WO2016051003A1

WO2016051003A1 - Heating cell, heater using same, heating system and use thereof

Info

Publication number: WO2016051003A1
Application number: PCT/ES2015/070712
Authority: WO
Inventors: José Francisco Fernandez Lozano; Elias DE LOS REYES DAVÓ; Ruth DE LOS REYES CÁNOVAS; Javier GARCÍA SEVILLA; Enrique VELA CARRASCOSA; Antonio JARA RICO
Original assignee: Consejo Superior De Investigaciones Científicas (Csic); Universitat Politècnica De València; Microbiotech, S.L.
Priority date: 2014-10-01
Filing date: 2015-09-30
Publication date: 2016-04-07
Also published as: ES2568749A1; EP3133348A4; ES2568749B1; EP3133348B1; EP3133348A1

Abstract

The present invention details a solution to the problem of transforming, with high efficiency, microwave energy into heat by means of heating units in the form of low-power heating cells that allow microwave energy to be propagated along transverse electromagnetic mode transmission lines to ceramic materials having high dielectric loss in the microwave region. Said low-power heating cells are integrated into a unit heater. A group of unit heaters forms a microwave-energy heating system that uses a low-electric-power line.

Description

HEATING CELL, HEATER THAT MAKES SAME USE, HEATING SYSTEM AND USE OF IT

D E S C R I P C I Ó N

OBJECT OF THE INVENTION

The present invention pertains to the field of heat generation systems, in particular to a heating system that uses ceramic pieces as heat emitting elements that are heated by microwave radiation distributed by planar technology.

More specifically, the present invention is directed to the heating element that employs ceramic compositions adapted as transducers containing microwave radiation susceptors capable of absorbing microwave radiation and transforming it into heat.

BACKGROUND OF THE INVENTION

Thermal radiation or heat radiation is the radiation emitted by a body due to its temperature. A radiator is a type of heat emitter whose function is to exchange heat from the heating system to yield it to the environment, and generally it is a device without moving parts or heat production. Radiators are discrete elements that are part of centralized heating installations. Originally, the first heating systems used steam and the high surface temperature of the radiators produced heat exchange by radiation. The replacement of water vapor radiators reduced operating temperatures and, given the small surface area of the radiators, causes most of the heat to be exchanged by convection. The heat emission or dissipation of a radiator depends on the difference in temperatures between its surface and the surrounding environment and the amount of surface in contact with that environment. The greater the exchange surface and the greater the temperature difference, the greater the exchange. In air conditioning installations and especially in heating installations, an emitter is a device that emits heat, giving it to the environment inhabited

A heater incorporates heat generating elements and a thermal radiator or thermal emitter. An example would be an apparatus that is heated by an electrical resistor incorporated inside the thermal emitter. In this example the term electric radiator is used, although the difference between a radiator and a heater is that the radiator does not produce energy, it is limited to being a heat sink that reaches the radiator usually through a network of pipes through a carrier fluid circulating that has been heated in a heat producing device located elsewhere.

An electric heater is generally a unit element that uses an electrical resistor to produce heat. The electrical resistors have a high energy consumption that requires an important electrical power. Typically a unit element consumes around 2 kW / h. According to ISO 7730, thermal comfort is defined as "That condition of the mind in which satisfaction with the thermal environment is expressed". This parameter is not easy to calculate since many factors are taken into account from location, orientation and ventilation of the house to activities carried out in it and clothing of its inhabitants. For usual conditions of use, it is estimated that the optimum comfort temperature is 22 ° C.

A heating system requires a set of heating elements that implies an important electric power supplied. The radiation efficiency depends fundamentally on the thermal inertia of the heat exchanger material. Normally this material is metallic, which makes it necessary a continuous power supply to maintain its high temperature, since the metallic materials have a very low specific heat. Portable heaters that incorporate a ceramic element have greater thermal inertia. A heater with ceramic element will need between 80-100 W per m ² , depending on the average insulation quality. A type house of 80 m ² would require at least 6 to 8 kW / h of minimum electrical power contracted to meet the demand of the heating system.

The advantages of electric heaters are related to the absence of emissions of gases or waste at the place of heat production, that is in the heater. In order to increase the efficiency of heating systems, heat accumulators are incorporated for their sustained and prolonged release for a certain time. One of the elements used as heat accumulators are ceramic blocks with high thermal inertia due to their low thermal conductivity and high density. An application of heaters with thermal accumulator is related to accumulating heat in hours of excess production of electric energy and releasing this without consumption of electric energy in hours of increased demand. Heating systems with ceramic accumulators have limitations related to the use of electrical resistors and their low efficiency, since due to the effect of the same thermal inertia that allows these ceramic materials to release their heat very slowly, one of the limitations of the state of The technique is related to the fact that ceramic materials require a very long time for heating when electrical resistors are used. Therefore, the state of the art requires new solutions that solve the aforementioned problems. Among the possible solutions, the use of microwave radiation as a heat generation system has been considered.

The electromagnetic waves defined in a given frequency range are called microwaves; generally between 300 MHz and 300 GHz, which implies an oscillation period of 3x 10 ^"9 s to 3x 10 ^{" 12} s and a wavelength in the range of 1 m to 1 mm. Other definitions, for example those of the IEC 60050 and IEEE 100 standards, place their frequency range between 1 GHz and 300 GHz, that is, wavelengths between 30 centimeters and 1 millimeters.

As in the case of other types of electromagnetic waves, microwaves can propagate through dielectric means and be transmitted or reflected in the interfaces formed by discontinuities between different media. Since the mid-twentieth century, some applications have appeared in which microwave energy has been used as a means to transfer energy to materials, taking advantage of their interaction with them.

One of the best known applications of microwaves is the microwave oven, which uses a magnetron to produce waves at a frequency of approximately 2.45 GHz. These waves make the water molecules vibrate or rotate generating heat. Because most foods contain a significant percentage of water, they can be easily cooked in this way. Water, fats and other substances present in food absorb microwave energy in a process called dielectric heating. Many molecules are electric dipoles, that is, they have a partial positive charge at one end and a partial negative charge at the other, and therefore, rotate in their attempt to align with the alternating electric field of microwaves. When rotating, the molecules collide with others and set them in motion, thus dispersing the energy. This energy, when dispersed as molecular vibration in solids and liquids, is transformed into heat.

Microwave applicators are usually multimodal cavities, and the interaction between the various electromagnetic modes that propagate in them and their multiple reflections encourage a very irregular field distribution that results in uneven heating, with the appearance of hot and cold spots. In addition, these techniques based on multimodal cavities are usually maladaptive techniques, an aspect that implies that a substantial part of the energy delivered to the load is reflected back to the source, thus reducing the efficiency of these methods.

Microwave energy cannot heat all materials: only those that, by their composition, are capable of absorbing electromagnetic energy and generating heat like water. Other materials, such as metals, reflect microwaves in the same way that a mirror reflects visible light. Finally, there are dielectric materials such as ceramics with compositions such as alumina that is not able to absorb microwave energy, letting it pass in the same way that light passes through a transparent crystal.

There is also a set of materials called "susceptors" because of its great capacity to absorb electromagnetic energy and convert it into heat see M. Gupta, Microwaves and metais. John Wiley & Sons, Singapore 2007. These are usually conductive metals such as graphite but alternatively stainless steel, molybdenum, silicon carbide, aluminum or other conductive materials can be used, which are embedded in a dielectric matrix. Different solutions for heating a microwave radiation absorbing material are widely described in the state of the art. In some cases, radiators with liquids are used, for example in document DE19949013 or ceramic elements such as in document R0117643 or in document US20060639602 to preferably absorb microwave radiation and store said energy in the form of heat in order to maintain the temperature of a more prolonged material. The problem not solved in the state of the art is that the transmission of microwaves to a dielectric medium even if this is a potential susceptor is not so immediate. A problem in the state of the art is that in many cases the interface between the air and the microwave susceptor is practically a Magnetic Wall, since these materials normally have a very high dielectric constant (for example water - ε = 76 , liver tissue - ε = 44 or silicon carbide (SiC) - ε = 10) while the air has ε = 1. The solutions used to transform microwave energy into heat are limited by the efficiency of the assembly formed by the microwave emitter and the dielectric medium that absorbs the microwaves. The problem is that the lack of adapted systems reduces efficiency, also generating problems of electric shocks at a first level that are also sources of uncontrolled microwave radiation. In this technical field, document EP2090869 is also known, which details a microwave heating element that makes use of an electric transmission line in the microwave band, a transmission line that is arranged on a dielectric material.

In the state of the art there are also solutions to homogenize the temperature by using stirrers of movable modes and elements. However, such solutions require methods that are appalling, introduce mechanical elements and are in any case undesirable in a domestic heating system.

This invention addresses a novel solution to the problem of transferring microwave energy in heat with high efficiency by means of heating units in the form of low-power heating cells that allow the propagation of microwave energy by means of electric transmission lines from electromagnetic transverse modes to ceramic materials. with high dielectric losses in the microwave region. This heat transduction is carried out in an adapted way in regions of high efficiency. These Low power heating cells are integrated in a unit heater that has autonomous operation and is characterized by generating heat in a non-reciprocal manner, that is, the heating time is clearly shorter than the heat release. The unit heater assembly forms a heating system in which they are fed sequentially with microwave energy between the unit heaters. The heating system thus constituted employs a line of low electrical power that allows considerably reducing the power supply requirements related to conventional high power heating systems. DESCRIPTION OF THE INVENTION

For a better understanding of the invention, a list is provided first with the corresponding definitions of the terms used throughout this document. · The term "heating cell" means a minimum structural unit of heat generation comprising an electric transmission line of electromagnetic transverse modes and a ceramic material with high dielectric losses.

• The term "microstrip based heating cell" means a heating cell based on microwave signal transmission that has a conduction strip separated from the mass strip by a dielectric substrate layer; said microstrip based heating cell and corresponds to an electric transmission line of electromagnetic transverse modes formed by a flat conductor placed on a thin substrate which in turn rests on a plane of mass capable of radiating electromagnetic waves to the high loss ceramic material that is interposes, thus transferring energy to it in an adapted and resonant way.

• The term "stripline heating cell" means a heating cell based on the type of transmission line for TEM (Electro-Magnetic Transverse) modes called stripline and corresponds to an electric transmission line of electromagnetic transverse modes formed by an embedded conductor in a high-loss ceramic material that absorbs the electromagnetic energy that propagates through said transmission line as it progresses. • The term "unit heater" means the heating device that integrates several heating cells of any of the classes described above and which is the minimum autonomous functional unit.

• The term "heating system" means the set of unit heaters controlled by a computer system.

• The term "power divider" means a device that distributes the power it receives at its input between n outputs, usually equally. The power dividers are used in radiofrequency and microwaves, optical communications, etc., to send to several devices the power received by a single door, keeping the impedances adapted to have a low level of reflected power.

• The term "microwave susceptor" means a material that has the capacity to absorb electromagnetic radiation in the microwave band and convert it into heat that is generally re-emitted in the form of infrared radiation.

• The term "high loss ceramic material" means an inorganic, non-metallic and shaped material that has the capacity to absorb electromagnetic radiation and convert it into heat that is generally re-emitted in the form of infrared radiation.

A first aspect of the present invention relates to a heating cell comprising an electric transmission line of single mode transverse electromagnetic modes, a power divider of single mode transverse electromagnetic modes and an electric charge in the form of high loss ceramic material that is coupled to said electric transmission line and characterized by presenting absorption of electromagnetic waves in the microwave frequency. The heating cells of the heating system are characterized by transforming electromagnetic radiation at the microwave frequency into thermal energy by heat generation.

In a preferred embodiment of the first aspect of the present invention, the electric transmission line of single mode transverse electromagnetic modes can be chosen from:

microstrip defined as a printed circuit board comprising a conductive metal sheet separated from a metal mass sheet by a dielectric sheet. This transmission line is terminated in a ground plane microstrip antenna designed to radiate energy directly to the high loss ceramic material in order to transfer the energy carrying the line.

- stripline defined as a metallic central conductor between two mass planes equidistant to it. The space between the ground planes and the conductor is filled with high loss ceramic material, so that the propagation of energy through the transmission line is transferred directly to said high loss ceramic material.

In a preferred embodiment of the first aspect of the present invention, the microstrip heating cell is characterized by presenting an electric transmission in single mode and resonant electromagnetic modes, based on the radiation of a microstrip antenna on a thick layer of microwave susceptor material or electric charge . The microwave susceptor material is placed in the reactive near-field zone of the antenna, which extends from the source of the excitation to a distance of approximately λ / (2π) where λ is the wavelength of microwave radiation and π is the constant pi with a value of 3.1416. The microstrip heating cell includes in the same metal structure, for example aluminum, a microstrip plane groove antenna supported on a dielectric substrate plate, fed by a transmission line and connected to an N-type input connector. keeps the load attached to the groove in the form of ceramic material with high losses to heat and between them a microwave-insulating thermal insulating material is placed. The heating cell comprises a reflective metal structure at its base which directs the radiation towards the load in the form of high loss ceramic material. The microstrip cell is shielded with electrical conductors on all its side walls and also on the free surface of the ceramic material with high losses to heat.

As is well known from the basic theory of antennas, any material placed in the near field of an antenna is susceptible to mismatch because the electromagnetic field radiated to this region, if reflected in some way, induces currents in the antenna with a determined phase relationship with the original excitation. This effect leads to a storage of energy in the free electrons of the antenna during a certain part of the oscillation cycle, followed by the consequent release of the antenna and creating the reactive effect that gives name to this region. This forces the impedance adaptation for the boundary condition presented, giving the microstrip heating cell its resonant character.

When the case of a flat wave is taken traveling in a medium with losses, linear, homogeneous and isotropic, all the information related to the power flow in the medium can be obtained from the Poyinting theorem. In a dielectric medium without internal electrical or magnetic sources, the dissipated power can be calculated using the following expression:

Pd = w / 2 JV (¾ ε "| E ^→ | ² -μ ₀ μ" | H ^→ | ² ) dV [1] Where P _{d is} the power dissipated in the material, ω the angular frequency of the excitation, ε ₀ the dielectric permittivity of the vacuum, ε "the complex component of the relative permittivity of the material, μ ₀ the magnetic permeability of the vacuum, μ" the complex component of the relative permeability of the material, E ^→ the electric field vector and H ^→ the vector field magnetic.

Equation [1] is suitable for calculating the power dissipation for a flat wave propagating inside a material, once inside it. Assuming that the material does not show magnetic losses (i? ^F "= ⁰ ), and taking the excitation as an approximation, the integral of the dissipated power can be calculated. To do so, the excitation will be taken as uniform in the XY plane (the plane coinciding with the face of the sample closest to the antenna) and approximated by a flat wave propagating in the direction of the Z axis. Although the electromagnetic problem is much more complex than this approximation, the dimensions of the sample are small compared to its high

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thermal conductivity ( ^'1 m), which makes the approximation error negligible for the thermal result. In this sense the dissipated power can be determined by the following relationship.

P _d -P _ent (1-e ⁽ - ^2ah) ) (1-r ² ) n _r [2] Where α is the loss coefficient or the real part of the complex propagation constant γ, which includes the dependence of the frequency of the excitation f and on the loss factor of the material struck; h is the thickness of the material sample; Γ is the reflection factor of the antenna; and η _Γ is the radiation efficiency of the antenna. The radiation efficiency and the reflection factor can be easily optimized by the design of the antenna, therefore having no effect on efficiency. The efficiency of energy absorption depends on the relationship between the depth of penetration (1 / a) and the thickness of the sample, in which the frequency of excitation is of key importance. The dissipated power will then depend on the frequency and the loss factor. These calculations allow to establish a range of characteristics of the high loss ceramic material required for its adaptation to high efficiency regimes.

In a preferred embodiment of the first aspect of the present invention, the electric charge in the form of a high loss ceramic material is coupled to the electric transmission line and is characterized by electromagnetic wave absorption at the microwave frequency. The absorption of microwaves in the ceramic material occurs due to the existence of dielectric losses in the same as, for example, a sintered ceramic of SiC or by the presence of susceptor particles such as, for example, SiC particles embedded in a matrix ceramics. The microwave radiation absorbing elements transform said microwave radiation into heat that will be transferred to the rest of the ceramic matrix by conduction and will be released into the environment by radiation with the thermal inertia corresponding to a ceramic material. Consequently, this new material behaves in a non-reciprocal manner as far as the heating time is concerned. And as will be seen later, the heating time is faster than the cooling time resulting in an advantage to obtain high efficiency heat generators.

In a preferred embodiment of the first aspect of the present invention the high loss ceramic material is characterized by a microwave frequency loss factor of at least 0.10.

In another preferred embodiment of the first aspect of the present invention, the high loss ceramic material used in the microstrip heating cell consists of a SiC ceramic plate with a surface area of 5x5 cm ² and a thickness of 0.7 cm, with a thickness of 0.7 cm. 99% density with respect to theoretical density, relative permittivity and high loss factor (ε = 10, tan5≤0.16). In the proposed scenario and for this dense SiC high loss ceramic material, the efficiency of the heating cell of the present invention depends only on the adaptation of the microstrip antenna and its radiation efficiency. In order to maximize radiation efficiencies and since near-field calculations in a material medium with losses can be very complex, a reasonable solution can be obtained by electromagnetic simulation. The simulation calculations show a radiation efficiency of η _Γ = 99.8% with an impedance adaptation better than Sn = -20 dB, achieving a total efficiency close to 99%. Such a high efficiency value is a clear advantage for the state of the art by enabling the transformation of electrical energy into heat with energy losses significantly lower than other systems available in the state of the art of heating systems. The high-density ceramic material of dense SiC of 50 grams of mass is characterized by increasing its temperature by 150 ° C when it is subjected to microwave radiation of 2.45 GHz for 30 seconds inside a conventional microwave oven of 1000 W. of power supplied per unit of mass to produce a ΔΤ-50 ° C required to act as a heater the sample consumes 55.6 kWh.Kg ^"1. The time required to decrease its temperature from the maximum temperature reached in 1/3 is 300 seconds The cooling rate in the temperature range of interest, that is, from 90 to 70 ° C to maintain the ΔΤ> 50 ° C required to heat is 0.08 ° Cs ^"1 . Where it is determined that the heating rates of the ceramic plate register values between 4.85 ° C / s and 6 ° C / s, while the cooling rates of said ceramic plate are lower by more than an order of magnitude with values below 0.267 ° C / s. The heating rates generated by microwave absorption and heat radiation cooling are different. The high loss ceramic material acts as a reciprocal heat generator since it absorbs microwave energy generating heat in a time significantly less than that required to transfer said heat to the medium. The difference between heating and cooling rates can be optimized by the composition of the high loss ceramic material. According to the determined temperature increase, the power transfer is not total due to the uneven distribution of the fields in the high loss ceramic material and in the microwave due to the small size of the ceramic compared to the size of the multimodal cavity.

In another preferred embodiment of the first aspect of the present invention the high loss ceramic material used in the microstrip heating cell is a composite material comprising at least 50% by weight of SiC particles and the rest is constituted by porosity and a compound silica-aluminous to keep the silicon carbide grains consolidated. The method of obtaining followed is to mix 50% by weight of SiC particles with 32.5% by weight of kaolinitic clay and 17.5% by weight of a talc mineral. The mixture is homogenized following known processes in the field of ceramic materials processing and the mixture is optimized to achieve a paste suitable for dry pressing, for example by wetting. Pressing is carried out by uniaxial pressure at a pressure of 250 kg / cm ² and the pieces obtained are dried for 24 hours in an oven at 80 ° C. Subsequently the ceramic plates are subjected to a heat treatment in an air atmosphere between 1100 and 1250 ° C, maintaining the maximum heating temperature for at least 30 minutes. Heating rates are higher than 3 ° C per minute and natural cooling, although not restricted to this thermal cycle. In another preferred embodiment of the first aspect of the present, the high loss ceramic material used in the microstrip heating cell consists of a porcelain ceramic plate of a composite material of the previously described composition of 8x3 cm ² of surface and a thickness of 0.7 cm , which comprises 50% by weight of SiC particles with an average particle size greater than 3 μηι and with a density of 85% with respect to theoretical density, relative permittivity and high loss factor (ε'≥13, tan5≥0.16 ).

In an alternative embodiment of the first aspect of the present, the high loss ceramic material used in the microstrip heating cell consists of a porcelain ceramic plate of a composite material of the previously described composition of 14.8 x 14.8 cm ² of surface and a thickness 1.1 cm thick. The high-loss porcelain ceramic material of the above-described composition of 1300 grams of mass is characterized by increasing its temperature by 120 ° C when subjected to microwave radiation of 2.45 GHz for 90 seconds in the inside a conventional microwave oven of 1600 W. In terms of power supplied per unit mass to produce an AT ~ 50 ° C required to act as a heater the sample consumes 3.9 kWh.Kg ^"1. The time required to decrease its temperature from The maximum temperature reached in 1/3 is 1260 seconds The cooling rate in the range of temperatures of interest, that is, from 90 to 70 ° C to maintain the AT> 50 ° C required to heat is 0.025 ° Cs ^"1 .

The high loss ceramic material formed as a composite material comprising SiC particles has an advantage for microwave energy absorption by requiring power consumption per unit mass significantly lower than those used for a dense SiC plate and the speed of cooling is also slower. Additionally, the composite materials are conformable according to procedures known in the ceramic industry thus providing high availability of shapes and dimensions within the limits of the technique that are advantageous to provide elements for the high loss ceramic material.

In another alternative embodiment of the first aspect of the present invention, the microstrip heating cell was modified by incorporating a conductive plane in the free face of the high loss ceramic material so that the surpluses of power absorption are not lost in free space by being reflected back to the sample and the antenna, and finally being absorbed after various reflections between the electrical walls of the cell. The conductive plane is constituted for example by a metallic material such as aluminum, brass, stainless steel or by a coating that has a metallic conduit such as a silver paint coating. In this way, the percentage of energy absorption of the material thickness is independent. This new design has a notable advantage since to avoid the inefficiency of the heating unit, a conductive plane is incorporated, which also has the advantage of improving the safety of the device by preventing microwave radiation from leaving the heating unit. Another advantage of the present invention by incorporating an electric charge in the form of a high loss ceramic material is that non-uniformity of the electric field is avoided given its small size with respect to the source and thermal conductivity of said ceramic charge. In an even more preferred embodiment of the first aspect of the present invention the conductive plane on the free face of the high loss ceramic material of the heating cell incorporates metal channels to increase its surface and more efficiently transfer heat to air. The increase of the heat exchange surface in the conductive plane allows the upward flow of heated air and thus acting as a heat sink and radiator element. The metallic conductor plane with a high surface has an advantage in acting as a heat sink element.

In a still more alternative embodiment of the first aspect of the present invention, the stripline heating cell is characterized by presenting an electric transmission of high-loss and very wide-band single mode electromagnetic modes. The stripline cell comprises a conductor housed inside a ceramic material of high dielectric losses that acts as a microwave or electric load susceptor, thus forming a very high loss transmission line that is absorbent in a frequency band comprising the region of microwave frequencies, including the 2.45 GHz ISM band.

The ceramic load used in the stripline cell is physically coupled to the electromagnetic energy transmission antenna to maximize energy absorption in the form of microwave radiation and its effective conversion into heat. This physical coupling is characteristic of the type of transmission line used in the present invention. Additionally, the dimensions and properties of the electric charge in the form of high loss ceramic material need to be adapted to the parameters of the electromagnetic energy transmission line.

In any of the possible embodiments of the first aspect of the invention, the heating cell where it is of the stripline type is characterized in that the central conductor is a metallic material whose resistance to the passage of electricity is very low. The best electrical conductors are metals, such as copper, gold, iron and aluminum, and their alloys, although there are other non-metallic materials that can also fulfill that function. The shape of the central conductor is chosen from the usual forms that are presented for metallic conductors such as circular section wires obtained by drawing techniques or rectangular section sheets obtained by rolling. The shape of the central conductor of the stripline is reproduced in negative inside the high loss ceramic material. While the high-loss ceramic material used in the stripline heating cell is a ceramic material such as the one described previously that comprises 50% by weight of SiC particles and the rest is constituted by porosity and a silica-aluminous compound to keep the grains of consolidated silicon carbide.

The ceramic material has a hole in its interior of sufficient dimensions to accommodate the central conductor. This hole is made prior to the sintering process of the material or is practiced on the sintered ceramic material by machining the ceramic pieces following methodologies known in the state of the art. It is possible that the high loss ceramic material used in the stripline heating cell comprises two pieces of high loss ceramic material so that a hollow or negative of dimensions corresponding to the length of the conductor has been made on one of the respective faces central and half of its section. The two pieces are joined so that the stripline is crimped inside. The ceramic pieces can be joined using for example an adhesive. The use of ceramic adhesives that withstand high temperatures results in an advantage for the correct operation of the device. The use of pieces of ceramic material that reproduce the stripline in a negative way results in a clear advantage for the manufacture of stripline heating cells, since it allows efficient and economical production of pieces of suitable dimensions based on the wide flexibility of forms of ceramic processes .

In those embodiments where the heating cell is stripline type, the shape of the central conductor and its thickness depends on the cross section of the stripline transmission line, being chosen according to it to obtain the desired propagation characteristics. The stripline heating cell becomes a transducer of electromagnetic radiation in heat with the ability to convert the power of the wave that propagates into heat by a minimum transmission line length. The stripline is short-circuited and has sufficient length to transduce all accepted power in a large bandwidth. As is known from the theory of transmission lines, the loss of power in a transmission line loaded with a short circuit can be determined by the following formula:

Pioss being the power loss in the material filling the transmission line, the value Vd wave incident on said line voltage,% and the characteristic impedance of the line, the coefficient defined above and loss length stripline transmission line.

Both the progressive and the regressive wave produced by the reflective load contribute to the loss of power following an exponential law that only depends on the length of the line (I) and the coefficient of losses (<½). Taking a possible form of the cylindrical central conductor, Figure 11 shows the characteristic impedance of the line for different diameters of the center line (D) at the frequency of 2.45 GHz.

The heating cell of the first aspect of the present invention based heating system may have a power divider of single mode transverse electromagnetic modes comprising n outputs, n being a positive natural number greater than 1. A non-standardized power splitter distributes the power in at least two transmission lines equally maintaining the impedances adapted to have a low level of reflected power. A particular case is the use of Wilkinson type power dividers that provide an even number of output elements.

The use of power dividers has the advantage of uniformly distributing the power in the different transmission lines of electromagnetic modes. The incorporation of a number of at least two microstrip heating cells or stripline heating cells allows the use of a magnetron-type microwave radiation source with powers of up to 1000 W. Microwave radiation is conducted through a coaxial guide, and the coupling between the microwave radiation generated by the magnetron and said distribution network is carried out by means of the use of a coupler, such as a guide-coaxial transition WR340. Since the power of a magnetron is clearly greater than the power that can be dissipated by a single heating cell, the use of power dividers allows it to be divided so that, for example, from a magnetron that produces 800 W of radiation Microwave and through 7 power dividers can be fed 8 heating units that can dissipate a maximum power in each of them of 100 W. The use of 15 Dividers also provide a microwave power supply for the supply of 16 heating units that can dissipate a maximum power, each of them, of 50 W. In a preferred embodiment of the second aspect of the present invention the high loss ceramic material used It corresponds to a single piece whose surface is sufficient to house the microstrip antennas in a number such that the power supplied by the magnetron can be dissipated. This aspect results in a clear advantage for the generation of heating units for heating systems since it allows the use of ceramic surfaces of a size larger than that described in the preferred embodiments of the first aspect of the present invention. Likewise, another advantage is that of heating in a homogeneous and efficient way a ceramic piece absorbing electromagnetic radiation in the microwave range of a surface of dimensions greater than that required for a unit cell.

In another preferred embodiment of the second aspect of the present invention, the high loss ceramic material used corresponds to two pieces in whose surface a gap or negative of dimensions corresponding to the length of the central conductor and half of its section has been made in such a way. that the stripline lines are housed in a number such that the power supplied by the magnetron can be dissipated.

In another preferred embodiment of the second aspect of the present invention, the high loss ceramic material used corresponds to a piece in which the stripline lines are housed in a number such that the power supplied by the magnetron can be dissipated.

In a third aspect of the present invention it consists of a heating system comprising heaters which in turn comprise the heating cells of the first aspect of the invention comprising an electric transmission line of transverse electromagnetic modes and a ceramic load coupled with dielectric losses. . Each unit heating system comprises a control system that allows synchronizing the electricity supply time so that only one of the heating systems is consuming electricity unitary and limited to the maximum power of the magnetron, for example 800 W. Each unit heating system uses a time to heat up by absorption of microwave energy which is a time significantly less than that required to dissipate the heat energy stored by said load. In this way, heating times of the heating cells sufficient to have a set of them at the temperature required to be used as a heating system can be available. For example, 6 heating systems that require 1 minute to be heated from 20 ° C to 80 ° C consuming 800 W can act synchronously by means of the corresponding control system to be heated with a total maximum power of 800 W. In this way, this embodiment results in a clear advantage over the state of the art in heating systems because it allows to have a high efficiency heating system limiting the power supplied by the electrical installation. The synchronization system between the different heating cells is carried out, for example, by means of a Wi-Fi or wired wireless system such as PLC, resulting in a clear advantage, as it allows different unit heating systems to be coupled without the need for them to be physically connected to each other. The heating system also includes a temperature data collection system, a programming system and an algorithm for efficiently distributing the heating times between the different heating unit systems so that the electric energy is used efficiently . The heating system thus designed has the advantage of being flexible in its configuration. In another preferred embodiment of the third aspect of the present invention the heating system comprising heating cells comprising an electric transmission line of transverse electromagnetic modes and a ceramic load coupled with dielectric losses is used to provide thermal comfort in the form of space heating. such as: domestic rooms, offices, commercial premises, industrial premises and in general inhabited spaces.

In any of the different aspects of the invention described herein, the thermal heaters may have an operating control to be integrated into a network of radiators that form a heating system with a marked improvement in the energy efficiency over a network of radiators of any other technology.

A fourth aspect of the invention relates to the use of the heating system using microwave radiation of the third aspect of the invention for thermal comfort in the form of heating for spaces such as domestic rooms, offices, commercial premises, industrial premises and in general inhabited spaces.

DESCRIPTION OF THE DRAWINGS To complement the description that is being made and in order to help a better understanding of the features of the invention, according to a preferred example of practical realization thereof, it is accompanied as an integral part of said description, a set of drawings in which, with an illustrative and non-limiting nature, the following has been represented:

Figure 1. It shows a scheme of the microstrip heating cell comprising an N-type input connector, a metal transmission line, a slot antenna in a microstrip mass plane, a microwave-insulating thermal insulating material, a ceramic material of high dielectric losses, a metal structure that completely encloses the cell whose base forms the reflector plane, and the dielectric substrate plate that supports the microstrip line).

Figure 2. It shows a graph called Smith's chart that represents the reflection factor of the stripline heating cell in both module and phase depending on the electrical length of the cell. From the figure it follows that both module and phase are decreasing with the electrical length, considering the responses contained within the dashed line are sufficient.

Figure 3. Shows a graph representing the Su parameter as the ratio between the reflected microwave signal with respect to the input signal or the efficiency parameter as a function of the microwave frequency for a microstrip heating cell when designed for space radiation free, loaded with a microwave susceptor and finally redesigned to be adapted using a high loss ceramic material. Figure 4a-4c. It shows graphs of heating of high loss ceramic materials in a multimodal microwave oven depending on the time of exposure to microwave radiation. Figure 5. It shows a scheme of the stripline heating cell comprising mass planes, high loss ceramic material and a transmission line formed by a central conductor.

Figure 6. Shows a graph representing the Su parameter as the ratio between the reflected microwave signal with respect to the input signal or the efficiency parameter as a function of the microwave frequency for a stripline heating cell for different lengths of the line transmission.

Figure 7. It shows a scheme of the stripline heating cell comprising transmission lines in a single piece comprising mass plane, high loss ceramic material and transmission lines formed by central conductors.

Figure 8. It shows a scheme of a cluster of microstrip heating cells for uniform heating over a larger piece of high-loss ceramic material, using microstrip lines in Wilkinson's splitter configuration, and groove antennas in the mass plane.

Figure 9. Shows a scheme of the components that make up a unit heater, namely the heating cells, Wilkilson power dividers, non-standardized power divider, coaxial guide transition, magnetron and relevant connections using coaxial cable.

Figure 10. It shows a diagram of the components that make up a heating system, namely different unit heaters, a control unit and a data connection for the control that could be PLC or PLC.

Figure 11. It shows a table that relates the diameter of the central conductor of the stripline cell with the characteristic impedance of the line seen from the excitation plane. Figure 12. It shows a table showing the value of the dispersion parameter S_11 and the corresponding percentage of power absorbed at the frequency of 2.45 GHz for different lengths of the transmission line, once the diameter of the center conductor of the line is set to D = 1 mm

PREFERRED EMBODIMENT OF THE INVENTION

As a practical case of realization of the invention, and without limitation thereof, several examples of embodiment of the heating cells, fed by a microstrip line or by a stripline transmission line, of one of the aspects of the invention are described below. invention that simply implement the main concepts object of this invention.

In the first aspect of the invention referred to a heating cell from microwave radiation, we have that said heating cell comprises in a metal structure (6) provided with a base at least one input connector (1), a transmission line electrical (2) of single-mode transverse electromagnetic modes acting as an antenna (3) and which is made of at least metal and / or ceramic material, a ceramic material with high dielectric losses (5.9) to which the antenna is fixed ( 3), and a reflector plane defined by the base of the metal structure (6) and located a of the electric transmission line (2), λ being the wavelength of the radiation incident in the cell, directing the said reflector plane microwave radiation to the ceramic material with high dielectric losses (5.9). Additionally, the heating cell can be equipped with a single mode transverse electromagnetic mode splitter.

In a possible first embodiment, the heating cell from microwave radiation can have the electric transmission line (2) defined by a conductive metal sheet separated from a metal mass sheet by a dielectric sheet and fixed a groove of the ceramic material of high dielectric losses (5) and supported by a dielectric substrate plate (7). In this possible embodiment, the heating cell additionally comprises a microwave-insulating thermal insulating material (4), located between the high dielectric loss ceramic material (5,9) and the antenna (3) when the electric transmission line (2) is a conductive metal sheet separated from a metal mass sheet by a dielectric sheet.

In a possible second embodiment the heating cell from microwave radiation can have the electric transmission line (2) defined by a metallic central conductor (10) which is located inside the ceramic material of high dielectric losses (9) an area comprised in an axis of symmetry of the ceramic material of high dielectric losses (9) between two planes of mass equidistant from the metallic central conductor (10).

In the case of choosing the second option, the metallic central conductor (10) is located in an area comprised in an axis of vertical symmetry of the ceramic material of high dielectric losses (9) which preferably divides the ceramic material of two equal parts high dielectric losses (9).

Likewise, the central conductor (10) is preferably located in a hollow of the ceramic material of high dielectric losses (9), which more preferably has dimensions respectively corresponding to the length of the central conductor, which is preferably greater than 10 cm and half of the central conductor section, the central conductor (10) being able to present, in any of the aforementioned examples, a circular, square or rectangular cross section.

The operation of one of the aspects of the invention can be seen in view of Figures 4a-4c where in Figure 4a it shows a graph referring to the heating-cooling of a ceramic SiC plate of 5x5 cm ² of surface and a thickness 0.7 cm, with a density of 99% with respect to the theoretical density; heating is carried out in a 1000 W microwave oven; Figure 4b shows a graph referring to the heating-cooling of the high loss ceramic material (5.9) with 50% by weight of SiC particles and the rest is constituted by porosity and a silica-aluminous compound to maintain the grains of consolidated silicon carbide; The plate of 14.8x14.8 cm ² of surface and a thickness of 1.1 cm thick has 1300 grams of mass; the heating is done in a 1600 W microwave oven; and Figure 4c shows a graph referring to the heating-cooling cycle curves consecutive.

The possibility is also provided that ceramic material with high dielectric losses (5.9) has a conductive plane, preferably of a material comprising aluminum, on at least one of its faces, preferably on a face where the line is not coupled. ..

Example 1. Heating cell with the groove antenna in the ground plane (3) microstrip using said ceramic material of high dielectric losses (5) of Figure 1 comprising SiC, preferably more than 50% by weight of SiC.

It refers to a microstrip type heating cell wrapped in the same metal structure (6), preferably aluminum, the antenna (3) is of the groove type in the mass plane, that is to say microstrip type fed by the transmission line (2) and connected to the input connector (1) type N. The groove antenna (3) is maintained in the ground plane, fixed by adhesive to an alumina fiber sheet that acts as a microwave-insulated thermal insulating material (4) at temperatures working of the heating cell and then the high loss ceramic material (5) which may preferably be a 100% SiC plate. The metal structure (6)

TO

It defines at its base an aluminum reflector plane located ai of the antenna (3), since it is in the aforementioned slot of the high loss ceramic material (5) at the same distance from the slot of the high loss ceramic material (5). The reflector plane is intended to direct a radiation of wavelength λ incident in the cell towards the ceramic material of high dielectric losses (5). The arrangement of the elements that make up the cell is schematized in Figure 1.

The power that can be supported by the groove antenna (3) in the ground plane fed by a microstrip line or a stripline transmission line can reach 300 W at this frequency. For safety reasons the power must be limited to 100 W on a surface of 5x5 cm ² assuming it is fully adapted (VSWR≥ 20 dB). For the dense SiC plate of 100% 6 mm thick and 5x5 cm ² surface the total mass would be: m ígr) = 3.153 / „„, 3 · 5 · 5 · 0.6 cnr * s? Lord

nt (jr) = 3.153 ¡ _cm3 ■ 5 · 5 ^■ 0.6 cm ^a ¾ Sojr

To increase the temperature of this piece from 20 ° C to 80 ° C, that is, an increase in temperature ΔΤ = 60 ° C in a time of 1 min = 60 s is required:

5060

Power =: = 35 W <100 W (power limit)

6th

50 ^■ 60 ^« 0.7

Power =: = 35 W <100 W (power limit}

6th

Therefore, adequate heating of the plate would be achieved so that said heating unit acts as a heat generating element.

Fig. 3 shows the measured responses of the antenna when it is designed for free space radiation, then loaded with a microwave susceptor and finally redesigned and reprinted for adaptation in the presence of high loss ceramic material (5). The simulations show a radiation efficiency of ¾ ^=! 99 ^, 8% as expected given the low losses of the substrate, and the measurements show an impedance adaptation better than S = -2Q dB _t achieving a total efficiency close to 99%.

Example 2. Stripline heating cell using high loss ceramic material (9) of Figure 5, high loss ceramic material (9) of porcelain with 50% by weight of SiC.

In this case we have a stripline heating cell comprising the said central conductor of a metallic material (10), preferably of 1 mm diameter copper that was machined from a copper sheet. The shape of the metallic central conductor (10) of the stripline line was reproduced negatively inside the high loss ceramic material (9) consisting of a porcelain type compound comprising 50% by weight of SiC particles and the rest is constituted porosity and a silica-aluminous compound to keep the carbide grains of consolidated silicon.

Fig. 7 shows the reflection factor versus frequency in this figure a metallic central conductor (10) can be observed. The stripline heating cell behaved like a broadband device. The energy transfer had an efficiency greater than 99%, since all the energy absorbed by the device is converted into heat without any leaks or reflections.

With any of the possible heating cell configurations, and adding power dividers, and at least one magnetron, we have a microwave radiation heater, since the magnetron would be responsible for generating the microwave radiation required by the heating cell. Additionally, the heater can be equipped with a communication unit and a control unit. If required according to installation, a heating system using microwave radiation can be defined by interconnecting a series of microwave radiation heaters as described in the previous paragraph. For this optimal operation, a control algorithm as well as an intelligent control system could be implemented.

Claims

1. Heating cell from microwave radiation characterized by comprising, enclosed in a metal structure (6) equipped with a base:

"At least one input connector (1),

• an electric transmission line (2) of single mode transverse electromagnetic modes acting as an antenna (3),

• a ceramic material with high dielectric losses (5.9) to which the antenna (3) is attached, and

«A reflecting plane defined by the base of the metal structure (6) and located at λ

ϊ of the electric transmission line (2), where λ is the wavelength of the radiation incident in the cell, said reflector plane directing said microwave radiation to the ceramic material of high dielectric losses (5.9).

2. Heating cell from microwave radiation according to claim 1 characterized in that it additionally comprises a divider of single-mode transverse electromagnetic modes.

3. Heating cell according to claim 2, characterized in that the electric transmission line (2) of single-mode transverse electromagnetic modes is selected from:

a conductive metal sheet separated from a mass metal sheet by a dielectric sheet and fixed to a groove of the high dielectric loss ceramic material (5) and supported by a dielectric substrate plate (7), and

- a metallic central conductor (10) which is located inside the ceramic material (9) between two mass planes equidistant from the metallic central conductor (10); Y

because the heating cell additionally comprises a microwave-insulating thermal insulating material (4), located between the ceramic material with high dielectric losses (5) and the antenna (3) when the electric transmission line (2) is a conductive metal sheet separated from a metal sheet of dough by a dielectric sheet.

4. Heating cell according to any one of claims 1 to 3 characterized in that the ceramic material with high dielectric losses (5.9) has a value of dielectric losses in the microwave electromagnetic radiation region ≥ 0.10.

5. Heating cell according to any one of claims 1 to 4 characterized in that the ceramic material with high dielectric losses (5) comprises a proportion of at least 50% by weight of SiC.

6. Heating cell according to any one of claims 1 to 5 characterized in that the ceramic material with high dielectric losses (5) comprises a conductive plane in at least one of its faces.

7. Heating cell according to claim 5 characterized in that the conductive plane is made of a material comprising aluminum.

8. Heating cell according to claim 3 characterized in that the metallic central conductor (10) is located in an area comprised in an axis of symmetry of the ceramic material of high dielectric losses (9).

9. Heating cell according to claim 8 characterized in that the axis of symmetry is a vertical axis of symmetry that divides the ceramic material of high dielectric losses (9) into two equal parts.

10. Heating cell according to claim 3 characterized in that the metallic central conductor (10) is located in a hollow of the ceramic material of high dielectric losses (9), a hollow having dimensions respectively corresponding to the length of the metallic central conductor (10 ) and in the middle of the conductor section metal center (10).

11. Heating cell according to claim 3 or any one of 8 to 10 characterized in that the metallic central conductor (10) has a length greater than 10 cm.

12. Heating cell according to claim 3 or any one of 8 to 11 characterized in that the metallic central conductor (10) has a cross-section that is selected from the group consisting of:

circular,

square, and

rectangular.

13. Microwave radiation heater characterized in that it comprises at least one heating cell as described in claims 1 to 12, power dividers, and at least one magnetron.

14. Microwave radiation heater according to claim 13 characterized in that it additionally comprises a communication unit and a control unit.

15. Heating system using microwave radiation characterized in that it comprises interconnected a series of microwave radiation heaters according to any one of claims 13 or 14, a control algorithm and an intelligent control system.

16. Use of the heating system using microwave radiation according to claim 16 for thermal comfort in the form of heating for spaces such as domestic rooms, offices, commercial premises, industrial premises and in general inhabited spaces.