Abstract:
Horticultural lighting system comprising a first string ( 22 ) of at least one light element ( 16, 40 ), a second string ( 24 ) of at least one light element ( 16, 40 ), both strings being electrically connected in parallel with each other, a power supply unit ( 12, 50, 60, 70 ) providing an AC current/voltage to the parallel circuit of the strings, and means for energizing ( 26, 42 ) the first string during a first half period of the AC voltage and the second string during a second half period of the AC voltage, so that each string radiates pulsed light, said first string and said second string being arranged such that different areas ( 32, 34 ) are irradiated. The lighting system is also to be used in an environment where living organisms are irradiated with light, or a greenhouse for exposing plants to pulsed light. A corresponding method is also claimed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a horticultural lighting system, the use of such a lighting system for exposing plants to pulsed light, and a method of irradiating plants in a greenhouse with pulsed light. 
     BACKGROUND OF THE INVENTION 
     Generally, lighting systems are used for horticultural applications in greenhouses or other environments where living organisms are to be irradiated with light, e.g. photo-bioreactors, to support plant growth. When developing new horticultural systems, one of the most important goals is the reduction of the total cost of ownership (TCO). 
     In the art, different documents report on the influence of pulsed light on the growth of plants, e.g. US 2004/0109302 A1. The described studies show that pulsed light (frequency, duty cycle), compared to continuous light, has a positive effect on energy consumption but no negative effect on plant growth. It is believed that a plant&#39;s pigment cannot directly absorb a second photon after absorption of a first photon. So, pulsed light decreases the TCO due to reduced power consumption, thereby increasing the overall efficiency of the system. A light source that can be easily pulsed is a light emitting diode (LED). The efficiency of today&#39;s LEDs (measured in micromole/J) is getting close to or is already above that of state-of-the-art high pressure sodium lamps. In addition, the emission spectra can be tuned to the absorption spectra of the pigments. A problem, however, is that drivers for LEDs do not offer a high efficiency, a high power density, high reliability and galvanic isolation at the same time. 
     Another problem relating to high pressure sodium lamps for horticultural applications is that they typically generate light in the green/yellow spectrum. However, the absorption spectra of most common pigments are in a different frequency range. 
     SUMMARY OF THE INVENTION 
     In view of the above, it is an object of the present invention to provide a lighting system which overcomes the deficiencies of prior art lighting systems, particularly systems using high pressure sodium lamps. A further object is to reduce the total cost of ownership of the lighting system. 
     These objects are achieved by a horticultural lighting system, preferably for greenhouses or other environments where living organisms are irradiated with light, like photo-bioreactors, comprising a first string of at least one light element, a second string of at least one light element, both strings being electrically connected in parallel with each other, a power supply unit providing an AC current/voltage to the parallel circuit of the strings, and means for energizing the first string during a first half period of the AC voltage and the second string during a second half period of the AC voltage, so that each string radiates pulsed light, said first string and said second string being arranged such that different areas are irradiated. 
     It is known from literature that pigments are not able to absorb a second photon directly after absorption of the first photon. This means that a break between two offered photons increases the efficiency of the system. The inventive horticultural lighting system uses this effect by providing two strings of light elements which are alternately energized by the same power supply unit, both strings of light elements being arranged to irradiate different areas of the greenhouse or the mentioned environment. Preferably, the output of the power supply unit is sinusoidal so that the resulting current through one of the two strings is the half cycle of a sinusoid. Hence, each string produces pulsed light, the frequency of which is dependent on the frequency of the AC current/voltage of the power supply unit. Since both strings do not irradiate the same area, the pulsed radiation is not balanced or equalized with respect to the illuminated area of the greenhouse. 
     In the context of the present invention, power supply unit means any component or circuitry being able to provide the required AC current/voltage. In the simplest case, the power supply unit only comprises electrical lines and input terminals which are connected with the mains or any other AC grid. Typically, the power supply unit comprises driver circuitry to provide an AC voltage with a predetermined frequency other than the mains frequency. 
     In the context of the present invention, means for energizing is any device or component which may enable or disable current flow in at least one direction, like switches which can block current flow in two directions, or diodes which can block current in one direction, 
     One of the advantages of this inventive lighting system is that it is very efficient in terms of energy consumption, since both half periods of the AC current/voltage are used for generating light which illuminates two different areas. Hence, the total cost of ownership may be reduced. In other words, the inventive lighting system combines pulsed light for irradiating plants or any other living organisms with modern driver technology for achieving a very energy-efficient system. 
     In a preferred embodiment, said means for energizing comprises a first diode assigned to the first string and a second diode assigned to the second string, both diodes being provided anti-parallel to each other. 
     In other words, each string comprises a diode connected in series with the at least one light element of the string, both diodes being provided anti-parallel to each other so that current of the power supply unit flows to both strings alternately and in different directions. The use of diodes as means for controlling the current flow is cheap and less complex compared to other solutions to control current flow, like electronic switches, etc. 
     In a further preferred embodiment, said light elements comprise light emitting diodes (LED) and/or organic light emitting diodes. 
     This measure has the advantage that light emitting diodes are very energy-efficient elements compared to high pressure sodium lamps generally used in horticultural applications. It is to be noted that “light emitting diode” in the context of the present invention also means organic light emitting diode. 
     A further advantage of light emitting diodes is that they also take over the function of the energizing means. It is only necessary to arrange the LEDs in both strings anti-parallel to each other. 
     A further preferred embodiment comprises electrical lamps. 
     It is to be noted that the light elements may also comprise any combination of light emitting diodes and lamps. Further, the strings may be asymmetrical, meaning that the number of light elements and/or the type of light elements are different in the strings. 
     Preferably, each string comprises a plurality of light elements, preferably LEDs, which are connected to each other in series, parallel or any combination of parallel and series connections. 
     The aforementioned measures have the advantage that the implementation of a very cost-effective and energy-efficient lighting system is achievable. 
     In a further preferred embodiment, said power unit comprises a DC-to-AC converter. More preferably, said converter comprises a resonant half-bridge converter or a resonant full-bridge converter. 
     The provision of a resonant converter offers a plurality of advantages, for example galvanic isolation which is often required if the power supply unit is fed by mains voltage. Further, the power supply unit has a high efficiency due to soft-switching. Further, high power density is achievable because a very high switching frequency can be used. Hence, the passive components can be designed small. Further, the reliability is very high due to reduced stress of the components. 
     In a further preferred embodiment, said light elements are selected such that their emission spectra match the absorption spectra of plants to be irradiated in the greenhouse. Preferably, some of the light elements have different wavelengths. More preferably, the frequency of said AC voltage is selected such that the pulse period of the light elements is between 2 microseconds and 20 milliseconds, preferably between 10 microseconds and 1 millisecond, preferably 100 microseconds. 
     The aforementioned measures have the advantage that the growth of the plants irradiated by the lighting system is additionally supported and enhanced. 
     An object of the present invention as mentioned above is also achieved by using the inventive lighting system in an environment where living organism can be irradiated with light, or a greenhouse for exposing plants to pulsed light. 
     The object mentioned above is also achieved by a method of irradiating plants in a greenhouse or other living organisms in a special environment with pulsed light from light elements arranged in two parallel-connected strings, which strings are both adapted to radiate light to different areas of the greenhouse or the environment, said method comprising the steps of:
         providing an AC current/voltage,   energizing one string during the first half period of the AC voltage for radiating light to a first area, and   energizing the other string during the second half period of the AC voltage for radiating light to a second area different to the first area.       

     This method achieves the same advantages as mentioned above, so that reference is made thereto. 
     It is further preferred that said AC voltage is selected such that the pulse period is between 2 microseconds and 20 milliseconds, preferably between 10 microseconds and 1 millisecond, preferably 100 microseconds. 
     This pulse period has proved to be advantageous. 
     Further features and advantages can be taken from the following description and the enclosed drawings. 
     It is to be understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are shown in the drawings and will be explained in more detail in the description below with reference to same. In the drawings: 
         FIG. 1   a  is a schematic block diagram of an inventive horticultural lighting system for greenhouses; 
         FIG. 1   b  is a voltage diagram illustrating the output of the power supply unit; 
         FIG. 1   c  is a schematic top view diagram of a greenhouse with different areas; 
         FIG. 2  is a preferred embodiment of the horticultural lighting system; 
         FIG. 3  is a schematic block diagram of the embodiment of  FIG. 2  using a resonant half-bridge converter; 
         FIG. 4  is a schematic block diagram of a further embodiment using diodes as means for energizing; 
         FIG. 5  is a schematic block diagram of a further embodiment using light elements arranged in series and parallel to each other; 
         FIG. 6  is a schematic block diagram of a further embodiment of the inventive lighting system with a resonant full-bridge converter; 
         FIG. 7  is a schematic block diagram of the embodiment shown in  FIG. 6  with a capacitor that is differently arranged; 
         FIG. 8  is a schematic block diagram of a further embodiment of an inventive lighting system with a full-bridge converter slightly different to that shown in  FIGS. 6 and 7 ; 
         FIG. 9  is a schematic block diagram of the embodiment shown in  FIG. 7  with a further capacitor; and 
         FIG. 10  is a schematic block diagram of the embodiment shown in  FIG. 8  with a further capacitor. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In  FIG. 1 , a horticultural lighting system is schematically shown and indicated with reference numeral  10 . It is to be noted that it is only a block diagram not showing all details of the electrical circuits. The lighting system  10  is used for horticultural applications in greenhouses to irradiate plants in the greenhouse with pulsed light. As already mentioned before, pulsed light enhances the growth of the irradiated plants. 
     The lighting system  10  comprises a power supply unit  12  which provides, at output terminals  14 , an AC voltage. The performance of the power supply unit  12  is adapted to the type and number of light elements supplied with power. The light elements are indicated with reference numeral  16  in  FIG. 1   a . The light elements  16  are arranged in two groups forming a first string  22  and a second string  24 , both strings being connected in parallel with each other and with the output  14  of the power supply unit  12 . Each string  22 ,  24  may comprise one or more light elements  16  that are serially connected as shown in  FIG. 1   a . However, it is to be noted that each string  22 ,  24  may also comprise parallel connections of light elements  16  or a combination of serially and parallel connected light elements  16 . 
     Further, each string  22 ,  24  comprises a current control means  26  which controls the current flow through the respective light elements  16  of the string. Particularly, the current control means  26  are adapted such that current flows through the light elements  16  of the first string  22  during a first period and through the light elements  16  of the second string  24  during a second period. It is preferred that the first period corresponds to the first half of the period of the AC voltage supplied by the power supply unit  12  and the second period corresponds to the second half of the AC voltage, as schematically shown in  FIG. 1   b.    
     Both current control means  26  cause the light elements  16  to generate pulsed light, such that the light pulses of the first string and the second string  22 ,  24  alternate. 
     In  FIG. 1   c , a top view of a greenhouse is schematically shown and indicated with reference numeral  30 . The greenhouse  30  generally has a surface area which may be divided in a plurality of sub-areas as shown in  FIG. 1   c . It is to be noted that the rectangular shape of the surface area and the sub-areas is only for illustration purposes. In this example, the surface area is divided into nine sub-areas, of which sub-area  1  and sub-area  2  are indicated with reference numeral  32  and  34 , respectively. Due to the fact that pulsed light is advantageous for plant growth, the light elements  16  of a string  22 ,  24  are used to irradiate sub-areas  32 ,  34 . In other words, for example the first string  22  of light elements  16  illuminates the sub-area  32 , and the second string  24  the sub-area  34 . The light elements  16  of both strings  22 ,  24  should not illuminate the same sub-area because the positive effect of pulsed light on plant growth would then be lost. 
     It is to be noted that the assignment of strings  22 ,  24  to adjacent sub-areas  32 ,  34  is only an example and not the only solution. The arrangement and orientation of the light elements  16  of both strings  22 ,  24  should only ensure that the strings  22 ,  24  irradiate different areas of the greenhouse  30  and hence different plants. 
     In  FIG. 2  one preferred embodiment of a horticultural lighting system  10  is shown in form of a block diagram. The power supply unit  12  comprises a DC-to-AC converter  50  and a DC source  52 . The DC-to-AC converter  50  is adapted to convert the DC voltage to an AC voltage supplied to the strings  22 ,  24 . 
     Further, in this embodiment the light elements  16  are light emitting diodes  40 , the LEDs  40  of the first string  22  being anti-parallel to the LEDs  40  of the second string  24 . 
     Due to the fact that light emitting diodes  40  have also the function of current control means, namely to allow current flow only in one direction, the LEDs  40  replace also the current control means  26 , shown in  FIG. 1   a.    
     By providing the LEDs  40  in the strings  22 ,  24  so as to be anti-parallel to each other, it is ensured that current I 1 , I 2  passes through the LEDs  40  only during a half period of the sinusoid of the current/voltage supply. 
     The frequency of the supplied AC voltage is selected such that the period of the light pulses generated by the LEDs  40  is between 2 microseconds and 1 millisecond. Preferably, the period is 100 microseconds. Hence, the AC frequency lies between 500 Hz and 250 kHz. 
     The LEDs  40  are preferably LEDs whose spectrum matches the absorption spectra of the plant&#39;s pigments. The adaptation of the LED spectrum to the absorption spectra of the plants further enhances the efficiency of the lighting system. 
     In  FIG. 2 , the AC supply voltage for the strings  22 ,  24  is generated by a DC-to-AC converter. In the art, there are multiple solutions as to how to implement such a converter  50 . In  FIGS. 3 to 8 , two conceivable circuits are shown. However, it is to be noted that other types of DC-to-AC converters are also possible to energize the light elements of the strings  22 ,  24 . 
     One preferred converter  50  is a so-called resonant half-bridge converter, which is indicated with reference numeral  60  in  FIG. 3 . The structure and function of such a half-bridge converter is known in the art, so that a detailed description may be refrained from. Generally, the half-bridge converter  60  comprises two switching elements  62  (transistors), which are coupled in series. Parallel to the series connection of the switching elements  62 , a series connection of two capacitors  64  is provided. Further, the half-bridge converter comprises a transformer  66 , the secondary side of which supplies the strings  22 ,  24  with energy via a capacitor  64 . 
     The primary side of the transformer  66  is connected via a capacitor with the centre tap of the transistor series connection and with the centre tap of the capacitor series connection. The transistors  62  are driven such that they alternately conduct so that an AC current passes the primary side of the transformer  66 . 
     The advantages of resonant converters, like the half-bridge converter  60 , are for example galvanic isolation, high power density, because very high switching frequencies can be used, and high reliability due to the reduced stress of the components. Further, high efficiency is achievable due to soft-switching, the efficiency being even higher than in standard cases because the output rectifier has been illuminated. 
     The embodiments shown in  FIGS. 4 and 5  also use resonant half-bridge converters  60 , but are different with respect to the implementation of the strings  22 ,  24 . 
     In the embodiment shown in  FIG. 4 , the first string  22  comprises a plurality of light elements  16  and a first diode  42  as current control means  26 . The second string  24  comprises also a plurality of light elements  16  and a second diode  42  as a current control means. Both diodes  42  are provided anti-parallel to each other so that AC current only flows through one string at the same time. Particularly, the light elements  16  of the first string  22  generate light during the first half of the sinusoid of the AC power supply (under ideal circumstances), whereas the light elements  16  of the second string  24  generate light during the second half of the sinusoid. 
     In  FIG. 5 , a further embodiment is shown which is similar to that shown in  FIG. 4  with the exception that both strings  22 ,  24  comprise light elements  16  which are also coupled parallel to each other. However, the function of this lighting system is the same as that of the aforementioned lighting systems. 
     In  FIGS. 6 to 8 , three further embodiments of a lighting system  10  are schematically shown which use so-called resonant full-bridge converters instead of half-bridge converters as described with respect to  FIGS. 3 to 5 . In  FIGS. 6 to 8 , the full-bridge converters are indicated with reference numeral  70 . 
     Generally, the structure and function of full-bridge converters is known in the art, so that a detailed description is refrained from. Contrary to the half-bridge converter, the full-bridge converter comprises two further switching elements (transistors)  62  which replace the series connection of the capacitors  64  in the half-bridge converter. To achieve current reversal on the primary side of the transformer  66 , diagonal pairs of transistors  62  are alternately switched and hence alternately conduct. 
     In the embodiment of  FIG. 6 , the primary side of the transformer  66  is coupled via a capacitor  64  with a centre tap of the first series connection of transistors  62  and directly with the centre tap of the second series connection of transistors  62 . Further, on the secondary side, a capacitor  64  is provided in series with the parallel connection of both strings  22 ,  24 . 
     The embodiment shown in  FIG. 7  differs from that shown in  FIG. 6  by the connection of the capacitor  64  on the secondary side. Here, this capacitor is provided parallel to the strings  22 ,  24 . 
     The embodiment shown in  FIG. 8  differs from that shown in  FIG. 7  by a direct connection of the primary side of the transformer  66  with the centre tap of the first series connection of transistors  62 . Hence, there is no capacitor in series with the primary side of the transformer  66 . 
     The embodiments shown in  FIGS. 9 and 10  are similar to the embodiments shown in  FIGS. 7 and 8 , respectively, with the difference that a further capacitor  68  is connected in series with the parallel connection of both strings  22 ,  24 . 
     As already mentioned before, the various embodiments shown in  FIGS. 3 to 8 , particularly with respect to the design of the power supply unit  12 , namely the AC to DC converter  50 , are only examples of conceivable solutions but shall not be considered as limiting the scope of the invention. Of course, there are other possibilities to provide an AC voltage with a desired frequency to the parallel-connected strings  22 ,  24 . Moreover, the inventive system and method can also be applied to multi-phase systems. 
     The main aspect of the invention is the provision of two strings of light elements connected parallel to each other and adapted such that both strings are energized alternately. Hence, the light elements of a string generate pulsed light, and the light pulses of the strings are generated alternately and hence not at the same time. Further, the light elements of the strings are arranged so that they irradiate different areas of the greenhouse. Consequently, the plants receive the light only from one string. Hence, there is always a “dark” phase between successive light pulses received by a plant.