GB2433185A

GB2433185A - Heating element comprising carbon fibres on an apertured base; a control system therefor

Info

Publication number: GB2433185A
Application number: GB0524936A
Authority: GB
Inventors: John Graham Bryant
Original assignee: HOTFOOT HEATED MEMBRANES Ltd
Current assignee: HOTFOOT HEATED MEMBRANES Ltd
Priority date: 2005-12-06
Filing date: 2005-12-06
Publication date: 2007-06-13
Anticipated expiration: 2025-12-06
Also published as: GB2433185B; GB0524936D0

Abstract

A heating element comprises a substantially planar base 20 of polyethylene terephthalate (PET) coated with a planar matrix of at least partially non-linearly orientated carbon fibres dispersed thereon, the base layer defining a series of apertures 15 there though for determining the resistance of the element. Copper foil terminal bus bars 30 extend beyond the base area, one of which electrically connects to the external aluminium layer of a foiled polyester layer 50,60 encapsulating the base layer 20 which aluminium forms a neutral layer. The internal face of the polyester layers carry a thermally activated adhesive. The element is controlled via an associated thermo sensor and a controller which determines the required voltage to be applied from a resistance verus temperature profile. The heater may be user in space heating applications or for example deicing roadways, runways, rigging or oil tanks.

Description

HEATING ELEMENT

Background to the invention

The present invention relates to a heating element. In particular, the present invention relates to a heating element for use in under-floor heating or wall-located heating applications

Summary of the Invention

Heating elements in the form of membranes are well known in aerospace and use conventional trace wires enclosed in a suitable outer skin. Other typical applications include tracers along pipes, anti-icing heaters on aircraft wings, gate-valve heated covers and most commonly, the electric blanket.

However, as the typical domestic room size is decreasing and the demands on wall space for pictures, shelving, mirrors, and the like increases and wall space in becoming increasing obscured by furniture, it is becoming more and more intrusive to free wall space to accommodate radiators and other heating means.

One well known solution is to use under-floor heating systems where the heating elements are located beneath the floorboards or within solid flooring, or to locate heating elements beneath the wall, out of sight. In this way, when heat is produced by such heating elements, the floor or wall immediately adjacent the element will become heated, thus transferring its heat to the air in the room via conduction.

Such heating elements conventionally comprise water filled elements or electrical ribbon elements located beneath the floor or within the wall cavity through which the heat must permeate before increased temperature benefits may be enjoyed within the room.

However, such conventional methods have the disadvantage that there is a significant delay between the change in temperature of the heating element being transferred into the room to be heated. Thus, an oscillation condition often occurs where the room is heated up, albeit slowly, until the desired temperature is exceeded. This is then detected by temperature sensors in the room and the heated element switched off. The room temperature will then decline until it falls below the desired room temperature at which point the temperature sensor will detect that the temperature has fallen below the desired threshold and the heating element will he switched on again. However, such constant cycling of the room temperature between temperatures which are below and above the desired temperature means that the room is only at the desired temperature for a fleeting moment in time each cycle, rather than being held at the desired temperature constantly.

There is therefore a need for an effective heating element which can be more efficiently operated to achieve a constant desired surrounding temperature.

The present invention seeks to address the problems of the prior art.

Statement of Invention

Accordingly, a first aspect of the present invention provides a heating element comprising a substantially planar base layer with a substantially planar matrix of at least partially non-linearly oriented carbon fibres dispersed thereon, the base layer defining a series of apertures therethrough.

In one embodiment, the carbon fibres are substantially non-linearly oriented.

In a further embodiment, the carbon fibres are substantially randomly oriented.

The carbon fibres may comprise carbon coated fibres, such as waste carbon fibre ends that are well coated in carbon. The carbon fibres may then be evenly dispersed over the entire surface area of the base layer.

The weight of the carbon fibre is the indicator that is directly related to the resistivity and conductivity of the material of the base layer and matrix.

The matrix of randomly oriented carbon fibres is adhered to the base layer by means of a relatively weak but flexible compound, such as PVA (polyvinyl alcohol) to form a paper-like structure which conducts electricity via the chance contact between fibres in the matrix, resulting in a resistive element.

Apertures are provided in the base layer. In the absence of such apertures, the resistance of the matrix is likely to be too low to allow practical use of the matrix within a heating element as the current density per unit area would be impractical.

The presence of apertures ensure that that the overall resistance of the matrix is increased such that the matrix can be practically used as a heating element as the current density is forced through the remaining pathways of the matrix around the apertures.

In one embodiment, the apertures are distributed substantially evenly throughout the base layer.

Thus the heated element of the present invention is a continuous veil-type membrane that conducts over its entire surface and offers a very short thermal path, giving high energy efficiencies and tight thermal control.

All or part of the base layer may comprise of polyester, although it will be appreciated that this is provided merely as an example and any other suitable material known to the skilled person could be used as an alternative to, or in combination with, polyester.

During manufacture, the carbon fibres are suspended in liquid at a suitable specific density and a planar member is passed through the liquid. The carbon fibres become randomly dispersed on the planar member and the planar member then passed squeezed between rollers as part of the drying process.

As a result of the method of manufacture, the connections between fibres results in a heating element which displays a resistance which is lower in the direction of manufacture, with higher resistance displayed in a direction transverse to the direction of manufacture.

The way the matrix is made modifies the resistance of the material enough to give a significant variation of conductivity across the material against that measured along its length. The result of this effect transforms the shape of the holes to achieve the same temperature change lengthwise to that across the matrix for the same potential difference. The result being an element with a uniform temperature across the whole surface for the voltage applied.

The calculation of a heating element for a particular operating voltage is done by measuring the resistance across and along the matrix before perforation, and by using a formula that calculates two paths at a time to replicate the function across and down the surface area of the element to produce a functional profile. The profile has to balance the required dissipation per unit area and also result in practical aperture sizes distributed evenly across the matrix. For special applications where the aperture size may need to be increased to allow fixatives, such as tile adhesive, to pass through the final element, this has to he taken into account also. Once the specifications have been determined for a particular heating element, formulated production is straightforward for all heating elements using

the same profile specifications.

In one embodiment, the heating element is further provided with, for example, a pair of bus bars provided by conductive foil such as copper foil, located adjacent the matrix at opposing edges of the base member, such that in use, the current is distributed evenly along the said opposing edges. It is preferred that the bus bars

S

are located adjacent the matrix at the two shorter opposing edges of a rectangular healing element.

The bus bars, are preferably coated with conductive glue to ensure a good electrical connection between the bus bars and the surface of the matrix-coated base layer.

Preferably, the bus bars further extend beyond the matrix at each of the said opposing edges. In this way, bus bars may be conveniently used to facilitate electrical connections to the heating element.

The electrical supply may be AC or DC with the supply voltage dependent upon the overall dimensions of the heating element. The heating element's length and therefore it overall resistance defines the design voltage, by controlling the current density per unit area.

As mentioned above, a heating element according to the present invention may be used with either AC or DC power supply. For example, a switched mode isolated power supply that provides from 48 V up to 110 V DC may be used. Alternatively, a 110 V centre tapped transformer AC power supply may be used. The heating element could even be operated using a 12 V to 48 V DC supply as found in a vehicle or aircraft.

The overall surface area also dictates the heat distribution across the heating element.

In one embodiment, the matrix and base layer are interposed between two layers of Polyester. However, it will he appreciated that any other material known to the skilled person and suitable for the purpose could be used in combination with or as an alternative to polyester.

Preferably, at least one of the polyester layers are foiled with aluminium on the side of the polyester layer not adjacent the matrix.

The opposing side of the each polyester layer may be coated with a thermally activated adhesive, such as polyurethane. Such a polyurethane adhesive may be used as an adhesive on all layers of the heating element, where required.

As a polyester layer is placed on either side of the base layer, thereby sandwiching the base layer and matrix therebetween, the adhesively coated side of each polyester layer will come into contact at positions corresponding to the locations of the apertures in the base layer, and will adhere to one another, thereby providing a strong, but flexible bond, holding the base layer and matrix coating firmly in position between the polyester layers.

The polyester layer may extend beyond the area of the base layer around at least a portion of the periphery of the base layer. Preferably, the polyester layer extends beyond the area of the base layer around the whole of the periphery of the base layer, resulting in a margin of polyester layer around the periphery of the base layer.

In such an arrangement around the periphery of the base layer where the adhesively coated surfaces of each polyester layer come into contact with one another, the polyester layers will adhere to one another, thereby encapsulating the base layer and matrix therebetween.

In one embodiment, the isolated bus bars at one end of the encapsulated base layer are connected with the aluminum foil coated polyester layer encapsulating the base layer adhering them to the aluminium foiled surface of the outer polyester layer, thereby producing the "neutral layer".

Preferably, the other bus bar becomes the live end of the heating element and therefore is insulated from the aluminium foiled surface of the neutral layer referred to above.

Conductive tapes may then be applied to both sides of the neutral layer near the live end of the heating element to produce the neutral connection at the same end as the live, and the whole arrangement encapsulated in a further layer of foiled polyester, such layer again extending beyond the neutral layer on all sides to produce the final earth layer.

The earth layer completes a final encapsulation of the heating element's construction and connects with the power cables by means of crimp connections that pass through the material, thereby resulting in a strong mechanical and electrical bond.

It will be appreciated that various techniques known to the skilled person may be used to encapsulate the crimp connection site dependant upon the type of application to which the heating element is to be put. For example, an insulation displacement crimp that passes through the polyester to make contact with the conductive layer(s) may be used.

In one embodiment of the present invention, the heating element is further provided with a thermally reflective barrier applied to at least a portion of one external planar side of the heating element. Such a thermally reflective barrier is preferably applied to the whole of one external planar side of the heating element.

In this way, the heat produced by the heating element is guided away from that side, towards and out of the side without the thermally reflective barrier. Thus, heat can be guided in the direction in which it is intended the generated heat should be supplied. This serves to prevent undesirable heat loss, for example, into the structure of the floor or the wall in which the heating element is located. Thus, undesirable heat loss can be minimised by effective use of such a thermal barrier as a significant proportion of the generated heat is reflected by the thermally reflective harrier. The resulting heating element can be a very efficient and cost-effective heating solution which minimises heat loss and thus wasted expense.

Such a thermally reflective barrier may comprise thermally insulating foam that will sustain the temperature gradients generated by the heated mat, phenolic, polyurethane, high temperature styrene, or any other material known to the skilled person and suitable for the purpose, either in combination or alone.

Installation of a heating element according to a first aspect of the present invention when used for under-floor heating is simple, as the heating element may be simply located below a carpet or any other finished floor surface, thereby removing the need for digging up concrete floors or lifting floorboards.

A further aspect of the present invention provides a heating system incorporating a heating element according to a first aspect of the present invention.

A further aspect of the present invention provides a heating system comprising a heating element as claimed in any one of Claims I to 16; and a controller for supplying electrical power at variable current and voltage levels to the heating element, the controller being operable to control the current and voltage levels in dependence upon a resistance-temperature characteristic of the heating element.

In one embodiment, the resistance-temperature characteristic is predetermined and stored by the controller.

A further aspect of the present invention provides a liquid heating system comprising a heating element according to a first aspect of the present invention suitable for wrapping around part or whole of a pipe of a reservoir through which the liquid to be heated passes. The temperature of the liquid may he controlled such that the liquid may be dispensed at a predetermined temperature and at a desired rate. It will be appreciated that the liquid may comprise any liquid whose temperature must be controlled, hut specifically includes water and aqueous solutions.

Brief Description of the Drawings

Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 illustrates the construction of an embodiment of a heating element according to a first aspect of the present invention; Figure 2A is a block diagram of a prior art heating system arrangement; Figure 2B is a block diagram of an embodiment of a heating element according to a first aspect of the present invention as part of a heating system; Figure 3 is a graph illustrating the relationship between resistance and temperature over time for a heating element embodying the present invention being heated to 50 C over the period of one hour; and Figure 4 is a graph illustrating the temperature profile (resistance versus temperature) for the heating element of figure 3.

Detailed Description of the Invention

Figure 1 shows a heating element 10 according to one specific embodiment of the present invention. Heating element 10 comprises a base layer 20 of polyethylene terephthalate (PET) coated with a matrix of waste carbon fibres of approximately 8 to 10 mm in length and 0.11 mm in diameter. The waste carbon fibres are dispersed evenly over the base layer 20 and monitoring for overall thickness by an electronic sensor (not shown) to ensure that the matrix layer thickness is accurately controlled.

Base layer 20 is provided with apertures 15 therethrough distributed evenly throughout base layer 20.

Apertures 15 may be created in base layer 20 by any suitable method known to the skilled person. However, one of the preferred methods is punching of the apertures 15. This is an efficient way of creating uniform apertures during the manufacturing process whilst creating a minimal amount of carbon fibre dust. Alternative methods include, but are not limited to, using a fine router to cut apertures in base layer 20 (preferably under compression to ensure a consistently clean cut), and roller cutting techniques.

The width of heating element 10 determines the current that determines the watts output for a given surface area. The length of heating element 10 determines the voltage requirement. The resistance is a function of the width and length of each the current pathway through the material, the density of current at the applied voltage will define the dissipation per unit area of material.

A copper bus bar 30 is provided at each end of base layer 20 i.e. in a rectangular heating element, a bus bar 30 is provided at each of the two opposing narrower edges. Bus bars 30 are provided to ensure even distribution of power across the entire base layer 20. Suitable connectors (not shown) with appropriate size wires provide input power to base layer 20.

In communication with the surface of the heating element I 0 is a thermo sensor 40 that feeds temperature information back to a controller (see figure 2).

Both sides of base layer 20 are further protected by membranes 50 which are composed of polyester coated on one side with a thermally activated adhesive and on the other side with aluminium foil. The two membranes 50 encapsulate base layer 20 therebetween, the two adhesive coated surfaces of each membrane 50 coming into contact with one another around the periphery of base layer 20 and at positions corresponding to the apertures 15 in base layer 20, thus holding base layer 20 firmly in place and forming a sealed earth layer.

A further aluminium foiled polyester layer 60 is then used to further encapsulate the base layer 20 to form a neutral layer'.

Finally, a thermally reflective layer (not shown) is applied to the exterior of one planar side of heating element 10. When heating element 10 is in use, for example, under finished flooring, the thermally reflective layer is opposite tot eh surface from which heat is to be output. For example, in a floor heating application, the thermally reflective barrier forms the lower layer of the element, with the carbon element located above it. In a wall heating application, the thermally reflective layer forms the layer furthest from the surface of the wall, with the carbon element being located between the thermally reflective layer and the wall surface. The heat produced by heating element 10 will be guided towards the surface of the floor/wall where it is desired, rather than a portion of the generated heat being lost in the opposite direction. Thus, the thermal layer increases the effective efficiency of heating element 10.

Figure 2A is a block diagram illustrating a conventional prior art heating system in which a controller I is in electrical connection with heating element 2. In use, a desired temperature is input into controller 1, which is then operable to activate heating element 2 to generate heat by controlling the supply voltage to heating element 2.

A value corresponding to the actual temperature (in the area being heated) is input into controller 1 as part of the feedback look and. Provided the actual temperature exceeds the desired temperature, the controller will continue to act to ensure a voltage supply to heating element 2. However, once the actual temperature equals the desired temperature, the controller 1 is operable to switch off the supply voltage to heating element 2. The actual temperature in the area being heated will then fall below the desired temperature over time (whether a short or a long time depends at least in part on the prevailing ambient temperature of the environment surrounding the area to be heated). Once this occurs, the controller 1 will them resume the supply voltage to the heating element 2 to generate heat and raise the actual temperature back up to a level equal to the desired temperature before switching off the supply voltage again. This oscillation between heat generation and switching off of the supply voltage to the heating element 2 will continue indefinitely. Thus the area to be heated only reaches the desired temperature fleetingly before exceeding or falling below the desired temperature on each oscillation. This is clearly not an ideal situation.

Figure 3 shows a graph illustrating the relationship between resistance and temperature over time for a heating element embodying the present invention being heated to 50 C over the period of one hour. As can be seen, a distinctive profile is generated for resistance when measured over time as the temperature of the heating element is raised from around 25 C to around 50 "C.

Figure 4 shows a graph illustrating the temperature profile (resistance versus temperature) for the heating element of figure 3. As can be seen, by plotting the resistance through the heating element as a function of temperature, a distinctive profile emerges which is specific for a heating element of specific matrix thickness, base layer dimension and aperture number, size and placement. Thus, generation of such a profile for a heating element of a particular specification will also apply to all other heating elements produced in accordance with such a specification.

Figure 2 is a block diagram showing the heating element 10 of figure 1 as part of a heating system. Controller 100 is in electrical contact with heating element 10.

The desired temperature value is input to controller 10, where it is recorded.

Controller 100 is operable to provide a supply voltage V to heating element 10.

As the controller 100 is controlling the voltage V supplied to the heating element and can detect the current I going through the circuit, the resistance can therefore be calculated by the controller using the simple equation of resistance equalling voltage divided by current. Once the resistance is known, the area within the resistance versus temperature profile for a heating element of corresponding specifications can be determined (using a profile such as that exemplified in figure 4). In this way, for any given current through the circuit, the controller can determine the temperature of the heating element. Thus, the heating element is acting as its own temperature sensor. Once the temperature of the heating element is known, the controller can determine whether the resistance through the heating element needs to be increased or decreased to achieve the desired temperature.

This can then be achieved by altering the effective supply voltage accordingly.

For example, if the measured resistance (Rm) corresponds to a measured temperature (Tm) which is less than the desired temperature (Td), the voltage supplied to the heating element can be adjusted to achieve the desired resistance (Rd) corresponding to the desired temperature (Td).

Thus, there is no need for an external temperature sensor as the heating element is acting as its own temperature sensor. In addition, the excessive oscillation of the temperature in the area to be heated is avoided as the heating element is producing heat at a constant temperature and is constantly monitored and adjusted to ensure that the desired temperature (Td) is sustained.

In a heating element according to the present invention, the voltage of the AC power is sampled by the controller and converted to a local value representing the instantaneous voltage.

The AC voltage is directed via a triac to the flexible mat via a sensing resistor of about 0.001 Ohms, a linear amplifier transforms this small voltage to a signal large enough to be processed by the controller.

The signal then is processed in software as a function of the dynamic AC voltage, that is to say that the instantaneous voltage and the instantaneous current are able to be used to measure the resistance oIthe load, by the use oiOhm's law.

So the method of reading the resistance of the mat is by monitoring the current through the element for the applied voltage.

The build of the mat and encapsulation of the carbon element modify the thermal response of the dynamic values received from the construction (the mat), the result being a combination of varying resistances that vary with the temperature of the mat resulting in the accompanying graph. This profile is the same for all sizes of construction designed for different working voltages. The controller operates to determine the part of the temperature curve the heating element is being driven in and is able to narrow down that position by iteration.

This process in practice takes very little time, resulting in high levels of accuracy and good recovery times if the power is interrupted.

The carbon has a negative temperature coefficient (which means as the temperature increases the resistance reduces), but the encapsulating layers produce a positive temperature effect, at AC this effect is modified by a variation in capacitance across the heating element due to temperature, making the change in dynamic resistance more complex.

Developing a profile of these effects in software allows the controller the ability to develop a characteristic as a function of time, as the time a heating element takes to heat up is modified by contact with an exterior surface. This surface may be a good or had conductor and the rate of thermal dissipation at that point will alter the rate S of change temperature in the heating element itself.

So rate of change is a significant portion of the controller's calculations to find the mats position on the temperature profile.

The difference is that the temperature of the mat is achieved via the same cables that drive the element as the construction doubles as it own non-linear temperature sensor, this simplifies construction and installation with its attendant cost advantages.

Although aspects of the invention have been described with reference to the embodiment shown in the accompanying drawings, it is to he understood that the invention is not limited to the precise embodiment shown and that various changes and modifications may be effected without further inventive skill and effort. For example, it will be appreciated that the present invention is not restricted to domestic and office heating applications, but can also be extended to dc-icing roadways, runways, rigging on ships, fuel oil tanks, industrial drying applications, and any other domestic, commercial or industrial application requiring the application of controlled heat into a selected environment. In other words, the present invention finds equal application in all circumstances where a tight thermal temperature source is required over a large surface area.

Claims

CLAIMS

1. A heating element comprising a substantially planar base layer with a substantially planar matrix of at least partially non-linearly oriented carbon fibres dispersed thereon, the base layer defining a series of apertures therethrough.

2. A heating element according to Claim 1, wherein the carbon fibres are substantially non-linearly oriented.

3. A heating element according to Claim I or Claim 2, wherein the carbon fibres are substantially randomly oriented.

4. A heating element according to any preceding Claim, wherein the carbon fibres are evenly dispersed on the base layer.

5. A heating element according to any preceding Claim, wherein the apertures are distributed substantially evenly throughout the base layer.

6. A heating element according to Claim 5, wherein the apertures are dimensioned in dependence upon the required relative resistance of the heating element.

7. A heating element accordingly to any preceding Claim, wherein a bus bar is provided adjacent the matrix at two opposing edges of the base member to distribute incoming current evenly along said opposing edges 8. A heating element according to Claim 7, wherein the bus bar extends beyond the matrix at each of said opposing edges.

9. A heating element according to Claim 7 or Claim 8, wherein the bus bar comprises conductive copper foil.

10. A heating element according to any preceding Claim, wherein the matrix is encapsulated between two layers of polyester.

Ii. A heating element according to Claim 10, wherein the polyester layer is foiled with aluminium on the side of the polyester layer furthest from the matrix. 1 (1

12. A heating element according to Claim 10 or Claim 11, wherein the polyester layer is coated with thermally activated adhesive.

13. A heating element according to any one of Claims 10 to 12, wherein the polyester layer extends beyond the area of the matrix around at least a part of the periphery of the matrix.

14. A heating element according to Claim 13, wherein the polyester layer extends beyond the area of the matrix around the whole of the periphery of the matrix.

15. A heating element according to any one of Claims 10 to 14, wherein an electrical contact is formed between the aluminium foil and a bus bar to form a neutral layer, with the opposing bus bar forming a live contact.

16. A heating element according to Claim 15, further encapsulated between two foiled polyester layers, the foiled polyester layers extending beyond the neutral layer around at least a periphery of the neutral layer.

17. A heating element according to Claim 16, further encapsulated between two foiled polyester layers, the foiled polyester layers extending beyond the neutral layer around the whole of the periphery of the neutral layer.

18. A heating system incorporating a heating element according to any preceding Claim.

19. A heating system comprising a heating element as claimed in any one of Claims I to 18; and a controller for supplying electrical power at variable current and voltage levels to the heating element, the controller being operable to control the current and voltage levels in dependence upon a resistance-temperature characteristic of the heating element.

20. A heating system according to Claim 20, wherein the resistance-temperature characteristic is predetermined and stored by the controller.

21. A heating element substantially as hereinbefore described and with reference to the accompanying drawings.

22. A heating system substantially as hereinbefore described and with reference to the accompanying drawings.