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Temperature dependence of thermophysical and rheological properties of seven vegetable oils in view of their use as heat transfer fluids in concentrated solar plants
Solar Energy Materials and Solar Cells 178 (2018) 129–138
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Temperature dependence of thermophysical and rheological properties of seven vegetable oils in view of their use as heat transfer fluids in concentrated solar plants
T
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J.-F. Hoffmanna,d,e, G. Vaitilingomb, J.-F. Henryc, M. Chirtocc, R. Olivesa, V. Goetza, , X. Pya a
PROMES-CNRS UPR-8521 Laboratory, University of Perpignan Via Domitia, Rambla de la Thermodynamique, Tecnosud, 66100 Perpignan, France CIRAD, unité de recherche BioWooEB, TA B-114/16, 73 rue JF Breton, 34398 Montpellier, France c GRESPI - EA 4694 Laboratory, Université de Reims Champagne Ardenne URCA, Moulin de la Housse BP 1039, 51687 Reims, France d AQYLON, 46-48 rue Renée-Clair, 75892 Paris, France e EDF - R&D, MFEE - Nouvelles Filières de Production et Thermochimie, 6 quai Waitier, 78401 Chatou, France b
A R T I C L E I N F O
A B S T R A C T
Keywords: Vegetable oils Thermophysical properties Thermal conductivity Specific heat capacity Dynamic viscosity Heat transfer fluid
Selecting a heat transfer fluid (HTF) depends on the value of thermophysical parameters. In response to the need for innovative heat transfer fluids in concentrated solar power (CSP) plants, vegetable oil offers a promising solution. The relevant thermophysical properties are density, thermal conductivity and specific heat capacity. Moreover, a rheological property, the dynamic viscosity is also relevant. The objective of this work was to study and compare these properties for seven different vegetable oils (rapeseed, soybean, sunflower, palm, copra, cotton and jatropha) in the temperature range from ambient to 250 °C. For all the properties studied, the values evolved with increasing temperature and were influenced by the fatty acid composition of each vegetable oil. Experimental results are compared with those in the literature and are found to be consistent. The temperature dependencies are correlated to temperature using polynomial equations. The correlations presented here may be useful as a database for the selection of innovative heat transfer fluids.
1. Introduction: vegetable oils as HTF 1.1. Selection criteria for heat transfer fluids Synthetic or mineral high temperature oils are widely used as industrial heat transfer fluids in many process applications. In concentrated solar power (CSP) plants, the heat transfer fluid (HTF) circulates in the solar receiver and transmits its energy to the heat engine cycle [1]. The thermodynamic performance of the working fluid is important in order to ensure the plant's efficiency. Most of the commonly used thermal fluids are suitable for a wide range of temperatures. According to the desired temperature range, the selection of a HTF takes into account certain criteria listed in Table 1. Important properties of HTF are a low lower temperature limit and a high upper temperature limit at low pressures. The high temperature is set by the CSP plant design, for instance plants with linear Fresnel reflector, reach a temperature of 250 °C. Thermophysical properties, like thermal conductivity, density and specific heat, must favor heat transfer increase and possibly thermal energy storage (TES), while dynamic viscosity has to be low to limit pressure drop and, as a result, pump power.
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Moreover, the thermal fluid must have a perfect chemical compatibility with the tank or thermal storage materials. Furthermore, a good HTF must be readily available at low cost while presenting the lowest toxicity, flammability and explosivity [2,3]. However, mineral or synthetic oils are petroleum-based and so are depleting resources. Synthetic organic fluids and mineral oils are very costly (6 €/L) and hazardous due to degradation by-products [4]. With a concern for sustainable development, there is a need to establish a set of safe, nontoxic HTF for use in CSP plants. Vegetable oils as heat transfer fluid are a promising solution, but there is an urgent need to determine its thermal properties from ambient temperature to 250 °C. 1.2. Usefulness of vegetable oils Vegetable oils are mostly used in the food sector, but also in other industries. These industrial applications, involving high temperatures, include insulators for the electrical industry, cutting oil for machining, biodiesel fuel and HTF (cooling fluid or thermal fluid). These different uses have prompted the characterization of the thermophysical and rheological properties of vegetable oils.
Corresponding author at: PROMES-CNRS UPR-8521 Laboratory, University of Perpignan Via Domitia, Rambla de la Thermodynamique, Tecnosud, 66100 Perpignan, France. E-mail address: [email protected] (V. Goetz).
https://doi.org/10.1016/j.solmat.2017.12.037 Received 19 July 2017; Received in revised form 30 November 2017; Accepted 19 December 2017 0927-0248/ © 2017 Published by Elsevier B.V.
Solar Energy Materials and Solar Cells 178 (2018) 129–138
J.-F. Hoffmann et al.
μ ρ
Nomenclature Cp T SSres SStot
Specific heat capacity, kJ kg−1 K−1 Temperature, °C Residual sum of squares Total sum of squares
Abbreviation CSP GHG HTF LCA TES
Greek symbols λ
Thermal conductivity, W m−1 K−1
• Density is an important thermophysical property for biodiesel fuel
•
•
•
Dynamic viscosity, mPa s Density, kg m−3
Concentrated Solar Power Greenhouse Gas Heat Transfer Fluid Life Cycle Assessment Thermal Energy Storage
temperature range 35–180 °C [27]. K. Miller measured the viscosity properties of frying oils from 170 °C to 190 °C in 1994 [37]. No data is published on the effect of temperature on the dynamic viscosity of vegetable oils from ambient temperature to 250 °C.
and HTF usages. Density is used in most fluid mechanics calculations. However, temperature dependence of density is difficult to establish. Timms measured the densities of palm, palm kernel and coconut oils from 50 °C to 200 °C in 1985 [5]. Noureddini et al. used a pycnometer to measure different oil densities from ambient temperature to 110 °C in 1992 [6]. More recently, Esteban et al., in 2012, [7] measured the density up to 140 °C. Many other studies have determined density, but all below 100 °C [8–13]. Thermal conductivity is a fundamental property that determines heat transfer inside the fluid. Measuring the thermal conductivity of liquids from ambient temperature to 250 °C remains a challenge. No published data are available on the thermal conductivity of vegetable oils in this temperature range. Usually, vegetable oils have been considered as food materials, and so their thermal conductivities were measured mainly for food processing purposes. Some research has been carried out on this important thermal property in relation to this use [14–22]. However, not many kinds of vegetable oils have been tested, and the temperature range rarely exceeded 100 °C. Also, their composition is usually not given. Specific heat capacity is an important thermal property for a heat transfer fluid. This parameter influences energy quantity during heat transfer. Compared with density and thermal conductivity, the specific heat of vegetable oil at high temperature has been more thoroughly investigated. In 1945, P. E. Clark measured the specific heat values of different vegetable oils from ambient temperature to 270 °C [23]. Except for Clark, researchers have not exceeded 180 °C, and not all common vegetable oils have been characterized [10,24–27]. The dynamic viscosity of fluids is a fundamental property that must be known for heat transfer and fluid flow operations. The design of heat transfer equipment, and the sizing and selection of pumps and pipes requires knowledge of the temperature dependence of viscosity. A number of studies have already reported the viscosity of vegetable oil from ambient temperature to 110 °C [5,8–11,19,20,28–36]. A few researchers have studied dynamic viscosity up to 180 °C. B. Esteban et al. determined dynamic viscosity from ambient temperature to 140 °C in 2012 for rapeseed, sunflower, soybean, palm, corn and grapeseed oil [7]. In 2008, O. O. Fasina et al. estimated the viscosity of vegetable oils in the
At present, there is a large gap in the experimental data for the thermophysical and rheological properties of vegetable oil. The industrial utilization of vegetable oils is evolving, and temperature range needs to be broadened to permit their use as heat transfer fluids. The objective of this work was to characterize the density, thermal conductivity, specific heat capacity and dynamic viscosity of rapeseed, soybean, sunflower, palm, copra, cotton and jatropha oils from ambient temperature to 250 °C. The experimental results are compared with those in the literature and all the data are correlated to temperature using polynomial equations. 2. Material and methods 2.1. Selected vegetable oils and pretreatment Vegetable oils are vegetable fat, also called plant oils, which are generally produced at industrial scale [10]. Oilseed plants are specifically cultivated for their seeds or fruits used to produce oil. All vegetable oils consist primarily of triglyceride fatty acids (or triacylglycerol), namely fatty acids triesters, a mixture of free fatty acids, mucilage, soaps, sterols, waxes, water, and impurities … [38]. These elements can be classified according to three different categories related to their relative proportions: triglycerides (95%), free fatty acids (5%) and minor constituents [39]. Fatty acids represent 90 − 96% of the triglyceride molar mass. The vegetable fat may contain many different fatty acids and a fatty acid can be found in several different vegetable fats. Each vegetable oil is characterized by its fatty acid composition. The differences between oils from different sources are directly related to the length of the fatty acid chains, typically ranging from 8 to 24 carbon atoms. In fatty acids, all the carbon atoms are tied up to hydrogen, oxygen and carbon. A saturated fatty acid has a carbon chain composed only of single bond. When a double bond is present between carbon atoms, it is described as monounsaturated fatty acid. If the fatty acid has several double bonds between carbons, it is called polyunsaturated fatty acids. These fatty acids are currently represented by a symbol such as C18:2 (Linoleic acid), which indicates a chain consisting of 18 carbon atoms and two double bonds. For the determination of thermal and thermophysical properties in the temperature range from 20 to 250 °C, seven different vegetables oils were selected. Commercial refined oils – rapeseed, soybean and sunflower – were acquired in French hypermarkets. The more exotic vegetables oils – palm, copra, cotton and jatropha – came from agricultural producers in Burkina Faso. The vegetable oils characterized in this study are listed in Table 2 according to their fatty acid composition determined by gas chromatography at CIRAD, France. Fatty acids composition is of utmost importance, because it reveals the degree of unsaturation of each oil, which is in relation with physical and chemical properties and may impact its thermal properties. The
Table 1 Criteria selection for HTF [2]. Low lower temperature limitation High upper temperature limitation Low pressure (< 1 atm) High heat storage capacity, or volumetric heat capacity ρ Cp High thermal conductivity Low dynamic viscosity Chemical compatibility with contact materials Low cost Industrial availability Acceptable life cycle assessment Low toxicity, flammability and explosivity
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Table 2 Fatty acid compositions of the studied vegetable oils [40]. Number of carbons
Fatty acid name
C 8: 0 Caprylic C 10: 0 Capric C 12: 0 Lauric C 14: 0 Myristic C 16: 0 Palmitic C 18: 0 Stearic C 18: 1 Oleic C 18: 2 Linoleic C 18: 3 Linolenic C 20: 0 Arachidic C 20: 1 Gadoleic Other minor fatty acids compounds Saturated Monounsaturated Polyunsaturated
Rapeseed
Soybean
Sunflower
Palm
Copra
Cotton
Jatropha
26.87% 2.52% 17.23% 47.88%
16.01% 6.05% 41.64% 32.53%
5.5% 29.39% 17.23% 47.88%
3.77% 22.06% 41.64% 32.53%
4.78% 1.35% 60.78% 19.22% 8.92%
11.32% 2.93% 23.3% 52.37% 5.84%
6.29% 3.44% 32.47% 55.36%
41.73% 5.56% 42.45% 6.71%
9.88% 7.87% 50.03% 16.13% 7.18% 2.03% 5.08% 1.21%
1.3% 3.65% 6.13% 62.08% 28.14%
4.24% 14.25% 23.3% 58.21%
2.44% 9.73% 32.47% 55.36%
3.55% 47.29% 42.45% 6.71%
0.59% 93.12% 5.08% 1.21%
instrument had a long-term relative error of 1.2% and absolute accuracy of 2%. The thermal conductivities of different vegetable oils were obtained from ambient temperature to 230 °C. Hoffmann et al. described the hot wire with the 3ω method for the determination of the thermal conductivity of vegetable oils [40].
selection was made to cover the world's most widely-produced vegetable oils. The major vegetable oils of worldwide production represent 176.7 Million Metric Tons in 2016 [41]. These oils account for more than 90% of production. A thermal pretreatment of one hour at 250 °C was carried out on each selected oil. This process avoids irreversible molecular recombination. This causes large variations in the density or specific heat capacity at around 150 °C [42]. A first temperature increase helps to stabilize the vegetable oils so that they can be characterized. The objective is their use as heat transfer fluids: a brief treatment does not damage the oils, and it is not detrimental to measurements of thermophysical and rheological properties.
2.4. Specific heat capacity measurement Specific heat capacity values of vegetable oils were characterized using a calorimeter (Setaram® C80, University of Reims). Specific heat capacity is a physical quantity that represents the amount of heat per unit mass required to raise the temperature by one degree Celsius. It is an important parameter when using vegetable oil as heat transfer fluid. The calorimeter was set to a temperature ramp of 1 °C min−1 with a measuring accuracy given by the manufacturer of ± 0.1% of the specific heat capacity values. However, a reproducibility value of 3.75% was determined after several measurements of specific heat capacity at different temperatures. The specific heats of the vegetable oils were measured from ambient temperature to 240 °C.
2.2. Density measurement The density of vegetable oils was determined using a pycnometer (CNRS PROMES, France). Density is a physical quantity that depends on temperature. A pycnometer measures the volume expansion of oil according to a temperature time course for a constant mass. The calibration of the measuring instrument was checked with a synthetic oil (Jarytherm DBT® [43]) over a range of temperatures between ambient and 250 °C. The synthetic oil, intented for use as a heat transfer fluid, possessed known thermal and thermophysical properties and temperature dependence. The calibration with this reference oil showed an instrument reproducibility of 0.63 kg m−3 and a theoretical error of 2.25% with a fixed confidence level set at 95% (k = 2). The density of the selected vegetable oil was determined from ambient temperature to 250 °C. Palm oil is in the solid state at ambient temperature, and so it was characterized from a starting temperature of 50 °C.
2.5. Dynamic viscosity measurement The dynamic viscosity of various vegetable oil was measured using a rheometer (ARES-G2, TA Instrument®, Rheonova, France). The dynamic viscosity is the ratio of shear stress to the velocity gradient perpendicular to the shear plane. It is an important fluid parameter when analyzing liquid behavior and fluid motion near solid boundaries. The rheometer was equipped with a cone-plane geometry for the viscosity measurement, and the temperature was regulated by a furnace with a measuring accuracy of 0.5 °C. The explored range of shear rates was limited for the low rate by the rheometer accuracy and for the highest rates by probable liquid ejection outside the geometry. Each experiment was performed isothermally, two times on different samples in order to assess the reproducibility. The sample volume required for the different measurements was 100 mL. The results are given with an accuracy of 10% considering the geometric imperfections and the measurement accuracy of the rheometer. The dynamic viscosity was measured for vegetable oils at 50 °C, 100 °C, 150 °C, 200 °C and 250 °C.
2.3. Thermal conductivity measurement Measuring the thermal conductivity of vegetable oils, or generally of liquids, as a function of temperature is a challenging operation. Thermal conductivity is a fundamental thermal parameter that determines heat transfer inside a fluid. The method used was based on a hot wire thermal probe with ac excitation and 3ω lock-in detection (University of Reims, France) [44]. The instrument is compact, inexpensive and ensures excellent measurement reproducibility. The fundamental frequency was set to 1 Hz. The test liquid volume was approximately 25 mL. The samples were heated in a furnace at 260 °C in glass bottles embedded in ceramic bowls with quartz sand. The signal amplitude and phase were continuously recorded as the liquid cooled down to ambient temperature. The average observed temperature ramp was about 2 K min−1. To assess the reference for determining the thermal conductivity, a well-known synthetic oil was selected and tested, Jarytherm DBT® oil, supplied by Arkema [43]. The measuring
3. Results and discussion 3.1. Density The density variation of different vegetable oils is plotted in Fig. 1. The density decreased linearly with increasing temperature due to thermal expansion. The fatty acid composition of vegetable oils influences their density at ambient temperature and also their density 131
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Fig. 1. Effects of temperature on density for vegetable oils. Fig. 3. Comparative density measurements of different soybean oils [6,7,9].
Fig. 2. Comparative density measurements of different palm oils [5,7,11,12].
Fig. 4. Effects of temperature on thermal conductivity for vegetable oils [40].
temperature profiles. For saturated fatty acids, the density decreases with increasing number of carbons [6]. This is confirmed for the density values at 25 °C: thus copra oil with 915.6 kg m−3 has the highest density. The effect of unsaturated fatty acids is also important, the expansion coefficient decreasing with the presence of monounsaturated and polyunsaturated fatty acid. Concerning the profiles of the densities for soybean and sunflower oils, polyunsaturated acids, such as linoleic (C18:2), appear to limit the thermal expansion more significantly. If vegetable oil is used as a heat transfer fluid, compositions with mainly unsaturated fatty acids are preferred, because the decrease in the density as a function of the temperature is lower. To compare the experimental results, palm (Fig. 2) and soybean (Fig. 3) oils were compared with the literature values of density as a function of temperature. Esteban et al. measured the density of several
vegetable oils up to 140 °C using a densitometer and a thermostat [7]. For palm oil, the first results were available as early as 1985 from Timms et al. at temperatures ranging from 50 °C to 200 °C [5]. Abollé et al. [12] in 2009 and Kibbey et al. [11] in 2014 determined the density of palm oil. In 1992, Noureddini et al. worked on the density of vegetable oils and fatty acids to a temperature of 110 °C, including soybean oil [6]. Franco et al. later determined the density of six vegetable oils as function of temperature. Using a standard glass hydrometer, they measured the density during a cooling phase from 80 °C to 20 °C [9]. Although a difference from the literature is visible for soybean oil, most experimental results are rather consistent. The variation in density depending on the temperature is adequately described for the two oils with different fatty acid compositions. 132
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Fig. 5. Comparative thermal conductivity measurements of different sunflower oils [17–19].
Fig. 7. Effects of temperature on specific heat capacity of vegetable oils.
Fig. 6. Comparative thermal conductivity measurements of different soybean oils [17–19].
Fig. 8. Comparative specific heat capacity measurements for different sunflower oils [26,27].
3.2. Thermal conductivity
temperature. For all the tested vegetable oils, the thermal conductivity decreased as the temperature increased. Focusing on copra and rapeseed oils, it is possible to see two different behaviors. For the rapeseed oil, which is unsaturated, the thermal conductivity ranged from 0.167 W m−1 K−1 at 20 °C to 0.141 W m−1 K−1 at 230 °C. By contrast, for copra oil, which is saturated, it ranged from 0.165 W m−1 K−1 at 20 °C to 0.126 W m−1 K−1 at 230 °C. The value for copra oil decreased most strongly as the temperature increased. In the intermediate case, for vegetable oils that contain approximately the same amounts of fatty acids, the absolute values of thermal conductivities and their temperature dependencies were similar [40]. To check the soundness of the experimental results, two vegetable
The thermal conductivity over the temperature range 25–230 °C of the selected vegetable oils is shown in Fig. 4. As this thermophysical property of vegetable oils mainly depends on temperature, a second degree polynomial equation was adopted to account for the correlation of the experimental conductivity with the temperature. The relative values of the polynomial curves were scaled to absolute values. They are calculated from the measurements of water at ambient temperature before and after each test. The plotted thermal conductivities are the results obtained after the correlation to temperature. The experimental and correlated results for rapeseed oil are shown in the Fig. 4. Palm oil was measured starting from 40 °C, because the oil is solid below this 133
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Fig. 9. Comparative specific heat capacity measurements for different soybean oils. [10,23,25–27].
Fig. 11. Comparative dynamic viscosity measurements for different sunflower oils. [9,19,27–29,33,34].
Fig. 10. Effects of temperature on dynamic viscosity for vegetable oils.
Fig. 12. Comparative dynamic viscosity measurements for different soybean oils. [5,9,19,27,28,32–34].
oils were compared with the literature. Figs. 5 and 6 compare the thermal conductivities of literature data for sunflower and soybean oils respectively. First, the available data do not go beyond 80 °C. Balderas Lopez et al. directly measured the thermal diffusivity and effusivity for different liquids with two photoacoustic methodologies in 2013, the thermal conductivity was obtained at ambient temperature. The photoacoustic method estimates a thermal conductivity for the sunflower oil of 0.15 W m−1 K−1 ± 0.03 and for the soybean oil of 0.17 W m−1 K−1 ± 0.02 [17]. In 2009, Turgut et al. evaluated the thermal conductivities of three different edible oils, namely sunflower oil, corn oil and olive oil at temperatures from 25 °C to 80 °C. The measurements were made using a hot wire probe method [18]. In 2008, Brock et al. reported the thermal conductivity values for sunflower and
soybean oils in the temperature range 20–70 °C using an analyzer of thermal properties (Decagon Inc., model KD2) and a thermostatic bath [19]. Brock et al. indicated that the thermal conductivity for the sunflower oil was 0.165 W m−1 K−1 ± 0.010 at 21 °C and 0.155 W m−1 K−1 ± 0.018 at 68.7 °C. For the soybean oil the thermal conductivity was 0.180 W m−1 K−1 ± 0.012 at 21 °C and 0.160 W m−1 K−1 ± 0.009 at 69.5 °C [19]. Turgut et al. do not clearly indicate the accuracy of the hot wire probe method. At 80 °C, the difference between the values of Turgut et al. and ours is less than 5% for thermal conductivity [18]. One may conclude that the results of Fig. 5 4 are validated by literature data below 80 °C.
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Table 3 Linear fit parameters for Eq. (2) and correlation coefficient r2. Oil sample
Rapeseed
Soybean
Sunflower
Palm
Copra
Cotton
Jatropha
a (kg m−3 K−1) b (kg m−3) r2 Temperature range (°C)
−0.6691
−0.5917
−0.6081
−0.7584
−0.7392
−0.6607
−0.6831
928.19
929.77
925.98
933.33
933.47
926.36
926.58
0.99961 25– 250
0.99809 25–250
0.99973 25–250
0.99183 50–250
0.99990 25–250
0.99883 25–250
0.99683 25–250
Table 4 Polynomial fit parameters for Eq. (3) and correlation coefficient r2 [40]. Oil sample
Rapeseed
Soybean
Sunflower
Palm
Copra
Cotton
Jatropha
c (× 10−7 W m−1 K−3) d (× 10−4 W m−1 K−2) e (W m−1 K−1) r2 Temperature range (°C)
2.00
0.92
3.50
2.58
2.44
1.06
2.80
−1.714
−1.563
−2.232
−2.113
−2.503
−1.629
−2.258
0.1698
0.1702
0.1706
0.1729
0.1703
0.1702
0.1736
0.99214 25–230
0.99486 25–230
0.99214 25–230
0.99382 40–230
0.99567 25–230
0.99214 25–230
0.99222 25–230
Table 5 Polynomial fit parameters for Eq. (4) and correlation coefficient r2. Oil sample
Rapeseed
Soybean
Sunflower
Palm
Copra
Cotton
Jatropha
f (× 10−9 kJ kg−1 K−5) g (× 10−7 kJ kg−1 K−4) h (× 10−5 kJ kg−1 K−3) i (× 10−3 kJ kg−1 K−2) j (kJ kg−1 K−1) r2 Temperature range (°C)
1.621
0.807
0.760
2.782
−0.830
1.373
2.262
−8.735
−3.705
−3.310
−16.443
4.647
−7.924
−10.423
14.933
5.491
4.147
30.534
−8.632
14.060
12.947
−5.976
−0.409
1.200
−16.320
8.856
−5.443
0.441
2.0985
1.9664
1.9506
2.4558
1.8447
2.1186
1.9608
0.99016 25–240
0.99508 25–240
0.99356 25–240
0.99582 40–240
0.99932 25–240
0.98861 25–240
0.98443 25–240
Table 6 Linear fit parameters for Eq. (5) in log-log representation and correlation coefficient r2. Oil sample
Rapeseed
Soybean
Sunflower
Palm
Copra
Cotton
Jatropha
l (dimensionless) m (dimensionless) r2 Temperature range (°C)
4.5966
4.1758
4.2454
4.9375
4.4986
4.4303
4.5168
−1.7645
−1.6966
−1.7295
−2.0299
−1.9365
−1.7978
−1.8371
0.99933 50–300
0.98421 50–250
0.99022 50–250
0.99984 50–250
0.99556 50–250
0.99255 50–250
0.99882 50–250
heat values of different vegetable oils in the range 1–270 °C [23]. In 1979 Formo et al. estimated a correlation for the specific heat of soybean oil from ambient temperature to 180 °C [10]. Tochitani et al. evaluated the specific heat capacity of soybean oil from the approximate melting point to 150 °C [25]. Although the tendencies with temperature of the experimental results appear to be consistent, there are still differences from the results of the two most recent authors, namely Santos et al. [26] and Fasina et al. [27]. This dispersion probably results from the difference between the equipment used. In their case, a differential scanning calorimeter (DSC) was used for the specific heat against temperature. Despite the capacity of both instruments (DSC and calorimeter) to measure this property, the level of accuracy can still vary. The calorimeter C80 uses a lower temperature ramp, 1 °C min−1 against 5 °C min−1 for DSC. With a larger quantity of oil and a more accurate heat flow-meter, the calorimeter theoretically meets all the conditions for more accurate
3.3. Specific heat capacity Fig. 7 illustrates the variation in the specific heat capacity of the seven vegetable oils, over the temperature range 25–240 °C. For all the tested vegetable oils, the specific heat capacity increased when the temperature increased, except in some temperature intervals. Unlike other materials, the variation of the specific heat is not linear and large variations are visible, e.g. for palm oil. To assess the experimental results, sunflower oils (Fig. 8) and soybean oils (Fig. 9) were compared with the literature for specific heat capacity. Recently, different authors have specifically worked on this thermal property for several vegetable oils. Santos et al. reported the specific heat of eight different vegetable oils including sunflower and soybean oils from 40 °C to 180 °C in 2005 [26]. In 2008 Fasina et al. measured the specific heats of twelve vegetable oil samples in the temperature range 35–180 °C [27]. In 1945 Clark measured the specific 135
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Therminol VP1
Vegetable oils
73.5% diphenyl oxide / 26.5% diphenyl 12 400
Fatty acid
differences are visible at 50 °C. For most authors, viscosity measurements are performed dynamically. For the experimental results, each data point was obtained at a constant temperature. Even so, the trend is highly useable and can provide dynamic viscosities values at 200 °C and 250 °C.
−11 to 20 250
4. Temperature correlations of properties
110
230 – 330
0.5 24 239
0
1.77 1.87
1.91 – 2.15 1.94 – 2.13
0.128 0.114
0.148 – 0.155 0.130 – 0.144
–
0.985 0.395 N/A
4.6 – 11.9 1.2 – 3.5 Good
€ kg−1 – kg CO2 eq kg−1 –
5.7 Low 3 High
0.4 – 1.2 High 0.85 – 1.87 Low
Table 7 Comparison of a synthetic oil (Therminol VP1®) and vegetable oils [4,39,46,47]. Thermal oil Composition
–
Pour point Temperature limit for use Flash point Relative pressure @ 100 °C @ 200 °C @ 300 °C Volumetric heat capacity @ 100 °C @ 200 °C Thermal conductivity @ 100 °C @ 200 °C Dynamic viscosity @ 100 °C @ 200 °C Chemical compatibility Cost Availability LCA - GHG Environmental hazard
°C °C °C kPa
The present study demonstrates an influence of the fatty acids composition on the thermophysical and rheological properties of vegetable oils. A general correlation between vegetable oil compositions and thermal properties was not established because experimental variations between samples were too low compared to the assumed overall level of errors. As a consequence, the characterization of thermophysical and rheological properties of each vegetable oils enables us to determine different correlations of these properties with temperature. These correlations use experimental data to develop a best representative equation. The quality and deviations of the points from the equations are defined by a correlation coefficient (r2) for each vegetable oil (Eq.(1)). These equations link the many characterizations to numerical models.
MJ m−3 K−1
W m−1 K−1
mPa s
(r 2 = 1 − SSres / SStot )
(1)
4.1. Temperature dependence of density The density evolves linearly with increasing temperature (in °C). This linear trend is obtained with different experimental values of the density.
measurements. Taking into account the error, repeatability and unknown composition of the oils in the literature data, the experimental results point to reasonable consistency.
ρ (T ) = aT + b
(2)
Table 3 presents the parameters a and b for each vegetable oil with the corresponding temperature range.
3.4. Dynamic viscosity The dynamic viscosity obtained over the temperature range 50–250 °C for all the vegetable oils is shown in Fig. 10. The viscosity decreases with increasing temperature. This effect of temperature on the viscosity of the vegetable oil has been attributed to the decrease of intermolecular forces, which makes its flow easier, thus reducing its viscosity [28]. The fatty acid composition has a certain influence on the viscosity profile. For saturated fatty acids, the dynamic viscosity increases with increasing number of carbons [32]. This influence is shown in the lower viscosity of coconut oil compared with palm oil. Vegetable oils also have monounsaturated and polyunsaturated fatty acids, mainly oleic acid (C18:1) and linoleic acid (C18:2). Studies have shown that these two fatty acids do not behave in the same way with changes in temperature, a decrease in the oil viscosity was distinctly observed with increasing portion of linoleic fatty acids. The authors suggest that the oils with more double bonds have a lower viscosity due to their freely filled structure [34]. Rapeseed oil, with 60.8% oleic acid, has a higher viscosity than soybean oil, with 52.4% linoleic acid. Literature data concerning the dynamic viscosity are more abundant than those for the other variables characterized here because this rheological property is necessary for research related to the use of vegetable oils as a biofuel. The measurement methods are largely identical. What differentiates the studies are the temperature ranges. Most research has focused on measuring the viscosity from ambient temperature to 110 °C [5,8,9,11,19,20,28–36]. Beyond this temperature, dynamic viscosity measurements were carried out by Esteban et al. to 140 °C [7] and by Fasina et al. to 180 °C [27] for many vegetable oils. Figs. 11 and 12 respectively show the comparison between our experimental data for sunflower and soybean oils and the values available in the literature. The multitude of available data allows a more meaningful analysis of the experimental results obtained for dynamic viscosity. Some small
4.2. Temperature dependence of thermal conductivity As the thermal conductivity of vegetable oils depends mainly on temperature, the following second-degree polynomial equation was adopted for the correlation of the experimental thermal conductivity.
λ (T ) = cT 2+ dT + e
(3)
Table 4 contains the polynomial coefficients c, d and e fitted for the different vegetable oils with the corresponding temperature range. 4.3. Temperature dependence of specific heat capacity The specific heat capacity is strongly temperature-dependent. To best fit the experimental profiles, a fourth degree polynomial equation was used to correlate the values of the specific heat.
Cp (T ) = fT 4 + gT 3 + hT 2 + iT + j
(4)
Table 5 gives the polynomial coefficients f, g, h, i and j for the vegetable oils corresponding the temperature range. 4.4. Temperature dependence of dynamic viscosity The dynamic viscosity has a linear evolution in log-log representation, depending on temperature. The function was correlated with the experimental values of the dynamic viscosity.
Log (μ (T )) = l + m (Log (T ))
(5)
Table 6 contains the power factor l and the exponent m for the investigated vegetable oils. 136
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5. Innovative heat transfer fluids
References
Up to now, published studies on potential use of vegetable oil as HTF are quite rare. According to criteria established in Table 1, it is possible to determinate if the vegetable oils are appropriate as heat transfer fluids. The Table 7 compares several criteria regarding synthetic and vegetable fluids. Synthetic oil Therminol VP1 ®, considered as the reference solar oil, is used as point of comparison. It has a 400 °C limit of operating temperature, with a very low dynamic viscosity. The vegetable oils selected in the present study seem to be appropriate fluids when compared with the reference: besides their experimentally determined 250 °C maximal operating temperature, their thermophysical properties (volumetric heat capacity and thermal conductivity) are on average 13% superior to the Therminol VP1 ® ones. Vegetable oils have a more enjoyable ecobalance and an important availability with prices 7 times cheaper than the traditional thermal oil. Therefore, these innovative fluids are promising candidates for heat transfer applications, and they make easier handling, transport and end of use treatment.
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6. Conclusions The use of vegetable oils as heat transfer fluids is proposed as a new potential industrial application of those products. According to IEA experts, 20 GWe of CSP are globally needed to be built every year until 2050 to meet the international energy policy targets. This would represent 400 Andasol plants every year [4]. If vegetable oils are used, with its characteristic properties, the global need of heat transfer fluid is 1,9% of the worldwide production. So, a use of vegetable oils in CSP would not lead to a "conflict of use", most of which are devoted to the food industry. Vegetable oils are differentiated mainly by their origin and by their fatty acid compositions. The influence of composition on thermophysical and rheological properties is clearly highlighted by the evolution of their parameters from ambient temperature to 250 °C. The innovative alternative of using these oils as a heat transfer fluid for concentrated solar power plants comes up against a lack of data on its thermal properties and its temperature behavior. Thermal conductivity, specific heat, density and dynamic viscosity were assessed for seven selected oils representing over 90% of the world production of vegetable oils. The investigated rapeseed oil, already applied to thermocline thermal energy storage tank fitted with an oil loop [45], offers the following properties at 210 °C: a thermal conductivity of 0.14 W m-1 K1 , a dynamic viscosity of 3.2 mPa s, a specific heat of 2.49 kJ kg-1 K-1, and a density of 788 kg m-3. The selected tested oils demonstrated very similar thermophysical and rheological properties. These values enable a range of possible applications as HTF depending on temperatures, costs and locations of implementation. The high availability of vegetable oils, regardless of location, is an advantage for renewable energy deployment requiring heat transfer fluids, especially in emerging countries.
Acknowledgments This project was supported financially by ANRT, a Ph.D. grant through a CIFRE program n° 2012/1516, AQYLON and EDF R&D Company and the Program “Investissements d′avenir” (Investment for the Future) of the Agence Nationale de la Recherche (National Agency for Research) of the French State under award number ANR-10-LABX22-01-SOLSTICE. The authors thank Jeremy Valette (CIRAD) and Didier Caron (University of Reims) for the many characterizations carried out. J.-F. H and M. C. acknowledge the support of SATT Nord, France through DISPOTHERM Project. The authors would like to thank T. Fasquelle for his help in the study. 137
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vegetable oils with rheological behaviour and oil uptake, Food Chem. 118 (2010) 398–402. C. Tangsathitkulchai, Y. Sittichaitaweekul, M. Tangsathitkulchai, Temperature effect on the viscosities of palm oil and coconut oil blended with diesel oil, J. Am. Oil Chem. Soc. 81 (2004) 401–405. M. Maskan, Change in colour and rheological behaviour of sunflower seed oil during frying and after adsorbent treatment of used oil, Eur. Food Res. Technol. 218 (2003) 20–25. K.S. Miller, R.P. Singh, B.E. Farkas, Viscosity and heat transfer coefficients for canola, corn, palm and soybean oil, J. Food Process. Preserv. 18 (1994) 461–472. T. Daho, G. Vaitilingom, S.K. Ouiminga, B. Piriou, A. Zongo, S. Ouoba, et al., Influence of engine load and fuel droplet size on performance of a CI engine fueled with cottonseed oil and its blends with diesel fuel, Appl. Energy 111 (2013) 1046–1053. S.S. Sidibé, J. Blin, G. Vaitilingom, Y. Azoumah, Use of crude filtered vegetable oil as a fuel in diesel engines state of the art: literature review, Renew. Sustain. Energy Rev. 14 (2010) 2748–2759. J.-F. Hoffmann, J.-F. Henry, G. Vaitilingom, R. Olives, M. Chirtoc, D. Caron, X. Py, Temperature dependence of thermal conductivity of vegetable oils for use in concentrated solar power plants, measured by 3omega hot wire method, Int. J. Therm. Sci. 107 (2016) 105–110.
138
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Temperature dependence of thermophysical and rheological properties of seven vegetable oils in view of their use as heat transfer fluids in concentrated solar plants
T
⁎
J.-F. Hoffmanna,d,e, G. Vaitilingomb, J.-F. Henryc, M. Chirtocc, R. Olivesa, V. Goetza, , X. Pya a
PROMES-CNRS UPR-8521 Laboratory, University of Perpignan Via Domitia, Rambla de la Thermodynamique, Tecnosud, 66100 Perpignan, France CIRAD, unité de recherche BioWooEB, TA B-114/16, 73 rue JF Breton, 34398 Montpellier, France c GRESPI - EA 4694 Laboratory, Université de Reims Champagne Ardenne URCA, Moulin de la Housse BP 1039, 51687 Reims, France d AQYLON, 46-48 rue Renée-Clair, 75892 Paris, France e EDF - R&D, MFEE - Nouvelles Filières de Production et Thermochimie, 6 quai Waitier, 78401 Chatou, France b
A R T I C L E I N F O
A B S T R A C T
Keywords: Vegetable oils Thermophysical properties Thermal conductivity Specific heat capacity Dynamic viscosity Heat transfer fluid
Selecting a heat transfer fluid (HTF) depends on the value of thermophysical parameters. In response to the need for innovative heat transfer fluids in concentrated solar power (CSP) plants, vegetable oil offers a promising solution. The relevant thermophysical properties are density, thermal conductivity and specific heat capacity. Moreover, a rheological property, the dynamic viscosity is also relevant. The objective of this work was to study and compare these properties for seven different vegetable oils (rapeseed, soybean, sunflower, palm, copra, cotton and jatropha) in the temperature range from ambient to 250 °C. For all the properties studied, the values evolved with increasing temperature and were influenced by the fatty acid composition of each vegetable oil. Experimental results are compared with those in the literature and are found to be consistent. The temperature dependencies are correlated to temperature using polynomial equations. The correlations presented here may be useful as a database for the selection of innovative heat transfer fluids.
1. Introduction: vegetable oils as HTF 1.1. Selection criteria for heat transfer fluids Synthetic or mineral high temperature oils are widely used as industrial heat transfer fluids in many process applications. In concentrated solar power (CSP) plants, the heat transfer fluid (HTF) circulates in the solar receiver and transmits its energy to the heat engine cycle [1]. The thermodynamic performance of the working fluid is important in order to ensure the plant's efficiency. Most of the commonly used thermal fluids are suitable for a wide range of temperatures. According to the desired temperature range, the selection of a HTF takes into account certain criteria listed in Table 1. Important properties of HTF are a low lower temperature limit and a high upper temperature limit at low pressures. The high temperature is set by the CSP plant design, for instance plants with linear Fresnel reflector, reach a temperature of 250 °C. Thermophysical properties, like thermal conductivity, density and specific heat, must favor heat transfer increase and possibly thermal energy storage (TES), while dynamic viscosity has to be low to limit pressure drop and, as a result, pump power.
⁎
Moreover, the thermal fluid must have a perfect chemical compatibility with the tank or thermal storage materials. Furthermore, a good HTF must be readily available at low cost while presenting the lowest toxicity, flammability and explosivity [2,3]. However, mineral or synthetic oils are petroleum-based and so are depleting resources. Synthetic organic fluids and mineral oils are very costly (6 €/L) and hazardous due to degradation by-products [4]. With a concern for sustainable development, there is a need to establish a set of safe, nontoxic HTF for use in CSP plants. Vegetable oils as heat transfer fluid are a promising solution, but there is an urgent need to determine its thermal properties from ambient temperature to 250 °C. 1.2. Usefulness of vegetable oils Vegetable oils are mostly used in the food sector, but also in other industries. These industrial applications, involving high temperatures, include insulators for the electrical industry, cutting oil for machining, biodiesel fuel and HTF (cooling fluid or thermal fluid). These different uses have prompted the characterization of the thermophysical and rheological properties of vegetable oils.
Corresponding author at: PROMES-CNRS UPR-8521 Laboratory, University of Perpignan Via Domitia, Rambla de la Thermodynamique, Tecnosud, 66100 Perpignan, France. E-mail address: [email protected] (V. Goetz).
https://doi.org/10.1016/j.solmat.2017.12.037 Received 19 July 2017; Received in revised form 30 November 2017; Accepted 19 December 2017 0927-0248/ © 2017 Published by Elsevier B.V.
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μ ρ
Nomenclature Cp T SSres SStot
Specific heat capacity, kJ kg−1 K−1 Temperature, °C Residual sum of squares Total sum of squares
Abbreviation CSP GHG HTF LCA TES
Greek symbols λ
Thermal conductivity, W m−1 K−1
• Density is an important thermophysical property for biodiesel fuel
•
•
•
Dynamic viscosity, mPa s Density, kg m−3
Concentrated Solar Power Greenhouse Gas Heat Transfer Fluid Life Cycle Assessment Thermal Energy Storage
temperature range 35–180 °C [27]. K. Miller measured the viscosity properties of frying oils from 170 °C to 190 °C in 1994 [37]. No data is published on the effect of temperature on the dynamic viscosity of vegetable oils from ambient temperature to 250 °C.
and HTF usages. Density is used in most fluid mechanics calculations. However, temperature dependence of density is difficult to establish. Timms measured the densities of palm, palm kernel and coconut oils from 50 °C to 200 °C in 1985 [5]. Noureddini et al. used a pycnometer to measure different oil densities from ambient temperature to 110 °C in 1992 [6]. More recently, Esteban et al., in 2012, [7] measured the density up to 140 °C. Many other studies have determined density, but all below 100 °C [8–13]. Thermal conductivity is a fundamental property that determines heat transfer inside the fluid. Measuring the thermal conductivity of liquids from ambient temperature to 250 °C remains a challenge. No published data are available on the thermal conductivity of vegetable oils in this temperature range. Usually, vegetable oils have been considered as food materials, and so their thermal conductivities were measured mainly for food processing purposes. Some research has been carried out on this important thermal property in relation to this use [14–22]. However, not many kinds of vegetable oils have been tested, and the temperature range rarely exceeded 100 °C. Also, their composition is usually not given. Specific heat capacity is an important thermal property for a heat transfer fluid. This parameter influences energy quantity during heat transfer. Compared with density and thermal conductivity, the specific heat of vegetable oil at high temperature has been more thoroughly investigated. In 1945, P. E. Clark measured the specific heat values of different vegetable oils from ambient temperature to 270 °C [23]. Except for Clark, researchers have not exceeded 180 °C, and not all common vegetable oils have been characterized [10,24–27]. The dynamic viscosity of fluids is a fundamental property that must be known for heat transfer and fluid flow operations. The design of heat transfer equipment, and the sizing and selection of pumps and pipes requires knowledge of the temperature dependence of viscosity. A number of studies have already reported the viscosity of vegetable oil from ambient temperature to 110 °C [5,8–11,19,20,28–36]. A few researchers have studied dynamic viscosity up to 180 °C. B. Esteban et al. determined dynamic viscosity from ambient temperature to 140 °C in 2012 for rapeseed, sunflower, soybean, palm, corn and grapeseed oil [7]. In 2008, O. O. Fasina et al. estimated the viscosity of vegetable oils in the
At present, there is a large gap in the experimental data for the thermophysical and rheological properties of vegetable oil. The industrial utilization of vegetable oils is evolving, and temperature range needs to be broadened to permit their use as heat transfer fluids. The objective of this work was to characterize the density, thermal conductivity, specific heat capacity and dynamic viscosity of rapeseed, soybean, sunflower, palm, copra, cotton and jatropha oils from ambient temperature to 250 °C. The experimental results are compared with those in the literature and all the data are correlated to temperature using polynomial equations. 2. Material and methods 2.1. Selected vegetable oils and pretreatment Vegetable oils are vegetable fat, also called plant oils, which are generally produced at industrial scale [10]. Oilseed plants are specifically cultivated for their seeds or fruits used to produce oil. All vegetable oils consist primarily of triglyceride fatty acids (or triacylglycerol), namely fatty acids triesters, a mixture of free fatty acids, mucilage, soaps, sterols, waxes, water, and impurities … [38]. These elements can be classified according to three different categories related to their relative proportions: triglycerides (95%), free fatty acids (5%) and minor constituents [39]. Fatty acids represent 90 − 96% of the triglyceride molar mass. The vegetable fat may contain many different fatty acids and a fatty acid can be found in several different vegetable fats. Each vegetable oil is characterized by its fatty acid composition. The differences between oils from different sources are directly related to the length of the fatty acid chains, typically ranging from 8 to 24 carbon atoms. In fatty acids, all the carbon atoms are tied up to hydrogen, oxygen and carbon. A saturated fatty acid has a carbon chain composed only of single bond. When a double bond is present between carbon atoms, it is described as monounsaturated fatty acid. If the fatty acid has several double bonds between carbons, it is called polyunsaturated fatty acids. These fatty acids are currently represented by a symbol such as C18:2 (Linoleic acid), which indicates a chain consisting of 18 carbon atoms and two double bonds. For the determination of thermal and thermophysical properties in the temperature range from 20 to 250 °C, seven different vegetables oils were selected. Commercial refined oils – rapeseed, soybean and sunflower – were acquired in French hypermarkets. The more exotic vegetables oils – palm, copra, cotton and jatropha – came from agricultural producers in Burkina Faso. The vegetable oils characterized in this study are listed in Table 2 according to their fatty acid composition determined by gas chromatography at CIRAD, France. Fatty acids composition is of utmost importance, because it reveals the degree of unsaturation of each oil, which is in relation with physical and chemical properties and may impact its thermal properties. The
Table 1 Criteria selection for HTF [2]. Low lower temperature limitation High upper temperature limitation Low pressure (< 1 atm) High heat storage capacity, or volumetric heat capacity ρ Cp High thermal conductivity Low dynamic viscosity Chemical compatibility with contact materials Low cost Industrial availability Acceptable life cycle assessment Low toxicity, flammability and explosivity
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Table 2 Fatty acid compositions of the studied vegetable oils [40]. Number of carbons
Fatty acid name
C 8: 0 Caprylic C 10: 0 Capric C 12: 0 Lauric C 14: 0 Myristic C 16: 0 Palmitic C 18: 0 Stearic C 18: 1 Oleic C 18: 2 Linoleic C 18: 3 Linolenic C 20: 0 Arachidic C 20: 1 Gadoleic Other minor fatty acids compounds Saturated Monounsaturated Polyunsaturated
Rapeseed
Soybean
Sunflower
Palm
Copra
Cotton
Jatropha
26.87% 2.52% 17.23% 47.88%
16.01% 6.05% 41.64% 32.53%
5.5% 29.39% 17.23% 47.88%
3.77% 22.06% 41.64% 32.53%
4.78% 1.35% 60.78% 19.22% 8.92%
11.32% 2.93% 23.3% 52.37% 5.84%
6.29% 3.44% 32.47% 55.36%
41.73% 5.56% 42.45% 6.71%
9.88% 7.87% 50.03% 16.13% 7.18% 2.03% 5.08% 1.21%
1.3% 3.65% 6.13% 62.08% 28.14%
4.24% 14.25% 23.3% 58.21%
2.44% 9.73% 32.47% 55.36%
3.55% 47.29% 42.45% 6.71%
0.59% 93.12% 5.08% 1.21%
instrument had a long-term relative error of 1.2% and absolute accuracy of 2%. The thermal conductivities of different vegetable oils were obtained from ambient temperature to 230 °C. Hoffmann et al. described the hot wire with the 3ω method for the determination of the thermal conductivity of vegetable oils [40].
selection was made to cover the world's most widely-produced vegetable oils. The major vegetable oils of worldwide production represent 176.7 Million Metric Tons in 2016 [41]. These oils account for more than 90% of production. A thermal pretreatment of one hour at 250 °C was carried out on each selected oil. This process avoids irreversible molecular recombination. This causes large variations in the density or specific heat capacity at around 150 °C [42]. A first temperature increase helps to stabilize the vegetable oils so that they can be characterized. The objective is their use as heat transfer fluids: a brief treatment does not damage the oils, and it is not detrimental to measurements of thermophysical and rheological properties.
2.4. Specific heat capacity measurement Specific heat capacity values of vegetable oils were characterized using a calorimeter (Setaram® C80, University of Reims). Specific heat capacity is a physical quantity that represents the amount of heat per unit mass required to raise the temperature by one degree Celsius. It is an important parameter when using vegetable oil as heat transfer fluid. The calorimeter was set to a temperature ramp of 1 °C min−1 with a measuring accuracy given by the manufacturer of ± 0.1% of the specific heat capacity values. However, a reproducibility value of 3.75% was determined after several measurements of specific heat capacity at different temperatures. The specific heats of the vegetable oils were measured from ambient temperature to 240 °C.
2.2. Density measurement The density of vegetable oils was determined using a pycnometer (CNRS PROMES, France). Density is a physical quantity that depends on temperature. A pycnometer measures the volume expansion of oil according to a temperature time course for a constant mass. The calibration of the measuring instrument was checked with a synthetic oil (Jarytherm DBT® [43]) over a range of temperatures between ambient and 250 °C. The synthetic oil, intented for use as a heat transfer fluid, possessed known thermal and thermophysical properties and temperature dependence. The calibration with this reference oil showed an instrument reproducibility of 0.63 kg m−3 and a theoretical error of 2.25% with a fixed confidence level set at 95% (k = 2). The density of the selected vegetable oil was determined from ambient temperature to 250 °C. Palm oil is in the solid state at ambient temperature, and so it was characterized from a starting temperature of 50 °C.
2.5. Dynamic viscosity measurement The dynamic viscosity of various vegetable oil was measured using a rheometer (ARES-G2, TA Instrument®, Rheonova, France). The dynamic viscosity is the ratio of shear stress to the velocity gradient perpendicular to the shear plane. It is an important fluid parameter when analyzing liquid behavior and fluid motion near solid boundaries. The rheometer was equipped with a cone-plane geometry for the viscosity measurement, and the temperature was regulated by a furnace with a measuring accuracy of 0.5 °C. The explored range of shear rates was limited for the low rate by the rheometer accuracy and for the highest rates by probable liquid ejection outside the geometry. Each experiment was performed isothermally, two times on different samples in order to assess the reproducibility. The sample volume required for the different measurements was 100 mL. The results are given with an accuracy of 10% considering the geometric imperfections and the measurement accuracy of the rheometer. The dynamic viscosity was measured for vegetable oils at 50 °C, 100 °C, 150 °C, 200 °C and 250 °C.
2.3. Thermal conductivity measurement Measuring the thermal conductivity of vegetable oils, or generally of liquids, as a function of temperature is a challenging operation. Thermal conductivity is a fundamental thermal parameter that determines heat transfer inside a fluid. The method used was based on a hot wire thermal probe with ac excitation and 3ω lock-in detection (University of Reims, France) [44]. The instrument is compact, inexpensive and ensures excellent measurement reproducibility. The fundamental frequency was set to 1 Hz. The test liquid volume was approximately 25 mL. The samples were heated in a furnace at 260 °C in glass bottles embedded in ceramic bowls with quartz sand. The signal amplitude and phase were continuously recorded as the liquid cooled down to ambient temperature. The average observed temperature ramp was about 2 K min−1. To assess the reference for determining the thermal conductivity, a well-known synthetic oil was selected and tested, Jarytherm DBT® oil, supplied by Arkema [43]. The measuring
3. Results and discussion 3.1. Density The density variation of different vegetable oils is plotted in Fig. 1. The density decreased linearly with increasing temperature due to thermal expansion. The fatty acid composition of vegetable oils influences their density at ambient temperature and also their density 131
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Fig. 1. Effects of temperature on density for vegetable oils. Fig. 3. Comparative density measurements of different soybean oils [6,7,9].
Fig. 2. Comparative density measurements of different palm oils [5,7,11,12].
Fig. 4. Effects of temperature on thermal conductivity for vegetable oils [40].
temperature profiles. For saturated fatty acids, the density decreases with increasing number of carbons [6]. This is confirmed for the density values at 25 °C: thus copra oil with 915.6 kg m−3 has the highest density. The effect of unsaturated fatty acids is also important, the expansion coefficient decreasing with the presence of monounsaturated and polyunsaturated fatty acid. Concerning the profiles of the densities for soybean and sunflower oils, polyunsaturated acids, such as linoleic (C18:2), appear to limit the thermal expansion more significantly. If vegetable oil is used as a heat transfer fluid, compositions with mainly unsaturated fatty acids are preferred, because the decrease in the density as a function of the temperature is lower. To compare the experimental results, palm (Fig. 2) and soybean (Fig. 3) oils were compared with the literature values of density as a function of temperature. Esteban et al. measured the density of several
vegetable oils up to 140 °C using a densitometer and a thermostat [7]. For palm oil, the first results were available as early as 1985 from Timms et al. at temperatures ranging from 50 °C to 200 °C [5]. Abollé et al. [12] in 2009 and Kibbey et al. [11] in 2014 determined the density of palm oil. In 1992, Noureddini et al. worked on the density of vegetable oils and fatty acids to a temperature of 110 °C, including soybean oil [6]. Franco et al. later determined the density of six vegetable oils as function of temperature. Using a standard glass hydrometer, they measured the density during a cooling phase from 80 °C to 20 °C [9]. Although a difference from the literature is visible for soybean oil, most experimental results are rather consistent. The variation in density depending on the temperature is adequately described for the two oils with different fatty acid compositions. 132
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Fig. 5. Comparative thermal conductivity measurements of different sunflower oils [17–19].
Fig. 7. Effects of temperature on specific heat capacity of vegetable oils.
Fig. 6. Comparative thermal conductivity measurements of different soybean oils [17–19].
Fig. 8. Comparative specific heat capacity measurements for different sunflower oils [26,27].
3.2. Thermal conductivity
temperature. For all the tested vegetable oils, the thermal conductivity decreased as the temperature increased. Focusing on copra and rapeseed oils, it is possible to see two different behaviors. For the rapeseed oil, which is unsaturated, the thermal conductivity ranged from 0.167 W m−1 K−1 at 20 °C to 0.141 W m−1 K−1 at 230 °C. By contrast, for copra oil, which is saturated, it ranged from 0.165 W m−1 K−1 at 20 °C to 0.126 W m−1 K−1 at 230 °C. The value for copra oil decreased most strongly as the temperature increased. In the intermediate case, for vegetable oils that contain approximately the same amounts of fatty acids, the absolute values of thermal conductivities and their temperature dependencies were similar [40]. To check the soundness of the experimental results, two vegetable
The thermal conductivity over the temperature range 25–230 °C of the selected vegetable oils is shown in Fig. 4. As this thermophysical property of vegetable oils mainly depends on temperature, a second degree polynomial equation was adopted to account for the correlation of the experimental conductivity with the temperature. The relative values of the polynomial curves were scaled to absolute values. They are calculated from the measurements of water at ambient temperature before and after each test. The plotted thermal conductivities are the results obtained after the correlation to temperature. The experimental and correlated results for rapeseed oil are shown in the Fig. 4. Palm oil was measured starting from 40 °C, because the oil is solid below this 133
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Fig. 9. Comparative specific heat capacity measurements for different soybean oils. [10,23,25–27].
Fig. 11. Comparative dynamic viscosity measurements for different sunflower oils. [9,19,27–29,33,34].
Fig. 10. Effects of temperature on dynamic viscosity for vegetable oils.
Fig. 12. Comparative dynamic viscosity measurements for different soybean oils. [5,9,19,27,28,32–34].
oils were compared with the literature. Figs. 5 and 6 compare the thermal conductivities of literature data for sunflower and soybean oils respectively. First, the available data do not go beyond 80 °C. Balderas Lopez et al. directly measured the thermal diffusivity and effusivity for different liquids with two photoacoustic methodologies in 2013, the thermal conductivity was obtained at ambient temperature. The photoacoustic method estimates a thermal conductivity for the sunflower oil of 0.15 W m−1 K−1 ± 0.03 and for the soybean oil of 0.17 W m−1 K−1 ± 0.02 [17]. In 2009, Turgut et al. evaluated the thermal conductivities of three different edible oils, namely sunflower oil, corn oil and olive oil at temperatures from 25 °C to 80 °C. The measurements were made using a hot wire probe method [18]. In 2008, Brock et al. reported the thermal conductivity values for sunflower and
soybean oils in the temperature range 20–70 °C using an analyzer of thermal properties (Decagon Inc., model KD2) and a thermostatic bath [19]. Brock et al. indicated that the thermal conductivity for the sunflower oil was 0.165 W m−1 K−1 ± 0.010 at 21 °C and 0.155 W m−1 K−1 ± 0.018 at 68.7 °C. For the soybean oil the thermal conductivity was 0.180 W m−1 K−1 ± 0.012 at 21 °C and 0.160 W m−1 K−1 ± 0.009 at 69.5 °C [19]. Turgut et al. do not clearly indicate the accuracy of the hot wire probe method. At 80 °C, the difference between the values of Turgut et al. and ours is less than 5% for thermal conductivity [18]. One may conclude that the results of Fig. 5 4 are validated by literature data below 80 °C.
134
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Table 3 Linear fit parameters for Eq. (2) and correlation coefficient r2. Oil sample
Rapeseed
Soybean
Sunflower
Palm
Copra
Cotton
Jatropha
a (kg m−3 K−1) b (kg m−3) r2 Temperature range (°C)
−0.6691
−0.5917
−0.6081
−0.7584
−0.7392
−0.6607
−0.6831
928.19
929.77
925.98
933.33
933.47
926.36
926.58
0.99961 25– 250
0.99809 25–250
0.99973 25–250
0.99183 50–250
0.99990 25–250
0.99883 25–250
0.99683 25–250
Table 4 Polynomial fit parameters for Eq. (3) and correlation coefficient r2 [40]. Oil sample
Rapeseed
Soybean
Sunflower
Palm
Copra
Cotton
Jatropha
c (× 10−7 W m−1 K−3) d (× 10−4 W m−1 K−2) e (W m−1 K−1) r2 Temperature range (°C)
2.00
0.92
3.50
2.58
2.44
1.06
2.80
−1.714
−1.563
−2.232
−2.113
−2.503
−1.629
−2.258
0.1698
0.1702
0.1706
0.1729
0.1703
0.1702
0.1736
0.99214 25–230
0.99486 25–230
0.99214 25–230
0.99382 40–230
0.99567 25–230
0.99214 25–230
0.99222 25–230
Table 5 Polynomial fit parameters for Eq. (4) and correlation coefficient r2. Oil sample
Rapeseed
Soybean
Sunflower
Palm
Copra
Cotton
Jatropha
f (× 10−9 kJ kg−1 K−5) g (× 10−7 kJ kg−1 K−4) h (× 10−5 kJ kg−1 K−3) i (× 10−3 kJ kg−1 K−2) j (kJ kg−1 K−1) r2 Temperature range (°C)
1.621
0.807
0.760
2.782
−0.830
1.373
2.262
−8.735
−3.705
−3.310
−16.443
4.647
−7.924
−10.423
14.933
5.491
4.147
30.534
−8.632
14.060
12.947
−5.976
−0.409
1.200
−16.320
8.856
−5.443
0.441
2.0985
1.9664
1.9506
2.4558
1.8447
2.1186
1.9608
0.99016 25–240
0.99508 25–240
0.99356 25–240
0.99582 40–240
0.99932 25–240
0.98861 25–240
0.98443 25–240
Table 6 Linear fit parameters for Eq. (5) in log-log representation and correlation coefficient r2. Oil sample
Rapeseed
Soybean
Sunflower
Palm
Copra
Cotton
Jatropha
l (dimensionless) m (dimensionless) r2 Temperature range (°C)
4.5966
4.1758
4.2454
4.9375
4.4986
4.4303
4.5168
−1.7645
−1.6966
−1.7295
−2.0299
−1.9365
−1.7978
−1.8371
0.99933 50–300
0.98421 50–250
0.99022 50–250
0.99984 50–250
0.99556 50–250
0.99255 50–250
0.99882 50–250
heat values of different vegetable oils in the range 1–270 °C [23]. In 1979 Formo et al. estimated a correlation for the specific heat of soybean oil from ambient temperature to 180 °C [10]. Tochitani et al. evaluated the specific heat capacity of soybean oil from the approximate melting point to 150 °C [25]. Although the tendencies with temperature of the experimental results appear to be consistent, there are still differences from the results of the two most recent authors, namely Santos et al. [26] and Fasina et al. [27]. This dispersion probably results from the difference between the equipment used. In their case, a differential scanning calorimeter (DSC) was used for the specific heat against temperature. Despite the capacity of both instruments (DSC and calorimeter) to measure this property, the level of accuracy can still vary. The calorimeter C80 uses a lower temperature ramp, 1 °C min−1 against 5 °C min−1 for DSC. With a larger quantity of oil and a more accurate heat flow-meter, the calorimeter theoretically meets all the conditions for more accurate
3.3. Specific heat capacity Fig. 7 illustrates the variation in the specific heat capacity of the seven vegetable oils, over the temperature range 25–240 °C. For all the tested vegetable oils, the specific heat capacity increased when the temperature increased, except in some temperature intervals. Unlike other materials, the variation of the specific heat is not linear and large variations are visible, e.g. for palm oil. To assess the experimental results, sunflower oils (Fig. 8) and soybean oils (Fig. 9) were compared with the literature for specific heat capacity. Recently, different authors have specifically worked on this thermal property for several vegetable oils. Santos et al. reported the specific heat of eight different vegetable oils including sunflower and soybean oils from 40 °C to 180 °C in 2005 [26]. In 2008 Fasina et al. measured the specific heats of twelve vegetable oil samples in the temperature range 35–180 °C [27]. In 1945 Clark measured the specific 135
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Therminol VP1
Vegetable oils
73.5% diphenyl oxide / 26.5% diphenyl 12 400
Fatty acid
differences are visible at 50 °C. For most authors, viscosity measurements are performed dynamically. For the experimental results, each data point was obtained at a constant temperature. Even so, the trend is highly useable and can provide dynamic viscosities values at 200 °C and 250 °C.
−11 to 20 250
4. Temperature correlations of properties
110
230 – 330
0.5 24 239
0
1.77 1.87
1.91 – 2.15 1.94 – 2.13
0.128 0.114
0.148 – 0.155 0.130 – 0.144
–
0.985 0.395 N/A
4.6 – 11.9 1.2 – 3.5 Good
€ kg−1 – kg CO2 eq kg−1 –
5.7 Low 3 High
0.4 – 1.2 High 0.85 – 1.87 Low
Table 7 Comparison of a synthetic oil (Therminol VP1®) and vegetable oils [4,39,46,47]. Thermal oil Composition
–
Pour point Temperature limit for use Flash point Relative pressure @ 100 °C @ 200 °C @ 300 °C Volumetric heat capacity @ 100 °C @ 200 °C Thermal conductivity @ 100 °C @ 200 °C Dynamic viscosity @ 100 °C @ 200 °C Chemical compatibility Cost Availability LCA - GHG Environmental hazard
°C °C °C kPa
The present study demonstrates an influence of the fatty acids composition on the thermophysical and rheological properties of vegetable oils. A general correlation between vegetable oil compositions and thermal properties was not established because experimental variations between samples were too low compared to the assumed overall level of errors. As a consequence, the characterization of thermophysical and rheological properties of each vegetable oils enables us to determine different correlations of these properties with temperature. These correlations use experimental data to develop a best representative equation. The quality and deviations of the points from the equations are defined by a correlation coefficient (r2) for each vegetable oil (Eq.(1)). These equations link the many characterizations to numerical models.
MJ m−3 K−1
W m−1 K−1
mPa s
(r 2 = 1 − SSres / SStot )
(1)
4.1. Temperature dependence of density The density evolves linearly with increasing temperature (in °C). This linear trend is obtained with different experimental values of the density.
measurements. Taking into account the error, repeatability and unknown composition of the oils in the literature data, the experimental results point to reasonable consistency.
ρ (T ) = aT + b
(2)
Table 3 presents the parameters a and b for each vegetable oil with the corresponding temperature range.
3.4. Dynamic viscosity The dynamic viscosity obtained over the temperature range 50–250 °C for all the vegetable oils is shown in Fig. 10. The viscosity decreases with increasing temperature. This effect of temperature on the viscosity of the vegetable oil has been attributed to the decrease of intermolecular forces, which makes its flow easier, thus reducing its viscosity [28]. The fatty acid composition has a certain influence on the viscosity profile. For saturated fatty acids, the dynamic viscosity increases with increasing number of carbons [32]. This influence is shown in the lower viscosity of coconut oil compared with palm oil. Vegetable oils also have monounsaturated and polyunsaturated fatty acids, mainly oleic acid (C18:1) and linoleic acid (C18:2). Studies have shown that these two fatty acids do not behave in the same way with changes in temperature, a decrease in the oil viscosity was distinctly observed with increasing portion of linoleic fatty acids. The authors suggest that the oils with more double bonds have a lower viscosity due to their freely filled structure [34]. Rapeseed oil, with 60.8% oleic acid, has a higher viscosity than soybean oil, with 52.4% linoleic acid. Literature data concerning the dynamic viscosity are more abundant than those for the other variables characterized here because this rheological property is necessary for research related to the use of vegetable oils as a biofuel. The measurement methods are largely identical. What differentiates the studies are the temperature ranges. Most research has focused on measuring the viscosity from ambient temperature to 110 °C [5,8,9,11,19,20,28–36]. Beyond this temperature, dynamic viscosity measurements were carried out by Esteban et al. to 140 °C [7] and by Fasina et al. to 180 °C [27] for many vegetable oils. Figs. 11 and 12 respectively show the comparison between our experimental data for sunflower and soybean oils and the values available in the literature. The multitude of available data allows a more meaningful analysis of the experimental results obtained for dynamic viscosity. Some small
4.2. Temperature dependence of thermal conductivity As the thermal conductivity of vegetable oils depends mainly on temperature, the following second-degree polynomial equation was adopted for the correlation of the experimental thermal conductivity.
λ (T ) = cT 2+ dT + e
(3)
Table 4 contains the polynomial coefficients c, d and e fitted for the different vegetable oils with the corresponding temperature range. 4.3. Temperature dependence of specific heat capacity The specific heat capacity is strongly temperature-dependent. To best fit the experimental profiles, a fourth degree polynomial equation was used to correlate the values of the specific heat.
Cp (T ) = fT 4 + gT 3 + hT 2 + iT + j
(4)
Table 5 gives the polynomial coefficients f, g, h, i and j for the vegetable oils corresponding the temperature range. 4.4. Temperature dependence of dynamic viscosity The dynamic viscosity has a linear evolution in log-log representation, depending on temperature. The function was correlated with the experimental values of the dynamic viscosity.
Log (μ (T )) = l + m (Log (T ))
(5)
Table 6 contains the power factor l and the exponent m for the investigated vegetable oils. 136
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5. Innovative heat transfer fluids
References
Up to now, published studies on potential use of vegetable oil as HTF are quite rare. According to criteria established in Table 1, it is possible to determinate if the vegetable oils are appropriate as heat transfer fluids. The Table 7 compares several criteria regarding synthetic and vegetable fluids. Synthetic oil Therminol VP1 ®, considered as the reference solar oil, is used as point of comparison. It has a 400 °C limit of operating temperature, with a very low dynamic viscosity. The vegetable oils selected in the present study seem to be appropriate fluids when compared with the reference: besides their experimentally determined 250 °C maximal operating temperature, their thermophysical properties (volumetric heat capacity and thermal conductivity) are on average 13% superior to the Therminol VP1 ® ones. Vegetable oils have a more enjoyable ecobalance and an important availability with prices 7 times cheaper than the traditional thermal oil. Therefore, these innovative fluids are promising candidates for heat transfer applications, and they make easier handling, transport and end of use treatment.
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6. Conclusions The use of vegetable oils as heat transfer fluids is proposed as a new potential industrial application of those products. According to IEA experts, 20 GWe of CSP are globally needed to be built every year until 2050 to meet the international energy policy targets. This would represent 400 Andasol plants every year [4]. If vegetable oils are used, with its characteristic properties, the global need of heat transfer fluid is 1,9% of the worldwide production. So, a use of vegetable oils in CSP would not lead to a "conflict of use", most of which are devoted to the food industry. Vegetable oils are differentiated mainly by their origin and by their fatty acid compositions. The influence of composition on thermophysical and rheological properties is clearly highlighted by the evolution of their parameters from ambient temperature to 250 °C. The innovative alternative of using these oils as a heat transfer fluid for concentrated solar power plants comes up against a lack of data on its thermal properties and its temperature behavior. Thermal conductivity, specific heat, density and dynamic viscosity were assessed for seven selected oils representing over 90% of the world production of vegetable oils. The investigated rapeseed oil, already applied to thermocline thermal energy storage tank fitted with an oil loop [45], offers the following properties at 210 °C: a thermal conductivity of 0.14 W m-1 K1 , a dynamic viscosity of 3.2 mPa s, a specific heat of 2.49 kJ kg-1 K-1, and a density of 788 kg m-3. The selected tested oils demonstrated very similar thermophysical and rheological properties. These values enable a range of possible applications as HTF depending on temperatures, costs and locations of implementation. The high availability of vegetable oils, regardless of location, is an advantage for renewable energy deployment requiring heat transfer fluids, especially in emerging countries.
Acknowledgments This project was supported financially by ANRT, a Ph.D. grant through a CIFRE program n° 2012/1516, AQYLON and EDF R&D Company and the Program “Investissements d′avenir” (Investment for the Future) of the Agence Nationale de la Recherche (National Agency for Research) of the French State under award number ANR-10-LABX22-01-SOLSTICE. The authors thank Jeremy Valette (CIRAD) and Didier Caron (University of Reims) for the many characterizations carried out. J.-F. H and M. C. acknowledge the support of SATT Nord, France through DISPOTHERM Project. The authors would like to thank T. Fasquelle for his help in the study. 137
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[35]
[36]
[37] [38]
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[40]
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