WO2013164557A2

WO2013164557A2 - Solar receiver with graphene foam thermal conduction core

Info

Publication number: WO2013164557A2
Application number: PCT/GB2013/000191
Authority: WO
Inventors: Gideon Stewan KUKARD
Original assignee: Kukard Limited
Priority date: 2012-05-01
Filing date: 2013-05-01
Publication date: 2013-11-07
Also published as: WO2013164557A3; GB2501713A; GB201207607D0

Abstract

The invention pertains to the field of solar heat exchangers, specifically solar receivers or solar absorbers used in solar thermal energy collectors. The solar receiver of the invention comprises a comprises a porous three-dimensional 'graphene foam' thermal conduction core (201) in direct three- dimensional contact with a heat transfer fluid (207) within an evacuated translucent inner tube (203) that is exposed to direct solar radiation. Alternatives without substantial differences exist in the disclosure. The receiver is integrated in a solar thermal energy collection system, such as a parabolic trough concentrated solar power system.

Description

SOLAR RECEIVER WITH GRAPHENE FOAM THERMAL CONDUCTION CORE

TECHNICAL FIELD

The present invention relates to the field of solar thermal energy collection. Specifically, the invention relates to an improved solar receiver or solar heat exchanger for use in solar thermal energy collectors.

BACKGROUND ART

A. Solar receiver

The first recorded use of a solar thermal energy collector was by Archimedes of Ancient Greece, who is said to have used a large concave mirror to concentrate sunlight onto water. The solar receiver is the heart of any modern solar thermal energy collector; within it heat from solar radiation is transferred either into a heat transfer fluid, such as oil or water, or directly into a working medium, such as in direct steam generation. When higher temperatures are desired, a concentrator may be used to focus incident solar radiation onto the receiver. Concentrated solar power systems are a type of solar thermal energy collector integrated with a steam- powered turbine for electricity generation. A design known to those skilled in the art is the parabolic trough concentrated solar power system. The concentrator of a parabolic trough system comprises a reflector shaped like an elongated parabola; it is linear in one dimension and shaped as a parabola in the other two dimensions. Variations of reflective materials may be used, such as silver, chromium, aluminium or high quality glass mirror. The parabolic trough concentrator focuses solar radiation onto a focal line; the focal line is in the center of the trough's two parabolic dimensions and along its linear dimension spaced marginally below the ends of the parabola. The concentrator moves along one axis to track the sun throughout the day with the aid of small motors.

The solar receiver in a parabolic trough system is in the shape of a tube that is placed along the focal line. The state of the art in such solar receivers comprises of a steel inner tube that is coated with a absorptive layer, principally to darken the color of the steel so that it is not reflective and instead is receptive to solar radiation. The steel inner tube is placed inside a glass outer tube that has an anti- reflective coating. A vacuum is maintained between the inner steel tube and the outer glass tube to stop any heat from escaping. A heat transfer fluid flows inside the steel inner tube which may be used to generate electricity by heating water for use in steam-powered turbine. Solar receivers of this nature boast high transmittance and absorptance factors and low emmitance factors. Transmittance is the measure of the amount of solar radiation that passes through the outer glass tube. Absorptance is the measure of solar radiation that is captured by the inner steel tube. Emmitance is the measure of the amount of heat that escapes the solar receiver. None of these factors measure the amount of solar radiation that is captured in the heat transfer fluid, i.e., the amount of solar thermal energy ultimately collected by the receiver - this can be described as the thermal conductivity, or heat-transfer capability, of the solar receiver and the collector as a whole.

Thermal conductivity of a solar receiver is dependent on the thermal conductivity of its component parts. In the state of the art solar receivers described above, thermal conductivity is limited to the thermal conductivity of the steel used in the construction of the inner tube. The advantage of using steel for the inner tube include that it can withstand high temperatures and is strong mechanically. A disadvantage of the steel tube is that it has low thermal conductivity. Although there are metals with higher thermal conductivities that could have been used for construction of the inner tube, such metals either could not withstand the heat of the heat transfer fluid or are costly precious metals. Another disadvantage of steel is that it is reflective and hence not absorptive of solar radiation; that is why such steel tubes are coated in an absorptive coating. However, the absorptive coating is less thermally conductive than the steel. Accordingly, while the coating may allow the steel to absorb heat, it diminishes the tube's ability to transfer heat. The effect is that the overall thermal conductivity of the receiver is limited both by the thermal conductivity of the absorptive coating and of the steel. A further disadvantage of the steel tube is that the heat transfer fluid only is in two dimensional contact with the inside of the steel tube. This results in a reduced surface area of steel to which the heat transfer fluid is exposed that reduces the efficiency of the thermal transfer even further. There is a need for a solar receiver that is both highly absorptive and highly thermally conductive.

Although parabolic trough concentrated solar power systems as are known in the art can achieve high temperatures of the heat transfer fluid, they require large physical space because the heat transfer fluid must remain in contact with the inner steel tube long enough to reach the desired temperature. The transfer of heat into the heat transfer fluid takes time because, as described above, the coated steel cannot efficiently transfer the heat that it collects and because in any case such non-efficient transfer is not maximized since there only is two dimensional contact with the heat transfer fluid. The result of the disadvantages of solar receivers used in parabolic trough concentrated solar power systems of this nature is that they only are viable on a large scale and in areas with a lot of open space. A need exists for a solar receiver that would make concentrated solar power systems viable on a small scale and in areas with limited space, such as residential rooftops. Further, there is a continuous need to improve overall thermal efficiencies of existing large scale systems.

Recent developments in solar receiver technologies have focussed on coating materials and alignment methods for placement within the focal line of a concentrator, for example, prior art documents US 20070209658 Al and WO 2011067294 A2. The present invention instead deals with thermal conversion efficiency of the entire system by concentrating on the efficiency of the heat transfer relationship between the solar receiver and the heat transfer fluid.

Developments in concentrator technologies have sought to prolong the time that the focal point or focal line of such concentrators is directed at the solar receiver. The linear Fresnel reflector and compact linear Fresnel reflector systems are examples of other concentrated solar power systems that are known generally and to those skilled in the art. Such systems use different concentrators to the parabolic trough but the same type of solar receivers. Accordingly, the disadvantages of solar receivers described in parabolic trough systems above, and the needs identified, apply equally to such other concentrated solar power systems. One of the setbacks in seeking to improve the thermal conversion efficiency of a system by improving the operation of the concentrator without improving the receiver is the cost. As the experiment of the dish Stirling technology has shown, multi-axis concentrator technologies include complicated and costly mechanics and software. One advantage of concentrating instead on improvements to the receiver is that such improvements could compensate for the lack of multi-axis solar tracking.

Non-concentrating solar thermal energy collectors come in a variety of designs, the principal two being flat plate collectors and evacuated tube collectors. Such collectors remain stationary and do not track the sun; accordingly they are able to work only limited hours of the day. The solar receiver of a flat plate collectors comprises of an absorber plate, usually constructed of a metal with high thermal conductivity. Solar absorption is achieved by coating the metal with an absorptive layer, typically a black paint, similar to the method described for the inner steel tubes in concentrated solar power systems described above. Several tubes containing a heat transfer fluid are located directly on top of the absorber plate. The absorber plate and tubes typically are housed within a translucent cover sheet designed to minimize solar radiation emission. The disadvantages of state of the art flat plate collectors is that the overall thermal efficiency of the system is limited by the thermal conductivity of the three elements that stand between the incident solar radiation and the heat transfer fluid: the absorber plate, the absorptive coating on such plate and the tubes carrying the heat transfer fluid. A further disadvantage is that the absorber plate is not in direct contact with the heat transfer fluid, unlike the solar receivers in concentrated solar power systems described above. The result is high convection and low thermal efficiency. A direct flow evacuated tube collector works in a similar way to the solar receivers of concentrated solar power systems but without the concentrator. They suffer from the same disadvantages of thermal efficiency limitation due to indirect contact between solar radiation and the heat transfer fluid and two dimensional contact between the heat transfer fluid and the tube through which it flows. There is a continuing need, and indeed great scope, for improvement in the thermal efficiency of non-concentrating solar thermal energy collectors. B. Graphene foam

Graphene is a single layer of carbon packed in a hexagonal lattice with a distance between carbon atoms of 0.142 nm. Its isolation in sufficient quantities for chemical testing by Andre Geim and Konstantin Novoselo first was published in October 2004 and earned them the Nobel Prize in Physics in 2010. Graphene's unique characteristics have been attributed to its two-dimensional structure. Its properties include near total transparency, record thinness, record mechanical strength, extreme denseness, extreme electrical conductivity and record thermal conductivity. As a transparent material, it is not absorptive of solar radiation.

As a two-dimensional material, graphene is not susceptible to many mechanical applications. "Graphene foam" is a structure whereby two-dimensional graphene can be presented in three dimensions. Since isolation of graphene in 2004, methods for creating three-dimensional structures that retain graphene's unique two-dimensional properties have been developed. One such method known to those skilled in the art is chemical vapor deposition (CVD) whereby a two-dimensional layer of graphene is synthesized onto a substrate, usually nickel or copper, through a combination of chemical reactions.

A three-dimensional graphene foam is comprised of a plurality of twisted two-dimensional layers with the result that it is porous. The level of porosity of the foam will depend on the technique used. Another difference with two-dimensional graphene is that three-dimensional graphene foam takes the color of the substrate used and is not transparent. Such coloration makes graphene foam absorptive of solar radiation. Further differences include that the three-dimensional structure of the graphene foam has an increased surface area when compared with two-dimensional graphene.

Research and development of graphene applications in the field of solar energy collection has been limited to photovoltaic, i.e., non-thermal, applications.

OBJECT OF THE INVENTION It is an object of the invention to provide an improved solar receiver that addresses the deficiencies in thermal conductivity in comparison with the state of the art. The invention concentrates on novel changes to the construction of a solar receiver as well as novel changes to the relationship between the solar radiation, thermal conduction core and heat transfer fluid within such a receiver.

SUMMARY OF THE INVENTION Accordingly, in a first aspect of the invention, there is provided a solar receiver or solar heat exchanger apparatus comprising a porous three-dimensional thermal conduction core disposed in a thermal transfer cavity defined by a housing, which is translucent to incident solar radiation, the apparatus configured to allow the passage of fluid through the cavity.

In a second aspect of the invention, there is provided a solar energy collection system comprising a solar receiver or solar heat exchanger as defined above and a solar concentrator for concentrating solar radiation onto the solar receiver or solar heat exchanger.

In a third aspect of the invention, there is provided a porous three-dimensional thermal conduction core configured for use in a solar heat exchanger by disposing it within a translucent tube arranged to allow the passage of a heat transfer fluid therethrough in direct contact with the thermal conduction core.

In a fourth aspect of the invention, there is provided a use of a porous three-dimensional thermal conduction core in a solar receiver or solar heat exchanger.

In a further aspect, there is provided a solar receiver that is characterized by the placement of a porous three-dimensional "graphene foam" thermal conduction core inside a translucent tube wherein a heat transfer fluid flows unidirectionally and in direct contact with the graphene foam. Optionally, the translucent tube is nested within an outer translucent rube of greater volume, such tubes joined so that a vacuum is maintained in the space between the tubes. Optionally, the translucent tube is coated on its inner side with a layer of two-dimensional graphene. Optionally, a concentrating and/or focussing means is provided to focus solar radiation onto the receiver.

DETAILED DESCRIPTION OF THE INVENTION

A solar radiation receiver for use in solar thermal energy collection systems is provided that is characterised by a porous three-dimensional thermal conduction core in direct three-dimensional contact with a heat transfer fluid so that the heat transfer fluid flows through and around the thermal conduction core.

The porous three-dimensional thermal conduction core may be of any suitable material. Typically, it is characterised by high thermal conductivity and low reflectivity. Optionally the porous three- dimensional conduction core comprises a ceramic or is a ceramic, but preferably comprises graphene. More preferably, the porous three-dimensional thermal conduction core is a graphene foam. Graphene foam has a thermal conductivity that is unmatched by any other known material and that is several times that of the materials used in state of the art solar receivers. The use of a graphene foam thermal conduction core allows rapid transfer of heat from either concentrated or direct solar radiation into a heat transfer fluid and increased overall efficiencies of the solar receiver and solar thermal energy collector as well as reduced spatial requirements of the overall system.

As used herein, the terms solar radiation receiver (solar receiver), solar absorber and solar heat exchanger are synonymous and may be used interchangeably. Hereinafter the invention will be described largely with reference to graphene foam as the thermal conduction core, but it should be understood where the context allows that the features described are applicable also to the general aspects of the invention.

The three-dimensional contact between the heat transfer fluid and the graphene foam thermal conduction core exposes the heat transfer fluid to a substantially larger surface area of the thermally conductive material (graphene foam) when compared to state of the art solar receivers. The shape and size of the graphene foam thermal conduction core may vary depending on the properties of the heat transfer fluid and can be constructed so at to maximize surface area while minimising resistance. By comparison, the solar receivers used in concentrated solar power systems described above maintain only two-dimensional contact between the thermally conductive material (steel) and the heat transfer fluid. Enhancement of the exposed surface area of the thermal conduction core to the heat transfer fluid also allows increased thermal efficiency and reduced sizes of solar thermal energy collection systems.

The invention is further characterized by direct contact between the incident solar radiation and the graphene foam thermal conduction core within heat transfer fluid. The graphene foam thermal conduction core and heat transfer fluid are disposed in a thermal transfer cavity defined by a housing translucent to incident solar radiation. The thermal transfer cavity defined by a housing may be provided by any suitable housing but is preferably provided by a panel element or an elongate element such as a tube, preferably a tube. Any suitable material may be used for the housing, which is preferably sufficiently translucent to enable a substantial portion of the incident solar radiation to transmit into the thermal transfer cavity. For example the material may be a plastic (such as Perspex, polycarbonate, etc.) or glass (e.g. a quartz glass, such as clear fused quartz glass). Preferably, the housing is of a material with high transparency, high mechanical strength and that is able to withstand high temperatures, such as fused quartz glass. This would allow high transmittance of solar radiation to the thermal conduction core while allowing the three-dimensional contact with the heat transfer fluid described above. State of the art solar receivers are not designed to expose the heat transfer fluid directly to solar radiation. This restricts the thermal efficiency of the receiver and increases the time needed to heat the heat transfer fluid to the desired temperature. Where the thermal transfer cavity defined by a housing is provided by a panel element, this may typically be formed of a first sheet of translucent material and a second sheet of material separated from the first sheet which together define a cavity within which is disposed a graphene foam. The edges of the panel should be sealed but configured to allow the passage of a heat transfer fluid therethrough.

Preferably the thermal transfer cavity defined by a housing is provided by a tube. Hereinafter, the invention will be described with reference to a tube (providing the housing defining a thermal transfer cavity), but it should be understood that further features described should be considered as also applying to the invention in general terms and to a panel where the context allows. Use of a translucent tube to house the heat transfer fluid, in conjunction with the placement of a thermal conduction core directly within the heat transfer fluid, is novel in the state of the art and eliminates barriers to absorption because the solar radiation is focused directly onto both the heat transfer fluid and the thermal conduction core.

The graphene foam thermal conduction core may be fixed in place inside the translucent tube by means of a pin or may be restrained within the translucent tube by means of a retaining ring. Alternative methods for holding the graphene foam thermal conduction core in place may be used.

The graphene foam may be provided in a block of a graphene foam formed or may be provided in sheets (being from 1-5 mm thick, typically about 2 mm thick) and rolled up to provide a roll or provided in layers stacked together. The translucent tube housing the graphene foam thermal conduction core and heat transfer fluid may be contained within a second translucent tube of larger volume. A vacuum may be maintained between the inner tube and the outer tube, which inner and outer tubes may be sealed by means of a sealing cap or by being sealed or fused together at their ends. The use of an evacuated tube is well known method to those skilled in the art for decreasing the solar radiation emmitance of the receiver. In one embodiment of the invention, the inner tube has a single layer of two-dimensional graphene coated on its inner surface that will further aid in transferring heat into the heat transfer fluid.

The tube or tubes may each be independently of any suitable section, for example circular (round), oval, elliptical, triangular, square, rectangular, pentagonal, hexagonal, heptagonal, octagonal or other regular or irregular section. The solar receiver may be used as part of integrated solar thermal energy collection systems, including concentrated solar power systems as are well known to those skilled in the art, e.g. comprising a solar concentrator such as a parabolic mirror arrangement or a Fresnel lens. The increased overall thermal efficiencies made possible by the invention would permit concentrated solar power systems to be economically viable on a small scale, such as for placement on residential rooftops.

The heat transfer fluid may be any suitable fluid capable of absorbing heat from a graphene foam. The heat transfer fluid is preferably a liquid at atmospheric conditions. The heat transfer fluid may be clear or coloured (e.g. light or dark), but is preferably light or clear. The heat transfer fluid may be organic/oil-based (e.g. a silicone oil , alcohol, acetone or ammonia) or aqueous (e.g. an aqueous solution), but is preferably alcohol or water and more preferably water such as de-aerated water.

In one embodiment of the invention, pumping a large amounts of de-aerated water through the receiver would prevent overheating and the creation of steam while making it possible to heat large volumes of water rapidly. In another embodiment, pumping a smaller (albeit still large by the standards of solar thermal collectors generally known to those skilled in the art) amounts of aerated water through the receiver could directly generate a plentiful supply of superheated steam and mechanical energy for conversion to electricity by means of a turbine. In one embodiment of the invention, a Fresnel lens may be used to concentrate the solar beams onto a heat exchanger of the invention; concentrating the sun's beams raises the temperature from ambient to several hundred degrees Celsius or even >1000 °C.

In one embodiment, the concentrated sunlight goes through two tubes of clear fused plastic quartz glass onto a graphene foam sheet, which together form the heat exchanger. This foam structure may be made by coating a single layer of graphene onto a metal foam made from copper or nickel using Chemical Vapor Deposition (CVD). The metallic foam base can then be melted out at high temperatures to leave just the graphene layer in place forming a foam structure. The graphene has a very high thermal conductivity and directly contacts the sunlight and the water, making the overall efficiency of the heat exchanger very high. Graphene is also dark allowing it to absorb more sunlight and therefore heat. There is preferably a vacuum maintained between the inner and outer tube to trap any heat and force it into the water. The inner tube may also have a single graphene layer coated on the inside again using CVD. This is done to transfer any heat from the inner glass tube into the water. The vacuum may be maintained by sealing the ends of the outer tube onto the inner tube. Water is typically pumped through the inner tube of the heat exchanger and around the graphene foam, to transfer the absorbed heat from the graphene into the water. Pumping the water through the heat exchanger also prevents overeating steam and makes it possible to heat large volumes of water fast. De-aerated water is used to allow the water to be heated to very high temperatures without it boiling or forming steam. A Fresnel lens and solar heat exchanger can be fitted onto a heliostat to track the sun and keep the lens's focal point on the graphene foam. The speed that the water is being pumped through the heat exchanger can be varied to create the desired temperature rise.

The invention will now be described in more detail, without limitation, with reference to the accompanying figures. Fig. 1 - shows inside the soiar heat exchanger. Outside tube (1) of clear fused quartz glass sealed onto the inner tube (2) to maintain vacuum (3). Inside tube of clear fused quartz glass in oval or round shape (2), vacuum between two layers of glass (3), de-aerated water (4) being pumped through heat exchanger, graphene foam (5) which is multi layered or rolled up to form multi layers and graphene layer (6) coated on the inside of inside tube (2). Fig. 2 - shows a Fresnel lens (7), focussing the sunlight onto the multi layered graphene foam (5) inside the heat exchanger while passing through outside tube (1) of fused quartz glass tube, the vacuum (3) and through the inside tube (6) of fused quartz glass. The graphene foam (5) will heat up and transfer heat to water (4) being pumped through heat exchanger by direct contact.

Fig. 3 - shows a side view of the heat exchanger with multi layers of graphene foam (5), stacked back to back. Where outer clear fused quartz layer (1) oval shaped inner clear fused quartz tube (2), de- aerated water (4) being pumped through heat exchanger, graphene coating (6) on inside of inner tube, vacuum (3) between inner and outer tubes.

Fig. 4 -outer fused quartz tube (1), inner tube of fused quartz in oval shape (2), vacuum (3), water (4) being pumped through heat exchanger, graphene coating (6) on inside of inner fused quartz tube (2), rolled up graphene foam sheet (5).

Fig. 5 - solar glass heat exchanger (8) as in Fig. 1, Fresnel lens (7) focussing sunlight on solar heat exchanger (8) metal plate heat exchanger (10) to separate solar circuit (9) and boiler circuits (15 and 16) and to facilitate heat transfer between them, pump (18) for circulating water through the solar circuit(9), gas or oil boiler (11), expansion vessel (12), central heating circuit (16) moving heated water to radiators for heating, heated water (15) being pumped through the cylinder coil (17) to produce domestic hot water, motorized valve or valves (14) diverting water between central heating and domestic hot water circuits, pumped solar water circuit (9) with de-aerated water, Radiator (19) on central heating circuit radiating heat. When the solar heat exchanger (8) is being used to heat the water of the circuits (15 and 16), the boiler will be off and when there is no sunlight available, the boiler (1 1) will heat the water on the circuit, because the solar heat exchanger (8) and solar circuit (9) will not be in use. Fig. 6 - solar heat exchanger (8) as in Fig. 1, pumped solar heating circuit (9) with de-aerated water, hot water cylinder with internal coils (20), internal coil (21) connected to gas or oil boiler, backup electrical immersion heater (22), cold water feed into cylinder (23), coil (24) connected to solar heat exchanger in cylinder used to produce domestic hot water while sun is shining, Fresnel lens (7), hot water (25) draw off to hot water taps. This layout will allow the cylinder to produce hot water in any conditions with the possibility of getting all or most of the heat of the sun.

Fig. 7 - In this Fig., sunlight enters from the top of the heat exchanger, through the outer layer (1) of fused quartz glass which will absorb minimal amounts of heat as it is clear glass and the focal point of the Fresnel lens is on the graphene foam (5) . The light then passes through the vacuum (3) and the inner tube (2) of fused quartz glass which is also clear, but will absorb more heat than the outer layer ( 1) because it is closer to the focal point . The heat that it does absorb, will be distributed into the water by graphene coating (6) on the inside of the tube, as it cannot go the other way being blocked by the vacuum (3). The sunlight continues down and hits the dark graphene foam (5), where most of the heat is absorbed this can be multi layers of graphene foam stacked back to back, or a large sheet of graphene foam rolled up. The heat is then distributed through the length of the graphene foam, because of the very high thermal conductivity of the graphene into the de-aerated water, by direct contact. The water is being pumped passed the graphene foam to keep it from overheating and forming steam. This also assists in heating large volumes of water.

Further and/or alternative embodiments are described in relation to the following figures. Referring to Fig. 8, a side view of a preferred embodiment of the invention is shown. The solar receiver comprises a porous three-dimensional graphene foam thermal conduction core (201) held by a retaining ring (202) inside a translucent inner tube (203) that is sealed to a translucent outer tube (204) by means of a sealing cap (205) in order to maintain a vacuum (206) between the outer tube (204) and the inner tube (203) wherein a heat transfer fluid (207) works unidirectionally within the inner tube (203) and in direct contact with the graphene foam thermal conduction core (201) as part of an integrated solar thermal energy collection system, such as a parabolic trough concentrated solar power system.

The inner and outer tubes may be of a cylindrical or rectangular shape configured to match the rate of flow required of the heat transfer fluid and the size of the system's piping. The receiver may be open at both ends or closed at one end depending on the required use. Generally, when a heat transfer fluid is used in a heat exchanger, the receiver will be open at both ends allowing the rapid heating of large volumes of fluid. When direct steam generation is required (i.e., not via a heat exchanger) the receiver may be sealed at one end and with other modifications to the remainder of the description of Fig. 8 that will be apparent to those skilled in the art.

Referring to Fig. 9, alternative configurations of graphene foam thermal conduction core are shown in a cross-sectional view of the receiver described in Fig. 8. The exact construction of the graphene foam thermal conduction core, including shape, size and porosity will depend on the intended use and the properties of the heat transfer fluid and will be configured to minimize resistance while maximizing surface area. Fig. 9A shows a graphene foam thermal conduction core in one piece. Fig. 9B shows a graphene foam thermal conduction core arranged as a multitude straight sections spaced marginally apart so that a dense heat transfer fluid may pass through the receiver with reduced obstruction. Fig. 9C shows a graphene foam thermal conduction core rolled into concentric circles.

Referring to Fig. 10, a side view of another embodiment of the invention is shown differentiated from that shown in Fig. 8 by the addition of a two-dimensional layer of graphene (208) on the inside of the inner tube. A graphene foam thermal conduction core (201) is held by a retaining ring (202) inside a translucent inner tube (203) that is sealed to a translucent outer tube (204) by means of a sealing cap (205) in order to maintain a vacuum (206) between the outer tube (204) and the inner tube (203) wherein a heat transfer fluid (207) works unidirectionally within the inner tube (203) and in direct contact with the graphene foam thermal conduction core (201).

Referring now to Fig. 1 1, one application of the invention is shown in a conventional domestic water heating system. A concentrating solar thermal collector system is shown comprised of a concentrator (207) and a receiver embodied from the invention (208). A heat transfer fluid circulates along the solar circuit (209) by means of a pump (218) towards a conventional metal plate heat exchanger (210) to separate the solar circuit (209) from the boiler circuits (215 and 216) and to facilitate heat transfer between such circuits. Such system can be installed together with conventional gas or oil boiler (211) expansion vessel (212) and central heating circuit (216) to move heated water to radiators (219) for spatial heating. Heated water (215) is pumped through the cylinder coil (217) to produce domestic hot water. A motorized valve or valves (214) diverts water between central heating circuit (216) and the domestic hot water circuits (215).

The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 - Front view of solar glass heat exchanger. Fig. 2 - Fresnel lens focussing sunlight on solar heat exchanger.

Fig. 3 - shows and side view of the heat exchanger with multi layers of graphene foam, stacked back to back.

Fig. 4 - shows an alternative version, where the graphene foam is rolled up, to form multi layers. Fig. 5 - shows the solar heat exchanger integrated into a standard central heating and hot water system.

Fig. 6 - shows the solar heat exchanger integrated into a hot water system.

Fig. 7 - Show the sunlight entering the heat exchanger and how heat is absorbed and transferred into the water. Fig. 8 - Side view of solar receiver with graphene foam.

Fig. 9 - Cross-sectional view of alternative configurations of graphene foam thermal conduction core. Fig. 10 - Side view of solar receiver graphene foam and graphene coating.

Fig. 11 - Schematic diagram of solar receiver integrated into a solar thermal collector system as part of a standard residential central heating and hot water system.

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the invention the embodiment described in Fig. 1 whereby the graphene foam thermal conduction core is fixed within an evacuated translucent inner tube wherein a heat transfer fluid flows unidirectionally and in direct contact with the graphene foam and incident solar radiation. Such inner translucent tube is sealed to an outer translucent tube so that a vacuum is maintained between them. Further, the best method comprises use of a concentrator so that the receiver is placed in the focal point or foal line of the concentrator. The solar thermal energy collector comprising of the receiver of the invention and a concentrator are a part of an integrated system used to heat water and/or generate steam for conversion into electricity.

Claims

I . A solar receiver or solar heat exchanger apparatus comprising a translucent tube having disposed therein a graphene foam, the apparatus arranged to allow the passage of a heat transfer fluid through the tube and in direct contact with the graphene foam.

2. An apparatus as claimed in claim 1, wherein the graphene foam is in sheet form.

3. An apparatus as claimed in claim 2, wherein the graphene foam comprises a rolled up sheet or stacked layers of graphene sheets.

4. An apparatus as claimed in any one of claims 1 to 3, which further comprises a second outer translucent tube arranged about the (first inner) translucent rube.

5. An apparatus as claimed in claim 4, wherein the first inner translucent tube and second outer translucent tube are arranged so that a vacuum may be maintained between the tubes.

6. An apparatus as claimed in claim 5, wherein the ends of the outer tube are sealed onto the inner tube.

7. An apparatus as claimed in any one of the preceding claims, wherein the translucent tube is coated on its inner side with a single (two-dimensional) layer of graphene.

8. An apparatus as claimed in any one of the preceding claims, wherein the tube(s) are independently of circular (round), oval, elliptical, triangular, square, rectangular, pentagonal, hexagonal, heptagonal, octagonal or other regular or irregular section.

9. An apparatus as claimed in any one of the preceding claims, wherein the tube(s) are glass tubes. 10. An apparatus as claimed in claim 9, wherein the tubes are clear fused quartz glass tubes.

I I . An apparatus as claimed in any one of the preceding claims wherein the heat transfer fluid is water, which optionally is de-aerated.

12. An apparatus as claimed in any one of the preceding claims, which is configured so that the heat transfer fluid flows unidirectionally through the tube.

13. An apparatus as claimed in any of the preceding claims, wherein a concentrating and/or focusing means are provided to focus solar radiation onto the receiver.

14. A solar heat exchanger comprising two tubes of clear fused quartz glass having a graphene foam sheet disposed therein.

15. A solar heat exchanger as claimed in claim 14 comprising a first inner tube of clear fused quartz glass and a second outer tube of clear fused quartz glass configured to maintain a vacuum therebetween and having disposed within the first inner tube a graphene foam sheet.

16. A solar energy collection system comprising a solar receiver or solar heat exchanger as defined in any one of claims 1 to 15 and a solar concentrator for concentrating solar radiation onto the solar receiver or solar heat exchanger.

17. A solar energy collection system as claimed in claim 16, wherein the solar concentrator is selected from a Fresnel lens.

18. A solar energy collection system as claimed in claim 16, wherein the solar concentrator is selected from a parabolic mirror.

19. A graphene foam configured for use in a solar heat exchanger by disposing it within a translucent tube arranged to allow the passage of a heat transfer fluid therethrough in direct contact with the graphene foam.

20. Use of graphene foam in a solar receiver or solar heat exchanger.

21. A use as claimed in claim 20, which is by disposing the graphene foam within a translucent tube of a solar receiver or solar heat exchanger configured to allow the passage of a heat transfer fluid therethrough in direct contact with the graphene foam, whereby the graphene foam may absorb incident solar radiation and transfer heat associated therewith to the passing fluid.