Patent Publication Number: US-2021194422-A1

Title: Solar Photovoltaic Panel Fog/Mist Cooling System

Description:
FIELD OF THE INVENTION 
     The present invention relates to cooling solar photovoltaic panels, and in particular to a solar photovoltaic panel fog/mist cooling system. 
     BACKGROUND OF THE INVENTION 
     Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art. 
     With nearly 400 Gigawatt (GW) of global installed capacity, and expected to be the world&#39;s largest source of energy in the next few decades, solar photovoltaics systems are set to become a largely influential part of power generation. Solar photovoltaic systems rely on sunlight striking panels of photovoltaic elements to produce electricity. The use of solar photovoltaic panels has been known to involve several drawbacks that hold them back from releasing their full potential. One of the drawbacks is high temperatures of the solar photovoltaic panels during working, especially in equatorial regions. The high temperature results in a decrease in the photoelectric conversion efficiency. High temperatures cause both short-term and long-term losses. This leads to the emergence of a variety of cooling systems, with a considerable amount of recent research was done on enhancing them. 
     In general, a solar photovoltaic panel system is coupled with a cooling system in order to prevent the overheating of the photovoltaic modules, e.g. solar photovoltaic panels. Various cooling methods were devised for use with a solar photovoltaic system, each with their own drawbacks that need to be considered. 
     One of the cooling strategies of the photovoltaic panels is described in U.S. Pat. No. 9,236,515 B2, which describes the use of a coolant fluid, which circulates in a heat exchanger to recuperate the heat from the photovoltaic panels. This system requires increased system design complexity, e.g. coolant loops, piping, valves etc. Besides, the cooling efficiency is very low. 
     Another approach for addressing the problem via an open loop system is described in US 2014/0299174, which describes the working principle to pump the water directly from a lake or a river, pass it through a system of nozzles and spray the underside of the solar panle module directly. In this open loop design, once the water is sprayed onto the photovoltaic panels, it drips back onto the ground and is lost. Further, it requires solutions for obtaining and providing water from the lake or river to the solar panels. 
     Another system which is being explored recently is the active water spraying system, which usually uses a normal water nozzle for spraying cooling fluid directly onto the back side of the solar panel, as described in U.S. Pat. No. 10,050,584 B2. This system also requires high design complexity. Further, it is hard to achieve a uniform and even distribution of the fluid on the photovaltaic by applying water spray. 
     The foregoing attempts to address the problem of heating of photovoltaic panels have their drawbacks which have to be considered. There has been a continuing need for a solar photovoltaic panel cooling system that reduces cost and complexity and which has great cooling effiency and low use of water. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide a solution for the drawbacks of the traditional solutions. In particular, it is the object of the present invention to provide a solar photovoltaics panel fog/mist cooling system which achieves faster and more efficient cooling of the solar photovoltaic panels and lower usage of water. 
     The present invention involves a solar photovoltaics panel fog/mist cooling system, comprising at least one solar photovoltaic panel, and at least one nozzle arranged at a back side of the at least one solar photovoltaic panel and configured to generate a fog/mist applied onto the back side of the at least one solar photovoltaic panel. 
     Unlike applying a normal nozzle as in the prior art technologies, which generates a water spray, the nozzle used in the present invention is specifically designed to generate a fog/mist, which is a phenomenon caused by small droplets of water suspended in the air. Because of this characteristic of a fog/mist, droplets of the fog/mist, unlike water spray, will not fall immediately through the air, but will suspend finely in the air. Droplets of the fog/mist are able to cover a larger surface area. The evaporation and cooling effectiveness can be enhanced. Further, according to the present application, at least one nozzle is arranged at a back side of the at least one solar photovoltaic panel. Thus, the fog/mist generated by the nozzles can be applied towards the back side of the at least one solar photovoltaic panel to achieve a more efficient cooling of the solar photovoltaic panel. 
     In another embodiment of the present invention, the system further comprises phase-change materials coupled to the back side of the at least one solar photovoltaic panel, and wherein the fog/mist generated by the at least one nozzle is applied directly onto a back side of the phase-change materials. 
     In another embodiment of the present invention, the system further comprises a husk mesh formed by husks, wherein the husk mesh is directly attached to the phase-change materials, and wherein the fog/mist generated by the at least one nozzle is applied directly onto the back side of the husk mesh and is absorbed by the husk mesh. 
     In another embodiment of the present invention, the system comprises a husk mesh formed by husks, wherein the husk mesh is directly coupled to the back side of the at least one solar photovoltaic panel, and wherein the fog/mist generated by the at least one nozzle is applied directly onto the back side of the husk mesh and is absorbed by the husk mesh. 
     In another embodiment of the present invention, the at least one nozzle is a small orifice nozzle or an impingement nozzle. 
     In another embodiment of the present invention, the system further comprises a distribution system including a manifold to distribute a fluid to the at least one nozzle, wherein the at least one nozzle is coupled to the manifold. 
     In another embodiment of the present invention, the at least one nozzle is statically configured to provide a single constant fog/mist spray pattern. 
     In another embodiment of the present invention, the at least one nozzle is dynamically configured to oscillate, rotate or move. 
     In another embodiment of the present invention, the at least one nozzle is driven by a motor and is controlled depending on a measured temperature of the at least one photovoltaic panel. 
     In another embodiment of the present invention, the distribution system is designed that the fog/mist generated by the at least one nozzle covers a whole surface of the back side of the at least one solar photovoltaic panel uniformly. 
     In another embodiment of the present invention, the system further comprises a closed loop system comprising a tank containing a fluid, and a pump configured to pump the fluid to the distribution system. 
     In another embodiment of the present invention, the fluid is water or a nanofluid. 
     In another embodiment of the present invention, a residue fluid collected from the at least one solar photovoltaic panel is returned to the tank and recirculated with the closed loop system. 
     In another embodiment of the present invention, the closed loop system further comprises a first filter arranged at an upstream side of the tank to filter the residue fluid collected from the at least one solar photovoltaic panel before being returned to the tank. 
     In another embodiment of the present invention, the closed loop system further comprises a second filter arranged at a downstream side of the tank to filter the water from the tank. 
     In another embodiment of the present invention, the closed loop system further comprises a third filter arranged at a downstream side of the pump to filter the fluid before being delivered to the distribution system. 
     In another embodiment of the present invention, the closed loop system further comprises a heat exchanger arranged at an upstream side of the tank. 
     In another embodiment of the present invention, a residue fluid collected from the at least one solar photovoltaic panel is delivered to the heat exchanger for heating purpose. 
     In another embodiment of the present invention, the residue fluid after passing through the heat exchanger is delivered to the tank and recirculated with the closed loop system. 
     In another embodiment of the present invention, the heat exchanger is a tank coil heat exchanger. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other aspects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  illustrates a schematic diagram of a solar photovoltaics panel fog/mist cooling system in accordance with the present invention. 
         FIG. 2  illustrates a schematic diagram of a solar photovoltaics panel fog/mist cooling system provided with phase-change materials in accordance with the present invention. 
         FIG. 3  illustrates a schematic diagram of a solar photovoltaics panel fog/mist cooling system provided with a husk mesh in accordance with the present invention. 
         FIG. 4  illustrates a schematic diagram of a solar photovoltaics panel fog/mist cooling system provided with phase-change materials and a husk mesh in accordance with the present invention. 
         FIG. 5A  illustrates a schematic diagram of a distribution system in accordance with the present invention. 
         FIG. 5B  illustrates an actual photo of a distribution system in accordance with the present invention. 
         FIG. 6  illustrates a schematic diagram of a solar photovoltaics panel fog/mist cooling system including a thermal resistance circuit in accordance with the present invention. 
         FIG. 7A  illustrates a schematic diagram of an impingement nozzle in accordance with the present invention. 
         FIG. 7B  illustrates a schematic diagram of a small orifice nozzle in accordance with the present invention. 
         FIG. 7C  illustrates a fog/mist spray pattern of the nozzle in accordance with the present invention. 
         FIG. 7D  illustrates the distribution of the fog/mist generated by the nozzle in accordance with the present invention. 
         FIG. 8  illustrates a schematic diagram of a solar photovoltaics panel fog/mist cooling system provided with a heat exchanger in accordance with the present invention. 
         FIG. 9  shows a comparison of the bottom temperature development in time between the solar photovoltaic panels with and without the fog/mist cooling system in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The aspects of a solar photovoltaics panel fog/mist cooling system will be described in conjunction with  FIGS. 1-9 . The main design of the solar photovoltaics panel fog/mist cooling system according to the present invention fully considers reduced and uniform module temperatures, and minimal power and resource consumption. In the detailed description, reference is made to the accompanying figures, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
       FIG. 1  is a schematic diagram illustrating a solar photovoltaics panel fog/mist cooling system according to an embodiment of the present invention. As shown in  FIG. 1 , a plurality of solar photovoltaic panels  101  for absorbing and converting sunlight into electricity are connected with each other and mounted within a support frame  100 . A glass cover  102  is mounted on an upper surface of the photovoltaic panels  101  and is used for transmitting as much sunlight as possible into the solar photovoltaic panels  101 . A solar irradiance  103  refers to the output energy of the photovoltaic panels  101 . 
     In the embodiment according to the present invention, a plurality of nozzles  110  are provided for cooling purpose. The nozzles  110  are specially designed nozzles that force fluid to flow out in a high pressure state which creates a fog/mist that effectively absorbs heat and cools the surface of the photovoltaic panels  101 . The nozzles  110  are selected from small orifice nozzles and impingement nozzles. The structure and work principles of the specially designed nozzles  110  will be described below in detail according to the  FIGS. 7A-7C . 
     As shown in the  FIG. 1 , the nozzles  110  are arranged at the back side of the solar photovoltaic panels  101  and are coupled with a distribution system  107 . The solar photovoltaic panel fog/mist cooling system includes an inlet  108  that coupled to a manifold of the distribution system  107  which distributes the cooling fluid to the nozzles  110 . The nozzles  110  are configurated to generate a fog/mist towards the back side of the solar photovoltaic panels  101 . The distribution system  107  is arranged in the support frame  100  under the solar photovoltaic panels. The structure of the distribution system  107  in accordance with the present invention will be described below in detail according to the  FIGS. 5A and 5B . 
     As shown in the  FIG. 1 , a water supply system comprising a tank  106  and a pump  105  is provided and connected with the support frame  100  of the solar photovoltaic panels  101  via the inlet  108 . The tank  106 , the pump  105  and the support frame  100  are in fluid communication with one another and form a closed loop system. The tank  106  contains a cooling fluid. The cooling fluid can be water or a nanofluid. A nanofluid is a fluid containing nanometer-sized particles, called nanoparticles. These fluids are engineered colloidal suspensions of nanoparticles in a base fluid. Nanofluids can provide an enhanced thermal conductivity. By applying nanofluids in the closed loop system, better thermal and electrical enhancement can be achieved. 
     The water supply system according to the  FIG. 1  of the present invention is a closed loop system which circulates the cooling fluid via the pump  105  towards the distribution system  107  and nozzles  110  through the inlet  108 . The pump  105  only requires a small fraction of the output energy of the solar photovoltaic panels  101 , which is sufficient for circulating the cooling fluid in the solar photovoltaic fog/mist cooling system. The distribution system  107  distributes the cooling fluid among the nozzles  110 . Then the nozzles  110  generate a fog or mist applied onto the back side of the solar photovoltaic panels  101 . According to the present invention, the numbers, the types, the positions of the nozzles  110  are configured to ensure that the generated fog/mist covers the complete surface of the back side of the solar photovoltaic panels  101  and distributes uniformly on the surface of the of the back side of the solar photovoltaic panels  101 . Hence, a better cooling performance can be achieved. By applying a fog/mist generated by the specifically designed nozzles  110  directly to the back side of the solar photovoltaic panels  101 , a more efficient cooling of the solar photovaltic panels can be achieved by applying a very simple design, without any water flowing circuit mounted on the solar photovoltaic panels. 
     During the cooling, a residue fluid will be produced on the surface of the back side of the solar photovoltaic panels  101 . The closed loop system according to the present invention is designed that the residue fluid leaving the solar photovoltaic panels  101  is collected and returned to the tank  106  through an outlet  109  at the support frame  100  and recirculated within the closed loop system. The closed loop system can further comprise a filter arranged at an upstream side of the tank  106  and configured to filter the residue fluid leaving the solar photovoltaic panels  101  before being returned to the tank  106 . The residue fluid will go through the filter, where the residue fluid is filtered before being sent back to the tank  106 . The clogging of the nozzles due to bigger particles can be avoided. 
     A further filter can be arranged at a downstream side of the tank  106  to filter the water from the tank  106 . Another filter can also be arranged at a downstream side of the pump  105  to filter the fluid before being delivered to the distribution system. The clogging of the nozzles can therefore further be avoided. 
     If the residue fluid collected from the surface of the solar photovoltaic panels  101  has a higher temperature, then a heat exchanger is provided in the closed loop system. The residue fluid with high temperature goes through the heat exchanger for heating purpose, which will be described in  FIG. 8  in detail. 
     As shown in the  FIG. 1 , the solar photovoltaic panels  101  can be provided with a Tedlar sheet  104  on the back side. Tedlar is the brand name of the US-American chemical company DuPont and refers to Polyvinyl fluoride (PVF). Tedlar sheet is therefore used as a backsheet of solar photovoltaic panels to enhance the longevity of the panels. 
     The cooling system according to the present invention is a low cost and low water usage system in nature. Further, it has the ability to cover and cool a much larger surface area as compared to the water jet used in the prior technologies. 
     In accordance with another embodiment of the present invention, and as shown in  FIG. 2 , a plurality of solar photovoltaic panels  201  are connected with each other and mounted with a support frame  200 . A glass cover  202  is mounted on an upper surface of the photovoltaic panels  201 . The solar photovoltaic panels  201  can be provided with a Tedlar sheet  204  on the back side. 
     Further, according to the embodiment, as shown in  FIG. 2  of the present invention, phase-change materials  211  are provided and coupled to the solar photovoltaic panels  201 . The phase-change materials  211  are filled within a closed box. Once the phase-change materials are filled within the box, the box will be closed tightly. As shown in the  FIG. 2 , the phase-change materials  211  are coupled to the Tedlar sheet  204  of the solar photovoltaic panels  201 . If no Tedlar sheet is provided, the phase-change materials are attached directly to the back side of the photovoltaic panels  201  in the closed box. The phase-change materials  211  are arranged between the back side of the solar photovoltaic panels  201  and the plurality of nozzles  210 . 
     A phase change material (PCM) is a substance with a high heat of fusion which, melting and solidifying at a certain temperature, is capable of storing and releasing large amounts of energy. Heat is absorbed or released when the material changes from solid to liquid and vice versa. Adding phase-change materials  211  on the back side of the solar photovoltaic panels  201  can further lower the panel&#39;s operating temperature and improve the performance of solar photovoltaic panels  201 . 
     Other modules/elements in the system according to  FIG. 2  of the present invention have the similar structure and similar work principle of that according to the  FIG. 1  of the present invention. 
     In the embodiment of the present invention according to  FIG. 2 , the specifically designed nozzles  210  generate a fog or mist applied onto the surface of the phase-change materials  211 . The generated fog/mist covers the complete surface of the phase-change materials  211  and distributes uniformly on the whole surface. A residue fluid leaving the surface of the phase-change materials  211  is collected and returned to the tank  206  and recirculated within the closed loop system. The water supply system in the  FIG. 2  is a similar closed loop system as the closed loop system according to the embodiment of the present invention according to the  FIG. 1 . 
     In accordance with another embodiment of the present invention, and as shown in  FIG. 3 , a plurality of solar photovoltaic panels  301  are connected with each other and mounted with a support frame  300 . A glass cover  302  is mounted on an upper surface of the photovoltaic panels  301 . The solar photovoltaic panels  301  can be provided with a Tedlar sheet  304  on the back side. 
     Further, according to the embodiment as shown in  FIG. 3  of the present invention, a husk mesh  312  formed by husks is provided and coupled to the solar photovoltaic panels  301 . The husks can be palm tree husks. As shown in the  FIG. 3 , the husk mesh  312  is coupled to the Tedlar sheet  304  of the solar photovoltaic panels  301 . If no Tedlar sheet is provided, the husk mesh is fixed and attached directly to the back side of the photovoltaic panels  301 . The husk mesh  312  is arranged between the back side of the solar photovoltaic panels  301  and the plurality of nozzles  310 . 
     Other modules/elements in the system according to  FIG. 3  of the present invention have the similar structure and similar work principle of that according to the  FIG. 1  of the present invention. 
     In the embodiment of the present invention according to  FIG. 3 , the specifically designed nozzles  310  generate a fog or mist applied onto the surface of the husk mesh  312 . The generated fog/mist covers the complete surface of the husk mesh  312  and distributes uniformly on the whole surface. The husk mesh  312  can absorb the fog/mist generated by the nozzles  310 . A residue fluid leaving husk mesh  312  is collected and returned to the tank  306  and recirculated within the closed loop system. The water supply system in the  FIG. 3  is a similar closed loop system as the closed loop system according to the embodiment of the present invention according to the  FIG. 1 . 
     In accordance with another embodiment of the present invention, and as shown in  FIG. 4 , a plurality of solar photovoltaic panels  401  are connected with each other and mounted with a support frame  400 . A glass cover  402  is mounted on an upper surface of the photovoltaic panels  401 . The solar photovoltaic panels  401  can be provided with a Tedlar sheet  404  on the back side. 
     Further, according to the embodiment as shown in  FIG. 4  of the present invention, both of phase-change materials  411  and a husk mesh  412  are provided. Similar as illustrated in the  FIG. 2  of the present invention, the phase-change materials  411  are filled within a closed box. Once the phase-change materials are filled within the box, the box will be closed tightly. As shown in the  FIG. 4 , the phase-change materials  411  are coupled to the Tedlar sheet  404  of the solar photovoltaic panels  401 . If no Tedlar sheet is provided, the phase-change materials  411  are attached directly to the back side of the photovoltaic panels  401  in the closed box. Further, the husk mesh  412  is attached to the back side of the box of the phase-change materials  411 . The husk mesh  412  will be sprayed by fog/mist generated by the specially designed nozzles  410 . 
     Other modules/elements in the system according to  FIG. 4  of the present invention have the similar structure and similar work principle of that according to the  FIG. 1  of the present invention. 
     In the embodiment of the present invention according to  FIG. 4 , the specifically designed nozzles  410  generate a fog or mist applied onto the surface of the husk mesh  412 . The generated fog/mist covers the complete surface of the husk mesh  412  and distributes uniformly on the whole surface. The husk mesh  412  can absorb the fog/mist generated by the nozzles  410 . A residue fluid leaving husk mesh  412  is collected and returned to the tank  406  and recirculated within the closed loop system. The water supply system in the  FIG. 4  is a similar closed loop system as the closed loop system according to the embodiment of the present invention according to the  FIG. 1 . 
       FIG. 5A  illustrates a schematic diagram of a distribution system  507  as in accordance with the present invention. The distribution system  507  comprises a manifold  513  on which the specifically designed nozzles according to the present invention are arranged. The nozzles can be all small orifice nozzles, or all impingement nozzles, or a combination of small orifice nozzles and impingement nozzles. The distribution system  507  is in fluid communication with the closed loop system as illustrated in  FIG. 1 -FIG,  4 . The manifold distributes the cooling fluid pumped from the pump to the nozzles  510 . As an example shown in the  FIG. 5A  of the present invention, the manifold  513  of the distribution system  507  has four columns of nozzles, with four nozzles being arranged on each column.  FIG. 5B  illustrates an actual photo of the distributions system in accordance with the present invention. 
     In the present application, the nozzles  510  can be designed according to either a static approach or a dynamic approach. According to the static approach, the nozzles  510  are fixed on the manifold  513  as shown in the  FIG. 5 . Each nozzle provides a single constant fog/mist spray pattern. According to the dynamic approach, only four nozzles will be placed in a line on the manifold, wherein the nozzles are dynamically configured to oscillate, rotate or move. The movement of the nozzles will be driven with help of motors that are temperature controlled. Therefore, in this approach, the temperature of the solar photovoltaic panels will be monitored. If the temperature is high, the motors and the nozzles will be signaled to be operated until the temperature of the photovoltaic panels is brought down to the desired levels. 
     The distribution system according to the present invention is designed to ensure that the fog/mist generated by the nozzles covers a whole surface of the back side of the solar photovoltaic panels uniformly. The exact number of the nozzles and their positions for a best cooling effect depend on different sizes of the solar photovoltaic panels. Furthermore, dust and other very small particles within the fluid circulated in the closed loop system could easily block the orifice of the nozzles, since the nozzles have very small diameter. Therefore, two different types of filters should be provided in the closed loop system of the cooling system, one after taking water from the tank, and the other one after pumping it into the mist system, to ensure that no particles penetrate through. 
       FIG. 6  illustrates a schematic diagram of a solar photovoltaics panel fog/mist cooling system including a thermal resistance circuit in accordance with the present invention. The right schematic diagram of the  FIG. 6  shows the flow of circulation through the cooling system, wherein a working fluid, which is water or nanofluid will be pumped via a pump  605  from a tank  606  to the distribution system  607 , where the nozzles  610  are placed.  603  represents incident solar radiation. The nozzles  610  generate the fog/mist uniformly over the whole surface of the back side of the solar photovoltaic panels  601 . The fluid will be collected back to the tank  606  after filtering. 
     In the right schematic diagram, reference number  640  refers to a radiation heat exchange between photovoltaic panels and the sky. Reference number  641  refers to a convection heat transfer between photovoltaic panels  641  and surrounding air. Reference number  644  refers to a convection heat transfer between the back side of the fog/mist cooling system and the fluid inside fog/mist cooling system. Reference number  645  refers to a convection heat transfer between the photovoltaic panels  601  and the fluid inside the fog/mist cooling system. The reference number  642  refers to the edge side heat loss. The referece number  643  refers to the bottom side heat loss. 
     The left schematic diagram of  FIG. 6  shows the thermal resistance network through the cooling system. There are two thermal resistances between the photovoltaic panel and the surrounding. One thermal resistance  652  refers to R r, PV-a  by convection with the ambient air and another thermal resistance  653  refers to R c, PV-a  by radiation with the sky. From the backside of the photovoltaic panels, there is one thermal resistance  655 , referring to R c, PV-f , by convection with the cooling fluid in the fog/mist cooling system. There is also another thermal resistance on the bottom of the cooling system  656 , referring to R c, b-f , which is mainly by conduction through the cover material. Reference number  650  refers to the equivalent blackbody sky temperature T sky . Reference number  651  refers to a temperature of air T a . Reference number  654  refers to a temperature of the photovoltaic panels T PV . Reference number  657  refers to a temperature of the cooling fluid in the fog/mist cooling system T f . Reference numbers  658  and  659  refer to radiation and convection between the backside of the photovoltaic panels and the surrounding. For the radiation heat transfer between the photovoltaic panel and the surrounding air, the temperature of surrounding air shoud be set to the equivalent blackbody sky temperature T sky . This equivalent backbody sky temperature accounts for the facts that the atmosphere is not a uniform temperature and the atmosphere radiates only in certain wavelength bands. As shown in the  FIG. 6 , the input temperature of the system is the cold temperature coming from the pump  605 . The output temperature of the system is the temperature of the hot water collected at the end of the cooling process. 
     The specific designs of the nozzles applied in the present invention are illustrated in the  FIGS. 7A-7D  in detail. 
     The specifically designed nozzles applied in the present invention are impingement nozzles and/or small orifice nozzles.  FIG. 7A  shows a form of an impingement nozzle  701  and  FIG. 7B  shows a form of a small orifice nozzle  704 . The specifically designed nozzles have the work mechanism that fluid is pumped through tubes or pipes to the specifically designed nozzles and that the specifically designed nozzles force the water out in micro-scale sized droplets. These water droplets are small enough to instantly flash-evaporate. This flash-evaporate reduces the temperature of surrounding material e.g. the surface of the back side of the solar photovoltaic panels according to the present invention. Fog/Mist cooling system applying the specifically designed nozzles are economical due to the greatly reduced amount of the fluid used and its ability to induce cooling with making the solar panels too wet. 
     Theoretically, smaller droplets cover a larger surface area. The evaporation and cooling effectiveness can be enhanced. 
     Both types of impingement nozzles and small orifice nozzle have different shapes and operate at different pressures, which in turn allows them to produce distinct spray characteristics and in turn changes in costs and effectiveness. The NFPA 750 Standards also introduce three categories to differentiate water mist generation technologies, sorted by pressure regions, namely low, intermediate, and high pressure systems. Low pressure systems run at 12 bar pressures or less. Intermediate pressure systems run within the range from 12 to 34 bar. High pressure systems run at 34 bar pressures or above. 
     The structure and work principle of an impingement nozzle  701  is described in detail based on the  FIG. 7A  of the present invention. The impingement nozzles use the traditional sprinklers and nozzles used in spray systems and have an orifice with a large diameter. This means that small droplets can be produced with high velocities. A deflector is also utilized to break up the fluid jet into droplets, where the velocity of the water jet and the deflector shape are the main factors governing the droplet size and distribution in the fog/mist, as well as the cone angle and spray momentum. 
     According to the present application, the specifically designed impingement nozzles use operating pressures spanning both low and intermediate pressures. The generated mist will have Sauter mean droplet size of between 30-60 microns. 
     As shown in the  FIG. 7A  of the present invention, fluid is ejected from the nozzle circular orifice, which initially produces a solid stream of fluid. Then the fluid impinges upon a pin  702  after exiting from the orifice. The pin  702  breaks up the spray into fine homogeneous fog/mist. After this impact, a full cone pattern  703  is formed which rapidly breaks down into a fog/mist. The specifically designed impingement nozzles according to the present invention can be operated under a pressure of minimum 4 bar and with a flow rate varying between 0.043 l/min to 5.35 l/min to generate a cone-shaped spray pattern fog with a spray angle of 90°. A dimension of a male connection is ⅛″ NPT or ¼″ NPT. 
     The structure and work principle of a small orifice nozzle  704  is described in detail based on the  FIG. 7B  of the present invention. The small orifice nozzle belongs to the type of pressure jet nozzles consisting of small diameter orifices, where high velocity water leaving the opening of the nozzle becomes unstable and breaks up through the swirl chambers used. As shown in the  FIG. 7B , liquid is forced through a very small orifice  705  at a high pressure breaking it up into small droplets. After exiting from the orifice a full cone pattern  706  is initially formed which rapidly breaks a homogenous fog/mist. Small orifice nozzles allow for the usage of multi-nozzle heads at low pressures, which provide larger cone angles. 
     Small orifice nozzles have the ability to produce various features like fine droplets, wide spray angles, and good spray projection. It is found that the size and distribution of the droplets in the small orifice nozzles depends on the operating pressure, as pressure increase the droplets get finer. Droplets momentum also depends on the operating pressure, as pressure increase the momentum increases. However, the operating pressure should not exceed an upper limit, which means any point beyond the limit affects the distribution negatively. The specifically designed impingement nozzles according to the present invention can be operated under a pressure of minimum 7 bar and with a flow rate varying between 0.043 l/min to 5.35 l/min to generate a cone-shaped spray pattern fog with a spray angle of 70°. The dimension of a male connection is ⅛″ or ¼″ BSP or NPT or ⅜″-24 UNF pipe size. 
     The droplet size of the fog/mist generated by the speficially designed nozzles according to the present invention depends on the structure of the atomizer, intensity of the liquids energy, liquid surface tension and density. The size of the atomized droplets is not uniform. Therefore, the average droplets size becomes an important factor. A Sauter Mean Diameter (SMD) is the most commonly used term refering to the droplet volume/surface area ration. It is often shown as D32, μm (micrometer) unit, wherein 1 μm is equal to 10 −3  mm. The fog/mist generated by the speficially designed nozzles according to the present invention will have a Sauter mean droplet size of between 30 and 60 micrometer. 
       FIG. 7C  illustrates a full cone spray pattern having a conical pattern generated by the inventive specifically designed nozzles according to the present invention. As can be seen in the  FIG. 7D , the full cone spray pattern provides a round impact area and uniform and even distribution. 
       FIG. 8  illustrates a schematic diagram of a solar photovoltaics panel fog/mist cooling system provided with a heat exchanger in accordance with the present invention. The modules/elements in the cooling system according to  FIG. 8  of the present invention have a similar structure and work principle of that according to the  FIG. 1  of the present invention. Moreover, as shown in the  FIG. 8 , a heat exchanger  820  is provided at an upstream side of the tank  806 . A residue fluid collected from the solar photovoltaic panes  801  usually has high temperature. Therefore, the residue fluid can be collected and delivered through an outlet  809  to the heat exchanger  820  for heating purpose. The heat exchanger  820  according to the present invention is a tank coil heat exchanger, in which a fluid is circulated inside the coil. The hot residue fluid collected from the solar photovoltaics panel fog/mist cooling system is delivered into the heat exchanger  820  and will exchange the heat with the fluid circulated inside the coil of the heat exchanger. A fluid  830  heated within the heat exchanger  820  is delivered to other devices/units for heating purpose. The residue fluid after passing through the heat exchanger has a low temperature and is delivered to the tank  806  to be recirculated in the closed loop system. 
     The systems according to  FIG. 2 - FIG. 4  can also be provided with a heat exchanger similar to the heat exchanger  820  in the  FIG. 8  of the present invention. 
       FIG. 9  of the present invention shows a comparison of the bottom temperature development in time between the solar photovoltaic panels with and without the inventive solar photovoltaics panel fog/mist cooling system. The blue line illustrates the bottom temperature reduction in time measured for a system with a fog/mist cooling system according to the present invention, whereas the red line illustrates the bottom temperature reduction development in time measured for a system without a fog/mist cooling system. The bottom temperature refers to the temperature measured on the back side surface of the solar photovoltaic panels. The experimental results show that it is possible to decrease panel temperature to a temperature below 35° C. or even below 30° C. in the case of applying the fog/mist cooling system according to the present invention. It can be observed that with the inventive fog/mist cooling system, the solar photovoltaic panels can be operated under a low temperature constantly. 
     The main advantages accompanying the present invention is that the solar photovoltaic panels can be cooled in a better way and can be operated at lower and uniform temperate with higher performance constantly. For cooling down the solar photovoltaics panels less water consumption is required due to the use of a closed loop system. More electricity production and better control of the system can be achieved. Moreover, the cooling efficiency of the system is unaffected by the external environmental conditions, e.g. the temperature of the solar photovoltaic panel, the amount of dust or particles to be removed, the angle through which sunlight strikes at the solar photovoltaic panels. 
     Many changes, modifications, variations and other uses and applications of the subject invention will become apparent to those skilled in the art after considering this specification and the accompanying drawings, which disclose the preferred embodiments thereof. All such changes, modifications, variations and other uses and applications, which do not depart from the spirit and scope of the invention, are deemed to be covered by the invention, which is to be limited only by the claims which follow.