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Comparative study of fluid-in-metal manifolds for heat extraction from single ended evacuated glass tubular collectors
Solar EnerA,y Vol. 35. No. I, pp. 81-91, 1985
0038-092X/85 $3.00 + .lY0 © 1985Pergamon Press Ltd.
Printed in the U.S.A.
C O M P A R A T I V E STUDY OF F L U I D - I N - M E T A L M A N I F O L D S FOR H E A T EXTRACTION FROM SINGLE E N D E D E V A C U A T E D GLASS T U B U L A R COLLECTORS YIN ZHIQIANG,t G. L. HARDING,~ S. CRAIG a n d R. E. COLLINS
School of Physics, University of Sydney, N.S.W. 2006. Australia. and B. WINDOW CSIRO National Measurement Laboratory, Lindfield. 2070. Australia [Received 19 A u g u s t 1983; revision received 6 D e c e m b e r 1984; accepted 10 D e c e m b e r 1984)
Abstract--An investigation is reported of heat transfer between the glass absorber tubes of all-glass evacuated collectors and fluid-in-metal manifolds designed for heat extraction from the glass asborber tubes. The heat transfer is studied using a novel solar simulator which heats a panel of glass tubes electrically to simulate solar input to a collector panel. Measurement of the temperatures at various points on the glass tubes and on the manifolds gives a measure of the efficiency of heat transfer for each manifold under various operating conditions and allows calculation of the efficiencies "qo of collectors incorporating the manifolds. The effect of fluid temperature, collector inclination and fluid flow rate has been investigated for four manifold designs of increasing simplicity. Experimental results for the manifolds are compared with calculations of heat transfer. Potential lifetime problems for the manifolds are also discussed. The simplest manifold design is shown to have good prospects for hightemperature (>I00°C) heat extraction.
thus allowing the possibility of draindown, as in the Sunmaster design[9] (see Fig. 2). In a recently deAll-glass single-ended concentric tubular evacuated collectors have been intensively researched and de- veloped simplification of the fluid-in-glass concept, collector tubes are horizontal or inclined with open veloped during the past ten years in several laboratories[I-5]. The collectors have associated re- ends up, allowing interconnection via a single header pipe of relatively low cost and complexflectors and a manifold which interconnects the ity[10-12] (see Fig. 3). In all these designs, extubes and circulates heat transfer fluid. The tubular tremely efficient heat transfer is achieved from abcollectors have been almost fully researched and sorbing surface of the evacuated collector tube to developed. Some of the most recent research aimed heat transfer fluid due to intimate contact of fluid at maximizing collector efficiency and cost effecwith the inner surface of the glass absorber tube. tiveness has included a detailed study of the effect of low cost antireflection coatings on the glass en- A major criticism of these designs is that collector velopes[6-8], the effect of selective surfaces of high operation may be interrupted in the event of breakabsorptance (a - 0.96) on collector perform- age of one or more glass collector tubes, with asance[8]. Diffuse and specular reflector designs have sociated loss of heat transfer fluid. In "fluid-in-metal" manifold designs, as in the been intensively studied in the past ten years, but General Electric collector[2] (see Fig. 1), heat transfurther work aimed at producing low-cost longfer fluid is contained within metal pipes which are lived reflectors with associated high collector effiinserted into the absorber tube of each collector, ciency is of considerable importance. Similarly, furand which are interconnected outside the tube. The ther development and testing of manifold concepts is needed with the aim of producing a most-cost- fluid circuit is therefore undamaged by breakage of the collector tubes, however, the heat transfer from effective design with acceptable lifetime. glass absorber tube to heat transfer fluid is less efTwo types of manifold have been developed for ficient due to the relatively high thermal impedglass tubular collectors. In "fluid-in-glass" designs, heat transfer fluid is contained directly in the glass ances associated with the air spaces and multiple interfaces in such designs. absorber tubes, as in the pioneering Owens-Illinois A quantitative indication of the efficiency of heat collector[l] (see Fig. 1). In a refinement of this contransfer can be obtained from measurement of cept, collectors are inclined with open ends down, A TTAv, the difference between the average temperature of the absorbing surface of the collector t Permanent address: Tsinghua University, Beijing, tube and average fluid temperature within the manChina. $ Present address: 29 Duff St. Turramurra 2074, ifold for particular incident sunlight or simulated Australia. sunlight intensity. A high absorber tube surface I. INTRODUCTION
81
82
Y.
ZHIQIANG
Owens Illinois Water-in-Glass
General Electric Water-in-Metal ,6--~
a cuated ollector Tube ~
Glass Feed Tube
Metal U-Tube with metal fin
Fig. 1. Schematic diagrams showing the Owens-Illinois "water-in-glass" manifold and the General Electric "water-in-metal" manifold. Both manifolds involve series connection of tubes.
UUU Collectors
Fig. 3. Schematic diagram of a novel water-in-glass manifold which consists of a single header pipe with connections to absorber tubes which are horizontal or inclined with open ends up. No channelling device is incorporated within the absorber tubes. Heat is transported to the header pipe by buoyancy effects.
temperature relative to fluid temperature results in increased heat loss from the collector tubes by mechanisms such as radiative heat loss from the absorber tube, conduction loss through gas contaminating the vacuum space and conduction loss along the glass tube at the interconnection point of absorber tube and envelope. Figure 4 illustrates the possible heat loss mechanisms from a collector tube. If the average radiation and conduction coefficients associated with a particular evacuated collector tube are known, the decrement in collector efficiency B'qo resulting from the increased heat losses can be determined from values of A TrAy, the number density of evacuated collector tubes and the incident solar flux[13] (A~qo is defined by "q0 ---"qOPT - - AT]owhere ~OPT is the collector optical efficiency and "qo is the collector efficiency for fluid near ambient temperature. The optical efficiency of a standard Sydney University collector is "qOPT = 0.63[6]). The required coefficients have been determined for the Sydney University evacuated collector and it has been shown that, when operating
A
B
D
:'. ,,-4),, // ',. ,. ..
7,-_'+
f Radiation <' Gas Conduction - - , Conductio.n thro: glass ano metal
Fig. 2. Schematic diagram of a drainable water-in-glass manifold developed by Sunmaster corp. U.S.A. A: copper inlet/outlet pipes and copper cup; B: O ring seals; C: small diameter tube functioning as an impedance for flow equalization in parallel connected tubes and as the drain tube; D: glass tube for fluid outlet; E: water; F: evacuated collector tube; G: insulation.
Fig. 4. Schematic diagram of a concentric tubular glass collector showing mechanisms of energy transfer from inner (absorber) tube to envelope. The radiation loss depends on the selective properties of the surface on the absorber tube. Radiation is larger in the region of the tube coated by gettering material. Gas conduction loss depends on the level of vacuum in the tube. A: Envelope: B: Absorber tube; C: Getter coated region of absorber tube; D: Stainless steel support for absorber tube and getter; E: Glass seal (absorber tube to envelope).
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Comparative study of fluid-in-metal manifolds for heat extraction in a solar flux of - 1 kWm 2, a value of A TTAVof 7 to 8°C results in a decrement A'qo = 0.01 in collector efficiency[13]. Thus the accurate determination ofA T+AVvalues for various manifolds allows the evaluation of the instantaneous efficiencies 11o or ~q(T) of collectors incorporating the manifolds[13]; this information together with information regarding materials and manufacturing costs of the entire collector, and manifold lifetimes, allows an estimate of the relative cost effectiveness of various manifold designs. Preliminary measurements by Window[ 14] have shown that for various fluid-in-metal designs, temperature differences of 20 to 50°C can occur between absorbing surface of the absorber tube, and fluid contained within metal pipes inserted into the absorber tube. Such temperature differences were far greater than those measured for water-in-glass manifolds (usually 5°C - 10°C). At Sydney University, a novel solar simulator has recently been designed and constructed exclusively for the testing of various manifold concepts[10-12]. Fifteen glass tubes, identical to the inner absorber tubes of evacuated collectors, are interconnected via the manifold under test and their outer surfaces directly heated by means of electrically powered strip heaters mounted above and below each tube. In these experiments the absorber tubes have no associated glass envelopes and require no coating with selective surfaces. Figure 5 shows schematically an absorber tube together with
r
m
t
-14m
Fig. 6. Schematic diagram of system used to study thermosiphon flow. A: 120-1 low-pressure storage tank; B: Connecting pipes; C: Collector manifold (M3); D: Glass tubes. C and D are enclosed within insulation in the solar simulator (see Fig. 5). The crosses denote temperature sensors on the manifold and in the tank, which are used to study thermosiphon flow.
heater elements. Heat input via the electric heater strips simulates solar fluxes 0.1- 1.0 kW/m. 2. Thermocouple temperature sensors are placed both on the outer surface of the glass absorber tubes and at various points on the manifold. Relative efficiency of heat transfer from absorber tubes to heat transfer fluid (water) is determined by measurement of temperature differences between glass tubes and the average fluid temperature in the manifold for various simulated solar fluxes, fluid flow rates, fluid inlet temperatures and collector inclinations. Fluid flow is established either by tbrced circulation or by thermosiphon circulation between the collector panel and a 120-1 storage tank (see Fig. 6). This simulator has been used successfully to examine the properties of the water-in-glass manifold shown in Fig. 3, both with forced circulation[ll] and thermosiphon circulation[12] through the header pipe. In this paper we report a similar study of four manifolds of the fluid-in-metal type. We show that most efficient heat transfer from glass absorber to fluid is obtained with a manifold design similar to that of General Electric (Fig. 1). Progressive atFig. 5. Schematic diagram of a portion of the solar sim- tempts to simplify this manifold concept (with the ulator, with glass absorber tube and fluid-in-metal mani- aim of reducing cost and complexity of the manifold. A: Polyurethane insulation; B: Upper stainless steel fold) result in progressively less efficient heat transheater strip; C: Lower stainless steel heater strip: D: Glass absorber tube; E: Manifold (M4); F: Thermocouple tem- fer and larger values of A'qo. Experimental results perature sensors on the surface of the glass tube: G: Ther- for the simplest manifolds are compared with a simple theoretical treatment of heat transfer, and telmocouple temperature sensors on the manifold.
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Y. ZHIQIANG
ative merits and demerits and potential lifetime problems for the manifolds are discussed.
2. MANIFOLDDESIGNS Manifold MI shown in Fig. 7 is similar in concept to the General Electric manifold shown in Fig. 1. The latter manifold consists of a copper U-tube with attached (welded) copper fin which clamps to the inner surface of the absorber tube. In M1 in the interests of reduced materials and assembly costs, the welded copper fin is replaced by a rolled aluminium fin which clips onto one arm of the copper U-tube. In addition, to reduce the flow impedence, collector tubes are connected in parallel with the inclusion of two header pipes, rather than the series connection of General Electric. Copper tubes O.D. = 6 mm are used in order to minimise the materials costs for the manifold. Manifold M2 (Fig. 7) is a simplified version of MI in which the rolled aluminium fin is replaced with a flat copper fin welded to one arm of the Utube. Thus less metal is utilized, the cost of rolling the metal fin is avoided and collector assembly is facilitated by the loose fitting fin. Manifold M3 (Fig, 7) is a simplified version of M2 in which the fin is removed completely, leaving only the copper U-tube within each absorber tube. A similar manifold design investigated by OwensIllinois[15] consists of a U-tube comprising considerably larger diameter metal tubes coated with a material of high thermal emittance (for example, a metal oxide) to maximise radiative heat transfer from glass to U-tube. Manifold M4 (see Fig. 7) consists of a single header pipe attached to metal " r i s e r s " (water-filled copper tubes closed at their lower ends) which are inserted into each absorber tube. Two situations were considered. In one case no mechanism was incorporated for locating the risers in a particular volume within the absorber tube. In this case each riser is assumed to locate (on average), close to the axis of the absorber tube (Fig. 7). In the second
M1
19mm~
M2
M3
M4 --
+. mm -12-5mm
,II I I
~i.l[li I
I
I Ii
© @ © © Fig. 7. Schematic diagrams showing manifolds M l, M2, M3 and M4. MI, M2 and M3 are identical except for the fin design.
case, each riser was clamped to the upper segment of an absorber tube by means of a spring loading mechanism at three positions on the 1.3 m length of each riser. The heat flow pattern expected is similar to that observed in the water-in-glass manifold shown in Fig. 3111, 12]. Buoyancy or thermosiphon effects should induce heat flow up the risers to the header pipe. Uni-directional flow of water occurs through the header pipe due to forced or thermosiphon circulation, but the fluid flow pattern within the risers is less well defined. This type of manifold involves series connection of collector tubes, but exhibits low impedance to fluid flow[l 1].
3. EXPERIMENTAL TECHNIQUES For each of the four manifolds a panel containing fifteen absorber tubes (1.35 m long, 30 mm O.D., 27.6 mm I.D.), was tested. This system is equivalent to a panel of Sydney University collectors of area 1.2 m 2. Two water-flow situations were investigated for each m a n i f o l d - - a forced flow of I 1/min with water inlet temperature 20-50°C, and thermosiphon flow produced with the collector panel connected to a storage tank (Fig. 6) with tank water temperature 20 to 50°C. Collector inclination was varied between 0 ° (horizontal) and 85 ° (near vertical), In each case the header pipes of the manifolds were uppermost, allowing air and exsolved gases to be easily cleared from the metal tubes and in the case of M4 allowing buoyancy effects to transport heat to the header pipe. Thick polyurethane insulation around the heaters and tubes results in 95% of the power input being removed via fluid in the manifold when steady state conditions are achieved. The heat balance in the simulator was determined by accurately measuring the water flow rate through a given manifold and measuring the inlet and outlet water temperatures when steady state was achieved for various manifolds and total electrical power inputs for collector tube surfaces running at -50°C above ambient, - 9 5 % of the electric power input was removed via the fluid in the manifolds, and - 5 % was lost by conduction through the thick polyurethane insulation around the heaters and tubes. Thus in most experiments where A TTAV is -<50°C, values of surface temperature of the glass tubes (and values of A~lo) should be accurate to within - 5 % . In most cases the random errors in the tube surface temperatures are greater than 5%. To simulate a solar flux of 1 kW/m 2 (as used in most experiments), a total electric power of 0.72 kW was applied to the heater strips in the simulator, with twice as much power supplied to the strips above the tubes, compared to the strips below the tubes (Fig. 5). This simulates the total absorbed energy and energy distribution around collector tubes in a panel with w h i t e diffuse rear reflector and efficiency ~q - 0.6[7, 11, 16]. Temperature sensors were located on the inlet
Comparative study of fluid-in-metal manifolds for heal extraction and outlet header pipes to determine water inlet/ outlet temperatures (T~,/To,t, respectively). In the case of thermosiphon induced flow, the temperature difference between inlet and outlet was used to determine the total flow rate through the panel[12]. To determine temperature distributions on the glass tubes, thermocouples were mounted on the top, side and bottom of the tubes and at six positions along their length (see Figs. 5 and 6). The parameter of interest for comparing heat extraction efficiencies is the difference in temperature (A T) between a given point on a glass absorber tube and the average fluid temperature in the manifold. In the case of M I-M3 (tubes parallel connected) the latter temperature is the mean of T~, and To,,. For M4 (tubes series connected), the latter temperature is that of water in the single header pipe at the connection point of the tube of interest, and may be calculated assuming a linear variation of temperature along the header pipe of M4. a T values obtained from positions around the circumference of the tubes were used to evaluate A TcAv--an average value around the circumference of a tube at a given distance along the tube. A TcAv values obtained from positions distributed along the tube length and around the tube circumference were used to evaluate A TTAv--an average value for the entire surface area of the tube. To investigate in detail the heat transfer along the risers of M4, temperature sensors were also placed at six positions on several of the metal risers within the absorber tubes.
4. RESULTS AND DISCUSSION
4.1.1 Manifold M1 with forced circulation Figure 8 shows the temperature (AT) distribution around the circumference of absorber tubes near their domed ends and average values around the circumference (A TCAV) as a function of position along the tubes. The total surface average (A TTAV) for an entire tube is listed in Table 1 together with
85
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L,
100
,L
AT(°C/
~__,
i
24-'3
~'
~"-~~M1~11_,
~50 <] 3~7
2 22~2
L
%
'
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1:2
Fig. 8. Graphs of A TC'AV ( = average temperature around the circumference of a glass tube--average fluid temperature in the manifold) as a function of position X on the absorber tube for manifolds MI. M2. M3. Also shown are AT values (= temperature at a point on the absorber tube--average fluid temperature in the manifold) on the top. side and bottom of the absorber tubes measured close to the domed ends. For MI. solid triangles: Forced flow: Open triangles: Thermosiphon flow.
the efficiency decrement Arlo. These results were obtained for flow rate I I/min and effective flux I kW/m 2 and were independent (within experimental uncertainties) of collector inclination (0 to 85 °) and water inlet temperature (20 to 54°C). Highest temperatures occur on the top surface of the tubes where there is highest energy input to the glass. Our results are similar to those reported by Window[ 14]. Window observed that introducing heat transfer compound into the gap between copper U-tube and aluminium fin reduced A Tc'Av by about 4°C indi-
Table 1. Values of A TT^v (average temperature of an absorber tube surface - average temperature of fluid in the manifold) and estimated efficiency decrement A-qofor fluid temperature near ambient, for manifolds MI-M4 and various operating conditions. Manifold M1 M2 M3 M4 Unsupported risers Spring loaded risers
Inclination (degrees)
Inlet temperature (°C)
Effective flux (kW/m2)
Flow rate (l/min)
0---, 85 0---~ 85 0--* 85 "0 45 45 45 84 45 45 45
20 ~ 50 20 ~ 50 20 ~ 50 20 20 50 25 20 20 50 20
1.0 1.0 1.0 1.0 1.0 1.0 0.5 1.0 1.0 1.0 0.5
0.35 and 1.0 0.35 and 1.0 0.35 and 1.0 1.0 0.6 and 1.0 0.6 and 1.0 0.43 1.0 1.0 1.0 1.0
ATTAV (°C) 21 54 66 122 102 98 54 120 71 72 48
= 3 _ 5 ___ 7 +_ 10 ~ 10 +__ 10 ± 5 - 10 _ 10 ± 10 ___ 5
A'qo 0.03 0.07 0.09 0.17 0.14 0.13 0.14 0.16 0.16 0.10 0.13
Y. ZHIQIANG
86
cating that the absence of a weld between U-tube and fin does not contribute greatly to A TeAr. 4.1.2 Manifold MI with thermosiphon circldation Thermosiphon flow rate as a function of inlet water temperature is shown in Fig. 9 for effective flux 1 kW/m 2 and collector inclination 45 °. The flow rate increases with increasing inlet temperature due to the effects of decreasing water viscosity and the increasing rate of density change of water as temperature increases. Thermosiphon flow occurs due to a pressure head developing in both the copper U-tubes and in the pipes connecting collector and tank (Fig. 6). The pressure head in the U-tubes should be approximately proportional to the vertical height of the collector which is proportional to sin 0, (where 0 is the inclination). Figure 10 shows the normalised flow rate f/f(O = 45 °) as a function of 0. Flow rate increases with 0 as expected. The temperature (2~ T) values measured on the tubes are similar to those obtained for forced flow (Fig. 8). 4.2. I Manifold M2 with fi~rced circulation Figure 8 shows the temperature (A T) distribution around the circumference of absorber tubes near their domed ends, and average values around the circumference (2~ TCAV) as a function of position along the tubes. The total surface average (A TTAV) and A'qo are listed in Table 1. These results, obtained for flow rate I l/min and effective flux I kW/ m z, were independent of collector inclination (0 to 85 °) and inlet water temperature (20 to 54°C). The temperatures z~Tare considerably higher than those measured for M I due to large air gaps between the inner surface of the absorber tube and copper Utube and fin. The temperature 2~TCAV peaks in the centre of the tube (Fig. 8) probably because the arms of the U-tube tend to be displaced furthest from the glass in this region. Near the domed end and open end of the absorber tube the U-bend in the copper tube and connections to the header pipes, respectively, tend to maintain closer proximity of the tube and glass, reducing the impedance to heat flow. The copper tube and fin have relatively low emit-
.c_.8
o.4 U.
0
20
30 4() ~n(°C)
5~0
Fig. 9. Graphs of thermosiphon flow rate versus manifold inlet water temperature (T~,) for manifolds MI, M2 and M4 with tube inclination 45° and effective flux I kW/m 2. Squares: M I; Circles: M2; Triangles: M4.
1.0 j.._t-------"--
:)
30
60
90
0 ° Fig. 10. Normalised thermosiphon flow rate f/f(O = 45°) versus collector inclination 0 for a collector with manifold M1, connected to the 120-1 storage tank. The results are appropriate for effective flux 1 kW/m2 and inlet water temperature -20°C. Similar results are obtained for manifolds M2, M3. tance (<0.05) therefore radiation plays a relatively small role in the heat transfer. 4.2,2 Manifold M2 with thermosiphon circulation Both thermosiphon flow rate as a function of water inlet temperature and flow rate as a function of angle are nearly identical to results for M 1 shown in Figs. 9 and 10. This is expected because the basic water flow path (header pipes plus U-tubes) are identical for M I and M2. The temperature distributions on the tubes are identical within experimental uncertainties to those obtained for forced flow. 4.3,1 Manifold M3 with forced circulation Figure 8 shows the temperature (AT) distribution around the circumference of absorber tubes near their domed ends and average values around the circumference (A TcAv) as a function of position along the tubes. The total surface average (A TVAV) and A'qo are listed in Table 1. These results, obtained for flow rate 1 l/min and effective flux 1 kW/ m 2, were independent of collector inclination (0 to 85 °) and water inlet temperature (20 to 50°C). Quantitatively similar results were reported by Window[ 14] for this manifold. The temperatures A TcAv for M3 are comparable to those for M2 near the domed end and open end of the glass tube where the arms of the U-tube are maintained close to the glass (see section 4.2.1). The slightly higher values for M3 in these positions are a result of the smaller surface area of manifold M3 compared with M2. The large peak in A TCAV in the centre of the tube occurs due to the absence of a support for the upper arm of the U-tube, with resulting increased physical separation of glass and metal tube, and consequently higher thermal impedance. These results suggest that the advantage of the copper fin in M2 is primarily that it supports the upper arm of the Utube relatively close to the glass. The average glass tube temperature A TVAV for M3 is therefore larger than for M2 (Table 1). 4.3.2 Manifold M3 with thermosiphon circulation Results for thermosiphon flow are similar to re-
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Comparative study of fluid-in-metal manifolds for heat extraction suits for M1 and M2 (Figs. 9 and 10) due to the basic similarity of the three manifold designs. 4.4.1 Manifold M4 with forced circulation A. No riser location mechanism. Figure l la shows the temperature (A T) distribution around the circumference of absorber tubes near their domed ends for flow 1 l/min, effective fluxes 1 kW/m 2 and 0.5 kW/m 2, collector inclination 45 ° and inlet water temperature 20°C. For effective flux 1 kW/m 2, and conditions such as higher inlet water temperature or inclinations near 0° or 85° the temperatures are higher but the temperature on the top surface of the tube is invariably 50°C higher than the side and bottom. Figure 12 shows the average temperatures around the circumference (A TCAV) as a function of position along the tubes for effective flux 1 kW/m 2, inclinations 0°, 45 ° and 84° with inlet water temperature 20° , and inclination 45 ° with inlet water temperature 50°C. For collector inclination 45 ° and inlet water temperature 20°C, an effective flux of 0.5 kW/m 2 has also been investigated. Table I summarises the total surface average temperatures A TTAV and Mqo for the operating conditions listed above. Figure 12 also shows the variation in temperature A TR ( = temperature of riser surface--temperature of water in the header pipe at the connection point of the riser), for the operating conditions described above. Negligible temperature variation
AT(°C) M a n i f o l d M 4 84"-10
140'-10
(a)
6 0"6
65-+4
•5 k W r r i 2
8 10
90+-10
(b)
95+-10
lkWm 2
Fig. 11. Temperatures (AT(°C) = temperature on glass surface--average fluid temperature in manifold M4) on the top, side and bottom of the glass tube, measured close to the domed end for collector inclination 45°, inlet water temperature 20°C, and two effective solar fluxes 0.5 and 1.0 kW/m2. (a) Unsupported risers; (b) spring loaded risers.
•
e"
j)
84,20 45,50 45,20 84,20 O, 20
AT ¢C )
45,20
50
0-5 kWm 2 ~I, 45,20
L
X(m) Fig. 12. Graphs of AT as a function of position along the glass absorber tubes (open symbols) and copper riser tubes (closed symbols) of M4 with unsupported risers. For the glass tubes AT = TcAv (= average temperature around the circumference of glass tube--average fluid temperature in the manifold). For the copper risers, AT = ATR (= temperature on the riser--average fluid temperature in the manifold). For each curve, the first number is the collector inclination in degrees, the second number is the inlet water temperature. All the results are for effective solar flux 1 kW/m2 except the circles with crosses, for which the flux is 0.5 kW/m2.
occurs around the circumference of the risers due to the high conductivity of copper. The results for A TR exhibit the same trends as temperature A TCAV of the glass tubes, but with absolute values about 55°C lower for effective flux 1 kW/m 2 and -30°C lower for effective flux 0.5 kW/m 2. The large temperature differences between glass absorber and copper riser tubes (particularly on the top surface of the glass where the temperature difference is -90°C for effective flux 1 kW/m 2) occur because the risers with no location mechanism are mounted approximately centrally within the absorber tube (see Fig. 7) with an air gap of - 7 mm between glass and riser. If heat transfer is assumed to occur across this gap exclusively via conduction, the average temperature difference expected between glass and riser is -150°D. The relatively low value measured ( - 5 5 ° C - - s e e Fig. 12) and the negligible contribution from radiative heat transfer, suggest that con-
88
Y. ZHIQIANG
vection currents in the air column enclosed within the absorber tube must contribute significantly to heat transfer. Recent work by Symons et a/.[17] has shown that convection currents are established in similar air columns. Convection may similarly play an important role in heat transfer for manifolds M2 and m3. B. Risers spring loaded against upper segment of absorber tube. Figure 1lb shows the temperature (A T) distribution around the circumference of absorber tubes near their domed ends for flow 1 1/ min, effective fluxes 1 kW/m 2 and 0.5 kW/m z, collector inclination 45 ° and inlet water temperature 20°C. For both 0.5 and 1 kW/m 2 the temperatures near the top and side surfaces of the absorber tube have been depressed (compared with Fig. ! la) due to the close proximity of the riser to the top of the tube where the highest energy flux occurs, and the temperatures near the bottom surface of the absorber tubes have increased due to the correspondingly larger spacing of riser and absorber tube surface. Figure 13 shows the average temperatures around the circumference (A TcAv) as a function of position along the tubes for effective flux 1 kW/m 2, inclination 45 ° and inlet water temperature 20°C and 50°C; and effective flux 0.5 kW/m 2, inclination 45 ° and inlet water temperature 20°C. The results are qualitatively similar to those in Fig. 12 for the un-
A TrR a r2
(
O°' n
i
~
45,50 45, 50
/
45, 20
.~
45,20
AT f°c) /
5O
(1)
11)
x
J
100
supported risers, but temperature A TCAV are generally 25 ° to 30 ° lower for effective flux 1 kW/m 2 and - 10°C lower for effective flux 0.5 kW/m 2. Table I summarises the total surface average temperatures A TTAV and A~qo values for the operating conditions listed above. The results show that A TTAV values are similarly 25-30°C lower than for the unsupported risers for effective flux 1 kW/m 2 and - 6 ° C lower for effective flux 0.5 kW/m 2. Figure 13 also shows the variation in temperature of the surface of the copper riser tube A TR for the operating conditions listed above. The results are qualitatively similar to those in Fig. 12 for 0 = 45 °, but are somewhat higher particularly for 50°C inlet temperature. At present we have insufficient data to explain this trend. A major problem with manifold M4 both in the unsupported and spring loaded designs, is the large temperature gradient along the riser (Figs. 12 and 13) which indicates relatively inefficient heat transfer to the header pipe, particularly for inclinations near horizontal and vertical. These results may be explained by reference to our previous research dealing with water-in-glass manifolds where heat flow (due to buoyancy effects) along water-filled glass tubes was studied in d e t a i l [ l l , 12]. The parameters which were varied included tube inner radius, inlet water temperature, collector inclination and effective solar flux. The tube length was constant at 1.35 m. For fixed inclination and low inlet water temperatures (Tin - 20°C), the relation
¢ Jr ./
l
P
J
I
•6
J
1.2
X(m) Fig. 13. Graphs of A T as a function of position along the glass absorber tubes (open symbols) and copper riser tubes (closed symbols) of M4 with spring loaded risers. For the glass tubes A T = TcAv and for the copper risers A T = ATR (see caption of Fig. 12). All the results are for effective solar flux 1 kW/mz except the triangles, for which the flux is 0.5 kW/m'-.
was derived, where A TrR is the total temperature drop along a tube (from closed end to fluid in the header pipe), r is the inner radius of the tube and E is the total energy absorbed by the tube (distributed evenly along the length of the riser). This equation derives from the assumption that laminar thermosiphon flow occurs in the inclined tubes. The linear relation A TrR a ~ was observed to break down for high inlet water temperatures (Tin ~ 50°C) where the thermosiphon flow probably becomes turbulent[8]. Figure 14 shows graphs of/x Trn.r 2 vs V ~ for glass tubes of inner radius 9.2 mm, with Tin = 20°C and inner radius 13.8 mm with T~, = 20°C and 50°C. The graphs for T~, = 20°C coincide suggesting that purely laminar flow occurs for both radii. The graph for Tm = 50°C is strongly non-linear as discussed above. Also shown are results obtained in the present work with unsupported and spring loaded copper risers of inner radius 5.6 mm inclined at 45 °, for inlet temperatures 20°C and 50°C. For inlet temperature 20°C, the graphs for both unsupported and spring loaded risers are reasonably linear, but their slopes are considerably larger than for the larger diameter glass tubes, suggesting that some turbulence has developed. The turbulence may develop
Comparative study of fluid-in-metal manifolds for heat extraction
89
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o 13"8
50
[
•
/ ,~/ o Y"/ /
v/
9"2
20
.._.
5'6 / ~ 5'6 L~.RO~ 5"6
20 50 20
E
~
~UR" r
o s.6
50
/ / ~ / /l /' / /, /
8//'/
10
Fig. 14. Graphs ofATTR × r2 vs ~/E where ATTR is the total temperature drop along a water-filled tube, r is the inner radius of the tube and E is the total energy absorbed by the tube. Results for 13.8 mm and 9.2 mm radius are appropriate for glass tubes. Results for 5.6 mm radius are appropriate for copper risers. UR: unsupported risers; SLR: spring loaded risers.
due to the small radius which results in a large temperature gradient along the riser, with consequent high temperatures in the lower portion of the riser. (The results for glass tubes suggest a total temperature difference A TrR of at least 34°C should occur for r -- 5.6 mm even in the case of pure laminar flow.) A further contribution to the relatively large A TrR may occur due to the uniformity of temperature around the circumference of the high thermal conductivity copper riser. Previous experiments with low thermal conductivity glass tubes showed that smaller values of A TrR were obtained when a higher proportion of heat was injected to the upper segment of the tube[l I]. Some recent experiments using low thermal conductivity stainless steel risers have indicated the existence of a temperature gradient around the circumference of the riser, and lower values of A TrR[18]. A higher temperature on the top surface of the tube apparently favours separation of water in the tube into two s t r e a m s - - a downward flowing cooler stream in the lower segment of the tube and an upward flowing warmer stream in the upper segment of the tube[ll]. The A TTR values for T i n = 50°C are higher than for Ti, = 20°C due to increased turbulence in the risers.
40
0~)
'
' 3()
'
e o
' 6'0
'
' 90
Fig. 15. Graphs of A T-rAv( = average surface temperature of a glass tube--average fluid temperature in the manifold) for manifold M4 and A Trn ( = total temperature drop along a copper riser of M4) as a function of collector inclination 0 for manifold inlet water temperature 20°C and effective solar flux 1 kW/m2.
Figure 12 (unsupported risers) shows that for collector inclinations near 0° and 85 °, larger values of ATrn were obtained than for 45 °, with consequent large values of surface temperature for the glass tubes. Figure 15 shows in more detail the variation of the total glass tube surface average temperature ATTAv, and the total temperature drop along the riser A TrR as a function of inclination 0 for flow rate 1 1/min and effective solar flux 1 kW/ m 2. The optimum collector inclination is 0 - 45°. Qualitatively similar results were obtained for water-in-glass manifolds[11]. Near horizontal, thermosiphon flow is impeded by the small vertical height of the collector. Near vertical, the fluid tends to stratify across the tube with resultant elimination of the inlet/outlet streams in the tube. 4.4.2 Manifold M4 with thermosiphon circulation
(unsupported and spring loaded risers) Thermosiphon flow rate as a function of inlet water temperature is shown in Fig. 9 for effective flux 1 kW/m 2 and collector inclination 45°. Considerably higher flow rates are obtained in comparison with M l-M3 due to the low flow impedance of M4. The flow rate is virtually independent of collector inclination 0 because thermosiphon flow through the header pipe is almost independent of the heat transport in the risers. The temperatures measured on the tubes for effective flux I kW/m 2 are similar to those obtained for forced flow (Fig. 11). 5. CONCLUDINGREMARKS A versatile solar simulator has been used to investigate four types of fluid-in-metal manifold used to extract heat from single ended evacuated collectors. The most complex manifold MI has been progressively simplified in various ways with associated progressive increases in thermal impedance
90
Y. ZHIQIANG
between glass absorber tube and heat transfer fluid and increases in efficiency decrement A"qo. Relatively efficient heat transfer and relatively low efficiency decrement I-0.03) is achieved in MI due to close contact of an aluminium fin with the inner surface of the glass tube. The absence of a closefitting fin in M2 and M3 results in considerably higher thermal impedance, and efficiency decrements - 0 . 0 7 and - 0 . 0 9 , respectively, but the results indicate that impedances may be reduced if the arms of the copper U-tube are maintained in contact with the glass tube, for example, by means of a spring metal fin welded to the arms of the Utube. Similar comments apply to manifold M4 which performs poorly (Avl0 -- 0.14) partly due to large air gaps between the metal risers and the glass. The temperature difference between riser and glass can be reduced by spring loading the riser against the upper surface of the glass tube with consequent reduction of A'qo to - 0 . 1 0 . The large temperature gradient along the riser of M4 also contributes significantly to the large temperature differences observed for this manifold. This problem can be reduced only by use of a riser of larger inner radius. The temperature drop along the riser varies (approximately) as I/r2, thus an inner radius of 8 to 10 mm should produce greatly improved heat transfer and efficiencies comparable to or better than M2. In addition, use of risers consisting of lower conductivity metal may reduce the temperature difference. If the water in the risers is allowed to boil, extremely efficient heat flow (with virtually zero temperature gradient along the risers, i.e. A TR 0) should occur up the risers to the header pipe. For the unsupported risers, for a flux of - I kW m 2 the value of b TTAV should thus decrease to -55°C, corresponding to the temperature difference between A TCAVand b TR in Fig. 12. For spring loaded risers, A TTAV should decrease further to -30°C (see Fig. 13 for the temperature difference between bTcAv and ATn). Consequently manifold M4 is a particularly simple and effective manifold for higher temperature (> 100°C) heat extraction. Heat transfer by conduction from glass to manifold within an absorber tube may be calculated relatively easily for M4. The results indicate that heat conduction alone is unable to explain the magnitude of temperature differences between glass and copper risers. Thus convection in the enclosed air contributes significantly to heat transfer for the three manifolds M2, M3 and M4. Radiative heat transfer could contribute to heat flow in M2, M3 and M4 if the copper manifold were coated with a high emittance material. However, the material utilised must withstand temperatures up to 300°C (the approximate stagnation temperature of evacuated collectors) in air, and continual expansion and contraction of the manifold without degrading or peeling from the copper manifold. All the manifolds behave acceptably in the thermosiphoning mode with flow rates - 0 . 3 5 l/min for
MI, M2, M3 and higher flow - 0 . 6 l/min for M4 which has lower impedance. A problem of considerable importance in relation to manifolds is their lifetime, which involves consideration of corrosion problems, tendency to become blocked by scale and sediment and the ease and reliability withh which manifolds can be protected in freezing conditions. Manifold M1 is most susceptible to corrosion, due to contact of the dissimilar metals copper and aluminium. We have observed evidence for corrosion in manifolds stored at ambient temperature less than three months after assembly and severe corrosion of the copper Utube after - 2 months stagnation of collectors. The gaps between glass and aluminium fin and between aluminium fin and copper U-tube could develop high thermal impedance with the accumulation of corrosion products, Use of a heat transfer paste between fin and U-tube may alleviate this problem. Corrosion should be a less serious problem for M2, M3 and M4 because there are no small gaps in which corrosion products may accumulate. Deposition of scale on the inner surfaces of the copper U-tube in M1, M2, M3 may be a serious problem due to the relatively small inner radius of the tube. Reduction of the inner radius by 1 mm or more due to scale, would greatly increase the impedance to fluid flow. Sediment deposition from impure water should not be a serious problem for the U-tubes of M i, M2, M3 because a relatively large uni-directional fluid flow occurs in the tubes, with fluid velocity 0.05 to 0.15 m/s for total flow rates 0.3 (thermosiphoning) to 1.0 1/min. Deposition of sediment in the risers of M4 is a potential problem because the fluid flow pattern is not uni-directional. In our experiments, sediment has been observed to deposit in the riser tubes from mains water in a relatively short time, however, this problem could be alleviated by recirculation of water and by occasionally allowing the water in the risers to boil with consequent probable expulsion of the sediment. Freeze protection of manifolds M l, M2, M3 may be achieved relatively easily by intermittent force flow of warm water through the manifold. In manifold M4 there is no well-defined fluid flow into and out of the risers, however, injection of warm water into the header pipe will achieve freeze protection due to thermosyphoning of near freezing water and ice particles up the copper risers into the header pipe. This effect is discussed in detail elsewhere[19]. At this stage a lack of information regarding mass production costs of the various manifolds and collectors prevents detailed comparisons of cost effectiveness of the manifolds. However, manifold MI is complex and most susceptible to corrosion. Manifold M4 has obvious advantages of extreme simplicity and extremely low flow impedence and manufacture using low cost metal (e.g. mild steel) should allow low cost fabrication of risers of larger diameter with consequent significantly increased
Comparative study of fluid-in-metal manifolds for heat extraction efficiency of heat e x t r a c t i o n w h i c h s h o u l d c o m p a r e f a v o u r a b l y with m a n i f o l d s M2 a n d M3.
Acknowledgements--The authors would like to thank H. Haldane for assistance with manifold construction, D. Mackey for assistance with data-logging and S. Chow for useful discussions. Financial support for the work described here was provided by the New South Wales State Government and by 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. REFERENCES
I. 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 Congr., Los Angeles, California, p. 220. 2. Advertising Literature, Advanced Energy Programs, General Electric Company, Philadelphia, Pennsylvania, U.S.A. 3. Department of Physics, University of Sydney. 4. C.E.N.G., Grenoble, France. 5. Department of Radio Electronics, Tsinghua University, Peking, China. 6. S. P. Chow, G. L. Harding, B. Window and K. J. Cathro, Effect of collector components on the collection efficiency of tubular evacuated collectors with diffuse reflectors. Solar Ener~,,y 32, 251 (1984). 7. S. P. Chow and G. L. Harding, Angular dependence of optical efficiency of evacuated tubular collectors with antireflection coatings and stationary specular reflectors. Solar Energy 34, 489-496. 8. S. P. Chow. G. L. Harding and K. J. Cathro, Optical
91
efficiency of evacuated tubular collectors with envelopes coated with antireflection layers and selective surface of high solar absorptance. Solar Energy tSubmitred (1984)). 9. Advertising Literature, Sunmaster Corporation, Coming, New York, U.S.A. 10. B. Window and G. L. Harding, Buoyancy effects and the manifolding of single ended absorber tubes. Solar Energy 31, 153-157 (1983). I I. Yin Zhiqiang, G. L. Harding and B. Window. Waterin-glass manifolds for heat extraction from evacuated solar collector tubes. Solar Ener~,y 32, 223-230 ( 1984)+ 12. G. L. Harding and Yin Zhiqiang, Thermosiphon circulation in solar water heaters incorporating evacuated tubular collectors and a novel water-in-glass manifold. Solar Ener~,y (To be published (1984)). 13. G. L. Harding, Yin Zhiqiang and D. Mackey, Heat extraction efficiency of concentric glass tubular evacuated collectors. Solar Energy 35, 7 1 - 7 9 (1985). 14. B. Window, Heat extraction from single ended glass absorber tubes. Solar Energy 31, 159-166 (1983). 15. U.S. Patent 4. 120, 285. Modular tubular solar energy collector apparatus. (1978). 16. B. Window and J. Zybert, Optical collection efficiencies of arrays of tubular collectors with diffuse reflectors. Solar Energy 26, 325 ( 1981 ). 17. J. G. Symons, M. K. Peck and L. H. Scott, Flat-plate solar collectors with slat convection suppression devices. Proc. ISES Conference (ANZ Section). University of Queensland (November 1982), p. 103. 18. R. Schmid, Department of Mechanical Engineering. University of Sydney. (Private communication (1983)). 19. G. L. Harding and S. Craig. Evaluation of a simple thermosiphoning hot water system incorporating evacuated collectors. (Submitted to Solar Energy (1983)).
0038-092X/85 $3.00 + .lY0 © 1985Pergamon Press Ltd.
Printed in the U.S.A.
C O M P A R A T I V E STUDY OF F L U I D - I N - M E T A L M A N I F O L D S FOR H E A T EXTRACTION FROM SINGLE E N D E D E V A C U A T E D GLASS T U B U L A R COLLECTORS YIN ZHIQIANG,t G. L. HARDING,~ S. CRAIG a n d R. E. COLLINS
School of Physics, University of Sydney, N.S.W. 2006. Australia. and B. WINDOW CSIRO National Measurement Laboratory, Lindfield. 2070. Australia [Received 19 A u g u s t 1983; revision received 6 D e c e m b e r 1984; accepted 10 D e c e m b e r 1984)
Abstract--An investigation is reported of heat transfer between the glass absorber tubes of all-glass evacuated collectors and fluid-in-metal manifolds designed for heat extraction from the glass asborber tubes. The heat transfer is studied using a novel solar simulator which heats a panel of glass tubes electrically to simulate solar input to a collector panel. Measurement of the temperatures at various points on the glass tubes and on the manifolds gives a measure of the efficiency of heat transfer for each manifold under various operating conditions and allows calculation of the efficiencies "qo of collectors incorporating the manifolds. The effect of fluid temperature, collector inclination and fluid flow rate has been investigated for four manifold designs of increasing simplicity. Experimental results for the manifolds are compared with calculations of heat transfer. Potential lifetime problems for the manifolds are also discussed. The simplest manifold design is shown to have good prospects for hightemperature (>I00°C) heat extraction.
thus allowing the possibility of draindown, as in the Sunmaster design[9] (see Fig. 2). In a recently deAll-glass single-ended concentric tubular evacuated collectors have been intensively researched and de- veloped simplification of the fluid-in-glass concept, collector tubes are horizontal or inclined with open veloped during the past ten years in several laboratories[I-5]. The collectors have associated re- ends up, allowing interconnection via a single header pipe of relatively low cost and complexflectors and a manifold which interconnects the ity[10-12] (see Fig. 3). In all these designs, extubes and circulates heat transfer fluid. The tubular tremely efficient heat transfer is achieved from abcollectors have been almost fully researched and sorbing surface of the evacuated collector tube to developed. Some of the most recent research aimed heat transfer fluid due to intimate contact of fluid at maximizing collector efficiency and cost effecwith the inner surface of the glass absorber tube. tiveness has included a detailed study of the effect of low cost antireflection coatings on the glass en- A major criticism of these designs is that collector velopes[6-8], the effect of selective surfaces of high operation may be interrupted in the event of breakabsorptance (a - 0.96) on collector perform- age of one or more glass collector tubes, with asance[8]. Diffuse and specular reflector designs have sociated loss of heat transfer fluid. In "fluid-in-metal" manifold designs, as in the been intensively studied in the past ten years, but General Electric collector[2] (see Fig. 1), heat transfurther work aimed at producing low-cost longfer fluid is contained within metal pipes which are lived reflectors with associated high collector effiinserted into the absorber tube of each collector, ciency is of considerable importance. Similarly, furand which are interconnected outside the tube. The ther development and testing of manifold concepts is needed with the aim of producing a most-cost- fluid circuit is therefore undamaged by breakage of the collector tubes, however, the heat transfer from effective design with acceptable lifetime. glass absorber tube to heat transfer fluid is less efTwo types of manifold have been developed for ficient due to the relatively high thermal impedglass tubular collectors. In "fluid-in-glass" designs, heat transfer fluid is contained directly in the glass ances associated with the air spaces and multiple interfaces in such designs. absorber tubes, as in the pioneering Owens-Illinois A quantitative indication of the efficiency of heat collector[l] (see Fig. 1). In a refinement of this contransfer can be obtained from measurement of cept, collectors are inclined with open ends down, A TTAv, the difference between the average temperature of the absorbing surface of the collector t Permanent address: Tsinghua University, Beijing, tube and average fluid temperature within the manChina. $ Present address: 29 Duff St. Turramurra 2074, ifold for particular incident sunlight or simulated Australia. sunlight intensity. A high absorber tube surface I. INTRODUCTION
81
82
Y.
ZHIQIANG
Owens Illinois Water-in-Glass
General Electric Water-in-Metal ,6--~
a cuated ollector Tube ~
Glass Feed Tube
Metal U-Tube with metal fin
Fig. 1. Schematic diagrams showing the Owens-Illinois "water-in-glass" manifold and the General Electric "water-in-metal" manifold. Both manifolds involve series connection of tubes.
UUU Collectors
Fig. 3. Schematic diagram of a novel water-in-glass manifold which consists of a single header pipe with connections to absorber tubes which are horizontal or inclined with open ends up. No channelling device is incorporated within the absorber tubes. Heat is transported to the header pipe by buoyancy effects.
temperature relative to fluid temperature results in increased heat loss from the collector tubes by mechanisms such as radiative heat loss from the absorber tube, conduction loss through gas contaminating the vacuum space and conduction loss along the glass tube at the interconnection point of absorber tube and envelope. Figure 4 illustrates the possible heat loss mechanisms from a collector tube. If the average radiation and conduction coefficients associated with a particular evacuated collector tube are known, the decrement in collector efficiency B'qo resulting from the increased heat losses can be determined from values of A TrAy, the number density of evacuated collector tubes and the incident solar flux[13] (A~qo is defined by "q0 ---"qOPT - - AT]owhere ~OPT is the collector optical efficiency and "qo is the collector efficiency for fluid near ambient temperature. The optical efficiency of a standard Sydney University collector is "qOPT = 0.63[6]). The required coefficients have been determined for the Sydney University evacuated collector and it has been shown that, when operating
A
B
D
:'. ,,-4),, // ',. ,. ..
7,-_'+
f Radiation <' Gas Conduction - - , Conductio.n thro: glass ano metal
Fig. 2. Schematic diagram of a drainable water-in-glass manifold developed by Sunmaster corp. U.S.A. A: copper inlet/outlet pipes and copper cup; B: O ring seals; C: small diameter tube functioning as an impedance for flow equalization in parallel connected tubes and as the drain tube; D: glass tube for fluid outlet; E: water; F: evacuated collector tube; G: insulation.
Fig. 4. Schematic diagram of a concentric tubular glass collector showing mechanisms of energy transfer from inner (absorber) tube to envelope. The radiation loss depends on the selective properties of the surface on the absorber tube. Radiation is larger in the region of the tube coated by gettering material. Gas conduction loss depends on the level of vacuum in the tube. A: Envelope: B: Absorber tube; C: Getter coated region of absorber tube; D: Stainless steel support for absorber tube and getter; E: Glass seal (absorber tube to envelope).
83
Comparative study of fluid-in-metal manifolds for heat extraction in a solar flux of - 1 kWm 2, a value of A TTAVof 7 to 8°C results in a decrement A'qo = 0.01 in collector efficiency[13]. Thus the accurate determination ofA T+AVvalues for various manifolds allows the evaluation of the instantaneous efficiencies 11o or ~q(T) of collectors incorporating the manifolds[13]; this information together with information regarding materials and manufacturing costs of the entire collector, and manifold lifetimes, allows an estimate of the relative cost effectiveness of various manifold designs. Preliminary measurements by Window[ 14] have shown that for various fluid-in-metal designs, temperature differences of 20 to 50°C can occur between absorbing surface of the absorber tube, and fluid contained within metal pipes inserted into the absorber tube. Such temperature differences were far greater than those measured for water-in-glass manifolds (usually 5°C - 10°C). At Sydney University, a novel solar simulator has recently been designed and constructed exclusively for the testing of various manifold concepts[10-12]. Fifteen glass tubes, identical to the inner absorber tubes of evacuated collectors, are interconnected via the manifold under test and their outer surfaces directly heated by means of electrically powered strip heaters mounted above and below each tube. In these experiments the absorber tubes have no associated glass envelopes and require no coating with selective surfaces. Figure 5 shows schematically an absorber tube together with
r
m
t
-14m
Fig. 6. Schematic diagram of system used to study thermosiphon flow. A: 120-1 low-pressure storage tank; B: Connecting pipes; C: Collector manifold (M3); D: Glass tubes. C and D are enclosed within insulation in the solar simulator (see Fig. 5). The crosses denote temperature sensors on the manifold and in the tank, which are used to study thermosiphon flow.
heater elements. Heat input via the electric heater strips simulates solar fluxes 0.1- 1.0 kW/m. 2. Thermocouple temperature sensors are placed both on the outer surface of the glass absorber tubes and at various points on the manifold. Relative efficiency of heat transfer from absorber tubes to heat transfer fluid (water) is determined by measurement of temperature differences between glass tubes and the average fluid temperature in the manifold for various simulated solar fluxes, fluid flow rates, fluid inlet temperatures and collector inclinations. Fluid flow is established either by tbrced circulation or by thermosiphon circulation between the collector panel and a 120-1 storage tank (see Fig. 6). This simulator has been used successfully to examine the properties of the water-in-glass manifold shown in Fig. 3, both with forced circulation[ll] and thermosiphon circulation[12] through the header pipe. In this paper we report a similar study of four manifolds of the fluid-in-metal type. We show that most efficient heat transfer from glass absorber to fluid is obtained with a manifold design similar to that of General Electric (Fig. 1). Progressive atFig. 5. Schematic diagram of a portion of the solar sim- tempts to simplify this manifold concept (with the ulator, with glass absorber tube and fluid-in-metal mani- aim of reducing cost and complexity of the manifold. A: Polyurethane insulation; B: Upper stainless steel fold) result in progressively less efficient heat transheater strip; C: Lower stainless steel heater strip: D: Glass absorber tube; E: Manifold (M4); F: Thermocouple tem- fer and larger values of A'qo. Experimental results perature sensors on the surface of the glass tube: G: Ther- for the simplest manifolds are compared with a simple theoretical treatment of heat transfer, and telmocouple temperature sensors on the manifold.
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Y. ZHIQIANG
ative merits and demerits and potential lifetime problems for the manifolds are discussed.
2. MANIFOLDDESIGNS Manifold MI shown in Fig. 7 is similar in concept to the General Electric manifold shown in Fig. 1. The latter manifold consists of a copper U-tube with attached (welded) copper fin which clamps to the inner surface of the absorber tube. In M1 in the interests of reduced materials and assembly costs, the welded copper fin is replaced by a rolled aluminium fin which clips onto one arm of the copper U-tube. In addition, to reduce the flow impedence, collector tubes are connected in parallel with the inclusion of two header pipes, rather than the series connection of General Electric. Copper tubes O.D. = 6 mm are used in order to minimise the materials costs for the manifold. Manifold M2 (Fig. 7) is a simplified version of MI in which the rolled aluminium fin is replaced with a flat copper fin welded to one arm of the Utube. Thus less metal is utilized, the cost of rolling the metal fin is avoided and collector assembly is facilitated by the loose fitting fin. Manifold M3 (Fig, 7) is a simplified version of M2 in which the fin is removed completely, leaving only the copper U-tube within each absorber tube. A similar manifold design investigated by OwensIllinois[15] consists of a U-tube comprising considerably larger diameter metal tubes coated with a material of high thermal emittance (for example, a metal oxide) to maximise radiative heat transfer from glass to U-tube. Manifold M4 (see Fig. 7) consists of a single header pipe attached to metal " r i s e r s " (water-filled copper tubes closed at their lower ends) which are inserted into each absorber tube. Two situations were considered. In one case no mechanism was incorporated for locating the risers in a particular volume within the absorber tube. In this case each riser is assumed to locate (on average), close to the axis of the absorber tube (Fig. 7). In the second
M1
19mm~
M2
M3
M4 --
+. mm -12-5mm
,II I I
~i.l[li I
I
I Ii
© @ © © Fig. 7. Schematic diagrams showing manifolds M l, M2, M3 and M4. MI, M2 and M3 are identical except for the fin design.
case, each riser was clamped to the upper segment of an absorber tube by means of a spring loading mechanism at three positions on the 1.3 m length of each riser. The heat flow pattern expected is similar to that observed in the water-in-glass manifold shown in Fig. 3111, 12]. Buoyancy or thermosiphon effects should induce heat flow up the risers to the header pipe. Uni-directional flow of water occurs through the header pipe due to forced or thermosiphon circulation, but the fluid flow pattern within the risers is less well defined. This type of manifold involves series connection of collector tubes, but exhibits low impedance to fluid flow[l 1].
3. EXPERIMENTAL TECHNIQUES For each of the four manifolds a panel containing fifteen absorber tubes (1.35 m long, 30 mm O.D., 27.6 mm I.D.), was tested. This system is equivalent to a panel of Sydney University collectors of area 1.2 m 2. Two water-flow situations were investigated for each m a n i f o l d - - a forced flow of I 1/min with water inlet temperature 20-50°C, and thermosiphon flow produced with the collector panel connected to a storage tank (Fig. 6) with tank water temperature 20 to 50°C. Collector inclination was varied between 0 ° (horizontal) and 85 ° (near vertical), In each case the header pipes of the manifolds were uppermost, allowing air and exsolved gases to be easily cleared from the metal tubes and in the case of M4 allowing buoyancy effects to transport heat to the header pipe. Thick polyurethane insulation around the heaters and tubes results in 95% of the power input being removed via fluid in the manifold when steady state conditions are achieved. The heat balance in the simulator was determined by accurately measuring the water flow rate through a given manifold and measuring the inlet and outlet water temperatures when steady state was achieved for various manifolds and total electrical power inputs for collector tube surfaces running at -50°C above ambient, - 9 5 % of the electric power input was removed via the fluid in the manifolds, and - 5 % was lost by conduction through the thick polyurethane insulation around the heaters and tubes. Thus in most experiments where A TTAV is -<50°C, values of surface temperature of the glass tubes (and values of A~lo) should be accurate to within - 5 % . In most cases the random errors in the tube surface temperatures are greater than 5%. To simulate a solar flux of 1 kW/m 2 (as used in most experiments), a total electric power of 0.72 kW was applied to the heater strips in the simulator, with twice as much power supplied to the strips above the tubes, compared to the strips below the tubes (Fig. 5). This simulates the total absorbed energy and energy distribution around collector tubes in a panel with w h i t e diffuse rear reflector and efficiency ~q - 0.6[7, 11, 16]. Temperature sensors were located on the inlet
Comparative study of fluid-in-metal manifolds for heal extraction and outlet header pipes to determine water inlet/ outlet temperatures (T~,/To,t, respectively). In the case of thermosiphon induced flow, the temperature difference between inlet and outlet was used to determine the total flow rate through the panel[12]. To determine temperature distributions on the glass tubes, thermocouples were mounted on the top, side and bottom of the tubes and at six positions along their length (see Figs. 5 and 6). The parameter of interest for comparing heat extraction efficiencies is the difference in temperature (A T) between a given point on a glass absorber tube and the average fluid temperature in the manifold. In the case of M I-M3 (tubes parallel connected) the latter temperature is the mean of T~, and To,,. For M4 (tubes series connected), the latter temperature is that of water in the single header pipe at the connection point of the tube of interest, and may be calculated assuming a linear variation of temperature along the header pipe of M4. a T values obtained from positions around the circumference of the tubes were used to evaluate A TcAv--an average value around the circumference of a tube at a given distance along the tube. A TcAv values obtained from positions distributed along the tube length and around the tube circumference were used to evaluate A TTAv--an average value for the entire surface area of the tube. To investigate in detail the heat transfer along the risers of M4, temperature sensors were also placed at six positions on several of the metal risers within the absorber tubes.
4. RESULTS AND DISCUSSION
4.1.1 Manifold M1 with forced circulation Figure 8 shows the temperature (AT) distribution around the circumference of absorber tubes near their domed ends and average values around the circumference (A TCAV) as a function of position along the tubes. The total surface average (A TTAV) for an entire tube is listed in Table 1 together with
85
• )
L,
100
,L
AT(°C/
~__,
i
24-'3
~'
~"-~~M1~11_,
~50 <] 3~7
2 22~2
L
%
'
' -6 ' X(ml
1:2
Fig. 8. Graphs of A TC'AV ( = average temperature around the circumference of a glass tube--average fluid temperature in the manifold) as a function of position X on the absorber tube for manifolds MI. M2. M3. Also shown are AT values (= temperature at a point on the absorber tube--average fluid temperature in the manifold) on the top. side and bottom of the absorber tubes measured close to the domed ends. For MI. solid triangles: Forced flow: Open triangles: Thermosiphon flow.
the efficiency decrement Arlo. These results were obtained for flow rate I I/min and effective flux I kW/m 2 and were independent (within experimental uncertainties) of collector inclination (0 to 85 °) and water inlet temperature (20 to 54°C). Highest temperatures occur on the top surface of the tubes where there is highest energy input to the glass. Our results are similar to those reported by Window[ 14]. Window observed that introducing heat transfer compound into the gap between copper U-tube and aluminium fin reduced A Tc'Av by about 4°C indi-
Table 1. Values of A TT^v (average temperature of an absorber tube surface - average temperature of fluid in the manifold) and estimated efficiency decrement A-qofor fluid temperature near ambient, for manifolds MI-M4 and various operating conditions. Manifold M1 M2 M3 M4 Unsupported risers Spring loaded risers
Inclination (degrees)
Inlet temperature (°C)
Effective flux (kW/m2)
Flow rate (l/min)
0---, 85 0---~ 85 0--* 85 "0 45 45 45 84 45 45 45
20 ~ 50 20 ~ 50 20 ~ 50 20 20 50 25 20 20 50 20
1.0 1.0 1.0 1.0 1.0 1.0 0.5 1.0 1.0 1.0 0.5
0.35 and 1.0 0.35 and 1.0 0.35 and 1.0 1.0 0.6 and 1.0 0.6 and 1.0 0.43 1.0 1.0 1.0 1.0
ATTAV (°C) 21 54 66 122 102 98 54 120 71 72 48
= 3 _ 5 ___ 7 +_ 10 ~ 10 +__ 10 ± 5 - 10 _ 10 ± 10 ___ 5
A'qo 0.03 0.07 0.09 0.17 0.14 0.13 0.14 0.16 0.16 0.10 0.13
Y. ZHIQIANG
86
cating that the absence of a weld between U-tube and fin does not contribute greatly to A TeAr. 4.1.2 Manifold MI with thermosiphon circldation Thermosiphon flow rate as a function of inlet water temperature is shown in Fig. 9 for effective flux 1 kW/m 2 and collector inclination 45 °. The flow rate increases with increasing inlet temperature due to the effects of decreasing water viscosity and the increasing rate of density change of water as temperature increases. Thermosiphon flow occurs due to a pressure head developing in both the copper U-tubes and in the pipes connecting collector and tank (Fig. 6). The pressure head in the U-tubes should be approximately proportional to the vertical height of the collector which is proportional to sin 0, (where 0 is the inclination). Figure 10 shows the normalised flow rate f/f(O = 45 °) as a function of 0. Flow rate increases with 0 as expected. The temperature (2~ T) values measured on the tubes are similar to those obtained for forced flow (Fig. 8). 4.2. I Manifold M2 with fi~rced circulation Figure 8 shows the temperature (A T) distribution around the circumference of absorber tubes near their domed ends, and average values around the circumference (2~ TCAV) as a function of position along the tubes. The total surface average (A TTAV) and A'qo are listed in Table 1. These results, obtained for flow rate I l/min and effective flux I kW/ m z, were independent of collector inclination (0 to 85 °) and inlet water temperature (20 to 54°C). The temperatures z~Tare considerably higher than those measured for M I due to large air gaps between the inner surface of the absorber tube and copper Utube and fin. The temperature 2~TCAV peaks in the centre of the tube (Fig. 8) probably because the arms of the U-tube tend to be displaced furthest from the glass in this region. Near the domed end and open end of the absorber tube the U-bend in the copper tube and connections to the header pipes, respectively, tend to maintain closer proximity of the tube and glass, reducing the impedance to heat flow. The copper tube and fin have relatively low emit-
.c_.8
o.4 U.
0
20
30 4() ~n(°C)
5~0
Fig. 9. Graphs of thermosiphon flow rate versus manifold inlet water temperature (T~,) for manifolds MI, M2 and M4 with tube inclination 45° and effective flux I kW/m 2. Squares: M I; Circles: M2; Triangles: M4.
1.0 j.._t-------"--
:)
30
60
90
0 ° Fig. 10. Normalised thermosiphon flow rate f/f(O = 45°) versus collector inclination 0 for a collector with manifold M1, connected to the 120-1 storage tank. The results are appropriate for effective flux 1 kW/m2 and inlet water temperature -20°C. Similar results are obtained for manifolds M2, M3. tance (<0.05) therefore radiation plays a relatively small role in the heat transfer. 4.2,2 Manifold M2 with thermosiphon circulation Both thermosiphon flow rate as a function of water inlet temperature and flow rate as a function of angle are nearly identical to results for M 1 shown in Figs. 9 and 10. This is expected because the basic water flow path (header pipes plus U-tubes) are identical for M I and M2. The temperature distributions on the tubes are identical within experimental uncertainties to those obtained for forced flow. 4.3,1 Manifold M3 with forced circulation Figure 8 shows the temperature (AT) distribution around the circumference of absorber tubes near their domed ends and average values around the circumference (A TcAv) as a function of position along the tubes. The total surface average (A TVAV) and A'qo are listed in Table 1. These results, obtained for flow rate 1 l/min and effective flux 1 kW/ m 2, were independent of collector inclination (0 to 85 °) and water inlet temperature (20 to 50°C). Quantitatively similar results were reported by Window[ 14] for this manifold. The temperatures A TcAv for M3 are comparable to those for M2 near the domed end and open end of the glass tube where the arms of the U-tube are maintained close to the glass (see section 4.2.1). The slightly higher values for M3 in these positions are a result of the smaller surface area of manifold M3 compared with M2. The large peak in A TCAV in the centre of the tube occurs due to the absence of a support for the upper arm of the U-tube, with resulting increased physical separation of glass and metal tube, and consequently higher thermal impedance. These results suggest that the advantage of the copper fin in M2 is primarily that it supports the upper arm of the Utube relatively close to the glass. The average glass tube temperature A TVAV for M3 is therefore larger than for M2 (Table 1). 4.3.2 Manifold M3 with thermosiphon circulation Results for thermosiphon flow are similar to re-
87
Comparative study of fluid-in-metal manifolds for heat extraction suits for M1 and M2 (Figs. 9 and 10) due to the basic similarity of the three manifold designs. 4.4.1 Manifold M4 with forced circulation A. No riser location mechanism. Figure l la shows the temperature (A T) distribution around the circumference of absorber tubes near their domed ends for flow 1 l/min, effective fluxes 1 kW/m 2 and 0.5 kW/m 2, collector inclination 45 ° and inlet water temperature 20°C. For effective flux 1 kW/m 2, and conditions such as higher inlet water temperature or inclinations near 0° or 85° the temperatures are higher but the temperature on the top surface of the tube is invariably 50°C higher than the side and bottom. Figure 12 shows the average temperatures around the circumference (A TCAV) as a function of position along the tubes for effective flux 1 kW/m 2, inclinations 0°, 45 ° and 84° with inlet water temperature 20° , and inclination 45 ° with inlet water temperature 50°C. For collector inclination 45 ° and inlet water temperature 20°C, an effective flux of 0.5 kW/m 2 has also been investigated. Table I summarises the total surface average temperatures A TTAV and Mqo for the operating conditions listed above. Figure 12 also shows the variation in temperature A TR ( = temperature of riser surface--temperature of water in the header pipe at the connection point of the riser), for the operating conditions described above. Negligible temperature variation
AT(°C) M a n i f o l d M 4 84"-10
140'-10
(a)
6 0"6
65-+4
•5 k W r r i 2
8 10
90+-10
(b)
95+-10
lkWm 2
Fig. 11. Temperatures (AT(°C) = temperature on glass surface--average fluid temperature in manifold M4) on the top, side and bottom of the glass tube, measured close to the domed end for collector inclination 45°, inlet water temperature 20°C, and two effective solar fluxes 0.5 and 1.0 kW/m2. (a) Unsupported risers; (b) spring loaded risers.
•
e"
j)
84,20 45,50 45,20 84,20 O, 20
AT ¢C )
45,20
50
0-5 kWm 2 ~I, 45,20
L
X(m) Fig. 12. Graphs of AT as a function of position along the glass absorber tubes (open symbols) and copper riser tubes (closed symbols) of M4 with unsupported risers. For the glass tubes AT = TcAv (= average temperature around the circumference of glass tube--average fluid temperature in the manifold). For the copper risers, AT = ATR (= temperature on the riser--average fluid temperature in the manifold). For each curve, the first number is the collector inclination in degrees, the second number is the inlet water temperature. All the results are for effective solar flux 1 kW/m2 except the circles with crosses, for which the flux is 0.5 kW/m2.
occurs around the circumference of the risers due to the high conductivity of copper. The results for A TR exhibit the same trends as temperature A TCAV of the glass tubes, but with absolute values about 55°C lower for effective flux 1 kW/m 2 and -30°C lower for effective flux 0.5 kW/m 2. The large temperature differences between glass absorber and copper riser tubes (particularly on the top surface of the glass where the temperature difference is -90°C for effective flux 1 kW/m 2) occur because the risers with no location mechanism are mounted approximately centrally within the absorber tube (see Fig. 7) with an air gap of - 7 mm between glass and riser. If heat transfer is assumed to occur across this gap exclusively via conduction, the average temperature difference expected between glass and riser is -150°D. The relatively low value measured ( - 5 5 ° C - - s e e Fig. 12) and the negligible contribution from radiative heat transfer, suggest that con-
88
Y. ZHIQIANG
vection currents in the air column enclosed within the absorber tube must contribute significantly to heat transfer. Recent work by Symons et a/.[17] has shown that convection currents are established in similar air columns. Convection may similarly play an important role in heat transfer for manifolds M2 and m3. B. Risers spring loaded against upper segment of absorber tube. Figure 1lb shows the temperature (A T) distribution around the circumference of absorber tubes near their domed ends for flow 1 1/ min, effective fluxes 1 kW/m 2 and 0.5 kW/m z, collector inclination 45 ° and inlet water temperature 20°C. For both 0.5 and 1 kW/m 2 the temperatures near the top and side surfaces of the absorber tube have been depressed (compared with Fig. ! la) due to the close proximity of the riser to the top of the tube where the highest energy flux occurs, and the temperatures near the bottom surface of the absorber tubes have increased due to the correspondingly larger spacing of riser and absorber tube surface. Figure 13 shows the average temperatures around the circumference (A TcAv) as a function of position along the tubes for effective flux 1 kW/m 2, inclination 45 ° and inlet water temperature 20°C and 50°C; and effective flux 0.5 kW/m 2, inclination 45 ° and inlet water temperature 20°C. The results are qualitatively similar to those in Fig. 12 for the un-
A TrR a r2
(
O°' n
i
~
45,50 45, 50
/
45, 20
.~
45,20
AT f°c) /
5O
(1)
11)
x
J
100
supported risers, but temperature A TCAV are generally 25 ° to 30 ° lower for effective flux 1 kW/m 2 and - 10°C lower for effective flux 0.5 kW/m 2. Table I summarises the total surface average temperatures A TTAV and A~qo values for the operating conditions listed above. The results show that A TTAV values are similarly 25-30°C lower than for the unsupported risers for effective flux 1 kW/m 2 and - 6 ° C lower for effective flux 0.5 kW/m 2. Figure 13 also shows the variation in temperature of the surface of the copper riser tube A TR for the operating conditions listed above. The results are qualitatively similar to those in Fig. 12 for 0 = 45 °, but are somewhat higher particularly for 50°C inlet temperature. At present we have insufficient data to explain this trend. A major problem with manifold M4 both in the unsupported and spring loaded designs, is the large temperature gradient along the riser (Figs. 12 and 13) which indicates relatively inefficient heat transfer to the header pipe, particularly for inclinations near horizontal and vertical. These results may be explained by reference to our previous research dealing with water-in-glass manifolds where heat flow (due to buoyancy effects) along water-filled glass tubes was studied in d e t a i l [ l l , 12]. The parameters which were varied included tube inner radius, inlet water temperature, collector inclination and effective solar flux. The tube length was constant at 1.35 m. For fixed inclination and low inlet water temperatures (Tin - 20°C), the relation
¢ Jr ./
l
P
J
I
•6
J
1.2
X(m) Fig. 13. Graphs of A T as a function of position along the glass absorber tubes (open symbols) and copper riser tubes (closed symbols) of M4 with spring loaded risers. For the glass tubes A T = TcAv and for the copper risers A T = ATR (see caption of Fig. 12). All the results are for effective solar flux 1 kW/mz except the triangles, for which the flux is 0.5 kW/m'-.
was derived, where A TrR is the total temperature drop along a tube (from closed end to fluid in the header pipe), r is the inner radius of the tube and E is the total energy absorbed by the tube (distributed evenly along the length of the riser). This equation derives from the assumption that laminar thermosiphon flow occurs in the inclined tubes. The linear relation A TrR a ~ was observed to break down for high inlet water temperatures (Tin ~ 50°C) where the thermosiphon flow probably becomes turbulent[8]. Figure 14 shows graphs of/x Trn.r 2 vs V ~ for glass tubes of inner radius 9.2 mm, with Tin = 20°C and inner radius 13.8 mm with T~, = 20°C and 50°C. The graphs for T~, = 20°C coincide suggesting that purely laminar flow occurs for both radii. The graph for Tm = 50°C is strongly non-linear as discussed above. Also shown are results obtained in the present work with unsupported and spring loaded copper risers of inner radius 5.6 mm inclined at 45 °, for inlet temperatures 20°C and 50°C. For inlet temperature 20°C, the graphs for both unsupported and spring loaded risers are reasonably linear, but their slopes are considerably larger than for the larger diameter glass tubes, suggesting that some turbulence has developed. The turbulence may develop
Comparative study of fluid-in-metal manifolds for heat extraction
89
30 b
•
('C /
? * 13"8
20
; ~/
~_
o 13"8
50
[
•
/ ,~/ o Y"/ /
v/
9"2
20
.._.
5'6 / ~ 5'6 L~.RO~ 5"6
20 50 20
E
~
~UR" r
o s.6
50
/ / ~ / /l /' / /, /
8//'/
10
Fig. 14. Graphs ofATTR × r2 vs ~/E where ATTR is the total temperature drop along a water-filled tube, r is the inner radius of the tube and E is the total energy absorbed by the tube. Results for 13.8 mm and 9.2 mm radius are appropriate for glass tubes. Results for 5.6 mm radius are appropriate for copper risers. UR: unsupported risers; SLR: spring loaded risers.
due to the small radius which results in a large temperature gradient along the riser, with consequent high temperatures in the lower portion of the riser. (The results for glass tubes suggest a total temperature difference A TrR of at least 34°C should occur for r -- 5.6 mm even in the case of pure laminar flow.) A further contribution to the relatively large A TrR may occur due to the uniformity of temperature around the circumference of the high thermal conductivity copper riser. Previous experiments with low thermal conductivity glass tubes showed that smaller values of A TrR were obtained when a higher proportion of heat was injected to the upper segment of the tube[l I]. Some recent experiments using low thermal conductivity stainless steel risers have indicated the existence of a temperature gradient around the circumference of the riser, and lower values of A TrR[18]. A higher temperature on the top surface of the tube apparently favours separation of water in the tube into two s t r e a m s - - a downward flowing cooler stream in the lower segment of the tube and an upward flowing warmer stream in the upper segment of the tube[ll]. The A TTR values for T i n = 50°C are higher than for Ti, = 20°C due to increased turbulence in the risers.
40
0~)
'
' 3()
'
e o
' 6'0
'
' 90
Fig. 15. Graphs of A T-rAv( = average surface temperature of a glass tube--average fluid temperature in the manifold) for manifold M4 and A Trn ( = total temperature drop along a copper riser of M4) as a function of collector inclination 0 for manifold inlet water temperature 20°C and effective solar flux 1 kW/m2.
Figure 12 (unsupported risers) shows that for collector inclinations near 0° and 85 °, larger values of ATrn were obtained than for 45 °, with consequent large values of surface temperature for the glass tubes. Figure 15 shows in more detail the variation of the total glass tube surface average temperature ATTAv, and the total temperature drop along the riser A TrR as a function of inclination 0 for flow rate 1 1/min and effective solar flux 1 kW/ m 2. The optimum collector inclination is 0 - 45°. Qualitatively similar results were obtained for water-in-glass manifolds[11]. Near horizontal, thermosiphon flow is impeded by the small vertical height of the collector. Near vertical, the fluid tends to stratify across the tube with resultant elimination of the inlet/outlet streams in the tube. 4.4.2 Manifold M4 with thermosiphon circulation
(unsupported and spring loaded risers) Thermosiphon flow rate as a function of inlet water temperature is shown in Fig. 9 for effective flux 1 kW/m 2 and collector inclination 45°. Considerably higher flow rates are obtained in comparison with M l-M3 due to the low flow impedance of M4. The flow rate is virtually independent of collector inclination 0 because thermosiphon flow through the header pipe is almost independent of the heat transport in the risers. The temperatures measured on the tubes for effective flux I kW/m 2 are similar to those obtained for forced flow (Fig. 11). 5. CONCLUDINGREMARKS A versatile solar simulator has been used to investigate four types of fluid-in-metal manifold used to extract heat from single ended evacuated collectors. The most complex manifold MI has been progressively simplified in various ways with associated progressive increases in thermal impedance
90
Y. ZHIQIANG
between glass absorber tube and heat transfer fluid and increases in efficiency decrement A"qo. Relatively efficient heat transfer and relatively low efficiency decrement I-0.03) is achieved in MI due to close contact of an aluminium fin with the inner surface of the glass tube. The absence of a closefitting fin in M2 and M3 results in considerably higher thermal impedance, and efficiency decrements - 0 . 0 7 and - 0 . 0 9 , respectively, but the results indicate that impedances may be reduced if the arms of the copper U-tube are maintained in contact with the glass tube, for example, by means of a spring metal fin welded to the arms of the Utube. Similar comments apply to manifold M4 which performs poorly (Avl0 -- 0.14) partly due to large air gaps between the metal risers and the glass. The temperature difference between riser and glass can be reduced by spring loading the riser against the upper surface of the glass tube with consequent reduction of A'qo to - 0 . 1 0 . The large temperature gradient along the riser of M4 also contributes significantly to the large temperature differences observed for this manifold. This problem can be reduced only by use of a riser of larger inner radius. The temperature drop along the riser varies (approximately) as I/r2, thus an inner radius of 8 to 10 mm should produce greatly improved heat transfer and efficiencies comparable to or better than M2. In addition, use of risers consisting of lower conductivity metal may reduce the temperature difference. If the water in the risers is allowed to boil, extremely efficient heat flow (with virtually zero temperature gradient along the risers, i.e. A TR 0) should occur up the risers to the header pipe. For the unsupported risers, for a flux of - I kW m 2 the value of b TTAV should thus decrease to -55°C, corresponding to the temperature difference between A TCAVand b TR in Fig. 12. For spring loaded risers, A TTAV should decrease further to -30°C (see Fig. 13 for the temperature difference between bTcAv and ATn). Consequently manifold M4 is a particularly simple and effective manifold for higher temperature (> 100°C) heat extraction. Heat transfer by conduction from glass to manifold within an absorber tube may be calculated relatively easily for M4. The results indicate that heat conduction alone is unable to explain the magnitude of temperature differences between glass and copper risers. Thus convection in the enclosed air contributes significantly to heat transfer for the three manifolds M2, M3 and M4. Radiative heat transfer could contribute to heat flow in M2, M3 and M4 if the copper manifold were coated with a high emittance material. However, the material utilised must withstand temperatures up to 300°C (the approximate stagnation temperature of evacuated collectors) in air, and continual expansion and contraction of the manifold without degrading or peeling from the copper manifold. All the manifolds behave acceptably in the thermosiphoning mode with flow rates - 0 . 3 5 l/min for
MI, M2, M3 and higher flow - 0 . 6 l/min for M4 which has lower impedance. A problem of considerable importance in relation to manifolds is their lifetime, which involves consideration of corrosion problems, tendency to become blocked by scale and sediment and the ease and reliability withh which manifolds can be protected in freezing conditions. Manifold M1 is most susceptible to corrosion, due to contact of the dissimilar metals copper and aluminium. We have observed evidence for corrosion in manifolds stored at ambient temperature less than three months after assembly and severe corrosion of the copper Utube after - 2 months stagnation of collectors. The gaps between glass and aluminium fin and between aluminium fin and copper U-tube could develop high thermal impedance with the accumulation of corrosion products, Use of a heat transfer paste between fin and U-tube may alleviate this problem. Corrosion should be a less serious problem for M2, M3 and M4 because there are no small gaps in which corrosion products may accumulate. Deposition of scale on the inner surfaces of the copper U-tube in M1, M2, M3 may be a serious problem due to the relatively small inner radius of the tube. Reduction of the inner radius by 1 mm or more due to scale, would greatly increase the impedance to fluid flow. Sediment deposition from impure water should not be a serious problem for the U-tubes of M i, M2, M3 because a relatively large uni-directional fluid flow occurs in the tubes, with fluid velocity 0.05 to 0.15 m/s for total flow rates 0.3 (thermosiphoning) to 1.0 1/min. Deposition of sediment in the risers of M4 is a potential problem because the fluid flow pattern is not uni-directional. In our experiments, sediment has been observed to deposit in the riser tubes from mains water in a relatively short time, however, this problem could be alleviated by recirculation of water and by occasionally allowing the water in the risers to boil with consequent probable expulsion of the sediment. Freeze protection of manifolds M l, M2, M3 may be achieved relatively easily by intermittent force flow of warm water through the manifold. In manifold M4 there is no well-defined fluid flow into and out of the risers, however, injection of warm water into the header pipe will achieve freeze protection due to thermosyphoning of near freezing water and ice particles up the copper risers into the header pipe. This effect is discussed in detail elsewhere[19]. At this stage a lack of information regarding mass production costs of the various manifolds and collectors prevents detailed comparisons of cost effectiveness of the manifolds. However, manifold MI is complex and most susceptible to corrosion. Manifold M4 has obvious advantages of extreme simplicity and extremely low flow impedence and manufacture using low cost metal (e.g. mild steel) should allow low cost fabrication of risers of larger diameter with consequent significantly increased
Comparative study of fluid-in-metal manifolds for heat extraction efficiency of heat e x t r a c t i o n w h i c h s h o u l d c o m p a r e f a v o u r a b l y with m a n i f o l d s M2 a n d M3.
Acknowledgements--The authors would like to thank H. Haldane for assistance with manifold construction, D. Mackey for assistance with data-logging and S. Chow for useful discussions. Financial support for the work described here was provided by the New South Wales State Government and by 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. REFERENCES
I. 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 Congr., Los Angeles, California, p. 220. 2. Advertising Literature, Advanced Energy Programs, General Electric Company, Philadelphia, Pennsylvania, U.S.A. 3. Department of Physics, University of Sydney. 4. C.E.N.G., Grenoble, France. 5. Department of Radio Electronics, Tsinghua University, Peking, China. 6. S. P. Chow, G. L. Harding, B. Window and K. J. Cathro, Effect of collector components on the collection efficiency of tubular evacuated collectors with diffuse reflectors. Solar Ener~,,y 32, 251 (1984). 7. S. P. Chow and G. L. Harding, Angular dependence of optical efficiency of evacuated tubular collectors with antireflection coatings and stationary specular reflectors. Solar Energy 34, 489-496. 8. S. P. Chow. G. L. Harding and K. J. Cathro, Optical
91
efficiency of evacuated tubular collectors with envelopes coated with antireflection layers and selective surface of high solar absorptance. Solar Energy tSubmitred (1984)). 9. Advertising Literature, Sunmaster Corporation, Coming, New York, U.S.A. 10. B. Window and G. L. Harding, Buoyancy effects and the manifolding of single ended absorber tubes. Solar Energy 31, 153-157 (1983). I I. Yin Zhiqiang, G. L. Harding and B. Window. Waterin-glass manifolds for heat extraction from evacuated solar collector tubes. Solar Ener~,y 32, 223-230 ( 1984)+ 12. G. L. Harding and Yin Zhiqiang, Thermosiphon circulation in solar water heaters incorporating evacuated tubular collectors and a novel water-in-glass manifold. Solar Ener~,y (To be published (1984)). 13. G. L. Harding, Yin Zhiqiang and D. Mackey, Heat extraction efficiency of concentric glass tubular evacuated collectors. Solar Energy 35, 7 1 - 7 9 (1985). 14. B. Window, Heat extraction from single ended glass absorber tubes. Solar Energy 31, 159-166 (1983). 15. U.S. Patent 4. 120, 285. Modular tubular solar energy collector apparatus. (1978). 16. B. Window and J. Zybert, Optical collection efficiencies of arrays of tubular collectors with diffuse reflectors. Solar Energy 26, 325 ( 1981 ). 17. J. G. Symons, M. K. Peck and L. H. Scott, Flat-plate solar collectors with slat convection suppression devices. Proc. ISES Conference (ANZ Section). University of Queensland (November 1982), p. 103. 18. R. Schmid, Department of Mechanical Engineering. University of Sydney. (Private communication (1983)). 19. G. L. Harding and S. Craig. Evaluation of a simple thermosiphoning hot water system incorporating evacuated collectors. (Submitted to Solar Energy (1983)).