- Email: [email protected]
Elastomer–metal–absorber: development and application
Pergamon
PII:
Solar Energy Vol. 67, Nos. 4–6, pp. 215–226, 1999 2000 Elsevier Science Ltd S 0 0 3 8 – 0 9 2 X ( 0 0 ) 0 0 0 7 2 – 4 All rights reserved. Printed in Great Britain 0038-092X / 99 / $ - see front matter
www.elsevier.com / locate / solener
ELASTOMER–METAL–ABSORBER: DEVELOPMENT AND APPLICATION BERND BARTELSEN*, GUNTER ROCKENDORF* , †, NORBERT VENNEMANN**, RAINER TEPE***, KLAUS LORENZ*** and GOTTFRIED PURKARTHOFER**** ¨ Solarenergieforschung GmbH, Am Ohrberg 1, D-31860 Emmerthal, Germany *Institut fur ¨ ¨ **Fachhochschule Osnabruck, Albrechtstr. 30, D-49076 Osnabruck, Germany ¨ ***Solar Energy Research Centre, Dalarna University College, S-78188 Borlange, Sweden ****Arbeitsgemeinschaft Erneuerbare Energie, A-8200 Gleisdorf, Austria Received 3 August 1999; revised version accepted 9 April 2000 Communicated by BRIAN NORTON
Abstract—A new principle of a solar collector that consists of appropriately shaped metal form plates as absorbers and clipped in elastomer fluid pipes, the so-called elastomer–metal–absorber, will be presented. The advantages are its freeze resistance, its seawater suitability and new possibilities for simplified collector installation and system techniques. The design parameters including a detailed analysis of the thermal resistance between the absorber and fluid will be discussed, where special regard is given to the development of an appropriate elastomer material with high thermal conductivity as one of the key items. The first development steps have shown that absorbers with a high thermal performance may be constructed. Finally, the idea of application of the principle of the elastomer–metal–absorber to metal roofs and fac¸ades will be presented. This idea is followed up within a development project. 2000 Elsevier Science Ltd. All rights reserved.
1. INTRODUCTION
The idea of a combined absorber with a metal absorber sheet for the absorption of the solar radiation and a flexible elastomer fluid pipe for the transport of the solar heat has been developed (Bartelsen et al., 1996). As shown in Fig. 1, a round shaped clip profile is integrated into a metal plate, which has an absorption layer for solar thermal conversion. In this profile an elastomer tube for the heat removal is clipped in. The application of this elastomer– metal–absorber in solar thermal collectors offers the following potential advantages and essential possibilities: • Due to its expected inherent freeze resistance, operation without an antifreeze additive seems to be possible. • System installation without heat exchanger in the solar loop may be discussed. • Operation with a corrosive fluid is possible, e.g. direct flow with sea or brackish water. • New and simplified techniques for the collector and system installation can be developed. The most promising application results from †
Author to whom correspondence should be addressed. Tel. 149-5151-999-521; fax 149-5151-999-500; e-mail: [email protected]
Fig. 1. Different design types of the elastomer–metal–absorber. 215
216
B. Bartelsen et al.
the new installation possibilities for the collector and the system. It is intended to integrate this new collector concept into roofs and fac¸ades made out of metal form sheet elements. Furthermore, the elastomer–metal–absorber concept seems to be an attractive collector for the solar desalination of brackish and sea water, as the collector may be operated directly with corrosive liquids without cost intensive corrosion protected heat exchangers. The desalination process should be designed to operate on a low temperature level (e.g. around 708C). The use of elastomer tubes in collectors requires appropriate absorber constructions. Different constructions have been developed and investigated with regard to the internal heat transfer resistance in combination with freeze–thaw cycles. These absorbers have been integrated in solar collector prototypes, and thus the thermal performance and the reliability have been examined. A high thermal efficiency may only be achieved with a low heat transfer resistance of the complete construction. In order to minimize this thermal resistance between absorber and fluid, the low thermal conductivity of standard elastomer material has to be improved. Therefore different elastomer mixtures with significantly higher thermal conductivities and acceptable mechanical properties have been developed and investigated. In the following, the results of the development and analysis work concerning the collector development will be presented, and future applications will be discussed. 2. COLLECTOR DESIGN
It is evident that the low thermal conductivity of normal elastomer material results in a high thermal resistance between absorber and fluid, which lowers the thermal performance of a collector with this design. For this reason, a theoretical study of the absorber heat transfer had to be performed first. The results of appropriate numerical calculations have led to the following conclusions (Bartelsen et al., 1996): • A direct contact of the metal absorber fin with the elastomer tube is necessary, no additional adhesive or contact material should be used. • The thermal conductivity of about 0.25 W/ mK for standard black elastomer material (Anderson, 1966; Hands, 1977) should be increased to a value of around 0.7–1.0 W/ mK. • The contact area between the absorber fin and elastomer tube must be large.
• The wall thickness of the tube should be small. • Finally, the diameter of the tube should be large. The last two requirements are in contradiction to the necessary strength of the elastomer tube at operation pressure. Of special importance for a high thermal performance is the contact between the metal absorber and the elastomer tube. Therefore, during the first development steps, different absorber fin– tube configurations, in the following called absorber strips, as well as different collector prototypes have been investigated with special regard to the heat transfer characteristics, thermal performance and reliability. Fig. 1 shows five different constructions of realized absorber shapes, which have been investigated up to now. Type ‘A’ is a typically soldered or welded absorber construction, which has been used for the first experiments. Type ‘B’ is an absorber construction out of roll bended aluminium sheets. The clip profile, which embraces the elastomer tube, is integrated in the sheet, thus no welding or soldering is necessary. Type ‘C’ and type ‘D’ are aluminium roll shaped constructions, which are used as absorbers in typical thermal collectors. Normally a copper fluid tube instead of the elastomer tube is used in the clip profile. Type ‘E’ is a specially developed absorber constructed out of roll bended aluminium sheets. This clip profile is an improvement of types ‘A’– ‘D’ and takes the capabilities of a roll-form machine for 1-mm thick aluminium sheets into account. These different constructions have been used as absorbers in collector prototypes for the measurement of the thermal performance and as single absorber strips for the investigation of the internal heat transfer conductance. 3. ANALYSIS OF INTERNAL THERMAL RESISTANCE
The efficiency of a solar collector mainly depends on the quality of the absorber. Beside the absorption and the emission of the coating, the capability to transfer the heat from the absorber to the fluid is important (Duffie and Beckmann, 1991; Rockendorf et al., 1996). Fig. 2 shows the thermal resistance network of a typical absorber strip. The heat has to pass four single resistances on its way to the fluid. These are the resistance of the
Elastomer–metal–absorber: development and application
217
Fig. 2. Simplified thermal steady-state model of absorber fin–tube configuration (heat losses are not included).
absorber fin and of the base connection, the tube wall resistance and the convective heat transfer resistance between the inner tube surface and the fluid. The serial connection of these single resistances is equivalent to the total resistance between the absorber and the fluid, (1 /Uint ). For the elastomer–metal–absorber the internal thermal resistance and the internal heat transfer conductance depend on: • the fin resistance, characterised by the tube distance W, the base diameter D, the fin thickness s f and the fin conductivity k f ; • the connection between fin and tube, characterised by the connection technique and its production quality; • the tube resistance, characterised by the conductivity of the elastomer k t , the tube diameter d t , the wall thickness of the tube, s t , and the contact area between the clip profile and the tube, i.e. the contact angle w ; • the convection between the inner tube surface and the fluid, characterised by the convective heat transfer coefficient afluid , which is a function of the fluid, its flow velocity, its temperature and the inner tube diameter. In metal absorbers, the resistance of the tube wall (1 /Utube ) is normally neglectable because of the high conductivity of the metal fluid tube. However, in the case of the elastomer–metal– absorber, the tube resistance is very important for the total resistance of the absorber construction. It may be summarized that the internal heat transfer resistance depends on the construction parameters of the absorber sheet and the fluid
tube, the connection technique between the absorber sheet and the fluid tube, its production quality and the operation parameters. The dependence of the internal thermal resistance of these different construction parameters has been determined by calculations, for which the base case parameters of the elastomer–metal– absorber in Table 1 have been used. Fig. 3 presents the results of Uint calculations carried out for different tube distances W; in Fig. 4 the tube diameter to wall thickness ratio is varied. In both figures the thermal conductivity of the elastomer material is used as the parameter. As Figs. 3 and 4 show, the internal heat transfer conductance of the elastomer–metal–absorber is mainly determined by the low conductivity of the elastomer material, which leads to a high thermal resistance of the tube. This tube resistance becomes even more important if the amount of collected heat transported over this resistance increases. Therefore, the second parameter of major importance is the tube distance. Furthermore, the tube diameter and the thickness of the tube as well as the contact area between the clip profile and the tube have a clear influence on the internal heat transfer resistance of the construction and therefore on the efficiency of the collector. As the collector efficiency factor F9 and thus the h0 value depend on the ratio Uint F9 5 ]]]] Uloss 1 Uint
(1)
the Uint value should be maximized by optimi-
Table 1. Base case parameters for the calculation of the internal thermal resistance of the elastomer–metal–absorber W (mm)
D (mm)
Fin sf (mm)
kf (W/ mK)
d t,i (mm)
Tube st (mm)
w (8)
Convection afluid (W/ m 2 K)
100
7
1
200
9
2
270
2000
218
B. Bartelsen et al.
Fig. 3. Internal heat transfer conductance Uint vs. tube distance, the parameter is the thermal conductivity of the elastomer material (calculated graphs).
zation of the complete absorber construction. For a non-selective single glazed flat plate collector a typical value for the heat loss coefficient Uloss , which is describing the heat losses from the absorber to the ambient air, of 5.5 W/ m 2 K is taken within the following discussion. This value is referred to the absorber area and the temperature difference between the absorber and ambient air at an air speed of 3 m / s. It has been derived from several collector tests according to the ISO standard (ISO9806-1, 1994) with typical commercial non-selective collector modules. In order to achieve an F9 value of at least 0.90, the Uint value of the non-selective flat plate collector should be
higher than 50 W/ m 2 K. For a typical unglazed collector (swimming pool absorber), a heat loss coefficient Uloss of 15 W/ m 2 K has been found to be a representative value for an air speed of 2 m / s. This value has been derived from different tests of unglazed collectors carried out in agreement with the ISO standard (ISO9806-3, 1995). In order to achieve an F9 value of at least 0.82, the Uint value for the unglazed collector should be above 70 W/ m 2 K. These relatively high Uint values may only be achieved with a thermal conductivity of the elastomer of at least 0.6 W/ mK, if a realistic tube distance of more than 80 mm is assumed. The
Fig. 4. Internal heat transfer conductance Uint vs. the ratio of the mean tube diameter to the tube wall thickness, the parameter is the thermal conductivity of the elastomer material (calculated graphs).
Elastomer–metal–absorber: development and application
alternative, to reduce the tube wall thickness, has a clear boundary: long-term reliability requires a wall thickness of at least 1.5 mm. Therefore, the thermal conductivity of standard black elastomer material (k t 50.25 W/ mK) has to be increased significantly. 4. DEVELOPMENT OF THE ELASTOMER MATERIAL
The department of material technology of the ¨ has University of Applied Science in Osnabruck developed an elastomer material based on ethylene-propylene-dien-terpolymer (EPDM) for application in the elastomer–metal–absorber. The main part of the development was to increase the low thermal conductivity of typical elastomer materials with a simultaneous improvement of the mechanical strength (Vennemann et al., 1997). In addition to the typically used filling material carbon black, particles of aluminium and graphite have been applied during the development steps. The selected filling materials show a high thermal conductivity if compared to the base material. The EPDM mixture is varied with different types of carbon black, two kinds of aluminium particles and various kinds of graphite powder. The measured thermal conductivity and the tensile strength of some of the elastomer mixtures are presented in Fig. 5. The mixture indication starting with a ‘V’ labels the first series with only one single additive, the indication ‘M’ is for laboratory mixtures with two conductive filling materials and the
219
indication ‘D’ stands for elastomer mixtures produced by using an industrial mixing device. For the elastomer mixtures in Fig. 5 the following filling materials have been used: V-0, pure polymer; V-1, addition of 100 phr 1 aluminium particles; V-2, addition of 40 phr carbon black; M-4, addition of 40 phr carbon and 80 phr aluminium; M-7, addition of 30 phr carbon and 80 phr graphite; M-13, addition of 10 phr carbon and 90 phr graphite; D-4, addition of 10 phr carbon and 90 phr graphite. The pure polymer without filling materials shows a thermal conductivity of about 0.2 W/ mK. Aluminium as a single filling material like in mixture V-1 improves only the thermal conductivity. If a conductive carbon black is added to the mixture (V-2), the tensile strength is raised by a factor greater than six and the thermal conductivity is doubled. The mixtures M-4 and M-7 contain two filling materials. Beside the conductive carbon black an aluminium or a graphite powder is added to the polymer. The thermal conductivity is raised up to around 0.8 W/ mK, four times the value of the pure polymer, and the tensile strength is at a high level too. For the efficiency measurement of the first improved collector prototypes, the elastomer mixtures M-5 (similar to M-4) and M-7 with a good thermal conductivity and a good tensile strength 1
phr means ‘per hundred rubber’, i.e. the number of weight parts of the filling material which is added to one hundred weight parts of the basis polymer material.
Fig. 5. Thermal conductivity and tensile strength of different elastomer mixtures (measured data), including the uncertainty intervals with a 95% level of confidence.
220
B. Bartelsen et al.
have been used. From these new elastomer mixtures tubes have been extruded and integrated into the test collectors. Due to the high carbon black content, the materials M-4, M-7 and M-13 have a very high viscosity during the mixing process and the extruded tubes show a high hardness and a low flexibility. Furthermore, the tube surface has a significant roughness. The conclusion of this first elastomer development step is that high thermal conductivity and tensile strength values have been achieved, but the material is not appropriate for an industrial production process and does not result in the desired properties of elastomer tubes. The second EPDM development step, therefore, focuses on the improvement of production parameters and final elastomer material data like hardness, stress relaxation, torsion pendulum and ageing resistance. For this purpose the content of carbon black has been reduced and the other components have been adjusted with regard to the special requirements. As a first result of this second development step the mixture D-4, produced in an industrial mixing device, shows clear progress and already meets some of the requirements. However, further efforts are necessary for the optimization of the elastomer material for use as the fluid tube in the elastomer–metal–absorber, especially with regard to the production parameters, costs and long-term reliability. This work is going on. The temperature resistance of EPDM is restricted to a short-term maximum temperature of 1608C, as the elastomer material presents a decreasing strength with increasing temperature and an accelerated degradation at such high temperatures. This has two consequences: • The stagnation temperature of the solar collector at an irradiance level of 1000 W/ m 2 , ambient temperature of 308C and low air speed has to be reduced to a value of 1608C. This condition leads to the requirement that the
linear heat loss coefficient a 1 (according to ISO 9806-1, referring to the mean fluid temperature and absorber area) must be higher than or equal to 5 W/ m 2 K. A typical commercial collector in the German market (a 1 value around 3.8 W/ m 2 K) attains about 2008C in stagnation conditions. • The system design has to avoid the simultaneous occurrence of high pressure and high temperature, which is the case for typical closed-loop solar systems. 5. EXPERIMENTS ON THERMAL AND RELIABILITY PROPERTIES
During the development steps of the elastomer– metal–absorber, the internal thermal conductivity between the solar absorber and the fluid, the Uint value, was determined by numerical calculations, measurements at single absorber strips and measurements at solar absorbers during the performance test procedure of complete test collectors. Table 2 presents some of the most important results. The five types of absorber profiles presented in Fig. 1 have been investigated with different construction and material parameters. The calculated and measured internal heat transfer conductance of the construction depends, as discussed in Section 3, on the thermal conductivity of the elastomer k t , the tube diameter d t , the thickness of the tube s t and on the contact angle of the clip profile. The tube distance is the same for each construction (W5115 mm) and the base diameter D is varied only in a small range. With the fin and tube construction parameters the internal heat transfer conductance Uint has been calculated. These theoretical values may be compared with the experimental results. Up to now the internal heat transfer conductance has been increased from around 20.7 W/ m 2 K to 58.7 W/ m 2 K (see Table 2), which is
Table 2. Internal heat transfer conductance Uint of different elastomer–metal–absorber constructions with aluminium absorber fin, calculated and measured values, including the uncertainty intervals with a 95% level of confidence Profile type A A A B B C D D E
Absorber fin W s abs (mm) (mm) 115 115 115 115 115 115 115 115 115
1.0 1.0 1.0 1.0 1.0 0.9 1.5 1.5 1.0
Clip profile w d cp (8) (mm)
kt (W/ mK)
285 285 250 255 255 290 255 270 275
0.25 0.78 0.78 0.78 0.75 0.78 0.75 0.6–0.9 1.0
12.0 12.0 13.2 13.0 13.0 11.0 13.0 12.2 12.2
Elastomer tube dt (mm) 12.0 11.7 13.2 13.2 13.2 11.7 13.2 12.2 12.8
Uint st (mm)
Calculated (W/ m 2 K)
Measured (W/ m 2 K)
2.0 1.5 1.5 1.5 1.5 1.5 1.5 1.5 2.0
19.3 58.1 55.5 57.4 55.1 57.7 57.5 50–62 61.5
20.762.1 39.763.6 47.363.3 52.062.3 52.562.6 55.363.5 50.262.0 58.761.9 –
Elastomer–metal–absorber: development and application
mainly caused by the rise of the elastomer conductivity k t . This higher Uint value results according to Eq. (1) in an increase of the collector efficiency factor from 0.79 to 0.91 for typical non-selective collectors (assumed heat loss coefficient between absorber plate and ambient air Uloss 55.5 W/ m 2 K, see Section 3). A realistic future aim for the internal heat transfer conductance is estimated to be around 75 W/ m 2 K. A typical value of Uint for typical metal absorbers (German market) is between 40 and 60 W/ m 2 K, ‘good’ absorbers attain 80 W/ m 2 K and more. If the construction type ‘C’ (measured value Uint 55.3 W/ m 2 K, see Table 2) was equipped with a metal fluid tube instead of the elastomer tube, the internal heat transfer conductance would be 115 W/ m 2 K, resulting in a collector efficiency factor of 0.95 instead of 0.91. The difference of 0.04 is the price for absorber construction using an elastomer fluid tube. Even with the future optimized constructions (expected Uint value of 75 W/ m 2 K) this difference will still be around 0.03. Table 2 shows that some calculated Uint values fit rather well to the measured ones, others show significantly lower measured values. The main reason for this difference is the thermal contact
221
between the outer tube surface and the metal profile. As some of the tubes showed a low flexibility, the contact to the absorber has been reduced, because the uneven and hard tube wall does not touch the whole embracing metal area (compare lines 3 and 4 in Table 2). Therefore, the tube flexibility and surface quality are important. For this reason, the fluid pressure normally has a positive influence on and it could furthermore be remarked that a heating-up under pressure also improves the thermal contact. Another important influence may also be derived from Table 2. If the outer diameter of the tube is too small in comparison to the clip profile, the measured values of Uint are significantly lower than the calculated ones (compare line 2 in Table 2). Therefore, the outer diameter of the tube should be around 0.5 mm larger than the clip profile diameter. Here the production tolerance has to be taken into account. Up to now, five different test collectors with integrated elastomer–metal–absorbers have been constructed and investigated. For the collector frame, insulation and cover standard flat plate collector components have been used. Fig. 6 shows two of the test collectors.
Fig. 6. Photograph of the test collectors EMA-3 and EMA-4.
222
B. Bartelsen et al.
The first test collector had a copper absorber plate with soldered clip profiles in the form of a type ‘A’ construction. With this collector the base case investigations and the first measurements with the improved elastomer tubes have been performed. For the base case investigations the absorber was equipped with a conventional rubber tube (low thermal conductivity of around 0.25 W/ mK) and a black painted non-selective surface (EMA1). For the second base case collector (EMA-2) an adhesive selective metal foil has been used instead of the black painted surface. The first improved test collector (EMA-3) contains the same absorber construction like EMA-2, but the rubber tube has been replaced by a tube made out of the improved elastomer mixture M-5 (similar to M-4, Fig. 5). Again an adhesive selective metal foil has been applied. The second improved test collector (EMA-4) was produced with a type ‘B’ absorber construction (roll bended absorber sheets) and an improved elastomer tube with a larger diameter. The absorber coating is again the adhesive selective foil. The performance investigations mainly consist of measurements of the thermal efficiency according to the ISO-standard 9806-1 (ISO9806-1, 1994) with different collector prototypes. These measurements have been carried out in an indoor test facility with a solar simulator under highly reproducible meteorological and collector inlet conditions, using instruments and sensors which are strictly controlled in order to achieve a low uncertainty of each single measured data point. The ambient temperature is kept constant during one test sequence within less than 61 K around the set temperature, the irradiance varies by less than 61% and the temporal air speed variations are below 60.3 m / s (mean values of each efficiency point). The sky temperature is 0–4 K below the ambient temperature and shows the same variations as the ambient air temperature. The fluid flow is automatically controlled to a constant collector inlet temperature with deviations below 60.05 K and to a flow-rate with deviations of less than 1% of the average value of one efficiency point. These strictly controlled test conditions lead to a small scatter of the measured efficiency points around the fitted efficiency graph within one test sequence. An uncertainty analysis of the total measurement system leads to a standard uncertainty of each single efficiency value of less than 0.01. This high accuracy in combination with the high
repeatability has been proven in several repeated collector efficiency measurements, where efficiency deviations between two independent tests at the same collector over the whole temperature range of below 0.01 have been stated. All collector tests discussed have been carried out within the same test facility, using the same sensors and measuring devices. The irradiance level was in the range 800–835 W/ m 2 and the ambient temperature was in the range 23–258C. The collector flow-rate was set to 80 l / m 2 h, the inlet temperature was varied from 208C to 808C in four steps. These are the same test conditions as used for standard efficiency measurements according to the ISO standard. It may be concluded that the uncertainty of the test results with regard to the repeatability and reproducibility is below 0.015. For comparison with a typical selective flat plate collector from series production (German market, 1998), the following collector parameters (referring to absorber area and mean fluid temperature) will be taken: h0 50.80, a 1 53.80 W/ m 2 K, a 2 50.012 W/ m 2 K 2 . These parameters describe a typical good (not the best) flat plate collector, representing the actual market situation. They have been derived from numerous collector investigations carried out in the test laboratory of ¨ Solarenergieforschung in recent the Institut fur years. The diagram in Fig. 7 presents the measured efficiency curves of the improved test collectors EMA-3 and EMA-4 in comparison with the typical selective flat plate collector as well as the selectively coated base case test collector EMA-2. The resulting low h0 value (T m 5T amb 5208C) of 0.68 of the base case collector EMA-2 is caused by the high thermal resistance of the rubber hose. For the first test collector with an improved elastomer tube (EMA-3 with elastomer mixture M-5) the h0 value was raised up to 0.78. The h0 value of the second improved test collector EMA4 attained 0.81. If compared to EMA-3 this prototype EMA-4 has been equipped with a more flexible elastomer tube made out of mixture M-7 and uses a different absorber construction, profile ‘B’. These improvements during the first development steps have shown that the proposed elastomer–metal–absorber construction attains a thermal performance close to that of typical flat plate collectors with selective metal absorbers in the present market situation. However, the h0 value of an absorber construction with an elastomer tube will remain at least
Elastomer–metal–absorber: development and application
223
Fig. 7. Measured efficiency curves of different test collectors compared with a typical selective flat plate collector, at an irradiance level of 800 W/ m 2 , ambient air temperature of 208C and air speed of 3 m / s, according to ISO 9806-1, referring to mean fluid temperature and absorber area.
0.03 smaller in comparison to the same construction with a metal fluid pipe (see previous discussion of F9 value in this chapter). As the long-term reliability of the absorber is the most important condition for any future applications, first reliability investigations have been carried out on the elastomer tube, the absorber construction and the collector prototypes. On the elastomer tubes burst pressure tests and long-term stability investigations at high temperature and high pressure have been performed. The tubes produced from the actual mixture D-4 (see Section 4) show a high pressure resistance both at ambient temperature (13 bar at 238C) and at high temperature (7 bar at 1308C). Also the long-term stability of the elastomer tubes seems to be sufficient. During tests the tubes were operated for more than 1500 h at a temperature of 1708C and an inner pressure of 3 bar. With four different absorber strips (type ‘B’ and ‘D’) 30 freeze–thaw cycles have been performed in a cold box. While the water content in the tube is frozen, the outer clip profile diameter
is enlarged by up to 0.2 mm and the upper opening width of the profile by 0.2–0.4 mm. This effect increases at the upper end of the absorber strip if the absorber is frozen with a tilt angle of about 308. After thawing, the original geometry has always been exactly found again with profile ‘D’, while profile ‘B’ showed a small irreversibility of the opening width of the profile (0.1 mm) during the first three cycles. New investigations on profile ‘E’ have also shown a complete reversibility of the clip profile dimensions. The reversibility of the freeze–thaw cycles is confirmed by the results of the internal heat transfer measurements carried out after the freeze–thaw cycles. Table 3 shows the measured heat transfer conductance Uint for four elastomer– metal–absorber constructions before and after 30 freeze–thaw cycles. The internal heat transfer conductance Uint after the freeze–thaw cycles attains the same value as the initial values for profile ‘B’ and even a better performance for profile ‘D’. It is assumed that the contact between the elastomer tube and the clip profile is improved
Table 3. Measured internal heat transfer conductance Uint including the uncertainty intervals with 95% level of confidence of four different elastomer–metal–absorber strips with aluminium absorber fins before and after 30 freeze–thaw cycles Profile type
B B D D
Absorber fin W sf (mm) (mm) 115 115 115 115
10 10 15 15
Clip profile w d cp (8) (mm) 255 255 255 270
130 130 130 122
Elastomer tube kt dt (W/ mK) (mm) 0.78 0.75 0.75 06–0.9
13.2 13.2 13.2 12.2
Uint Initially measured measured (W/ m 2 K)
Measured after freeze–thaw cycles (W/ m 2 K)
52.062.3 52.562.6 50.262.0 58.761.9
52.362.2 53.163.6 54.862.1 62.962.0
224
B. Bartelsen et al.
by setting effects caused by the high pressure in the tube in the frozen state. On a complete collector prototype an exposition test on an outdoor test roof has been carried out. After the exposition during winter and spring the collector showed no changes. As the thermal expansion coefficients of the elastomer material (18310 25 l / K) and the aluminium absorber plate (2.4310 25 l / K) show a great difference, this item was observed during the exposition test. Even in stagnation conditions in the collector prototype (1508C), i.e. a temperature increase of 130 K and therefore a theoretical elongation difference of about 20 mm per meter, no effect had been recognised. Also with a 5.2-m long uncovered absorber (stagnation temperature 758C), no prolongation of the tube with respect to the metal plate has been identified. It may be concluded that the friction of the elastomer tube in the clip profile is high enough to ensure that the tube will nearly not be moved by thermal expansion in the longitudinal direction of the clip profile. It may be summarized that the results of the first reliability investigations are encouraging. However, still more reliability tests on collector elements are necessary. They will include further freeze–thaw cycles and the long-term reliability under stagnation conditions. 6. APPLICATION AS FAC ¸ ADE AND ROOF ELEMENT
Industrially produced roofs and fac¸ades often consist of corrugated metal form sheets made out of steel or aluminium. These roof or fac¸ade constructions are widely used for industrial, public or residential buildings. The elastomer–metal–absorber concept will transform these metal form sheets into uncovered or transparently covered roof and fac¸ade absorbers by integrating an appropriate clip profile into the form sheets during the production pro-
cess. The elastomer tube can then easily be clipped into these profiles after the installation of the roof or fac¸ade. Fig. 8 shows the conversion of a typical metal form sheet element into an unglazed or transparently covered solar collector. The first step of the conversion is the integration of the clip profile into the metal form sheet during the roll form process. The form sheet is covered with a paint of high solar absorptivity, with or without selective properties. The sheet will be mounted on the roof or fac¸ade by normal roofing or metal processing companies. The optical and technical properties will be the same as those of normal metal roofs. The next step is the integration of the elastomer tubes into the form sheets. The elastomer tubes will be connected via the manifold tubes to the solar system. Thus, an uncovered absorber results, where the technical properties of the metal roof or fac¸ade remain unchanged. As an additional option for systems with a higher demand temperature a transparent cover may be added, using single glass panes or transparent plastic covers. In this way glazed collectors may be produced, which are specially suited for large systems. Because of the use of the metal roof or fac¸ade elements as absorbers, it is expected that the additional costs for the transformation into an uncovered solar absorber or a single glazed collector should be small. At this early stage of the product development, it is not possible to present a cost comparison worked out in detail. Therefore, in the following only some ideas are discussed, which should give an impression of the cost reduction potential. The cost figures are manufacturing costs (German market, not including VAT and employer’s profit) for a collector field completely installed on the roof without storage tank, pump, control etc. and the piping from the roof through the building. For the uncovered elastomer–metal–absorber mounted on the roof the cost shares for the
Fig. 8. Steps from a metal roof and fac¸ade element to a solar collector.
Elastomer–metal–absorber: development and application
elastomer tubes, the manifolds, the additional metal consumption due to the clip profiles, the installation, the connection from the manifolds to the roof and miscellaneous costs are summarized. They are estimated to be between 13 and 17 $ per m 2 . This cost summary is based on the assumption that the costs for metal form sheet elements are charged to the roofing costs, and that the depreciation of the metal processing machine is not enlarged by the profile modification. The production costs therefore consider only the additional parts for the use of the roof as an active solar absorber. Compared to these additional costs the manufacturing costs for typical unglazed collector fields for swimming pool heating mounted on a flat roof are between 30 and 35 $ per m 2 . This rough estimate gives, therefore, a potential cost reduction of about 50% of the completely installed absorber field if compared to the relevant standard solar technology. The additional effort to transform the uncovered solar absorber into a glazed collector includes the glass pane, the glass mounting profiles and their installation. If it is possible to develop a costeffective solution for the fixing and mounting of the glass, the same cost reduction rate of 50% may be expected for the glazed collector. However, this cost estimation is still very uncertain, it has to be intensified during the next development steps. The idea of this building integrated collector type shows further advantages, which are considered to have the same importance as the expected cost reduction: • Metal form sheets are a common and well proven technology. • The transformation into the elastomer–metal– absorber does not affect the reliability of the original roof or fac¸ade. • The additional effort to transform metal roofs into unglazed absorbers seems to be low; on the other hand, the metal roof and fac¸ade elements gain by their additional property as active solar absorbers. • The extension to glazed collector roofs for a higher demand temperature is possible. • The integration may be performed with a high aesthetic quality and architectural acceptance. Typical examples for a future application of this concept are buildings with a high demand for low-temperature heat in the range below 808C, e.g. hotels, sports halls, hospitals etc. using glazed collectors or outdoor swimming pools and heatpump systems using unglazed absorbers. Furthermore, domestic hot water and residential room
225
heating applications may also be taken into account. A research and development project, funded by the European Commission, has started to develop and investigate the integrated elastomer–metal– absorber in roof and fac¸ade metal form sheet elements. The main tasks within this cooperation between industrial partners from various fields of activity and research institutions are the improvement of the elastomer material and absorber construction, the production and installation technology, the long-term reliability and thermal performance, heat use concepts and the system integration. Although the first results of this project are encouraging, further problems have to be solved on the way via first test systems and medium-sized pilot and demonstration plants to an industrial product. 7. CONCLUSION AND OUTLOOK
The principle of the elastomer–metal–absorber with its clip profile contact opens up new possibilities with regard to the heat transfer fluid, the collector and system design and the architectural integration. The development steps have shown that the proposed elastomer–metal–absorber construction already has a thermal performance close to that of typical flat plate collectors, with only a slightly lower h0 value. The essential results up to now are the increase of the thermal conductivity of the elastomer material from 0.25 W/ mK up to 1.0 W/ mK, which, in combination with an optimized absorber construction, leads to an internal heat transfer conductance of 60 W/ m 2 K, a value comparable to standard flat plate collectors. The first reliability investigations, especially the freeze–thaw cycles, have led to encouraging results. However, further development work has to be focused on this issue. It is proposed to apply this design to metal roofs and fac¸ades. The special attraction for this is given using the following reasons: • expected cost reduction for unglazed absorbers or glazed collectors due to synergetic effects; • significant reduction of the energetic payback time; • enlargement of the solar market by new manufacturers and new solar thermal applications, especially in large commercial and public buildings, as well as in residential buildings. The development work will be continued within an ongoing R&D project, where special regard is given to the industrial production of the tube
226
B. Bartelsen et al.
and the absorber profile, the long-term reliability and thermal performance, the installation questions and the way to operate the collector field in extreme situations like start, stagnation and frost conditions and thermal system integration including appropriate heat use concepts.
Uloss W X Xs
construction between absorber and fluid, referred to absorber area (W/ m 2 K) heat loss coefficient of the absorber plate, referred to absorber area and (T abs 2 T amb ) (W/ m 2 K) tube distance (mm) absorber location variable in width direction, measured from the border of the fin (mm) position on the fin, where the mean absorber temperature occurs, measured from the border of the fin (mm)
NOMENCLATURE a1 a2
afluid D d cp dt d i,t d t,m F9 G h
h0 kf kt
w sf st smax T abs T amb T base Tm T t,out T t,in Ufin Ubase Utube Uconv Uint
constant part of collector heat loss coefficient (ISO9806-1, 1994), referred to absorber area and 2 (T m 2 T amb ) (W/ m K) linear temperature dependence of collector heat loss coefficient (ISO9806-1, 1994), referred to absorber 2 2 area and (T m 2 T amb ) (W/ m K ) convective heat transfer coefficient between the inner tube surface and the fluid (W/ m 2 K) base diameter (projection width of visible part of the tube, clipped in the profile) (mm) inner diameter of the clip profile (mm) outer tube diameter (mm) inner tube diameter (mm) arithmetic mean of outer and inner tube diameter (mm) collector efficiency factor (–) solar irradiance (W/ m 2 ) thermal collector efficiency, referred to absorber area and T m (–) thermal collector efficiency, referred to absorber area and T m , at T m 5 T amb (–) thermal conductivity of the fin material (W/ mK) thermal conductivity of elastomer tube material (W/ mK) contact angle of the clip profile (8) thickness of the fin (mm) wall thickness of the tube (mm) tensile strength (N / mm 2 ) mean temperature on the absorber fin (8C) ambient air temperature (8C) temperature of the absorber base (8C) mean temperature of heat transfer fluid (8C) temperature on the outer surface of the tube (8C) temperature on the inner surface of the tube (8C) internal heat transfer conductance of the fin, referred to 2 absorber area (W/ m K) internal heat transfer conductance of the base, referred 2 to absorber area (W/ m K) internal heat transfer conductance of the tube wall, referred to absorber area (W/ m 2 K) convective heat transfer conductance between tube wall and fluid, referred to absorber area (W/ m 2 K) internal heat transfer conductance of total absorber
Acknowledgements—The work is partially funded by the European Commission, within the project ‘Fac¸ade and Roof Integrated Solar Collectors with a Combination of Elastomer Tubes and Metal Form Sheet Elements’, Contract No. JOR3CT98-0236, organized in the framework of the Non-Nuclear Energy Research and Technological Development Programme JOULE III.
REFERENCES Anderson D. R. (1966) Thermal conductivity of polymers. Chem. Reviews 66, 677–690. Bartelsen B., Rockendorf G. and Vennemann N. (1996). Development of an elastomer–metal–absorber for thermal solar collectors. In Proceedings of the EuroSun ’96, Freiburg, Germany, 16–20 September, 1996, pp. 495–499. ¨ DGS-Sonnenenergie Verlag, Munchen. Duffie J. A. and Beckmann W. A. (1991). In Solar Engineering of Thermal Processes, 2nd ed., pp. 268–276, Wiley Interscience, New York. Hands D. (1977) The thermal diffusivity and conductivity of natural rubber compounds. Rubber Chem. Tech. 50, 253– 265. ISO9806-1 (1994). ISO Standard 9806 -1. Test methods for solar collectors. Part 1: Thermal performance of liquid heating collectors including pressure drop. ISO, Switzerland. ISO9806-3 (1995). ISO Standard 9806 -3. Test methods for solar collectors. Part 3: Thermal performance of unglazed liquid heating collectors (sensible heat transfer only) including pressure drop. ISO, Switzerland. Rockendorf G., Falk S. and Wetzel W. (1996). Bedeutung und bestimmung des kollektorwirkungsgradfaktors bei sonnenkollektoren (effect and determination of the collector efficiency factor at solar collectors). 6. Symposium Thermische Solarenergie, Staffelstein, Germany, 8–10 May, 1996, pp. 196–201. OTTI, Regensburg. ¨ Vennemann N., Bokamp K., Wallach I., Bartelsen B. and Rockendorf G. (1997). EPDM compounds with improved thermal conductivity for thermal solar collectors. In Proceedings of the International Rubber Conference, ¨ Nurnberg, Germany, 30 June–3 July, 1997.
PII:
Solar Energy Vol. 67, Nos. 4–6, pp. 215–226, 1999 2000 Elsevier Science Ltd S 0 0 3 8 – 0 9 2 X ( 0 0 ) 0 0 0 7 2 – 4 All rights reserved. Printed in Great Britain 0038-092X / 99 / $ - see front matter
www.elsevier.com / locate / solener
ELASTOMER–METAL–ABSORBER: DEVELOPMENT AND APPLICATION BERND BARTELSEN*, GUNTER ROCKENDORF* , †, NORBERT VENNEMANN**, RAINER TEPE***, KLAUS LORENZ*** and GOTTFRIED PURKARTHOFER**** ¨ Solarenergieforschung GmbH, Am Ohrberg 1, D-31860 Emmerthal, Germany *Institut fur ¨ ¨ **Fachhochschule Osnabruck, Albrechtstr. 30, D-49076 Osnabruck, Germany ¨ ***Solar Energy Research Centre, Dalarna University College, S-78188 Borlange, Sweden ****Arbeitsgemeinschaft Erneuerbare Energie, A-8200 Gleisdorf, Austria Received 3 August 1999; revised version accepted 9 April 2000 Communicated by BRIAN NORTON
Abstract—A new principle of a solar collector that consists of appropriately shaped metal form plates as absorbers and clipped in elastomer fluid pipes, the so-called elastomer–metal–absorber, will be presented. The advantages are its freeze resistance, its seawater suitability and new possibilities for simplified collector installation and system techniques. The design parameters including a detailed analysis of the thermal resistance between the absorber and fluid will be discussed, where special regard is given to the development of an appropriate elastomer material with high thermal conductivity as one of the key items. The first development steps have shown that absorbers with a high thermal performance may be constructed. Finally, the idea of application of the principle of the elastomer–metal–absorber to metal roofs and fac¸ades will be presented. This idea is followed up within a development project. 2000 Elsevier Science Ltd. All rights reserved.
1. INTRODUCTION
The idea of a combined absorber with a metal absorber sheet for the absorption of the solar radiation and a flexible elastomer fluid pipe for the transport of the solar heat has been developed (Bartelsen et al., 1996). As shown in Fig. 1, a round shaped clip profile is integrated into a metal plate, which has an absorption layer for solar thermal conversion. In this profile an elastomer tube for the heat removal is clipped in. The application of this elastomer– metal–absorber in solar thermal collectors offers the following potential advantages and essential possibilities: • Due to its expected inherent freeze resistance, operation without an antifreeze additive seems to be possible. • System installation without heat exchanger in the solar loop may be discussed. • Operation with a corrosive fluid is possible, e.g. direct flow with sea or brackish water. • New and simplified techniques for the collector and system installation can be developed. The most promising application results from †
Author to whom correspondence should be addressed. Tel. 149-5151-999-521; fax 149-5151-999-500; e-mail: [email protected]
Fig. 1. Different design types of the elastomer–metal–absorber. 215
216
B. Bartelsen et al.
the new installation possibilities for the collector and the system. It is intended to integrate this new collector concept into roofs and fac¸ades made out of metal form sheet elements. Furthermore, the elastomer–metal–absorber concept seems to be an attractive collector for the solar desalination of brackish and sea water, as the collector may be operated directly with corrosive liquids without cost intensive corrosion protected heat exchangers. The desalination process should be designed to operate on a low temperature level (e.g. around 708C). The use of elastomer tubes in collectors requires appropriate absorber constructions. Different constructions have been developed and investigated with regard to the internal heat transfer resistance in combination with freeze–thaw cycles. These absorbers have been integrated in solar collector prototypes, and thus the thermal performance and the reliability have been examined. A high thermal efficiency may only be achieved with a low heat transfer resistance of the complete construction. In order to minimize this thermal resistance between absorber and fluid, the low thermal conductivity of standard elastomer material has to be improved. Therefore different elastomer mixtures with significantly higher thermal conductivities and acceptable mechanical properties have been developed and investigated. In the following, the results of the development and analysis work concerning the collector development will be presented, and future applications will be discussed. 2. COLLECTOR DESIGN
It is evident that the low thermal conductivity of normal elastomer material results in a high thermal resistance between absorber and fluid, which lowers the thermal performance of a collector with this design. For this reason, a theoretical study of the absorber heat transfer had to be performed first. The results of appropriate numerical calculations have led to the following conclusions (Bartelsen et al., 1996): • A direct contact of the metal absorber fin with the elastomer tube is necessary, no additional adhesive or contact material should be used. • The thermal conductivity of about 0.25 W/ mK for standard black elastomer material (Anderson, 1966; Hands, 1977) should be increased to a value of around 0.7–1.0 W/ mK. • The contact area between the absorber fin and elastomer tube must be large.
• The wall thickness of the tube should be small. • Finally, the diameter of the tube should be large. The last two requirements are in contradiction to the necessary strength of the elastomer tube at operation pressure. Of special importance for a high thermal performance is the contact between the metal absorber and the elastomer tube. Therefore, during the first development steps, different absorber fin– tube configurations, in the following called absorber strips, as well as different collector prototypes have been investigated with special regard to the heat transfer characteristics, thermal performance and reliability. Fig. 1 shows five different constructions of realized absorber shapes, which have been investigated up to now. Type ‘A’ is a typically soldered or welded absorber construction, which has been used for the first experiments. Type ‘B’ is an absorber construction out of roll bended aluminium sheets. The clip profile, which embraces the elastomer tube, is integrated in the sheet, thus no welding or soldering is necessary. Type ‘C’ and type ‘D’ are aluminium roll shaped constructions, which are used as absorbers in typical thermal collectors. Normally a copper fluid tube instead of the elastomer tube is used in the clip profile. Type ‘E’ is a specially developed absorber constructed out of roll bended aluminium sheets. This clip profile is an improvement of types ‘A’– ‘D’ and takes the capabilities of a roll-form machine for 1-mm thick aluminium sheets into account. These different constructions have been used as absorbers in collector prototypes for the measurement of the thermal performance and as single absorber strips for the investigation of the internal heat transfer conductance. 3. ANALYSIS OF INTERNAL THERMAL RESISTANCE
The efficiency of a solar collector mainly depends on the quality of the absorber. Beside the absorption and the emission of the coating, the capability to transfer the heat from the absorber to the fluid is important (Duffie and Beckmann, 1991; Rockendorf et al., 1996). Fig. 2 shows the thermal resistance network of a typical absorber strip. The heat has to pass four single resistances on its way to the fluid. These are the resistance of the
Elastomer–metal–absorber: development and application
217
Fig. 2. Simplified thermal steady-state model of absorber fin–tube configuration (heat losses are not included).
absorber fin and of the base connection, the tube wall resistance and the convective heat transfer resistance between the inner tube surface and the fluid. The serial connection of these single resistances is equivalent to the total resistance between the absorber and the fluid, (1 /Uint ). For the elastomer–metal–absorber the internal thermal resistance and the internal heat transfer conductance depend on: • the fin resistance, characterised by the tube distance W, the base diameter D, the fin thickness s f and the fin conductivity k f ; • the connection between fin and tube, characterised by the connection technique and its production quality; • the tube resistance, characterised by the conductivity of the elastomer k t , the tube diameter d t , the wall thickness of the tube, s t , and the contact area between the clip profile and the tube, i.e. the contact angle w ; • the convection between the inner tube surface and the fluid, characterised by the convective heat transfer coefficient afluid , which is a function of the fluid, its flow velocity, its temperature and the inner tube diameter. In metal absorbers, the resistance of the tube wall (1 /Utube ) is normally neglectable because of the high conductivity of the metal fluid tube. However, in the case of the elastomer–metal– absorber, the tube resistance is very important for the total resistance of the absorber construction. It may be summarized that the internal heat transfer resistance depends on the construction parameters of the absorber sheet and the fluid
tube, the connection technique between the absorber sheet and the fluid tube, its production quality and the operation parameters. The dependence of the internal thermal resistance of these different construction parameters has been determined by calculations, for which the base case parameters of the elastomer–metal– absorber in Table 1 have been used. Fig. 3 presents the results of Uint calculations carried out for different tube distances W; in Fig. 4 the tube diameter to wall thickness ratio is varied. In both figures the thermal conductivity of the elastomer material is used as the parameter. As Figs. 3 and 4 show, the internal heat transfer conductance of the elastomer–metal–absorber is mainly determined by the low conductivity of the elastomer material, which leads to a high thermal resistance of the tube. This tube resistance becomes even more important if the amount of collected heat transported over this resistance increases. Therefore, the second parameter of major importance is the tube distance. Furthermore, the tube diameter and the thickness of the tube as well as the contact area between the clip profile and the tube have a clear influence on the internal heat transfer resistance of the construction and therefore on the efficiency of the collector. As the collector efficiency factor F9 and thus the h0 value depend on the ratio Uint F9 5 ]]]] Uloss 1 Uint
(1)
the Uint value should be maximized by optimi-
Table 1. Base case parameters for the calculation of the internal thermal resistance of the elastomer–metal–absorber W (mm)
D (mm)
Fin sf (mm)
kf (W/ mK)
d t,i (mm)
Tube st (mm)
w (8)
Convection afluid (W/ m 2 K)
100
7
1
200
9
2
270
2000
218
B. Bartelsen et al.
Fig. 3. Internal heat transfer conductance Uint vs. tube distance, the parameter is the thermal conductivity of the elastomer material (calculated graphs).
zation of the complete absorber construction. For a non-selective single glazed flat plate collector a typical value for the heat loss coefficient Uloss , which is describing the heat losses from the absorber to the ambient air, of 5.5 W/ m 2 K is taken within the following discussion. This value is referred to the absorber area and the temperature difference between the absorber and ambient air at an air speed of 3 m / s. It has been derived from several collector tests according to the ISO standard (ISO9806-1, 1994) with typical commercial non-selective collector modules. In order to achieve an F9 value of at least 0.90, the Uint value of the non-selective flat plate collector should be
higher than 50 W/ m 2 K. For a typical unglazed collector (swimming pool absorber), a heat loss coefficient Uloss of 15 W/ m 2 K has been found to be a representative value for an air speed of 2 m / s. This value has been derived from different tests of unglazed collectors carried out in agreement with the ISO standard (ISO9806-3, 1995). In order to achieve an F9 value of at least 0.82, the Uint value for the unglazed collector should be above 70 W/ m 2 K. These relatively high Uint values may only be achieved with a thermal conductivity of the elastomer of at least 0.6 W/ mK, if a realistic tube distance of more than 80 mm is assumed. The
Fig. 4. Internal heat transfer conductance Uint vs. the ratio of the mean tube diameter to the tube wall thickness, the parameter is the thermal conductivity of the elastomer material (calculated graphs).
Elastomer–metal–absorber: development and application
alternative, to reduce the tube wall thickness, has a clear boundary: long-term reliability requires a wall thickness of at least 1.5 mm. Therefore, the thermal conductivity of standard black elastomer material (k t 50.25 W/ mK) has to be increased significantly. 4. DEVELOPMENT OF THE ELASTOMER MATERIAL
The department of material technology of the ¨ has University of Applied Science in Osnabruck developed an elastomer material based on ethylene-propylene-dien-terpolymer (EPDM) for application in the elastomer–metal–absorber. The main part of the development was to increase the low thermal conductivity of typical elastomer materials with a simultaneous improvement of the mechanical strength (Vennemann et al., 1997). In addition to the typically used filling material carbon black, particles of aluminium and graphite have been applied during the development steps. The selected filling materials show a high thermal conductivity if compared to the base material. The EPDM mixture is varied with different types of carbon black, two kinds of aluminium particles and various kinds of graphite powder. The measured thermal conductivity and the tensile strength of some of the elastomer mixtures are presented in Fig. 5. The mixture indication starting with a ‘V’ labels the first series with only one single additive, the indication ‘M’ is for laboratory mixtures with two conductive filling materials and the
219
indication ‘D’ stands for elastomer mixtures produced by using an industrial mixing device. For the elastomer mixtures in Fig. 5 the following filling materials have been used: V-0, pure polymer; V-1, addition of 100 phr 1 aluminium particles; V-2, addition of 40 phr carbon black; M-4, addition of 40 phr carbon and 80 phr aluminium; M-7, addition of 30 phr carbon and 80 phr graphite; M-13, addition of 10 phr carbon and 90 phr graphite; D-4, addition of 10 phr carbon and 90 phr graphite. The pure polymer without filling materials shows a thermal conductivity of about 0.2 W/ mK. Aluminium as a single filling material like in mixture V-1 improves only the thermal conductivity. If a conductive carbon black is added to the mixture (V-2), the tensile strength is raised by a factor greater than six and the thermal conductivity is doubled. The mixtures M-4 and M-7 contain two filling materials. Beside the conductive carbon black an aluminium or a graphite powder is added to the polymer. The thermal conductivity is raised up to around 0.8 W/ mK, four times the value of the pure polymer, and the tensile strength is at a high level too. For the efficiency measurement of the first improved collector prototypes, the elastomer mixtures M-5 (similar to M-4) and M-7 with a good thermal conductivity and a good tensile strength 1
phr means ‘per hundred rubber’, i.e. the number of weight parts of the filling material which is added to one hundred weight parts of the basis polymer material.
Fig. 5. Thermal conductivity and tensile strength of different elastomer mixtures (measured data), including the uncertainty intervals with a 95% level of confidence.
220
B. Bartelsen et al.
have been used. From these new elastomer mixtures tubes have been extruded and integrated into the test collectors. Due to the high carbon black content, the materials M-4, M-7 and M-13 have a very high viscosity during the mixing process and the extruded tubes show a high hardness and a low flexibility. Furthermore, the tube surface has a significant roughness. The conclusion of this first elastomer development step is that high thermal conductivity and tensile strength values have been achieved, but the material is not appropriate for an industrial production process and does not result in the desired properties of elastomer tubes. The second EPDM development step, therefore, focuses on the improvement of production parameters and final elastomer material data like hardness, stress relaxation, torsion pendulum and ageing resistance. For this purpose the content of carbon black has been reduced and the other components have been adjusted with regard to the special requirements. As a first result of this second development step the mixture D-4, produced in an industrial mixing device, shows clear progress and already meets some of the requirements. However, further efforts are necessary for the optimization of the elastomer material for use as the fluid tube in the elastomer–metal–absorber, especially with regard to the production parameters, costs and long-term reliability. This work is going on. The temperature resistance of EPDM is restricted to a short-term maximum temperature of 1608C, as the elastomer material presents a decreasing strength with increasing temperature and an accelerated degradation at such high temperatures. This has two consequences: • The stagnation temperature of the solar collector at an irradiance level of 1000 W/ m 2 , ambient temperature of 308C and low air speed has to be reduced to a value of 1608C. This condition leads to the requirement that the
linear heat loss coefficient a 1 (according to ISO 9806-1, referring to the mean fluid temperature and absorber area) must be higher than or equal to 5 W/ m 2 K. A typical commercial collector in the German market (a 1 value around 3.8 W/ m 2 K) attains about 2008C in stagnation conditions. • The system design has to avoid the simultaneous occurrence of high pressure and high temperature, which is the case for typical closed-loop solar systems. 5. EXPERIMENTS ON THERMAL AND RELIABILITY PROPERTIES
During the development steps of the elastomer– metal–absorber, the internal thermal conductivity between the solar absorber and the fluid, the Uint value, was determined by numerical calculations, measurements at single absorber strips and measurements at solar absorbers during the performance test procedure of complete test collectors. Table 2 presents some of the most important results. The five types of absorber profiles presented in Fig. 1 have been investigated with different construction and material parameters. The calculated and measured internal heat transfer conductance of the construction depends, as discussed in Section 3, on the thermal conductivity of the elastomer k t , the tube diameter d t , the thickness of the tube s t and on the contact angle of the clip profile. The tube distance is the same for each construction (W5115 mm) and the base diameter D is varied only in a small range. With the fin and tube construction parameters the internal heat transfer conductance Uint has been calculated. These theoretical values may be compared with the experimental results. Up to now the internal heat transfer conductance has been increased from around 20.7 W/ m 2 K to 58.7 W/ m 2 K (see Table 2), which is
Table 2. Internal heat transfer conductance Uint of different elastomer–metal–absorber constructions with aluminium absorber fin, calculated and measured values, including the uncertainty intervals with a 95% level of confidence Profile type A A A B B C D D E
Absorber fin W s abs (mm) (mm) 115 115 115 115 115 115 115 115 115
1.0 1.0 1.0 1.0 1.0 0.9 1.5 1.5 1.0
Clip profile w d cp (8) (mm)
kt (W/ mK)
285 285 250 255 255 290 255 270 275
0.25 0.78 0.78 0.78 0.75 0.78 0.75 0.6–0.9 1.0
12.0 12.0 13.2 13.0 13.0 11.0 13.0 12.2 12.2
Elastomer tube dt (mm) 12.0 11.7 13.2 13.2 13.2 11.7 13.2 12.2 12.8
Uint st (mm)
Calculated (W/ m 2 K)
Measured (W/ m 2 K)
2.0 1.5 1.5 1.5 1.5 1.5 1.5 1.5 2.0
19.3 58.1 55.5 57.4 55.1 57.7 57.5 50–62 61.5
20.762.1 39.763.6 47.363.3 52.062.3 52.562.6 55.363.5 50.262.0 58.761.9 –
Elastomer–metal–absorber: development and application
mainly caused by the rise of the elastomer conductivity k t . This higher Uint value results according to Eq. (1) in an increase of the collector efficiency factor from 0.79 to 0.91 for typical non-selective collectors (assumed heat loss coefficient between absorber plate and ambient air Uloss 55.5 W/ m 2 K, see Section 3). A realistic future aim for the internal heat transfer conductance is estimated to be around 75 W/ m 2 K. A typical value of Uint for typical metal absorbers (German market) is between 40 and 60 W/ m 2 K, ‘good’ absorbers attain 80 W/ m 2 K and more. If the construction type ‘C’ (measured value Uint 55.3 W/ m 2 K, see Table 2) was equipped with a metal fluid tube instead of the elastomer tube, the internal heat transfer conductance would be 115 W/ m 2 K, resulting in a collector efficiency factor of 0.95 instead of 0.91. The difference of 0.04 is the price for absorber construction using an elastomer fluid tube. Even with the future optimized constructions (expected Uint value of 75 W/ m 2 K) this difference will still be around 0.03. Table 2 shows that some calculated Uint values fit rather well to the measured ones, others show significantly lower measured values. The main reason for this difference is the thermal contact
221
between the outer tube surface and the metal profile. As some of the tubes showed a low flexibility, the contact to the absorber has been reduced, because the uneven and hard tube wall does not touch the whole embracing metal area (compare lines 3 and 4 in Table 2). Therefore, the tube flexibility and surface quality are important. For this reason, the fluid pressure normally has a positive influence on and it could furthermore be remarked that a heating-up under pressure also improves the thermal contact. Another important influence may also be derived from Table 2. If the outer diameter of the tube is too small in comparison to the clip profile, the measured values of Uint are significantly lower than the calculated ones (compare line 2 in Table 2). Therefore, the outer diameter of the tube should be around 0.5 mm larger than the clip profile diameter. Here the production tolerance has to be taken into account. Up to now, five different test collectors with integrated elastomer–metal–absorbers have been constructed and investigated. For the collector frame, insulation and cover standard flat plate collector components have been used. Fig. 6 shows two of the test collectors.
Fig. 6. Photograph of the test collectors EMA-3 and EMA-4.
222
B. Bartelsen et al.
The first test collector had a copper absorber plate with soldered clip profiles in the form of a type ‘A’ construction. With this collector the base case investigations and the first measurements with the improved elastomer tubes have been performed. For the base case investigations the absorber was equipped with a conventional rubber tube (low thermal conductivity of around 0.25 W/ mK) and a black painted non-selective surface (EMA1). For the second base case collector (EMA-2) an adhesive selective metal foil has been used instead of the black painted surface. The first improved test collector (EMA-3) contains the same absorber construction like EMA-2, but the rubber tube has been replaced by a tube made out of the improved elastomer mixture M-5 (similar to M-4, Fig. 5). Again an adhesive selective metal foil has been applied. The second improved test collector (EMA-4) was produced with a type ‘B’ absorber construction (roll bended absorber sheets) and an improved elastomer tube with a larger diameter. The absorber coating is again the adhesive selective foil. The performance investigations mainly consist of measurements of the thermal efficiency according to the ISO-standard 9806-1 (ISO9806-1, 1994) with different collector prototypes. These measurements have been carried out in an indoor test facility with a solar simulator under highly reproducible meteorological and collector inlet conditions, using instruments and sensors which are strictly controlled in order to achieve a low uncertainty of each single measured data point. The ambient temperature is kept constant during one test sequence within less than 61 K around the set temperature, the irradiance varies by less than 61% and the temporal air speed variations are below 60.3 m / s (mean values of each efficiency point). The sky temperature is 0–4 K below the ambient temperature and shows the same variations as the ambient air temperature. The fluid flow is automatically controlled to a constant collector inlet temperature with deviations below 60.05 K and to a flow-rate with deviations of less than 1% of the average value of one efficiency point. These strictly controlled test conditions lead to a small scatter of the measured efficiency points around the fitted efficiency graph within one test sequence. An uncertainty analysis of the total measurement system leads to a standard uncertainty of each single efficiency value of less than 0.01. This high accuracy in combination with the high
repeatability has been proven in several repeated collector efficiency measurements, where efficiency deviations between two independent tests at the same collector over the whole temperature range of below 0.01 have been stated. All collector tests discussed have been carried out within the same test facility, using the same sensors and measuring devices. The irradiance level was in the range 800–835 W/ m 2 and the ambient temperature was in the range 23–258C. The collector flow-rate was set to 80 l / m 2 h, the inlet temperature was varied from 208C to 808C in four steps. These are the same test conditions as used for standard efficiency measurements according to the ISO standard. It may be concluded that the uncertainty of the test results with regard to the repeatability and reproducibility is below 0.015. For comparison with a typical selective flat plate collector from series production (German market, 1998), the following collector parameters (referring to absorber area and mean fluid temperature) will be taken: h0 50.80, a 1 53.80 W/ m 2 K, a 2 50.012 W/ m 2 K 2 . These parameters describe a typical good (not the best) flat plate collector, representing the actual market situation. They have been derived from numerous collector investigations carried out in the test laboratory of ¨ Solarenergieforschung in recent the Institut fur years. The diagram in Fig. 7 presents the measured efficiency curves of the improved test collectors EMA-3 and EMA-4 in comparison with the typical selective flat plate collector as well as the selectively coated base case test collector EMA-2. The resulting low h0 value (T m 5T amb 5208C) of 0.68 of the base case collector EMA-2 is caused by the high thermal resistance of the rubber hose. For the first test collector with an improved elastomer tube (EMA-3 with elastomer mixture M-5) the h0 value was raised up to 0.78. The h0 value of the second improved test collector EMA4 attained 0.81. If compared to EMA-3 this prototype EMA-4 has been equipped with a more flexible elastomer tube made out of mixture M-7 and uses a different absorber construction, profile ‘B’. These improvements during the first development steps have shown that the proposed elastomer–metal–absorber construction attains a thermal performance close to that of typical flat plate collectors with selective metal absorbers in the present market situation. However, the h0 value of an absorber construction with an elastomer tube will remain at least
Elastomer–metal–absorber: development and application
223
Fig. 7. Measured efficiency curves of different test collectors compared with a typical selective flat plate collector, at an irradiance level of 800 W/ m 2 , ambient air temperature of 208C and air speed of 3 m / s, according to ISO 9806-1, referring to mean fluid temperature and absorber area.
0.03 smaller in comparison to the same construction with a metal fluid pipe (see previous discussion of F9 value in this chapter). As the long-term reliability of the absorber is the most important condition for any future applications, first reliability investigations have been carried out on the elastomer tube, the absorber construction and the collector prototypes. On the elastomer tubes burst pressure tests and long-term stability investigations at high temperature and high pressure have been performed. The tubes produced from the actual mixture D-4 (see Section 4) show a high pressure resistance both at ambient temperature (13 bar at 238C) and at high temperature (7 bar at 1308C). Also the long-term stability of the elastomer tubes seems to be sufficient. During tests the tubes were operated for more than 1500 h at a temperature of 1708C and an inner pressure of 3 bar. With four different absorber strips (type ‘B’ and ‘D’) 30 freeze–thaw cycles have been performed in a cold box. While the water content in the tube is frozen, the outer clip profile diameter
is enlarged by up to 0.2 mm and the upper opening width of the profile by 0.2–0.4 mm. This effect increases at the upper end of the absorber strip if the absorber is frozen with a tilt angle of about 308. After thawing, the original geometry has always been exactly found again with profile ‘D’, while profile ‘B’ showed a small irreversibility of the opening width of the profile (0.1 mm) during the first three cycles. New investigations on profile ‘E’ have also shown a complete reversibility of the clip profile dimensions. The reversibility of the freeze–thaw cycles is confirmed by the results of the internal heat transfer measurements carried out after the freeze–thaw cycles. Table 3 shows the measured heat transfer conductance Uint for four elastomer– metal–absorber constructions before and after 30 freeze–thaw cycles. The internal heat transfer conductance Uint after the freeze–thaw cycles attains the same value as the initial values for profile ‘B’ and even a better performance for profile ‘D’. It is assumed that the contact between the elastomer tube and the clip profile is improved
Table 3. Measured internal heat transfer conductance Uint including the uncertainty intervals with 95% level of confidence of four different elastomer–metal–absorber strips with aluminium absorber fins before and after 30 freeze–thaw cycles Profile type
B B D D
Absorber fin W sf (mm) (mm) 115 115 115 115
10 10 15 15
Clip profile w d cp (8) (mm) 255 255 255 270
130 130 130 122
Elastomer tube kt dt (W/ mK) (mm) 0.78 0.75 0.75 06–0.9
13.2 13.2 13.2 12.2
Uint Initially measured measured (W/ m 2 K)
Measured after freeze–thaw cycles (W/ m 2 K)
52.062.3 52.562.6 50.262.0 58.761.9
52.362.2 53.163.6 54.862.1 62.962.0
224
B. Bartelsen et al.
by setting effects caused by the high pressure in the tube in the frozen state. On a complete collector prototype an exposition test on an outdoor test roof has been carried out. After the exposition during winter and spring the collector showed no changes. As the thermal expansion coefficients of the elastomer material (18310 25 l / K) and the aluminium absorber plate (2.4310 25 l / K) show a great difference, this item was observed during the exposition test. Even in stagnation conditions in the collector prototype (1508C), i.e. a temperature increase of 130 K and therefore a theoretical elongation difference of about 20 mm per meter, no effect had been recognised. Also with a 5.2-m long uncovered absorber (stagnation temperature 758C), no prolongation of the tube with respect to the metal plate has been identified. It may be concluded that the friction of the elastomer tube in the clip profile is high enough to ensure that the tube will nearly not be moved by thermal expansion in the longitudinal direction of the clip profile. It may be summarized that the results of the first reliability investigations are encouraging. However, still more reliability tests on collector elements are necessary. They will include further freeze–thaw cycles and the long-term reliability under stagnation conditions. 6. APPLICATION AS FAC ¸ ADE AND ROOF ELEMENT
Industrially produced roofs and fac¸ades often consist of corrugated metal form sheets made out of steel or aluminium. These roof or fac¸ade constructions are widely used for industrial, public or residential buildings. The elastomer–metal–absorber concept will transform these metal form sheets into uncovered or transparently covered roof and fac¸ade absorbers by integrating an appropriate clip profile into the form sheets during the production pro-
cess. The elastomer tube can then easily be clipped into these profiles after the installation of the roof or fac¸ade. Fig. 8 shows the conversion of a typical metal form sheet element into an unglazed or transparently covered solar collector. The first step of the conversion is the integration of the clip profile into the metal form sheet during the roll form process. The form sheet is covered with a paint of high solar absorptivity, with or without selective properties. The sheet will be mounted on the roof or fac¸ade by normal roofing or metal processing companies. The optical and technical properties will be the same as those of normal metal roofs. The next step is the integration of the elastomer tubes into the form sheets. The elastomer tubes will be connected via the manifold tubes to the solar system. Thus, an uncovered absorber results, where the technical properties of the metal roof or fac¸ade remain unchanged. As an additional option for systems with a higher demand temperature a transparent cover may be added, using single glass panes or transparent plastic covers. In this way glazed collectors may be produced, which are specially suited for large systems. Because of the use of the metal roof or fac¸ade elements as absorbers, it is expected that the additional costs for the transformation into an uncovered solar absorber or a single glazed collector should be small. At this early stage of the product development, it is not possible to present a cost comparison worked out in detail. Therefore, in the following only some ideas are discussed, which should give an impression of the cost reduction potential. The cost figures are manufacturing costs (German market, not including VAT and employer’s profit) for a collector field completely installed on the roof without storage tank, pump, control etc. and the piping from the roof through the building. For the uncovered elastomer–metal–absorber mounted on the roof the cost shares for the
Fig. 8. Steps from a metal roof and fac¸ade element to a solar collector.
Elastomer–metal–absorber: development and application
elastomer tubes, the manifolds, the additional metal consumption due to the clip profiles, the installation, the connection from the manifolds to the roof and miscellaneous costs are summarized. They are estimated to be between 13 and 17 $ per m 2 . This cost summary is based on the assumption that the costs for metal form sheet elements are charged to the roofing costs, and that the depreciation of the metal processing machine is not enlarged by the profile modification. The production costs therefore consider only the additional parts for the use of the roof as an active solar absorber. Compared to these additional costs the manufacturing costs for typical unglazed collector fields for swimming pool heating mounted on a flat roof are between 30 and 35 $ per m 2 . This rough estimate gives, therefore, a potential cost reduction of about 50% of the completely installed absorber field if compared to the relevant standard solar technology. The additional effort to transform the uncovered solar absorber into a glazed collector includes the glass pane, the glass mounting profiles and their installation. If it is possible to develop a costeffective solution for the fixing and mounting of the glass, the same cost reduction rate of 50% may be expected for the glazed collector. However, this cost estimation is still very uncertain, it has to be intensified during the next development steps. The idea of this building integrated collector type shows further advantages, which are considered to have the same importance as the expected cost reduction: • Metal form sheets are a common and well proven technology. • The transformation into the elastomer–metal– absorber does not affect the reliability of the original roof or fac¸ade. • The additional effort to transform metal roofs into unglazed absorbers seems to be low; on the other hand, the metal roof and fac¸ade elements gain by their additional property as active solar absorbers. • The extension to glazed collector roofs for a higher demand temperature is possible. • The integration may be performed with a high aesthetic quality and architectural acceptance. Typical examples for a future application of this concept are buildings with a high demand for low-temperature heat in the range below 808C, e.g. hotels, sports halls, hospitals etc. using glazed collectors or outdoor swimming pools and heatpump systems using unglazed absorbers. Furthermore, domestic hot water and residential room
225
heating applications may also be taken into account. A research and development project, funded by the European Commission, has started to develop and investigate the integrated elastomer–metal– absorber in roof and fac¸ade metal form sheet elements. The main tasks within this cooperation between industrial partners from various fields of activity and research institutions are the improvement of the elastomer material and absorber construction, the production and installation technology, the long-term reliability and thermal performance, heat use concepts and the system integration. Although the first results of this project are encouraging, further problems have to be solved on the way via first test systems and medium-sized pilot and demonstration plants to an industrial product. 7. CONCLUSION AND OUTLOOK
The principle of the elastomer–metal–absorber with its clip profile contact opens up new possibilities with regard to the heat transfer fluid, the collector and system design and the architectural integration. The development steps have shown that the proposed elastomer–metal–absorber construction already has a thermal performance close to that of typical flat plate collectors, with only a slightly lower h0 value. The essential results up to now are the increase of the thermal conductivity of the elastomer material from 0.25 W/ mK up to 1.0 W/ mK, which, in combination with an optimized absorber construction, leads to an internal heat transfer conductance of 60 W/ m 2 K, a value comparable to standard flat plate collectors. The first reliability investigations, especially the freeze–thaw cycles, have led to encouraging results. However, further development work has to be focused on this issue. It is proposed to apply this design to metal roofs and fac¸ades. The special attraction for this is given using the following reasons: • expected cost reduction for unglazed absorbers or glazed collectors due to synergetic effects; • significant reduction of the energetic payback time; • enlargement of the solar market by new manufacturers and new solar thermal applications, especially in large commercial and public buildings, as well as in residential buildings. The development work will be continued within an ongoing R&D project, where special regard is given to the industrial production of the tube
226
B. Bartelsen et al.
and the absorber profile, the long-term reliability and thermal performance, the installation questions and the way to operate the collector field in extreme situations like start, stagnation and frost conditions and thermal system integration including appropriate heat use concepts.
Uloss W X Xs
construction between absorber and fluid, referred to absorber area (W/ m 2 K) heat loss coefficient of the absorber plate, referred to absorber area and (T abs 2 T amb ) (W/ m 2 K) tube distance (mm) absorber location variable in width direction, measured from the border of the fin (mm) position on the fin, where the mean absorber temperature occurs, measured from the border of the fin (mm)
NOMENCLATURE a1 a2
afluid D d cp dt d i,t d t,m F9 G h
h0 kf kt
w sf st smax T abs T amb T base Tm T t,out T t,in Ufin Ubase Utube Uconv Uint
constant part of collector heat loss coefficient (ISO9806-1, 1994), referred to absorber area and 2 (T m 2 T amb ) (W/ m K) linear temperature dependence of collector heat loss coefficient (ISO9806-1, 1994), referred to absorber 2 2 area and (T m 2 T amb ) (W/ m K ) convective heat transfer coefficient between the inner tube surface and the fluid (W/ m 2 K) base diameter (projection width of visible part of the tube, clipped in the profile) (mm) inner diameter of the clip profile (mm) outer tube diameter (mm) inner tube diameter (mm) arithmetic mean of outer and inner tube diameter (mm) collector efficiency factor (–) solar irradiance (W/ m 2 ) thermal collector efficiency, referred to absorber area and T m (–) thermal collector efficiency, referred to absorber area and T m , at T m 5 T amb (–) thermal conductivity of the fin material (W/ mK) thermal conductivity of elastomer tube material (W/ mK) contact angle of the clip profile (8) thickness of the fin (mm) wall thickness of the tube (mm) tensile strength (N / mm 2 ) mean temperature on the absorber fin (8C) ambient air temperature (8C) temperature of the absorber base (8C) mean temperature of heat transfer fluid (8C) temperature on the outer surface of the tube (8C) temperature on the inner surface of the tube (8C) internal heat transfer conductance of the fin, referred to 2 absorber area (W/ m K) internal heat transfer conductance of the base, referred 2 to absorber area (W/ m K) internal heat transfer conductance of the tube wall, referred to absorber area (W/ m 2 K) convective heat transfer conductance between tube wall and fluid, referred to absorber area (W/ m 2 K) internal heat transfer conductance of total absorber
Acknowledgements—The work is partially funded by the European Commission, within the project ‘Fac¸ade and Roof Integrated Solar Collectors with a Combination of Elastomer Tubes and Metal Form Sheet Elements’, Contract No. JOR3CT98-0236, organized in the framework of the Non-Nuclear Energy Research and Technological Development Programme JOULE III.
REFERENCES Anderson D. R. (1966) Thermal conductivity of polymers. Chem. Reviews 66, 677–690. Bartelsen B., Rockendorf G. and Vennemann N. (1996). Development of an elastomer–metal–absorber for thermal solar collectors. In Proceedings of the EuroSun ’96, Freiburg, Germany, 16–20 September, 1996, pp. 495–499. ¨ DGS-Sonnenenergie Verlag, Munchen. Duffie J. A. and Beckmann W. A. (1991). In Solar Engineering of Thermal Processes, 2nd ed., pp. 268–276, Wiley Interscience, New York. Hands D. (1977) The thermal diffusivity and conductivity of natural rubber compounds. Rubber Chem. Tech. 50, 253– 265. ISO9806-1 (1994). ISO Standard 9806 -1. Test methods for solar collectors. Part 1: Thermal performance of liquid heating collectors including pressure drop. ISO, Switzerland. ISO9806-3 (1995). ISO Standard 9806 -3. Test methods for solar collectors. Part 3: Thermal performance of unglazed liquid heating collectors (sensible heat transfer only) including pressure drop. ISO, Switzerland. Rockendorf G., Falk S. and Wetzel W. (1996). Bedeutung und bestimmung des kollektorwirkungsgradfaktors bei sonnenkollektoren (effect and determination of the collector efficiency factor at solar collectors). 6. Symposium Thermische Solarenergie, Staffelstein, Germany, 8–10 May, 1996, pp. 196–201. OTTI, Regensburg. ¨ Vennemann N., Bokamp K., Wallach I., Bartelsen B. and Rockendorf G. (1997). EPDM compounds with improved thermal conductivity for thermal solar collectors. In Proceedings of the International Rubber Conference, ¨ Nurnberg, Germany, 30 June–3 July, 1997.