WO2014032684A1

WO2014032684A1 - Electrostatic harvester utilizing spatial thermal gradients

Info

Publication number: WO2014032684A1
Application number: PCT/EP2012/003645
Authority: WO
Inventors: Shankar Karanilam Thundiparambu Ravindran; Peter Woias; Prashant Nilkund; Michael KRÖNER
Original assignee: Albert-Ludwigs-Universität Freiburg
Priority date: 2012-08-30
Filing date: 2012-08-30
Publication date: 2014-03-06
Also published as: EP2890891A1

Abstract

The present application relates to a micromechanical generator system for generating power from a spatial thermal gradient. In particular, it relates to such a micromechanical generator being based on an electrostatic harvester which produces a change in capacity for transforming the thermal energy into electric energy. According to the present application, the micromechanical generator comprises a micro motor arranged between two reservoirs having different temperatures, said micro motor 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. The transducing member is thermally coupled to a capacitive generator for transforming said temporal temperature variation into electric power via a variation of a dielectric constant of the capacitive generator.

Description

ELECTROSTATIC HARVESTER UTILIZING SPATIAL THERMAL GRADIENTS

The present invention relates to a micromechanical generator system for generating power from a spatial thermal gradient. In particular, the present invention relates to such a micromechanical generator being based on an electrostatic harvester which produces a change in capacity for transforming the thermal energy into electric energy.

Energy harvesting from spatial thermal gradients is currently accomplished using thermoelectric generators. However, they have relatively low efficiency. Electrical energy can be generated from thermal fields using micro heat engines.

An example of a known system for energy harvesting with micro heat engines consisting of bimetallic strips and pyroelectric materials is shown in WO 2012/025137 A1. The heat engine produces thermal oscillations and the pyroelectric material converts these periodic thermal transients to electrical charges. The oscillating thermal fields generated by micro heat engines open a new possibility in the field of electrical power generation.

According to the present invention, the same thermal transients are used to generate electrical power using ferroelectric or ferrielectric materials. All ferroelectric materials transform into their ferrielectric phase, when heated above their Curie temperature. These materials exhibit exceptionally high dielectric constants near their Curie temperature and are commonly found in ceramic capacitors. These materials used as capacitors currently suffer from the high variation in their dielectric constant with temperature. For example, a 10 pF capacitor at room temperature would have its capacitance reduced to nearly 2 pF when heated to nearly 90 °C.

The present invention is based on the surprising idea that this 'disadvantage' (i. e. the change in the capacitance of these high capacitance materials) can be employed to generate electrical energy in the order of tens of millijoule from an available spatial thermal gradient.

US patent US 6,936,994 discloses an assembly for generating electrical energy from fluctuating thermal fields or mechanical vibrations. Although mechanical vibrations are abundant in nature and can be used to generate electrical power using electrostatic devices, thermal transients are quite rare in nature. Most of them, if any, have exceptionally long time constants, rendering their conversion to electrical energy impractical. However, thermal spatial gradients are abundant in nature and are currently exploited for power generation using thermoelectric generators only. US patents US 4,087,735 and US 3,971 ,938 relate to electrostatic energy conversion from a temporally varying radiant energy in combination. The capacitance used is light sensitive. The arrangement according to these documents is considered as a type of solar cell.

The present invention aims at utilizing electrostatic harvesters to generate power from an available spatial thermal gradient.

Electrostatic energy generators according to the present invention make use of the change in their capacitance to generate electrical power. Figure 1 depicts the basic electrostatic energy harvesting principle according to the present invention. When the switch is in position 1 , the capacitor gets charged up. The switch is in position 2 when the capacitance is altered and energy is generated. The switch is in position 3 after the maximum energy is generated and stored in the capacitor.

The capacitor is charged to attain ' V volts by connecting it to a voltage source. The source is disconnected and the capacitance is now decreased. In current electrostatic harvesters utilizing vibrations and/or mechanical motion to generate electrical power, the capacitance,

is reduced either by increasing the gap between the plates of the area (cf), or reducing the effective electrode area {A) or by changing the dielectric medium (ε_Γ) with the help of a mechanical pump. As charge generated is

Q = CV (2) and is kept constant, the voltage has to increase. If the capacitance reduces by a factor of 'n', then the final voltage across the capacitor is nV and the net energy generated is

Consequently, generated energy is (n-1 ) times the initial energy in the capacitor. During discharging the energy generated is dissipated across the load resistor. The capacitance is charged up again for the next half cycle of energy generation. However, by utilizing the change in dielectric property of the material involved, one can accomplish the same and generate electrical power, albeit from a thermal field. Alternate heating and cooling the material instead of mechanical vibrations, i.e. thermal vibrations create an oscillating capacitance. This opens up a new window in the field of thermal energy har- vesting using micro heat engines which have the inherent nature of generating thermal transients.

The typical variation of the dielectric constant of a material is shown in Figure 2. Based on a commercially available capacitor, the following estimation can be performed:

• Capacitance at room temperature (23 °C) : 25 F

· Capacitance when heated to 100 °C: 5 F

• Charging voltage: 10 V

• n = 25 MF / 5 pF = 5

• Ε = (5-1)^*0.5^*22 μ *10² = 4.4 mJ.

This energy yield is comparable to that of conventional thermoelectric generators operating from similar temperature differences.

Materials like Triglycine sulfate (TGS) - a common pyroelectric material - exhibit a huge variation in their dielectric constants around the Curie temperature. The dielectric constant of TGS varies from 100 at 60°C to 5000 or more at 49 °C (Curie temperature). Hence, energy in the order of tens of millijoule can be generated using this method from temperature changes as small as 11 K.

A 4J thin film Li ion battery can be used as the charging source. Such batteries have recharging cycles in the order of 100000, with a 10 % depth of discharge. With trickle charging the recharging cycles could go up even further. With 20 years life expectancy this would correspond to at least 5000 recharging cycles a year or 1 cycle every 2 hours. From the above calculations, it is clear that the principle according to the present invention can generate at least 2 J per hour assuming a frequency of ¼ Hz. With the help of suitable circuits, the battery can be charged back and the excess energy generated can be stored in a capacitor for driving a sensor or even an actuator. Such a hybrid system is much more reliable than a battery alone or harvester alone system. A critical problem faced by most of the thermoelectric generators is the low voltages available across their output. The change in the capacitance of the materials and the subsequent increase in the voltage can be used to create a transformer to step-up ultra high voltages. The voltage source used in Figure 1 has to be replaced with the thermoelectric generator. A switch utilizing the same thermal gradient (principle similar to those of a micro heat engine) to heat up and cool down the capacitor as well as making and breaking electrical contacts, can boost up the voltage level by a factor of 'n'. This corresponds to conversion of 20 mV to 1 V using materials like TGS. The power generated at a higher voltage, if small, could drive other energy extracting circuits. These extracting circuits in turn could provide electrical power from TGS at high voltages. Else the power generated by the capacitor could be utilized directly as a power multiplier. The voltage source could also be replaced with a pyroelectric generator (PEG). The PEG as well as the capacitor is subject to thermal fluctuations. The power generated by the PEG could be multiplied by the change in capacitance. Again, the proposed generator can be used as power multiplier.

In internal combustion engines or electromechanical systems where both mechanical vibra- tions and thermal gradients are present, the above mentioned capacitor and micro heat engine with a piezoelectric harvester could improve the power output drastically.

According to a further advantageous embodiment thermally stable electrets can be integrated into the capacitor in order to generate electric power from thermal transients, either naturally available or generated by micro heat engine (micro thermomechanic generator). Thus, a battery in the harvester can be dispensed with.

The presence of heat engines also favor the incorporation of micro switches in the system (thermally/magnetically actuated or contact based) to simplify the complex electronic switching system existing in current electrostatic generator based systems.

The accompanying drawings are incorporated into and form a part of the specification to il- lustrate several embodiments of the present invention. These drawings together with a description 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 invention. Further features and advantages will become apparent from the following more particular 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 the basic principle of electrostatic harvesting;

Fig. 2 shows the variation of dielectric constant with temperature for a conventional SMD

Y5 V capacitor 22 μ F 50 V;

Fig. 3 shows a bimetallic engine with a generator in a sandwich configuration in a first position;

Fig. 4 shows the arrangement of Fig. 3 in a second position;

Fig. 5 shows a partially exploded view of the arrangement of Figs. 3 and 4;

Fig. 6 shows a bimetallic engine and generator in a lateral configuration in a first position;

Fig. 7 shows the arrangement of Fig. 6 in a second position;

Fig. 8 shows a partially exploded view of the assembly of Figs. 6 and 7;

Fig. 9 shows a bimetallic engine and generator with two beams in a first position;

Fig. 10 shows the arrangement of Fig. 9 in a second position;

Fig. 11 shows the arrangement of Figs. 9 and 10 and in a partially exploded view;

Fig. 12 shows a bimetallic engine and generator comprising a bimetallic cantilever and magnets in a first position.

Fig. 13 shows the arrangement of Fig. 12 in a second position;

Fig. 14 shows the arrangement of Fig. 12 in a perspective view;

Fig. 15 shows the arrangement of Fig. 12 in a perspective and partially exploded view;

Fig. 16 shows a further bimetallic cantilever and magnetic engine in a first position;

Fig. 17 shows the arrangement of Fig. 16 in a second position;

Fig. 18 shows a perspective view of the arrangement of Fig. 16; Fig. 19 shows a perspective partially exploded view of Figs. 16 and 18;

Fig. 20 shows a perspective view of a further embodiment of a micro heat engine in a first position;

Fig. 21 shows a side view of the micro heat engine according to Fig. 20;

Fig. 22 shows a side view of the micro heat engine according to Fig. 20 in a second position;

Fig 23 shows a perspective view of still a further embodiment of a micro heat engine in a first position;

Fig. 24 shows a perspective partially exploded view of the arrangement of Fig. 23;

Fig. 25 shows a view of still a further embodiment of a micro heat engine in a first position;

Fig. 26 shows a perspective view of the arrangement of Fig. 25;

Fig. 27 shows a view of still a further embodiment of a micro heat engine in a first position;

Fig. 28 shows a perspective view of the arrangement of Fig. 27;

Fig. 29 shows a perspective, partially exploded view of the arrangement of Fig. 28;

Fig. 30 shows a view of still a further embodiment of a micro heat engine in a first position;

Fig. 31 shows a perspective view of the arrangement of Fig. 30;

Fig. 32 shows a perspective, partially exploded view of the arrangement of Fig. 31 ;

Fig. 33 shows an air-encapsulated micro heat engine in the first position;

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

Fig. 35 shows the operation of a switch in a first position according to an advantageous embodiment;

Fig. 36 shows the operation of the switch of Fig. 35 in a second position;

Fig. 37 shows a switched diode charge pump circuit; Fig. 38 shows a switch transistor charge pump circuit;

Fig. 39 shows a bootstrapping circuit for two generators;

Fig. 40 shows a side view of a micro engine with two complementary generators;

Fig. 41 shows a perspective view of the arrangement of Fig. 40;

Fig. 42 shows a schematic sectional view of a further embodiment of a micro engine with more than one generator in a first position;

Fig. 43 shows the arrangement of Fig. 42 in a second position;

Fig. 44 shows a schematic sectional view of still a further embodiment of a micro engine with more than one generator in a first position;

Fig. 45 shows the arrangement of Fig. 44 in a second position;

Fig. 46 shows a battery recharger with feedback circuit that is used to make the system self sustaining;

Fig. 47 shows the circuit of Fig. 46 with the feedback circuit according to a first embodiment;

Fig. 48 shows the circuit of Fig. 46 with the feedback circuit according to a second embodiment;

Fig. 49 shows the circuit of Fig. 46 with the feedback circuit according to a further embodiment;

Fig. 50 shows an alternative generator design involving a change in physical state at low temperature;

Fig. 51 shows the arrangement of Fig. 50 at an elevated temperature;

Fig. 52 shows a further alternative generator design at low temperature;

Fig. 53 shows the arrangement of Fig. 52 at an elevated temperature;

Fig. 54 shows several different designs for hinging a beam to its support. The invention will now be explained in more detail referring to the figures.

Different possible embodiments of the micromechanical generator according to the invention will be explained in the following with reference to Figures 3 to 32. Similar devices incorporating different kinds of micro heat engines attached to electrostatic harvesters are also possi- ble.

Suitable electronic circuits to charge the capacitors or store the generated energy in them are depicted in Figures 37 to 39 and 46 to 49. The circuits can be modified to charge the battery, thereby prolonging the life of the battery and the harvester. The switches involved can be realized via low power loss/consumption electronic switches. However, the electronic switches can be replaced with mechanical contact switches or reed switches or thermally actuated switches, which are integrated in the harvester itself, similar to the ones shown in the International Application WO 2012/025137 A1.

Table 1 gives an overview over materials having a permittivity that varies significantly with temperature. Table 1 :

List of materials with a permittivity that varies significantly with temperature

1. Rochelle salt (NaKC₄H₄0₆.4H₂0)

2. KH₂P0₄ type ferroelectrics

3. (NH₄)₂S0₄ and (NH₄)₂BeF₄ type ferroelectrics

4. Colemanite CaB₃0₄(OH)₂.H₂0

5. Thiourea

6. Glycin sulphate and derivatives like TGS

7. Glycin selenate and derivatives

8. Potassium Diphosphate KH₂P0

9. Barium Titanate (BaTi0₃)

10. Ti0₂ based condenser elements with various doping ions : CaO, SrO, BaO, MgO, Fe₂0₃

11. Complex pervoskites such as PZT

12. Ferroelectric polymers, such as Polyvinylidene Diflouride PVDF

13. Salts like (Bi,Na)Ti0₃, (K,Na).(Ta,Nb).0₃ 14. (NH₄)2Cd₂(S0 )2 type ferroelectrics

15. Cd₂Nb₂0₇ type ferroelectrics

16. PbNb₂0₆ type ferroelectrics

17. Certain alums like (CH₃NH₃).AI(S0₄)₂.12H₂0

18. Antiferroelectrics/ferrielectrics like PbZr0₃, PbHf0₃,NaNb0₃, W0₃, NH₄H₂P0₄ and isomorphous ammonium salts

In addition to the materials listed in Table 1 , the change in dielectric constant of liquids or solids on transformation to vapor / gaseous state can be utilized to build a generator similar to the one discussed above. For example, the dielectric constant of Acetone is approximately 20 in liquid state at 25 °C whereas the same in vapor state exhibits a dielectric constant of nearly 1 at 56 °C. Various generators incorporating this principle are illustrated in Figures 50 and 51. A list of liquids whose dielectric strength varies substantially above and below their melting and/or boiling points are given in Table 2.

Table 2: List of materials having a permittivity that varies below and above melting or/ and boiling points

In the following, different mechanisms according to the present invention are described for generating thermal oscillations on the electrostatic generator. Figures 3 to 5 show an arrangement having a bimetallic engine and a generator in a sandwich configuration. Figure 3 shows the micro heat engine 100 with an electrostatic capacitor 106 getting heated up and Figure 4 shows the state where it is cooled down. The two states are taken in a periodic fashion, from an available spatial thermal gradient. Figure 5 shows the partially exploded view of the assembly. The heat engine 100 consists of a buckled bimetallic strip 110 positioned in a thermally insulating beam support 108 between two heat reservoirs 102, 104 at different temperatures. When heated up, bending moments generated inside the bimetallic beam 110 cause the beam to flip its state from the one shown in Figure 3 to the state of Figure 4. The beam cools down after it comes in contact with the heat sink 104. The bending moments generated due to cooling are in the opposite direction compared to that of Figure 3. After the moment generated reaches a threshold, the beam flips back to its original state. The process continues whereby the capacitive generators 106 attached are also getting heated up and cooling down periodically, leading to a periodic change in the dielectric constant of the generator material. This temporal variation of dielectric constant is utilized for power generation and the genera- tors 106 are connected via terminals 112 to a corresponding circuitry.

Another possible embodiment incorporating the same working principle is shown in Figures 6 to 8. These Figures show an arrangement having a bimetallic engine 100 with a capacitive generator 106 in a lateral configuration. In particular, Figures 6 and 7 show the engine with the generator in the down and up position, respectively, and Figure 8 shows the partially ex- ploded view of the assembly.

Attaching the generator sidewise (laterally) increases the net thermal power transfer, compared to the generic design (sandwich configuration) discussed before. This is owing to the elimination of the two bimetal-generator thermal interfaces.

A further advantageous embodiment of a bimetallic engine and a generator according to the present invention having two beams 110, 110 is shown in Figures 9 to 11. In particular, Figure 9 shows the engine with the generator 106 attached to its sides in contact with hot reservoir 102 / downstate, whereas Figure 10 depicts the contact with the cold reservoir 104 / up state. Figure 11 shows the arrangement in a partially exploded view.

A further advantageous embodiment is shown in Figures 12 to 19. According to this em- bodiment, a bimetallic engine is provided with the generator according to the present invention in a cantilever arrangement and, furthermore, magnets 114 are provided at the hot and cold reservoirs 102, 104 for attracting the cantilever beam. In particular, Figure 12 shows the engine with the generator attached to its sides in contact with hot reservoir 102 / downstate and Figure 13 in contact with the cold reservoir 104 / up state. Figures 14 and 15 show the isometric and partially exploded views of the assembly.

According to this embodiment, the bistability is achieved by incorporating magnets 114 into the hot and cold reservoirs 102, 104. A cantilever 110 replaces the doubly clamped and supported beam described in the previous embodiments. The beam is ferromagnetic in nature. Hence, it is attracted by both the magnets. Depending on the temperature and initial deflection of the beam 110 it will get attached either the heat sink 104 or heat source 102. For a beam which is inherently not ferromagnetic or ferrimagnetic in nature, a small ferromagnetic or ferrimagnetic member could be attached to it.

For example, if this_. is attached to the heat source 102, as shown in Figure 12, it gets heated up due to convection as well as through conduction via the generator 106. Once the bending moment generated inside the bimetallic beam 110 exceeds a threshold, it flips its state and attains the state shown in Figure 13. Now the beam 110 is getting cooled down in a similar fashion. The beam 110 deflects downwards after the bending moments generated exceed a threshold and goes back to its state shown in Figure 12. The whole process repeats and the generator 106 is getting heated up and cooled down periodically. This leads to a periodic variation in the dielectric constant of the generator 106, which can be utilized to generate electric power. A modified bimetallic cantilever and magnet engine is shown in Figures 16 to 19.

In particular, Figure 16 shows the engine 100 with the generator 106 attached to its sides in contact with hot reservoir 102 / downstate and Figure 17 shows the state in contact with the cold reservoir 104 / up state. Figures 18 and 19 show the isometric view partially exploded views of the assembly, respectively. According to this modified version of the magnet engine, an additional heat conducting block 116 is arranged on the heat sink 104 and on the heat source 102. The modifications increase the heat transfer to and from the beam 110 significantly. The additional block 116 can be fabricated in different shapes and sizes and is not limited to the wedge-shaped one illustrated above. Yet another embodiment of a micromechanical generator to generate electric power from an available spatial thermal gradient is shown in Figure 20. The generator according to this embodiment comprises a bimetallic bistable beam 110, supported on either ends on bearings 108, sandwiched between two generators 106 and 106'. The beam 110 is ferromagnetic in nature. Embedded magnets 114 inside the reservoirs 102 and 104 ensure that the engine chamber is always in contact with either of the reservoirs 102 or 104 at any given point of time. For a beam that is inherently not attracted to a magnet, a suitable magnetic member which is ferromagnetic or ferrimagnetic or even a magnet itself with appropriate polarity could be attached to the beam.

The two states are depicted in Figures 21 and 22. As shown in Figure 21 , when the chamber 140 is in contact with the heat source 102, due to the magnetic force of attraction between the magnet 114 embedded inside the source 102, and the beam 110, generators 106, 106' and the bearings 108 are getting heated up. Bending moments generated inside the beam 1 0 due to heating causes the beam to flip upwards when it exceeds a threshold. Now the beam is closer to the magnet embedded inside the heat sink 104. As the force of attraction between the beam 110 and magnet 114 embedded in the heat sink 104 is more than that between the beam 110 and the magnet 114 embedded in the source 102, the engine chamber 140 is pulled towards the heat sink 104.

This new state is depicted in Figure 22. As the heat sink 104 is at a lower temperature than that of the source 102, the engine chamber 140 cools down. The beam 110 flips downwards when the bending moments generated inside the beam 110 exceeds a threshold. The cham- ber 140 is now in contact with the heat source 102. The whole process repeats. During this process the generators 106 and 106' are getting heated up and cooled down periodically. Energy can be harvested from these generators 106 and 106' in ways similar to that discussed before.

Figure 23 depicts yet another embodiment that is an enhanced version of the mechanism described in Figure 20. Figure 24 shows the exploded view of the embodiment. The heat transfer rate throughout the engine chamber 140 is enhanced by replacing the air inside the cavity 1 16 with a fluid of high thermal conductivity. The fluid is kept in the location with the help of sealing elements 130 and 130'. The working principle of the embodiment is quite similar to that of the embodiment described in Figure 20, albeit at a higher operational fre- quency and with a higher power output.

Figure 25 depicts an improved design of the embodiment described in Figure 20. Figure 26 shows the partially exploded view of the embodiment. The chamber 140 is supported on fine beams 118 and 118' onto spacers 120 and 120', sandwiched between the heat source 102 and sink 104. This enables the unit 100 to be mounted in any orientation with respect gravitational force or any other external forces like centrifugal/centripetal forces acting on the chamber. The beams 118 and 118' can be tuned to compensate the effects of external forces or unwanted forces present in the installation area. Figure 27 depicts an improved design of the embodiment described in Figure 23. The chamber 140 is supported on fine beams 118 and 118' onto spacers 120 and 120'. The spacers 120 and 120' are in turn sandwiched between the heat source 102 and sink 104. This enables the unit 100 to be mounted in any orientation with respect gravitational force or any other external forces like centrifugal/centripetal forces acting on the chamber. The beams 118 and 118' can be tuned to negate the effects of external forces or unwanted forces present in the installation area.

Figures 28 and 29 depict the isometric and partially exploded views, of the embodiment detailed in Figure 27, for the purpose of clarification.

Figure 30 shows yet another form of the embodiment albeit with integrated pyroelectric gen- erators 128 and 128'.

Figures 31 and 32 depict the isometric and partially exploded views of the aforementioned embodiment, respectively. The pyroelectric generators 128 and 128', capable of generating charges from thermal transients, replace the battery used to pre-charge the capacitor. The electric power/charges generated in the pyroelectric generators 128 and 128' can be fed di- rectly to the capacitive generators 106 and 106' or through other rectifiers or SSHI or SECE circuits disclosed in WO 2012/025137 A1. This eliminates the need for a battery, to pre- charge the capacitor prior to electric power.

Furthermore, also an air encapsulated engine as disclosed in WO 2012/025137 A1 can be used together with the capacitive generator 106 according to the present invention. The prin- ciples described in this document can be used in an analogous way, however, replacing the pyroelectric generator according to this document by a capacitive generator 106 according to the present invention. In Figures 33 and 34, the following components are shown, which are described in detail in Figures 1 to 9 and the belonging description of the published International Application WO 2012/025137 A1 :

104 Heat sink 132 Bistable membrane

128 Thermal transport 134 Air gap for thermal insulation

112, 112' Electrical contact 136 Thermal interface 102 Heat source Engine chamber

130 Hermetic sealing Cavity

As shown in Figures 33 and 34, the micro heat engine with an electrostatic capacitor 106 is getting heated up (Figure 33) and cooled down (Figure 34) in a periodic fashion from an available spatial thermal gradient.

As shown in Figure 33, when the engine chamber is in contact with the heat source, the en- gine chamber comprising the electrostatic generator 106 gets heated up. Simultaneously, the encapsulated air inside the chamber is also getting heated up and the pressure inside the chamber increases. Once this pressure reaches a threshold, the bistable membrane 132 flips and the engine chamber comes in contact with the heat sink 104, as depicted in Figure 34. When the engine is cooling down, the air inside the chamber is also cooling down, leading to a pressure reduction inside the chamber. Once the pressure falls below a threshold, the chamber flips down to its initial state shown in Figure 33. The whole process continues ensuring an intermittent transfer of heat from the heat source to the heat sink. Simultaneously, the electrostatic generator 106 also experiences periodic changes in its permittivity. This can be utilized to generate electric power. The embodiments mentioned above are for illustrative purpose only. Various other mechanisms to alternatively heat and cool the electrostatic generator viz. using shape memory alloys, alternatively pumping hot and cold fluids around the generators with passive or active control or a combination of both will also accomplish similar harvesters. Yet another method of heating and cooling the generator can be achieved with the help of a thermal switch which is operated by an external power supply or integrated battery. Such embodiments which have been disclosed to function as a micro heat engine to generate mechanical energy from a spatial thermal gradient and convert the mechanical energy to electric energy using a piezoelectric generator or electromagnetic generator. The present invention however utilizes the thermal transients generated by the aforementioned mechanisms to generate electrical power directly. This results in increased efficiency and power output of the harvester.

In the following, several advantageous electronic circuits which can be used for the power management, in particular the pre-charging and extraction of the capacitive generator according to the present invention will be explained in detail.

Figures 35 and 36 show the operation of switches which are embedded in or attached onto the heat source 102 of one of the embodiments aforementioned in Figures 3 to 34. When the engine is in down state, the bimetallic beam is in contact with the plunger 148 of the micro switch 146. The switch is now in its ON' state. When the engine attains its up state, the force on plunger vanishes and the switch goes back to its 'OFF' state. Depending on the nature of the switch a plurality of electrical circuits can be made or broken. It is even possible to i.e. close a few circuits and open up others, if the switch is SPDT, DPDT etc. One or more similar switches can be embedded in or attached onto the sink 104 to get a complimentary switching action. These switches are not limited to the type described in Figures 35 and 36. Similar functionality can be attained with the help of magnetic reed switches, thermally actuated switches etc. with little modifications in the embodiment 100. These switches are utilized in the following electrical circuits used to store and harvest the electrical energy generated by the embodiment 100.

Figure 37 shows as a first circuit design a switched diode charge pump.

As shown in Figure 37, the battery source V_Dc is used to pre-charge the capacitor C_VA via diode D1 to a voltage

at the end of the cooling cycle (up state of engines men- tioned above or the like). The capacitor is then subjected to heat so that the temperature change reduces its capacitance, in the down state of the embodiments discussed in or those similar to those discussed above. The reduction in capacitance introduces a corresponding rise in voltage across capacitor plates from This leads to charges getting pushed through diode D2 to storage capacitor C_ST_> to a voltage V₂-V_D2f. Each cycle introduces more and more charge accumulation on CST, leading an increase in voltage and energy stored over time. A super-capacitor used for storage could be, for example, a very reliable storage.

A switched transistor charge pump is shown in Figure 38. According to this embodiment, the battery source V_DC is used to pre-charge the capacitor C_VAR via transistor connected in diode configuration to a voltage of V^Voc-V™ at the end of the cooling cycle (up state of en- gines mentioned in or similar to those shown in Figures 3 to 34). The capacitor is then subjected to heat so that the temperature change reduces its capacitance, in the down state of the embodiments discussed in or those similar to those explained above. The reduction in the capacitance C_ST introduces corresponding rise in voltage across capacitor plates from \ '. The charges at this elevated voltages are transferred to the storage capacitor C_ST via another transistor T₂ in diode configuration to a voltage V₂'-VT2f. Each cycle introduces more and more charge accumulation on CST- The circuit in Figure 39 shows a charge/current/energy mirroring circuit where the alternate switching of the capacitors is used to gain maximum voltage swing possible. Both capacitors are heated and cooled out of phase i.e. while C_vari is getting heated up C_var2 is getting cooled down and vice-versa. When the capacitor C ARI is getting heated up while C A 2 is in the cooling phase, charges flow from C_VAR1 to C_var2-Once CVARI attains its minimum value, i.e. at the end of its heating phase the switches flip their position. Now as C_var2 is getting heated up, charges flow to C_vari - The switches flip again at the end of its heating phase. The process repeats. As the charges deposited on each generator capacitor increases in each cycle, the voltages across them as well as energy stored also increase with each cycle. This circuit utilizes both heating and cooling cycles of the device and improves efficiency considerably.

Figure 40 shows an embodiment to attain the alternate heating and cooling of the pair of generators in a complimentary fashion, to realize the generators mentioned in Figure 39.

An isometric view of the embodiment is shown in Figure 41.

The bistable beam 1 10 is now operated in its second mode of buckling. As one half 110" of the beam 1 10 is getting heated up, the other half 10"' is getting cooled down. Opposite bending moments are generated in each of the halves 110" and 1 0"', due to heating and cooling, respectively. Once the moments exceed a threshold, the beam flips its state. Half of the beam 1 10", previously in contact with the heat source comes in contact with the heat sink. The opposite holds true for the other half 110"'. So the attached generators to these two halves are getting heated up and cooled down in a complimentary fashion. Mechanical switches embedded into the system 100 appropriately, as described in Figure 35 and 36, realize the SPDT switch illustrated in Figure 39. For additional reliability the midpoint of the beam 1 10 can also be hinged.

Figures 42 and 43 depict yet another embodiment consisting of two sets of generators. This is essentially a modified version of the embodiments aforementioned. An additional heat sink 104' is stacked adjacently to the heat source 102 of the system 100 described in Figures 3 to 34. Generator 106" is connected to beam through the member 134. The generator 106 is cooling down while the generator 106" is heating up. As the beam cools down and flips to its second stable state, the generators also flip their positions, and attain the state shown in Figure 43. Generator 106 is now getting heated up, whereas generator 106" is cooling down. The entire process repeats ensuring a pair of generators in operation, which are getting heated up and cooled down in a complimentary fashion. Electrical energy is generated from the embodiment by utilizing the circuit described in Figure 39 and micro switches embedded inside or attached to the micro heat engine 100, as depicted in Figures 35 and 36.

Figures 44 and 45 depict yet another mechanism to heat up and cool down a plurality of generators, part of which is getting heated up, while the other part is getting cooled down. As the bimetallic beam 1 10 is getting heated up, bending moments are generated inside it. Simultaneously, generators 106 and 106" are getting heated up and cooled down, respectively. Once the bending moments reaches a threshold, the beam flips its state. The generators which are connected via a member 134, which itself is hinged to another member 134' at its midpoint. This mechanism ensures that, when the beam flips its state to that shown in Figure 45, the generators 106 and 106" flip their states too, like in a See-saw. As a result, generator 106 is cooling down while generator 106" is heating up. Simultaneously beam 1 10 is also cooling down inducing bending moments inside it. Once the bending moment exceeds a threshold, the beam 110, along with the generators 106 and 106", flips back to its state shown in Figure 44. The whole process repeats ensuring a complimentary heating/cooling pattern for generators 106 and 106".

The circuit in Figure 46 shows a feedback circuit which is used to recharge the battery from charge of capacitor C_ST- This feedback mechanism ensures that battery operation is self sustained and life of energy harvester as well as that of the battery is prolonged. Various circuits to achieve the feedback are detailed in the following diagrams. Figure 47 shows a circuit where in the switches can be electronic type or micro switches integrated into the system 100 as described by Figures 35 and 36. Battery V_SOURCE is closed when the beam 1 10 in system 100 is in contact with heat sink 104. Charges flow from VSOURCE to capacitors C_ST as well as C_VAR, depending on the voltage difference across the diodes D and D₂. When the beam 1 10 has flipped and is in contact with the heat source 102, switch S_Wc is opened and switch SWH is closed. As the generators 106, 106' or/and 106" heat up, the voltage across C_VAR increases. Once this voltage exceeds the sum of the voltages across C_ST, V_BAT and V_D3, charges flow into the battery. The voltage across C_Sj rises due the same charges or current flowing into it as well. Once cooled down, the beam 1 10 flips back to its other state. Now, switch S_Wc is closed whereas switch S_WH is open. As the voltage across C_VAR is low, charges flow from C_ST to C_VAR. Charges cannot flow from VSOURCE to CST as the diode D, is reverse biased. The whole process repeats ensuring that charges are deposited into battery V_BAT, cyclically. This system essentially converts a small number of charges at a high potential into an increased number of charges at lower potential, or in other words it works like a charge transformer. In other words, the circuit mirrors the charges and consequently the current flowing into the battery onto the capacitor C_ST- These charges stored in C_ST flow into the generator, to pre-charge it during the next cooling phase. The potential of the charges are changing due to the switching, leading to a charge-transformer act. A modified version of the circuit mentioned in Figure 47 is depicted in Figure 48. The circuit is advantageous as there is only one battery V_BAT- In comparison to the circuit in Figure 47, VBAT acts as both V_SOURCE and V_BAT_> though not simultaneously. When the beam 1 10 is in contact with heat sink 104, switches S_Wc are closed whereas SWH are opened. Both the capacitors could be charging, depending on the biased state of the diodes Di and D₂. When the beam flips its state after cooling down sufficiently, the capacitance of the generator/s 106, 106' and/or 106" is maximum. Once flipped, switches S_Wc are opened and S_WH are closed. As the generator/s gets heated up, the voltage across it/them increases. Once this voltage exceeds a threshold, the diode D₃ is forward biased and charges flow into the battery and CST- Once heated up sufficiently, beam 1 10 flips its state and the whole process repeats. Figure 49 depicts yet another configuration to recharge the battery V_SOURCE from the generator itself. The operation of the circuit is quite similar to that of circuits described in Figures 47 and 48. The circuits depicted here to recharge the battery are only for illustrative purpose. Different versions of the circuits, incorporating charge transformation or mirroring principle, can be devised by those familiar with the art. For example, a plurality of generators could be charged in parallel and later discharged in series or a parallel-series combination using switches to get the desired amount of charges at a specified voltage.

Referring now to Figures 50 and 51 , a further capacitive generator design involving a change in physical state - solid to liquid, liquid to gas or solid to gas - is explained in more detail. In particular, according to the present invention, the generator comprises an electrode structure 220 that work as a sort of micro wicks 222 based on capillary forces.

Figure 50 shows a liquid filled chamber with micro wicks 222 at low temperature (lower than the boiling point of the fluid 224) and Figure 51 shows the same assembly at an elevated temperature. At elevated temperatures the liquid 224 vaporizes, filling the chamber with vapor 226 and leading to a reduction in dielectric constant of the medium in between the walls 220 of the wicks. The walls 220 act like electrodes as well, either by virtue of their own conductivity or due to a very fine layer of electrode deposited on their inner surface walls. When cooled down, the liquid 224 is drawn back through the wicks. Condensation in other regions can be prevented with the help of hydrophobic surfaces 218. In a similar fashion, hydrophilic surfaces/coatings 228 on the inside surfaces of the wicks can enhance the drawing of fluid back into the wicks 222 during the cooling period. Depending on the fluid used, the coating properties vary. For example with water as a fluid, Teflon works well for the hydrophobic surface and Titanium dioxide for the hydrophilic surface. However, if acetone is used instead of water, different kinds of coatings are necessary.

Figures 52 and 53 show another arrangement of dielectric materials 304 with a pair of eiectrets 302 and 302'. Eiectrets are materials with permanent embedded charges which last for 50 years or more. Conventionally, such eiectrets based capacitive generators utilize change in effective area or gap to generate electrical power from ambient mechanical vibrations. However, the embodiment described here utilizes the temperature dependence of the dielectric constant of the medium between the eiectrets to generate electrical power from thermal transients or oscillations. As depicted, the dielectric material whose dielectric constant and relative permittivity depends on temperature, is sandwiched between a pair of dielectric layers with opposite polarity. These charges polarize the dielectric material just like a voltage applied across it.

However, these charges are immobile and would not power a load resistor connected across the metal electrodes. If this embodiment is heated up, the dielectric constant of the material sandwiched in between reduces. So the polarization inside the material also reduces. Since the charge density has to reduce, to account for the change in polarization, negative charges flow into the positive electrode 330 and positive charges to the negative electrode 330'. This constitutes a current through the load resistor 320, as shown in Figure 53. When the dielectric material 304 is cooled down, the charges are neutralized by a current flow in the reverse direction, as shown in Figure 52. Thus electrical power can be generated using such a generator without a pre charge source. With alternate heating and cooling of the arrangement according to this embodiment using the embodiments mentioned previously, a cyclic power generation is possible. The device works, in principle, like a pyroelectric generator. However, it must be noted that a pyroelectric effect does not exist in such materials as they operate above their Curie temperature. Moreover, in pyroelectric materials the power produced is lower for materials with high values of dielectric constants whereas in this embodiment it is higher for materials with high dielectric constants.

Figure 54 depicts a detailed view of the bearing profile. The special profiles 150 or 160 depicted here enable the beam to be flipped upwards and downwards for very low temperature differences. For example, when the beam is heated up, axial forces are generated inside the beam in addition to the bending moments. If the beam is clamped or hinged at the ends with conventional hinges, these axial forces themselves create a moment which opposes the bending moments generated due to difference in coefficient of thermal expansion of the two constituent layers of the beam 1 10. The beam 1 10 supported on the bearings 108 is such that the ends of the beams are sliding while on the bearing surface 150 or 160 when the beam 1 10 is getting heated up. The motion of the beam in this fashion reduces the axial forces to a great extend resulting in the beam flipping with less heating up. The same holds true for the flipping down action also. The profiles are for illustrative purpose only. Similar advantageous profiles can be designed by those familiar with the art.

Claims

Micromechanical generator system comprising: a micro motor arranged between two reservoirs having different temperatures, said micro motor being operable to move at least one 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 transducing member is thermally coupled to a capacitive generator for transforming said temporal temperature variation into electric power via a variation of a dielectric constant of the capacitive generator.

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

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

Micromechanical generator system according to at least one of the preceding claims, wherein said micro motor comprises at least one flexible beam with a ferromagnetic member, or at least one bimetallic member.

Micromechanical generator system according to at least one of the preceding claims, wherein said micro motor comprises a flexible beam that is clamped on two sides or that is arranged as a cantilever which is fixed only on one side, or a combination of both.

Micromechanical generator system according to at least one of the preceding claims, wherein two capacitive generators are provided on two opposing surfaces of a bimetallic flexible beam.

7. Micromechanical generator system according to at least one of the claims 1 to 3, wherein said micro motor comprises a bistable membrane stretched over a compart- ment comprising a working fluid, said working fluid actuating the bistable membrane, thereby moving said transducing member.

8. Micromechanical generator system according to at least one of the preceding claims, wherein said capacitive generator is connected to a supply battery, an electromagnetic generator, a pyroelectric generator, a thermoelectric generator or a piezoelectric generator for being charged.

9. Micromechanical generator system according to at least one of the preceding claims, wherein said capacitive generator comprises a dielectric material exhibiting ferroelectric and/or ferrielectric characteristics, around its Curie temperature.

10. Micromechanical generator system according to at least one of the preceding claims, wherein said capacitive generator comprises a dielectric material that changes its phase between said first and second position.

11. Micromechanical generator system according to claim 10, wherein said capacitive generator comprises a sealed compartment containing said phase change material, and a plurality of electrodes which are arranged in a distance to each other which is chosen to exert capillary forces on the phase change material.

12. Micromechanical generator system according to at least one of the preceding claims, wherein a clamping profile is provided to oscillate from low thermal gradients.

13. Micromechanical generator system according to at least one of the preceding claims, wherein integrated switches are provided that are activated or deactivated synchronously, depending on the state of the device.

14. Micromechanical generator system according to claim 13, wherein the integrated switches are actuated by mechanical force, thermal fields, changing thermal fields, proximity to a magnet or magnetic material, and/or proximity to a charged material, preferably an electret.

15. Micromechanical generator system according to at least one of the preceding claims, wherein said capacitive generator comprises a dielectric material whose dielectric constant depends on temperature is sandwiched between a plurality of bound charges, of either polarity, capable of producing a permanent electric field/ polarization in the dielectric material.

16. Micromechanical generator system according to at least one of the preceding claims, wherein a voltage source is attached to the electrostatic generator via switches in one position to pre-charge the generator and connected in a second position to recharge itself from the electric power generated by the electrostatic generator.

17. Micromechanical generator system according to at least one of the preceding claims, wherein one electrostatic generator is heated up while another one is getting cooled down and vice versa, by operating a beam in its second mode of buckled state.

18. Micromechanical generator system according to at least one of the preceding claims, wherein multiple electrostatic generators are linked such that part of them are getting heated up while the remaining are getting cooled down and vice versa.

19. Micromechanical generator system according to claim 18, wherein in the link incorporates mechanical, magnetic, thermal or electrostatic systems to flip the two sets of generators in a complimentary fashion.

20. Micromechanical generator system according to at least one of the preceding claims, wherein the system is operable as a power generator amplifying the power generated by a plurality of pyroelectric, piezoelectric, electromagnetic or thermoelectric generators existing externally and operating independent of the generator itself.

21. Micromechanical generator system according to at least one of the preceding claims, wherein the system is operable as a power generator amplifying the power generated by a plurality of pyroelectric, piezoelectric, electromagnetic or thermoelectric generators integrated into the generator itself.

22. Micromechanical generator system incorporating a plurality of electrostatic generators, wherein the energy generated in one or more generators is transferred to the other generators in a periodic fashion to enhance power generation.

23. Micromechanical generator system according to at least one of the preceding claims, wherein special surface profiles, coatings or interfacial materials; in solid, liquid, gas state or a combination of two or more of the three states; are utilized to enhance the heat transfer at the contact or mating regions.

24. Micromechanical generator system according to one of the claims 16, 18 and 22, wherein an alternatively switching voltage source and alternatively heated or cooled down generators are combined in a complimentary fashion for enhancing the power generated, and for storing the generated power in the battery.

25. Micromechanical generator system according to at least one of the preceding claims, wherein charges at a given voltage can be elevated to another level thereby functioning as a voltage transformer, for harvesters based on thermoelectric, electromagnetic, electrostatic, piezoelectric, pyroelectric or triboelectric principles or a multiple combination of them, with low output voltages.

26. Micromechanical generator system according to at least one of the preceding claims, wherein a material, whose dielectric constant varies with temperature, is sandwiched between bound or static charges in the form of electrets, to create a permanent electric field inside the dielectric material, so as to eliminate the need of a battery or any other voltage source to pre-charge the dielectric.

27. Micromechanical generator system according to at least one of the claims 10 and 11, wherein the electrode surfaces are tailored to enhance the capillary forces.

28. Micromechanical generator system according to at least one of the claims 10, 11 and 27, wherein the inner surfaces of the chamber are modified such that the phase change material is unable to attach itself onto the walls via surface forces.

29. Micromechanical generator system according to at least one of the preceding claims, wherein the amplified power or voltage itself is utilized to activate or operated other energy harvesting circuits of another harvester.

30. Micromechanical generator system according to at least one of the preceding claims, wherein charges and energy form one electrostatic generator is transferred to the other, which is operating in a complimentary fashion, and vice-versa using switches.