WO2013165249A1

WO2013165249A1 - Device comprising a transparent structure, solar collector or greenhouse provided with such device

Info

Publication number: WO2013165249A1
Application number: PCT/NL2013/050334
Authority: WO
Inventors: Ronald GRIESSEN; Marinus Johannes Slaman
Original assignee: Stichting Vu-Vumc
Priority date: 2012-05-04
Filing date: 2013-05-03
Publication date: 2013-11-07
Also published as: NL2008752C2

Abstract

The invention relates to a device for transmission of incoming radiation comprising a plate of a transparent material provided with a first flat side and a second side opposed to the first side, the second side being provided with a prismatic structure with certain magic apex angle which has a first and a second angle, wherein the apex angle of the prismatic structure is equal to a magic apex angle. The device can be applied advantageously in greenhouses and solar collectors to maintain a substantially constant transmission of incoming radiation from the sun during a long period of the day. To adjust the level of the limited transmission between a minimum transmission of 50% and the Fresnel transmission the prismatic structure can be provided with flat portions between the prisms.

Description

DEVICE COMPRISING A TRANSPARENT STRUCTURE, SOLAR COLLECTOR OR GREENHOUSE

PROVIDED WITH SUCH DEVICE

Field

The invention relates to a device for transmission of incoming radiation comprising a body of a transparent material. The invention further relates to a solar collector or a greenhouse provided with such device.

Background

The device can be used, for example, in greenhouses in tropical areas at or near the equator or in more moderate areas in the northern or southern hemisphere. For optimal growing conditions it is desirable to have an approximately constant irradiation level in the greenhouse during a relatively long period of the day, without exceeding a predetermined maximum level of irradiation. Too high irradiation levels could harm crop or flowers growing in the greenhouse. Presently, one of the solutions to avoid high irradiation levels is to spray chalk on the transparent roof of the greenhouse. A disadvantage of the chalk layer on the greenhouse is that the attenuation of the irradiation level is constant during the day and may become too low at low elevation angles of the sun in the morning and afternoon. This may slow down growth.

Furthermore, the chalk layer should be removed from the greenhouse when it is not required anymore. Another solution is to use moveable indoor screens. A

disadvantage of these moveable indoor screens is that they require mechanical activation and do not keep sun radiation and heat out of the greenhouse.

The device can also be used, for example, in solar collectors. A solar collector comprises a vessel provided with a transparent plate and an absorbing plate with a circumferential wall connecting the transparent plate and the absorbing plate. The vessel is provided with an inlet and an outlet for transport of a liquid for transferring heat from the solar collector to an end user. The transparent plate, the circumferential wall and the absorbing plate of the vessel can be made of plastic, which is less expensive than conventional used materials such as glass, aluminum and copper. For high elevation angles of the sun around noon, the temperature of these parts can become too high, so that the plastic can deteriorate. To avoid a high temperature at high elevation angles of the sun, the solar collector should be protected against sun light around noon by a mechanical light shutter, which requires maintenance.

Summary of the invention

It is an object of the invention to provide a device with a transparent structure that has a substantially constant transmission for a large range of angles of incoming radiation.

According to a first aspect of the invention this object is achieved by a device for transmission of incoming radiation comprising a body of a transparent material provided with a first flat side and a second side opposed to the first side, the second side being provided with a prismatic structure, wherein the apex angle Ψ of the prismatic structure is substantially equal to

Ψ = 180 - 2 arcsin

(1) wherein Ψ represents the apex angle in degrees, and n represents the index of refraction of the transparent material. The invention is based on the insight that in order to keep the transmitted solar radiation as constant as possible the prismatic structure should transmit about 50% of the incoming radiation around noon and gradually more before and after noon, whereas in the morning and afternoon at low elevation angles of the sun a maximum transmission should be required. The prismatic structure with an apex angle that equals the above defined angle in formula (1) is the so-called magic angle Ψ of the prism which provides for incidence angles infinitesimally close to zero a transmission of 50%. The relation for this apex angle can be found using geometrical optics and Snell's law. In order to have total reflection on one side of the prism the angle of incidence should satisfy k l < β (2) wherein a represents the angle of incidence with respect to the normal to the basis of the prism and β represents the critical angle of incidence given by β = arcsin

Furthermore, it has been found that the prism with the defined magic apex angle Ψ transmits the incoming radiation proportionally with the angle of incidence between 0 and β. This leads to a substantially constant transmission of incoming sunlight. The body of transparent material can comprise, for example, a plate or a foil which make it suitable to build for example greenhouses, wherein a roof structure is provided with the plates or the foil with the prismatic structure.

JP2010085905 discloses a light guide plate capable of suppressing a temperature rise in a vehicle due to solar light in the summer season and capable of taking light into the vehicle and increasing a temperature in the vehicle, by controlling an outgoing angle of incident light rays. The light guide plate includes a prism surface having a

configuration made by continuously juxtaposing trigonal unit prisms in the width direction of the unit prism on the lower surface of a transparent resin plate, wherein an apex angle of the unit prism constituting the prism surface is in the range between 60 and 120 °. That document does not disclose the specific values of the apex angle of the prismatic structure according to formula (1). Therefore, the transmission of the known light guide plate is varying substantially during the day depending of the angle of incidence.

In an embodiment the prismatic structure comprises a plurality of parallel prisms. In this arrangement the prisms of the structure are arranged parallel to each other. The size of the prisms is such that the laws of geometric optics hold. Practically, the prisms can have a height larger than approximately 5 μπι.

In a further embodiment the prismatic structure is provided with a flat portion between two adjacent prisms. This arrangement of a mixed structure made of prism and flat portions enable a higher transmission than the 50% of a pure prismatic structure for angle of incidence close to zero or between zero and the critical angle β. For certain applications a transmission of more than 50% can be required. By adjusting the width of the flat portion the transmission of such mixed prismatic structure can be adjusted between 50% and the maximum obtainable transmission, i.e. the Fresnel transmission.

In an embodiment the transmission of incoming radiation for an angle of incidence smaller than the critical angle β of the device, depends on a fraction of the flat portion, a transmission coefficient of the prismatic structure without the flat portion, and a Fresnel transmission coefficient of the transparent material. The optical transmission coefficient Tf of a device comprising the prismatic structure with an apex angle that equals the magic angle and prisms separated by the flat portions can be approximately given by

7>(α) = (1 - f T_f=0 (a) + / x (a) (4) wherein 7 ( ) represents the transmission coefficient for an angle of incidence a of incoming radiation;

/ represents the fraction of a flat portion and is equal to the ration of the width x of the flat portion and the distance d between adjacent prisms; and F( )F( ) represents a Fresnel transmission coefficient.

For example, a prism structure with a fraction /= 0 is equivalent to a simple prism structure with adjacent prisms.

In further embodiment the flat portion is provided with a coating to reflect at least partly the incoming radiation.

In an embodiment the index of refraction is a value in the range between 1.45 and 1.70 with a corresponding magic angle of substantially 93° and 1.75 with a corresponding magic apex angle of substantially 108°..T he index of refraction of materials in this range enables a prismatic structure which can be applied

advantageously in various applications such as, for example, greenhouses and solar collectors and can be obtained commercially from manufacturers.

This range of msgic apex angle is related to commercially available materials the can be applied in the device. For example, polymetamethylacrylaat (PMMA), polycarbonate, like Lexan or flint glass.

In an embodiment the device comprises a further transparent plate positioned at a predetermined distance opposite the first flat side or the second side for forming a gap between the body and the further transparent plate. This gap can be filled with air. This arrangement can improve the thermal isolation of the device, for example, in a greenhouse provided with such device, a higher temperature can be obtained. In a further embodiment the device is integrated in a channel plate. The device can be then manufactured as a single piece for example by extruding the body with the prismatic structures and the further flat plate. Partitions between the body and the further flat plate maintain the gap.

The invention further relates to a greenhouse provided with a device according to any one of the claims 1 to 11. The device according to the invention comprises the body of a transparent material. The body can be a plate or a foil. The plate or foil can be provided with a first flat side and a second side opposed to the first side with a prismatic structure, wherein the apex angle of the prismatic structure is equal to the defined magic angle Ψ. This arrangement prevents that irradiation in the greenhouse becomes too high during periods of the day wherein the sun is at high elevation angles and the device prevents damage to growing crops in the greenhouse.

The invention further relates to a solar collector comprising a device according to one of the claims 1 to 11. The solar collector may comprise a vessel provide with a front side and an absorbing back side and a circumferential wall connecting the front side with the back side. The front side can comprise the device for transmission of incoming radiation. The body can comprise a plate or foil of a transparent material provided with a first flat side and a second side opposed to the first side, provided with a prismatic structure, wherein the apex angle of the prismatic structure is substantially equal to the defined magic angle Ψ. Furthermore, the solar collector can be provided with an inlet and an outlet for transporting a fluid, for example water or oil, which can transfer heat from the solar collector to an end user or boiler. The device according to the invention comprising a body of a transparent material provided with a first flat side and a second side opposed to the first side with a prismatic structure wherein the apex angle of the prisms in the prismatic structure is substantially equal to the defined magic angle¹?, prevents that the temperature in the solar collector becomes too high during periods of the day wherein the sun is at high elevation angles.

Brief description of drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

Fig. 1 shows a diagram of irradiation versus angle of incidence on a horizontal plane at the equator; Fig. 2 shows typical light ray paths in a prism with a magic apex angle for three typical direction of incoming radiation;

Fig. 3 shows a diagram of a transmission coefficient, without Fresnel contribution, versus angle of incidence a for a device with a prismatic structure with a magic apex angle;

Fig. 4 show a diagram of the dependence on the index of refraction of the critical angle, the transmission at critical angle and the magic apex angle;

Fig. 5 shows a first embodiment of a prismatic structure;

Fig. 6 shows a diagram of a transmission coefficient, including Fresnel contribution, versus angle of incidence a for a device with a prismatic structure with a magic apex angle Ψ;

Fig. 7a shows a diagram of solar light transmission versus angle of incidence a for a conventional prismatic structure provided with prisms with an index of refraction of 1.59 and an apex angle of respectively 62, 90 and 118°.

Fig. 7 b shows a diagram of solar light transmission versus angle of incidence a for a prismatic structure provided with prisms with a magic apex angle;

Fig. 8 shows a schematic drawing of a second embodiment of a prismatic structure comprising a flat portion between adjacent prisms;

Fig. 9 shows a diagram showing the solar transmission versus angle of incidence for a prismatic structure provided prisms with a magic apex angle and flat portions between adjacent prisms;

Fig. 10 shows a diagram with a first graph representing the radiation transmitted by the device according to the invention and a second graph representing the radiation transmitted by a transparent plate with a chalk layer;

Fig. 11 shows a schematic drawing of a greenhouse provided with a device according to the invention;

Fig. 12 shows a schematic drawing of a solar collector provided with a device according to the invention; and

Fig. 13 shows an embodiment of the device comprising a flat plate with a prismatic structure and a further plate separated by a gap.

Detailed description In the figures the same reference numerals indicate like elements.

The invention relates to a device for transmission of incoming radiation comprising a body of transparent material. The body can be a plate. The plate can be provided with a first flat side and a second side opposed to the first side. The incoming radiation can be light coming from the sun. The angle of incidence of the incoming sun light varies with the elevation of the sun during the day. Instead of the plate a foil of transparent material can be used. The transparent material can be plastic, for example, polycarbonate like Lexan. The prisms of the device are directed for example North-South. The device can be used, for example, in a roof structure of a greenhouse. For optimal growing conditions of crop or flowers in the greenhouse the irradiation levels should be substantially constant during a long period of the day.

Fig. l shows a diagram with a curve 10 representing an irradiation level versus angle of incidence a on a horizontal plane at the equator. For certain crops or flowers the maximum irradiation levels should be below certain values. For example below 700 W/m². This value is represented by a second curve 12. In order to meet this condition the prism should transmit at least 50% at high elevation angles of the sun around noon and maximum transmission at low elevation angle early in the morning and late in the afternoon. This requires an angular dependent transmission which can be obtained via prisms with an apex angle Ψ and an index of refraction n.

Fig. 2 shows light ray paths in a prism 20 with an apex angle Ψ for three typical angles of incoming radiation a. Fig. 2a shows light rays 21 at an angle of incidence a = 10° on the prism. The prism 20 transmits a first portion 22 of the light rays 21 and reflects a second portion 23 of the light rays 21. Fig. 2b shows incoming light rays 21 at an angle a = 30° on the prism. The prism 20 transmits a first portion 24 of the light rays 21 and reflects a second portion 25 of the light rays 21, wherein the second portion 25 is larger than the second portion 23 described with relation to Fig. 2a. Fig. 2c shows incoming light rays 21 at an angle a = 60° on the prism. The prism 20 transmits all the incoming light rays as transmitted light rays 26.

Fig. 3 shows a schematic diagram of the transmission coefficient, neglecting Fresnel contribution, versus the angle of incidence a for a prism with the defined magic apex angle Ψ. In Fig.3 the transmission is 50% for an angle of incidence a close to zero indicated in point 31. The transmission of the prism increases proportionally with the angle of incidence a until the critical angle β is reached as is represented by the graph 32. For the critical angle β the transmission is defined and indicated as Τ_β. For angles of incidence larger than the critical angle β the transmission becomes close to 100%. The critical angle β, the magic apex angle Ψ and the transmission coefficient depend on the index of refraction n. The critical angle β is given by formula (3), the magic apex angle Ψ given by formula (1).

Fig. 4 shows a diagram of the dependence between the critical angle β, the transmission Ύ_β and the magic angle Ψ with the index of refraction n. A first graph 41 represents the relation between the critical angle and the index of refraction n. A second graph 42 represents the relation between the transmission at the critical angle β and the index of refraction n. A third graph 43 represents the relation between the defined magic apex angle Ψ and the index of refraction n. Fig.4 can be used to determine the magic angle of prisms in a prismatic structure made of a material with an index of refraction n. Once the magic angle is determined the plate with the prismatic structure can be manufactured with the determined apex angle. For example, by extruding the prismatic structures together with the plate material.

Fig. 5 shows an embodiment of a device comprising a prismatic structure with an index of refraction n= 1.59 and a magic apex angle of 102.06 °. The device may comprise a plate 50 of transparent material for example polycarbonate. The plate is provided with a first flat side 51. The flat side can be directed to the sun. The other side 52 of the plate is provided with a prismatic structure comprising multiple parallel prisms 53 positioned adjacent to each other. The width W oi the base plane of the prism can be for example 2 mm. The thickness / of the plate can be 3 mm. The size of the plate can be 100x60 cm².

Fig. 6 shows a diagram comprising a graph 60 representing the transmission coefficient of a device with a prism structure 50 made of polycarbonate with n = 1.59 and an apex angle of the prism that is equal to the defined magic angle Ψ =102.06°.

Fig. 7a shows a graph representing the total transmission of light in W/m² for varying angles of incidence during the day for a light guide device provided with a conventional prismatic structure with apex angles of respectively 62, 90 and 118°. Graph 73 can be obtained by multiplying the solar radiation at an angle of incidence represented by the graph 10, also shown in Fig. 1 with the transmission of a prismatic structure with an apex angle of 62 ° for the angle of incidence varying during the day . Graph 74 can be obtained by multiplying the solar radiation at an angle of incidence represented by the graph 10, also shown in Fig. 1 with the transmission of a prismatic structure with an apex angle of 90 ° for the angle of incidence varying during the day . Graph 75 can be obtained by multiplying the solar radiation at an angle of incidence represented by the graph 10, also shown in Fig. 1 with the transmission of a prismatic structure with an apex angle of 118 ° for the angle of incidence varying during the day

Fig. 7b shows a graph 70 representing the total transmission of light in W/m² through the device 50 for varying angles of incidence during the day. The graph 70 can be obtained by multiplying the solar radiation at an angle of incidence represented by the graph 10, also shown in Fig. 1 with the transmission for the angle of incidence represented by graph 60 shown in Fig. 6. Graph 70 shows that the transmitted solar radiation is substantially more constant during a long period of the day compared to that related to the conventional device shown in Fig. 7a.

The peaks 71 and 72 have no deleterious effect on plant growth as they last for a short period in the morning and late afternoon.

Fig. 8 shows a further embodiment of device comprising a prismatic structure 80 provided with multiple prisms 81 and a flat portion 82 between two adjacent prisms 81. In certain applications more transmission is required than the 50% transmission of the prismatic structure 50 provided with adjacent prisms 53. For example, in moderate climate areas of the hemisphere wherein the maximum irradiation is lower than in tropical areas. The higher transmission can be obtained by providing the flat portion 82 of the transparent material 83 in between the adjacent prisms 81. The transmission of this spaced prismatic structure 80 is approximately given by formula (4) for an angle of incidence a smaller than the critical angle β.

Fig. 9 shows a diagram of a graph 90 representing the irradiation transmitted by a device with a spaced prismatic structure 80 with flat portions 82 between the prisms 81 versus the angle of incidence a of incoming radiation. The fraction / defined as the ratio of the width x of the flat portion 82 and the distance d between two adjacent prism 81 is in this example 0.3. The device 80 can also be manufactured of polycarbonate. For polycarbonate the index of refraction is n = 1.59 and the magic apex angle Ψ is 102.06 ° . The graph 90 shows a substantially constant sunlight transmission around 745 W/m². The graph 91 shows the power of direct incoming sunlight on a horizontal surface for comparison. The graph 91 is the same as the graph 10 in Fig. l . The improvement of the device according to the invention can be showed by a comparison of the transmission of sunlight via the spaced prismatic structure 80 and the transmission via a conventional chalk layer which can be provided on the roof of a greenhouse.

Fig 10 shows a first graph 100 representing a transmission of a device 80 with a spaced prismatic structure manufactured of polycarbonate with an index of refraction « of 1.59, an magic apex angle Ψ of 102.06°, and a fraction / of 0.3, and a second graph 101 representing the transmission for a chalk layer on a transparent plate with a constant transmission coefficient of 62% versus the angle of incidence a. The third graph 102 shows the power of direct incoming sunlight on a horizontal surface for comparison.

Fig.10 shows qualitatively that the total energy transmitted by the device comprising the spaced prismatic structure represented by the area under the first graph 100 is significantly higher than the total energy that is transmitted by the chalked layer, represented by the area under the second graph 101.

In the above described embodiments of the device comprising the prismatic structures and space prismatic structure materials can be applied with an index of refraction between 1.45 and 1.7. Examples of materials with an index of refraction « in that range are:

Polymetamethylacrylate « = 1.49

Polycarbonate « = 1.59

Extra dense flint glass « = 1.7

This range of index of refraction provides magic apex angles in the range between 93 and 108 degrees according to formula (1).

In an embodiment the device can be applied in, for example, a greenhouse.

Fig.11 shows a schematic drawing of greenhouse that can be used in tropical area. The greenhouse 104 is provided with a roof structure 105 and poles 106 which support the roof structure. The roof structure 105 can be curved.

The roof structure 105 carries the device provided with the plate or foil provided with the prismatic structure 50 or the space prismatic structure 80. The plate and foil provided with the prismatic structure can be made of a transparent material, for example, polycarbonate. For example Lexan with an index of refraction « of 1.59 and a magic apex angle of 102.06 °. The size of the greenhouse can be, for example, lOx 40 m. The height can be about 3 m.

In a further embodiment the device can be applied in a solar collector.

Fig. 12 shows a schematic drawing of an example of solar collector. The solar collector 120 may comprise a vessel 121 provided with a front panel 122 and an absorbing back panel 123. The vessel is further provided with an inlet 125 and an outlet 126 for transporting a fluid, for example, water or oil, respectively to and from the vessel to a water boilerl29 via pipes 127,128 and a pump 130. The water circuit can be used to transfer heat from the vessel 121 to water boiler 129.

The front and back panel 122, 123 of the vessel 121 are connected together by a circumferential wall 124. The front panel 122 comprises the plate or foil provided with the prismatic structure 50 or the spaced prismatic structure 80 as described in relation to Fig. 5 and Fig. 8. The solar collector may be provided with a further flat transparent plate or foil 131 without prismatic structure facing the sun. This further flat plate 131 is positioned at a small distance, for example 5 mm from the flat side of the plate or foil 122 with the prismatic structure to form a gap 132 between the first front panel 122 and the further flat plate 131. The function of this further plate 131 and the gap 132 is to create a layer of static air which thermally insulates the solar collector from the surrounding. The two parallel plates 122, 131, with and without the prismatic structure, might be produced as a single element via extrusion, for example as a channel plate.

Fig. 13 shows an example of a channel plate 133. The channel plate 133 comprises the first front plate 122 and the further plate 131. The gap 132 can be maintained via partitions 134. The prismatic structure or the spaced prismatic structure can be made of polycarbonate, for example Lexan, the index of refraction n of Lexan is 1.59 and the related magic apex angle is 102.06 °. The device comprising the plate or the foil provided with this prismatic structure avoids excessive heating of the vessel 121 at high elevation angles of the sun around noon in tropical or moderate climate areas and provides a substantial constant irradiation of the absorbing plate 123. It will be apparent to the person skilled in the art that other embodiments of the invention can be conceived and reduced to practice without departing from the true spirit of the invention, the scope of the invention being limited only by the appended claims. The description illustrates the invention and is not intended to limit the invention.

Claims

1. Device for transmission of incoming radiation comprising a body of a transparent material provided with a first flat side and a second side opposed to the first side, the second side being provided with a prismatic structure, wherein an apex angle ψ of the prismatic structure is substantially equal to

1

Ψ = 180 - 2arc sin - n wherein Ψ represents the apex angle in degrees, and n represents the index of refraction of the transparent material.

2. Device according to claim 1, wherein the body comprises a plate or a foil.

3. Device according to claim 1 or 2, wherein the prismatic structure comprises a plurality of parallel prisms.

4. Device according to any one of the claim 1 to 3, wherein the second side of the body is provided with a flat portion between two adjacent prisms.

5. Device according to claim 4, wherein the transmission for the incoming radiation for an angle of incidence smaller than a critical angle of the prismatic structure depends on a fraction of the flat portion, a transmission coefficient of the prismatic structure without the flat portion, and a Fresnel transmission coefficient of the transparent material.

6. Device according to claim 4, wherein the flat portion is provided with a coating to reflect at least partly the incoming radiation.

7. Device according to one of the preceding claims wherein the index of refraction is a value in the range between 1.45 with a corresponding magic apex angle of substantially 93° and 1.75 with a corresponding magic apex angle of substantially 108°.

8. Device according to one of the preceding claims wherein the transparent material comprises a polycarbonate or polymetamethylacrylate.

9. Device according to one of the preceding claims wherein the device comprises a further transparent plate positioned at a predetermined distance opposite the first flat side or the second side for forming a gap between the body and the further transparent plate.

10. Device according to claim 9, wherein the device is integrated in a channel plate.

11. Greenhouse provided with a device according to any one of the claims 1 to 10

12. Solar collector provided with a device according to any one of the claims 1 to 10.