WO2016132125A1

WO2016132125A1 - Heaters

Info

Publication number: WO2016132125A1
Application number: PCT/GB2016/050394
Authority: WO
Inventors: Matthew Sharpe; Martin Benson
Original assignee: Xefro Ip Limited
Priority date: 2015-02-18
Filing date: 2016-02-17
Publication date: 2016-08-25
Also published as: GB2535499A; GB201602793D0; GB2537214A; EP3259322A1; GB201502739D0

Abstract

A heating element comprising a layer of a carbon-based electrically conductive ink on an electrically insulating substrate panel and which forms part of an electric circuit between electrodes, the ink being of such volume conductivity and thickness, and the panel being so dimensioned, that an applied mains voltage across the electrodes dissipates between 200 and 1200 watts per square metre of panel.

Description

Heaters This invention relates to electrically powered heaters, including space heaters for heating domestic, commercial and other premises, and fluid heaters for example for heating water.

Electrically powered heaters for space heating generally comprise a flat panel, which might be sealed and contain an oil, or which might be a casing open to the flow of air, in each case containing an electric wire resistance element, which might be a curled wire or a sealed lance. Heaters having completely enclosed heating elements so that no really hot surface is exposed to the air are necessarily expensive. The heating element itself reaches a high temperature and can fail on that account, and cannot be replaced in a sealed unit. Heaters in which the element is exposed to the air are less expensive, but when not used for a while collect dust particles which, when the heater is next turned on, burn off, producing unpleasant smells. They, too, can fail because of the high temperatures reached by the element. Heaters are known in which the resistance element itself has a high surface area, and so delivers a desired heat output without itself reaching a high temperature.

Carbon based inks are known which have appropriate electrical resistance to be useful as the conductive component of heating elements.

US2007/0284557 discloses a transparent film comprising a network of graphene flakes with, optionally, other carbon nanostructure such as nanotubes. WO2012/062572 discloses a domestic appliance heater in which a coating comprising carbon nanotubes and/or graphite is applied to a substrate.

WO2012/000580 discloses a heating pad for applying external heat to a hot water tank, which has a face of a carbon containing polymer through which current is passed to heat it. A more recent heater is known from US2014/021195 comprising a flexible substrate with a graphene layer on at least one face, and electrical connection to the graphene layer. The substrate is transparent and has a graphene layer formed on at least one side, and an electrode supplies electrical energy to heat the graphene. The graphene is formed on the surface of the substrate, or on a metallic electrode deposited on the substrate, by transfer from another substrate or by deposition from a carbon-containing gaseous medium.

Graphene is, of course, a new form of carbon that is available in thin - a few molecules thick - sheets or plates of carbon atoms arranged in a hexagonal lattice. It is a good conductor of electricity. Its properties are most apparent when in extended sheet form, as prescribed in US2014/021195. These heaters require carbon, particularly when in the form of graphene, to be

expensively incorporated in polymer film or applied to surfaces.

Carbon-based electrically conductive inks are known, which can be used to print electric resistance heating elements, and this is a more straightforward and inexpensive method for making resistive, extended area heating elements.

The present invention provides carbon-ink-based resistive, extended area heating elements which are inexpensive and robust.

The invention comprises a heating element comprising a layer of a carbon-based electrically conductive ink on an electrically insulating substrate panel and which forms part of an electric circuit between electrodes, the ink being of such volume conductivity and thickness, and the panel being so dimensioned, that an applied mains voltage across the electrodes dissipates between 200 and 1200 watts per square metre of panel.

The panel may be so shaped that the heat dissipation is uniform, or substantially so, over the whole panel, and this is true of rectangular panels when the electrodes extend the whole length of opposite edges. However, instead of being solid, the ink may be printed in a sinuous path over the surface, giving an extended resistive element.

Panels may be made in different sizes, viz. 600 x 600, 800 x 600, 1000 x 600 and 1200 x 600, all in millimetres, to give a range of panel heaters adapted for heating different spaces.

Provided due regard is paid to heat dissipation, the panel temperature will not exceed 70°C, and a target operating temperature may be between 45°C and 55°C.

The substrate panel may comprise heat conductive material with a good emissivity, and may be a carbon-fibre loaded polymer. If the panel material is electrically conductive, the ink may be applied to an electrically insulating film on the surface of the panel. An insulating film may be applied over the exposed ink surface and that, again, covered with another panel of for example carbon-fibre loaded polymer. A heater, which may be free-standing or wall-mounted, may comprise a stack of two or more panels, with a gap between the, or each pair of, panels. The gap may be profiled to have a venturi effect encouraging airflow to carry heat away by convection. The stack may be held in a framework. Outer faces of panels may be profiled with a dimple or other low-relief pattern to increase emissive area.

At least one face of the heater may additionally have, over some or all of its area, a compartment containing a phase change material such as a paraffinic or salt hydrate material that can melt while the heater is on power, for release when off power. While this may be useful in maintain a constant level of heat output with a long switching cycle, it may be of more interest for an external water tank heating application, where heat can be stored in the phase change material for release to the tank when it cools, the phase change material being capable of storing a lot more heat than water in the tank, enabling the use of a smaller tank for cost and space savings.

For wall mounted heaters, the face against the wall may have an insulating layer which prevents local heating of the wall that can give rise to increased heat loss through the wall at that location, as well as discolouring or degradation of paint or other wall decoration behind the heater, of and such layer may contain an aerogel and/or retroreflective microbeads. The heater can be configured as a ceiling panel heater or as a floor block or flag heater for underfloor heating. And the ink may be printed on to a flexible substrate such as a wallpaper or a lining paper for direct heating of building walls.

The electrodes may be printed on to the edges of the carbon-based ink using an ink which is a good conductor of electricity, such as a silver-based ink. The electrodes may have endwise tabs or other connector arrangements for attachment of mains leads.

The carbon-based ink may contain graphene, which may be comprised in graphene flakes, which may comprise graphene oxide flakes or be otherwise functionalised as by attachment of molecules at points of the lattice, which may serve to 'tune' the graphene to emit mid- to long-wavelength infra-red, which may be for example of wavelength 9μιη. If the substrate panelling is transparent to such wavelength, or reasonably so, then the infra-red emissions will be appropriate for absorption by people within the space. Or the substrate panelling may absorb infra-red at any wavelength, and re-emit it

preferentially at 9μιη.

The ink may comprise graphene and carbon black as the electrically conductive components. Graphene and carbon black may be present in a weight ratio between 8: 1 and 14: 1, a ratio of 11.25: 1 being found particularly effective. The ink may comprise, by weight:

Graphene 14 - 22%, preferably 16 - 20%, with 18% being found particularly effective Conductive carbon black 0.7. - 2.5%, preferably 1 to 2%, more preferably 1,6%

A resin, e.g a copolyester resin 35 - 45%, preferably 39%

Dispersant 0.4 - 0.8%, preferably 0.6%

As an excipient, a solvent such as diacetone alcohol 18 - 26%, preferably 22%.

There may be other benefits to graphene-containing inks for use in the present application, which may derive from its rheological properties, serving to achieve an optimum viscosity, and from its electrical conductivity, which allows for good control over the conductivity of the ink. This will allow for panels of a given size to be more easily designed with maximum emissive area that will operate at a low, safe temperature that will not stress the ink, with resulting low rate of failure. A metre square panel with an ink thickness of 0.1mm adapted to dissipate IkW will have, with a mains voltage of 230V, a resistance of about 53 ohms. The volume resistance Ω of the ink will therefore be 0 05 ohms. For a screen printed or ink jet printed ink, the thickness will be substantially lower, and the volume resistance will be correspondingly lower. However, at low ink layer thicknesses, graphene morphology may result in a nonlinear relationship between ink thickness and dry resistance will be different from liquid resistance, and the resistance may change with first use if heating drives off excipient.

The ink may, after application to the substrate, simply be left to dry, or, depending, of course, on the excipient, be heat-dried or heat- or ultraviolet-cured. The heating element may be controlled in any of the usual ways, which can include, of course, no control at all, the heater being so specified as to reach comfortable

equilibrium, or thermostatic control, either from a thermostat measuring panel temperature, maintaining it at between 49°C and 51°C, or from a space thermostat powering on when the space is too cold and off when the space has reached a higher temperature.

Heating elements and heaters containing them will now be described with reference to the accompanying drawings, in which:

Figure 1 is a face-on view of a heating panel;

Figure 2 is a cross-section through a first heater;

Figure 3 is a cross-section through a second heater;

Figure 4 is a section through a ceiling heating panel;

Figure 5 is a section through a heated floor block or flag; and

Figure 6 is a diagrammatic illustration of a process for making heated wallpaper.

The drawings illustrate a heating element 11 comprising a layer 12 of a carbon-based electrically conductive ink on an electrically insulating substrate panel 13 and which forms part of an electric circuit 14 between electrodes 15, the ink being of such volume conductivity and thickness, and the panel 13 being so dimensioned, that an applied mains voltage across the electrodes 15 dissipates between 200 and 400 watts per square metre of panel.

The panel 13 is so shaped that the heat dissipation is uniform, or substantially so, over the whole panel, and is a rectangular panel, the electrodes 15 extending the whole length of opposite edges.

Panels 13 may be made in different sizes, e.g. 600 x 600, 800 x 600, 1000 x 600 and 1200 x 600, all in millimetres, to give a range of panel heaters adapted for heating different spaces. Figure 1 shows a 600 x 600 mm panel, and, in dashed lines, how it may be extended in length, and extension to 1200 mm in length being of twice the power of one of 600 mm.

The substrate panel 13 comprises heat conductive material with a good emissivity, and a carbon-fibre loaded polymer is very suitable. The ink is applied to an electrically insulating film 16 on the surface of the panel 13. The same film 16 is applied over the exposed ink surface and that, again, covered with another panel 17 of carbon-fibre loaded polymer. The ink layer 12 may be solid or may, as illustrated by dashed breaks 12a, printed in a sinuous pattern, forming a longer, narrower resistor.

The heaters shown in Figures 2 and 3, which may be free-standing or wall-mounted, comprise a stack of two panels 13, with a gap 18 between them. The gap 18 could be profiled to have a venturi effect encouraging airflow to carry heat away by convection. The stack is be held in a framework comprising separators 19. Assembly is completed with upper and lower end caps 21. Outer faces of panels 13 can be profiled with a dimple or other low-relief pattern to increase emissive area.

Figure 3 shows a heater of which one face has additionally a compartment 31 containing a phase change material 32 such as a paraffinic or salt hydrate material that can melt while the heater is on power, for release when off power. The phase change material compartment can also be used in an external heating arrangement for a water tank.

Placed in thermal contact with the outside of a copper water tank, the phase change material will release heat to the cooling tank, which, because of the greater heat capacity of the phase change material, can be made smaller than otherwise, saving on capital cost and space. Figure 4 illustrates a ceiling panel heater 11 in which the encapsulated ink layer 12 is on a heat insulating panel 41 mounted on a ceiling mounting plate 42. Figure 5 illustrates a floor block or flag 11 with the encapsulated ink layer 12 embedded in the plastics or concrete floor tile or flag. Figure 6 illustrates a method for making a heated wallpaper 61. A flexible substrate 62, which might be paper with an electrically insulating sealant coating or an insulating laminated film, is unwound from a roll 63 and passed to a printed 64, which could be a screen printer or an ink jet printer, for application of the carbon-based ink layer and the electrodes. An electrically insulating film 65 is applied over the ink layer and bonded thereto. A thermally insulating coating, for example, of an aerogel material such as Aero- Therm™, and hollow microbeads is applied from dispensers 66, 67, the wallpaper 61 (which may be lining paper) being wound up on roll 68.

For wall mounted heaters, the face against the wall can have an insulating layer 22, Figure 2, which prevents local heating of the wall that can give rise to increased heat loss through the wall at that location, as well as discolouring or degradation of paint or other wall decoration behind the heater, of and such layer may contain an aerogel and/or retroreflective microbeads.

The electrodes 15 are printed on to the edges of the carbon-based ink layer 12 using an ink which is a good conductor of electricity, such as a silver-based ink. The electrodes 15 have endwise tabs 23 or other connector arrangements for attachment of mains leads.

The carbon-based ink contains graphene, comprised in graphene flakes, which can comprise graphene oxide flakes or be otherwise functionalised as by attachment of molecules at points of the lattice, which may serve to 'tune' the graphene to emit mid- to long-wavelength infra-red, which can be for example of wavelength 9μιη. If the substrate panelling is transparent to such wavelength, or reasonably so, then the infra-red emissions will be appropriate for absorption by people within the space. Or the substrate panelling may absorb infra-red at any wavelength, and re-emit it preferentially at 9μιη.

The ink comprises graphene and carbon black as the electrically conductive components. Graphene and carbon black are present in a weight ratio of 11.25 : 1 , though could be between 8: 1 and 14: 1, a ratio of being found particularly effective. The ink comprises, by weight:

A resin, e.g a copolyester resin 35 - 45%, preferably 39%

Dispersant 0.4 - 0.8%, preferably 0.6%

As an excipient, the solvent diacetone alcohol 18 - 26%, preferably 22%.

There are other benefits to graphene-containing inks for use in the present application, deriving from its rheological properties, serving to achieve an optimum viscosity, and from its electrical conductivity, which allows for good control over the conductivity of the ink. This allows for panels of a given size to be more easily designed with maximum emissive area that will operate at a low, safe temperature that will not stress the ink, with resulting low rate of failure. The ink is, after application to the substrate, simply left to dry, or, depending, of course, on the excipient, be heat-dried or heat- or ultraviolet-cured.

The heating element 12 can be controlled in any of the usual ways, which can include, of course, no control at all, the heater being so specified as to reach comfortable

equilibrium, or thermostatic control, either from a thermostat 24 measuring panel temperature, maintaining it at between 49°C and 51°C, or measuring ambient temperature and powering on when the space is too cold and off when the space has reached a higher temperature.

Claims

Claims:

1 A heating element comprising a layer of a carbon-based electrically conductive ink on an electrically insulating substrate panel and which forms part of an electric circuit between electrodes, the ink being of such volume conductivity and thickness, and the panel being so dimensioned, that an applied mains voltage across the electrodes dissipates between 200 and 1200 watts per square metre of panel.

2 A heating element according to claim 1, in which the panel is so shaped that the heat dissipation is uniform, or substantially so, over the whole panel.

3 A heating element according to claim 2, in which the panel is rectangular and the electrodes extend the whole length of opposite edges. 4 A heating element according to claim 3, so configured that its temperature will not exceed 70°C in use.

5 A heating element according to any one of claims 11 to 4, in which the substrate panel comprises heat conductive material with a good emissivity.

6 A heating element according to claim 5, in which the heat conductive material comprises a carbon-fibre loaded polymer.

7 A heating element according to any one of claims 1 to 6, in which the ink is applied to an electrically insulating film on the surface of the panel.

8 A heating element according to any one of claims 1 to 7, in which an insulating film is applied over the exposed ink surface and that covered with another panel. 9 A heater comprising a stack of two or more panels, with a gap between the, or each pair of, panels.

10 A heater according to claim 9, in which the gap is profiled to have a venturi effect encouraging airflow to carry heat away by convection.

11 A heater according to claim 9 or claim 10, in which the stack is held in a framework.

12 A heater according to any one of claims 9 to 11, in which outer faces of panels are profiled with a dimple or other low-relief pattern to increase emissive area.

13 A heater according to any one of claims 9 to 12, in which at least one face of the heater additionally has, over some or all of its area, a compartment containing a phase change material such as a paraffinic or salt hydrate material that can melt while the heater is on power, for release when off power. 14 A heater according to any one of claims 9 to 13, being a wall mounted heater, in which the face against the wall has an insulating layer which prevents local heating of the wall. 15 A heater according to claim 14, in which such layer contains an aerogel and/or retroreflective microbeads.

16 A heating element according to any one of claims 1 to 8, in which the electrodes are printed on to the edges of the carbon-based ink using an ink which is a good conductor of electricity, such as a silver-based ink.

17 A heating element according to claim 16, in which the electrodes have endwise tabs or other connector arrangements for attachment of mains leads. 18 A heating element according to any one of claims 1 to 8, in which the carbon- based ink contains graphene.

19 A heating element according to claim 18, in which the graphene is comprised in graphene flakes.

20 A heating element according to claim 19, in which the graphene flakes comprise graphene oxide flakes or are otherwise functionalised as by attachment of molecules at points of the lattice, which serve to 'tune' the graphene to emit mid- to long-wavelength infra-red.

21 A heating element according to claim 20, in which the graphene flakes emit infrared with a wavelength of 9μιη.

22 A heating element according to claim 20 or claim 21, in which the substrate panelling is transparent to such wavelength, or reasonably so.

23 A heating element according to claim 20, in which the substrate panelling absorbs infra-red at any wavelength, and re-emits it preferentially at 9μιη. 24 A heating element according to any one of claims 18 to 21, in which the carbon- based ink comprises conductive carbon black.

25 A heating element according to claim 24, in which the graphene/carbon black ratio by weight is between 8: 1 and 14: 1.

26 A heating element according to claim 25, in which the graphene/carbon black ratio by weight is 11.25: 1.

27 A heating element according to any one of claims 1 to 8 in which the ink after application to the substrate has been left to dry, or has been heat-dried or heat- or ultraviolet-cured. 28 A heating element according to any one of claims 1 to 27, being so specified as to reach, without any thermostatic control, a comfortable equilibrium. 29 A heating element according to any one of claims 1 to 27, together with a thermostatic control, either from a thermostat measuring panel temperature, maintaining it at between 49°C and 51°C, or from a space thermostat powering on when the space is too cold and off when the space has reached a higher temperature. 30 A heating element according to any one of claims 1 to 29, configured as a ceiling heating panel.

31 A heating element according to any one of claims 1 to 29, configured as a floor heating panel.

32 A heating element according to any one of claims 1 to 29, comprised in a wallpaper.

33 A carbon-based electrically conductive ink adapted for printing a heating element according to any one of claims 1 to 32, comprising graphene flakes, conductive carbon black, a resin binder and a dispersant in an excipient.

34 An ink according to claim 33, in which the graphene comprises graphene oxide. 35 An ink according to claim 33 or claim 34, in which the graphene comprises functionalised graphene.

36 An ink according to any one of claims 33 to 35, in which the ratio by weight of graphene to carbon black is between 8: 1 and 14: 1.

37 An ink according to claim 36, in which the ratio is 11.25: 1.

38 An ink according to any one of claims 33 to 37, in which the excipient is diacetone alcohol.