Method and Apparatus for Energy Harvesting and/or Generation, Storage, and Delivery

BACKGROUND OF THE INVENTION

1. Field of the Invention (Technical Field)

The present invention is related to miniature systems for harvesting, generating, storing, and delivering energy, and implementation thereof.

2. Background Art

Note that the following discussion refers to a number of publications and references. Discussion of such publications herein is given for more complete background of the scientific principles and is not to be construed as an admission that such publications are prior art for patentability determination purposes.

Wireless sensors and networks of wireless sensors will be used to 1) monitor the structural health of buildings, bridges and aircraft, etc.; 2) monitor the environment, such as in domestic and commercial buildings, and military and homeland security installations; and 3) control industrial processes for increased autonomy, as well as for other tasks. These systems will find use in factory automation, process and environmental control, security, medicine, and condition-based maintenance, as well as in defense applications and intelligence gathering. Widespread use of wireless sensors will improve safety, increase security, lower heating, ventilation and cooling (HVAC) costs, and increase manufacturing efficiency.

Such wireless sensor systems will typically: 1) require numerous individual devices (known as nodes or motes) to provide comprehensive monitoring capability; 2) be located in inaccessible places and 3) require long intervals between scheduled maintenance. Periodic maintenance, such as replacing batteries, would clearly increase operating costs (often to prohibitive levels), and could be inconvenient, at best, if it required interruption of a continuous process. For some remote, hostile, or inaccessible locations, any maintenance may be impossible to perform. In the immediate future, energy management and improved battery technologies may mitigate some of these issues, but in the long term there is clearly a need to develop an energy source that can last years with little or no maintenance.

Miniaturized turbines and micro-fuel cells have been proposed as means of meeting long term energy delivery needs for wireless devices. While these systems exploit the high energy density of hydrocarbon fuels, for example, these systems are inherently limited by the need for a means of storing and delivering a consistent fuel supply, as well as advanced thermal management to safely remove waste heat. These challenges can be overcome; however, the plumbing and additional system engineering (also known as the balance of plant) adds considerably to the overall size and complexity of such systems.

There are additional challenges with micro-fuel cells. Most types use hydrogen fuel, as protons diffuse through an electrolyte more easily than other ions. Storing molecular hydrogen presents significant scientific and engineering challenges, and so most systems use other fuels such as hydrocarbons, methanol or formic acid, or natural gas that must be reformed with steam at high temperature (600° C.) to yield hydrogen and CO. These reformers again add engineering complexity and require extensive insulation for both safety and efficient operation. Furthermore, reported data for micro-fuel cells indicate maximum peak power densities on the order of 50 mW/cm²but with a duration of less than 100 ms. These challenges ensure that combustion and micro-fuel cell power systems will be unable to meet the volumetric energy and power densities needed for severely volume-constrained applications.

Energy harvesters that garner ambient environmental energy (such as light, vibrations, etc.) and convert it to electrical energy are attractive solutions for wireless sensors as they do not need to be replaced, recharged or refueled. Of course, they do not function in the absence of ambient energy (analogous to solar cells at night), and so an energy harvesting power supply must be designed to include some kind of energy storage that can provide back-up power in such situations.

Storage of the energy generated may be accomplished using conventional capacitors, which have very limited energy storage capability (E=½CV², where the capacitance, C, is typically on the order of a few hundred microfarads, and V=3-5V). This approach leaves the system vulnerable to interruptions in the ambient energy source. Although batteries and or supercapacitors have been proposed as alternative storage devices, no design has effectively accommodated the low power available from small energy harvester devices. Conventional battery chargers, for example, will not operate at the low power levels delivered by energy harvesters, and, besides, even in they could, they would waste too much of the input energy. Further, no existing system discloses the use of optimum energy storage elements for different functions (e.g. back-up power, pulse power, etc.).

Finally, a major challenge that faces wireless sensor nodes is the asymmetry of the power demands of sensing, processing, communication and sleep functions—on the order of four orders of magnitude. Because communication functions draw relatively high power levels (typically from tens to a few hundred milliWatts), wireless sensor nodes are designed to communicate infrequently (for example, once a minute to once an hour), reverting to a low-power sleep state to extend battery life. In order to meet high power communications loads, the usual approach is to design a power source large enough to handle the highest power load. Unfortunately, energy harvesting devices and batteries typically have low power densities, and so power sources are typically oversized for most of the operating needs of the system.

There is, therefore, a need for a simple and compact system which combines energy generation via harvesting of ambient energy sources with energy storage to provide back-up power, and deliver high power pulses as needed.

SUMMARY OF THE INVENTION
Disclosure of the Invention

The present invention is a method for delivering power to a load, the method comprising the steps of collecting charge from an energy source, boosting the voltage of the collected charge with a charge pump, controllably charging a supercapacitor to a threshold voltage, controllably charging a battery from the supercapacitor, and delivering power from the supercapacitor to a load. The voltage of the collected charge is preferably less than half of the threshold voltage, and optionally approximately one fourth of the threshold voltage. The threshold voltage is preferably programmable. The method preferably further comprises the step of switching a second supercapacitor from being in parallel with the supercapacitor to being in series with the supercapacitor, which step preferably comprises delivering power at a delivery voltage, preferably approximately twice the threshold voltage, higher than the threshold voltage to the load or to charge the battery. The method preferably further comprising the step of switching at least two capacitors in the charge pump from being in parallel to being in series. The step of controllably charging a supercapacitor preferably comprises adjusting the rate of the switching step based on a parameter selected from the group consisting of the charging rate of the supercapacitor and the power level of the energy source. The method preferably further comprises the step of conditioning charge pump gate voltages, preferably comprising saturating one or more FETs in the charge pump. The method preferably further comprises the step of recharging the supercapacitor from the charge pump when power from the energy source is available, and preferably further comprises the step of recharging the supercapacitor from the battery when power from the energy source is unavailable.

The present invention is also an apparatus for delivering power to a load, the apparatus comprising at least one energy source, a charge pump for boosting a voltage of charge collected from the energy source, a control module, a battery, and one or more supercapacitors for charging the battery and delivering power to a load, wherein the control module controls charging of the supercapacitors to a threshold voltage by the charge pump. The voltage of the collected charge is preferably less than half of the threshold voltage, and optionally approximately one fourth of the threshold voltage. The control module is preferably programmable. The supercapacitors are preferably switched to being arranged in series for delivering power at a delivery voltage higher than the threshold voltage to the load or to charge the battery. The delivery voltage is optionally approximately twice the threshold voltage. The control module preferably adjusts the rate of switching capacitors in the charge pump based on a parameter selected from the group consisting of the charging rate of the supercapacitors and the power level of the energy source. The apparatus preferably further comprises a FET gate conditioner circuit for saturating one or more FETs in the charge pump. The supercapacitors are preferably recharged from the charge pump when power from the energy source is available, and preferably recharged from the battery when power from the energy source is unavailable or insufficient.

The present invention is also a system for providing energy to a load, the system comprising at least one energy source, one or more supercapacitors electrically connected to the energy source, and a battery electrically connected to the supercapacitors such that the battery is charged by the supercapacitors and not directly by the energy source. The battery preferably comprises a volumetric rechargeable Li polymer battery. The battery is preferably not charged by low current charging above its damage voltage threshold. The battery is preferably charged up to its maximum voltage. The system preferably comprises sufficient energy reserves to enable the load to remain in an “on” state during time periods when the energy source is expected not to be generating energy. The system preferably further comprises a first voltage step up stage comprising a low voltage oscillator and an AC charge pump stage for stepping up a voltage of charge from an energy source initially having a voltage less than approximately one volt and a second voltage step up stage for further stepping up a voltage of the higher voltage charge. The system preferably comprises a circuit system for automatically adjusting an input voltage for a high impedance energy source to half of an open circuit voltage of the energy source, independent of a frequency of the energy source, wherein the energy source preferably comprises a piezoelectric generator. The system preferably comprises a battery charging circuit for pulse charging the battery.

The present invention is also a method of providing power to a load, the method comprising the steps of charging one or more supercapacitors with energy from an energy source and charging a battery from the supercapacitors and not the energy source. The step of charging the battery preferably comprises not low current charging the battery above a damage voltage threshold, charging a battery up to its maximum voltage, and/or charging the battery using controlled pulses, in which case the pulsed charging is preferably performed at fixed voltage with current limiting. The method preferably further comprises the step of recharging the supercapacitors with the battery when energy from the energy source is absent or insufficient for powering the load. The method preferably further comprises the steps of stepping up the voltage of charge from an energy source initially having a voltage less than approximately one volt, transferring the higher voltage charge to a second voltage step up stage, and further stepping up the voltage of the charge. The method preferably further comprises the step of automatically adjusting an input voltage for a high impedance energy source to half of an open circuit voltage of the energy source, independent of a frequency of the energy source, wherein the energy source preferably comprises a piezoelectric generator. The load preferably remains in an “on” state during time periods when the energy source is expected not to be generating energy, and preferably enters a “sleep” state during time periods when the energy source does not generate energy for an unexpectedly long time.

An object of the present invention is to provide a system for harvesting, generating, storing, and delivering energy, particularly for remote applications.

An advantage of the present invention is that it can effectively store charge from very low power or low voltage energy sources while still providing power to a load at higher voltages.

Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a part of the specification, illustrate several embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating a preferred embodiment of the invention and are not to be construed as limiting the invention. In the drawings:

FIG. 1 is a schematic diagram of an embodiment of the present invention;

FIG. 2 is a comparison of battery discharge under i) pulsed load, ii) continuous averaged load, and iii) pulsed load with supercapacitor;

FIG. 3 is a block diagram showing control electronics;

FIG. 4(
a) is a graph showing solar cell output voltage under increasing pulse loads;

FIG. 4(
b) is a graph showing the output voltage from a solar cell and supercapacitor in parallel;

FIG. 4(
c) is a graph comparing voltage droop under load for a lithium battery and a lithium battery and supercapacitor in parallel;

FIG. 5 displays circuits for implementing the present invention;

FIG. 6 depicts Voltage Charge Pump and FET Gate Conditioner circuits of the present invention;

FIG. 7 shows a voltage output and doubling circuit of the present invention;

FIG. 8 is a circuit diagram showing a backup battery charging module;

FIG. 9 is a schematic of an embodiment of a low-power oscillator charge pump driver comprising an astable oscillator;

FIG. 10 is a schematic of an embodiment of a low-power oscillator charge pump driver comprising a transistor-driven oscillator;

FIG. 11 is a schematic of an embodiment of a low-power oscillator charge pump driver comprising a dual output amplified crystal oscillator;

FIG. 12 shows schematics of low power charge pump drive circuits and a schematic of a low power AC charge pump circuit of the present invention;

FIG. 13 is a schematic of another voltage measurement circuit according to the present invention;

FIGS. 14-15 are schematics depicting an additional stage of the FET Gate Conditioning controlled by an additional CPU port pin;

FIG. 16A is a schematic of a microcontroller VCC Supervisory Circuit;

FIG. 16B is a schematic of a voltage measurement circuit;

FIG. 17 shows schematics of FET Gate Conditioning and High Voltage Charge Pump circuits;

FIG. 18 is a schematic of a Voltage Output Doubling/Voltage Output Control circuit;

FIG. 19 shows schematics of Battery Recharge and SCap Recharge circuits;

FIG. 20 is a schematic of a Input Voltage Limit, Voltage Reference, and VCC Regulation circuit; and

FIG. 21 is a schematic of an optional Piezo Dynamic Loading Interface.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
Best Modes for Carrying Out the Invention

The system of the present invention is a compact long lifetime power source which will enable the implementation of many applications, including but not limited to wireless sensor networks, that will no longer require battery changes or replacement. The present invention preferably comprises several components including energy storage (including but not limited to batteries and supercapacitors), energy harvesting and/or generation sources (including but not limited to vibrational (piezoelectric, capacitive or inductive), thermal (thermoelectric), radioisotope (betavoltaic), solar (photovoltaic), fuel cells, microcombustion, or biochemical sources), and ultra-low power electronics for functions including but not limited to charging of energy storage devices, power conditioning, rectification, power management, monitoring state of charge, and/or voltage step-up. The present invention may be configured using different combinations of the above components depending on the intended application. The energy harvesting component will generate power from the ambient environment, which power is then preferably stored in the energy storage components. The ultra-low power electronics preferably control how this power is used or delivered. The present invention can be packaged in many common form factors (battery sizes such as AA, AAA, C, D, 9V, etc), as well as in custom sizes, and are preferably capable of plugging directly into a wireless system/component, just like a battery, in order to provide a long lifetime power source.

The present invention preferably combines energy harvesting, energy storage in (for example) batteries and supercapacitors, and ultra-low power electronics to condition the output from the harvester and carefully charge the storage devices. Batteries (or microbatteries) provide back-up energy to accommodate periods without ambient energy, while supercapacitors (or microsupercapacitors) provide excellent power delivery capabilities to handle high and/or pulsed loads. The electronics circuitry is preferably designed to draw only a very small portion of the harvested energy so that the remainder may be directed for storage, as described above, or for immediate use by the system it is powering. Furthermore, once it is fully charged, the battery can be switched out of the circuit to serve as a back-up source, and the energy generated by the harvester is then preferably used to power the application load (i.e., sensor) and accommodate any charge leakage in the supercapacitor.

Because it is harvesting energy from its environment, the present invention provides a long-lived energy source, far out-lasting conventional batteries. The system may be designed to have a custom form factor or to fit a standard battery configuration, including but not limited to AA, AAA, 9V, Li123, C, D, and others, so it can be used in systems already configured for these types of batteries with little to no modification. Moreover, as advanced packaging and integration technologies deliver smaller sensor nodes, the present invention is also expected to evolve through similar miniaturization strategies and will, therefore, be compatible with future generations of miniaturized sensors.

As used throughout the specification and claims, the term “battery” means battery, microbattery, thin film battery, and the like, whether planar or volumetric. As used throughout the specification and claims, the term “supercapacitor” means capacitor, supercapacitor, microsupercapacitor, microcapacitor, ultracapacitor, electric double layer capacitor (EDLC), and the like. As used throughout the specification and claims, the term “energy source” means a transducer, energy harvester or energy generator, such as vibrational (including piezoelectric, electrostatic and inductive methods), thermal (thermoelectric), solar (various forms of photovoltaics), radioisotope, chemical, biochemical, fuel cell, microturbine, adenosine triphosphate (ATP) converters (ATP is a biomolecule able to store and transport chemical energy within cells), magnetic, electromagnetic (or RF) induction (EMI), and microcombustion generators and the like.

FIG. 1 shows a broad schematic of the present system, which preferably comprises three sections: energy harvesting and/or generation (E/H), energy storage, and control electronics to rectify the input power (if necessary) and safely charge the storage devices. Energy generation and/or harvesting may be accomplished using a hardware component which is capable of converting ambient energy into electrical energy. The present invention may contain one or more units of the same type of energy harvesting hardware (for example, two vibration harvesters), or contain a combination of two or more pieces of hardware capable of different types of energy harvesting (for example one piece of hardware harvests vibrational energy while another piece of hardware harvests thermal energy). Using two or more energy harvesting components may provide higher levels of available power and/or may decrease the likelihood that the system will be subjected to periods of time with no available harvestable ambient energy.

The electrical energy from the harvester(s) is stored, initially, in a supercapacitor (or supercapacitors), then in a rechargeable battery. These two energy storage devices provide complementary features: supercapacitors deliver energy efficiently (high specific power), while batteries store energy efficiently and so provide back-up power when the harvester is not providing enough power. The supercapacitors in the system of the present invention are preferably used as the first stage energy storage component because they can be discharged and recharged more efficiently and more often than batteries, and are also preferably used for energy delivery because of their low impedance.

A low-power digital control module (DCM) preferably monitors the state of charge (SOC) of both the battery and supercapacitor(s), and simultaneously dynamically adjusts the operation of the charging module to accommodate any fluctuations in the level of energy delivered by the harvester(s). The Digital Control Module is preferably programmable and preferably comprises a microcontroller. As used throughout the specification and claims, the term “control module” means a DCM, an ultra-low power microcontroller, a low power Field Programmable Gate Array (FPGA), a low power microprocessor, programmable logic designed into an Application Specific Integrated Circuit (ASIC), and the like. The charging module in the present invention preferably comprises a charge pump, but could include other devices and architectures such as piezoelectric transformers, dc-dc converters and the like. Should the incoming energy not be sufficient to recharge the supercapacitors, the control module preferably switches in the battery to maintain charge on the supercapacitor. Because energy harvesters only produce very small amounts of power, this circuitry (the charge pump, in particular) preferably operates extremely efficiently to transfer as much of the available power as possible to the energy storage devices without wasting it in the charger.

The use of a control module imparts capabilities to the system that conventional power sources such as batteries and other energy harvesting power supplies do not provide. For example, in addition to monitoring the SOC and the input energy source, it can provide state of health monitoring for the whole system, as well as communication with the host sensor or application. For example, with appropriate sensor hardware, the control module may optionally detect a sharp temperature excursion or an unusual vibration pattern that may indicate a tamper event, optionally triggering power down. For sensors, communications with the host sensor mote could enable sophisticated system management capabilities not possible with other power solutions. For example, the complex interaction of SOC and input energy variables may be used to determine whether the sensor should enter power conservation or power down modes.

As discussed, energy storage is preferably accomplished using at least one secondary (rechargeable) battery (including but not limited to lithium-ion, lithium polymer, thin film lithium ion, nickel metal hydride (NiMH), and nickel/cadmium (NiCd)) preferably in combination with at least one supercapacitor (also known as an ultracapacitor or electrochemical double layer capacitor (EDLC), comprising, for example, aqueous or organic based electrolyte chemistries and symmetric or asymmetric types). The energy storage components of the present invention preferably store the energy generated by the harvester components for use at a later time. The different characteristics of batteries and supercapacitors make them suitable for different functions of energy storage and delivery. If the energy must be stored for a long time, and released slowly, for example as back-up in case there is no harvestable ambient energy (e.g., at night when the energy harvester is a solar cell), a battery would be the preferred energy storage device. If the energy must be delivered quickly, as in a pulse for a radio frequency (RF) communication module, but long term storage is not critical, use of a supercapacitor is preferable. The system can employ i) a battery (or several microbatteries) alone, ii) a supercapacitor (or supercapacitors) alone, or iii) any combination of batteries, microbatteries and supercapacitors appropriate for the application of interest.

Improvements in materials, construction, etc. for lithium ion and some lithium polymer batteries are resulting in devices that are capable of delivering pulsed loads as well as steady state loads. These devices may be capable of fulfilling both back up and pulse load delivery functions which are currently preferably accomplished using a combination of microbatteries and supercapacitors. However, as shown in FIG. 2, in order to maximize the life of the battery, separate devices preferably fulfill these functions. FIG. 2 shows pulsed load delivery from a CR2320 lithium coin cell; the pulses are 105 mW with a 1.5 second pulsewidth and 5 second repetition rate. The lifetime of the battery when used with a supercapacitor is greatly increased over that of a battery when used alone.

The present invention may use commercially available (off-the-shelf) batteries and supercapacitors or custom designed devices depending on the anticipated usage parameters of the system. Parameters that may influence energy storage selection include the power profile, availability and consistency of ambient energy and environmental conditions such as temperature and humidity. Moreover, some novel thin film batteries exist, both as research devices and commercial products. These devices tend to have high discharge rate (high power) capability, but their capacity values on a per unit area basis are low, meaning that large footprint devices are needed to meet useful energy storage capabilities. For certain applications, these devices may prove to be appropriate. One embodiment of the present invention may combine a volumetric battery for energy back-up with a thin film battery in place of the supercapacitor to provide pulse power.

FIG. 3 shows a more detailed block diagram of the device. The circuitry will operate for both AC (e.g., vibration, rotational) and DC (e.g., solar, thermal) sources. An incoming AC signal is rectified preferably using low forward-bias-voltage diodes to minimize losses. These may comprise silicon Schottky diodes, germanium p-n diodes or other specially engineered components. The control electronics circuitry primarily fulfills two functions: determining how the energy generated by the energy-harvesting component is to be directed (to storage and/or directly to the load) and safely and efficiently charging the energy storage devices without overloading, overcharging, or otherwise damaging them or the generator. It is preferred that the circuits consume very little of the incoming power. Small scale energy harvesters that are of interest for use in wireless sensors and other microsystems generally deliver low levels of power (typically on the order of a few hundred microWatts). It is important that the charging and power management circuitry delivers as much of that power as possible to the energy storage or the sensor device rather than consuming it in its own operation.

The circuitry preferably comprises a charge-switching module for charging a supercapacitor energy storage device and a battery charging circuit, controlled by the control module. Energy for operation of the control module is preferably stored in an intermediate energy storage device, for example a conventional capacitor (typically a few μF to a few hundred μF). The charge switching module preferably prevents irreversible damage to the supercapacitors (for example C19 and C21 in FIG. 7) by carefully controlling their charging in order to avoid overcharging them past a desired threshold voltage, preferably approximately 3.3V. During charging, supercapacitor C21 optionally provides low-voltage power to the load, or alternatively the control module may optionally switch output power off until the supercapacitors reach the desired charging threshold.

Connecting a supercapacitor directly to an energy source delivering voltage V will draw a current I equal to V/ESR, where ESR is the equivalent series resistance of the supercapacitor (typically a few Ohms, and ideally <<1Ω). As a low ESR is a desirable characteristic of supercapacitors for power delivery, the supercapacitor would draw a high instantaneous current, overloading the energy source. As the system of the present invention may sit on the shelf for some time before being deployed, it is not possible to store the supercapacitor in the charged state, and the charge-switching module preferably controls the current draw in order to prevent damaging the generator on initial deployment.

Once they are fully charged, the supercapacitors then preferably serve as an energy “cache” for charging the battery. This keeps the average current draw very low. Before charging the battery, the parallel supercapacitors are preferably intermittently switched in series to increase the voltage. Then the battery, for example a Li-Ion battery, is charged, preferably in two stages. First, a constant current charge increases the battery voltage until a target voltage, preferably monitored by the control module, is reached. This stage usually delivers about 70% of the total capacity of the battery. The battery is preferably then pulse charged at a constant voltage. Charging is typically terminated once the current level falls below 10% of the initial charging current.

By employing a control module that operates at extremely low power, various functions or conditions of the present system can be periodically monitored both during the charging cycles and during operation, including but not limited to the state of charge of both the battery and supercapacitor and whether or not the generator is producing power. Once the battery is fully charged it may be removed from the circuit so that the present invention provides power to the sensor or other load via the energy source and supercapacitor(s) only. If the control module detects that the energy cell has ceased to provide power (e.g. during periods of darkness in the case of a solar cell, or in the case that ambient vibrations cease), then the control module preferably switches in the battery to provide back-up power. In this case, the battery preferably charges or trickle charges the supercapacitor and provides quiescent power to the sensor until the ambient energy source is restored. The battery is preferably designed to be able to store sufficient energy to provide back-up power for the entire time the system is anticipated to be without ambient energy. This time would vary for different applications, but may range from a few hours to several days or weeks. In the event that the stored energy in the battery drops below a predetermined level, a low battery condition is preferably communicated to the sensor, allowing the sensor to shut down or go into a power conservation mode, for example.

Finally, current is preferably delivered to the load through the supercapacitor which can supply both steady state and high power pulse current, depending on the demand from the load. For steady state operation, the energy source (or the battery if there is no energy from the generator or harvester) preferably delivers energy to charge or trickle charge the supercapacitor, and compensates for the energy delivered to the load or lost through the supercapacitor's internal leakage. As discussed previously, the supercapacitors' low impedance makes them well suited for delivering high power bursts when needed by the load.

Thus, the present invention preferably utilizes one or more supercapacitors as primary energy storage devices, and one or more batteries as secondary energy storage devices, which are preferably used to recharge the supercapacitor(s). The primary energy storage device preferably provides a higher power output than the secondary storage device, while the secondary storage device provides a greater total energy storage than the primary device. That is, the load is preferably powered only from the supercapacitor(s), and not from the battery. The supercapacitors can deliver both pulsed and steady state power, depending on the requirements of the load, and have higher power delivery and lower Equivalent Series Resistance than the battery, thereby increasing efficiency (especially for pulsed power applications). In the present invention, the battery (secondary storage device) is used to charge the supercapacitor(s) (primary storage device). This configuration also permits the use of supercapacitors with reduced capacitances, and thus reduced leakages, enabling charging from low power energy sources.

Preferred features, advantages and benefits of the present invention, listed in Table 1, address concerns that manufacturers and end users of wireless sensors and other applications have identified as potential barriers to adoption of this kind of technology. For example, while not having to change batteries periodically is desirable for many wireless sensor applications, the vulnerability of energy harvesting to a lack of ambient energy (e.g., darkness for solar cells) is also concern for many users. The present invention addresses these concerns at low cost.

TABLE 1

Feature
Advantage
Benefit

Harvests ambient energy
No need to change batteries, recharge, or refuel
Reduced system maintenance

costs

Provides steady state,
Enables cold start, offers redundancy in the
Reduces system vulnerability and

pulse and back-up power
case of ambient energy failure, and efficiently
system size

delivers pulse power

Small form factor
Can be designed into standard or custom
Can be plugged in to customers'

shapes
existing products

Flexible source of input
Similar products can be used in different
Flexible, broad product line that

energy
applications with little reengineering
can be used in different

environments - e.g., industrial,

commercial, and residential

As discussed above, the present invention preferably utilizes a rechargeable volumetric battery to greatly increase the reserve energy. The present invention preferably uses a standard Li polymer package, such as the 20 mm×25 mm×4 mm (2000 mm³) PowerStream GM042025, which has a capacity of 150 mAhr or about 2 kJ. In contrast, many existing systems use a thin film battery, which typically utilize solid state electrolytes. One such battery is the Infinite Power Solutions MEC101, which is 25.4 mm×25.4 mm×0.17 mm (110 mm³), with a capacity of one mAhr (14 J). Thus, on a volumetric basis, conventional Li polymer cells, which have a smaller surface footprint, are nearly eight times more energy dense than a thin film battery (1 J/mm³vs. 0.127 J/mm³). So, if we compare surface footprints of the two batteries, using Li polymer cells will provide more than 110 times the back-up endurance as a thin film battery. Complex sensors, such as mesh networked wireless sensors or sensors for vibration, fluid flow or other time dependent sensing, often require over two orders of magnitude more power than low power sensors during operation. For such higher power sensors, the larger storage capacity of Li polymer cells can mean the difference between being able to continuously power the sensor overnight rather than requiring it to shut down after an hour or two (for example in a system using a solar energy source so that no energy is harvested overnight). In addition, Li polymer cells are orders of magnitude less expensive.

Thus it is preferable that the battery preferably comprises an energy density of greater than 0.127 J/mm³, more preferably greater than 0.2 J/mm³, and even more preferably greater than 0.5 J/mm³.

However, Li polymer cells have a disadvantage that other types of batteries (such as solid state thin film, Ni-chemistry, or lead-acid) don't have: they cannot be charged at low currents all the way up to their maximum voltage (typically 4.2 V). Any low current charging above 4.05 V causes plating of metallic lithium on the anode that can lead to instabilities and compromise safety. Specifically, dendritic lithium plated during low current charging can lead to the creation of an internal short circuit, localized heating, and, potentially, to combustion or explosion of the battery. Thus, for Li polymer cells, low current charging must be limited to below 4 V (a practical limit) to prevent the battery from undergoing irreversible structural changes. This limit doesn't enable the full use the entire capacity of the battery. As used throughout the specification and claims, the term “low current charging” means charging using a current rate less than approximately 10% of the current rate that the fully charged battery would produce if it were discharged in one hour. As used throughout the specification and claims, “damage voltage threshold” means the following: a battery charged using low current charging to a voltage above its “damage voltage threshold”, which is less than the battery's maximum voltage, would suffer irreversible structural changes or other irreversible damage.

Therefore the preferred battery of the present invention is preferably not charged directly from an energy source, which due to its low current output would either prevent the battery from being fully charged (if the battery is charged to less than the damage voltage threshold), or cause damage to the battery (if the battery is charged to more than the damage voltage threshold). Instead, the battery is preferably charged from one or more supercapacitors as or after the supercapacitor(s) are charged from the energy source. The use of supercapacitors ensures that the battery can be fully charged above to its maximum voltage without damage to the battery. The supercapacitor(s) may in return be charged by the battery when the power from the energy source is unavailable or insufficient to maintain a constant output voltage.

Because of the large energy reserves of the present system, the power the system provides to a load such as a sensor can always be left “on”. In contrast, other systems require the power to the load to be turned off by switching the load out of the circuit (i.e. disconnecting the load) when there is insufficient harvested energy to power the load, and switching the load back into the circuit (i.e. turning on the power supply output) only when there is enough energy available. Being “always on” enables a sensor, for example, to avoid rebooting every time it needs to take a measurement, allows sensors to maintain a mesh network wherein they all talk to their neighbors to hop data packets to the gateway, and avoids missing an event of interest. In the rare instance where the present system cannot provide sufficient power to a load, the sensor can transition to a “sleep” mode while remaining connected to the present system, and “awaken” when the power to the load is sufficient. Alternatively the load can be switched out of the circuit until sufficient power is stored.

Circuit Diagrams

Electronics of the present invention preferably convert ultra-low power from environmental sources into higher voltage, high power output power with smart, programmable control. The present invention preferably accepts either AC or DC power input with voltages as low as approximately 1.5V, and current as low as approximately 5-15 μA, and preferably employ dynamic self-modification of the conversion control to efficiently convert and store the input energy in high energy density supercapacitors and backup batteries. The voltage regulation of the present invention is preferably not dependent on the maximum voltage supplied by the energy source used, and is preferably programmable to provide steady state DC output voltages up to 5 VDC or greater with current output many orders of magnitude greater than that of the input after storage. The present invention is preferably programmable to provide any desired voltage (not only one or two discrete voltages), either at the time of delivery or in the field. Thus the same system can be used for a wide range of loads and energy storage elements suitable for use in different applications.

As shown in FIG. 5, the present invention preferably accepts and rectifies AC or DC current input into an initial charge storage capacitor. The present device also preferably uses a charge pump with dynamically self-modifying switch rates to very efficiently transfer the charge into one, two, or more high-energy-density capacitors (or supercapacitors) at up to about four times the highest input voltage. The dynamic switch rate of the charge pump is preferably controlled by the control module. The control module preferably measures the relevant voltages and controls the switching rate of the charge pump, the final voltage level of the stored energy, and the charging of the backup battery module.

The control module preferably dynamically controls the operation of the charge pump to efficiently store very low power energy from the energy source(s). For dynamic control of the charge pump switch rate, the control module preferably periodically measures the supercapacitor voltage and computes the rate-of-change of the voltage. If the rate is above a specified level, the switch rate is preferably kept sufficiently high to pump as much charge as is available. If the voltage rate-of-change falls below that level, the switch rate is preferably correspondingly decreased so that the charge pump supercapacitors can maintain 90% of the highest voltage level prior to switching. As the supercapacitor voltage increases, the potential differences in the charge pump decrease, and so the control module preferably further decreases the switch rate in order to accumulate as much charge as possible in the switching capacitors prior to the switch cycle. Also, if the current available at the source increases, the control module preferably senses this as the voltage storage rate increases, and preferably increases the switch rate dynamically as before, in order to take advantage of the greater available quantity of charge flow.

If more than one supercapacitor is used, the supercapacitors, when charged to the programmed final voltage level, may optionally be switched in series to provide twice the voltage for subsequent backup battery charging via the battery charging circuit, or for higher voltage output. Power output (on or off) and battery charging is preferably controlled by the control module. When input power is unavailable, the control module may be programmed to turn off the output or recharge the supercapacitors from the backup battery module for continued operation.

In general, the charge pump of the present invention stores low level charge obtained from the energy source(s) and boosts the voltage, enabling the supercapacitor to charge at the higher voltage level. In contrast, the charging voltage of many existing systems is limited to the output voltage of the energy source, whatever that may be. For low power energy sources, for example a solar cell exposed only to low light, this voltage may not be sufficient to charge the supercapacitor to its threshold voltage (for example due to leakage of the supercapacitor). Thus these other systems simply won't work with low voltage or low power energy sources. The charge pump preferably incorporates intermediate charge storage for increased efficiency, and preferably does not utilize resistors for signal control or limiting. Other designs as described in the prior art use resistors or inductors for voltage control between the energy source(s) and the energy storage components, resulting in limited current flow and therefore lower efficiency, or use no voltage control at all. In addition, it is preferable that the charge pump comprises MOSFETs, which require voltage, not current, for switching.

The embodiment of the high voltage capacitive charge pump pictured in FIG. 6 preferably follows an input voltage rectifier. The operation of the charge pump is preferably dynamically controlled by the control module (FIG. 5), based on the voltage level of the supercapacitors C19 and C21 measured by the control module.

The capacitors C6, C8, C10, C11 in the charge pump preferably have a low capacitive value and low leakage, allowing the system to scavenge low charge levels presented by both high impedance, low power environmental generators such as piezoelectric vibration sources, as well as low impedance higher available charge generators such as photovoltaic cells under a high intensity source. In order to present the supercapacitors with a high charging voltage, the flux of current from the charge pump capacitors during switching is preferably controlled by the switching rate, which is preferably dynamically changed with time by the control module. For example, when little charge is available from the generators, the switch rate is preferably decreased. When there is ample charge available, the switch capacitors achieve their final value very quickly, and the switch rate is preferably increased. This dynamic control allows the system to take advantage of very low levels of ambient energy while maintaining a high charge voltage into the supercapacitors.

If desired, the storage capacitors C6, C8, C10 and C11 in the charge pump are switched from parallel to series preferably by using low resistance, low gate-source voltage MOSFETS. To increase the efficiency of the transistors, the gate signals are preferably conditioned by ultra-low current op-amps U15 and U16, thereby increasing transistor saturation and decreasing the channel resistance. When the control module determines that the desired system voltage level has been obtained, or conversely that no energy is available at the input, switching of the charge pump is preferably halted to conserve power.

In greater detail, an embodiment of the Voltage Charge Pump circuit of the present invention, shown in FIG. 6, is preferably operated by switching off FET Q45 and switching on FETs Q36 and Q37, thereby switching capacitors C8 and C6 from parallel into series and providing a current flow path to charge capacitors C11 and C10 in parallel with FET Q46 on and FETs Q39 and Q38 off. The gates of Q36, Q45 and Q37 are preferably exactly out of phase with the gates of Q39, Q46, and Q38.

After the charge is transferred into C11 and C10 at the higher voltage, the process is preferably reversed, with FETs Q36 and Q37 turning off and Q45 turning on so that capacitors C8 and C6 are recharged in parallel, while simultaneously FET Q46 is turned off and Q39 and Q38 are turned on, thereby switching capacitors C11 and C10 from parallel into series and providing a current flow path discharging C11 and C10 into the supercapacitors through diode D27 at the higher voltage. Diodes D22 and D23 block the back-flow of charge from capacitors C6 when in series with C8, and C10 when in series with C11. Diode D21 blocks the back-flow of charge from capacitors C8 and C6.

An FET Gate Conditioner circuit is also depicted in FIG. 6. Saturation of the charge pump switching FETs is typically only possible with gate-source voltage levels that go as high as the highest levels seen by the sources of the respective transistors. This is preferably accomplished with gate voltage conditioning provided by the FET Gate Conditioner circuit. This circuit preferably takes the digital voltage switching signals provided by the control module, utilizing ultra-low-power operational amplifiers in the saturated or comparator mode, or alternatively transistors in a totem-pole configuration, to swing the signals to the positive rail provided by either (1) VCC when the supercapacitors are at low voltage and charging, or (2) the supercapacitor voltage when it exceeds VCC. This higher gate-source voltage differential preferably saturates the FETs in the charge pump when switching, allowing for the low channel impedances necessary for highly efficient switching with very low current drain. This action preferably allows the charge pump to operate on very small quiescent current, enabling the pump to charge to higher voltages with less input power.

When necessary, the control module may switch the energy storage supercapacitors into series, at least doubling the output voltage at high power levels. This action can drive higher voltage loads, or provide the higher voltage necessary for backup battery charging. In this case, a Voltage Output and Doubling Circuit, shown in FIG. 7, is preferably employed. The signal preferably drives low the gates of FETs Q32 and Q43, thereby switching the capacitors C21 and C19 into series. Diode D28 blocks the back-flow of current from C19 to C21. An output signal from the control module preferably drives high the gate of FET Q44, pulling low the gate of FET Q50, turning on the output of power from C19 to the output pin.

FIG. 8 depicts an embodiment of a Backup Battery Charging Module circuit, which preferably provides the voltages and currents necessary to charge the backup battery module as described above. Unlike other implementations of charging circuits, which often use power hungry transformers, this circuit preferably comprises low power op-amps.

Low Power Charge Pump Driver Circuits

FIG. 9 is a schematic of an embodiment of a low-power oscillator charge pump driver comprising an astable oscillator wherein the transistors Q54 and Q55 are preferably very-low gate-source threshold FET transistors, where the Gate-Source threshold voltages may be specified as low as 0V. As the voltage at Vsc1 causes the voltage at the gate of Q55 to exceed threshold, Q55 is turned on and pulls the voltage at the R5-C16 node down to Vds(sat). As Q55 turns off, C16 recharges past the threshold voltage at the gate of Q54, turning it on, and pulling low the voltage at R6-C17. This embodiment preferably works with very low AC input voltages. The system is preferably kept from achieving steady-state equilibrium by the phase offsets caused by C16 and C17, provided the input voltages are greater than Vgs(threshold)(Q54 or Q55)+Vdrop(R+C(ω)). The frequency is preferably limited by the complex impedance of the capacitors and the switching speed of the FET gates.

FIG. 10 is a schematic of another embodiment of a low-power oscillator charge pump driver comprising an LC pair oscillator, where the LC pairs C6-L1 and C7-L2 oscillate and are driven by the transistor Q58. The inductor and capacitor in each pair each preferably store energy out of phase with the other. As the C6-C7 node is pulled high and low, the base of Q58 is driven above, and below, the threshold voltage. This action causes Q58 to turn on and off, in turn causing current to increase through L1 to the Q58 collector, through L2 from the Q58 emitter, and then decrease through both. The reverse emf due to the current differential through the inductors L1 and L2 preferably creates the necessary dV/dt for each capacitor. As the Q58 collector voltage drops, it provides the base drive for transistor Q59, which provides the output drive. The input perturbation is preferably stabilized by the RC pair C9-R1.

FIG. 11 is a schematic of yet another embodiment of a low-power oscillator charge pump driver in a standard dual output crystal oscillator configuration, wherein the transistor Q56 provides amplification, and Q57 provides an additional inverted output.

These charge pump driver circuits preferably control the charge rate dynamically with a digital processor, based on the rate of change of the accumulated charge. The voltage level of the accumulated charge is preferably programmable with the digital processor. If no charge is available for charging the supercapacitors from the ambient environment, the digital processor preferably stops the charging process and sleeps, thereby conserving the stored energy of the system, and will then operate on the backup battery. The digital processor preferably replenishes the stored charge in the supercapacitors to a sufficient operating level from the lithium ion backup battery if no charge is available for charging the supercapacitors from the ambient environment. If the supercapacitors are charged to their maximum programmed level, and charge is available from the ambient environment, the digital processor preferably charges the lithium ion backup battery. The digital processor preferably charges the backup battery to a programmed voltage level during the charging process. The digital processor preferably senses the voltage levels of the supercapacitors and backup battery and determines the state of the system.

Any of the instances of the low-voltage oscillators shown in FIGS. 9-11 can drive either of the Charge Pump Drive circuits/preamps at the bottom of FIG. 12, providing the resulting output to the AC charge pump shown at the top of FIG. 12. This then outputs charge to the charge pump of FIG. 6. The low-voltage oscillator signal is input to the gate of either N-channel MOSFET Q2 or Q6, which then drives either the transformer T1 or the P-channel MOSFET ‘MOSFET P’, causing the low-voltage input to oscillate from ground to Low-Vin max. as a sinusoid at the oscillator frequency. This sinusoidal input is input to the AC charge pump wherein each stage of capacitor C5 and Diode D1 and capacitor C1, shown repeated four times in C6 through C8 and D2 through D4 and C2 through C4, amplifies the peak-to-peak voltage and stores charge in the capacitors C1 through C4 at successively higher voltages. The final AC charge pump output capacitor, C4, is the charge input to the existing Enerpak system at the amplified pre-amp voltage.

The system preferably acquires charge from the ambient environment at a level below one volt DC with a low voltage DC-to-DC converter, preferably represented by one of these charge pump driver circuits. These charge pump driver circuits comprise low voltage oscillators which work in conjunction with AC charge pump stages as a preliminary charge store, thereby performing very low power step up for voltages less than 1.0V. The higher voltage charge is then preferably transferred to the charge pump circuit of FIG. 6 for further voltage step-up. This enables the system to work more efficiently at these sub-one volt inputs, thereby increasing the flexibility of the present invention so it can efficiently operate with a variety of energy sources. In contrast, existing systems typically employ only one main step-up circuit or method, requiring the main voltage step-up system to work over a wider range, rendering it less efficient.

Additional Circuits

FIG. 13 is a schematic of a modification of the Voltage Measurement circuit described above. The MSP430 comparator has two inputs, CA0 and CA1, which may be set up for internal or external operation. These dedicated input pins may also be used exclusively as external outputs of the comparators internal reference circuit. The voltage measurement circuit preferably uses CA1 as a dedicated voltage reference input, and uses CA0 as a multiplexed voltage input, and also as a voltage reference output for other parts of the system, depending on the operation currently undertaken. In order for this to occur, the battery voltage supplied as a reference input must be switched out of the circuit. This is accomplished by providing a logic low signal to the gate of transistor Q14, shown in the diagram labeled “Voltage Measurement”. This action turns off Q14, thus pulling high the gate of transistor Q52 through resistor R14, turning off Q52 and removing the Battery voltage from CA0. After that action, CA0 may be re-directed through software to act as a voltage reference output. To supply Battery voltage as a reference input for measurement, a logic high signal is supplied to the gate of Transistor Q14, reversing the above action. Thus switching the battery voltage out of the circuit allows the battery voltage and a reference output to both share the CA0 line at different times. Since one pin can be used for both a precision voltage reference supply and the input to comparator A, this eliminates the need for a separate voltage reference circuit, saving both space and power.

To keep the processor in reset during brownout conditions, a supervisory circuit may be added. This monitors Vcc and keeps the microcontroller in reset until Vcc rises above 2.125 volts, which prevents the processor from executing spurious locations and allows the CPU to initiate the internal registers in a known state once the voltage has risen to an appropriate level for CPU operation. Depending on the controller used, this function could also be performed by the microprocessor without requiring the separate supervisory circuit.

FIGS. 14-15 depict an additional stage of the FET Gate Conditioning, preferably controlled by an additional CPU port pin. The CPU preferably times the gating action for the transistors in the charge pump so that the flying capacitors will be disconnected prior to making connections, i.e., break before make. This preferably prevents current from leaking to ground by having both sets of transistors switched on simultaneously for a brief transition period. FIG. 14 shows the transistors in the charge pump with their new switching signal designations at the gates. This preferably results in lower power consumption and higher efficiency.

FIG. 16A is a schematic of a VCC supervisory circuit which modifies FIG. 5 and preferably keeps the microcontroller in a reset state when the VCC voltage falls below 2.25V to prevent spurious execution of programming when the program counter is unreliable. The IC is preferably a Texas Instruments TPS3838J25 DBVT.

FIG. 16B is a schematic of a modified Voltage Measurement. The “BattMeas” signal selects the Battery as the source to be measured. When “BattMeas” is not active, the supercapacitor is preferably the source to be measured. When the source is selected, the Vtst signal preferably goes active, and a timer measures the RC decay from C40/R46 until the voltage trips the microcontroller comparator A.

FIG. 17 is a schematic of a modified FET Gate Conditioning circuit of FIG. 6. There are now two stages of double comparators, which provide another set of signals controlled by the CPU, allowing for different periods for the FET switches in the charge pump. This preferably increases efficiency by decreasing the charge leakage during the switching cycle.

The High Voltage Charge Pump circuit of FIG. 6 may be modified, as also shown in FIG. 17. Diodes are added in each stage between the drain of Q36 and the Q35-drain/C6 node and between the drain of Q39 and the Q46-drain/C10 node. These diodes preferably prevent charge leakage during the switch cycle, making the charge pump more efficient. Another alteration is the addition of software code in the digital control circuit, which stops clocking the charge pump, and configures its transistors to pass charge directly to the supercapacitors when the input power is high enough. This is called “Coast Mode”.

FIG. 18 shows a modification to the Voltage Output Doubling circuit of FIG. 7 comprising an added diode between the drain of Q32 and the Q43-drain/C19 node. This preferably prevents charge leakage during the switch cycle, making this circuit more efficient.

FIG. 19 shows a modification to the Battery Charge circuit of FIG. 8. The voltage doubling circuit is activated, bringing the voltage at SCap2 up over 6V, and power is supplied to the charge circuit, both when the BC_cntrl signal goes active. Op amp U18A is powered, regulating the voltage at the emitter of Q21 to 5.66V. Transistor Q23 regulates the current, with the Q23-gate voltage limited with negative feedback from the voltage drop across R19 driving Q4 to cause a voltage increase across R30. This preferably provides the maximum limit to the charging current, protecting the Li-ion battery during the recharge. In addition, they system preferably continuously measures the status of the backup Li ion polymer battery, and uses charging pulses, controlled by the microcontroller at fixed voltage with current limiting, to provide optimum charging characteristics. The battery is never over-charged, nor charged at low currents over a damage threshold voltage, and thus will be optimally charged without danger of overcharge or overheating.

FIG. 20 shows an Input Voltage Limit, Voltage Reference, and VCC Regulation circuit modifying the VCC level circuit of FIG. 9. This circuit comprises a precision 1.2V reference, U23, preferably an Analog Devices Inc. ADR280. This precision reference is used in the Battery Charge circuit, and also the Input Voltage Limit circuit and the VCC Regulation circuit. The Input Voltage Limit circuit clamps the input through Q22, fed back to op amp U11A, exceeding the reference set by the R68/R69 divider pair and causing transistor Q13 to shut off. The VCC Regulation circuit similarly opens or shuts transistor Qnew8 with a voltage feedback from VCC to the voltage divider pair R66/R67 reference for op amp U11B.

FIG. 21 shows a Piezo Dynamic Loading Interface useful when input power to FIG. 14 is optionally provided by a piezo vibration cantilever source. Typically the power from a piezoelectric cantilever is optimized when the loaded output voltage is approximately ½ the unloaded output voltage. The Dynamic Loading Interface circuit preferably accomplishes this by using the voltage measurement portion of the charge pump cycle, where the charge pump is shut off and the input is consequently unloaded, to shut off all input from the rectifier by closing transistor Q19 when the measurement signal “Vtst” goes active. At this point, the input voltage at “Pwr_In0” is unloaded and floats to its maximum, recharging a reference capacitor C7 through diode D27. When charge pump operation resumes, this reference voltage at C7 provides a reference for op amp U19A through the low-leakage voltage divider R29/R21. This reference-driven op amp regulates the current through Q23, keeping the loaded voltage at ½ of the unloaded reference stored in C7. Should the voltage, “Pwr_In1” be outside the acceptable limits for the input to the system, the DC-DC converter U20 will provide voltage within those limits.

The circuit in FIG. 21 can optionally automatically adjust the input voltage for piezoelectric generators, as seen by the rest of the voltage step-up circuit, to one half of the piezo generator open circuit voltage for any piezoelectric generator frequency, and automatically adjusts if the frequency changes. This enables the piezoelectric generator to operate at an optimum efficiency level. In contrast, other existing systems typically set the step-up circuit to a set frequency and match the piezoelectric generator permanently to that frequency. Because piezoelectric harvesters are high impedance devices, matching the impedance is critical to maximizing the harvested energy. Thus the present invention accomplishes the desired impedance matching even when only very limited power is available for different piezoelectric generator frequencies. This increases the flexibility of the present invention, enabling it to efficiently operate with a variety of energy harvester transducers.

Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover all such modifications and equivalents. The entire disclosures of all patents and publications cited above are hereby incorporated by reference.

Number	Date	Country
61081993	Jul 2008	US
60756876	Jan 2006	US
60824755	Sep 2006	US

	Number	Date	Country
Parent	11620017	Jan 2007	US
Child	12506191		US

Method and Apparatus for Energy Harvesting and/or Generation, Storage, and Delivery

Information

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Date Filed

Date Published

Inventors

Original Assignees

CPC

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International Classifications

Abstract

Description

Claims

CROSS-REFERENCE TO RELATED APPLICATIONS

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Provisional Applications (3)

Continuation in Parts (1)