WO2006056809A1 - Solid state electrocaloric cooling devices and methods - Google Patents

Abstract

A cooling device comprises several electrocaloric working elements (200, 210, 220, 230) with working temperatures covering a required temperature range. The elements are separated by heat switches (100, 110, 120, 130, 140) . The number of electrocaloric elements and heat switches can be different in other embodiments. The device can be cascaded or paralleled to provide greater cooling. Thermoelectric heat switches are provided for switching from a heat conducting to a heat insulating state. In some embodiments, the thermoelectric switches are replaced by microelectromechanical switches. The thermoelectric switches can operate in passive heat conducting mode. In some embodiments, the switches can operate in active heat conducting mode, with the reverse current passed through the couples in the off heat conduction state, electrocaloric elements can take the form of multilayer capacitors comprised of ferroelectric thin films .

Description

M&C Folio: WPP290547

Solid State Electrocaloric Cooling Devices and Methods

The present invention relates to solid state electrocaloric cooling devices with heat switches, and to related methods of cooling.

Some observers believe the physical limits of silicon will one day slow or halt the steady advance of the computing industry. Today though, a more immediate threat to Moore's law has emerged. The same forces that enable exponential improvements in processor performance are creating enormous problems in another dimension: power. As the transistor density increases, processors consume more power and generate more heat, at an accelerating rate. If past trends continue, with no change in power management, then, over the next decade, high-performance computing devices will surpass the industry's ability to cooi them economically (www.intel.com/labs). Despite the many novel approaches that are being tried now to develop alternative cooling methods, there is a need for improved techniques.

Common refrigeration systems use greenhouse gases that are hazardous to the environment. We describe a new reliable active solid state device without moving parts proposed which can offer some advantages over the common approach to refrigeration, both in the electronics and computer industries, and in home and industrial refrigeration systems. Being more efficient, the proposed refrigeration method can help to save energy spent on numerous refrigeration facilities all around the world.

An eSectrocaloric cooling system comprised of a plurality of electrocaloric (EC) working elements (an example of one is depicted in Fig. 1 ) with a range of working temperatures covering a wide temperature interval was described in Y.V. Sinyavsky and V. M. Brodyansky. Ferroelectrics. 131: p. 321 , 1992. and A.E. Romanov, Ju.V. Sinyavskij, N. D. Pashkov, and G. E. Luganskij. "Method of cooling and device for realization of this method", SU Patent No 2075015, 1993. The heat exchange between adjacent electrocaioric working elements as well as heat flow to the ambient and from the cooled body was organised with fluid and gas heat exchangers. The prototype schematic is shown in Fig. 5. The working elements were made of PbSco.₅Tao.₅O₃ with various thermal treatment so that their working temperatures were different. Variations of ECE versus temperature for different working elements are shown in Fig. 2. The prototype comprised two blocks of working elements 1 and 2 (see Fig. 5), heat load 3, and heat sinks 4, The liquid heat exchanger was pumped through the system with a pump 5. Four blocks of PST have been combined into a single shell. Each block had a sandwich type structure comprising a number of PST plates 20 x 10 x 0.3 mm³ each. Each block was 55 mm long with a cross section of 10 x 5 mm². The total mass of the PST working material was 35 g. The results of the experiments with the prototype are shown in Fig. 6. Curve 1 shows the ECE of a single EC element, and curve 4 shows the temperature difference between the heat load and the heat sink. The maximum field for Curve 1 is 30 kVcm^"1 and could not be increased due to surface arc (the experiment was carried out in air). Due to the same reason the disclosed prototype with a gas heat exchanger, He, was studied only up to 30 kVcrrf ¹ (see Curve 2). The ECE effect of a working element placed in pentane, a heat exchanger liquid, is shown by Curve 3. The ECE value is lower due to higher heat capacity of pentane compared to that of a gas. However, the net temperature difference developed between the ends of the whole device with liquid pentane as a heat exchange liquid is much higher and shown by Curve 4.

Electrocaioric refrigeration systems for cryogenic temperature ranges (from 20 K and below) are disclosed in L.W. Lawless. "Paraelectric refrigeration method and apparatus", US Patent No 3436924, 1969. and L.W. Lawless. "Closed- cycle electrocaioric refrigerator and method", US Patent No 3638440, 1972. An electrocaioric refrigeration system comprising a number of working elements separated by passive unidirectional heat pipes was proposed in A. Basiulis and B. L. Robert. "Solid-state electrocaioric cooling system and method", US Patent No 4757688, 1986. The proposed unidirectional heat pipes work by principle of unidirectional heat transfer accomplished by evaporation of a liquid heat carrier, e.g. N₂ in a cryogenic temperature range. However, such a process is slow and has a large inertia, which makes the realization of such a device difficult.

Summary of the Invention

In a first aspect the invention provides a device for transferring heat from a heat source to a heat sink, the device comprising: at least two heat switches, a first to receive heat from the heat source, a second to pass heat to the heat sink; at least one electrocalohc element between two said heat switches and in thermal contact with said two heat switches; and a control input for controlling said at least two heat switches and said electrocaϊoric element to transfer heat from said source to said sink; and wherein one or both of said heat switches comprise a thermoelectric heat switch, tunnelling heat switch or an electromechanical heat switch.

Preferably the device for transferring heat from a heat source to a heat sink is a part of a cascaded system having a plurality of devices for transferring heat from a heat source to a heat sink. More preferably the device for transferring heat from a heat source to a heat sink it is a stacked cascaded system and comprises a parallel system. The control input may comprise one or more eiectrical connections.

Preferred embodiments further comprise a controller to control said heat switches and said at least one electrocaloric element, the controller comprising control means for: cooling said electrocaloric element to a temperature of less than said source or equal to said source; turning said first heat switch on to transfer heat from said source to said electrocaloric element; turning said first heat switch off; warming said electrocaloric element to a temperature of greater than said sink or equal to said sink; turning said second heat switch on to transfer heat from said electrocaloric element to said sink; and turning said second heat switch off. In another aspect the invention provides a method of controlling a heat transfer device for transferring heat from a heat source to a heat sink, the heat transfer device comprising at least one electrocaloric element disposed between a pair of thermoelectric heat switches, a first switch being in thermal contact with said source and a second switch with said sink, the method comprising: cooling said electrocaloric element to a temperature of less than said source or equal to said source; turning said first heat switch on to transfer heat from said source to said electrocaloric element; turning said first heat switch off; warming said electrocaloric element to a temperature of greater than said sink or equal to said sink; turning said second heat switch on to transfer heat from said eiectrocaloric element to said sink; and turning said second heat switch off; and wherein turning said heat switches off comprises applying a voltage in a first, forward direction to said switches; and wherein turning said switches on comprises applying a voltage in a second, reverse direction to said forward direction to a said switch.

There is also provided a controller configured to implement this method.

The invention further provides processor control code to implement the above- described controllers and methods, in particular on a data carrier such as a disk, CD- or DVD-ROM, programmed memory such as read-only memory (Firmware), or on a data carrier such as an optical or electrical signal carrier. Code (and/or data) to implement embodiments of the invention may comprise source, object or executable code in a conventional programming language (interpreted or compiled) such as C, or assembly code, code for setting up or controlling an ASIC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array), or code for a hardware description language such as Verilog (Trade Mark) or VHDL (Very high speed integrated circuit Hardware Description Language). As the skilled person will appreciate such code and/or data may be distributed between a plurality of coupled components in communication with one another. Broadly speaking, we will describe a cooling device comprising several electrocaloric working elements with working temperatures covering a required temperature range. The elements are separated by heat switches. The number of electrocaloric elements and heat switches can be different in other embodiments. The device can be cascaded or paralleled to provide greater cooling. Thermoelectric heat switches are provided for switching from a heat conducting to a heat insulating state. In some embodiments, the thermoelectric switches are replaced by microelectromechanica! switches. The thermoelectric switches can operate in passive heat conducting mode. In some embodiments, the switches can operate in active heat conducting mode, with the reverse current passed through the couples in the off heat conduction state. electrocaloric elements can take the form of multilayer capacitors comprised of ferroelectric thin films. In some embodiments, the electrocaloric elements can be made from bulk materials. Thermoelectric heat switches can be energized by a direct electrical coupling to a current source or by induction coupling.

These and other aspect of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:

Figure 1 Shows a schematic of an electrocaloric element.

Figure 2 Shows results of the electrocaloric effect measurement of four different samples of PbSco.₅Tao.₅O3 used in the prototype shown in Fig. 5. The Curie point, taken as the temperature of the maximum of ECE, can be moved, which allows a much wider temperature interval to be covered than with a single element.

Figure 3 Shows the results of the electrocaloric effect measurement of

Pbo.99Nbo.o2(Zro.75Sno.2Tio,o5)o.9βθ3.

Figure 4 Shows a thermoelectric couple. Figure 5 Shows a schematic of an electrocaioric fridge prototype. Some characteristics of the prototype are shown in Fig. 6.

Figure 6 Shows experimental results obtained with the prototype shown in Figure 5.

Figure 7 Shows schematic design of one embodiment of the proposed cooling device comprising one electrocaloric element 200 and two heat switches 100 and 110. Electronic circuits that supply the electrocaloric element and heat switches are not shown here for the sake of clarity.

Figure 8 Shows schematic design of one embodiment of the proposed cooling device, a cascaded system comprising 2 electrocaSoric elements 200 and 210; 3 heat switches 100, 110 and 120; and electronic circuits that supply the eSectrocaioric elements and heat switches. The circuits are not shown here for the sake of clarity.

Figure 9 Shows the working cycles of the cooϋng sandwich shown in Figure 8. Other embodiments of the proposed device (e.g. cascaded devices comprising 4, 6, etc. electrocaloric elements) can use working cycles of the same principle. Heat exchange between 200 and 210 takes τ seconds and between 200 and 10 {210 and 20) - 2τ seconds. A label ΔT| stands for a polarisation of an electrocaloric element (either 200 or 210) that increases its temperature by ΔT via the electrocaloric effect and a blue label ΔT| - a depolarisation of an electrocaloric element (either 200 or 210) accompanied by its cooling with AT. See also Fig. 15, later.

Figure 10 Shows the schematic design of another embodiment of the proposed cooling device, a cascaded system comprising 4 electrocaloric elements 200, 210, 220 and 230; 5 heat switches 100, 110, 120, 130, and 140; and electronic circuits that supply the electrocaloric elements and heat switches. The circuits are not shown here for the sake of clarity, Figure 11 Shows an example thermodynamic cooling cycle of an electrocaloric element (e.g. 200, 210 in Fig. 8).

Figure 12 Shows an example design of a micromechanical heat switch which can be fabricated with a method presented in Fig. 13.

Figure 13 Shows microfabrication of a ladder micromechanical heat switch shown in Fig. 12.

Figure 14 Shows a schematic design of still another embodiment of the proposed cooling device, a cascaded system comprising 6 electrocaloric elements 200, 210, 220, 230, 240, and 250; 7 heat switches 100, 110, 120, 130, 140, 150, and 160; and electronic circuits that supply the electrocaloric elements and heat switches. The circuits are not shown here for the sake of clarity.

Figure 15 Shows results of an example calculation of an example cooling cycle of one embodiment of the device - the one comprising 2 electrocaloric elements, shown in Fig. 8. The temperatures of the heat source 20 (T₂₀), EC elements 200, 210 [T_2Oo, T₂₁₀), and the heat sink 10 [T₁₀) are shown. The picture represents equilibrium - 600 cycles have been simulated before these curves are taken. Four stages of a possible refrigeration cycle are highlighted (see Fig. 9). The electrocaloric effect is taken to be the same in both elements: AT₁ = M₂.

Figure 16 Shows efficiency and cooling power of one embodiment of the device comprising 4 electrocaloric elements (shown in Fig. 10) versus heat conductivity of the switches, τ is the time for heat exchange between any two electrocaloric elements (A.3).

Figure 17 Shows efficiency and cooling power of some embodiments of the device (shown in Figs. 7, 8, 10 and 14): cascaded devices comprising 2, 4, and 6 electrocaloric elements in passive (the reverse current in heat switches is iRev = 0 A) and active (IRΘV = 0.05, 0.1 and 0.2 A) heat conducting modes. The influence of the reverse current on the performance shown in Fig. 18 is calculated at the highlighted temperature differences: 3 K for a cascaded device comprising 2 electrocaloric elements, 6 K for a cascaded device comprising 4 electrocaloric elements, and 10 K for a cascaded device comprising 6 electrocaloric elements.

Figure 18 Shows the influence of the heat switches' reverse current on the performance of some embodiments of the cooling device: cascaded devices comprising 2, 4, and 6 electrocaloric elements. The particular current values can be reduced by increasing the number of couples N and decreasing the geometrical factor G - a standard approach in design of thermoelectric devices. It should be understood that the values used in the catcuiations are not fixed.

Figure 19 Shows estimated efficiency of some embodiments of the proposed device: cascades comprising 2, 4, and 6 electrocaloric elements and a commercially available thermoelectric cooler. The devices operate in both passive (W = O A) and active (l_Rev = 0.05, 0.1 and 0.2 A) heat conducting modes. The average temperature difference between the hot and the cold ends is about 6— -8⁰C for all curves. Data for the Melcor device is taken from fwww.melcor.com, for a device No PT8-7-30 with G = 0.17 cm, N = 71 , heat resistance r_Hβat = 0.04⁰CW^"1).

Figure 20 Shows I. Temperature of an electrocaloric element with higher initial temperature (T_Hot) and an electrocaloric element with lower initial temperature (Tco_td) interspaced with a thermoelectric device with a current of 0, 0.05 and 0.2 A versus time. The current is applied at t = 0 s, when Tπot = 305 K and Tcai_d = 300 K; and II. As above for a current of 0.2 A. Joule heating linear with time manifests on a 10 s timescale.

Figure 21 Shows a schematic design of an embodiment of a parallel configured device for transferring heat from a heat source 20 to a heat sink 10. Introduction

it is helpful to first outline features of some preferred embodiments of the invention.

We describe a device for transferring heat from a heat source 20 to a heat sink 10, see Fig. 20. The device comprises at least two heat switches 100 and 110, the former to receive heat from the heat source 20, the latter to pass heat to the heat sink 10; at least one electrocaloric (EC) element 200 between the heat switches 100 and 110 and in thermal contact with the heat switches; and a control input for controlling 100, 110, and 200. One or both of the heat switches comprise a thermoelectric heat switch, tunnelling heat switch or an electromechanical heat switch.

Preferably, a device described above is a part of a cascaded system having a plurality of the devices for transferring heat from a cold end to a hot end. Some embodiments of a cascaded system are presented in Figs. 7, 9, and 13. Referring to Fig. 7, a cascaded system comprising two electrocaloric elements 200 and 210 and three heat switches 100, 110 and 120 is presented. An electrocaloric element 210 of an additional stage is utilised as a heat source in the previous stage of the cascaded system. In other words, the embodiment of a cascaded system shown in Fig .8 comprises two cooling devices: a first device comprising an electrocaloric element 200 and heat switches 100 and 110 and a second device comprising an eiectrocaloric element 210 and heat switches 110 and 120.

Referring to Fig. 9, a cascaded system comprising four electrocaloric elements 200, 210, 220, and 230 and five heat switches 100, 110, 120, 130, and 140 is presented. Electrocaloric element 210 is utilised as a heat source in the previous stage comprising the electrocaloric element 200. Electrocaloric element 220 is utilised as a heat source in the stage comprising electrocaloric elements 200 and 210. Electrocaloric element 230 is utilised as a heat source in the stage comprising electrocaloric elements 200, 210, and 220. In other words, the system comprises four cooling devices: a first device comprising an electrocaloric element 200 and heat switches 100 and 110, a second device comprising an eiectrocaloric element 210 and heat switches 110 and 120, a third device comprising an electrocaloric element 220 and heat switches 120 and 130, and a fourth device comprising an electrocaloric element 230 and heat switches 130 and 140. The hot end of a fourth device is the cold end of the third device, the hot end of the third device is the cold end of the second device, and the hot end of the second device is the cold end of the first device.

Referring to Fig. 13, a cascaded system comprising six electrocaloric elements 200, 210, 220, 230, 240, and 250 and seven heat switches 100, 110, 120, 130, 140, 150, and 160 is presented. It should be understood that cascaded systems comprising any other number of electrocaloric elements and heat switches can be used.

Another embodiment of the present invention, a parallel configured device for transferring heat from a heat source 20 to a heat sink 10, is presented in Fig. 21. A plurality of parallel positioned stacks 500, 510, 520 and 530 transfer heat from a heat source 20 to a heat sink 10. The number of stacks can be different. Each stage in each stack 500, 510, 520 and 530 comprises an electrocaloric cooling system presented in one or several embodiments of the present invention. Other configurations and orientations of the elements shown in Fig. 21 may be used.

An off state of heat switches comprises applying a voltage in a first, forward direction to the switches, and an on state of the switches comprises applying a voltage in a second, reverse direction to the forward direction to the switches. However, the heat switches do not need to fully reject heat in an off state. Embodiments of a device of the present invention also comprise a control input that comprises one or more electrical connections, and means to control heat switches and electrocaloric elements (e.g. a computer program).

As previously mentioned, we also describe a method of controlling a heat transfer device for transferring heat from a heat source to a heat sink. As an example, a method is considered for an embodiment comprising one eiectrocaioric element 200 and two heat switches 100 and 110, see Fig. 20. The method comprises: cooling electrocaloric element 200 to a temperature of less than source 20 or equal to the source 20; turning the first heat switch 110 on to transfer heat from the source 20 to the electrocaloric element 200; turning the first heat switch 110 off; warming the electrocaloric element 200 to a temperature of greater than the sink 10 or equal to the sink 10; turning the second heat switch 100 on to transfer heat from the electrocaloric element 200 to the sink 10; and turning the second heat switch 100 off.

Turning the heat switches 100 and 110 off comprises applying a voltage in a first, forward direction to the switches, and turning the switches 100 and 110 on comprises applying a voltage in a second, reverse direction to said forward direction to a said switch.

We also describe a method of controlling a cascaded system having a plurality of devices for transferring heat from a cold end to a hot end and wherein the method is repeated for each device for transferring heat from a heat source to a heat sink in the cascaded system. Also, we disclose a method of controlling a parralel and stacked cascaded system having a plurality of devices for transferring heat from a cold end to a hot end wherein the method is repeated for each device for transferring heat from a heat source to a heat sink in said systems.

A device embodying aspects of the present invention also comprises a controller configured to implement methods explained above, and a carrier to carry a computer program code (e.g. host or disk) to implement methods explained above.

The Electrocaloric Effect and Electrocaloric Working Elements

The electrocaloric effect is, broadly speaking, a change of a material's temperature upon an application or removal of an electric field. There are some obvious facts that make the electrocaloric effect (ECE) more attractive for applications than magnetocaloric effect (MCE):

• it is easier to generate and maintain electric rather than magnetic fields,

• it is easier to organise the cycle of energy return after each cycle of the refrigeration,

• it is easier to build a cascaded cooling device comprising several working elements separated by heat switches, etc.

A standard geometry of an electrocaloric (EC) working element is shown in Fig. 1. It comprises a pair of electrodes 300 and a slab of electrocaloric material 310. So an electrocaloric element is basically a capacitor with an electrocaloric material between its electrodes. The element is energised from a DC or AC voltage source 330, and the voltage is applied by switching 320. So when the switch 320 is turned on, the electrocaloric material 310 polarises and its temperature changes due to the electrocaloric effect. When 320 is turned off, the temperature of 310 returns to its initial value. It should be understood that the Figure is a schematic only and different circuitry can be used, as well as different structure of the electrodes (e.g. multilayer capacitor structure, similar to that described in M. Togashi. "Multilayer ceramic electronic device", US Patent No 2003/0026059, 2003, for example; layers of electrocaloric material in the multilayer capacitor can be made with thin film deposition techniques, e.g. sol- gel, as disclosed in D.A. Barrow, T.E. Petroff, and M. Sayer. "Method for producing thick ceramic films by a sol gei coating process", US Patent No 5585136, 1996, for example).

Electrocaloric Materials

The best known electrocaioric materials are perovskite ferroelectric materials. The highest electrocaloric effect at room temperature was found in PbSco_.5Tao.₅O₃ (PST). For a more detailed description of the material see E.H.Birks L.A.Shebanov, K.J.Borman, and A.R.Sternberg. Ferroelectrics. 94: p. 305, 1989, for example. The value of the ECE peaks at the critical temperature Tc but remains within 80% of the maximum value within about 10⁰C region around Tc. The critical temperature can be adjusted by a special heat treatment of the material during the annealing phase of its manufacturing E. K, H. Birks, L.A. Shebanov, K.YA Bormanis, and M. Dambekalne. "Lead scanotantalate as active element of microcryogenic systems", SU Patent No 147944O₁ 1989. A typical dependence of ECE for PST after different heat treatments at electric field 25 kVcm^"1 is shown in Fig. 2., which is adapted from Y.V. Sinyavsky, N. D. Pashkov, Y.M. Gorovoi, G. E. Lugansky, and L.A. Shebanov. Ferroelectrics. 90: p. 213, 1989.

The highest ECE was found in PbZrTiO₃ (PZT) doped with Sn and Nb: Pbo.₉₉Nbo_.o₂{Zro_.75Sn₀.₂Tio.o₅)o.9sθ3 (BA Turtle. "Polarization reversal and electrocaloric measurements for field-enforced transitions in the system lead zircon ate-lead titanate-lead oxide:tin oxide", PhD Thesis at University of Illinois at Urbana-Champaign, 1982). The ECE peaks at about 160⁰C and has a maximum value of 2.6⁰C at 30 kVcm^"1. Its dependence upon temperature for samples obtained with different treatments is shown in Fig. 3.

Due to the nature of the electrocaloric effect, it is natural that its value grows with the electric field. In practice the maximum field that can be applied to a material is determined by the breakdown voltage. For people acquainted with the state of the art it is well known that thin film materials have much higher breakdown voltages than same compounds in bulk form. A typical breakdown value for thin films is 2 MVcrrT¹ while that for bulk materials is about 30 kV cm^"1. This gives grounds to the prediction that thin film materials will have a much higher eiectrocaloric effect. Some experimental evidence of this was provided in (L. Shebanovs, K. Borman, W.N. Lawless, and Kaivane A. Ferroelectrics. 273: p. 2515, 2002). Thus, a multilayer capacitor system comprised of a plurality of ferroelectric thin films (deposited by sol gel, for example, as disclosed in D.A. Barrow, T. E. Petroff, and M, Sayer. "Method for producing thick ceramic films by a sol gel coating process", US Patent No 5585136, 1996) separated with electrodes (with a geometry described in M. Togashi. "Multilayer ceramic electronic device", US Patent No 2003/0026059, 2003, for example) can manifest a much higher ECE than bulk material of the same composition due to the higher electric field applied. The compounds described above can be used in embodiments of the invention either in the form of a multilayer capacitor or in bulk form. However, it should be understood, that any materials with ECE can be used in embodiments of the invention.

The inherent efficiency of an eiectrocaloric element depends on several factors. Among them are hysteresis losses if the materia! used is ferroelectric and losses due to Joule heating due to finite electrical resistance of the elements. As numerous experiments showed, the inherent efficiency of EC elements is remarkably high - around 98% of the Carnot cycle (Y.V. Sinyavsky, N. D. Pashkov, Y.M. Gorovoi, G. E. Lugansky, and Shebanov L. Ferroelectrics. 90: p. 213, 1980). The efficiency η was estimated using the formula:

,_{7 =}J ^Qv^_{, {1 )}

ΗID &1N ^~ Qairr where QE_CE is the amount of heat generated in an EC element due to ECE, Qw is electrical energy supplied for the polarization if the EC element, Q_OUT is electrical energy withdrawn from the EC element at its depolarization, and η_iD is the efficiency of the ideal Carnot refrigeration cycle. The Carnot efficiency is given by:

„ - ^Tcoi.D (2)

¹ IIOT ¹COLD where THOT and T_COLD are temperatures of the hot and cold end respectively. Thus it is supposed in the following that the electrical circuits of the cooϋng device allow for saving of the electrical energy coming from a depolarization of the working elements. This is easy and straightforward to implement by building RC - circuits for example, where the capacitance element is an electrocaloric element.

Heat Switches

A heat switch is a device that can be switched between two states - heat conducting and heat insulating. The heat insulating state does not need to be fully insulating. Some examples are given below.

Thermoelectric heat switches are important parts of the proposed device in some embodiments, so they will be discussed in more detail here. An example of a thermoelectric couple is presented in Fig. 4. As a rule, it comprises two legs _ 440 {n-type semiconductor) and 450 {p-type semiconductor), several layers of electrical conducting material 410, several layers of material 400 which is an electrical insulator and heat conductor. When the element is energised with the voltage source 460, it pumps heat from heat source 420 to heat sink 430. The heat pumped at the cold surface 420 Q_c (W) is equal to:

where N is the number of couples (there is only one in Fig. 4), T_Coid is the cold surface temperature (K), α is the Seebeck coefficient (VK^"1), / is the electrical current (A), p is the resistivity of 440 and 450 (Ωcm), k is the heat conductivity of 440 and 450 (WcrrT¹K^*1), ΔT - the temperature difference between 430 and 420 (K), and G is the ratio of an element's cross-sectiona! area to its height (cm). The first term in the brackets in (3) describes the Peltier effect, the second term describes Joule heating and the third describes parasitic heat flow from 430 to 420.

A thermoelectric element can work as a heat switch, i.e. it can either conduct heat (when / = 0) from the hot to the cold surface (an on state of a heat switch) or not conduct heat (when / = lort, at which the Peltier cooling just offsets the thermal conductivity and Joule heating — an off state of a heat switch). The value of I o_ff can be found from the equation Qc - 0:

It should be understood that the value given by (4) is an example only, and any other close value can be used. Also, a heat insulating state (off state) does not need to be fully heat insulating.

A magnetocaloric cooling system using thermoelectric and microelectromechanical (MEMS) heat switches is disclosed in U.S. Ghoshal. "Apparatus and methods for performing switching in magnetic refrigeration systems using thermoelectric switches", US Patent No 6595004, 2003. Various arrangements of heat switches and working magnetocaloric elements are mentioned. However due to the difficulty in generating a magnetic field, construction of such a device would be complicated. Additionally its efficiency and cooling power would be questionable due to large heat ieaks. One of the significant differences between the device and method disclosed in U. S.6595004 and embodiments of the invention is the nature of the working elements - they are electrocaloric in embodiments of the invention, not magnetocaloric as in US6595004. This makes the whole system simpler and allows for direct estimations of the device efficiency with parameters of available materials.

High thermodynamic efficiency of thermoelectric heat switches was discussed in length in US6595004. All the discussion along with the conclusions are relevant in the present context. For people acquainted with the field it is well known that efficiency of a thermoelectric materia! sharply increases with decrease of the temperature difference between the hot and cold contacts of a thermocouple. This difference is kept small throughout the whole cooling cycle of the proposed device.

An electrocaloric cooling system based on a cascade of electrocaloric working elements separated with piezoelectric heat switches is disclosed in V. M. Brodyansky, Yu.V. Sinyavsky, and N. D. Pashkov. "Thermal switch (its versions)", SU Patent No 918770, 1982. A piezoelectric heat switch is similar to the electrocaloric element shown in Fig. 1 , but material 310 has high piezoelectric effect in this case, i.e. significant contraction or elongation upon application of electric field. If placed between two electrocaloric elements so that upon elongation it touches both electrocaloric elements, and upon contraction touches none of them, such a piezoelectric element can work as a heat switch. The proposed device does not use bulk piezoelectric heat switches. In some embodiments of the proposed device, microelectromechanical heat switches are used, but their major feature is a complex surface that increases the surface contact area, which is absent in V. M. Brodyansky, Yu.V. Sinyavsky, and N. D, Pashkov. "Thermal switch (its versions)¹¹, SU Patent No 918770, 1982.

Electrocaloric Cooling Technology

We next describe some embodiments of the our device and estimation of their characteristics.

Description of some embodiments of the device

The device comprises at least one electrocaloric working element 200 placed between at least two heat switches 100 and 110, see Fig. 7. Heat switches 100, 110 have thermal contact with a heat sink 10 and heat source 20 respectively. There is also a thermal contact between 100 and 200, 110 and 200. We use heat switches which are active elements and require electrical energy to function.

In another embodiment, a cascaded system comprising two electrocaloric working elements is proposed. An electrocaloric element 210 of an additional stage is utilised as a heat source in the previous stage of the cascaded system, see Fig. 8. In other words, the embodiment of a cascaded system shown in Fig.8 comprises two cooling devices: a first device comprising an electrocaloric element 200 and heat switches 100 and 110 and a second device comprising an electrocaloric element 210 and heat switches 110 and 120.

Referring to Fig. 8, a heat source 20 is connected mechanically to the heat switch 120 so that there is a good thermal contact between them. The heat switch 120 is mechanically connected to the working element 210, so that there is good thermal contact between them. However, there is no direct electrical connection between 120 and 210. In a similar fashion electrocaloric element 200 is thermally connected to heat switches 100 and 110 and electrically isolated from them. Elements 210 and 110 as well as 110 and 200 do not contact each other in Fig. 8 for the sake of clarity. Buffer layers providing thermal contact and electrical insulation between 10 and 100, 100 and 200, 200 and 110, 110 and 210, 210 and 120, and 120 and 20 are also not shown here for clarity.

In some embodiments, thermoelectric elements can be used as heat switches, i.e. they can be switched between two states - heat insulating and heat conducting. In some embodiments, an on state corresponds to heat flow from the hot to the cold elements by virtue of heat conduction through the thermocouples in passive mode. In other embodiments, the heat flow in an on state is assisted by the application of a reversed current so that the time of heat exchange decreases, and this mode of operation is called active mode. An off state is defined as zero or nearly zero heat transfer to the cold end of a thermoelectric couple/couples. In this state a direct current is passed through the couples so that the thermoelectric cooling effect precisely or almost precisely offsets the heat conduction. In some embodiments the current passed through the thermocouples in an off mode can be higher than that required to offset the heat conduction process in order to reduce the time of heat exchange.

A thermoelectric heat switch incorporated into the proposed device can be energised either directly or by electromagnetic induction, in a similar way to that described in U.S. Ghoshal. "Apparatus and methods for performing switching in magnetic refrigeration systems using thermoelectric switches", US Patent No 6595004, 2003 in relation to magnetocaloric systems.

In another embodiment, thermoelectric switches are replaced by tunneling cooling devices, disclosed in A. Tavkhelidze, L. Koptonashvili, Z. Berishvili, and G. Skhiladze. "Method for making a diode device", US Patent No 6417060, 2002, for example. In these embodiments a proposed device operates in a similar way to that described in the previous paragraph.

Preferably, thermoelectric heat switches can be manufactured by means of electrolysis disclosed in J.-P. Fleurial, M.A. Ryan, A. Borshchevsky, W. Phillips, E.A. Koiawa, G.J. Snyder, T. Caillat, T. Kascich, and P. Mueller. "Microfabricated thermoelectric power-generation devices", US Patent No 6388185, 2002, incorporated by reference in its entirity. The cited patent proposes a thermoelectric device for electrical energy generation, but it can be used for the reverse purpose, i.e. heat pumping, due to Onsager relations. Thermocouples made by electrolysis are with a flat geometry, e.g. with 1 mm x 1 mm cross section and 100 μm thickness. BiTe is the material used in the cited patent, and it can be used in embodiments of the invention. However, other thermoelectric materials can be used for heat switches as well. From the analysis below it follows that the higher the figure of merit of used thermoelectric materials the higher the efficiency of the whole cooling device.

The following estimations show a preferable range of sizes of thermoelectric heat switches. However it should be understood that this is just an example and heat switches of other sizes can be used. For the cooling device to work efficiently, there are preferably two conditions to be satisfied: (/) the heat capacity of each heat switch should be much less than that of an electrocaloric element, and (//) heat transfer should be accomplished for a reasonable amount of time. Although there is no strict objective limit on this time, I = 1 s seems a plausible upper value. A time scale of the device operation can be set by the polarisation time (0.1 s), so a heat transfer taking, say, 10 s, is far too slow. These conditions can be expressed as

C_HmtSw = 2Nh²Gc^_n « C,

C - <⁵>

≤ ϊ ,

VlNIkG where C is the heat capacity of an EC element, C_Hβatsw is total heat capacity of a heat switch, Cβi-τ_e is the volume heat capacity of bismuth teiluride, a possible material used in thermocouples, N is the number of thermocouples, h is the height of the thermocouples, G is the ratio of the area of one leg of a thermocouple to its height h, k is the heat diffustvity of BiTe. After a simplification one has two inequalities:

^C r < G « — S ■ (6)

4NkI 2Nh²c_Bi^Te

A necessary condition for this inequality to be true is simply that it should remain true if we take G out. Then C and 2Λ/ cancel out and we get a condition for the height h:

Substitution of the values for bismuth teliuride (c_Bj-τe = 544 Jkg^"1K^'\ k = 0.366*10^"6 m² s^'1) and 1 s for t gives h « 1 mm. Thus, a height of 10 - 100 μm is appropriate, and this gives a hint at a preferable method to use for the fabrication of the couples: electrochemical deposition (US6388185), as described above. A typical value of the G factor is around 1 cm, so a cross sectional size of one leg is about -JGh = 0.32 - 1.0 mm and a total size of a typical number of thermocouples in a heat switch 2N ~ 2 * 540 legs (this value is used in the efficiency estimation described below, however a different number of thermocouples can be used) is -JlNGh = 10 - 33 mm. If a free space between the legs is of about the same size, the cross sectional size of the whole heat switch is 20 - 66 mm. It is important to note that different values of G factor and other geometrical features can be used in the proposed device.

In some embodiments, microelectromechanical systems (MEMS) can switch from a heat conducting to a heat insulating state. With this embodiment, when a first working element is subject to electric field, it generates heat and its temperature is raised above the temperature of the heat sink. At this time a heat switch between the first EC element and a heat load is in an off state, and a heat switch between the EC element and a heat sink is in an on state, thus allowing for the heat flow from the EC element to the heat sink. The heat flow goes on until the temperatures of the EC element and the heat sink are the same. In the next stage, the electric field is removed and the EC element cools down and its temperature drops below the temperature of the heat load. At this time the heat switch between the EC element and heat load is in an on state, and the heat switch between the EC element and the heat sink is in an off state. This configuration allows for the heat flow from the heat load to the EC element. The heat flow goes on until the temperatures of the EC element and the heat load are the same. By repeating these steps the device comprising only one EC element and two heat switches can pump heat from the cold to the hot end. Embodiments comprising more than one EC element can work in a similar fashion.

Operation of an embodiment comprising one electrocaloric element and two heat switches shown in Fig. 7 comprises:

• cooling electrocaloric element 200 to a temperature of less than source 20; • turning heat switch 110 on to transfer heat from the source 20 to the electrocaloric element 200;

• turning the heat switch 110 off;

• warming electrocaloric element to a temperature of greater than sink 10;

• turning heat switch 100 on to transfer heat from electrocaloric element 200 to sink 10; and

• turning the heat switch 10 off.

In other embodiments of the proposed device, e.g. cascaded or paralleled systems having a plurality of devices for transferring heat from a heat source to a heat sink, the operation can be repeated in every device. Some examples are given below.

An example of an operation of a cascaded system comprising 2 electrocaloric elements with heat switches depicted in Fig. 8 is schematically shown in Fig. 9. After all transition processes finish and the device reaches equilibrium, electrocaloric elements 200 and 210 are at their working temperatures T_w∑oo and Tw₂w respectively, what corresponds to an optimum electrocaloric effect of the materia! used. The working temperatures are chosen so that Tw2io < Tw∑oo, Tw₂io is close to the heat source (20) temperature T20 and Tw2oo is close to the ambient (10) temperature T₁₀. An optimum difference between adjacent working temperatures depends on the value of the eiectrocaloric effect among other factors. The difference between adjacent working temperatures of different pairs of working elements can be different. Referring to Fig. 9, an initial configuration of a cooling device is: 210 is depolarised, 200 is polarised, both are at their working temperatures, and all heat switches (100, 110, and 120) are in an off state, i.e. they do not conduct heat. This configuration can be achieved by a proper biasing of the heat switches. For example, to cool down 200, the heat switch 100 should pump some heat from 200 to the ambient 10, etc. Stable cooling at constant cooling power is achieved by running elementary cooling cycles. Each cycle comprises four stages: • (/) Application of bias to 210. Its temperature rises due to the electrocaloric effect. At the same time 200 is depolarised. Its temperature decreases due to electrocaloric effect. All heat switches 100, 110, and 120 are in an off state.

• (//) Heat switch 110 is turned on, i.e. it conducts heat. Switches 100 and 120 are off. Heat is transferred from 210 to 200.

• (///) 210 is depolarised. It cools down by the electrocaloric effect and its temperature is now lower than that of the heat source 20. 200 is polarised. It heats up and its temperature becomes higher than that of the heat sink 10. All switches are in an off state.

• (/V) Switches 120 and 100 are on, switch 110 is off, heat transfers from the source 20 to element 210 and from 200 to the ambient 10.

The cycle described can be generalised and used in embodiments with cascades comprising any number of electrocaloric elements. However, the described cycle is just an example and other cycles can be run on devices according to embodiments of the invention.

Another embodiment of the proposed device, a cascaded system comprising four electrocaloric elements: 200, 210, 220, 230 and five heat switches: 100, 110, 120, 130, 140 is presented in Fig. 10. Electrocaloric element 210 is utilised as a heat source in the previous stage comprising the electrocaloric element 200. Electrocaloric element 220 is utilised as a heat source in the stage comprising eiectrocaloric elements 200 and 210. Electrocaloric element 230 is utilised as a heat source in the stage comprising electrocaloric elements 200, 210, and 220. !n other words, the system comprises four cooling devices: a first device comprising an electrocaioric element 200 and heat switches 100 and 110, a second device comprising an electrocaloric element 210 and heat switches 110 and 120, a third device comprising an electrocaloric element 220 and heat switches 120 and 130, and a fourth device comprising an electrocaloric element 230 and heat switches 130 and 140. The hot end of a fourth device is the cold end of the third device, the hot end of the third device is the cold end of the second device, and the hot end of the second device is the cold end of the first device.

Referring to Fig. 10, the end working element 200 has the highest working temperature of all elements and is separated from the heat sink 10 by the heat switch 100. The end working element 230 has the lowest working temperature of all elements and is separated from the heat source 20 by the heat switch 140. As an example, the working elements 200, 210, 220, and 230 of embodiments of the invention can have the working temperatures Tw2oo, Tw_∑w, Tw220, and Twzw shown in Table 1.

Table 1. An example of working temperatures of a cascaded device comprising four electrocaloric elements in ⁰C.

It should be understood that the number of cooling devices in a cascaded or parallel system is variable and a cascade comprising four electrocaloric elements is shown for example only. However, as the following analysis shows, devices comprising an even number of EC elements are in general more efficient and therefore preferable. It is assumed that the energy of a discharge after a deporarization of an EC element is kept in the circuit and used afterwards. Due to the periodic character of the device operation, it can be designed rather easily as an RC circuit. Electrical circuitry is not depicted in Figs. 8, 10, 14 for the sake of clarity.

A giant electrocaioric effect in thin film PbZro.₉₅Tio.₀₅O₃, is described in more detail in arXiv:cond-mat/0511487 v1 19 Nov 2005, A. Mischenko, Q. Zhang, J. F. Scott, R.VV. Whatmore and N. D. Mathur, to which reference may be made, and this material may also be used in embodiments of the present device. Some aspects of efficiency and cooling power

Device efficiency and cooling power depend on the time required to fulfil each stage of a refrigeration cycle. The time of polarisation/depolarisation of an EC element τp_oι is rather difficult to estimate. It depends on the electrical capacitance of an EC element CEI and on the circuit resistivity p: τp_oι = pc&. In theory it could be made very small by reducing p at a given value of Ce- But in practice there is a minimum value of p at which the ferroelectric material does not break down on the application of voltage. So, this time cannot be adjusted easily. A reasonable experimental value for τp_oi is about 0.01 - 0.1 s (Y.V. Sinyavsky and V. M. Brodyansky. Ferroelectrics. 131 : p. 321 , 1992).

The time of a heat exchange process τ can be varied by changing the heat switches' geometry. However, the faster the heat exchange, the more power is required to offset the heat conductivity between the elements when the heat switches are in an off state. A much more promising approach for reducing the heat exchange time is applying a reverse current to the Peltier elements, and this idea is explored later. For the sake of clarity and in order to consider the working principle of the device, let r be about 1 s until the end of the next paragraph. The time required for a heat exchange between an end element (200 or 230 in Fig. 10, for example) and a heat sink or heat load (10 or 20 in Fig. 10) is 2 τ if neither heat sink nor heat load change their temperature upon the heat exchange.

Referring to Fig. 9, each stage (/, ii, Hi and /V) comprises either fast polarisation/depolarisation processes or slow heat conduction processes, but never both at the same time. Moreover, each heat exchange process is either between 200 and 210 or between 200 and 10 or 210 and 20. The former takes r seconds, the latter two take 2 τ seconds, and the polarisation takes negligible time comparing to τ. That is why it is preferable to use even numbers of electrocaioric elements in the proposed device - otherwise each of the four stages would involve heat transfer to the heat sink/heat load, thus slowing down the whole cycle and reducing the cooling power. If τ is close to the polarisation time τpoi, then it changes nothing in the principle of operation and this discussion, but the time notation in Fig. 9 (r, 3r, 4r, . . . ) will be different. It should be understood that embodiments comprising an odd number of electrocaloric elements can also be used.

Moreover, it is possible to reduce the cycle period even more. Stage /7 (see Fig. 9) can be started before stage / is finished (the same holds with stages /V and Hi, respectively). As soon as temperatures of 200 and 210 are equal at some moment during stage /, heat switch //^' can be turned on to start the heat conduction. Also, applying this concept will allow a thermodynamic process to run within the working elements that is close to a Carnot cycle. A working thermodynamic cycle of an electrocaioric element is depicted in Fig. 11 using the variables T and S, where T is temperature and S is entropy. If stage // (and /V) start after stages / (and Hi respectively) finish, then an element passes through the points 1-2 -3-4¹ of the diagram. But if stage H (or JV) starts after point 2 (or 4) on the diagram, then the cycle will be closer to the ideal Carnot cycle 1- 2-3-4. This approach will allow an increase in the inherent efficiency of the energy transfer inside the electrocaloric elements. Examples of such energy interconversions are: electrical energy at the electrodes <→ energy of the ferroelectric, and structural ordering <→ heat energy of the the crystal lattice.

In some embodiments, the time required for heat exchange between two adjacent EC elements τ can be decreased by using an extra heat conductor parallel to the heat switch. It effectively increases the heat switch's heat conductivity and lowers its thermoelectric figure of merit. In some embodiments, these shunts are microactuators fabricated by microelectromechanica! systems technology. One of the possible designs for additional heat conductors is shown in Fig. 12, a. The thermoelectric elements of the heat switch between 200 and 210 are not shown in that Figure for the sake of clarity. Only two electrocaloric elements are shown in the Figure for the sake of clarity. However, this concept can be applied to any number of working elements. Microactuators can be made of a piezoelectric material, e.g. PZT - lead zirconium titanate. They can be either compressed {voltage applied), so that the device does not conduct heat, or stretched (no voltage applied) so that there is a physical contact between the two parts to allow heat transfer. The efficiency of such a device depends on the contact surface area S_Comp- It is compared to the plain surface area S_Pι_ain in Fig. 12, b. The larger the surface area, the more efficient the microelectromechanica! heat switch. Three geometries (ladder - 61 , conical - 62, and spherical - 63) for the heat transfer elements are shown in Fig. 12, b along with their effective contact surface area normalised to the plain surface area. The size of such a device can vary, but for the sizes considered in the current research, a reasonable value is 10-100 μm in both diameter and depth. Although conical (62) and spherical (63) shapes could be hard to produce, the ladder type projection (61 ) can be produced by a conventional microfabrication technique depicted in Fig. 13. in some embodiments, such a device can be used as a mechanical heat switch between the working elements and end working elements and heat sink/load.

Referring to Fig. 13, a preferable way to manufacture a mechanical heat switch is presented. For those acquainted with the art, the procedure is commonplace. In the first stage I, a layer of photoresist is deposited on top of Si or other heat conducting materia!. A part of its surface 30 is exposed to the UV radiation. Then the exposed surface is etched away by diluted acetic acid, for example, so that a trench is formed in the photoresist. Then at stage Ii a strong etchant (e.g. HF) is applied to the surface. It removes the remaining photoresist and makes trench 31 in S/ (or other heat conducting material used). At stage IH the photoresist is deposited again and a part of its surface 32 is exposed to the UV light, and similarly to step II, a strong etchant is applied at step IV to make a next trench. Finally a part of a complex surface 61 is obtained. Only one trench is shown in Fig. 13, however such a procedure can be realised to produce a number of trenches on the same surface.

in some embodiments, the work of a heat switch based on a thermoelectric element is modified in the following way. A reverse voltage is applied across the element when it is in the on state (that is no current, heat transfer goes on - for example switch 110 at the stage //^' in Fig. 9). In this case the Peltier effect will effectively pump heat from the hot to the cold eiectrocaloric element that it connects (for example from 210 to 200 in Fig. 9). In other words, the application of a voltage with reverse polarity increases the effective heat conductivity of the heat switch, decreases the time required for the heat transfer and increases the cooling power and efficiency. To find out the influence on the cooling efficiency, a couple of equations for the heat transfer can be considered (for more details see the Appendix). One of the main results of the calculations is that even a rather small current (0.05 A, for example) decreases τ by several orders of magnitude. This method will be referred to as the active heat conductivity method.

Example 1: Calculation of efficiency and cooling power of the proposed device in some embodiments

The operation of some embodiments of the proposed cooling device with thermoelectric heat switches has been simulated using MS Excel^®. The eiectrocaloric effect of each electrocaloric element versus temperature was described by a Gaussian curve with a maximum of 3⁰C. The peaks of the Gaussians are centered at the appropriate working temperatures for each element TW,- and have a width of about 1O⁰C. Embodiments of cascades comprising 2, 4, and 6 electrocaloric elements have been simulated. The working temperatures (Tw₂oo, Tmw for a cascaded device comprising 2 electrocaloric elements, Tw2oo, Tmio, Tw∑≥o, Twzzo for a cascaded device comprising 4 electrocaloric elements, and Tw∑aa, TWaio, TW220, Tw≥w, Tw24o_* Tw₂₅o for a cascaded device comprising 6 electrocaloric elements) used in the simulation are listed in Table 2. It should be understood that embodiments of the invention are not limited to these temperatures. Any temperatures may be used. An embodiment of the proposed device comprising a cascaded system of 6 electrocaloric elements and 7 heat switches is shown in Fig. 14. The system comprises additional electrocaloric elements that are utilised as heat sinks/sources in the previous stages of the cascaded system. This embodiment comprises 6 cooling devices that are cascaded in analogy with cascaded systems described above (shown in Figs. 8 and 10), As an example, the initial temperature of the heat load 20 is 16⁰C for all devices, the initial temperature of the heat sink 10 is 20⁰C for the cascaded device comprising 2 electrocaioric elements, 24⁰C for the cascaded device comprising 4 electrocaioric elements, and 28⁰C for the cascaded device comprising 6 electrocaioric elements.

Table 2. Working temperatures of some embodiments of cascaded cooling devices comprising various numbers of electrocaioric elements in ⁰C.

Other important characteristics of the model are listed in Table 3. The efficiency of the proposed device depends on how fast the heat is dissipated from a heat sink (component 10 in Figs. 7, 8, 10, and 14, for example). The rate depends mainly on the design of the heat sink. Usually it is a structure with a large surface area and a fan that creates the air flow through it. As a rule, applied heat (that is to be dissipated) increases the temperature of a heat sink. As a first approximation, the relation between the power to be dissipated W and temperature change <5Tof the heat sink is linear, and the coefficient is called the heat resistivity: pπeai - 5T/W. A typical value for a metallic plate without a fan is P_Heat - 0.3⁰CW^"1. However, in the calculations presented below, the heat resistivity is assumed to be much less (0.001-0.05⁰CW^"1). This simpiification allows us to study the intrinsic thermodynamic properties of some embodiments of the proposed device. The efficiency and cooling power obtained from the simulations are compared with the parameters of a commercial Peltier cooler with the heat resistivity of the same order of magnitude: 0.04⁰CW^"1.

Table 3. The parameters of cooling devices used in the calculation.

The simulation results for a cascaded device comprising 2 electrocaloric elements at equilibrium (after 600 cycles have passed) are shown in Fig. 15. Four stages of the cooling cycle {/, /7, /77, and /V) are highlighted (they are the same as in Fig. 9). The temperature of the heat load (component 20 in Figs. 7, 8, 10, and 14, for example) is raised artificially at the end of stage ;/^' of every cycle simulating the heat generation in a real device. The simulated temperature of the heat sink (component 10 in Figs. 7, 8, 10, and 14) is higher than that of the ambient because of its finite heat resistivity (its temperature should rise in order to dissipate the incoming heat). It should be understood that other cycles can be run in the proposed device, and this particular cycle is simulated for illustration only. Increasing the heat conductivity of heat switches while ail other parameters are held constant produces a visible effect on the overall cooling performance. Fig. 16 shows some important characteristics (for a cascaded device comprising 4 electrocaloric elements shown in Fig. 10), namely the cooiing power, efficiency and the average time r for heat transfer between two adjacent electrocaloric elements (200 and 210, 210 and 220, 220 and 230 in Fig. 10) as a function of the heat conductivity of the switches. So the approach of changing the heat switches' heat conductivity when the device is working can give more options and allows for the optimisation of cooling power versus efficiency. That is, additional heat conductors provide an adjustable option for the design of a particular cooling device so that it could be used either to pump more heat less efficiently or to work at the highest efficiency but with Sower cooling power. In some embodiments, additional shunts made of MEMS described above can fulfil the function to change the effective heat conductivity of the heat switches. Example 2: Calculation of efficiency and cooling power of some embodiments of the proposed device comprising heat switches in active heat conductivity mode

Embodiments comprising heat switches in active heat conductivity mode are more preferable as the calculation gives more striking results. Some of them are presented in Figs. 17 and 18. Referring to Fig. 17, the efficiency and cooling power of the proposed cooling device versus a difference between heat sink and heat load temperatures, T₁₀ - T_∑o, are presented for some embodiments: a cascaded device comprising 2 electrocaloric elements, a cascaded device comprising 4 electrocaloric elements, and a cascaded device comprising 6 electrocaloric elements at different values of the reversed current passed through heat switches in active heat conductivity mode. The efficiency is expressed in percentage of the idea! Carnot efficiency given by formula (2). Application of quite a small reverse current (when the heat switches are in an on state, i.e. conducting heat) significantly improves the device performance. The device parameters used in the calculation are listed in Table 3. Referring to Fig. 18_» efficiency and cooling power versus the reverse current of the same devices are presented. It should be understood that the parameters used are only for the sample calculation, as well as geometric sizes of the electrocaloric elements and heat switches. Any other set of parameters and sizes can be used in the proposed cooling device. Moreover, electrocaloric elements can have different sizes, preferably the size of the working element which is closest to the heat sink should be biggest to account for the heat losses in the previous electrocaloric elements and heat switches.

A comparison of a commercial fully optimised thermoelectric device with an embodiment of the electrocaloric cooling device proposed here is made in Fig. 19. The data for thermoelectric cooling device is taken from www.melcor.com using device reference number PT8-7-30 with G = 0.17 cm, N = 71 , P_Heat ⁼ 0.04^DCW^"1, the temperature difference between its cold and hot ends is about 6 K). It should be understood that the comparison has been made with just one thermoelectric solution taken from Melcor. Other solutions provided by this source can have different parameters. The heat resistance of the thermoelectric cooler is chosen to be very low (close to that of the cooling device suggested): 0.04⁰CW^"1. As seen from Fig. 19, the efficiency of the considered embodiment of the present invention rises with the cooling power, unlike for the thermoelectric cooler. A possible reason is as follows. In order to provide more cooling power, the current through the thermoelectric heat switch should be increased. However, this also increases Joule heating, which is proportiona! to the current squared. As a result, the efficiency falls. In the case of the considered embodiment, if more heat is pumped through the it, the temperature of the cold surface rises, but the energy consumption remains the same. If the heat dissipation at the hot end is fast enough, the efficiency rises with cooling power.

Active heat conductivity mode in more detail

Some facts are presented below to clarify the idea of the active heat conductivity mode. The equations for the heat transfer between two electrocaloric elements interspaced with a thermoelectric heat switch are considered here {for instance, consider two electrocaloric elements 200 and 210 interspaced with a heat switch 110 in Fig. 8, with the rest of the Figure omitted). In the first approximation, the process can be described by two differential equations:

and the boundary conditions are:

where N is the number of couples, C (JK^'1) is the heat capacity of an electrocaloric element, T_Coid (K) is the temperature of an electrocaloric (EC) element with lower initial temperature T_Coid (O)(K), Tπot (K) is the temperature of another EC element with higher initial temperature Tcoi_d (O)(K), / (A) is the current passed through the thermoelectric heat switch, a is the Seebeck coefficient (VK^"1), pis the resistivity (Ωcm), k is the heat conductivity (Wcm^"1K^'1), G (cm) is the ratio of a heat switches' cross-sectional area to its height. These equations are genera! and include the "reverse" current in thermocouples that accelerate the heat exchange process. However, if we assume / = 0, these equations will describe the passive heat transfer process between two electrocaloric elements.

A typical size of an electrocaloric element used in the calculations was 30 * 30 * 2 mm³ with a total heat capacity of C = 4.86 JK^"1. Initially, one of the electrocaloric elements was at Tc_oω(O) = 300 K, and the other was at Tπo_t(0) - 305 K. Let 540 thermoelectric couples made out of bismuth telluride, which are connected electrically in series and thermally in parallel (see Fig. 4) have a height of 10 μm, and let the tota! cross section correspond to G = 1 cm: 10.3 ^* 10.3 mm² (small enough to fit between the electrocaloric elements). Then two electrocaloric elements will exchange heat for about r- 0.4 s, which is close to the time constant of the EC element polarisation. The total heat capacity of the couples is 0.44 JK^"1, around ten times smaller than that of the electrocaloric elements. Let us now apply a reverse voltage to the couples in order to reduce r . The result is depicted in Fig. 20, I for currents of 0, 0.05 and 0.2 A. As can be seen from the Figure, the application of even a 0.05 A current is enough to influence r. Larger current of 0.2 A makes a larger effect. The Joule heating is small for both currents on the timescale of one second. At a larger timescale, the Joule heating becomes important and both temperatures rise linearly versus time (see Fig. 20, H). The difference between temperatures in the linear region is determined by the competition of the Peltier heat transfer and heat conductance. The solution for a passive heat exchange (at / = 0 A) can be deduced from (A.1 ) and (A.2). The temperature difference T_Hot - T_Coid decays exponentially with time: T_Hot - Tcoid ~ exp (-t/τ), where

_τ = 4NkGZC. (A.3)

This formula is used to estimate τ presented in Fig. 16.

No doubt many effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims

1. A device for transferring heat from a heat source to a heat sink, the device comprising: at least two heat switches, a first to receive heat from the heat source, a second to pass heat to the heat sink; at least one electrocaloric element between two said heat switches and in thermal contact with said two heat switches; and a control input for controlling said at least two heat switches and said electrocaloric element to transfer heat from said source to said sink; and wherein one or both of said heat switches comprise a thermoelectric heat switch, tunnelling heat switch or an electromechanical heat switch.

2. A device as claimed in claim 1 , wherein said device for transferring heat from a heat source to a heat sink is a part of a cascaded system having a plurality of devices for transferring heat from a heat source to a heat sink.

3. A device as claimed in claim 2, wherein said device for transferring heat from a heat source to a heat sink is a stacked cascaded system.

4. A device as claimed in claim 2, wherein said device for transferring heat from a heat source to a heat sink comprises a parallel system.

5. A device as claimed in claim 1 , 2, 3 or 4 wherein said control input comprises one or more electrical connections.

6. A device as claimed in claim 1 , 2, 3 or 4 further comprising a controller to control said heat switches and said at least one electrocaloric element, the controller comprising control means for: cooling said eiectrocaloric element to a temperature of less than said source or equal to said source; turning said first heat switch on to transfer heat from said source to said electrocaloric element; turning said first heat switch off; warming said electrocaloric element to a temperature of greater than said sink or equal to said sink; turning said second heat switch on to transfer heat from said electrocaloric element to said sink; and turning said second heat switch off.

7. A method of making a device for transferring heat from a heat source to a heat sink comprising: providing at least two heat switches, a first to receive heat from the heat source, a second to pass heat to the heat sink; providing at least one electrocaloric element between two said heat switches and in thermal contact with said two heat switches; and providing a control input for controlling said at least two heat switches and said electrocaloric element to transfer heat from said source to said sink; and wherein providing one or both of said heat switches comprise providing a thermoelectric heat switch, tunnelling heat switch or an electromechanical heat switch,

8. A method as claimed in claim 7, wherein said device for transferring heat from a heat source to a heat sink is provided as part of a cascaded system having a pluraiity of devices for transferring heat from a heat source to a heat sink.

9. A method as claimed in claim 8, wherein said cascaded system is a stacked cascaded system.

10. A method as claimed in claim 8, wherein said system also comprises a parallel system.

11. A device as claimed in claim 6 wherein turning said heat switches off comprises applying a voltage in a first, forward direction to said switches; and wherein turning said switches on comprises applying a voltage in a second, reverse direction to said forward direction to a said switch.

12. A method of controlling a heat transfer device for transferring heat from a heat source to a heat sink, the heat transfer device comprising at least one electrocaloric element disposed between a pair of thermoelectric heat switches, a first switch being in thermal contact with said source and a second switch with said sink, the method comprising: cooling said electrocaloric element to a temperature of less than said source or equal to said source; turning said first heat switch on to transfer heat from said source to said electrocaloric element; turning said first heat switch off; warming said electrocaloric element to a temperature of greater than said sink or equal to said sink; turning said second heat switch on to transfer heat from said electrocaloric element to said sink; and turning said second heat switch off; and wherein turning said heat switches off comprises applying a voltage in a first, forward direction to said switches; and wherein turning said switches on comprises applying a voltage in a second, reverse direction to said forward direction to a said switch.

13. A method as claimed in claim 12, wherein said device for transferring heat from a heat source to a heat sink is part of a cascaded system having a plurality of devices for transferring heat from a heat source to a heat sink and wherein the method is repeated for each device for transferring heat from a heat source to a heat sink in said cascaded system.

14. A method as claimed in claim 13, wherein said cascaded system is a stacked cascaded system.

15. A method as claimed in claim 14, wherein said system also comprises a parallel system.

16. A controller configured to implement the method of claim 12.

17. A carrier carrying computer program code to implement the method of claim 12.