Patent Publication Number: US-2010115837-A1

Title: Tray for growing organic material and a nursery assembly

Description:
FIELD OF THE INVENTION 
     The present invention relates to a tray for growing organic material. The invention further relates to a nursery assembly. 
     BACKGROUND OF THE INVENTION 
     A nursery assembly is known from U.S. Pat. No. 4,914,858. Said patent discloses a nursery assembly which is built up of a rack in which trays are accommodated. The trays are filled with a substrate into which seeds for germinating are inserted. Above each tray, a group of LEDs is situated. The heat emission from LEDs is minimal in comparison with that of light bulbs. This low radiation of heat enables the nurseryman to position the LEDs very close to the plant material. 
     U.S. Pat. No. 6,680,200 discloses an assembly for promoting the growth of plant tissues that includes a plurality of trays each defining an array of wells, wherein each well contains a tissue sample. A plurality of LED arrays mounted on a circuit board above each tray provides the light for the tissue samples in said trays. Each of the LEDs is centered above a respective one of the wells. 
     JP 09-098665 discloses a plant cultivation apparatus capable of reducing electric power cost, such as illumination cost and refrigeration cost, while having an excellent growing performance and being useful for vegetable cultivation. 
     In the known assemblies, in a stable situation the air temperature and the soil temperature are equal. Furthermore, the soil is heated via the air. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved tray for growing organic material. 
     This object is achieved by a tray for growing organic material comprising a plurality of LEDs, the tray having an upper side and a lower side, the upper side being arranged to carry a substrate for growing organic material and the lower side being arranged to radiate the light generated by the LEDs, the tray comprising a structure enabling the tray to radiate more heat, generated by the LEDs, to the upper side of the tray than to the lower side of the tray. Organic material could be any plant or seed material, but also a bacterial culture or cell culture. 
     The present invention is based on the recognition that the growth of plants can be improved by keeping the soil in which the roots are positioned above a certain temperature. For this reason black plastic is used in agriculture to cover the soil so as to enable a more efficient heating up of the soil below the plastic by the sunlight. This increases the average soil temperature, which improves the growth of plant material. According to the invention, the heat generated by the LEDs is not wasted. The tray according to the invention mainly transfers the generated heat to the substrate and not to the air. The tray functions as a floor heating device of the substrate. In this way the average temperature of the substrate could be higher than the air temperature. This enables the nurseryman to reduce further costs of heating or cooling the air, while the temperature of the substrate with the roots is kept above the desired substrate temperature. The embedding of LEDs in the tray enables the nurseryman to position more trays in an assembly. The floor heating function in combination with the function of lighting device is also advantageous in an embodiment of a substrate with a cell culture and bacterial culture, wherein the substrate is heated from below and not via the air above the culture. 
     In an embodiment of the invention the structure comprises a first part and a second part, a side of the first part forming the upper side and a side of the second part forming the lower side of the tray, wherein the LEDs are attached on the first part and the first part has a higher thermal conductivity than the second part. In this way the heat will be transferred mainly to the upper side of the tray. Furthermore, the higher the thermal conductivity the more the heat will be spread over a bigger surface of the upper side of the tray. This reduces the temperature variations in the substrate. 
     In an embodiment of the invention a gap separates the first part and the second part. The gap, which could be filled with air or another gas, functions as a thermal resistance between the first and the second part. In this way the heat generated by an LED will be transferred through the first part to the upper side of the tray. 
     In an embodiment of the invention the material of the first part is at least one of the materials: metal, glass, Plexiglass, or plastic. In a further embodiment of the invention the material of the second part is at least one of the transparent materials: glass, Plexiglass, or plastic. These features enable to provide a tray that can easily be cleaned after each growth period and that protects the LEDs against humidity and dust. 
     In an embodiment of the invention the surface of the first part facing the second part is reflective. This feature increases the lighting efficiency of the tray. 
     In an embodiment of the invention the structure comprises one or more heat pipes, and the LEDs are thermally coupled to the one or more heat pipes. This feature enables to transfer quickly the heat from the LED to the lowest temperature of the heat pipe. In this way the tray evenly heats the substrate. This improves the growing of roots. 
     In an embodiment of the invention the heat pipes are horizontally positioned. This feature enables to embed the heat pipes in a plate. By mounting the LEDs near to a heat pipe, the heat generated by the LED will be spread more evenly over the contact area of the tray and the substrate. Furthermore, this feature enables to use one heat pipe for a tray. The heat pipe could meander in a sinuous, spiral, or circular course over the tray. In another embodiment the heat pipe could extend horizontally from one side to another side of the tray. 
     In an embodiment of the invention the heat pipes are positioned above a bottom layer, which bottom layer comprises areas for allowing passage of the light emitted by the LEDs. This embodiment enables to improve the efficiency of transfer of heat from the LED to the substrate. By positioning the heat pipe above a bottom layer, the contact area of the heat pipe and the soil is increased. The LEDs have to be attached to the heat pipe to enable the heat pipe to transfer efficiently the generated heat from an LED to the soil. The bottom layer comprises transparent areas for allowing passage of the light generated by the LEDs to the plant material positioned below the tray. 
     In an embodiment of the invention the heat pipes are vertically positioned. This feature reduces the temperature difference between the top of the substrate and the bottom of the substrate. A smaller temperature variation in the substrate results in a more uniform growth of roots in the substrate, irrespective of the depth of the substrate. 
     A further aspect of the invention relates to a nursery assembly comprising a plurality of trays according to the claims, the trays being positioned above each other. 
     Yet another aspect of the invention relates to a method of growing organic material, which method comprises the steps of: 
     providing a nursery area containing organic material in a substrate with trays according to the invention comprising a plurality of LEDs; and 
     energizing the LEDs so that they emit light onto said organic material in said substrate and heat up the substrate via a structure of said trays. In this way the heat generated by the LEDs is not wasted by heating the air, but is used to directly heat up the substrate. The invention provides a floor heating system for a substrate. By means of the invention the temperature of the substrate can be higher than the air temperature. This means that the air temperature can be reduced, while the optimal substrate temperature for the growing of roots is maintained. In this situation an energy saving can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described in detail, by way of example and with reference to the accompanying drawings, in which: 
         FIG. 1  shows a perspective view of a nursery assembly provided with trays according to the invention; 
         FIG. 2  shows schematically a cross section of a first embodiment of the invention; 
         FIGS. 3   a  and  3   b  show schematically cross sections of a second embodiment of the invention; 
         FIGS. 4   a  and  4   b  show schematically cross sections of a third embodiment of the invention; and, 
         FIGS. 5   a  and  5   b  show schematically cross sections of a fourth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  shows a perspective view of a nursery assembly  100  built up of a rack  102  in which trays  104 ,  104 ′ according to the invention are accommodated. The trays  104 ,  104 ′ are detachably positioned in the rack  102 . The trays  104 ,  104 ′ in  FIG. 1  are filled with a substrate. In the substrate, seeds for germinating could be inserted or cultivated plants  106  could grow. Examples of a substrate are soil, potting compost, mineral wool or rock wool. The trays  104 ,  104 ′ comprise a plurality of Light Emitting Diodes (LEDs)  108  at the bottom of a tray  104 ,  104 ′. The LEDs embedded in a tray  104 ′ radiate light upon the plant material in the tray  104  positioned below said tray  104 ′. LEDs have the advantage that the heat generated by the LED is not radiated in the form of Infra Red light (IR) in the beam. This has the advantage that the light source can be positioned close to the plant material. In  FIG. 1  can be seen that the LEDs are uniformly distributed over the bottom of the tray  104 ′. This enables to provide uniform lighting conditions for the plant material  106 . According to the invention, the heat generated by the LEDs is mainly transferred to the substrate to keep the roots of the plants above a certain temperature or at an optimal temperature for growing plants or germinating seeds. The structure of the tray is such that it functions as a floor-heating device and a lighting device. As the heat from the LEDs heats up mainly the substrate material and not the air between the trays  104 ,  104 ′, the air temperature can be reduced. This can lead to a reduction of the energy which is normally used to heat up the air in the space where plants grow. This will reduce the costs for producing plant material. 
     It should be noted that the LEDs can be any “light-emitting element” that emits radiation in any region or combination of regions of the electromagnetic spectrum, for example, the visible region, the infrared and/or ultraviolet region, when activated by applying a potential difference across it or passing a current through it, for example. Therefore, the LED can have monochromatic, quasi-monochromatic, polychromatic or broadband spectral emission characteristics. Also a combination of LEDs with different wavelengths can be used in a single tray. One type with a wavelength optimized for the growth and another type with a wavelength optimal for e.g. the blooming stage of the plants. Examples of light-emitting elements include semiconductor, organic, or polymer/polymeric light emitting diodes, optically pumped phosphor coated light emitting diodes, optically pumped nano-crystal light-emitting diodes or any other similar light-emitting devices, as will be readily understood by a person skilled in the art. 
     Furthermore, it should be noted that the rack and trays are provided with supply wires. A person skilled in the art knows how to install the supply wires and corresponding connections in the rack  102  and the trays  104 ,  104 ′. Therefore, this subject matter is not discussed in further detail. 
       FIG. 2  shows schematically a cross section of a first embodiment of a tray  200  according to the invention. The tray comprises a top layer  202  and a bottom layer  206 . LEDs  204  are embedded between the top layer  202  and the bottom layer  206 . Preferably, the LEDs  204  are attached to the top layer  202 . This provides a good thermal coupling between a LED  204  and the top layer  202 . The top layer  202  could be made of glass, metal, Plexiglass, plastic or any other material suitable for providing a tray for growing plants and conducting heat. A suitable metal is for example aluminum, which is a light material with a good thermal conductivity. In a preferred embodiment, the top layer  202  comprises a light-reflective surface  214 . The light-reflective surface  214  increases the amount of light radiated via the bottom of the tray. The efficiency of the system can be even further improved by applying a white plastic foil around the plant (organic material). This improves the lighting efficiency of the tray  200 . The bottom layer  206  is made of a transparent material such as glass, Plexiglass or plastic. Also a diffuser material could be used resulting in a homogenous emitting surface. The top layer  202  and bottom layer  206  are spaced from each other by means of a gap  203 . The gap  203  is filled with air or any other suitable gas or liquid that forms a thermal insulation between the top layer  202  and the bottom layer  206 . In this way, the heat dissipated by the LED  204  towards the top layer  202  will not easily be transferred to the bottom layer, but rather be passed to the substrate  210  in which plant material  208  is growing. If the top layer  202  is made of a good heat conductive material, the top layer  202  will pass heat, evenly distributed over the contact area  216  with the substrate, to the substrate  210 . In this way, more heat will be radiated to the substrate  210  above the tray  200  than to the air below the tray  200 . The technology for manufacturing Metal Core Printed Circuit Boards could be used for mounting the LEDs on a layer of metal. 
     Another advantage of the thermal conductivity of the top layer is that the average operating temperature of the LEDs will decrease. A reduction of the operating temperature will increase the lighting efficiency of the LEDs. Furthermore, reduction of the average operating temperature will increase the lifetime of an LED. 
     The space between the LED  204  and the bottom layer  206  could be minimal. The heat generated by the LEDs is dissipated at the rear side of the LEDs and transferred to the top layer  202 . The space between the LEDs  204 , which now forms the gap  203 , forms a thermal resistance between the top layer  202  and the bottom layer  206 . The bottom layer  206  could comprise collimators (not shown) provided in the beams of the LEDs. A collimator provides a light beam of a LED with the desired optical characteristics, such as beam angle and beam axis. 
     In  FIG. 2 , the tray comprises a rim  212 , to ensure that the substrate or water provided to the substrate will not spill from the tray  200 . The flat surface of the tray as well as the top side and bottom side provide the tray with easy cleaning characteristics. Furthermore, the two layers  202 ,  206  provide a waterproof environment for the embedded LEDs  204  and corresponding electronics. Furthermore, the top and bottom surface of the tray comprises no electrical parts. 
       FIGS. 3A and 3B  show schematically cross sections of a second embodiment of a tray according to the invention.  FIG. 3B  shows a cross section, which is perpendicular to the cross section in  FIG. 3A . The tray  300  comprises a layer  302  provided with heat pipes  304 . An LED  306  is attached to the bottom side of a heat pipe  304 . It should be noted that more than one LED could be mounted to the bottom side of the heat pipe  306 . 
     A heat pipe is a simple device that can quickly transfer heat from one point to another. They are often referred to as the “superconductors” of heat as they possess an extraordinary heat transfer capability with almost no heat loss. The principle of a heat pipe is well known and is described in, among other documents, “Experimental investigations of micro heat pipes fabricated in silicon wafers”, Peterson G. P. et al, Journal of Heat Transfer, vol. 115. pp 751-756, 1993. In short, a heat pipe is a sealed elongated channel of small cross sectional dimensions. A liquid is applied inside the heat pipe that is otherwise evacuated. A first end of the elongated heat pipe is used as the heat-receiving end at which heat is absorbed and the liquid evaporated. A second end of the elongated heat pipe is used as a condenser and is connected to a heat sink, in our application the substrate. At the second end, the vapor is condensed to recuperate the liquid. The rate of vapor travel from the first end to the second end is governed by the difference in vapor pressure between the two ends. With a proper design of the size and shape of the channel, the liquid formed at the second end of the heat pipe is transported by capillary force to the first end where it is evaporated again. Typically, the heat pipes are provided with angular sections or capillary structures to enhance the capillary force. The capillary structures could comprise at least one of: grooves, a screen mesh, a fine fiber or a sintered powder at the inner side of the heat pipe. The liquid thus works as an efficient circulating heat-transporting medium. The heat pipe principle provides an efficient cooling device without the need for any pump or similar device, since the cooling liquid is transported by means of capillary force only. Suitable cooling liquids include methanol, liquid ammonia and water. The cross sectional dimensions and the detailed design of heat pipes are optimized for the application in question in order to provide a suitable capillary pressure. WO2006/114726A2 and WO2006/105638 disclose the application of cooling a LED with a heat pipe. 
     The heat pipes  304  in the second embodiment are vertically positioned. A heat pipe will normally conduct the heat to the coldest place of the external surface of the heat pipe. Consequently, the heat radiated by the heat pipe will be evenly distributed over the contact area with the substrate. The roots of the plant material grow between the vertically positioned heat pipes. Therefore, the roots will be heated from aside. In this embodiment, the temperature of the substrate could vary in the horizontal direction and will minimally vary in the vertical direction. This means that the growing conditions of the roots with respect to the depth of a root in the substrate are more or less similar. 
     In the embodiment of  FIG. 3 , the heat pipes  304  pass through the substrate  310  from the bottom to the top layer of the substrate  310 . The heat pipes  304  provide means for anchoring the substrate  310  to the tray  300 . If the substrate is made of e.g. Rock wool, the Rock wool could comprise preformed holes to receive the heat pipes  304 . In another embodiment, the top of the heat pipes is covered with the substrate. 
     The envelope of the heat pipes may have, inter alia, a cylindrical, rectangular, or other custom shape, based on the design. For example, the upper surface of the tray could form an array of wells, each of the wells being arranged to contain a substrate for growing one or more nursery plants. In this embodiment, the heat pipes could be embedded just below the surface of a sidewall of each well. 
       FIGS. 4   a  and  4   b  show schematically cross-sections of a third embodiment of a tray according to the invention.  FIG. 4   b  is a cross section perpendicular to the cross section of  FIG. 4   a . In this embodiment, a heat pipe  404  extends horizontally over a bottom layer  402  of the tray  400 . LEDs  406  are attached to the heat pipe  404 . In an embodiment, the heat pipes could extend parallel to one another from one side of the tray to the other side of the tray, or one heat pipe could meander across the whole surface of the tray  400 . The bottom layer  402  comprises transparent areas  414  to pass the light generated by the LEDs  406  to the plant material positioned below the tray  400 . The other areas  412  could be of transparent or non-transparent material. In an embodiment the transparent areas  414  comprise collimators which shape the light beams of the LEDs. Preferably, the LEDs are placed in a regular pattern across the tray, to provide the tray with uniform lighting conditions for the plant material situated below the tray. 
     In this embodiment, the heat generated by the LEDs  406  is evenly radiated across the outer surface of the heat pipe  404 . The substrate  410  wherein the plant material grows is heated up from below. 
       FIGS. 5A and 5B  show schematically cross-sections of a fourth embodiment of a tray according to the invention.  FIG. 5B  is a cross section perpendicular to the cross section of  FIG. 5A . The tray comprises a top layer  502  of thermally conductive material. LEDs  504  are attached to the bottom side of the top layer  502 . The heat generated by the LEDs  504  will be distributed across the top layer  502 . Furthermore, a thermal isolation layer  506  is attached to the bottom side of the top layer  502 . To pass the light emitted by the LEDs to the plant material below the tray  500 , the isolation layer  506  comprises holes  514  at the position of the LEDs  504 . By means of this structure, the heat generated by the LEDs is mainly radiated via the upper surface of the tray to the substrate  510  to heat up the substrate  510 . The more thermally conductive the material of the top layer  502 , the more the heat is spread over the upper surface of the tray. In an embodiment, the holes  514  comprise collimators which shape the light beams of the LEDs. 
     Although the invention is described by means of preferred embodiments thereof, it is to be understood that these are non-limitative examples. Thus, various modifications are conceivable to those skilled in the art, without departing from the scope of the invention, as defined by the claims. For example the heat pipes could be embedded in the top layer  502  of the fourth embodiment to improve the thermal conductivity of the top layer  502 . Furthermore, the contact area of the top layer with the substrate could be increased by for example a wavelike surface instead of a flat surface or by providing cooling structures, for example ribs, to the top layer which extend the upper surface. Furthermore, the embodiments given above show trays with plant material. The invention could also be advantageously used in for example a laboratory for cultivating cell cultures or bacterial cultures. 
     The use of the verb “to comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Furthermore, the use of the article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. In the claims, any reference signs placed between parentheses shall not be construed as limiting the scope of the claims. Furthermore, the invention resides in each and every novel feature or combination of features.