Abstract:
A receiver for use with parabolic solar concentrators. The receiver consists of a heat conductive tube helically coiled into a hollow cone shape. Heat conductive fluid is pumped into an entry point in the tube and exits at an exit point from the tube. A casing is provided to cover the cone. An insulator between the cone and casing prevents heat from escaping from the coil. When deployed, the receiver is positioned such that the aperture of the cone is directed towards the parabolic solar concentrator. The receiver can be positioned so that the focus of the parabolic concentrator is outside of the cone but adjacent to the aperture. Alternatively, the receiver can be positioned so that focus of the parabolic concentrator is inside the cone but still adjacent to the aperture. To assist in the heat transfer, the inside of the cone can be colored black.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to solar energy. More specifically, the present invention relates to a receiver for use with a parabolic solar concentrator. 
       BACKGROUND OF THE INVENTION 
       [0002]    Interest in alternative energy sources, including solar energy, dates back to the 1970s when energy shortages due to oil embargoes and unstable political situations occurred. Nowadays, high oil prices, pollution and global warming due to the burning of fossil fuels, and the dangers associated with nuclear energy have rekindled an interest in these alternatives. Solar power has always been of interest to alternative energy advocates. The idea that the energy of the sun, through the light and heat it provides, can be harvested is an attractive one. 
         [0003]    It is well known that solar energy can be used to heat water and other fluids. The heated fluid can then be used as the conduit by which energy, in the form of heat, can be siphoned off to other uses. Research into solar energy has provides many ways by which solar energy can be harvested. In a concentrated solar thermal system (CST), the incident solar radiation is focused into an enclosure through optical devices where the concentrated solar radiation is absorbed by the receiver and converted into heat. CST systems are classified based on the optical configurations. These configurations include a parabolic trough, a parabolic dish and a central tower. 
         [0004]    Parabolic dish concentrators (PDC) are considered to be the most efficient among solar technologies (Tyner, et al., 2001). The high collection efficiency of the parabolic dish CST system is attributed to the high concentration ratio of up to 10,000 suns (1 Sun=1000 W/m 2 ), which is substantially higher than the other CST configurations; 100 Suns for the parabolic trough CST system and 1000 Suns for the central tower CST system (see Steinfeld A. “Solar thermochemical production of hydrogen-a review”, Solar Energy, 2005, Vol. 78, pp. 603-615). A parabolic-dish CST system consists of a parabolic dish with a highly reflective surface, a receiver located at the focal point of the paraboloid, support mechanisms for the dish and receiver, and a tracker to track the sun&#39;s movement to keep the parabolic dish aligned with the sun throughout the day. The receiver is the heart of any parabolic dish CST system where the conversion of solar radiation into heat takes place. The receiver is the component that regulates the overall efficiency of the system. Despite its critical role, there is a scarcity of studies investigating receiver geometry and performance to improve its efficiency. The literature shows few studies that have been conducted to understand and improve the performance of some pre-existing receivers. Most of the references are theoretical in nature and do not devote much to practical applications. However, due to the escalated power demand and lack of efficient solar thermal technologies, there is a need to develop efficient and affordable thermal receivers for CST-PDC systems. 
       SUMMARY OF INVENTION 
       [0005]    The present invention provides a receiver for use with parabolic solar concentrators. The receiver consists of a heat conductive tube helically coiled into a hollow cone shape. Heat conductive liquid is pumped into an entry point in the tube and exits at an exit point from the tube. A casing is provided to cover the cone. An insulator between the cone and casing prevents heat from escaping from the coil. The insulation can be replaced by a vacuum gap between the cone and casing. When deployed, the receiver is positioned such that the aperture of the cone is directed towards the parabolic solar concentrator. The receiver can be positioned so that the focus of the parabolic concentrator is outside of the cone but adjacent to the aperture. Alternatively, the receiver can be positioned so that focus of the parabolic concentrator is inside the cone but still adjacent to the aperture. To assist in the heat transfer, the inside of the cone can be colored black. 
         [0006]    In one aspect, the present invention provides a receiver for use with a concentrated thermal system, the receiver comprising:
       at least one thermally conductive tube, said at least one tube being helically coiled into a hollow cone shape, said at least one tube being for passing a thermally conductive liquid from an entry point on said tube to an exit point on said tube;   an outer casing for covering an outside of said cone shape;   wherein said receiver is placed adjacent to a focal point of a parabolic dish solar concentrator.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The embodiments of the present invention will now be described by reference to the following figures, in which identical reference numerals in different figures indicate identical elements and in which: 
           [0011]      FIG. 1  is a diagram illustrating the environment in which the invention would be deployed; 
           [0012]      FIG. 2  illustrates a receiver according to one aspect of the invention; 
           [0013]      FIG. 3  is a diagram of an outer casing for use with the receiver illustrated in  FIG. 2 ; 
           [0014]      FIG. 4  is a diagram illustrating the convergence and divergence of solar rays reflected off of a parabolic dish concentrator; 
           [0015]      FIG. 5  is a cross-sectional diagram of one embodiment of the invention using a cylindrical shaped outer casing; 
           [0016]      FIG. 6  is a cross-sectional diagram of one implementation illustrating the measurements of the receiver; 
           [0017]      FIG. 7  is a cross-sectional diagram of another implementation illustrating the measurements of the receiver; 
           [0018]      FIG. 8  illustrates the performance curve for the receiver illustrated in  FIG. 6 ; 
           [0019]      FIG. 9  illustrates the performance curve for the receiver illustrated in  FIG. 7 ; and 
           [0020]      FIG. 10  illustrates the different positions of the receiver relative to the surface of the parabolic concentrator used in experiments to determine a suitable distance between the surface and the aperture of the receiver. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Referring to  FIG. 1 , a diagram illustrating the environment for the invention is illustrated. A parabolic solar reflector or concentrator dish  10  receives and reflects sunlight  20  from the sun  30 . The reflected sunlight  20  is reflected towards a receiver  40 . The heat from the sunlight heats the receiver and this heat can be siphoned by the receiver to be used by other machines or for other uses. 
         [0022]    Referring to  FIG. 2 , an illustration of a receiver according to one aspect of the invention is provided. The receiver is formed from tubing which has been formed into a conic helix. The hollow cone formed by the hollow tubing has an apex and an opening directly opposite to the apex. When deployed, a casing (see  FIG. 3 ) covers the outside of the cone from the elements as well as providing insulation to prevent heat loss to the surrounding environment. Further insulation may also be used and placed between the cone and the casing. A vacuum can be created between the cone and the casing for insulation. If a vacuum is used, the vacuum can be created between the tubing and the sleeve or casing. A glass cover can also be placed at the aperture of the sleeve. 
         [0023]    As noted above, the receiver is deployed at or near the focus of a parabolic solar reflector or concentrator. Solar rays are reflected from the reflective surface of the parabolic concentrator towards the focus of the parabola. By placing the receiver at or near the focus, the cone receives most of the heat reflected by the parabolic concentrator. 
         [0024]    In use, a heat conductive liquid is pumped continuously through the tubing by way of an entry point and an exit point on the tubing. The liquid is heated by the solar heat reflected by the parabolic concentrator. As can be seen from  FIG. 2 , it is preferred that the entry point be at or near the opening of the cone while the exit point be at or near the apex of the cone. As an alternative to the liquid, any heat conductive fluid (including a suitable liquid or gas) may be used. 
         [0025]    It is preferable that that coil forming the cone be tightly wound such that there is minimal spacing between adjacent sections of the coil. This will prevent heat loss in the coil. It is also preferable that the inner portion or the internal section of the coil be colored black to maximize absorption and to minimize reflection of solar heat reflected by the parabolic concentrator. It is also preferred that the tube forming the coil be continuous and have only one hole at each end by which liquid can enter and exit the tube. It is also preferred that the tube uses one end near the opening of the cone as the entry point and the other end, near the apex of the cone, as the exit point. 
         [0026]    The reasoning behind the conical shape of the receiver stems from the reflection of the sunlight by the parabolic concentrator. Theoretically, when rays reflect from the surface of a parabolic reflector, the rays converge at the focal point of the reflector and diverge after passing the focal point as illustrated in the left diagram in  FIG. 4 . In real-world applications, due to an optical aberration, the reflected sunlight from the concentrator&#39;s parabolic surface converges at a focal plane, thereby forming a concentrated sunlight beam rather than a point (see right illustration in  FIG. 4 ). As the reflected sunlight passes the focal plane of the concentrator, the sunlight starts to diverge. This convergence and divergence of the reflected sunlight takes a shape similar to a cone. To maximize the reflected sunlight being received, the receiver was shaped as a cone with an almost closed end (apex) to trap all of the diverged sunlight. The dimensions of the receiver have an impact on the effective surface area (i.e. the area that is exposed to the concentrated sunlight). For best results, it is preferred that the receiver&#39;s aperture be as small as possible and preferably slightly larger than the sunlight beam waist at the focal point or focal plane of the concentrator dish so as to maximize the parabolic-dish sunlight exposure. 
         [0027]    The use of a cone shape and the use of a helically coiled tube increases the effective surface area of the receiver and increases the residence time of the working fluid inside the receiver. The design, as a result, maximizes the direct sunlight exposure for the receiver and will, in turn, maximize the heat extraction by the working fluid from the receiver&#39;s internal portion. 
         [0028]    In one implementation (implementation A), 60 feet of 0.25″ copper tubing was used for the hollow tubing while the outer casing was constructed of steel. The outer casing can be conical in shape (as in  FIG. 3 ) or it can be tubular in shape with a rectangular cross-sectional shape. This tubular configuration for the outer casing can be seen in  FIG. 5 .  FIG. 5  is a cross-sectional view of an embodiment of the invention using a tubular outer casing. The insulation between the helically coiled tube and the outer casing can be clearly seen. In this implementation a 6 ft (1.8 m) diameter parabolic dish was used. The focal distance of the dish was 26.5″. The receiver measurements for this implementation is shown in  FIG. 6 . 
         [0029]    In a second implementation (implementation B), 100 feet of 0.25″ copper tubing was used for the hollow tubing while the outer casing was constructed of steel. In this implementation, a 10 ft (3.05 m) diameter dish was used. The focal distance of the dish was 43″. The receiver measurements for this implementation is shown in  FIG. 7 . 
         [0030]    The positioning of the receiver relative to the parabolic concentrator and the focus of the parabolic concentrator requires careful planning. As is known, the focus of a parabola depends on the dimensions of the parabola. Another dimension of note is the angle of the cone formed by the helically coiled tube. The receiver&#39;s geometry depends on the focal area, focal distance and the angle of reflection at the outer rim of the parabolic dish. The cone angle of the receiver is crucial as it determines the length/height of the receiver for a given length which might affect the receiver&#39;s performance. Earlier tests which compared different cone angles for receivers with the same surface area (60 feet of 0.25″ copper tubing) show that a receiver with a cone angle of 38° had better performance than a receiver with a cone angle of 20.5°. In two implementations described above and illustrated in  FIGS. 6 and 7 , a cone angle of 38° was used. 
         [0031]    To fabricate these receivers, two jigs were made according to the selected geometries. The copper tubing was then helically coiled on each jig to form the conical shape. To keep the conical shape and to minimize the gap between the coils, the helical coils of each receiver were tied or welded to one another. As well, once the outer casing was used, each receiver was also firmly attached to the outer casing, further reducing any tendency for gaps between the coils. 
         [0032]    The following dimensions were used for the two implementations:
       Implementation A:   Aperture distance: 8.5″   Receiver height: 11″   Cone angle: 38   Implementation B:   Aperture distance: 14″   Receiver height: 20″   Cone angle: 38       
 
         [0041]      FIGS. 6 and 7  illustrate the various dimensions for the two implementations of the receiver. 
         [0042]    For field testing, both receivers were tested independently to determine their operating characteristics. Each receiver was mounted on a different parabolic dish collector by the receiver mount. The receiver was adjusted so that its aperture is slightly away from the focal point. A water pump was used to pump the water through system. Data acquisition device (USB 9211A) was used to collect the data throughout the experiments. Two thermocouples were used to measure the temperatures of the water outlet and inlet. 
         [0043]    The outlet and inlet temperatures of the working fluid were measured during each experiment. Assuming steady-state conditions, the heat flux inside the receiver was computed using the following steady-flow energy equation: 
         [0000]        {dot over (q)}={dot over (m)}C   p ( T   out   −T   in )   (equation 1)
       In the equation,
           {dot over (q)} is the heat transfer rate to the working fluid inside the receiver in Watts,   {dot over (m)} is the mass flow rate in kg/s,   C p  is the specific heat of water which is 4118 J/kg·K at 25°-100° C.,   T out  is the working fluid outlet temperature,   T in  is the working fluid inlet temperature.   
               
 
         [0050]    Knowing the solar radiation at the day and time of the experimentation and the projection area of the concentrator (2.54 m 2  or 27.4 ft 2  for implementation A and 7.3 m 2  or 78.5 ft 2  for implementation B), the input energy from the sun was computed as: 
         [0000]        E   s   =I×A   p ×Cos(α)×ρ   (equation 2)
       where   E s  represents the solar energy incident on the parabolic dish in Watts,   I is the local solar radiation in W/m 2 ,   A p  is the projection area of the collector in m 2 ,   α is the tilt angle of the collector (equal to zero in PDC as it is always oriented towards the sun by using a solar tracking system, and   ρ is the reflectivity of the parabolic dish surface which is assumed to be close to 1.       
 
         [0057]    Thus, equation 2 can be written as: 
         [0000]        E   s   =I×A   p    (equation 3)
 
         [0058]    Then the overall efficiency (which will be referred to as the conversion efficiency) is quantified as: 
         [0000]    
       
         
           
             
               
                 
                   
                     η 
                     c 
                   
                   = 
                   
                     q 
                     
                       E 
                       s 
                     
                   
                 
               
               
                 
                   ( 
                   
                     equation 
                      
                     
                         
                     
                      
                     4 
                   
                   ) 
                 
               
             
           
         
       
       
         
           
             where η c  is the conversion efficiency of the system. 
           
         
       
     
         [0060]    The performance of implementation A of the receiver is summarized below in Table 1: 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Flow rate 
                   
                 Efficiency 
               
               
                 (GPH) 
                 W.F. 
                 % 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 6.7 
                 1 
                 69 
               
               
                 7.3 
                 1 
                 92 
               
               
                 7.6 
                 1 
                 96 
               
               
                 10 
                 1 
                 96 
               
               
                 13.3 
                 1 
                 86 
               
               
                   
               
             
          
         
       
     
         [0061]    It should be noted that, for Table 1, GPH is Gallons per hour, W.F. is the weather factor (where 1 represents clear sky conditions). The results for Table 1 are also plotted in  FIG. 8 . 
         [0062]      FIG. 8  and this table show that the heat transfer rate increases with an increase in the flow rate reaching its optimum value, 96% in the flow rate range 7.6 to 10 GPH. As the flow rate further increased, the receiver&#39;s efficiency drops as the residence time of the working fluid inside the receiver decreased. This parametric study is crucial as it identifies the optimum flow rate for this implementation. 
         [0063]    The performance of implementation B of the receiver is summarized below in Table 2 and is also plotted in  FIG. 9 . 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Flow rate 
                   
                 Efficiency 
               
               
                 (GPH) 
                 W.F. 
                 % 
               
               
                   
               
             
             
               
                 10 
                 1 
                 57 
               
               
                 12 
                 1 
                 76 
               
               
                 15 
                 1 
                 92 
               
               
                 21 
                 1 
                 93 
               
               
                   
               
             
          
         
       
     
         [0064]    Comparing the performance of the two implementations, it can be seen that the trends are similar and that the optimal efficiencies in both implementations are almost identical. From this, it can be concluded that the performance of the invention is not size dependent and that, in both the receivers, efficiency exceeded 90% at the optimum flow rates. 
         [0065]    As noted above, placement of the receiver relative to the parabolic concentrator may affect the receiver&#39;s performance. To test this factor, implementation A was used. The tests detailed below were performed to determine a suitable distance between the receiver and the parabolic concentrator. As mentioned earlier, the receiver frame was designed so that the aperture distance can be adjusted. The receiver was tested at two additional positions as shown in illustration (a) and (c) in  FIG. 10 . Illustration (b) in  FIG. 10  corresponds to the distance used for the test detailed above in implementation A. The results are compared with the results from the above testing where the aperture distance was 27.5 inches. In case (a), the receiver&#39;s aperture distance was equal to the focal distance of the parabolic dish, i.e. 26.5 inches. In this case the receiver effective surface area (the receiver&#39;s inner surface area where the reflected sunlight incidents) was decreased. In case (c), the aperture distance was set to 25.5, i.e. 1″ less than the parabolic dish focal distance. For this position the effective surface area also decreased. The experiments were conducted at the flow rates of 6, 10, 12 GPH using the same setup as for the implementation A. The test results for case (a) are shown in the Table 3 below: 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Flow 
                   
                   
               
               
                 rate 
                   
                 Efficiency 
               
               
                 (GPH) 
                 W.F. 
                 % 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 6 
                 1 
                 76 
               
               
                 8 
                 1 
                 76.5 
               
               
                 12 
                 1 
                 94 
               
               
                   
               
             
          
         
       
     
         [0066]    From the above, if one compares this performance with the results from the previous tests in Table 1 (where the aperture was 27.5″ from the surface of the parabolic concentrator), it can be seen that the performance of the receiver has degraded. It should also be noted that the conversion efficiency was low at lower flow rates, 6 GPH and 8 GPH. As the flow rate increases, the conversion efficiency increases. However the conversion efficiency recorded for this position, with the cone aperture being 26.5″ away from the parabolic concentrator surface, was lower than when the cone aperture was 27.5″ away from the parabolic concentrator surface. 
         [0067]    For the case where the aperture was only 25.5″ from the parabolic concentrator surface (case (c)), the results are shown in Table 4 below: 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Flow 
                   
                   
               
               
                 rate 
                   
                 Efficiency 
               
               
                 (GPH) 
                 W.F. 
                 % 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 6 
                 1 
                 75 
               
               
                 8 
                 1 
                 84 
               
               
                 12 
                 1 
                 85 
               
               
                   
               
             
          
         
       
     
         [0068]    The conversion efficiency at this position (25.5″ away from the concentrator surface) was lower than when the aperture was 27.5″ or 26.5″ away from the concentrator surface. However, the conversion efficiency did not reach 90% in this configuration. The previous tests confirm that the placement of the receiver relative to the parabolic concentrator surface has an effect on the receiver&#39;s performance. 
         [0069]    The receiver has a number of advantages. For this implementation of the receiver,
       (i) the entire concentrated sunlight is incident on the inner portion of the tightly spaced tube coil and this allows most of the sunlight to be absorbed by the tube surface (the tube surface is black coated to maximize absorption and minimize reflection),   (ii) the tight spacing of the tube coil also minimizes the amount of sunlight escaping,   (iii) copper is an excellent conductor of heat which increases the heat transfer to the working fluid,   (iv) the working fluid will receive heat continuously from the inlet to the exit which will increase the heat transfer to the fluid,   (v) the tightly spaced small diameter tubing in helical loops will allow more number of turns of tubing. This provides a larger surface area of tubing to reflected sunlight as well as a higher exposure time for the fluid in the coil. These features will maximize the heat transfer,   (vi) the helical loop increases fluid mixing and hence enhances the heat transfer,   (vii) the insulation on the outer side of tubing (i.e. between the tubing and casing) will minimize heat loss,   (viii) the receiver design is compact.       
 
         [0078]    It should be noted that the measurements provided above, especially with relation to  FIGS. 5 to 7 , are examples and should not be taken as limiting the scope of the invention. Other dimensions for the receiver, the coil, the placement and distance of the coil, and the other parts of the invention are, of course, possible. It should also be noted that while the above contemplates a single helical tube for transferring heat from concentrated sunlight to the liquid inside the tube, two or more tubes are also possible. For such a configuration, the tubes may be side-by-side as the tubes are helically coiled. 
         [0079]    A person understanding this invention may now conceive of alternative structures and embodiments or variations of the above all of which are intended to fall within the scope of the invention as defined in the claims that follow.