Thermoelectric energy harvesting device and method of harvesting environmental energy

A thermoelectric energy harvesting device including a first thermal-coupling interface, a second thermal-coupling interface, and a membrane. The membrane arranged between the first thermal-coupling interface and the second thermal-coupling interface and connected to the first thermal-coupling interface by a supporting frame. A thermal bridge between the second thermal-coupling interface and a thermal-coupling portion of the membrane. A thermoelectric converter on the membrane configured to supply an electrical quantity as a function of a temperature difference between the thermal-coupling portion of the membrane and the supporting frame.

BACKGROUND

Technical Field

The present disclosure relates to a thermoelectric energy harvesting device and to a method of harvesting environmental energy.

Description of the Related Art

As is known, systems for harvesting energy from environmental-energy sources have aroused and continue to arouse considerable interest in a large number of technological fields. Typically, energy harvesting systems (also known as “energy-scavenging systems”) are designed to harvest, store, and transfer energy generated by mechanical, thermal, or chemical sources to a generic load of an electrical type. In this way, the electrical load does not need batteries or other power-supply systems, which are frequently cumbersome and not very resistant to mechanical stresses and entail maintenance costs for interventions of replacement.

Environmental energy may be harvested from different available sources and converted into electrical energy by appropriate transducers. For instance, available energy sources may be mechanical or acoustic vibrations or, more in general, forces or pressures, sources of chemical energy, electromagnetic fields, environmental light, and thermal-energy sources. For harvesting and conversion it is possible to use, for example, electrochemical, electromechanical, piezoelectric, electroacoustic, electromagnetic, photoelectric, electrostatic, thermoelectric, thermoacoustic, thermomagnetic, or thermionic transducers.

Systems based upon thermal-energy sources may use thermopile thermoelectric devices, which exploit a temperature difference between a hot body and a cold body to produce an electrical quantity. Known thermopile thermoelectric converters, albeit presenting a satisfactory efficiency, generally have a rather complex and cumbersome three-dimensional structure. For instance, the thermopile may be formed of an array of semiconductor pillars with different types of doping that extend between two plates, perpendicular thereto. Each pillar with a first type of doping has its ends electrically coupled to corresponding ends of two adjacent pillars, which both have a second type of doping. On the one hand, the complex structure renders the known thermoelectric devices costly to produce and not very sturdy from the mechanical standpoint. On the other, miniaturization, which is becoming an increasingly determining aspect, is hindered.

BRIEF SUMMARY

An aim of the present disclosure is to provide a thermoelectric energy harvesting device and a method of harvesting environmental energy that will enable the limitations described to be overcome or at least attenuated.

DETAILED DESCRIPTION

With reference toFIG. 1, a system for harvesting environmental energy, designated as a whole by the reference number1, comprises a thermoelectric energy harvesting device2, a driving interface3, a storage element5, and a voltage regulator6. In addition, an output of the voltage regulator6supplies an electrical load7.

The thermoelectric energy harvesting device2is thermally coupled between a hot body8(or heat source) at a higher temperature and a cold body9(or heat sink) at a lower temperature and uses the temperature difference between the hot body8and the cold body9for supplying harvesting electrical quantities, in particular a harvesting voltage VHand a harvesting current IH.

The driving interface3receives the harvesting voltage VHand the harvesting current IHfrom the thermoelectric energy harvesting device2and supplies charging electrical quantities, in particular a charging voltage VCHand a charging current ICH, to the storage element5. The energy stored in the storage element5increases as a result of the energy transferred thanks to the charging voltage VCHand to the charging current ICHand determines a storage voltage VSTthat follows charging profile of the storage element5itself.

The voltage regulator6receives the storage voltage VSTand supplies a regulated supply voltage VDDto the electrical load7as required.

FIGS. 2 and 3show in greater detail the thermoelectric energy harvesting device2, which, in one embodiment, comprises a package structure10, a heat-conveying structure11and a thermoelectric converter12.

FIG. 2further illustrates the hot body8and the cold body9, which may, by way of non-limiting example, be a microprocessor or other integrated circuit, and a heat dissipater, respectively.

The package structure10comprises a framelike lateral structure13, closed on opposite sides by a first thermal-coupling interface14and by a second thermal-coupling interface15. In one embodiment, the first thermal-coupling interface14is a thermally conductive ceramic plate arranged on which are both the lateral structure13and the heat-conveying structure11. The second thermal-coupling interface15is a metal lid arranged on the lateral structure13. In this case, the first thermal-coupling interface14is in contact with the cold body9, and the second thermal-coupling interface15is in contact with the hot body8. This is not in any case to be considered limiting in so far as the device may operate also with a flow of heat in the opposite direction. The inside of the lateral structure13may be sealed from the first thermal-coupling interface14and from the second thermal-coupling interface15.

The heat-conveying structure11, in the presence of a temperature difference between the hot body8and the cold body9, conveys heat between the first thermal-coupling interface14and the second thermal-coupling interface15. In greater detail, the heat-conveying structure11includes a membrane17, a supporting frame18, connecting the membrane17to the first thermal-coupling interface14, and a thermal bridge20between the second thermal-coupling interface15and a thermal-coupling portion17aof the membrane17. In one embodiment, the thermal-coupling portion17amay be a central portion of the membrane17.

In one embodiment, the membrane17and the supporting frame18are integrated in a same semiconductor die. The membrane has a peripheral portion17bconnected to the supporting frame18, which is fixed to the first thermal-coupling interface14. The supporting frame18keeps the membrane17suspended over a cavity19delimited by the membrane17itself on one side and by the first thermal-coupling interface14on the other. In some embodiments, the cavity19may be filled with a thermally insulating material, whether solid (for example, a polymeric material) or gaseous (for example, air). In one embodiment, a vacuum may be formed around the membrane17. Thanks to the first thermal-coupling interface14, the supporting frame18is at a temperature close to the temperature of the cold body9. The supporting frame18thus couples the membrane17to the first thermal-coupling interface14both mechanically and thermally.

In one embodiment, the thermal bridge20is defined by a continuous element with high thermal conductivity, for example of metal, arranged in contact with the second thermal-coupling interface15and with the thermal-coupling portion17aof the membrane17. The thermal bridge20is arranged symmetrically with respect to the membrane17. In the example illustrated inFIG. 3, the membrane17is quadrangular and the thermal bridge20extends along a median line parallel to the sides. The region of contact between the thermal bridge20and the membrane17defines the thermal-coupling portion17a. The high thermal conductivity and the contact with the second thermal-coupling interface15causes the thermal bridge20to be substantially at the temperature of the hot body8.

Consequently, in the configuration represented inFIG. 2heat flows from the hot body8to the cold body9through the second thermal-coupling interface15, the thermal bridge20, the membrane17, the supporting frame18, and the first thermal-coupling interface14. As has been said, however, the flow of heat may follow the reverse path, if the hot body8and the cold body9are coupled to the first thermal-coupling interface14and to the second thermal-coupling interface15, respectively.

The thermoelectric converter12comprises a plurality of thermopiles22arranged symmetrically with respect to the thermal bridge20and electrically coupled, for example in series. In the embodiment ofFIGS. 2 and 3, in particular, two thermopiles22are present. With reference also toFIG. 4, the thermopiles22have respective first junctions23, thermally coupled to the thermal-coupling portion17aof the membrane17, and respective second junctions24away from the thermal-coupling portion17aof the membrane17along the path of the flow of heat between the hot body8and the cold body9. For instance, the second junctions24may be located on the supporting frame18. In the embodiment illustrated, the first junctions23and the second junctions24define hot junctions and cold junctions of the thermopiles22, respectively. In one embodiment not illustrated, the second junctions22may be located on a peripheral portion of the membrane.

In greater detail, each thermopile22comprises a plurality of respective first semiconductor strips25, which have a first type of doping, and a plurality of respective second semiconductor strips26, which have a second type of doping, opposite to the first type of doping. The first semiconductor strips25and the second semiconductor strips26extend parallel to one another on a face of the membrane17between the thermal-coupling portion17aand the supporting frame18. The first semiconductor strips25and the second semiconductor strips26are consecutive to one another, alternate with one another, and have respective first ends in the proximity of the first or thermal-coupling portion17aof the membrane17and respective second ends in the proximity of the supporting frame18. The first ends of adjacent first semiconductor strips25and second semiconductor strips26(close to the thermal-coupling portion17a) are connected to one another and form respective first junctions23. The second ends of adjacent first semiconductor strips25and second semiconductor strips26close to the supporting frame18are connected to one another and form respective second junctions24. In practice, the first semiconductor strips25and the second semiconductor strips26form a plurality of thermocouples arranged electrically in series and thermally in parallel between the region of thermal coupling17aof the membrane17and the supporting frame18.

Through vias28in the supporting frame18connect terminals29of the thermopiles22to respective connection lines30(FIG. 2) that run on the first thermal-coupling interface14and traverse the framelike lateral structure13. In turn, the connection lines30are coupled to respective pads31, which enable the electrical quantities generated by the thermoelectric converter12to be transferred onto the outside of the package structure10. In one embodiment (not illustrated), the through vias28may be replaced by wire bondings, if the distance between the membrane17and the second thermal-coupling interface15so allows.

The device described enables efficient exploitation of thermopiles with a basically two-dimensional configuration. In particular, the heat-conveying structure11with the membrane17favors both provision of efficient, sturdy, and compact devices and containment of the manufacturing costs. In addition to offering a plane surface suited to housing the thermopiles22, which are two-dimensional, the membrane17has a thermal resistance much greater than both the thermal bridge20and the supporting frame18on account of the small thickness and, as regards the thermal bridge20, also on account of the lower thermal conductivity. The membrane17supports almost entirely or in any case to a preponderant extent the temperature difference between the hot body8and the cold body9. The thermal jump between the hot junctions and the cold junctions is thus very high as compared to the maximum thermal jump available, and the thermopiles22work efficiently. Furthermore, the efficiency benefits from the fact that, in a direction perpendicular to the faces of the membrane17, the dispersion due to conduction of heat is very contained. Outside the thermal-coupling portion17a, in fact, the membrane17is set facing thermally insulating regions both on the side of the cavity19(which may contain air or another solid or gaseous insulating material), and on the opposite side. The heat-conveying structure11thus enables the flow of heat between the hot body8and the cold body9to be guided basically along the membrane17, enabling effective exploitation of the available thermal jump.

As has been mentioned, the device described is also sufficiently sturdy. In particular, the strips that form the thermopiles22do not have to support mechanical loads and are adequately protected within the package structure10also in regard to stresses of a certain degree.

FIG. 5illustrates a different embodiment of the present disclosure. In this case, a thermoelectric energy harvesting device102comprises a package structure110, a heat-conveying structure111, and a thermoelectric converter112. The package structure110is substantially made as already described and comprises a lateral framelike structure113, closed on opposite sides by a first thermal-coupling interface114and by a second thermal-coupling interface115(here illustrated only in part).

The heat-conveying structure111includes a membrane117, a supporting frame118, connecting a peripheral portion117bof the membrane117to the first thermal-coupling interface114, and a thermal bridge120between a central thermal-coupling portion117aof the membrane117and the second thermal-coupling interface115.

In this case, the thermal bridge120is defined by a metal drop or ball in contact both with the thermal-coupling portion117aof the membrane117and with the second thermal-coupling interface115.

The thermoelectric converter112comprises four thermopiles122, arranged symmetrically with respect to the thermal bridge120. The thermopiles122extend in directions perpendicular to one another from the coupling portion117atowards a respective side of the membrane117, which in this case has a quadrangular shape. The hot junctions and cold junctions (not illustrated) of the thermopiles122are arranged, respectively, in the proximity of the coupling portion117aof the membrane117and in the proximity of the supporting frame118. The thermopiles112may further be electrically coupled to one another, for example in series by connection lines between respective terminals129.

In the embodiment illustrated inFIG. 6, a thermoelectric energy harvesting device202has a modular structure. In greater detail, the thermoelectric energy harvesting device202comprises a package structure210, a heat-conveying structure211, and a plurality of thermoelectric converters212.

The package structure210is substantially built as already described and comprises a framelike lateral structure213, closed on opposite sides by a first thermal-coupling interface214and by a second thermal-coupling interface (here not illustrated; the connection to the lateral structure213and to the heat-conveying structure211is in any case obtained substantially in line with what has already been described and illustrated with reference toFIGS. 1-5).

The heat-conveying structure211comprises a plurality of membranes217connected to the first thermal-coupling interface214by respective supporting frames218, which, in one embodiment, are connected to one another and form a single piece. More precisely, the membranes217and the frames218are provided in a single semiconductor die, and the membranes217are arranged in an orderly fashion to form an array for optimizing the area occupied. The heat-conveying structure211further comprises one thermal bridge220for each membrane217. The thermal bridges220are in contact, on one side, with thermal-coupling portions of respective membranes217aand, on the other, with the second thermal-coupling interface (which, as in the embodiments already described, may be a thermally conductive plate that closes the lateral structure213on a side opposite to the first thermal-coupling interface214).

Arranged on each membrane217is a respective thermoelectric converter212, which comprises a plurality of thermopiles222, arranged symmetrically with respect to the corresponding thermal bridge220. In the example described, each thermoelectric converter212comprises two thermopiles222.

The thermoelectric energy harvesting device202is thus modular, and each module250includes a membrane217, with the respective frame218, and a thermoelectric converter212.

In one embodiment, terminals of the thermopiles222are connected to respective pads231, which are in turn coupled to respective terminals of a configurable selective-connection stage251, for example a bank of switches or another electronic circuit with equivalent functions. The connection may be obtained, for example, by through vias in the frames218and connection lines (not illustrated) on the first thermal-coupling interface214. The selective-connection stage251has output terminals232for supplying the harvesting voltage VHand the harvesting current IHand may be accessible from outside for setting an operating mode of the thermoelectric energy harvesting device202. In particular, the selective-connection stage251may be configured to select one or more of the modules250, possibly excluding others, and for connecting the selected modules250in series or in parallel. The configuration of the selective-connection stage251may be arranged, for example, manually, by positioning jumpers as required, or else via software, by electronic interfaces. In one embodiment, the configuration of the selective-connection stage may be modified without restrictions throughout the service life of the thermoelectric energy harvesting device2and, possibly, also during its operation.

The modular structure enables efficient exploitation of the surface available, increasing the number of the thermoelectric converters housed and, in practice, the power that may be supplied. At the same time, it is not necessary to extend the surface of the individual membranes, which might otherwise be weakened.

In the embodiment illustrated inFIG. 6, the thermoelectric energy harvesting device202comprises six modules250that form a 2×3 array. The number and arrangement of the modules may, however, be determined in a flexible way. The membranes, the frames, and the thermoelectric converters may in fact be all easily integrated in a single semiconductor wafer, which is divided into dice containing the desired number of modules.FIGS. 7aand 7billustrate dicing of a wafer300into dice containing respectively four (2×2) and nine (3×3) modules350.

The membrane, arranged on which are the thermoelectric converters, does not necessarily have to present a quadrangular shape. According to the embodiment ofFIG. 8, for example, in a thermoelectric energy harvesting device402, a heat-conveying structure411may comprise a circular membrane417, having a peripheral portion417bconnected to a supporting frame418, which, for convenience, on the outside may be quadrangular. The thermal bridge420may include a drop or ball in contact with the membrane417at a central thermal-coupling portion417a. In this case, the thermoelectric converter may also comprise a single thermopile422with conductive strips425,426arranged radially and at a uniform distance apart over the entire membrane417. Hot junctions423and cold junctions424of the thermopile422are arranged, respectively, in the proximity of the coupling portion417aof the membrane417and in the proximity of the supporting frame418.

Finally, it is evident that modifications and variations may be made to the thermoelectric device and to the method described herein, without thereby departing from the scope of the present disclosure.