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Water-in-glass manifolds for heat extraction from evacuated solar collector tubes
$olarEnergyVol. 32. No..2, pp. 223-230, 19M
0038--,092X184 S3.00§ Perg:x,"non Press Ltd.
Printed in Great Britain.
WATER-IN-GLASS MANIFOLDS FOR HEAT EXTRACTION FROM EVACUATED SOLAR COLLECTOR TUBES YIN ZHIQIANGt and G. L. HARDING School of Physics, University of Sydney, N.S.W. 2006, Australia and B. WINDOW CSIRO National Measurement Laboratory, Lindfleld, N.S.W. 2070, Australia {Receited 24 November 1982; accepted 15 March 1983)
Abstract--Measurements are reported on three novel manifolds of the water-in-glass type for evacuated all-glass single-ended tubular collectors. The manifolds provide for series connection of tubes, but because there is virtually no partitioning of the inner volume of the collector tubes, the manifolds are extremely simple and exhibit low impedance to fluid flow. The efficiency of heat extraction from the tubes has been determined by measuring temperatures at various points on the surface of glass tubes in a panel of area - 1.2 m2 while heating the tubes electrically to simulate solar energy input. Measurements have been made for a range of tube inclinations (0-80~ water flow rates (0.5-5 Imin-t, water inlet temperatures (13-70~ and effective solar fluxes (100-1000Wire2) for two absorber tube diameters. The results show that Ior a wide range of operating conditions buoyancy effects alone result in efficient heat transfer to the tops of the tubes. The manifold designs described offer a possible low cost solution to the problem of manifolding evacuated collectors for sub-100oC heat extraction for domestic and industrial applications. I. LNTRODUCTION Evacuated collectors achieve high heat extraction efficiencies relative to fiat plate collectors at temperatures above 80~ due to the combined effects of a h i g h l y selective surface and vacuum insulation of the absorber element [I-10]. Comparative testing of evacuated and fiat plate collectors has illustrated that evacuated collectors also offer considerable performance advantages over conventional fiat plate collectors for relatively low temperature applications such as the provision of domestic hot water [11, 12], but at present they are considerably more expensive than fiat plate collectors. The interconnection of collector tubes via a manifold for circulation of heat transfer fluid contributes substantially to the cost of a complete collector and, in Order to reduce evacuated collector costs, there is a need to develop simpler manifolds using cheaper materials and low cost construction techniques. Besides the obvious requirements of simplicity and low cost, a collector manifold should exhibit features such as the ability to withstand collector stagnation (270-300~ during load removal) for long periods, resistance to internal or external corrosion, and low impedance for minimum pumping requirements. In the case of concentric glass tubular collectors [3, 5, 6, 9, 10] thermal energy must be efficiently transferred from the glass absorber tube to a circulating fluid, and the manifold should result in a low average absorbing surface temperature relative to the fluid temperature in order to minimise losses from the collector tubes. Heat transfer
~Permanent address: Tsinghua University, Peking, China.
fluid may be contained directly in the glass absorber tubes, as in the pioneering Owens-Illinois collector [3] or in metal tubes in thermal contact with the glass absorber, as in the General Electric collector [6]. Variations on fluid-in-glass and fluid-in-metal manifolds have been developed by Sunmaster U.S.A. [13] and Sydney University, Australia [14-18]. Some recent studies of various manifolds for concentric tubular collectors has shown that metal manifolds of the type developed by General Electric result in temperature differences of up to 30~ between fluid and the glass absorber tube surface, whereas water-in-glass manifolds of the type developed by Owens-Illinois result in temperature differences < 10~ [14]. Thus while fluid-in-glass manifolds suffer from the disadvantages of high heat capacity and the possibility of fluid leakage in the event of tube breakage, they possess the potential for achieving greatest simplicity, lowest cost and highest efficiency for collectors providing sub-boiling water for domestic and industrial applications. In this paper we describe measurements .on three closely related novel manifolds of the fluid-in-glass type, in which partitioning of the inner volume of each absorber tube to define fluid flow paths, is eliminated or virtually eliminated. The manifolds allow series connection of collector tubes yet exhibit extremely low impedance for fluid circulation, and the measurements show that buoyancy or thermosiphon effects are primarily responsible for highly efficient heat extraction from the tubes. This detailed study was stimulated by results of preliminary studies of such manifolds which indicated the efficacy of tl~ermosiphon effects in producing heat transfer in single-ended collectors [14, 171. 22_I
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2. M.-kNIFOLDBE.SIGNS Figure 1 shows the design of two conventional manifolds. The Owens-Illinois "water-in-glass' manifold [3] utilises a relatively complex coupling at the open ends of pairs of absorber tubes, and incorporates a long glass feeder tube for fluid circulation. The General Electric "water-in-metal" manifold [6] consists of a copper Utube with attached copper fin which clamps to the inner surface of the absorber tube. In the present work, the basic manifold design (A) consists of a single copper header pipe with copper connectors soldered at equal intervals along the header pipe. Each connector is sealed to a collector tube by means of an O-ring seal. Figure 2 shows the construction and dimensions of manifolds used in the present experiments in conjunction with glass absorber tubes of O.D. 22 or 30 mm. In practice, the connectors may seal either to the glass envelope of a collector (see Fig. 3, manifold (A1), or to the absorber tube of a collector (see Fig. 3, manifold A2). In the present experiments, the absorber tubes have no envelopes (see Section 3), so the latter sealing technique was utilised. Short partitions (manufactured from copper sheet), are press fitted into each connector to divide it equally into fluid inlet and fluid outlet segments (see Figs. 2, 3A1, 3A2). The partitions protrude into the header pipe to encourage some fluid circulation around the partitions into the upper regions of the absorber tubes. The absorber tubes were mounted with axes in a North-South plane, which maximises their daily solar
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Fig. 3. Schematic diagrams showing manifold designs and seals onto evacuated collector tubes. (Upper: longitudinal sections, lower: cross sections). Ah Manifold containing short partitions (P) and external O-ring seal; A2: Manifold containing short partitions (P) and internal O-ring seal: B: Manifold containing no partitions and internal O-ring ; C: Manifold containing elongated partitions (P) and internal O-ring seal.
collection efficiency [19], and with open ends up, allowing rapid filling of the tubes, and unimpeded escape for exsolved gases. The small gaps between partitions and inner surface of the header pipe (Fig. 3) serve an important function in allowing gas to be flushed from the header pipe. Figure 4 shows schematically a pumped collector system incorporating the manifold. This manifold provides for series connection of collector tubes, but because there is no channelIing device which partitions the inner
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volume of each tube into inlet and outlet segments, the manifold exhibits extremely low impedance to fluid flow. Two modifications of this basic manifold have also been investigated (see Fig. 3). (B)--in which the partitions are completely removed from the header pipe and connectors. This design results in slightly lower impedance, but there is less tendency for pumped fluid to circulate in the upper region of each absorber tube. (C)--in which the partitions are elongated to extend a distance 200 mm into the absorber tube. This results in slightly higher impedance, but fluid is encouraged to circulate to 9 a greater depth in each absorber tube. 3. EXPERIMENq'ALTECHNIQUES The experimental technique for investigation of heat transfer in the tubes involves study of the temperatures at various points on the surfaces of the glass absorber tubes in a panel, under various conditions which simulate operating conditions for a collector [17]. In these experiments the absorber tubes have no surrounding glass envelopes, and are heated electrically to simulate the solar input. The electrical heaters were made using 20 mm wide, 0.5 mm thick strips of stainless steel foil mounted in recesses in -100mm thick sheet of polyurethane foam. One strip heats the upper part of each tube, and another heats the lower part. The polyurethane foam insulates each heater and tube from its neighbours. Experiments have been carried out using absorber tubes of two diameters and heaters have been constructed for collector panels containing twenty 22ram O.D. tubes, and fifteen 30 mm O.D. tubes. The tube length is 1.32 m. The effective area of each panel is - 1 . 2 m2. Figure 5 shows a cross section of the heater for 30ram O.D. tubes. Variacs were used to adjust the powers Pu and Pt. into the upper and lower heaters separately to simulate the imbalance of solar energy input to the tubes which occurs in most collector designs. In most of the
Fig. 5. Sectionalview of apparatus for heating 30.ram O.D. glass tubes of length 1.32m. A: Glass tubes; B: Upper heater elements; C: Lower heater elements; D: Upper segment of polyurethane foam; E: Lower segment of polyurethane foam; F: coppercenstantan thermocouple junctions mounted on sides of glass tubes.
experiments the power input was adjusted to simulate the solar insolation on an array of collector tubes spaced by two absorber tube diameters (45 or 60 ram) and which incorporated a diffuse reflector. Ray tracing calculations and measurement of optical efficiency r/o for such collectors have shown that such a design has r/o-0.6, with twice as much energy striking the top as the bottom of the absorber tubes [19, 20]. Thus, for the various incoming fluxes investigated (I = 0.1 ~ 1.0 kWm-2) the total power per unit area of panel was adjusted to equal 0.6 x I and the ratio PoIPL was 2: 1, Other flux distributions were also investigated and are discussed later. Temperatures were measured on each of the tubes in a panel by copper.-constantan thermocouples mounted under glass tape on the side of each tube ~ 80 mm from the domed end (Figs. 5 and 7) and on each of three tubes in the panel six thermocouples were mounted at equally spaced intervals (see Section 4). It was found that mounting on the side gave more consistent results than mounting on the top of the tube because variability in the heater strip to tube spacing produces more variability in a top thermocouple near the heater strip than a side thermocouple. (Independent measurements on a range of manifolds has established that temperatures measured here are at most 2~ lower than the average temperature around the circumference.) The manifold inlet and outlet water temperatures were also recorded. Water flow rate through the collector panel could be varied from 0.5 to 5 lmin-1. For an effective solar flux of 1 kWm-2, these flows produced a temperature difference across the panel of - 1 7 to ~ 1.5~ The water inlet temperature to the manifold was varied between 13 and -70~ using a commercial "instamatic" water heater powered by a variac. The inclination angle, 0, of the collector (measured with the horizontal) was varied between 0 and 80~ to investigate collector performance at the optimum inclination for solar collection at various latitudes.
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226 4, R E ~ t L T S
Figure 6 illustrates some typical results obtained for temperatures near the bottom of each of the 15 tubes in the panel of 30 mm diameter tubes incorporating manifold A. The temperature difference between the tube bottom and the water inlet to the manifold is plotted for each tube, 1-15, for three inclinations of the collector panel. These results are appropriate to a solar flux of 1 kW/m2, flow rate 1.01/min and inlet temperature 13~ In each case the temperature increase across t~e panel was ~9~ Also plotted is the expected linear variation of fluid temperature along the manifold. These results indicate that on average for inclinations 15, 45 and 70~ the glass temperature near each glass tube bottom is respectively 6, 5.5 and 20~ above the inlet water temperature for that tube. Figure 7 illustrates typical temperature distributions observed along the lengths of the 30 mm tubes. AT is the difference between the glass tube temperature and the water inlet temperature to that tube. Results are shown for water flow ll/min, inlet temperature lYC and three inclinations 0 = 15, 45 and 70~for manifolds A and B. {Manifold C behaves similarly to A). These results indicate that the temperature distribution along the collectors depends strongly on inclination and manifold design. Large temperature gradients are obtained for large inclinations 0, particularly for manifold B. These results suggest that temperature measured at the bottom of a tube is the most useful parameter to monitor, being close to the maximum surface temperature of the glass tube, and thus providing an indication of the efficiency of heat transfer to the header pipe. The length-averaged absorber tube temperature, T (average), which is of interest for deriving heat losses in operational co/lectors, may be evaluated from results such as those in Fig. 7. To allow evaluation of T(average) from T (bottom), we plot in Fig. 8 some typical values of (T (bottom)-T(average)) as a function of collector inclination 0 for manifolds A, B, C (30 mm
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Fig. 8. Graph of 8T = T(bottomFT(avcrage) vs tube inclination Ofor manifolds A. B, C. The results are appropriate for effective solar flux I kWm-'s and flow rate 1 Imin- Iresults are not strongly dependent on flow rate). Open symbols: Manifold inlet temperature = 15~ Circles: Manifold A; Triangles: Manifold B; Squares: Manifold C. Solid Iine with bars: Mar,ifords A. B, C with water inlet temperature = 50~
Water-in-glassmanifolds for heat extraction from evacuated solar collector tubes tubes) for fluid inlet temperatures 15 and 50~ Results for the three manifolds are almost identical for high fluid inlet temperatures. Values obtained for AT(bottom), (temperature near tube bottom-water inlet temperature for that tube), averaged over all tubes in the panel for a wide range of collector operating conditions, and for the three manifold designs are summarised in Figs 9-12. Results obtained for manifold A (incorporating 22 or 30 mm tubes) for various tube inclinations and water inlet temperatures, (flow rate constant at llmin -t are shown in Fig. 9). Except at very low and very high inclinations, AT values increase as fluid inlet temperature is increased. For all inlet temperatures, AT rises sharply as tube inclination approaches 90~ and rises slightly for inclinations approaching 0~ Higher ATs are observed for 22 mm tubes than for 30 mm tubes except at large inclinations. Results obtained for manifold A (incorporating 30 mm tubes) for various water inlet temperatures and flow rates (with 0 constant 45 ~ are shown in Fig. I0. The increase in AT with increasing inlet temperature is again clearly evident. In each case increasing the flow rate by a factor of two increases AT by about I~ Qualitatively similar results are obtained for other inclinations 0. The relative behaviour of the three manifolds A, B, C (30 mm tubes) for various collector inclinations and two inlet temperatures (13 and 50~ for constant flow rate 1 lmin-I is shown in Fig. 11. For inclinations greater than 10~ manifold A is superior to B, particularly for low inlet
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Fig. 9. Graph of AT(bottom) vs tube inclination 0 for manifold A. Effective solar flux was I kWm-z and water flow rate = l lmin-k Solid symbols: 30ram tubes; Open symbols: 22ram tubes; Circles: inlet temperature 13~ Triangles: inlet temperature 50~ Squares: inlet temperature 70~
temperatures, while manifold C is superior to A particularly for high inlet temperatures. This graph shows clearly (see Fig. 9) that for all three manifolds, AT is smaller for high inlet temperature (50~ compared to low inlet temperature (13~ for inclinations near 0 and 800. Figure 12 shows the variation of AT(bottom) with effective solar flux for manifold A incorporating either 22
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I(kWm -~) Fig. 12. Graph of AT(bottom)vs effective solar flux I for manifold A incorporating 30 or 22 mm tubes at inclination 45~ The flow rate was I Imin-k Circles and triangles: heater power ratio PoIPL = 2. Solid circles: 30ram tubes, inlet temperature I3~ Open circles: 30ram tubes, inlet temperature 50~ Solid triangles: 22 mm tubes, inlet temperature 13~ Crosses: 30 mm tubes with PdPL = 3. and inlet temperature 13~ (lower) and 50~ (upper).
or 30 mm tubes. Results are appropriate for inclination 45~ flow rate l lmin -t, 70=0.6 and Po/PL=2, corresponding to normally incident sunlight on a collector incorporating a diffuse reflector. Some results are also shown for the 30 mm tubes when the ratio Pu/PL equals 3, corresponding to sunlight incident at 45+ with the normal [21]. Similar experiments have shown that AT values are decreased when a greater proportion of heat is introduced to the upper segment of the tube. Introducing the entire power input to the lower heater results in the converse--an increase of 2 to 3cC in AT. The transient behaviour of the manifold schemes have been tested for favourable situations, i.e. where the panel is mounted at 45~. In particular, the change in outlet temperature for a system of fifteen 30 mm tubes attached to manifold A mounted at 45~ with a flow of I I/min was measured for an abrupt switch-on of the heater (ikW/m 2) and an abrupt switch-off of the heater (1 kW/m2). The rise (fall) times observed were 36 and 27 rain respectively.
5. DISCUSSIONANDCONCLUSION The simulation of solar input to tubular collectors by electrical heating is an extremely versatile technique which has allowed a detailed investigation of some novel manifolds of the water-in-glass type. For the three manifolds studied, the maximum temperature difference between the surface of a glass absorber tube and the fluid inlet temperature at the top of the tube is < 10~C for a wide range of operating conditions, indicating that buoyancy forces alone are sufficient to produce efficient heat transfer to the top of the tube.
The mechanism of heat transfer proposed involves the separation of a colder stream of water which is injected into the tube, from a hotter outgoing stream, by buoyancy forces. This fluid flow and heat transfer pattern is generally favoured by lower flow rates and lower water temperatures (as illustrated in Figs. 9-11), both of which contribute to a lower Reynolds number and less turbulence in the fluid, with consequent lower values of AT. The fluid stream separation is also favoured by a greater proportion of power on the top surface of the tubes (Fig. 12). As the inclination of the tubes approaches vertical, buoyancy forces are no longer able to keep the cold and warm streams separate, and longitudinal heat flow breaks down leading to the fluid stratifying across the tube and considerably higher glass surface temperatures (Figs. 7, 9 and 11).The buoyancy effects also become less efficient for promoting fluid flow for inclinations close to horizontal, resulting in slightly increasing values of AT (Figs. 9 and ii). For inclinations close to horizontal or vertical, tube surface temperatures decrease as inlet temperature increases. At either of these inclinations, where buoyancy effects are greatly retarded, increased buoyancy at higher temperatures resulting from larger density changes for a given temperature increment may play a more important role than at intermediate values of 0. Better definition of fluid inlet/outlet streams at the top of each collector tube leads to more efficient heat extraction from the tubes, as indicated by the superiority of manifold A (containing short partitions) compared with B (containing no partitions) (Fig. 11). The extended partiiions of manifold C further improve fluid circulation by defining inlet and outlet streams at greater depth in the tubes (although partitions divide the tubes vertically, not horizontally), with consequent improved heat extraction and reduction in surface temperature of the absorber tubes. The effect of absorber tube diameter on AT may be explained by the following theory. Assuming an effective fluid flow [ occurs in the ingoing and outgoing streams within each tube due to buoyancy forces, and that the temperature difference between the streams at the top of the tube is proportional to AT, then conservation of energy requires that f . ATaE
(1)
where E is the energy absorbed by each tube. The driving force responsible for the flow within each tube is a buoyancy force which we assume to be proportional to AT. Thus f a A TIR
(2)
where R is the impedance to fluid flow in each tube. Equations (1) and (2) give (AT)z a R.E.. In Fig. 13, (AT)z is plotted against E using the results presented in Fig. 12 for absorber tubes of outer diameter 30 and 22ram with inlet water temperature 13~ and
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Fig. 13. Graph of (AT)2 vs energy absorbed by an absorber tube (E) for two absorber tube diameters and two water inlet temperatures. The lengths of the absorber tubes are in each case 1.32m. outer diameter 30 mm with inlet water temperature 50~ Linear graphs are obtained for the lower water inlet temperature suggesting that in this case the two tubes can be characterised by impedances R3o and Rz2 independent of the energy E absorbed by each tube (i.e. independent of the thermosiphon flow [). Furthermore the ratio R2zlR~o determined from Fig. 13 is (4.9-+0.1) whereas the ratio of the fourth powers of the inner diameter of the tubes is 4.8, indicating that the thermosiphon flow within the tubes can be characterised by Poiseuille's equation and is therefore laminar. No experimental results for the dependence of AT on absorber tube length have been obtained but the applicability of Poiseuille's equation suggests that since flow impedance RaLIr 4 where L is the tube length and r the inner radius, then (fiT) z a LIr4• For all other parameters constant, E a L thus AT cz L. This simple theory may allow AT values to be predicted for absorber tubes of other dimensions in the case of low water inlet temperatures. By contrast, a strongly non-linear relationship is obtained between (~T) ~ and E for the 30 mm tube with water inlet temperature 50~ This indicates that Poiseuille's equation is not appropriate for thermosiphon flow at relatively high temperatures and suggests that the flow has become turbulent due to the considerably lower water viscosity. The transient response of the system is satisfactory. In the simplest possible model of water flowing through a collector of low heat capacity and no heat exchange between various parts, the rise and fall times should be 80% of the residence time; this is equal to 10 min for the configuration and flow rate mentioned earlier. Actual rise times are approximately three times this, due to finite thermal capacities and conductivities, and, more importantly, a reduction in flow through the tubes below the nominal flow. This reduction is due to the short circuiting of flow past the metal partition in the manifold, and the short circuiting of flow around the top of the tube rather than along the length of the tube and back. We believe time constants of this magnitude to be acceptable in most applications below 100~ The three manifold concepts are all extremely simple and offer potential for manufacture at low cost for example, using moulded plastic [22]. Our results suggest
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that the manifold design utilised may depend on the standard operating conditions for the collector, particularly collector inclination and maximum operating temperature. If inclinations > 60~ are required, manifold C with extended partitions may be favoured. For inclinations 0~ ~ manifold B with greatest simplicity, lowest cost, and the advantage of lowest impedance may be favoured, except for high temperature operation where mahifolds A and C have some advantages. Our results suggest that for water inlet temperatures > 90~ boiling may occur in the collectors under some conditions. Boiling may be suppressed by increasing fluid pressure in the collectors, but in extreme cases where vigorous boiling occurs increased heat transfer to the tops of the tubes may be expected. In the present experiments, forced circulation through the header pipes of the manifolds has been investigated. Fluid circulation by thermosiphoning has also been investigated [23].
Aeknowledgenients--Tbe authors wish to thank Dr. S. Craig and Professor R. E. Collins for useful discussions; D. Mackey for assistance with data-logging;S. Chow for ray tracing results and J. White for designing some of the apparatus. Financial support for the work described here was provided by CSIRO and by the New South Wales State Government and His Royal Highness Prince Nawaf Bin Abdul Aziz of the Kingdom of Saudi Arabia through the Science Foundation for Physics within the University of Sydney. Yin Zhiqiang thanks the Australia-China Council for financial support during the course of this work.
RE~CES I. E. Speyer, Solar energy collector with evacuated tubes. Trans. ASME. J. Engng Power 87, 270 (1965). 2. U. Ortabasi and W. Buehl. Analysis and performance of an evacuated tubular collector. Ext. Abstr. 1975 ISES Cong. p. 222, Los Angeles, California. 3. D. C. Beekley and G. R. Mather, Analysis and experimental tests of solar collector arrays based on evacuated tubular solar collectors. Ext. Abstr. 1975 ISES Cong. p. 220, Los Angeles, California. 4. R. Bruno, W. Herman, H. Horster, R. Kersten and F. Mahdjuri, High efficiency solar collectors. Ext. Abstr. 1975 ISES Cong., p. 256, Los Angeles, California. 5. B. Window, D. R. McKenzie, G. L. Harding and A. R. Collins, The Sydney University Evacuated Collector Program. Proc. 1979 ISES Conf. (ANZ Section), Solar Realities in the 1980's,p. 87 Perth, Australia. 6. Advertising literature, Advanced Energy Programs, General Electric Company, Philadelphia, Pennsylvania, U.S.A. 7. K. Hirotani, K. Kanatani ~md M. Osumi, An evacuated glass tube solar collector and its application to a solar cooling, heating and hot water supply system for the hospital in Kinki University. Solar Energy 22, 535 (1979). 8. Advertisingliterature, Philco Italiana, Brembate Sopra, Italy. 9. C. C. Yin, J. C. Woo and T. Y. Tao, Research and Development of all-glass vacuum tubular collectors. Abstr. 1981 ISES Cong., p. 482. Brighton, U.K. 10. A. Keller, State of Development of a solar collector of glass. Abstr. 1981 ISES Cong., p. 484. Brighton, U.K. 11. R. Bruno, W. Hermann, H. Horster, R. Kersten and F. Mahdjuri, Analysis and optimisation of solar hot water systems. Proc. ISES Cong. New Delhi, p. 1487.Pergamon Press, New York (1978). 12. G. L. Morrison, D. R. McKenzie, 1. C. Onley, R. E. Collins and N. H. Tran. Long-term performance of evacuated tubular solar water heaters in Sydney, Australia. Solar Energy. To be published.
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13. Adtenising Literature. Sunmaster Corp., Corning, New York. U.S.A. 14. B. Window, Heat extraction from single-ended glass absorber tubes. Solar Energy 31, 159 (1983). 15. B. Window and G. L. Harding, Progress in the materials science of all-glass evacuated collectors. Solar Energy To be published. 16. G. L. }larding, B. Window and R. Gammon, High performance evacuated solar energy collectors---design, applications and viability. Int. J. Ambient Energy 3, 171 (1982). 17. B. Window and G. L. Harding, Buoyancy effects and the manifolding of single-ended absorber tubes. Solar Energy 31, 153 (1983). 18. R. C. McPhedran, D. W. J. Mackey, D. R. McKenzie and R. E. Collins, Flow distribution in parallel connected manifolds for evacuated tubular solar collectors. Austr. L Phys. (1982).
19. B. Window and J. Zybert. Optical collection efficiencies of arrays of tubular collectors with diffuse reflectors. Solar Energy 26, 325 (1981). 20. S. P. Chow, G. L. }larding. 13. Window and K. Cathro, Effect of collector components on the collection efficiency of tubular evacuated collectors with diffuse reflectors. Solar Energy 32. 251 (1984). 21. S. P. Chow, School of Physics, University of Sydney. Private communication (1982). 22. S. Craig. Soltek, 49 Eastview Ave., N. Ryde, Sydney 2113. Private Communication (1983). 23. G. L. Harding and Yin Zhiqiang, Thermosiphon circulation in solar water heaters incorporating evacuated tubular collectors and a novel water-in-glass manifold. Solar Energy To be published.
0038--,092X184 S3.00§ Perg:x,"non Press Ltd.
Printed in Great Britain.
WATER-IN-GLASS MANIFOLDS FOR HEAT EXTRACTION FROM EVACUATED SOLAR COLLECTOR TUBES YIN ZHIQIANGt and G. L. HARDING School of Physics, University of Sydney, N.S.W. 2006, Australia and B. WINDOW CSIRO National Measurement Laboratory, Lindfleld, N.S.W. 2070, Australia {Receited 24 November 1982; accepted 15 March 1983)
Abstract--Measurements are reported on three novel manifolds of the water-in-glass type for evacuated all-glass single-ended tubular collectors. The manifolds provide for series connection of tubes, but because there is virtually no partitioning of the inner volume of the collector tubes, the manifolds are extremely simple and exhibit low impedance to fluid flow. The efficiency of heat extraction from the tubes has been determined by measuring temperatures at various points on the surface of glass tubes in a panel of area - 1.2 m2 while heating the tubes electrically to simulate solar energy input. Measurements have been made for a range of tube inclinations (0-80~ water flow rates (0.5-5 Imin-t, water inlet temperatures (13-70~ and effective solar fluxes (100-1000Wire2) for two absorber tube diameters. The results show that Ior a wide range of operating conditions buoyancy effects alone result in efficient heat transfer to the tops of the tubes. The manifold designs described offer a possible low cost solution to the problem of manifolding evacuated collectors for sub-100oC heat extraction for domestic and industrial applications. I. LNTRODUCTION Evacuated collectors achieve high heat extraction efficiencies relative to fiat plate collectors at temperatures above 80~ due to the combined effects of a h i g h l y selective surface and vacuum insulation of the absorber element [I-10]. Comparative testing of evacuated and fiat plate collectors has illustrated that evacuated collectors also offer considerable performance advantages over conventional fiat plate collectors for relatively low temperature applications such as the provision of domestic hot water [11, 12], but at present they are considerably more expensive than fiat plate collectors. The interconnection of collector tubes via a manifold for circulation of heat transfer fluid contributes substantially to the cost of a complete collector and, in Order to reduce evacuated collector costs, there is a need to develop simpler manifolds using cheaper materials and low cost construction techniques. Besides the obvious requirements of simplicity and low cost, a collector manifold should exhibit features such as the ability to withstand collector stagnation (270-300~ during load removal) for long periods, resistance to internal or external corrosion, and low impedance for minimum pumping requirements. In the case of concentric glass tubular collectors [3, 5, 6, 9, 10] thermal energy must be efficiently transferred from the glass absorber tube to a circulating fluid, and the manifold should result in a low average absorbing surface temperature relative to the fluid temperature in order to minimise losses from the collector tubes. Heat transfer
~Permanent address: Tsinghua University, Peking, China.
fluid may be contained directly in the glass absorber tubes, as in the pioneering Owens-Illinois collector [3] or in metal tubes in thermal contact with the glass absorber, as in the General Electric collector [6]. Variations on fluid-in-glass and fluid-in-metal manifolds have been developed by Sunmaster U.S.A. [13] and Sydney University, Australia [14-18]. Some recent studies of various manifolds for concentric tubular collectors has shown that metal manifolds of the type developed by General Electric result in temperature differences of up to 30~ between fluid and the glass absorber tube surface, whereas water-in-glass manifolds of the type developed by Owens-Illinois result in temperature differences < 10~ [14]. Thus while fluid-in-glass manifolds suffer from the disadvantages of high heat capacity and the possibility of fluid leakage in the event of tube breakage, they possess the potential for achieving greatest simplicity, lowest cost and highest efficiency for collectors providing sub-boiling water for domestic and industrial applications. In this paper we describe measurements .on three closely related novel manifolds of the fluid-in-glass type, in which partitioning of the inner volume of each absorber tube to define fluid flow paths, is eliminated or virtually eliminated. The manifolds allow series connection of collector tubes yet exhibit extremely low impedance for fluid circulation, and the measurements show that buoyancy or thermosiphon effects are primarily responsible for highly efficient heat extraction from the tubes. This detailed study was stimulated by results of preliminary studies of such manifolds which indicated the efficacy of tl~ermosiphon effects in producing heat transfer in single-ended collectors [14, 171. 22_I
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2. M.-kNIFOLDBE.SIGNS Figure 1 shows the design of two conventional manifolds. The Owens-Illinois "water-in-glass' manifold [3] utilises a relatively complex coupling at the open ends of pairs of absorber tubes, and incorporates a long glass feeder tube for fluid circulation. The General Electric "water-in-metal" manifold [6] consists of a copper Utube with attached copper fin which clamps to the inner surface of the absorber tube. In the present work, the basic manifold design (A) consists of a single copper header pipe with copper connectors soldered at equal intervals along the header pipe. Each connector is sealed to a collector tube by means of an O-ring seal. Figure 2 shows the construction and dimensions of manifolds used in the present experiments in conjunction with glass absorber tubes of O.D. 22 or 30 mm. In practice, the connectors may seal either to the glass envelope of a collector (see Fig. 3, manifold (A1), or to the absorber tube of a collector (see Fig. 3, manifold A2). In the present experiments, the absorber tubes have no envelopes (see Section 3), so the latter sealing technique was utilised. Short partitions (manufactured from copper sheet), are press fitted into each connector to divide it equally into fluid inlet and fluid outlet segments (see Figs. 2, 3A1, 3A2). The partitions protrude into the header pipe to encourage some fluid circulation around the partitions into the upper regions of the absorber tubes. The absorber tubes were mounted with axes in a North-South plane, which maximises their daily solar
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Fig. 3. Schematic diagrams showing manifold designs and seals onto evacuated collector tubes. (Upper: longitudinal sections, lower: cross sections). Ah Manifold containing short partitions (P) and external O-ring seal; A2: Manifold containing short partitions (P) and internal O-ring seal: B: Manifold containing no partitions and internal O-ring ; C: Manifold containing elongated partitions (P) and internal O-ring seal.
collection efficiency [19], and with open ends up, allowing rapid filling of the tubes, and unimpeded escape for exsolved gases. The small gaps between partitions and inner surface of the header pipe (Fig. 3) serve an important function in allowing gas to be flushed from the header pipe. Figure 4 shows schematically a pumped collector system incorporating the manifold. This manifold provides for series connection of collector tubes, but because there is no channelIing device which partitions the inner
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volume of each tube into inlet and outlet segments, the manifold exhibits extremely low impedance to fluid flow. Two modifications of this basic manifold have also been investigated (see Fig. 3). (B)--in which the partitions are completely removed from the header pipe and connectors. This design results in slightly lower impedance, but there is less tendency for pumped fluid to circulate in the upper region of each absorber tube. (C)--in which the partitions are elongated to extend a distance 200 mm into the absorber tube. This results in slightly higher impedance, but fluid is encouraged to circulate to 9 a greater depth in each absorber tube. 3. EXPERIMENq'ALTECHNIQUES The experimental technique for investigation of heat transfer in the tubes involves study of the temperatures at various points on the surfaces of the glass absorber tubes in a panel, under various conditions which simulate operating conditions for a collector [17]. In these experiments the absorber tubes have no surrounding glass envelopes, and are heated electrically to simulate the solar input. The electrical heaters were made using 20 mm wide, 0.5 mm thick strips of stainless steel foil mounted in recesses in -100mm thick sheet of polyurethane foam. One strip heats the upper part of each tube, and another heats the lower part. The polyurethane foam insulates each heater and tube from its neighbours. Experiments have been carried out using absorber tubes of two diameters and heaters have been constructed for collector panels containing twenty 22ram O.D. tubes, and fifteen 30 mm O.D. tubes. The tube length is 1.32 m. The effective area of each panel is - 1 . 2 m2. Figure 5 shows a cross section of the heater for 30ram O.D. tubes. Variacs were used to adjust the powers Pu and Pt. into the upper and lower heaters separately to simulate the imbalance of solar energy input to the tubes which occurs in most collector designs. In most of the
Fig. 5. Sectionalview of apparatus for heating 30.ram O.D. glass tubes of length 1.32m. A: Glass tubes; B: Upper heater elements; C: Lower heater elements; D: Upper segment of polyurethane foam; E: Lower segment of polyurethane foam; F: coppercenstantan thermocouple junctions mounted on sides of glass tubes.
experiments the power input was adjusted to simulate the solar insolation on an array of collector tubes spaced by two absorber tube diameters (45 or 60 ram) and which incorporated a diffuse reflector. Ray tracing calculations and measurement of optical efficiency r/o for such collectors have shown that such a design has r/o-0.6, with twice as much energy striking the top as the bottom of the absorber tubes [19, 20]. Thus, for the various incoming fluxes investigated (I = 0.1 ~ 1.0 kWm-2) the total power per unit area of panel was adjusted to equal 0.6 x I and the ratio PoIPL was 2: 1, Other flux distributions were also investigated and are discussed later. Temperatures were measured on each of the tubes in a panel by copper.-constantan thermocouples mounted under glass tape on the side of each tube ~ 80 mm from the domed end (Figs. 5 and 7) and on each of three tubes in the panel six thermocouples were mounted at equally spaced intervals (see Section 4). It was found that mounting on the side gave more consistent results than mounting on the top of the tube because variability in the heater strip to tube spacing produces more variability in a top thermocouple near the heater strip than a side thermocouple. (Independent measurements on a range of manifolds has established that temperatures measured here are at most 2~ lower than the average temperature around the circumference.) The manifold inlet and outlet water temperatures were also recorded. Water flow rate through the collector panel could be varied from 0.5 to 5 lmin-1. For an effective solar flux of 1 kWm-2, these flows produced a temperature difference across the panel of - 1 7 to ~ 1.5~ The water inlet temperature to the manifold was varied between 13 and -70~ using a commercial "instamatic" water heater powered by a variac. The inclination angle, 0, of the collector (measured with the horizontal) was varied between 0 and 80~ to investigate collector performance at the optimum inclination for solar collection at various latitudes.
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Figure 6 illustrates some typical results obtained for temperatures near the bottom of each of the 15 tubes in the panel of 30 mm diameter tubes incorporating manifold A. The temperature difference between the tube bottom and the water inlet to the manifold is plotted for each tube, 1-15, for three inclinations of the collector panel. These results are appropriate to a solar flux of 1 kW/m2, flow rate 1.01/min and inlet temperature 13~ In each case the temperature increase across t~e panel was ~9~ Also plotted is the expected linear variation of fluid temperature along the manifold. These results indicate that on average for inclinations 15, 45 and 70~ the glass temperature near each glass tube bottom is respectively 6, 5.5 and 20~ above the inlet water temperature for that tube. Figure 7 illustrates typical temperature distributions observed along the lengths of the 30 mm tubes. AT is the difference between the glass tube temperature and the water inlet temperature to that tube. Results are shown for water flow ll/min, inlet temperature lYC and three inclinations 0 = 15, 45 and 70~for manifolds A and B. {Manifold C behaves similarly to A). These results indicate that the temperature distribution along the collectors depends strongly on inclination and manifold design. Large temperature gradients are obtained for large inclinations 0, particularly for manifold B. These results suggest that temperature measured at the bottom of a tube is the most useful parameter to monitor, being close to the maximum surface temperature of the glass tube, and thus providing an indication of the efficiency of heat transfer to the header pipe. The length-averaged absorber tube temperature, T (average), which is of interest for deriving heat losses in operational co/lectors, may be evaluated from results such as those in Fig. 7. To allow evaluation of T(average) from T (bottom), we plot in Fig. 8 some typical values of (T (bottom)-T(average)) as a function of collector inclination 0 for manifolds A, B, C (30 mm
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Fig. 6. Results for temperature rise T (measured near the bottom of glass tubes) above the water inlet temperature ( - 13~ for fifteen 30 mm tubes connected to a manifold of type A, for three inclinations of the tubes. Circles: 15"; Triangles; 450; Squares: 70~ The effective solar flux was l kWm-2, and the flow rate I Imin-~. The dashed line illustrates the expected temperature variation in the header pipe of the manifoldand corresponds to the temperature rise T for perfect heat transfer from tubes to header pipe.
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Fig. 8. Graph of 8T = T(bottomFT(avcrage) vs tube inclination Ofor manifolds A. B, C. The results are appropriate for effective solar flux I kWm-'s and flow rate 1 Imin- Iresults are not strongly dependent on flow rate). Open symbols: Manifold inlet temperature = 15~ Circles: Manifold A; Triangles: Manifold B; Squares: Manifold C. Solid Iine with bars: Mar,ifords A. B, C with water inlet temperature = 50~
Water-in-glassmanifolds for heat extraction from evacuated solar collector tubes tubes) for fluid inlet temperatures 15 and 50~ Results for the three manifolds are almost identical for high fluid inlet temperatures. Values obtained for AT(bottom), (temperature near tube bottom-water inlet temperature for that tube), averaged over all tubes in the panel for a wide range of collector operating conditions, and for the three manifold designs are summarised in Figs 9-12. Results obtained for manifold A (incorporating 22 or 30 mm tubes) for various tube inclinations and water inlet temperatures, (flow rate constant at llmin -t are shown in Fig. 9). Except at very low and very high inclinations, AT values increase as fluid inlet temperature is increased. For all inlet temperatures, AT rises sharply as tube inclination approaches 90~ and rises slightly for inclinations approaching 0~ Higher ATs are observed for 22 mm tubes than for 30 mm tubes except at large inclinations. Results obtained for manifold A (incorporating 30 mm tubes) for various water inlet temperatures and flow rates (with 0 constant 45 ~ are shown in Fig. I0. The increase in AT with increasing inlet temperature is again clearly evident. In each case increasing the flow rate by a factor of two increases AT by about I~ Qualitatively similar results are obtained for other inclinations 0. The relative behaviour of the three manifolds A, B, C (30 mm tubes) for various collector inclinations and two inlet temperatures (13 and 50~ for constant flow rate 1 lmin-I is shown in Fig. 11. For inclinations greater than 10~ manifold A is superior to B, particularly for low inlet
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Fig. 9. Graph of AT(bottom) vs tube inclination 0 for manifold A. Effective solar flux was I kWm-z and water flow rate = l lmin-k Solid symbols: 30ram tubes; Open symbols: 22ram tubes; Circles: inlet temperature 13~ Triangles: inlet temperature 50~ Squares: inlet temperature 70~
temperatures, while manifold C is superior to A particularly for high inlet temperatures. This graph shows clearly (see Fig. 9) that for all three manifolds, AT is smaller for high inlet temperature (50~ compared to low inlet temperature (13~ for inclinations near 0 and 800. Figure 12 shows the variation of AT(bottom) with effective solar flux for manifold A incorporating either 22
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I(kWm -~) Fig. 12. Graph of AT(bottom)vs effective solar flux I for manifold A incorporating 30 or 22 mm tubes at inclination 45~ The flow rate was I Imin-k Circles and triangles: heater power ratio PoIPL = 2. Solid circles: 30ram tubes, inlet temperature I3~ Open circles: 30ram tubes, inlet temperature 50~ Solid triangles: 22 mm tubes, inlet temperature 13~ Crosses: 30 mm tubes with PdPL = 3. and inlet temperature 13~ (lower) and 50~ (upper).
or 30 mm tubes. Results are appropriate for inclination 45~ flow rate l lmin -t, 70=0.6 and Po/PL=2, corresponding to normally incident sunlight on a collector incorporating a diffuse reflector. Some results are also shown for the 30 mm tubes when the ratio Pu/PL equals 3, corresponding to sunlight incident at 45+ with the normal [21]. Similar experiments have shown that AT values are decreased when a greater proportion of heat is introduced to the upper segment of the tube. Introducing the entire power input to the lower heater results in the converse--an increase of 2 to 3cC in AT. The transient behaviour of the manifold schemes have been tested for favourable situations, i.e. where the panel is mounted at 45~. In particular, the change in outlet temperature for a system of fifteen 30 mm tubes attached to manifold A mounted at 45~ with a flow of I I/min was measured for an abrupt switch-on of the heater (ikW/m 2) and an abrupt switch-off of the heater (1 kW/m2). The rise (fall) times observed were 36 and 27 rain respectively.
5. DISCUSSIONANDCONCLUSION The simulation of solar input to tubular collectors by electrical heating is an extremely versatile technique which has allowed a detailed investigation of some novel manifolds of the water-in-glass type. For the three manifolds studied, the maximum temperature difference between the surface of a glass absorber tube and the fluid inlet temperature at the top of the tube is < 10~C for a wide range of operating conditions, indicating that buoyancy forces alone are sufficient to produce efficient heat transfer to the top of the tube.
The mechanism of heat transfer proposed involves the separation of a colder stream of water which is injected into the tube, from a hotter outgoing stream, by buoyancy forces. This fluid flow and heat transfer pattern is generally favoured by lower flow rates and lower water temperatures (as illustrated in Figs. 9-11), both of which contribute to a lower Reynolds number and less turbulence in the fluid, with consequent lower values of AT. The fluid stream separation is also favoured by a greater proportion of power on the top surface of the tubes (Fig. 12). As the inclination of the tubes approaches vertical, buoyancy forces are no longer able to keep the cold and warm streams separate, and longitudinal heat flow breaks down leading to the fluid stratifying across the tube and considerably higher glass surface temperatures (Figs. 7, 9 and 11).The buoyancy effects also become less efficient for promoting fluid flow for inclinations close to horizontal, resulting in slightly increasing values of AT (Figs. 9 and ii). For inclinations close to horizontal or vertical, tube surface temperatures decrease as inlet temperature increases. At either of these inclinations, where buoyancy effects are greatly retarded, increased buoyancy at higher temperatures resulting from larger density changes for a given temperature increment may play a more important role than at intermediate values of 0. Better definition of fluid inlet/outlet streams at the top of each collector tube leads to more efficient heat extraction from the tubes, as indicated by the superiority of manifold A (containing short partitions) compared with B (containing no partitions) (Fig. 11). The extended partiiions of manifold C further improve fluid circulation by defining inlet and outlet streams at greater depth in the tubes (although partitions divide the tubes vertically, not horizontally), with consequent improved heat extraction and reduction in surface temperature of the absorber tubes. The effect of absorber tube diameter on AT may be explained by the following theory. Assuming an effective fluid flow [ occurs in the ingoing and outgoing streams within each tube due to buoyancy forces, and that the temperature difference between the streams at the top of the tube is proportional to AT, then conservation of energy requires that f . ATaE
(1)
where E is the energy absorbed by each tube. The driving force responsible for the flow within each tube is a buoyancy force which we assume to be proportional to AT. Thus f a A TIR
(2)
where R is the impedance to fluid flow in each tube. Equations (1) and (2) give (AT)z a R.E.. In Fig. 13, (AT)z is plotted against E using the results presented in Fig. 12 for absorber tubes of outer diameter 30 and 22ram with inlet water temperature 13~ and
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Fig. 13. Graph of (AT)2 vs energy absorbed by an absorber tube (E) for two absorber tube diameters and two water inlet temperatures. The lengths of the absorber tubes are in each case 1.32m. outer diameter 30 mm with inlet water temperature 50~ Linear graphs are obtained for the lower water inlet temperature suggesting that in this case the two tubes can be characterised by impedances R3o and Rz2 independent of the energy E absorbed by each tube (i.e. independent of the thermosiphon flow [). Furthermore the ratio R2zlR~o determined from Fig. 13 is (4.9-+0.1) whereas the ratio of the fourth powers of the inner diameter of the tubes is 4.8, indicating that the thermosiphon flow within the tubes can be characterised by Poiseuille's equation and is therefore laminar. No experimental results for the dependence of AT on absorber tube length have been obtained but the applicability of Poiseuille's equation suggests that since flow impedance RaLIr 4 where L is the tube length and r the inner radius, then (fiT) z a LIr4• For all other parameters constant, E a L thus AT cz L. This simple theory may allow AT values to be predicted for absorber tubes of other dimensions in the case of low water inlet temperatures. By contrast, a strongly non-linear relationship is obtained between (~T) ~ and E for the 30 mm tube with water inlet temperature 50~ This indicates that Poiseuille's equation is not appropriate for thermosiphon flow at relatively high temperatures and suggests that the flow has become turbulent due to the considerably lower water viscosity. The transient response of the system is satisfactory. In the simplest possible model of water flowing through a collector of low heat capacity and no heat exchange between various parts, the rise and fall times should be 80% of the residence time; this is equal to 10 min for the configuration and flow rate mentioned earlier. Actual rise times are approximately three times this, due to finite thermal capacities and conductivities, and, more importantly, a reduction in flow through the tubes below the nominal flow. This reduction is due to the short circuiting of flow past the metal partition in the manifold, and the short circuiting of flow around the top of the tube rather than along the length of the tube and back. We believe time constants of this magnitude to be acceptable in most applications below 100~ The three manifold concepts are all extremely simple and offer potential for manufacture at low cost for example, using moulded plastic [22]. Our results suggest
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that the manifold design utilised may depend on the standard operating conditions for the collector, particularly collector inclination and maximum operating temperature. If inclinations > 60~ are required, manifold C with extended partitions may be favoured. For inclinations 0~ ~ manifold B with greatest simplicity, lowest cost, and the advantage of lowest impedance may be favoured, except for high temperature operation where mahifolds A and C have some advantages. Our results suggest that for water inlet temperatures > 90~ boiling may occur in the collectors under some conditions. Boiling may be suppressed by increasing fluid pressure in the collectors, but in extreme cases where vigorous boiling occurs increased heat transfer to the tops of the tubes may be expected. In the present experiments, forced circulation through the header pipes of the manifolds has been investigated. Fluid circulation by thermosiphoning has also been investigated [23].
Aeknowledgenients--Tbe authors wish to thank Dr. S. Craig and Professor R. E. Collins for useful discussions; D. Mackey for assistance with data-logging;S. Chow for ray tracing results and J. White for designing some of the apparatus. Financial support for the work described here was provided by CSIRO and by the New South Wales State Government and His Royal Highness Prince Nawaf Bin Abdul Aziz of the Kingdom of Saudi Arabia through the Science Foundation for Physics within the University of Sydney. Yin Zhiqiang thanks the Australia-China Council for financial support during the course of this work.
RE~CES I. E. Speyer, Solar energy collector with evacuated tubes. Trans. ASME. J. Engng Power 87, 270 (1965). 2. U. Ortabasi and W. Buehl. Analysis and performance of an evacuated tubular collector. Ext. Abstr. 1975 ISES Cong. p. 222, Los Angeles, California. 3. D. C. Beekley and G. R. Mather, Analysis and experimental tests of solar collector arrays based on evacuated tubular solar collectors. Ext. Abstr. 1975 ISES Cong. p. 220, Los Angeles, California. 4. R. Bruno, W. Herman, H. Horster, R. Kersten and F. Mahdjuri, High efficiency solar collectors. Ext. Abstr. 1975 ISES Cong., p. 256, Los Angeles, California. 5. B. Window, D. R. McKenzie, G. L. Harding and A. R. Collins, The Sydney University Evacuated Collector Program. Proc. 1979 ISES Conf. (ANZ Section), Solar Realities in the 1980's,p. 87 Perth, Australia. 6. Advertising literature, Advanced Energy Programs, General Electric Company, Philadelphia, Pennsylvania, U.S.A. 7. K. Hirotani, K. Kanatani ~md M. Osumi, An evacuated glass tube solar collector and its application to a solar cooling, heating and hot water supply system for the hospital in Kinki University. Solar Energy 22, 535 (1979). 8. Advertisingliterature, Philco Italiana, Brembate Sopra, Italy. 9. C. C. Yin, J. C. Woo and T. Y. Tao, Research and Development of all-glass vacuum tubular collectors. Abstr. 1981 ISES Cong., p. 482. Brighton, U.K. 10. A. Keller, State of Development of a solar collector of glass. Abstr. 1981 ISES Cong., p. 484. Brighton, U.K. 11. R. Bruno, W. Hermann, H. Horster, R. Kersten and F. Mahdjuri, Analysis and optimisation of solar hot water systems. Proc. ISES Cong. New Delhi, p. 1487.Pergamon Press, New York (1978). 12. G. L. Morrison, D. R. McKenzie, 1. C. Onley, R. E. Collins and N. H. Tran. Long-term performance of evacuated tubular solar water heaters in Sydney, Australia. Solar Energy. To be published.
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13. Adtenising Literature. Sunmaster Corp., Corning, New York. U.S.A. 14. B. Window, Heat extraction from single-ended glass absorber tubes. Solar Energy 31, 159 (1983). 15. B. Window and G. L. Harding, Progress in the materials science of all-glass evacuated collectors. Solar Energy To be published. 16. G. L. }larding, B. Window and R. Gammon, High performance evacuated solar energy collectors---design, applications and viability. Int. J. Ambient Energy 3, 171 (1982). 17. B. Window and G. L. Harding, Buoyancy effects and the manifolding of single-ended absorber tubes. Solar Energy 31, 153 (1983). 18. R. C. McPhedran, D. W. J. Mackey, D. R. McKenzie and R. E. Collins, Flow distribution in parallel connected manifolds for evacuated tubular solar collectors. Austr. L Phys. (1982).
19. B. Window and J. Zybert. Optical collection efficiencies of arrays of tubular collectors with diffuse reflectors. Solar Energy 26, 325 (1981). 20. S. P. Chow, G. L. }larding. 13. Window and K. Cathro, Effect of collector components on the collection efficiency of tubular evacuated collectors with diffuse reflectors. Solar Energy 32. 251 (1984). 21. S. P. Chow, School of Physics, University of Sydney. Private communication (1982). 22. S. Craig. Soltek, 49 Eastview Ave., N. Ryde, Sydney 2113. Private Communication (1983). 23. G. L. Harding and Yin Zhiqiang, Thermosiphon circulation in solar water heaters incorporating evacuated tubular collectors and a novel water-in-glass manifold. Solar Energy To be published.