Abstract:
Hybrid system of parametric solar thermal cylinder ( 14 ) and photovoltaic receiver ( 3 ), which comprises a thermal absorber receiver ( 2 ) through which a heat-carrier fluid circulates, and, additionally at least one spectral separation filter ( 4 ), situated between the photovoltaic receiver ( 3 ) and the thermal absorber receiver ( 2 ), which receives the light reflected from the primary mirror ( 1 ) of the parametric cylinder ( 14 ) and which permits the selective separation of the solar spectrum, directing a part thereof towards the photovoltaic receiver ( 3 ) and the remainder towards the thermal absorber receiver.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention refers to a hybrid system of parametric cylinder solar thermal receivers and photovoltaic receivers in an integrated solar concentration system. 
       BACKGROUND OF THE INVENTION 
       [0002]    Solar concentration technology is divided into two main blocks: solar thermal concentration and photovoltaic solar concentration. 
         [0003]    The operating principle of both systems is based on the same concept: using an optical system that concentrates light. Said concentrated light can be directed to heat a fluid which enters a turbine cycle in the case of thermal systems, or directly generates electricity through photoelectric effect on a semiconductor in the case of photovoltaic systems. 
         [0004]    Photovoltaic concentration technology consists of concentrating levels of solar radiation on cells of very reduced size. The use of a much cheaper optical element to concentrate incident light allows the use of more efficient solar cells (which are also generally more expensive). These systems can be potentially more competitive in terms of cost compared with conventional photovoltaic systems, as they largely replace the semiconductor area with standard optical elements such as lenses or mirrors, as well as being more efficient. 
         [0005]    The combination of both reflexive and refractive elements gives rise to a wide variety of solar concentration systems. Nevertheless, in most photovoltaic concentration systems Fresnel lenses have been used with secondary concentration elements, double-mirror systems with light homogenisers or dielectric or, either mirrored or dielectric CPC (compound parabolic concentrators) systems. 
         [0006]    Therefore, in the field of solar thermal concentration systems, two technologies currently dominate the market, namely the parabolic cylinder and the tower. 
         [0007]    In particular, parabolic cylinder technology consists of a parabolic mirror which focuses the light on an absorber tube through which a heat-carrier fluid circulates. Said fluid heats up and is subsequently used to heat the steam used in a traditional turbine cycle. 
         [0008]    At the present time, electricity production using solar thermal concentration plants is not, from a financial point of view, among the most competitive of the renewable technologies designed to obtain electrical power from the sun. Plants with solar thermal concentration (either parabolic cylinders or towers) have higher energy production costs than those associated with photovoltaic or wind energy systems. However, solar thermal concentration systems have a competitive advantage in that they are able to operate with thermal storage. Said storage provides manageability, as well as electricity supply during the time slots when electricity consumption is at its highest. This presupposes a differentiating factor in solar thermal technology compared with, for example, wind or photovoltaic technologies. 
         [0009]    In the case of photovoltaic solar concentration, more efficient and economical systems are obtained than those of solar thermal concentration, however they have the disadvantage of being less manageable and they cannot store energy in a cost-efficient and effective manner, compared with solar thermal technology. 
         [0010]    The document described in US 2009/0283144 A1 provides a device with one or several photovoltaic solar cells and at least one solar concentration mirror situated in the vicinity of the solar cells. The mirror comprises a multilayer optical film and a layer of UV protection applied on the previous film in such a way that the solar concentrator reflects a band of the visible spectrum (that corresponding to the absorption bandwidth of the solar cell) towards the solar cell, and transmits the wavelengths that may degrade or negatively affect the solar cell. Thus, improved functioning of photovoltaic solar cells is obtained. 
         [0011]    In respect of hybrid photovoltaic-solar thermal systems, scientific literature contains several concepts which use spectral separation in order to obtain improved use of the solar spectrum by combining solar thermal collectors and solar cells. However, integrating solar thermal and photovoltaic technologies in a single hybrid solar concentration system is not an easy task. Some proposals refer to spectral separation systems applied to a system with Stirling dishes and engines. 
         [0012]    It is therefore necessary to obtain a hybrid photovoltaic-solar thermal system which resolves the aforementioned disadvantages, so that the integrated system is characterised by lower cost than standard solar thermal technology, due to the inclusion of the photovoltaic part working very efficiently in the selective spectral range, and which, in addition, is manageable, providing energy in a stable manner and during the time slots when it is really needed. 
       SUMMARY OF THE INVENTION 
       [0013]    Therefore, the purpose of the invention is to provide a hybrid system of parametric solar thermal cylinder and photovoltaic receiver which allows storage and manageability of the energy with lower production costs than standard solar thermal technology, providing a more efficient integrated solar concentration system. 
         [0014]    The hybrid system of parametric solar thermal cylinder and photovoltaic receiver in this invention comprises a thermal absorber receiver through which a heat-carrier fluid circulates, and additionally at least one spectral separation filter between the photovoltaic receiver and the thermal absorber receiver, which receives the light reflected from the primary mirror of the parametric cylinder and which allows selective separation of the solar spectrum, directing a part thereof towards the photovoltaic receiver and the rest towards the parametric cylinder. 
         [0015]    According to an embodiment of the invention, the part of the solar spectrum directed towards the photovoltaic receiver is the part reflected by the spectral separation filter and the part of the solar spectrum directed towards the thermal absorber receiver is the part transmitted by the spectral separation filter. 
         [0016]    According to another embodiment of the invention, the part of the solar spectrum directed towards the photovoltaic receiver is the part transmitted by the spectral separation filter and the part of the solar spectrum directed towards the thermal absorber receiver is the part reflected by the spectral separation filter. 
         [0017]    The spectral separation filter therefore allows selective separation of the solar spectrum in such a way that the radiation bands are directed selectively to each of the receivers (thermal and photovoltaic). The spectral range of the part focused on the photovoltaic receiver is situated specifically in the band where the solar cell operates with the best performance. This results in a more efficient integrated system in which costs are optimised. 
         [0018]    The hybrid system in this invention has the following advantages:
       Better use is made of solar radiation with respect to traditional photovoltaic systems or those with separate solar thermal concentration.   The integrated system obtained is more manageable compared with other traditional photovoltaic systems or those with photovoltaic concentration.       
 
         [0021]    Therefore, the result is a solution that is economically more profitable, in which power is manageable and all the advantages of renewable energies are retained (reduction of greenhouse gases, pollution etc.). 
         [0022]    Other characteristics and advantages of this invention will be disclosed in the following detailed description of the illustrative and non-limiting embodiment of its purpose in relation to the figures attached hereto. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0023]      FIG. 1 : shows a diagram of the invention according to an embodiment, referred to hereinafter as direct configuration. 
           [0024]      FIG. 2 : shows a diagram of the invention according to another embodiment, referred to hereinafter as inverse configuration. 
           [0025]      FIG. 3 : shows a diagram of another embodiment of the invention. 
           [0026]      FIG. 4 : shows a diagram of another embodiment of the invention. 
           [0027]      FIG. 5 : shows a diagram of the functioning of the spectral separation filter. 
           [0028]      FIG. 6 : shows a diagram of the architecture of the spectral separation filter. 
           [0029]      FIG. 7 : System of thermal management using extruded heat sink with transversal orientation. 
           [0030]      FIG. 8 : System of thermal management using extruded heat sink with parallel orientation. 
           [0031]      FIG. 9 : System of thermal management using extruded heat sink which encourages internal air flow with parallel orientation. 
           [0032]      FIG. 10 : System of thermal management using a heat pipe. 
           [0033]      FIG. 11 : System of thermal management by means of active cooling circuit. 
           [0034]      FIG. 12 : Transmission curve of the spectral separation filter in direct configuration. 
           [0035]      FIG. 13 : Transmission curve of the spectral separation filter in inverse configuration. 
           [0036]      FIG. 14 : Alternative system of thermal management by means of an active cooling circuit comprising pumps, fans, and pipes through which the cooling fluid flows. 
           [0037]      FIG. 15 : Pipes or ducts through which the cooling fluid of the system flows in  FIG. 14   
           [0038]      FIGS. 16 and 17 : Photovoltaic receiver with the pipe of  FIG. 15  incorporated. 
           [0039]      FIG. 18 : Transmission curve of the spectral separation filter with aperiodic design. 
           [0040]      FIG. 19 : Diagram of the architecture of the spectral separation filter in  FIG. 18   
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]      FIG. 1  shows an embodiment of the invention. The hybrid system in this embodiment of the invention comprises:
       A primary mirror  1  which focuses light and is preferably in a parabolic shape.   A spectral separation filter  4 , which will reflect a specific band of the useful visible spectrum for the photovoltaic receiver  3  which will be described below and which will transmit the remaining wavelengths.  FIG. 5  conceptually shows the functioning of the spectral separation filter  4 . This element shall be manufactured preferably by depositing multi-layers of transparent oxides with a low/high refractive rate, which would filter the spectrum as shown in the diagram in  FIG. 6 .   A thermal absorber receiver  2 , which will preferentially capture the spectrum bands with shorter and longer wavelengths than the visible band reflected.   A photovoltaic receiver  3 , which will preferentially capture wavelengths in the visible spectrum. Said photovoltaic receiver  3  may comprise:
           Re-concentrating mirrors  8 .   Interconnected photovoltaic solar cells  12 .   Glass substrates for protecting the cells and internal elements of the photovoltaic receiver  3 .   Encapsulating materials   Thermal management systems, comprising:
               a material which insulates the system electrically and transfers the heat generated in the cell  12  towards the heat sink, and   a thermal management system. There are various options such as heat sinks manufactured by extrusion, heat pipes, or active cooling circuits.   
               
               
 
         [0053]    The parametric cylinder  14  will comprise as a minimum the primary mirror  1  and the thermal absorber receiver  2 . 
         [0054]    In the embodiment of  FIG. 1  the thermal absorber receiver  2  is placed on the spectral separation filter  4 , and the photovoltaic receiver  3  below said filter  4 . This arrangement allows the light reflected by the primary mirror  1  of the parametric cylinder  14  to selectively separate in the spectral separation filter  4 , such that the part reflected by said filter  4  is directed towards the photovoltaic receiver  3 , and the part transmitted by said filter  4  is directed towards the thermal absorber receiver  2 . 
         [0055]      FIG. 3  shows an embodiment in which the photovoltaic receiver  3  is placed on the primary mirror  1  of the parametric cylinder  14 . 
         [0056]    As a design alternative there is an inverse option in which the part transmitted is directed to the photovoltaic receiver  3  (see  FIG. 2 ). In the embodiment of  FIG. 2  the thermal absorber receiver  2  is placed beneath the spectral separation filter  4 , and the photovoltaic receiver  3  on top of said filter  4 . This arrangement allows the light reflected by the primary mirror  1  of the parametric cylinder  14  to be selectively separated in the spectral separation filter  4 , so that the part reflected by said filter  4  is directed towards the photovoltaic receiver  3 , and the part transmitted by said filter  4  is directed towards the thermal absorber receiver  2 . 
         [0057]      FIG. 4  shows an embodiment of the invention in which the system optionally includes a reflector or secondary collector  5  which re-concentrates light directly onto the thermal absorber receiver  2 , and which permits an increase in the area of aperture of the primary and, consequently, the temperature at which said receiver could operate. For the purposes of simplification, primary mirror  1  has not been represented. 
         [0058]      FIG. 3  also shows the reflector or secondary collector  5  of  FIG. 4 , although for the purpose of simplification some elements of the invention in  FIG. 3  have been omitted. 
         [0059]    The system in this invention may present several filters of spectral separation  4  and various photovoltaic receivers  3 . In this way the range of light to the thermal absorber receiver  2  would be removed, which could be less efficient. 
         [0060]    The primary mirror  1  of the parametric cylinder  14  may be made from curved glass and have a reflecting surface manufactured in silver or aluminium. It may also be made from any reflective material. 
         [0061]    The spectral separation filter  4  may be curved, flat or faceted, and shall be optimised in order to work on transferring/reflecting light in the range which maximises the efficiency of the photovoltaic receiver  3 . 
         [0062]      FIG. 5  shows a diagram of the functioning of a spectral separation filter  4 , in which part of the light is reflected and part is transmitted, and  FIG. 6  shows a diagram of its architecture, with various layers  6  situated on a substrate  7 . 
         [0063]    The preferred method for manufacturing filters  4  is through sputtering. Said technique is a physical process in which atoms are vaporised from a solid material known as a “target”, which is bombarded with energetic ions. The layers of transparent oxides are combined with different refractive index and thickness. Said refractive index and thickness of layers allows determination of the optical path that the light will take, thus determining the transmission/reflectance conduct presented, depending on the incident wavelength. 
         [0064]    There are other filter manufacturing methods such as evaporation or immersion techniques (dip coating) using sol-gel techniques. Evaporation consists of the passage from liquid phase to gas of a material deposited in the substrate, normally a solar grade glass. In dip coating, the substrate is submerged in solutions, depositing the desired layer, and subsequently subjecting it to partial curing. This process of deposition and immersion may be repeated as often as necessary to obtain the desired structure. 
         [0065]    The cooling or heat management system of the photovoltaic receiver  3  may be passive (aluminium heat sinks  9  produced through different manufacturing processes, heat pipes or product transfer pipes (circulation of a fluid). 
         [0066]      FIGS. 7, 8 and 9  show various configurations of heat management systems using heat sinks  9  manufactured through extrusion processes. These heat sinks may be oriented transversally towards the cell  12  ( FIG. 7 ) or in parallel ( FIGS. 8 and 9 ). 
         [0067]      FIG. 10  shows a system of thermal management using heat pipes. A heat pipe is a passive system of heat evacuation. It consists of a sealed pipe  10  with a material that adheres to its walls and which contains a fluid inside. The heat is evacuated through a change in the vapour-liquid phase. The pipe consists of three differentiated sections: the evaporation section, the adiabatic section and the condensation section. The heat is transferred from the surface to be cooled to the evaporator where the fluid contained inside the pipe  10  is vaporised and the steam rises through the same, passing through the adiabatic section until it enters a condensation section in which said vapour is condensed. The material adhering to the walls of the pipe  10  generates capillarity forces on the working fluid of the heat transfer pipe and encourages movement of the fluid from the condenser to the evaporator. By means of this system, a very effective heat transmission is guaranteed from the heat generating source (photovoltaic cell) to the heat dissipation fins  11 . This is a sealed means of contention, with no need for pumping, and in which the fluid used will preferably be water. It may be designed to cool solar cells which are required to work at temperatures below a maximum temperature of some 60°-70° C. in order to maintain its efficiency. 
         [0068]    The fluid to be chosen for use inside will depend on the temperature range of the functioning of the heat transfer pipe and also the material used for construction of the pipe  10  material, those most frequently used include: helium, nitrogen, ammonia, acetone, methanol, ethanol, water, toluene, mercury, sodium, lithium and silver. 
         [0069]    The most frequently used materials for construction of the pipes  10  are copper, nickel, steel, aluminium, niobium, tungsten, and alloys of these metals. 
         [0070]      FIG. 11  shows a system of thermal management by means of active cooling circuit  13 . Said circuit  13  would use water or other cooling liquid which circulates in contact with the back part of the photovoltaic receiver  3 . Said fluid heats up as it passes through the length of pipe, cooling the solar cell  12 . This is the most efficient system of thermal dissipation, however it implies fluids in permanent movement in addition to the apparatus needed to move them, such as pumps. 
         [0071]    The pipe used in the circuit  13  of  FIG. 11  may be steel or aluminium and with a diameter between 50 and 200 mm. With respect to length, this will be defined according to the length of the photovoltaic receiver  3 . The pipe could have a flat face designed to improve heat transmission in the area of contact with the photovoltaic receiver  3 . 
         [0072]    Another preferred embodiment of a system of heat management using an active cooling circuit is shown in  FIGS. 14 to 17 . The system is formed by at least one pump, at least one fan  15  and at least one pipe  16  through which the cooling fluid flows ( FIG. 14 ). In an embodiment, the pipe would have a diameter of 45 mm and a cooling fluid flow of 1.8 kg/sec. would circulate through its interior 
         [0073]    Preferably the cooling system will be active. The pump (or pumps) propels the cooling fluid that circulates through the pipes  16  and the fans  15  cooling the hot fluid proceeding from the cooling circuit. By way of example, the cooling fluid could be a mixture of water and glycol at 50% in weight with the following temperatures in a specific application: 
         [0000]    T1 (outlet of the fan/entrance of the circuit): 60° C.
 
T2 (halfway through the circuit): 71.5° C.
 
T3 (outlet of the fan/entrance of the circuit): 75.2° C.
 
         [0074]      FIG. 15  shows the pipes  16  (which may be made from aluminium, for example) which allow circulation of the cooling fluid in closed circuit. As this is a linear system formed by discrete parts, it is necessary to use sealing means for their connection such as for example, sealing gaskets. A discrete part has a finite length. As a parabolic cylinder collector is relatively long, it is necessary to join these discrete parts using sealed joints if active dissipation is used. 
         [0075]    The pipes  16  may be extruded parts, and they may be multi-channel, that is with at least one channel through which the cooling fluid flows. 
         [0076]      FIGS. 16 and 17  show the complete apparatus of the pipe  16  with the photovoltaic receiver. These figures show the photovoltaic cell  17  of the receiver. The lateral elements consist of the homogeniser  18  and the filter  19  is situated in the lower part. All of which is joined and sealed with an adhesive (for example, silicone). 
         [0077]    In respect of the spectral separation filters  4 , their design is determined by the direct or inverse configuration of the system. 
         [0078]    The materials of the filters  4  are preferably transparent glass substrates, multi layers of transparent oxide conductors such as silica oxide or niobium oxide, transparent materials in the visible part and reflectors in the infra-red (by way of example we could cite layers of transparent oxides such as ITO), passivation layer and anti-reflecting layer. The filter  4  may include all or some of the aforementioned layers. 
         [0079]    The passivation layer is a barrier layer which reduces diffusion of the glass component (impurities such as NA) to the multi-layer of transparent oxides. 
         [0080]    Preferably, the number of layers of the complete filter  4  (of the oxide part) would be 1 to 200. It would be even more preferable for these to be 4 to 100, and ideally from 5 to 20. 
         [0081]    The anti-reflective layer may be found at the two ends of the filter  4  or only at one end. Normally it will be on the side where the solar radiation occurs. Preferably these shall be silica oxide layers as they have a low refractive index. It is desirable for the refractive index of these layers to be intermediate between the glass and the air. 
         [0082]    In the event of direct configuration a filter  4  should be obtained which provides:
       Maximum reflectance in a range (reflection band) 550-950 nm.   Maximum transmittance in a range 300-550 nm and 950-2500 nm.       
 
         [0085]      FIG. 12  shows the design and optical conduct at different angles of incidence of a spectral separation filter in accordance with the requirements described. 
         [0086]    As may be seen in the figure, the design of the filter  4  is defined by the following sequence
       200L/V/100L/48H/(145L/85H)×3/280L       
 
         [0088]    With V being glass, H transparent niobium oxide with a high refractive index (n_H=2.30) and L transparent silica oxide with a low refractory index (n_L=1.43). The numbers preceding each of these materials refer to the thickness of the layer of material (in nanometres). In this particular design, the layer (145L/85H) is repeated three times. 
         [0089]    The oxide layers may vary in the following ranges of thickness: from 1 to 1000 nm and, preferably from 5 to 400 nm. 
         [0090]    The two outermost layers  200 L and  280 L are proposed as anti-reflecting layers of the structure. In the event of inverse configuration, a filter  4  should be obtained which provides:
       Maximum transmittance in a range (reflection band) 550-950 nm.   Maximum reflectance between 400-550 nm and 950-2500 nm       
 
         [0093]      FIG. 13  shows the design and optical conduct of a spectral separation filter in accordance with the requirements described.
       V/M/90L/25H/(75L/42H)×3/150L       
 
         [0095]    With V being glass, H transparent niobium oxide with a high refractive index (n_H=2.30) and L transparent silica oxide with a low refractory index (n_L=1.43) and M is a transparent material in the visible and reflecting parts in the IR. By way of example there are many transparent oxides such as the ITO which may comply with this function, without being restricted to use of these materials. 
         [0096]    There is also the alternative of removing the material M and using simply transparent glass. In this case a multi-layer L/H would need to be added in order to reflect radiation in the range of 950-1300 nm. 
         [0097]    The outermost 150L is proposed as a passivation and anti-reflecting layer (AR) of the structure. The passivation layer is a barrier layer which reduces diffusion of the glass component to the multi-layer of transparent oxides. 
         [0098]      FIG. 18  shows the optical conduct of another spectral separation filter  4 , which is applicable in concentration systems subject to high temperatures and solar degradation because it does not degrade. In this case the filter is an aperiodic design defined by the following sequence:
       130L/(162H/262L/164H/261L/159H/250L)/(156H/212L/123H/208L/127H/196L)/142H1 249L/Glass/114L   in which L is SiO 2 , H is Nb 2 O 5 , and in which the numbers that precede each of the materials refer to the thickness of the layer of said material in nanometres. The outer layer, SiO 2  with a thickness of 114 nm is proposed as an anti-reflecting layer of the structure.       
 
         [0101]    The filter is very efficient in the IR and transmits between 500-950 nm. In the zone of 550 nm downwards there are third order interferences. This means that the curve oscillates between high and low transmission values. 
         [0102]    Tertiary interferences below 500 nm would not be a problem, as it is light which could be adequately made use of by both receivers, thus it does not affect the global efficiency of the system. 
         [0103]      FIG. 19  shows a diagram of the spectral separation filter architecture the optical conduct of which is represented in  FIG. 18 , where DM 2  is (156H/212L/123H/208L/127H/196L) and DM 1  is (162H/262L/164H/261LJ159H/250L). 
         [0104]    The use of an aperiodic design improves the conduct of the filter. Basically, making aperiodic layers enables secondary interferences that occur below 550 nm to be mitigated. The aim is to ensure that the filter behaves as in IR below 550 nm (reflecting above) and this is obtained due to the aperiodic design with a reduced number of layers. 
         [0105]    Below a preferred embodiment is described for a hybrid photovoltaic-solar thermal system, which would correspond to  FIG. 2 . 
         [0106]    Said system would be formed by a parametric cylinder  14 , with an aperture of 8.2 m and a focal distance of 2.235 m (with the focal distance being the distance between the primary mirror  1  of the parametric cylinder  14  and the photovoltaic receiver  3 ). The spectral separation filter  4  would reflect the light in an absorber receiver pipe  2  of 70 mm diameter. The spectral separation filter  4  is positioned at a distance of less than 100 mm from the pipe, which would preferably transmit light within a range of 500 to 950 nm to the photovoltaic receiver  3 . 
         [0107]    A thermal receiver  2  would be positioned in the reflected light focus, with a diameter of 90 mm. 
         [0108]    With respect to the spectral separation filter  4 , a structure defined by the following sequence is proposed
       M/90L/25H/(75L/42H)×3       
 
         [0110]    With H being transparent niobium oxide with a high refractive index (n_H=2.30) and L transparent silica oxide with a low refractory index (n_L=1.43) and M is a transparent material in the visible and reflecting part in the IR. 
         [0111]    There is also the alternative of removing the material M and simply using transparent glass. In this case a multi-layer L/H would need to be added in order to reflect radiation in the range of 950-1300 nm. 
         [0112]    For the material H, TiO 2  y Nb 2 O 5 , could preferably be used in order to be deposited with SiO 2 , due to the difference of refractive index between them. The structure of layers may vary between 5 and 100, depending on the performance that can be obtained from the filter. 
         [0113]    Optionally at the ends of the multilayer structure, a barrier layer of diffusion of components can be deposited between the glass and the multilayer, in addition to anti-reflecting layers in the other end, together with a hard layer which protects the structure from abrasive agents. 
         [0114]    The glass on which the multilayer is to be deposited could be composed of an anti-reflecting layer deposited on one or two faces. 
         [0115]    Although this invention has been described here entirely in connection with preferred embodiments, it is evident that modifications could be introduced within the scope of the following claims, without said scope being limited by the aforementioned embodiments.