Although only very slight improvements have been made to this figure of merit in the past few decades [ 12 ], the expected progress in the development of new materials with higher figures of merit could increase the efficiency of thermoelectric generators.
Furthermore, it should be remembered that the efficiency of such devices depends on the temperature difference between the body and the surroundings, and therefore the greater the difference, the greater the increase in the efficiency and vice versa Figure 1. Thermoelectric device efficiency as a function of the environment temperature and the figure of merit. Another important consideration in harvesting energy from the human body is the mechanisms through which heat is lost to the surroundings.
The total sensible heat that is released into the atmosphere by a person walking at natural speed is approximately W [ 13 ]. If we could capture all this energy and convert it into electricity with an efficiency of 2. However, to harvest this energy, it would be necessary to cover the body with a thermoelectric material perhaps a jacket or a garment like a diving suit.
The design of an item of clothing with an embedded thermoelectric material that would cover part of the body or the whole body is obviously a challenge. Since in cold weather, the device would have to function as thermal insulator; however, currently available thermoelectric materials have a much higher thermal conductivity than typical coat material. This would result in a coat that would be too heavy to wear or in a need for an additional layer of thermal insulation material, thereby reducing the temperature difference along the device.
In addition, such a device would have to allow sweat evaporation; however, this would mean that some of the sensible heat would flow out through the openings, causing a loss of available energy. The above data suggest that this technology would be more practical for low power applications, for which it would be necessary to cover only a small part of the body.
One such example is the Seiko Thermic watch, which uses a thermoelectric material to generate its own power [ 14 ]. The relatively low power output of thermoelectric technology led us to consider the exploitation of the mechanical energy that can be derived from the body during motion to produce electrical energy.
When considering a particular motion as a candidate for energy harvesting, the following main factors must be taken into consideration. First, muscles perform positive and negative mechanical work within each motion: During the positive work phase, the muscles generate the motion, and in negative work phases, the muscles absorb energy and act as brakes to retard or stop the motion.
Winter [ 8 ] defined negative and positive muscle work as follows: Positive work is the work performed by the muscles during a concentric contraction, i. When the muscle performs positive work, it generates motion. Therefore, the use of positive energy e. On the other hand, negative work is the work done during an eccentric contraction, i. An energy harvesting device should therefore replace part of the muscle action during negative work and create resistance to retard the motion, similar to "generative braking" in hybrid cars.
Theoretically, such a device will allow energy generation with minimal or no interference with natural motions. In this paper, we explore the option of generating energy during activities that are performed naturally throughout the day, with particular emphasis on walking.
The choice of walking as a candidate movement for the study of energy harvesting is based on the fact that it is a natural movement, performed without conscious thought and involving a range of relative motions between different body segments and between different segments and the ground.
When assessing the potential power harvesting capability of an energy-harvesting device, we must consider five main factors: the muscle's negative work phases during each motion, the means by which the device is attached to the body, the convenience of use of the device, the effect of the additional weight of the device on the amount of effort expended by the wearer, and finally the effect of the harvesting energy device on the body.
For example, during walking, in the heel strike phase, energy is converted into heat in the shoe sole [ 15 ], and harvesting this energy should not affect the normal gait pattern. In the following section, we will analyze the main body motion segments during natural walking to facilitate assessment of the potential power-harvesting capability during each motion segment.
The major body motions during walking that we considered as potential energy sources were heel strikes, center of mass motion, shoulder and elbow joint motion during arm swings, and leg motions, i.
To estimate the potential power of each motion, we performed an integrative analysis using data available in the literature. In addition, for the upper body joints we conducted our own experiment to calculate the power of each motion. For the analysis of the energy produced during the above-described motions, we used two definitions of work: 1 the force acting through a displacement, and 2 the product of torque and angular displacement.
Heel strike refers to the part of the gait cycle during which the heel of the forward limb makes contact with the ground. Several researchers, e. It is, however, generally agreed that energy is lost during the collision.
A number of researchers have tried to estimate the amount of energy dissipated in the collision. For example, Shorten [ 18 ] calculated the energy loss in a running shoe and related it to a force acting through a linear displacement.
Using a viscoelastic model for the midsole, he determined the part of the energy is stored as elastic energy in the sole of the shoe and the part that is dissipated. He predicted that for a typical runner moving at 4. To gain a better understanding of the source of energy, let us consider a simple model in which an external force acts on the sole of the shoe over a complete stride.
The maximum ground reaction force acting on the shoe is approximately equal to 1. Since a complete stride at natural walking speed has a frequency of approximately 1 Hz two steps per second , the theoretical maximal power will be 4 W. While it is possible to construct a device that will have a larger displacement during the heel strike, such a design may impair stability and manoeuvrability [ 7 ]. Intuitively speaking, this will result in the wearer of the device feeling as if s he is walking on soft sand.
During walking, muscles generate torques at the ankle, knee, and hip joints. These torques acts along three axes 3-D , and their magnitude changes during the gait cycle Figure 2. The most significant torques in terms of the work that is performed during the walking cycle are those acting in the axes normal to the sagittal plane [ 19 ]. Winter and colleagues [ 20 ] calculated the work performed at different leg joints during a single step and normalized it by the subject's weight.
In addition, they divided the net work done by the muscles at the joints into several phases of motion. Their classification was based on the negative and positive muscle work performed at the joints during walking Table 2.
We used these findings to estimate the total work and the negative work performed during a gait cycle at the hip, knee, and ankle joints. Typical kinematics and kinetics during a walking cycle.
For an kg person walking at normal speed, the joint work for each step is calculated by using the following equation:. Another motion that could be utilized to generate energy is the motion of the center of mass. The center of mass performs a motion similar to a 3-D wave i. The total motion of the vertical wave from the lowest to the highest point is approximately 5 cm [ 8 ].
For an external mass e. To facilitate energy harvesting, there must be a relative motion between the mass and the person carrying it. We used the following model to estimate an upper bound on the total amount of energy required to generate this motion, based on changes in the height of the mass in each gait cycle i. By applying this equation for a center of mass motion of 5 cm during walking, we find that for a device of 20 kg there is a potential of 20 W to be harvested.
Arm motion refers to the backward and forward swinging movement of the arm that occurs during walking and running. The arm motion is composed of two sub-motions: the relative motion between the forearm and the upper arm change of angle of the elbow and the relative motion between the trunk and the upper arm change of angle at the shoulder.
To calculate the net muscle joint torque during the during the gait cycle, we used a recursive inverse dynamic top down. Then, using the angular displacement and the joint torque equation 3 , we calculated the work at the shoulder and elbow joints during the gait cycle, according to the method applied by Winter and his colleagues for leg joints [ 20 ].
To calculate the energetics of the arm joints, we performed an experiment with three male subjects of average weight 82 kg range kg and average height 1.
Motion data were obtained using a six-camera motion capture system at a sampling rate of Hz Vicon , Lake Forest, CA. Marker motion data were low-pass filtered Butterworth fourth-order forward and backward passes with a cut-off frequency of 6 Hz. The arm was represented by a two-link system, consisting of the upper arm and the forearm including the hand.
The segmental properties mass, center of mass, and moment of inertia were calculated on the basis of De Leva's adjustments [ 21 ] to the work of Zatiorsky-Seluyanov. The measurements from our experiment were used to calculate arm energetics. A summary of our analyses is given in Table 3.
This summary presents the amount of work performed in each joint or body part and of the portion that is negative work. Further, it shows the maximum joint torque during these motions; this information is required because for harvesting maximum energy, an energy conversion device should be able to withstand torques similar in magnitude to the maximum joint torque.
We obtained results showing the amount of positive and negative muscle work in each motion, and motion where energy is lost to the surroundings e. The importance of these results is that they will affect the design of energy-harvesting devices. It is possible to consider the harvesting of energy during positive work; for example, a user rotating a crank to generate energy. This type of generation of electrical energy would require an additional metabolic cost.
A better way to generate energy from human motion would be to use energy that would otherwise be lost to the surroundings.
This would ideally enable the generation of electricity from human motion with minimal or no additional load. There are two types of motion relevant to energy harvesting: 1 motion in which energy is lost directly to the surroundings e. Exploiting the latter type of motion in an energy-conversion device might not cause an additional load to the user.
The idea explored in this paper is that in these phases the muscles act as brakes to slow down the motion of the limb. By replacing the negative work done by the muscle with an electric generator, we can reduce the load on the muscles and generate electricity at the same time. Another important consideration is the way in which this motion is utilized. For example, while the knee and elbow joint motions are mostly single-degree-of-freedom movements, the shoulder and the hip joints perform much more complex movements, and, therefore, much more complex mechanisms would be required to exploit the energy generated from these joints.
Consequently, we focus on joints with one-degree-of-freedom motion. In addition, it is important to know the maximum joint torque during these motions, since for maximum energy harvesting, an energy-conversion device should be able to withstand torques of similar magnitude to maximum joint torque.
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Nano Letters , 11 3 , A First Principles Investigation. Nano Letters , 11 2 , Nano Letters , 10 12 , Quinto-Su , S. Nano Letters , 10 10 , The Journal of Physical Chemistry Letters , 1 19 , Paci and Horacio D. Nano Letters , 10 9 , ACS Nano , 4 7 , Nano Letters , 10 6 , Nano Letters , 10 5 , Journal of the American Chemical Society , 13 , Snyder and Zhong Lin Wang.
ACS Nano , 4 2 , The currents in the individual sections combine to form one large current. This current is the electricity that moves from generators through power lines to consumers. Electromagnetic generators driven by kinetic mechanical prime movers account for nearly all of U.
Most of U. In a turbine generator, a moving fluid—water, steam, combustion gases, or air—pushes a series of blades mounted on a rotor shaft. The generator, in turn, converts the mechanical kinetic energy of the rotor to electrical energy. Different types of turbines include steam turbines, combustion gas turbines, hydroelectric turbines, and wind turbines. Most steam turbines have a boiler in which a fuel is burned to produce hot water and steam in a heat exchanger, and the steam powers a turbine that drives a generator.
Nuclear power reactors use nuclear fuel rods to produce steam. Solar thermal power plants and most geothermal power plants use steam turbines. Most of the largest U. Combustion gas turbines , which are similar to jet engines, burn gaseous or liquid fuels to produce hot gases to turn the blades in the turbine.
Steam and combustion turbines can be operated as stand-alone generators in a single-cycle or combined in a sequential combined-cycle. Combined-cycle systems use combustion gases from one turbine to generate more electricity in another turbine. Most combined-cycle systems have separate generators for each turbine.
In single-shaft combined cycle systems, both turbines may drive a single generator. Learn more about different types of combined-cycle power plants. Combined-heat-and-power CHP plants , which may be referred to as cogenerators , use the heat that is not directly converted to electricity in a steam turbine, combustion turbine, or an internal combustion engine generator for industrial process heat or for space and water heating.
Most of the largest CHP plants in the United States are at industrial facilities such as pulp and paper mills, but they are also used at many colleges, universities, and government facilities. CHP and combined-cycle power plants are among the most efficient ways to convert a combustible fuel into useful energy. Hydroelectric turbines use the force of moving water to spin turbine blades to power a generator.
Most hydroelectric power plants use water stored in a reservoir or diverted from a river or stream. Pumped-storage hydropower plants use the same types of hydro turbines that conventional hydropower plants use, but they are considered electricity storage systems see below. Discount will be applied automatically at checkout. Your account has been created successfully, and a confirmation email is on the way. Why glass recycling in the US is broken Scientists synthesize large borophene crystals Glowing dyes could store digital data for the long term Every time we move our body, we expend a lot of energy.
Researchers would love to harvest this energy to generate electricity to power portable electronic devices such as cell phones or medical implants. The triboelectric effect arises because some materials attract and retain electrons better than others do. When two such mismatched surfaces touch or rub against each other, one surface can grab electrons from the other, leaving one positively charged and the other negative.
If the charged surfaces then separate, a voltage develops between them. An electric current flows if the two then reconnect, for instance, through a wire.
Zhong Lin Wang and his colleagues at Georgia Institute of Technology build devices that convert kinetic energy to electricity using the triboelectric effect. Their new design uses a layer of polydimethylsiloxane and one of aluminum, both patterned with bumps to increase their contact area.
The researchers joined the sheets at one pair of opposite edges and allowed the sheets to bow away from each other in the center. External pressure can force their centers together, allowing the polymer to grab electrons from the aluminum. When the researchers remove pressure, the two sheets spring apart, creating a voltage across the sheets. To test a generator with an area of 9 cm 2 , the researchers used an electric motor to rhythmically tap the device at a frequency of 6 Hz.
This tapping could power a light-emitting diode. The team also could charge an empty cell-phone battery enough to make a call, although the charging process took 85 hours.
The researchers calculate that increasing the area of the generator to cm 2 and stacking four devices on top of each other should reduce the charging time to around two hours.
Kinetic energy is freely available on a person, Wang says. Contact the reporter.
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