Near-instantaneous heating device with electric cord

Abstract

Electric heating cord for dynamic radiant heating The present invention relates to a device for the electrical heating of premises by radiation using an electrically heated cord, and capable of reaching substantially its nominal temperature level in a very short time of the order of a fraction of a second to a few seconds. The invention is characterized by a device making it possible to supply the heating cord in parallel mode making it possible to inject into the heating cord a high power during the duration Tau making it possible to reach the nominal level over this duration and in series mode outside the interval Tau, that is to say during continuous operation where preferentially the choice is made of a heating cord in a situation of thin thickness.The invention is particularly suitable for heating premises with long infrared around lambda = 10 microns corresponding to the low temperature range, for example between 300K and 400K Figure to be published for the abstract: [Figure 7].

Description

Title of the invention: Near-instantaneous heating device using an electric cord

[0001] FIELD OF THE INVENTION. The present invention relates to a device for the electrical heating of premises, preferably by radiation using an electrically heated cord, and capable of substantially reaching its nominal temperature level, i.e. its nominal thermal power level, in a very short time of the order of a fraction of a second to a few seconds. The invention is particularly suitable for the low temperature range, for example between 300K and 400K.

[0002] BACKGROUND OF THE INVENTION Heating premises using electrical energy essentially uses the Joule effect in a heating cord which is an electrical resistive element powered by electrical connectors from an electrical energy source. The transfer of heat to the premises, the air and the occupants occurs by thermal conduction and / or convection and / or infrared radiation emitted by the heating cord due to its temperature.

[0003] In the current and future context of energy savings it may be particularly interesting to be able to activate a heater from the non-activated level to its nominal power level in a very short time such as one second.

[0004] Heating a room through the phenomena of thermal conduction and / or thermal convection of the air is difficult to reconcile with response times of the order of a second. Only heating by radiation, in particular infrared, is capable of enabling designs compatible with such short response times. For this reason, the device which is the subject of the invention is preferably adapted to infrared heating. The time required to reach from zero thermal power level at least 90% of the nominal thermal power level, which we will call the dynamic rise time Tau, is dependent on the design of the heating cord and its thermal interaction with the surrounding system, whether it is the support, the power supply, the gaseous environment, the environment from the point of view of optical and infrared properties.

[0005] The operation of a heater generally comprises two types of phases: a so-called continuous phase where the electrical power consumed and the heating power restored are similar and stationary. The continuous phase represents the majority of the duration of use of the heater. The electrical power consumed during this continuous phase and therefore the heating power supplied by radiation, for a given temperature, is directly related to the surface temperature of the heating cord. The Stefan-Boltzmann law indicates that the flux Phi of radiated power, i.e. the surface density of power radiated in the half-space by a surface heated to the absolute temperature T is obtained by multiplying the fourth power of T by the constant Sigma and a coefficient Eps between 0 and 1, called emissivity, depending on the nature of the material and its surface: Phi=Eps*Sigma*T4. The constant Sigma is equal to 5.671*E-8 W *m2*K 1 . The emissivity of materials in various forms and surface conditions is easily accessible in open databases. For example, the emissivity of a shiny aluminum surface is between 0.039 and 0.057, anodized aluminum has an emissivity of around 0.8, the emissivity of oxidized steel is between 0.7 and 0.9. For example: a 600mm*600mm surface of anodized aluminum heated to 127 degrees Celsius or 400K radiates a power of 418W. A so-called transient phase, particularly at the time of start-up during the temperature rise time (Tau). This phase must be as short as possible. In the hypothesis of a heating cord in thermal contact with a thick support, during the transient phase of duration Tau, the thermal diffusivity parameter D(m2 / s) determines the extension of the depth affected by the temperature rise. The law of the temperature rise as a function of depth can be modeled analytically from the heat diffusion equations called Fick equations or can be calculated numerically. But a simple rule makes it possible to dimension with an accuracy of the order of 30% the phenomena. This rule is as follows: from a surface temperature T during the duration Tau the quantity of thermal energy having diffused towards the depth is the same as that which is necessary to reach the temperature T in a homogeneous manner over a depth equal to the diffusion length L where L=RACINE(D.Tau) For example, for an aluminum heating cord where D=9.8*E-5 m2*s1 and for Tau = ls, L is 0.01m or ICm Another example for a polyethylene layer for which D=0.17*E-6 m2*s 1 and for Tau =ls, L is 4.12 *E4 m or 0.4mm. Another example for a layer of quartz glass for which D=8.7*lE-7 m2*s1 and for Tau =ls, L is 9.31 *E4 m or 0.93mm. This means that for a heating cord in the form of an aluminum ribbon with a thickness significantly less than 1 cm, for example 50 microns, the temperature rise is homogeneous throughout the thickness. The same applies to a 50 micron thick polyethylene layer which is low compared to 0.4mm. In practice, we encounter two types of situations: the so-called infinite thickness situation, where the heating cord is in thermal contact with a very thick material the so-called thin thickness situation where the heating cord and its possible associated layers have a resulting thickness much lower than the thermal diffusion length during the duration Tau. If there are several layers, we will take as diffusion length with an excellent approximation the square root of the number obtained by multiplying Tau by the sum of the thermal diffusion coefficients of each of the layers.

[0006] The thin thickness situation can be encountered at the level of the parts of the heating cord which are not supported either because they are stretched between two support elements, or because they are left to freely take their equilibrium shape between two support elements. In the thin thickness situation we can make the approximation that heating during the duration Tau is essentially adiabatic in nature and therefore the bulk of the power to be supplied is made up of the quantity of energy to be stored by the heat capacity to reach the temperature T, to which we add the average power flow lost PhiTrans by radiation assuming a linear growth of the temperature during the duration Tau. The temperature T reached almost adiabatically at the end of the duration Tau is such that T-Ta=Tau*(Pvt-PhiTrans / ep) / Cv (where Ta is the initial temperature, Cv the volumetric heat capacity, Pvt the volumetric power density deposited during the duration Tau)

[0007] In the situation of infinite thickness we will make the same approximation by replacing ep by the diffusion length L. During the continuous phase the surface density of deposited power Psc is substantially equal to the radiated flux Phi. Now the relationship between T and the flux Phi radiated during the continuous phase is as follows T=(Phi / (Eps* sigma) )1 / 4

[0008] Hence the equation linking the parameters so that at the end of the duration Tau, the temperature reached is the temperature chosen for the continuous phase: -in the case of thin thickness: (Phi / (Eps*sigma) )1 / 4 =Tau*(Pvt-PhiTrans / ep) / Cv +Ta -in the case of infinite thickness: (Phi / (Eps*sigma) )1 / 4 =Tau*(Pvt-PhiTrans / L) / Cv +Ta Examples for a heating cord in the form of an aluminum ribbon with a thickness of ep = 40 microns, emissivity eps = 0.8, heat capacity Cv = 2484 * 1E3 J / m3, heated to T=400K from an initial temperature of 290K over the duration Tau=lS, 1st calculation gives Phi = 1161 W / m2 - Pvt = 2.73 * lE8 W / m3 , which translates to a surface power density Pst = l.09 * lE4 W / m2 The ratio of injected power between the transient phase and the continuous phase is equal to: Pst / Psc=Pst / phi= l.09*lE4W / m2 / 1161 W / m2=9.41

[0009] The same type of calculation carried out for the hypothesis of infinite thickness, in the case of a 20 micron aluminum ribbon supported by a thick quartz glass support and for an initial temperature of 290 K is illustrated by the curve in [Fig.l]. It is easy to see that the ratio between the power required for the temperature rise phase of duration Tau = 1S and the continuous operating phase reaches the value of 140 between 375K and 400K The graph in [Fig.2] shows the relationship of this ratio between the power required for the temperature rise phase of duration Tau = 1S and the continuous operating phase, for a temperature range up to 850K, and for thicknesses of 10, 40 and 80 microns of an aluminum ribbon cord, and for an initial temperature of 290 K. It will be noted that this ratio presents, in a similar way to what is observed in the infinite thickness situation, a maximum for temperatures between 350K and 400K. On the other hand, in the thin thickness situation this maximum varies between 2 and 19 for the chosen parameter values, which is of the order of 10 times less than for the infinite thickness case. Note that the temperature range in which the maximum is located corresponds to a low temperature domain, a temperature domain which is however particularly interesting for long infrared heating (around lambda 10 microns) corresponding to an emission temperature close to the temperature of the human body.

[0010] The geometry of the heating cord can be in the form of a two-dimensional winding (flat winding) on a support surface, for example in the form of a nested counter-rotating spiral or double spiral or any other geometric design such as Zig-Zag or combination of one and other geometric figures. The support surface may possibly be in the form of a grid formed for example by a network, square, or hexagonal or star-shaped for example of mini-beams supporting the heating cord at the level of the mini-beams. The heating cord may be in the form of a three-dimensional winding (coil) of a heating cord on a core, for example made of ceramic, glass, or polymeric material. A core may be structured, comprising for example at its periphery longitudinal beams distributed radially at the vertices of a polygon, thus allowing support of the heating cord by the intermediary of the contact of the tape with the beams and the absence of contact of the heating cord with the core in the inter-beam spaces The surface of the heating cord emits infrared radiation due to the temperature of the cord. It is therefore favorable for it to have a maximum surface oriented towards the room to be heated. For this reason, an oblong cord section where the thickness is less than the lateral extension will be preferably chosen: for example, a flattened elliptical, rectangular, or rectangular section with rounded edges. In these last two cases, we speak of a ribbon. The oblong shape is more favorable or even essential for the configuration to correspond to the thin thickness situation as defined above.

[0011] Various layers can be associated with the heating part of the cord such as an electrically insulating layer and or a so-called black layer intended to guarantee high emissivity. These associated layers have an impact on the rise dynamics during the Tau phase. It is necessary to take them into account in particular through the overall thermal diffusion coefficient which can be evaluated as indicated above. Another example of a heating cord is a 30mm wide, 20 micrometer thick stainless steel or aluminum alloy tape. Another example of a heating cord is a 50 micrometer thick, 25mm wide, polymeric material tape such as polypropylene, aluminized on at least one of its faces to a thickness of 5 micrometers. The heating cord necessarily has at least partial physical and therefore thermal conductive contact with support elements; these can be thick or massive, which creates a significant loss of heat between the heating cord and the support elements, which is critical in particular in transient phases including the dynamic rise phase Tau. Indeed, if these contact zones represented a significant proportion of the total surface area, this would result in a local overall thermal inertia much greater than that of the heating element alone, which is unfavorable to the possibility of having a rapid dynamic rise time Tau; in fact, the dynamic rise time is essentially a function of the ratio between the available electrical power and T thermal inertia.The temperature rise time, which will be designated by Taulong, of these parts in thermal contact with the support elements, under the effect of the injected power level corresponding to the level of the continuous phase, is by nature rather of the order of one or a few minutes than of the second. Consequently, to obtain a sufficiently short rise time (Tau) it could be necessary to create a transient phase of extremely high electrical power injected during the duration of the rise time (Tau). The results illustrated by [Fig.l] show that this objective is ir. realistic because the power level that would be required is 140 times the power level required for the continuous phase.

[0012] DETAILED DESCRIPTION OF THE INVENTION The device which is the subject of the invention presented below makes it possible to heat a room in particular by infrared radiation while avoiding the drawback mentioned above in the paragraph "Background of the invention" in the sense that its design according to the invention makes it possible to achieve "quasi-instantaneous heating", that is to say that the invention makes it possible to choose a low value for the rise time Tau, typically between 0.1 seconds and 20 seconds, without the need to create an unreasonably high transient phase of injected electrical power.

[0013] . The device which is the subject of the invention is characterized in that:

[0014] _the heating cord comprises, in addition to the two end connections, a number C of electrical connections called intermediate connections, distributed along the heating cord, preferably arranged so as to define C+1 sections of cord whose resistance between two consecutive connection points is substantially the same regardless of the number of the connection point

[0015] -One of the end connections is intended to be connected to the pole defined by convention as the +V pole of a generator while the other end connection is intended to be connected to the pole defined as the -V pole of the generator. Each intermediate connection point is connected, through a switch, called a controlled series-parallel switch, alternately to the +V pole and to the -V pole while respecting the alternation with respect to the end connections. The series-parallel switches are preferably of the "normally open" type because they correspond to the greatest electrical positioning duration. These series-parallel switches can be made by means of electrotechnical type relays, i.e. with contact actuated by a coil or with a semiconductor such as a MOS transistor, IGBT, TRIAC - a device called a switching device allows the series-parallel switches to be operated simultaneously so that they are all simultaneously either in the open position, the C+1 sections of cord are then electrically in series between the +V pole and the -V pole, this is called a series supply of the cord, or they are all simultaneously in the closed position, the C+1 sections of cord are then electrically in parallel between the +V pole and the -V pole, this is called a parallel supply of the cord.

[0016] When the two poles +V and -V are switched on, the switching device switches all the series-parallel switches into the closed circuit position (assuming their rest position is the open position). After a time interval Tau, the switching device switches all the series-parallel switches into the open circuit rest position.

[0017] It will be noted that the resistance seen between the +V and -V poles is divided by (C+l)2 when the series-parallel switches move from the open position to the closed position, which translates for the same supply voltage into a power injected into the heating cord multiplied by (C+l)2 For example, for C=2, i.e. only for two intermediate connections and therefore only two series-parallel switches, for the same supply voltage, it is possible to inject during the phase of duration Tau a power nine times greater than that which is injected during the continuous phase. The Tau time interval results from: Either because the measured temperature of the heating cord (for example by measuring the relative variation in its resistance due to the variation in the resistivity of the material which is generally of the order of a few 1E-6 per degree) reaches the temperature chosen for the continuous phase, i.e. for the series supply phase -Either from the signal provided by a timer, activated by switching on the +V, -V poles

[0018] When the cord is in the form of a wound winding, each of the C+1 sections of the cord preferably corresponds to an integer or half-integer number of turns.

[0019] Preferably, the device which is the subject of the invention will be designed in such a way that the majority, for example at least 75%, of the heating surface of the radiating cord towards the volume to be heated is capable of reaching the temperature of the continuous phase during the duration Tau, that is to say is in a situation of thin thickness, which implies that the cord is not in contact with an element external to the cord and that the overall thickness of the heating cord, that is to say the thickness of the electrically conductive part plus the thicknesses of any associated non-conductive layers, is lower than the overall thermal diffusion length during the duration Tau by a factor k, where k is at least equal to 2 and where the overall diffusion length is the square root of the product of Tau by the sum of the diffusion coefficients of the layers constituting the cord.Correlatively, this means that only a small part, for example less than 25%, of the heating surface of the radiating cord towards the volume to be heated is in thermal contact with a support element. Optionally, the device which is the subject of the invention comprises an electrical energy storage system physically located in the vicinity of the cord supplying power to the heating cord via a switch controlled by it. The electrical energy storage system can be implemented using one or more batteries and / or one or more supercapacitors. The storage system of electrical energy is sized in terms of storage capacity at the level of the quantity of energy necessary to supply the heating cord for the duration Tau while being able to deliver the instantaneous power necessary to supply the heating cord in the parallel supply situation, multiplied by a safety factor, for example of a value between 2 and 5 - The interest of the optional choice of the electrical energy storage system lies in the following remarks. In the absence of the electrical storage system, as the required transient power level is significantly higher than that of the continuous phase, it would be necessary to dimension the electrical supply wires from the alternating current network for the transient level, that is to say with very high conductor cross-sections, while this need only represents a small fraction of the time. Furthermore, if the heating cords are used to equip radiant heating slabs, for example with dimensions of 600mm*600mm, slabs intended to be assembled, for example in a square network on the ceiling of the room to be heated, all the large section connections that would necessarily be implemented would lead to a technically and economically penalizing situation. In addition, multiplying the current draw from the alternating current network during the Tau phase by the number of heating slabs could imply the need to resize the alternating current network.

[0020] DESCRIPTION OF THE DRAWINGS Other objects, characteristics and advantages of the invention will be better understood in light of the detailed description which follows, with reference to the appended drawings in which [Fig.l] shows the graph linking the ratio between the power demand during the transient phase Tau and the power demand during the continuous phase as a function of the temperature T of the continuous phase in the situation of infinite thickness for a temperature range up to 850K, for a 20 micron thick aluminum ribbon cord on a thick quartz glass support and for an initial temperature of 290 K.

[0021] [Fig.2] shows the graph linking the ratio between Power called during the transient phase Tau and the power called during the continuous phase as a function of the temperature T of the continuous phase in the situation of thin thickness for a temperature range up to 850K, and for thicknesses of 10, 40 and 80 microns of a ribbon-cord, in aluminum, and for an initial temperature of 290 K. [Fig.3] illustrates an example of three-dimensional winding of a heating cord where the latter is a strip (11) and the core (12) is a cylinder of hexagonal section with rounded corners comprising a hollow (121) in four zones intended to be oriented towards the volume of the room to be heated, and in which the heating cord has no thermal contact with the core and is therefore likely in these areas to be able to rise in temperature very quickly, whereas the parts of the heating cord in the rear part of the core without hollows will have a much slower temperature rise time. [Fig.4] illustrates an example of a two-dimensional planar winding of a heating cord in the form of a spiral (21) and where the support structure (22) comprises support beams of the ribbon distributed radially in a star on which the heating cord has thermal contact while the heating cord has no thermal contact between the beams. This architecture offers a very low proportion of the surface of the heating cord in thermal contact with the support and therefore correlatively a large part of the surface of the cord is likely to be able to rise in temperature very quickly, [Fig.5], [Fig.6] and [Fig.7] illustrate an example of an embodiment where: the heating cord visible in [Fig.5] is in the form of a flat winding of 12 turns of metal tape 51 around a core whose central part is not shown here, comprising two uprights 52 on either side. The rear part of each turn of tape is shunted by a conductive element 511 whose electrical resistance is very low so that the rear part of each turn is almost equipotential. [Fig.6] shows the winding 51 and in symbolic form the electrical supply device designed for a number of intermediate contacts equal to 2. The intermediate connection points 612 and 613 determine, in combination with the connection points +V and -V, three equivalent portions of the ribbon cord. The switches 611 for switching from the series supply position to the parallel supply position and vice versa are shown with their common control 614. [Fig.7] illustrates on the left an upright 52 out of its functional position, in the center the winding 51 with the two uprights 52 in the functional position and on the right an enlarged section of a part of the other upright out of the functional position. These three illustrations explain the structure of each upright. Each upright has an internal conductive tube 521 extending over the entire length of the upright and its functions are the mechanical strength of the upright and the electrical conduction from the end of the tube to the intermediate connection on the winding at the level of the turn, i.e. contact turn. Each upright has on its periphery sections 523n of conductive tube each corresponding to a turn n of ribbon and establishing an electrical contact with it. Washers 52201 electrically insulate the sections 523n from each other. Between the outer diameter of the inner tube 521 and the inner diameter of the sections 523n are two sections of insulating tube 52200 which occupy the entire length except at the level of the section 523n where n corresponds to the contact turn. At this level a section of conductive tube 5221 establishes the contact between the tube 521 and the section 523n and thus with the contact turn. The end sections of 523n are marked as 5230, because they are longer and part of their length is uncovered allowing the +V and -V connection on the ends of the strip.

Claims

Claims

1. Heating device consisting of an electric heating cord, capable of reaching its nominal power level from the switched-off level in a short rise time Tau between 0.1 and 20 seconds, characterized in that: the heating cord is powered through an electrotechnical or electronic device making it possible to power C+l consecutive lengths of the cord in parallel for the duration Tau and to power the C+l sections of the heating cord in series beyond the duration Tau

2. Claim according to 1 wherein the date of the change from parallel power supply to series power supply is determined by a timer

3. Claim according to 1 where the date of the changeover from parallel power supply to series power supply is determined by a device comparing the temperature reached by the heating cord with the reference temperature corresponding to the desired temperature of the cord during the continuous phase

4. Claim according to 1 to 3 where the majority of the heating surface of the cord, radiating towards the volume to be heated, has no thermal contact with support elements, and the thickness of the electrically conductive part plus the thicknesses of any associated non-conductive layers is smaller than the overall thermal diffusion length during the duration Tau by a factor k, where k is at least equal to 2 and where the overall diffusion length is the square root of the product of Tau by the sum of the diffusion coefficients of the layers constituting the cord.

5. - Claim according to 1 to 4 wherein the heating cord is electrically powered from a buffer electrical energy storage system located near the heating cord.

6. claim according to 1 to 5 wherein the cord is in the form of a wound winding and each of the C+1 sections of the cord corresponds to an integer or half-integer number of turns.

7.

8. Claim according to 1 to 6 or the number C is 2 Claim according to 1 to 7 where the heating cord has the shape of a ribbon of rectangular or rectangular section with rounded edges.

9. Claim according to 5 wherein the energy storage system is one or more batteries and or one or more supercapacitors.

10. Claim according to 1 to 9 wherein the heating cord is in the form of a winding of tape on a core having two end posts and where the electrical contacts with the tape take place via sections of conductive tubes located at the periphery of the posts.

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