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Thermal performance study of a solar collector using a natural vegetable fiber, coconut coir, as heat insulation
Energy for Sustainable Development 14 (2010) 297–301
Contents lists available at ScienceDirect
Energy for Sustainable Development
Thermal performance study of a solar collector using a natural vegetable fiber, coconut coir, as heat insulation H.Y. Andoh a,⁎, P. Gbaha a, B.K. Koua b, P.M.E. Koffi a, S. Touré b a b
Laboratoire d'Energies Nouvelles et Renouvelables, Institut National Polytechnique Félix Houphouët Boigny, B.P. 1526 Yamoussoukro, Ivory Coast Laboratoire d'Energie Solaire, Université de Cocody, 22 B.P. 582, Abidjan 22, Ivory Coast
a r t i c l e
i n f o
Article history: Received 21 March 2009 Revised 22 September 2010 Accepted 23 September 2010 Keywords: Solar water heater Natural circulation Coconut coir Polyurethane Glass wool Fiber glass Polystyrene
a b s t r a c t There are few solar water heaters in Africa because of their acquisition cost, which makes it difficult for the populations to afford them. This article presents a solar water heater designed with a local vegetable material as insulating material, coconut coir, widespread in tropical countries. The study focuses on the comparative thermal performance of this collector and another collector, identical in design, fabrication, and operating under the same conditions, using glass wool as heat insulation, as well as with eight other designs, chosen randomly, using various materials as heat insulation, with performance data from the literature. The materials cost of the coconut coir collector is 25% less than the glass wool one. The results of the study show very good thermal performance of the collector using coconut coir compared to the traditional ones. For example, the outlet hot water temperature of the coconut coir collector was more than 80 °C. Internal hot water temperature rise was more than 40 °C. The thermal efficiency was a little over 51% although it was generally below 50% for the collectors using traditional insulation. It has a good pair of values of FR(τα) and FRUL. So, the lower cost of this type of solar water heater and its performance suggest that this design may be more suitable for application in tropical countries where coconut coir is commonly available. © 2010 International Energy Initiative. Published by Elsevier Inc. All rights reserved.
Introduction Conventional natural circulation flat plate solar water heaters are the most economical and large-scale use of solar energy all over the world. It has been designed, developed, and tested in detail by Kalogirou et al. (1999), Karaghouli and Alnaser (2001), Chang et al. (2002), and Kudish et al. (2003), among many others. Its thermal performance, which depends on its design parameters, thickness, type of insulation, number and type of glass covers, spacing between absorber and inner glass, has also been studied by Nahar (2003), Abdullah et al. (2003), and Esen and Esen (2005). Apart from these parameters, its performance also depends on climatic and operational parameters (Nahar, 2003). Although very widespread, the solar water heater remains rare in Africa, because of its acquisition cost, which is relatively high. The objective of the study is to conceive a cheap yet efficient flat plate solar water heater. The work focuses mainly on the comparison of the thermal performance and the fabrication costs of two identical solar water heaters of the same dimensions, design, manufactured in the same manner, operating in the same conditions. One of the two collectors of these solar water heaters uses the glass wool (GW), and the other uses a natural vegetable fiber, coconut coir (CC), as heat
⁎ Corresponding author. E-mail address: [email protected] (H.Y. Andoh).
insulation. The comparison extends to other solar water heaters from the literature, using traditional thermal insulation. In our analysis, we took into account (1) the rise of water temperature through the absorbers, (2) the efficiency of the collectors (ability of the collector to convert the solar energy into heat), (3) the pair of values that characterizes the collector, like FR(τα) and FRUL., in which, FR is the heat removal efficiency, UL, the total heat loss coefficient, and (τα) the optical effectiveness. Materials and methods For this study, the flat plate solar collector we used (Fig. 1) for the production of domestic hot water is primarily made of (1) a blackpainted network of piping in which circulates the working fluid towards a storage tank; (2) a reflective aluminum plate located below the tubes, which allows the reduction of the thermal losses towards the back of the collector by reflecting the thermal radiation inside it; (3) a glass cover to reduce the losses at the absorber by radiation (greenhouse effect) and by convection (effect of motionless air layer); and (4) a framework containing the whole with walls covered with an insulating layer of material to reduce the losses at the absorber back and sides. The general experimental device is composed of two solar water heaters each with a storage tank (each equipped with a coil heat exchanger), a pyranometer, and a data acquisition system. The pyranometer also includes a thermopile, a digital integrator for the
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Fig. 1. Longitudinal section of a flat plate solar collector used for the study.
digital display reading of instantaneous power, and energy received over a specified period. To avoid the use of a circulating pump for the fluid, the collector is positioned at a lower level than the storage tank to allow a convective thermal loop between the exchanger and the collector. Thus, when the sun beams strike the collector surface, the density of the fluid in the collector becomes lower than the one contained in the exchanger. The hot water of the collector goes up into the exchanger, and the cold water of the exchanger goes down into the lower header tube. For this thermosyphon effect to begin, a difference in height between the upper end of the collector and the center of the storage tank is needed (Cardonnel, 1983). For this study, the storage tank is positioned 0.5 m above the collector. One of the solar collectors is shown schematically in Fig. 2. The list of the materials used in the manufacture of each of them and their costs are presented in Appendix A. The comparative costs of these materials show that the glass wool only represents 27.3% of the total manufacturing cost for the glass wool solar water heater, when the coconut coir represents 2.81% for the total cost of the coconut coir solar water heater. The materials cost of the solar water heater using coconut coir as heat insulation is 25% cheaper than that of the glass wool one. In addition, the glass wool is imported and not always available in all countries like in Ivory Coast. Coconut coir has a life span of more than 10 years when protected (Koffi, 2008). The research was performed in Yamoussoukro, the political capital city of Ivory Coast, a West African country, located between 5° and 11° Northern latitude. Yamoussoukro is located at 6.54° Northern latitude (Bassigny and Gougou, 1996). The annual solar energy received in this area lies between 1650 and 1950 kWh/m2/day (Benallou and Bougard, 1990). In this country, coconut coir is considered as a waste because it is not useful and is generally discarded. To determine the optimal position of the collectors, preliminary measurements of temperature and insolation were realized while varying the inclination angle of the collectors. The results make it
possible to determine the optimal tilt angle, at 10° ± 2° compared to the horizontal (Nanga et al., 1998). The study compares the thermal performance of solar water heaters using traditional heat insulation with one using an alternative material, coconut coir. Table 1 lists the references for the data on the solar collectors whose performance data are compared. Table 2 shows some physical characteristic parameters of each collector. The type of insulation used in the collectors, the thermal conductivity, and the thickness of the insulation are also presented. Among the ten solar water heaters, the data for eight of them are taken from the published literature while the data of two (collectors C7 and C8), designed, manufactured, and tested in our laboratories under the same operating and climatic conditions, are obtained from our experimental studies (Koffi, 2008). The two collectors have the same area (2 m2), the same insulation thickness (50 mm), the same optical efficiency (τα = 0.83), and the same fluid flow rate. The collector C7 is equipped with coconut coir (CC), while the collector C8 is equipped with glass wool (GW). A complete and detailed description of these devices can be found in Koffi (2008), and the eight others, in the various references cited in Table 1. Theoretical analysis The thermal performance of a flat plate solar collector relates the solar radiation input (IT), the useful energy gain (Q U), and the heat losses (Q L), expressed as: Q u = A c IT ðταÞ−Q L
ð1Þ
with Q L = A c UL ðTabs −Ta Þ
ð2Þ
Here, τα represents the fraction of the solar radiation absorbed by the collector and depends mainly on the transmittance of the transparent covers and on the absorbance of the absorber. The higher
Table 1 Presentation of the references relating to each collector.
Fig. 2. Front view of one of the solar water heaters, the storage tank, and the connection of pipes used for the study.
Collectors
References
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10
Huang and Hsieh (1985) Karaghouli and Alnaser (2001) Chuawittayawuth and Kumar (2002) Abdullah et al. (2003) Esen and Esen (2005) Kürklü et al. (2002) Koffi (2008) (our study) Koffi (2008) (our study) Saha and Mahanta (2001) Kalogirou and Papamarcou (2000)
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Table 2 Some characteristic parameters of the ten collectors. Collector
Insulation Insulation thermal conductivity (W/m K) Insulation thickness (mm) Fluid flow rate (kg/s)
C1
C2
C3
C4
Calcium silicate 0.828 30 0.023
Glass wool Polyurethane Polyurethane Glass wool 0.040 0.035 0.035 0.040 40 30 40 100 0.022 0.033
this parameter is, the better the collector. UL is the collector overall heat loss coefficient. The smaller this parameter is, the best the collector. Ac is the collector area in meter squared, Tabs and Ta are absorber and ambient temperatures. Hottel and Whillier (1958) showed how Eqs. (1) and (2) can be combined and rewritten as: Q u = Ac F ′½IT ðταÞ−UL ðTabs −Ta Þ
ð3Þ
F' is the collector efficiency factor. It depends on the details of the construction. The useful heat gain can also be expressed from the working fluid flow rate (m) ˙ through the collector and the inlet (Tfi) and outlet (Tf0) working fluid temperatures as: ˙ p Tf 0 −Tfi Q u = mC
ð4Þ
The collector instantaneous efficiency (η(t) is influenced by several factors such as the material used, the design of the absorber, the properties of glass, weather and operating conditions (Nahar, 2003). It is defined as the ratio of the useful heat gain (Q U) to the solar radiation intensity (IT(τα)). η=
Qu Ac IT ðταÞ
ð5Þ
Or by η = FR ðταÞ−FR UL
Tabs −Ta IT ðταÞ
ð6Þ
Results and discussion The thermal performance depends on the design parameters, thickness and type of insulation, number and type of glass covers, spacing between absorber and inner glass, glazing material, weather, and operating conditions (Nahar, 2003). For the study, eight collectors of different configurations were randomly selected from the literature. The study focuses particularly on the ability of each system to convert solar energy into heat. The data presented for the comparisons are those obtained for the best operating conditions (ideal case with clear sunshine), that is, neither cloud nor haze obscures the sun, highest heat flux, highest useful energy, and highest water temperature rise in the collectors, at a given time (t). It is also assumed that no water is drawn from the storage tanks during the day for all the collectors. The thermal performance of a solar collector is based particularly on a good heat insulation of the absorber, resulting in very low thermal losses. Thus, the thickness and the thermal conductivity of the insulation constitute determining factors. It is what one observes from the various results presented below in Figs. 3 and 4. So, Fig. 3 presents the comparative hot water temperature rise in the collectors. This parameter varies from the highest value of 62 °C, for the collector C10 to the lowest value of 24 °C for the collector C1. That of collectors C7 (with coconut coir), C8 (with glass wool) are 43 and 49 °C, respectively. This is to be expected, since glass wool has a lower thermal
C5
C6
C7
C8
C9
C10
Glass wool 0.040 50 0.0023
Coconut Coir 0.074 50 0.0085
Glass wool 0.040 50 0.0085
Glass fiber 0.035 50 0.0083
Glass fiber 0.035 50 0.002
conductivity (0.040 W/m °C) (Bejan, 1993) than coconut coir (0.074 W/ m °C) (Appendix B). The temperature rise for collector C7 is higher than those of collectors C1 (24 °C), C6 (25 °C), C2 (28 °C) C9 (37 °C), and C4 (40 °C). These results are obtained with an error margin of ±0.5 °C. As Eq. (4) shows, the water temperature rise in the absorber is proportional to the useful energy, on the one hand, and inversely proportional to the fluid flow rate, on the other hand. When analyzing the results of the ten collectors, it can be seen that only the useful heat gain (QU) influences notably these results. The fluid flow rate does not have practically any influence on these results. For example, for the collectors C7 (with coconut coir) and C8 (with glass wool), designed, manufactured, and tested under the same operating and climatic conditions, the fluid flow rate is the same for the two collectors (0.0085 kg/s); however, the water temperature rise is 43 °C for the collector C7 and 49 °C for the collector C8. The second example concerns the collectors C6 and C10 where the fluid flow rates are, respectively, 0.0023 kg/s and 0.0020 kg/s, two values almost identical; however, the collector C10 presents a water temperature rise twice that of the collector C6. Third example, when comparing the results of the collector C4 with those of the collector C2, the fluid flow rate in the collector C4 is approximately 1.5 times that of the collector C2; however, the water temperature rise in the collector C4 is approximately 1.5 times higher than that of this collector. Fig. 4 compares the efficiency calculated by Eq. (5). The highest efficiency of 58% corresponds to the collector C8, while the lowest value of 32% corresponds to the collector C6. That of the collector C7 is 52%, which constitutes a relative good value, considering that in general, collector's efficiencies are lower than 50%. For the collectors C1 (thermal conductivity = 0.828 W/m K; efficiency = 37%), C7 (thermal conductivity = 0.074 W/m K; efficiency = 52%), and C8 (thermal conductivity = 0.040 W/m K; efficiency = 58%), the logic is respected. Since the higher the thermal conductivity of the insulation, the higher the thermal losses through the sides and the bottom of the collectors, consequently the weaker is the thermal effectiveness. Eq. (6) indicates that efficiency (η ) values could be determined over a range of temperature differences between the absorber (Tabs),
70 60 50
To-Ti (°C)
Parameters
40 30 20 10 0
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
Collectors Fig. 3. Hot water temperature rise in the collectors: C7 and C8 are from our study.
300
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Table 3 Characteristic parameters and efficiency equations for the ten collectors.
Efficiency (%)
60 50 40 30 20 10 0
C1
C2
C3
C4
C5
C6
C7
C8
C9
Collectors
FR(τα)
FRUL
Efficiency equations
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10
0.78 0.80 0.80 0.68 0.74 0.87 0.80 0.80 0.66 0.79
8.88 6.55 9.00 7.76 3.55 12.33 5.76 5.40 8.00 6.65
η = 0. 78 − 8. 88(Τabs − Τα) / ΙΤ(τα) η = 0. 80 − 6. 55(Τabs − Τα) / ΙΤ(τα) η = 0. 80 − 9. 00(Τabs − Τα) / ΙΤ(τα) η = 0. 68 − 7. 76(Τabs − Τα) / ΙΤ(τα) η = 0. 74 − 3. 55(Τabs − Τα) / ΙΤ(τα) η = 0. 87 − 12. 33(Τabs − Τα) / ΙΤ(τα) η = 0. 80 − 5. 76(Τabs − Τα) / ΙΤ(τα) η = 0. 80 − 5. 40(Τabs − Τα) / ΙΤ(τα) η = 0. 66 − 8. 00(Τabs − Τα) / ΙΤ(τα) η = 0. 79 − 6. 65(Τabs − Τα) / ΙΤ(τα)
C10
Collectors Conclusion Fig. 4. Comparative efficiencies of the ten collectors: C7 and C8 are from our study.
ambient air (Ta) and the solar radiation intensity (IT), in order to draw an efficiency curve for a collector. A straight line will result where the slope is equal to FRUL and the y-intercept is equal to FR(τα). So, for each collector, an efficiency curve, i.e., efficiency η against (Tabs − Ta)/ IT, is plotted. An equation for each efficiency curve was obtained to compute collector characteristic parameters. Fig. 5 shows efficiency curves of the ten collectors. The respective FR(τα), FRUL values and efficiency equations are presented in Table 3. Considering linear regression equations of the collectors, the optical effectiveness (τα) varies from 0.66 for the collector C9 to 0.87 for the collector C6. That one of the collector C7 is 0.78. The overall heat loss coefficient varies from 12.33 W/m2 °C for the collector C6 to 3.55 W/ m2°C for the collector C5. That of the collector C7 is 5.76 W/m2 °C. By analyzing these results, the following comments can be made: according to Duffie and Beckman (1991) for a good collector, the pair of values of the intercept FR(τα) and the slope FRUL, are 0.8 and 4.5 W/ m2°C. These parameters are 0.6 and 8.5 W/m2°C, for a poor collector (Tiwari et al., 1991). Those of collector C7 are 0.78 and 5.76 W/m2 °C, which are good values for this collector among the ten.
An experimental study is conducted to evaluate the thermal performance of a solar water heater using a vegetable fiber, coconut coir (collector C7), as heat insulation. This thermal performance is then, firstly, compared with that of a collector (C8), using glass wool as heat insulation, designed, manufactured, and tested under the same operating and climatic conditions and, secondly, to that of eight others taken randomly from the published literature, using various materials as heat insulation. The comparison mainly focuses on three fundamental parameters: (1) the water temperature rise through the absorber, (2) the efficiency of the collector, and (3) the pair of values of FR(τα) and FRUL. So, the water temperature rise through the absorber of the collectors, in decreasing order of performance, was determined to be collector C10, collector C8, collector C5, collector C3, collector C7, and so on. In terms of efficiency, the decreasing order of performance is collector C8, collector C3, collector C5, collector C7, and so on. Concerning the absorptivity and the transmittivity just as the thermal losses, the collector C7 (with coconut coir) is classified among the good collectors. It can be said that the results of this study are original and important when compared with those of previous works. So, the low cost of this type of solar water heater and its attested good thermal performances show that this material is a good alternative material to conceive now solar water heater at reasonable prices.
Appendix A. The cost of materials used for the manufacture of the two types of collectors including tanks and labor, in US dollars (US$)
90 80
Efficiency (%)
70
C1
C2
C3
C4
C5
Quantity Part
Cost (US$) glass wool
Cost (US$) coconut coir
C6
4
129
129
C7
C8
1
19
19
C9
C10
3
94
94
194 2.4 220
194 2.4 17
84 20.7
84 20.7
18 7.5 19.5
18 7.5 19.5
60 50 40
3 1 1
30 20
1 1
10 0
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
(Ti-Ta)/IT (°C.m2/W) Fig. 5. Collector comparative efficiencies with the reduced temperature parameter to estimate FR(τα) and FRUL of the ten collectors: C7 and C8 are from our study.
1 1 1
Galvanized steel tubes of 0.5-in. diameter and 6 m long (standard length) Galvanized steel tube of 1-in. diameter and 6 m long Galvanized steel sheet (2 × 1 m standard dimensions) 1 mm thick Galvanized steel sheet (2 × 1 m) 2 mm thick Aluminum foil (2.5 × 1.5 m) Roll of glass wool, or coconut coir, 0.2 m3 (standard dimension) (50 mm thickness) Glass plate of 4 mm thick (2.2 × 1 m) Tin (5 kg) of non glossy black paint + painting brush + thinner Tin of silicone Bag of Steel rivets 10 m flexible plastic hose and clips (for the piping) Labor cost Total
50 808
50 605
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Appendix B. Determination of the thermal conductivity of the coconut coir The measurement of the coconut coir thermal conductivity was carried out in a laboratory of Poitiers University, in France. B.1. Principle of the measurement The method used in our case is what is known as the box method. It consists of establishing a one-way heat flux, normal to the surface of the sample to be tested, as presented in Fig. B. The sample is placed between a hot source and a cold one. The variation in temperature generates a heat flux proportional to the temperature difference. Once steady state is reached, one can record the average temperature on the cold and hot sides of the sample. The air temperature of the hot box and that of ambient air was also measured. Knowing the power delivered by the hot source, it then becomes possible to calculate the thermal conductivity of the material by reporting the values obtained in the Fourier equation.
Fig. B. Experimental setup of coconut coir thermal conductivity measurement.
B.2.2. Results The result obtained after 10 h of maintaining the device in a steady state is k = 0.074 W/m K, with a result precision of ±5%. References
dQ ∂T = −kdS dt ∂n
ðB1Þ
After simplification of the equation according to the experimental conditions, one obtains:
Q =
k SðTh −Tc Þ + Q L e
ðB2Þ
with: Q L = C ðTb −Ta Þ
ðB3Þ
Q is the power provided by the hot source, Q L corresponds to the lateral losses that occur at the level of the walls of the box, e is the thickness of the sample, k is the thermal conductivity of the sample, S is the surface of the sample, C is a constant of the measurement device. Th, Tc, Tb, and Ta are, respectively, the temperatures of the hot surface of the material, the cold surface of the material, the box, and ambient. B.2. Experimental procedure The characteristics of the sample tested are summarized in the table below:
Thickness
Surface
Volume
Mass
Volume mass
0.04 m
0.042 m2
0.00168 m3
0.037 kg
22 kg/m3
B.2.1. Installation of the sample The material tested is very heterogeneous. So, we laid out 9 thermocouples. The thermocouples are fixed using adhesive tape and the contact with the sample is optimized thanks to silicone grease.
Abdullah AH, Abou-Ziyan HZ, Ghoneim AA. Thermal performance of flat plate solar collector using various arrangements of compound honeycomb. Energy Convers Manage 2003;44:3093–112. Bassigny A, Gougou A. Rapport sur le plan directeur de l'assainissement de Yamoussoukro, Decom; 1996. Bejan A. Heat Transfer. Wiley: New York; 1993. 675 pp. Benallou A, Bougard J. Guide de l'énergie solaire: le solaire thermique au service du développement durable, (Guide of solar energy: the thermal solar to the service of sustainable development). Québec: IEPF; 1990. 166 pp. Cardonnel C. Solaire actif et passif, conception, bilan thermique, habitat. Editions Parisiennes, CFP, chaud froid, plomberie; 1983. 237 pp. Chang JM, Shen MC, Huang BJ. A criterion study of solar irradiation patterns for the performance testing of thermosyphon solar water heaters. Solar Energy 2002;73: 287–92. Chuawittayawuth K, Kumar S. Experimental investigation of temperature and flow distribution in a thermosyphon solar water heating system. Renewable Energy 2002;26:431–48. Duffie JA, Beckman WA. Solar Engineering of Thermal Processes. New York: Wiley; 1991. p. 197–249. Esen M, Esen H. Experimental investigation of a two-phase closed thermosyphon solar water heater. Solar Energy 2005;79:459–68. Hottel HC, Whillier A. Evaluation of flat plate solar collector performance. Trans., of Conference on the use of Solar Energy, University of Arizona; 1958. p. 74-104. Huang BJ, Hsieh CT. A simulation method for solar thermosyphon collector. Solar Energy 1985;35(1):31–43. Kalogirou S, Panteliou S, Dentsoras A. Modelling of solar domestic water heating systems using artificial neural networks. Solar Energy 1999;65(6):335–42. Kalogirou SA, Papamarcou C. Modelling of a thermosyphon solar water heating system and simple model validation. Renewable Energy 2000;21:471–93. Karaghouli AA, Alnaser WE. Experimental study on thermosyphon solar water heater in Bahrain. Renewable Energy 2001;24:389–96. Koffi PME. Conception et réalisation d'un chauffe eau solaire thermosiphon—Etude comparative d'un isolant classique et d'un isolant à fibre végétale (Design and realization of a thermosyphon solar water heater—Comparative study of a traditional insulation with a vegetable fiber insulation). Thesis, Institut National Polytechnique Houphouët Boigny Yamoussoukro-Côte d'Ivoire. 2008. 115 pp. Kudish AI, Evseev EG, Walter G, Priebe T. Coaxial tubular solar collector constructed from polymer materials: an experimental and transient simulation study. Energy Convers Manage 2003;44:2549–66. Kürklü A, Özmerzi A, Bilgin S. Thermal performance of a water-phase change material solar collector. Renewable Energy 2002;26:391–9. Nahar NM. Year round performance and potential of a natural circulation type of solar water heater in India. Energy Build 2003;35:239–47. Nanga S, Siaka D, Kodjo D. Optimisation de l'angle d'inclinaison d'un capteur solaire en Côte d'Ivoire (Optimisation of inclination angle of a solar collector in Côte d'Ivoire). Projet de Fin d'Etude, INP-HB Yamoussoukro, Côte d'Ivoire; 1998. p. 150. Saha SK, Mahanta DK. Thermodynamic optimization of solar flat-plate collector. Renewable Energy 2001;23:181–93. Tiwari RC, Kumar A, Gupta SK, Sootha GD. Thermal performance of flat plate solar collectors manufactured in India. Energy Convers Manage 1991;31:309–13.
Contents lists available at ScienceDirect
Energy for Sustainable Development
Thermal performance study of a solar collector using a natural vegetable fiber, coconut coir, as heat insulation H.Y. Andoh a,⁎, P. Gbaha a, B.K. Koua b, P.M.E. Koffi a, S. Touré b a b
Laboratoire d'Energies Nouvelles et Renouvelables, Institut National Polytechnique Félix Houphouët Boigny, B.P. 1526 Yamoussoukro, Ivory Coast Laboratoire d'Energie Solaire, Université de Cocody, 22 B.P. 582, Abidjan 22, Ivory Coast
a r t i c l e
i n f o
Article history: Received 21 March 2009 Revised 22 September 2010 Accepted 23 September 2010 Keywords: Solar water heater Natural circulation Coconut coir Polyurethane Glass wool Fiber glass Polystyrene
a b s t r a c t There are few solar water heaters in Africa because of their acquisition cost, which makes it difficult for the populations to afford them. This article presents a solar water heater designed with a local vegetable material as insulating material, coconut coir, widespread in tropical countries. The study focuses on the comparative thermal performance of this collector and another collector, identical in design, fabrication, and operating under the same conditions, using glass wool as heat insulation, as well as with eight other designs, chosen randomly, using various materials as heat insulation, with performance data from the literature. The materials cost of the coconut coir collector is 25% less than the glass wool one. The results of the study show very good thermal performance of the collector using coconut coir compared to the traditional ones. For example, the outlet hot water temperature of the coconut coir collector was more than 80 °C. Internal hot water temperature rise was more than 40 °C. The thermal efficiency was a little over 51% although it was generally below 50% for the collectors using traditional insulation. It has a good pair of values of FR(τα) and FRUL. So, the lower cost of this type of solar water heater and its performance suggest that this design may be more suitable for application in tropical countries where coconut coir is commonly available. © 2010 International Energy Initiative. Published by Elsevier Inc. All rights reserved.
Introduction Conventional natural circulation flat plate solar water heaters are the most economical and large-scale use of solar energy all over the world. It has been designed, developed, and tested in detail by Kalogirou et al. (1999), Karaghouli and Alnaser (2001), Chang et al. (2002), and Kudish et al. (2003), among many others. Its thermal performance, which depends on its design parameters, thickness, type of insulation, number and type of glass covers, spacing between absorber and inner glass, has also been studied by Nahar (2003), Abdullah et al. (2003), and Esen and Esen (2005). Apart from these parameters, its performance also depends on climatic and operational parameters (Nahar, 2003). Although very widespread, the solar water heater remains rare in Africa, because of its acquisition cost, which is relatively high. The objective of the study is to conceive a cheap yet efficient flat plate solar water heater. The work focuses mainly on the comparison of the thermal performance and the fabrication costs of two identical solar water heaters of the same dimensions, design, manufactured in the same manner, operating in the same conditions. One of the two collectors of these solar water heaters uses the glass wool (GW), and the other uses a natural vegetable fiber, coconut coir (CC), as heat
⁎ Corresponding author. E-mail address: [email protected] (H.Y. Andoh).
insulation. The comparison extends to other solar water heaters from the literature, using traditional thermal insulation. In our analysis, we took into account (1) the rise of water temperature through the absorbers, (2) the efficiency of the collectors (ability of the collector to convert the solar energy into heat), (3) the pair of values that characterizes the collector, like FR(τα) and FRUL., in which, FR is the heat removal efficiency, UL, the total heat loss coefficient, and (τα) the optical effectiveness. Materials and methods For this study, the flat plate solar collector we used (Fig. 1) for the production of domestic hot water is primarily made of (1) a blackpainted network of piping in which circulates the working fluid towards a storage tank; (2) a reflective aluminum plate located below the tubes, which allows the reduction of the thermal losses towards the back of the collector by reflecting the thermal radiation inside it; (3) a glass cover to reduce the losses at the absorber by radiation (greenhouse effect) and by convection (effect of motionless air layer); and (4) a framework containing the whole with walls covered with an insulating layer of material to reduce the losses at the absorber back and sides. The general experimental device is composed of two solar water heaters each with a storage tank (each equipped with a coil heat exchanger), a pyranometer, and a data acquisition system. The pyranometer also includes a thermopile, a digital integrator for the
0973-0826/$ – see front matter © 2010 International Energy Initiative. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.esd.2010.09.006
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Fig. 1. Longitudinal section of a flat plate solar collector used for the study.
digital display reading of instantaneous power, and energy received over a specified period. To avoid the use of a circulating pump for the fluid, the collector is positioned at a lower level than the storage tank to allow a convective thermal loop between the exchanger and the collector. Thus, when the sun beams strike the collector surface, the density of the fluid in the collector becomes lower than the one contained in the exchanger. The hot water of the collector goes up into the exchanger, and the cold water of the exchanger goes down into the lower header tube. For this thermosyphon effect to begin, a difference in height between the upper end of the collector and the center of the storage tank is needed (Cardonnel, 1983). For this study, the storage tank is positioned 0.5 m above the collector. One of the solar collectors is shown schematically in Fig. 2. The list of the materials used in the manufacture of each of them and their costs are presented in Appendix A. The comparative costs of these materials show that the glass wool only represents 27.3% of the total manufacturing cost for the glass wool solar water heater, when the coconut coir represents 2.81% for the total cost of the coconut coir solar water heater. The materials cost of the solar water heater using coconut coir as heat insulation is 25% cheaper than that of the glass wool one. In addition, the glass wool is imported and not always available in all countries like in Ivory Coast. Coconut coir has a life span of more than 10 years when protected (Koffi, 2008). The research was performed in Yamoussoukro, the political capital city of Ivory Coast, a West African country, located between 5° and 11° Northern latitude. Yamoussoukro is located at 6.54° Northern latitude (Bassigny and Gougou, 1996). The annual solar energy received in this area lies between 1650 and 1950 kWh/m2/day (Benallou and Bougard, 1990). In this country, coconut coir is considered as a waste because it is not useful and is generally discarded. To determine the optimal position of the collectors, preliminary measurements of temperature and insolation were realized while varying the inclination angle of the collectors. The results make it
possible to determine the optimal tilt angle, at 10° ± 2° compared to the horizontal (Nanga et al., 1998). The study compares the thermal performance of solar water heaters using traditional heat insulation with one using an alternative material, coconut coir. Table 1 lists the references for the data on the solar collectors whose performance data are compared. Table 2 shows some physical characteristic parameters of each collector. The type of insulation used in the collectors, the thermal conductivity, and the thickness of the insulation are also presented. Among the ten solar water heaters, the data for eight of them are taken from the published literature while the data of two (collectors C7 and C8), designed, manufactured, and tested in our laboratories under the same operating and climatic conditions, are obtained from our experimental studies (Koffi, 2008). The two collectors have the same area (2 m2), the same insulation thickness (50 mm), the same optical efficiency (τα = 0.83), and the same fluid flow rate. The collector C7 is equipped with coconut coir (CC), while the collector C8 is equipped with glass wool (GW). A complete and detailed description of these devices can be found in Koffi (2008), and the eight others, in the various references cited in Table 1. Theoretical analysis The thermal performance of a flat plate solar collector relates the solar radiation input (IT), the useful energy gain (Q U), and the heat losses (Q L), expressed as: Q u = A c IT ðταÞ−Q L
ð1Þ
with Q L = A c UL ðTabs −Ta Þ
ð2Þ
Here, τα represents the fraction of the solar radiation absorbed by the collector and depends mainly on the transmittance of the transparent covers and on the absorbance of the absorber. The higher
Table 1 Presentation of the references relating to each collector.
Fig. 2. Front view of one of the solar water heaters, the storage tank, and the connection of pipes used for the study.
Collectors
References
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10
Huang and Hsieh (1985) Karaghouli and Alnaser (2001) Chuawittayawuth and Kumar (2002) Abdullah et al. (2003) Esen and Esen (2005) Kürklü et al. (2002) Koffi (2008) (our study) Koffi (2008) (our study) Saha and Mahanta (2001) Kalogirou and Papamarcou (2000)
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Table 2 Some characteristic parameters of the ten collectors. Collector
Insulation Insulation thermal conductivity (W/m K) Insulation thickness (mm) Fluid flow rate (kg/s)
C1
C2
C3
C4
Calcium silicate 0.828 30 0.023
Glass wool Polyurethane Polyurethane Glass wool 0.040 0.035 0.035 0.040 40 30 40 100 0.022 0.033
this parameter is, the better the collector. UL is the collector overall heat loss coefficient. The smaller this parameter is, the best the collector. Ac is the collector area in meter squared, Tabs and Ta are absorber and ambient temperatures. Hottel and Whillier (1958) showed how Eqs. (1) and (2) can be combined and rewritten as: Q u = Ac F ′½IT ðταÞ−UL ðTabs −Ta Þ
ð3Þ
F' is the collector efficiency factor. It depends on the details of the construction. The useful heat gain can also be expressed from the working fluid flow rate (m) ˙ through the collector and the inlet (Tfi) and outlet (Tf0) working fluid temperatures as: ˙ p Tf 0 −Tfi Q u = mC
ð4Þ
The collector instantaneous efficiency (η(t) is influenced by several factors such as the material used, the design of the absorber, the properties of glass, weather and operating conditions (Nahar, 2003). It is defined as the ratio of the useful heat gain (Q U) to the solar radiation intensity (IT(τα)). η=
Qu Ac IT ðταÞ
ð5Þ
Or by η = FR ðταÞ−FR UL
Tabs −Ta IT ðταÞ
ð6Þ
Results and discussion The thermal performance depends on the design parameters, thickness and type of insulation, number and type of glass covers, spacing between absorber and inner glass, glazing material, weather, and operating conditions (Nahar, 2003). For the study, eight collectors of different configurations were randomly selected from the literature. The study focuses particularly on the ability of each system to convert solar energy into heat. The data presented for the comparisons are those obtained for the best operating conditions (ideal case with clear sunshine), that is, neither cloud nor haze obscures the sun, highest heat flux, highest useful energy, and highest water temperature rise in the collectors, at a given time (t). It is also assumed that no water is drawn from the storage tanks during the day for all the collectors. The thermal performance of a solar collector is based particularly on a good heat insulation of the absorber, resulting in very low thermal losses. Thus, the thickness and the thermal conductivity of the insulation constitute determining factors. It is what one observes from the various results presented below in Figs. 3 and 4. So, Fig. 3 presents the comparative hot water temperature rise in the collectors. This parameter varies from the highest value of 62 °C, for the collector C10 to the lowest value of 24 °C for the collector C1. That of collectors C7 (with coconut coir), C8 (with glass wool) are 43 and 49 °C, respectively. This is to be expected, since glass wool has a lower thermal
C5
C6
C7
C8
C9
C10
Glass wool 0.040 50 0.0023
Coconut Coir 0.074 50 0.0085
Glass wool 0.040 50 0.0085
Glass fiber 0.035 50 0.0083
Glass fiber 0.035 50 0.002
conductivity (0.040 W/m °C) (Bejan, 1993) than coconut coir (0.074 W/ m °C) (Appendix B). The temperature rise for collector C7 is higher than those of collectors C1 (24 °C), C6 (25 °C), C2 (28 °C) C9 (37 °C), and C4 (40 °C). These results are obtained with an error margin of ±0.5 °C. As Eq. (4) shows, the water temperature rise in the absorber is proportional to the useful energy, on the one hand, and inversely proportional to the fluid flow rate, on the other hand. When analyzing the results of the ten collectors, it can be seen that only the useful heat gain (QU) influences notably these results. The fluid flow rate does not have practically any influence on these results. For example, for the collectors C7 (with coconut coir) and C8 (with glass wool), designed, manufactured, and tested under the same operating and climatic conditions, the fluid flow rate is the same for the two collectors (0.0085 kg/s); however, the water temperature rise is 43 °C for the collector C7 and 49 °C for the collector C8. The second example concerns the collectors C6 and C10 where the fluid flow rates are, respectively, 0.0023 kg/s and 0.0020 kg/s, two values almost identical; however, the collector C10 presents a water temperature rise twice that of the collector C6. Third example, when comparing the results of the collector C4 with those of the collector C2, the fluid flow rate in the collector C4 is approximately 1.5 times that of the collector C2; however, the water temperature rise in the collector C4 is approximately 1.5 times higher than that of this collector. Fig. 4 compares the efficiency calculated by Eq. (5). The highest efficiency of 58% corresponds to the collector C8, while the lowest value of 32% corresponds to the collector C6. That of the collector C7 is 52%, which constitutes a relative good value, considering that in general, collector's efficiencies are lower than 50%. For the collectors C1 (thermal conductivity = 0.828 W/m K; efficiency = 37%), C7 (thermal conductivity = 0.074 W/m K; efficiency = 52%), and C8 (thermal conductivity = 0.040 W/m K; efficiency = 58%), the logic is respected. Since the higher the thermal conductivity of the insulation, the higher the thermal losses through the sides and the bottom of the collectors, consequently the weaker is the thermal effectiveness. Eq. (6) indicates that efficiency (η ) values could be determined over a range of temperature differences between the absorber (Tabs),
70 60 50
To-Ti (°C)
Parameters
40 30 20 10 0
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
Collectors Fig. 3. Hot water temperature rise in the collectors: C7 and C8 are from our study.
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Table 3 Characteristic parameters and efficiency equations for the ten collectors.
Efficiency (%)
60 50 40 30 20 10 0
C1
C2
C3
C4
C5
C6
C7
C8
C9
Collectors
FR(τα)
FRUL
Efficiency equations
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10
0.78 0.80 0.80 0.68 0.74 0.87 0.80 0.80 0.66 0.79
8.88 6.55 9.00 7.76 3.55 12.33 5.76 5.40 8.00 6.65
η = 0. 78 − 8. 88(Τabs − Τα) / ΙΤ(τα) η = 0. 80 − 6. 55(Τabs − Τα) / ΙΤ(τα) η = 0. 80 − 9. 00(Τabs − Τα) / ΙΤ(τα) η = 0. 68 − 7. 76(Τabs − Τα) / ΙΤ(τα) η = 0. 74 − 3. 55(Τabs − Τα) / ΙΤ(τα) η = 0. 87 − 12. 33(Τabs − Τα) / ΙΤ(τα) η = 0. 80 − 5. 76(Τabs − Τα) / ΙΤ(τα) η = 0. 80 − 5. 40(Τabs − Τα) / ΙΤ(τα) η = 0. 66 − 8. 00(Τabs − Τα) / ΙΤ(τα) η = 0. 79 − 6. 65(Τabs − Τα) / ΙΤ(τα)
C10
Collectors Conclusion Fig. 4. Comparative efficiencies of the ten collectors: C7 and C8 are from our study.
ambient air (Ta) and the solar radiation intensity (IT), in order to draw an efficiency curve for a collector. A straight line will result where the slope is equal to FRUL and the y-intercept is equal to FR(τα). So, for each collector, an efficiency curve, i.e., efficiency η against (Tabs − Ta)/ IT, is plotted. An equation for each efficiency curve was obtained to compute collector characteristic parameters. Fig. 5 shows efficiency curves of the ten collectors. The respective FR(τα), FRUL values and efficiency equations are presented in Table 3. Considering linear regression equations of the collectors, the optical effectiveness (τα) varies from 0.66 for the collector C9 to 0.87 for the collector C6. That one of the collector C7 is 0.78. The overall heat loss coefficient varies from 12.33 W/m2 °C for the collector C6 to 3.55 W/ m2°C for the collector C5. That of the collector C7 is 5.76 W/m2 °C. By analyzing these results, the following comments can be made: according to Duffie and Beckman (1991) for a good collector, the pair of values of the intercept FR(τα) and the slope FRUL, are 0.8 and 4.5 W/ m2°C. These parameters are 0.6 and 8.5 W/m2°C, for a poor collector (Tiwari et al., 1991). Those of collector C7 are 0.78 and 5.76 W/m2 °C, which are good values for this collector among the ten.
An experimental study is conducted to evaluate the thermal performance of a solar water heater using a vegetable fiber, coconut coir (collector C7), as heat insulation. This thermal performance is then, firstly, compared with that of a collector (C8), using glass wool as heat insulation, designed, manufactured, and tested under the same operating and climatic conditions and, secondly, to that of eight others taken randomly from the published literature, using various materials as heat insulation. The comparison mainly focuses on three fundamental parameters: (1) the water temperature rise through the absorber, (2) the efficiency of the collector, and (3) the pair of values of FR(τα) and FRUL. So, the water temperature rise through the absorber of the collectors, in decreasing order of performance, was determined to be collector C10, collector C8, collector C5, collector C3, collector C7, and so on. In terms of efficiency, the decreasing order of performance is collector C8, collector C3, collector C5, collector C7, and so on. Concerning the absorptivity and the transmittivity just as the thermal losses, the collector C7 (with coconut coir) is classified among the good collectors. It can be said that the results of this study are original and important when compared with those of previous works. So, the low cost of this type of solar water heater and its attested good thermal performances show that this material is a good alternative material to conceive now solar water heater at reasonable prices.
Appendix A. The cost of materials used for the manufacture of the two types of collectors including tanks and labor, in US dollars (US$)
90 80
Efficiency (%)
70
C1
C2
C3
C4
C5
Quantity Part
Cost (US$) glass wool
Cost (US$) coconut coir
C6
4
129
129
C7
C8
1
19
19
C9
C10
3
94
94
194 2.4 220
194 2.4 17
84 20.7
84 20.7
18 7.5 19.5
18 7.5 19.5
60 50 40
3 1 1
30 20
1 1
10 0
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
(Ti-Ta)/IT (°C.m2/W) Fig. 5. Collector comparative efficiencies with the reduced temperature parameter to estimate FR(τα) and FRUL of the ten collectors: C7 and C8 are from our study.
1 1 1
Galvanized steel tubes of 0.5-in. diameter and 6 m long (standard length) Galvanized steel tube of 1-in. diameter and 6 m long Galvanized steel sheet (2 × 1 m standard dimensions) 1 mm thick Galvanized steel sheet (2 × 1 m) 2 mm thick Aluminum foil (2.5 × 1.5 m) Roll of glass wool, or coconut coir, 0.2 m3 (standard dimension) (50 mm thickness) Glass plate of 4 mm thick (2.2 × 1 m) Tin (5 kg) of non glossy black paint + painting brush + thinner Tin of silicone Bag of Steel rivets 10 m flexible plastic hose and clips (for the piping) Labor cost Total
50 808
50 605
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Appendix B. Determination of the thermal conductivity of the coconut coir The measurement of the coconut coir thermal conductivity was carried out in a laboratory of Poitiers University, in France. B.1. Principle of the measurement The method used in our case is what is known as the box method. It consists of establishing a one-way heat flux, normal to the surface of the sample to be tested, as presented in Fig. B. The sample is placed between a hot source and a cold one. The variation in temperature generates a heat flux proportional to the temperature difference. Once steady state is reached, one can record the average temperature on the cold and hot sides of the sample. The air temperature of the hot box and that of ambient air was also measured. Knowing the power delivered by the hot source, it then becomes possible to calculate the thermal conductivity of the material by reporting the values obtained in the Fourier equation.
Fig. B. Experimental setup of coconut coir thermal conductivity measurement.
B.2.2. Results The result obtained after 10 h of maintaining the device in a steady state is k = 0.074 W/m K, with a result precision of ±5%. References
dQ ∂T = −kdS dt ∂n
ðB1Þ
After simplification of the equation according to the experimental conditions, one obtains:
Q =
k SðTh −Tc Þ + Q L e
ðB2Þ
with: Q L = C ðTb −Ta Þ
ðB3Þ
Q is the power provided by the hot source, Q L corresponds to the lateral losses that occur at the level of the walls of the box, e is the thickness of the sample, k is the thermal conductivity of the sample, S is the surface of the sample, C is a constant of the measurement device. Th, Tc, Tb, and Ta are, respectively, the temperatures of the hot surface of the material, the cold surface of the material, the box, and ambient. B.2. Experimental procedure The characteristics of the sample tested are summarized in the table below:
Thickness
Surface
Volume
Mass
Volume mass
0.04 m
0.042 m2
0.00168 m3
0.037 kg
22 kg/m3
B.2.1. Installation of the sample The material tested is very heterogeneous. So, we laid out 9 thermocouples. The thermocouples are fixed using adhesive tape and the contact with the sample is optimized thanks to silicone grease.
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