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Measurement of wind speed distributions across a solar collector
Solar Encr~! r Vol. 24. pp. 403 to 405
0038-092X/80 0401 0403/$02.00/0
© Pergamon Press Ltd., 1980. Printed in Great Britain
TECHNICAL NOTE Measurement of wind speed distributions across a solar collector M. V. OLIPHANT Institute for Energy Studies, School of Physical Sciences, The Flinders University of South Australia, Bedford Park, South Australia, 5042.
(Received 17 August 1979: accepted 1 November 1979)
INTRODUCTION
EXPERIMENT
in order to calculate the expected heat loss and efficiency of a flat plate collector it is necessary to have a knowledge of the convective heat transfer coefficient from the top cover to the surroundings. The coefficient used extensively to date is that referenced by M c A d a m s [1]
The solar collector test rig consisted of two flat plate collectors each with outside dimensions 1320 × 6 0 0 m m s and separated by a gap of 370 mms. Because the collectors were not mounted directly on the roof, but were on an elevated frame without any backing (Fig. 1), a board was occasionally placed across the gap to make a continuous surface of 132 x 1570mms. Experiments were made with and without the board in position. The collectors faced North and were inclined at an angle of 31 ° (latitude of Adelaide is 35°S). Ten 3 0 m m . dia. cup anemometers were placed at various positions around the two collectors--each anemometer had its own mechanical counter. The cups were mounted so that the centre of each was 100 m m s above the plate surface. The mean free wind speed was measured by a single 50 m m dia. cup anemometer, placed at a height of about 6 m above the roof (3 m above collector plates). Mean wind direction measurements were taken every half. minute during a 10-min test period.
hw = 5.7 + 3,8v[Wm -2 K - t ]
(1)
f o r 0 < v < 5 m s t where v is the wind speed. Watmuff et al. [2] propose that the following relation is more accurate. hw = 2.8 + 3.0v[Wm -2 K - t ]
(2)
for 0 < v < 7 m s t. They suggest that eqn (1) includes a radiative as well as convective component, whereas eqn (2) more properly accounts for convection only. In evaluating hw, the average value of the wind speed across the collector plate surface, v, is required. The influence of wind direction was not considered. In 1932 Rowley and Eckley [3] performed experiments to find the relationship between the "surface coefficients" of glass for different angles of incidence between the wind and the surface. They concluded that for all practical purposes the surface coefficients obtained for air flow parallel to the surface could be used without any correction for wind direction. These authors indicated that such a correction would be difficult to make and could not be justified due to other uncertainties surrounding the problem. Nevertheless, their graph of surface coefficients for glass shows a significant variation with angle of incidence, e.g. at v = 4.5 m s - t there is a 20 per cent difference between the surface coefficients for angles of incidence of 0 and 60 °. It is also felt that the geometry, area and surface properties of the sample on which tests are conducted to determine h,,, must have some influence on the values of its coefficients. At collector test sites wind speed measurements are generally taken at either
R ES U L T S
These are shown in Figs. 2-6. The only results selected for analysis were those for which the wind direction remained predominantly constant over the 10-rain test period. Figures 2-5 were obtained with the gap between the two collectors blocked by the backing board. The profiles changed quite significantly with wind direction and as expected were influenced by local obstructions. There were no obstructions to the North and West of the collector, however, to the South there was a storage tank and to the East an instrument shed. The results given in Figs. 5 and 6 show that the contour pattern is quite sensitive to local flow conditions. Figures 2, 5 and 6 demonstrate that the wind speeds near the collector surface were consistently lower than the free wind speed. CONCLUSIONS
1. Several metres above the collector surface, in the free flow region where the conditions would not be representative of those prevailing at the collector surface, or, 2. Near the collector frame where local turbulence would make a single wind speed measurement unreliable.
The conclusions to be drawn from this experiment are
In order to gauge the validity of these procedures, an experiment was performed to measure variations in wind speed across the collector cover surface and to observe any dependence on wind direction. In addition any significant difference between the mean wind speeds, measured at the plate surface and in the free flow region, could be determined. 403
1. The wind pattern across a collector is a function of wind direction. 2. It is not valid to take a single wind speed measurement either near the collector or at a height of several metres above it and expect this value to give an accurate indication of the average wind speed across the plate surface. Hence it would be expected that this procedure would lead to inconsistencies in the evaluation of convection losses from the collector.
404
Technical Note
Fig. I. Solar collector test rig, showing positions of anemometers.
4"2ms-=
~
2lms~
2.0 ms ~
N Fig. 2. Wind speed contours across the solar collector plates, Wind direction from the North,
Fig. 3. Wind speed contours across the solar collector plates. Wind direction from the North-West. Free mean wind speed -~ 5.3 ms-1.
Technical Note
405
Z--/ i Fig. 4. Wind speed contours across the solar collector plates. Wind direction from the West.
2.5__ ms'l
J'I J
S
f
f
t.3 ms "~ I
/
Fig, 5. Wind speed contours across the solar collector plates. Wind direction from the South. Free mean wind speed -~ 4.8 ms- ~. The errors in hw may also be compounded by inaccuracies in the coefficients "'a" and "b" in a relation of the form h,. = a + by. This is brought about by not accounting for variations in wind direction and by relying on results obtained from test samples having different physical characteristics from present day collectors.
S.E.24/4--
~
Fig. 6. Wind speed contours across the solar collector plates. (Board covering gap between the collectors has been removed). Wind direction from the South. Free mean wind speed -~ 5.0 ms- i. In solar collector efficiency tests which conform to, say, ASHRAE standards, a high degree of accuracy is required in measuring parameters such as temperature, insolation and flow rate. The present results indicate that the procedure as presently laid down does not adequately account for the influence of wind on collector efficiencies.
Acknowledgements--Thanks are given to David Proctor of the Department of Mechanical Engineering, CSIRO, Higherr, Victoria, for his encouragement and for pointing out useful references. Thanks also to Professor H. A. Blevin and Dr. E. L. Murray for many helpful discussions and under whose supervision this work was carried out. The loan of the RIMCO anemometers by Dr. J. Bennett of the Earth Sciences Discipline at Flinders University is also gratefully acknowledged. This work was supported by a South Australian Government Energy Research Grant. REFERENCES
1. W. C. McAdams, Heat Transmission, 3rd Edn. McGraw Hill, N.Y. (1954). 2. J. H. Watmuff, W. W. S. Charters and D. Proctor, Solar and wind-induced external coefficients solar collectors. Comples Intl Revue d'Hellio Technique, p. 56 (Sept. 1977). 3. F. B. Rowley and W. A. Eckley, Surface coefficients as affected by direction of wind. A.S.H.V.E. Transactions, 38, 33-45 (1932).
0038-092X/80 0401 0403/$02.00/0
© Pergamon Press Ltd., 1980. Printed in Great Britain
TECHNICAL NOTE Measurement of wind speed distributions across a solar collector M. V. OLIPHANT Institute for Energy Studies, School of Physical Sciences, The Flinders University of South Australia, Bedford Park, South Australia, 5042.
(Received 17 August 1979: accepted 1 November 1979)
INTRODUCTION
EXPERIMENT
in order to calculate the expected heat loss and efficiency of a flat plate collector it is necessary to have a knowledge of the convective heat transfer coefficient from the top cover to the surroundings. The coefficient used extensively to date is that referenced by M c A d a m s [1]
The solar collector test rig consisted of two flat plate collectors each with outside dimensions 1320 × 6 0 0 m m s and separated by a gap of 370 mms. Because the collectors were not mounted directly on the roof, but were on an elevated frame without any backing (Fig. 1), a board was occasionally placed across the gap to make a continuous surface of 132 x 1570mms. Experiments were made with and without the board in position. The collectors faced North and were inclined at an angle of 31 ° (latitude of Adelaide is 35°S). Ten 3 0 m m . dia. cup anemometers were placed at various positions around the two collectors--each anemometer had its own mechanical counter. The cups were mounted so that the centre of each was 100 m m s above the plate surface. The mean free wind speed was measured by a single 50 m m dia. cup anemometer, placed at a height of about 6 m above the roof (3 m above collector plates). Mean wind direction measurements were taken every half. minute during a 10-min test period.
hw = 5.7 + 3,8v[Wm -2 K - t ]
(1)
f o r 0 < v < 5 m s t where v is the wind speed. Watmuff et al. [2] propose that the following relation is more accurate. hw = 2.8 + 3.0v[Wm -2 K - t ]
(2)
for 0 < v < 7 m s t. They suggest that eqn (1) includes a radiative as well as convective component, whereas eqn (2) more properly accounts for convection only. In evaluating hw, the average value of the wind speed across the collector plate surface, v, is required. The influence of wind direction was not considered. In 1932 Rowley and Eckley [3] performed experiments to find the relationship between the "surface coefficients" of glass for different angles of incidence between the wind and the surface. They concluded that for all practical purposes the surface coefficients obtained for air flow parallel to the surface could be used without any correction for wind direction. These authors indicated that such a correction would be difficult to make and could not be justified due to other uncertainties surrounding the problem. Nevertheless, their graph of surface coefficients for glass shows a significant variation with angle of incidence, e.g. at v = 4.5 m s - t there is a 20 per cent difference between the surface coefficients for angles of incidence of 0 and 60 °. It is also felt that the geometry, area and surface properties of the sample on which tests are conducted to determine h,,, must have some influence on the values of its coefficients. At collector test sites wind speed measurements are generally taken at either
R ES U L T S
These are shown in Figs. 2-6. The only results selected for analysis were those for which the wind direction remained predominantly constant over the 10-rain test period. Figures 2-5 were obtained with the gap between the two collectors blocked by the backing board. The profiles changed quite significantly with wind direction and as expected were influenced by local obstructions. There were no obstructions to the North and West of the collector, however, to the South there was a storage tank and to the East an instrument shed. The results given in Figs. 5 and 6 show that the contour pattern is quite sensitive to local flow conditions. Figures 2, 5 and 6 demonstrate that the wind speeds near the collector surface were consistently lower than the free wind speed. CONCLUSIONS
1. Several metres above the collector surface, in the free flow region where the conditions would not be representative of those prevailing at the collector surface, or, 2. Near the collector frame where local turbulence would make a single wind speed measurement unreliable.
The conclusions to be drawn from this experiment are
In order to gauge the validity of these procedures, an experiment was performed to measure variations in wind speed across the collector cover surface and to observe any dependence on wind direction. In addition any significant difference between the mean wind speeds, measured at the plate surface and in the free flow region, could be determined. 403
1. The wind pattern across a collector is a function of wind direction. 2. It is not valid to take a single wind speed measurement either near the collector or at a height of several metres above it and expect this value to give an accurate indication of the average wind speed across the plate surface. Hence it would be expected that this procedure would lead to inconsistencies in the evaluation of convection losses from the collector.
404
Technical Note
Fig. I. Solar collector test rig, showing positions of anemometers.
4"2ms-=
~
2lms~
2.0 ms ~
N Fig. 2. Wind speed contours across the solar collector plates, Wind direction from the North,
Fig. 3. Wind speed contours across the solar collector plates. Wind direction from the North-West. Free mean wind speed -~ 5.3 ms-1.
Technical Note
405
Z--/ i Fig. 4. Wind speed contours across the solar collector plates. Wind direction from the West.
2.5__ ms'l
J'I J
S
f
f
t.3 ms "~ I
/
Fig, 5. Wind speed contours across the solar collector plates. Wind direction from the South. Free mean wind speed -~ 4.8 ms- ~. The errors in hw may also be compounded by inaccuracies in the coefficients "'a" and "b" in a relation of the form h,. = a + by. This is brought about by not accounting for variations in wind direction and by relying on results obtained from test samples having different physical characteristics from present day collectors.
S.E.24/4--
~
Fig. 6. Wind speed contours across the solar collector plates. (Board covering gap between the collectors has been removed). Wind direction from the South. Free mean wind speed -~ 5.0 ms- i. In solar collector efficiency tests which conform to, say, ASHRAE standards, a high degree of accuracy is required in measuring parameters such as temperature, insolation and flow rate. The present results indicate that the procedure as presently laid down does not adequately account for the influence of wind on collector efficiencies.
Acknowledgements--Thanks are given to David Proctor of the Department of Mechanical Engineering, CSIRO, Higherr, Victoria, for his encouragement and for pointing out useful references. Thanks also to Professor H. A. Blevin and Dr. E. L. Murray for many helpful discussions and under whose supervision this work was carried out. The loan of the RIMCO anemometers by Dr. J. Bennett of the Earth Sciences Discipline at Flinders University is also gratefully acknowledged. This work was supported by a South Australian Government Energy Research Grant. REFERENCES
1. W. C. McAdams, Heat Transmission, 3rd Edn. McGraw Hill, N.Y. (1954). 2. J. H. Watmuff, W. W. S. Charters and D. Proctor, Solar and wind-induced external coefficients solar collectors. Comples Intl Revue d'Hellio Technique, p. 56 (Sept. 1977). 3. F. B. Rowley and W. A. Eckley, Surface coefficients as affected by direction of wind. A.S.H.V.E. Transactions, 38, 33-45 (1932).