Solar Energy for a Sustainable Future June, 14-19, 2003

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1 NEW LOW CONCENTRATION CPC TYPE COLLECTOR WITH CONVECTION CONTROLED BY A HONEYCOMB TIM MATERIAL: A COMPROMISE WITH STAGNATION TEMPERATURE CONTROL AND SURVIVAL OF CHEAP FABRICATION MATERIALS Manuel Collares Pereira and Maria João Carvalho Department of Renewable Energies, INETI, Estrada do Paço do Lumiar, Lisboa, Portugal , , collares.pereira@ineti.pt João Correia de Oliveira AO SOL, Energias Renováveis, Lda, Apartado 173, 2135 Samora Correia, Portugal , , jco@aosol.pt Abstract Stationary non evacuated CPC collectors have the potential to perform close to evacuated tubular collectors at high temperatures, yet they can be manufactured with the simplicity and low cost of good flat plate collectors. However taking full advantage of the concentration factor achievable with CPCs without the use of more expensive materials is a real challenge, as there is always a risk of too high stagnation temperatures. But more and more applications come forward requiring operating temperatures around or above 100ºC, as, for instance, solar cooling or desalination. In this paper modifications on a standard CPC 1.12X collector were made in order to obtain efficiencies arround 50% at an inlet temperature above 100ºC. These modifications include the addition of honeycomb type TIM, calculated in order to comply with the following constraints: 1) be able to fit in the available space; 2) have the cell size and thickness which will achieve the desired convection reduction; 3) keep healthy distances from the hottest places in the collector; 4) produce a stagnation temperature not above the threshold for the survivability of the different collector components. This latest requirement is crucial. Its solution depends on many factors but advantage is taken of the fact that the CPC collector seems to have a stronger convection regime once the temperature of the absorber rises above a certain value. In the present paper the solution is presented and discussed. Collector test results are presented. This solution constitutes the basis for implementation of the solar collector field in the AQUASOL project, financed by the EU, and which will be installed at Plataforma Solar. 1. INTRODUCTION Stationary non evacuated CPC collectors have the potential to perform close to evacuated tubular collectors at high temperatures, yet they can be manufactured with the simplicity and low cost of good flat plate collectors. One example is the CPC 1.12 X produced by the company Ao Sol. In the past extensive testing was made with a modified commercial CPC collector with 1.12X concentration factor (Collares Pereira M. et al, 1994), modified to accommodate first a thin teflon acting as a double transparent cover and after a 10cm high, 5mm cell, honeycomb type TIM. The heatloss factor F'U L was reduced by about 1.3W/ºC m 2 in the first case and about 1W/ºC m 2 in the second one. The second one had, on top, a substantial penalty (17%) of the optical efficiency, due to the height of the honeycomb and its small cell size. These results were an indication that it might be possible to have a CPC of very low concentration, come even closer in thermal performance to the evacuated tubular collector types, keeping the cost low. The first idea was to increase the concentration a little, without compromising full stationarity. However taking full advantage of the concentration factor achievable with CPCs without changing the present fabrication techniques and without resorting to more expensive materials is a real challenge, as the resulting higher stagnation temperatures create a real survival problem in case of a circulation pump failure occurring under high solar radiation.. But more and more applications come forward requiring operating temperatures around or above 100ºC, as, for instance, solar cooling or desalination and the challenge is worth considering. Also because there are better solar selective coatings 1, better TIM materials, etc. To meet this challenge starting from the present standard product without substantial changes in its production, ruled out the teflon film option. Thus the work was carried out with the honeycomb TIM solution, with the following initial constraints: 1) use of a standard product 2, without any adaptations as was previously done (Collares Pereira M. et al, 1994), in order not to have a different fabrication line in case of success. 2) use of a honeycomb TIM which will not reduce in any significant way the collector's optical efficiency 3) use of a TIM that will not enhance the thermal performance to a point that would make stagnation temperature a real problem 1 Sunstrip Tinox 2 CPC 1.12 X Company Ao Sol

2 In the present paper the solution implemented is presented, tested and discussed. This study is the basis for the decisions made for the solar collector field in the AQUASOL project, financed by the EU, and which will be installed at Plataforma Solar, Almeria. 2. DESCRIPTION OF THE CPC COLLECTOR AND ADAPTATIONS. 2.1 Standard CPC collector used - description. The collector used in the testing is the standard AO SOL CPC 1.12X product, with the modifications explained below. Figure 1 shows a cross-section and a photograph of that commercial standard product. It consists of seven troughs with an inverted V fin (Patent nº PE ), aluminum and copper of the Sunstrip type, selectively coated. Total concentration is 1.12X and the design half acceptance angle is 56.4º while the truncation angle is 78º (Carvalho, M.J et al, 1985, 1987). The mirrors are from an aluminum sheet of 0.4 mm thickness, anodized, produced by ALANOD. It has a solar clear tempered glass cover with 3 mm thickness produced by COVINA.. The insulation is of the poliurethane injected foam type. Total glass area is 1.98 m 2 (aperture area). (a) photograph (b) cross section of CPC (commercial product) Fig.1 Collector CPC Ao Sol obtained after an exposure test of 42 days with daily solar radiation above 14 MJ/m 2. The official reported results, obtained in a N.S. orientation (i.e. with the troughs as in Fig. 1) are: F'η 0 =0.70 ± 0.01; F'U L =3.4 ± 0.2 These results consider the efficiency calculated with respect to the solar radiation incident on the collector. If efficiency is determined considering the solar radiation seen by the collector, which is beam radiation and 1/C (1/1.12) of diffuse radiation, the results would be: F'η 0 =0.71 ± 0.01; F'U L =3.5 ± 0.2 The N.S. orientation is the one used in thermosyphon systems, in which the CPC is combined with a solar tank placed above. However, in forced circulation systems the most common orientation of the collector is E.W.( i.e. with the troughs horizontal and running from East to West) 2.2 The CPC and the honeycomb TIM In Fig.1(b), it can be seen that the distance between the highest points on the aluminum mirrors and the glass cover is small. The value is ~7mm. That is the space available to accommodate de TIM. Most studies (Holland, K.G..T and K. Iynkaran (1...); Hellstrom, B and Karlsson (1991)) with honeycomb type TIMs indicated the need to have at least 15 mm and preferably more in terms of TIM thickness, albeit in relation with cell size, in order to have true convection supression. The 7mm available looked a bit short of the values referred above, i.e., probably yielding a response not quite so perfect in terms of providing a layer of stagnant air. However this was exactly what we needed in order to keep under control the stagnation temperature risk. As for cell size we decided to use a large one in comparison with the one described in reference (Collares Pereira, M. et al, 1995); in this case about 10 mm, in order to affect the collector optical efficiency as little as possible. Another constraint was the survivability of the honeycomb material at high temperature. Wacotech Timax CA 50 9/s was the solution found. A few sheets were simply laid under the cover and against the mirror tips as can be seen in Fig.2. A note should be made to the easy way of this addition of the TIM sheet to the CPC collector, not requiring any special fixing means. It has been tested under the European Standard EN at LECS - Laboratório de Ensaio de Colectores Solares of INETI, an accredited testing laboratory for solar collectors. The results reported below 3 were 3 In this paper we will use the standard linear fit to the measured data points Fig.2 cross section of CPC with honeycomb TIM

3 η η ISES Solar World Congress 2003 Prior to the efficiency measurements with the TIM selected, the absorber was substituted by an absorber made with a different selective coating, that of TINOX. The reason was the expectation of slightly better behavior at high temperatures, given the data 4 provided by both SUNSTRIP and TINOX for their respective selective coatings. This was not a good decision as will be explained below. The fins were all copper and only coated on one side, requiring bending in order to have a situation like in Fig2, simulating a coating on both faces. Fig. 4 (b) photo of collector with honeycomb 3. TEST RESULTS AND DISCUSSION The same collector was tested in both configurations, with and without the honeycomb TIM, under forced convection of air, as prescribed in the European Standard. The results can be seen both in Fig.5 and Fig.6 Colector CPC Ao Sol sem TIM 1.0 Fig.3 Tinox coated copper fin, displaying the bended copper sheet, in order to simulate a fin coated on both sides Fig.4 shows two photographs of the collector with and without the TIM, in different stages of the testing reported in this paper and with the Tinox coated absorber (Tf-Tamb)/Icol Fig.5 efficiency test of the CPC, without TIM Ensaio Ajuste Linear Colector CPC Ao Sol com TIM Ensaio Ajuste Linear (Tf-Tamb)/Icol Fig. 4 (a) photo of collector without TIM Fig 6 efficiency test of the CPC with TIM The parameters of the linear fit are: Without TIM: F'η 0 = 0.72; F'U L = 3.8 W/ºC m 2 With TIM F'η 0 = 0.72; F'U L = 3.2 W/ºC m 2 4 Sunstrip (ε~0.1, 0.94< α<.95) and Tinox (ε>0.05 and 0.94<α<0.96) As can be seen the results without the TIM are worst than the ones reported for the commercial collector with

4 the Sunstrip absorber. The explanation must be connected with the performance of the fin, much longer in the case of the Tinox absorber, thus transferring heat in a different, probably worst, manner to the fluid circulating in the tube, that in turn raising fin temperature for the same fluid temperature, resulting in higher losses. This is why we commented above that the choice of absorber was not the most convenient for the test. However the Laboratory with its heavy testing schedule did not yet allow for other tests with the standard AO SOL absorber. The optical efficiency is not seriously affected by the TIM as expected from the large cell size, which is a very positive result. The action of the TIM can be observed to be one of reduction of the heat loss factor by W/m2/ºC. This result is certainly below the one reported in reference (Collares Pereira, M. (1994)) as expected given the reduced TIM height! But this is in tune with the objectives as stated in the introduction: to reduce losses but not as much as to risk destroying the collector under eventual stagnation. In Figure 7 the results of the measured angular response are also presented. The measurement is done only in one direction with respect to the optical axis of symmetry of the collector. As can be seen the results agree very well with the design angles, all the more so as the measurement is made with the glass cover on and the optical response of the glass transmissivity is superimposed on the optical performance of the mirrors, penalizing the larger angles. Fig.7 Measurement of the CPC half acceptance angular characteristic Stagnation temperature was also measured in both situations. The result was obtained by tilting the collector so that it would be near normal incidence at noon, due south, but leaving it stationary. This procedure simulates a real worst situation. It is clear that a higher stagnation temperature can be obtained in both cases if the collector is made to track the sun at near normal incidence, a situation not likely to be encountered in practice, since it is mounted in a fixed position. Solar radiation was around 1000W/m 2 on the collector plane and ambient temperature was around 31ºC in the first day (no TIM) and 32ºC the second day (with TIM). The first day was a little windier, with wind from the same N.W. direction (wind velocity not measured). T/I= ±.004 0C m2/w without TIM and T/I= ± C m2/w with TIM Interesting to note is that the temperatures measured on the TIM itself were not higher than 142 ºC which ensures its survivability, according to its manufacturer (140ºC is the limit). As can be seen the stagnation temperature was higher with TIM but not very high in either case, as desired. Two factors explain this: 1) on one hand (Collares Pereira et al, 1995) these CPCs are known to have higher heat losses above a certain temperature threshold (around 120ºC - in any case a temperature above the highest temperature of the efficiency measurements performed, and where the linear fit to the data would not apply), quite likely attributed to a change in convective regime generated by the special geometry of the collector's internal volume 5. This effect is likely present in the case with TIM but it will certainly be different, better or worst, it is hard to say 2) the introduction of the TIM reduces optical efficiency for angles larger then normal incidence, and that, in turn favors a reduction in stagnation temperature as measured above. With an ambient temperature of 35ºC the results above point to a fin temperature which might be around 200ºC. That estimate is already close to the limited tolerated by AO SOL 6, and thus fulfills the usefulness criteria stated at the start. In particular, extrapolating this result to the one obtained with the standard collector, it is reasonable to expect an F'U L on the order of 2.8W/ºC m 2 and that may be already too low. Besides regularly higher ambient temperatures can be expected in some sunny regions of the World, further aggravating the problem. Thus the idea now is to continue this study by using cells with a larger size, which, presumably, would yield a slightly higher heat loss factor, thus alleviating the problem. 5 This is a very convenient fact- although not yet thoroughly studied- and which is behind the present choice of materials and fabrication technique employed by AO SOL 6 For instance at 200ºC fin temperature, the temperature gradient to the insulating foam still keeps its temperature around 100ºC, thus in a survival situation

5 4. CONCLUSIONS The addition of honeycomb TIM to the standard CPC produced by AO SOL seems to enhance its thermal performance significantly, but perhaps not as much as to question its survivability under stagnation temperature. The reduction on heat loss factor was between 0.6 and 0.7W/m2/ºC, a factor on the order of 20% of the total heat loss measured for the standard product without TIM. In any case it is interesting to note that non evacuated CPC collectors - fully stationary - are still far from the thermal performance limit they may aspire, i.e. if one is ready to change the fabrication technique to have higher resistance to temperature from the different components (namely insulation) then the CPC can have a little higher concentration, the TIM can be thicker and values around 2 W/ºC m 2 might be possible, a truly remarkable achievement for a non evacuated device. And all that at a much lower cost and higher durability. In fact AO SOL has already explored the effect of increasing concentration, without modifying its production procedures, manufacturing several 1.5XCPCs with 2.38 m 2, with teflon film and with honeycomb TIM, obtaining values of F'U L =2.8 W/ºC m 2 and 2.6W/ºC m 2, to be used in absorption cooling, as explained elsewhere (Correia de Oliveira, J. et al, 2003), a clear step in the direction above. Thus AO SOL will be ready to introduce a new product on the market, once higher temperature applications create the proper demand. Farinha Mendes J., Carvalho M.J., Horta P. (Eds) CD ROM, ISBN Covina glass SGG Securit 3 mm EN :2001, Thermal solar system and components Solar collectors Part 2: Test methods, ICS PE Hellström B. and Karlsson B. (1991). Evaluation of a honeycomb glazing for high temperature solar collectors. TIM Workshop, Birmingham Hollands K.G.T. and Iynkaran K. (19..). Proposal for a Compound-Honeycomb Collector. Solar Energy, 34, Sunstrip Test Report number n.8/ /2002, LECS-INETI Tinox Wacotech Timax CA 50 9/s REFERENCES AO SOL - Alanod Quality 320 G Collares-Pereira M., Carvalho M.J., Farinha Mendes J.,Oliveira J.,Haberle A.,Wittwer V.(1995). Optical and Thermal Testing of a new 1.12X CPC Solar Collector Solar Energy Materials, and Solar Cells, 37, Carvalho M.J., Collares Pereira M., Gordon J.M. (1987). Economic optimisation of stationary non-evacuated CPC solar collectors. Journal of Solar Energy Engineering, A.S.M.E., 109, Carvalho M.J., Collares Pereira M., Gordon J.M., Rabl A. (1985). Truncation of CPC solar collectors and its effect on energy collection. Solar Energy. 35, Correia de Oliveira J., Branco R., Collares Pereira M., Carvalho M.J. (2002). Novo colector do tipo CPC sem vácuo para aplicações de aquecimento e arrefecimento ambiente, Actas do XI Congresso Ibérico e VI Iberoamericano de Energia Solar, Vilamoura, Algarve,