Considerations for device design
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.g., heel strike). 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. Typically, muscle efficiencies during positive work are approximately 25%, which means that if all the mechanical work were converted into electricity, there would be an increase of approximately 4 J of metabolic cost for every 1 J of energy generated. 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.g., heel strike) in the form of heat, plastic deformation, sound, or other forms, and 2) motion in which the muscles perform negative work. 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. A torque of higher magnitude on the device would require stronger transmission and would therefore lead to an increase in the device weight, which would, in turn, increase the energy expenditure. Moreover, the lower the additional mass mounted on the leg, the higher the energetic cost of carrying it [22, 23].
From our analysis of human motions during walking (Table 3), we can see that all the motions examined include some negative-energy phase. For an energy-hungry application, we need to maximize the total amount of energy to be harvested, and, therefore, heel strike, and knee and ankle motions seem to be good candidates for energy harvesting devices, since a relatively large part of their total energy can be recovered. Furthermore, these motions are almost all single-degree-of-freedom movements, which simplifies the device design.
Efficiency of harvesting electrical power
The magnitude of the power that can be harvested is not the sole consideration for choosing a movement or designing a device; the other important parameter for an energy-harvesting device is its efficiency.
where Δelectrical_power is the electrical power output and Δmetabolic_power is the difference in metabolic cost of a particular activity with and without a device (e.g., walking with a device and without it). The change in metabolic cost is made up of two main components: 1) the energy spent to generate the electrical power, and 2) the energy spent by the user in carrying the device, which is a function of the device weight and the location of the device on the body. Therefore, in a comparison of two devices, the efficiency of harvesting might be a better metric than the maximum power output. For example, for two devices of equal weight producing the same amount of energy, a knee device will have better efficiency than an ankle device because the cost of carrying the knee device mass is lower. Note that a reduction of the device weight by the use of lighter materials (e.g., carbon fibers) and an optimized design will also reduce the cost of carrying the device and will lead to the development of more efficient devices. The first component of the change in human metabolic power derives from the generation of electrical power. This addition in metabolic power is affected by muscle work and device conversion efficacy [24].
Where ΔMetabolic power
g
is the change in metabolic power due to the change in muscle work resulting from the energy generation component alone, η
device
is the device efficiency, and η
muscle
is the muscle efficiency in the given motion.
The change in metabolic cost due to the change in muscle work is dependent on the type of work done by the muscles, since the efficiencies of positive and negative work at the joint are not the same. For positive work, the efficiency ranges between 15% and 25% [8], while for negative work, the values range from 28% to 160% [25, 26]. The parameters that affect muscle efficiencies are: the nature of the performed motion, the particular muscles involved, and the activation forces and velocity of these muscles. This means that when the energy harvester replaces the muscle work during negative work, the predicted reduction in metabolic cost will be less than the predicted reduction for replacing positive work phases. In addition, in some cases, the negative work is performed using passive elements such as connective tissue, which store elastic energy like springs and return it back to the gait cycle [27]. In these cases, harvesting this energy might mean that the muscles will have to perform extra work in order to replace the energy that is lost to the device. For devices based on generative braking, we used the joint net power as a criterion to determine which joints are good candidates for energy-harvesting devices. It is, however, difficult to interpret the contribution of each muscle to the net joint torques, for the following reasons: 1) muscles work across multiple joints, and therefore, theoretically, it is possible that a particular muscle will contribute to negative work at one joint and positive work at another; and 2) the net joint torque is a function of all the activity of agonist and antagonist muscles and as such cannot account for simultaneous generation of energy by a certain muscle group and absorption by the antagonist group, or vice versa. As a result, it is possible that when the generator resists motion during positive power, it will help the muscle that is doing negative work. Therefore, recommendations as to the appropriate joint to be exploited for generative braking based on the amount of negative work done at the joint should be considered only as guidelines, and the final evaluation must be based on experimental work.
Comparing the cost of energy harvesting to carrying batteries
While ideally the energy-harvesting device should not increase the metabolic cost, it is possible that in some cases it will do so. In these cases, the user may have to consume extra food to cover the additional metabolic cost for electricity generation. Hence, for a given mission, the best option should be chosen on the basis of a comparison between the metabolic cost for generating energy and carrying extra food versus carrying batteries with the equivalent amount of energy. In the case of a backpack device [7], the user carries the food and batteries on his/her back, and thus the cost of carrying the weight is the same for both. In this regard, Rome et al. [7] reported a device that achieved 19.5% efficiency in converting metabolic energy to electrical power. Since the specific energy of food is typically 3.9 × 107 J kg-1[28], which is much greater than the specific energy for lithium batteries (4.1 × 105 J kg-1) and zinc-air batteries (1.1 × 106 J kg-1) [29], the weight of food would be 19 times lighter than that of lithium batteries and 7 times lighter than that of zinc-air batteries. Therefore, they concluded that the addition of food weight is negligible. This means, for example, that walking at 1.5 m/s (while generating 5 W) for 10 h would save approximately 0.4 kg of lithium batteries and 0.15 kg of zinc-air batteries, meaning the longer the expedition, the greater the weight savings.
Now that we have estimated the potential of energy to be harvested from each of the body motions and discussed considerations in utilizing energy sources from human motion, we believe it is important to include a review of existing devices. These devices are classified according to the motions used to harvest the energy and their location on the body.
Review of the state of the art in energy-harvesting devices
Center of mass
Currently available center-of-mass devices use the motion of the center of mass relative to the ground during walking to generate energy. For example, when carrying a backpack, the body applies forces on the backpack or any other mass in order to change the direction of its motion. Rome and colleagues used these forces in a spring-loaded backpack that harnesses vertical oscillations to harvest energy [7]. This device, with a 38 kg load, generates as much as 7.4 W during fast walking (approximately 6.5 km/h). The device is a suspended-load backpack (Figure 3) that is interposed between the body and the load, resulting in relative motion movement. For this device, the relative motion was approximately 5 cm, and this linear motion was converted into rotary motion that drove a generator (a 25:1 geared motor). Generation of this energy was achieved with the small amount of extra metabolic cost of 19 W, which is 3.2% more than carrying a load in regular backpack mode (with no relative motion). This additional cost is less than 40% of that required by conventional human power generation (e.g., hand-crank generators or wind-up flashlights). While the mechanism of this energy harvesting is not fully understood, from the above results it seems reasonable to believe that there is contribution of both negative and positive muscle work.
Another approach to harvesting energy using a backpack was taken by Granstrom and colleagues [30], who mounted a piezoelectric material in the shoulder strap of a 44-kg backpack and used the stress in the straps to generate 50 mW. A different class of device that uses the motion of the center of mass to harness energy is based on oscillations of a floating magnet due to this motion. Niu and colleagues built a linear electrical generator (1 kg) that used the motion of the body during walking to produce 90-780 mW, depending on the walking conditions [31]. They optimized the electrical circuits and linear generator design to produce the highest power output from the walking motion.
Heel strike
Several devices have been built to generate energy from heel-strike motion. Some devices use the energy from the relative motion between the foot and the ground during the stance phase (the phase in which the foot is on the ground). Others use the energy from the bending of the shoe sole. In both cases, the device aims to use the energy that would otherwise have been lost to the surroundings. An example of such a device is a hydraulic reservoir with an integrated electrical magnetic generator that uses the difference in pressure distribution on the shoe sole to generate a flow during the gait cycle. This prototype produces an average power of 250-700 mW during walking (depending on the user's gait and weight); its drawback is that it is quite bulky and heavy [32]. Paradiso and his colleagues [33] built a shoe that harvests energy using piezo-electric materials from heel strike and the toe off motions. The average power during a gait cycle is 8.3 mW. Another device that was built by the same group is a shoe with a magnetic rotary device that produces a maximum power of 1.61 W during the heel strike and an average power of 58.1 mW across the entire gait.
A different approach was taken by Kornbluh and his collaborators [34] at SRI International, who developed electrostatic generators based on electroactive polymers (EAPs). Such materials can generate electricity as a function of mechanical strain. Their technology provides energy densities for practical devices of 0.2 J/g. In addition, these materials can "cope" with relatively large strains (50-100%). The SRI team incorporated an elastomer generator into a boot heel. Their generator design was based on a membrane that is inflated by the heel strike. They achieved 0.8 J/step (800 mW) with this device. The energy was harvested during a compression of 3 mm of the heel of the boot onto which the device was mounted [34]. A key advantage in the construction of such devices is that they can be mounted on an existing shoe, thereby obviating the need for a special external device to generate energy. The power output of these devices is relatively low, with a maximum of approximately 2 W at normal walking speed. However, there are many applications (e.g., MP3 players, PDA, cellular telephones) for which this energy would be sufficient to operate the device.
Knee
A device for the knee joint based on negative work of the muscles was proposed by Niu and colleagues [35] and subsequently developed by Donelan et al. [24, 36]. This 1.6-kg device comprised an orthopedic knee brace configured such that knee motion drove a gear train (113:1) through a unidirectional clutch, transmitting only knee extension motion to a DC brushless motor that served as the generator (Figure 4). The generated electrical power was dissipated by a load resistor. This method generated 2.5 W per knee at a walking speed of 1.5 m/s. The additional metabolic cost of generating energy (not including the cost of carrying the device) was 4.8 W, i.e., 12.5% of the metabolic cost required by conventional human power generation. However, there were certain drawbacks associated with this device in that it used only a small part of the motion of the knee (end of the swing phase) to generate energy: During the gait cycle, the muscle net work in the knee joint is approximately 90% negative work, which is approximately 34 W, but the device harvested energy only at the end of swing phase and with an efficiency of 65%. Based on this data, we calculated the difference between the power of the current device and that of an ideal device (that would harvest all the negative work during walking). The power that is still available = (total power - current power output/efficiency)*device efficiency = (33.5-5/0.65) × 0.65 = 16.8 W. The main challenge in harvesting energy from the knee movement is that as more energy is harvested, the resistance to the motion as generated by the device will increase, thereby increasing the motion controls by the device at the expense of the muscles.
Method for energy conversion
A key component of the energy-harvesting devices reviewed above is the method they use to convert the mechanical work to electricity. The main technologies in current use are based on piezoelectrics, EAPs, and electrical induction generators. Piezoelectric materials, which generate a voltage when compressed or bent [38], have been used mainly for heel strike devices. Their main advantage is that they are simple to incorporate into a shoe. However, due to the small displacement and the high generated voltage, the power output of this technology is limited to approximately 100 mW [35]. EAPs also generate electricity when under mechanical stress, but they have a low efficiency (compared to magnetic machines) and a relatively high operation voltage, both of which can make the electrical circuit complicated and expensive. Yet, due to the excellent strain properties of EAPs when compared to piezoelectrics, more energy can be harvested from the former. Furthermore, EAPs are much lighter and easier to shape than magnetic materials. Therefore, we conclude that EAPs are a good alternative to piezoelectrics for biomechanical applications [38]. Of the three technologies discussed above, magnetic machines, which are low in cost, have the highest conversation efficiency. However, the higher efficiency levels are generally achieved at high speeds and in rotary implementations. Human motions, in contrast, are relatively slow, and, as a result, the application of electromagnetic energy conversion needs an addition of transmission to increase the rotation speed. While for the backpack, the transmission adds only a small percentage to the total weight, in the knee device, the transmission construction added approximately 650 g, which was 40% of the total weight of the device. Moreover, when using a rotary magnetic-based generator, the input should ideally have a constant rotation direction and speed. However, human joint angles change speed and direction during the walking cycle, which adds complexity to the use of rotary magnetic devices to harvest energy.
Possible directions for future research are the innovative design of magnetic machines that reduce the need for high rotary speeds, improvement of the power density of elastomers and magnetic-based generators (e.g., using stronger magnets), and improvement of the efficiency of energy harvesting by using elastomers.