SOLAR ENERGY SYSTEM

Abstract

A solar panel (302) for heating a target fluid using incident solar radiation is described, the solar panel (302) includes: three major edges (306) arranged so that the solar panel (302) can be inscribed in a triangle with each major edge (308) of the panel (302) lying along at least a portion of a side of the triangle; a cavity for retaining the target fluid; and an inlet and an outlet for the target fluid, for exchanging the target fluid with adjacent solar panels (302).

Claims

1-25. (canceled)

26. A solar energy system for heating a target fluid using incident solar radiation, the system comprising: a protective upper layer; a target fluid layer comprising the target fluid; a light transmissive inter-fluidic dividing layer; a working fluid layer comprising a working fluid; and a lower retaining layer.

27. The system according to claim 26, wherein the inter-fluidic dividing layer is transparent.

28. The system according to claim 26, wherein the inter-fluidic dividing layer is translucent or transmits light in a diffusive manner.

29. The system according to claim 26, wherein the inter-fluidic dividing layer is transmissive to light in both the infrared and visible parts of the electromagnetic spectrum.

30. The system according to claim 26, wherein both the target fluid and the working fluid are liquid.

31. The system according to claim 26, further comprising at least one of a pump or a siphon for circulating the target fluid through the target fluid layer of the system.

32. The system according to claim 26, wherein the target fluid and the working fluid are arranged such that incoming solar radiation passes through the target fluid prior to passing into the working fluid.

33. The system according to claim 26, wherein the target fluid comprises at least one of oil or water.

34. (canceled)

35. The system according to claim 26, wherein the working fluid comprises a colloid comprising a dispersed phase of nanoparticles.

36. The system according to claim 35, where the nanoparticles comprise carbon nanoparticles.

37. The system according to claim 35, wherein the colloid comprises a dispersion medium comprising at least one of water or oil.

38. (canceled)

39. The system according to claim 35, wherein the colloid comprises a dispersion medium and a high viscosity fluid for thickening the dispersion medium.

40. The system according to claim 26, wherein the absorbance of the incident radiation in the working fluid is greater than the absorbance of the incident radiation in the target fluid for at least a portion of the spectrum of the incident solar radiation.

41. The system according to claim 40, wherein the portion of the spectrum comprises visible wavelengths.

42. The system according to claim 26, wherein the working fluid emits absorbed radiation at infrared wavelengths.

43. The system according to claim 26, further comprising an upper insulating layer.

44. The system according to claim 26, wherein the lower retaining layer comprises a reflective layer for reflecting radiation back through the overlying layers.

45. The system according to claim 26, wherein the transfer of energy between the working fluid and the target fluid predominantly comprises radiant transfer.

46-80. (canceled)

81. The system according to claim 26, further comprising at least one solar panel for heating the target fluid using incident solar radiation, the solar panel comprising: three major edges arranged so that the solar panel can be inscribed in a triangle with each major edge of the panel lying along at least a portion of a side of the triangle; a cavity for retaining the target fluid; and an inlet and an outlet for the target fluid, for exchanging the target fluid with adjacent solar panels.

82. The system according to claim 26, further comprising a network of solar panels for heating the target fluid using incident solar radiation, the network comprising: a first solar panel; and a second solar panel; wherein the first and second solar panels each include a cavity for retaining the target fluid, and the first and second solar panels are coupled together to allow the target fluid to flow between them; and further comprising one or more inlets, each respective inlet coupled with a corresponding one of the first and second solar panels, the one or more inlets configured for selectively enabling fluid flow into each panel.

Description

[0116] Embodiments of the apparatus and methods described herein will now be described in more detail with reference to the drawings in which:

[0117] FIG. 1 is a schematic diagram of a solar thermal system according to one embodiment;

[0118] FIG. 2 is a schematic diagram of a solar thermal system according to a further embodiment;

[0119] FIG. 3 is a plan view of a solar panel according to an embodiment;

[0120] FIGS. 4A and 4B are plan views of a network of solar panels according to an embodiment;

[0121] FIGS. 5A and 5B are plan views of solar panels according to another embodiment;

[0122] FIG. 6 is a schematic diagram of a high shear mixer;

[0123] FIGS. 7A and 7B are schematic views of high shear mixers according to yet another embodiment;

[0124] FIG. 8 illustrates a step in the manufacture of an impedance matching layer according to one embodiment;

[0125] FIG. 9 illustrates an impedance matching layer according to one embodiment;

[0126] FIG. 10 illustrates a calendering process in the manufacture of an impedance matching layer according to one embodiment;

[0127] FIG. 11A is a schematic illustration of a diaphragm according to one embodiment; and

[0128] FIGS. 11B and 11C provide a schematic illustration of the diaphragm of FIG. 11A in use as a valve in a pipe.

[0129] FIG. 1 illustrates schematically an embodiment of solar thermal system. The system comprises a number of layers stacked in a flat-panel arrangement. The first, or upper layer comprises a protective glass layer 118 which provides some structural stability and protection for the panel. Glass is rigid, scratch resistant, and is transparent to a broad spectrum of the incident electromagnetic radiation, so it is a suitable material to use for the upper layer of the solar thermal system. However, the skilled person will appreciate that other materials such as a transparent plastic may also be suitable. It will be appreciated that the upper layer need not be optically clear, but may diffuse the light.

[0130] Below the upper glass layer there is provided a layer of the target fluid 120. The target fluid is the fluid into which the solar thermal system is designed to transfer the solar energy. Typically, the target fluid comprises water. Water is transparent to some wavelengths of the incident solar radiation, in particular the radiation at visible wavelengths and higher. Therefore, a large proportion of the incident radiation will pass directly through the water 134. However, water is opaque to radiation at infrared wavelengths. Therefore, the incident radiation at infrared wavelengths and lower is absorbed 132 into the target fluid on entry into the system and hence can be used to heat the water directly. This minimises the number of layers that this useful part of the spectrum of the incident radiation must pass through before absorption by the target fluid.

[0131] A further advantage of enabling the incident radiation to pass first through the target fluid, at least in some embodiments, is that the incident ultraviolet radiation will have a pasteurising or anti-microbial effect on the target fluid. This may be helpful, for example, in a system designed to pasteurise water.

[0132] The target fluid is supported by a further glass layer 122, which also separates the target fluid 120 from the layer of working fluid 124. In fact, so long as light is transmitted through this further layer 122 (also called an inter-fluidic dividing layer), it is possible to use both the working fluid 124 and the target fluid 120 to extract energy from sunlight. Therefore, any material, such as light-transmissive plastics may be used for the inter-fluidic dividing layer 122.

[0133] Since a large proportion of the solar energy incident on the surface of the Earth is in the infrared, visible, and ultraviolet regions of the electromagnetic spectrum, it is preferable that the inter-fluidic dividing layer 122 is transmissive to light in any one of these regions. Preferably, the layer 122 is transmissive to light in the infrared and visible portions of the spectrum, as these two regions combined account for a majority of the sun's energy which reaches the earth's surface.

[0134] In this context, transmissive means that the light is not blocked by the layer. For example, the light may pass through almost entirely unaffected, if the layer is transparent. Alternatively, the layer may be diffusive or translucent, in which case, the incident light rays are substantially scattered in an incoherent manner, but nonetheless pass through the layer.

[0135] The working fluid is designed to have a high absorption of incident radiation at wavelengths at which the target fluid has a poor absorption. In particular, as described in more detail below, the working fluid is designed to absorb a high proportion of the radiation incident at wavelengths within the visible part of the spectrum. Absorption of the radiation causes the working fluid to heat up and to emit infrared radiation 136, 138. This infrared radiation is transmitted 136 or reflected 138 back to the target fluid 120 for absorption.

[0136] Below the working fluid there is provided a lower glass layer 126, which covers a reflective and insulating layer 130 shown in FIG. 1 as a single layer, but which may be provided as two separate layers. In particular, the reflective layer may be provided by adding a reflective coating to the lower surface of the glass layer 126. The insulting layer can be formed of any suitable organic or inorganic material such as polystyrene, polyester or another plastics-based material or insulating felt or wool.

[0137] The target fluid from any of the systems described herein may be used directly for drinking, cooking, heating or washing, or may be used for sterilising, pasteurising other fluids or items. The heated fluid may also be used for space or water heating, drying, cleaning or solar-driven cooling. The systems described herein may also be used as part of a distillation process, for example for the desalination of salt water. While the target fluid is preferably used directly, it is possible to transfer the energy from the target fluid into another medium, such as another fluid, or into another form, such as electrical energy, outside the solar thermal system illustrated in FIG. 1.

[0138] FIG. 2 is a schematic diagram of a solar thermal system according to a second embodiment. The system of FIG. 2 incorporates all of the elements of FIG. 1 described above, but includes a number of additional layers to improve the performance or robustness of the system.

[0139] In particular, the upper glass surface of the panel is covered with additional layers to decrease the heat loss from the panel and provide additional strength and resilience to the panel. The panel is covered with an outer glass or plastic-based layer 210, embodiments of which will be discussed in more detail below.

[0140] Below the outer layer 210 is an air gap comprising two sealed sections 212, 216 filled with air and separated by a thin plastic film 214 or membrane. The thin plastic layer stratifies the air within the air gap 212, 216, reducing movement of the air in this space and therefore reducing the movement of heat between the outer layer 210 and the glass surface 218. The plastic film can be manufactured from any suitable plastics material, for example polyethylene (HDPE or LDPE) or polyvinyl chloride.

[0141] This outer structure, including the outer layer and the stratified air gap, reduces loss of heat from the target material in the solar panel.

[0142] The outer structure is followed by the layers already described above in relation to FIG. 1; an upper glass layer 218, the target fluid in this case comprising water 220, a second glass layer 222, the working fluid in this case comprising a carbon nanofluid 224 and a third glass layer 226.

[0143] Advantageously, a further air gap 228 is arranged below the third glass layer 226 to provide insulation and reduce heat loss from the lower surface of the panel. The final layers comprise a reflective layer 230 and an insulating layer, as in the embodiment of FIG. 1, but these are provided as two separate layers in this embodiment.

[0144] It is desirable to make each layer in the panels described in FIGS. 1 and 2 as thin and light as possible while still enabling it to maintain its functionality. Typically, each glass layer will be around 1 mm thick and the layers of the target and working fluids will be 0.5 mm-1 mm in thickness. Other layers may be much thinner, however; in particular, it is not necessary to make the thin film layer thicker than around 0.01 mm and the reflective layer may simply comprise a thin layer of foil that is also around 0.01 mm in thickness, or simply a coating painted onto an adjacent glass or insulating layer. The total thickness of a panel such as that illustrated in FIG. 2 should be less than 20 mm, preferably less than 15 mm, preferably around 10 mm in thickness.

[0145] Reducing the thickness of the panel reduces its cost since it reduces the amount of material required to manufacture the panel. It also results in a lighter-weight panel, making the panel easier and cheaper to transport, install and use.

[0146] It is further noted that none of the solid layers in the panels described above with reference to FIGS. 1 and 2 need to be optically clear and, in fact, providing diffuse optical layers can reduce the loss of unabsorbed electromagnetic energy from the panel, since it is more difficult for the energy to reflect back out of the panel. In addition, there are many appropriate materials from which these layers could be made, for example light-transmissive plastics or glass. As has been emphasised above, transmission in the infrared, visible, and/or ultraviolet parts of the electromagnetic spectrum is preferable, as these regions collectively account for the vast majority of incident solar radiation at the earth's surface.

[0147] While the panels may be made in any shape, a particularly advantageous shape for the panels is based on triangular shapes. For example, FIG. 3 shows a solar panel 302 having a substantially triangular shape. In fact, the shape is that of an irregular hexagon, or a truncated triangle, that is a triangle with the corners removed. The shape of the panel is such that it has three major edges 306, and three minor edges 308. The major edges 306 are arranged so that they lie along (that is, they are aligned with) the sides of a triangle 304, shown schematically by dashed lines in this figure. It is important to note that the triangle 304 is not part of the solar panel, but is shown solely for illustrative purposes, to highlight the triangular basis for the shape of the solar panel. Of course, it is possible that the major edges 306 of the solar panel extend all the way to the corners of the triangle 304, so that the panel 302 is itself triangular.

[0148] The panel shown in FIG. 3 is based on an equilateral triangle, but may be based instead on other triangles, depending on the desired application.

[0149] Turning now to FIGS. 4A and 4B, a plurality of solar panels 402, 403 are shown in a network 400. In FIG. 4A, a central panel 402 is surrounded by adjacent panels. Here, the panels are triangular in shape. In this figure, the panels 402, 403 can have inlets and outlets for exchanging target fluid with adjacent panels which are situated in the corners of the triangles. This means that there are regions 407, where many of the panels may couple to one another to exchange fluid. In this example, each region 407 has 6 panels linking to it. This means that the central panel 402 is able to link to 12 neighbours in total to exchange target fluid. This high degree of connectivity means that the resulting network is very flexible and can conform to convex, concave or circular shapes.

[0150] In FIG. 4B, the panels 402 each have the truncated triangle (or irregular hexagonal) shape. In addition, the network 400 is an irregular shape. It will be apparent that the overall shape of the network can be adapted to fit a wide range of shapes and sizes, by simply adding more panels where needed. In addition, the panels can be connected to one another so that the distance and/or angle between the panels can be adjusted. This allows the network to conform to the underlying geometry, even when it is not flat. It will further be appreciated that not all of the panels may be interconnected along their edges so that a larger gap can be left between some adjacent panels if this is necessary to conform to the underlying geometry.

[0151] At the locations where 6 panels 402 meet, there is a gap, due to the truncation of the triangular shape. This allows more space for a fluid interchange hub 407 that was possible using regular triangular shaped panels. In this example, the minor edges 408 include inlets and outlets for the panels, so that the panels can each exchange target fluid with adjacent panels, in any flow direction.

[0152] The flow of the target fluid around the network 400 can be controlled for a variety of reasons. Most importantly, there may be situations when a particular panel is significantly cooler than the others, perhaps because the sun has moved in the sky and that panel is now in shade. If the target fluid flows to that panel, then the overall heating power of the network will be reduced, and the fluid output at the end will be at a lower temperature. Therefore, it can be advantageous to divert the flow of the target fluid around the network by selectively closing inlet valves to particular panels so that cooler panels are bypassed.

[0153] Turning now to FIGS. 5A and 5B, some internal structure of the solar panels 502 is shown. In particular, FIG. 5A shows solar panel 502 with three internal walls 510, shaped to direct the flow of target fluid towards the centre of the panel 502. Once again, the inlets and outlets for this panel 502 are located in the minor edges 508 of the panel. A flow path 512 shows how fluid entering at the top left corner progresses through the panel. Instead of being able to travel along one major edge of the panel, and exit as soon as it reaches another minor edge, the fluid must flow almost the entire length of a major edge before making it to the centre of the panel. In order to exit the panel, the fluid must travel almost two further major edge's distance.

[0154] Considering now FIG. 5B, in which a similar internal structure is shown. However, in this case, there are six internal walls, 510a and 510b. In effect, this is the same design as that shown in FIG. 5A, but with a smaller version of the same design nested within the original one. It is clear that the path length 512 for the fluid to arrive at the centre of the panel in this figure is even longer than the one in FIG. 5A. Once at the centre, the fluid must traverse the same lengthy path in reverse, in order to exit the panel. It will be apparent to the skilled person that this process of nesting similar, but smaller versions of the internal structure of FIG. 5A can be repeated as often as desired, for example, forming a spiral structure.

[0155] It will be apparent to the skilled person that there are many different designs which could be used to increase the path length of the fluid as it travels through a panel. For example, while the examples in FIGS. 5A and 5B each use straight walls, curved walls could be used as well or instead.

[0156] As the incident solar radiation transitions from air into each of the layers of the solar panel system described herein, around 20% of the energy is lost through reflection. In order to reduce the loss of energy at each air/solid interface, a matching layer can be provided. The matching layer has a profile such that the impedance of the matching layer to electromagnetic radiation varies through the layer from a value close to the impedance of air to close to the impedance of the solid into which the radiation is passing. The matching layer can be attached to or formed on the surface of each solid layer, where the solid layer interfaces with air such that the matching layer enables the electromagnetic radiation to transition into and out of each layer while minimizing the loss of energy due to reflection and sudden changes in refractive index.

[0157] As a specific example, the outer or upper surface of the top glass layer of the solar panel is preferably coated with a matching layer to smooth the transition of electromagnetic radiation from the air into the glass layer. A further matching layer may be formed on the lower exit side of the glass layer to smooth the transition of the electromagnetic radiation back into the first insulating air gap.

[0158] The matching layer comprises a plurality of composites, strata or sub-layers, each of which has a gradually increasing impedance to the passage of electromagnetic radiation. A matching layer according to embodiments described herein is illustrated schematically in FIG. 9. The first composite 900 encountered by the incident radiation 906 has an impedance close to that of air. The final composite 904 prior to the incident radiation reaching the glass layer 908, has an impedance similar to that of glass. Intervening composites, and in the embodiment illustrated in FIG. 9 this is illustrated as a single intervening layer 902, have an impedance between that of air and glass.

[0159] Preferred embodiments for use with the solar panel system described herein include at least 2, preferably at least 3 composites or sub-layers. However, for other embodiments in which the transmission of a very high proportion of incident energy is important, a larger number of layers may be used. An impedance matching layer comprising 10, 20, 50 layers or more may be manufactured in accordance with the methods described herein. The difference in impedance between each layer in such an embodiment is very small such that the incident radiation would not perceive boundaries for reflection or refraction as it passed through each composite of the matching layer.

[0160] The composition of each of the composite layers is illustrated schematically in FIG. 8. In general, each composite is formed of a base carrier material 800 with an impedance similar to that of the material into which the electromagnetic energy is passing. In the solar panel embodiment, a suitable base carrier material is liquid silicone, which is transparent, inexpensive and easy to work with.

[0161] The base carrier material is divided into portions and each portion of the base carrier material is doped or mixed with different proportions of a dopant material 802 that has an impedance significantly lower than that of the carrier material. For the solar panel embodiment described herein, an aerogel based on carbon or silica would be a suitable dopant material. Use of an aerogel provides the added advantages that these materials are highly thermally insulating and the hydrophobic properties of aerogel also impart self-cleaning properties to the layer.

[0162] The aerogel is provided, or manufactured, in powdered form, which can be mixed directly into the carrier material, for example in a high shear mixing system such as that described herein.

[0163] The addition of aerogel to the liquid silicone reduces the impedance of the carrier material portions or composites. Each composite within the matching layer is doped in varying and increasing proportions with the aerogel to decrease the impedance of the carrier material until the impedance of the final composite is close to that of the air from which the radiation is received. In contrast, the impedance of the composite close to the solid, such as glass, into which the electromagnetic radiation is passing, is close to that of the solid layer.

[0164] The impedance of the composite positioned next to the solid surface (for example the glass layer) is within 30%, preferably within 20%, of the value of the impedance of the solid surface itself. The impedance of the composite positioned next to the air is within 30%, preferably within 20%, of the value of the impedance of the air.

[0165] In a particular embodiment, a silicone rubber is mixed with a silica-based aerogel in gradually increasing quantities to form composites for the matching layer. This results in an optically clear material with a hydrophobic surface and with a refractive index close to 1.

[0166] In addition, the silicone rubber and the silica aerogel are a particularly suitable combination, as the silicone rubber optically grabs the silica aerogel. That is, the boundary between these materials is not sharp, but behaves as if it were a more gradual boundary. This causes the silica aerogel to act more like a dopant, and less like a region of impurity which has a different impedance.

[0167] Once composites with different impedance values have each been mixed, each layer is rolled, or calendered into sheets or strips. The layers of varying impedance are then stacked on top of each other in decreasing order of impedance. A further calendering process is then used to compress the layers together to form the complete matching layer.

[0168] Apparatus for calendering is illustrated schematically in FIG. 10.The composite 1000 is fed into the calendering apparatus, which comprises two pairs of opposing rollers 1002a, 1002b, each roller in the pair rotating in opposite directions to cause the composite 1000 to pass between the rollers and to be extruded or rolled out into a flat layer or stratum. As will be appreciated by the skilled person, FIG. 10 is a schematic diagram of one embodiment of the calendering apparatus and many different types of calendering apparatus would be suitable for converting the composites into layers. In particular, 3 or more sets of rollers may be provided, each set of rollers having a smaller inter-roller spacing than the preceding set to calender the composites into thinner layers.

[0169] It will be appreciated that other types of calendering or extrusion equipment may also be used to form layers from the doped composites.

[0170] While two composites of different impedances may be sufficient in the matching layer to increase significantly the proportion of electromagnetic energy passing into the solar panel system, the matching layer preferably comprises at least 3 composites. In an exemplary embodiment, the impedance of the first composite in the matching layer is around 120% of the impedance of air, the impedance of the third layer is around 80% of that of the glass layer into which the radiation is passing, and the impedance of the second, middle layer lies roughly mid-way between the impedance of the first and third layers.

[0171] Once multiple composites with different doping levels have been formed and passed through a first calendering process, the impedance matching layer itself is formed by stacking the calendered composites in decreasing order of impedance and passing the stacked layers through another calendering apparatus.

[0172] Each composite may be formed to have a thickness which is equal to an odd multiple of a quarter of the wavelength of light in a part of the electromagnetic spectrum in which the solar energy received at the earth's surface is a maximum. This can help to transmit more of the energy at that wavelength through the layer. In practice, forming a layer to exactly this dimension is difficult, and more commonly a range of desired wavelengths is chosen, so that when the layer is produced it will have a thickness which is optimal for transmission of a wavelength within that range, even though a specific wavelength in that range would be hard to select.

[0173] Once formed, the impedance matching layer comprises a thin, substantially transparent film which can be covered in a pressure-sensitive adhesive for attaching to the surfaces of the solar panel layers where there is an interface from solid into fluid, including the interfaces between the glass/plastic and air layers and the interfaces between the glass/plastic and target/working fluid layers. Suitable adhesives include thin layers of acrylic or silicone adhesives.

[0174] In an alternative embodiment, however, the matching layer can be formed integrally with the solid layer when the solid layer is manufactured. This enables the matching layer to be formed and secured to the solid layer without the need for an intervening adhesive layer, which can introduce discontinuities in the impedance encountered by the incident radiation. Such integrally-formed embodiments may be particularly useful where the materials need to be optically-clear, such as those described in more detail below.

[0175] It will be appreciated by the skilled person that the matching layer described above has many applications beyond forming a matching layer for the transition of electromagnetic energy into and out of the layers of a solar panel. In particular, an optical impedance matching layer can be used to increase the transmissivity of any material designed to enable the transmission of electromagnetic radiation. In particular, the addition of an impedance matching layer to the inbound, and preferably outbound, surfaces of glass that forms a window or windscreen can increase the transmission of energy through the glass, which reduces reflections from the glass, hence increasing visibility through the glass itself. In particular embodiments, matching layers such as those described herein may be attached to windscreens of cars or aeroplanes. Similar matching layers may be applied as coatings for spectacles, sunglasses, visors or goggles. Matching layers such as those described herein can also be used to reduce reflections from glass surfaces, such as screens of electronic or computer equipment.

[0176] Another application of the impedance matching layer is in optical, astronomical, medical imaging and photographic equipment, where it may be applied to imaging equipment such as lenses, microscopes, telescopes, mirrors and light sources such as lasers.

[0177] In a further embodiment, a matching layer such as that described above can be used to create one way glass, which has a high transmissivity in one direction, but which presents a significant difference in optical impedance to light incident from the other direction, significantly reducing the transmissivity of light in that direction.

[0178] While the solar thermal system described above may be implemented with a number of different types of working fluids and target fluids, in a particular embodiment the working fluid advantageously comprises a nanofluid, that is a fluid in which are suspended nanoparticles. This embodiment will be described in more detail below.

[0179] The nanoparticles used in the present embodiment comprise carbon nanoparticles. These can be manufactured relatively inexpensively and possess useful properties with regard to absorption of light at visible wavelengths. In particular, an unmodified and unfiltered sample of carbon nanoparticles appears black as it contains a wide range of sizes of carbon nanoparticles, and in particular different diameters of carbon nanotubes, which each preferentially absorb different wavelengths of light across the visible spectrum.

[0180] Nanoparticle of Nickel Chromium Oxide, Nickel Oxide or Nickel Chromium can also be used in embodiments of the present system, either in place of the carbon nanoparticles or preferably in a mixture together with a larger proportion of the carbon nanoparticles. Alloys of nickel such as those listed are particularly useful in the working fluid of the present system since they have a low emissivity and readily absorb the incident radiation.

[0181] Use of nanoparticles within a fluid significantly increases the surface area within the fluid that is available to absorb the sun's energy. Increasing or maximising the absorbent surface area increases the capacity of the fluid to absorb the sun's energy. 1 m.sup.2 of black plastic, which is traditionally used for absorbing the sun's energy in a thermal solar panel, has an absorbent surface area of 1 m.sup.2. In contrast, 1 cm.sup.3 of nanofluid has a surface area of 1300 m.sup.3.

[0182] Hence, the nanoparticles within the fluid create a lot of surface area over which the sun's energy can be absorbed. In fact, each nanoparticle has the capacity to absorb so much energy that the absorbent capacity of a thin layer of the nanofluid is greater than the amount of radiation that is typically incident from the sun in a flat panel solar thermal system and the proportion of incident radiation absorbed is high. It will therefore be appreciated by the skilled person that the nanofluid described herein may also advantageously be used in a concentrated solar thermal system.

[0183] The nanofluid comprises a plurality of nanoparticles formed from carbon. It has been found that, in order to operate as an effective nanofluid in embodiments described herein, it is not necessary to use nanoparticles that are particularly uniform in shape or size. However, certain types of nanoparticles have been found to be effective for the systems described herein and these are highlighted and discussed in more detail below. Typically, however, a mixture of carbon particles of dimensions around 30-300 nm formed as tubes, spheres, fullerenes, plates or as irregular shapes can be used in the production of the working fluid described herein.

[0184] Such carbon nanoparticles can be purchased from a commercial source or can be formed using processes such as chemical vapour deposition of acetylene to form graphene, from which the carbon nanoparticles can be formed. In a particular process, a surfactant is mixed with the graphene using a mechanical mixing process, such as a pestle and mortar. As the surfactant coats the graphene, it functionalises the graphene by causing the graphene sheets to break into layers and to wrap into carbon nanotubes and nanoparticles, such as fullerene particles. The carbon nanotubes can either be closed tube structures, or may simply comprise a graphene layer rolled into a tube shape. The precise shapes, dimensions and degree of uniformity of the nanoparticles is not considered to be important in embodiments of the present system. In fact, providing a variety of shapes and dimensions of nanoparticles can assist in the absorption of electromagnetic energy across a broad spectrum. Further details of the surfactant are set out below.

[0185] To form the nanofluid to use as the working fluid within the solar thermal application described above, the carbon nanoparticles are mixed with a fluid. The nanoparticles should be mixed evenly throughout the fluid and dispersed within the fluid to maximise the surface area of the particles available for absorption of radiation.

[0186] As described in more detail below, the nanoparticles are mixed to form a colloid of nanoparticles, forming the dispersion phase, suspended in a fluid dispersion medium.

[0187] The nanoparticles can be mixed within the fluid using chemical additives to reduce their surface tension and surface energy and to overcome Van der Waals forces and intermolecular forces from hydrogen bonds sufficiently to enable dispersion within the fluid.

[0188] In particular, a surfactant has been found to be helpful to neutralise the hydrophobic properties of the nanoparticles and enable the particles to be dispersed more evenly throughout the fluid. Rather than adding additional surfactants at this processing stage, it has been found that the surfactant already present with the nanoparticles that remains from the functionalisation of the particles is sufficient to assist in the dispersion of the nanoparticles throughout the fluid when the particles are dispersed using the methods described below.

[0189] Suitable surfactants for use in functionalising and dispersing the nanoparticles within the fluid include simple soaps, particularly for use in water-based fluids, or lecithin, particularly with oil-based fluids.

[0190] It is also noted that, advantageously, benzene rings often remain attached to the carbon nanoparticles in the carbon residue, which increase the solubility of the carbon nanoparticles in oil-based dispersion fluids. The existence of such benzene rings reduces the need to add other agents to increase the solubility of the nanoparticles.

[0191] Therefore, while further chemicals could be added to actively promote dispersion within the fluid, surprisingly, it has been found that good dispersion of the nanoparticles within the fluid can be achieved using mechanical dispersion methods without the need for further chemical additives. An approach to this problem in which the number of chemicals used is minimised can improve the environmental impact of the nanofluid and can simplify and reduce costs in manufacturing.

[0192] Mechanical dispersion of the nanoparticles within the fluid can be achieved in particular using a very high shear mixing system or high shear homogeniser. There are many embodiments of such systems, but they typically include at least one rotating blade and at least one fixed stator for generating high shear forces within the fluid by causing adjacent portions of the fluid to move at different rates relative to each other. A simplified diagram of a basic high shear mixer is illustrated schematically in FIG. 6 to illustrate the principles of such a mixing method. In such a mixer, a shaped rotating blade or rotator 610 draws fluid upwards towards it and causes high rotational motion of the fluid that is nearest to it, relative to the body of the fluid as a whole. The fluid is then propelled outwards from the rotator through a static element, called a stator 612, which comprises a number of orifices 614. As the fluid passes through the orifices 614, elements within the fluid, in this case groups of nanoparticles, are broken up into smaller aggregations, or individual elements, hence increasing the dispersion of the particles through the fluid.

[0193] A more effective high shear mixer can be implemented using two sets of intermeshed concentric blades. Each blade is circular and comprises a number of teeth. One of the sets of blades is caused to rotate at high speed within the other set such that a rotating blade (rotator) is next to a static blade (stator), followed by further pairs of rotating and static blades to the edge of the mixer. Fluid is input to the centre of the rotating blades and drawn outwards through the rotating and static blades. As it passes through the mixer, particles within the fluid are disaggregated and dispersed.

[0194] The skilled person will appreciate that, while elements of a high shear mixing system have been described above, a number of different embodiments of such systems may be implemented to adequately disperse the particles within the fluid.

[0195] Very high shear mixing systems can provide an even dispersion of nanoparticles within the fluid and the particles are so dispersed and disaggregated that fluids mixed in this way have settlement times of many thousands, or even millions, of years.

[0196] Another high shear mixing system is shown in FIGS. 7A and 7B. Starting with FIG. 7A, a plan view of the mixing system 700 is shown. This comprises a central axis 706 to which means 704 for mounting a blade is rotatably mounted. Beneath the means 704 for mounting a blade is a surface 702. The surface is surrounded by a wall 710. In operation, fluid to be mixed is placed on the surface 702. The fluid is prevented from leaving the surface by the wall 710. The means for mounting a blade is then rotated around the central axis 706, as shown by the arrow 708.

[0197] In order to better show the effect of the movement of the means for mounting a blade while the fluid is on the surface 702, the arrangement 700 is shown from the side in FIG. 7B. Here a blade 705 is shown mounted on the means 704 for mounting a blade. The means 704 for mounting a blade is shown held a predetermined distance from the surface 702. The blade 705 contacts the surface, and bends, exerting a force on the surface 702. When the blade moves in the direction indicated by the arrow 708, it compresses the fluid 712a. Most of fluid 712a is pushed ahead of the blade, but a small proportion of the fluid 712a is squeezed under the blade 705, and forms a thin layer 712b behind the blade. As the fluid is squeezed under the blade, it is mixed in a high shear manner.

[0198] The smaller the distance between the means 704 for mounting the blade and the surface 702, the greater the force exerted by the blade (as it bends more), which causes higher shear mixing, while making the layer 712b behind the blade thinner. In an alternative arrangement, the blade does not contact the surface, and is instead held a short distance from the surface. The fluid is then mixed as it passes through the small gap.

[0199] Since the blade is rotatably mounted on the central axis, multiple rotations cause the fluid to be repeatedly mixed in this way. Alternatively, the system could comprise a linear movement of the blade relative to the surface, but it is preferable to have a rotational motion, since this allows repeated mixing in a simple manner.

[0200] The blade can be made of any material, depending on the elasticity requirements, and the shear force required. Similarly, the surface can be made of any material so long as it is hard and relatively smooth. There is no need for the surface to be flat or planar, so long as the blade conforms to the profile of the surface as it moves across it. In the specific application of making a working fluid for the solar panels described herein, it has been found that small metal particles bind to the carbon nanoparticles, reducing their ability to absorb radiation. Therefore for this use, non-metal blades and surfaces are recommended in such an embodiment. For example plastic or rubber blades and glass surfaces are suitable for this use.

[0201] Once the nanoparticles have been dispersed within the fluid dispersion medium, it is important that they are retained in suspension within the fluid and resist sedimentation and aggregation or flocculation. Viscous properties of the fluid within which the particles are dispersed can be used to slow these processes and maintain the particles in solution.

[0202] Fluids having a viscosity of greater than 500 cP (where 1 centipoise, 1 cP=1 mPa.Math.s), preferably greater than 800 cP, are advantageous.

[0203] However, it is also beneficial if the nanoparticles are able to flow within the fluid dispersion medium to some extent, since this flow or mixing of nanoparticles can help in enabling the particles to remain suspended in the fluid. Therefore, the fluid should have a viscosity of less than 2500 cP, preferably less than 2000 cP, further preferably less than 1200 cP.

[0204] The viscosities set out above relate to the viscosities of the dispersion medium when the working fluid is cool, at around 20 C. The viscosity of an oil-based working fluid will decrease as its temperature rises. However, it is beneficial if the viscosity of the fluid at higher temperatures, when the fluid is exposed to incoming radiation, is lower than the viscosity at cool temperatures. Having a lower viscosity at the higher operating temperatures can enable the nanoparticles to move more freely as they absorb incident photons. This movement of the nanoparticles can help to distribute the heat energy more evenly throughout the fluid and can also help to stir the nanoparticles within the fluid, reducing sedimentation and keeping the particles distributed throughout the fluid. For this reason, the viscosity of the working fluid may fall to 500 cP or lower when the working fluid is absorbing incident radiation and is at its operating temperature.

[0205] Maintaining a relatively high fluid viscosity is more important, however, at times when there is no incident radiation to stir the nanoparticles within the fluid and sedimentation of the particles is more likely to occur. Therefore, discussion in the present application focusses on the viscosity of the fluid at cool temperatures. As discussed in more detail below, the addition of a wax to the dispersion medium may assist in both raising the viscosity of the dispersion medium and increasing the variation in viscosity between the cool fluid and the working fluid in operation.

[0206] Suitable fluids within which the nanoparticles are suspended can be water-based or oil-based. The lower specific heat capacity of oil compared to that of water means that the increase in temperature for an oil-based fluid will be greater than that for a water-based fluid for the same incident radiation. This enables the working fluid to heat up and cool down more quickly, reducing the amount of heat stored in the working fluid and increasing the speed with which the target fluid can start to be heated once the panel is exposed to incident radiation. Oils also tend to have a lower freezing point and a higher boiling point than water and many are outside the temperature ranges that a solar thermal panel would expect to encounter under normal environmental conditions. Oils also tend to have higher viscosity than water, which is helpful in retaining the nanoparticles in suspension. There are therefore a number of advantages to using an oil-based fluid. However, water is readily available, non-polluting and can be adapted as described herein to carry the nanoparticles in the working fluid as described herein.

[0207] Suitable oils may include hydrocarbon oils, but natural or organic oils are preferred including castor oil, soybean oil, coconut oil or palm oil. In particular, castor bean oil, or castor oil, has a viscosity of around 1000 cP and would be a suitable dispersion medium to use even without the addition of additives.

[0208] Whether the working fluid is oil or water based, it may be necessary to increase its viscosity to bring it within the range described above. A number of additives can be mixed with the base fluid in order to increase its viscosity to above 500 cP, preferably to around 1000 cP. High viscosity fluids that may be used include silicones or heavier oils; however, such materials are less environmentally desirable and may increase problems for the end of life disposal of the solar panels. Therefore, an organic material is preferred and suitable materials include agar, gum aribica, extracted natural products such as barnacle extract. Another material that may be used to increase the viscosity of the carrier fluid is lecithin, in particular soy lecithin or a synthetic version.

[0209] In a particular embodiment, a wax is mixed with the dispersion medium of the working fluid. The wax can be used to increase the viscosity of the dispersion medium, where this is necessary. However, use of a wax within the dispersion medium is also helpful to increase the viscosity differential of the fluid between a cool non-operating temperature and a hot operating temperature. That is, the wax can be used to increase the viscosity of the fluid at cool temperatures and hence hold the nanoparticles more firmly in suspension when the solar panel is not receiving incoming radiation, for example at night. In preferred embodiments, the viscosity of the wax may increase to around 2000-2500 cP when the system is cool, at around 20 C.

[0210] Suitable waxes include naturally-occurring waxes such as beeswax and carnauba wax or a hydrocarbon wax such as paraffin wax. Such waxes mix well with the oil-based dispersion media described above and operate to change their viscosity in an even and predictable way. Carnauba wax is particularly suitable for use in systems described herein since it is a high temperature wax with a melting point of around 90 C. Therefore, the viscosity of the dispersion medium as a whole decreases only at a higher temperature and the nanoparticles are held more securely in suspension until higher energy levels are reached within the dispersion medium.

[0211] The addition of waxes as described above also enables the manufacturer of the working fluid to fine-tune the viscosity of the working fluid for a particular temperature-range that a particular solar panel is expected to encounter. For example, a solar panel for deployment at higher latitudes or altitudes is likely to have a cooler resting temperature than a solar panel deployed at lower latitudes. Therefore, the amount of wax in the dispersion medium can be reduced for lower temperature panels.

[0212] While a high viscosity is desirable to reduce sedimentation of the nanoparticles, the viscosity should not be so high as to preclude all convective flow within the working fluid. As noted above, convective flow is helpful in mixing the nanoparticles through the fluid in use and in maintaining an even heat distribution within the working fluid.

[0213] A further property of a suitable fluid for carrying the nanoparticles is that it should have high optical coupling with the nanoparticles themselves. That is, the incident electromagnetic energy should be able to pass from the fluid into the nanoparticles without encountering a significant interface between the two media. Matching of the optical impedance of the two materials, preferably to within 20% of the impedance value, is one factor that increases the optical coupling between the two elements of the nanofluid. A high optical coupling can enable the incident light, or photons, to be reflected around within the carrier fluid until they are absorbed by a nanoparticle, such that absorption by nanoparticles is enabled over the 360 surrounding the nanoparticle.

[0214] It has been found that it is advantageous to use nanoparticles within the nanofluid that contain a high proportion of nanotubes, for example more than 50% nanotubes. It has been found by the present inventor that the shape of the nanotubes and the way that the electrons are able to move within the nanotubes increases the ability of this particular shape of nanoparticle to absorb electromagnetic radiation. In particular, the nanotubes can act as antennae tuned to, and resonating with, the incident electromagnetic radiation. These resonant properties of the nanotubes increase the likelihood of a photon interacting with and being absorbed by a nanotube as it passes through the working fluid.

[0215] Further additives to the working fluid can also increase its capability for the absorption of incident energy. In particular, the addition of small quantities of a high transmissivity material has been found to significantly increase the ability of the nanofluid to absorb incident radiation.

[0216] The addition of a high transmissivity material tunes the circuit formed by the nanoparticles such that the resonant frequency of the circuit matches the resonant frequency of the incoming electromagnetic signal. The addition of particular additives in particular proportions can tune the nanoparticles to absorb incoming radiation more strongly at particular frequencies, for example at visible and ultraviolet frequencies. It can be helpful to tune the nanoparticles to absorb at these frequencies since the working fluid (water) is more highly absorbent in other bands of the spectrum, for example around the infrared frequencies.

[0217] High transmissivity materials encompass any material with a transmissivity greater than at least about 70% and suitable materials include Calcium Fluoride, CaF.sub.2 (also known as fluorspar), Silicon Dioxide in a crystalline form, SiO.sub.2 (also known as silica), Indium Tin Oxide, ITO, Aluminium-doped Tin Oxide, AZO, and Indium-doped Cadmium Oxide.

[0218] Crystalline forms of SiO.sub.2 that are particularly useful as an additive in the present system are those with higher densities, which have corresponding high refractive indices. In particular, crystalline forms with densities of 2.5-3 g/cm.sup.3 or greater have been found to be most effective. These forms have a refractive index of around 1.5 to 1.55.

[0219] Particularly suitable materials, including CaF.sub.2 and SiO.sub.2, have both high transmissivity and high refractive indices.

[0220] The additive is required only in small quantities, in particular less than 0.1%, preferably around 0.01% of the amount of nanoparticles present in the fluid, and should be substantially homogeneously dispersed throughout the carrier fluid.

[0221] The additive alters the optical properties of the nanofluid, in particular due to the different mass of its particles and alters the optical connection between the carrier fluid and the particles, increasing the ability of the nanoparticles to absorb the incident radiation. In particular, materials with high transmissivity and high refractive index may help to increase the effective path length of the light through the nanofluid working fluid layer, hence increasing the likelihood of a particular photon of light being absorbed by the carbon nanoparticles.

[0222] Incident solar radiation is absorbed into the target fluid, either directly or via absorption and retransmission by the working fluid, as described above. In view of the increased absorption capacity of the carbon-nanoparticle-carrying working fluid, the fluid heats up quickly, and a significant difference in temperature between the working fluid and the target fluid is beneficial in facilitating heat transfer into the target fluid. However, the system should not be designed simply to maximise the temperature difference between the fluids. In fact, it is advantageous to transfer heat from the working fluid into the target fluid as quickly as possible as any increase in temperature of the working fluid will decrease its ability to absorb further solar radiation (since the difference in temperature between the sun and the working fluid will decrease). The working fluid reaches temperatures of greater than 100 C., typically around 160-190 C. Such temperatures allow heating of the target fluid to above 150 C., preferably to around 160 C.

[0223] As will be appreciated by the skilled person, heat loss from the panel can become significant at these temperatures, since the insulating properties of the panel are necessarily limited. Further, degradation of components of the panel will also start to occur particularly if elements of the panel are held at very high temperatures for an extended period. Therefore, it is important to enable efficient transfer of the heat energy to be transferred into the target fluid.

[0224] As described above, the working fluid absorbs visible and infrared energy and converts both into infrared energy (converting the wavelength from 300-1000 nm to 5000 nm-10,000 nm). Since water is opaque to infrared energy, the target fluid comprising water readily absorbs the infrared energy emitted by the working fluid. Hence absorption of the energy by the nanoparticles in the working fluid and retransmission of this energy as radiant infrared energy enables efficient collection of the energy in the target fluid.

[0225] It is noted that the energy is largely transferred into the target fluid using radiant transfer from the surface of one fluid to the other, rather than by thermal conductivity (although a small amount of conductive transfer of the energy would be expected). Placing an extended surface of the working fluid close to that of the target fluid, as described in the solar panels illustrated in FIGS. 1 and 2 enables efficient radiant transfer of the infrared energy from one fluid to the other.

[0226] FIGS. 11A to 11C show a diaphragm 1100. In particular, FIG. 11A shows a diaphragm in two states, a normal state 1100 and an inverted state 1101. In the normal state, the diaphragm is below a particular temperature, called the inversion temperature. Once the diaphragm has been heated to the inversion temperature, thermal expansion stresses build up and cause the diaphragm to invert, resulting in the inverted state 1101. This process is entirely reversible; when the temperature drops below the inversion temperature again, the diaphragm returns to the normal state.

[0227] This may find use in valves to control flow into certain panels in a network. For example, in the description of FIGS. 4A and 4B, the flow path may be selected to bypass cooler panels. Therefore, by placing a temperature sensitive valve at the inlet to such panels, the network can have an inherent bypassing functionality built into it.

[0228] This is demonstrated schematically in FIGS. 11B and 11C. It will be appreciated that the size of the pipe and the curvature of the diaphragm have been exaggerated in FIGS. 11B and 11C to enable the principles of operation to be more easily demonstrated. In FIG. 11B, a pipe 1102 is shown in which a diaphragm is mounted. In this example, the temperature is higher than the inversion temperature, and the diaphragm is in the inverted state 1101, which leaves the pipe 1102 is clear, and fluid is able to flow. In this case, the flow path which is open is an inlet to a solar panel. The diaphragm is mounted in the pipe 1102 by mounting means 1104 which do not affect the diaphragm's ability to invert.

[0229] In FIG. 11C, the temperature has dropped below the inversion temperature, and the diaphragm has returned to the normal state 1100. In this example, this means that it extends into the pipe 1102 and blocks it, thereby preventing fluid flow into the solar panel. When the solar panel heats up again, the hot target fluid will cause the diaphragm to heat up, and eventually invert and open the pipe to enable fluid to flow. As long as the target fluid flowing through the pipe stays hotter than the inversion temperature, the diaphragm remains in the inverted state, and fluid can flow.

[0230] As the skilled person will appreciate, other designs of thermal valve are possible. For example, a valve could be constructed which uses thermal expansion to exert a linear motion to press a pipe closed. While the diaphragms in FIGS. 11A to 11C are shown as having a constant thickness, the diaphragms may have a thickness which varies. Changing the geometry of the diaphragm is one way of changing the temperature at which the diaphragm inverts. Suitable materials for making the diaphragm from are plastics materials, for example polyvinylidene fluoride (PVDF) or polypropylene. Diaphragms can even be constructed from two or more different materials, and the differing thermal responses of the two materials can be used to tailor the inversion temperature. Plastics are suitable materials as they have good resistance to fatigue.

[0231] The inversion temperature is a parameter of the network that can be selected, depending on the intended use. For example, 40 C. is a comfortable temperature for washing, 60 C. is a temperature at which water-borne pathogens start to die.

[0232] It is also possible to design diaphragms so that they have two inversion temperatures, an upper one and a lower one. This introduces a degree of hysteresis into the system, and helps prevent unstable oscillations of the diaphragm valves near to the inversion temperature. For example, the temperature at which the diaphragm changes from the normal to inverted state may be 10 C. higher than the temperature at which it changes from the inverted state to the normal state. In the 10 C. window between these two states, the diaphragm takes on the configuration it most recently had.

[0233] The panel and systems described herein may be used directly for drinking, cooking, heating or washing, or may be used for sterilising, pasteurising other fluids or items. For example, the system can be used as a domestic hot water heating system or a home heating or cooling system. Alternatively, one or more panels may be used in an industrial preheater for processes including distillation, desalination or drying of materials such as food crops.

[0234] In a particular embodiment, the system may have medical uses in the cleaning and sterilisation of surgical or medical equipment or in the preparation of medicines and vaccines.

[0235] The system may also be deployed for example in the pre-treatment of sewage or in the preheating of fluids for use in a power plant.