EP2609317A1

EP2609317A1 - Micromechanical pyroelectric generator

Info

Publication number: EP2609317A1
Application number: EP10752307.8A
Authority: EP
Inventors: Peter Woias; Till Huesgen; Shankar Karanilam Thundiparambu Ravindran
Original assignee: Albert Ludwigs Universitaet Freiburg
Current assignee: Albert Ludwigs Universitaet Freiburg
Priority date: 2010-08-27
Filing date: 2010-08-27
Publication date: 2013-07-03
Also published as: WO2012025137A1

Abstract

The present invention relates to a micromechanical power generator which can be applied in a wide range of applications and, in particular, in the field of energy harvesting. The power generator comprises of a micro heat engine arranged between two reservoirs having different temperatures, the said micro heat engine being operable to move a transducing member between a first position where it is in thermal contact with the first reservoir and a second position where it is in thermal contact with the second reservoir, thereby transforming the spatial temperature difference between the two reservoirs into a temporal temperature variation. According to the present invention, the said transducer member is thermally coupled to a pyroelectric member for transforming the said temporal temperature variation into electric power.

Description

MICROMECHANICAL PYROELECTRIC GENERATOR

The present invention relates to an electric power generator based on a micro heat engine which can be applied in a wide range of applications and, in particular, in the field of energy harvesting. In particular, the engine may generate periodic thermal oscillations from a static thermal field to run a pyroelectric generator coupled to it and to produce electrical energy. Moreover, the system may have an in-built electrical power enhancement and conditioning circuitry.

Specifically, autonomous sensor nodes represent an essential part of wireless sensor networks which are gaining increasing relevance in many areas. Such sensor nodes are pres- ently powered by means of batteries or wired connections to a central power source. These batteries, however, have a limited life time and therefore cause additional maintenance expenditure due to periodic replacements, not to mention the environmental problems associated with their disposal. Also, wire-based connections suffer from the high costs of their manual installation and maintenance and of several failure sources, e.g. by mechanical flex- ure. An alternative to providing locally available energy is the so-called energy harvesting. This concept implies that the energy that is present in the vicinity of the sensor node can be transformed into electrically usable energy by means of an energy conversion mechanism. As is generally known, spatial temperature gradients which provide large energy densities show high development potential for this purpose. It is known to use thermoelectric generators (TEG), based on the Seebeck effect, for energy generation from temperature gradients in the micro scale. These generators can be fabricated in micro electro mechanical systems (MEMS) specific batch processes and are already commercially available, for instance from Micropelt, Nextreme, ThermoLife etc. However, thermoelectric conversion has the disadvantage of a low efficiency with the existing technology. Under normal operating temperature differences, the efficiency is about 1 to 2 % of the Carnot efficiency. A further problem can be seen in the thermal coupling of these generators, because they typically have a low thermal resistance, compared to that of the attached heat sink. This in turn reduces the effective temperature difference at the generator and thus also reduces the output power. Else, a large heat flow is required to maintain a sufficient temperature drop across the generators alone. In summary, there is a need for improving the efficiency as well as the thermal resistance for an improved thermal coupling. As an alternative, thermomechanical generators are known. These generators are based on thermodynamic cycle processes similar to those utilized in macroscopic power plants. Usually, in a first step mechanical energy is generated, which in a second step is converted by means of a mechanoelectric generator into electrical energy. These known concepts, how- ever, are still inefficient at micro scale.

On the other hand, the pyroelectric effect is used to convert temporal thermal variations into electrical signals, hence, electrical energy. At the moment, this phenomenon is mainly used in infrared detectors, motion detectors, fire alarms etc. The utilization of pyroelectric effect for a solid state cooler/generator, is described in the published patent application US 2010/0037624 A1. However, the generator according to this document is based on actively switched liquid crystals for transforming the spatial temperature gradient into a temporal temperature variation. Consequently, this arrangement needs an additional integrated power supply for operating the liquid crystal thermal switches, making them bulky, complex and inefficient for an energy-harvesting concept. US patent 7,034,41 1 is based on the idea of directly coupling a pyroelectric material to a heat source in order to generate power. However, this document remains silent on the temperature profile of the heat source and the system will function only if these sources have a time-dependent temperature profile. Such natural variations are too slow in the case of most heat sources, leading to a highly inefficient energy harvesting system. Hence, there exists a requirement for a mechanism to convert spatial thermal gradients to temporal thermal variations with a sufficiently fast dynamics and without the need of external electrical energy.

The problem underlying the present invention is to provide a micromechanical generator for use in low power electronics, such as wireless sensor networks, which is able to generate energy from spatial temperature gradients in a particularly effective and economic way. This problem is solved by the subject matter of the independent claims. Advantageous embodiments of the present invention are the subject matter of the dependent claims.

The present invention relates to a thermomechanic generator which converts such spatial temperature differences into utilizable electric energy.

The present invention is based on the idea that by combining a micro motor arranged be- tween two reservoirs having different temperatures and being operable to move a transducing member with a pyroelectric member that transforms the temporal temperature variation caused by the movement of the transducer member into electric power, a particularly effective and economic micromechanical generator, which can be used in a wide field of applications, can be provided. The first reservoir may be a heater, a heated fluid or any heat source wherefrom energy is to be harvested, whereas the second reservoir may be ambient air or any other suitable medium with a lower temperature than the first reservoir.

In particular, the micro engine moves the transducing member between a first position, where it is in thermal contact with the first reservoir, and a second position, where it is in thermal contact with the second reservoir, thereby transforming the spatial temperature difference between the two reservoirs into a temporal temperature variation. Consequently, in contrast to all known micro heat engines, the temporal temperature variation is used for the direct energy generation and it is not the mechanical energy present from the movement of the transducing member, which is converted. With the present invention, output powers could be reached which are comparable to the best thermoelectric generators presently on the market. By further optimizing the charge extraction circuitry and the material combina- tions, far better converting efficiencies can be achieved. Theoretically, this efficiency is only limited by the Carnot factor. In particular, multi layered thin film pyroelectric materials will lead to improved efficiencies.

According to the present invention, the micro motor is directly driven by the spatial temperature difference. By dispensing with all further intermediate energy conversions, the efficiency can be maximized.

By generating a periodic temperature oscillation from a spatial temperature variation, a conversion efficiency can be reached which is coming close to the theoretical limit.

Energy losses can be further reduced by a motor design, wherein the pyroelectric generator itself forms the transducing member. Furthermore, this is the most space-saving manner of constructing the micromechanical generator according to the present invention.

According to the present invention, the micro motor comprises a bistable membrane stretched over a compartment filled with a working fluid actuating the bistable membrane, thereby moving the transducing member, in this case the motor itself.

Such a motor structure is for instance known from US patent 7,329,959. Here, an engine chamber has a stiff plate on one side and a bistable membrane on the other. The hollow chamber is filled with a working medium, either a gas or phase-changing fluid. By means of a mechanically fixing connector, the membrane is connected to the heat source. When the working fluid is in a state with lower temperature, the chamber is in contact with the heat source and the membrane is arched into the cavity. By this direct contact with the heat source, heat flows into the engine chamber, thereby heating up the working fluid. Consequently, the pressure inside the cavity rises. When a predetermined positive threshold pressure is exceeded, the bistable membrane snaps to the outside, thereby moving the complete motor chamber in a direction towards the heat sink. The engine chamber gets into contact with the heat sink and therefore heat is dissipated to the heat sink, thereby decreasing the pressure within the working medium. When the pressure falls below a second predetermined threshold, the membrane snaps back to the inside and the motor element again gets into contact with the heat source. Now, the cycle starts again.

A further conventional mechanism for generating a reciprocating motion from a spatial thermal gradient is described in the published US patent application 2009/0315335 A. A f erro- magnetic material is attached to a beam suspended between two heat reservoirs kept at two different temperatures. The heat reservoir with the highest temperature is magnetic or has a magnet embedded into it. The ferromagnetic material is attracted and attaches itself to this heat reservoir. The beam bends, due to the movement of the ferromagnetic material. The ferromagnetic material gets heated up and once its temperature exceeds its Curie tempera- ture, it becomes paramagnetic and the force of attraction disappears. The stretched beam pulls back the magnetic material and attaches it to the other reservoir, kept at a lower temperature. The magnetic material cools down and once its temperature falls below its Curie temperature it becomes ferromagnetic again. It gets attracted towards the other heat reservoir and the cycle is repeated. The periodic movement of the magnetic material results in periodic oscillations of the beam. Piezoelectric elements attached to the beam are also stretched and relaxed periodically, resulting in a periodic electric power generation.

In order to generate electrical energy using this mechanism, it is proposed to use the periodic motion of magnetic material to periodically close and open a magnetic circuit, comprising of a permanent magnet. A coil enclosing this magnetic path (wound over this circuit) ex- periences periodic changes in the magnetic flux linked with it and results in a periodic electrical energy generation. So the mechanism utilizes a thermal gradient to change the phase of the magnetic material, uses this phase transition to drive a beam and finally converts this elasto-mechanical energy to electrical energy, using a piezoelectric generator or using the opening and closure of a magnetic path via electromagnetic conversion. It does not utilize the temporal thermal variations produced by the described embodiment to generate electrical energy from them via a direct thermoelectrical conversion mechanism. There exist a lot of intermediate steps in the aforementioned known arrangement to convert a spatial thermal gradient to electrical energy.

In contrast thereto, the mechanism according to the present invention utilizes the generated temporal thermal variations to generate electrical energy from a spatial thermal gradient, thereby eliminating the low efficient intermediate energy transfer stages. An alternative micro engine according to the present invention is based on at least one bimetallic member acting as a fixed end beam which can move between the first and second reservoir. The bimetallic member carries the pyroelectric generator on at least one surface or can even be fabricated as a whole from pyroelectric material. The bimetallic member is heated up when in contact with the reservoir having the higher temperature and conse- quently snaps into a position where it comes into contact with the other reservoir, dissipating heat thereto. The bimetallic mechanism thus cools down until it snaps back to the first position and the cycle re-starts.

In order to keep down the thermal capacitance of the bimetallic member, same can comprise a bimetallic two-layer arrangement only at those regions of the moving mechanism where a corresponding bending moment is needed to induce a mechanical deformation.

US Patent 3,339,077 describes an arrangement utilizing a bistable bimetallic membrane, operating on a spatial thermal gradient, to generate mechanical energy and motion. This energy is converted to electrical energy in a further step, using an electromagnetic generator. The advantage of the mechanism according to the present invention lies in the use of a pyroelectric generator which converts thermal transients directly to electrical energy, thereby eliminating the intermediate energy conversion steps needed in the known system, effectively resulting in a highly efficient micro power generator.

According to the present. invention, at least one pyroelectric member is interacting with said transducing member in a way that said pyroelectric member experiences a temporal tem- perature change. For instance, the pyroelectric member can be attached to the moving transducing member itself. Then, the heat flow from the transducing member towards the heat sink, and also from the heat source towards the transducing member, can be applied to the pyroelectric member in a time variable manner.

According to an advantageous embodiment of the present invention, the pyroelectric member comprises at least a first and a second pyroelectric element, the first pyroelectric ele- ment being thermally coupled with the first reservoir when the transducing element is in the first position, and the second pyroelectric element being thermally coupled with a second reservoir when the transducing element is in the second position. This arrangement allows for an enhancement of charge generation and thereby, improved power generation.

The working fluid advantageously comprises a gas, a phase changing material which changes between a liquid phase and a vapour phase, or a phase changing material which changes between a solid phase and a vapour phase. In any case, the material must be able to move the membrane in a manner of bistable behaviour by means of volume increase and volume decrease depending on the temperature.

The pyroelectric member can be chosen from a plurality of materials, such as mono crystal- line, bulk ceramic, thin film, polymer and/or composite pyroelectric materials.

According to a further advantageous embodiment, the micromechanical generator comprises liquid metal droplets attached to the heat source and heat sink. Thereby the thermal resistance at the mating interfaces is enhanced, and thus the operation frequency and thereby the power output can be increased. Such metal droplets can be provided for any kind of micromechanical generators irrespective of the actual transducing mechanism. Suitable materials for these liquid metal droplet are for instance mercury or a eutectic alloy of gallium, indium and tin, Galinstan™, a registered trademark of Geratherm Medical AG.

The accompanying drawings are incorporated into and form a part of the specification to illustrate several embodiments of the present invention. These drawings together with a de- scription serve to explain the principles of the invention. The drawings are merely for the purpose of illustrating the preferred and alternative examples of how the invention can be made and used and are not to be construed as limiting the invention to only the illustrated and described embodiments. Furthermore, several aspects of the described embodiments may form— individually or in different combinations— solutions according to the present in- vention. Further features and advantages will become apparent from the following more par- ticular description of the various embodiments of the invention, as illustrated in the accompanying drawings, in which like references refer to like elements, and wherein:

Fig. 1 shows a micromechanical generator in a first position according to a first embodiment;

Fig. 2 shows the micromechanical generator of Figure 1 in a second position;

Fig. 3 shows the time dependent temperature variation of the pyroelectric element in Figures 1 and 2;

Fig. 4 shows an alternative embodiment of the micromechanical generator according to the present invention;

Fig. 5 shows time dependent voltage variation of the arrangement of Figure 4;

Fig. 6 shows the relationship between the operation frequency and the temperature difference, between the two reservoirs, for the arrangement of Figure 4;

Fig. 7 shows the dependence of power generated on the temperature difference, with the micromechanical generator according to Figure 4;

Fig. 8 shows the energy stored in a storage capacitor when using the micromechanical generator according to Figure 4;

Fig. 9 shows still another advantageous embodiment of the micromechanical generator;

Fig. 10 shows the electric output power depending on the temperature difference resulting from the arrangement of Figure 9;

Fig. 11 shows a first equivalent circuit diagram of a pyroelectric member;

Fig. 12 shows a simplified equivalent circuit diagram of a pyroelectric member;

Fig. 13 shows a circuit diagram for synchronized electrical charge extraction, SECE;

Fig. 14 shows a typical wave form for the circuit according to Figure 13;

Fig. 15 shows a circuit diagram of the continuous rectification harvesting on capacitor,

CRHC, method; Fig. 16 shows a circuit diagram for synchronized switch harvesting on inductor, SSHI;

Fig. 17 shows a typical wave form for the circuit according to Figure 16;

Fig. 18 shows a circuit diagram implementing the SECE method, where mechanical switches are integrated into the thermal interfaces on the upper and lower side of the engine chamber;

Fig. 19 shows a circuit diagram of a circuit implementing the SSHI method with mechanical switches integrated into thermal interfaces on the upper and lower side of the engine chamber;

Fig. 20 shows the specific output power of the improved pyroelectric generator in compari- son to a conventional Micropelt MPG-701 thermoelectric generator;

Fig. 21 shows a schematic cross-section of a part of the micromechanical generator having metallic switches in a first position;

Fig. 22 shows the micromechanical generator of Figure 21 in a second position;

Fig. 23 shows the arrangement of Figure 21 with additional liquid metal drops in the first position;

Fig. 24 shows the arrangement of Figure 23 in the second position;

Fig. 25 shows a schematic representation of a bimetallic switch provided on the heat sink in a first position of the motor;

Fig. 26 shows the arrangement of Figure 25 in the second position of the motor. Fig. 27 shows a micromechanical generator based on a ferromagnetic motor in a first state;

Fig. 28 shows the arrangement of Figure 27 in a second state.

Fig. 29 shows the concept of Figure 27 with the pyroelectric generator fixed to the moving beam;

Fig. 30 shows the arrangement of Figure 29 in the second state; Fig. 31 shows still a further embodiment of the arrangement of Figure 27 in the first state; Fig. 32 shows the arrangement of Figure 31 in a second state;

Fig. 33 shows still a further embodiment of the arrangement of Figure 27 in a first state;

Fig. 34 shows the arrangement of Figure 33 in a second state;

Fig. 35 shows a micromechanical generator based on a bimetallic motor in a first state; Fig. 36 shows the arrangement of Figure 35 in a second state;

Fig. 37 shows a further micromechanical generator based on a bimetallic motor in a first state;

Fig. 38 shows the arrangement of Figure 37 in a second state.

The micromechanical generator, according to the present invention, will be described with reference to the figures in the following. According to the present invention, thermal temporal variations generated by a micro motor are converted into electric energy by means of a pyroelectric member. In the following, the micro motor is also referred to as a micro heat engine.

Pyroelectricity describes the effect of a charge accumulation in a material due to transient temperature changes. This effect is well known and commercially utilized in many different sensing applications, such as infrared detectors, fire alarms, motion detectors etc. Pyroelectric power generation, however, has seen less interest than thermoelectric energy harvesting in the recent years. Thermoelectric generators require a spatial temperature gradient, whereas pyroelectric generators rely on temporal temperature changes, which are less frequently found in nature. The micro heat engine, according to the present invention, however, generates a periodic temperature variation from a spatial temperature gradient. This temperature variation can be used to drive an integrated pyroelectric generator.

The following section reviews the fundamentals of pyroelectric power generation.

The pyroelectric effect links the separation of electric charges to a temporal temperature change. The charge generated on the surface, of area A, of a pyroelectric material plate, subject to a temperature variation of ΔΓ, is

AQ = pAAT (1) where p is the pyroeiectric coefficient. Two forms of the pyroeiectric effects can be distinguished. The primary pyroeiectric effect is based only on a temperature-induced change in the lattice geometry. The secondary pyroeiectric effect is based on the volume change of the crystal lattice, which causes a piezoelectric charge separation. Measurements of the pyroeiectric coefficient under constant strain include only the primary effect. However, the coefficient p is more commonly measured under constant stress, which includes both, the primary and secondary effect. Literature data include both contributions to the pyroeiectric coefficient, unless otherwise specified. For a continuous time varying temperature, using equation (1 ), the pyroeiectric current is expressed by:

This pyroeiectric current leads to the equivalent circuit representation of a pyroeiectric material plate, which is composed of a current source l_p, a capacitance C_p, and an internal resistor R_p, as shown in Figure 1 1. The capacitance of the material plate is given by the equation for a parallel plate capacitor, hence C_p = ₀e_r (3) where εο = 8.8542 x 10^"12 F/m is the permittivity of free space, ε_Γ the relative permittivity of the material, and t the material plate thickness. In most practical applications, however, the internal resistance R_p may be neglected, as the time constant τ_ρ = R_PC_P is significantly higher than the thermal oscillation period. Therefore, the equivalent circuit is simplified, as shown in Figure 12. The energy that is generated in the pyroeiectric material upon a temperature change AT s then given by

^ ₌ Ρ^_{ΑΤ2 (4)} p 2C 2ε₀ε_Γ

Thus, the energy converted directly depends on the ratio p²/^, which represents the figure of merit (FOM) for energy conversion. As the efficiency of the pyroeiectric effect is mainly based on the properties of the active materials, it is worthwhile reviewing the literature. A brief summary of some pyroeiectric material properties is listed in Table 1. Among these materials, single crystalline (1-x)Pb(Mg_1/3Nb₂/3)-xPbTi0₃ (PMN-PT) provides the best pyroelectric properties. These single crystals are typically grown by the Bridgman technique, and are expensive and fragile. Bulk ceramics in the same material system, however, are significantly less efficient, though robust. A comparable performance to the bulk PMN-PT material is provided by bulk PZT ceramics, that are commonly used in piezoelectric devices. This material is widely available and technologies for structuring are well elaborated. Bulk ceramics in the barium- strontium-calcium-oxide system are rather uncommon, but provide improved pyroelectric figures of merit. Also thin film materials have been investigated, but tend to show degraded pyroelectric properties compared to bulk ceramics. Another interesting material for pyroelectric devices is polyvinylidene fluoride, PVDF, an electro-active polymer. Despite a low figure of merit, this material shows advantages as it is not fragile and potentially available at low costs. Excellent pyroelectric properties are provided by composite materials, based on polymers and ceramics. Table 1 : Pyroelectric properties of some selected materials

The theoretical fundamentals of pyroelectric energy conversion were presented above. Equation (4) represents the electric energy that is generated in a pyroelectric material plate. As the electric energy should be delivered to an external load, an efficient extraction of the charge stored in the pyroelectric capacitor is necessary. In the following, several known methods of charge extraction are presented. Originally, these methods were developed for efficient charge extraction from piezoelectric generators. Recently, these methods have been investigated for pyroelectric energy harvesting. The following paragraphs briefly introduce three different methods and present basic equations for the work output.

The first method described here is known as synchronized electric charge extraction (SECE). In principle, the pyroelectric generator is connected with a switch S to a resistive load R_L. Figure 13 shows the ideal circuit diagram and Figure 14 illustrates the operation principle with three schematic waveforms; one shows the temperature profile, the next the corresponding voltage profile, and the last the switching scheme. The switch remains open, when the temperature is rising . At the peak temperature, the charge stored in the pyroelectric material and the voltage become maximum. Here, the switch is closed and electric work is performed in the resistive load. Assuming a generator capacitance C_p = 16 nF and a load resis- tancer R_L < 1 kQ the charge is drained of the capacitor in less than a millisecond, as the time constant =R_LC_P would be less than 16 isec. Thus, discharging the capacitor C_p is roughly a factor 10000 faster than the thermal oscillation period, which is in the range of 1 Hz for the micro heat engine. The switch is opened, when the charge is completely removed. Then, a negative voltage builds up, due to cooling. Again, the switch is closed at minimum temperature to drain of the accumulated charge. Compared to Equation (4) the electric work is doubled, as heating and cooling both contribute to the charge generation:

^_£ = 2¾ = -^ = ^ΔΓ² (5)

C ε₀ε_Γ

In many applications, the energy is to be stored in a capacitor to provide a constant voltage to a load. This can be achieved by connecting the pyroelectric generator to a load capacitor d using a full-bridge rectifier, as shown in Figure 15. In the following, this method is called continuous rectification harvesting on capacitor (CRHC). The load capacitor is charged when the rectified pyroelectric voltage exceeds the voltage of the load capacitor. Due to the periodic nature of the pyroelectric voltage, the voltage on the load capacitor increases with the number n of half cycles of the thermal oscillation, hence

The total electrical energy stored on the load capacitor is then given by w_CRHc (7) and the electrical energy added to the load capacitor in the half cycle n is then given by

AW(n) = W_CRHC(n)-W_CRHC(n-l) (8) dAWjn)

Solving = 0 with respect to n allows to find the optimal operation point n_opt, which

dn

corresponds to an optimal load capacitor voltage V_L(n_opt).

The third method presented here is called synchronized switch harvesting on inductor (SSHI). The operation principle is illustrated in Figure 16 and Figure 17. The pyroelectric generator is connected with a switch to an inductor, which is located on the AC side of a full-bridge rectifier. A battery which is modelled as ideal voltage source is connected to the DC side of the rectifier. Each time the temperature, and thus the pyroelectric voltage, reach a maximum or minimum, the switch is turned on for a time

. This time period corresponds to the half oscillation period of the LC circuit and thus causes a voltage reversal on the pyroelectric generator. Due to non-ideal conditions, a part of the electric energy is dissipated and the value of the reversed voltage is reduced by a factor β. Thus β ranges from 0, when all energy is dissipated, to 1 for an ideal inductor. As the temperature further changes, the reversed voltage increases until the battery voltage is reached. Then charge is transferred to the battery, for storage. As for the CRHC method, an optimum working point exists for the battery voltage, which is pA

bat, opt AT (9)

2C_p(l-y0)

At this operation point, the maximum electrical power is delivered to the load, which is

For a typical inversion ratio of β= 0.8, the SSHI method generates 2.5 times more energy per cycle than the SECE method. However, one general problem exists for both methods, as they require an exact detection of the maximum and minimum of the voltage at Cp. This is, following the state of the art, done with electronic detection circuits that require a certain amount of electrical power. In contrast to that, the present invention therefore presents concepts for the integration of the SECE and SSHI methods using mechanical switches integrated into the thermal contact areas. This concept is self-controlled, does not require an electronic detection of maxima and minima and does not consume electrical power. In the following, a first embodiment of the micromechanical pyroelectric generator will be explained with reference to Figure 1 and 2. The micromechanical generator 100, according to the present invention comprises a micro motor (also referred to as micro heat engine) 105, which is arranged between two reservoirs having different temperatures. In Figures 1 and 2, heat source 102 is in contact with the reservoir having a higher temperature than the second reser- voir being thermally coupled to the heat sink 104. According to the present invention, the micro motor 105 comprises a cavity 106 which is filled with a working fluid. The cavity 106 is formed by means of for instance an isotropic etching step in a substrate 108. W^'rthin the substrate 108, a membrane 1 10 is formed which is connected to the heat source 102 by means of a mechanical connector 1 12. The mechanical connector 112 is surrounded by a thermal insulation trench 1 14, which thermally insulates the bistable membrane 1 10 against the heat source 102.

Figure 1 shows the situation, where the micro motor 105 is at a low temperature and the pressure of the working fluid in the cavity 106 is low. In this situation, the bistable membrane 1 10 is arched towards the cavity 106. Consequently, the micro motor 105 rests on the heat source 102. Heat flows from the heat source 102 to the micro motor 105, as indicated by arrows 116. Thereby, the working fluid in the cavity 106 warms up and the pressure within the cavity 106 increases.

After a predetermined threshold pressure is reached, the membrane 110 snaps to the outside, as this is shown in Figure 2. The deformation of the bistable membrane 110 leads to a movement of the whole motor structure 105 so that an upper plate 1 18 is moved towards the heat sink 104 and touches same. As indicated by arrows 120, heat is now flowing from the motor to the heat sink 104, thereby causing the working fluid to cool down and reducing the pressure within the cavity 106. After a second threshold pressure is reached, the bistable membrane 1 10 snaps back into the position shown in Figure 1 , the motor structure 105 moves to be in contact with the heat source 102 and the cycle starts again. According to the present invention, the rigid plate 118 is formed by a pyroelectric ceramic. This pyroelectric ceramic thereby experiences a temporal temperature variation, as this is shown in Figure 3.

As is generally known, for a sinusoidal temperature variation, the conversion efficiency is where k² represents the pyroelectric coupling coefficient, which is a function of the material characteristics and T]_camot signifies the Carnot efficiency factor.

As this is schematically shown in Figures 1 and 2, the charge distribution within the pyroelectric ceramic 1 18 also periodically changes its polarity. Electrodes on the upper and lower surfaces of the pyroelectric ceramic allow the extraction of these charges via bond wires 122. In order to allow a simple wire bond connection, the outward surface of the pyroelectric ceramic can be directly contacted by the bond wire, whereas the inner surface is contacted to the substrate via a conductive bond layer 124. The bond wire 122 can then be attached to the conductive bond layer 124. One of the previously described circuitries can be used for extracting the charge.

For a generator having a surface area of 9 x 9 mm² and an operating point of 315 K, a theoretical operating frequency of 2.8 Hz can be assumed for a threshold temperature difference of 20 K and an external temperature difference of 30 K. In this case per cycle, a heat energy of 1.65 J is transported leading to a heat flow rate of 4.6 W. The ceramic was assumed to be <1 1 1) PMN-0.25PT having a pyroelectric coupling factor of 4.79 %. With a Carnot efficiency factor of 9.1 %, the output power amounts to 15.46 mW. These results can be compared to simulation results for a commercially available thermoelectric generator, as this is shown in Table 2. Table 2: Comparison of the Characteristics

An alternative embodiment of a micromechanical generator according to the present invention is shown in Figure 4. Here, the pyroelectric member 118 is not attached to the moving micro motor 105, but to the heat sink 104. This may for instance be effected by means of an adhesive tape or a glue layer 126.

According to the embodiment shown in Figure 4, an implementation of a pyroelectric generator into the micro heat engine was demonstrated using Johnson Matthey Piezoproducts VIBRIT1100, a PZT ceramic. The manufacturer of the material does not provide data on the pyroelectric coefficient of the material, hence an experimental characterization was neces- sary. In the following, the measurement procedure to determine the dielectric constant and the pyroelectric coefficient are detailed together with the results. Then, the experimental setup with integrated pyroelectric generator is presented. Measurements of the open-circuit voltage are performed as well as charging a storage capacitor with the CRHC method.

The VIBRIT1100 material is delivered as 47 x 22 mm² sheets with a thickness of 200 pm. To investigate the pyroelectric properties, electrical contacts are directly soldered to the metal electrodes. The capacitance of the generator sheet is directly measured with a Fluke 187 digital multimeter. This measurement yields the dielectric constant, which is e_r = ^- = 3642.2 (12) ε₀Α with A = 1064 mm², t = 200 μητι, and C_p = 171 nF. This value is comparable to the nominal relative permittivity of 4750, which is provided by the manufacturer.

A simple and straightforward method to measure the pyroelectric coefficient is the pyroelectric voltage method. Here, a defined temperature change ΔΤ is applied to the pyroelectric ma- terial sheet and the change in the open-circuit voltage Δ V is measured. The pyroelectric coefficient is then given by

C_PAT

(13) AAV

To conduct the experiment, the pyroelectric material is placed on a closed-loop controlled thermo-chuck. To enable a high impedance voltage measurement, the pyroelectric material is connected to a unity gain amplifier circuit based on a National Semiconductor LMC 6484 operational amplifier. The input impedance of this amplifier is 10 ΤΩ. A voltage rise of 0.786 V is measured for a 2 K temperature change, hence the pyroelectric coefficient is 409.1 pC/m²K.

As shown in Figure 4, for demonstration purposes and to validate the theoretical models, the pyroelectric generator 1 18 was attached to the heat sink 104, and only touches the micro motor 105 when same is in contact with the heat sink 104. The pyroelectric material sheet is laser cut with a ACI Laser-Components DPL Magic Maker to a size of 10 x 10 mm² with a 2 x 2 mm² extension for soldering of the electrical contact wires. This generator plate 1 18 is attached to the heat sink 104 with tesafix 4900 double side adhesive tape 126. The transient temperature variation is generated with a micro engine as described in Figure 1 , that operates between the heat source and the generator. A Tektronix TDS2014 digital oscilloscope measures the output voltage of the pyroelectric generator. To account for the generator, the lumped parameter network model of the micro heat engine is adapted by adding additional resistors and capacitors between the heat sink and the upper thermal switches. The thermal conductivity of Vibrit 1100 is 1.2 W/Km. As no data are available for the tape material, a thermal conductivity of 0.2 W/Km was assumed.

Figure 5 shows the recorded waveform of the engine operated at a temperature difference of 79.5 K. Due to the additional thermal resistance of the generator material on the cold side, cooling takes more time than heating. An operation frequency of 0.42 Hz is observed and the output voltage oscillates between -12 V and +12 V. To calculate the temperature difference in the pyroelectric material, Equation (1 ) is inserted in Q = C V, which yields

(14)

_PA wherein V is the measured voltage. In the given case, the peak to peak voltage of 24 V corresponds to a temperature variation of 8.75 K. Assuming charge extraction with the SECE method, the electrical energy generated in a full cycle (heating and cooling) is given by Equation (4) and is 8.65 μϋ. This energy would result in an output power of 3.60 pW. The measurement of the operation frequency and output voltage is repeated for lower temperature differences.

As shown in Figure 6, engine operation starts at a minimum temperature difference of 43 K. Then, the operation frequency increases linearly. However, the operation frequency is significantly reduced, compared to operation without generator, due to the additional thermal resis- tance of the pyroelectric material.

In the investigated operation range, the temperature variation of the active material increases from 7.1 K at 47 K temperature difference to 8.75 K at 79.5 K temperature difference. These thermal oscillations yield a theoretical minimum and maximum power output of 1.24 MW and 3.60 pW, as shown in Figure 7.

To demonstrate energy harvesting, the CRHC method was implemented in an additional experiment. Here, the rectifier is set up with Fairchild Semiconductor FDLL 300A diodes. These diodes incorporate a reverse current of less than 1 nA up to 125 V. A 10 MF load capacitor is connected to the DC side of the rectifier. Both, polymer-based capacitors (from Evox Rifa) as well as tantalum-based (from Kemet) were used for these experiments, due to their low leakage currents. Prior to the actual measurement, the capacitor is short-circuited to remove any charge present in it. Subsequently, it is connected to the pyroelectric generator and the engine is operated at a constant temperature difference of 80 K. After a defined period, the capacitor is disconnected from the generator and the voltage is measured with a digital oscilloscope. The energy stored on the capacitor is calculated with Equation (7).

This measurement is repeated several times with an increasing operation time to generate the data summarized in Figure 8. Here, the energy stored on the load capacitor is plotted versus the number of full thermal oscillations. After 250 full cycles, 380 μϋ are stored in the 10 pF load capacitor. The figure also includes the theoretical results obtained with Equation (6) and Equation (7). The average deviation between theory and experiment is 17.9 % for the tantalum capacitor and 5.4 % for the polymer capacitor. Figure 9 shows a third advantageous embodiment of the present invention, wherein not only one surface of the micro motor 105 is carrying a pyroelectric generator 1 18, but both surfaces are equipped with these generators.

The following subsection analyzes the improved engine design with two pyroelectric genera- tors integrated into the engine chamber according to a further advantageous embodiment. Figure 9 schematically illustrates this design. The engine chamber consists of two <1 1 1) PMN-0.25PT pyroelectric material plates, that are bonded with an adhesive layer to both sides of the silicon membrane chip. The overall chip dimensions are 10 x 10 mm², with a membrane size of 3.0 x 3.0 mm². The membrane is composed of 9.0 pm thick Si and 3.0 pm thick SiO₂, which yields threshold pressures of ±6.0 kPa and a volume stroke of 120 nl. With a cavity volume of 5.78 μΙ and air as working fluid, the engine operates with 30 K cycle temperature difference. The pyroelectric materials have the same outer dimension and a thickness of 150 μητι. A cut-out with the size of the membrane is structured into the lower generator plate. Both generator plates are bonded with a 10 pm thick layer of conductive epoxy (Heraeus PC3001) to the silicon chip. Due to the low thermal resistance of the material, the temperature amplitudes in the generator layers are maximized. To further enhance heat transfer, a liquid metal droplet array, preferably of Hg, Galinstan™ etc., is attached to the heat source 102 and heat sink 104. These independent metallic drops drastically reduce the thermal resistance of the interface region, compared to that of a rigid plane-plane con- tact. This leads to higher heat transfer rate and thereby higher operation frequencies, leading to a higher electrical power output.

The engine performance is analyzed in a temperature range up to 80 K. Operation starts at a temperature difference of 45 K. At this temperature difference, the frequency is 0.33 Hz and rises to 0.99 Hz at 80 K. Figure 10 shows the corresponding output power, assuming an SECE harvesting circuit, and the thermal resistance in the investigated temperature range. The output power increases from 1.87 mW to 6.13 mW, while the thermal resistance decreases from 27.2 K/W to 21.0 K/W. The overall theoretical thermal to electric conversion efficiency increases 0.28 % to 0.40%.

Table 3 shows parameters for the theoretical performance analysis of the improved micro heat engine with <1 1 1> PMN-0.25PT pyroelectric generators. Table 3: Lumped parameters of the improved micromechanical generator

Component Material Geometric parameters Lumped parameters

Membrane Si/Si0₂ t_Si = 9.0 Mm Δζ = ±35.2 Mm

tsi02 = 3.0 pm A V = ±0.12μΙ

a_mem = 3.0 Mm Δρ = ±6.0kPa

Membrane Si t_Si = 510 pm C_Si = 20.0 mJ/K

substrate achio = 10 mm

a_mem = 3.0 mm

Upper generaPMT-PT t_ug = 150 p C_ug = 37.5mJ/K

tor achip = 10 mm Kug = 5.77 K/W

Lower generaPMT-PT tig = 150 M C|g = 34.12 mJ/K

tor K|g = 6.34 K/W

Upper bond PC3001 tbi = 10pm bl = 2.19 mK/W

layer A_b, = 86.3 mm² Cb| = 0.17mJ/K

Vcav = 5.78μΙ

lower bond PC3001 tbi = 10pm bl = 2.19 mK W

layer A_b| = 91 .0 mm² Cbl = 0.18mJ/K

cav = 5.78MI

Working fluid air tl_s = 510 Mm Kair = 2000 K/W

t_bi = 10μπη Cair = 4.00pJ/K

a_mem = 3.0 mm

Wbi =3.15 mm

Heat source liq. Hg achip = 10 mm K|s,on = 0.61 K/W

contact a_mem = 3.0 mm Kis.off = 33.81 K

Heat sink liq. Hg achip = 10 mm Kus,on =0.56 K/W

contact Kus.off = 30.76 K/W

The improved engine also opens up new possibilities to implement the SECE and SSHI charge extraction circuits. Here, a major problem is the peak detection for controlling the switches. Also, transistor-based switches incorporate leak currents that deteriorate the per- formance of the circuit. Mechanical switches integrated into the thermal contact interfaces between the engine chamber and the heat sink S_UP as well as between the engine chamber and the heat source S_d0wn could provide a solution.

Figure 18 shows a possible implementation of the SECE method using two switches in each thermal interface. At peak temperature, the engine chamber snaps up and the upper switches S_up,i and S_UPI2 are closed. Then, diode D1 is forward biased. A current flows through R_L and S_up,₂ until the charge is drained off the pyroelectric material. The time constant for discharging is much lower than the thermal oscillation period, hence the energy is transferred approximately at a constant temperature. The engine chamber temperature de- creases afterwards, generating a negative voltage in C_p. Now, D1 is reversely biased. When the engine moves down, the upper switches are opened and the lower switches close. At this time, D₂ is forward biased, allowing electrical charge dissipation in R_L. Subsequent heating causes a positive voltage in the generator, thus D₂ is reverse biased and C_p is charged. Also the SSHI method could be implemented using two switches and two diodes, as illustrated in Figure 19. The positive voltage stored on the pyroelectric generator is reversed by the coil when S_Up closes contact. The diode prevents discharging afterwards. When the temperature minimum is reached, the engine chamber moves back to the heat sink and S_up is opened and Sdown closed. The energy is transferred into the coil and the voltage is reversed. Again, the diode prevents discharging of the capacitor. In both cases, however, the forward drop voltage across the diode introduces additional losses, which reduce the inversion ratio β. These diodes could be omitted, when the switch is turned on only for a defined time period. Such a switch would require a trigger signal at the peak amplitudes. This trigger signal could be generated by an integrated electro-magnetic harvester. The voltage peak is detected based on the motion of the engine chamber, which begins at the respective peak temperatures.

Such an electromagnetic harvester converts the mechanical energy of the engine chamber motion into electric energy using electro-magnetic induction. For instance, a permanent magnet can be attached to the engine chamber and an induction coil can be integrated into the heat sink. Electric pulses with a peak open-circuit voltage of up to 2.5 mV can be generated for use as a trigger signal.

The present invention relates electrical power generation using the micro heat engine based on the pyroelectric effect. For an experimental proof -of-concept, a PZT ceramic plate was attached to the heat sink. A cyclic temperature variation of approximately 8 K was generated by the heat engine. This simple demonstrator already provides up to 4 pW of electric power. An optimized engine design with (1 1 1 ) PMN-0.25PT single crystals is able to generate an output power of up to 6 mW. To investigate the potential of this optimized engine for practical energy harvesting, an application scenario is considered. Here, the pyroelectric engine is attached to a heat source, which is at temperature Τ^+ΔΤ. Passive cooling fins with a thermal resistivity of 50 cm²K/W connect the cold side of the engine to the surrounding air at Tamb- The thermal resistivity of the engine is in the range of 25 cm²K/W. Due to the relatively higher thermal resistance of the cooling fins, the effective temperature difference across the generator is significantly reduced. Figure 20 shows the electric output power of the pyroelectric engine, compared to a conventional micro-scale thermoelectric generator. With SSHI charge extraction, the output power of the pyroelectric engine exceeds the thermoelectric generator by a factor 2, in the investigated temperature range. The switches which are needed for instance for the circuits according to Figures 18 and 19, can be implemented according to the present invention by contact switches which are embedded into the micro generator. As shown in Figures 21 and 22, these switches can be contact switches made from metallic contacts. Figure 21 shows the switches in an OFF- state and Figure 22 shows same in an ON-state. In order to improve the electrical contact interface, the contact switches may additionally comprise liquid metal drops 103, as shown in Figures 23 and 24.

The switches can also be realized by magnetically operated Reed contacts. In this case, a permanent magnet is mounted at or inside the transducing member to trigger these Reed switches that are located at suitable distances and positions with respect to the transducing member.

Another advantageous switch arrangement is shown in Figures 25 and 26. Here, the heat sink 104 comprises a metallic contact and at least one bimetallic shorting contact 107. When the motor 105 is in thermal contact with the heat sink, heat is dissipated to the heat sink 104 and the metallic contacts are linked by means of the bimetallic shorting, which due to the heat influence moves. When the motor 105 returns into the first position, the heat sink 104 cools down again and the bimetallic shorting contact 107 opens.

With reference to Figures 27 to 34, still a further embodiment of a generator according to the present invention will be explained. According to this particular embodiment, a ferromagnetic material and permanent magnet heat source according to the known thermal mechanical engine of US 2009/0315335 A is used in order to generate a temporal temperature variation from a spatial temperature difference.

The Figures 27, 29, 31 and 33 show the state, where the flexible beam 200 is in contact with the heat sink, whereas Figures 28, 30, 32 and 34 show the second state where the flexible beam is attracted towards the permanent magnet heat source 204. When the temperature of the ferromagnetic material 202 is above its Curie temperature, the ferromagnetic material is not attracted to the permanent magnet, whereas, when the temperature of the ferromagnetic material 202 is below its Curie temperature, same is attracted to the magnet and the flexible beam 200 is bent towards the permanent magnet heat source 204. Once it is heated above its Curie temperature the material becomes paramagnetic and the elastic forces inside the beam 200 pull back the ferromagnetic material 202 towards the heat sink 206.

The pyroelectric generator 118 experiences a temporal temperature variation which corresponds to the embodiments described above and transforms same into electric energy according to the principles explained above.

As shown in the Figures 27 to 34, the pyroelectric generator 1 18 may be located at various positions: it may be attached directly to the permanent magnet heat source 204 or to the movable ferromagnetic material 202. The pyroelectric generator 1 18 may also be attached to the flexible beam 200 in order to be in direct contact to the heat sink 206. Alternatively, the pyroelectric generator 1 18 may also be attached to the heat sink 206.

An alternative heat engine 105 to convert a spatial thermal gradient into a temporal thermal gradient is depicted in Figures 35 and 36. The important component of this engine is a bimetallic member 300, which can be a beam or a membrane, which is bistable in nature. When the heat engine 105 is not connected to any heat reservoirs, the beam or membrane 300 and the attached pyroelectric generators 1 18 are in contact with the heat source 102. Once the engine is attached to the two heat reservoirs, with the heat source 102 to the hot- ter reservoir, heat flows to the bistable bimetallic beam 300 through the pyroelectric generators. When heated up, the bending moment developed in the beam 300 increases and when this moment exceeds a threshold value, the beam 300 flips and attains its second stable position, shown in figure 36.

The generator 1 18 as well as the beam 300 are now in contact with the heat sink 104, at- tached to the other heat reservoir. Heat flows out of the beam and generator. Once the beam 300 starts cooling down, a bending moment is developed therein and when this moment exceeds a threshold value, the beam 300 flips back to its initial position shown in Fig. 35 and comes in contact with the heat source. The whole cycle is repeated.

During the reciprocating motion of the beam, the pyroelectric generator experiences a ther- mal transient, which is cyclic in nature. This leads to electrical power generation inside the pyroelectric generator, which can be utilized to power micro sensors, sensor networks etc. as set forth above with respect to the alternative micro motors.

An advanced embodiment of the aforementioned engine-generator combination is shown in Figures 37 and 38. The added magnets 306 and the ferromagnetic plate 304 help to control the threshold moments nearly independent of the beam geometry. Furthermore, the magnets 306 improve the thermal interface by increasing the contact force between the pyroelectric generator and the heat sink 104 or heat source 102.

According to present invention, at least one generator can be located at a plurality of locations, like the heat source 102, heat sink 104 etc., due to the presence of thermal transients in these locations as well. The bistable beam or membrane 300 can be also designed in different shapes and from different materials, as is within the knowledge of a person skilled in the art.

Claims

Micromechanical generator comprising: a micro motor (105) arranged between two reservoirs having different temperatures, said micro motor (105) being operable to move a transducing member between a first position where it is in thermal contact with the first reservoir and a second position where it is in thermal contact with the second reservoir, thereby transforming a spatial temperature difference between the two reservoirs into a temporal temperature variation, wherein said transducer member is thermally coupled to a pyroelectric member (1 18) for transforming said temporal temperature variation into electric power.

Micromechanical generator according to claim 1 , wherein said micro motor (105) is directly driven by the spatial temperature difference.

Micromechanical generator according to claim 1 or 2, wherein said temporal temperature variation is a periodic oscillation.

Micromechanical generator according to one of the preceding claims, wherein said micro motor (105) as such forms the transducing member.

Micromechanical generator according to one of the preceding claims, wherein said micro motor (105) comprises at least one flexible beam (200) with a ferromagnetic member (202).

Micromechanical generator according to one of the claims 1 to 4, wherein said micro motor (105) comprises at least one bimetallic member (300).

Micromechanical generator according to one of the claims 1 to 4, wherein said micro motor (105) comprises a bistable membrane (110) stretched over a compartment (106) comprising a working fluid, said working fluid actuating the bistable membrane (110), thereby moving said transducing member. Micromechanical generator according to claim 7, wherein said bistable membrane (1 10) is formed within a substrate (108) and wherein said substrate (108) and said pyroelectric member (1 18) are bonded to each other to form said cavity (106) there between.

Micromechanical generator according to claim 7 or 8, further comprising a heat source (102) being thermally coupled to said first reservoir, and a heat sink (104) being thermally coupled to said second reservoir, wherein said membrane (1 10) is mechanically supported by a connecting point (1 12) arranged on said heat source (102).

Micromechanical generator according to one of the preceding claims, wherein said pyroelectric member (118) is coupled to said transducing member and is moved with the transducing member.

Micromechanical generator according to one of the preceding claims, further comprising a heat source (102) being thermally coupled to said first reservoir, and a heat sink (104) being thermally coupled to said second reservoir, wherein said pyroelectric member (1 18) is coupled to said heat sink (104) and is not moved with the transducing member.

Micromechanical generator according to one of the preceding claims, wherein said pyroelectric member (118) comprises at least a first and a second pyroelectric element, said first pyroelectric element being thermally coupled with the first reservoir when the transducing element is in the first position, and said second pyroelectric element being thermally coupled with the second reservoir when the transducing element is in the second position.

Micromechanical generator according to one of the claims 7 to 12, wherein said working fluid comprises a gas, a phase-changing material which changes between a liquid phase and a vapour phase, or a phase-changing material which changes between a solid phase and a vapour phase.

Micromechanical generator according to one of the preceding claims, wherein said pyroelectric member (1 18) comprises a mono-crystalline, a bulk ceramic, a thin film, a polymer and/or a composite pyroelectric material.

15. Micromechanical generator according to one of the preceding claims, further comprising at least one switch that is operable to make and break an electrical circuit when actuated by the moving transducing member.

16. Micromechanical generator according to claim 15, wherein said switch comprises a metallic contact switch or a Reed switch.

17. Micromechanical generator according to claim 15, wherein said switch comprises a liquid metal, preferably mercury or a eutectic alloy of gallium, indium and tin, to enhance the electrical contact interface of the switch.

18. Micromechanical generator according to claim 15, wherein said switch comprises a thermally actuated bimetallic micro switch.

19. Micromechanical generator according to one of the preceding claims, wherein said pyroelectric member (118) is electrically contacted by means of wire bond connections (122).

20. Micromechanical generator according to one of the preceding claims, wherein liquid metal droplets are attached to the heat source (102) and heat sink (104) to reduce the thermal resistance at the mating interfaces.