Electric heating cord for dynamic radiant heating.

Abstract

Electric heating cord for dynamic radiant heating The present invention relates to a device for the electrical heating of premises by radiation by means of 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 the choice of a heating cord in a situation of thin thickness and by an electronic regulator system 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 a lower power level outside the Tau interval, corresponding to the level of continuous operation.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 4].

Description

Title of the invention: Electric heating cord for dynamic radiant heating.

[0001] FIELD OF THE INVENTION 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, 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 likely to allow for designs compatible with such short response times. 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 close and stationary. The continuous phase represents the majority of the duration of use of the heater. The electrical power consumed during this phase continues and therefore the heating power provided 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 known as Fick's equations or can be calculated numerically. But a simple rule makes it possible to size the phenomena with an accuracy of the order of 30%. 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 worth 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 polyethylene layer with a thickness of 50 microns, 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 = 1S , 1e 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 should 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. A core may be structured, for example comprising at its periphery longitudinal beams distributed radially at the vertices of a polygon, thus allowing support of the heating cord by means of the contact of the tape with the beams and the absence of contact of the heating cord with the core in the inter- spaces. beams 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 may be associated with the heating portion 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 stainless steel or aluminum alloy tape 30mm wide and 20 micrometers thick. Another example of a heating cord is a polymeric material tape such as polypropylene 50 micrometers thick and 25mm wide 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 Tau rise phase. From a dynamic point of view, the existence of this thermal flux in the contact zones results in a local overall thermal inertia which can be 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 the thermal inertia. The temperature rise time, which will be designated 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 more 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 in [Fig.l] show that this objective is unrealistic because the power level which . would be needed is 140 times the power level needed 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 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 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 radiant cord towards the volume to be heated is in thermal contact with a support element.

[0014] The device which is the subject of the invention comprises an electronic regulation device making it possible to inject into the heating cord a power, high during the duration Tau making it possible to reach over the duration Tau the desired temperature for the continuous phase and then decreasing to reach the level corresponding to that of the continuous phase. Optionally, the device which is the subject of the invention comprises an electrical energy storage system physically located in the vicinity of the cord and electrically connected to it via an electronic regulation device and capable of supplying the electrical power necessary to the cord during the transient phase Tau. The electrical energy storage system can be implemented using one or more batteries and / or one or more supercapacitors. The electrical energy storage system is sized in terms of storage capacity at the level of the quantity of energy necessary to power the heating cord during the duration Tau, multiplied by a safety factor, for example of a value between 2 and 5. Preferably, said electronic regulation device makes it possible to power the heating cord from the electrical energy storage system during the transient phase (high power) and during the continuous phase (reduced power). Said electronic regulation device uses classic know-how known in the field of electronics. These regulators can be for example of analog type (regulation ballast) or switching of the current passage at a frequency high enough so that the temperature fluctuations due to switching are minimal. For example, we will choose a working rate and a switching frequency such that the duration between two current pulses is less than Tau / 100. Alternatively, the heating cord may not be powered during the continuous phase from the electrical energy storage system. In this case, the heating cord is powered during the continuous phase from the alternating current network through an electronic adaptation and regulation device. - 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 size 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 on the alternating current network by the number of heating slabs during the Tau phase could imply the need to resize the alternating current network.

[0015] 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.

[0016] [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 thin thickness situation for a temperature range up to 850K, and for thicknesses of 10, 40 and 80 microns of a ribbon cord, made of 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 zones 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] illustrates the two possible types of power supply architectures. The so-called series architecture, illustrated by the elements whose number has the index 1 in the third position. -The so-called parallel architecture, illustrated by the elements whose number has the index 2 in third position. It will be noted that to facilitate the understanding of the electrical diagrams any type of regulator is symbolically suggested as being a switching electronics.

[0017] In the series architecture a regulator 411 transforms the power from the alternating current network 401 into storable power in the storage element 421. This regulator 411 is dimensioned a little above the power level of the continuous phase to take into account the power necessary for the charge of 421. A regulator 431 transforms the energy stored in 421 into electrical power at the transient level during the transient phases and at the continuous phase level the rest of the time. Its dimensioning in terms of power is that corresponding to the transient phase, therefore high. In the parallel architecture a regulator 4122 transforms the power from the alternating current network 402 into storable power in the storage element 422. This regulator 4122 is sized slightly above the power level of the continuous phase. The regulator 4121 transforms the alternating energy 402 into power suitable for powering the heating winding 442 during the continuous phase. The sizing is that of the continuous phase. A regulator 432 transforms the energy stored in 422 into electrical power to power 442 at the transient level during the transient phases. Its sizing in terms of power is that corresponding to the transient phase, therefore high.

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10. Claims Heating device consisting of an electric heating cord, capable of reaching its nominal power level from the switched-off level in a short Tau rise time between 0.1 and 20 seconds, characterized in that: -the majority of the heating surface of the cord, radiating towards the volume to be heated, has no thermal contact with support elements. - 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, - the heating cord is powered through an electronic device allowing the injection of high electrical power for the duration Tau, allowing the nominal temperature level to be reached over this duration, then decreasing to reach the level corresponding to the continuous phase. - Claim according to 1 wherein the heating cord is electrically powered during the transient phase Tau from a buffer electrical energy storage system located near the heating cord Claim according to 2 wherein the heating cord is electrically powered during the continuous phase from said buffer electrical energy storage system Claim according to 1 to 3 or k is greater than 10 Claim according to 4 where the section of the heating cord is oblong Claim according to 1 to 5 wherein the heating cord has the shape of a ribbon of rectangular or rectangular section with rounded edges. Claim according to 2 to 6 wherein the energy storage system is one or more batteries and or one or more supercapacitors. Claim according to 2 to 7 wherein the capacity of the energy storage system is 2 to 5 times the amount of energy required to power the heating cord for the duration Tau Claim according to 6 wherein the ribbon is made of aluminum alloy or iron alloy or aluminized polymeric material. Claim according to 6 and 9 where the width of the ribbon is between 25mm and 50mm and where its thickness is between 15 micrometers and 100 micrometers.

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