracted more attention. Since antiquity, human-powered vehicle has been a kind of transportation for people or goods using human muscle power. Based on the merits of being convenient, low cost, physical exercise, environmental protection and energy conservation, human-powered transportation is also becoming popular nowadays. And at sometimes, human-powered transportation is the only available choice, especially in inaccessible or underdeveloped regions. In addition, under the modern technology, human-powered transportation is driven directly by human power. The performance of the devices, which directly scavenges human power, depends on the condition of human body and the structure of the devices. So improving the efficiency of the device is very important. Moreover, introducing human kinetic energy to power transportation is not only saving energy and protecting environment but also keeping people healthy.

Human-powered land vehicle

Human-powered land vehicles mainly include human-powered bicycle and car. Almost all the human-powered vehicles are driven directly by human kinetic energy. As the energy source, human kinetic energy is changed into mechanical energy through the transmission mechanism, when it is directly used to drive the machine. The transmission mechanism includes the wheel gear system, the wheel hub, the chain and the connecting rod. So the efficiency of human-powered machine depends on the efficiency of the muscle and the transmission mechanism.

Human-powered bicycle

As the most common human-powered vehicle, the bicycle is a pedal-driven and single-track vehicle with two wheels attached to a frame one behind the other [37]. The bicycle, invented in the 19th century, is not only utilized in transportation but also as a popular form of recreation such as children’s toy, and adult fitness device etc.

In 2010, Google Inc. developed a personal efficient, cost-effective, recreation and fitness bicycle vehicle called Shweeb. The idea of Shweeb is “Drive innovation in public transportation” which is one of the five winning ideas in Google Inc. sponsored Project 10^100 in 2008. Geoff Barnett, the inventor of Shweeb, said that Shweeb originated from the desire to solve the traffic problem and allow people to move around the city quickly and safely. Shweeb, running on sleek ultra-thin monorails which were set 6m above the ground level based on the towers, can reach a speed of over 50 km an hour [38]. In the future, the Shweeb transit network would be expanded to connect high density residential areas with employment and business zones. Mills, the Shweeb general manager, said that Shweeb was in a research and development phase and really tested out for a transportation system. The structure of Shweeb is revealed in Fig. 9.

Human-powered car

It is well-known that automobile is the most common motor vehicle in peoples’ daily life. With at least four wheels, the typical modern automobile scavenges oil. There are over 600 million passenger cars around the world [39]. In addition, the number of car is increasing rapidly, which leads to the dramatic growth of oil consumption and CO₂ emission. In this case, human-powered car emerges as the time requires. Among them, HumanCar is one of the most classical and successful human-powered cars. As the showcase creation, HumanCar is developed by Seattle company HumanCar^® which is founded by Charles S. Greenwood PE. With four wheels, HumanCar is a four passenger vehicle that can achieve a speed of 60 miles per hour [40]. Up to now, HumanCar has a history of one hundred years. When swing the hand shank in HumanCar, it will run. The HumanCar contains an arrangement of mechanical parts which are used to convert oscillatory motion into rotational motion to propel the wheels of this full body muscle car. Based on the bi-directional power interface, the drivers can face forward or backward or in any combination. HumanCar mainly features complete non-dependence on foreign fuel sources, and low weight. As a plug-in hybrid electric vehicle, now HumanCar can power its affiliated electronic equipments by itself, such as GPS, bluetooth^®, iPod^® and so on [40]. The structure of HumanCar is denoted as Fig. 10. Based on the features of HumanCar, its application may include teamwork training, developing country transportation solution, hybrid power inputs, city grid lock solution, campus shuttle, racing and so on. The future development of this kind of car focuses on the enhancement of the efficiency and the promotion in the third world.

Human-powered watercraft

Human-powered watercraft is one which is propelled by human power based on paddles, poles, and a crank or treadle through hands or feet. Currently, human-powered watercraft includes canoe, hydrofoil, kayak, paddleboarding, pedal, rowing, surfboard, hydrocycle and so on. The traditional human-powered watercraft consumed power from human hands. Now, with the demand of high speed, feet or cooperation of hands- and feet-powered watercrafts are popular. Based on Derwent Innovations Index, there are 38 patents on the topical of human powered watercraft from 1991 to 2011. The early device was mainly comprised of floats, bicycle-type frame, handlebar, rudder, sprocket, chain, and propeller. The floats were used to support the bicycle-type frame which was employed to drive the propeller through the help of sprocket and chain [41,42]. In recent years, based on deck, rocker, push-bar, pedal and propeller, human-powered watercraft can be operated by hand or feet effectively [43,44].

In addition, several conceptual human-powered watercrafts were presented. For instance, in 2008, the River Gym concept, designed by architect Mitchell Joachim and trainer Douglas Joachim, would provide the user with floating around Manhattan by capturing the energy expended at the gym [45]. Furthermore, in the same year Jonathan Mahieddine, a French designer, designed the Solar and Human Powered concept boat which can be powered both by human kinetic energy and solar energy [46]. The pictures of these human-powered boats are shown in Fig. 11. Although the speed and efficiency of the present human-powered watercraft need to be improved, it is certain that human-powered watercraft in the future will be efficient, comfortable, safe and healthy.

Human-powered aircraft

The dream of flying in the sky for human beings has not discontinued all the time. At the same time, fully human-powered aircraft also draws peoples’ attention especially in the modern society. Since the first officially authenticated human-powered aircraft made by Piggott the Man Powered Aircraft (SUMPAC) in Southampton University at Lasham Airfield in 1961, various kinds of human-powered aircrafts have been developed [47]. The history of human-powered aircraft is listed in Table 3 [48].

Human-powered aircraft is driven by human power directly to overcome gravity which emerged from aerodynamics, engineering and pure physical stamina. Through the pedals, chains and belts, human power can be changed into the mechanical energy of aircrafts. While, for driving a propeller, the pedaling is more effective than the rowing motion [49,50]. In addition, through the leg muscles, the full cruising power of the pilot is limited by the ability to use oxygen but not the number of the used muscles [51]. Shenstone [52] expressed the power required for driving the aircraft as

(10)P={\displaystyle \frac{W}{L/D}}{\displaystyle \frac{V}{\mathrm{550}}},

where W is the weight of the aircraft and pilots, L is the height of the lift, D is the length of the drag, and V is the air speed. So if a low weight, low speed and a high L/D (good gliding angle) are gained, the power for flight will be the minimum amount. The design of human-powered flight as a sport was summarized by Win [53]. The human-powered helicopter has been studied since 1989 when the first human-powered helicopter flied successfully, which was reviewed by Naito [54].

Human-powered home appliances

With the living standard improving, the type and number of household appliances increase gradually. In USA, the number of households grew by 34.5 million from 1978 to 2009 [55]. There is also a great increase in the production of household appliances in China. The yield of washing machine, television and refrigerator in China from 2001 to 2009 is illustrated in Fig. 12 [56-58]. The increasing home appliances lead to huge growth in energy consumption. For example, the power consumption of a typical television is 100-200 watts, so the power consumed by these television sets produced in China in 2009 could be 3.64-7.27 TW•h (Assuming the TV time is one hour a day). Based on this calculation, the power consumption of all the household appliances worldwide could be huge. For the sake of saving energy, protecting environment and making home appliances available in remote area or even at an emergency, human power is an alternative to power home appliances.

The combination of electromagnetic generator and bicycle, pioneer of bicycle generator, appeared in the 1970s [59-61]. Although some of the early generators were just used for lighting, the commercial bicycle generator can provide a power of over 100 watts [62]. The structure of the typical bicycle generator (Pedalpowergenerator) is depicted in Fig. 13. Since 2007, gyms with human power generation in fitness facilities have become popular. These gyms, such as the California Fitness facility in Hong Kong, employ fitness facilities to harvest the power produced during exercises. In addition, Recreational Sports Facility (RSF) at the University of California would supply a power of approximately 10 MW•h into the electric grid over a year based on averages over 2800 patrons per year using 28 elliptical machines [63]. So, with the bicycle generator available, utilization of human power to propel most of the home appliances is possible. Besides, the home appliances driven by human power directly based on the modern machine technology, such as washing machine, are popular in the third world, especially in India.

Human-powered washing machine

Washing clothes by hand not only wastes time but also makes people tired. Currently, based on the bicycle generator, human-powered washing machine is available. Some companies even began to produce human-powered washing machine by connecting the washing machine with a bicycle generator. For example, in 2010, Haier showcased its bike-powered washing machine which combined an exercise bike with a lithium-ion battery and washing machine [64]. Although human-powered washing machines are possible, they are impractical in rural regions, especially in developing countries due to the high price of machine and expensive electricity. Therefore, it would be desirable to have a kind of washing machine which is directly driven by human kinetic energy. In 2009, the pedal-powered washing machine of MIT was tested successfully [65]. In 2005, the prototype of this machine was built by Raduta, a mechanical engineering graduate student at MIT. The pedal-powered washing machine is comprised of a bicycle chain, oil drums and a bicycle frame. This pedal-powered washing machine, as shown in Fig. 14 [65,66], is not expensive at all, which is a prior choice to the people in developing countries. In addition, with the merits in saving time, labor and fossil fuels, the proposed human-powered washing machines generally can be denoted in Table 4.

Human-powered television

Since 1920s, as a device for advertising, entertainment, medium of news, the television set has become commonplace in homes and business institutions. In order to meet the varying demands of the people, the number of TV is increasing dramatically. In USA, the average household had 2.5 televisions in 2009, which was 2.5 times of that in 1978. Although energy saving methods have been considered, the power consumption of TVs could not be ignored. The energy consumption of TVs in 2004-2009 in the world is revealed in Fig. 15 [72]. Since watching TV for too long a time may affect peoples’ health, the utilization of human power to power TV is a plausible alternative.

Based on the electricity inverter, controller, battery and other attached devices, the power produced by the bicycle generator can be used to power television effectively. The structure of TV powered by a bicycle generator is illustrated in Fig. 16. Furthermore, pedal-generator-powered TV is also very popular. Compared with a bicycle generator, using a pedal generator to power television can save more space. Meanwhile, a pedal generator is much cheaper and simpler. The Pedal-powered TV was first proposed by Allison, an obesity researcher at St. Luke’s-Roosevelt Hospital in New York [73]. The research on TVcycle demonstrated that it can help people lose weight, especially leg fat [74,75]. At present, the commercial pedaled generator with simple operation is practicable [76]. In fact, through bike generators, movie can be powered by a group of people. On 28 August 2011, an activity named Summer Cycle-in Cinema was held in Hackney City Farm in London. On that day, the movie of “The Princess Bride (1987)” was powered by riders via the bike generator [77].

Discussion

Clearly, the role of direct or indirect application of energy from a human individual in the total energy system is rather limited. As an important remedy, it is suggested in this paper that the potential application of human power from people in a whole city or over the world should be considered. In fact, the number of existing devices drivable by human power is surprisingly large, which includes almost all the necessary electric appliances in guaranteeing a good quality of daily life. This promises the future of pervasive human power. The only problem restricting the wide application of human power most probably lies in the inconvenience during the utilization. If such technical barrier can be broken, the human power can definitely be used as an idealistic electricity source to fulfill the basic needs of modern life. Three novel basic ways, termed as HPG, HPC and HPS, as shown in Fig. 17, to utilize human power, are proposed in this paper.

HPG

Establishment of this new conceptual grid will serve to realize a highly efficient administration and utilization of the electricity produced by human body. The HPG is capable of harvesting the electric power produced by each individual at different regions and thus can supply flexible power to the customers as demanded, without relying on the conventional electricity grid. Furthermore, compared with the existing electricity grid, the HPG can better guarantee the electricity supply, in particular when encountering natural disaster or emergency. Structurally, the HPG is comprised of the energy and the information network. The energy network performs the main functions of electricity production, modulation, transmission, distribution and consumption. Powered by the energy network, the information network provides an intelligent control of the energy network to make the entire electricity grid run safely and optimally. With the set up of the hardware, electricity can be supplied flexibly to the HPG via human-powered generators in the community from such places as the gymnasium and even workplaces, thus making electricity supply much more reliable and convenient. Applications of the new system is rather wide such as lighting a home LED, charging a mobile phone, powering a machine, enabling the communication system and keeping the internet in operation etc. Evidently, the realization of the HPG will trigger many new fundamental researches for solving energy problems and broadening the electric power market in the near future.

Depicted in Fig. 18 is a basic illustration of the structure of the HPG. It can be observed that the HPG is mainly composed of a human-powered generator, an electricity input port, an input-current conditioning module, an electricity input path, a low-level energy storage device, a secondary energy storage device, a high-level energy storage device, an electricity output path, an output-current conditioning module, an electricity output port, an electric equipment, a central control system, an electric power dispatching system and a information transmission path etc. With these basic elements, human power from different sources can be well collected, administrated and utilized.

HPC

To harvest human power, it is quite necessary to develop devices capable of immediate charging. Such a charger is typically comprised of a a HPG and a power adjustment chip. The HPG, which could be obtained by a variety of fitness-generators, is employed to convert human motion to electricity. The power adjustment chip is used to regulate the electricity produced by the HPG, which could include a voltage converter and a rectifier or power filter. The human power charger is mainly set in public places, such as streets, office buildings, airports, railway stations, hospitals, schools, emporium and so on. Meanwhile, it could be set at home as one desires. Like the vending machine, the main advantage of such a charger is to provide instant service. Such function can even be realized via a wireless way by using mechanical, electromagnetic, microwave energy conversion etc. In that case, there would be no physical barrier to restrict the utilization of the charger just caused by varied connector.

HPS

The purpose of human power storage device is to immediately store the electricity generated by an individual. Such storage device mainly contains a voltage transformer and an electrical storage. The voltage transformer is to transform different grades of electricity generated by human power into the electricity which can be stored in the electrical storage. The electrical storage is to store the electricity coning from the voltage transformer, which can be realized by an ultracapacitor or an accumulator etc. Furthermore, like a human-powered charger, human-powered storage device can also be set in public places or at home, especially in gymnasiums and playgrounds. And the electricity delivery and storage can even be completed wirelessly which is to provide a generalized energy source. With the help of such a storage device, even the small power from human motion will play a role in tackling “electricity shortage”, when the amount gathered is sufficient enough.

To sum up, making the full use of “green” human power would give more chances to the globe and promisingly satisfy most of the needs in peoples’ daily life. This is especially critical when the society encounters “electricity shortage” due to either insufficient energy supply or disaster. The 21 century would be delightful to see a world full of human power appliances in the near future. It is the right time now to think more and make our further endeavor to fully explore and utilize the powerful human energy.

Conclusions

In this paper, the application of the human power was comprehensively summarized. The basic features and harvesting principles of human power were presented first. Then human-powered transportations, which include human-powered land vehicle, watercraft and aircraft, were summarized. In addition, human-powered home appliances were also presented, such as human-powered washing machine and television. Finally, based on the technology of harvesting human power, the HPG, HPC and HPS were proposed to significantly extend the application range of human power from a group of people. The concept, feature and application of these three ways of using human power are discussed in detail. It is held in this paper that as a kind of renewable energy, human power has a huge potential for future growth.

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