Abstract:
A solar thermal collector is provided. The collector comprises a housing defining an inlet, and an outlet for a heat transfer fluid, said housing comprising a window to allow sunlight to pass there through; a heat transfer core disposed within the interior of the housing said housing designed to be heated by exposure to said sunlight; and a heat absorbing component in proximity to the heat transfer core, said heat absorbing component designed to at least partially absorb heat losses from the heat transfer core; wherein a positioning the components within the housing defines at least one path for the heat transfer fluid for preheating of the heat transfer fluid prior to said heat transfer fluid passing through the heat transfer core.

Description:
[0001]    This application claims the benefit of priority to U.S. Provisional Patent Application No. 61/732,685, the entire specification and drawing of which are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    Embodiments of the invention relate to devices and methods to harness solar radiation as an energy source. 
       BACKGROUND 
       [0003]    Solar thermal collectors are devices for converting sunlight into heat that can be used to perform work. They can fall into one or more categories including tracking (wherein the collector physically moves in order to follow the path of the sun to increase efficiency), non-tracking (wherein the collector does not track the path of the sun), and concentrating (wherein optics are used to concentrate the rays of the sun to increase performance). 
         [0004]    One other characteristic of solar thermal collectors is their use of heat retention strategies to reduce heat loss to the environment to increase efficiency. 
       SUMMARY 
       [0005]    According to one aspect of the invention, there is provided a method for heating a fluid, comprising: 
         [0006]    allowing sunlight to pass through into an interior of a housing for a collector to a heat transfer core disposed within the interior of the housing, thereby to heat the heat transfer core; 
         [0007]    transporting heat from the heat transfer core to an exterior of the housing by a heat transfer fluid entering the housing through an inlet and exiting through an outlet; 
         [0008]    positioning a heat absorbing component in proximity to the heat transfer core, and at least partially absorbing heat losses from the heat transfer component by said heat absorbing component; and 
         [0009]    preheating the heat transfer fluid using heat captured by the heat absorbing component. 
         [0010]    According to another aspect of the invention, there is provided a solar thermal collector comprising: 
         [0011]    a housing defining an inlet, and an outlet for a heat transfer fluid, said housing comprising a window to allow sunlight to pass there through; 
         [0012]    a heat transfer core disposed within the interior of the housing said housing designed to be heated by exposure to said sunlight; and 
         [0013]    a heat absorbing component in proximity to the heat transfer core, said heat absorbing component designed to at least partially absorb heat losses from the heat transfer core; wherein a positioning the components within the housing defines at least one path for the heat transfer fluid for preheating of the heat transfer fluid prior to said heat transfer fluid passing through the heat transfer core. 
         [0014]    Other aspects of the invention will be apparent from the detailed description below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a schematic drawing showing various paths for a heat transfer fluid through an induced flow solar thermal collector, in accordance with one embodiment of the invention. 
           [0016]      FIG. 2  shows a cross-section through an induced flow solar thermal collector with a heat transfer core immersed within a nano-porous medium, in accordance with one embodiment of the invention. 
           [0017]      FIG. 3  shows a cross-section through an induced flow solar thermal collector with the heat transfer core adjacent to the nano-porous medium, in accordance with one embodiment of the invention. 
           [0018]      FIG. 4  is a plot that juxtaposes the IR emission of a blackbody against the IR absorption of aerogel. 
           [0019]      FIG. 5  is a plot the thermal conductivity of aerogel at versus air pressure. 
           [0020]      FIG. 6  is a table comparing the thermal conductivity of several gasses at atmospheric pressure. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these specific details. 
         [0022]    Reference in this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not others. 
         [0023]    In one embodiment, an induced flow solar thermal collector is disclosed. The collector includes a heat transfer core designed to be heated by sunlight, and a heat transfer fluid to transport heat from the heat transfer core to an external loop. Advantageously the collector includes a nano-porous medium which in cooperation with the heat transfer fluid may be used to capture heat lost through radiation and conduction, as will be described. 
         [0024]      FIG. 1  shows a schematic drawing of an embodiment  100  of the induced flow solar thermal collector. As will be seen, the collector includes a housing  102  to house a preheat array  104 , a nano-porous medium  106 , and a heat transfer core  108 . The construction and characteristics of each of these components will be described later.  FIG. 1  is intended to illustrate various paths for a heat transfer fluid as it traverses the collector between an inlet  110  and an outlet  112 . These paths include: 
         [0025]    Path  114 : which flows directly to the heat transfer core  108  from inlet  110  from where it exits the collector  100  via the outlet  112 . 
         [0026]    Path  116 : which flows from the inlet  110  to the preheat array  104  and then to the heat transfer core  108  from where it exits the collector  100  via the outlet  112 . 
         [0027]    Path  118 : which flows from the inlet  110  firstly through the preheat array  104 , secondly through the nano-porous medium  106 , and thirdly through the heat transfer core  108  from where it exits the collector  100  via the outlet  112 . 
         [0028]    Path  120 : which flows form the inlet  110 , to the nano-porous medium  106 , and then to the heat transfer core  108 , from where it exits the collector  100  via the outlet  112 . 
         [0029]      FIG. 2  shows a cross-sectional schematic drawing of an embodiment  200  of the induced flow solar thermal collector. Components of the collector  200  that are the same or at least similar to components of the collector  100  have been indicated with like or the same reference numerals. Referring  FIG. 2 , the collector  200  includes a collector housing  102  made from a material that can endure exposure to the natural environment for long periods of time. Housing  102  is thermally insulating by virtue of the materials it is comprised of as well as insulating materials which may be incorporated onto its interior. Such materials include but are not limited to, rock wool, granular aerogel, vacuum insulated panels, reflective metal films or metal coated plastics and some combination of these or other materials whose function is to minimize the escape of heat via conduction, convection, and radiation from the interior of the collector housing  102 . 
         [0030]    In one embodiment, housing  102  may be bonded and hermetically sealed to a transparent faceplate  122  such that air, moisture, or other elements from the natural environment many not enter the housing  102 . In one embodiment, the transparent faceplate  122  may include a material such as a glass or a polymer that is substantially transparent to visible light and to some degree near infrared radiation. In one embodiment, said material may be low-iron glass. In some cases, polymers that are inherently resistant to UV exposure or UV resistant by virtue of a coating or the incorporation of UV inhibiting additives may also be used. In one embodiment, the transparent faceplate  122  may be coated with an anti-reflection coating to enhance the transmission of visible light. 
         [0031]    In one embodiment, a heat transfer component  124  is located within the housing  102 . The heat transfer component  124  includes a preheat array  104 , a heat absorbing component in the form of a nano-porous medium  106 , and a heat transfer core  108 . Two inlets  110  are configured to allow a heat transfer fluid  126  into the housing  102 . The heat transfer fluid  126  traverses the collector  200  by the various paths described in  FIG. 1  and then exits the device through an outlet  112 . 
         [0032]    In one embodiment, the nano-porous medium  106  may comprise a silica aerogel, which is a porous glass-like medium characterized by a matrix of open cellular pores. The pores may have an average size ranges from 10&#39;s of nm to 100&#39;s of nm, in accordance with various embodiments. The aerogel is highly thermally insulating due to the fact that it substantially impedes the propagation of gas molecules through its volume and the fact that it is partially to substantially absorbing in mid to far infrared frequencies. In one embodiment, the required average pore size is determined by the operational pressure of the heat transfer fluid, the properties of the heat transfer fluid, and the desired properties of the aerogel. 
         [0033]    The nano-porous medium  106  defines paths by which heat transfer fluid  126  may flow within the medium. The paths may include the pores of the medium itself, which are formed during its manufacture. In one embodiment, the paths may include fabricated pathways formed in the medium. In some embodiments, the fabricated pathways may be created at the same time that the medium is manufactured but are distinct and separate from the pores that are inherent to nano-porous medium  106 . In other embodiments, the pathways may be formed in the nano-porous medium  106  after its manufacture. 
         [0034]    In one embodiment, the preheat array  104  may comprise an array of physical structures which are configured to enhance the transfer of heat to the heat transfer fluid  126  flowing through the array and the nano-porous medium  106  with which it is in thermal contact (i.e. it is bonded to the preheat array  104  with minimal thermal resistance). Structurally, the preheat array  104  may comprise a plurality of fins, pins, or other features which increase the surface area of the array. In one embodiment, the preheat array  104  may be comprise a portion of the nano-porous medium  106 , which has been molded during manufacture such that these features result. In some cases, the preheat array  104  may include a transparent material which is fabricated separately and then bonded to the nano-porous medium during assembly. 
         [0035]    Functionally, the heat transfer core  108  includes a light absorption element and a heat transfer element. In one embodiment, the heat transfer core  108  comprises a corrugated element. In one embodiment, the corrugated element may comprise corrugated copper film into which fins have been embossed. The corrugated element defines the light absorption element. The corrugations or fins define the heat transfer element and has a geometry that increases the total surface area exposed to the heat transfer fluid  126  as it passes through. 
         [0036]    In one embodiment, the fins may be coated with a light absorbing coating, which defines the light absorption element. The light absorbing coating, which can be made via many techniques well known in the art, serves to increase the absorption of visible light while simultaneously minimizing the amount of infrared radiation that is emitted by the heat transfer core  108 . 
         [0037]    During operation sunlight  128 , is transmitted through the transparent faceplate  122  and is subsequently incident on the heat transfer core  108  where it is absorbed, resulting in heating of the heat transfer core  108 . The heat transfer fluid  126  is admitted through the inlets  110  into the interior volume of the collector  200 . 
         [0038]    In one embodiment, the heat transfer fluid  126  is nominally a gas, which can include air, carbon dioxide or other candidates with suitable cost, and toxicity characteristics. The nominal pressure is slightly above atmospheric pressure though higher and lower pressures are possible. 
         [0039]    As discussed in the flow diagram of  FIG. 1 , the heat transfer fluid  126  may traverse the collector  200  the various paths described with reference to  FIG. 1 . 
         [0040]    When the heat transfer fluid  126  encounters the heat transfer core  108 , the temperature of the heat transfer fluid  126  rises, the extent of which is determined by the strength of incident sunlight  128 , the flow rate of the fluid, the geometry of heat transfer core  108 , and other factors. The heat transfer fluid  126  functions to extract heat from the heat transfer core  108  and transports said heat to an external loop (not shown) as is flows out of the collector via the outlet  112 . 
         [0041]    One of ordinary skill in the art would appreciate that the heat transfer core  126  loses heat to the environment via conduction, convection, and radiation through the nano-porous medium  106  at a rate that will increase as the temperature of the core increases. In one embodiment, the collector  200  in configured to recover the lost heat by the incoming heat transfer fluid  126  in at least two ways. 
         [0042]    Firstly, as the heat transfer fluid  126  passes through the preheat array  104 , some portion of the lost heat which is transferred from the nano-porous medium  106  to the preheat array  104  will subsequently be transferred to the heat transfer fluid  126  via conduction. 
         [0043]    Secondly, portions of the heat transfer fluid  126  which passes through the preheat array  104  and portions which do not flow through the preheat array  104  will both recover heat directly from the nano-porous medium  106  by virtue of flowing through the pores and through the fabricated pathways of the medium due to conduction. In this fashion the heat transfer fluid  126  undergoes preheating by retaining some of the heat that would normally be lost. The result is that the overall efficiency of the collector  200  is increased and higher operational temperatures may be maintained. 
         [0044]      FIG. 3  shows a cross-sectional schematic drawing of an embodiment  300  of the induced flow solar thermal collector. Components of the collector  300  that are the same or at least similar to components of the collectors  100  and  200  have been indicated with like or the same reference numerals. One difference between the embodiments  300 , and  200  is that in the case of the embodiment  200 , the heat transfer core  108  is no longer immersed within the nano-porous medium  106 . Instead the heat transfer core  108  is located adjacent to the preheat array  104 . With the embodiment  300 , flow only occurs through the top of the nano-porous medium  106  via some combination of the paths described with reference to  FIG. 1 . 
         [0045]    The embodiment  300  has the advantage of relying on the insulating properties of the housing  102  in conjunction with or without other insulating materials incorporated onto the interior of the housing  102 , to minimize heat losses through the bottom of the collector. In all other ways the operation of the collector  300  is similar to the operation of the collector  200 . 
         [0046]    One of the characteristics of nano porous mediums is that they can act as thermally driven pumps under the appropriate circumstances. In particular if the average pore size is comparable to the mean free path of the gas suffusing the medium, the medium is sufficiently thermally insulating, and there is a thermal gradient applied to the medium, spontaneous pumping of the gas from the cold side to the hot side of the medium can occur. This phenomenon is known as thermal transpiration. Advantageously, the induced flow collectors disclosed herein can benefit from this pumping because of the similarities between its architecture and that of a transpiration pump. By absorbing the sunlight in the heat transfer core  108 , a thermal gradient is established which can induce pumping or flow through the nano porous medium  106  towards the heat transfer core  108 . The overall result and attribute is that the power required to pump the heat transfer fluid  126  through the collector is supplemented to some extent by the transpiration pump effect. The extent of the supplement is determine by factors which include the intensity of the thermal gradient, the size and size distribution of the pores within the nano-porous medium, and the characteristics and pressure of the heat transfer fluid  126  among other properties. In one embodiment the solar collector is thus designed and operated to enhance the effect of thermal transpiration in the pumping of the heat transfer fluid  126 . 
         [0047]    Referring now to  FIG. 4 , a plot is shown which juxtaposes the emission characteristics of a black radiator,  400 , at approximately 150° C. against the infrared absorption characteristics,  402 , of aerogel. As can be seen, while the two curves do not overlap perfectly, the indication is that radiation from the emitter at wavelengths longer than approximately 7 microns will undergo significant absorption thus enhancing the aerogel&#39;s ability to capture radiative losses from an emitter, or in the case of the induced flow solar collector, from the heat transfer core. 
         [0048]    In another embodiment, the solar thermal collector may be operated at a pressure which is lower than that of atmospheric pressure. The characteristics of the induced flow solar collector may be improved by reducing the pressure of the heat transfer fluid  126 . This is due to the fact that within a nano-porous medium the overall thermal conductivity is dominated, within certain pressure limits, by the thermal conductivity of the heat transfer fluid  126 . Thus reducing the pressure of the heat transfer fluid  126  and therefore its density has the effect of reducing the overall thermal conductivity of the medium and therefore heat losses to the environment. 
         [0049]    Referring now to  FIG. 5 , a plot is shown which reveals the relationship between the thermal conductivity of aerogel and the pressure of the heat transfer fluid  126 . It can be seen that reduced thermal losses can be realized down to pressure of approximately 0.015 mbar. Thus a lower operating pressure can improve both efficiency and operational temperatures of the collector. 
         [0050]    In yet another embodiment the collector may be operated using a heat transfer fluid  126  in the form of a gas other than air. 
         [0051]    Referring now to  FIG. 6 , a table  600  is shown which reveals the thermal conductivities of several candidate gasses for the heat transfer fluid  126 . As can be seen there are a number of gasses with thermal conductivities which are lower than air. The utilization of one of these candidate gasses would consequently result in the improvement of the efficiency and operational temperature of the solar collector. 
         [0052]    In yet another embodiment the solar collector may not include a preheat array  104  or an inlet  110  in direct fluid communication with the heat transfer core  108 .