A NEW CONCEPT OF BUILDING INTEGRATED THERMAL SOLAR COLLECTOR Fabrice Motte, Gilles Notton, Christian Cristofari, Jean Louis Canaletti University of Corsica-Centre de Vignola-Route des Sanguinaires-20000 Ajaccio, France E-mail:
[email protected] ;
[email protected];
[email protected];
[email protected] Abstract: We present a new flat plate solar collector integrated into a drainpipe. The innovation is based on a geometrical approach. The collector is made of several serial modules. The drainpipe keeps its water evacuation function. After a presentation of the energy situation in France, the new concept of solar collector is described; the experiment and the collected data are shown and the first experimental results are presented and discussed. Key words: Flate plate solar water collector, Building-integration, Drainpipe
1. Introduction There is no doubt that the development of the renewable energies is necessary for economical and environmental reasons. The building integration may be an important development parameter for practical and esthetical reasons. We present a new patented concept of flat plate solar water collector totally integrated into a drainpipe. The main objective is to get a minimum visual impact combined with maximum performances. An experimental wall was build in order to characterize and to model the thermal behavior of this collector in view of future improvements. 2. French building energy situation Massive exploitation of fossils energies leads to stocks rarefaction but also to environmental problems. In December 2008, the European Union fixed ambitious objectives for 2020 [1]: - to decrease the emission of green house gases of 20%; - to decrease the energy consumption of 20%; - to cover 20% of consumed energy using renewable energies. 180 160
Transport Agriculture Résidential-Tertiary Industry Iron and steel metallurgy
p = prevision
140 120
MTOE
100 80
42.18% 41.07% 42.29%
60
41.23% 42.46%
42.20% 42.97% 42.50% 42.21% 43.10% 43.42% 43.63% 43.59%
40 20 0 1973
1979
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2004
2005
Figure 1 .French Final Energy Consumption by sector [2]
2006
2007 (p)
Cooking 6%
Specific electricity utilisation 11%
Water heating 11%
Heating 72%
Figure 2.. Repartition of the energy consumption in housing sector [3]
In France, 30 M-housings use 50% of the final energy and produce 25% of green house gazes (2nd sector). In Europe, 500 M-inhabitants in 60 M-housings consume half the energy. The residential and tertiary sector is the first consumer (Fig.1) with 70.6 MTOE in 2007 (43.6%) [2]. In the housing sector, the heating and the water heating represent in term of energy consumption 72% and 11% (Fig. 2). Renewable energies improve the energy efficiency and reduce fuel or electricity consumption. Flat-plate solar collectors can provide the domestic hot water. The two main limiting factors in their development are the price and the visual impact. 3. New solar collector presentation The new concept of flat-plate solar water collector patented and named H2OSS® presents a high building integration without any visual impact. This collector is available in various sizes both for individual and collective habitations. The solar collector is arranged so it can also be used on north oriented walls (the collector being oriented south into the drainpipe). This product is totally invisible from the ground level thanks to the drainpipe integration (Fig.3).
Figure 3. Integration of the solar collector and simplified internal conception of H2OSS® module
The drainpipe preserves its role of rain water evacuation. The canalizations connecting the house to the collector are hidden in the drainpipe evacuation. An installation includes several serial connected modules. Each module is about one meter length and around ten centimeters in width (for individual houses). The number of modules depends on the drainpipe length.
From top to bottom, the collector is composed by a flat glass, an air space, a highly selective absorber and an insulation layer. First, the cold fluid from the storage tank flows through the inferior insulated tube and then in the upper tube in thermal contact with the absorber. 4. First experimental results 4.1. The experimental wall A meteorological station and an experimental wall are installed in the laboratory located in Ajaccio (France), a seaside Mediterranean site. The experimentation has 3 main objectives: - testing the thermal behavior and collecting experimental data in view to calculate performances; - validating a numerical thermal model - adjusting some parameters to improve performances and efficiency. An experimental drainpipe was built comprising 18 serial modules (about 2m²) split in two rows (Fig.4).
Figure 4. Experimental wall
Figure 5. Temperature control loop and experimentation..
We keep a constant input fluid temperature using a control loop which heats the fluid if it is too cold and cools it in the other case (Fig.5). The data collected every minute are: global solar irradiance, ambient temperature and humidity, wind speed and direction, fluid flow rate
and input and output fluid temperatures (for each module). The flow rate was fixed at 0.120 m3.h-1 (the flow rate influence will be study through a future work). 4.2. The thermal behaviour Fig. 6 shows inlet and outlet collector temperatures, ambient temperature, wind speed, solar irradiance and instantaneous efficiency defined by equation [4]:
η = ρ .Q.Cp.(Toutlet − Tinlet ) I . Ac T_outlet T_inlet Instantaneous efficiency Ambiant temperature Wind speed Solar irradiance
Temperature (°C), Efficiency (%) Wind speed (km/h)
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50
1000 900 800 700 600
40
500 30
400 300
20
Solar irrandiance (W/m²)
70
200 10 100 0 06:00
0 08:24
10:48
13:12 Time (h)
15:36
18:00
Figure.6: Experimental results for the H2OSS® collector
The maximum difference between inlet and outlet fluid temperatures is 9°C. The inlet fluid temperature is not constant in this experiment. The instantaneous efficiency, up to 60% at the steady-state, decreases rapidly after noon (Fig. 6). In fact, the wall is south-east oriented and the solar irradiance absorbed, different the morning and the afternoon, is given by [4]:
q abs = I . Ac .τ .α . cos(θ i ) We studied the incidence angle of the direct solar radiation. The collector is tilted by 25° with an azimuth of -28°. There are numerous steps of calculation to obtain the following results [5] (Fig 7). The product of the glass transmittance by the collector absorptivity is assumed constant until 60° of incidence [4]. Concerning the cosine part of the second equation, we note that cos(10°)=0.985. Then it is sensible to say that for an incidence beam lower than 10°, the irradiance received by the collector is equal to the global solar irradiance. In Fig. 7, we see that the incidence angle falls under 10° in May, June July and August. Furthermore, on the wall, the upper row starts shadowing the downer row at 1 p.m in summer what explain why the performances are better in the morning. The results must be interpreted only when the system is in a steady state [4], with an incidence angle lower than 10° and before the first row begins to shadow the other one. This short time interval is around solar midday in summer, the temperature evolution in the fluid flow direction has been made with an input temperature of 50°C (Fig.8). The measured temperatures correspond to the temperature sensors located on the upper tube between each module. The profile is linear and the small variations around the straight line are due to the resolution of thermal sensor (0.2°C). The maximal useful length has not been reached. Into each module, the temperature continues to increase, thus we can install efficiently more than 18 serial modules. If the output temperature finishes increasing and begins to stabilize, it will be necessary to modify the configuration in using parallel modules. In the input tube located into the insulation, the fluid temperature increases by 1°C for 18 m
length before entering in the absorber. 40 35
Incidence angle (degrees)
30
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October 25 20 15 10 5
14
13.5
13
12.5
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0 Local French Hour
Fig. 7: Solar beam angle on collector 40 39.5 T = 0.3367L + 33.129
Temperature (°C)
39 38.5 38 37.5 37 36.5 36 10
11
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15 Lenght (m)
16
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20
Fig.8: Temperature evolution versus the length of the drain pipe.
4.3. Energy performances In Fig. 9, the efficiency (η) at stationary state is plotted, according to the European standard [6] versus the reduced temperature (Tr) defined by [7]:
Tr = (Tm − Tamb ) I We calculated the linear regression and we obtain with a correlation coefficient equal to 0.96:
η = −15.085.Tr + 0.83 = − K .Tr + B with B the optical efficiency and K the thermal looses [6] higher than for conventional solar collectors with simple glass and highly selective absorber (Fig 8). The average value of K is usually about 5 W.m-1.K-1 [7]. This difference is due to the shape of the H2OSS® modules. The thermal looses on the sides of the modules are more important and so the performances decrease rapidly when the reduced temperature increases. This point will be optimized through a future work, using numerical calculations.
0.7 y = -15.085x + 0.83 2 R = 0.9783
Instantaneous efficiency
0.6
0.5
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0.3
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0.1 0.015
0.02
0.025
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0.035
0.04
0.045
Reduced temperature (K.m²/W)
Figure 9. Efficiency for our solar collector and for conventional solar collectors without glass (a); with one (b) and two glasses (c), with a selective absorber and one glass (d) and vacuum collector (e)
5. Conclusion A new concept of flat plate solar water collector highly building integrated was presented. The collector is made of several modules in serial position. The particularities of the collector are that it is integrated into a drainpipe and totally invisible from the ground level. It can be installed on both new and old buildings, and on individual or collective habitations. An experiment was implemented and promising first experimental results were presented. However it is necessary to modify the shape of this collector in order to improve the thermal insulation. The next characterisation step will be to study the fluid flow rate influence, which affects the thermal looses. References [1]. http://ec.europa.eu/climateaction/eu_action/index_fr.htm [2]. French Ministry of ecology and economy; 2007 L’énergie en France – Chiffres Clés. [3]. Besson, D : Consommation d’énergie : autant de dépenses en carburants qu’en énergie domestique. ISEE Première, n°1176, 02/2008. [4]. Chasseriaux ; 1993 ; Conversion thermique du rayonnement solaire ; France ; Dunod. [5]. Perrin de Brichambaut Ch, Vauge Ch,;1982, Le Gisement Solaire, France ; Lavoisier [6]. European Standards PrEN 12975-1, 2005, Solar installation and components. [7]. Duffie J.A, Beckman W.A; 1980; Solar engineering on thermal processes; John Wiley&Sons. New York.
