GB2514617A

GB2514617A - Energy converter

Info

Publication number: GB2514617A
Application number: GB1309756.3A
Authority: GB
Inventors: Bernd W Gotsmann; Fabian Menges
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 2013-05-31
Filing date: 2013-05-31
Publication date: 2014-12-03
Also published as: WO2014191858A3; WO2014191858A2; GB201309756D0

Abstract

An energy converter 10 for converting a stationary spatial thermal gradient between a warm reservoir 20 and a cold reservoir 30 into electric and/or magnetic energy is proposed. The energy converter 10 includes a thermal oscillator 11 for creating an oscillating heat flux from the stationary spatial gradient by means thermal relaxation oscillations, and a converting layer 12 coupled to the thermal oscillator 11 and configured to provide electric and/or magnetic energy by changing its electric and/or magnetic polarization due to the created oscillating heat flux. Where the converting layer 12 can be either pyroelectric or pyromagnetic.

Description

ENERGY CONVERTER

FIELD OF THE INVENTION

The invention relates to an energy converter for converting a stationary spatial thermal gradient between a warm reservoir and a cold reservoir into electric andlor magnetic energy and to a device including at least one thermal oscillator.

i0 BACKGROUND

The present invention is directed to converting waste heat into electric or magnetic energy.

While there are technical solutions implemented to convert heat from high temperature sources, there is only marginal energy conversion from low-temperature heat sources, so-iS called waste heat. World-wide, industry discharges over 1014 joules annually of low-grade waste heat, of about 10°C to 250°C, from electric power stations, pulp and paper mills, steel and other metal foundries, &ass manufacturers, petrochemical plants and the like.

A technology to recover or convert this low-grade waste heat to usable electricity may save industrial sectors tens of millions of dollars annually. Waste heat conversion may be interesting not only in order to reduce reduction of carbon dioxide emissions from buming fossil fuels. In principle, it may be also available in abundance in almost any location of the world, and therefore attractive to power mobile or distributed devices, such as sensor arrays, mobile computers, phones, smartphones and the Uke.

Document US 20i i10298333 Al describes a direct conversion of nanoscale thermal radiation to electrical energy using pyroelectric materials. In particular, document US 20i i10298333 Al shows a converter comprising a hot source, a cold source and a pyroelectric plate. The hot source and the cold source are in parallel, wherein the pyroelectric plate is configured to oscillate between the hot source and the cold source. For example, the pyroelectric plate is comprised of a pyroelectric material film sandwiched between electrodes of the pyroelectric plate used to collect electric charges and to app'y an electric field.

A pyroelectric material can turn heat into electricity if the temperature changes over time. For example, the temperature fluctuations of ambient temperatures in a room are sufficient to extract about I RW in 1 cm3 of pyroelectric material. Although this conversion efficiency is remarkable, it is too low to be considered relevant for technological applications.

Conventional pyroelectric energy generators rely on the property that the spontaneous polanzation (and hence dielectnc constant) of certain materials is temperature dependent.

Cycling the material's temperature induces an alternating current in an external circuit when the pyroelectric material is made the dielectric in a capacitor. The intrinsic dipole moment of lO the pyroelectric material is made part of a capacitor and an ammeter is connected between the two capacitor electrodes. At constant temperature, no culTent flows in the circuit. When the capacitor temperature is increased, the polarization decreases, effectively reducing the capacitor's dielectric constant, and causing a current to flow in the external circuit to compensate for the decrease in the bound charge in the capacitor. This property can be used iS to generate electricity where the electrical current and energy conversion efficiency depends on the rate of change, and on the magnitude of the temperature change in the capacitor.

Conventional waste heat to electrical energy conversion techniques (thermoelectric, piezoelectric and pyroelectric) all suffer from low energy conversion efficiencies, limited partly by the Carnot efficiency, but also by the inherent limitations of the conversion technologies themselves.

Further, in reference [i]. a pyroelectric energy converter for harvesting waste heat is described. Therein, simulations versus experiments are shown. The simulated prototypical device consists of a hot and cold source separated by a series of vertical microchannels supporting pyroelectric thin films made of co-polymer P (VDF-TRFE) and undergoing the Olsen cycle.

In reference [2], an improved pyroelectric energy converter for waste heat energy harvesting using co-polymer P (VDF-TRFE) and Olsen cycle is described.

Document US 2012/0056504 Al shows an MEMS-based (MEMS; micro-electro-mechanical system) pyroelectric thermal energy harvester. The pyroelectric thermal energy harvester is adapted for generating an electric current. It includes a cantilevered layered pyroelectric capacitor extending between a first surface and a second surface, where the first surface includes a temperature difference from the second surface. The layered pyroelectric capacitor includes a conductive, bimetal top electrode layer, an intermediate pyroelectric dielectric layer and a conductive bottom electrode layer. In addition, a pair of proof masses is affixed at a distal end of the layered pyroelectric capacitor to face the first surface and the second surface, wherein the proof masses oscillate between the first surface and the second surface such that a pyroelectric cunent is generated in the pyroelectric capacitor due to temperature cycling when the proof masses alternately contact the first surface and the second surface.

lO Document US 7,081,682 B2 shows a system for converting heat generated by a component to electrical energy. The system includes a pyroelectric converter which may be implemented to convert heat energy and/or temperature changes into dectrical energy. The pyroelectric converter may comprise pyroelectric material, such as stacks of vinylidine fluoride and trifluoroethylene copolymer film. iS

Document EP 2,295,736 BI shows an apparatus for converting waste heat from a production process into electrical energy. The production process comprises a number of sub-processes in which the waste heat from at least two sub-processes is simultaneously extracted from the production process by a single closed work-producing cooling circuit, in which a power source is incorporated to which an electricity generator is connected to supply electrical energy and in which there is at least one heat exchanger for each sub-process in the form of an evaporator.

Moreover, document US 2008/0295879 Al describes thermal electric and pymelectric energy conversion devices. WO 2007/099229 Al shows thick and thin films for power generation and cooling comprising a pyroelectric thin film element having a thickness of less than i Mm.

Furthermore, in reference [3]. pyroelectric waste heat energy harvesting using relaxor ferroelectric 8/65/35 PLZT and the Olsen cycle are shown. In reference [4], ORNL energy harvesters are described which transform waste into electricity. Moreover, reference [5] describes improving the efficiency of pyroelectric conversion. There, the pyroelectric converter uses copolymer of poly-vinylidene fluoride with trifluoroethylene.

These conventional solutions for energy conversion using waste heat are not very efficient.

The reason for this is the limited efficiency andlor scalability of the energy conversion technologies. For example, heat engines, such as sterling engines, require a minimum system size in order to run efficiently. Even the best sterling engines designed for large temperature differences only reach about 50% of the Carnot efficiency. Conventional thermoelectric converters are not efficient enough to be considered for waste-heat recovery. Furthermore, the only wide-spread technical solution for powering mobile devices is using batteries today.

Some of the above-mentioned solutions for energy conversion use pyroelectric materials.

lO These conventional pyroelectric converters produce charge in response to a change in temperature. One of the problems associated with these conventional pyroelectric converters is the lack of rapidly varying temperatures. A conventional technical solution using the mechanical oscillation of a bimetallic strip suffers from fatigue and wear.

iS Accordingly, it is an aspect of the present invention to provide an improved energy converter.

BRIEF SUMMARY OF THE INVENTION

According to an embodiment of a first aspect, an energy converter for converdng a stationary spatial thermal gradient between a warm reservoir and a cold reservoir into electric and/or magnetic energy is suggested. The energy converter includes a thermal oscillator and a converting layer. The thermal oscillator is configured to create an oscillating heat flux from the stationary spatial gradient by means of thermal relaxation oscillations. The converting layer is coupled to the thermal oscillator and configured to provide electric and/or magnetic energy by changing its electric andlor magnetic polarization due to the created oscillating heat flux.

The converting layer may include a pyroelectric andlor a pyromagnetic material. According to embodiments of the invention the converting layer can turn the heat of the spatial temperature gradient between the warm reservoir and the cold reservoir into electric and/or magnetic energy if the temperature changes over time. The thermal oscillator may be adapted to rapidly vary the temperature of the heat flux directed to the converting layer. The thermal oscillator creates the oscillating heat flux and may therefore create a great change of temperature over time. In other words, the thermal oscillator may boost the effIciency of the converting layer. Thus, energy converters according to embodiments of the invention may enhance the efficiency of converting waste-heat into electric and/or magnetic energy. As a result, energy converters according to embodiments of the invention may provide large reductions in waste-heat production and subsequent cooling requirements, together with the generation of above-mentioned high-quality electrical energy from a wide range of waste-heat sources.

The energy conversion efficiency of embodiments of the invention may be given by the temperature swing, the oscillation period, and the material properties, e.g. pyroelectric lO coefficient, thermal resistance and thermal capacitance. In particular, the energy converter may be adapted to have a conversion efficiency that is within a factor of two of the theoretically possible conversion efficiency of pyroelectrics (see reference [6]). The efficiency may therefore reach up to 25% of the Carnot efficiency using dedicated circuitry according to embodiments of the invention. iS

in an embodiment, the converting layer is a pyroelectric layer which is configured to provide electric energy by changing its electric polarization due to the created oscillating heat flux.

Here. pyroelectric layer particularly refers to a piece of matter comprising a pyroelectric material, i.e. a material exhibiting a pyroelectric effect. To allow extraction of electrical energy from the pyroelectric effect, the pyroelectric layer is advantageously positioned in between two electrodes from which electrical cunent can be drawn through the pyroelectric effect. A pyroelectric layer may for example consist of one of the following materials: PZT, PVDF. TGS. Lithiumtantalat. Bariumtitanat, Natriumnitrid or those mentioned in reference [6].

In a further embodiment, the converting layer is a pyromagnetic layer which is configured to provide magnetic energy by changing its magnetic polarization due to the created oscillating heat flux.

Here. pyromagnetic layer particularly refers to a piece of matter comprising a pyromagnetic material, i.e. a material exhibiting a pyromagnetic effect. To allow extraction of electrical energy from the pyromagnetic effect, the pyromagnetic layer is advantageously positioned in proximity of electrical coils from which electrical current can be drawn through the pyromagnetic effect. Materials that can be applied are mentioned in reference [iO].

In a further embodiment, the thermal oscillator is sandwiched between the converting layer and a further converting layer.

In a further embodiment, the converting layer and the further converting layer both are pyroyelectric layers which are configured to provide electnc energy by changing their electric polanzations due to the created oscillating heat flux.

In a further embodiment, the converting layer and the further converting layer both are lO pyromagnetic layers which are configured to provide magnetic energy by changing their magnetic polarizations due to the created oscillating heat flux.

In a further embodiment, the thermal oscillator includes a thermal conductor and a thermal switch. The thermal conductor is configured to conduct a heat flux from the warm reservoir iS towards the cold reservoir. The thermal switch may be coupled to the thermal conductor for receiving the heat flux. The thermal switch has two different states of thermal conductance for providing the thermal relaxation oscillations such that the oscillating heat flux is created from the received heat flux.

The thermal conductor combined with the thermal switch can form a solid-state thermal oscillator that creates an oscillating heat flux (alternating culTent) from an available stationary spatial thermal gradient (direct current). The thermai oscillator may be a self-sustaining device without mechanical moving parts that requires no active control advantageously. The thermal oscillator has a certain difference between its two states of conductance.

The underlying physicai mechanism is based on alternating phase transitions leading to thermal relaxation oscillations between the two states of thermal conductance, which create said oscillating heat flux from the direct heat flux as provided by the warm reservoir, e.g. a heat source. The thermal oscillator may be considered an analog to an electronic relaxation oscillator. The thermal oscillator may have numerous applications, as the technical creation and utilization of a non-stationary, alternating heat flux is applicable to technical systems operating in response to periodic temperature variations, like energy-harvesting devices, sensing devices, switching devices or clocking devices. Self-sustaining thermal oscillations may directly allow modulating any subsequent temperature dependent properties, e.g. electrical, magnetic, optical and mechanical effects of materials in thermal contact, e.g. via radiation, conduction or convection.

in particular, the thermal switch is according to embodiments of the invention an element that changes its thermal conductance as a function of temperature between two states in a step-like manner, for example as consequence of a material phase change in a small temperature interval. For example, the thermal switch may be made from a matenal showing metal-insulator phase transitions, such as vanadium dioxide. In particirlar. the phase change may be tunable, for example around 333 K. The switching material itself may show said difference in lO thermal conductance between the two states, hi the example of vanadium dioxide, this difference is moderate, i.e. factor 2.

For example, the warm reservoir is embodied as a warm or hot bath. In an analogous way, the cold reservoir may be embodied by a cold bath. Depending on the application, the thermal iS conductor may be an active device. For example, the thermal conductor may include a pyroelectric material which is configured for energy-harvesting upon cycling its temperature.

If an oscillating temperature source is needed, one end of the thermal conductor or the thermal switch may be attached to a thermal electrode.

In a further embodiment, the thermal switch is sandwiched between the thermal conductor and a further thermal conductor, so that the thermal oscillator particularly consists of a senes of a thermal conductor connected to both the warm reservoir and the thermal switch, and the further thermal conductor connected to both the cold reservoir and the thermal switch. The further thermal conductor may be connected to the cold reservoir. The sandwich of the thermal switch between two thermal conductors may be advantageous for adjusting the temperature profile in the thermal oscillator. The two thermal conductors are attached between two temperature baths, for example. The thermal conductors show a much smaller dependence of thermal conductance on temperature than the thermal switch. in this embodiment, the switching temperature interval can be chosen freely between the temperatures of the warm and cold reservoir through suitable choice of the magnitude of thermal conductance of the first and second conductor.

In a further embodiment, the thermal switch is sandwiched between the thermal conductor and a further thermal conductor which is connectable to the cold reservoir.

In a further embodiment, a vacuum gap is arranged between the thermal conductor and the thermal switch, in particular in such a way that heat transfer by thermal radiation is dominant over thermal conduction via potential gas molecules. Here, vacuum may include a rarefied gas that has a thermal conductance through the gas molecules on the same order or preferably smaller than the electro-magnetic theimal radiation. For example, the vacuum gap has a width between mm and 200nm. Advantageously, the switching of thermal conductance is enhanced by the combination of the metal-insulator transition of the thermal switch with the thermal near-field radiation across the vacuum gap or vacuum layer. In one embodiment, the lO thermal conductor is an electrical insulator. The thermal conductance through the vacuum layer between the thermal conductor and the switch in its electrically insulating state is several times. e.g. 100 times, larger than the conductance through the vacuum layer between the thermal conductor and the switch in its electrically conducting state. The width of the vacuum gap is preferably on the order of tens of nanometers. The thermal conductance across iS a vacuum gap as a function of separation between vanadium dioxide and silicon oxide was calculated for the two different states in literature, see reference [7].

For the example of vanadium dioxide as the material for the thermal switch and silicon dioxide as the material for the thermal conductor, the thermal transmission through the vacuum gap of JO nm decreases by a factor of 100 upon switching the vanadium dioxide from its first state metallic) to its second state (electrically insulating), although the thermal conductivity of the thermal switching layer is slightly increasing by the factor of two. In this case, the variation of conductance through the vacuum gap is larger than the variation of conductance through the thermal switch. Therefore, the thermal switch can either be connected directly to the cold reservoir, or to a further conductor.

In a further embodiment, the said vacuum gap is sandwiched between the thermal switch and a thermal conductor in a metallic state. In this case, the thermal conductance of the vacuum gap is larger when the thermal switch is in its electrically conducting state as compared to the case of the switch being in its electrically insulating state. In this case, the thermal conductor is preferably connected to either the hot reservoir or to a first conductor.

In a further embodiment, the thermal switch is configured to switch at a first switching temperature from a first state of the two states in which the thermal switch has a first conductance to a second state of the two states in which the thermal switch has a second conductance. Further, the thermal switch is configured to switch at a second switching temperature from the second state to the first state.

In a further embodiment, the thermal switch is configured such that its two different states of thermal conductivity are adapted to provide penodic metal-insulator phase transitions leading to the thermal relaxation oscillations such that the oscillating heat flux is created from the received heat flux.

lO Thus, the combination of the thermal conductor and the thermal switch to a thermal oscillator leads to thermal relaxation oscillations such that the oscillating heat flux is created from the received heat flux. Such oscillation can for example be based on alternating metal-insulator phase transitions.

iS For example, the first thermal conductance corresponds to a thermal conductance of a metal and the second thermal conductance corresponds to a thermal conductance of an insulator.

In a further embodiment, the first and the second switching temperatures of the thermal switch lie between the temperature of the warm reservoir and the temperature of the cold reservoir. In particular. a switching material of the thermal switch is selected such that the first and the second switching temperatures lie between the temperature of the warm reservoir and the temperature of the cold reservoir.

Thus, the switching temperatures of the switching material of the thermal switch are between the temperatures of the two reservoirs. Furthermore, for a large temperature swing, the thermal conductance of the thermal conductor may be between the magnitudes of the thermal conductance of the thermal switch in its two states.

In general embodiments, the temperature of the warm reservoir is just higher than the temperature of the cold reservoir. According to a preferred embodiment the temperature of the cold reservoir is between 0 degree Celsius and 25 degree Celsius and the temperature of the warm reservoir is between 35 degree Celsius and 200 degree Celsius. These temperature ranges provide the advantage that they offer the possibility to use or reuse heat as warm reservoir that is broadly available and so far often not used/reused and just wasted. This may be denoted as a waste heat application. As an example, the human body or electronic devices such as mobile phones and computers may be used as a warm reservoir for these preferred temperature ranges and the ambient or room temperature may be used as cold reservoir.

According to further embodiments high temperature applications with a temperature range of the warm reservoir between 200 degree Celsius and 600 degree Celsius may be envisaged.

In a further embodiment, the thermal switch is configured to undergo the periodic metal-insulator phase transitions in time intervals which are smaller than a thermal equilibration lO time or thermal time constant of the thermal conductor, preferable smaller than nanoseconds.

In a further embodiment, the thermal switch has a state of larger thermal conductance at temperatures above the switching temperature interval, and a state of lower thermal conductance at temperatures below the switching temperature interval. hi this case, the switch iS is preferably connected to the cold reservoir, and the thermal conductor is connected to both the switch and the hot reservoir.

In a further embodiment, the thermal switch has a state of smaller thermal conductance at temperatures above the switching temperature interval, and a state of larger thermal conductance at temperatures bethw the switching temperature interval. In this case, the switch is preferably connected to the hot reservoir, and the thermal conductor is connected to both the switch and the cold reservoir.

In a further embodiment, a switching material of the thermal switch is configured to have single domain behaviour during the phase transitions. in this regard, the switching material of the thermal switch has a thickness which is smaller than i0Onm. In particular, the switching material has a thickness between iOnm and i0Onm.

In a further embodiment, a lateral dimension of the switching material andlor the theimal conductors is restricted such that single domain behaviour during the phase transitions is ensured. This can be done for thermal impedance matching. The requirements for the application may be such that a certain maximum heat flux between the warm and the cold reservoir through the thermal oscillator can be tolerated to maintain the temperature difference. At the same time, the materials of choice of the switch and the thermal conductors have a given thermal conductivity. With a suitable choice of the cross-section-length ratio, the thermal conductance of the thermal switch or the thermal conductors can be tuned.

Recapitulating the above three embodiments, the entire switching material, e.g. vanadium dioxide, of the thermal switch undergoes a phase transition in veiy short time intervals, e.g. picoseconds or nanoseconds, instead of gradually throughout the switching layer. Thus, it is ensured that the thermal oscillator never finds a stationary state. So, the thermal oscillator swings between its two states for providing said oscillating heat flux. Thus, the switching material shows a single domain behavior during its phase transition leading to a step-like lO response in it thermal conductance versus temperature dependence. For the example of vanadium dioxide as the material for the thermal switch, this happens if the layer of the thermal switch is thin, for example between 10 nm to 100 nm with a crystallographic c-axis orientation in parallel to the incident heat flux direction. Furthermore, the lateral dimension of the active switching layer of the thermal switch may be restricted to avoid the formation of iS a multi-phase system with the lateral direction of the film. By applying these geometrical restrictions, e.g. tuning area and thickness of the switching material, single-phase behavior of the phase change layer of the thermal switch can be ensured.

Moreover, the oscillation frequency depends on the thermal capacitances of the thermal switch and the thermal conductors. To optimize both, the magnitude of the temperature swing and the oscillation frequency, the geometries of the different layers may be adjusted. For example, the thermal resistance scales with length over cross-sectional area, while the thermal capacitance scales with the product of length and cross-sectional area.

In a further embodiment, the thermal conductance of the thermal conductor is between a first magnitude of conductance of the thermal switch in its first state and a second magnitude of conductance of the thermal switch in its second state.

In a further embodiment, the thermal conductor has a plurality of spacers for defining a certain distance to the thermal switch.

In a further embodiment, the thermal switch includes at least one of the following switching materials: vanadium(II)-oxide, titanium-doped vanadium(III)-oxide, silicon-phosphor, silicon-arsenic, silicon-boron, and silicon gallium.

In particular, the thermal switch is made from a material showing metal-insulator phase transitions, such as vanadium dioxide. In particular, the phase change temperatures may be tunable, for example around 333 K. The switching material itself may show a difference in thermal conductivity between the two states. In the example of vanadium dioxide, this difference is moderate, i.e. factor 2.

As a switching material, vanadium dioxide maybe an adequate material. Its switching temperature is slightly above room temperature, about 333 K, and tunable in a certain range.

lO This makes it suitable for applications using low temperature reservoirs, such as from waste heat or in energy-harvesting applications. The preferred thickness is in the nanometer range to endure spontaneous switching of the entire film of the vanadium dioxide. Furthermore, other possible materials exhibiting Mott transitions are listed in reference [8].

iS In a further embodiment, the thermal conductor includes silicon-dioxide. According to some implementations, the phase transition may be a Mott transition.

According to some implementations, the phase transition may not be a Mott transition. Other phase transitions are possible that lead to step-like variations of thermal conductance when crossing the transition temperature.

Further, according to some implementations, the switching direction may be inverted by replacing the oxide material, e.g. silicon dioxide, facing the phase change material, e.g. vanadium dioxide, by a metal, for example gold.

In a further embodiment, the converting layer is sandwiched between the thermal oscillator and the further thermal oscillator. This embodiment is particularly advantageous for the case that the two temperatures of the warm reservoir and the cold reservoir are given to some precision. In this case, the two thermal switches of the two oscillators may be implemented so that the temperature of the pyroelectric layer is varied stronger.

In a further embodiment, the thermal oscillator includes a thermal conductor connectable to the warm reservoir and a thermal switch coupled to the thermal conductor and the converting layer, wherein the thermal switch is adapted to switch at a switching temperature corresponding to a temperature of the warm reservoir. Moreover, the further thermal oscillator may include a further thermal conductor connectable to the cold reservoir and a further thermal switch coupled to the further thermal conductor and the converting layer, wherein the further thermal switch is adapted to switch at a further switching temperature corresponding to a temperature of the co'd reservoir.

In a further embodiment, the thermal oscillator is embodied as a solid-state thermal oscillator.

An electronic equivalent to said solid-state thermal oscillator maybe a relaxation oscillator.

lO In a further embodiment, the thermal oscillator is configured to create the oscillating heat flux from the stationary spatial thermal gradient by means of periodic metal-insulator phase transitions leading to the thermal relaxation oscillations.

Any embodiment of the first aspect may be combined with any embodiment of the first iS aspect to obtain another embodiment of the first aspect.

According to an embodiment of a second aspect. a system for converting a stationary spatial gradient between a warm reservoir and a cold reservoir into electric and/or magnetic energy is suggested. The system includes a stack of energy converters, wherein each of the energy converters is embodied according to above first aspect.

According to an embodiment of a third aspect, a device is suggested which includes at least one energy converter of the above discussed first aspect and/or at least one system of the above discussed second aspect.

In an embodiment, the device is embodied as an energy harvesting device. Energy converters based on pyroelectric effects may hold the promise to outperform existing thermal electric energy converters (see reference [9]) in terms of energy conversion efficiency. The thermal oscillator according to embodiments of the invention may enhance the efficiency, because the conversion principle of such an energy converter is based on temporary changes in temperature which may be provided by said thermal oscillator. By means of said thermal oscillator for creating temperature oscillations, the efficiency of the energy converter based on pyroelectric and/or pyromagnetic effects may be boosted.

According to an embodiment of a fourth aspect there is provided a method for converting a stationary spatial thermal gradient between a warm reservoir and a cold reservoir into electric and/or magnetic energy. The method comprises a step of creating by a thermal oscillator an oscillating heat flux from the stationary spatial thermal gradient by means of thermal relaxation oscillations. The method further comprises a step of providing dectric and/or magnetic energy by changing the electric andior magnetic polarization of a converting layer due to the created oscillating heat flux, wherein the converting layer is coupled to the thermal oscillator.

lO Any embodiment of one aspect of the invention may be combined with any embodiment of another aspect of the invention.

In the following, exemplary embodiments of the present invention are described with iS reference to the enclosed figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a schematic block diagram of a first embodiment of an energy converter; Fig. 2 shows a schematic block diagram of a second embodiment of an energy converter; Fig. 3 shows a schematic block diagram of an embodiment of a thermal oscillator of the energy converter of Fig. I or 2; Fig. 4 shows the conductance of the thermal switch of the thermal oscillator of Fig. 3 as a function of temperature; Fig. 5 shows a temperature oscillation at 375 K for a point between a pyroelectric layer and a thermal oscillator of an energy converter as a function of time; Fig. 6 shows a temperature oscillation at 380 K for a point between a pyroelectric layer and a thermal oscillator of an energy converter as a function of time; Fig. 7 shows a temperature oscillation at 385 K for a point between a pyroelectric layer and a thermal oscillator of an energy converter as a function of time; Fig. 8 shows a schematic block diagram of a third embodiment of an energy converter; Fig. 9 shows the temperature as a function of position in the energy converter of Fig. 8; and Fig. 10 shows a schematic block diagram of a device induding an energy converter.

lO Similar or functionally similar elements in the figures have been allocated the same reference signs if not otherwise indicated.

DETAILED DESCRIPTION OF THE EMBODIMENTS

iS In Fig. 1, a schematic block diagram of a first embodiment of an energy converter 10 for converting a stationary spatial gradient between a warm reservoir 20 and a cold reservoir 30 into dectric and/or magnetic energy is depicted.

The energy converter 10 comprises a thermal oscillator 11 and a converting layer i2 coupled to the thermal oscillator II. The thermal oscillator II is configured to create an oscillating heat flux from the stationary spatial gradient by means thermal relaxation oscillations. In particular, the thermal oscillator 11 is configured to create the oscillating heat flux from the stationary spatial gradient by means of periodic metal-insulator phase transitions leading the thermal rdaxation oscillations. The thermal oscillator is particularly embodied as a solid-state thermal oscillator 11. Further, the converting layer 12 is configured to provide electric andlor magnetic energy by changing its electric and/or magnetic polarisation due to the created oscillating heat flux.

Regarding the converting layer 12, there are two possible alternatives. In a first alternative, the converting layer 12 is a pyroelectric layer which is configured to provide electric energy by changing its electric polarization due to the oscillating heat flux created by the thermal oscillator 11. As a second alternative, the converting layer 12 is a pyromagnetic ayer which is configured to provide magnetic energy by changing its magnetic polanzation due to the oscillating heat flux created by the thermal oscillator 11.

In Fig. 2, a schematic block diagram of a second embodiment of an energy converter 10 is shown. In Fig. 2, the thermal oscillator ills sandwiched between a converting layer 12 and a further converting layer 13. Both converting layers 12, 13 are configured to provide electric and/or magnetic energy by changing their electric and/or magnetic polarizations due to the oscillating heat flux created by the thermal oscillator 11.

Also for the embodiment of Fig. 2, two main alternatives are possible. According to a first alternative, the converting layer 12 and the further converting layer 13 are both pyroelectric layers which are configured to provide electrical energy. According to a second alternative.

both the converting layer 12 and the further converting layer 13 are pyrornagnetic layers for providing magnetic energy.

Fig. 3 shows a schematic block diagram of an embodiment of a thermal oscillator Ii of the iS energy converter 10 of Fig. 1 or 2. In other words, the thermal oscillator ii of Fig. 3 may be used for the energy converter of Fig. I or Fig. 2.

The thermal oscillator 11 of Fig. 3 includes a thermal conductor 14, a thermal switch 15 and a further thermal conductor 16. The thermal switch 15 of Fig. 3 is sandwiched between the thermal conductor 14 and the further thermal conductor 16. Each of the therma' conductors 14, 16 is configured to conduct a heat flux created from the stationary spatial gradient between the warm reservoir 20 and the cold reservoir 30 from the warm reservoir 20 towards the cold reservoir 30.

The thermal switch 15 is configured to receive the heat flux. Further, the thermal switch 15 has two different states SI, S2 of thermal conductance for providing the periodic metal-insulator phase transitions leading to the thermal relaxation oscillation such that the oscillating heat flux is created from the received heat flux.

In this regard, Fig. 4 shows the conductance k of the thermal switch 15 of the thermal oscillator 10 of Fig. 3 as a function of temperature T. The thermal switch ISis configured to switch at a first switching temperature TI from a first state SI in which the thermal switch has a first conductance ki to a second state S2 in which the thermal switch 15 has a second conductance k2. The first conductance ki and the second conductance k2 are different values between the thermal switch 15 switches according to the hysteresis H. The first conductance Id may be a constant or nearly a constant slightly and linearly changing with temperature T. In an analogous way, the second conductance k2 may be a smaller constant or nearly a constant also slightly and linearly changing with temperature T. At the second switching temperature T2, the thermal switch 15 switches from the second state S2 to the first state SI according to the hysteresis H. The hysteresis H descnbes the temperature-driven phase transitions in the thermal switch 15. In particular, the first conductance ki colTesponds to the conductance of a metal and the second conductance k2 corresponds to the conductance of an insulator.

Further, the first and the second switching temperatures TI, T2 of the thermal switch 15 lie between the temperature of the warm reservoir 20 and the temperature of the cold reservoir (see Figs. 1 and 2).

Hereby, the thermal switch 15 may be configured to undergo the periodic metal-insulator phase transitions in time intervals which are smaller than nanoseconds, preferably smaller than picoseconds.

Moreover, the switching direction of the thermal switch 15 may be inverted. In such a case, the thermal switch 15 may switch from a state of higher thermal conductance at lower temperatures to a state of lower thermal conductance at higher temperatures.

The oscillation behaviour of an energy converter as shown in Fig. 1 or 2 was simulated and is shown in the following Figs. 5-7.

In particular, Figs. 5-7 show a temperature oscillation at 375 K, 380 K and 385 K for a point between the pyroelectric layer 12 and the thermal oscillator 11 of the energy converter 10 as a function of time t.

Figs. 5-7 show that the oscillation frequency increases as the temperature difference between the hot reservoir 20 and the cold reservoir 30 increases, while the amplitude of the oscillation stays constant. The amplitude is given by the switching temperatures of the layers of the thermal oscillator 11, which may be adjusted by doping and geometncal constraints.

The following material properties were used for the simulations of Figs. 5-7.

Pyroelectric layer: density = 7500 kg/mA3, specific heat = 500 J/kg-K, thermal conductance = W/mK, thickness = 2.5mm, permittivity = 961, polarization = l.79e-3 V02 for the thermal switch 15: density = 4340 kg/m'3, specific heat = 690/ (750) J/kg-K, thermal conductance = 3.5/ (5) W/mK, thickness = lOOnm This implementation as illustrated in Figs.-7 may produce an average power of 2 nW per cm2 using no dedicated circuitry. This corresponds to 1.5% of the efficiency.

Fig. 8 shows a schematic block diagram of a third embodiment of an energy converter 10.

The energy converter 10 of Fig. 8 comprises two thermal oscillators 11 sandwiching one pyroelectric layer 12 which is relatively large compared to the thermal oscillators 11. Each of the thermal oscillators 11 includes at least one thermal conductor 14 and a thermal switch 15.

The embodiment of Fig. 8 is particularly advantageous for the case that the two temperatures of the warm reservoir 20 and the cold reservoir 30 are given to some precision. In this case, the two thermal switches 15 of the two oscillators 11 may be implemented so that the temperature of the pyroelectric layer 12 is varied stronger. For this, the two switching temperatures have to be chosen accordingly. In this regard, Fig. 9 shows the temperature T as a function of position Pin the energy converter of Fig. 8.

Further, in order to optimize for the conversion efficiency of the pyroelectric layer 12 for the use with large thermal gradients, a stack of energy converters 10 (not shown) may be used so that each individual energy converter 10 runs in its optimum temperature range.

Fig. 10 shows a schematic block diagram of a device 40 including an energy converter 10 for converting a stationary spatial gradient between a warm reservoir 20 and a cold reservoir 30 into electric andlor magnetic energy. The energy converter 10 of Fig. 10 may be embodied as one of the three embodiments of the energy converter 10 described with reference to Figs. 1.

2 and 8. The device 40 of Fig. 10 may be an energy-harvesting device.

More generally. while the present invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims.

REFERENCE SIGNS

energy converter 11 thermal oscillator 12 converting ayer 13 further converting layer 14 thermal conductor thermal switch 16 further thermal conductor lO 17 vacuum gap warm reservoir cold reservoir device P position is T temperature time S I first state of thermal switch S2 second state of thermal switch

REFERENCES

[I] PYROELECTRIC ENERGY CONVERTER FOR HARVESTING WASTE HEAT:

SIMULATIONS VERSUS EXPERIMENTS

Author: Raylene C. Moreno. Brian A. James. Ashcon Navid, Laurent Pilon Source: International Journal of Heat and Mass Transfer, Volume 55, Issues 15-16, July 2012, Pages 4301-4311 [2] IMPROVED PYROELECTRIC ENERGY CONVERTER FOR WASTE HEAT ENERGY HARVESTING USING CO-POLYMER P(VDF-TRFE) AND OLSEN

CYCLE

Author: E-liep Nguyen, Ashcon Navid, Laurent Pilon Source: 2010 14th International Heat Transfer Conference (IHTC14), August 8-13, [3] PYROELECTRIC WASTE HEAT ENERGY HARVESTING USING RELAXOR FERROELECTRIC 8/65/35 PLZT AND TUE OLSEN CYCLE Author: Felix Y Lee, Sam Goijahi. Ian M McKinley, Christopher S Lynch and Laurent Pilon Source: lOP PUBLISUING SMART MATERIALS AND STRUCTURES, 26 January 2012 [4] ORNL ENERGY HARVESTERS TRANSFORM WASTE INTO ELECTRICITY Source: OAK RIDGE, Tenn., May 16, 2011 [5] IMPROVING THE EFFICIENCY OF PYROELECTRIC CONVERSION Author: Lia Kouchachvili, Michio ilcura Source: International Journal of Energy Research, Volume 32, Issue 4. pages 328- 335, 25, March 2008 [6] Sebald et al.. "Pyroelectric Energy Conversion: Optimization Principles", IEEE transactions on ultrasonics. ferroelectrics, and frequency control, vol.55, no. 3, March [7] P.J. van Zwol, PHONON POLAR1TONS ENHANCE NEAR-FIELD THERMAL TRANSFER ACROSS THE PHASE TRANSITION OF VO2, Phys. Rev. B 84, 161413(R), 2011 [8] Imada eta]., Reviews of Modern Physics, Vol. 70, No. 4, October 1998 [9] G. Sebald et al., ON THERMOELECTRIC AND PYROELECTRIC ENERGY HARVESTING, SMART MATER, Struct. 18. 2009, 125006 [10] M-H. Phan et at, Review of the magnetocaloric effect in manganite materials, Journal of Magnetism and Magnetic Materials 308, 325-340, 2007

Claims

CLAIMS1. An energy converter (10) for converting a stationary spatial thermal gradient between a warm reservoir (20) and a cold reservoir (30) into electric and/or magnetic energy. the energy converter (10) comprising: a thermal oscillator (11) for creating an oscillating heat flux from the stationary spatial thermal gradient by means of thermal relaxation oscillations, and a converting ayer (12) coupled to the thermal oscillator (11) and configured to provide electric andlor magnetic energy by changing its electnc andlor magnetic polarization i0 due to the created oscillating heat flux.
2. The energy converter of claim 1, wherein the converting layer (12) is a pyroelectric layer which is configured to provide electric energy by changing its electric polarization due to the created oscillating heat flux. iS
3. The energy converter of claim 1, wherein the converting layer (12) is a pyromagnetic layer which is configured to provide magnetic energy by changing its magnetic polarization due to the created oscillating heat flux.
4. The energy converter of claim 1, wherein the thermal oscillator (Ii) is sandwiched between the converting layer (12) and a further converting layer (i3).
5. The energy converter of claim 4, wherein the converting layer (12) and the further converting layer (i3) both are pyroyelectric layers which are configured to provide electric energy by changing their electric polarizations due to the created oscillating heat flux.
6. The energy converter of claim 4, wherein the converting layer (12) and the further converting layer (i3) both are pyromagnetic layers which are configured to provide magnetic energy by changing their magnetic polanzations due to the created oscillating heat flux.
7. The energy converter of one of claims 1 to 6, wherein the theimal oscillator (Ii) includes: a thermal conductor (i4) which is configured to conduct a heat flux from the warm reservoir (20) towards the cold reservoir (30), and a thermal switch (15) coupled to the thermal conductor (14) for receiving the heat flux and having two different states (Si, S2) of thermal conductance for providing the periodic metal-insulator phase transitions leading to the thermal relaxation oscillations such that the oscillating heat flux is created from the received heat flux.lO
8. The energy converter of claim 7, wherein the thermal switch (15) is sandwiched between the thermal conductor (i4) and a further thermal conductor (16).
9. The energy converter of claim 7 or 8, iS wherein a vacuum gap (17) is arranged between the thermal conductor (14) and the thermal switch (15).
10. The energy converter of one of claims 7 to 9.wherein the thermal switch (15) is configured to switch at a first switching temperature (Ti) from a first state (SI) of the two states (SI, S2) in which the thermal switch (15) has a first conductance tkl) to a second state (S2) of the two states (S 1, S2) in which the thermal switch (15) has a second conductance k2). and wherein the thermal switch (15) is configured to switch at a second switching temperature (T2) from the second state (S2) to the first state (S I).
11. The energy converter of one of claims 7 to II, wherein the theimal switch (15) includes at least one of the following switching materials: vanadium(ll)-oxide. titanium-doped vanadium(lH)-oxide, silicon-phosphor, silicon-arsenic, silicon-boron, silicon gallium.
12. The energy converter of claim 1, wherein the converting layer is sandwiched between the thermal oscillator and a further thermal oscillator.
13. The energy converter of claim 12, wherein the thermal oscillator includes a thermal conductor connectable to the warm reservoir and a thermal switch coupled to the thermal conductor and the converting layer, wherein the thermal switch is adapted to switch at a switching temperature corresponding to a temperature of the warm reservoir, and wherein the further thermal oscillator includes a further thermal conductor connectable to the cold reservoir and a further thermal switch coupled to the further thermal conductor and the converting layer. wherein the further thermal switch is adapted to switch at a further switching temperature corresponding to a temperature of the cold reservoir.
14. The energy converter of one of claims Ito 13, wherein the thermal oscillator is embodied as a solid-state thermal oscillator.
15. The energy converter of one of claims Ito 14, wherein the theimal oscillator (Ii) is configured to create the oscillating heat flux from the stationary spatial thermal gradient by means of periodic metal-insulator phase transitions leading to the thermal relaxation oscillations.
16. A system for converting a stationary spatial gradient between a warm reservoir (20) and a cold reservoir (30) into electric and/or magnetic energy. the system comprising: a stack of energy converters, each of the energy converters being embodied as claimed by one of claims ito 15.
17. A device (40) comprising at least one energy converter (10) of one of claims I to 15 or at least one system of claim 16.
18. A method for converting a stationary spatial thermal gradient between a warm reservoir (20) and a cold reservoir (30) into electric andlor magnetic energy, the method comprising: -creating by a thermal oscillator an oscillating heat flux from the stationary spatial thermal gradient by means of thermal relaxation oscillations, and -providing dectric and/or magnetic energy by changing the electric and/or magnetic polanzation of a converting layer being coupled to the thermal oscillator (11) due to the created oscillating heat flux.