WO2013021397A1

WO2013021397A1 - Absorber for concentrated solar power system

Info

Publication number: WO2013021397A1
Application number: PCT/IN2012/000520
Authority: WO
Inventors: Nitin Goel
Original assignee: Sunborne Energy Technologies Pvt Ltd; Imdea Energy Institute; Steinfeld, Aldo; Romero, Manuel; POKKUNURI, Prasad; Goswami, Dharendra Yogi; STEFANAKOS, Elias Kyriakos
Priority date: 2011-08-08
Filing date: 2012-07-26
Publication date: 2013-02-14

Abstract

The present subject matter relates to an absorber (100) for a Concentrated Solar Power (CSP) system. The absorber (100) includes an enclosure (102), where the enclosure (102) includes an incident wall (106) having an aperture (110) for access of sunlight. The enclosure (102) includes a first heat transfer structure (114) provided in a cavity (108) of the enclosure (102) to absorb a first portion of thermal energy from the sunlight. Furthermore, the incident wall (106) includes a second heat transfer structure (112) to absorb a second portion of thermal energy from the sunlight.

Description

ABSORBER FOR CONCENTRATED SOLAR POWER SYSTEM

TECHNICAL FIELD

[0001] The present subject matter relates, in general, to concentrated solar power (CSP), and in particular, to an absorber for a CSP system.

BACKGROUND

[0002] Concentrated Solar Power (CSP) systems utilize sunlight for the purpose of power generation, process heat applications, or to generate fuel, such as through thermo-chemical or gasification processes. Generally, a reflector system, including mirrors or lenses, is used in the CSP system to receive incident sunlight and then to concentrate the incident sunlight onto a smaller area. The incident sunlight is directed onto a heat exchanger known as an absorber. In the absorber, thermal energy from the incident sunlight is absorbed and transferred to a heat transfer fluid, such as water, molten salt, gas or air. Thereafter, the heat transfer fluid can either be conveyed for power generation purposes, process heat applications, solar fuels or be stored for later use.

[0003] The absorbers may be classified based on geometrical configuration, heat transfer fluid, or heat transfer structures. Based on the geometrical configurations, the absorbers can be either cavity type absorbers or external type absorbers. The selection of a particular absorber type in the CSP system depends on a plurality of parameters, such as configuration of the reflector system and geographical location of the CSP system. A cavity type absorber generally includes an enclosure having a cavity, which receives the incident sunlight from the reflector system through an aperture in the enclosure. The incident sunlight, after entering the cavity, impinges on heat transfer structures located inside the cavity to transfer the thermal energy to the heat transfer fluid.

[0004] External type absorbers, on the other hand, are typically designed as flat plate or cylindrically shaped tubular panels. In these types of absorbers, the heat transfer structures are external and exposed to the sunlight which is focused thereon by the reflector system.

[0005] Conventional absorbers employ different types of heat transfer structures, such as tube type heat transfer structures and volumetric type heat transfer structures. In a tube type heat transfer structure, the thermal energy absorbed by one or more channels of the tube type structure is transferred to the heat transfer fluid flowing in the channels. In a volumetric type heat transfer structure, the thermal energy is absorbed inside the volume of a porous structure and is transferred to the heat transfer fluid, such as gas or air, from the volumetric heat transfer structure.

[0006] Furthermore, the conventional absorbers can use different types of heat transfer fluid. Generally, in case of the tube type heat transfer structure, fluids, such as water, molten salt, gas or air, may be used as the heat transfer fluid, and in case of the volumetric type heat transfer structure, air, gas or water may be used as the heat transfer fluid.

SUMMARY

[0007] The present subject matter described herein relates to an absorber for a Concentrated Solar Power (CSP) system. In one implementation, the absorber includes an enclosure having an incident wall. The incident wall includes an aperture for access of concentrated sunlight. The enclosure further includes a first heat transfer structure provided in a cavity of the enclosure to absorb a first portion of thermal energy from the concentrated sunlight. Moreover, the incident wall includes a second heat transfer structure to absorb a second portion of thermal energy from the concentrated sunlight.

[0008] These and other features, aspects, and advantages of the present subject matter will be better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in simplified form. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The detailed description is described with reference to the accompanying figure(s). In the figure(s), the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The use of the same reference number in different figure(s) indicates similar or identical items. The features, aspects and advantages of the subject matter will be better understood with regard to the following description, and the accompanying drawings.

[00010] Fig. 1 shows an absorber, according to an embodiment of the present subject matter. [00011] Fig. 2 shows a concentrated solar power (CSP) system implementing the absorber, according to an embodiment of the present subject matter.

DETAILED DESCRIPTION

[00012] The subject matter described herein relates to an absorber for a concentrated solar power (CSP) system.

[00013] Generally, CSP systems include a reflector system and an absorber. The reflector system reflects and concentrates incident sunlight onto the absorber, which absorbs and transfers thermal energy contained in the concentrated sunlight to a heat transfer fluid flowing therein.

[00014] The absorber design plays an important role in determining an overall thermal efficiency of the CSP system and, therefore, is designed to be as efficient as possible in transferring thermal energy to the heat transfer fluid. Generally, absorbers are selected based on various factors, such as the type of reflector system, heat transfer fluid, operating temperature regime, and thermal storage requirement of the CSP system. Further, the absorbers can be classified based on geometrical configuration, heat transfer fluid, or heat transfer structures.

[00015] Based on the geometrical configuration, the absorbers are classified into cavity type absorbers and external type absorbers. Further, the absorbers may have either a tube type heat transfer structure or a volumetric type heat transfer structure. The cavity type absorber generally includes a well insulated enclosure with a small opening referred to as an aperture to allow sunlight to enter. The concentrated sunlight, on passing through the aperture, impinges on a heat transfer structure in a cavity of the enclosure. The heat transfer structure, thereafter, absorbs the thermal energy from the sunlight. For example, in a cavity type absorber having a volumetric type heat transfer structure, the concentrated sunlight impinges on a porous structure, and the thermal energy of the sunlight is absorbed inside the volume. The thermal energy is transferred to the heat transfer fluid flowing through the porous structure.

[00016] Cavity type absorbers have intrinsic advantages of higher absorptivity and lower re-radiation losses in comparison to external type absorbers. Because of multiple internal reflections, the cavity type absorber behaves like a blackbody. With the absorption of the concentrated sunlight, re-radiation of energy occurs when the heat transfer structure in the cavity gets heated to such an extent that the heat transfer structure itself begins to radiate part of the absorbed energy back to the atmosphere instead of transferring the energy to the heat transfer fluid. The enclosure design enables lower re-radiation losses, as re-radiation occurs only through the aperture.

[00017] Generally, in order to increase the apparent absorptivity of the enclosure, the ratio of enclosure size to aperture size is increased. The ratio of enclosure size to aperture size can be increased by either increasing the enclosure size or decreasing the aperture size. However, the increase in enclosure size beyond a limit may be inefficient due to disproportionate increase in cost and increased heat losses from the enclosure walls. On the other hand, decreasing the size of the aperture beyond a limit may reduce the ability of the reflector system to concentrate sunlight on the aperture. This may result in a phenomenon known as energy spillage around the aperture.

[00018] The phenomenon of energy spillage occurs when concentrated sunlight falls outside the aperture and is lost to the atmosphere instead of being absorbed inside the enclosure. The concentrated sunlight that is focused by the reflector system onto the absorber can be classified into a centered high concentration region sunlight and peripheral low concentration region sunlight depending on the area of concentration of the concentrated sunlight. The high concentration region sunlight is that portion of the concentrated sunlight that falls within the aperture of the absorber. Therefore, the high concentration region sunlight generally enters the enclosure and is absorbed by the heat transfer structures therein. The low concentration region sunlight is that portion of the concentrated sunlight that falls outside the aperture of the absorber. Therefore, the low concentration region sunlight generally strikes the surfaces around the aperture and is reflected or lost to the atmosphere. [00019] In certain other cases, to reduce energy spillage from the absorber, the size of the aperture can be increased to increase the quantity of sunlight entering the enclosure. However, the increased size of the aperture results in greater losses due to energy re-radiation from internal surfaces of the enclosure through the aperture. Therefore, the size of the aperture is determined by achieving a trade-off between losses due to energy spillage and those due to re- radiation from the absorber.

[00020] The subject matter described herein relates to an absorber for a concentrated solar power (CSP) system.

[00021] In one embodiment, the absorber includes an enclosure formed by an inner wall and an incident wall. The incident wall receives the concentrated sunlight reflected by the reflector system. Furthermore, the incident wall has an aperture for access of concentrated sunlight into the enclosure. In an embodiment, the aperture may be covered by a transparent surface, which allows passage of solar radiations into the enclosure through the aperture. In this embodiment, there will be no flow of heat transfer media across the aperture. In an example, the transparent surface may be glass.

[00022] In one embodiment, the absorber includes a first heat transfer structure. In an implementation, the first heat transfer structure is provided in a cavity of the absorber and is exposed to the concentrated sunlight entering the cavity through the aperture. Further, the first heat transfer surface absorbs the thermal energy contained in the concentrated sunlight. In one example, the inner wall is formed by a frame with the first heat transfer structure provided within the frame.

[00023] The first heat transfer structure can be a volumetric type heat transfer structure, a tube type heat transfer structure, or a combination thereof. In one example, the volumetric type heat transfer structure includes a porous structure. Further, a heat transfer fluid flows through the first heat transfer structure and carries away the thermal energy absorbed by the first heat transfer structure. In one implementation, the heat transfer fluid is air. In another example, the first heat transfer structure is tube type heat transfer structure having an array of channels interconnected in series or parallel configuration, or in a combination of the two configurations. In yet another example, the first heat transfer structure is a combination of the volumetric type heat transfer structure and the tube type heat transfer structure. In said example, the combination of the volumetric type heat transfer structure and the tube type heat transfer structure may be provided in series or parallel configuration, or any other combination thereof.

[00024] Further, in an embodiment, the absorber includes a second heat transfer structure provided in the incident wall. In one implementation, the incident wall is formed by the second heat transfer structure. In another implementation, the incident wall is formed by a frame with the second heat transfer structure provided within the frame. The second heat transfer structure in the incident wall is exposed to the concentrated sunlight and absorbs the thermal energy from the sunlight. A heat transfer fluid car ries thermal energy from the concentrated sunlight absorbed by the second heat transfer structure and can be stored or utilized further.

[00025] In an example, the second heat transfer structure can be a volumetric type heat transfer structure, a tube type heat transfer structure, or a combination thereof. The volumetric type heat transfer structure can be a porous structure, and the tube type heat transfer structure, on the other hand, can be one or more channels interconnected in series, parallel or in combination of both series and parallel arrangements allowing flow of the heat transfer fluid. In yet another example, the second heat transfer structure is a combination of the volumetric type heat transfer structure and the tube type heat transfer structure. In said example, as previously mentioned, the combination of the volumetric type heat transfer structure and the tube type heat transfer structure may be provided in series or parallel configuration, or any other combination thereof.

[00026] By providing the second heat transfer structure on the incident wall, even the portion of sunlight not falling within the diameter of the aperture is absorbed by the second heat transfer structure provided in the incident wall. In this manner, energy spillage is reduced and the overall thermal efficiency of the absorber is improved. Furthermore, by this configuration, the aperture size may be decreased with respect to known systems to further reduce the re-radiation losses as energy spillage can be captured by the second heat transfer structure.

[00027] Further, the absorber may include an outer insulating layer surrounding the inner wall to reduce conductive heat losses. In an embodiment, the outer insulating layer is configured at a predetermined distance from the inner wall of the enclosure and a channel can be formed between the outer insulating layer and the inner wall of the enclosure. The channel may create the passage for guiding the heat transfer fluid to absorber outlet. In an example, the predetermined distance between the outer insulating layer and the inner wall, i.e., width of the channel may be varied along a length of the channel, according to a pressure drop across the enclosure. In said example, the width of the channel may be reduced proportionately as the pressure increases across the absorber. Furthermore, in an example, the channel may be provided in order to equilibrate a pressure drop of a flow of the heat transfer fluid across the first heat transfer structure.

[00028] According to an embodiment, the flow of the heat transfer fluid is assisted by a pressure gradient mechanism that provides a pressure gradient across the enclosure. In one example, the pressure gradient is provided across the enclosure such that an aperture side of the absorber is at higher pressure than an absorber outlet side of the absorber. It will be understood that a side of the absorber on which the incident wall is provided is referred to as an aperture side of the absorber, and a side of the absorber on which an absorber outlet is provided for outflow of the heat transfer fluid, is referred to as an absorber outlet side of the absorber.

[00029] By providing the pressure gradient, the heat transfer fluid can flow across the absorber, i.e., through the second heat transfer structure in the incident wall, into the enclosure through the aperture, through the first heat transfer structure into the channel, and out through the absorber outlet. The heat transfer fluid then flows in the channel formed between the inner wall and the outer insulating layer, and exits the absorber via the absorber outlet. While flowing across the absorber, the thermal energy absorbed by the second and the first heat transfer structures is transferred to the heat transfer fluid.

[00030] In one embodiment, the pressure gradient mechanism may include an external pump which provides a pressure gradient across the enclosure. The thermal energy stored in the heat transfer fluid can be further utilized for power , generation, or process heat applications, or to generate solar fuels through, for example, thermo-chemical or gasification processes. [00031] In another embodiment, a first heat transfer fluid enters the cavity of the absorber and flows through the first heat transfer structure and absorbs thermal energy from the first heat transfer structure. The first heat transfer fluid can then be directly used or stored for further purpose. Further, a second heat transfer fluid flows through the second heat transfer structure. While flowing through the second heat transfer structure, the second heat transfer fluid absorbs thermal energy from the second heat transfer structure and can be either directly used or stored for further purpose. In one example, the second heat transfer fluid may transfer thermal energy to the first heat transfer fluid either through direct or indirect contact. In said example, the first heat transfer fluid is used for further purposes or stored for later use.

[00032] In an example where the first heat transfer structure, or the second heat transfer structure, or both the first and the second heat transfer structures are a combination of the volumetric and the tube type heat transfer structure, the heat transfer media flowing through each of them is referred to as the first and the second heat transfer fluid, respectively. In said example, there can be two different heat transfer fluids flowing through the first heat transfer structure, or the second heat transfer structure, or both the first and the second heat transfer structures. The two heat transfer media flowing' through the tube type heat transfer structure and the volumetric type heat transfer structure may not mix, i.e., they may be mutually exclusive. In an example, the first heat transfer fluid is pressurized air and the second heat transfer fluid is ambient air, which does not mix with the pressurized air.

[00033] According to the present subject matter, the aperture size may be decreased with respect to the cavity size to effectively reduce re-radiation. The resulting energy spillage will now be incident on the second heat transfer structure installed around the aperture on the incident wall of the absorber, thus resulting in effective energy absorption.

[00034] Fig. 1 shows an absorber 100 for a concentrated solar power (CSP) system, according to an embodiment of the present subject matter.

[00035] In an embodiment, the absorber 100 includes an enclosure 102. The enclosure 102 includes an inner wall 104. In an example, the enclosure 102 can be a box-shaped structure and, in such a case, the inner wall 104 may be formed as a plurality of walls. The enclosure 102 further includes an incident wall 106, which receives concentrated sunlight focused by a reflector system (not shown in figure) of the CSP system. The incident wall 106 and the inner wall 104 define a cavity 108 of the enclosure 102. Further, in said embodiment, the incident wall 106 has an aperture 1 10 for the access of incident sunlight into the enclosure 102.

[00036] In an embodiment, the aperture 1 10 may be covered by a transparent surface, which is made of a material transparent to solar radiations. In an implementation, the transparent surface may be quartz or glass. In another embodiment, the aperture 1 10 may be open to the atmosphere.

[00037] In an embodiment, a secondary concentrator may be provided in the aperture 1 10 of the absorber 100. The secondary concentrator provides a secondary focusing means for the concentrated sunlight. [00038] According to an embodiment, the absorber 100 includes a first heat transfer structure 1 14 provided in the enclosure 102. In an implementation, the first heat transfer structure 1 14 is provided along the inner wall 104 of the enclosure 102. In another implementation, the first heat transfer structure forms the inner wall 104. In said implementation, a frame is provided in the enclosure 102 and the first heat transfer structure 1 14 is provided within the frame. The frame being made of a conducting material, such as metal, or of an insulating material. The first heat transfer structure 1 14 absorbs a first portion of thermal energy, for example, contained in a high concentration region of the concentrated sunlight entering the cavity 108 of the absorber 100 through the aperture 1 10.

[00039] In an embodiment, the first heat transfer structure 1 14 may be a volumetric type heat transfer structure. In said embodiment, the volumetric type heat transfer structure includes a porous structure, such as wire mesh or foam made of metal or ceramics, in monolithic channel configuration, honeycomb configuration, or as a packed bed. A heat transfer fluid flows through the volumetric type heat transfer structure and transports the thermal energy from the porous structure. In said embodiment, the heat transfer fluid is air or gases.

[00040] In another embodiment, the first heat transfer structure 1 14 is a tube type heat transfer structure. In said embodiment, the heat transfer fluid is a fluid, such as water, air, carbon dioxide (C0₂), molten salts, or gases. Further, the tube type heat transfer structure of the first heat transfer structure 1 14 may include one or more fins or inserts to increase heat transfer from the first heat transfer structure 1 14. In an implementation, the heat transfer fluid flows through the first heat transfer structure 1 14. The heat transfer fluid flows through the channels of the tube type heat transfer structure and absorbs thermal energy from the concentrated sunlight impinging on the first heat transfer 1 14.

[00041] In yet another embodiment, the first heat transfer structure 1 14 is a combination of the volumetric type heat transfer structure and the tube type heat transfer structure. The combination may be provided such that the volumetric type heat transfer structure and the tube type heat transfer structure are disposed in series or parallel configuration, or any other combination thereof. In one example, the combination may be provided in such a way that the porous structure of the volumetric type heat transfer structure is disposed in gaps in the array of channels of the tube type heat transfer structure. In another example, the channels of the tube type heat transfer structure pass through the porous structure volumetric type heat type structure. In above examples* the heat transfer media in the volumetric and the tube type heat transfer structures do not mix; however, for simplicity, the heat transfer media flowing through the first heat transfer structure 1 14 can collectively be referred to as the first heat transfer fluid. In a further example, the combination may be provided in such a way that the volumetric type heat transfer structure is adjacent to the tube type heat transfer structure. In the latter type of combination, the heat transfer fluid flows from the volumetric type heat transfer structure to the tube type heat transfer structure or vice versa.

[00042] According to the embodiment shown in fig. 1 , the absorber 100 includes a second heat transfer structure 1 12. In one implementation, the second heat transfer structure 1 12 forms the incident wall 106. In another implementation, second heat transfer structure 1 12 is provided within a frame forming the incident wall 106. In an implementation, the second heat transfer structure 112 may be a tube type heat transfer structure. In said implementation, the second heat transfer structure 1 12 includes one or more channels interconnected in series, parallel or in combination of both series and parallel configurations, and the heat transfer fluid flows through the channels of the second heat transfer structure 1 12. Further, the heat transfer fluid used in the second heat transfer structure 1 12 is a fluid, such as water, organic fluids, molten salts, or air, carbon dioxide (C0₂), or other gases. Further, the tube type heat transfer structure of the second heat transfer structure 1 12 can include one or more fins or inserts to increase heat transfer from the second heat transfer structure 1 12. [00043] In another implementation, the second heat transfer structure 1 12 may be a- volumetric type heat transfer structure. In said implementation, the second heat transfer structure 1 12 includes a porous structure, such as metallic or ceramic wire mesh, metallic or ceramic foam, in monolithic channel configuration, honeycomb configuration, or as a packed bed. The heat transfer fluid flows through the body of the second heat transfer structure 1 12 and heat absorption takes place throughout the volume of the heat transfer structure. Further, in said implementation, the porous structure of the second heat transfer structure 112 can have a variable porosity. Further, the porous structure can be provided with thermal conductive inserts. With the provision of such inserts, heat transfer coefficient associated with the porous structure may be substantially improved. In an implementation, the heat transfer fluid flows through the second heat transfer structure 1 12. The heat transfer fluid flows through the channels of the tube type heat transfer structure and absorbs thermal energy from the concentrated sunlight impinging on the second heat transfer 1 12.

[00044] In yet another implementation, the second heat transfer structure 1 12 may be a combination of the volumetric and the tube type heat transfer structure. The manner in which the combinations of the second heat transfer structure 1 12 are provided, and the heat transfer fluid flows, are similar to the combinations described with respect to the first heat transfer structure 1 14. As mentioned earlier, the combination may be provided such that the volumetric type heat transfer structure and the tube type heat transfer structure are disposed in series or parallel configuration, or any other combination thereof. In one example, the channels of the tube type heat transfer structure may be provided within the porous structure of the volumetric type heat transfer structure. In another example, the porous structure of the volumetric type heat transfer structure may be provided in gaps in the array of channels of the tube type heat transfer structure. Furthermore, it will be understood that the heat transfer media flowing through the second heat transfer structure 1 12 can flow in either series or parallel or its combination arrangements through tube and volumetric type of heat transfer structures.

[00045] The concentrated sunlight, focused by the reflector system impinges on the second heat transfer structure 1 12, and thermal energy contained therein is transferred through the second heat transfer structure 1 12 and to the heat transfer fluid flowing therein. The heat transfer fluid is then transported to a further area of interest.

[00046] Further, by the provision of the second heat transfer structure 1 12 around the aperture 1 10, a second portion of thermal energy is absorbed by the second heat transfer structure 1 12. The second portion of thermal energy may be the thermal energy contained in a low concentration region of the concentrated sunlight. In this manner, energy spillage is mitigated and the overall thermal efficiency of the absorber 100 is improved. Furthermore by this configuration, the size of the aperture 1 10 may be decreased with respect to known systems to reduce the re-radiation losses. In addition, the incident wall 106 of the absorber can be covered by a transparent surface, such as quartz or glass. The covering of the incident wall 106 with the transparent surface assists in reduction of re-radiation losses through wavelength selective transmissivity of the transparent surface. As a result, re-radiation losses of thermal energy are reduced to a large extent.

[00047] In an embodiment, the first and the second heat transfer structures 1 14 and 1 12 are coated with a wavelength selective coating. The wavelength selective coating reduces re- radiation losses of certain wavelengths of radiation from the first and the second heat transfer structures 1 14 and 1 12, thereby increasing the thermal energy available to be transferred to the heat transfer fluid.

[00048] Further, in one embodiment, the absorber 100 may include an outer insulating layer 1 16 to accommodate the enclosure and reduce heat losses from the inner wall 104. The outer insulating layer 1 16 is provided at a predetermined distance from the inner wall 104 to form a channel 1 18. The channel 1 18 serves as a passage for the flow of the heat transfer fluid. In one example, the channel 1 18 creates the passage for guiding the heat transfer fluid from the aperture 1 10 and out of the absorber 100 through an absorber outlet 120.

[00049] In an example, the distance between the outer insulating layer 1 16 and the inner wall 104 may be varied according to a pressure drop across the enclosure 102. In said example, the distance, i.e., width of the channel 1 18 may be reduced proportionately along its length, as the pressure increases across the absorber 100.

[00050] Further, the absorber 100 can include a pressure gradient mechanism (not shown in figure), which assists the flow of the heat transfer fluid across the enclosure 102. The pressure gradient is provided such that pressure at a side of the aperture 1 10 of the absorber 100 is lower than a side of the absorber outlet 120 of the absorber 100.

[00051] The pressure gradient mechanism provides a pressure gradient across the absorber 100. In an example, with the side of the aperture 1 10 provided with a high pressure, the heat transfer fluid is drawn through the second heat transfer structure absorbing the low concentration region of the concentrated sunlight. The heat transfer fluid then enters the aperture 1 10. The heat transfer fluid flows through the first heat transfer structure 1 14 absorbing thermal energy of the high concentration region of the concentrated sunlight absorbed by the first heat transfer structure 1 14. It later flows through the channel 1 18 formed between the inner wall 104 and the outer insulating layer 1 16 to exit the absorber 100. The thermal energy, i.e., the low and high concentration regions of the concentrated sunlight, thus absorbed by the heat transfer fluid, can be further utilized for power generation, or process heat applications, or to generate solar fuel. In one example, the solar fuel can be obtained through thermo-chemical or gasification processes. In one embodiment, the pressure gradient mechanism may include an external pump which provides the pressure gradient across the enclosure.

[00052] During operation of the absorber 100, both the low and high concentration regions of the sunlight are incident on the incident wall 106 of the absorber 100. In an example, the high concentration sunlight enters the enclosure 102 of the absorber 100 through the aperture 1 0 and the low concentration sunlight is incident over the second heat transfer structure 1 12. The pressure gradient mechanism causes the heat transfer fluid to flow from the second heat transfer structure 1 12 towards the first heat transfer structure 1 14 in the enclosure 102 along the direction shown by arrows 122.

[00053] In an example, the second heat transfer structure 1 12 absorbs the low concentration region of the concentrated sunlight and transfers the thermal energy contained therein to the heat transfer fluid, as the heat transfer fluid enters the absorber 100 through the incident wall 106. As the heat transfer fluid is drawn through the cavity 108 of the absorber 100 and into the first heat transfer structure 1 14, the heat transfer fluid may absorb the thermal energy absorbed from high concentration region of the concentrated sunlight by the first heat transfer structure 1 14. Thereafter, the heat transfer fluid, now carrying the thermal energy from both the low and high concentration regions of the sunlight flows across the absorber 100. Further, the heat transfer fluid flows through the channel 1 18 formed between the inner wall 104 and the outer insulating layer 1 16, in case the first heat transfer structure 1 14 is provided along the inner wall 104. Further, the heat transfer fluid may exit the absorber 100 through the absorber outlet 120.

[00054] In another example, the high concentration region of sunlight enters the enclosure 102 of the absorber 100 through the aperture 1 10 and is partially incident over the second heat transfer structure 1 12 as well. The low concentration region of sunlight, as described earlier, may be incident on the second heat transfer structure 1 12. In said example, the high concentration region of sunlight that enters the enclosure 102 may be absorbed by the first transfer structure 1 14, and the high concentration region of sunlight that is incident on the second heat transfer structure 1 12 is absorbed therein along with the low concentration region of sunlight.

[00055] In yet another example, the high concentration region of sunlight may enter the enclosure 102 along with a portion of the low concentration region of sunlight, and both are absorbed by the first heat transfer structure 1 14 therein. The remaining portion of the low concentration region of sunlight may be incident over the second heat transfer structure 1 12, and is absorbed therein as described earlier.

[00056] In another embodiment, the first heat transfer structure 1 14 is provided with a first heat transfer fluid flowing through the absorber 100 by entering the cavity 108 through the aperture 1 10. The first heat transfer fluid can absorb the first portion of thermal energy, which is absorbed by the first heat transfer structure 1 14 from the concentrated sunlight in the manner as described above. The first heat transfer fluid can then be further used or stored for later use. In addition, a second heat transfer fluid can be provided to further flow through the second heat transfer structure 1 12. In an example, while flowing through the second heat transfer structure 1 12, the second heat transfer fluid absorbs the second portion of thermal energy, absorbed by the second heat transfer structure 1 12 from-the concentrated sunlight in the manner as described above. The second heat transfer fluid can either be directly used or stored for further purpose. In one example, the first heat transfer fluid may transfer thermal energy to the second heat transfer fluid either through direct or indirect contact. In said example, the second heat transfer structure 1 12 is used for further purposes or stored for later use.

[00057] In an implementation, the heat transfer fluid entering the second heat transfer structure 1 12 is recirculated air. For example, the recirculated air may be circulated by means of the external pump as described earlier. Furthermore, in an example, the heat transfer fluid may be pressurized before entering the second heat transfer structure 1 12. In another example, the heat transfer fluid may be pressurized after passing through the second heat transfer structure, before entering the first heat transfer structure 1 14.

[00058] According to the present subject matter, the size of the aperture 1 10 may be decreased with respect to a size of the cavity 108 to effectively reduce re-radiation. The low concentration region of concentrated sunlight will be incident on the second heat transfer structure 1 12 installed around the aperture 1 10 on the incident wall 106 of the absorber 100, thus resulting in effective energy absorption.

[00059] Fig. 2 shows a CSP system 200 implementing the absorber 100, according to an embodiment of the present subject matter. In said embodiment, the CSP system 200 includes a reflector system 202, an absorber system 204, an energy storage system 206, and an energy conversion system 208, along with the absorber 100.

[00060] The reflector system 202 may include a heliostat field, having a plurality of lenses or mirrors. These heliostats receive, reflect, and concentrate incoming sunlight onto the absorber system 204. Further, the concentrated sunlight can be broadly divided into two regions, viz., low and high concentration regions of sunlight.

[00061] The absorber system 204 may include one or more of the absorbers 100 according to the present subject matter. The lenses or mirrors of the reflector system 202 may be configured to receive, reflect, and concentrate incident sunlight onto the absorbers 100 of the absorber system 204.

[00062] According to the present subject matter, in one example, the first portion of thermal energy contained in the high concentration region of the concentrated sunlight on entering the enclosure 102, impinges on the first heat transfer structure 114 and is absorbed by the first heat transfer structure 1 14. The second portion of thermal energy contained in the low concentration region of the concentrated sunlight, on the other hand is incident on the incident wall 106 of the absorbers 100 is absorbed by the second heat transfer structure 1 12.

[00063] Further, as described earlier, the heat transfer fluid entering the absorbers 100 at the incident wall 106, receives the second portion of thermal energy absorbed from the low concentration region of concentrated sunlight, as the heat transfer fluid passes through the second heat transfer structure 1 12. Further, the heat transfer fluid enters the cavity 108 of the enclosure 102 and passes through the first heat transfer structure 1 14. While passing through the first heat transfer structure 1 14, the heat transfer fluid absorbs the first portion of thermal energy absorbed from the high concentration region of concentrated sunlight by the first heat transfer structure 1 14 before exiting the absorbers 100.

[00064] In an implementation, the energy storage system 206 is connected to the absorber system 204 through a control valve. The control valve selectively allows the heat transfer fluid to flow from the absorber system 204 to the energy conversion system 208 or to the energy storage system. For example, in one case, the control valve is regulated to selectively guide the flow of the heat transfer fluid from the absorber system 204 to the energy storage system 206. In such a case, the thermal energy in the energy storage system 206 may be harnessed for further use by the energy conversion system 208. In another case, the control valve may selectively allow the heat transfer fluid to bypass the energy storage system 206, and may be transported from the absorber system 204 directly to the energy conversion system 208. In said example, the thermal energy in the heat transfer fluid may be directly utilized by the energy conversion system 208.

[00065] In an embodiment, the absorber 100 is mounted atop a tower. The tower is built at a predetermined height according to various factors, such as geographical location and reflector system characteristics. [00066] In an embodiment, the absorber 100 may be located at a lower height than the reflector system 202, the height being measured, for example, from a ground level. In this case, the reflector system 202 has two components, for example, a first reflector component to receive incident sunlight and focus the incident sunlight onto a second reflector component. The second reflector component is positioned at a relatively higher position than the absorbers 100 and is configured to receive, reflect and concentrate the sunlight from the first reflector component onto the absorbers 100.

[00067] In an example, the energy conversion system 208 may be embodied as a power generation system, a system to process heat applications, or to generate fuel, such as through thermo-chemical or gasification processes.

[00068] By the provision of the second heat transfer structure 1 12 according to the present subject matter, the thermal efficiency of the CSP system 200 is substantially improved. Therefore, energy spillage is mitigated and the overall thermal efficiency of the absorbers 100 is improved.

[00069] According to the present subject matter, a method to absorb thermal energy from sunlight using the absorber 100 of the CSP system 200 is provided. The method includes providing a flow of the heat transfer fluid through the second heat transfer structure 1 12 in an incident wall 106 of an enclosure 102 of the absorber 100, where the heat transfer fluid in an example, may absorb the portion of thermal energy in the low concentration region of the concentrated sunlight. Furthermore the method includes directing a transfer of the portion of thermal energy in the low concentration region of the concentrated sunlight from the second heat transfer structure to the heat transfer fluid. The method also includes providing a flow of the heat transfer fluid through a first heat transfer structure 1 14 in a cavity 108 of the absorber 100, where the heat transfer fluid absorbs the portion of thermal energy in the high concentration region of the concentrated sunlight. Moreover the method includes directing a transfer of the portion of thermal energy in the high concentration region of the concentrated sunlight from the first heat transfer structure 1 14 to the heat transfer fluid.

[00070] According to the present subject matter, the heat transfer fluid is preheated in the second heat transfer structure 1 12 by absorbing a portion of thermal energy from the concentrated sunlight. Furthermore, the heat transfer fluid is further heated in the first heat transfer structure 1 14 situated in the cavity 108 of the absorber 100.

[00071] Although the present subject matter has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible. As such, the spirit and scope of the appended claims should not be limited to the description of the embodiments contained therein.

Claims

I/We Claim:

1. An absorber (100) for a Concentrated Solar Power (CSP) system, the absorber (100) comprising: an enclosure (102), wherein the enclosure (102) comprises, a cavity (108); a first heat transfer structure (1 14) provided in the cavity (108) to absorb a first portion of thermal energy from sunlight; and an incident wall (106) having an aperture (1 10) for access of sunlight, wherein the incident wall (106) comprises a second heat transfer structure (1 12) to absorb a second portion of thermal energy from the sunlight.

2. The absorber (100) as claimed in claim 1 , wherein the incident wall (106) and an inner wall (104) of the enclosure (102) define the cavity (108).

3. The absorber (100) as claimed in claim 1, wherein the first heat transfer structure (1 14) transfers the, first portion of thermal energy to a heat transfer fluid, and wherein the second heat transfer structure (1 12) transfers the second portion of thermal energy to the heat transfer fluid.

4. The absorber (100) as claimed in claim 3, wherein the heat transfer fluid is one of air, water, carbon dioxide, molten salts, and organic salts.

5. The absorber (100) as claimed in claim 3, wherein the first heat transfer structure (1 14) transfers the first portion of thermal energy to a first heat transfer fluid.

6. The absorber (100) as claimed in claim 3, wherein the second heat transfer structure (1 12) transfers the second portion of thermal energy to a second heat transfer fluid.

7. The absorber (100) as claimed in claim 1, further comprising an outer insulating layer (1 16) surrounding the inner wall (104) to form a channel (1 18) for passage of the heat transfer fluid.

8. The absorber (100) as claimed in claim 7, wherein a width of the channel (1 18) varies along a length of the channel (1 18) to equilibrate a pressure drop of a flow of the heat transfer fluid across the first heat transfer structure (1 14).

9. The absorber (100) as claimed in claim 1, wherein the first heat transfer structure (1 14) comprises at least one of a volumetric type heat transfer structure and a tube type heat transfer structure.

10. The absorber (100) as claimed in claim 1, wherein the second heat transfer structure (1 12) comprises at least one of a volumetric type heat transfer structure and a tube type heat transfer structure.

1 1. The absorber (100) as claimed in claim 1 , wherein the first heat transfer structure (1 14) is made different from second heat transfer structure (1 12).

12. The absorber (100) as claimed in claim 9 or 10, wherein the volumetric type heat transfer structure is formed of a porous structure.

13. The absorber (100) as claimed in claim 12, wherein the porous structure is selected from at least one of a ceramic material and a metal.

14. The absorber (100) as claimed in claim 12, wherein the porous structure is formed by one of a wire mesh, a foam, monolithic channels, and a honeycomb configuration.

15. The absorber (100) as claimed in claim 12, wherein the porous structure comprises metal inserts.

16. The absorber (100) as claimed in claim 12, wherein the porous structure exhibits variable porosity.

17. The absorber (100) as claimed in claim 9 or 10, wherein the tube type heat transfer structure comprises a plurality of channels in at least one of a parallel configuration and a series configuration.

18. The absorber (100) as claimed in claim 9 or 10, wherein the tube type heat transfer structure comprises an array of fins.

19. The absorber (100) as claimed in claim 1 , wherein the aperture (1 10) is covered by a transparent surface.

20. The absorber (100) as claimed in claim 1 , wherein the aperture (1 10) is open to the atmosphere.

21. The absorber (100) as claimed in claim 1 , wherein the incident wall (106) is covered by a transparent surface.

22. The absorber (100) as claimed in claim 1 , wherein at least one of the first heat transfer structure (1 14) and the second heat transfer structure (1 12) are coated with a wavelength selective coating.

23. The absorber (100) as claimed in claim 1 further comprising a pressure gradient mechanism to provide a pressure gradient across the absorber (100) for allowing flow of the heat transfer fluid across the absorber (100).

24. The absorber (100) as claimed in claim 1, wherein the aperture (1 10) is provided with a secondary concentrator.

25. A Concentrated Solar Power (CSP) system (200), comprising:

an absorber system (204), wherein the absorber system (204) comprises at least one absorber (100) comprising, an enclosure (102), wherein the enclosure (102) comprises, a first heat transfer structure (1 14) provided in a cavity (108) of the enclosure (102) to absorb a first portion of thermal energy from the sunlight; and an incident wall (106) having an aperture (1 16) for access of sunlight, wherein the incident wall (106) comprises a second heat transfer structure (1 12) to absorb a second portion of thermal energy from the sunlight; a reflector system (202) to focus the sunlight on the absorber system (204); and an energy conversion system (208) coupled to the absorber s ystem (204), wherein the energy conversion system (208) utilizes the thermal energy from the absorber system (204).

26. The CSP system (200) as claimed in claim 25, wherein the system (200) further comprises an energy storage system (206) connected to the absorber system (204) and to the energy conversion system (208), and wherein the energy storage system (206) stores the energy absorbed by the absorber (100).

27. A method to absorb thermal energy from sunlight, the method comprising: providing a flow of a heat transfer fluid through a first heat transfer structure (1 14) in a cavity (108) of an absorber (100) subsequent to a flow of the heat transfer fluid through a second heat transfer structure (1 12), wherein the first heat transfer structure (1 14) absorbs a first portion of thermal energy from the sunlight, and the second heat transfer structure (112) absorbs a second portion of thermal energy from the sunlight, and wherein the heat transfer fluid is preheated in the second heat transfer structure (1 12) and further heated in the first heat transfer structure (1 14);

directing a transfer of the first portion of thermal energy from the first heat transfer structure (1 14) to the heat transfer fluid; and

directing a transfer of the second portion of thermal energy from the second heat transfer structure (1 12) to the heat transfer fluid.

28. The method as claimed in claim 27, wherein the directing comprises transferring the first portion of thermal energy to a first heat transfer fluid and transferring the second portion of thermal energy to a second heat transfer fluid.

29. The method as claimed in claim 27, wherein the heat transfer fluid is at least one of ambient air and recirculating air.

30. The method as claimed in claim 27, wherein the heat transfer fluid is pressurized before entering one of the first heat transfer structure (1 14) and the second heat transfer structure (1 12).

3il . The method as claimed in claim 27, wherein the flow of the first and the second transfer media is provided by providing a pressure on an outlet side of the absorber (100).