The invention relates to a backpack based system for energy generation that generates electricity from the wearer's movement.
Man has become more dependent on portable technology in all arenas of life from business, to medicine, to recreation, to first response/disaster relief, to exploration, to field scientists, and to the military. This dependence requires electricity, which is presently provided by batteries. Total dependence on batteries is problematic as people working off the electric grid may have to carry up to 25 lbs of batteries. A device that could generate significant electrical energy to recharge batteries would provide much more freedom and operational ability.
Until recently, electrical energy harvesting from body movements has been extremely limited in terms of wattage (10 or 20 milliwatts). The present inventor has previously disclosed a backpack device which can generate electricity when carrying very heavy loads (U.S. Pat. No. 6,982,497 “A Backpack for Harvesting Electrical Energy During Walking and for Minimizing Shoulder Strain”). That device suspends the load so that it operates as a force-damped oscillator that generates electricity from a geared generator via a rack and pinion arrangement. The description in U.S. Pat. No. 6,982,497 is hereby incorporated by reference.
Exemplary applications of such a power generating backpack include backpacks for soldiers. Combat units operating on foot in remote regions are highly dependent on electrical energy for communications, navigation and sensing. As the use of electronic devices on the battlefield has increased, so has the need for electrical energy independence. At present, power is provided by carrying a large weight of batteries (as much as 20 lbs), which when added to their usual 801b packs, represents a crushing burden, resulting in reduced performance and back/joint injuries for the soldiers. The electricity-generating backpack described in U.S. Pat. No. 6,982,497 addresses this problem by providing significant levels of electricity generation from human movement. For example, during normal walking, the backpack originally generated over 2 watts of electricity and was later modified to generate up to 7.5 watts of electricity. This is significantly more energy than previous devices. At the same time, the backpack provides ergonomic benefits as the soldiers walk. This electricity production will permit soldiers to replace up to 20 lbs of disposable batteries with a small rechargeable battery.
Use of backpacks of the type described in U.S. Pat. No. 6,982,497 will permit longer and better performance in the field by combat soldiers and special forces. The backpack will enable military units to go into the field and essentially generate their own electrical power as they need it. This will permit them to stay longer in the field, and because they will not need to carry 20 lbs of batteries, the soldiers will be more mobile. Further, because of the reduction in forces on the body, the soldier will be able to move faster (i.e., run) than they can presently, with an expected reduction in orthopedic injury. Hence, use of the ergonomic backpacks described in U.S. Pat. No. 6,982,497 is expected to lead to not only lower incidence of orthopedic injury during deployment, but also fewer orthopedic problems following deployment and in later years. Finally, at present Marines use disposable batteries because of their higher energy density. The 1 kg batteries used presently cost $70-90 each and Marines may use 1 or 2 a day that are disposed of after use. The electricity generating backpack of U.S. Pat. No. 6,982,497 will greatly reduce this recurring battery cost and reduce the environmental cost of manufacturing and disposing of millions of lithium-ion batteries. Such a backpack will also save considerable funds by generating the electricity for free.
However, there are a number of limitations that limit the practical use of the backpack of U.S. Pat. No. 6,982,497. For example, a significant limitation involves the necessity for damping of the mechanical dynamics and hence the need for appropriate management of energy drawn from the mechanical system by the generator. The backpack also has several other limitations including: 1) it is very heavy (14.5 lbs of extra weight over and above a normal backpack); (2) if the weight carried in the backpack has to be changed or the speed of the wearer changes significantly, then the springs (and/or spring placement) have to be modified, which is very difficult to accomplish in the field; (3) the efficiency of the generator is only 30-40%; (4) the electrical power generated, though much larger than other devices, is modest; and (5) the electricity is unusable in its present form for attaching a typical electronic device will overdamp the system and prevent sufficient movement of the load.
A next generation electricity generation backpack addressing these problems is desired. The present application is directed to such a backpack.
A next generation electricity-generating backpack is disclosed that is substantially lighter in weight, has the multiple springs replaced with one large spring whose spring constant can be adjusted in the field in seconds, and the DC generator is replaced by a geared brushless AC generator that permits approximately 70%-80% overall efficiency and the generation of up to 20 W of electrical power during walking and 25 W during running. The invention also addresses potential limitations to the function of this backpack. For example, in the backpack described in U.S. Pat. No. 6,982,479, the electrical power generated by the generator (which is measured by the voltage and current across a power resistor) is converted to heat, and is essentially unusable. The problem is that when an electronic device such as a battery charger is connected across the output of the generator, it draws too much current, and overdamps the movement of the load. The overdamping prevents the large excursions and high velocities of the load (with respect to the backpack frame), which are necessary to generate high electrical powers. Although removal of too much power is problematic, removing too little power is also problematic. Because the spring constant of the backpack is set so that the natural frequency is matched to the walking frequency, if electricity is not removed from the system, the load would resonate uncontrollably making it difficult to walk. A device is thus provided that addresses this potential problem by limiting the amount of electrical power removed from the system.
A device that always removes some electricity, but not too much, is necessary to extract large levels of the electricity while controlling damping. Embodiments of the invention are provided that perform this function using electrical circuitry. In a first embodiment, electrical damping circuits are provided including a flyback converter designed to emulate a desired load at its input terminals.
The electricity generating backpack also includes numerous design efficiencies that have been implemented to decrease weight and improve electricity generation from mechanical movement. For example, the position of the generator has been moved to a position internal to the backpack that is safer for the generator and reduces noise. Movement of the generator also permits the suspension components to be enclosed in a dust cover to keep the components safe and clean so as to facilitate efficient operation. The gears and generator are also reengineered for efficiency, and special grease and gear coatings are preferably used to prolong the life of the electricity generating mechanism and to reduce noise.
In particular embodiments of the electricity generating backpack, additional electricity generating devices are added to the backpack for generating additional power. For example, a so-called E-Mod device hooks a generator to the backpack belt at the wearer's hip and includes a wand that fits against the wearer's femur so as to move through a range of motion as the wearer walks. The movement of the wearer's femur moves the wand, which, in turn, cranks the generator gears to generate power. Another embodiment permits the wearer, when not walking, to crank the generator using pedals that are connected to a separate generator. In this embodiment, the device includes a harness for restraining the user with respect to the backpack and the pedals so as to effectively form a recumbent ergometer that generates, for example, up to 100 W.
The electricity generated and stored by the backpack may be used to charge batteries and to power a number of devices that may be carried by the backpack. For example, the backpack may include a commercial Sterling Cooler System that is powered by the backpack's stored power to provide cooling power for cooling items carried by the backpack. In a particularly useful embodiment, the power generated by the backpack powers a cooling system that enables the refrigeration of vaccines and medications that are to be delivered to remote areas.
These and other characteristic features of the invention will be apparent to those skilled in the art based on the following detailed description of the invention.
The above and other objects and advantages will become apparent to one skilled in the art based on the following detailed description of the invention, of which:
a) and 2(b) respectively illustrate a side view and a front view of the assembled backpack of
a) and 4(b) illustrate alternative embodiments of the electricity generating mechanism of
a) and 12(b) illustrate bottom and top views of a lightweight bicycle ergometer used to generate larger levels of power by converting the linear motion of the legs into the rotary motion of an AC generator.
a) illustrates a backpack with an internal frame.
b) illustrates a dust cover for the embodiment of
A detailed description of illustrative embodiments of the present invention will now be described with reference to
The electricity generating backpacks described in U.S. Pat. No. 6,982,479 and in related U.S. patent application Ser. No. 11/957,222, filed Dec. 14, 2007, entitled “Suspended Load Ergonomic Backpack” (the contents of both of which are incorporated herein by reference) have been further refined to introduce several novel features as will be described herein. General improvements to the backpack will first be described and then alternate embodiments of supplemental electricity generating devices and uses of the generated power will be described.
I. Improvements to Electricity Generating Backpack
Protection from Physical and Environmental Damage
In accordance with an exemplary embodiment, the electricity generator within the backpack is moved from the top of the backpack to a safer place where the electricity generator is less prone to damage, the rack on which the load is mounted need not stick up so as to injure the wearer, and less noise is generated. In the exemplary embodiment, the electricity generator and rack are placed between the backpack frame and the suspensions system within the boundaries of the pack body where it may be completely protected within a box formed by the top and bottom structures and the bearing tubes on the side. This design keeps all moving elements within a frame such that nothing sticks out and so that the frame may be encased within a protective casing that preferably keeps out undesirable elements such as dirt, dust, and/or water. For military applications, for example, the casing material may be formed of a resilient material such as plastic such that the surface bends to avoid restricting movement of a helmeted wearer.
a) illustrates a side view of the assembled backpack, including thermoplastic polyurethane side bumpers 34 that cover the bearings 18 and rails 20 and stick out from the sides of side walls 12 to protect the backpack body 10, particularly the electricity generating mechanism, from impact and abrasion, while
In the embodiment of
a) and 4(b) illustrate alternative embodiments of the electricity generating mechanism of
Those skilled in the art will appreciate that, when working in areas off of the electric grid, there is the potential to be exposed to extreme environments. This is particularly the case for soldiers who may submerge their backpacks in fresh and potentially salt water. This is problematic for the electrical system and the bearings. To address this problem, the generator of the backpack of
Furthermore, when the backpack is worn in desert environments, there is the problem of sand, dirt, and dust being lodged in surfaces between moving parts, which would reduce efficiency and potentially damage the parts. Such areas include between the gears of the rack and pinion system 28, between the bearings 18 and the rails 20, and in the AC generator 36. Instead of encapsulating individual components, a dust cover 23 as illustrated in
Increasing Efficiency of Conversion of Mechanical to Electrical Energy
As noted above, the backpack described in U.S. Pat. No. 6,982,497 had an efficiency of conversion of mechanical to electrical energy of 30-40%. Theoretically, generators should have approximately 90% efficiency. On the surface, this suggests that there is a large mechanical inefficiency in the system of driving the rotor of the backpack of U.S. Pat. No. 6,982,497 at high speeds, which includes a 25:1 planetary gear set and a rack and pinion arrangement. Upon examination of the specifications for the DC gear motor used in the backpack of U.S. Pat. No. 6,982,497 as a generator, the efficiency of the planetary gear was discovered to be only 68%. The present inventors have thus replaced the DC gear motor with a geared/generator system that uses a brushless AC motor (as opposed to the brushed DC motor). The efficiency of the gearing is greatly improved by utilizing a single stage gear design. The planetary gear system used in the backpack of U.S. Pat. No. 6,982,497 had multiple stages and hence losses at each stage. To obtain the required high gear ratio in a single stage, the following approach was used. The rack 28a interfaces with a very small diameter pinion gear 28b. On the same axle is a large diameter spur gear that meshes with a small gear cut into the axle of the generator 36. By having only one stage, the gear system is very efficient. Although the required high gear ratio can be achieved, this requires the spur gear to be quite large and orthogonal to the generator axis, giving the generator 36 a large foot print and making it difficult to fit in between the pack body 10 and the frame of the backpack. This problem has been overcome by essentially carving out space in the backpack body for the generator 36 to project into in a direction perpendicular to the wearer's back and then protecting the generator 36 with a vacuum formed plastic shell incorporated into the backpack body 10 as noted above.
The average efficiency of the new geared generator has been found to be about 75% when driven by an actuator or during walking experiments, and the efficiency has been found to be 85-90% when traveling unidirectionally at a constant speed. The measured improvements in power generating efficiencies were typically higher at slow walking speeds than at high walking speeds, which had the important advantage of producing a significant amount of power at low walking speeds. At high weights and the fastest walking speeds, this leads to a remarkable average electrical power output of up to 20 W of electricity. To put this in perspective, before the electricity-generating backpack of U.S. Pat. No. 6,982,497, the highest reported power output that was generated during walking was between 20-200 mW. Moreover, this property permits the electricity generating system of the invention to be used even in daypacks with relatively small (e.g., 20-30 pound) loads while still producing several watts of power.
By way of example,
The inventors do not believe that these improvements are due solely to using a more efficient generator. Rather, some of the improved electrical power and efficiency was achieved by reducing frictional losses, particularly at the rack-pinion gear interface using coatings and greases that also significantly extend the life of the gears. In the backpack of U.S. Pat. No. 6,982,497, the rack was not positioned exactly, and pressure and position were achieved by using a fixed Teflon bearing surface. In the backpack described herein, the fixed Teflon bearing surface is replaced by roller bearings used to position the rack with respect to the pinion gear. Furthermore, the rack 28a is now connected to the backpack plate by a semi-flexible aluminum rod 39 (
Weight-Reduction
The parasitic weight of the backpack of U.S. Pat. No. 6,982,497 has been reduced by some 70%, from 14.23 lbs to 4.5 lbs (4.3 lbs plus 0.2 lbs for the power electronic circuitry). The major weight savings involved using a tube structure, rather than a plate, to hold the backpack body 10 and using electro-less nickel-coated aluminum tubes rather than solid steel for the vertical bearing rods 20. Weight was also saved by reducing the size of many of the structural components. Finally, the AC generator 36 and gears of rack and pinion system 28 are also much lighter than the DC generator and gears of the backpack of U.S. Pat. No. 6,982,497.
Spring Design and Control of the Spring Constant.
A disadvantage of the backpack of U.S. Pat. No. 6,982,497 is that it used up to 5 springs (and their respective hardware), and the springs (or attachment points) had to be changed for different weights carried. The present backpack replaces the multiple springs with only 1 stainless steel spring 26 between the fixed frame 25 and moving H frame 14 having an adjustable spring constant that adjusts to loads ranging from 40 to 100 lb loads by engaging more or less active coils. Although one stainless steel spring is considerably lighter than the 5 springs used in U.S. Pat. No. 6,982,497, the extra hardware associated with the spring constant adjustment makes the total weight about the same as with the backpack of U.S. Pat. No. 6,982,497. However, the adjustment system is a significant functional improvement, as in a matter of seconds the spring constant (k) of the system can be changed. The advantage is that the resonant frequency of the system
can be adjusted to match the step frequency during walking and thereby improve the power output of the backpack system for a given walking speed and weight of the load.
Circuit Design of Power Management and Battery Charging
As noted above, the electrical output of the geared DC servomotor generator used in the electricity generating backpack described in U.S. Pat. No. 6,982,479 was run through a power resistor so that the electrical power generated could be easily measured. However, for the backpack to function effectively, the damping, which is determined largely by the current output of the generator, must be controlled. Too much current flow, as would be observed when attaching most electronic devices (e.g., battery charger), overdamps the load, leading to little relative movement of the load with respect to the backpack frame, and thus low electrical power output On the other hand, too little current removed results in the load undergoing large vertical oscillations making it difficult to walk (this is a special concern as the resonant frequency of the backpack is matched to walking frequency). Accordingly, it is desired to provide a circuit that: 1) controls current output from the generator to optimize harvesting and ergonomics, 2) directs current to power the portable devices in real-time, 3) sends excess power to either ultracapacitors or rechargeable batteries or both, and 4) in the special case where there is no damping because all storage devices are full and no portable devices connected, then damping is obtained by switching a load resistor across the output of the generator. In this case the electrical energy is converted to heat and lost, but this load resistor is usually out of the circuit and only switched in during this special case.
In addition to the damping considerations, there are important electrical considerations as well. A characteristic of charging and recovering energy from batteries is that there can be a significant loss of electrical energy, as much as 30-50%. This makes it advantageous to power devices in real-time directly from the backpack. In addition, storing electrical energy in high specific energy capacitors can avoid some of these losses. The disadvantage of capacitors is that they have a smaller energy density than batteries. It is likely therefore that after powering devices in real time, the highest priority will be to store at least some of the extra electrical energy on high specific energy capacitors (high priority), after which the remainder will be stored on lower-priority batteries. Hence, the circuits described below are designed to maximize the amount of usable electricity from the mechanical power removed from the human-backpack system.
As illustrated in
The output of the brushless AC generator 36 is rectified through a full-bridge diode rectifier circuit 46. Should the output voltage of the rectifier circuit 46 attempt to exceed a preset value, a variable resistive load 48 is enabled which limits the voltage to this value. This feature protects the power electronic circuit 40, as well as provides some minimum amount of damping to the backpack to avoid excessive displacements. The output voltage from the rectifier circuit 46 is then converted by a flyback DC-DC converter 50 designed to emulate a desired (e.g., resistive) load at its input terminals. As explained below, the brushless AC generator 36 in the backpack application of the invention, where the speed of the AC generator is constantly changing, is more efficient if the current drawn from the AC generator 36 is a certain function of the rotor speed. In particular, a resistive-type load is more efficient than a constant voltage or constant current load. This resistive emulation also provides a linear damping to the mechanical dynamics of the backpack, which was found to be necessary to avoid excessive displacements of the backpack that occur in an underdamped condition.
Ultracapacitors 52 at the output of the flyback converter 50 provide an energy storage mechanism from which power can be stored and extracted efficiently. This is desired for the backpack application, as the output voltage of the rectifier circuit 46 is a rectified sinusoid at the relatively low frequency of approximately 2 Hz, and the power drawn from the rectifier circuit 46 is proportional to the square of this voltage. The ultracapacitors 52 therefore provide constant power to the electronic device load 44 through a DC/DC converter 54 and to battery pack 42 via battery charger circuit 56 when the power output of the rectifier circuit 46 is low, and store the excess power available when the power output of the rectifier circuit 46 is high. The ultracapacitor voltage powers the lithium-technology battery charger 56, plus the separate DC/DC converter 54 that provides a regulated output voltage to electronic device load 44. When the output voltage of the flyback converter 50 is lower than the battery voltage, the diode 58 begins to conduct, and the battery pack 42 provides power to the electronic device load 44.
In an exemplary embodiment, 7.2V lithium ion batteries were used for battery pack 42, and the ultracapacitor energy storage system included four ultracapacitors, rated at 2.5 F and 5.4V, with two sets of parallel ultracapacitors connected in series. The output voltage of the flyback converter 50 in this embodiment was therefore limited to 9.5V to ensure that the combined rated voltage of the ultracapacitors was not exceeded. It is noted that in many applications, particularly military, voltages of 12-30 Volts will be desired and the system can be adjusted to achieve this. The battery charger circuit 56 was designed to charge a 7.2V lithium-ion battery at charging currents up to 1 A. This current decreases as the battery voltage gets close to its fully charged value. The minimum voltage experienced by the ultracapacitors 52 in this embodiment is therefore approximately 6.9V while the battery pack 42 is charged (7.2V minus approximately 0.3V across the diode 58, in this case a Schottky rectifier). The voltage regulator 54 is designed to provide a constant 5V output voltage for the electronic load 44 at current levels up to 2 A.
Structure of the AC Generator
As noted above, the AC generator 36 functions to converting mechanical energy into electrical energy. Those skilled in the art will appreciate that this can be achieved using one of the following methods:
Magnetic-field-based devices tend to be preferable to electric-field-based devices due to the fact that significantly higher forces and torques can be practically generated with magnetic fields as opposed to electric fields. Energy conversion devices based upon materials with electromechanical constitutive properties tend to be desirable in applications where the power levels are relatively low (i.e., on the order of milliwatts). As the performance and reliability of magnetic-field-based devices are well known, an exemplary embodiment of the AC generator 36 is a magnetic-field-based device.
Magnetic-field-based electro-mechanical energy conversion devices tend to work best when the electrical frequencies associated with the device are high (i.e., in the tens or hundreds of Hz). There are two main reasons for this: power density and energy conversion efficiency.
Power Density: The size of an electromechanical device tends to determine the maximum force or torque that the device is capable of generating. The power converted by the device, however, is given by the expression:
P=ffldν
or
P=τfldω
where ffld and τfld are the electromagnetic force or torque, respectively, ν is the linear velocity and ω is the rotational velocity. Hence, for high power density, it is desirable that the velocity be high. This corresponds directly to a high electrical frequency of the voltage and current waveforms in the device.
Efficiency: High electrical frequency is also necessary to achieve a high efficiency. For example, in a permanent-magnet-based device, the RMS AC voltage generated by the magnets in a machine winding can be written as:
E=ωeλ,
where ωe is the electrical frequency in radians/sec, and λ is the RMS flux-linkage of the permanent magnets in the device's windings. This peak flux-linkage is fixed for a given design. The converted electromechanical power in a device winding is given by:
Pconv=EI,
where I is the RMS current in the winding. The electrical losses in the device will often be dominated by conduction losses. The conduction losses in a winding are given by:
Pcloss=RI2
where R is the electrical resistance of the winding. The efficiency is therefore given by:
Inspection of this expression makes it clear that a high electrical frequency is necessary for high efficiency. It is therefore typically desirable to operate electromechanical devices in the range of 10's to 100's of Hz, with the higher frequencies corresponding to higher performance.
Generally, the mechanical oscillation frequencies associated with human-based energy harvesting are on the order of 1 Hz, a frequency much too low for efficient direct energy conversion with magnetic-field-based devices. Therefore, the need in accordance with the invention to boost the electrical operating frequency of the device leads to two proposed approaches, namely linear and rotary converters.
Linear Device: For a linear device, the recommended structure for use in accordance with the invention includes a relatively short stator that resides on the stationary part, and a relatively long “mover” consisting of a shaft covered with a series of alternately poled permanent magnets. The magnets should be placed along the entire stroke length of the shaft in order to harvest as much energy as is available. This can make the design of a linear generator with high power density challenging. The relationship between the linear velocity ν of the shaft and the electrical frequency ω of the device is given by:
where λ is the “wavelength” of the device, corresponding to the length of a north-south pole-pair on the shaft. In order to achieve a high electrical frequency, the wavelength of the resulting flux waveform generated by the permanent magnets will need to be small to convert the slow mechanical velocity into a high electrical frequency. The shaft design would therefore need to consist of a large number of small magnets. Under these circumstances, in order to achieve good magnetic coupling between the stator and mover with such a design, the “gap” between the mover and stator will also need to be relatively small.
Rotating Device: A rotating electromechanical device could also be used as the AC generator 36. This would require a system that would convert the linear motion of the system into rotational motion. Two approaches to achieving this conversion include:
It can be shown that the magnetic-field-based generator under the conditions of a backpack application, where the speed of the generator is constantly changing, is more efficient if the current drawn from the AC generator 36 is a certain function of the rotor speed. In particular, a resistive-type load can be shown to be more efficient than a constant voltage or constant current load. In such case, the DC-DC converter 54 that interfaces with the rectifier circuit 46 and the ultracapacitor energy storage system 52 can be designed in such a way as to mimic a resistive load. Although a buck-boost converter topology may implement this function, flyback converter 50 (which is essentially a buck/boost converter with the inductor replaced by a transformer) was selected since the electrical isolation provided by the flyback transformer of the flyback converter 50 eliminated the need for isolated gate driver circuitry and since the turns ratio of the flyback transformer created an extra degree of freedom that allowed the design of a converter that would operate in discontinuous current conduction mode (DCM) over all operating points with reasonable efficiency.
In the case of both the buck-boost and flyback converters, when operating in DCM mode the input voltage of the converter mimics a resistive load. It can be shown that, under these conditions, the power drawn by the converter is proportional to the square of the input voltage, i.e.:
where Vin is the input voltage of the converter, fs is the switching frequency, L is the converter inductance (in the case of the flyback converter 50, L corresponds to the magnetizing inductance of the flyback transformer on the primary side), and D is the duty cycle. Hence, if the flyback converter 50 is operating at a fixed duty cycle it will emulate a constant resistance. The duty cycle that emulates a desired resistance Rdes can be determined from the following equation:
Design of Flyback Converter in Discontinuous Conduction Mode
In an exemplary embodiment, the flyback converter 50 emulates a resistive load of, for example, 10Ω over a voltage range of up to 24Vpk from the AC generator 36. This corresponds to a maximum power P=V2/R=57.6 W that is drawn from the AC generator 36 by the flyback converter 50. The flyback converter 50 is thus designed to operate at this operating point, at which the components in the flyback converter 50 will experience the maximum voltage and current levels, and DCM will be most difficult to accomplish.
As illustrated in
A switching frequency fs=100 kHz was chosen for the flyback converter 506. At a power level of 57.6 W, the peak energy stored in the flyback converter 50 is Epk=P/fs=576 μJ. The peak current seen in the primary winding is therefore:
The peak magnetic flux density in the core is therefore given by:
well within the saturation flux density of the core material. For an input voltage of 24V, the duty cycle which achieves this peak current is:
In order to ensure that the flyback converter 50 stays in discontinuous current conduction mode, the energy stored in the flyback converter 50 is completely discharged through the secondary winding before the switching period ends. As the minimum possible output voltage of the converter is 3.3V, the number of secondary turns that will guarantee discontinuous current conduction over all operating points is given by:
As this number of turns is low, it was achieved by wrapping three separate coils of 18 AWG wire at three turns apiece, and connecting them in parallel. This design also mitigates the skin effect and its adverse impact on winding resistance in the secondary winding. The peak value of the secondary current is therefore:
In the approach described above, a fixed duty cycle will cause the flyback converter 50 to appear as a resistive load to the AC generator 36, and hence the power drawn from the AC generator 36 will be proportional to the square of the generator voltage. However, this approach should be modified if the ultracapacitor energy is at its maximum allowable voltage, and the battery charger circuit 56 and/or electronic device load 44 are drawing less power than that prescribed by these conditions.
This property is achieved in the circuit implementation of
In the embodiment of
SEPIC Converter Topology
A SEPIC (Single-Ended Primary Inductor Converter) topology also may be used in an exemplary embodiment of the flyback converter 50 to interface the AC generator 36 to the ultracapacitor energy storage system 52. An exemplary design of such a flyback converter is illustrated in
The circuitry associated with the PI regulator and the PWM generator is shown in
The current regulator and PWM generator circuitry of
Energy Storage, Power Loads
The power extracted by the SEPIC converter 70 of
Should the ultracapacitor energy storage system 100 be full, and the amount of power provided by the main SEPIC converter exceed that required by the battery charger 102 and electronic load, the 10 W load resistor 106 will be connected in parallel to the ultracapacitor voltage to absorb this excess power. This is achieved by connecting the load resistance when the ultracapacitor voltage exceeds 25V.
In addition to its ability to emulate a resistive load, another important feature of the flyback converter 50 in the embodiment of
Another feature of the embodiment of
Pem=ΛPMIωr,
where ΛPM is the flux-linkage generated by the permanent magnets, I is the armature current, and ωr is the angular velocity of the rotor. If it is assumed that mechanical losses in the machine can be modeled by a constant torque τloss and that electrical losses are limited to I2R winding losses, the efficiency of the generator can be written as follows:
The current drawn from the generator that maximizes the generator efficiency can therefore be determined using basic optimization theory, and is as follows:
Hence, the optimal current drawn from the generator can be a complex function of rotor speed. When optimizing efficiency, however, it should be noted that the efficiency of the power electronic circuitry should also be taken into account.
It should be noted that the optimal load is not necessarily the load which maximizes efficiency. The mechanical power drawn by the generator results in a mechanical damping on the backpack's spring-mass system. If the amount of damping is too low, a suboptimal amount of power is harvested. However, if the amount of damping is too high, the system will be overdamped, which will limit the excursion of the backpack and hence also result in a suboptimal amount of power harvested. It can therefore be shown that an optimal amount of damping exists which will maximize the amount of power harvested. Furthermore, it is important that the effects of the AC generator 36 not have an undesirable ergonomic effect on the backpack wearer. The optimal load that is to be emulated is therefore determined through a weighted optimization of the amount of mechanical power withdrawn from the backpack, the combined efficiency of the AC generator 36 and the power electronic circuitry 40, and the ergonomic effects on the wearer.
In addition to determining this optimal load, the converter design may be optimized to maximize its efficiency through, for example, synchronous rectification. The diodes of the rectifier circuit 46 and DC-DC converter 54 of
The AC generator 36 exhibits its best efficiency when it is connected to a load whose power draw increases with generator voltage, such as a resistive load. However, the efficiency of the flyback converter circuit 50 may not be maximized under these conditions. Typically, power electronics can best be designed to perform optimally when the operating range is somewhat limited. By placing an appropriately sized capacitance between the AC generator 36 and the energy harvesting circuit, a compromise can be achieved between the desired operation of the AC generator 36 and the flyback converter 50 so that overall efficiency is maximized. The capacitor will limit the voltage excursions at the input of the power electronics, allowing them to operate more efficiently.
The energy harvesting circuit of
The inventors have found that, when run on an actuator, the backpack generates differing levels of mechanical power depending on the stiffness of the spring 26. One approach would be to simply adjust the spring constant as noted above so that one maximizes the amount of power generated. This reasonable approach, however, could be at the expense of the ergonomics and physiology of the wearer. The inventors have found that by using an elastic coupling between the backpack frame and the load, one can change the timing and magnitude of forces from the load onto the body. This has major consequences for joint injury and metabolic cost. A key factor is the compliance of the elastic structure. For example, with the springs in the electricity-generating backpack described by Rome et al. in Science (2005), the accelerative forces were reduced by 30%, whereas by using a much more compliant coupling (bungee cords) in the ergonomic backpack described by Rome et al. in Nature (2006), there was a much larger effect-a reduction by some 82% during walking. This not only makes walking with the ergonomic backpack more comfortable, but actually reduces the metabolic cost, permitting the wearer to carry 12 lbs of extra weight for the same metabolic cost. This compliance also permits individuals to actually run comfortably with the ergonomic backpack without injury.
Another embodiment of the backpack may take advantage of the greater ergonomic benefits of the bungee backpack described in related U.S. patent application Ser. No. 11/957,222, and be able to field convert it to an electricity generating backpack. The bungee backpack described therein obtains its superior ergonomic benefits by having very long bungee cord giving a very compliant coupling. By bypassing the pulleys as illustrated in
II Additional Electricity Generating Elements for Backpack
E-MOD
The backpack described herein may be further modified to include an Electric Generation Module (E-MOD), which is a wearable human-driven electricity-generation device 130 that may generate from 200 mW up to more than 1 Watt of electricity by utilizing the rotation of the leg with respect to the hip (or the knee) to generate electricity. This approximately 45° rotary movement of the leg (
The E-Mod 130 is particularly effective when it is integrated with a very stiff belt 132 and held on by a belt clip 134 that can withstand the torque generated by a wand on the leg without rotating the shell of the generator. Certain backpack belts and military belts are particularly stiff, preventing rotation.
In an exemplary embodiment, the energy conversion system of the E-Mod 130 is located at the hip as shown in
Specially designed clothing such as trousers (
Although the hip is the preferred location, another possible location for the generating device is the knee, as illustrated in
Different types of energy conversion systems can be used. One method is using a geared electromagnetic generator (or motor) 130 to generate the electricity. The motor could be either in-line with the axle axis or at right-angles to it as shown in
The devices illustrated in
Power Generation when not Walking
If one depends on the electricity generation capabilities of the backpack when not on the electric grid, then the wearer must have the option of generating electricity when he/she is not actually walking. For instance, rain, wind, or darkness can prevent the wearer from walking around. If he/she is critically dependent on electricity to, for example, keep vaccines cool, provide communications, or GPS capabilities, then there must be an alternate way to generate electricity.
In a first embodiment, the electricity generation of the backpack can be used by holding the backpack vertically and using one's hands to rhythmically push the pack body down with respect to the frame while letting the spring return the load to its appropriate position and then push down again. No additional equipment is required in this embodiment.
In a second embodiment, one can use the stronger leg muscles to achieve the same result by having the backpack lay horizontally with its frame down and sitting with his/her feet against the pack body. The load could then be pushed by the feet in a rhythmic way so as to generate electricity. Of course, the frame must be prevented from moving, which can be achieved by lodging it against a tree, using tent stakes, or using a harness 112 and foot pads 113 that accommodate the bottom and feet of the user to keep the frame from moving away (
If a larger level of power is needed, then the linear motion of the legs can be converted to rotary motion of the generator by attaching a lightweight bicycle ergometer 110 to the frame of the backpack as illustrated in
III. Power Uses for the Electricity Generating Backpack
Providing Power in Remote Areas
As noted above, the power generating backpack of the invention may be used to generate power for operating electronic devices of all types. Alternatively, the device of the invention may be used to generate small levels of electricity for use in remote villages in developing countries. In a report for the United Nations entitled “New Village—“Leapfrogging the Grid” on a Micro Scale,” the author's thesis is that new portable devices, coupled with a micropower sources such as the backpack, can result in dramatic benefits with only small financial investments. For example, a scourge in remote villages is the contaminated drinking water. SteriPen is a low-power portable UV light device that can kill the microbes in a 0.5 liter of water using only 225 Joules of electricity (5 W for 45 s). The backpack of the invention may be used to power such devices as well as communication devices (cell/satellite phones) that can be used in case of natural disaster or medical emergency to summon help. Furthermore, low-power medical instrumentation can permit the ability to conduct rudimentary medical tests in the village.
Another potentially extremely important use of the electricity-generating backpack of the invention is to provide electricity for powering a small refrigeration system to keep vaccines and medicines cool while being delivered in remote areas. It is difficult to deliver and dispense vaccines (and certain medications) to remote areas, as these must remain refrigerated to retain their potency. As will be explained below, a small refrigeration system may be integrated into the electricity-generating backpack to keep vaccines and medications cool during delivery. In the embodiments described below, it was assumed that the cooling system must be able to maintain a temperature of 4° C., regardless of the external environment. Secondary considerations are the minimization of power consumption and weight of the cooling mechanism. As will be described below, several different approaches for cooling medicine using the electricity generated by the energy harvesting backpack of the invention may be used, including:
It is noted that the vapor-compression refrigeration technique is widely used in domestic and commercial refrigerators. This technique is very mature and relatively inexpensive. However, vapor-compression refrigeration uses refrigerants which are typically undesirable due to their negative environmental impact. The weight of the compressor is also a disadvantage. In addition, the efficiency of this approach is not as high as that of the Stirling engine cooler. Therefore, the vapor-compression refrigeration technique is not considered as a potential feasible solution for the self-sustainable medical cooling system.
Thermoelectric Cooling
Thermoelectric cooling uses the Peltier effect to create a heat flux between the junctions of two different types of material. Heat is then transferred from one junction to the other by the application of a voltage, and hence current, between the junctions. As a result, electrical energy is consumed by the device. Peltier devices have no moving parts, are maintenance free, and are relatively light. However, their overall cooling efficiency is generally only around 5-10% of that of the ideal Carnot cycle. This is very low compared with the 40-60% that can be achieved by conventional vapor-compression cooling systems.
When a current I is made to flow through the circuit, as shown in
{dot over (Q)}=ΠABI=(ΠB−ΠA)I,
where ΠAB is the Peltier coefficient of the device, and ΠA and ΠB are the coefficients of each junction material. Therefore, by controlling the current through the Peltier device, the heat absorbed by the lower junction per unit time can be controlled. As long as this value is larger than the heat transferred into the lower junction from the environment, the temperature of the lower junction can be lowered with respect to the higher junction. The lower junction is put in a confined space (e.g., the internal space of a cooler), and the desired temperature is the internal space of the cooler (i.e., not the lower junction of the Peltier device). Therefore, the performance of a thermoelectric cooler is a function of ambient temperature, the insulation capabilities of the cooler, the Peltier module geometry, and the electrical parameters of the Peltier device.
In an exemplary embodiment, the S28™ Thermoelectric Cooler by Smartparts, Inc. was used as the Peltier device. This cooler was chosen as it had the lowest power consumption of thermoelectric coolers that were commercially available. The rated parameters of this cooler are provided in Table 1 below.
During testing of the Peltier device, it was discovered that the internal temperature of the cooler rises more quickly when the device is off than it falls when the device is on. As a result, one conjectured approach, involving operating the Peltier device at a duty cycle to reduce power consumption, would not be very effective. These results also suggest that the thermoelectric cooler would not be a particularly effective cooling approach with the energy harvesting backpack of the invention in that the thermoelectric cooler was not able to achieve the desired temperature (4° C.) even under relatively benign ambient conditions. Furthermore, the power required by the device exceeded by far the power currently available from the energy harvesting backpack.
Evaporative Cooling
Evaporative cooling was also considered as one of the potential candidates for the self-sustainable medical cooler because of its high efficiency and simple operation mechanism. Evaporative cooling is a physical phenomenon in which the evaporation of a liquid (here water is used), typically into surrounding air, cools an object or a liquid in contact with it. There are different evaporative cooling methods, such as direct evaporative cooling, indirect evaporative cooling, indirect/direct evaporative cooling, and indirect/indirect evaporative cooling. The simplest one is direct evaporative cooling, in which case outside air is blown through a water-saturated medium (usually cellulose) and cooled by evaporation. The cooled air is circulated by a blower. Direct evaporative cooling adds moisture to the air stream until the air stream is close to saturation.
For direct evaporative cooling, the wet-bulb temperature (WBT) stays the same, while the dry bulb temperature (DBT) is reduced. For a starting dry-bulb temperature (DBT_start) and wet-bulb temperature (WBT_start):
temperature drop achievable(TDA)=(DBT_start−WBT_start)*(efficiency of the media), achievable temperature=DBT_start−TDA=DBT_final
A direct evaporative cooler was built and tested using a CELdek® pad as shown in
The air flow through the cooling media is implemented through a DC fan whose input voltage is 12 V, input power is 1.44 W, and maximum air flow is 63.5 cubic feet per minute in an exemplary embodiment. In the case of an evaporative cooler built as a confined air system as shown in
It was found in this system that the desiccant does not dry the air quickly enough, which makes the relative humidity of the air cycled in the cooling system very high. This means that the difference between the dry-bulb temperature and the wet-bulb temperature is very small, and so the temperature drop will be very small, which makes this structure not feasible.
In an alternate structure, the desiccant container 230 is omitted and the desiccant is put inside of the cooler 240.
In another alternate structure, the cooler is not sealed, which means that the air in the cooler can be exchanged freely with the ambient air. In this case, the working air is from the environment, which can be considered to have infinite capacity, and the dry-bulb and wet-bulb temperature can be considered as constant at one specific testing condition. Thus, the equations (2) and (3) can be used to calculate the temperature drop and achievable temperature.
which is very close to the actual measured final temperature.
In this example, the evaporative cooling technique is very efficient (with only 1.44 W input power, the achieved temperature drop is 8.2° C.) and simple. However, there are serious intrinsic disadvantages with this approach in the backpack application of the invention. From the above equations, it can be seen that the final temperature of the cooler is determined solely by the status of the working air (i.e., the initial dry-bulb temperature and the relative humidity). Even when the efficiency of the media is 100% and the initial relative humidity of the air is 0%, to achieve 4° C. final temperature the initial dry-bulb temperature has to be less than 17° C. When the ambient temperature is above 17° C., it is theoretically impossible to lower the air temperature to 4° C., no matter the rate of air flow of the fan, or how perfectly the dehumidification method can work. This was partially confirmed by changing the fan to a more powerful one, whose input power is 6.0 W. There was no improvement seen for the achievable temperature. Therefore, the evaporative cooling technique does not appear to have the ability to achieve the desired cooler temperature over the range of ambient conditions that can be expected, which eliminates its feasibility in the proposed application.
Stirling Engine Cooling
Unlike internal combustion engines, Stirling engines are closed-cycle, which means that the working fluid is permanently contained within the system. The Stirling engine has the potential to achieve the highest efficiency of any real heat engine, theoretically up to the full Carnot efficiency. When a motion is applied to the shaft, a temperature difference appears between the two reservoirs of the working fluid, and this makes the Stirling engine work as a refrigerator. This technology can be 70% more energy efficient and 40% more thermally efficient than thermoelectric or vapor compressor-based coolers.
In an exemplary embodiment, a Coleman® 26 Quart Stirling Power Cooler (Model # 5726-750) was used. Another existing Stirling cooler product, the Kodiak® AC/B5 Active-Cool Container, which appears to be based upon the same Stirling engine as the Coleman cooler, and targeted towards medical applications was also used. There are 5 different temperature settings available for the Coleman cooler, 10° C., 6° C., 3° C., −7° C., and −18° C. The target temperature of the cooler is 4° C., therefore 3° C. was chosen as the setting. The ratings of this cooler suggested that the power consumption of the cooler may be too high for the backpack application; however, power measurements revealed that the rated power consumption only occurred during the initial cool-down. Once the setting temperature is reached (in this case, 3° C.), a much lower power consumption is needed to keep the cooler at that temperature.
Of the three cooling techniques described herein, the Stirling cooler was felt to be the most appropriate choice for this application, due to its ability to control the cooler temperature independently of ambient temperature, and its relatively low power consumption when compared to thermoelectric and vapor-compression cooling. Of course, the other cooling techniques may be more appropriate for other applications. The weight and power consumption of the Stirling cooler could feasibly be used in conjunction with an energy harvesting backpack like that described above, but a smaller, lighter cooler with a lower power consumption would probably be desired.
The power consumption of the Coleman® Stirling cooler while it sustains the desired temperature is comparable to the output power capabilities of the energy harvesting backpack, depending upon total backpack mass, the speed of walking, and the ambient temperature. As the wearer would not likely be walking nonstop, however, an electrochemical battery, the bicycle ergometer 110 described above, and the like would likely be required to provide power when the backpack is not in motion, as well as to provide power when the cooler requirements exceed the backpack's generating capability. Should the average power of the backpack exceed the cooling requirements, such a system could be operated indefinitely. Otherwise, the backpack power would extend the operating time of the cooler, as opposed to a battery-powered-only system.
Those skilled in the art will also appreciate that numerous other modifications to the invention are possible within the scope of the invention. For example, the efficiency of the electricity generating mechanism makes it possible to generate electrical power on the order of several watts from a small backpack which would only carry 20-30 lbs. Thus, the backpack of the invention is not limited by size. Also, those skilled in the art will appreciate that the spring 26 can be replaced by short, relatively stiff bungee cords that bypass pulleys as described in U.S. patent application Ser. No. 11/957,222. Accordingly, the scope of the invention is not intended to be limited to the preferred embodiment described above, but only by the appended claims.
The present application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. Nos. 60/921,239, 60/921,240, and 60/921,241, filed Mar. 29, 2007, the contents of which are incorporated herein by reference.
This invention was made with government support under N00014-06-M-0309 awarded by the Office of Naval Research and under Grant No. 1R43-HD55110-01A1 awarded by the National Institutes of Health. The government has certain rights in the invention.
Number | Name | Date | Kind |
---|---|---|---|
3672306 | Dehne et al. | Jun 1972 | A |
3677357 | Baumgartner | Jul 1972 | A |
3760905 | Dower | Sep 1973 | A |
3914669 | Holtz | Oct 1975 | A |
3914670 | Holtz | Oct 1975 | A |
4194656 | Zufich | Mar 1980 | A |
4887751 | Lehman | Dec 1989 | A |
5136173 | Rynne | Aug 1992 | A |
5443247 | Polites et al. | Aug 1995 | A |
5503314 | Fiscus | Apr 1996 | A |
5552695 | Schwartz | Sep 1996 | A |
5564612 | Gregory | Oct 1996 | A |
5613665 | Lund | Mar 1997 | A |
5617984 | Fabel | Apr 1997 | A |
5628443 | Deutsch | May 1997 | A |
5682353 | Eitan et al. | Oct 1997 | A |
5696413 | Woodbridge et al. | Dec 1997 | A |
5762243 | McMaster et al. | Jun 1998 | A |
5769431 | Cordova | Jun 1998 | A |
5806740 | Carlson | Sep 1998 | A |
5818132 | Konotchick | Oct 1998 | A |
5890640 | Thompson | Apr 1999 | A |
5902073 | Eungard et al. | May 1999 | A |
5904282 | Gleason | May 1999 | A |
5955904 | Kawasaki | Sep 1999 | A |
5984157 | Swetish | Nov 1999 | A |
6020653 | Woodbridge et al. | Feb 2000 | A |
6179186 | Blanking | Jan 2001 | B1 |
6202907 | Higgins | Mar 2001 | B1 |
6276584 | McLachlan | Aug 2001 | B1 |
6351137 | Hariton | Feb 2002 | B1 |
6360534 | Denniss | Mar 2002 | B1 |
6423412 | Zhang et al. | Jul 2002 | B1 |
6545384 | Pelrine et al. | Apr 2003 | B1 |
6548993 | Rutyna et al. | Apr 2003 | B1 |
6600346 | Macaluso | Jul 2003 | B1 |
6607107 | Dexheimer | Aug 2003 | B2 |
6619523 | Duckworth | Sep 2003 | B1 |
6622483 | Denniss | Sep 2003 | B2 |
6626342 | Gleason | Sep 2003 | B1 |
6637631 | Lafoux et al. | Oct 2003 | B2 |
6646463 | Hariton | Nov 2003 | B1 |
6651853 | Higgins et al. | Nov 2003 | B2 |
6801027 | Hann et al. | Oct 2004 | B2 |
6802442 | Thompson | Oct 2004 | B1 |
6876135 | Pelrine et al. | Apr 2005 | B2 |
6982497 | Rome | Jan 2006 | B2 |
7046528 | Sankman et al. | May 2006 | B2 |
7131534 | Enes | Nov 2006 | B2 |
7155979 | Lasalandra et al. | Jan 2007 | B2 |
7212932 | Taylor | May 2007 | B1 |
7230838 | Xu | Jun 2007 | B2 |
7287677 | Reid | Oct 2007 | B2 |
7345407 | Tanner | Mar 2008 | B2 |
7361999 | Yeh | Apr 2008 | B2 |
7365953 | Biros et al. | Apr 2008 | B2 |
7391123 | Rome | Jun 2008 | B2 |
7461553 | Lasalandra et al. | Dec 2008 | B2 |
7638889 | Yeh | Dec 2009 | B2 |
7703562 | Kalik | Apr 2010 | B2 |
20010035723 | Pelrine et al. | Nov 2001 | A1 |
20020170932 | Higgins et al. | Nov 2002 | A1 |
20020171213 | Kim | Nov 2002 | A1 |
20020190699 | Mueller-Fiedler et al. | Dec 2002 | A1 |
20030025983 | Lasalandra et al. | Feb 2003 | A1 |
20030062723 | Mancl et al. | Apr 2003 | A1 |
20040183306 | Rome | Sep 2004 | A1 |
20060011689 | Reid | Jan 2006 | A1 |
20060046907 | Rastegar et al. | Mar 2006 | A1 |
20060192386 | Rome | Aug 2006 | A1 |
20070096469 | Yeh | May 2007 | A1 |
20070107492 | Lasalandra et al. | May 2007 | A1 |
20080185411 | Rome et al. | Aug 2008 | A1 |
20080203128 | Bass et al. | Aug 2008 | A1 |
20080245835 | Reid | Oct 2008 | A1 |
Number | Date | Country |
---|---|---|
2385481 | Nov 2002 | CA |
29615526 | Oct 1996 | DE |
815541 | Mar 1981 | SU |
1684237 | Oct 1991 | SU |
WO 0019862 | Apr 2000 | WO |
WO 2004082427 | Sep 2004 | WO |
WO 2007016781 | Feb 2007 | WO |
Number | Date | Country | |
---|---|---|---|
20090015022 A1 | Jan 2009 | US |
Number | Date | Country | |
---|---|---|---|
60921239 | Mar 2007 | US | |
60921240 | Mar 2007 | US | |
60921241 | Mar 2007 | US |