Heat pump, heat transfer arrangement, cooling module, turbomachine, method of pumping heat

Abstract

The heat pump comprises an assembly of thermoelectric modules 100 with high thermal conductivity metallic plates or heat pipes thermal interfaces 31, 32 arranged between layers of thermoelectric modul

Description

The present disclosure relates to a heat pump and associated methods and devices, including cooling modules for turbomachines. Heat pumps are devices that spend energy transferring heat from a cooler place to a warmer place. They are a very efficient means of heating or cooling. A familiar example of a heat pump is found in refrigerators, such as domestic refrigerators. A refrigerator circulates a fluid called a refrigerant around a closed system. Heat is absorbed into the fluid at a first part of the system, and is expelled from the fluid at a second part of the system. Thus, the first part is cooled by the refrigerant and the second part is heated by the refrigerant. The first part is called an evaporator. Prior to entering the evaporator, the refrigerant in a saturated liquid state passes through an expander which causes its pressure to drop, which in turn causes partial evaporation of the fluid and absorption of heat. As the fluid passes through the evaporator it absorbs heat from the warmer environment of the evaporator, and thus cools the environment of the evaporator. Absorbing the heat causes further evaporation of the remaining liquid component of the refrigerant, until it reaches a saturated vapour state. The second part is called a condenser. Prior to entering the condenser, the refrigerant, still in a saturated vapour state, passes through a compressor which causes an increase in pressure and thus in temperature. Now in a superheated vapour state, the fluid passes into the condenser, around which the ambient temperature is lower than that of the superheated vapour. Consequently, heat escapes from the refrigerant, warming the environment of the condenser and cooling the refrigerant, until it returns to a saturated liquid state, ready to return to the expander and repeat the cycle. Compressors are typically mechanical devices driven by either internal or external motors, containing a variety of moving parts. The maintenance of so many working parts is one of the design and engineering challenges associated with typical heat pumps of this kind. It is common for the heat absorption by the environment of the condenser and heat extraction from the environment of the evaporator to be aided by air-flow generators, such as fans, directing a flow of air towards each of the condenser and the evaporator. A heat pipe is a heat transfer device which employs some similar principles to those described above for a refrigeration cycle, relying on the evaporation and condensation of a working fluid to absorb and expel heat. Rather than spending energy transferring heat from a lower temperature place to a higher temperature place, a heat pipe is a very efficient conductor of heat from a higher temperature place to a lower temperature place. A heat pipe is typically a closed fluid conduit partly filled with a saturated liquid, and otherwise filled with the gas phase of the same fluid. When the heat pipe is placed across a temperature differential, that is, when a first end of the heat pipe is placed in a higher temperature environment and the second end is placed in a lower temperature environment, the first end absorbs heat from the higher temperature environment, causing the working fluid at the first end to evaporate. The vapour at the first end travels to the second end. The cooler environment at the second end absorbs heat from the working fluid, causing it to condense. The liquid is then returned to the first end, either by gravity, by a wick structure, or by some other means known to persons skilled in the art. Heat pipes are typically used in situations where the rapid conduction of heat is desirable. For example, they are often used in cooling systems in which it is desirable to rapidly remove heat from a heat source and draw it to a second location from which it might more easily dissipate. While they are extremely efficient, each heat pipe is only efficient within a limited range of temperatures, and so each one must be carefully designed for a particular purpose. The working fluid and other parameters must be selected according to the intended operating temperature range. Thermoelectric modules and assemblies are semiconductor devices which can be used as heat pumps to provide cooling or heating in domestic, commercial and industrial applications. The use of thermoelectric modules and assemblies to transfer heat by the Peltier method and generate power by the Seebeck method is well known for many applications. Thermoelectric modules and assemblies are incorporated into systems to transfer heat and / or to generate or reclaim electrical energy. It is an object of the present disclosure to provide improved heat pumps and / or methods of using the same. It is also an object of the disclosure to improve cooling in turbomachines. According to an aspect of the invention, there is provided a heat pump, comprising: a plurality of thermoelectric layers and one or more thermal interface bodies, stacked along an axial direction with the thermal interface body or bodies being interleaved between respective thermoelectric layers, wherein: each thermoelectric layer comprises one or more thermoelectric modules electrically drivable to pump heat parallel to the axial direction, the thermoelectric modules in each thermoelectric layer being arranged to operate thermally in parallel with each other when the thermoelectric layer comprises more than one thermoelectric module; and each thermal interface body has a thickness parallel to the axial direction selected such that when heat is being pumped by the thermoelectric modules a maximum lateral temperature difference, defined as a maximum temperature difference in the thermal interface body perpendicular to the axial direction, is lower than a maximum axial temperature difference, defined as a maximum temperature difference in the thermal interface body parallel to the axial direction. Providing a heat pump with such a stacked arrangement of thermoelectric layers interleaved with thermal interface bodies allows the heat pump to pump heat efficiently over a large overall temperature difference. The stacking allows individual thermoelectric modules to be operated between smaller temperature differences, thereby promoting more efficient operation of those thermoelectric modules. Arranging for each thermal interface body to have a thickness such that a maximum lateral temperature difference is lower than a maximum axial temperature difference ensures that heat spreads laterally sufficiently quickly that all of the thermoelectric modules can operate with high efficiency and uniformity, with minimal variation in the temperature differences across different thermoelectric modules in the same thermoelectric layer. Providing the thermal interface bodies between the thermoelectric layers provides lateral heat spreading that is superior to what can be achieved by stacking thermoelectric modules directly on top of each other (e.g., with semiconducting pillars in different layers separated only by ceramic module walls). The thermal interface bodies also facilitate scaling to larger heat pumping powers by allowing thermoelectric layers to contain multiple distinct thermoelectric modules while still operating efficiently. Increasing the thicknesses of the thermal interface bodies may seem counter-intuitive because it increases a thermal resistance in respect of heat conduction from one thermoelectric layer to the next thermoelectric layer in the stacked arrangement. However, the inventors have found that the reduced thermal resistance in respect of lateral heat flows outweighs any disadvantages associated with increased axial thermal resistance and provides improvements in overall efficiency. In an embodiment, each of at least a subset of the thermal interface bodies comprises a solid block of thermally conductive material, optionally having a volume averaged thermal conductivity at room temperature that is at least 50 Wm~1K~1. Configuring the thermal interface bodies to have such high thermal conductivities improves lateral heat spreading and thermal efficiency. In an embodiment, the thermally conductive material is metallic, optionally comprising aluminium. Metallic materials, which are not typically used for module walls of thermoelectric modules due to their electrical conductivity, have been found to be particularly well suited to forming thermal interface bodies with high thermal performance. In an embodiment, each of at least a subset of the thermal interface bodies has an average thickness in the axial direction that is larger than an average thickness of the thermoelectric modules in each of one or more of the thermoelectric layers. Configuring thermal interface bodies to satisfy this lower bound has been found to enhance lateral heat spreading sufficiently to improve overall heat pumping efficiency. In an embodiment, each of at least a subset of the thermal interface bodies has a thickness equal to or greater than 1mm. The inventors have found that configuring the thermal interface bodies to be at least 1mm thick enhances lateral heat spreading sufficiently to improve overall heat pumping efficiency. In an embodiment, the heat pump comprises a controller configured to allow selective variation of a number of thermoelectric modules powered in each of one or more of the thermoelectric layers. This provides enhanced flexibility for the heat pump, allowing the heat pump to be optimised for different operating scenarios and / or different directions of heat flow. According to an aspect of the invention, there is provided a heat pump, comprising: a plurality of thermoelectric layers and one or more thermal interface bodies, stacked along an axial direction with the thermal interface body or bodies being interleaved between respective thermoelectric layers, wherein: each thermoelectric layer comprises one or more thermoelectric modules electrically drivable to pump heat parallel to the axial direction, the thermoelectric modules in each thermoelectric layer being arranged to operate thermally in parallel with each other when the thermoelectric layer comprises more than one thermoelectric module; and each thermoelectric module comprises: a pair of electrically insulating module walls separated from each other along the axial direction; and a plurality of semiconductor pillars connected electrically in series, each pillar extending between the module walls and configured such that, when an electrical current is driven through the pillars, heat is pumped along each pillar in the axial direction via the Peltier effect. Each thermal interface body may take any of the forms disclosed herein. Each of at least a subset of the thermal interface bodies may comprise a solid block of thermally conductive material, optionally having a volume averaged thermal conductivity at room temperature that is at least 50 Wm~1K~1. The thermally conductive material may be metallic, optionally comprising aluminium. Each of at least a subset of the thermal interface bodies may have an average thickness in the axial direction that is larger than an average thickness of the thermoelectric modules in each of one or more of the thermoelectric layers. Each of at least a subset of the thermal interface bodies may have a thickness equal to or greater than 1mm. Module walls on the same axial side of different thermoelectric modules in the same thermoelectric layer may be separated from and / or non-integral with each other, and / or may be non-integral with the thermal interface bodies. Providing thermal interface bodies that are distinct from the structure, e.g. module walls, of the thermoelectric modules provides flexibility for optimizing thermal properties of the thermal interface bodies and / or accommodating multiple distinct thermoelectric modules in the same thermoelectric layer. In an embodiment, each of at least a subset of the thermal interface bodies comprises a solid block of thermally conductive material and the thermally conductive material has a volume averaged thermal conductivity at room temperature that is higher than the volume averaged thermal conductivity at room temperature of the module walls. Configuring the thermal interface bodies to have such high thermal conductivities improves lateral heat spreading and thermal efficiency. In an embodiment, the heat pump is configured to pump heat away from a target region and is configured to drive the thermoelectric modules such that the amount of heat pumped by each thermoelectric layer increases progressively from thermoelectric layer to thermoelectric layer in a direction away from the target region. This configuration takes account of heat dissipation in the thermoelectric layers and improves overall efficiency of the heat pump. In an embodiment, the number of thermoelectric modules, and / or total number of pillars of the thermoelectric modules, increases progressively from thermoelectric layer to thermoelectric layer in the direction away from the target region, each of the thermoelectric modules optionally comprising the same number of pillars. Providing different numbers of thermoelectric modules in different layers makes it possible to achieve the progressive increase in heat pumping from thermoelectric layer to thermoelectric layer with less or no need to drive individual thermoelectric modules differently. The heat pump can simultaneously drive a plurality, optionally all, of the thermoelectric modules with the same power in each of at least two of the thermoelectric layers. This allows individual thermoelectric modules to be operated close to or in an optimal operating range. In an embodiment, the heat pump comprises at least three of the thermoelectric layers with at least two respective interleaved thermal interface bodies and the at least two respective interleaved thermal interface bodies increase progressively in lateral size in the direction away from the target region. Progressively increasing lateral sizes of the thermal interface bodies may reduce cost and weight by saving material, as well as facilitating prevention of unwanted conduction of heat between different thermal interface bodies (e.g., towards the target region to be cooled). In an embodiment, the heat pump comprises at least three of the thermoelectric layers with at least two respective interleaved thermal interface bodies and the at least two respective interleaved thermal interface bodies have substantially the same size and shape when viewed perpendicularly to the axial direction. Thus, despite having different numbers of thermoelectric modules in the different thermoelectric layers, the size and shape of the interleaved thermal interface bodies may be kept the same. The inventors have found that keeping thermal interface bodies the same size achieves comparable or improved performance relative to alternative arrangements in which the thermal interface bodies are reduced in size as the number of thermoelectric modules in adjacent layers reduces. Without wishing to be bound by theory, the inventors believe it may be that maintaining the lateral size of the thermal interface bodies increases their heat capacity and that this helps to maintain near uniform temperature differences across thermoelectric modules in the same thermoelectric layer, thereby allowing the thermoelectric modules to all operate with high efficiency. The inventors have also found that keeping the thermal interface bodies the same size and shape facilitates robust construction, as well as mechanical stability and balance. Keeping the thermal interface bodies the same size and shape therefore achieves an improved balance of properties relative to alternative arrangements. In an embodiment, the heat pump comprises a controller configured to allow selective variation of a number of thermoelectric modules powered in each of one or more of the thermoelectric layers. The controller is configured to select the number of thermoelectric modules in each of the one or more of the thermoelectric layers such that the number of powered thermoelectric modules increases progressively from thermoelectric layer to thermoelectric layer in a direction away from the target region. This allows the controller to take account of heat dissipation in the thermoelectric layers and improves overall efficiency of the heat pump without requiring different numbers of thermoelectric modules to be provided in different thermoelectric layers. The controller provides enhanced flexibility of operation, allowing the same stacked arrangement to be used in different scenarios and / or to be less sensitive to installation orientation. In an embodiment, the controller is configured to allow switching of the heat pump between: a first mode in which heat is pumped away from a first target region by pumping heat in a first direction parallel to the axial direction, the controller being configured in the first mode to select the number of thermoelectric modules in each of the one or more of the thermoelectric layers such that the number of powered thermoelectric modules increases progressively from thermoelectric layer to thermoelectric layer in the first direction; and a second mode in which heat is pumped away from a second target region by pumping heat in a second direction opposite to the first direction, the controller being configured in the second mode to select the number of thermoelectric modules in each of the one or more of the thermoelectric layers such that the number of powered thermoelectric modules increases progressively from thermoelectric layer to thermoelectric layer in the second direction. The ability to switch between these two modes allows the same stack configuration to be used to selectively pump in different directions without compromising on efficiency. This may be particularly convenient in situations where the heat pump is large and / or installed in a way that is difficult to adjust when it is needed to switch a direction of pumping. For example, where the heat pump is configured to provide a cooling function in summer and a heating function in winter, this can be done without altering a position or fixation of the heat pump. Instead, a user can simply manipulate the controller to switch the pumping direction. According to an aspect of the disclosure, there is provided a cooling module for a turbomachine, wherein: the turbomachine comprises: a fan; one or more turbines enclosed by a turbine casing; a main conduit system configured to guide a main flow of air from the fan through the one or more turbines; and a bypass conduit outside of the main conduit system, the bypass conduit configured to guide a bypass flow of air outside of the main conduit system, and the cooling module comprises: the heat pump of any embodiment of the present disclosure configured to be mounted in a region between the bypass conduit and the turbine casing and to apply cooling to one or more components in the region. The region may be an under-cowl region adjacent to the turbine casing, which may be at around 500 degrees C during use. Relatively cool air may be diverted from the bypass flow to cool the under-cowl region. However, bleeding air out of the bypass flow removes gas that would otherwise be able to contribute to thrust and thus reduces efficiency. Using heat pumps of the present disclosure to provide cooling in the under-cowl region may reduce or avoid the need to bleed air out of the bypass flow for the purposes of cooling and thereby improve overall efficiency of the turbomachine. According to an aspect of the invention, there is provided a method of pumping heat, comprising: providing a plurality of thermoelectric layers and one or more thermal interface bodies, stacked along an axial direction with the thermal interface body or bodies being interleaved between respective thermoelectric layers, wherein each thermoelectric layer comprises one or more thermoelectric modules electrically drivable to pump heat parallel to the axial direction, the thermoelectric modules in each thermoelectric layer being arranged to operate thermally in parallel with each other when the thermoelectric layer comprises more than one thermoelectric module; and driving the thermoelectric modules to pump heat in the axial direction, wherein for each thermal interface body a maximum lateral temperature difference, defined as a maximum temperature difference in the thermal interface body perpendicular to the axial direction, is maintained lower than a maximum axial temperature difference, defined as a maximum temperature difference in the thermal interface body parallel to the axial direction. Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings. Figure lisa schematic side sectional view of an example thermoelectric module. Figure 2 is a schematic side sectional view of a heat pump comprising a stacked arrangement of thermoelectric modules and thermal interface bodies consisting of solid blocks of material. The number of thermoelectric modules increases progressively from thermoelectric layer to thermoelectric layer. Figure 3 depicts a variation on the arrangement of Figure 2 in which the thermal interface bodies in different layers have the same size and shape. Figures 4 and 5 are schematic side sectional views of a variation on the heat pump of Figure 3 in which the same number of thermoelectric modules are provided in each thermoelectric layer and a controller is used to select different numbers of thermoelectric modules to power in different thermoelectric layers. Figure 6 is a graph demonstrating performance of an arrangement of the type depicted in Figure 2 as a function of time. Figure 7 is a schematic side sectional view of a heat pump comprising a stacked arrangement of thermoelectric modules and a thermal interface body consisting of a solid block of material. Figure 8 is a schematic side sectional view of a variation on the heat pump of Figure 7 in which the thermal interface body defines a chamber containing a fluid. Figure 9 is a schematic side sectional view of a variation on the heat pump of Figures 7 and 8 in which individual thermoelectric layers have multiple thermoelectric modules. Figure 10 is a schematic side sectional view of a turbomachine. Figure 11 is a schematic expanded view of a portion of the turbomachine of Figure 10. Figure 12 depicts a reservoir or block and a heat pipe coupled to respective sides of a thermoelectric module. Figure 13 depicts a reservoir or block and two heat pipe arrangements arranged around two thermoelectric modules. Figure 14 depicts a heat pipe arrangement with two integral reservoirs interfacing with a first and a second plurality of thermoelectric modules. Figure 15 depicts a heat pipe arrangement with a single integral reservoir interfacing with a first plurality of thermoelectric modules. Figure 16 depicts a thermosyphon and two thermoelectric assemblies. The present disclosure relates to heat pumps using thermoelectric modules. An example of a thermoelectric module 100 is depicted in Figure 1. An example of a heat pump is depicted in Figure 2. Referring to Figures 1 and 2, the heat pump may comprise a stacked arrangement. The stacked arrangement comprises a plurality of layers stacked along an axial direction. The axial direction is a vertical direction in the plane of the page in the orientation of the figures. The plurality of layers comprises a plurality of thermoelectric layers 61-63 and one or more thermal interface bodies 31-32. The thermoelectric layers 61-63 and thermal interface body or bodies 31-32 are stacked along the axial direction. The thermal interface body or bodies 31-32 are interleaved between respective (pairs of) thermoelectric layers 61-63. In the example of Figure 2, thermal interface body 31 is interleaved between thermoelectric layers 61 and 62, and thermal interface body 32 is interleaved between thermoelectric layers 62 and 63. As exemplified in Figure 2, further thermal interface bodies 71 and 72 may be provided, for example on one or both axial ends of the stacked arrangement. The further thermal interface bodies 71 and 72 may take any of the forms described herein for the interleaved thermal interface bodies 31-32. Each thermoelectric layer 61-63 comprises one or more thermoelectric modules 100. Each thermoelectric layer 61-63 is depicted one-dimensionally in Figure 2, such that thermoelectric layer 61 appears to contain a single thermoelectric module 100, thermoelectric layer 62 appears to contain three thermoelectric modules 100, and thermoelectric layer 63 appears to contain five thermoelectric modules 100. However, it will be understood that the thermoelectric layers 61-63 extend two-dimensionally (e.g., into and out of the plane of the page) and can contain further thermoelectric modules 100 at positions into or out of the page. In this embodiment, the number of thermoelectric modules 100 in the thermoelectric layer 61 should, however, be less than the number of thermoelectric modules 100 in the thermoelectric layer 62, and the number of thermoelectric modules 100 in the thermoelectric layer 62 should be less than the number of thermoelectric modules 100 in the thermoelectric layer 63 (as will be explained in further detail below). Each thermoelectric module 61-63 is electrically drivable to pump heat parallel to the axial direction (e.g., upwards in Figure 2). The thermoelectric modules 100 in each thermoelectric layer 62-63 are arranged to operate thermally in parallel with each other when the thermoelectric layer 62-63 comprises more than one thermoelectric module 100. Different thermoelectric layers 61-63 and interleaved thermal interface bodies 31-32 are arranged thermally in series. In use, heat flows in sequence through the thermoelectric layers 61-63. The thermoelectric layers 61-63 and / or thermal interface bodies 31-32 may have a substantially planar form. The axial direction may be perpendicular to planes of the thermoelectric layers 61-63 and / or thermal interface bodies 31-32. Providing thermoelectric modules 100 separated from each other by thermal interface bodies 31-32 in the stacked arrangement allows the heat pump to operate over a relatively large temperature difference while individual thermoelectric modules 100 operate over smaller temperature differences. Operating the thermoelectric modules 100 over smaller temperature difference allows the thermoelectric modules 100 to operate with higher efficiency. For example, the heat pump may operate between around -20 degrees Celsius and room temperature while individual thermoelectric modules 100 operate with temperature differences that are approximately N times smaller, where N is the number of thermoelectric layers in the stacked arrangement (e.g., within about 10-20 degrees Celsius for N = 3). When heat is being pumped through the stacked arrangement in a desired direction, the temperature differences across the interleaved thermal interface bodies will be in the opposite direction to the temperature differences across the thermoelectric layers 31-32. This is because the temperatures of the hot and cold sides of the thermoelectric modules 100 will be driven until heat is flowing through the neighbouring thermal interface bodies 31-32 in the desired direction, which requires the temperature difference (also referred to herein as “delta T”) across each thermal interface body 31-32 to be in the appropriate sense (i.e., in a cooling mode, higher on the side nearer to the region that is being cooled than on the other side) such that heat flows in the desired direction, whereas the delta T across the thermoelectric modules 100 themselves will always be in the opposite direction to the intended heat flow (e.g., when in cooling mode, the hot side of the thermoelectric module 100 will always be on the opposite side of the thermoelectric module 100 to the region that is being cooled. The unfavourable delta T across the thermoelectric modules 100 intrinsically limits efficiency of the thermoelectric modules 100 but is mitigated by arranging for the thermal resistance between the hot and cold sides of the thermoelectric modules 100 to be high. Each thermal interface body 31-32 has a thickness parallel to the axial direction selected such that when heat is being pumped by the thermoelectric modules 100 a maximum lateral temperature difference is lower than a maximum axial temperature difference. Lateral temperature differences are defined as temperature differences between positions in the thermal interface body that are displaced relative to each other only laterally. The maximum lateral temperature difference is the maximum of these lateral temperature differences considering all possible pairs of different lateral positions. Axial temperature differences are defined as temperature differences between positions in the thermal interface body that are displaced relative to each other only axially. The maximum axial temperature difference is the maximum of these axial temperature differences considering all possible pairs of different axial positions. This configuration ensures that heat spreads laterally sufficiently quickly that all of the thermoelectric modules 100 can operate with high efficiency and uniformity, with minimal variation in the temperature differences across different thermoelectric modules 100 in the same thermoelectric layer 61-63 (such as thermoelectric modules 100 that are at different lateral positions in different thermoelectric layers 61-63, even where the different thermoelectric layers 61-63 have different numbers of thermoelectric modules 100). The use of thermoelectric layers 61-63 configured in this manner allows the heat pump to be scaled up to include large numbers of individual thermoelectric modules 100 while maintaining high levels of energy efficiency. Increasing the thicknesses of the thermal interface bodies 31-32 may seem counterintuitive because it increases a thermal resistance in respect of heat conduction from one thermoelectric layer 61-63 to the next thermoelectric layer 61-63 in the stacked arrangement. However, the inventors have found that the reduced thermal resistance in respect of lateral heat flows outweighs any disadvantages associated with increased axial thermal resistance and provides improvements in overall efficiency. Referring to Figure 1, each thermoelectric module 100 may comprise a pair of electrically insulating module walls 102 and a plurality of semiconductor pillars 105-106. The module walls 102 are separated from each other along the axial direction. The pillars 105-106 extend between the module walls 102 (e.g., from an interconnect 104 adjacent to one of the pair of module walls 102 to an interconnect 104 adjacent to the other of the pair of module walls 102). The pillars 105-106 are configured such that when an electrical current is driven through the pillars 105-106 heat is pumped in parallel along the pillars 105-106 in the axial direction via the Peltier effect. In the example shown, the pillars 105-106 are connected electrically in series via interconnects 104. A single row of pillars 105-106 are depicted in Figure 1 but it will be understood that plural rows may be provided with endmost interconnects 104A providing connections between the row depicted and respective rows in planes into or out of the page. A row of seven pillars 105-106 is depicted but it will be understood that rows containing more than seven pillars or fewer than seven pillars may be provided. The pillars 105-106 may be doped differently to obtain the Peltier effect. For example, the pillars 105-106 may alternate along the series between being n-doped and p-doped. The module walls 102 are electrically insulating and may be formed from a ceramic material such as AI2O3. Ceramics are relatively poor thermal conductors but can be made sufficiently thin to allow relatively efficient heat exchange through the module walls 102 in the axial direction. For each of one or more (optionally all) of the thermoelectric modules 100, the insulating module walls 102 thermally (and / or physically) contact respective faces of different thermal interface bodies 31-32. For example, for the thermoelectric modules 100 in the thermoelectric layer 62, the uppermost module walls 102 will contact a lowermost face of the thermal interface body 32 and the lowermost module walls 102 will contact an uppermost face of the thermal interface body 31. The thermoelectric modules 100 may be provided as physically separate and / or interchangeable and / or individually controllable units. Thus, module walls 102 on the same axial side of different thermoelectric modules 100 in the same thermoelectric layer 61-63 may be separated from and / or non-integral with each other and the thermal interface bodies 31-32. Optionally, at least two (optionally all) of the thermoelectric modules 100 comprise the same number of pillars 105-106. Providing thermoelectric modules 100 with the same number of pillars may facilitate efficient manufacture and / or simplify control. The thermal interface bodies may in principle comprise or consist of gaseous material, liquid material, solid material or any combination thereof. In some arrangements, however, each of at least a subset (optionally all) of the thermal interface bodies 31-32 comprises a solid block of thermally conductive material. The thermally conductive material may have a volume averaged thermal conductivity at room temperature that is higher than the volume averaged thermal conductivity at room temperature of the module walls 102 and / or is at least 50 optionally at least 75 IVoptionally at least 100 Wm^K-1, optionally at least 150 Wm~1K~1, optionally at least 200 Wm^K-1. The thermally conductive material may be metallic. Metals are extremely good thermal conductors. In contrast to the module walls 102 of the thermoelectric modules 100, the thermal interface bodies can be metallic because they are insulated from the pillars 105-106 of the thermoelectric modules 100 by the module walls 102. In some arrangements, the thermally conductive material comprises, consists essentially of, or consists of aluminium. Aluminium has a desirable balance of low density, high thermal conductivity and high specific heat capacity. In some arrangements, additionally or alternatively, the thermally conductive material may have a volume averaged specific heat capacity at room temperature of at least 0.1 kJ kg^K^1, optionally at least 0.25 kJ kg^ K~\ optionally at least 0.5 kJ kg^K^1, optionally at least 0.75 kJ kg^K-1. Having a high specific heat capacity may make it possible for the thermal interface bodies 31 -32 to absorb larger amounts of heat without excessively changing temperature, thereby improving stability of the heat pump. In some arrangements, an average thickness of each of one or more of the thermal interface bodies 31-32 in the axial direction is larger than an average thickness of the thermoelectric modules 100 in the thermoelectric layers. Each of at least a subset of the thermal interface bodies 31-32 may for example have a thickness equal to or greater than 1mm, optionally equal to or greater than 2mm, optionally equal to or greater than 5mm, optionally equal to or greater than 10mm, optionally equal to or greater than 50mm. Alternatively or additionally, each of at least a subset (optionally all) of the thermal interface bodies 31-32 may define a chamber 56 comprising a fluid, as exemplified in Figure 8 discussed below. The chamber 56 may be configured such that in operation the fluid is simultaneously present in both liquid and gas phases of the fluid and the thermal interface body 31-32 stores or releases energy by changing relative proportions of the liquid and gas phases. In some arrangements, the fluid comprises, consists essentially of, or consists of, ammonia. The chamber 56 may be defined by a thermally conductive casing. The thermally conductive casing may have a volume averaged thermal conductivity at room temperature of at least 50 Wm^K'1, optionally at least 75 Wm~1K~1, optionally at least 100 lVm~1K~1, optionally at least 150 Wm^ K^1, optionally at least 200 Wm^ K^1. In some arrangements, the thermally conductive casing comprises, consists essentially of, or consists of aluminium. As depicted in Figure 2, the heat pump may comprise a controller 40. The controller 40 is configured to control driving of the thermoelectric modules 100. The controller 40 may comprise any suitable combination of electrical hardware and software capable of providing the desired functionality, including for example a power source or a connection to a power source and / or data processing hardware, firmware and / or software. In some arrangements, the heat pump is configured to pump heat away from a target region 51, for example to apply cooling to the target region 51 and / or to heat a region 52 on the opposite side of the stack to the target region 51. In this case, the heat pump may be configured to drive the thermoelectric modules 100 such that the amount of heat pumped by each thermoelectric layer 61-63 increases progressively from thermoelectric layer to thermoelectric layer in a direction away from the target region 51. This is necessary because the TEMs are not 100% efficient, which means that not all of the power input to the TEMs is used to pump energy through the TEMs. Some of the input power will be dissipated as heat, for example via Joule heating in the pillars, which will add to the total amount of heat that needs to pumped away by the next thermoelectric layer in the sequence. The situation will be worse for the next thermoelectric layer in the sequence because it will need to be driven at a higher power and will dissipate even more heat, leading to a cumulative effect along the sequence of thermoelectric layers. Figure 2 depicts an example heat pump configured to address the above challenges by having a number of thermoelectric modules 100, and / or total number of pillars 105-106 of the thermoelectric modules 100, that increases progressively from thermoelectric layer to thermoelectric layer in the direction away from the target region 51 (e.g., with each of the thermoelectric modules 100 comprising the same number of pillars 105-106 such that the total number of pillars 105-106 in each thermoelectric layer 61-63 is proportional to the number of thermoelectric modules 100 in each thermoelectric layer 61-63). Thermoelectric modules 100 are represented in the figures by solid rectangles in each of three example thermoelectric layers 61-63. The numbers of thermoelectric modules 100 depicted in each thermoelectric layer 61-63 are for illustrative purposes. Providing different numbers of thermoelectric modules 100 in different thermoelectric layers, as exemplified in Figure 2, makes it possible to achieve the progressive increase in heat pumping from thermoelectric layer to thermoelectric layer with less or no need to drive individual thermoelectric modules 100 differently. The heat pump can simultaneously drive a plurality, optionally all, of the thermoelectric modules 100 with the same power in each of at least two of the thermoelectric layers. This allows individual thermoelectric modules 100 to be operated close to or in an optimal operating range. As shown in Figure 2, as the number of thermoelectric modules 100 in each thermoelectric layer 61-63 increases, lateral sizes of the thermal interface bodies 31-32 may also increase. Interleaved thermal interface bodies 31-32 may increase progressively in lateral size in the direction away from the target region 51. For example, each thermal interface body 31-32 may be configured to be large enough when viewed along the axial direction to contact all of the axially outwardly facing surfaces of module walls 102 of thermoelectric modules 100 in adjacent thermoelectric layers 61-63. Thus, in the example of Figure 2, thermal interface body 31 needs to be large enough to extend laterally beyond the three thermoelectric modules 100 in thermoelectric layer 62 but does not need to be as large as the thermal interface body 32 that is adjacent to thermoelectric layer 63, which has more thermoelectric modules 100 than thermoelectric layer 62. Thermal interface body 32 is thus laterally larger than thermal interface body 31. Progressively increasing lateral sizes of the thermal interface bodies 31-32 may reduce cost and weight by saving material, as well as facilitating prevention of unwanted conduction of heat between different thermal interface bodies 31-32 (e.g., towards the target region 51 to be cooled). Figure 6 is a graph demonstrating performance of an arrangement of the type depicted in Figure 2 as a function of time. Temperatures T1-T6 were measured as a function of time at respective different points in the stack. The temperatures were obtained using thermocouples positioned as schematically indicated in Figure 2. The ambient temperature curve is labelled TO. The curve corresponding to T6 shows how the cold side adjacent to target region 51 is rapidly and smoothly cooled to below -10 degrees C in less than 30 minutes and to nearly -20 degrees C after 80-100 minutes. In the meantime, a steady delta T is seen to be established across thermal interface bodies 31 and 32. Delta T across the upper thermal interface body 32 was maintained at around 1.5 degrees C. Delta T across the lower thermal interface body 31 was maintained at around 3 degrees C. In both cases, a large and stable delta T is established immediately after the heat pump is switched on and is maintained during the whole operation of the heat pump, thus demonstrating a stable heat pumping action during the whole period of observed operation. It is furthermore noted that the three thermoelectric layers 61-63 contribute evening to the cooling, with T6, T4, T2 and T1 being roughly evenly spaced. This is achieved by arranging for the amount of heat pumped by each thermoelectric layer to increase progressively from thermoelectric layer to thermoelectric layer in a direction away from the target region 51 being cooled, as well as laterally spreading heat within the thermal interface bodies 31-32 as discussed above. Figure 3 depicts a variation on the arrangement of Figure 2 in which thermal interface bodies 31-32 are configured to have the same size and shape when viewed perpendicularly to the axial direction instead of increasing progressively in size, despite the progressive increase in the number of thermoelectric modules 100 from layer to layer. Thus, Figure 3 is an example of an embodiment of the disclosure in which the heat pump comprises at least three thermoelectric layers 61-63 with at least two respective interleaved thermal interface bodies 31-32 having substantially the same size and shape when viewed perpendicularly to the axial direction. This approach may seem counterintuitive because it is desirable to thermally isolate opposite sides of each thermoelectric module 100 to minimize adverse flow of heat towards the target region 51. However, the inventors have found that keeping the thermal interface bodies 31-32 the same size achieves comparable or improved performance relative to alternative arrangements in which the thermal interface bodies 31-32 increase progressively in size. Without wishing to be bound by theory, the inventors believe it may be that maintaining the lateral size of the thermal interface bodies increases their heat capacity and that this helps to maintain near uniform temperature differences across thermoelectric modules in the same thermoelectric layer, thereby allowing the thermoelectric modules to all operate with high efficiency. The inventors have also found that keeping the thermal interface bodies the same size and shape facilitates robust construction, as well as mechanical stability and balance. In some arrangements, regions of the thermal interfaces outside of the thermoelectric modules 100 are thermally insulated to reduce unwanted heat transfer between thermal interface bodies 31-32 in regions outside of the thermoelectric modules 100 where there is no direct contact between module walls 102 of thermoelectric modules 100 and the thermal interface bodies 31-32. As depicted schematically in Figures 2-5, an insulating coating or tape 65 may be applied to relevant surfaces and / or cavities may be filled with thermally insulating material (not shown). Although not shown in Figure 3, thermoelectric modules 100 in thermoelectric layers 61, 62 having fewer thermoelectric modules 100 than other thermoelectric layers 63 may be more spread out spatially. For example, an average lateral spacing between thermoelectric modules 100 in thermoelectric layers 61, 62 having fewer thermoelectric modules 100 than other thermoelectric layers 63 may be larger. Thermoelectric modules 100 in thermoelectric layer 61 (in cases where more than one thermoelectric module 100 is present) may thus be more spread out than thermoelectric modules 100 in thermoelectric layer 62 and / or thermoelectric modules 100 in thermoelectric layer 62 may be more spread out than thermoelectric modules 100 in thermoelectric layer 63. In some arrangements, as exemplified in Figures 4 and 5, the controller 40 is configured to allow selective variation of a number of thermoelectric modules 100 powered in each of one or more of the thermoelectric layers 61-63. The controller 40 may thus allow each of one or more of the thermoelectric layers 61-63 to be selectively operated in different modes, each mode involving powering of a different respective number of thermoelectric modules 100 in the respective thermoelectric layer 61-63. The controller 40 allows selection of the number of thermoelectric modules 100 to be powered in each of one or more of the thermoelectric layers 61-63. The controller 40 may additional or alternatively be configured to allow selective variation of a power applied to one or more thermoelectric modules 100 in each of one or more of the thermoelectric layers 61-63. The number of thermoelectric modules 100 in each thermoelectric layer 61-63 may, as illustrated in Figures 4 and 5, be the same but the controller 40 allows different numbers of thermoelectric modules 100 to be powered in different layers. The controller 40 may thus selectively power a plurality of different numbers of the thermoelectric modules 100 in each thermoelectric layer 61-63, optionally including all of the thermoelectric modules 100 and one or more subsets of the thermoelectric modules 100. The controller 40 allows functionality to be achieved that is the same as or similar to the functionality achieved by arrangements having different numbers of thermoelectric modules 100 in different thermoelectric layers 61 -63. In particular, the controller 40 may be configured to select the number of thermoelectric modules 100 in each of the one or more of the thermoelectric layers 61-63 such that the number of powered thermoelectric modules 100 increases progressively from thermoelectric layer to thermoelectric layer. In Figures 4 and 5, thermoelectric modules 100 that are currently being powered are indicated by solid black rectangles in the respective thermoelectric layers 61-63 and thermoelectric modules 100 that are not being powered are indicated by solid white rectangles. Figure 4 thus shows a selection of thermoelectric modules 100 to power that allows functionality equivalent to that of the arrangement of Figure 3 to be achieved. Figure 5 shows an alternative selection of thermoelectric modules 100 that allows the heat pumping of the arrangement of Figure 3 to be performed in the opposite sense (i.e., cooling region 52 as the target region instead of region 51) without changing the orientation of the stack. Configuring the controller 40 to be able to selectively drive different sets of thermoelectric modules 100 in this way provides enhanced flexibility of operation, allowing the same stacked arrangement to be used in different scenarios and / or to be less sensitive to installation orientation. Providing the same numbers of thermoelectric modules 100 in each thermoelectric layer 61-63 also improves structural robustness, mechanical stability and / or balance. In some arrangements, as depicted schematically in Figures 4 and 5, the controller 40 is configured to select the number of thermoelectric modules 100 in each of the one or more of the thermoelectric layers 61-63 such that the number of powered thermoelectric modules 100 increases progressively from thermoelectric layer to thermoelectric layer in a direction away from the target region 51. In some arrangements, the controller 40 is configured to allow switching of the heat pump between a first mode and a second mode. The first mode, exemplified by Figure 4, is a mode in which heat is pumped away from a first target region 51, for example to apply cooling to the first target region 51, by pumping heat in a first direction parallel to the axial direction (e.g., upwards in the example of Figure 4). The controller 40 is configured in this mode to select the number of thermoelectric modules 100 in each of the one or more of the thermoelectric layers 61-63 such that the number of powered thermoelectric modules 100 increases progressively from thermoelectric layer to thermoelectric layer in the first direction (e.g., such that the number of thermoelectric modules 100 increases from thermoelectric layer to thermoelectric layer going from bottom to top in the orientation of Figure 4). The second mode, exemplified by Figure 5, is a mode in which heat is pumped away from a second target region 52, for example to apply cooling to the second target region 52, by pumping heat in a second direction opposite to the first direction. The controller 40 is configured in this mode to select the number of thermoelectric modules 100 in each of the one or more of the thermoelectric layers 61-63 such that the number of powered thermoelectric modules 100 increases progressively from thermoelectric layer to thermoelectric layer in the second direction (e.g., such that the number of thermoelectric modules 100 increases from thermoelectric layer to thermoelectric layer from top to bottom in the orientation of Figure 5). The ability to switch between these two modes allows the same stack configuration to be used to selectively pump in different directions without compromising on efficiency. This may be particularly convenient in situations where the heat pump is large and / or installed in a way that is difficult to adjust when it is needed to switch a direction of pumping. For example, where the heat pump is configured to provide a cooling function in summer and a heating function in winter, this can be done without altering a position or fixation of the heat pump. Instead, a user can simply manipulate the controller 40 to switch the pumping direction. The arrangements described above with reference to Figures 4 and 5 allow for particularly effective switching between different directions of pumping of heat through the stack but various other arrangements are possible. For example, the controller 40 may be configured to vary powers applied to thermoelectric modules 100 in different thermoelectric layers 61-63 to change the sense of the progressive increase in heat pumping power from thermoelectric layer to thermoelectric layer when the direction of pumping of heat through the stack is switched, without necessarily selecting different numbers of thermoelectric modules 100 to power in any thermoelectric layer (or in addition to that) and / or without necessarily providing different numbers of thermoelectric modules 100 in different thermoelectric layers (and / or in addition to that). In further embodiments, the controller 40 is configured, and / or the heat pump is operated manually, to vary how thermoelectric layers 61-63 in the stack are driven as a function of time. For example, each of the thermoelectric layers may be configured to establish a larger temperature difference, or larger temperature difference averaged over time, across the thermoelectric layer in the axial direction during a first time period and to establish a smaller temperature difference, or smaller temperature difference averaged over time, across the thermoelectric layer in the axial direction during a second time period subsequent to the first time period. An overall temperature difference across the stack may be larger during the first time period than during the second time period. Driving larger temperature differences across some or all of the thermoelectric layers during a first time period relative to a second time period subsequent to the first time period may be appropriate for example shortly after turning the heat pump on to increase a speed at which the heat pump drives a target region 51 to a target temperature. For example, if the heat pump is being used to cool a room to a target temperature, this approach may allow colder air to be output by the heat pump during the first time period than during the second time period, optionally colder than a target temperature of the target region being cooled. This allows the target region as a whole to be cooled more quickly towards the target temperature than in alternative configurations in which air is output at or near to the target temperature. Figures 7-9 depict further examples of the stacked arrangement in which a single interleaved thermal interface body 31 is provided between two thermoelectric layers 61-62. Further thermal interface bodies 71 and 72 are provided on the axial ends of the stacked arrangement. Figures 7 and 9 exemplify arrangements comprising a thermal interface body in the form of a solid block of material. Figure 8 exemplifies an arrangement comprising a thermal interface body 31 defining a chamber 56 containing a fluid. Referring to Figure 7, the thermal interface body 31 extends continuously between thermoelectric modules 100 that are separated from each other by the thermal interface body 31. The uppermost thermoelectric module 100 is in thermoelectric layer 62. The lowermost thermoelectric module 100 is in thermoelectric layer 61. As described above, the thermoelectric modules 100 may be driven by a controller 40. The lowermost thermoelectric module 100 has a first side 211 and a second side 212. The uppermost thermoelectric module 100 has a first side 221 and a second side 222. The thermal interface body 31 is between the uppermost and lowermost thermoelectric modules 100. The thermal interface body 31 has a first face 311 and a second face 312. The thermoelectric modules 100 may be substantially planar and the sides of the thermoelectric modules 100 referred to above may be the largest planar sides of the thermoelectric modules 100. The second side 212 of the lowermost thermoelectric module 100 is thermally coupled (e.g., in thermal contact with, such as directly in contact with or in contact with via a thermally conductive medium such as a thermally conductive adhesive) to the first face 311 of the thermal interface body 31. The first side 221 of the uppermost thermoelectric module 100 is thermally coupled to the second face 312 of the thermal interface body 31. Thus, the thermal interface body 31 is interleaved between the lowermost and uppermost thermoelectric modules 100. In some arrangements, the first and second sides 311 and 312 of the thermal interface body 31 (and, optionally, any of the thermal interface bodies in embodiments of the present disclosure) are parallel to each other. The thermal interface body 31 may, for example, have an outer profile that is substantially planar. The thermal interface body 31 may have a thickness, defined by the separation between the first and second faces 311,312, that is smaller than one or more (or all) dimensions of the thermal interface body 31 in directions perpendicular to the thickness of the thermal interface body 31. These dimensions perpendicular to the thickness may be referred to as in-plane or lateral dimensions. A cross-sectional shape of the thermal interface body 31 in the plane of the thermal interface body 31 is not particularly limited by may, for example, be substantially rectangular, for example square. In some arrangements, the thermal interface body 31 occupies at least a majority (optionally all) of a volume directly between the second side 212 of the lowermost thermoelectric module 100 and the first side 221 of the uppermost thermoelectric module 100 (indicated by the region between the dotted lines in Figure 7 and labelled 54). The volume 54 may be formed by projecting a shape of the second side 212 of the lowermost thermoelectric module 100 (or, if different, a shape of the first side 221 of the uppermost thermoelectric module 100) along the axial direction (e.g., vertically in the plane of the page in the orientations shown in Figures 7-9). For example, if the shapes of the respective sides of the thermoelectric modules 100 are rectangular, the volume would be a cuboid having dimensions defined by the rectangular shapes of the sides of the thermoelectric module 100 and the thickness of the thermal interface body 31. The thermal interface body 31 may or may not extend beyond the volume 54. In the examples shown, the thermal interface body 31 does extend beyond volume 54 (i.e., laterally outwards). As mentioned above, the thermal interface bodies may comprise solid blocks of material. Accordingly, the thermal interface body 31 may comprise a solid block of material. The solid block may fill at least a majority (optionally all) of the volume 54 between the second side 212 of the lowermost thermoelectric module 100 and the first side 221 of the uppermost thermoelectric module 100. Figure 8 depicts a variation on the arrangement of Figure 7 in which the thermal interface body 31 defines a chamber 56 (which may be closed and / or sealed). The chamber 56 may be positioned between the second side 212 of the lowermost thermoelectric module 100 and the first side 221 of the uppermost thermoelectric module 100 (e.g., between the first and second sides of the thermal interface body 31). The chamber 56 may contain a fluid (a liquid and / or a gas). Heat exchange to and / or from the thermal interface body 31 may cause a phase change in the fluid. The fluid in this case may be referred to as a phase change material. The fluid may be simultaneously present in both liquid and gas phases of the fluid and the thermal interface body 31 may store and / or release energy by changing relative proportions of the liquid and gas phases of the fluid in the chamber 56. In the examples of Figures 7-9, the stacked arrangement comprises a further thermal interface body 71-72 on each axial end. The uppermost further thermal interface body 72 is thermally coupled to the second side 222 of the uppermost thermoelectric module 100. The thermal coupling may take any of the forms discussed above in relation to the thermal coupling between the lowermost and / or uppermost thermoelectric modules 100 and the thermal interface body 31. The lowermost further thermal interface body 71 is in thermal contact with the first side 211 of the lowermost thermoelectric module 100. Figure 9 depicts a variation on the arrangement of Figure 7 in which each thermoelectric layer 61-62 comprises a plurality of the thermoelectric modules 100 configured to operate thermally in parallel with each other. The thermoelectric modules 100 in each thermoelectric layer 61-62 are separated from each other and / or configured to be separately drivable. The thermal interface body 31 extends (or spans) continuously between at least a subset (optionally all) of the plurality of thermoelectric module 100 in thermoelectric layer 61 and at least a subset (optionally all) of the plurality of thermoelectric modules 100 in thermoelectric layer 62. In arrangements of this type, each of the thermal interface bodies may separate and extend continuously between thermoelectric modules 100 of one or more pairs of thermoelectric modules 100 that are thermally in series with respect to each other. Any of the details described with reference to Figures 7-9 may be applied in the context of the embodiments described with reference to Figures 1-6. Any of the heat pumps discussed above may be used in a heat transfer arrangement comprising a thermosiphon. The thermosiphon may be configured as described below with reference to Figure 16 or in other ways. The thermosiphon comprises an evaporator 14 and a condenser 16. The thermal siphon comprises a closed loop circuit 17 for circulation of a fluid between the evaporator 14 and the condenser 16. The evaporator 14 may comprise a heat pump according to any of the arrangements discussed herein, including the arrangements discussed with reference to Figures 1-9. The heat pump of the evaporator 14 transfers heat into the fluid in an evaporator region of the circuit 17. The condenser 16 may comprise a heat pump according to any of the arrangements discussed herein, including the arrangements discussed with reference to Figures 1-9. The heat pump of the condenser 16 transfers heat out of the fluid in a condenser region of the circuit 17. In the example of Figure 16, described in further detail below, a first plurality of thermoelectric modules 1 of the heat pumps of both the evaporator 14 and the condenser 16 may be configured in the same way as the thermoelectric modules 100 in the thermoelectric layer 61 discussed with reference to Figure 9. The reservoir blocks 3 are configured in the same way as the thermal interface body 31 of Figure 9. A second plurality of thermoelectric modules 5 of the heat pumps in Figure 16 may be configured in the same way as the thermoelectric modules 100 of the thermoelectric layer 62 discussed with reference to Figure 9. In the example of Figure 16, a finned heat sink 15 is provided at the position of the lowermost further thermal interface body 71 of Figure 9. Figures 10 and 11 depict a further class of embodiment in which the ideas described above are applied in the context of a turbomachine 70. Figure 10 is a schematic side sectional view of a turbomachine. Figure 11 is a schematic expanded view of a portion of the turbomachine generally in the region 72 depicted by the broken line box in Figure 10. The turbomachine 70 comprises a fan 74 and one or more turbines 75 enclosed by a turbine casing 76. The turbomachine 70 further comprises a main conduit system 78. The main conduit system 78 is configured to guide a main flow of air from the fan 74 through the one or more turbines 75. A bypass conduit 80 is provided outside of the main conduit system 78. The bypass conduit 80 guides a bypass flow 82 of air outside of the main conduit system 78. A cooling module 84 is provided that comprises a heat pump according to any of the embodiments described herein. In the example shown, the cooling module comprises five of the stacks 86 of the heat pump along with associated heat exchangers 90. The heat pump is mounted in an under-cowl region 88 between the bypass conduit 80 and the turbine casing 76. For example, the heat pump may be mounted to a radially inner duct casing 81 defining the bypass conduit 80. The heat pump applies cooling to one or more components in the under-cowl region 88. The components to be cooled may comprise any of the components that typically need to be cooled in this region. This may include sensors, electronics, cabling etc. The under-cowl region 88 is adjacent to the turbine casing 76 which may be at around 500 degrees C during use. Relatively cool air (e.g., at a temperature in the region of 170 degrees C) is typically diverted from the bypass flow 82 to provide a temperature in the under-cowl region 88 in the region of 300 degrees C. However, a large proportion of the thrust provided by the turbomachine is provided by the bypass flow. Bleeding air out of this flow removes gas that would otherwise be able to contribute to the thrust and reduces efficiency. Using heat pumps of the present disclosure to provide cooling in the under-cowl region may reduce or avoid the need to bleed air out of the bypass flow 82 for the purposes of cooling and thereby improve overall efficiency of the turbomachine. Further arrangements of the disclosure In the discussion below, the following terms should be understood as defined here: a. thermoelectric semiconductors, or semiconductors, are individual chips, stacks or pieces of thermoelectric semiconductor material, such as may be incorporated into a module as defined below; b. thermoelectric modules (TEMs) are electronic circuit components comprising one or more thermoelectric semiconductors, typically in a casing with electrical contact points or terminals, usually for use either in heat pumping or power generation; c. thermoelectric assemblies (TEAs) are devices or constructions which incorporate one or more TEMs and other components to achieve a purpose, such as heat pumping or electricity generation; d. thermoelectric coolers (TECs) are TEMs or TEAs particularly configured, through the selection of materials, components and so on, for efficient use as heat pumps; e. thermoelectric generators (TEGs) are TEMs or TEAs particularly configured, through the selection of materials, components and so on, for efficient use in electrical power generation. It will be appreciated that TECs and TEGs are essentially interchangeable, since any thermoelectric module will demonstrate the Peltier effect when a current is applied across it, and the Seebeck effect when a temperature difference is applied across it. Nevertheless, it is common for TEGs and TECs to be formed differently, for example being optimized for operation in different environments or for different efficiency priorities. Applying a current through a thermoelectric semiconductor creates a heat flux by the Peltier effect, drawing heat from one side and radiating heat on the other. Electrical energy is also converted into heat energy by Joule heating. A temperature differential existing across a thermoelectric semiconductor can cause an electrical current to be generated by the Seebeck effect. The greater the temperature difference between the hot and cold sides, the greater the electrical current that is generated. Thermoelectric semiconductors or modules can operate individually, can be grouped together in stacks, or can form part of a system. The number of semiconductors or modules in a system, the application and variation of electrical current (both in terms of magnitude and direction), and the temperature differential across modules can be controlled to provide flexible operation. Typically, a controllable power source connected to the terminals of a thermoelectric module will pass an electrical current through the semiconductors therein, which are arranged electrically in series, to transfer heat across the thermoelectric module, resulting in a hot side and a cold side. Reversing the polarity of the electric current changes the direction of the heat transfer and reverses the hot side and the cold side. TECs are solid state heat pumps having hot and cold sides in use. Electrical energy is applied to transfer heat from the cold side to the hot side. When the applied current is switched off, the heat will return from the hot side to the cold side by thermal conduction within the module or assembly, until thermal equilibrium is achieved across the device. Furthermore, as the temperature difference increases across a TEM / TEA, it becomes less efficient as a heat pump and the heat transfer slows until the device itself heats up and transfers heat back to the cold side. Because the hot and cold sides are close together, and thermally coupled, it is difficult to thermally insulate them in order to stop or reduce the reverse heat transfer. Heat sinks can be used with a TEA / TEM. A typical heat sink may comprise a plate formed of a thermally conductive material, such as a metal. Fins may be provided that protrude from their external faces to increase their surface area in contact with the surrounding fluid, such as air. They may be provided with fans to further increase the contact between the surrounding air and the heat sink fins. Embodiments of the disclosure seek to improve the efficiency of TEMs / TEAs as heat pumps, so that they can operate across a wider range of temperatures before reaching their efficiency limits. This aim may be achieved by slowing the increase in the temperature difference across a TEA / TEM by one or more of a. More rapidly extracting heat from the hot side of a TEA / TEM being operated as a heater or cooler, by employing improved heat sinks. b. More rapidly providing heat to the cold side of a TEA / TEM being operated as a heater or cooler, by employing improved heat sinks to the external environment, in thermal communication with the cold side, or by employing a heat source in thermal communication with the cold side. c. Providing stepped heat pumping with a plurality of TEAs / TEMs, stacked with interposing thermal conductors and / or heat storage reservoirs (which may also be referred to as thermal interface bodies). One particularly useful set of embodiments relates to solid state heat pumps for use in refrigeration systems, making use of heat pipes and refrigerant cycles. There are many advantages to such embodiments, not least the removal of the need for moving parts, such as those required by prior art compressors. These may provide TEAs / TEMs with conditions in which they are able to operate over a much wider range of temperatures than in prior art conditions. In a first class of arrangement of the present disclosure, there is provided a heat pump comprising a first thermoelectric module connected to a controllable source of electrical power, and a first heat pipe containing a phase change fluid in thermal communication at a first end with a first side of the first thermoelectric module. A heat pipe is a much more effective heat sink than the heat sinks typically associated with thermoelectric assemblies of the prior art. If the heat pipe is coupled to the hot side, in use, of the thermoelectric module, the phase change fluid at the first end will evaporate, rapidly cooling the thermoelectric module’s hot side, and transporting the heat energy extracted to the second end of the heat pipe, where the fluid will condense and rapidly release the heat energy. If the heat pipe is coupled to the cold side, in use, of the thermoelectric module, the phase change fluid at the first end will condense, rapidly releasing heat to the cold side of the thermoelectric module, and causing evaporation due to pressure change at the second end, rapidly drawing heat from the external environment around the second end. Thus, the heat pipe draws heat from the external environment to the cold side of a thermoelectric module much more rapidly than a conventional heat sink. The principles of both of the options described above can be combined to form a particularly effective heat pump using two heat pipes. Thus, some embodiments further comprise a second heat pipe, having a first end in thermal communication with the second side of the first thermoelectric module. If a heat pipe is positioned so that its condensing end is above its evaporating end, the condensed fluid will return to the evaporating end under gravity. Wicks or similar provisions can be made for the return of the fluid otherwise. Some embodiments of the disclosure further comprise a second thermoelectric module connected to a controllable source of electrical power, wherein the first heat pipe is disposed between the first and second thermoelectric modules, in thermal communication at its second end with a second side of the second thermoelectric module. Such an assembly is extremely versatile. In one possible mode of operation, both the first and second thermoelectric modules can be powered simultaneously, with the same polarity, providing a particularly efficient heat pump in which the range of temperatures across which the two thermoelectric modules are able to operate effectively is considerably increased. For example, when the thermoelectric modules are powered so that their first sides are their hot sides and their second sides are their cold sides, the first thermoelectric module heats the first end of the heat pipe, causing rapid extraction of the heat from the first thermoelectric module’s hot side, and rapid heating of the second thermoelectric module’s cold side. Because the first hot side is being rapidly cooled and the second cold side is being rapidly heated, a great deal more heat is transferred by the heat pump before either thermoelectric module experiences a heat differential sufficient to significantly impair its effectiveness. In another mode of operation, only one of the thermoelectric modules might be powered at any one time. For example, the second thermoelectric module could be powered so that its first side is its cold side, thus extracting heat from the environment around the heat pipe. This might lend itself to an efficient refrigeration system, in which the heat pipe and first thermoelectric module protrude from the roof of a refrigeration chamber, while the second thermoelectric module’s hot side is positioned outside of the chamber to allow the extracted heat to escape. Although the first thermoelectric module is not being powered in this mode, since it would not be desirable to activate a heat source within a cooling chamber, the provision of both thermoelectric modules in this embodiment provides a very efficient general purpose heat pump that can easily be used to meet a variety of needs. In some embodiments, the first heat pipe may comprise a first plurality of heat pipes, each heat pipe of the first plurality of heat pipes having a respective first end and second end, wherein all of the first ends are in fluid communication with each other by means of a first fluid passageway section of pipe. Such embodiments lend themselves particularly well to heating or cooling an external environment, since the plurality of heat pipes in mutual fluid communication increases the surface area of the heat pipe system, thus providing more rapid transfer of heat. In some such embodiments, it may be advantageous to provide a plurality of first thermoelectric modules in thermal communication at their respective first sides with the first end of the first heat pipe. In this case, the plurality of first thermoelectric modules may be in direct thermal communication with the first fluid passageway section of pipe. At least one adjacent pair of heat pipes of the first plurality of heat pipes may additionally be in fluid communication with each other at their second ends by means of a fluid bridging section of pipe. The inventor has found, that by bridging the heat pipes of the first plurality of heat pipes at their second ends, the incidence of unwanted hot spots within the heat pipe system is considerably reduced. Furthermore, by bridging two or more of the heat pipes of the first plurality of heat pipes at their respective second ends, a second fluid passageway section of pipe is formed. In embodiments with a second thermoelectric module, or a plurality of second thermoelectric modules, at the second end of the first heat pipe arrangement, such a second fluid passageway section of pipe provides a suitable interface with the second thermoelectric module(s). Alternatively or additionally, the first heat pipe may comprise a first reservoir section, in fluid communication with and having a greater cross section than other sections of the first heat pipe, wherein the first reservoir section is the section of heat pipe closest to and in most direct thermal communication with the first thermoelectric module. The inventor has found, surprisingly, that providing a first reservoir of phase change material at the interface between the first thermoelectric module(s) and the first heat pipe, still greater efficiencies of heat transfer can be achieved. The reservoir acts as both an effective heat sink and an effective heat store. In some such embodiments, in which the first heat pipe comprises a first plurality of heat pipes, the first reservoir may provide the first fluid passageway section. In other such embodiments, a second reservoir may be provided at the second end of the first heat pipe. This may be particularly advantageous in embodiments having a second thermoelectric module or a plurality of second thermoelectric modules, which can interface with the second reservoir. In such embodiments in which the first heat pipe comprises a first plurality of heat pipes, the second reservoir may form the second fluid passageway section. In some alternative embodiments, the first heat pipe may form a closed loop. A looped heat pipe, also called a thermosyphon, provides a conduit around which the phase change fluid can circulate, driven by pressure changes and, in some orientations, gravity. Powering the first thermoelectric module to heat or cool the first end of the first heat pipe in a loop configuration causes the circulation of fluid around the thermosyphon, providing for rapid heat transfer from the first end to the second end of the thermosyphon. Some such embodiments comprise a second thermoelectric module at the second end of the thermosyphon. Since closed loops do not have ‘ends’ in the sense of extreme opposing vertices, references to the first and second ends of a thermosyphon in the context of this specification should be taken to mean any two points around the loop which are separated from and substantially opposed to one another. This may take different meanings depending on the shape of the loop. In such embodiments, having two thermoelectric modules, the system operates in a similar way to a refrigeration cycle described earlier in this specification. The first thermoelectric module may be operated to heat the thermosyphon, and thus perform the function of the evaporator. Similarly, the second thermoelectric module may be operated to cool the thermosyphon, and thus perform the function of the condenser. In some thermosyphon embodiments having only a first thermoelectric module, said first thermoelectric module may be arranged so that its second side is in thermal communication with the second end of the thermosyphon. In this way, one side can operate as an evaporator while the other side operates as a condenser. In such embodiments it may be advantageous to position the condenser offset with the evaporator so that the condenser is positioned higher than the evaporator. In some embodiments, the first thermoelectric module comprises a first stack of thermoelectric modules, each successive one of the thermoelectric modules of the first stack being rated for optimal operation across a different successive temperature range. Alternatively or additionally, the second thermoelectric module may comprise a second stack of thermoelectric modules, each successive one of the thermoelectric modules of the second stack being rated for optimal operation across a different successive temperature range. In a second class of arrangement, there is provided a heat pump comprising a first thermoelectric module, connected to a controllable source of electrical power, thermally coupled at a first side to a first face of a first block of a thermally conducting solid. The word ‘block’ should be understood in this context to refer to any suitable three dimensional shape. In some embodiments, for example, the block may be a non-planar shape, that is, a three dimensional shape with a thickness which is not significantly smaller than its height and width. In some embodiments, portions of the block may be hollow or may define recesses, fins, or cavities, depending on the particular purpose. The inventor has found that a block of thermally conducting material is a more effective heat sink than the prior art heat sinks which are typically finned plates of thermally conducting material. Unlike the heat sinks of the prior art, it can also act as a store of thermal energy, which may be useful in some instances. Different choices of material and dimensions will determine whether the block is better suited to retain heat, or release it, for example when coupled to the cold side of a thermoelectric module. In some embodiments, a second block of thermally conducting solid is thermally coupled to the second side of the first thermoelectric modules, providing an improved heat sink to both sides of the thermoelectric module. Additionally or alternatively, a second thermoelectric module, connected to a controllable source of electrical power, may be thermally coupled, by a second side, to a second face of the first block of thermally conducting solid. If the characteristics of the block are selected appropriately, for improved heat extraction and release, and if successive thermoelectric modules are powered with the same polarity, such an arrangement provides an improved stack of thermoelectric modules over prior art stacks which typically interpose thin thermally conducting plates between each layer of thermoelectric semiconductors. The improvement is due at least to more rapid heat extraction from a thermoelectric module’s hot side thermally coupled to the first block, which allows more heat to be transferred by the stack before said thermoelectric module reaches its efficiency limits as described above, when compared with a prior art stack containing thermoelectric modules with the same configuration. In a third class of arrangement, there is provided a heat pump comprising a first thermoelectric module thermally coupled at a first side to a first face of a first reservoir of a phase change fluid. The inventor has found that a reservoir of phase change fluid is a more effective heat sink than the prior art heat sinks which are typically finned plates of thermally conducting material. This is because the phase change fluid evaporates in thermal communication with a hot side of a thermoelectric module, leading to a very rapid extraction of heat. Unlike the heat sinks of the prior art, it can also act as a store of thermal energy, which may be useful in some instances. In some embodiments, a second reservoir of phase change fluid is thermally coupled to the second side of the first thermoelectric modules, providing an improved heat sink to both sides of the thermoelectric module. Additionally or alternatively, a second thermoelectric module may be thermally coupled, by a second side, to a second face of the first reservoir of phase change fluid. Such an arrangement provides a significantly improved stack of thermoelectric modules over prior art stacks which typically interpose thin thermally conducting plates between each layer of thermoelectric semiconductors. The improvement is due at least to more rapid heat extraction from a thermoelectric module’s hot side thermally coupled to the first block, and faster release of heat to another thermoelectric modules cold side. These effects allow more heat to be transferred by the stack before said thermoelectric modules reach their efficiency limits as described above, when compared with a prior art stack containing thermoelectric modules with the same configuration. In any one of the arrangements disclosed herein, some or all of the thermoelectric modules may be additionally switchably connected to an electrical load, such that when the thermoelectric modules so connected are situated with an existing heat differential across them, they can be disconnected from the controllable source of electrical power and connected to the electrical load. In this way, the electrical energy generated by the Seebeck effect can power the electrical load. The electrical load may be an electrical energy storage system, such as a battery. Figure 12 depicts an example configuration compatible with the first, second and third classes of arrangement discussed above. A first thermoelectric module 1 is thermally coupled on its first side to a first end of a heat pipe 2. The thermoelectric module is thermally coupled on its second side to a first face of a reservoir block 3. The thermoelectric module 1 is a conventional stack of semiconductors sandwiched between electrically conductive plates as is well known to the skilled person. It is connected to a controllable source of electrical power, in particular to a source of direct current (not shown). The power source is controllable to apply current across the thermoelectric module in either a first direction or a second direction, so as to pump heat across the module in either a first direction or a second direction by the Peltier effect. The power source should also be controllable to apply no current at all in some modes of operation. The thermoelectric module 1 may also be switchably connectable to an electrical load (not shown). Thus, if the thermoelectric module has an existing heat differential across it, it can supply power to the load by the Seebeck effect. The control circuitry is preferably arranged so that the controllable source of electrical power cannot be controlled to supply current to the thermoelectric module while the module is also connected to the electrical load. The electrical load may be an electrical energy storage system, such as a battery. It may alternatively be a water heating system, a domestic appliance, or a metered feed to a utility grid. Appropriate conversion circuitry should be provided according to the particular case. The heat pipe 2 is a closed, elongate fluid conduit containing a phase change fluid, such as water. It is partly filled with the fluid in the liquid state, and otherwise filled with the same fluid in a gaseous state. It is common to fill heat pipes of this kind by adding the fluid in liquid form and boiling it so as to evacuate any air in the tube, replacing it with the fluid in gaseous form, and then sealing the pipe. The pipe is made of a thermally conductive material, such as copper. The dimensions of the pipe, as well as the material from which it is formed and the fluid with which it is filled, all determine the particular heat transfer characteristics of the heat pipe, and these should be selected carefully according to the particular intended purpose. In this embodiment, the heat pipe 2 comprises an interface region 4 at its first end. This has a greater cross section than the rest of the pipe 2, but a shorter length than the rest of the pipe 2. It is to the first end of the interface region 4 that the thermoelectric module is coupled. Since the width of the heat pipe 2 is a significant contributing factor to its heat transfer characteristics, and since the area of the thermoelectric module 1 is a significant contributing factor to its heat transfer characteristics, an interface region may well be advantageous in many configurations in which the cross section of the heat pipe 2 is smaller than the area of the thermoelectric module 1. The reservoir block 3 may be a solid block of thermally conducting material, such as aluminium. The material and dimensions should be selected according to the particular purpose. Alternatively, the reservoir block 3 may be a reservoir of phase change fluid, preferably comprising a chamber defined by a thermally conductive casing, such as a copper casing, filled with a suitable fluid, such as ammonia. The ammonia will partially fill the reservoir in liquid form, and fill the rest of the reservoir in gaseous form. The chamber is typically sealed. The assembly depicted in Figure 1 can be operated in a variety of modes. In one exemplary mode, power is applied to the thermoelectric module 1 while all elements are at room temperature, such that the hot side is coupled to the heat pipe 2 and the cold side is coupled to the reservoir block 3. In this mode, heat is extracted quickly from the hot side of the thermoelectric module 1 by the evaporation of fluid in the heat pipe 2. This heat is then dissipated to the surrounding environment when the fluid condenses at the second end of the heat 5 pipe 2. At the same time, heat is extracted from the reservoir block 3 by the cold side of the thermoelectric module. Because heat is extracted so quickly from the hot side by the heat pipe 2, more heat is transferred by the assembly before the temperature difference between the hot side and the cold side of the thermoelectric module reaches the point at which it can no longer operate effectively. In another exemplary mode of operation, the reservoir block 3 has been preconditioned by another heat source, so that it is warmer than room temperature. Power is then applied to the thermoelectric module 1 so that the hot side is coupled to the heat pipe 2 and the cold side is coupled to the reservoir block. Because the cold side is warmer than the hot side to begin with, and the warmed reservoir block 3 acts as a source of heat as it releases its stored heat to the cold side of the thermoelectric module 1, more heat is transferred by the assembly before the temperature difference between the hot side and the cold side of the thermoelectric module reaches the point at which it can no longer operate effectively. Figure 13 depicts another assembly combining a more complicated embodiment of the first class of arrangement with a more complicated embodiment of the second or third classes of arrangement. A first thermoelectric module 1 is coupled, on its first side, to a first end of a first heat pipe arrangement 2, and to a first face of a reservoir block 3 on its second side. A second thermoelectric module 5 is coupled, on its first side, to a second face of the reservoir block 3, and to a first end of a second heat pipe arrangement 6 on its second side. Both of the thermoelectric modules 1, 5 are controllably connected to power circuitry and load circuity, as in the arrangement of Figure 1 described above. The first heat pipe assembly 2 comprises a first plurality of heat pipes, all protruding from the first interface region 4, which is similar to the arrangement of Figure 12 described above. The second heat pipe assembly 6 similarly comprises a second plurality of heat pipes, all protruding from a second interface region 7. The second ends of all of the heat pipes of the first plurality of heat pipes are joined in fluid communication with each other by means of a first bridging conduit 8. The second ends of two pairs of adjacent heat pipes of the second plurality of heat pipes are similarly joined to each other in fluid communication by means of second 9 and third 10 bridging conduits. The skilled reader will appreciate that, more generally, all of the heat pipes, or any set or sets of heat pipes, or no heat pipes, may be joined at their second ends in this way. The inventor has found that joining at least some of the heat pipes at their second ends in this way prevents the formation of unwanted hot spots within the heat pipe arrangements. In some embodiments, it will be appreciated that these fluid conduits 8-10 may form additional interface regions for additional thermoelectric modules coupled to the second ends of the heat pipes. As is indicated by apparent relative depth in Figure 13, the interface regions 4, 7 each have a greater volume than the fluid conduits 8-10. This optional characteristic is selected to form reservoirs 4, 7 of phase change fluid which are integral to the heat pipe arrangements 2, 6, and which are directly coupled to the thermoelectric modules 1,5. The inventor has found, surprisingly, that the heat extraction characteristics of the heat pipe arrangements are improved by the provision of an integral reservoir of phase change fluid. For the purposes of this specification, an integral reservoir should be understood to be any portion of a heat pipe arrangement with a greater volume per unit distance in any direction, than the rest of the heat pipe arrangement. Figure 14 shows an embodiment of the first class of arrangement, comprising a single heat pipe arrangement 2, with an integral reservoir 4 at the first 5 end and an integral reservoir 11 at the second end. The usefulness of the integral reservoirs as interface regions can be seen in the connection of a first plurality of thermoelectric modules 1 to the first integral reservoir 4, across more than one face of the reservoir 4. A second plurality of thermoelectric modules 12 is provided at the second integral reservoir 11, on only a single face. The skilled reader will appreciate that the heat pipe and integral reservoir arrangements can take a vast array of forms, such as that depicted in Figure 15, which shows a first plurality of heat pipes 2 protruding in a first direction from an integral reservoir 4, and a second plurality of heat pipes 13, protruding in a second direction from the same integral reservoir 4. All of the heat pipes in the two pluralities of heat pipes 2, 13 are in fluid communication with each other via the integral reservoir. A plurality of thermoelectric modules 1 is coupled to at least one face of the integral reservoir 4. Figure 16 depicts a loop thermosyphon system. A heat pipe arrangement 2 is formed into a closed loop conduit of a suitable phase change fluid. A first section of the heat pipe 2 passes through an evaporator 14. The evaporator may be an integral reservoir, or it may simply be a length of pipe similar in cross section to the rest of the pipe 2. It may be a heat transfer block which encases or surrounds the first section of pipe. The skilled reader will appreciate that many heat transfer mechanisms will be suitable for this purpose. A first plurality of thermoelectric modules 1 is thermally coupled to the evaporator 14. The hot sides of the first plurality of thermoelectric modules are coupled to the evaporator. In this embodiment, the cold sides of the first plurality of thermoelectric modules are coupled to a first face of a reservoir block 3, similar to those in previous embodiments. The hot sides of a second plurality of thermoelectric modules 5 are coupled to a second face of the reservoir block 3. The cold sides of the second plurality of thermoelectric modules 5 are coupled to another heat sink 15, in this case a conventional finned heat sink but the skilled person will readily be able to apply the principles of the present disclosure to select an improved heat sink if required. A second section of heat pipe 2 is thermally coupled to a condenser. The condenser is arranged similarly to the evaporator, with a combination of thermoelectric modules, a reservoir block, and a heat sink, but with the hot sides and cold sides of the thermoelectric modules reversed. In a preferred mode of operation, the evaporator is heated and the condenser is cooled, causing the circulation of the working fluid as described above in relation to a refrigeration cycle. Preferably, the condenser is arranged above the evaporator so that gravity will aid the return of the condensed liquid to the evaporator. The phase change fluid rapidly extracts heat from the evaporator, and rapidly expels heat to the condenser, thus transferring heat very efficiently from the evaporator to the condenser. Because of the principles of the present disclosure applied to the thermoelectric modules of the condenser and the evaporator, considerably more heat will be transferred before they reach the limits of their usefulness, when compared with identically rated thermoelectric modules in prior art assemblies. Some embodiments of the disclosure are defined in the following numbered clauses. 1. A heat pump comprising a first thermoelectric module connected to a controllable source of electrical power, and a first heat pipe containing a phase change fluid in thermal communication at a first end with a first side of the first thermoelectric module. 2. A heat pump according to clause 1 comprising a second thermoelectric module connected to a controllable source of electrical power, wherein the first heat pipe is disposed between the first and second thermoelectric modules, in thermal communication at its second end with a second side of the second thermoelectric module. 3. A heat pump according to clause 1 or clause 2, wherein the first heat pipe comprises a first plurality of heat pipes, each heat pipe of the first plurality of heat pipes having a respective first end and second end, wherein all of the first ends are in fluid communication with each other by means of a first fluid passageway section of pipe. 4. A heat pump according to clause 3, wherein at least one pair of adjacent heat pipes of the first plurality of heat pipes are also in fluid communication with each other at their second ends by means of a fluid bridging section of pipe. 5. A heat pump according to any preceding clause, wherein the first heat pipe comprises a first reservoir section, in fluid communication with and having a greater cross section than other sections of the first heat pipe, and wherein the first reservoir section is the section of heat pipe closest to and in most direct thermal communication with the first thermoelectric module. 6. A heat pump according to clause 5, further comprising a second reservoir at the second end of the first heat pipe. 7. A heat pump according to any preceding clause, wherein the first heat pipe forms a closed loop. 8. A heat pump comprising a first thermoelectric module, connected to a controllable source of electrical power, thermally coupled at a first side to a first face of a first block of a thermally conducting solid. 9. A heat pump according to clause 8, wherein a second block of thermally conducting solid is thermally coupled to the second side of the first thermoelectric modules. 10. A heat pump according to clause 8 or clause 9, further comprising a second thermoelectric module, connected to a controllable source of electrical power, thermally coupled by a second side, to a second face of the first block of thermally conducting solid. 11. A heat pump comprising a first thermoelectric module, connected to a controllable source of electrical power, thermally coupled at a first side to a first face of a first reservoir of a phase change fluid. 12. A heat pump according to clause 11, further comprising a second reservoir of phase change fluid, thermally coupled to the second side of the first thermoelectric modules. 13. A heat pump according to clause 11 or clause 12, comprising a second thermoelectric module, connected to a controllable source of electrical power, thermally 5 coupled by a second side, to a second face of the first reservoir of phase change fluid. 14. A heat pump according to any preceding clause in which the first thermoelectric module is switch-ably connected to an electrical load. 15. A heat pump according to clause 14, in which the electrical load is a battery.

Claims

1. A heat pump, comprising:a plurality of thermoelectric layers and one or more thermal interface bodies, stacked along an axial direction with the thermal interface body or bodies being interleaved between respective thermoelectric layers, wherein:each thermoelectric layer comprises one or more thermoelectric modules electrically drivable to pump heat parallel to the axial direction, the thermoelectric modules in each thermoelectric layer being arranged to operate thermally in parallel with each other when the thermoelectric layer comprises more than one thermoelectric module; andeach thermal interface body has a thickness parallel to the axial direction selected such that when heat is being pumped by the thermoelectric modules a maximum lateral temperature difference, defined as a maximum temperature difference in the thermal interface body perpendicular to the axial direction, is lower than a maximum axial temperature difference, defined as a maximum temperature difference in the thermal interface body parallel to the axial direction.

2. The heat pump of claim 1, wherein each of at least a subset of the thermal interface bodies comprises a solid block of thermally conductive material, optionally having a volume averaged thermal conductivity at room temperature that is at least 50 Wm^K^1.

3. The heat pump of claim 2, wherein the thermally conductive material is metallic, optionally comprising aluminium.

4. The heat pump of any preceding claim, wherein each of at least a subset of the thermal interface bodies has an average thickness in the axial direction that is larger than an average thickness of the thermoelectric modules in each of one or more of the thermoelectric layers.

5. The heat pump of any preceding claim, wherein each of at least a subset of the thermal interface bodies has a thickness equal to or greater than 1mm.

6. The heat pump of any preceding claim, comprising a controller configured to allow selective variation of:a number of thermoelectric modules powered in each of one or more of the thermoelectric layers; and / ora power applied to one or more thermoelectric modules in each of one or more of the thermoelectric layers.

7. The heat pump of any preceding claim, wherein each thermoelectric module comprises:a pair of electrically insulating module walls separated from each other along the axial direction; anda plurality of semiconductor pillars connected electrically in series, each pillar extending between the module walls and configured such that, when an electrical current is driven through the pillars, heat is pumped along each pillar in the axial direction via the Peltier effect.

8. A heat pump, comprising:a plurality of thermoelectric layers and one or more thermal interface bodies, stacked along an axial direction with the thermal interface body or bodies being interleaved between respective thermoelectric layers, wherein:each thermoelectric layer comprises one or more thermoelectric modules electrically drivable to pump heat parallel to the axial direction, the thermoelectric modules in each thermoelectric layer being arranged to operate thermally in parallel with each other when the thermoelectric layer comprises more than one thermoelectric module; andeach thermoelectric module comprises:a pair of electrically insulating module walls separated from each other along the axial direction; anda plurality of semiconductor pillars connected electrically in series, each pillar extending between the module walls and configured such that, when an electrical current is driven through the pillars, heat is pumped along each pillar in the axial direction via thePeltier effect.

9. The heat pump of claim 7 or 8, wherein each of at least a subset of the thermal interface bodies comprises a solid block of thermally conductive material and the thermally conductive material has a volume averaged thermal conductivity at room temperature that is higher than the volume averaged thermal conductivity at room temperature of the module walls.

10. The heat pump of any of claims 7 to 9, wherein, for each of one or more of the thermoelectric modules, the module walls thermally contact respective faces of different thermal interface bodies.

11. The heat pump of any of claims 7 to 10, wherein module walls on the same axial side of different thermoelectric modules in the same thermoelectric layer are separated from and / or non-integral with each other, and / or are non-integral with the thermal interface bodies.

12. The heat pump of any of claims 7 to 11, wherein at least two of the thermoelectric modules comprise the same number of pillars.

13. The heat pump of any of claims 7 to 12, wherein the heat pump is configured to pump heat away from a target region and is configured to drive the thermoelectric modules such that the amount of heat pumped by each thermoelectric layer increases progressively from thermoelectric layer to thermoelectric layer in a direction away from the target region.

14. The heat pump of claim 13, wherein the number of thermoelectric modules, and / or total number of pillars of the thermoelectric modules, increases progressively from thermoelectric layer to thermoelectric layer in the direction away from the target region, each of the thermoelectric modules optionally comprising the same number of pillars.

15. The heat pump of claim 14, comprising at least three of the thermoelectric layers with at least two respective interleaved thermal interface bodies and the at least two respectiveinterleaved thermal interface bodies increase progressively in lateral size in the direction away from the target region.

16. The heat pump of claim 14, comprising at least three of the thermoelectric layers with at least two respective interleaved thermal interface bodies and the at least two respective interleaved thermal interface bodies have substantially the same size and shape when viewed perpendicularly to the axial direction.

17. The heat pump of any of claims 13 to 16, wherein:the heat pump comprises a controller configured to allow selective variation of a number of thermoelectric modules powered in each of one or more of the thermoelectric layers; andthe controller is configured to select the number of thermoelectric modules in each of the one or more of the thermoelectric layers such that the number of powered thermoelectric modules increases progressively from thermoelectric layer to thermoelectric layer in a direction away from the target region.

18. The heat pump of claim 17, wherein the controller is configured to allow switching of the heat pump between:a first mode in which heat is pumped away from a first target region by pumping heat in a first direction parallel to the axial direction, the controller being configured in the first mode to select the number of thermoelectric modules in each of the one or more of the thermoelectric layers such that the number of powered thermoelectric modules increases progressively from thermoelectric layer to thermoelectric layer in the first direction; anda second mode in which heat is pumped away from a second target region by pumping heat in a second direction opposite to the first direction, the controller being configured in the second mode to select the number of thermoelectric modules in each of the one or more of the thermoelectric layers such that the number of powered thermoelectric modules increases progressively from thermoelectric layer to thermoelectric layer in the second direction.

19. The heat pump of any preceding claim, wherein the heat pump is configured to simultaneously drive a plurality, optionally all, of the thermoelectric modules with the same power in each of at least two of the thermoelectric layers.

20. The heat pump of any preceding claim, wherein each of at least a subset of the thermal interface bodies defines a chamber comprising a fluid.

21. The heat pump of claim 20, configured such that in operation the fluid is simultaneously present in both liquid and gas phases of the fluid and the thermal interface body stores or releases energy at least partly by changing relative proportions of the liquid and gas phases.

22. A heat transfer arrangement, comprising:a thermosiphon comprising an evaporator, a condenser, and a closed loop circuit for circulation of a fluid between the evaporator and the condenser, wherein:the evaporator comprises a heat pump according to any of claims 1 to 21 configured to transfer heat into the fluid in an evaporator region of the circuit; and / orthe condenser comprises a heat pump according to any of claims 1 to 21 configured to transfer heat out of the fluid in a condenser region of the circuit.

23. A cooling module for a turbomachine, wherein:the turbomachine comprises:a fan;one or more turbines enclosed by a turbine casing;a main conduit system configured to guide a main flow of air from the fan through the one or more turbines; anda bypass conduit outside of the main conduit system, the bypass conduit configured to guide a bypass flow of air outside of the main conduit system, and the cooling module comprises:the heat pump of any of claims 1 to 21 configured to be mounted in a region between the bypass conduit and the turbine casing and to apply cooling to one ormore components in the region.

24. A turbomachine, comprising:a fan;one or more turbines enclosed by a turbine casing;a main conduit system configured to guide a main flow of air from the fan through the one or more turbines;a bypass conduit outside of the main conduit system, the bypass conduit configured to guide a bypass flow of air outside of the main conduit system; andthe heat pump of any of claims 1 to 21 mounted in a region between the bypass conduit and the turbine casing and configured to apply cooling to one or more components in the region.

25. A method of pumping heat, comprising:providing the heat pump of any of claims 1 to 21; anddriving the thermoelectric modules such that the maximum lateral temperature difference is lower than the maximum axial temperature difference in each of the thermal interface bodies.

26. A method of pumping heat, comprising:providing a plurality of thermoelectric layers and one or more thermal interface bodies, stacked along an axial direction with the thermal interface body or bodies being interleaved between respective thermoelectric layers, wherein each thermoelectric layer comprises one or more thermoelectric modules electrically drivable to pump heat parallel to the axial direction, the thermoelectric modules in each thermoelectric layer being arranged to operate thermally in parallel with each other when the thermoelectric layer comprises more than one thermoelectric module; anddriving the thermoelectric modules to pump heat in the axial direction, wherein for each thermal interface body a maximum lateral temperature difference, defined as a maximum temperature difference in the thermal interface body perpendicular to the axial direction, is maintained lower than a maximum axial temperature difference, defined as amaximum temperature difference in the thermal interface body parallel to the axial direction.

27. The method of claim 25 or 26, wherein:each of at least a subset of the thermal interface bodies defines a chamber comprising5 a fluid; andthe fluid is simultaneously present in both liquid and gas phases of the fluid and the thermal interface body stores or releases energy by changing relative proportions of the liquid and gas phases.10Application No: GB2312348.2Examiner: Mr Steve MorganClaims searched: 1-7 &9-27Date of search: 30 January 2024Patents Act 1977: Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular relevance X 1,4, 5-7, 10,13-15 &19-27 US4011104 A (HUGHES) See line 14, column 3 - line 62, column 6 X 1-3, 5-7, 9-12, 19, 22-25 &26 US2022 / 0238777 A (LG) See paragraphs 190-213 X 1-3, 5-7, 9-11, 13-15, 17, 19 &23-26 IP2003078177 A (YAMAHA) Note paragraph 24 in particular A - US11665963 B (HAMILTON SUNDSTRAND) See figure 3 &Lines 5-53, column 5 A - US2017 / 0145892 A (BOEING)Categories:X Document indicating lack of novelty or inventive step A Document indicating technological background and / or state of the art. Y Document indicating lack of inventive step if P Document published on or after the declared priority date but combined with one or more other documents of same category. before the filing date of this invention. & Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:International Classification:Subclass Subgroup Valid From Hl ON 0010 / 81 01 / 01 / 2023 Hl ON 0019 / 00 01 / 01 / 2023

Images