Study of the Energetic, Exergetic, and Thermal Balances of a Solar System in comparison with a Conventional System during the Distillation of Rosemary Leaves


 The solar energy produced by Scheffler parabola (10 m2), is not fully exploited by solar distillation system of Aromatic and Medicinal Plants. In this work, the optical losses in the primary and secondary reflectors, and the thermal losses at each part of this system (distillation still, steam line, condenser) were determined. A thermal energetic and exergetic analysis was also performed for a solar system distillation of rosemary leaves. For average intensity radiation of 849.1W/m2 and 6 Kg of rosemary leaves during 4 hours of distillation, exergy and optical efficiencies of the system achieved up to 26.62% and 50.97%, respectively. The thermal efficiency of the still, steam line and condenser is about 94.80%, 93.08%, and 87.76%, respectively. Total efficiency of the solar distillation system, taking into account the heat losses in the still, steam line, and condenser, as well as the optical losses in the two reflectors, is 39.49%. The efficiency can be as high as 42.42% and if the steam line is insulated. Moreover, the comparison between the Solar Steam Distillation and Conventional Steam Distillation shows that solar distillation is much more efficient since it gives better results, and especially it avoids emission of 12.10 kg of CO2 during extraction.


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The world's energy relies heavily on fossil fuels; according to researchers, each year the energy consumption increases by 79 1% in the developed countries and 5% in developing countries. With those expectations, fossil fuel resources will not be able 80 to meet the rising energy demand (Herez, Ramadan, and Khaled 2018). The unsustainable and non-renewable nature of fossil 81 fuels coupled with environmental issues resulting from the use of these sources such as pollution, greenhouse effect, and 82 global warming led to the alternative green source investigation. Presently, renewable energies have gained remarkable 83 interest word widely, and it will play an important role in the world's future. According to the global renewable energy 84 scenario, the proportion of solar thermal applications will be about 480 million tons of oil equivalent by 2040 (Kralova and 85 Sjöblom 2010). Solar energy is one of the most promising sources in this category, currently used in numerous applications; 86 several of them rely on the conversion of energy into thermal energy such as solar cooking, drying, and extraction. The 87 ranges of all these processes lie between 60 and 280 °C (Munir, Hensel, and Scheffler 2010). 88 Aromatic and Medicinal Plants (AMPs) are strongly linked to human civilization. Many AMPs contain antioxidant 89 compounds used for food preservation instead of synthetic antioxidants, which have been the subject of numerous 90 epidemiological studies on the negative impact of these synthetic products on human health (Giacometti et al. 2018). These 91 plants can also be used to extract essential oils (EOs); with more than 3000 valorised species (Lubbe and Verpoorte 2011). 92 The extraction techniques have been used, according to researchers, since the discovery of fire. Traditional technologies for 93 treating essential oils are necessary and widely used in many parts of the world. Hydro-distillation, steam-distillation, and 94 maceration are the most used traditional methods. 95 The distillation of medicinal and aromatic plants by a decentralized solar system is an innovative technology that allows 96 the use of solar energy for the extraction of EOs, and facilitates the access of small farmers to this technique against the 97 centralized exploitation with high investment and operating costs. Since, the energy costs for solar heat are 0.015 to 0.028 C£ 98 / kWh and the annual energy gains is from 550 to 1100 kWh/m 2 (Kalogirou 2003). Indeed, the exploitation of solar energy in 99 this sector limits the consumption of conventional energies, encourages the use of renewable energies that reduce 100 environmental pollution, and the emission of carbon dioxide (Nandwani 1996). 101 Rosemary (Rosmarinus officinalis L.) is a perennial shrub native to the Mediterranean region. The plant is also cultivated 102 in Spain, Morocco, Tunisia and South-Eastern Europe. Rosemary leaves have an intense aromatic flavor and a bitter, slightly 103 spicy taste. Rosemary is widely used in seasonings and flavours, as a preservative and as an antioxidant. Pharmaceutical 104 applications are also known (Wollinger et al. 2016). 105 Wolfgang Scheffler first developed the Scheffler reflector in 1986 in India and Kenya (Scheffler 2006), it was used for the 106 distillation of AMPs to extract EOs by adding a second reflector, a still of distillation, and a condenser. Moreover, an 107 auxiliary biomass system has also been coupled to the distillation unit to complete the system in case of unfavorable climatic 108 conditions (Afzal et al. 2017). Several studies to determine the thermal power and the efficiency of the system have been 109 made. For an 8 m 2 Scheffler reflector made of aluminium and for solar irradiation in the range of 700-800 W/m 2 , the average 110 power and the efficiency of the solar distillation system were found to be 1.54 kW and 33.21%, respectively (Munir and 111 Hensel 2010). In addition, the use of this solar system for the production of steam is now an economically attractive 112 possibility since the payback period of such a system does not exceed 2 years (Jayasimha 2006). However, the system has 113 many losses that were ignored in previous studies (Kumar et  Based on the aforementioned considerations, this work aims to identify and determine the power lost by convection and 116 radiation at the still, the power lost in the steam line and condenser of a 10 m 2 solar system for distillation to establish thermal 117 balances of the solar system studied during the distillation of rosemary leaves. The optical losses (at the primary and 118 secondary reflectors) and thermal analysis (energetic and exergetic) were also performed. Moreover, a comparison of the 119 energy level and composition of essential oils with a conventional butane-based system was also made. This solar distillation system includes a 10 m 2 fixed focal length Scheffler concentrator, a secondary reflector, a 126 distillation still, a condenser, and a Florentine vase (Fig. 1). The axes of rotation of the reflector are set to the local latitude 127 angle (31° 37' 46) so that the axis of rotation of the reflector and the axis of rotation of the earth are parallel to each other. 128 The still has been insulated with 70 mm of rock wool to minimize heat loss. The 10 m 2 parabolic reflector is equipped 142 with an electronic small photovoltaic plate (PV) and mechanical system for the daily and seasonal monitoring of the sun. A 143

Photovoltaic tracking device
Motor Alembic Secondary reflector

Steam line Condenser
pyranometer and thermocouples were used to register solar radiation and temperature, respectively, and were connected to a 144 computer via a data recorder. 145

Energy distribution at the first reflector 146
The energy at the level of the first reflector is distributed in the form of absorbed radiation and reflected radiation. The 147 reflected radiation of energy (Erp) depends upon the reflectivity of the used material. Hence, the energy produced by the first 148 reflector is in the form of the following equation ( Where Gb (W/m 2 ) is the direct irradiation measured by a pyranometer, At is the reflector surface (10 m 2 ) and δ is the solar 154 declination (Munir 2010 Where Rs is the reflectivity of the secondary reflector (0.83). A concrete foundation has been built to fix the secondary 164 reflector in an optimal position in relation to the focal point. 165

Energy distribution at the distillation unit 166
The secondary reflector components are designed to reflect and distribute all the rays toward the bottom of the distillation 167 unit, the energy available at the bottom of the distillation unit (Ebot) is given by (Munir 2010 The useful energy and the energy losses of distillation unit include the reflectivity due to the incomplete absorbance and 170 the heat losses of different parts of the still by conduction, convection and by radiation. The thermal energy available to 171 operate the distillation system (Econd,b) is given by (Munir 2010 Where αb = 0.90 is the absorbance of the vessel. To determine the losses at the still, an electrical diagram will be evaluated 174 as shown in figure 2. Still line and the ambient air, because no Nusselt correlation is found for a pipe bent at 45° (Fig. 3), this work may present a key to 181 remedy this problem by making a total assessment between the condenser and the steam line. 182 In the condenser, the cooling water and the steam coming from the steam line circulate against the current. A simple 183 calculation shows that this relative flow system is more efficient. Additionally, the inlet temperature of the condenser cooling 184 water must be as cold as possible so that the amount of heat transferred is maximized. 185 In this case, the temperature difference between the water inlet and outlet of the condenser is minimized, resulting in 186 cooling energy (MacPhee and Dincer 2009): 187 The following diagram presented in Figure 3 has been used for the determination of heat losses in the solar system. 189 Where ηo is the optical efficiency of the two reflectors, ηstill is the thermal efficiency of the still, ηpipe is the thermal 203 efficiency of the steam line and ηcond is the thermal efficiency of the condenser. The thermal efficiencies are calculated using 204 the following relationships: 205 In addition, the efficiency of the distillation system is the volume of essential oil recovered per unit of energy consumed 209 (mL/kWh); this efficiency links useful solar energy to useful thermal energy through by the following equation: 210 ..
Where Tamb, and Ts, are the ambient temperature, and the temperature of the sun (Ts = 5762 K) (Venkatachalam and 220 Cheralathan 2019). Tout, and Tin is the final water temperature in the still is the initial water temperature in the still, ∆t is the 221 distillation time (s), m is the mass of water in (kg) and Cp is the heat capacity of water in (kJ. kg -1 .K -1 ). 222 The exergetic efficiency of the AMPs solar distillation system represents the ratio between the exergy required by the

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The conventional system used is made of the same components as the solar system, except that a butane gas cylinder 227 linked to an injector that burns this gas with a spark replaces the Scheffler dish and the secondary reflector. In this system, the 228 steam produced passes through the rosemary leaves and is charged with essential oil; it is then condensed and recuperated in 229 a Florentine flask. Extraction continues until no more essential oil is obtained. The recovered essential oil is dried with 230 anhydrous sodium sulphate and stored at 4°C until it is used. The energy produced by this system is calculated by equation Scheffler's parabola reflects 85% (reflectivity of the mirrors used) of the direct radiation collected by its surface, of which 244 only a fraction of 85% will reach the second reflector due to adjustment errors. The second reflector will also reflect a portion 245 of 83% of the radiation it has received, of which only 85% will reach the bottom of the still. The optical efficiency ηo = 246 50.97% higher than 47.3% obtained by Veynandt (Veynandt 2008) using an 8 m 2 Scheffler parabola for an irradiance of 850 247 W/m 2 . Moreover, this rate is in agreement with Scheffler (Scheffler 2006) who showed that about half of the solar energy 248 collected by the reflector finally becomes available at the bottom of the still. 249

Thermal losses in the still 250
For the calculation of the different powers, it was considered that the system works by natural convection around the still. 251 For this purpose, it was necessary to calculate the side surface, bottom and cone of the alembic while taking into account the 252 geometrical shape of each part in order to choose the correct Nusselt correlation (Munir 2010 (Table 3), the useful energy is 9.78 kWh (Table 1). While the energy consumed by water is 6.49 kWh, which means 261 that the useful energy can cover not only the needs demanded by water, but also produce energy that will be consumed by the 262 AMPs, as well as an additional energy that the system does not use. 263 Thermal losses in the steam line and the condenser 264 Figure 3 shows the different thermal energies lost at the level of the alembic, steam line and the condenser. The 265 temperature difference between the inlet and outlet water of the condenser is about 0.7°C, resulting in cooling energy 266 Elc=0.75 kWh. The energy required for the condensation of the steam produced in the still is Eoc=5.36 kWh. Therefore the 267 energy received by the condenser from the steam line is the sum of both Eop=6.11 kWh (Fig. 5). 268

Efficiency of the solar distillation system 273
The results, as shown in Table 3, indicate that the thermal efficiency of the steam line and the condenser were ηpipe = 274 93.08% and = 87.76%, respectively, for the distillation of 6 Kg of rosemary leaves with 15 L of water. 275 It was found that losses in the steam line present 6.92% of the energy consumed, while the efficiency of the system, 276 assuming that the AMPs will process all the energy produced by the still, sys = 39.49%. However, if the steam line is 277 insulated; the efficiency becomes 42.42%, which allows the system to benefice up to 2.93% of the energy consumed (Table  278 3). 279 For a mass of 6 Kg of rosemary, the EO extracted in this experiment is about 50 mL, while the energy consumed by the 280 water has been calculated at 6.49 kWh. This gives an important essential oil yield per unit of consumed energy ηEO = 6.26 281 mL/ kWh, much more effective than 1.13 mL / kWh obtained by Munir (Munir and Hensel 2010) for a quantity of 3 Kg of 282 rosemary leaves, the essential oil extracted volume is 4.6 ml via 8 m² solar reflector and for a consumed energy of 4.04 kWh. 283 While the mass yield in EO is 0.83%, slightly higher than 0.82% that found by Hilali with the same solar system (Hilali et al. In fact, three experiments were made, with the aim of comparing the effect of increasing the quantity of AMPs. It has been 297 proven that the more the quantity of AMPs is important, the time to produce steam increases. Table 2 shows the time needed  298 to obtain the first floral water drop. 299 In Table 3, for the same amount of water, an increase in plant mass increases the time needed for the evaporation of water 308 from 40 to 50 min, which increases the energy of sensitization and decreases the energy of the latent phase from 1.00 and 309 5.65 to 1.14 and 5.37 kWh, respectively. I.e. decrease in the energy consumed by water and AMPs from 6.66 to 6.49 kWh 310 and consequently decrease in the losses in the steam line from 0.75 to 0.37 kWh. As already mentioned, the plants prevent the 311 dispersion of the steam, which consequently increase the time needed for the boiling. Therefore This means that 49.03% of the energy collected has been wasted, but the useful energy and the efficiency of the still after 317 calculating the losses by conduction, convection and radiation are 11.86, 9.78, 9.78 kWh and 96.07%, 94.73%, 94.83%, 318 respectively, for 2, 4, and 6 Kg of rosemary leaves. 319 On the other hand, the thermal energy gained by water and AMPs during the two phases of sensitization and latency 320 during 4 hours is 6.66, 6.51, and 6.49 kWh for 2, 4, and 6 Kg of AMPs, respectively. While Munir found 9.13 kWh for the 321 distillation of 20 Kg of water for 6 hours with an 8 m 2 parabola Scheffler (Munir et al. 2014). 322 As shown in Table 3, the quantities of EOs recovered are 17, 35, and 50 mL, and essential oil extraction yields are 0.85%, 323 0.88%, and 0.83%, respectively, for 2, 4, and 6 Kg of AMPs, slightly higher than 0.82% that found by Hilali with the same 324 solar system (Hilali et al. 2018). 325 The total efficiency of the distillation system, taking into account the heat losses (at the still, the steam line, and the 326 condenser) and the optical losses at the level of the two reflectors is 40.61%, 38.90%, and 39.49%, higher to 33.21% found 327 by Munir (Munir et al. 2014). In addition, if the steam line has been insulated, an efficiency of 46.86%, 44.12%, and 42.42% 328 will be obtained, respectively. In addition, the energy gain can reach 6.25%. 329 Table 3 gives a total overview of the different energies, powers, and yields calculated for the three experiments. However, 330 this result has not previously been described. 331     Table 4 summarizes the average values of the exergetic quantities calculated for the distillation system for an average 365 radiation intensity between 838.02 and 863 W/m 2 . It is clear that the input exergy (19.43 kWh) and output exergy (5.17 kWh) 366 are significant and almost equal to the input energy (20.95 kWh) and output energy (5.715 kWh), respectively. However, the 367 major losses are optical losses at the two reflectors (first and secondary) which are 49.03% of the energy produced, but the 368 heat loss in the still by conduction, convection and radiation is only 5.2% confirmed by Veynandt (Veynandt 2008). These 369 losses reduce the energy produced, 20.95 kWh to the energy consumed 5.71 kWh, and consequently reduce the input exergy 370 19.59 kWh to the output exergy 5.17 kWh. Therefore, the exergetic efficiency of the solar system for the distillation of AMPs 371 is about 26.62% and is more efficient than 1.25% obtained by Ozturk (Öztürk 2004), when he showed that energy output 372 varied between 20.9 (0.084 kWh) and 78.1 W (0.31 kWh), whereas the exergy output was in the range 2.9-6.6 W (0.011-373 0.026 kWh) in four hours of operation (Öztürk 2004). The exergetic efficiency obtained in this work is also higher than 374 0.027% and 0.028% using an truncated pyramid type solar cooker and an solar box cooker respectively (Kumar et al. 2011  Comparison between the SSD and the CSD. 383 As shown in Table 5, the energies produced in 4 hours of distillation of 2 kg of rosemary leaves are 24.21 and 15.13 kWh 384 respectively for solar steam distillation (SSD) and conventional steam distillation (CSD). Moreover, with the SSD, energy 385 consumption has been completely eliminated as it is a sustainable and free source, while the CSD consumes about 1.2 kg of 386 butane gas (in 4 hours) or 12.10 kg of CO2 released during the extraction period (Li et al. 2013). It can be said that the SSD is 387 a sustainable and economical process for the extraction of essential oils. 388 Fifty-two compounds were identified in rosemary essential oils using both SSD and CSD techniques. As shown in Table  390 6, essential oil compounds such as monoterpenes and sesquiterpenes were ranked according to their retention time. In 391 general, oxygenated compounds are more valuable because of their high odor characteristic, thus adding to the essential oil's 392 fragrance. The essential oils obtained by the two processes are qualitatively similar. α-Pinene was mainly detected in both 393 essential oils: 42.27% for CSD and 46.98% for SSD, eucalyptol, also in similar amounts of 22.66% for CSD and 22.63% for 394 SSD. Therefore, the identification rate of compounds by the GC-MS analysis method is in favors of SSD 97.42% against 395 96.07% for CSD. SSD method can be considered as a more efficient procedure because it uses a green and renewable energy. 396 In this work, optical losses at two solar reflectors and thermal losses at the still, the steam line, and the condenser were 400