Abstract:
A solar collector with external reflector. A solar collector includes a glass housing having a heat pipe disposed within the housing and a light reflector disposed external to the housing.

Description:
[0001]    The invention is directed to a collector having an externally disposed nonimaging reflector and more particularly is directed to a solar collector with a heat pipe positioned within an evacuated glass tube with an externally disposed nonimaging reflector.  
         BACKGROUND OF THE INVENTION  
         [0002]    It was recognized more than 20 years ago, that combining selective absorbers, vacuum insulation and nonimaging concentration (using Compound Parabolic Concentrator, or “CPC”, type optics as shown in FIG. 9A-9C) enabled stationary mid-temperature collectors to have a useful operating range approaching 300 degrees Celsius”. Following the early proof-of-concept experiments, a commercial collector was developed in the last 5-years with good performance up to 250 degrees Celsius. These configurations integrated all the optics within the vacuum envelope. For this reason we refer to them as ICPC&#39;s (integrated CPC&#39;s). Their cost of manufacture is presently too high for widespread applications. On the other hand, the advent of very low-cost evacuated tubes allows us now to consider these as candidates for low-cost mid-temperature applications. One can combine various of these features to use such low-cost tubes (intended as stand-alone low-temperature collectors for providing domestic hot water) as receivers and now combined with external nonimaging reflectors. Since these glass tubes were originally intended for low-temperature (domestic hot water) use, their use at higher temperatures raised issues such as providing for efficient heat transfer to a working fluid, and assuring against thermal-induced tube breakage.  
           [0003]    A solar collector which is efficient at temperatures in the 125 to 150 degree Celsius above ambient range would therefore be of great utility for many high-value applications. For example, operating temperatures for solar cooling in conjunction with double-effect chillers are in this range. At the same time the collector component would need to be low-cost, have minimal operation and maintenance cost and long life. The external reflector form of a CPC has the potential for satisfying these criteria. The vacuum receiver has intrinsically long-life, being protected from the environment. The impressive commercial development of vacuum solar collectors in China over the last decade and more demonstrates that these can be manufactured and sold at low-cost. To give an example; in the year 2000 the all-glass dewar type solar tube made in China was available at an OEM cost of $3 US. Since the volume of manufacturing has been rising, prices are not increasing. It is significant to observe that a wide-angle CPC reflector will “unwrap” the cylindrical solar tube to an aperture of approximately 0.2 square meters. Therefore the vacuum component contributes $15 per square meter to the cost. The heat extraction device which may be a manifold likely adds a similar amount. The nonimaging reflector can be estimated at $20 per square meter, which is dominated by the material cost for a high quality aluminum mirror. An installed cost of approximately $100 per square meter would be a reasonable goal. The availability of an efficient mid-temperature solar collector for $100 per square meter would have a broad vista of applications.  
         SUMMARY OF THE INVENTION  
         [0004]    A solar collector system is directed to a combination of a heat pipe disposed within a housing which is at least partially transparent to light with the housing preferably evacuated. The heat pipe includes a copper pipe and coupled aluminum heat transfer fins disposed about the heat pipe. The fins are molded to optimize thermal contact with the heat pipe and interior surface of the housing. The solar collector further includes a reflector assembly externally disposed to the housing to simplify construction and costs of manufacture. Preferably the reflector is a nonimaging design. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 shows XCPC thermal model performance and measured performance of a test panel with dewar tubes;  
         [0006]    [0006]FIG. 2 shows instantaneous solar to thermal conversion efficiency for a heat pipe embodiment for mid temperature performance ranges;  
         [0007]    [0007]FIG. 3 shows performance limits of a commercial VAC 2000 solar collector;  
         [0008]    [0008]FIG. 4A shows a disassembled embodiment of a portion of a solar receiver and FIG. 4B shows a cross section of an assembled unit;  
         [0009]    [0009]FIG. 5 shows a partially assembled collector system with the manifold and heat pipe in position;  
         [0010]    [0010]FIG. 6 shows a first collector configuration with external reflector;  
         [0011]    [0011]FIG. 7 shows a second collector configuration with external reflector;  
         [0012]    [0012]FIG. 8 shows a third collector configuration with external reflector;  
         [0013]    [0013]FIG. 9A shows a CPC shape for various incidence angles, FIG. 9B shows 0° (normal) incidence and FIG. 9C SHOWS 30° incidence;  
         [0014]    [0014]FIG. 10A shows a plot of thermal performance of collector test number C444 with wind; FIG. 10B shows the performance without wind;  
         [0015]    [0015]FIG. 11A shows a plot of thermal performance of collector test number C500 with wind; FIG. 11B shows the performance without wind; and  
         [0016]    [0016]FIG. 12A shows a plot of thermal performance of collector test number C370 with wind; FIG. 12B shows the performance without wind. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0017]    In accordance with the invention, two types of preferred combination of solar collectors  12  (concentrators or receivers) are described, including an all glass dewar-type tube  11  and a heat-pipe  10  in a conventional evacuated tube  13  (see FIGS. 4A, 4B and  5 ). The dewar-type  11  is very low-cost since it is made in large quantities by a large number of manufacturers and uses a very low-cost borosilicate glass tubing. Good heat transfer poses technical challenges, and our experiments with a heat transfer compound to couple the tube  11  to a manifold  20  gave encouraging results. The preliminary mid-temperature performance obtained with a test panel with dewar tubes is compared with that predicted by a simple model shown in FIG. 1. The heat-pipe evacuated tube  13  (see FIG. 4B), uses the same very low-cost glass tubing. The heat transfer is accomplished in an elegant way by the incorporation of the heat pipe  10  within the evacuated tube  13  which in turn is disposed in a full panel array  15  (see FIGS. 4A, 4B and  5 ). The heat pipe  10  of FIGS. 4A and 4B includes a copper heat pipe  16  and contoured aluminum heat transfer fins  18  with the heat pipe  10  inserted into the glass tube  14  sandwiched between two aluminum fins  18 . The fins  18  are molded to maximize contact with the heat pipe  10  and the inside surface of the evacuated glass tube  14 . The heat pipe  10  transfers heat to the manifold  20  shown in FIG. 5 via heat transfer liquid inside the hollow heat pipe  10 . The hollow centre of the heat pipe  10  includes a vacuum, so that at even at temperatures of around 25-30° C. the well known heat transfer compound will vaporize. When heated the vapor rises to the tip (condenser) of the heat pipe  10  where the heat is transferred to the water flowing through the manifold  20 . The loss of heat causes the vapor to condense and flow back down the heat pipe  10  where the process is once again repeated. The preliminary mid-temperature performance obtained with the prototype heat-pipe version is shown in FIG. 2. The performance limit of known CPC-type vacuum solar collectors (not shown) can be gauged from FIG. 3. In this type of solar device both absorber and nonimaging concentrating optics are encased in an integral glass envelope, and this is called the integrated CPC or 1CPC. Commercial collectors of this type have a higher cost than the all glass dewar type with external CPC reflectors  22  of FIGS. 6-8. However, it does indicate a practical and realizable performance upper limit for the stationary nonimaging solar collectors  12 . One can further combine the advantages of the low-cost all-glass evacuated receiver with the heat pipe. As shown in FIGS. 4A, 4B and  5 , the heat pipe  10  and absorber fin assembly is inserted in the double-walled evacuated tube  14  and the heat pipes  10  are inserted into the simple flow-through heat exchanger manifold  20 . There is no fluid connection which is one of the chief advantages of a heat application, but appears sufficiently robust to withstand stagnation temperatures. Various examples of performance of a conventional evacuated tube but externally disposed reflector (without the heat pipe  10 ) are shown in Examples 1-111 wherein collector test results are shown in FIGS. 6-8 for the collector configurations. These tests were made by Solartechnik Prüfung Forschung, located in Bern, Switzerland.  
         [0018]    While preferred embodiments have been illustrated and described, it should be understood that changes and modifications can be made therein in accordance with one of ordinary skill in the art without departing from the invention in its broader aspects.  
       EXAMPLES  
       [0019]    The following non-limiting examples describe various embodiments and associated performance test results.  
       Example I  
       [0020]    Collector Test No. C444. The embodiment of FIG. 6 is described in Table 1 and was subjected to various tests as set forth in Table 2. Note there was no stagnation temperature for standard values ISO 9806-2 and EN 12975-2 are 30° C./1000 W/m 2 . The thermal performance (flowrate at test: 204 l/h) is shown in FIGS. 10A and 10B, with and without wind, respectively.  
                   TABLE 1                           Contact   Ritter Solar GmbH, D-72135 Dettenhausen           Tel. +49 (07157) 5359-0,           Fax +49 (07157) 5359-20       Distributed in*   DE       Type   ETC, cylindrical absorbers, CPC,           direct heat transfer       Assembly       Installation*   Installation on sloping roof,           Flat root with support       Rated flowrate*   180 l/h       Absorber coating*   Al/Al N       Dimensions   2.010 m 2 , 1.988 m 2 , 2286 m 2         (absorber, aperture, gross)       Gross dimensions:   1.640 × 1.394 × 0.105       l, w, h (in m)       Weight including glazing*   35 kg                          
 
         [0021]    [0021]                                             TABLE 2                           Carried               Test   out   Section   Report*                                Durability test according to ISO   No   3   LTS C444       Durability test according to EN   No   3   C444LPEN       Measurement of stagnation temperature   No   3.1       Efficiency measurement acc. SPF   Yes   4.1       Efficiency measurement acc   Yes   4.1       ISO, DIN, EN       Incidence angle modifier (IAM)   Yes   4.4       Measurement of pressure drop   No   4.5       Measurement of thermal capacity   Yes   4.6       Measurement of time constant   Yes   4.6                            
         [0022]    Tables 3A and 3B illustrate characteristic efficiency values (normal incidence, G=800 W/m 2 ) for efficiency with and without wind, respectively. Tables 4A and 4B show power output (power in watts per collector, normal incidence, beam irradiation) with and without wind, respectively.  
                                                                 Tables 3A and 3B            Reference area   Absorber   Aperture   Gross   Reference area   Absorber   Aperture   Gross               η (T* m  = 0.00)   0.62   0.62   0.54   η (x = 0.00)   0.62   0.62   0.54       η (T* m  = 0.05)   0.56   0.57   0.49   η (x = 0.05)   0.56   0.57   0.50       η (T* m  = 0.10)   0.50   0.51   0.44   η (x = 0.10)   0.50   0.51   0.44                  
 
         [0023]    [0023]                                                                 Tables 4A and 4B            Irradiation   400 W/m 2     700 W/m 2     1000 W/m 2     Irradiation   400 W/m 2     700 W/m 2     1000 W/m 2                 t* m  − t e  = 10K   474   846   1′218   t* m  − t e  = 10K   475   847   1′219       t* m  − t e  = 30K   429   801   1′173   t* m  − t e  = 30K   431   803   1′175       t* m  − t e  = 50K   382   754   1′126   t* m  − t e  = 50K   385   757   1′129                    
         [0024]    Table 5 shows incidence angle modifier (IAM), Table 6 shows pressure drop in Pascals (test fluid 33.3% Ethylenglykol) and Table 7 shows thermal capacity and time constant.  
                                                                                                         TABLE 5                                   0°   10°   20°   30°   40°   50°   60°   70°   80°   90°                                    K(Θ), long     1.0                   0.90               0.0       K(Θ), trans     1.0       1.01   1.0   1.01   1.01   1.05   1.16       0.0                  
 
         [0025]    [0025]                                                 TABLE 6                                   100 l/h   150 l/h   250 l/h   350 l/h   500 l/h                                        20° C.           60° C.           80° C.                        
         [0026]    [0026]                           TABLE 7                                   Thermal capacity (kJ/K)   Time constant (s)                           16.2   202                        
         [0027]    These tests were performed by SPF, Hochschule Rapperswil (HSR) at Oberseestr. 10, CH-8640 Rapperswil.  
       Example II  
       [0028]    Collector Test No. C500. (Consolar GmbH, TUBO 11 CPC) The embodiment of FIG. 7 is described in Table 8 and the tests of Table 9 were performed. There was no stagnation temperature for standard values ISO 9806-2 and EN-12975-2 were 30° C./1000 W/m 2 . The thermal performance (flowrate at test: 100 l/h) is illustrated in FIGS. 11A and 11B, with and without wind, respectively.  
                   TABLE 8                           Contact   Consolar GmbH, D-60489 Frankfurt/M.           Tel. +49 (069) 61 99 11 30,       Fax +49 (069) 61 99 11 28       Distributed in*   DE, AT, *EU*       Type   ETC, cylindrical absorbers, CPC,           direct heat transfer       Assembly       Installation*   Installation on sloping root,           flat roof with support       Rated flowrate*   100 l/h       Absorber coating*   Metal carbide       Dimensions   0.873 m 2 , 0.967 m 2 , 1.163 m 2         (absorber, aperture, gross)       Gross dimensions:   1.860 × 0.625 × 0.045       l, w, h (in m)       Weight including glazing*   13 kg                          
 
         [0029]    [0029]                                             TABLE 9                           Carried               Test   out   Section   Report*                                Durability test according to ISO   No   3   LTS C500       Durability test according to EN   No   3   C500LPEN       Measurement of stagnation temperature   No   3.1       Efficiency measurement acc. SPF   Yes   4.1       Efficiency measurement acc   Yes   4.1       ISO, DIN, EN       Incidence angle modifier (IAM)   Yes   4.4       Measurement of pressure drop   Yes   4.5       Measurement of thermal capacity   No   4.6       Measurement of time constant   No   4.6                            
         [0030]    Tables 10A and 10B illustrate characteristic efficiency values (normal incidence, G=800 W/m 2 ) for efficiency with and without wind, respectively. Tables 11A and 11B show power output (power in watts per collector, normal incidence, beam irradiation) with and without wind, respectively.  
                                                                 Tables 10A and 10B            Reference area   Absorber   Aperture   Gross   Reference area   Absorber   Aperture   Gross               η (T* m  = 0.00)   0.73   0.66   0.55   η (x = 0.00)   0.73   0.66   0.55       η (T* m  = 0.05)   0.66   0.59   0.49   η (x = 0.05)   0.67   0.60   0.50       η (T* m  = 0.10)   0.59   0.53   0.44   η (x = 0.10)   0.61   0.55   0.46                  
 
         [0031]    [0031]                                                                 Tables 11A and 11B                400   700   1000       400   700   1000       Irradiation   W/m 2     W/m 2     W/m 2     Irradiation   W/m 2     W/m 2     W/m 2                 t m  − t e  = 10K   241   431   622   t m  − t e  = 10K   244   434   624       t m  − t e  = 30K   217   407   597   t m  − t e  = 30K   224   414   604       t m  − t e  = 50K   192   383   573   t m  − t e  = 50K   204   394   584                    
         [0032]    Table 12 shows incidence angle modifier (IAM), and Table 13 shows pressure drop in Pascals (test fluid 33.3% Ethylenglykol).  
                                                                                                         TABLE 12                                   0°   10°   20°   30°   40°   50°   60°   70°   80°   90°                                    K(Θ), long     1.0                   0.93               0.0       K(Θ), trans     1.0   1.0   1.0   0.95   0.82   0.84   0.90   1.02   1.03   0.0                  
 
         [0033]    [0033]                                                                     TABLE 13                                   50 l/h   100 l/h   150 l/h   175 l/h   200 l/h                                        20° C.   6400   13300   21400   26000   30700           60° C.           80° C.                        
       Example III  
       [0034]    Collector Test No. C370. (Paradigma-Schweiz, CPC 14 Star) The embodiment of FIG. 8 is described in Table 14, and the tests of Table 15 were performed. The stagnation temperature for standard values ISO 9806-2 and EN 12975-2 were for 30° C./1000 W/m 2 , 269° C. The collector also passed a durability test. The thermal performance (flowrate at test: 179 l/h) is shown in FIGS. 12A and 12B, with and without wind, respectively.  
                   TABLE 14                           Contact   Paradigma-Schweiz, CH-6201 Sursee           Tel. +41 (041) 925 11 22,           Fax +41 (041) 925 11 21       Distributed in*   CH, DE, AT, *EU*, PL, HR       Type   Evacuated tube collector,           cylindrical absorbers, CPC,           direct heat transfer       Installation*   Installation on sloping root,           Flat root with support,           Facade installation       Rated flowrate*   180 l/h       Absorber coating*   Al/Al N       Dimensions   2.332 m 2 , 2.325 m 2 , 2.618 m 2         (absorber, aperture, gross)       Gross dimensions: l, w, h (in m)   1.613 × 1.623 × 0.120       Weight including glazing*   42 kg                          
 
         [0035]    [0035]                                             TABLE 15                           Carried               Test   out   Section   Report*                                Durability test according to ISO   Yes   3   C370QPISO       Durability test according to EN   Yes   3   C370QPEN       Measurement of stagnation temperature   Yes   3.1   C370QPEN       Efficiency measurement acc. SPF   Yes   4.1   LTS C370       Efficiency measurement acc   Yes   4.1   C370LPEN       ISO, DIN, EN       Incidence angle modifier (IAM)   Yes   4.4       Measurement of pressure drop   No   4.5       Measurement of thermal capacity   Yes   4.6       Measurement of time constant   No   4.6                            
         [0036]    Tables 16A and 16B illustrate characteristic efficiency (normal incidence, G=800 W/m 2 ) for efficiency with and without wind, respectively. Table 17A and 17B show power output (power in watts per collector, normal incidence, beam irradiation) with and without wind, respectively.  
                                                                 Tables 16A and 16B            Reference area   Absorber   Aperture   Gross   Reference area   Absorber   Aperture   Gross               η (T* m  = 0.00)   0.68   0.68   0.60   η (x = 0.00)   0.68   0.68   0.60       η (T* m  = 0.05)   0.59   0.60   0.53   η (x = 0.05)   0.60   0.60   0.54       η (T* m  = 0.10)   0.50   0.51   0.45   η (x = 0.10)   0.52   0.52   0.46                  
 
         [0037]    [0037]                                                                                                   Tables 17A and 17B            Irradiation   400 W/m 2     700 W/m 2     1000 W/m 2     Irradiation   400 W/m 2     700 W/m 2     1000 W/m 2                      t m  − t e  = 10K   593   1′065   1′537   t m  − t e  = 10K   597   1′069   1′541       t m  − t e  = 30K   517   989   1′461   t m  − t e  = 30K   528   1′000   1′472       t m  − t e  = 50K   437   909   1′381   t m  − t e  = 50K   455   928   1′400                    
         [0038]    Table 18 shows incidence angle modifier (IAM).  
                                                                                                         TABLE 18                                   0°   10°   20°   30°   40°   50°   60°   70°   80°   90°                                    K(Θ), long     1.0                   0.90               0.0       K(Θ), trans     1.0       1.01   1.00   1.01   1.01   1.05   1.16       0.0