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Thermal and optical performance test results for compound parabolic concentrators (CPCs)
Solar EnergyVol. 44, No. 5. pp. 257-270, 1990
0038-092X/90 $3.00 + .00 Copyright © 1990 Pergamon Press pie
Printed in the U.S.A.
THERMAL AND OPTICAL PERFORMANCE TEST RESULTS FOR COMPOUND PARABOLIC CONCENTRATORS (CPCs) D. SURESH,* J. O'GALLAGHER,and R. WINSTON The University of Chicago, The Enrico Fermi Institute, Chicago, IL 60637, U.S.A. Abstract--The primary objectiveof the present study was to evaluatethe performancecharacteristics(thermal and optical) of a properly truncated CPC that could be used in two-stage solar thermal power generation systems. The CPCs selected for testing were the 5 : l cones with a 25* acceptance angle and an untruncated concentration ratio of 5.6×. Experiments were carded out at the Advanced Components Test Facility of the Georgia Tech Research Institute. Severalcones ofthe same dimensions hut with different shell materials, reflector surfaces, and employing various heat removal methods were tested. It has been demonstrated • experimentally for the first time that the CPCs with high reflectivitysurfaces can have optical et~cienciesin the range of 90% and above. In order to verifythese results,a computer ray-trace analysiswas also performed. These tests have shown that passive cooling alone is adequate for small-scale,low-power systems.
I. INTRODUCTION
duce the high temperature required for more efficient power generation. In this paper we report both the thermal and optical performance results of a third experimental test of nonimaging secondary concentrators whose design was chosen to be compatible with the latter approach. Some preliminary results were reported elsewhere[7].
Physical conservation principles set a limit to the maximum achievable concentration of any optical design. Traditional approaches based on the image-forming techniques of focussing optics fall short of the so-called "thermodynamic limit". However about a decade ago a new class of optical devices referred to as nonimaging concentrators which can closely approach this limit was proposed for solar energy collection[l-3]. The most familiar of these devices is the family of compound parabolic concentrators (CPCs) which achieve moderate concentration levels with much wider acceptance angles than had previously been thought possible. Two-stage designs employing nonimaging devices in tandem with a focussing element was proposed for solar thermal power generation. In a two-stage system, the primary is a conventional focussing element such as a parabol0idal mirror and the secondary is a nonimaging device, which when properly designed can augment the overall concentration of the hybrid system to approach the thermodynamic limit. Applications of two-stage designs lie in the higher concentration, small angular acceptance regime[4]. Two previously reported experiments have successfully demonstrated the practicality of this concept for point-focus dish concentrators by providing substantial increases in concentration ratio and intercept factor, respectively, on a JPL (Jet Propulsion Laboratory) i l-m dia. Test Bed Concentrator then at the Edwards Air Force Base[5], and on a 6-m alia. Omnium-G concentrator[6]. As an alternative design approach, rather than increasing the concentration ratio, the secondaries can instead be used to relax the optical tolerance requirements for the primary dish while maintaining the same concentration ratio and performance. Such an approach may result in substantial cost savings in a compound system which can still pro-
* Present Address: D.L.R. (EN-TT), Pfaffenwaldring3840, D-7000 Stuttgart 80, F.R.G. (West Germany).
2. BASIC DESIGN CONSIDERATIONS The resources and scope of the present effort would not permit construction of a full-scale primary ofeither of the two candidate designs originally proposed[8], to test the various possible secondaries. Therefore it was decided to fabricate several secondary elements corresponding to a representative base-line design and to evaluate them at the Advanced Components Test Facility (ACTF) operated by the Georgia Tech Research Institute (GTRI), Atlanta. In fact, an optimized twostage system with good optical coupling of a secondary to a specific primary requires the design of a proper CEC (Compound Elliptic Concentrator). Since we could not design hardware for a well-defined primary the CPC was chosen as the best candidate geometry representative of a 'generic' second-stage suitable for any moderately long focus primary. We structured our design around a secondary. CPC with an acceptance angle of 25 ° which corresponds to an untruncated concentration of 5.6× and a minimum focal ratio of I. 13. The physical dimensions of the entrance and exit aperture are as shown in the profile drawing (Fig. l). This CPC design could be used with any long focal length primary whose rim angle does not exceed 25* and whose focal spot dia does not exceed 48.3 cm (19.0 in.).
Fabrication technique for the CPC shell The CPC test cones were fabricated to the desired profile by metal spinning technique. This procedure was proven successful in an earlier project of ours to develop "trumpet-type" secondaries[5]. First, a brass template was cut in a numerically controlled machine
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Thermal loads and cooling methods
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according to the computer-generated set of coordinates defining the exact profile of the CPC. A steel mandril was then machined using the template and the individual cones were spun over it. Twelve spinnings were made, six each of copper and aluminum. Nine of spun shells (five copper and four aluminum) were then used to make complete secondaries; and the remaining cones were retained as spares.
Reflector surface preparations Copper cones. The inner surface of the cone was polished and buffed to remove the surface irregularities and tool marks. The outer surface was masked and the inner surface electroplated with commercial silver to a thickness of approximately 10-t /.tin. This surface was then polished and buffed again to a high gloss. Two different commercial facilities were available locally for silver-plating large size copper cones. Small, flat sample pieces of silver plated copper by each vendor were used to obtain the reflectivity characteristics of each surface. We found average values of reflectivity, p = 0.96 _+ 0.02 in one case, and p = 0.92 __ 0.02 in another. Three of the five copper cones used plating with high reflectivity while the other two had the lower value. Aluminum cones. As with the copper pieces, the inside surfaces of the shells were first polished and buffed to prepare a smooth finish. Two different reflector surfaces were used for the aluminum cones. Two of the four aluminum cones had a thin film of vacuumdeposited silver which was then overcoated with a thin layer of magnesium fluoride for protection. The other two aluminum cones had a coating of vacuum-deposited aluminum which was then overcoated with a layer of silicon oxide. No direct measurements of the reflectivities of these surfaces were available.
Thermal stresses on a CPC secondary depend on the stagnation temperatures reached by its walls. The temperature profile of the reflecting surface, in turn, will be decided by the amount of energy incident on it, and the manner in which it is distributed. The amount of energy absorbed by the reflecting surface will depend on its absorptance, a (= I - p). This absorbed energy will be dissipated through convection, radiation, and conduction. Our preliminary calculations based on a simple model showed that at the relatively low concentration ratios temperatures will remain under acceptable limits of about 200°C with passive cooling alone. However, at high concentrations, prohibitively high temperatures could result and some form of active or semiactive heat removal may be necessary. The results of a similar study based on a detailed heat transfer analysis on the thermal control for trumpet-type secondaries have been presented elsewhere[9]. To measure the effectiveness of a simple passive thermal design, selected CPCs were prepared with a high emissivity black paint (PYROMARK 2400) applied to the outside shell surface. One such version was fabricated for each of the different reflector surfaces-two copper cones with silver plating by the two different vendors; and two aluminum cones, one with silver and the other with aluminum coating. For these four cones no other additional provisions were made for enhancing heat removal. One additional cone of each material was prepared with longitudinal fins of the same material tack-welded to the outer surface. Each cone had eight fins spaced at 45 degree intervals around the circumference. It was found that in spite of the great care taken welding had a tendency to distort the contour of the inner surface. Therefore the tacks were spaced relatively far apart. Good thermal contact along the entire fin was assured by soft-soldering for the copper fins and applying a fillet of thermally conducting epoxy along the length of the aluminum fins. The remaining two copper CPCs and one aluminum cone (with overcoated silver) were fabricated with active water cooling. Each had 0.95 cm (] in.) dia. copper or aluminum tubing wound around the entire outer surface of the cone. The coils were tightly wound near the exit aperture and spaced at 3.8 cm (1.5 in.) intervals for the remainder of the surface. To avoid distorting the reflector surface the coils were attached using soft-solder for the copper coils and tack welding plus thermal epoxy for the aluminum coils (as in the case of the fins). Thus, we prepared a variety of copper and aluminum test cones with different reflector surfaces and with a variety of heat-removal techniques, such as entirely passive, enhanced passive, and completely watercooled.
Design and fabrication of cold water calorimeter In order to complement the energy flux measurements at the CPC exit aperture using the scanning flux
Thermal and optical performance test results for compound parabolic concentrators
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calorimeter (SFC) available at the site, a cold water calorimeter (CWC) was designed and built by the University of Chicago group. It was constructed as follows: a right circular cylinder ofdia. 27.9 cm (! 1.0 in.) and height 43.2 cm (17.0 in.) was formed from commercially available copper sheet of thickness 0.16 cm ( in.). Copper tubing of 1.6 cm (~ in.) dia. was wound closely around the outside with somewhat greater pitch. For illustration, a schematic diagram of the CWC mounted on a water-cooled CPC is shown in Fig. 2. One end of the cylinder was closed with a circular copper disc with a center hole whose diameter was exactly that of the exit aperture. The other end was closed with a 1.3 cm (½ in.) thick brass plate with spiral channels cut in it for coolant flow which were sealed by a 0.3 cm (~ in.) thick copper plate. Cooling water was fed from the inlet through the back plate and then to the top end of the inside coil, through it to the bottom of the outside coil and then up through the outer coil to the outlet. The innerside of the calorimeter was coated black with PYROMARK 1500. The entire outside of the cylinder was wrapped with a 5.0 cm (2.0 in.) thick FIBER-FRAX spun ceramic insulation which was held
in place by an aluminum jacket. There were 12 thermocouples on the body of each secondary, arranged as shown in Fig. 2. In addition, two thermocouples were spot-welded on to the surface of the inlet and outlet tubes of the CWC and one each was placed near the exit and the entrance. One immersion-type thermocouple was inserted in the water flow at the inlet and another at the outlet. Finally there was one thermocouple spot-welded to the body of the calorimeter and another left "floating" inside the insulation material. 3. EXPERLMENTAL TESTS
Description of the facility The ACTF uses a tracking array of about 550 circular heliostats, each 111 cm (43.7 in.) in diameter and made of 0.3 cm (~ in.) thick glass to deliver a concentrated solar flux to a work platform atop a central tower, 22.8 m (74.8 ft.) high. A complete description of the facility can be found in reference[10]. Individual mirror focussing is achieved by tightening a tension wire around the circumference of the mirror.
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Alternately, i f o n e wishes to use unfocussed plane mirrors, this can be achieved by removing the tension wire. To define the aperture for our tests, a large water-cooled plate with a circular opening of 45.7 cm (18.0 in.) in diameter was mounted horizontally on the tower at a
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height of 21.2 m (69.5 ft.) above the level of the mirror plane. Immediately below the test platform are two hydraulically actuated, water-cooled shutters which can be used to block the solar flux from the test area so the equipment adjustments can be made while the mirror field continues to track. The layout of the mirror field is as shown schematically in Fig. 3. Out of all the available mirrors, two field configurations were chosen for our tests-one consisting of 53 mirrors and the other 38. Viewed from the target plane, these two sets of mirrors filled two different annular zones that subtended effective rim angles between 15 ° and 22 °, respectively. The terms "'wide field" and "narrow field" are used in this paper to refer to these two fields. Although the design cutoffangle of the CPC was 25 °, it is well known that there is some rounding of the optical throughput curves for CPC cones to be expected due to skew ray losses and mirror slope errors in the CPC profile shape itself[l]. These two different field configurations were chosen to probe the actual throughput characteristics in this cutoffregion. There were no mirrors directly under the tower or immediately adjacent to its base so that the central portion of the CPC's field-of-view was unfilled. Thus these tests weigh the effects of the eutoffre~on relatively more heavily than would be characteristic of an actual primary dish. In fact, as will be discussed in a later section, the measurements set a lower limit on the ac-
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tual optical efficiency ofthe secondary when used with a dish primary. At the top of the tower the concentrated solar flux was measured using the SFC bar. It was essentially a scanning bar containing 31 Gardon gauge calorimeter transducers (HYCAL) spaced equally at 2.5-cm (l.0in.) intervals. The SFC was mounted along the eastwest direction and swept in the north-south direction across the desired measurement plane over a guide rail, driven by a motor. The total energy in the scanned plane was found by integrating the measured instantaneous flux from each calorimeter gauge during the time of the scan and then adding the contributions from all the gauges. This technique was used to measure the flux at the entrance plane of each CPC and again at its exit plane to cross-check with the CWC results and to determine the corresponding throughput with better possible accuracy than using either one of these devices.
261
location on the 28, 29 of September, and 5, 6, 7, and 9 of October. On the other days hardware preparation, testing and calibration of the sensors and preliminary data analysis were carried out. The first CPC tested was the water-cooled copper cone with silver reflecting surface on a clear day with an ambient temperature around 20°C. Tests were carried out with the 53-mirror wide field configuration, having unfocussed (fiat) mirrors. First, two successive entrance plane flux measurements were made by moving the scanning bar across the plane immediately above the water-cooled aperture plate. The shutters were then closed and the test CPC was mounted flush with the top of the plate with its entrance centered directly over the aperture. The water flow rate through the cooling coils was kept high enough (about 100 ml s-') to keep the surface temperature of the CPC shell close to ambient. The shutters were then opened and two successive scans of the energy in the exit plane of the CPC were carried out. Then the CWC was mounted in place, and shutter was opened. After equilibrium temperature was reached, measurements were carried out manually by monitoring the inlet and outlet temperatures of water through the CWC using two different high precision digital voltmeters while measuring the flow rate with a measuring jar and a stopwatch. Finally after removing the CWC two more exit plane scans
Chronology of tests Our experiments took place over a three-week period beginning the last week of September 1985, during which the facilities and personnel of the ACTF were allocated for support of our measurements for a total of six "sunny days" of operation. Solar availability provided satisfactory insolation levels to fulfill this al-
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were done in quick succession. The direct beam insolation as well as the measurements from the therme,couple sensors on the test cones were continuously recorded by the ACTF data acquisition system (DAS). Subsequently, on the following day, two passively cooled copper cones (silver plated by two different vendors) were tested in a similar manner. After a period of inclement weather, testing was resumed on the 5th of October and two more tests were carded out. First, the finned copper CPC with silver reflecting surface was tested with the wide field then with the narrow field (still using unfocussed mirrors). In addition, a tightly wound spiral of copper tubing which fitted exactly on to the opening in the aperture plate was used to redefine a smaller aperture of 21.6 cm (8.5 in.) at the entrance plane. Both the scanning bar and the CWC were used to measure the energy emerging through this aperture for further cross-calibration purposes. Next day, two different aluminum CPCs (one with silver and another with aluminum reflecting surfaces, both overcoated with magnesium fluoride) were tested with the wide field and unfocussed mirrors. Since no thermal problems had been encountered during the first four days of testing it was decided to increase the power by using focussed mirrors while at the same time reducing the effects due to being near the edge of the acceptance angle by using the narrow
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field only. All of the tests carried out on the last two days of testing were done with this configuration. At the completion of this period all the major tests which we had planned were successfully completed so that in one configuration or another data had been obtained for each design alternative incorporating all relevant variations in shell material, reflective surface and cooling strategy, including stand-alone, passiveonly cooling. 4. DATA R E D U C T I O N AND A N A L Y S I S
Insolation data
Direct insolation was continuously monitored; the analog voltage signals were digitized and converted in energy flux units (W m -2) which were printed out (in real time) along with scan data as well as temperatures. In addition, the digitized raw data were also available on magnetic tapes. After the 5th October, the insolation data were found to "drift" due probably to some trouble in that particular DAS channel. However, before noticing this drift the insolation values available on print (lj) had already been used in our analysis (described in the following sections). Therefore, to correct for this drift, it was necessary to read the raw electrical data from the tape, convert them into W m -~. These values were called 12. A regression plot for several days (October 5, 6, 7, and 9) showed the drift, as shown in (xi00)
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Scan data Each calorimeter on the scanning bar (SFC) was individually calibrated. The scanning was done with a scanning speed in the vicinity of 0.8 cm s-~ (0.3 in. per sec.). The digitized data with the flux values in converted units (W cm -z) were available along with the scanning bar position. The flux map thus had approximately 2.5 cm × 0.8 cm (1.0 in. × 0.3 in.) grids. All those flux values from the "active" sensors (19 for entrance scan and 9 for exit scan) were added to give the total flux over the grid points. This total flux was converted into actual power (in watts) by multiplying it by the area ofthe grid. Again, these values were normalized with respect to 1.0 kW m -2 insolation, knowing the average direct insolation during a scan. However, as mentioned in the previous section, due to problems of drift in the insolation channel of the DAS, the raw voltages from the tape had to be read, converted into appropriate units and then these values were used to correct the already normalized energy. There were other problems in estimating energy from the scan data as well. The digitization error in each channel varied from 0.14 to 0.2 W cm-:. Some of these values had to be subtracted from the flux in-
263
tegral over the entire grid. These would introduce otherwise an error of about 1%. A serious error was introduced due to malfunctioning and intermittent failure of one of the sensors, almost in the middle region of the scanning bar. In such cases, it needed a strenuous effort to estimate (by comparison and guesswork) the total flux measured by it, with a penalty of still introducing some error in the energy integral. Another possible serious error in estimating the power could be due to selecting a "proper" value for the mean scan-speed. If one is not careful in selecting the starting and stoppage points of scan, errors up to 5% might be introduced in the estimation of the average scan speed. The error that might creep in due to the use of "untrimmed grid area" in the calculations is only a second-order effect.
Calorimetric data Temperature measurements from the E-type thermocouples at the inlet and outlet of the CWC were monitored through the DAS, while these were also measured directly using two different devices--a Hewlett Packard and a FLUKE. The measured output voltages (mV) were converted into temperature units (Celsius) using a conversion chart provided by the thermocouple manufacturer, and the average temperature was used in our calculations. There was always (xlOO)
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264
an offset of about 0.5°C between these two inlet temperature measurements, possibly due to the lack of proper calibration before the start of the experiments. Other temperature measurements Due to some problems in the junction compensator (kept at 65°C), the startup temperatures never agreed with one another. It is difficult to estimate the actual error involved in the interpretation of the final temperatures measured by those thermocouples on the CPC and inside the CWC. Therefore, these temperatures could be used only as qualitative representation of the actual temperatures. However, it has to be pointed out that this has no serious consequence in the interpretation of the actual thermal performance Of the CPCs.
5. RESULTS AND DISCUSSIONS The results obtained from our tests are mainly on two areas: • thermal performance of the CPC • optical performance of the CPC. Some preliminary results were reported earlier[7]. The following sections present the full results of the final analysis of these data.
Thermal perJbrmance The entire series of tests have been totally successful because of the excellent thermal performance of all the CPCs° both actively and passively cooled. The single most important conclusion to be drawn from these results is that the passive cooling alone is adequate enough for using the CPCs designed specifically for two-stage systems of moderate concentration. The thermal performance of silver coated copper and aluminum CPCs° as well as the aluminum on aluminum CPCs are illustrated in Figs. 5-9. Table I gives a concise summary of the maximum temperature rise (above the ambient) of the CPCs tested under different field and climatic conditions. It is quite apparent that an interpretation of these results does need a more careful analysis than one-to-one comparison of various results. The major difficulty is the inadequacy of the number of data available. The results obtained indicate that active cooling helps maintain the CPC shell temperature very close to the ambient. It is possible, therefore, to safely operate a two-stage device with concentration ratios exceeding several thousands by utilizing a water-cooled secondary. Although operated at a comparatively low input power, the finned CPC seems to give next best result as far as the thermal control is concerned. It is not clear enough to establish at this stage whether the tern-
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perature rise is linearly related to increase in input power (for any particular reflecting surface), however, the limited results available suggest such a possibility. By the same token, one may assume that the nature of the primary mirrors (focussed or unfocussed) does not affect the thermal performance of the CPC (except at a higher power output). It is rather difficult to estimate the effect due to wind direction (and speed) and also the nature of the sky (hazy, clear, etc.) on the temperature profile. Overcoated passively cooled alum i n u m CPC with aluminized reflecting surface receiving an estimated input power of about 14.4 kW
registered a temperature rise of 105.5°C under hazy sky conditions. Surprisingly, a similar CPC, but with silvered reflecting surface, reached a higher temperature rise o f about 116 °. 0°C with a lower input of about 12.9 kW. However, in this case the sky was clear and its effect on the performance, if any, is not well understood. In spite of all the vagaries involved in temperature measurements, especially the compensator box which was undependable, the tests proved to be successful. This is mainly due to the fact that all the temperatures remained well below any dangerous limit so as to threaten the short-term survival of the sec-
Table 1. Summary of thermal behavior under different conditions
CPC reflecting surface Silver on copper
Overcoated silver on aluminum Overcoated aluminum on aluminum
Temperature control mode
Effective concentration
Equilibrium temperature (°C)
Water-cooled Finned Passive Passive
400 x 290x 400× 380x
25 45 78 130
Passive
360x
125
266
D. SURESH et al.
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Thermal and optical performance test results for compound parabolic concentrators
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D. SURESHel al.
268
ondaries. Long-term survival in a high flux/concentration e n v i r o n m e n t remains a question for future research.
Table 3. Comparison of experimental and ray-trace results CPC reflector material
Reflectivity (percent)
~ (percent)
Average ~m~ (percent)
Silver on copper Silver on aluminum Aluminum on aluminum
96 92 88
93 - 3 90 _+ 3 86 +- 4
90 - 3 89 +- i 82 _+ 1
Optical performance As described in an earlier section, different CPCs were tested under various conditions. The first set of data presented in Fig. l0 represents the power measurements at the entrance and exit of a silver-on-copper, finned CPC using the scanning bar as well as the CWC. It also contains the coil scan data. This is essentially a low power measurement, with unfocussed mirrors in both the wide and narrow field cases, taken on the October 5, 1985. Results obtained for the silvered- and also the aluminized-aluminum CPCs with the wide, unfocussed field are represented in Fig. I I. The next two figures, 12 and 13, represent the test results with narrow, focussed mirror field. In one case (Fig. 12), it is for a water-cooled silver-on-copper CPC, and in the other (Fig. 13) results shown are for two cases: silvered a l u m i n u m and aluminized aluminum. All the power measurements have been normalized with respect to a direct insolation of 1.0 kW m -E, in order to avoid the effects due to fluctuations in average insolation during different test runs. A c o m m o n striking feature of these four figures is the time dependence of the power measured. The rise in measured power in the mornings through solar noon is less steep than the drop in the afternoon. This effect is exaggerated in the case of focussed beam (higher power levels) and more so in the narrow field cases. More data are needed to confirm the above statements. One probable reason for this could be due to the asymmetry in the distribution of useful mirror field around the CPC axis. A close examination of one set of scan data around noon time shows interesting characterist i c s - a n almost flat input profile at the entrance and a double-humped output profile with a dip in the middle at the exit of the CPC. Due to lack of time and more data a deeper analysis and a correct interpretation of these results are made difficult. Precise measurements of the optical efficiencies had been made complicated by several factors including the strong time-of-day effects, unreliable sensitivity of the radiometer elements of the SFC at the low intensity levels, and the other uncertainties pointed out earlier. However, over a relatively short interval of time it is possible to estimate and correct for the time-of-day
variations. The relative uncertainties in the estimation of optical efficiencies in different cases vary between +0.2% and __.4.0%. Table 2 gives the estimated values for the throughput and the uncertainty in each case based on the scatter of the data. Silver-on-copper CPCs are very promising with an average throughput of 90.0% _ 3.0% in the case of a narrow field with both focussed and unfocussed mirrors. Next best is the silver-on-aluminum CPC with an optical efficiency of 89.0% ___ 1.0%. Aluminum-ona l u m i n u m CPC gave only 82.0% _ 1.0%. These results confirm the fact that a secondary with good reflecting surface can be used to give an efficiency well above 90.0%. Corresponding results for the wide field cases show lesser optical efficiencies. They are 84.0% + 2.0%, 81.0% ___2.0%. and 75.0% _+ 0.2% respectively for silvered copper, silvered aluminum, and aluminized alum i n u m CPCs. The drop in throughput for the large field is attributed to the fact that all the additional mirrors in the wide field are at the edge of the acceptance of the CPC. In fact, the CPC was designed for an acceptance angle of 25 ° which corresponds to a m a x i m u m concentration of 5.6× (for the untruncated cone). However, after these cones were fabricated, laboratory tests showed that the cones did have a cut-off angle of about 23 °. Even the narrow field did include a number of mirrors just near this cut-offangie. Coupled with this, a possible misalignment of the secondar3" could have caused the overall optical efficiency to drop by a few percent. One can normally hope to achieve a throughput of over 95.0% with a configuration which has its center field completely filled as opposed to the A C T F which provided a donut-type field with no mirrors at the central region, while quite a few of them almost at cutoff angles. A computer ray-trace analysis was performed to identify a "properly" truncated CPC with a lesser ac-
Table 2. Summary of optical test results CPC tested
Field type
r/opu~ (percent)
A~ (percent)
Silver on copper Silver on copper Silver on copper Silver on aluminum Silver on aluminum Aluminum on aluminum Aluminum on aluminum
Narrow, Focussed Narrow, Unfocussed Wide, Unfocussed Narrow, Focussed Wide, Unfocussed Narrow, Focussed Wide, Unfocussed
90.0 90.0 84.0 89.0 81.0 82.0 75.0
__.2.0 ___4.0 ---0.2 _+1.0 __.2.0 +-1.0 ---0.2
Thermal and optical performance test results for compound parabolic concentrators
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ceptance angle than 25 ° o f the original design, whose behaviour would agree with our experimental data. Results of ray-tracing of a 21 ° CPC almost agreed, although our experimental points lie slightly to the left of that curve. For all practical purposes we could take that our 25 ° CPC behaved more like a 21 ° one, although the theoretical cutoffangle was around 23 °. In fact, this was an unexpected result and is still difficult to forward exact reasoning for this apparent discrepancy. The optical efficiency results predicted by the model are given in Table 3 and also in Fig. 14 for comparison purposes with the experimental results obtained.
6. CONCLUSIONS
It has been experimentally demonstrated for the first time that with high reflectivity surfaces CPC efficiencies can be in the 90% range, although one would have expected efficiencies in the high nineties. One or several reasons could have caused such a drop in optical efficiency--such as the slope error, misalignment, and particularly the donut-shaped mirror field, nonuniformly distributed around the axis of the CPC. Until further experiments are conducted with various mirror fields, it appears that the exact cause may not be identified. Currently (1989) there are a few ongoing projects to build different CPCs at Stuttgart, West Germany, in which one of us (D.S.) is involved. Testing of these
might give some clue as to how some of the errors identified as causing a drop in the optical efficiency could be rectified in a production unit. These experiments have also shown conclusively that there are no serious thermal problems in the use of secondaries. Passive cooling could be adequate for concentration ratios up to about 100Ox. At higher concentrations simple active cooling methods can be employed, if necessary. There are several avenues still to be investigated before such two-stage systems can be scaled up for use in large-scale power generation. Although the cold water calorimeter acted as a cavity receiver in these experiments, the heat transfer characteristics of the system under extremely high flux/temperature conditions have to be studied thoroughly. The effect of secondary on the receiver heat loss and hence its performance is currently being studied and the preliminary results show an improvement in the efficiencies. The complete results will be published elsewhere. The thermal behavior of a Hexagonal CPC under severe operating conditions has been analyzed recently based on a detailed computer modelling[ 11 ].
Acknowledgments--Weacknowledge with thanks the financial support for this work by the U.S. Department of Energy, through the Solar Energy Research Institute, under subcontract No. XK-4-04070-3. Our sincere thanks are due to the Central Development Shop personnel at the Research Institutes of the University of Chicago and the staffat the ACTF. Special men-
270
D. SURESH et aL
tion must be made of the excellent technical support by Martin Lentz throughout this work and Ejaz Ahmad for the ray-trace analysis. We are particularly grateful to the reviewers for their valuable comments and suggestions which indeed have helped improve the presentation of this paper.
REFERENCES
1. R. Winston, Principles of solar concentrators of a novel design, Solar Energy 16(2), 89-95 (1974). 2. A. Rabl, Comparison ofsolar concentrators, Solar Energy 18(2), 93-111 (1976). 3. W.T. Welford and R. Winston, The optics ofnonimaging concentrators-light and solar energy. Academic Press, New York (1978). 4. R. Winston and W. T. Welford, Design of nonimaging concentrators as second stages in tandem with image forming first-stage concentrators, Appl. Opt. 19(3), 347351 (1980). 5. J. O'Gallagher and R. Winston, Test of a 'trumpet' secondary concentrator with a paraboloidal dish primary, Solar Energy 36( l ), 37--44 (1986).
6. U. Ortabasi, E. M. Gray and J. O'Gallagher, Deployment of a secondary concentrator to increase the intercept factor of a dish with large slope errors, DOE/JPL-1060-69, (1984); Proc. Fifth Parabolic Dish Solar Thermal Power Review, Palm Springs, pp. 170-178, (1983). 7. J. O'Gallagher, R. Winston, D. Suresh, and C. T. Brown, Design and test of an optimized secondary concentrator with potential cost benefits for solar energy conversion, Energy" 12(3--4), pp. 217-226 (1987). 8. D. Suresh, J. O'Gallagher, and R. Winston, Compound optical systems with maximal concentration for solar thermal conversion, Final Report (Phase I), DOE-SERI (1986). 9. D. Suresh, J. O'Gallagher, and R. Winston, A heat transfer analysis for passively cooled "trumpet" secondary concentrators, A S M E J. Solar Energy Engg. 109(4), 289297 (1987). 10. D. H. Neale, User's manual, ACTF, Georgia Tech Research Institute, Atlanta ( 1981 ). 11. D. Suresh, Some studies related to a new hexagonal compound parabolic concentrator (HCPC) as a secondary in tandem with a solar tower. DLR/EN-TT, Report no. lB90102 (1990).
0038-092X/90 $3.00 + .00 Copyright © 1990 Pergamon Press pie
Printed in the U.S.A.
THERMAL AND OPTICAL PERFORMANCE TEST RESULTS FOR COMPOUND PARABOLIC CONCENTRATORS (CPCs) D. SURESH,* J. O'GALLAGHER,and R. WINSTON The University of Chicago, The Enrico Fermi Institute, Chicago, IL 60637, U.S.A. Abstract--The primary objectiveof the present study was to evaluatethe performancecharacteristics(thermal and optical) of a properly truncated CPC that could be used in two-stage solar thermal power generation systems. The CPCs selected for testing were the 5 : l cones with a 25* acceptance angle and an untruncated concentration ratio of 5.6×. Experiments were carded out at the Advanced Components Test Facility of the Georgia Tech Research Institute. Severalcones ofthe same dimensions hut with different shell materials, reflector surfaces, and employing various heat removal methods were tested. It has been demonstrated • experimentally for the first time that the CPCs with high reflectivitysurfaces can have optical et~cienciesin the range of 90% and above. In order to verifythese results,a computer ray-trace analysiswas also performed. These tests have shown that passive cooling alone is adequate for small-scale,low-power systems.
I. INTRODUCTION
duce the high temperature required for more efficient power generation. In this paper we report both the thermal and optical performance results of a third experimental test of nonimaging secondary concentrators whose design was chosen to be compatible with the latter approach. Some preliminary results were reported elsewhere[7].
Physical conservation principles set a limit to the maximum achievable concentration of any optical design. Traditional approaches based on the image-forming techniques of focussing optics fall short of the so-called "thermodynamic limit". However about a decade ago a new class of optical devices referred to as nonimaging concentrators which can closely approach this limit was proposed for solar energy collection[l-3]. The most familiar of these devices is the family of compound parabolic concentrators (CPCs) which achieve moderate concentration levels with much wider acceptance angles than had previously been thought possible. Two-stage designs employing nonimaging devices in tandem with a focussing element was proposed for solar thermal power generation. In a two-stage system, the primary is a conventional focussing element such as a parabol0idal mirror and the secondary is a nonimaging device, which when properly designed can augment the overall concentration of the hybrid system to approach the thermodynamic limit. Applications of two-stage designs lie in the higher concentration, small angular acceptance regime[4]. Two previously reported experiments have successfully demonstrated the practicality of this concept for point-focus dish concentrators by providing substantial increases in concentration ratio and intercept factor, respectively, on a JPL (Jet Propulsion Laboratory) i l-m dia. Test Bed Concentrator then at the Edwards Air Force Base[5], and on a 6-m alia. Omnium-G concentrator[6]. As an alternative design approach, rather than increasing the concentration ratio, the secondaries can instead be used to relax the optical tolerance requirements for the primary dish while maintaining the same concentration ratio and performance. Such an approach may result in substantial cost savings in a compound system which can still pro-
* Present Address: D.L.R. (EN-TT), Pfaffenwaldring3840, D-7000 Stuttgart 80, F.R.G. (West Germany).
2. BASIC DESIGN CONSIDERATIONS The resources and scope of the present effort would not permit construction of a full-scale primary ofeither of the two candidate designs originally proposed[8], to test the various possible secondaries. Therefore it was decided to fabricate several secondary elements corresponding to a representative base-line design and to evaluate them at the Advanced Components Test Facility (ACTF) operated by the Georgia Tech Research Institute (GTRI), Atlanta. In fact, an optimized twostage system with good optical coupling of a secondary to a specific primary requires the design of a proper CEC (Compound Elliptic Concentrator). Since we could not design hardware for a well-defined primary the CPC was chosen as the best candidate geometry representative of a 'generic' second-stage suitable for any moderately long focus primary. We structured our design around a secondary. CPC with an acceptance angle of 25 ° which corresponds to an untruncated concentration of 5.6× and a minimum focal ratio of I. 13. The physical dimensions of the entrance and exit aperture are as shown in the profile drawing (Fig. l). This CPC design could be used with any long focal length primary whose rim angle does not exceed 25* and whose focal spot dia does not exceed 48.3 cm (19.0 in.).
Fabrication technique for the CPC shell The CPC test cones were fabricated to the desired profile by metal spinning technique. This procedure was proven successful in an earlier project of ours to develop "trumpet-type" secondaries[5]. First, a brass template was cut in a numerically controlled machine
257
258
D. SURESHet al.
Thermal loads and cooling methods
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according to the computer-generated set of coordinates defining the exact profile of the CPC. A steel mandril was then machined using the template and the individual cones were spun over it. Twelve spinnings were made, six each of copper and aluminum. Nine of spun shells (five copper and four aluminum) were then used to make complete secondaries; and the remaining cones were retained as spares.
Reflector surface preparations Copper cones. The inner surface of the cone was polished and buffed to remove the surface irregularities and tool marks. The outer surface was masked and the inner surface electroplated with commercial silver to a thickness of approximately 10-t /.tin. This surface was then polished and buffed again to a high gloss. Two different commercial facilities were available locally for silver-plating large size copper cones. Small, flat sample pieces of silver plated copper by each vendor were used to obtain the reflectivity characteristics of each surface. We found average values of reflectivity, p = 0.96 _+ 0.02 in one case, and p = 0.92 __ 0.02 in another. Three of the five copper cones used plating with high reflectivity while the other two had the lower value. Aluminum cones. As with the copper pieces, the inside surfaces of the shells were first polished and buffed to prepare a smooth finish. Two different reflector surfaces were used for the aluminum cones. Two of the four aluminum cones had a thin film of vacuumdeposited silver which was then overcoated with a thin layer of magnesium fluoride for protection. The other two aluminum cones had a coating of vacuum-deposited aluminum which was then overcoated with a layer of silicon oxide. No direct measurements of the reflectivities of these surfaces were available.
Thermal stresses on a CPC secondary depend on the stagnation temperatures reached by its walls. The temperature profile of the reflecting surface, in turn, will be decided by the amount of energy incident on it, and the manner in which it is distributed. The amount of energy absorbed by the reflecting surface will depend on its absorptance, a (= I - p). This absorbed energy will be dissipated through convection, radiation, and conduction. Our preliminary calculations based on a simple model showed that at the relatively low concentration ratios temperatures will remain under acceptable limits of about 200°C with passive cooling alone. However, at high concentrations, prohibitively high temperatures could result and some form of active or semiactive heat removal may be necessary. The results of a similar study based on a detailed heat transfer analysis on the thermal control for trumpet-type secondaries have been presented elsewhere[9]. To measure the effectiveness of a simple passive thermal design, selected CPCs were prepared with a high emissivity black paint (PYROMARK 2400) applied to the outside shell surface. One such version was fabricated for each of the different reflector surfaces-two copper cones with silver plating by the two different vendors; and two aluminum cones, one with silver and the other with aluminum coating. For these four cones no other additional provisions were made for enhancing heat removal. One additional cone of each material was prepared with longitudinal fins of the same material tack-welded to the outer surface. Each cone had eight fins spaced at 45 degree intervals around the circumference. It was found that in spite of the great care taken welding had a tendency to distort the contour of the inner surface. Therefore the tacks were spaced relatively far apart. Good thermal contact along the entire fin was assured by soft-soldering for the copper fins and applying a fillet of thermally conducting epoxy along the length of the aluminum fins. The remaining two copper CPCs and one aluminum cone (with overcoated silver) were fabricated with active water cooling. Each had 0.95 cm (] in.) dia. copper or aluminum tubing wound around the entire outer surface of the cone. The coils were tightly wound near the exit aperture and spaced at 3.8 cm (1.5 in.) intervals for the remainder of the surface. To avoid distorting the reflector surface the coils were attached using soft-solder for the copper coils and tack welding plus thermal epoxy for the aluminum coils (as in the case of the fins). Thus, we prepared a variety of copper and aluminum test cones with different reflector surfaces and with a variety of heat-removal techniques, such as entirely passive, enhanced passive, and completely watercooled.
Design and fabrication of cold water calorimeter In order to complement the energy flux measurements at the CPC exit aperture using the scanning flux
Thermal and optical performance test results for compound parabolic concentrators
INLET
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calorimeter (SFC) available at the site, a cold water calorimeter (CWC) was designed and built by the University of Chicago group. It was constructed as follows: a right circular cylinder ofdia. 27.9 cm (! 1.0 in.) and height 43.2 cm (17.0 in.) was formed from commercially available copper sheet of thickness 0.16 cm ( in.). Copper tubing of 1.6 cm (~ in.) dia. was wound closely around the outside with somewhat greater pitch. For illustration, a schematic diagram of the CWC mounted on a water-cooled CPC is shown in Fig. 2. One end of the cylinder was closed with a circular copper disc with a center hole whose diameter was exactly that of the exit aperture. The other end was closed with a 1.3 cm (½ in.) thick brass plate with spiral channels cut in it for coolant flow which were sealed by a 0.3 cm (~ in.) thick copper plate. Cooling water was fed from the inlet through the back plate and then to the top end of the inside coil, through it to the bottom of the outside coil and then up through the outer coil to the outlet. The innerside of the calorimeter was coated black with PYROMARK 1500. The entire outside of the cylinder was wrapped with a 5.0 cm (2.0 in.) thick FIBER-FRAX spun ceramic insulation which was held
in place by an aluminum jacket. There were 12 thermocouples on the body of each secondary, arranged as shown in Fig. 2. In addition, two thermocouples were spot-welded on to the surface of the inlet and outlet tubes of the CWC and one each was placed near the exit and the entrance. One immersion-type thermocouple was inserted in the water flow at the inlet and another at the outlet. Finally there was one thermocouple spot-welded to the body of the calorimeter and another left "floating" inside the insulation material. 3. EXPERLMENTAL TESTS
Description of the facility The ACTF uses a tracking array of about 550 circular heliostats, each 111 cm (43.7 in.) in diameter and made of 0.3 cm (~ in.) thick glass to deliver a concentrated solar flux to a work platform atop a central tower, 22.8 m (74.8 ft.) high. A complete description of the facility can be found in reference[10]. Individual mirror focussing is achieved by tightening a tension wire around the circumference of the mirror.
260
D. SURESHet al.
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Alternately, i f o n e wishes to use unfocussed plane mirrors, this can be achieved by removing the tension wire. To define the aperture for our tests, a large water-cooled plate with a circular opening of 45.7 cm (18.0 in.) in diameter was mounted horizontally on the tower at a
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height of 21.2 m (69.5 ft.) above the level of the mirror plane. Immediately below the test platform are two hydraulically actuated, water-cooled shutters which can be used to block the solar flux from the test area so the equipment adjustments can be made while the mirror field continues to track. The layout of the mirror field is as shown schematically in Fig. 3. Out of all the available mirrors, two field configurations were chosen for our tests-one consisting of 53 mirrors and the other 38. Viewed from the target plane, these two sets of mirrors filled two different annular zones that subtended effective rim angles between 15 ° and 22 °, respectively. The terms "'wide field" and "narrow field" are used in this paper to refer to these two fields. Although the design cutoffangle of the CPC was 25 °, it is well known that there is some rounding of the optical throughput curves for CPC cones to be expected due to skew ray losses and mirror slope errors in the CPC profile shape itself[l]. These two different field configurations were chosen to probe the actual throughput characteristics in this cutoffregion. There were no mirrors directly under the tower or immediately adjacent to its base so that the central portion of the CPC's field-of-view was unfilled. Thus these tests weigh the effects of the eutoffre~on relatively more heavily than would be characteristic of an actual primary dish. In fact, as will be discussed in a later section, the measurements set a lower limit on the ac-
MEASUREMENTS
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Thermal and optical performance test results for c o m p o u n d parabolic concentrators
tual optical efficiency ofthe secondary when used with a dish primary. At the top of the tower the concentrated solar flux was measured using the SFC bar. It was essentially a scanning bar containing 31 Gardon gauge calorimeter transducers (HYCAL) spaced equally at 2.5-cm (l.0in.) intervals. The SFC was mounted along the eastwest direction and swept in the north-south direction across the desired measurement plane over a guide rail, driven by a motor. The total energy in the scanned plane was found by integrating the measured instantaneous flux from each calorimeter gauge during the time of the scan and then adding the contributions from all the gauges. This technique was used to measure the flux at the entrance plane of each CPC and again at its exit plane to cross-check with the CWC results and to determine the corresponding throughput with better possible accuracy than using either one of these devices.
261
location on the 28, 29 of September, and 5, 6, 7, and 9 of October. On the other days hardware preparation, testing and calibration of the sensors and preliminary data analysis were carried out. The first CPC tested was the water-cooled copper cone with silver reflecting surface on a clear day with an ambient temperature around 20°C. Tests were carried out with the 53-mirror wide field configuration, having unfocussed (fiat) mirrors. First, two successive entrance plane flux measurements were made by moving the scanning bar across the plane immediately above the water-cooled aperture plate. The shutters were then closed and the test CPC was mounted flush with the top of the plate with its entrance centered directly over the aperture. The water flow rate through the cooling coils was kept high enough (about 100 ml s-') to keep the surface temperature of the CPC shell close to ambient. The shutters were then opened and two successive scans of the energy in the exit plane of the CPC were carried out. Then the CWC was mounted in place, and shutter was opened. After equilibrium temperature was reached, measurements were carried out manually by monitoring the inlet and outlet temperatures of water through the CWC using two different high precision digital voltmeters while measuring the flow rate with a measuring jar and a stopwatch. Finally after removing the CWC two more exit plane scans
Chronology of tests Our experiments took place over a three-week period beginning the last week of September 1985, during which the facilities and personnel of the ACTF were allocated for support of our measurements for a total of six "sunny days" of operation. Solar availability provided satisfactory insolation levels to fulfill this al-
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262
D. SURESH
were done in quick succession. The direct beam insolation as well as the measurements from the therme,couple sensors on the test cones were continuously recorded by the ACTF data acquisition system (DAS). Subsequently, on the following day, two passively cooled copper cones (silver plated by two different vendors) were tested in a similar manner. After a period of inclement weather, testing was resumed on the 5th of October and two more tests were carded out. First, the finned copper CPC with silver reflecting surface was tested with the wide field then with the narrow field (still using unfocussed mirrors). In addition, a tightly wound spiral of copper tubing which fitted exactly on to the opening in the aperture plate was used to redefine a smaller aperture of 21.6 cm (8.5 in.) at the entrance plane. Both the scanning bar and the CWC were used to measure the energy emerging through this aperture for further cross-calibration purposes. Next day, two different aluminum CPCs (one with silver and another with aluminum reflecting surfaces, both overcoated with magnesium fluoride) were tested with the wide field and unfocussed mirrors. Since no thermal problems had been encountered during the first four days of testing it was decided to increase the power by using focussed mirrors while at the same time reducing the effects due to being near the edge of the acceptance angle by using the narrow
et al.
field only. All of the tests carried out on the last two days of testing were done with this configuration. At the completion of this period all the major tests which we had planned were successfully completed so that in one configuration or another data had been obtained for each design alternative incorporating all relevant variations in shell material, reflective surface and cooling strategy, including stand-alone, passiveonly cooling. 4. DATA R E D U C T I O N AND A N A L Y S I S
Insolation data
Direct insolation was continuously monitored; the analog voltage signals were digitized and converted in energy flux units (W m -2) which were printed out (in real time) along with scan data as well as temperatures. In addition, the digitized raw data were also available on magnetic tapes. After the 5th October, the insolation data were found to "drift" due probably to some trouble in that particular DAS channel. However, before noticing this drift the insolation values available on print (lj) had already been used in our analysis (described in the following sections). Therefore, to correct for this drift, it was necessary to read the raw electrical data from the tape, convert them into W m -~. These values were called 12. A regression plot for several days (October 5, 6, 7, and 9) showed the drift, as shown in (xi00)
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Thermal and optical performance test results for compound parabolic concentrators Fig. 4. All the normalization in our calculations were done with respect to the latter.
Scan data Each calorimeter on the scanning bar (SFC) was individually calibrated. The scanning was done with a scanning speed in the vicinity of 0.8 cm s-~ (0.3 in. per sec.). The digitized data with the flux values in converted units (W cm -z) were available along with the scanning bar position. The flux map thus had approximately 2.5 cm × 0.8 cm (1.0 in. × 0.3 in.) grids. All those flux values from the "active" sensors (19 for entrance scan and 9 for exit scan) were added to give the total flux over the grid points. This total flux was converted into actual power (in watts) by multiplying it by the area ofthe grid. Again, these values were normalized with respect to 1.0 kW m -2 insolation, knowing the average direct insolation during a scan. However, as mentioned in the previous section, due to problems of drift in the insolation channel of the DAS, the raw voltages from the tape had to be read, converted into appropriate units and then these values were used to correct the already normalized energy. There were other problems in estimating energy from the scan data as well. The digitization error in each channel varied from 0.14 to 0.2 W cm-:. Some of these values had to be subtracted from the flux in-
263
tegral over the entire grid. These would introduce otherwise an error of about 1%. A serious error was introduced due to malfunctioning and intermittent failure of one of the sensors, almost in the middle region of the scanning bar. In such cases, it needed a strenuous effort to estimate (by comparison and guesswork) the total flux measured by it, with a penalty of still introducing some error in the energy integral. Another possible serious error in estimating the power could be due to selecting a "proper" value for the mean scan-speed. If one is not careful in selecting the starting and stoppage points of scan, errors up to 5% might be introduced in the estimation of the average scan speed. The error that might creep in due to the use of "untrimmed grid area" in the calculations is only a second-order effect.
Calorimetric data Temperature measurements from the E-type thermocouples at the inlet and outlet of the CWC were monitored through the DAS, while these were also measured directly using two different devices--a Hewlett Packard and a FLUKE. The measured output voltages (mV) were converted into temperature units (Celsius) using a conversion chart provided by the thermocouple manufacturer, and the average temperature was used in our calculations. There was always (xlOO)
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D. SURESHet al.
264
an offset of about 0.5°C between these two inlet temperature measurements, possibly due to the lack of proper calibration before the start of the experiments. Other temperature measurements Due to some problems in the junction compensator (kept at 65°C), the startup temperatures never agreed with one another. It is difficult to estimate the actual error involved in the interpretation of the final temperatures measured by those thermocouples on the CPC and inside the CWC. Therefore, these temperatures could be used only as qualitative representation of the actual temperatures. However, it has to be pointed out that this has no serious consequence in the interpretation of the actual thermal performance Of the CPCs.
5. RESULTS AND DISCUSSIONS The results obtained from our tests are mainly on two areas: • thermal performance of the CPC • optical performance of the CPC. Some preliminary results were reported earlier[7]. The following sections present the full results of the final analysis of these data.
Thermal perJbrmance The entire series of tests have been totally successful because of the excellent thermal performance of all the CPCs° both actively and passively cooled. The single most important conclusion to be drawn from these results is that the passive cooling alone is adequate enough for using the CPCs designed specifically for two-stage systems of moderate concentration. The thermal performance of silver coated copper and aluminum CPCs° as well as the aluminum on aluminum CPCs are illustrated in Figs. 5-9. Table I gives a concise summary of the maximum temperature rise (above the ambient) of the CPCs tested under different field and climatic conditions. It is quite apparent that an interpretation of these results does need a more careful analysis than one-to-one comparison of various results. The major difficulty is the inadequacy of the number of data available. The results obtained indicate that active cooling helps maintain the CPC shell temperature very close to the ambient. It is possible, therefore, to safely operate a two-stage device with concentration ratios exceeding several thousands by utilizing a water-cooled secondary. Although operated at a comparatively low input power, the finned CPC seems to give next best result as far as the thermal control is concerned. It is not clear enough to establish at this stage whether the tern-
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perature rise is linearly related to increase in input power (for any particular reflecting surface), however, the limited results available suggest such a possibility. By the same token, one may assume that the nature of the primary mirrors (focussed or unfocussed) does not affect the thermal performance of the CPC (except at a higher power output). It is rather difficult to estimate the effect due to wind direction (and speed) and also the nature of the sky (hazy, clear, etc.) on the temperature profile. Overcoated passively cooled alum i n u m CPC with aluminized reflecting surface receiving an estimated input power of about 14.4 kW
registered a temperature rise of 105.5°C under hazy sky conditions. Surprisingly, a similar CPC, but with silvered reflecting surface, reached a higher temperature rise o f about 116 °. 0°C with a lower input of about 12.9 kW. However, in this case the sky was clear and its effect on the performance, if any, is not well understood. In spite of all the vagaries involved in temperature measurements, especially the compensator box which was undependable, the tests proved to be successful. This is mainly due to the fact that all the temperatures remained well below any dangerous limit so as to threaten the short-term survival of the sec-
Table 1. Summary of thermal behavior under different conditions
CPC reflecting surface Silver on copper
Overcoated silver on aluminum Overcoated aluminum on aluminum
Temperature control mode
Effective concentration
Equilibrium temperature (°C)
Water-cooled Finned Passive Passive
400 x 290x 400× 380x
25 45 78 130
Passive
360x
125
266
D. SURESH et al.
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D. SURESHel al.
268
ondaries. Long-term survival in a high flux/concentration e n v i r o n m e n t remains a question for future research.
Table 3. Comparison of experimental and ray-trace results CPC reflector material
Reflectivity (percent)
~ (percent)
Average ~m~ (percent)
Silver on copper Silver on aluminum Aluminum on aluminum
96 92 88
93 - 3 90 _+ 3 86 +- 4
90 - 3 89 +- i 82 _+ 1
Optical performance As described in an earlier section, different CPCs were tested under various conditions. The first set of data presented in Fig. l0 represents the power measurements at the entrance and exit of a silver-on-copper, finned CPC using the scanning bar as well as the CWC. It also contains the coil scan data. This is essentially a low power measurement, with unfocussed mirrors in both the wide and narrow field cases, taken on the October 5, 1985. Results obtained for the silvered- and also the aluminized-aluminum CPCs with the wide, unfocussed field are represented in Fig. I I. The next two figures, 12 and 13, represent the test results with narrow, focussed mirror field. In one case (Fig. 12), it is for a water-cooled silver-on-copper CPC, and in the other (Fig. 13) results shown are for two cases: silvered a l u m i n u m and aluminized aluminum. All the power measurements have been normalized with respect to a direct insolation of 1.0 kW m -E, in order to avoid the effects due to fluctuations in average insolation during different test runs. A c o m m o n striking feature of these four figures is the time dependence of the power measured. The rise in measured power in the mornings through solar noon is less steep than the drop in the afternoon. This effect is exaggerated in the case of focussed beam (higher power levels) and more so in the narrow field cases. More data are needed to confirm the above statements. One probable reason for this could be due to the asymmetry in the distribution of useful mirror field around the CPC axis. A close examination of one set of scan data around noon time shows interesting characterist i c s - a n almost flat input profile at the entrance and a double-humped output profile with a dip in the middle at the exit of the CPC. Due to lack of time and more data a deeper analysis and a correct interpretation of these results are made difficult. Precise measurements of the optical efficiencies had been made complicated by several factors including the strong time-of-day effects, unreliable sensitivity of the radiometer elements of the SFC at the low intensity levels, and the other uncertainties pointed out earlier. However, over a relatively short interval of time it is possible to estimate and correct for the time-of-day
variations. The relative uncertainties in the estimation of optical efficiencies in different cases vary between +0.2% and __.4.0%. Table 2 gives the estimated values for the throughput and the uncertainty in each case based on the scatter of the data. Silver-on-copper CPCs are very promising with an average throughput of 90.0% _ 3.0% in the case of a narrow field with both focussed and unfocussed mirrors. Next best is the silver-on-aluminum CPC with an optical efficiency of 89.0% ___ 1.0%. Aluminum-ona l u m i n u m CPC gave only 82.0% _ 1.0%. These results confirm the fact that a secondary with good reflecting surface can be used to give an efficiency well above 90.0%. Corresponding results for the wide field cases show lesser optical efficiencies. They are 84.0% + 2.0%, 81.0% ___2.0%. and 75.0% _+ 0.2% respectively for silvered copper, silvered aluminum, and aluminized alum i n u m CPCs. The drop in throughput for the large field is attributed to the fact that all the additional mirrors in the wide field are at the edge of the acceptance of the CPC. In fact, the CPC was designed for an acceptance angle of 25 ° which corresponds to a m a x i m u m concentration of 5.6× (for the untruncated cone). However, after these cones were fabricated, laboratory tests showed that the cones did have a cut-off angle of about 23 °. Even the narrow field did include a number of mirrors just near this cut-offangie. Coupled with this, a possible misalignment of the secondar3" could have caused the overall optical efficiency to drop by a few percent. One can normally hope to achieve a throughput of over 95.0% with a configuration which has its center field completely filled as opposed to the A C T F which provided a donut-type field with no mirrors at the central region, while quite a few of them almost at cutoff angles. A computer ray-trace analysis was performed to identify a "properly" truncated CPC with a lesser ac-
Table 2. Summary of optical test results CPC tested
Field type
r/opu~ (percent)
A~ (percent)
Silver on copper Silver on copper Silver on copper Silver on aluminum Silver on aluminum Aluminum on aluminum Aluminum on aluminum
Narrow, Focussed Narrow, Unfocussed Wide, Unfocussed Narrow, Focussed Wide, Unfocussed Narrow, Focussed Wide, Unfocussed
90.0 90.0 84.0 89.0 81.0 82.0 75.0
__.2.0 ___4.0 ---0.2 _+1.0 __.2.0 +-1.0 ---0.2
Thermal and optical performance test results for compound parabolic concentrators
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ceptance angle than 25 ° o f the original design, whose behaviour would agree with our experimental data. Results of ray-tracing of a 21 ° CPC almost agreed, although our experimental points lie slightly to the left of that curve. For all practical purposes we could take that our 25 ° CPC behaved more like a 21 ° one, although the theoretical cutoffangle was around 23 °. In fact, this was an unexpected result and is still difficult to forward exact reasoning for this apparent discrepancy. The optical efficiency results predicted by the model are given in Table 3 and also in Fig. 14 for comparison purposes with the experimental results obtained.
6. CONCLUSIONS
It has been experimentally demonstrated for the first time that with high reflectivity surfaces CPC efficiencies can be in the 90% range, although one would have expected efficiencies in the high nineties. One or several reasons could have caused such a drop in optical efficiency--such as the slope error, misalignment, and particularly the donut-shaped mirror field, nonuniformly distributed around the axis of the CPC. Until further experiments are conducted with various mirror fields, it appears that the exact cause may not be identified. Currently (1989) there are a few ongoing projects to build different CPCs at Stuttgart, West Germany, in which one of us (D.S.) is involved. Testing of these
might give some clue as to how some of the errors identified as causing a drop in the optical efficiency could be rectified in a production unit. These experiments have also shown conclusively that there are no serious thermal problems in the use of secondaries. Passive cooling could be adequate for concentration ratios up to about 100Ox. At higher concentrations simple active cooling methods can be employed, if necessary. There are several avenues still to be investigated before such two-stage systems can be scaled up for use in large-scale power generation. Although the cold water calorimeter acted as a cavity receiver in these experiments, the heat transfer characteristics of the system under extremely high flux/temperature conditions have to be studied thoroughly. The effect of secondary on the receiver heat loss and hence its performance is currently being studied and the preliminary results show an improvement in the efficiencies. The complete results will be published elsewhere. The thermal behavior of a Hexagonal CPC under severe operating conditions has been analyzed recently based on a detailed computer modelling[ 11 ].
Acknowledgments--Weacknowledge with thanks the financial support for this work by the U.S. Department of Energy, through the Solar Energy Research Institute, under subcontract No. XK-4-04070-3. Our sincere thanks are due to the Central Development Shop personnel at the Research Institutes of the University of Chicago and the staffat the ACTF. Special men-
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tion must be made of the excellent technical support by Martin Lentz throughout this work and Ejaz Ahmad for the ray-trace analysis. We are particularly grateful to the reviewers for their valuable comments and suggestions which indeed have helped improve the presentation of this paper.
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