Abstract:
An invention proposes a heat sink system for large-size photovoltaic receivers of tower-type solar power stations with application of an array of heliostats intended to concentrate solar radiation on the photovoltaic receiver. 
     The heat sink system is designed as a two-phase thermo-siphon and it can ensure a stable temperature on all photovoltaic cells installed on the large-size receiver with very small deviations of the temperatures from one photovoltaic cell to another.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0001]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0002]    This invention proposes a heat sink system for large-size photovoltaic receivers of tower-type solar power stations with application of an array of heliostats intended to concentrate solar radiation on the photovoltaic receiver. 
         [0003]    The problem of effective chilling of photovoltaic cells installed on a large-size receiver presents a serious technical challenge. Significant deviations in temperatures of different PV cells cause significant decrease in efficiency of the entire photovoltaic receiver. 
         [0004]    US patent application No. 20070089775 describes a photovoltaic cell module for a receiver of solar radiation-based electrical power generating system. The module includes an assembly for extracting heat from the photovoltaic cells. The assembly includes a coolant chamber positioned behind and in thermal contact with the exposed surface of the photovoltaic cells. The coolant chamber includes an inlet for a coolant and an outlet for heated coolant. The assembly also includes a plurality of beads, rods, bars or balls of high thermal conductivity material in the coolant chamber that are in thermal contact with the photovoltaic cells and each other and together have a large surface area for heat transfer and define a three dimensional labyrinth that can conduct heat therethrough away from the photovoltaic cell or cells. 
         [0005]    In particular, the applicant of this patent application has found that the above-described cell module makes it possible to extract sufficient heat generated by incident concentrated solar radiation so that the temperature difference between the inlet coolant temperature and the front faces of the photovoltaic cells is less than 40.degree. C, typically less than 30 degree C., more typically less than 25 degree C., and in recent test work less that 20 degree C., and that this result can be achieved with a low pressure drop of coolant, typically less than 100 kPa, typically less than 60 kPa, and more typically less than 40 kPa across the coolant inlet and coolant outlet of the cell module. The low pressure drop is an important consideration because it means that it is possible to minimize the energy requirements for circulating coolant through the module. 
         [0006]    However, this technical solution cannot provide the temperature difference in the range of some Celsius degrees. In addition, there is a need in pumping means for circulating the coolant. 
         [0007]    US patent application No. 20080314437 describes a multi heliostat concentrating (MHC) system for utilizing sun energy, which has at least on MHC module. A MHC module has at least one optical concentrator having a focusing reflective surface, aperture and an optical axis. A plurality of heliostats, which are preferably located symmetrically relative to the optical axis of an optical concentrator simultaneously reflect sun radiation towards its aperture. Flux error correcting a flux homogenizing device disposed at the focal region of an optical concentrator provides for further concentrating and homogenizing the flux of the focused sun radiation. A receiver preferably comprising concentrated photovoltaic cells and an optional passive heat-sink provides for efficiently and economically generating electrical power. 
         [0008]    This patent application does not give description of the passive heat sink system. 
         [0009]    Therefore, application of two-phase thermo-siphon system for cooling the large-size photovoltaic receiver seems an attractive technical solution. 
         [0010]    However, there are some technical problems to be solved for such two-phase thermo-siphon system: 
         [0000]    1. It is necessary to provide permanent wetting of the rear side of an entire large-size metal plate with the photovoltaic cells installed on its forward side. This wetting should be by a low-boiling working medium (for example—acetone).
 
2. It is necessary that the working pressure in the evaporation chamber is very close to atmospheric pressure in order to ensure mechanical intact of this evaporation chamber.
 
3. The working pressure in the evaporation chamber should remain very close to atmospheric pressure during the night time without solar radiation despite heat losses from the evaporation chamber of the two-phase thermo-siphon into the surroundings via the entire large-size metal plate with the photovoltaic cells installed on its forward side, and heat losses via the other walls of the evaporation chamber.
 
         [0011]    Detailed description of thermo-siphons with regulation of their working pressure is presented in a book: EFFECTIVE HEAT EXCHANGERS WITH APPLICATION OF TWO-PHASE THERMO-SIPHONS, I. L. Pioro et al., Naukova Dumka, Kiev 1991 (in Russian). However, this book does not propose a simple and reliable design of a two-phase thermo-siphon with the required features. 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    The invention provides design of a large-size heat sink for photovoltaic power stations constructed as a photovoltaic receiver mounted on a tower and an array of heliostats directing reflected solar radiation on the photovoltaic receiver. 
         [0013]    The design has all required features described in the background of this invention. 
         [0014]    An evaporation chamber of the two-phase thermo-siphon includes following elements: 
         [0000]    1. a large-size metal plate with the photovoltaic cells installed on its forward side, the rear side of this metal plate is provided with a capillary structure;
 
2. a set of trays is mounted on the rear side of the large-size metal plate in such a way that the forward walls of these trays are formed by sections of the large-size metal plate with its capillary structure; the trays provide wetting of the adjacent areas of the capillary structure; in addition, there are overflow elements in construction of these trays, which ensure filling all the trays with the liquid working medium.
 
         [0015]    The evaporation chamber includes as well: 
         [0000]    1. lateral and a rear walls with an outlet connection for removal of vapors of the working medium to a condenser and an inlet connection for return of condensate of the working medium from the condenser into the upper tray;
 
2. an array of a vertical sealed containers mounted on the lower lateral wall of the evaporation chamber; these sealed containers are filled with phase change material (PCM) with melting point some Celsius degree lower than the operating temperature of the working medium; the outer surfaces of these sealed containers are provided with capillary coatings;
 
3. a safety valve (or valves), which provides fluid communication of the evaporation chamber interior with the atmosphere in the case of significant deviation of pressure in the interior of the evaporation chamber from the atmospheric pressure;
 
4. layers of thermal insulation of the lateral and rear walls of the evaporation chamber.
 
5. some inlet and outlet connections for supply and removal of the working medium in its liquid and vaporous states.
 
         [0016]    In addition, the lower section of the evaporation chamber can be provided with an electrical heater in order to maintain required pressure in the interior of the evaporation chamber in hours without solar radiation. 
         [0017]    An auxiliary pumping means is supplying the liquid working medium from the bottom section of the evaporator chamber into the inlet connection serving for feeding the liquid working medium into the upper tray. It allows to compensate for the condensate deficiency caused by condensation of the vaporous working medium on the walls of the evaporation chamber itself. 
         [0018]    In another version of this invention, the liquid working medium fills the evaporation chamber until such level, that the lower section of the capillary coating is submerged into the liquid working medium. For certain technical parameters of the evaporation chamber pumping ability of the capillary coating compensates for the condensate deficiency caused by condensation of the vaporous working medium on the walls of the evaporation chamber itself. 
         [0019]    The outer side of the large-size metal plate can be provided with stiffening ribs. 
         [0020]    Vapors of the working medium are removed from the evaporation chamber via the outlet connection and enter into a condenser, which is designed as a heat exchanger of a recuperation type with condensing the vapors of the working medium by a surrounding air or by cooling water. 
         [0021]    The evaporation chamber is provided with a sensor of internal pressure, which sends a signal to a control block. This control block in accordance with a value of difference between the internal pressure and atmospheric pressure regulates heat exchange rate in the condenser by adjusting flow rate of the cooling medium (surrounding air or cooling water). 
         [0022]    Condensate is returning from the condenser into the evaporation chamber via its inlet connection. 
         [0023]    In addition, there are a vacuum pump and a cooler-separator, which are in a fluid communication with the condensing side of the condenser and serves for removal of non-condensable gases from the interior of the two-phase thermo-siphon and recovery of the condensed working medium from the cooler-separator into the evaporation chamber. 
         [0024]    Another version of design of the evaporation chamber allows to diminish or obviate at all supply of electrical energy for heating the liquid working medium at night time; this version allows at the same time to maintain internal pressure in the evaporation chamber very close to the atmospheric pressure. 
         [0025]    Maintaining the evaporation pressure in this version is based on two technical solutions: a mechanical heat pump in combination with heat storage and discharging by PCM (phase change materials) with such temperature of melting that it is somewhat higher than the operating temperature of the working medium in the evaporation chamber. 
         [0026]    There is a condensation-evaporation vessel, which is packed with vertical sealed containers of small diameter; these containers are filled with PCM with melting temperature that somewhat higher than the operating temperature of the evaporation chamber. The walls of the condensation-evaporation vessel can be provided with layers of thermal insulation. 
         [0027]    The outer surfaces of the vertical sealed containers are provided with capillary coatings, which ensure constant wetting these surfaces by the liquid working medium. 
         [0028]    The condensation-evaporation vessel is provided with an inlet connection for supply of compressed and saturated vapors of the working medium into the tank during solar hours of operation and with an outlet connection serving for return flow of the evaporated working medium into the evaporation chamber during the night time. 
         [0029]    A mechanical compressor and a desuperheater are arranged in line and serve for pressurizing the vapors from the evaporation chamber and bringing them in the saturation state. 
         [0030]    The desuperheater is fed by the liquid working medium pumped from the evaporation chamber by a second auxiliary pump. 
         [0031]    The outlet connection of condensation-evaporation vessel is in fluid communication with the evaporation chamber through a control valve regulating a desirable pressure in the evaporation chamber. 
         [0032]    The photovoltaic cells, which are mounted on the outer surface of the large-size metal plate, can be provided with a displaceable screen allowing diminishment of heat losses by radiation and natural convection to the surroundings during the night time. 
         [0033]    The heat sink system operates in following manner: the liquid working medium, which wets the rear side of the large-size metal plate, is evaporating and obtained vapors are directed into the condenser of the recuperative type with their following condensing. The condensate is returned into the evaporation chamber and fed into the upper tray; thereafter this condensate passes in zigzag flow the complete array of the trays with wetting by capillary effect the most part of the rear surface of large-size metal plate. The pressure in the interior of the evaporation chamber is regulated by adjusting the cooling rate in the condenser. 
         [0034]    The auxiliary pumping means is supplying the liquid working medium from the bottom section of the evaporator chamber into the inlet connection serving for feeding the liquid working medium into the upper tray. It allows to compensate for the condensate deficiency caused by condensation of the vaporous working medium on the walls of the evaporation chamber itself. 
         [0035]    In the other version of this invention, the liquid working medium fills the evaporation chamber until such level, that the lower section of the capillary coating is submerged into the liquid working medium. For certain technical parameters of the evaporation chamber pumping ability of the capillary coating compensates for the condensate deficiency caused by condensation of the vaporous working medium on the walls of the evaporation chamber itself. 
         [0036]    It should be noted that energizing the electrical heater can ensure this compensation as well; however, from an energetic point of view, application of the auxiliary pumping means is preferable. 
         [0037]    At the night time the desired pressure in the evaporation chamber is established by latent heat of soldering of PCM in the vertical containers and, if it is needed, by an electrical heater installed in the bottom section of the evaporation chamber. 
         [0038]    Operation of the condensation-evaporation vessel in the second version is outlined above. 
         [0039]    There is a third version of the two-phase thermo-siphon design; this version comprises additionally an intervening container with a level gauge. The liquid working medium is pumped by the auxiliary pumping means into the intervening container until a certain height in it, and is discharged into a distributor of the evaporation chamber via a control cock. The rate of pumping the liquid working medium is significantly lower, than the rate of the condensed working medium supplied from the condenser. However, in the beginning period of operation of the photovoltaic receiver since advent of solar radiation, the control cock is open maximally in order to provide intensive wetting of the entire capillary coating of the large-size metal plate. In such a way, application of the auxiliary pump with the control valve allows to obviate usage of trays and ensures wetting the entire capillary coating of the large-size metal plate even for deviations in equal distribution of the liquid working medium across the width of the large-size metal plate and the condensate deficiency caused by condensation of the vaporous working medium on the walls of the evaporation chamber itself. 
         [0040]    The rear side of the large-size metal plate of the evaporation chamber can be provided with an array of vertical metal ribs, which allow to diminish deviations in distribution 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0041]      FIG. 1  shows a general view of a solar power station with photovoltaic cells installed on a tower and an array of heliostats, which concentrate solar radiation on the photovoltaic cells. 
           [0042]      FIG. 2  shows a rear side of a large-size metal plate; the forward side of this metal plate serves for installation of the photovoltaic cells. 
           [0043]      FIG. 3   a  shows a vertical cross-section of a first version of an evaporation chamber, condenser and an auxiliary appliances, which serve as a heat sink for cooling the photoelectrical cells. 
           [0044]      FIG. 3   b  shows a vertical cross-section of a first version of an evaporation chamber, condenser and an auxiliary appliances, which serve as a heat sink for cooling the photoelectrical cells; this first version includes an auxiliary pumping means arranged outside the evaporation chamber. 
           [0045]      FIG. 4   a  shows a vertical cross-section of a second version of an evaporation chamber, condenser and an auxiliary appliances, which serve as a heat sink for cooling the photoelectric cells. 
           [0046]      FIG. 4   b  shows a vertical cross-section of a second version of an evaporation chamber, condenser and an auxiliary appliances, which serve as a heat sink for cooling the photoelectric cells; this second version includes an auxiliary pumping means arranged outside the evaporation chamber. 
           [0047]      FIG. 5  shows a vertical cross-section of a third version of the evaporation chamber, condenser, an intervening container, the auxiliary pump and auxiliary appliances, which serve as a heat sink for cooling the photoelectric cells. 
           [0048]      FIGS. 6   a  and  6   b  show a back view of the large-size metal plate and its vertical transverse cross-section. 
           [0049]      FIGS. 7   a  and  7   b  show a back view of the large-size metal plate and its vertical transverse cross-section; the back side of the large-size metal plate is provided with vertical ribs. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0050]      FIG. 1  shows a general view of a solar power station with photovoltaic cells installed on a tower and an array of heliostats, which concentrate solar radiation on the photovoltaic cells. 
         [0051]    The solar power station comprises: heliostats  101 ; tower  102 ; a two-phase thermo-siphon  100 , which consists of an evaporation chamber  103  with photovoltaic cells  106  installed on the outer surface of the forward wall of the evaporation chamber  103 , fan  105  and condenser  104 . 
         [0052]      FIG. 2  shows a rear side of a large-size metal plate; the forward side of this metal plate serves for installation of the photovoltaic cells. 
         [0053]    It comprises: the metal plate  201  with a porous capillary coating  202  on its rear side; a multistage array of trays  203  with downcomers  204 ; an inlet connection  205 . 
         [0054]      FIG. 3   a  shows a vertical cross-section of a first version of an evaporation chamber, condenser and an auxiliary appliances, which serve as a heat sink for cooling the photoelectrical cells. 
         [0055]    It comprises: the evaporation chamber  103 ; fan  105 ; condenser  104 ; a vacuum pump  312 ; cooler-separator  313 ; control block  315 . 
         [0056]    The evaporation chamber consists of following components: the forward metal plate  201  with photovoltaic cells  106  installed on its external side and capillary coating  202  on its internal side; the multistage array of trays  203  with downcomers  204  mounted on the rear side of the forward metal plate  201 ; 
         [0000]    a lateral wall  304 , an upper and bottom walls  303  and  302 , and a rear wall  301 ; an inlet connection for supply of liquid working medium into the upper tray  203 ; an outlet connection for removal of the gaseous working medium from the evaporation chamber  103 ; an inlet connection  311  for recovery of the condensed working medium into the evaporation chamber; manometer  316 ; an outlet connection  310  with a safety valve  309 , which is mounted on this outlet connection  310 ; an electrical heater  308  arranged in the lower section of the evaporation chamber  103 ; an array of oblong sealed containers  306  filled with PCM  305  and a capillary coatings  307  on their outer surfaces. This PCM has a melting point somewhat lower than the operating temperature of the evaporation chamber  103  at sunny hours. 
         [0057]    Condenser  104 , which is in fluid communication with the outlet connection  317  and the inlet connection  205 , serves for condensing vapors of the working medium removed from the evaporation chamber  103 . 
         [0058]    Fan  105  supplies a cooling air into condenser  104 . 
         [0059]    A vacuum pump  312  and condenser-separator  313  are energized during beginning operation of the heat sink system with allowing to withdraw non-condensable gases (air) via valve  318  from the interior of the evaporation chamber  103  and condenser  104 . 
         [0060]    Operation of the entire heat sink system and its elements such as the electrical heater  308 , fan  105 , and valve  318  are regulated by a control block according to value of internal pressure measured by manometer  316  and presence of non-condensable gases in the interior of the evaporation chamber  103  and condenser  104 . 
         [0061]    The heat sink system operates in a following manner: supply of the liquid working medium into the upper tray  203  allows wetting entire capillary coating  202  of the metal plate  201 . It ensures effective cooling of photovoltaic cells  106  installed on the outer surface of this metal plate  201 . 
         [0062]    The evaporated working medium is expelled via the outlet connection  318  into condenser  104 . 
         [0063]    The condensed working medium is returning into the evaporation chamber  103  via the inlet connection  205 . 
         [0064]    PCM  305 , which is filled in the oblong sealed containers  306 , is melting during sunny hours at the expense of latent heat condensation of the working medium vapors on the outer surface of the oblong sealed containers  306 . At the night time molten PCM is solidifying with release of the latent heat of solidification and evaporating the liquid working medium from the external capillary coatings  307 . 
         [0065]      FIG. 3   b  shows a vertical cross-section of a first version of an evaporation chamber, condenser and an auxiliary appliances as in  FIG. 3   a  with two additional units: a lower outlet connection  319  and a first auxiliary pump  320 . 
         [0066]      FIG. 4   a  shows a vertical cross-section of a second version of an evaporation chamber, condenser and an auxiliary appliances, which serve as a heat sink for cooling the photoelectrical cells. 
         [0067]    It comprises: the evaporation chamber  103 ; fan  105 ; condenser  104 ; a vacuum pump  312 ; cooler-separator  313 ; control block  315 . 
         [0068]    The evaporation chamber consists of following components: the forward metal plate  201  with photovoltaic cells  106  installed on its external side and capillary coating  202  on its internal side; the multistage array of trays  203  with downcomers  204  mounted on the rear side of the forward metal plate  201 ; 
         [0000]    a lateral wall  304 , an upper and bottom walls  303  and  302 , and a rear wall  301 ; an inlet connection for supply of liquid working medium into the upper tray  203 ; an outlet connection for removal of the gaseous working medium from the evaporation chamber  103 ; an inlet connection  311  for recovery of the condensed working medium into the evaporation chamber; manometer  316 ; an outlet connection  310  with a safety valve  309 , which is mounted on this outlet connection  310 ; an electrical heater  308  arranged in the lower section of the evaporation chamber  103 . 
         [0069]    Fan  105  supplies a cooling air into condenser  104 . 
         [0070]    A vacuum pump  312  and condenser-separator  313  are energized during beginning operation of the heat sink system and allows to withdraw non-condensable gases (air) via valve  318  from the interior of the evaporation chamber  103  and condenser  104 . 
         [0071]    There is a sealed vessel  417  with an array of oblong sealed containers  419  filled with PCM  422  and a capillary coatings  427  on their outer surfaces. This PCM has a melting point somewhat higher than the operating temperature of the evaporation chamber  103  at sunny hours. 
         [0072]    The vapors of the working medium are removed at the sunny hours via the outlet connector  424  from the evaporation chamber  103  and compressed by compressor  420 . The superheated pressurized vapors are brought into saturation state in desuperheater  421  by atomization of the liquid working medium, which is supplied by pump  418  from an inlet connection  319  in the bottom section of the evaporation chamber  103 . 
         [0073]    In such a way, the latent condensation heat of the working medium vapors is transformed into the latent heat of melting of PCM in the oblong sealed containers  419 . 
         [0074]    At the night time the molten PCM  422  is solidifying with release of the latent heat of solidification and evaporating of the liquid working medium from the external capillary coatings  427 . The working medium vapors are entering into the evaporation chamber via the outlet connection  425  of vessel  417 , a control valve  416  and an inlet connection  423  of the evaporation chamber  103 . 
         [0075]    The heat sink system operates in a following manner: supply of the liquid working medium into the upper tray  203  allows wetting entire capillary coating  202  of the metal plate  201 . It ensures effective cooling of photovoltaic cells  106  installed on the outer surface of this metal plate  201 . 
         [0076]    The evaporated working medium is expelled via the outlet connection  318  into condenser  104 . 
         [0077]    The condensed working medium is returning into the evaporation chamber  103  via the inlet connection  205 . 
         [0078]    PCM  422 , which is filled in the oblong sealed containers  419 , is melting during sunny hours at the expense of latent heat condensation of the working medium vapors on the outer surface of the oblong sealed containers  419 . At the night time molten PCM is solidifying with release of the latent heat of solidification and evaporating of the liquid working medium from the external capillary coatings  427 . 
         [0079]    Entrance of hot vapors of the working medium into the evaporation chamber  103  at the night time allows to keep such pressure in the evaporation chamber  103 , which is very close to atmospheric pressure. 
         [0080]      FIG. 4   b  shows the same as  FIG. 4   a  with an additional pump  320  for supplying the liquid working medium in the inlet connection  205 . 
         [0081]      FIG. 5  shows a vertical cross-section of a third version of the evaporation chamber, condenser, an intervening container and auxiliary appliances, which serve as a heat sink for cooling the photoelectrical cells. It comprises: the evaporation chamber  103 ; fan  105 ; condenser  104 ; a vacuum pump  312 ; cooler-separator  313 ; control block  315 . 
         [0082]    The evaporation chamber consists of following components: the forward metal plate  201  with photovoltaic cells  106  installed on its external side and capillary coating  202  on its internal side; distributor  534  of the liquid working medium; 
         [0000]    a lateral wall  304 , an upper and bottom walls  303  and  302 , and a rear wall  301 ; an inlet connection for supply of liquid working medium into the upper tray  203 ; an outlet connection for removal of the gaseous working medium from the evaporation chamber  103 ; an inlet connection  311  for recovery of the condensed working medium into the evaporation chamber; manometer  316 ; an outlet connection  310  with a safety valve  309 , which is mounted on this outlet connection  310 ; an electrical heater  308  arranged in the lower section of the evaporation chamber  103 . 
         [0083]    Fan  105  supplies a cooling air into condenser  104 . 
         [0084]    A vacuum pump  312  and condenser-separator  313  are energized during beginning operation of the heat sink system and allows to withdraw non-condensable gases (air) via valve  318  from the interior of the evaporation chamber  103  and condenser  104 . 
         [0085]    There is a sealed vessel  417  with an array of oblong sealed containers  419  filled with PCM  422  and a capillary coatings  427  on their outer surfaces. This PCM has a melting point somewhat higher than the operating temperature of the evaporation chamber  103  at sunny hours. 
         [0086]    The vapors of the working medium are removed at the sunny hours via the outlet connector  424  from the evaporation chamber  103  and compressed by compressor  420 . The superheated pressurized vapors are brought into saturation state in desuperheater  421  by atomization of the liquid working medium, which is supplied by pump  418  from an inlet connection  319  in the bottom section of the evaporation chamber  103 . 
         [0087]    The additional pump  320  supplies the liquid working medium into an intervening container  531  with a level gauge  533 . Supply of the liquid working medium into the inlet connection  295  is regulated by a control cock  532 . 
         [0088]      FIGS. 6   a  and  6   b  show a back view of the large-size metal plate and its vertical transverse cross-section. 
         [0089]    It comprises the photovoltaic cells  106 , the large-size metal plate  201  with the capillary coating  202 , a metal strip  634  with a lower toothed edge  635 , face plane wall  636 ; the upper section of the large-size metal plate  201 , a metal strip  634  with a lower toothed edge  635  and face plane wall  636  form distributor  534 . 
         [0090]      FIGS. 7   a  and  7   b  show a back view of the large-size metal plate and its vertical transverse cross-section having the same parts as in  FIGS. 6   a  and  6   b  with vertical ribs  701 .