TECHNICAL FIELD

This invention relates to the absorption of thermal radiation. In particular, though not exclusively, it relates to a heat absorption material, and a heat exchanger and heater comprising such a material. Aspects of the invention also relate to a method of transferring thermal radiation and to the use of a carbon material for absorbing thermal radiation.

BACKGROUND

Combustion is an exothermic redox chemical reaction which occurs between a fuel and an oxidant, resulting in the emission of heat. The heat produced by combustion can be harvested for useful purposes. For example, in a boiler heat is produced by combustion and then transferred into a working fluid for downstream use.

Combustible fuels, such as hydrocarbon fuels, are well known in the art. A typical example in in the context of a heating system is natural gas. The oxidant in combustion is frequently gaseous oxygen from the surrounding atmosphere, though can be a separate oxidiser.

Once produced, it is known that heat can be transferred as convective heat, conductive heat, or radiant heat (also known as thermal or infra-red radiation).

The high temperature conditions achieved within many heating devices today accompany the appearance of a flame during combustion. This can be detrimental in circumstances where the flammability of surrounding materials is of concern, or where there is a need to minimise the harmful by-products of combustion produced at higher temperatures.

Catalytic heaters provide a means of generating heat in the absence of a flame. Catalytic heaters can be used with a variety of fuels (Applied Catalysis, 1 (1983) 249-282). This means they can be used in modern, domestic applications that run on natural gas and can also use hydrogen gas, or combinations of natural and hydrogen gases as fuel sources.

In a catalytic heater, instead of allowing combustion to occur naturally, a catalyst is provided to lower the activation barrier of oxidising the fuel. Importantly, the process of combusting fuels catalytically does not consume the catalyst; the catalyst is regenerated following oxidation of the fuel ready to oxidise further fuel.

The presence of the catalyst allows combustion to take place at lower temperatures. For example, combustion without a catalyst often starts at 500°C, or an even higher temperature; with a catalyst present, in some implementations, flameless combustion can occur at a temperature as low as 250°C.

The operating temperatures of catalytic heaters can be below the ignition temperatures of fuel, improving safety. Furthermore, catalytic heaters can have a much higher thermal efficiency than conventional heaters. Not least due to the lower operating temperature, compared to conventional combustion, heating of the atmosphere surrounding combustion can be reduced, as is the emission of visible light.

The process of combusting fuel catalytically produces mostly radiant heat in the form of thermal radiation. The emitted thermal radiation is directional and can be harvested by an appropriately positioned heat exchanger. In this way, the radiant heat of catalytic combustion can be transferred to a working fluid within the heat exchanger, such as water, for onward transmission.

However, harvesting thermal radiation is a challenge. Metals are commonly used in conventional combustion systems to transfer heat due to their high thermal conductivity, however metals are ineffective absorbers of thermal radiation. Polymer materials have poor thermal conductivity and in high temperature, oxidative environments tend to rapidly degrade.

There remains a need in the art for new materials and methods to facilitate harvesting thermal radiation.

SUMMARY OF INVENTION

It has now been found that particulate carbon materials can offer advantages in the context of harvesting thermal radiation. This has been found to be of particular benefit in the context of thermal radiation produced by catalytic heaters.

From a first aspect, the invention provides a heat absorption material for absorbing thermal radiation, the heat absorption material comprising a particulate carbon material.

From a second aspect, the invention provides a heat exchanger for harvesting thermal radiation, the heat exchanger comprising a heat absorption material according to the first aspect of the invention and a heat transfer arrangement for transferring heat absorbed by the heat absorption material to a working fluid.

From a third aspect, the invention provides a heating device comprising a heating element for emitting thermal radiation and a heat exchanger according to the second aspect of the invention positioned to absorb the thermal radiation.

From a fourth aspect, the invention provides a method of enhancing absorption of thermal radiation in a heat absorption material, the method comprising incorporating a particulate carbon material into a matrix of the heat absorption material.

From a fifth aspect, the invention provides a method of harvesting thermal radiation, the method comprising absorbing the thermal radiation into a particulate carbon material. From a sixth aspect, the invention provides the use of a particulate carbon material for absorbing thermal radiation.

DETAILED DESCRIPTION

Aspects and embodiments of the invention relate to, or make use of, a heat absorption material for absorbing thermal radiation, the heat absorption material comprising a particulate carbon material.

Advantageously, the particulate carbon material may comprise detectable sp ² -bonded carbon atoms.

Suitably the ratio of sp ² -bonded carbon to sp ³ -bonded carbon may be at least 1 : 1, optionally at least 3: 1, or even at least 3: 1. The measurement of such ratios may be performed with X-ray photoemission spectroscopy, for example as described in sp ² /sp ³ Carbon Ratio in Graphite Oxide with Different Preparation Times, D. W. Lee and J. W. Seo, The Journal of Physical Chemistry C 2011 115 (&), 2705-2708 DOI: 10.1021/jpl07906u.

Notably, sp ² -bonded carbon atoms can be approximated to aromatic C=C bonds, which have wavenumbers of 1400 - 1700 cm' ¹ . This lends itself to absorbing thermal radiation, for example from catalytic heaters. Typically, catalytic heaters emit thermal radiation with a wavelength in the range of 3.5 microns (medium wavelength) to 10 microns (long wavelength), corresponding to wavenumbers of 1000 to 3300 cm' ¹ .

Optionally, carbon content of the carbon material determined to DIN 51903 at 800° C. for 20 hours may advantageously be more than 90% by weight, more advantageously more than 95% by weight and yet more advantageously more than 97% by weight.

The residual moisture content of the carbon material determined to DIN 51904:2012-11 at 110° C. for 8 hours may advantageously be less than 5% by weight, suitably less than 3% by weight, or even less than 2% by weight.

The sheet resistance of the carbon material measured by a four-point probe measurement of a 25 pm film collected on membrane filter may advantageously be less than 10 Ohm/square.

The tapped density of the carbon material measured according to ASTM D7481 may advantageously be in the range of from 0.01 to 0.5 gem' ¹ , for example in the range of from 0.05 to 0.4 gem' ¹ or even in the range of from 0.06 to 0.3 gem' ¹ .

In some embodiments, impurities or functionalisation of the carbon material may enhance performance. For example, suitably, the carbon material may comprise, for example due to functionalisation of a base material, one or more functional groups selected from alcohols (typical wavenumber 3100 - 3500 cm' ¹ ), carboxylic acids (typical wavenumber 2600 - 3200 cm' ¹ ), and amines (3100 - 3500 cm' ¹ ). This can further facilitate the absorption of longer wavelength thermal radiation. The presence of some sp ³ -bonded carbon atoms may also be of use in this context.

However, the extent of impurities and any functionalisation may need to be balanced with desired levels of thermal conductivity or other material properties.

Advantageously, the particulate carbon material may have a thermal conductivity at 25 °C of at least 100 W/mK, optionally at least 500 W/mK, such as at least 1000 W/mK, or even at least 2500 W/mK. A high thermal conductivity of the carbon material can help transfer absorbed heat by conduction. Such levels of thermal conductivity may be achieved, for example, with graphene or graphite materials.

The carbon material is particulate in the sense that it comprises particles. Particle sizes referred to herein are volume based diameters, unless otherwise specified. Typically, the carbon material comprises particles having a particle size distribution. Optionally, the carbon material may have a monomodal particle distribution.

A particle size distribution may be defined based upon established D50 and D90 parameters. D50 means that 50% of the particles have a particle diameter smaller than or equal to the value specified. D90 means that 90% of the particles have a particle diameter smaller than or equal to the value specified.

The D50 and D90 values, and particle sizes in general, can be ascertained by sieve analysis in accordance with DIN 51938:2015-09 or by laser diffraction according to ISO 13320:2009. Suitably, the particle size distribution may be determined by sieve analysis according to DIN 51938:2015-09 for materials having a D50>150 pm or by laser diffraction according to ISO 13320:2009 for materials having a D50<150 pm. It should be noted that, in the case of sieve analysis according to DIN 51938:2015-09, the indices D50 and D90 are based on weight (% by weight), not, as in the case of laser diffraction, on the number of particles analysed in the sample volume.

The carbon material may suitably comprise or consist of particles having a median particle size (D50) of less than 800 pm, or less than 600 pm, or less than 220 pm, or even less than 150 pm.

In some embodiments, the carbon material comprises or consists of particles having a median particle size (D50) in the range of from 1 pm to 100 pm, for example in the range of from 2 pm to 50 pm, such as in the range of from 5 pm to 20 pm.

Optionally, the carbon material may comprise or consist of particles having a D90 of less than 600 pm, suitably less than 300 pm. In some embodiments, the carbon material comprises or consists of particles having a D90 in the range of from 1 pm to 100 pm, for example in the range of from 15 pm to 50 pm, such as in the range of from 10 pm to 40 pm.

The carbon material may comprise one or more materials. It may be favourable to use a single material or to blend a plurality of materials.

Optionally, the carbon material may comprise or consist of a graphite material.

The graphite material may, for example, take the form of fibres, rods, spheres, hollow spheres, platelets, in each case either in aggregated or agglomerated form.

Suitably, the graphite material may comprise platelets. The height of such particles is typically distinctly smaller compared to the breadth or length of the particles. Such flat particles may in turn be agglomerated or aggregated into constructs. The thickness of platelet-shaped primary particles may suitably be less than 500 nm, preferably less than 200 nm and more preferably less than 100 nm. As a result of the small sizes of these primary particles, the shape of the particles may be bent, curved, waved or deformed in some other way. The length dimensions of the particles can be determined by standard methods, for example electron microscopy.

Optionally, the graphite material may have a relatively high specific surface area, determined as the BET surface area by means of nitrogen adsorption to DIN ISO 9277:2010. Suitably, the graphite material may have a BET surface area of more than 5 m ² /g, more preferably more than 10 m ² /g or even more than 18 m ² /g.

The particle diameter D50 of the graphite material may suitably be less than 800 pm, or less than 600 pm, or less than 220 pm, or even less than 150 pm. The D90 of the graphite material may advantageously be less than 600 pm, suitably less than 300 pm.

Optionally, the graphite material may comprise expanded graphite, alone or in a mixture with unexpanded graphite.

In expanded graphites, individual basal planes of the graphite have been driven apart by a special treatment which results in an increase in volume of the graphite, preferably by a factor of 200 to 400. The production of expanded graphites is described inter alia in documents U.S. Pat. Nos. 1,137,373 A, 1,191,383 A and 3,404,061 A.

Commercially available expanded and unexpanded graphites include Sigratherm® GFG 5, Sigratherm® GFG 50, Sigratherm® GFG 200, Sigratherm® GFG 350, Sigratherm® GFG 500 (formerly available under the Ecophit® product name) from SGL Carbon GmbH, TIMREX® BNB90, TIMREX® KS5-44, TIMREX® KS6, TIMREX® KS150, TIMREX® SFG44, TIMREX® SFG150, TIMREX® C-THERM™ 001 and TIMREX® C-THERM™ Oil, and C- THERM™ 002 from TIMCAL Ltd. or Imerys Graphite & Carbon Switzerland Ltd.

Suitably, the carbon material may comprise or consist of carbon black. Carbon back is a material produced by the incomplete combustion of coal and coal tar, vegetable matter, or petroleum products, including fuel oil, fluid catalytic cracking tar, and ethylene cracking. Carbon black is a form of paracrystalline carbon.

Optionally, the particulate carbon material may comprise or consist of a carbon nanomaterial.

A carbon nanomaterial is sized, in at least one dimension, in the range of from the thickness of a single graphene layer to about 100 nm. Optionally, a carbon nanomaterial may be sized, in at least one dimension, up to about 50 nm, or even up to about 20 nm.

The carbon nanomaterial may comprise or consist of one or more graphene layers. The term "graphene layer" is used herein to refer to a single-atom-thick sheet of hexagonally arranged sp ² -bonded carbon atoms, either occurring within a multi-layer structure or by itself, optionally comprising impurities.

Advantageously, the carbon nanomaterial may comprise or consist of graphene, bilayer graphene, trilayer graphene, few-layer graphene, multi-layer graphene, or combinations thereof.

The term "graphene" is used herein to refer to a single graphene layer.

The term "bilayer graphene" is used herein to refer to two stacked graphene layers.

The term "trilayer graphene" is used herein to refer three stacked graphene layers.

The term "few-layer graphene" is used herein to refer to 2 to 5 stacked graphene layers.

The term "multi-layer graphene" is used herein to refer to 2 to 10 stacked graphene layers.

Optionally, the carbon material may have from 1 to 200 graphene layers, in particular 1 to 100 graphene layers, such as in the range of from 1 to 30 graphene layers, or even in the range of from 1 to 20 or in the range of from 1 to 10 graphene layers.

The term "graphene nanomaterial" is used herein to refer to a material comprising or consisting of one or more graphene layers. Thus, in some embodiments, the carbon nanomaterial comprises or consists of a graphene nanomaterial. The graphene nanomaterial particles may, for example, be a graphene quantum dots, graphene nanoflakes, graphene nanoribbons, graphene nanosheets, or combinations thereof.

The term "graphene quantum dots" is used herein to refer to graphene nanomaterial particles with a maximum dimension of less than 30 nm.

The term "graphene nanoflakes" is used herein to refer to graphene nanomaterial particles with a maximum dimension of less than 100 nm.

The term "graphene nanoribbons" is used herein to refer to ribbons of graphene or multilayer graphene with a width of less than 50 nm and a length greater than the width.

The term "graphene nanosheet" is used herein to refer to graphene nanomaterial particles with a maximum dimension of less than 2000 nm, preferably less than 1000 nm.

Graphene layers may be present in the carbon nanomaterial alongside amorphous carbon. Alternatively, the carbon nanomaterial may be substantially free from amorphous carbon.

The graphene layers may display a crystalline order or may be turbostratic, i.e. lacking any observable registry of the graphene layers.

The graphene layers, or indeed the carbon nanomaterial as a whole, may be pristine. The term "pristine" is used herein to describe graphene layers or materials substantially free from impurities.

However, the graphene layers, or indeed the carbon nanomaterial, may comprise one or more impurities. For example, the layers or material may be oxidised.

Typical impurities are heteroatoms e.g. defined as O, S, N, and P. Such impurities may be helpful in the context of absorbing and transmitting heat. Indeed, the carbon nanomaterial may be "functionalised" to deliberately increase or modify the type, amount and location of functional groups on the surface. Suitable groups for functionalisation include amine groups, a hydroxyl groups, peroxide groups, or carboxylic acid groups.

The extent of the impurities may be defined by a C/heteroatom atomic ratio. Suitably, a graphene layer or indeed the carbon nanomaterial as a whole, may have a C/heteroatom ratio of at least 2, in particular of at least 3, or even of at least 5 or 10. In some embodiments of the invention, the C/heteroatom atomic ratio is in the range of from 2 to 10. Pristine graphene layers or carbon material may, for example, have a C/heteroatom atomic ratio of at least 20, or even of at least 50 or of at least 100. Partially oxidised graphene layers are particularly common and may lead to desirable properties in the carbon nanomaterial, in particular in the context of absorbing thermal radiation. Suitably, a graphene layer or indeed the carbon nanomaterial as a whole, may have a C/O atomic ratio of at least 2, in particular of at least 3, or even of at least 5 or 10. In some embodiments of the invention, the C/O atomic ratio is in the range of from 2 to 10. Pristine graphene layers or carbon material may, for example, have a C/O atomic ratio of at least 20.

Since, impurities may be desirable, the graphene layer, or indeed the carbon nanomaterial as a whole, may suitably have a C/heteroatom ratio (and/or a C/O atomic ratio) of at most 20, in particular of at most 10, or even of at most 5.

Suitable carbon nanomaterials may be obtained, for example, by exfoliation of graphite. Such materials are commercially available, for example under the name PureGRAPH® from First Graphene Ltd. For a characterisation of such nanomaterial, reference is made to "Accounting Carbonaceous Counterfeits in Graphene Materials Using the Thermogravimetric Analysis (TGA) Approach", Dusan Losie, Farzaneh Farivar, Pei Lay Yap, and Afshin Karami, Analytical Chemistry 2021 93 (34), 11859-1186, DOI: 10.1021/acs.analchem.lc02662, which is incorporated herein by reference.

Alternatively, carbon nanomaterials, including functionalised variants, may be obtained via a bottom-up synthesis as described, for example in GB2570753.

Suitably, the carbon nanomaterial may comprise the following Raman spectroscopy bands: one peak appears at ~1580 cm-1 (G band), and another at ~2700 cm-1 (2D band). Suitably, the G band peak may be between 1565 cm-1 and 1585 cm-1. Optionally, the 2-D band peak may be between 2690 cm-1 and 2705 cm -1. The PureGRAPH ® materials mentioned above show these bands.

The heat absorption material may advantageously be a composite material.

Advantageously, the loading of the carbon material in the heat absorption material may be at least 1%, at least 2%, at least 5%, at least 7%, at least 10%, at least 12%, or even at least 15% by weight of the total heat absorption material.

Suitably, the loading of the carbon material in the heat absorption material may be at most 50%, at most 40%, at most 35%, at most 30%, at most 25%, at most 20%, or even at most 18% by weight of the total heat absorption material.

For example, the carbon material may be present in the heat absorption material in an amount in the range of from 1 to 25% by weight, or in the range of from 5 to 20 % by weight of the total heat absorption material. Suitably, the heat absorption material may be capable of absorbing thermal radiation with a wavelength in the range of from 1 to 100 pm, such as in the range of from 2 to 50 pm, or even in the range of from 3 to 20 pm. Advantageously, for example to work well with catalytic heaters, the heat absorption material may be capable of absorbing thermal radiation at a wavelength in the range of from 3.5 pm to 10 pm.

Advantageously, the heat absorption material may have a thermal conductivity at 25 °C of at least 0.5 W/mK, optionally at least 0.7 W/mK, such as at least 1 W/mK, or even at least 1.5 W/mK. A high thermal conductivity can help transfer absorbed heat by conduction.

Advantageously, the heat absorption material may comprise a matrix.

The matrix may, in principle, be of any suitable type, provided that it can conduct heat away from the carbon material and is sufficiently stable to withstand exposure to thermal radiation.

In some embodiments, the matrix has a lower thermal conductivity than the particulate carbon material. Thus, heat transfer may be improved by incorporating the particulate carbon material into the matrix.

In some embodiments, the matrix has a lower thermal radiation absorption, optionally in the range of 3.5 microns (medium wavelength) to 10 microns (long wavelength), than the particulate carbon material. Thus, absorption may be improved by incorporating the particulate carbon material into the matrix.

The carbon material may suitably be dispersed in the matrix. This can facilitate incorporation or application of the carbon material into or onto a heat exchanger or the like.

In various embodiments, the matrix may comprise an organic material. Suitably, the matrix may comprise a polymeric material, or a monomer or other precursor thereof.

In some embodiments, the heat absorption material comprises a polymeric matrix with the particulate carbon material dispersed therein.

Suitably, the matrix may comprise a thermoplastic or thermoset polymer. This can, for example, facilitate casting the heat absorption material into a desired shape.

A common limitation of polymeric materials is poor thermal conductivity. In various embodiments, this can be improved by the incorporation of the carbon material.

Suitable polymers are those sufficiently stable to withstand exposure to thermal radiation. Examples of suitable polymeric materials may include one or more of acrylics, urethanes, nylons, styrenes, polyamides (PA6, PA12, PA66, polyphthalamide), polyolefins and epoxies. Optionally, the polymer may be a thermoplastic or thermoset polymer.

In some embodiments, the polymer is extruded. Suitably, the heat absorption material constitutes one or more tubes or flat plates suitable for incorporation into a heat exchanger.

It may also be advantageous, in some applications, for the polymeric material to be suitable for containing a heat transfer fluid, such as water.

Optionally, the matrix may comprise one or more of Acrylonitrile-Butadiene-Styrene (ABS), Acetal Copolymer (AC), Chlorinated PolyVinylChloride (CPVC), Nylon Polyamide (PA), PolyButylene (PB), PolyButylene Terephthalate (PBT), Polycarbonate (PC), PolyEthylene (PE), Cross linked Polyethylene (PEX), Polysulfone (PLS), Polypropylene (PP), PolyPhthalAmide (PPA), PolyPhenylene Ether (PPE), PolyPhenylene Oxide (PPO), PolyPhenylene Sulphide (PPS), Polystyrene (PS), Polyurethane (PUR), PolVinylChloride (PVC) and PolyVinyliDineFluoride (PVDF), Poly-TetraFluoroEthylene (PTFE), PerFluoro Alkoxy alkane (PFA), perFluoro Ethylene-Propylene (FEP) and PolyEther-Ether Ketone (PEEK), Polyethersulfone (PES), Polyether ketone (PEK), or monomers or precursors thereof.

The carbon material may be incorporated into the matrix in any suitable manner. Suitably, the carbon material may simply be mixed into the matrix.

Optionally, the matrix may comprise a coating material. This can facilitate application of the heat absorption material onto a substrate.

Conveniently, the matrix may comprise a paint formulation. This may be suitably heat resistant. Such paint formulations are known in the art. They may, for example, be referred to as "enamel paints".

In various embodiments, the paint formulation may comprise an organic solvent (e.g. xylene), one or more resins (e.g. selected from silicone resins, acrylic resins, epoxy resins), optionally a heat resistant pigment (e.g. selected from manganese ferrite, mica), and a drier (e.g. zinc octoate).

Optionally, the paint formulation may be a spray paint formulation.

Some aspects and embodiments of the invention relate to, or make use of, a heat exchanger comprising a heat absorption material in accordance with the invention, for example as described anywhere hereinabove, and a heat transfer arrangement for transferring heat absorbed by the heat absorption material to a working fluid.

The heat exchanger may be of any suitable type. Optionally, the heat transfer arrangement may comprise a channel for a working fluid. The channel may be defined by the heat absorption material.

Conveniently, the heat transfer arrangement may configured to work via conduction. Optionally, the heat transfer arrangement may be configured to work via radiation.

Optionally, the heat exchanger may comprise a working fluid inlet and a working fluid outlet, with one or more channels therebetween, optionally defining a heat exchange manifold.

The heat transfer arrangement may be formed of the heat absorption material. In some embodiments, the heat exchanger can consist of the heat absorption material. Conveniently, this may be achieved by forming the heat absorption material into the heat exchanger, for example by casting or extruding a polymeric heat absorption material as hereinabove described.

Additionally or alternatively, the heat exchanger may comprise a conductive structure in contact with the heat absorption material. The heat exchanger may be arranged such that heat absorbed by the heat absorption material can be conducted, via the conductive structure, to a working fluid. Suitably, the conductive structure may define one or more channels for a working fluid, optionally defining a heat exchange manifold.

The conductive structure may comprise one or more conductive materials, as is known in the art, for example, copper, aluminium, stainless steel or the like. Suitably, the conductive structure may be substantially free from the carbon material.

Advantageously, the heat absorption material may be coated onto at least part of the conductive structure. Conveniently, this may be achieved by applying a heat absorption material in the form of a coating, such as a paint.

Some aspects and embodiments of the invention relate to, or make use of, a heating device comprising a heat exchanger in accordance with the invention, for example as described anywhere hereinabove. The heating device comprises a heating element for emitting thermal radiation and the heat exchanger positioned to absorb the thermal radiation.

The heating device may be of any suitable type. Suitably, the heating element may emit thermal radiation at a wavelength in the infrared range, i.e. 700 nm - 1 mm.

In some embodiments, the heating element may emit thermal radiation at a wavelength in the range of from 1 to 100 pm, such as in the range of from 2 to 50 pm, or even in the range of from 3 to 20 pm. Optionally, the heating element may emit thermal radiation with a wavelength in the range of from 3.5 pm to 10 pm. Carbon materials have been found to be of particular benefit in the context of absorbing thermal radiation of such wavelengths.

Advantageously, the heating device may comprise a catalytic heating element.

A catalytic heating element may suitably comprise a catalyst pad, optionally a heat source for activating the catalyst pad, and a fuel source for combustion on the activated catalyst pad. The heat source for activating the catalyst pad may, for example be electric.

Conveniently, the catalytic heating element may comprise a metal catalyst. The catalyst may comprise, for example, platinum, palladium, or other metals and alloys thereof capable of promoting the oxidation of fuel.

The fuel may suitably comprise hydrocarbons, for example natural gas or methane. Additionally or alternatively, the fuel may comprise hydrogen gas.

Suitably, the catalytic heating element may be arranged to combust fuel at a temperature below 1000 °C, optionally below 800°C, or below 700°C, or even below 600°C. At such temperatures, heat absorption and transfer are more challenging than at higher temperatures. In some embodiments, the heat absorption material can help to address this by enabling absorption of infra-red radiation emitted at such low temperatures.

The heating device may for use as a domestic heating device, for example a boiler used in a central heating system. Alternatively, the heating device may be for use as an industrial heating device.

Suitably, the heating device may be fuelled by natural gas and/or hydrogen.

Some aspects and embodiments of the invention relate to, or make use of, a method of enhancing absorption of thermal radiation in a heat absorption material, the method comprising incorporating a particulate carbon material into a matrix of the heat absorption material.

The thermal radiation carbon material, and matrix may be as hereinabove described.

Some aspects and embodiments of the invention relate to, or make use of, a method of harvesting thermal radiation, the method comprising absorbing the thermal radiation into a particulate carbon material.

The thermal radiation and carbon material may be as hereinabove described.

Suitably, the method may comprise conducting heat from the carbon material to a working fluid, for example water. Optionally, the heat may be conducted to the working fluid by flowing the working fluid through a heat exchanger comprising the carbon material, for example a heat exchanger as hereinabove described. Optionally, the method may comprise generating the thermal radiation. Suitably, the method may comprise catalytically reacting a fuel, for example as hereinabove described, with an oxidant, for example oxygen, to generate the thermal radiation.

Some aspects and embodiments of the invention relate to the use of a carbon material for the purpose of absorbing thermal radiation.

The use may be in a heat absorption material, heat exchanger, or heating device, or in accordance with a method according to any aspect of embodiment of the invention described herein.

In one particularly beneficial embodiment, the use is for absorbing thermal radiation generated by a catalytic heating element.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and do not exclude other components, integers or steps. Moreover the singular encompasses the plural unless the context otherwise requires: in particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Preferred features of each aspect of the invention may be as described in connection with any of the other aspects. Within the scope of this application, it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible.

EXAMPLES

One or more embodiments of the invention will now be described, by way of example only:

Example 1 - Graphene as a coating on metal heat exchanger

Formulations of a heat resistant coating were prepared at graphene nanomaterial loadings of 5%, 10%, 15%, 17.5%, and 20% by mass in a commercially available paint (Coo-Var® Heat Resistant Satin Black Enamel), by mixing the samples in a planetary centrifugal mixer (DAC 400.1 FVZ).

The following graphene grades were used:

PureGRAPH® 5 with a D50 of 5 pm measured by laser diffraction, sheet resistance of < 10 Ohm/square measured by a four-point probe measurement of a 25 pm film collected on membrane filter and a tapped density measured according to ASTM D7481 of 0.062 gem' ¹

PureGRAPH®20 with a D50 of 20 pm measured by laser diffraction, sheet resistance of < 10 Ohm/square measured by a four-point probe measurement of a 25 pm film collected on membrane filter and a tapped density measured according to ASTM D7481 of 0.251gcm ^_1

A sample of the coating was used to coat a copper heat exchanger, which consisted of a 10 mm coiled copper pipe, with metallic fins. The coating was applied by spray coating, with each layer being allowed to dry for at least 8 hours prior to adding the next layer. The final layer thickness was 700 pm. This process was repeated with all formulations.

The heat exchanger was placed in a catalytic heater unit - with no water added. After 2 minutes the external temperature was 180 DEG C. After 4 minutes it rose to 230 DEG C. This process worked for all the formulations.

Without the coating the copper heat exchanger did not absorb any thermal radiation from the heater and hence did not heat up. The copper heat exchangers with the graphene coating did heat up. This confirms the graphene coated unit is capable of heating water in a closed loop system, with the graphene-enhanced coating both absorbing the thermal radiation and efficiently transferring it to the metallic heat exchanger.

Next, the thermal diffusivity of the coatings was investigated by applying a 700 micron coating layer onto an aluminium substrate for laser flash analysis (LGA). This was repeated at a range of loadings (5%, 10%, 15%, 17.5%, and 20%, along with a control (0% loading)) using PureGRAPH®5 as the filler material. After 8 hours, when the samples were fully cured, 6.3mm x 6.3mm squares were cut from each sample. A graphite coating (thickness 1-2 pm) was sprayed onto the aluminium side of the samples in to improve absorption of the flash. These cut-outs were loaded into appropriately sized graphite holders and inserted into a Linseis LFA 500+ carousel.

The flash was applied to each sample at 21 °C in vacuum. A two-layer curve fitting calculation was applied to the data to find thermal diffusivity of the coating using known thermal constants of aluminium.

The results were obtained are shown in Table 1 and Figure 1.

Table 1

The data from Example 1 confirm that the graphene enhanced coating will have two advantages:

1. Absorption of radiation in the medium to far infrared range which is suitable for use with catalytic heaters

2. Enhanced thermal conductivity, which promotes heat transfer from the coating into the metallic heat exchanger.

There is an optimal coating level, which is likely to be in the range above. Above this range, the adhesion and mechanical properties of the coating could deteriorate, for example the coating becomes too mechanically stiff. In addition, there is a cost implication. Therefore, it is expected that a PureGRAPH® 5 graphene loading level of 15 - 20% would represent the optimal level.

Example 2 - Graphene vs black paint

A set of trials were carried out using a catalytic heating system.

The system consisted of two catalytic heaters, fed with liquified petroleum gas (LPG) and operating at a catalyst temperature of approximately 450 DEG C. Water was circulated through the system and the system temperatures were monitored. The system was connected to a series of domestic radiators, which were used to heat up a room.

The following heat exchanger variants were tested:

Uncoated copper heat exchanger.

Graphene coated copper heat exchanger. The coating was made using PureGRAPH® 20 graphene loaded at 20% w/w into a proprietary paint formulation, provided by Lysis Technologies Ltd (code 173-21481). It was spray coated onto the copper heat exchanger, using butyl acetate thinner to achieve the desired consistency. The PureGRAPH® 20 graphene had a D50 of 21 pm, a D10 of 9 pm and a D90 of 51 pm all measured by laser diffraction, a sheet resistance of < 10 Ohm/square measured by a four-point probe measurement of a 25 pm film collected on membrane filter, and a tapped density measured according to ASTM D7481 of 0.233 gem' ¹ .

Black spray-painted heat exchanger. An off the shelf "stove paint" was used to spray paint the heat exchanger.

The "stove paint" formulation contained black pigment and was substantially carbon-free. The graphene-coated system gave the best performance, in terms of temperature gain in the system:

- After 60 minutes of operation, the graphene coated system had a water temperature of about 60 DEG C leaving the heater unit, compared to 50 DEG C for the "stove paint" variant and 45 DEG C for the uncoated system.

- The graphene-coated heat exchanger had the highest surface temperature (90 DEG C) after 30 min, compared to about 60 DEG C for the uncoated and "stove paint" variants.

Example 3 - Heat flux trials

A set of coatings was further assessed by measuring heat flux through an identical aluminium sheet (RS Components - SIC 1050A Aluminium sheet, 300x200x1.2mm) coated with each coating. Measurements were made with a high temperature heat flux sensor, supplied by Hukseflux, serial number 5016 and product code HFS01-1-3.

Carbon particulate coatings were formed by incorporating 20% w/w of the following carbon particulates into a proprietary paint formulation, provided by Lysis Technologies Ltd (code 173-21481):

Carbon Black

PureGRAPH® 10 graphene with a D50 of 11 pm, a D10 of 5 pm and a D90 of 27 pm all measured by laser diffraction, a sheet resistance of < 10 Ohm/square measured by a four-point probe measurement of a 25 pm film collected on membrane filter, and a tapped density measured according to ASTM D7481 of 0.116 gem' ¹ .

PureGRAPH® 20 graphene with a D50 of 21 pm, a D10 of 9 pm and a D90 of 51 pm all measured by laser diffraction, a sheet resistance of < 10 Ohm/square measured by a four-point probe measurement of a 25 pm film collected on membrane filter, and a tapped density measured according to ASTM D7481 of 0.233 gem' ¹ .

PureGRAPH® 50 graphene with a D50 of 48 pm, a D10 of 17 pm and a D90 of 132 pm all measured by laser diffraction, a sheet resistance of < 10 Ohm/square measured by a four-point probe measurement of a 25 pm film collected on membrane filter, and a tapped density measured according to ASTM D7481 of 0.233 gem' ¹ .

As controls, an uncoated sheet and a sheet coated with carbon-free black "stove paint", as used in Example 2, were also tested. The results are given in Table 2 below:

Table 2

The data confirms that the application of carbon materials (graphene and carbon black) gives a significant improvement over the control sample and even with the commercially available "stove paint"

The PureGRAPH® 10 variant gives the best performance, in terms of the maximum heat flux, although it is closely matched by PureGRAPH® 20 and carbon black.

Example 4 - Comparison with anodised aluminium

A set of tests was performed to determine the performance of graphene-coated aluminium heat exchangers against those coated with anodised aluminium.

The heat exchangers consisted of flat sheets containing water channels for heat transfer.

- One heat exchanger was coated with a carbon particulate coating formed by incorporating 20% w/w of PureGRAPH® 20 with properties as defined in Example 3 into a proprietary paint formulation, provided by Lysis Technologies Ltd (code 173- 21481).

- Another heat exchanger was coated with a commercially available black anodising dye available from Caswell Inc, 7696 Route 31 Lyons, NY 14489.

A third heat exchanger was left uncoated as a control. The heat exchangers were tested in a catalytic heating system consisted of two catalytic heaters, fed with liquified petroleum gas (LPG) and operating at a catalyst temperature of approximately 480 DEG C. For the purpose of the trials water was not passed through the exchanger and the surface temperature was measured after 8 minutes. The external (side exposed to heater) and internal (side away from heater) temperatures were measured for each heat exchanger.

The results are shown in Table 3:

Table 3

The results indicate that the graphene-based coating was the most effective absorber of infra-red radiation from the catalytic heater, reaching the highest temperature (90 DEG C).

Example 5 - graphene / graphite in an engineering polymer

A polymer loaded with graphene (5% PureGraph®50) and 15% graphite was prepared with the following composition:

• 15% graphite (Sigratherm® GFG200)

• 5% PureGraph® 50 graphene with a D50 of 50 pm measured by laser diffraction

• Zytel® HTN 8200 NO 010 (PPA - polyphthalamide)

The loaded polymer was prepared by twin screw extrusion of the materials at a temperature of 330°C. Small shapes were extruded and placed in a glass jar between two catalytic heating elements.

The heaters were supplied with liquified petroleum gas (LPG) as the hydrocarbon feedstock. The catalytic heating elements were operated at a temperature of approximately 400°C

A control of pure Zytel®HTN 8200 NO 010 was also placed between the two catalytic heating elements in an identical glass jar.

The graphene I graphite loaded engineering polymer absorbed heat without changing form or melting. There was good heat distribution and high thermal conductivity. The temperature was measured as a function of time and the data for the graphene-loaded sample is shown in Figure 1.

This is confirmed by hot disk conductivity testing of equivalent plaques:

By contrast, the control suffered from discolouration and melting - where it faced the catalytic heater, it discoloured and stuck to the glass jar. This shows that, without the graphene and graphite filler, the polymer will melt and decompose, making it unsuitable for use in a heat exchanger for a catalytic heater. However, with the filler, the material will absorb thermal radiation, transfer heat efficiently and did not melt or decompose at the operating temperatures in the catalytic heater unit.