Patent Application
20200348055
The present disclosure relates to the thermal management of devices and systems, specifically to a magnetocaloric refrigerator or heat pump comprising a thermal switch for controlling the heat flux between a heat sink and the heat source, in particular for being used in a magnetic heating and/or cooling apparatus and respective operation methods thereof, particularly as a thermal diode.
In order to maintain device operational temperature, at its optimum thermal performance, several technologies have been developed. Vapour chamber, phase change material (PCM) heat sink, synthetic air-jet pump, piezoelectric fan and thermal switches, are some examples of those technologies [1].
Thermal switches present several advantages when compared with the mentioned technologies. They provide a larger range of thermal control, maintain a uniform temperature during oscillation of external thermal conditions, be adopted to pulsed heat addition/rejection systems, and allow localized cooling or heating.
Thermal switches are usually described as a particular kind of thermal diode that has the ability to enable and break the heat flux, allowing the control of the heat flux direction and intensity, between a heat sink and a heat source.
Thermal diodes can be divided in three main categories:
The active solid-state thermal switch category requires an external activation to operate. This is achieved applying a voltage, pressure, magnetic or electric field, etc. Within the active solid-state thermal switches family, thermoelectric and active mechanical contact thermal diodes are the most commonly used and commercially available for room temperature applications. However, thermionic devices can pose as alternative to thermolectrics for high temperature purposes.
The thermoelectric thermal switches of U.S. Pat. No. 1,695,103A, also known as Peltier modules, use the Peltier effect to transfer heat from the hot to the cold junction. Thermoelectrics are well known and wide spread devices. They can be used in several applications as thermal switches, heat valves, refrigerators, heaters or electricity generators (using the associated Seebeck effect). Using a mature technology in its production, thermoelectrics are relatively cheap and provide a fast heat switching. However they present low energy conversion efficiency, making them less attractive for some applications.
Active and passive mechanical contact thermal diodes exploit the intrinsic materials properties. Through the magnetostriction, piezoelectric, thermal expansion-contraction effects, etc it is possible to mechanically establish or break the contact between the hot source and the cold sink, controlling the heat flux. This category is one of the most used in thermal switches, mostly due to the thermo-actuated bimetallic strip (see U.S. Pat. No. 3,617,971A) and shape memory thermal switch (see U.S. Pat. No. 6,239,686 B1). The bimetallic strip thermal switch, is made by joining two metallic strips with different thermal expansion coefficient. Thereby, the strip is forced to bend when a certain temperature is reached, establishing contact between the heat sink and heat source. Shape memory are materials (alloys or polymers) that possess the ability to return to their original form when plastically deformed. The main types of shape-memory alloys are copper-aluminium-nickel, nickel-titanium and the emergent Ni—Mn—Ga. In these alloys, the austenitic-martensitic transitions are responsible for the shape-memory effect. However, glass or melting transitions are the responsible for the shape-memory effect in polymers.
The main disadvantage of the mechanical contact thermal diodes is the thermal contact resistance between the surfaces of the thermal switch device and the heat sink or heat source. The thermal contact resistance of these devices can narrow the operational working speed and cause significant heat losses.
Thermionic devices (see U.S. Pat. No. 4,747,998A), use a voltage potential to force the passage of electrons in a vacuum cavity, localized between a cold anode and a hot cathode. Here, the electrons are considered heat carriers. Despite of their higher efficiency when compared to thermoelectrics, they present a high operating temperature (<230° C.) making them unfit for room temperature applications [2].
Passive solid-state thermal diodes or thermal rectification devices are composed by materials or structures which transfer heat asymmetrically. This means that, for a given temperature difference, the heat rate in one direction through the material/structure is not the same as the heat rate when the temperature difference is reversed. There are several mechanisms at play in the thermal rectification phenomenon, including surface roughness/flatness at material contacts, difference in temperature dependence of thermal conductivity between dissimilar materials at the contact, thermal potential barrier between material contacts, nanostructured asymmetry, anharmonic lattices (typically 1D) and quantum thermal systems. This anisotropic thermal behaviour can be found in graphene and some perovskite cobalt oxides (LaCoO3 and La0.7Sr0.3CoO3), as well as in carbon nanotubes and boron nitride nanotubes as patented by Chang et al. (see US 20100167004A1). However at room temperature the reported rectification was of only 2%.
Fluidic thermal diodes are systems that use the properties of fluids to control the flux of heat. Within this category, microfluidic systems have gathered most of the researcher attention and several novel thermal diodes have been developed. Most of these systems use the fluid's motion to manipulate the heat. This can be made using pumps, the effect of gravity or the manipulation of fluids properties through an external activation. Exploiting this feature there are three main ways to put fluids in motion: through the appliance of an electric field, the appliance of a magnetic field and through the simultaneous appliance of electric and magnetic fields.
In example examining
Magnetic nanofluids (MNF) also known as ferrofluids offer the possibility to control and promote the fluid flow and consequently the heat transfer process, through the appliance of an external magnetic field. MNF are colloidal mixtures of ferromagnetic (i.e. iron, nickel, cobalt) or ferrimagnetic (Fe2O3, Fe3O4, CoFe2O4) nanoparticles dispersed in water, ethylene glycol (EG), and/or various types of oils [3]. To prevent the aggregation caused by London-van der Waals or magnetic dipole-dipole interactions, the suspended solid particles can be coated with a surfactant layer i.e. oleic acid, tetramethylammonium hydroxide [4,5]. As conventional NFs, the properties of MNFs can be engineered thought the manipulation of their composition. Therefore, their properties (i.e. viscosity, thermal conductivity, thermal energy storage capacity, heat transfer coefficients, etc.) can be tailored to meet the specific requirements of the intended application.
Several devices using MNFs were developed to control the heat flux, as the Heat Pipe Technologies presented in documents JPH11183066A and U.S. Pat. No. 5,005,639A. These proposals exploited the magnetic induced flow properties of ferrofluids, where the appliance of the magnetic field to the MNF has the only purpose of putting in a continuous motion the fluid (instead of a mechanic fluid pump), getting it close to the heat source and evacuating the heat close to the heat exchanger.
In document JP2012233627, the heat transport is achieved using a non-magnetic solid floating or sinking in a magnetic fluid. In this case, a bulk material (BM), is the heat carrier and establishes the thermal contact between the hot and cold sides, while the magnetic fluid acts as the bulk material carrier, forcing its movement inside a container.
These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure.
It is disclosed an externally activated thermal switch (herewith, EATS), which is a device that has the purpose to control the heat flux between a heat sink and a heat source. An embodiment comprises a thermally insulator cage, open in the top and in the bottom, allowing the sequential contact of the thermal bridge or thermal carrier with the two sides. The thermal bridge, or thermal carrier, is sealed inside the insulator cage with a heat conductor plate. Interrupting (OFF mode) and establishing (ON mode) contact with the heat sink and source, it is possible to control the flux of heat through it. The embodiments of the present disclosure offer the capability to work with a vast number and nature of thermal bridges (TB) and thermal carriers (TC), as well as with different nature of external activations, such as magnetic, allowing it to be used in a wide range of operating conditions.
In the embodiments of the present disclosure, we make use of two operating principles different from the above mentioned:
Compared to Heat Pipe technologies, the presently disclosed EATS geometry can provide a large improvement in the heat transport efficiency, this effect is can be substantiated by the enhancement of the MNF thermal conductivity as explained by Philip et al. [7]. It was observed that, when an external magnetic field is applied, parallel to the temperature gradient, the MNF thermal conductivity (k) is enhanced up to 300%, corresponding to a thermal conductivity ratio (k/kf) of 4, where k and kf are the thermal conductivities of the nanofluid and the base fluid, respectively. This is evidence that the potential use of this phenomenon in thermal switches has a significant effect. The k enhancement was attributed to uniformly dispersed chain-line aggregates of nanoparticles formed under the influence of the magnetic field. Inversely, the decrease in k is expected to be due to the zippering of these chains. Moreover, these observations showed that the clusters morphology and distribution have a strong effect in the effective transport of heat through percolating interfacial structures, since they play an important role in the fluid convection velocity and heat conduction.
Regarding document JP2012233627A, the presently disclosed EATS eliminates the use of the BM and improves the thermal contact at the hot and cold side, since the thermal resistance in the interface between the two solid surfaces is much higher than between a liquid-solid interface [6]. This allows EATS to increase the working frequencies and potentially its efficiency.
The present disclosure (in particular, the EATS embodiments) provides an apparatus and method for performing heat switching between heat source and heat sink. As depicted in
With a simple design, this system has the potential to be downsized to the micro scale. The capability to work with a vast number and nature of thermal bridges and thermal carriers, allows this equipment to be used in a wide range of operating conditions. The EATS can also be applied in different configurations, e.g. series, parallel or tubular. Copulating these devices in series (i.e. the source of a first switch connected directly or indirectly with the sink of a second switch) may increase the operating efficiency. By installing this device in parallel (i.e. the source of a first switch connected, directly or indirectly, with the source of a second switch, and the sink of a first switch, connected directly, or indirectly with the sink of a second switch) it is possible to cover bigger areas and to independently establish the thermal contact between the heat sources and eat sink, allowing localized cooling to be performed.
For applications were the EATS needs to work in the horizontal or in the absence of gravity, the OFF state can also be achieved applying an external magnetic field to attract the TB or TC. For example, alternating the appliance of a magnetic field on the two sides of the EATS device, can force the movement of the TB or TC and therefore overcome the misdirected gravity (in relation to the device needs) or its absence.
Therefore, the presented externally activated thermal switch can represent a versatile, reliable and inexpensive device to control heat fluxes. Besides of the above mentioned advantages, this invention also offers the possibility to use small particles or bulk magnetocaloric materials (e.g. Gd), allowing the synchronization of magnetization/demagnetization cycles with the contact with the cold reservoir/heat sink, the principle of a magnetocaloric refrigerator/heat pump.
It is described an externally activatable thermal switch for transferring heat from a heat source to a heat sink, said switch comprising:
It is disclosed a magnetocaloric refrigerator or heat pump, comprising an externally activatable thermal switch as described.
In an embodiment of the magnetocaloric apparatus, the activatable thermal switch is arranged such that, when the thermal switch is activated, the apparatus alternates between the following two states:
An embodiment of the magnetocaloric apparatus comprises an electronic circuit or electronic controller configured to alternate the apparatus between the following two states when the thermal switch is activated:
It is to be noted that if the material of the heat source or of the heat sink is fluid-tight (e.g. not porous), the thermally conductive windows which are part of the insulator cage can be simply holes in the insulator cage. Otherwise, they can be fluid-tight material which is suitably able to conduct heat.
In a embodiment, the first activatable magnet is an electromagnet.
In a embodiment, the first activatable magnet is a permanent magnet movable between two positions in respect of its thermally conductive window: a proximal position and a distal position.
In a embodiment, the magnetic nanofluid is a colloidal mixture of ferromagnetic nanoparticles, further in particular of iron, nickel, or cobalt nanoparticles, or is a ferromagnetic nanoparticle dispersion, further in particular of Maghemite (Fe2O3), Magnetite (Fe3O4) or Cobalt Ferrite (CoFe2O4) nanoparticles, or more generally Iron oxides (Fe2O3 or Fe3O4) or Cobalt Ferrite (CoFe2O4).
An embodiment further comprises a second activatable magnet, placed at the other of the thermally conductive windows in respect of the thermally conductive window of the first activatable magnet, such that the produced magnetic field is aligned substantially parallel to the temperature gradient from heat source to heat sink.
In a embodiment, the second activatable magnet is an electromagnet.
In a embodiment, the second activatable magnet is a permanent magnet movable between two positions in respect of its thermally conductive window: a proximal position and a distal position.
In a embodiment, the insulator cage is tubular.
In a embodiment, the insulator cage is made of a polymer, ceramic or any other material suitable to limit the thermal contact between the two windows of the thermal switch.
In a embodiment, the thermally conductive windows are made of a thermally conductive or thermally semi-conductive material, metal, alloy, ceramic or composite.
It is also described a magnetic thermal, namely magnetocaloric, apparatus comprising a plurality of the externally activatable thermal switches according to any of the previous embodiments, wherein said switches are connected in series, parallel, or combinations thereof.
It is also described a magnetic thermal, namely magnetocaloric, apparatus comprising one or more of the externally activatable thermal switches according to any of the previous embodiments, for thermal energy storage, for refrigeration or for heating, or combinations thereof.
It is also described a magnetic thermal, namely magnetocaloric, apparatus comprising an externally activatable thermal switch according to any of the previous embodiments, layered between two magneto caloric material layers.
It is also described a magnetic thermal, namely magnetocaloric, apparatus comprising a plurality of the externally activatable thermal switches according to any of the previous embodiments, layered in alternating layers with a magneto caloric material layer.
It is also disclosed a method for operating the magnetocaloric apparatus comprising an externally activatable thermal switch according to any of the previous embodiments, said method comprising:
It is also disclosed a method for operating the magnetocaloric apparatus comprising an externally activatable thermal switch according to any of the previous embodiments, said method comprising, when the switch is activated, alternating between the following two states:
In an embodiment, the frequency of the alternating between the two states is between 5 and 30 Hz, further in particular between 10 and 20 Hz, or between 5 and 20 Hz, or between 10 and 30 Hz.
In an embodiment, an external predefined level of electric current, electric field, pressure or light is used to trigger the externally activatable thermal switch.
It is also described a magnetic thermal, namely magnetocaloric, apparatus according to any of the previous embodiments, comprising an electronic circuit or electronic controller configured to carry out the method of any of the previous method embodiments.
It is also described a non-transitory storage media including computer program instructions for implementing a magnetic thermal, namely magnetocaloric, apparatus, the program instructions including instructions executable to carry out the method of any of the previous method embodiments.
The following figures provide preferred embodiments for illustrating the description and should not be seen as limiting the scope of invention.
With reference to the drawings and more specifically
The interior of the PMMA thermal insulator, contains magnetic nanofluid (MNF) 54. The MNF used is a colloidal mixtures of 4.35 vol. % of 10 nm Fe3O4 nanoparticles dispersed in poly-α-olefin oil.
The present device uses a permanent magnet 61 to apply a magnetic field to the MNF 65, as depicted in
Adjusting the quantity of the MNF inside the insulator cage and the magnetic field, it is possible to have two different mechanisms of thermal transport:
These two mechanisms are following described for further understanding.
To assess the performance of the present invention the apparatus schematized in
By decreasing the thermal switch thickness to 1 cm, it was possible to increase the device operational frequency, with no loss of efficiency. Therefore, it was assessed a reduction in the temperature span of ˜80% for frequencies of 5 Hz and ˜70% for frequencies between 10-30 Hz.
Using the apparatus described above (
While specific embodiments of this invention have been shown and described, it should be understood that many variations thereof are possible. The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form released. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.
The term “comprising” whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
It will be appreciated by those of ordinary skill in the art that unless otherwise indicated herein, the particular sequence of steps described is illustrative only and can be varied without departing from the disclosure. Thus, unless otherwise stated the steps described are so unordered meaning that, when possible, the steps can be performed in any convenient or desirable order.
It is to be appreciated that certain embodiments of the invention as described herein may be incorporated as code (e.g., a software algorithm or program) residing in firmware and/or on computer useable medium having control logic for enabling execution on a computer system having a computer processor, such as any of the servers described herein. Such a computer system typically includes memory storage configured to provide output from execution of the code which configures a processor in accordance with the execution. The code can be arranged as firmware or software, and can be organized as a set of modules, including the various modules and algorithms described herein, such as discrete code modules, function calls, procedure calls or objects in an object-oriented programming environment. If implemented using modules, the code can comprise a single module or a plurality of modules that operate in cooperation with one another to configure the machine in which it is executed to perform the associated functions, as described herein.
The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof.
The above described embodiments are combinable.
The following claims further set out particular embodiments of the disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 109315 | Apr 2016 | PT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2017/052006 | 4/7/2017 | WO | 00 |
