Patent Application
20090293929
The present disclosure relates to power supplies and energy scavengers for autonomous and wearable devices. More in particular it relates to hybrid energy scavengers comprising a thermopile unit and photovoltaic cells.
A thermoelectric generator (TEG) utilises a temperature difference occurring between a hot (warm) object, i.e. a heat source, and its colder surrounding, i.e. a heat sink, or vice versa, and is used to transform a consequent heat flow into a useful electrical power. The necessary heat can for example be produced by radioactive materials, as e.g. in space applications, or by sources available in the ambient, like e.g. standard cooling/heating systems, pipe lines including pipe lines with warm waste water, surfaces of engines, parts of machines and buildings or by endotherms such as warm-blooded animals or human beings. Natural temperature gradients could be used to provide the heat for the TEGs as well, such as geothermal temperature gradients, temperature gradients on ambient objects when naturally cooling/heating at night/day, temperature differences between a fluid in a pipeline and its surrounding, heated machinery, engines, transport and ambient air, between window glass and air indoor or outdoor, etc.
TEGs can be characterised by an electrical and a thermal resistance and by both voltage and power generated per unit temperature difference between the hot and cold sides of the TEG. The relative significance of these factors depends on the specific application. In general, the electrical resistance is preferably low and, obviously, voltage or power output should be maximised (in particular in applications with small temperature difference, i.e. a few degrees C. or a few tens degrees C.). If a constant temperature difference is imposed at the boundaries of a TEG, e.g. by means of hot and cold plates at fixed temperatures relative to each other, the value of its thermal resistance is not crucial, because the output voltage and the output power are proportional to the temperature difference, which is fixed.
Contrary thereto, if the boundary condition is a constant heat flow or a limited heat flow through the device, then the thermal resistance, on one hand, has to be large enough to generate a reasonable temperature drop over the device, but on the other hand, has to be small enough to avoid a dramatic decrease in the heat flow through the TEG, for example by more than a factor of 2. The term “constant heat flow” means that in the considered range of TEG thermal resistances the heat flow through the device is constant (limited by the ambient). However, this does not mean that the heat flow stays at the same value over time in a practical application. The term “limited heat flow” means that, when decreasing the thermal resistance of the TEG, the heat flow through the device increases till a certain value, at which the conditions of constant heat flow are reached. In the case of “limited heat flow” the heat flow through the device is not limited by the ambient objects/fluids only, but also for example by the thermal resistance of the TEG.
The basic element of a TEG is a thermocouple 10 (
In
There is an increasing interest in low-cost TEGs, which could replace batteries in consumer electronic products operating at low power, and in wearable and thus miniaturised TEGs. For example, TEGs mounted in a wristwatch have been used to generate electricity from wasted human heat, thus providing a power source for the watch itself. MEMS technology has been used to fabricate miniaturised TEGs and thin film technology has been used to fabricate miniaturised TEGs on a thin polymer tape.
In EP 1 612 870 and US 2006/0000502, a micromachined TEG is proposed specially suited for application on heat sources having large thermal resistance, e.g. on living beings. It is shown that a good TEG for such applications preferably comprises a hot plate and a radiator, both having a larger size than the thermopile. A TEG 40 according to EP 1 612 870 and US 2006/0000502 is depicted in
In US 2008 0271772, freestanding and membrane-type film-based thermopiles are proposed. A film-based freestanding or membrane-based thermopile with thermal shunts according to US 2008 0271772 is illustrated in
In US 2008 0314429, a TEG 40 thermally matched to the heat source and the heat sink is proposed wherein a good efficiency can be reached in case the TEG operates under conditions of non-constant heat flow and non-constant temperature difference between hot and cold plates. A multi-stage thermopile according to US 2008 0314429, the entirely which is hereby incorporated by reference, is shown in
The measured output power of a device fabricated according to US 2008 0314429 and with a human body as a heat source is shown in
From
The present disclosure provides wearable or autonomous hybrid energy scavengers with a good output power. More in particular a hybrid energy scavenger comprising a thermopile unit and photovoltaic cells is provided, wherein the hybrid energy scavenger may generate a good output power when operating under conditions of small temperature difference between a heat source and a heat sink, and/or when receiving the heat from a heat source with high thermal resistance or dissipating the heat into a heat sink with high thermal resistance.
In a preferred embodiment, it is an advantage of the hybrid energy scavengers according to the present disclosure that their output power may be larger than the output power of prior art scavengers comprising a thermopile unit, for a same size and/or weight. Moreover, the output power of a hybrid energy scavenger according to the preferred embodiment may be less dependent on ambient temperature than the output power of prior art thermopile-based scavengers.
A hybrid energy scavenger for connection between a heat source and a heat sink is disclosed, wherein the hybrid energy scavenger comprises a hot plate for connection to the heat source, a heat dissipating structure for connection to the heat sink and a thermopile unit being mounted between the hot plate and the heat dissipating structure, wherein the heat dissipating structure comprises at least one photovoltaic cell. It is preferred that the thermal resistance of the heat dissipating structure is substantially lower, e.g. a factor of 10 lower, than the thermal resistance of the thermopile unit. The at least one photovoltaic cell may have a thermal conductance of substantially greater than 0.5 W/cm·K The contact area between the heat dissipating structure and the heat sink may be substantially larger, e.g. a factor of two larger, than the area of the thermopile unit in a plane parallel to the hot plate.
In a preferred embodiment, a hybrid energy scavenger includes a thermoelectric generator (TEG) or a thermopile unit and more specifically a thermopile unit operating in conditions of small temperature difference between a heat source and a heat sink, and/or either receiving the heat from a heat source with high thermal resistance or dissipating it into a heat sink with high thermal resistance such as a human body or a body of any other endotherm, or a fluid such as air.
The term ‘heat dissipating structure’ is intended to encompass a wide range of physical arrangements. In a simplest form, the heat dissipating structure can comprise a single photovoltaic cell mounted to the thermopile unit. It is advantageous that the interface, or contact area, between the heat dissipating structure and the heat sink is greater than the area of the thermopile unit in a plane parallel to the hot plate. Advantageously, the heat dissipating structure comprises a plurality of structural elements. The structural elements may increase the contact area between the heat dissipating structure and the heat sink.
In embodiments of the present disclosure the at least one photovoltaic cell may be mounted on an element of the heat dissipating structure. The at least one photovoltaic cell is preferably thermally connected to the element of the heat dissipating structure to which it is mounted. This allows the at least one photovoltaic cell to contribute to the heat dissipating properties of the heat dissipating structure. Preferably the at least one photovoltaic cell is mounted to an outer face of the heat dissipating structure.
In embodiments of the present disclosure the at least one photovoltaic cell may be a structural element of the heat dissipating structure. This avoids the need to provide both a structural element and a photovoltaic cell, which can reduce the overall size, weight and cost of the heat dissipating structure. Every element of the heat dissipating structure may comprise or consist of a photovoltaic cell.
The heat dissipating structure of the hybrid energy scavenger of the present disclosure may comprise a cold plate, the cold plate being positioned in between the thermopile unit and the at least one photovoltaic cell. The heat dissipating structure may comprise a radiator, the radiator for example being positioned in between the thermopile unit and the at least one photovoltaic cell. The radiator can comprise a plurality of thermally conducting structural elements
The hybrid energy scavenger may further comprise a thermally conductive spacer positioned between the thermopile unit and at least one of the hot plate and the heat dissipating structure.
The thermopile unit may be thermally matched according to US 2008 0314429, the entire disclosure of which is incorporated here by reference.
The hot plate may be shaped as a comb for providing a thermal shunt between the scavenger and the heat source.
The heat source may have a thermal resistance in the range between 10 cm2K/W and 1500 cm2K/W and/or the heat sink may have a thermal resistance in the range between 10 cm2K/W and 1500 cm2K/W.
The heat source may be an animal, a human being, a clothed human being or ambient air and the heat sink may be ambient air, an animal, a human being or a clothed human being. The heat source may for example be a space object or an artificial space object and the heat sink may be interplanetary space, a space object or an artificial space object. The heat source may for example be a distant radiating object or a plurality of distant radiating objects like space objects or ambient objects on earth.
The hybrid energy scavenger of the present disclosure may be in the form of a wearable device or an autonomous device.
Another aspect of the present disclosure provides a wearable device or an autonomous device comprising a hybrid energy scavenger of the present disclosure.
A further aspect of the disclosure provides a method for generating electrical energy using the hybrid energy scavenger of the present disclosure.
These and other characteristics, features, peculiarities and advantages of the present disclosure will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the disclosure. This description is given for the sake of example only, without limiting the scope of the disclosure. The reference figures quoted below refer to the attached drawings.
The presently preferred embodiments are described below in conjunction with the appended figures, where like reference numerals refer to like elements in the various figures, and wherein:
In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the disclosure and how it may be practiced in particular embodiments. However it will be understood that the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures and techniques have not been described in detail, so as not to obscure the present disclosure. While the present disclosure will be described with respect to particular embodiments and with reference to certain drawings, the reference is not limited hereto. The drawings included and described herein are schematic and are not limiting the scope of the disclosure. It is also noted that in the drawings, the size of some elements may be exaggerated and, therefore, not drawn to scale for illustrative purposes.
Furthermore, the terms first, second and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the disclosure described herein are capable of operation in other sequences than described or illustrated herein.
Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the disclosure described herein are capable of operation in other orientations than described or illustrated herein.
It is to be noticed that the term “comprising”, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a device comprising means A and B” should not be limited to devices consisting only of components A and B. It means that with respect to the present disclosure, the only relevant components of the device are A and B.
In the embodiment shown in
This is illustrated in
In embodiments of the present disclosure it is preferred that the area of the interface between the heat dissipating structure 49 and the heat sink, e.g. ambient air, is substantially larger, preferably at least a factor of two, still more preferred at least a factor of three, than the area of the thermopile unit 50 in a plane parallel to the hot plate 37. Increasing the contact area between the hot plate 37 and the heat source may be done by e.g. increasing the size of the hot plate 47. However, e.g. when using the human body as a heat source, adapting the shape of the hot plate 37 without increasing its size may offer a significant improvement of the generated power. For example, the human wrist has a non-uniform temperature and the thermal resistance varies on its circumference. Therefore a hot plate of 1×4 cm2 placed along the radial or ulnar artery according to US 2008 0314429, provides better heat flow than a hot plate of 2×2 cm2, the better heat flow resulting in larger generated power.
In embodiments of the present disclosure it is preferred that the thermal resistance of the heat dissipating structure 49 is substantially lower, e.g. at least a factor of 10 lower, than the thermal resistance of the thermopile unit 50. Therefore, as the photovoltaic cells 56 are part of the heat dissipating structure 49, it is preferred that their thermal resistance is low (i.e. substantially lower, e.g. at least a factor of 10 lower, than the thermal resistance of the thermopile unit 50). For example, it is preferred to use photovoltaic cells 56 made from a material with a good thermal conductivity (e.g. silicon, having a thermal conductivity of 1.49 W/cm·K at 300 K).
In embodiments of the present disclosure a thermal isolation 51 may be present at one or more sides of the thermopile unit 50, as for example illustrated in
It is an advantage of a hybrid energy scavenger 60 according to embodiments of the present disclosure that the heat dissipating structure 49, comprising photovoltaic cells 56, can generate electrical energy from ambient light, in addition to the energy generated by the thermopile unit 50 from a heat source (e.g. human body), without increasing the size or the weight as compared to prior art TEGs (e.g. as illustrated in
In prior art documents, such as for example WO 2000/05769, hybrid power sources are described which comprise both photovoltaic cells and a thermopile unit. These power sources were developed for increasing the electrical power generated by photovoltaic cells. It is known that only a small portion of the sun's spectrum is converted by photovoltaic cells, while unused heat is dissipated in the cells. In the hybrid power sources described in the prior art the unused heat is directed partially into an attached thermopile unit to generate additional energy.
The prior art teaches that cooling the thermopile unit at a side opposite to the side where the photovoltaic cell is located, i.e. providing a radiator with a low thermal resistance, is advantageous as this may lead to a higher electrical output. Contrary to the teachings of the prior art, instead of cooling the thermopile unit at the side opposite to the side where the photovoltaic cells are located, in embodiments of the present disclosure this side of the thermopile unit is connected to a heat source, more in particular a heat source with a high thermal resistance, such as for example a human body.
The main area of application of the prior art devices is the generation of electricity at a level of kilowatts and more, in outdoor applications where direct sunlight is available. Therefore, in prior art devices it is preferred to provide a lens or a reflector for concentrating sunlight on the photovoltaic cell. Concentration of sunlight leads to a higher output form the photovoltaic cell and to higher photovoltaic cell temperatures and thus to a higher temperature difference between both sides of the hybrid power source, resulting in a higher power output. Contrary thereto, the main area of application of devices according to the present disclosure is the generation of electricity at a level of milliwatts or less, e.g. for powering wearable and autonomous devices, both for indoor and outdoor applications. Concentration of light on the photovoltaic cells 56 would be a disadvantage in embodiments of the present disclosure, as concentration of light would lead to an increase of the temperature of the photovoltaic cells, which would counteract the heat dissipating function of the photovoltaic cells 56. Furthermore, as the hybrid energy scavengers of the present disclosure may mainly be used in indoor applications, concentration of light is not an issue, as generally only diffuse light (which is difficult to effectively concentrate) is available indoor.
Therefore, the design and the requirements of the hybrid energy scavenger 60 of the present disclosure are substantially different from prior art hybrid power sources. For example, in the present disclosure the photovoltaic cells 56 preferably have a low thermal resistance, as they function as a heat dissipating element being part of the heat dissipating structure 49, the heat dissipating structure 49 being for enhancing heat transfer from the thermopile unit 50 to the ambient, e.g. ambient air. In hybrid energy scavengers 60 according to embodiments of the present disclosure, the thermal conductivity of the substrate material used for the fabrication of the photovoltaic cells 56 is preferably high, for example higher than 0.5-1 W/cm·K. In prior art devices, the thermal resistance of the photovoltaic cells is not an issue and any type of substrate for the photovoltaic cells may be appropriate. Furthermore, in prior art devices a good thermal contact between the photovoltaic cells and the thermopile unit is preferred, for avoiding losses of thermal energy in between the photovoltaic cells and the thermopile unit. In embodiments of the present disclosure it is preferred to provide e.g. a radiator 48 between the thermopile unit 50 and the photovoltaic cells 56 to enhance heat transfer to the ambient. Providing such a radiator 48 in prior art devices would lead to less efficient devices, as less heat energy would be transferred from the photovoltaic cells to the thermopiles.
Calculations were performed related to the indoor power generation by hybrid energy scavengers comprising a thermopile unit 50 composed of a single thermopile 21 and photovoltaic cells 56, wherein the scavengers are located on the body 61 of a human being. It was assumed that the hybrid energy scavenger occupies 1 cm2 on the skin.
The two basic designs according to the prior art that were analysed are shown in
In the calculations it is assumed that in all devices the photovoltaic cells 56 are fabricated on a 0.5 mm-thick silicon substrate. The radiator 48 is assumed to be made of Al with a thickness of 0.5 mm and comprising four fins with a fin height of 5 mm, wherein the height is the size in a direction perpendicular to the surface of the photovoltaic cell. This is a typical radiator design used in wearable devices. The thermal resistance of the thermopile 21 is assumed to be 50 cm2K/W. The hybrid energy scavenger is attached to the skin of a person with a body thermal resistance of 300 cm2K/W, an average value according to V. Leonov et al. in “Thermoelectric converters of human warmth for self-powered wireless sensor nodes”, IEEE Sensors Journal, v. 7, no. 5 (2007), pp. 650-657. A core body temperature of 37° C. and an ambient air temperature of 22° C. are used in the calculations. The heat transfer from the hybrid energy scavenger into the ambient air also depends on the thermal resistance of the body 61 to which it is attached. A device attached to a wrist of a person is considered. The combined convection and radiation heat transfer from a unit surface of a wrist of 5 cm in diameter is described by a thermal resistance of the environment of 1010 cm2K/W at a skin temperature of 30° C. This value was used for calculation of the heat transfer from the hybrid energy scavengers to the ambient air.
The thermal circuit used for modelling the prior art device of
In
The second prior art device, shown in
Next a hybrid energy scavenger according to embodiments of the present disclosure is modelled, the scavenger comprising a heat dissipating structure 49 composed of photovoltaic cells 56 as shown in
By adding a cold plate shaped as a radiator 48 in between the thermopile 21 and the photovoltaic cells 56 of the heat dissipating structure 49, a device according to embodiments of the present disclosure can be further improved. Such a device is illustrated in
In the present document the description of a hybrid energy scavenger 60 according to the present disclosure relates to an energy scavenger, preferably a wearable energy scavenger, wherein the heat source is a human body, for example a hybrid energy scavenger attached to a wrist or to a forehead of a person. However, this is only by means of an example and is not intended to be limiting for the disclosure, which is applicable for all ambient heat sources and heat sinks with high thermal resistance (e.g. between 10 cm2 K/W and 1000 cm2 K/W), like, e.g. endotherms and ambient air, i.e. in case of limited heat flow.
The power conditioning electronics of a hybrid energy scavenger 60 according to embodiments of the present disclosure may have a charge storage element, such as a capacitor, a supercapacitor, an ultracapacitor or a rechargeable battery, e.g. a NiMH cell, to avoid power shortages. This storage element may be charged from two parallel electrical circuits, i.e. a thermopile circuit and a photovoltaic cells circuit. In conditions of an office with good illumination conditions, both units, the photovoltaic cells 56 and the thermopile unit 50, can be more or less equal in power supplying abilities. Depending on the particular positioning of the hybrid energy scavenger 60 on a human body, the illumination conditions may change; simultaneously, the available heat flow and therefore the power generated by the thermopile unit 50 may change. On a head of a person working in the office at an ambient temperature of e.g. 20-22° C., a power of 2-6 μW/cm2 can for example be generated by the photovoltaic cells 56, while up to 30 μW/cm2 can be generated in the same conditions by the thermopile unit 50, as can be verified experimentally.
In such conditions, the thermopile unit 50 dominates the power production. However, at higher ambient temperatures, e.g. 29-30° C., e.g. as a result of incident sunlight, the power generated by the thermopile unit 50 may decrease to less than 10 μW/cm2, while the photovoltaic cells 56 may generate more power than 10 μW/cm2, i.e. more than the thermopile unit 50. At night and at home, i.e. at low light levels, the photovoltaic cells 56 do not generate any significant power. When a person is walking outdoor at 36° C. air temperature, the thermopile unit 50 may produce power only periodically (See: V. Leonov, T. Torfs, N. Kukhar, C. Van Hoof, and R. Vullers in “Small-size BiTe thermopiles and a thermoelectric generator for wearable sensor nodes”, Proceedings of the 5th European Conf. on Thermoelectrics, Odessa, Ukraine, Sep. 10-12, 2007, pp. 76-79.). Therefore, the photovoltaic cells 56 in such conditions completely dominate, e.g. providing 1-2 mW/cm2 and more.
In winter, when it is cold, the thermopile unit 50 works better, while the illumination conditions are usually worse due to a lack of sunlight. In summer, on the contrary, the thermopile unit 50 can be outperformed by the photovoltaic cells 56. An example of the expected power generated, on 24-hour average, from the thermopile unit 50 and from the photovoltaic cells 56 of the hybrid energy scavenger 60 according to embodiments of the present disclosure, is plotted in
Using the hybrid energy scavenger 60 of the present disclosure on a head of a person or on animals can be complicated by the presence of their hair/coat, thermally isolating the hot plate 37 from the skin. Therefore, a pin-featured or a fin-featured hot plate 37 as shown in
Positioning the hybrid energy scavenger 60 on areas of the body of an endotherm proximal to the body inner organs referred to as a core of the body, e.g. the brain, may allow further increase of the generated power. In case of a human being, the optimal position of the hybrid energy scavenger 60 is on the temples of his head and on his forehead. In this case, the hybrid energy scavenger 60 becomes effective also during nocturnal time, i.e. on a sleeping person. By providing a pin-featured or fin-featured hot plate 37 (
When a self-powered system (i.e. a system not connected to a power line and not comprising a non-rechargeable battery) powered by a thermoelectric generator is not in use, the energy storage element 92 can be discharged. When positioning the (discharged) system comprising the thermoelectric generator on a heat source such as a human body, a certain time is needed to charge the energy storage element 92 to a voltage level that is sufficiently high for powering the load 93. This may lead to long waiting times.
Relatively large-size self-powered systems can start from the fully discharged state in seconds or in a few tens of seconds, e.g. with a supercapacitor as a charge storage element 92.
Small-size self-powered systems, e.g. comprising watch-size scavengers or smaller scavengers, may require several minutes or even several tens of minutes to charge the small supercapacitor or other energy storage element 92 to an energy level that is sufficiently high for powering a load 93, thus leading to long start-up waiting times. This could be a competitive disadvantage as compared to systems with primary batteries as power supply and could prohibit using self-powered systems in market-oriented products.
However, if ambient illumination is available during storage of devices powered by a hybrid energy scavenger 60 according to the present disclosure, the photovoltaic cells 56 may provide sufficient power to prevent discharging of the energy storage element 92 and to keep the energy storage element 92 in a charged state. When putting such scavenger on a heat source, e.g. a human body, there is no need for fully charging the energy storage element 92 from the body and thus the start-up waiting time can be significantly reduced as compared to systems only comprising a thermoelectric generator.
In embodiments of the present disclosure, the photovoltaic cells 56 can be used for fully charging the energy storage element 92 during storage. The photovoltaic cells 56 may prevent discharge of the energy storage element 92, they may keep the energy storage element 92 at a fully charged state and they may provide enough power for any power consumption that may happen during storage of the system. In such a case the device powered by the hybrid energy scavenger 60 is preferably switched off or put into a waiting regime, thereby consuming minimal power. In such a system some decisions on device level are required, related to starting the device or switching it off. It is an advantage that there is no waiting time between putting the system on a heat source, e.g. a human body, and the device start. Instead of using the photovoltaic cells 56 of the hybrid energy scavenger 60, additional photovoltaic cells may be provided (e.g. not as part of the radiator but as separate photovoltaic cells) for preventing discharge of the energy storage element 92 during storage and thus for significantly reducing or avoiding a start-up waiting time.
In other embodiments of the present disclosure the photovoltaic cells 56 (or separate photovoltaic cells provided nearby the system) may not fully charge the energy storage element 92 during storage. Instead they may charge the energy storage element 92 to a voltage level that is lower than the voltage level required for powering the load. Thus, the photovoltaic cells provide only a pre-charging of the energy storage element 92. For example, a wireless pulse oximeter, when put on the human body, may require 10 to 20 minutes to be charged by a thermoelectric generator. The system starts when the voltage on a supercapacitor 92 reaches 1.4 V. The system switches off when the voltage drops below 0.4 to 0.5 V. If a photovoltaic cell 56 is added as part of the radiator of the thermoelectric generator (or a separate photovoltaic cell nearby), during the storage period of the pulse oximeter the photovoltaic cell can provide e.g. 1.2 V on the supercapacitor 92 and the system does not start because of the too low voltage. When the pulse oximeter is put on the human body, the voltage required for the system to start (i.e. 1.4V) is only 0.2 V higher than the voltage level already provided by the photovoltaic cells in a standby regime. Therefore, the waiting time after putting the system on may be decreased from 10 to 20 minutes to 1 to 2 minutes.
In preferred embodiments of the present disclosure the charge storage element 92 may be a rechargeable battery. When using a capacitor, a supercapacitor or an ultracapacitor as an energy storage element 92, shortage of power can be avoided only for short periods, e.g. milliseconds to minutes, in case there is no energy scavenging by the photovoltaic cells 56 (i.e. in the absence of ambient illumination) and the thermopiles 21. Moreover, when such an energy storage element 92 is used in combination with photovoltaic cells 56, during energy scavenging the charge storage element 92 may be quickly saturated and all additional power generated (by the photovoltaic cells 56 or by the thermopiles 21) that is not directly used by the load is lost. When a rechargeable battery is used as the energy storage element 92, it can provide the required power for a longer period, e.g. days or weeks, even without energy scavenging. Moreover, a rechargeable battery may store substantially more energy as compared to a capacitor and may be saturated substantially less quickly.
As the hybrid energy scavenger 60 according to embodiments of the present disclosure may also be used for outdoor applications at temperatures above body core temperature and with large radiant heat from the sun, the hybrid energy scavenger 60 may be used in reverse mode of operation, i.e. when the heat flow direction is from the ambient into a body, or to another surface, on which the device is mounted.
| Number | Date | Country | Kind |
|---|---|---|---|
| 08154194.8 | Apr 2008 | EP | regional |
This application claims priority to U.S. Provisional Application Ser. No. 61/034,047 filed Mar. 5, 2008 and European Patent Application No. EP 08154194.8 filed Apr. 8, 2008, the contents of which are incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61034047 | Mar 2008 | US |
