Thermoacoustic device, heat transfer system, and method of operating said thermoacoustic device

Abstract

The invention relates to a heat transfer system, a thermoacoustic device, a heat pump system, a heat engine system, and a method for operating a thermoacoustic device, which provides the advantage of compactness, low-noise and high-efficiency.

Description

[0001] TITLE OF THE INVENTION

[0002] THERMOACOUSTIC DEVICE, HEAT TRANSFER SYSTEM, AND METHOD OF OPERATING SAID THERMOACOUSTIC DEVICE

[0003] BACKGROUND OF THE INVENTION

[0004] The invention relates to a heat transfer system for instance configured to provide heating for spaces and / or heating for domestic hot water. In order to use relatively low- temperature heat sources or sinks, e.g. ground, water, commercial and industrial waste heat, solar thermal energy and ambient air, but also for instance district heating, a heat transfer system typically comprises a heat pump that uses a vapor-compression cycle to allow for the transmission of heat from one location at a lower temperature to another location at a higher temperature, i.e. to transport heat against a temperature gradient.

[0005] To use the vapor-compression cycle, the heat pump includes two heat exchangers, one being the condenser to release heat and the other being the evaporator to accept heat. A compressor is used to increase the pressure and thus temperature of a low-temperature low-pressure gas that is subsequently passed through the condenser where it releases heat to the surroundings as it cools and condenses completely. The now cooled high- pressure liquid passes through an expansion valve which reduces the pressure abruptly causing the temperature to drop dramatically. The low-temperature low-pressure mixture of liquid and vapor now travels through the evaporator where it vaporizes completely as it accepts heat from the surroundings before returning to the compressor to start the cycle again.

[0006] Many working fluids that are suitable for the vapor-compression cycle are not environmentally friendly. Recent regulations have pushed the industry to use low greenhouse warming potential working fluids such as propane as working fluid. However, propane heat pumps have quite some challenges, including but not limited to flammability of propane, noisy compressors, limited working temperature ranges and / or limited lifetimes, and size and weight of the system. SUMMARY OF THE INVENTION

[0007] In view of the above it is an object of the invention to provide an improved heat transfer system that mitigates at least one of the abovementioned problems.

[0008] According to a first aspect of the invention, there is provided a heat transfer system comprising: a first fluid circuit with a pump to circulate fluid in the first fluid circuit, a second fluid circuit with a pump to circulate fluid in the second fluid circuit, a control unit, a thermoacoustic device comprising: o an acoustic network with a loop and a working fluid provided in the acoustic network, o an acoustic driver to provide acoustic energy to the acoustic network, and o a thermal core arranged in the loop, said thermal core including a hot heat exchanger, a cold heat exchanger, and a regenerator arranged in between the hot heat exchanger and the cold heat exchanger, wherein the control unit is configured to operate the acoustic driver, wherein the hot heat exchanger is part of the first fluid circuit, wherein the cold heat exchanger is part of the second fluid circuit, wherein the acoustic driver includes two actuators, wherein the control unit is configured to synchronously operate the two actuators, wherein the two actuators are arranged such that synchronized operation of the two actuators by the control unit results in forces applied to a housing or frame of the thermoacoustic device that at least partially cancel each other, and wherein the acoustic driver and acoustic network are configured such that synchronized operation of the two actuators results in the formation of a traveling wave at the location of the thermal core allowing to move heat from the cold heat exchanger to the hot heat exchanger.

[0009] Thermoacoustic devices provide many advantages over vapor-compression cycles, mainly based on the use of environmentally friendlier working fluids such as helium, argon, and nitrogen which gases are inert and non-flammable. However, prior art thermoacoustic devices until now have not been considered a good alternative for the vapor-compression cycle due to the low efficiency of the devices, the relatively large size, relatively high costs, and / or the inherent noise generated by the acoustic driver.

[0010] However, the thermoacoustic device used in the invention according to the first aspect uses two actuators that each generate acoustic waves, but which are arranged and operated synchronously such that vibrations and / or reaction forces resulting from these waves cancel each other at least partly when interacting with the housing of the thermoacoustic device such that the noise level externally of the thermoacoustic device is reduced.

[0011] In an embodiment, operated synchronously means that the actuators move in phase, i.e. driving signals provided to the actuators are in phase. This is for instance the case when the moving parts of the two actuators are arranged such that they move in opposite directions when moving the working fluid towards the loop.

[0012] In an embodiment, operated synchronously means that the actuators move out of phase, i.e. the driving signals provided to the actuators are shifted 180 degrees relative to each other. This is for instance the case when the moving parts of the two actuators are arranged such that they move in the same direction when moving the working fluid towards the loop.

[0013] In an embodiment, each of the two actuators is arranged such that a nearly standing wave can be generated at one side of each actuator that branches in two directions to travel in opposite directions through the loop to subsequently interfere with each other and create a wave having traveling wave characteristics at the thermal core.

[0014] The nearly standing wave, which may be referred to as standing wave throughout the description, generated by an actuator may be a pure standing wave but in practice may have a more complex form, e.g. a superposition of a pure standing wave and a pure traveling wave, wherein the contribution of the pure standing wave is the largest. Because of the traveling wave characteristics, the wave at the thermal core will be referred to as traveling wave throughout the description, but it will be clear to the skilled person that the wave at the thermal core may not necessarily be a pure travelling wave, but may have a more complex form, e.g. a superposition of a pure standing wave and a pure traveling wave, wherein the contribution of the pure travelling wave is the largest. Hence, in simplified terms, the traveling wave expands at the cold heat exchanger and travels though the regenerator to compress at the hot heat exchanger. The advantage of the traveling wave is that the corresponding cycle best matches a Stirling cycle which is more efficient than a Brayton cycle using a pure or nearly standing wave and that the thermal contact between the working fluid and the regenerator is improved thereby further improving the efficiency.

[0015] Hence, each of the two actuators is able to generate a nearly standing wave at an entry location of the loop, which nearly standing wave branches in a first direction along a first loop part, which may alternatively be referred to as forward length, towards a hot heat exchanger side of the thermal core, and branches in a second direction along a second loop part, which may alternatively be referred to as back length, towards a cold heat exchanger side of the thermal core to interfere with each other. Each of the two actuators may interact with its own acoustic loop or the two actuators may interact with the same loop. In case of the latter, the two actuators may for instance be arranged in respective side arms of a T-connection.

[0016] In other words, each loop of the acoustic network includes a first loop part extending between the acoustic converter and the hot heat exchanger, and a second loop part extending between the acoustic converter and the cold heat exchanger. The first loop part and the second loop part are connected to each other at the acoustic driver such that during operation of the acoustic driver, the first loop part and the second loop part are in fluid communication with each other for the most part of the actuation cycle, possibly throughout the entire actuation cycle. This means that for at most a relatively short period of the actuation cycle, the connection between the first and second loop parts may be closed off by the moving parts of the acoustic driver. In an embodiment, a spring-type partitioning element is provided in the loop, e.g. between the acoustic driver and the cold heat exchanger of the thermal core, wherein the spring-type partitioning element is configured to close off the cross-section of the loop and to be impermeable for the working fluid while allowing transmission of pressure waves in the working fluid through the spring-type partitioning element.

[0017] The spring-type partitioning element, i.e. a partition comprising an element with mechanical properties defined by a spring constant thereof, is preferably configured to enforce larger volume flows through the regenerator without adding fluid volume to the device. Besides increasing the volume velocities and therewith power density, the springtype partitioning element may further improve the phasing between pressure and velocity of the working fluid in the regenerator and therefore has a beneficial effect on the efficiency as well. The spring-type partitioning element can further be used to suppress DC flow in the loop, i.e. to suppress time-averaged flow in the loop.

[0018] The spring-type partitioning element may be or comprise a membrane, in which case the spring-type partitioning element may be referred to as booster membrane.

[0019] In an embodiment, a partitioning element is provided in the loop, preferably between the hot heat exchanger and the acoustic driver, more preferably arranged closer to the hot heat exchanger than to the acoustic driver, which partitioning element is configured to close off the cross-section of the loop to suppress DC flow in the loop, while at the same time the partitioning element is acoustically transparent and allows waves to pass between the hot heat exchanger and the cold heat exchanger and vice versa.

[0020] In this manner, the DC flow is reduced, preferably blocked, and the convective heat transfer between the hot heat exchanger and other portions of the loop is strongly reduced. As a result, less heat leaks away from the hot heat exchanger and thus more heat can be transferred between the cold and hot heat exchangers, thereby improving the efficiency of the thermoacoustic device. The partitioning element may be or comprise a membrane, in which case the partitioning element may be referred to as an anti-streaming membrane.

[0021] The control unit may be dedicated to controlling the thermoacoustic device and may for instance be part of the thermoacoustic device but may also be configured to control other components of the heat transfer system, such as the pumps and / or valves in the water circuit and / or the fluid circuit.

[0022] In an embodiment, the control unit is configured to operate the acoustic driver based on a control signal that is representative for the power demand of the heat transfer system. This control signal may be determined by the control unit itself, for instance based on user input and / or one or more sensor signals and / or one or more control rules and / or one or more databases. The control signal may also be received from a parent controller.

[0023] The control signal may be in the form of an absolute value or a percentage of a maximum power demand. The control unit is then configured to operate the acoustic driver at the desired power demand or closest achievable value.

[0024] In an embodiment, a sensor may be provided, e.g. arranged in the first fluid circuit, to measure a parameter like power, pressure, temperature and / or flow, e.g. of the fluid in the first fluid circuit, and wherein the control unit is configured to operate the acoustic driver in dependence of an output of the sensor. Hence, for instance, the control signal may be based on the output of the sensor. The sensor may thus be a power sensor, a pressure sensor, a temperature sensor, or a flow sensor.

[0025] In an embodiment, the thermoacoustic device comprises additional acoustic control elements which can be used, e.g. actively by the control unit or passively, to adjust the specific power within the thermal core. An example of a passive additional acoustic control element may be a side-branched Helmholtz resonator to enforce larger volume flows through the regenerator similar to the booster membrane. In an embodiment, the control unit is configured to operate the acoustic driver to adjust the power output of the thermoacoustic device to short-term variations, e.g. represented by the control signal, and to operate the additional acoustic control elements to adjust the power output of the thermoacoustic device to long-term variations, e.g. due to wear.

[0026] In an embodiment, said loop surrounds a central space, wherein the acoustic driver is at least partially arranged in the central space. Main advantage thereof is that the device can be smaller in design.

[0027] In an embodiment, a moveable portion or part of at least one actuator of the acoustic driver has a moving range that lies completely or at least partially in said central space, such that the moveable portion / part is moved at least partially in and out of the central space or the moveable portion / part is arranged entirely or for the most part in said central space.

[0028] In an embodiment, an entry location to the loop at one side of at least one of the actuators is arranged in or faces towards said central space.

[0029] In an embodiment, the loop defines a central plane running through a center of the loop and parallel to the loop, wherein the two actuators are arranged on either side of the central plane such that the acoustic driver extends substantially perpendicular to the central plane.

[0030] In an embodiment, the actuators of the two actuators of the acoustic driver comprise pistons that are moveable in parallel directions.

[0031] In an embodiment, each of the two actuators separates the acoustic loop from a bounce chamber preferably such that positive and negative pressure differences can be created with a fixed total volume of the loop and bounce chambers. In an embodiment, the two actuators reciprocate around an equilibrium position, wherein a pressure inside the bounce chamber is equal to a pressure inside the acoustic loop when the two actuators are in the equilibrium position.

[0032] In an embodiment, the heat transfer system comprises a plurality of thermoacoustic devices arranged in series and / or in parallel.

[0033] In an embodiment, the thermoacoustic device includes a cooling circuit to control a temperature of components of the thermoacoustic device other than the thermal core, wherein the cooling circuit is connected to or part of the second fluid circuit. This may have the benefit that the temperature of the thermoacoustic device can be stabilized using the cooling circuit thereby preventing damage due to overheating and / or an increase in efficiency. The connection between the cooling circuit and the second fluid circuit may be upstream or downstream of the cold heat exchanger. Such a connection may be a fluid connection but may also include a thermal connection only where the fluids in the circuits are separated but heat can be transferred using a separate heat exchanger.

[0034] In an embodiment, the thermoacoustic device includes a cooling circuit to control a temperature of components of the thermoacoustic device other than the thermal core, wherein the cooling circuit is connected to or part of the first fluid circuit. This may have the benefit that the temperature of the thermoacoustic device can be stabilized using the cooling circuit thereby preventing damage due to overheating and / or an increase in efficiency. The connection between the cooling circuit and the first fluid circuit is preferably upstream of the hot heat exchanger. The other components of the thermoacoustic device are then used to preheat the first fluid thereby improving the efficiency. Such a connection may be a fluid connection but may also include a thermal connection only where the fluids in the circuits are separated but heat can be transferred using a separate heat exchanger.

[0035] In an embodiment, the thermoacoustic device is configured to allow heat generated in the acoustic driver to flow towards the cold heat exchanger using thermal conduction. This provides a passive way of cooling of the acoustic driver to prevent overheating and / or an increase in efficiency / performance.

[0036] In an example, a cooling circuit may be arranged externally on the device outer wall, e.g. using a coil like shape wrapper around the device wall, which cooling circuit is part of the second fluid circuit upstream of the cold heat exchanger, thereby being able to receive heat transferred through the device wall (provided via thermal conduction and / or thermal convection), thereby preheating the fluid in the second fluid circuit, which will stabilize the temperature of the device and increase efficiency of the device.

[0037] Hence, heat generated because of inefficiencies is used as an additional input to the thermoacoustic process.

[0038] In a preferred embodiment, the net forces applied by the two actuators on the housing or frame are substantially equal in magnitude, substantially extending in opposite directions and are preferably aligned relative to each other. This has the advantage that the forces cancel each other substantially completely and do not introduce moments into the housing or frame.

[0039] In an embodiment, the acoustic network includes two loops and a thermal core per loop. The acoustic driver is then configured to provide acoustic energy to both loops and thus the formation of a traveling wave at the location of each thermal core.

[0040] The loops may be interconnected, e.g. via a connecting tube / conduit / channel, wherein the acoustic driver is arranged to generate a nearly standing wave in the connecting tube / conduit / channel that subsequently branches in four directions (namely along the first loop part and second loop part of one loop and along the first loop part and second loop part of the other loop) to travel in two opposite directions through each loop, i.e. parallel through the first and second loop parts, to subsequently interfere with each other and create a wave having traveling wave characteristics at the thermal core of each loop. Alternatively, the loops are arranged separately, wherein one of the two actuators is arranged to provide acoustic energy to one loop of the acoustic network and the other one of the two actuators is arranged to provide acoustic energy to the other loop of the acoustic network.

[0041] In an embodiment, a first and / or second loop part may include two or more loop portions extending in parallel to each other. These loop portions may be separate from each other. In an example, the first loop part or the second loop part includes two, three, four or more loop portions running in parallel between the thermal core and the acoustic driver.

[0042] According to a second aspect of the invention, there is provided a thermoacoustic device comprising: an acoustic network with a loop and a working fluid provided in the acoustic network, a thermal core arranged in the loop, said thermal core including a hot heat exchanger, a cold heat exchanger, and a regenerator in between the hot heat exchanger and the cold heat exchanger, an acoustic converter for converting acoustic energy in the acoustic network into another type of energy or vice versa, wherein the loop surrounds a central space, and wherein the acoustic converter is arranged at least partially in the central space.

[0043] In an embodiment, the loop further includes a first loop part extending between the acoustic converter and the hot heat exchanger, and a second loop part extending between the acoustic converter and the cold heat exchanger, wherein the first loop part and the second loop part are connected to each other at the acoustic converter. In other words, the first loop part and the second loop part are connected to each other at the acoustic converter such that during operation of the acoustic converter, the first loop part and the second loop part are in fluid communication with each other for the most part of the actuation cycle, preferably throughout the entire actuation cycle. This means that for at most a relatively short period of the actuation cycle the connection between the first and second loop parts may be closed off by the moving parts of the acoustic converter. Preferably, fluid communication is possible throughout the entire actuation cycle.

[0044] In an embodiment, a moveable portion or part of the acoustic driver has a moving range that lies completely or at least partially in said central space, such that the moveable portion / part is moved at least partially in and out of the central space or the moveable portion / part is arranged entirely or for the most part in said central space.

[0045] In an embodiment, the loop defines a central plane running through a center of the loop and parallel to the loop, and wherein the acoustic converter extends perpendicular to the central plane.

[0046] In an embodiment, the acoustic converter comprises one or more pistons that are moveable in a moving direction, which moving direction may be substantially perpendicular to a plane defined by the central space.

[0047] In an embodiment, a piston of the acoustic converter separates the loop from a bounce chamber. Preferably, the piston reciprocates around an equilibrium position, wherein a pressure inside the bounce chamber is equal to a pressure inside the acoustic loop when the piston is in the equilibrium position.

[0048] In an embodiment, the thermoacoustic device further comprises a cooling circuit to control a temperature of components of the thermoacoustic device other than the thermal core, wherein the cooling circuit is connected to or part of a first or second fluid circuit connected to the hot heat exchanger and the cold heat exchanger, respectively. Alternatively, or additionally, a thermal cooling path may be provided between the cold heat exchanger and the part of the thermoacoustic device to be cooled to allow cooling via thermal conduction. For instance, the thermoacoustic device may be configured to allow heat to flow from the acoustic converter to the cold heat exchanger.

[0049] In an example, a cooling circuit may be arranged externally on the device outer wall, e.g. using a coil like shape wrapper around the device wall, which cooling circuit is part of the second fluid circuit upstream of the cold heat exchanger, thereby being able to receive heat transferred through the device wall (provided via thermal conduction and / or thermal convection, thereby preheating the fluid in the second fluid circuit, which will stabilize the temperature of the device and increase efficiency of the device.

[0050] Hence, heat generated because of inefficiencies is used as an additional input to the thermoacoustic process.

[0051] In an embodiment, the acoustic converter separates the loop from one or more bounce chambers to allow positive and negative pressure differences using a fixed total volume of loop and one or more bounce chambers.

[0052] In an embodiment, the acoustic converter includes two converter units arranged such that forces applied by the two converter units to a housing or frame of the thermoacoustic device are or can be synchronously applied to at least partially cancel each other.

[0053] According to a third aspect of the invention, there is provided a heat pump system comprising a thermoacoustic device according to the second aspect of the invention and a control unit, wherein the acoustic converter includes an actuator to provide acoustic energy to the acoustic network, and wherein the control unit is configured to operate the acoustic converter to transfer heat from the cold heat exchanger to the hot heat exchanger.

[0054] According to a fourth aspect of the invention, there is provided a heat engine system comprising a thermoacoustic device according to the second aspect of the invention, a heat source and a heat sink, wherein the heat source is connected to the hot heat exchanger and the heat sink is connected to the cold heat exchanger to provide a temperature difference between the hot heat exchanger and the cold heat exchanger, wherein the acoustic network is configured to generate acoustic energy using the temperature difference, and wherein the acoustic converter is configured to convert the acoustic energy into another type of energy. According to a fifth aspect of the invention, there is provided a method for operating a thermoacoustic device according to the first aspect of the invention, wherein the two actuators are driven synchronously such that forces applied to a housing or frame of the thermoacoustic device at least partially cancel each other.

[0055] In an embodiment, the method includes determining a total system resonance and operating the two actuators at or near the total system resonance. This may have the advantage that a change of total system resonance can be determined, e.g. due to a change in operating temperature or changes to the surrounding conditions, and the operation of the thermoacoustic device can be adjusted accordingly to maintain a desired efficiency level. The total system resonance may have different contributions but may be dominated by the mechanical resonance.

[0056] As will be clear to the skilled person, to avoid unduly repetition of features and embodiments, features and embodiments relating to one aspect of the invention may readily be applied to other aspects of the invention where appropriate.

[0057] BRIEF DESCRIPTION OF THE DRAWINGS

[0058] The invention will now be described in a non-limiting way by reference to the accompanying drawings in which like parts are indicated by like reference symbols, and in which:

[0059] Fig. 1 schematically depicts a heat transfer system according to an embodiment of the invention,

[0060] Fig. 2 schematically depicts a thermoacoustic device according to an embodiment of the invention,

[0061] Fig. 3A schematically depicts a cross-sectional view of a thermoacoustic device according to another embodiment of the invention,

[0062] Fig. 3B schematically depicts another cross-sectional view of the thermoacoustic device of Fig. 3A,

[0063] Fig. 4 schematically depicts a heat transfer system according to another embodiment of the invention, Fig. 5 schematically depicts a cross-sectional view of a thermoacoustic device according to a further embodiment of the invention,

[0064] Fig. 6 schematically depicts a cross-sectional view of a thermoacoustic device according to yet another embodiment of the invention,

[0065] Fig. 7 schematically depicts another cross-sectional view of the thermoacoustic device of Fig. 6, and

[0066] Fig. 8 schematically depicts a cross-sectional view of an alternative embodiment of the thermoacoustic device of Fig. 5.

[0067] DETAILED DESCRIPTION OF THE INVENTION

[0068] Fig. 1 schematically depicts a heat transfer system 100 according to an embodiment of the invention. The heat transfer system 100 comprises a first fluid circuit 110 with a pump 111 to circulate the fluid in the first fluid circuit 110. The heat transfer system 100 further comprises a second fluid circuit 120 with a pump 121 to circulate the fluid in the second fluid circuit.

[0069] The heat transfer system 100 comprises a thermoacoustic device 130 having an acoustic network 131 with a loop 132, an acoustic driver 133 and a thermal core 134 arranged in the loop 132. The acoustic network 131 is filled with a working fluid like Helium under pressure, e.g. a pressure of 40-80 bar, e.g. 60 bar. The acoustic driver 133 is configured to provide acoustic energy to the acoustic network.

[0070] The thermal core 134 includes a hot heat exchanger 134a, a cold heat exchanger 134b, and a regenerator 134c arranged in between the hot heat exchanger 134a and the cold heat exchanger 134b. The hot heat exchanger 134a is part of the first fluid circuit 110 and the cold heat exchanger 134b is part of the second fluid circuit 120. The thermoacoustic device 130 can be operated, as will be explained below in more detail, to move heat from the cold heat exchanger 134b to the hot heat exchanger 134a thereby moving heat from the second fluid circuit 120 to the first fluid circuit 110.

[0071] The first fluid circuit 110 may for instance be arranged in a building to heat one or more spaces using a heat exchanger 112, e.g. in the form of a radiator, convector, fan coil or underfloor heating, and / or may be used to provide domestic hot water. Although a single heat exchanger 112 is shown in Fig. 1, it is envisaged that a plurality of heat exchangers 112 are provided, which may be arranged in series or in parallel. The fluid in the first fluid circuit may be a liquid, e.g. water or brine, but may also be a gas, e.g. air.

[0072] The second fluid circuit 120 may for instance be arranged to extract heat from an environment around a building. This can for instance be done directly by circulating air from the environment through the cold heat exchanger of the thermoacoustic device. The pump 121 may then be embodied in the form of a fan to circulate the air in the second fluid circuit 120.

[0073] However, it is also possible that the extraction of heat from the environment is done indirectly using a heat exchanger 122 provided in the second fluid circuit as shown in Fig. 1. A fan 123 may then for instance be provided to allow the extraction of heat from air blown through the heat exchanger 122 by the fan 123 and to provide the heat to the fluid in the second fluid circuit. The heat can subsequently be extracted from the fluid in the second fluid circuit in the cold heat exchanger 134b of the thermoacoustic device 130 to transfer this heat to the first fluid circuit 110 via the hot heat exchanger 134a.

[0074] The fluid in the second fluid circuit may thus be a gas, e.g. air, or a liquid, e.g. water or brine. In an embodiment, the fluid in the first fluid circuit is identical to the fluid in the second fluid circuit, e.g. both fluids are water. An advantage thereof is that the fluid in the first and second fluid circuit are interchangeable allowing to include valves and pipes to change the function of the heat transfer system. In the abovementioned example, the second fluid circuit was used to extract heat from an environment and the first fluid circuit was used to heat spaces in a building. Additional valves and pipes may allow to connect the pump 111 and the heat exchanger 112 to the cold heat exchanger 134b of the thermoacoustic device 130, and to connect the pump 121 and the heat exchanger 122 to the hot heat exchanger 134a of the thermoacoustic device 130, thereby reversing the function from heating to cooling. Heat can now be extracted from the spaces in the building and transferred to the environment. Alternatively, or additionally, this functionality can be used to defrost the outdoor heat exchanger, in this case the heat exchanger 122. In the winter, in the situation that the second fluid is below 0 degrees Celsius in the heat exchanger 122, water present in the air passing the heat exchanger 122 will condense on the fins that are generally present in the heat exchanger 122 and subsequently freezes. This ice inhibits heat transfer. By changing the functionality of the heat transfer system, the ice can be melted as heat is not extracted from but now delivered to the air in the heat exchanger 122. However, other means of defrosting the heat exchanger may alternatively or additionally be used, including but not limited to the use of electric resistive heating to heat the second fluid and / or the fins or other components of the heat exchanger 122.

[0075] When the first fluid and the second fluid are not identical, e.g. water as first fluid and brine as second fluid, the changing between heating and cooling as described above may not be possible without allowing mixing of the first and second fluid and / or using devices to prevent or reverse the mixing of the first and second fluid. However, when changing the functionality is not possible or desired, the defrosting functionality may still be required. As mentioned above, this may be provided by electric resistive heating, but it is alternatively, or additionally, possible to have a buffer vessel with second fluid that is for instance heated indirectly using the first fluid. When defrosting is necessary, the relatively warm second fluid in the buffer vessel can be used, e.g. by circulating the second fluid from the buffer vessel through the heat exchanger, to defrost the heating exchanger 122. Fluid communication between the buffer vessel and the heating exchanger 122 is then not a problem as both contain the same second fluid.

[0076] The acoustic driver 133 includes two actuators, in this embodiment including two pistons 135 that are moveable back and forth in a direction parallel to the arrow AD in Fig. 1. Each actuator / piston 135 is arranged in a respective side arm of a T-connection connected to the loop. Operation of the actuators is controlled by a control unit 150, which control unit 150 is configured to synchronously operate the two actuators. In this example, the synchronous operation is characterized by the pistons 135 moving in opposite directions. As a result, the forces applied to a housing or frame of the thermoacoustic device 130 by the actuators are also in opposite directions and thus cancel each other at least partially.

[0077] The acoustic driver 133 and acoustic network 131 are configured such that synchronized operation of the two actuators results in the formation of a traveling wave at the location of the thermal core 134 allowing to move heat from the cold heat exchanger 134b to the hot heat exchanger 134a.

[0078] In the example of Fig. 1, the control unit 150 is not only configured to control operation of the actuators, but also to control the pumps 111 and 121 and / or any valves (not shown) in the first and / or second fluid circuit. Also, in the example of Fig. 1, the first fluid circuit comprises a temperature sensor 113 to measure a temperature of the fluid in the first fluid circuit, preferably after exiting the hot heat exchanger 134a. An output of the temperature sensor 113 is provided to the control unit 150 to operate the acoustic driver 133 and / or the pumps 111 and 121 in dependence of the measured temperature of the fluid in the first fluid circuit. In an embodiment, the output of the temperature sensor 113 is used by the control unit 150 to determine a control signal representative for a desired power demand for the thermoacoustic device 130, which control signal is then used to operate the acoustic driver 133. The control signal may alternatively or additionally be based on other signals or information, e.g. user input, other sensor signals, etc.

[0079] In the example of Fig. 1, the pistons 135 are on one side in communication with the loop 132, and on the opposite side in communication with a respective back volume 137, which may also be referred to as bounce chamber. By filling the back volumes 137 with a compressible fluid, preferably the same working fluid used in the loop, the back volumes act as gas springs for the pistons 135 allowing to minimize the work to be done by the pistons. Alternatively, or additionally, mechanical springs may be provided to provide the same functionality as the back volumes.

[0080] The pistons 135 may have a center position, and it is preferred that the pressure in the back volumes is equal to the pressure in the loop when the pistons 135 are in the center position, so that (mean) forces applied by the fluid in the back volumes and the working fluid are in equilibrium, and / or leakage of fluid over the pistons 135 is minimized, and / or electrical power to maintain the center position of the pistons are minimized.

[0081] In this example, the loop 132 is provided with two partitioning elements 132a and 132b. In this embodiment, the thermal core 134 is arranged in between the two partitioning elements 132a, 132b. Both partitioning elements suppress DC flow in the loop 132. By providing the partitioning elements close to the thermal core it is also prevented that circular flow patterns exist in or near the thermal core.

[0082] The partitioning element 132a at the hot heat exchanger side of the thermal core 134 is preferably acoustically transparent and allows waves to pass between the hot heat exchanger and the cold heat exchanger and vice versa. In this manner, the convective heat transfer is strongly reduced compared to a thermoacoustic device without partitioning element 132a at the hot heat exchanger side of the thermal core.

[0083] The partitioning element 132b at the cold heat exchanger side of the thermal core 134 is preferably a spring-type partitioning element with mechanical properties defined by a spring constant thereof, and configured to enforce larger volume flows through the regenerator 134c without adding fluid volume to the device.

[0084] The loop includes a first loop part LP1 extending between the acoustic driver and the hot heat exchanger 134a. Hence, the partitioning element 132a is arranged in the first loop part LP1. The loop further includes a second loop part LP2 extending between the acoustic driver and the cold heat exchanger 134b. hence, the partitioning element 132b is arranged in the second loop part LP2. The first loop part LP1 and the second loop part LP2 convene at the T-connection where the actuators / pistons 135 are provided such that the first and second loop parts LP1, LP2, are connected to each other, i.e. in fluid communication, at the acoustic driver. A substantially standing wave generated by the synchronous operation of the two actuators 135 is thus able to branch in two directions, namely through the first loop portion LP1 and through the second loop portion LP2, to interfere with each other at the thermal core 134. As schematically depicted in Fig. 1, the loop 132 of the acoustic network surrounds a central space 136, and the acoustic driver 133 is at least partially arranged in the central space 136, in this case by having the actuators / pistons 135 arranged moveably in the central space, i.e. the moving parts moving entirely inside the central space 136. This saves valuable space and makes the thermoacoustic device smaller in size.

[0085] Fig. 2 schematically depicts a thermoacoustic device 130 according to an embodiment of the invention and that may alternatively be used in a heat transfer system 100 of Fig. 1. The thermoacoustic device 130 has an acoustic network 131 with a loop 132 and a working fluid provided in the acoustic network 131. An acoustic driver 133 is provided to provide acoustic energy to the acoustic network 131. Arranged in the loop 132 is a thermal core 134 having a hot heat exchanger 134a, a cold heat exchanger 134b and a regenerator 134c arranged in between the hot heat exchanger 134a and the cold heat exchanger 134b.

[0086] The acoustic driver 133 comprises two actuators, each including a piston 135 that are moveable in a direction parallel to arrow AD. Each actuator is arranged in a respective side branch of the loop defining an entry location EL of the loop where the actuators provide acoustic waves to the loop. A control unit (not shown in Fig. 2) is configured to operate the acoustic driver 133 by synchronously operating the two actuators such that the pistons substantially move in opposite directions. As a result, thereof forces applied to a housing or frame of the thermoacoustic device 130 by the two actuators at least partially cancel each other thereby reducing vibrations emitted to the surroundings.

[0087] The synchronized operation of the two actuators results in a nearly standing wave in between the two actuators, i.e. in the space between the two entry locations EL, that branches into the loop in two opposite directions, namely along a first loop part LP1 towards the hot heat exchanger 134a and along a second loop part LP2 towards the cold heat exchanger 134b. Subsequent interference at the thermal core 134 results in the formation of a traveling wave allowing to move heat from the cold heat exchanger 134b to the hot heat exchanger 134a. The loop 132 surrounds a central space 136. The acoustic driver 133 is arranged such that a portion thereof is arranged in the central space 136, namely the right actuator in Fig. 2. In other words, the entry location EL of the loop corresponding to the right actuator in Fig. 2 faces towards the central space 136. The piston 135, which is the moveable part thus moves in and out of the central space 136 or entirely (or at least for the most part) inside the central space 136.

[0088] Fig. 3A and 3B depict different cross-sectional views of a thermoacoustic device 130 according to another embodiment of the invention. The thermoacoustic device 130 is suitable to be used in the heat transfer system 100 of Fig. 1. Fig. 3A is a cross-sectional view of plane A-A depicted in Fig. 3B, and Fig. 3B is a cross-sectional view of plane B-B depicted in Fig. 3A.

[0089] The thermoacoustic device 130 has an acoustic network 131 with a loop 132 and a working fluid provided in the acoustic network 131. An acoustic driver 133 is provided to provide acoustic energy to the acoustic network 131. Arranged in the loop 132 is a thermal core 134 having a hot heat exchanger 134a, a cold heat exchanger 134b and a regenerator 134c arranged in between the hot heat exchanger 134a and the cold heat exchanger 134b. A first loop part LP1 is arranged between the acoustic driver 133 and the hot heat exchanger 134a. A second loop part LP2 is arranged between the acoustic driver 133 and the cold heat exchanger 134b.

[0090] The acoustic driver 133 comprises two actuators, each including a piston 135 that is moveable in a direction parallel to arrow AD. A control unit (not shown in Figs. 3A and 3B) is configured to operate the acoustic driver 133 by synchronously operating the two actuators such that the pistons substantially move in opposite directions. As a result, thereof forces applied to a housing or frame of the thermoacoustic device 130 by the two actuators at least partially cancel each other thereby reducing vibrations emitted to the surroundings.

[0091] The synchronized operation of the two actuators results in a nearly standing wave in between the two actuators that branches into the loop in two opposite directions, namely towards the hot heat exchanger 134a via the first loop part LP1 and towards the cold heat exchanger 134b via the second loop part LP2. Interference at the thermal core 134 results in the formation of a traveling wave allowing to move heat from the cold heat exchanger 134b to the hot heat exchanger 134a.

[0092] The loop 132 surrounds a central space 136 and defines a plane 138 running through a center of the loop and parallel to the loop 132. In the previous embodiments shown in Figs. 1 and 2, the acoustic driver 133 was arranged in the same plane as the loop 132 or in a plane parallel thereto, such that the two actuators were arranged in a plane parallel to the plane defined by the loop 132. In the embodiment of Figs. 3A and 3B, the acoustic driver 133 extends perpendicular to the plane 138 such that an actuator is arranged on either side of the plane 138. In other words, the acoustic driver extends through the loop 132.

[0093] The acoustic loop 132 is arranged in a middle section MS of the device 130 as can be best seen in Fig. 3A. An advantage of the two actuators 135 being arranged perpendicular to the plane 138 is that back volumes 137, which may alternatively be referred to as bounce chambers, can be arranged on either side of the middle section MS, which will be referred to as the right section RS and the left section LS according to the directions in the drawing itself. The two actuators 135 separate the respective back volume 137 from the acoustic loop 132. The total volume of the loop 132 and the two back volumes 137 is then constant independent of position of the actuators 135. Hence, positive and negative pressure differences can be obtained for a constant total volume. By being able to arrange the back volumes 137 in the right and left sections next to the middle section, a compact thermoacoustic device can be obtained.

[0094] An advantage of the configuration of Figs. 3A and 3B is that the housing of the device 130 can also be divided into three parts corresponding to the middle section MS, the right section RS and the left section LS. The middle section MS is then a first housing part including space for the acoustic driver which is surrounded by the loop 132. The right section and the left section are then formed by respective second and third housing parts connected to the first housing part, thereby forming the back volumes 137 next to the loop and acoustic driver. This provides a compact design.

[0095] Fig. 4 schematically depicts a heat transfer system 100 according to another embodiment of the invention. The heat transfer system 100 comprises a first fluid circuit 110 with a pump 111 to circulate the fluid in the first fluid circuit 110. The heat transfer system 100 further comprises a second fluid circuit 120 with a pump 121 to circulate the fluid in the second fluid circuit.

[0096] The heat transfer system 100 comprises a thermoacoustic device 130, which may be a thermoacoustic device similar to an embodiment shown in Figs. 1, 2, 3A and 3B. As described in relation to the Figs. 1-3B, the thermoacoustic device includes a hot heat exchanger 134a and a cold heat exchanger 134b and the thermoacoustic device is operable to move heat from the cold heat exchanger 134b to the hot heat exchanger 134a, thereby moving heat from the second fluid circuit to the first fluid circuit.

[0097] The first fluid circuit 110 includes two branches 110a, 110b. The branch 110a includes two heat exchangers 112 arranged in parallel to heat one or more spaces. Each of the two heat exchangers 112 can be controlled using valve 116.

[0098] The branch 110b may be used for domestic hot water supply. Two three-way valves 115 allow to switch between the branches 110a and 110b. The branch 110b is thermally connected to a water circuit 119 by a heat exchanger 114. The water circuit 119 includes a water pump 120 to circulate water between the heat exchanger 114 and a water storage tank 117. The water from the water storage tank 117 is then directed through the heat exchanger 114 to be heated using the fluid in the branch 110b and returned to the water storage tank 117. Fresh water can be supplied to the water storage tank 117 using a water supply WS. Tap or sanitary water can then be extracted from the water storage tank using valves 118. As the water storage tank 117 only needs to be filled now and then, it is possible to temporarily switch from space heating using branch 110a to domestic water supply using branch 110b. In an alternative embodiment, not shown in Fig. 4, the heat exchanger 114 is arranged inside the water storage tank 117 directly transferring heat from the branch 110b to the water inside the water storage tank 117. In this way, the water pump 120 and water circuit 119 may be omitted. Heat exchanger 114 may then for instance be embodied as a coil (serpentine) of pipe.

[0099] In a further alternative embodiment, the heat exchanger 114 may be omitted when the heat exchanger 134a is configured such that inside the heat exchanger 134a the flow for heating and the flow for domestic hot water supply are separated from each other, i.e. the branches 110a and 110b are separated from each other but both travel through the heat exchanger 134a simultaneously. In other words, the heat exchanger 134a has two separate flow channels, one for the domestic hot water supply and one for the heating. In this way, the water supply WS can be directly connected to branch 110b and heat exchanger 134a takes over the function of heat exchanger 114.

[0100] In yet another alternative embodiment, the heat exchanger 114 is arranged parallel to the heat exchangers 112 without the use of valves 115. In this way, water inside the water storage tank 117 may be heated simultaneously while doing space heating using heat exchangers 112.

[0101] A potential benefit of using the thermoacoustic device 130 is that, for a given supply temperature and thermal power output, the efficiency of the device may remain mostly unaffected or improve with decreasing mass flow rate and subsequently increasing temperature difference over the hot heat exchanger. Such flexibility may enable lower mass flow rates to be used to achieve the same heating power, decreasing the circulation pump power consumption and / or enabling slammer, existing diameter water piping to be used without replacement. Further, the heat transfer system 100 may therefore be designed to increase the temperature of the water from e.g. 10 or 20 degrees Celsius to 40 to 60 degrees Celsius in a single pass of the water.

[0102] Fig. 5 schematically depicts a cross-sectional view of a thermoacoustic device 130 according to a further embodiment of the invention. In this specific example, the thermoacoustic device 130 has a substantially symmetrical design with a plane of symmetry indicated by a dashed line and reference symbol SD. A substantially symmetrical design may thus include some non-symmetries.

[0103] The device 130 includes an acoustic network with a first loop 132.1 and a second loop 132.2. The first loop 132.1 is connected to a back volume 137a via conduit SB1. The second loop 132.2 is connected to back volume 137b via a conduit SB2.

[0104] An advantage of the symmetrical design is that a slender cigar-like design can be obtained allowing to arrange the acoustic network in a pipe or conduit construction. In this embodiment, the device 130 has five pipe elements P1-P5. The first loop 132.1 is arranged in pipe element Pl. Conduit SB1 is arranged in pipe element P2. The back volumes 137a and 137 are arranged in pipe element P3. Conduit SB2 is arranged in pipe element P4, and the second loop 132.2 is arranged in pipe element P5.

[0105] When the device 130 has a cigar-like shape, the length is much longer than the width and thickness and the width and thickness are substantially identical, e.g. the width and thickness may commonly be referred to as the diameter. However, other ratios are also possible. The width may for instance be longer than the thickness of the device 130, and the length may be larger or smaller. As an example, the aspect ratio L / D, with the L being the length and D the diameter of the design 130, may be in the range of 2-10, e.g. 5.

[0106] The acoustic network is filled with a working fluid like Helium under pressure, e.g. a pressure of 40-80 bar, for instance 60 bar.

[0107] The first and second loop 132.1, 132.2 have a coaxial design wherein both halves of the respective loop share a common axis.

[0108] The device 130 further includes an acoustic driver with a first actuator 135a arranged in the conduit SB1 and a second actuator 135b in the conduit SB2. The acoustic driver is configured to provide acoustic energy to the acoustic network. The first actuator 135a is configured to provide acoustic energy to the first loop 132.1 and the second actuator 135b is configured to provide acoustic energy to the second loop 132.2. The acoustic driver is to be operated using a control unit (not shown). The control unit is configured to synchronously operate the first and second actuator 135a, 135b.

[0109] The first and second loops 132.1, 132.2 each include a thermal core 134.1, 134.2. Each thermal core 134a, 134b includes a hot heat exchanger, a cold heat exchanger, and a regenerator arranged in between the hot heat exchanger and the cold heat exchanger. The hot and cold heat exchanger can be connected to first and second fluid circuits for cooling and / or heating purposes. This has already been described in relation to other embodiments and will not be unduly repeated here.

[0110] In the cross-sectional view of Fig. 5, the first loop 132.1 and the second loop 132.2 are each formed by four loop portions indicated using reference symbols LPA, LPB, LPC and LPD. The loop portions LPA and LPB extend from the respective conduit along a sidewall of the respective pipe element to meet at an opposite side of said respective pipe element and form one half of the respective loop. The loop portion LPC extends from the loop portions LPA and LPB to the hot heat exchanger while loop portion LPD extends from the cold heat exchanger to the loop portions LPA and LPB. Both loop portions LPC and LPD extend through the center portion of said respective pipe element and together form the other half of the respective loop. The loop portions LPA, LPB and LPC together form a first loop part between the actuator and the thermal core. The loop portion LPD forms a second loop part between the actuator and the thermal core. The loop portions LPA, LPB may be separate loop portions that extend in parallel or may be two parts of a single loop portion.

[0111] The first and second actuator 135a, 135b are to be operated to generate nearly standing wave that branches to travel along the first loop part and the second loop part thereby traveling in opposite directions through the loop. The wave traveling through the first loop part will interfere with the wave traveling through the second loop part to create a wave having traveling characteristics at the respective thermal core. The thermal core is arranged such that the traveling wave expands at the cold heat exchanger and travels though the regenerator to compress at the hot heat exchanger thereby transferring heat from the cold heat exchanger to the hot heat exchanger.

[0112] The first and second actuators 135a, 135b may include pistons reciprocating around an equilibrium position. During operation, actuation of the pistons results in reaction forces applied to the pipe elements. The control unit is therefore configured to synchronously operate the two actuators, i.e. the pistons move in phase and thus simultaneously towards and away from the first and second loop, respectively. This has the benefit that the forces applied to the housing of the thermoacoustic device at least partially cancel each other.

[0113] The actuators 135a, 135b are on one side in communication with the respective loop of the acoustic network, and on the opposite side in communication with a respective back volume. The back volumes 137a, 137b are preferably filled with the same working fluid as in the respective loop and preferably at the same pressure as the working fluid in the loop when the actuators are in the equilibrium position.

[0114] In the embodiment of Fig. 5, a spring-type partitioning element 132b is arranged in the loop part LPD of the first and second loop at the cold heat exchanger side of the respective thermal core. The spring-type partitioning element 132b is configured to enforce larger volume flows through the regenerator of the respective thermal core without adding fluid volume to the device.

[0115] Figs. 6 and 7 schematically depict different cross-sectional views of a thermoacoustic device 130. The thermoacoustic device includes a housing 200 with a space for an acoustic converter 135 that is able to convert acoustic energy in an acoustic network 131 into another type of energy or vice versa.

[0116] The acoustic network comprises a loop 132 and arranged inside the loop 132 a thermal core 134, said thermal core including a hot heat exchanger, a cold heat exchanger, and a regenerator in between the hot heat exchanger and the cold heat exchanger. The loop defines a first loop part extending between the acoustic converter 135 and the thermal core, i.e. the hot heat exchanger side of the thermal core 134. A second loop part LP2 extends between the acoustic converter 135 and the thermal core, i.e. the cold heat exchanger side of the thermal core 134. The first loop part includes four separate portions of which three are visible in Figs. 6 and 7 and denoted LPA, LPB and LPD.

[0117] Acoustic waves generated by the acoustic converter are able to travel along the first loop part and the second loop part LP2 to the thermal core, or acoustic waves generated by the thermal core are able to travel along the first loop part and the second loop part LP2 to the acoustic converter.

[0118] Fig. 8 schematically depicts a cross-sectional view of a thermoacoustic device 130 according to an embodiment of the invention and which is described as an alternative embodiment of the thermoacoustic device 130 of Fig. 5. The pipe elements Pl and P5 in Fig. 8 are identical to the corresponding pipe elements Pl and P5 of Fig. 5. These elements will not be described in full here to avoid duly repetition, and also not all reference symbols are shown to keep Fig. 8 clear. The only reference symbols depicted for pipe elements Pl and P5 are the first loop 132.1 and the second loop 132.2. Reference is made to Fig. 5 for all other reference symbols, where applicable.

[0119] The pipe elements Pl and P5 are connected to each other by pipe element P6. This pipe element is depicted as a single pipe element but may be formed using multiple pipe subelements (not shown). The first and second loops 132.1, 132.2 are connected to each other via conduit SB. Extending from the conduit SB are a first side branch SB1 and a second side branch SB2 connecting the conduit SB to a back volume 137a and back volume 137b, respectively.

[0120] The device 130 further includes an acoustic driver with a first actuator 135a arranged in the first conduit SB1 and a second actuator 135b in the second conduit SB2. The acoustic driver is configured to provide acoustic energy to the acoustic network and thus to both loops 132.1 and 132.2. A nearly standing wave can be generated in the space SPA of the conduit SB in between the first actuator 135a and the second actuator 135b. This nearly standing wave subsequently branches in four directions (namely along the first loop part and second loop part of the first loop 132.1 and along the first loop part and second loop part of the second loop 132.2) to travel in two opposite directions through each loop 132.1, 132.2, i.e. parallel through the first and second loop parts, to subsequently interfere with each other and create a wave having traveling wave characteristics at the thermal core 134.1, 134.2 of the first and second loops 132.1, 132.2, respectively.

[0121] The first and second actuators 135a, 135b may include pistons reciprocating around an equilibrium position. During operation, actuation of the pistons results in reaction forces applied to the pipe element P6. A control unit is therefore configured to synchronously operate the two actuators, i.e. the pistons move in phase and thus simultaneously towards and away from the conduit SB, respectively. This has the benefit that the forces applied to the housing of the thermoacoustic device at least partially cancel each other.

[0122] Although the examples include a single thermoacoustic device, it is explicitly mentioned here that it is also possible to use more than one thermoacoustic device in a heat transfer system, heat pump system or heat engine system according to the invention. The multiple thermoacoustic devices may be arranged in parallel in which the hot heat exchangers of the multiple thermoacoustic devices are connected to each other and in which the cold heat exchangers of the multiple thermoacoustic devices are connected to each other. Alternatively, the multiple thermoacoustic devices may be arranged in series in which the cold heat exchanger of one thermoacoustic device is connected to the hot heat exchanger of the next thermoacoustic device. Combinations of series and parallel connections are also possible.

[0123] The benefit of multiple thermoacoustic devices is that either more fluid can be given the same temperature difference and / or that the temperature difference can be increased for a given flow rate. Although in the above specification, the working fluid is not always specified, the working fluid is preferably a gas, e.g. a noble gas. The working fluid may for instance be Helium, Neon, Argon, Nitrogen or Hydrogen. It is preferred that the working fluid is an inert gas, preferably non-flammable. Alternatively, the working fluid is a compressible liquid.

[0124] Although in the above specification and the examples, use is made of two actuators, e.g. pistons, it is envisaged that the invention can also be applied to embodiments using an acoustic driver with three or more actuators arranged to provide acoustic energy to the acoustic network. In case of three actuators, the actuators may be arranged to work in a corresponding direction that is 120 degrees rotated relative to a direction of another one of the actuators. In case of an even number of actuators, the actuators may be arranged in pairs, wherein the actuators of each pair work in opposite direction, i.e. 180 degrees rotated relative to each other.

[0125] Although examples have been given in which the first loop part may include one, two or four separate loop portions, any number of loop portions is envisaged as well as a plurality of loop portions for the second loop part.

[0126] The invention may also be summarized by the following clauses:

[0127] 1. A heat transfer system comprising: a first fluid circuit with a pump to circulate fluid in the first fluid circuit, a second fluid circuit with a pump to circulate fluid in the second fluid circuit, a control unit, a thermoacoustic device comprising: o an acoustic network with a loop and a working fluid provided in the acoustic network, o an acoustic driver to provide acoustic energy to the acoustic network, and o a thermal core arranged in the loop, said thermal core including a hot heat exchanger, a cold heat exchanger, and a regenerator arranged in between the hot heat exchanger and the cold heat exchanger, wherein the control unit is configured to operate the acoustic driver, wherein the hot heat exchanger is part of the first fluid circuit, wherein the cold heat exchanger is part of the second fluid circuit, wherein the acoustic driver includes two actuators, wherein the control unit is configured to synchronously operate the two actuators, wherein the two actuators are arranged such that synchronized operation of the two actuators by the control unit results in forces applied to a housing or frame of the thermoacoustic device that at least partially cancel each other, and wherein the acoustic driver and acoustic network are configured such that synchronized operation of the two actuators results in the formation of a traveling wave at the location of the thermal core allowing to move heat from the cold heat exchanger to the hot heat exchanger.

[0128] 2. A heat transfer system according to clause 1, wherein a sensor is arranged in the first fluid circuit to measure a parameter of the fluid in the first fluid circuit, and wherein the control unit is configured to operate the acoustic driver in dependence of an output of the sensor.

[0129] 3. A heat transfer system according to clause 1 or 2, wherein said loop surrounds a central space, and wherein the acoustic driver is at least partially arranged in the central space.

[0130] 4. A heat transfer system according to clause 3, wherein the loop defines a central plane running through a center of the loop and parallel to the loop, and wherein the two actuators are arranged on either side of the central plane such that the acoustic driver extends substantially perpendicular to the central plane.

[0131] 5. A heat transfer system according to any of clauses 1-4, wherein the actuators of the two actuators of the acoustic driver comprise pistons that are moveable in parallel directions.

[0132] 6. A thermoacoustic device comprising: an acoustic network with a loop and a working fluid provided in the acoustic network, a thermal core arranged in the loop, said thermal core including a hot heat exchanger, a cold heat exchanger, and a regenerator in between the hot heat exchanger and the cold heat exchanger, an acoustic converter for converting acoustic energy in the acoustic network into another type of energy or vice versa, wherein the loop surrounds a central space, and wherein the acoustic converter is arranged at least partially in the central space.

[0133] 7. A thermoacoustic device according to clause 6, wherein the loop defines a central plane running through a center of the loop and parallel to the loop, and wherein the acoustic converter extends perpendicular to the central plane.

[0134] 8. A thermoacoustic device according to clause 6, further comprising a cooling circuit to control a temperature of components of the thermoacoustic device other than the thermal core, wherein the cooling circuit is connected to or part of the second fluid circuit.

[0135] 9. A thermoacoustic device according to clause 6, further comprising a cooling circuit to control a temperature of components of the thermoacoustic device other than the thermal core, wherein the cooling circuit is connected to or part of the first fluid circuit.

[0136] 10. A thermoacoustic device according to clause 6, wherein the acoustic converter separates the loop from one or more bounce chambers to allow positive and negative pressure differences using a fixed total volume of loop and one or more bounce chambers.

[0137] 11. A heat pump system comprising a thermoacoustic device according to clause 6 and a control unit, wherein the acoustic converter includes an actuator to provide acoustic energy to the acoustic network, and wherein the control unit is configured to operate the acoustic converter to transfer heat from the cold heat exchanger to the hot heat exchanger.

[0138] 12. A heat engine system comprising a thermoacoustic device according to clause 6, a heat source and a heat sink, wherein the heat source is connected to the hot heat exchanger and the heat sink is connected to the cold heat exchanger to provide a temperature difference between the hot heat exchanger and the cold heat exchanger, wherein the acoustic network is configured to generate acoustic energy using the temperature difference, and wherein the acoustic converter is configured to convert the acoustic energy into another type of energy.

[0139] 13. A method for operating a thermoacoustic device according to clause 6, wherein the two actuators are driven synchronously such that forces applied to a housing or frame of the thermoacoustic device at least partially cancel each other. 14. A method according to clause 13, further including determining a total system resonance and operating the two actuators at or near the total system resonance.

Claims

C L A I M S1. A heat transfer system (100) comprising: a first fluid circuit (110) with a pump (111) to circulate fluid in the first fluid circuit, a second fluid circuit (120) with a pump (121) to circulate fluid in the second fluid circuit, a control unit (150), a thermoacoustic device (130) comprising: o an acoustic network (131) with a loop (132) and a working fluid provided in the acoustic network, o an acoustic driver (133) to provide acoustic energy to the acoustic network, and o a thermal core (134) arranged in the loop, said thermal core including a hot heat exchanger (134a), a cold heat exchanger (134b), and a regenerator (134c) arranged in between the hot heat exchanger and the cold heat exchanger, wherein the control unit is configured to operate the acoustic driver, wherein the hot heat exchanger is part of the first fluid circuit, wherein the cold heat exchanger is part of the second fluid circuit, wherein the acoustic driver includes two actuators, wherein the control unit is configured to synchronously operate the two actuators, wherein the two actuators are arranged such that synchronized operation of the two actuators by the control unit results in forces applied to a housing or frame of the thermoacoustic device that at least partially cancel each other, and wherein the acoustic driver and acoustic network are configured such that synchronized operation of the two actuators results in the formation of a traveling wave at the location of the thermal core allowing to move heat from the cold heat exchanger to the hot heat exchanger.

2. A heat transfer system according to claim 1, wherein a sensor (113) is arranged in the first fluid circuit to measure a parameter of the fluid in the first fluid circuit,and wherein the control unit is configured to operate the acoustic driver in dependence of an output of the sensor.

3. A heat transfer system according to claim 1 or 2, wherein said loop surrounds a central space (136), and wherein the acoustic driver is at least partially arranged in the central space.

4. A heat transfer system according to claim 3, wherein the loop defines a central plane running through a center of the loop and parallel to the loop, and wherein the two actuators are arranged on either side of the central plane such that the acoustic driver extends substantially perpendicular to the central plane.

5. A heat transfer system according to any of claims 1-4, wherein the actuators of the two actuators of the acoustic driver comprise pistons (135) that are moveable in parallel directions.

6. A thermoacoustic device comprising: an acoustic network with a loop and a working fluid provided in the acoustic network, a thermal core arranged in the loop, said thermal core including a hot heat exchanger, a cold heat exchanger, and a regenerator in between the hot heat exchanger and the cold heat exchanger, an acoustic converter for converting acoustic energy in the acoustic network into another type of energy or vice versa, wherein the loop surrounds a central space, wherein the loop further includes a first loop part extending between the acoustic converter and the hot heat exchanger, and a second loop part extending between the acoustic converter and the cold heat exchanger, wherein the first loop part and the second loop part are connected to each other at the acoustic converter, and wherein the acoustic converter is arranged at least partially in the central space.

7. A thermoacoustic device according to claim 6, wherein the loop defines a central plane running through a center of the loop and parallel to the loop, and wherein the acoustic converter extends perpendicular to the central plane.

8. A thermoacoustic device according to claim 6, further comprising a cooling circuit to control a temperature of components of the thermoacoustic device other than the thermal core, wherein the cooling circuit is connected to or part of the second fluid circuit.

9. thermoacoustic device according to claim 6, further comprising a cooling circuit to control a temperature of components of the thermoacoustic device other than the thermal core, wherein the cooling circuit is connected to or part of the first fluid circuit.

10. A thermoacoustic device according to claim 6, wherein the acoustic converter separates the loop from one or more bounce chambers to allow positive and negative pressure differences using a fixed total volume of loop and one or more bounce chambers.

11. A heat pump system comprising a thermoacoustic device according to claim 6 and a control unit, wherein the acoustic converter includes an actuator to provide acoustic energy to the acoustic network, and wherein the control unit is configured to operate the acoustic converter to transfer heat from the cold heat exchanger to the hot heat exchanger.

12. A heat engine system comprising a thermoacoustic device according to claim 6, a heat source and a heat sink, wherein the heat source is connected to the hot heat exchanger and the heat sink is connected to the cold heat exchanger to provide a temperature difference between the hot heat exchanger and the cold heat exchanger, wherein the acoustic network is configured to generate acoustic energy using the temperature difference, and wherein the acoustic converter is configured to convert the acoustic energy into another type of energy.

13. A method for operating a thermoacoustic device according to claim 1, wherein the two actuators are driven synchronously such that forces applied to a housing or frame of the thermoacoustic device at least partially cancel each other.

14. A method according to claim 13, further including determining a total system resonance and operating the two actuators at or near the total system resonance.

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