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Reply to Paul Theunissen
294
Letters to the Editor
Comments on "Optical transmittance measurements on a solar collector panel of cylindrical glass tubes Dear Sir, In the paper "Optical transmittance measurements on a solar collector cover of cylindrical glass tubes," Solar Energy 28(1), 5-11 (1982), Mr. Herrick reports that the transversal transmittance of an Evacuated Tube Cover (ETC) "exhibits a pronounced maximum at incidence angle near 70°" . As written also in the conclusions, "no explanation for this maximum can be offered at the present time...", Table 1 shows that the experimental set up overpredicts the transmittance of a double glass cover beyond the incidence angles of 40°, but Mr. Herrick writes "This good agreement demonstrates the proper functioning of the apparatus..." This disagreement raises questions about the accuracy of the results for the ETC at large incidence angles and in particular about the existence of the transversal transmittance maximum around 700. I computed the deviation, •, of Herrick's experiments for the double glass transmittance, "r~,from the transmittance values, r2, given in Ref. [8] of the paper, using the following relation:
Table 1. Experimental error, e, of Herrick's double glass transmittance and corrected ETC transversal transmittance, ~'t.c Angle of incidence degree 0 10 20 30 40 50 60 70 80
e
7t.c
0.010 0.001 0.006 0.016 0.037 0.042 0.099 0.208 0.920
0.876 0.884 0.882 0.878 0.869 0.876 0.885 0.828 0.458
• = (zl - r2)/~,_. These results are given in the enclosed table as a function of incidence angle. Assuming the same experimental errors occur during the measurements of r±, The ETC "Hemispherical Optical Transmittance" (Table 1, column A), a corrected set of values for the transversal transmittance, ~'_.c is easily computed:
"r~.c = "r±/(1+ •). The resulting ETC transversal transmittance, presented in the enclosed Table does not show a maximum and, moreover, has a
shape similar to those found by Bayazitoglu and Aggarpour (Ref.[6] of paper). I believe that the maximum of the ETC transversal transmittance near 70° claimed by Mr. Herrick is a result of an experimental error. PAUL H. THEUNISSEN
University of Wisconsin-Madison College of Engineering Engineering Experiment Station Solar Energy Laboratory Madison, WI 53706 U.S.A.
Research Assistant
Reply to Paul Theunissen Dear Sir, Paul Theunissen may possibly be correct in his assertion of error in my paper in Solar Energy 28, 5 (1982). However, if he is correct it will be for reasons other than those cited in his letter of 27 May. His assumption that the same errors occurred in the Evacuated Tube Cover (ETC) measurement as occurred in the 2 Flat Glass Cover (FGC) measurement is difficult to justify in light of the statements on pg 10 of my article. There I identify an important source of error for the FGC case which was stated to be absent for the ETC case. Additionally there is the problem of comparing unlike quantities without accounting for their differences. I refer to Th~unissen's comparison of my FGC transmittance measurements on a unit 2 feet wide with those from Ref. [8] (ASHRAE Handbook 1974) for units of infinite width. Edge effects were clearly acknowledged in my article. Theunissen's assumption is not justified by the facts. This discussion may have a useful result in that it helps call
attention to the probable differences in angular dependence of transmittance arising from the dimensions of the cover. It offers an opportunity to record my growing conviction that it is very difficult to mount a solar collector cover in a way that does not affect its transmittance. Most actual collector covers, I have come to believe, have angular dependence properties quite different from those of infinitely wide covers. Probably this difference is unimportant for most present fiat plate collectors. However when new collectors with significantly broader specific temperature rise capabilities (cloudy weather performance capability) arise on the scene then actual angular transmittance measurements will be needed. CARLYLES. HERRIEK
GeneralElectric Company CorporateResearch and Development Schenectady, N Y 12345
Letters to the Editor
Comments on "Optical transmittance measurements on a solar collector panel of cylindrical glass tubes Dear Sir, In the paper "Optical transmittance measurements on a solar collector cover of cylindrical glass tubes," Solar Energy 28(1), 5-11 (1982), Mr. Herrick reports that the transversal transmittance of an Evacuated Tube Cover (ETC) "exhibits a pronounced maximum at incidence angle near 70°" . As written also in the conclusions, "no explanation for this maximum can be offered at the present time...", Table 1 shows that the experimental set up overpredicts the transmittance of a double glass cover beyond the incidence angles of 40°, but Mr. Herrick writes "This good agreement demonstrates the proper functioning of the apparatus..." This disagreement raises questions about the accuracy of the results for the ETC at large incidence angles and in particular about the existence of the transversal transmittance maximum around 700. I computed the deviation, •, of Herrick's experiments for the double glass transmittance, "r~,from the transmittance values, r2, given in Ref. [8] of the paper, using the following relation:
Table 1. Experimental error, e, of Herrick's double glass transmittance and corrected ETC transversal transmittance, ~'t.c Angle of incidence degree 0 10 20 30 40 50 60 70 80
e
7t.c
0.010 0.001 0.006 0.016 0.037 0.042 0.099 0.208 0.920
0.876 0.884 0.882 0.878 0.869 0.876 0.885 0.828 0.458
• = (zl - r2)/~,_. These results are given in the enclosed table as a function of incidence angle. Assuming the same experimental errors occur during the measurements of r±, The ETC "Hemispherical Optical Transmittance" (Table 1, column A), a corrected set of values for the transversal transmittance, ~'_.c is easily computed:
"r~.c = "r±/(1+ •). The resulting ETC transversal transmittance, presented in the enclosed Table does not show a maximum and, moreover, has a
shape similar to those found by Bayazitoglu and Aggarpour (Ref.[6] of paper). I believe that the maximum of the ETC transversal transmittance near 70° claimed by Mr. Herrick is a result of an experimental error. PAUL H. THEUNISSEN
University of Wisconsin-Madison College of Engineering Engineering Experiment Station Solar Energy Laboratory Madison, WI 53706 U.S.A.
Research Assistant
Reply to Paul Theunissen Dear Sir, Paul Theunissen may possibly be correct in his assertion of error in my paper in Solar Energy 28, 5 (1982). However, if he is correct it will be for reasons other than those cited in his letter of 27 May. His assumption that the same errors occurred in the Evacuated Tube Cover (ETC) measurement as occurred in the 2 Flat Glass Cover (FGC) measurement is difficult to justify in light of the statements on pg 10 of my article. There I identify an important source of error for the FGC case which was stated to be absent for the ETC case. Additionally there is the problem of comparing unlike quantities without accounting for their differences. I refer to Th~unissen's comparison of my FGC transmittance measurements on a unit 2 feet wide with those from Ref. [8] (ASHRAE Handbook 1974) for units of infinite width. Edge effects were clearly acknowledged in my article. Theunissen's assumption is not justified by the facts. This discussion may have a useful result in that it helps call
attention to the probable differences in angular dependence of transmittance arising from the dimensions of the cover. It offers an opportunity to record my growing conviction that it is very difficult to mount a solar collector cover in a way that does not affect its transmittance. Most actual collector covers, I have come to believe, have angular dependence properties quite different from those of infinitely wide covers. Probably this difference is unimportant for most present fiat plate collectors. However when new collectors with significantly broader specific temperature rise capabilities (cloudy weather performance capability) arise on the scene then actual angular transmittance measurements will be needed. CARLYLES. HERRIEK
GeneralElectric Company CorporateResearch and Development Schenectady, N Y 12345