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Gunasekara, S N., Stalin, J., Marçal, M., Delubac, R., Karabanova, A. et al. (2017) Erythritol, Glycerol, their Blends, and Olive Oil, as Sustainable Phase Change Materials.
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Energy Procedia 00 (2017) 000–000
www.elsevier.com/locate/procedia
1876-6102 © 2017 The Authors. Published by Elsevier Ltd.
Peer-review under the responsibility of EUROSOLAR - The European Association for Renewable Energy.
11th International Renewable Energy Storage Conference, IRES 2017, 14-16 March 2017, Düsseldorf, Germany
Erythritol, glycerol, their blends, and olive oil, as sustainable phase change materials
Saman Nimali Gunasekara
a,*, Joseph Stalin
a, Mariana Marçal
a, Regis Delubac
a,b, Anastasiya Karabanova
a,c, Justin NingWei Chiu
a,Viktoria Martin
aaKTH Royal Institute of Technology, Department of Energy Technology, Brinellvägen 68, SE-100 44 Stockholm, Sweden
bUniversity of Pau and Pays de l'Adour, BP 576 64012, Pau, France.
cGubkin Russian State University of Oil and Gas, 65 Leninsky Prospekt, Moscow, 119991, Russia.
Abstract
In searching for new candidates to be used in latent heat storage, it is desirable to explore food-grade materials of renewable origin.
Here, erythritol, glycerol, and olive oil have been characterized as PCMs. An assessment of the production process of erythritol (melting between 117-120 °C with an enthalpy around 300 kJ/kg) indicates its renewable aspects as a PCM. In addition, a simplified cost assessment of high-purity erythritol production, using glycerol, indicates a potential cost reductions up to 130-1820 times lower than the current laboratory-grade prices. Glycerol already is cost-effective. However, the glycerol-erythritol system, evaluated using the Temperature-history (T-history) method, did not exhibit phase change suitable for PCMs. Glycerol, and up to 30 mol% erythritol compositions had no phase change due to glass transition; the remainder froze but with large supercooling; and the system underwent thermally activated change. Hence, to realize glycerol or the glycerol-erythritol system as PCMs, further research is needed primarily to device fast-crystallization. Olive oil is a cost-effective food commodity, with potential for further cost reductions by mass-production. An olive oil sample, containing the fatty acids: linoleic, palmitic, oleic, margaric, and stearic was evaluated using the T-history method. This olive oil melted and froze between -4.5 to 10.4 °C and -8 to -11.9 °C respectively, with the respective enthalpies 105 and 97 kJ/kg. As the specific heat (cp) profiles of olive oil displayed two peaks, the composition adjustment of olive oil could yield a eutectic or confirm a polymorph. In either case, olive oil has a potential to be a PCM e.g. in chilling applications, while its properties such as thermal conductivity need to be determined. As a whole, this study exemplifies the potential of renewable organic materials, in pure and blend forms, as sustainable PCMs, making TES with PCMs sustainable.
© 2017 The Authors. Published by Elsevier Ltd.
Peer-review under the responsibility of EUROSOLAR - The European Association for Renewable Energy.
Keywords: Phase change materials (PCMs); Thermal energy storage (TES); Renewable materials; Cost-effective; Temperature-history (T- history) method
1. Introduction
Phase change materials (PCMs) are an attractive alternative today, for thermal energy storage (TES). The energy management and saving opportunities provided by PCMs include load shifting, peak shaving [1]-[3], industrial surplus energy harvesting [4]- [6] and mobile TES [7]- [9]. Apart from the water-ice system which is the cheapest PCM, several materials (e.g. paraffins, salt-hydrates and fatty acids [10], [11]) have been considered for TES applications.
For sustainability, the PCMs themselves must also be renewable, non-toxic, environmental-friendly, and cost- effective. Some materials, e.g. paraffins and salt-hydrates are cost-effective, yet non-renewable. Some others, e.g.
polyols, and fatty-acids, are renewable, bio-degradable and non-toxic, though are not as cost-effective as salt hydrates or paraffins [12]-[14].
Finding a PCM that fulfils sustainability criteria, in addition to the TES design requirements (a suitable melting temperature, large melting enthalpy and high thermal conductivity) is challenging. Polyols and fatty acids are materials emerging as PCMs since they come with attractive TES properties. They are also non-toxic, some even being food- grade (e.g. erythritol, xylitol, glycerol, and oleic, linoleic and palmitic fatty acids) [15], [16]. Olive oil, a blend of fatty acid-derivatives, has so far not been considered in the PCM-context. If suitable, olive oil would be a candidate PCM of renewable origin. Contributing to identify and develop sustainable PCM materials, this study aims to evaluate: the potential for cost-reduction of erythritol as a bulk PCM; the melting temperatures and enthalpies of glycerol and the erythritol-glycerol system; and food-grade olive oil as a PCM. Here, the Temperature-history (T-history) method is used [15], [17]. Finally, the potential of these materials as being sustainable PCMs for TES is assessed as a whole.
Nomenclature
Avg. average
cp specific heat (kJ/(kg·K)) DSC differential scanning calorimetry
Gal gallon
ΔH enthalpy change (kJ/kg)
Er erythritol
Frz freezing
HT high-temperature
HPLC high-pressure liquid chromatography
L liquid
Mel melting
NA not available
PCM phase change material
T temperature (°C)
TAG triacylglycerol/triglyceride TES thermal energy storage T-history temperature-history RBC repeated batch cultures
S solid
SS stainless steel
S1, S2 different (olive oil) samples
1.1. Background
Presently, erythritol (C4H6(OH)4), glycerol (C3H5(OH)3) and olive oil are primarily food ingredients. Erythritol is a low-calorie sugar substitute, while both erythritol and glycerol are used in pharmaceutical and cosmetics applications [18], [19]. Erythritol functions as a flavour enhancer, formulation aid, humectant, stabilizer and a thickener [18].
Glycerol functions as a food-additive, and in chemical industries as a solvent and a precursor [19]. Commercial-grade glycerol of more than 95% purity is termed glycerine [20], while, the lower-purity glycerol is generally called ‘crude glycerol’. The main constituents of olive oil are triacylglycerols (TAGs, also called triglycerides). A TAG is formed by the esterification of three fatty acid molecules with one glycerol molecule [21], [22]. Hence, glycerol and olive oil are inevitably connected. The main fatty acid constituents of the TAGs in olive oil are palmitic, palmitoleic, stearic, oleic, linoleic and linolenic acids [22], comprising of about 94-96% of the total weight of the TAGs [21]. In addition, olive oil contains smaller amounts of free fatty acids and glycerol, among others [22].
Erythritol is produced by hydrogenation of starch, primarily from maize [23]- [25], as further detailed in section 3.1. Glycerol is a by-product of: biodiesel or bioethanol production by transesterification of vegetable oils[19],[20];
saponification of fats or oils during soap production; and hydrolysis of fats and oils [26]. During these processes, about 10-20 % (in biodiesel production) or 10% (in the other processes) of the total production volume is made of glycerol [20], [26]. With an increase of biodiesel and bioethanol production globally, glycerol production is also on the rise [20], thus cost reductions could be expected. Olive oil is produced by: crushing the olives, mixing this paste, and separating the oil from it, followed by the addition of additives, as desired [27]. Erythritol has experienced considerable attention as a PCM ( [15], [17], [28]- [36]), whereas glycerol and olive oil are novel to the PCM-context.
Their available TES-specific thermal properties are summarized in Table 1.
Table 1. Thermal properties of erythritol, glycerol and olive oil, relevant to TES design (S: Solid, L: Liquid, NA: Not available)
Material Melting
temperature (°C)
Melting enthalpy (kJ/kg)
Thermal conductivity (W/(m·K))
Viscosity (Pa s) Density (kg/m3) Sources
Meso-erythritol 117-121/112- 120/102-112
284-370 0.73/0.59 (S), 0.33/0.32 (L)
NA NA [15]
Commercial grade erythritol
119 204 0.35 (S),
0.39 (L)
NA 1480 (S), 1300
(L)
[37]
Glycerol 18/ 18.2 ~199 0.14, 0.28 (25
°C)
1.41, 1.5 1260, 1261 [15]
Crude glycerol NA NA NA 8.46-8.8a, 16 1060 (25 °C) [20], [38]
Commercial grade virgin olive oil
~-15 to 10 ~90c NA 0.128-0.13 (10 °C),
0.0373-0.0384 (40 °C)
NA [39], [40]
aat 40 °C, for different feedstock [20], branges obtained for several olive crops, cand a freezing enthalpy of 82 kJ/kg [39]
As seen, a number of TES design parameters are still unknown, particularly necessary in designing the heat charging and discharging process (e.g., the heat exchanger design). Hence, these candidate materials need to be fully characterized in terms of physical properties in order to comprehend their PCM-suitability from a TES systems perspective.
2. Experimental study- materials and methods
Below is an example which the authors may find useful. Meso-erythritol (C4H10O4, CAS number 149-32-6) of 99%
purity [41], and glycerol (CAS number 56-81-5) of more than 99% purity [42], were used in determining the thermophysical properties of relevance to the PCM context. Commercial-grade virgin olive oil (origin: Spain) was bought from a grocery store.
For the erythritol-glycerol system, several blend compositions, expressed in mol% erythritol (referred to as mol%
Er) were prepared, as shown in Table 2. From these, the 10, 40, 60 and 90 mol% Er were evaluated in pre- heating/cooling evaluations. There, these were subjected to step-wise pre-heating in an oven (Termarks TS4000) from room temperature to up to 135 °C, and pre-cooling to room-temperature, and to -17 °C using a freezer (Whirlpool AFG 649-B). By visual observations and photographing, a pre-assessment of their phase change behaviour was done, as discussed in section 3.3. Afterwards, the compositions 0, 10-30, and 70-93 mol% Er, were evaluated with the T- history method (section 3.3). Their containing stainless-steel (SS316) test-tubes’ weights are also given in Table 2.
Once the corresponding amounts were weighed and mixed, these samples were melted in the oven at 135 °C for 5 hours, to obtain a clear, single liquid.
Table 2. The evaluated erythritol-glycerol blend compositions and the corresponding weights Composition,
mol% Era
0c 10d 10c 20c 30c 40d 60d 70c 74c 80c 85c 90c 90d 93c
Erythritol (g)b 0 2.57 3.85 7.45 10.87 9.38 13.31 26.45 27.79 29.5 30.97 32.29 18.45 33.15 Glycerol (g)b 30 17.43 26.15 22.53 19.13 10.62 6.69 8.55 7.21 5.46 4.03 2.71 1.55 1.85 Total evaluated
sample (g)b
14.24 20 13.42 13.85 13.66 20 20 10.13 10.68 8.19 10.98 10.60 20 8.82
SS Test-tubes (T-history) (g)b
60.36 -- 60.37 60.23 60.43 -- -- 60.39 60.12 60.40 60.21 60.25 -- 60.35
athe expanded uncertainty of the molar composition is 1.3%, with 0.95 confidence
bthe expanded uncertainty of mass is 0.04 g, with 0.95 confidence
cin the T-history evaluations, din the pre-heating/cooling evaluations
Olive oil was evaluated as it is, with no purification or pre-preparation. There, two identical samples, S1 and S2 were evaluated with the T-history method. The weights of these samples and the SS test-tubes containing them, are given in Table 3. The corresponding results are discussed in section 3.4.
Table 3. The weights of the olive oil samples (S1 and S2) and their containment stainless-steel (SS) test tubes
S1 S2
Sample weight (g)a 9.71 9.71
Test-tube weight (g)a 60.10 60.01
awith0.04 g expanded uncertainty with 0.95 confidence
2.1. Temperature-history method
The T-history method was employed to determine the phase change temperatures and enthalpies of erythritol- glycerol system and olive oil. The method is explained elsewhere ( [15], [17]), while the details specific for this study are presented here. A solid stainless steel (type SS316) reference, of an identical geometry to the test-tubes and a weight of 145.80 ±0.04 g, was used. The specific heat and thermal conductivity of SS316 employed in the calculations were: 0.5 kJ/(kg·K) and 16.2 W/(m·K), respectively [43].The temperatures were measured using calibrated T-type thermocouples, with 0.1 °C expanded uncertainty with 0.95 confidence. The sample test-tubes and the references, once the thermocouples were connected, were insulated with high-temperature (HT) Armaflex insulations. These were then placed inside a stainless steel containment, within a temperature programmable climate chamber. In this chamber, the samples were thermally cycled for four cycles, each. There, controlled heating/cooling rates were used, allowing isothermal steps at the lowest and the highest cycle temperatures for a given time, as shown in Table 4.
Table 4. Thermal cycling programs used in the erythritol-glycerol and olive oil T-history testing
Test Lowest Temperature
(°C)
Highest Temperature (°C)
Heating rate (°C/min) Isothermal holding time (hours)
Erythritol-Glycerol -20 138 0.05 8
Olive oil -30 80 0.1 3
3. Erythritol, erythritol-glycerol and olive oil as cost-effective, safe and renewable PCMs
This section discusses the erythritol production process in detail, aiming to identify potential in cost reductions that can yield lower market prices of erythritol. In addition, thermophysical properties of erythritol-glycerol blends as well as of olive oil are presented and discussed.
3.1. Erythritol production process - opportunities
Erythritol is commercially produced mainly by a biotechnological method [44]. There, starch extracted from maize is hydrolyzed to produce D-glucose, which is then fermented using a fungus [44]. The fungi used are e.g. Moniliella
pollinis [45]-[47], Aureobasidium sp., Trichosporonoides sp., Torula sp. [46], or Candida magnoliae [44]. The fermented broth is then purified using adsorbents and filtration [45], [47], followed by crystallization under controlled cooling [45]. To obtain higher (e.g. more than 99.5%) purities of erythritol, the crystallization process is repeated several times [45], [47].
By using more productive microorganisms, the erythritol yield can be increased up to 37-61% (w/w per carbon source used) [46]. The yield can also be optimized by: controlling the glucose concentration; using alternative carbon sources; including vitamins and minerals supplements; and removing inhibitors and by-products [46], [48]. An increased erythritol yield, as well as substituting maize or glucose with more cost-effective carbon sources can significantly reduce the erythritol production costs. For example, erythritol yield has increased by ten-fold, by using wheat straw instead of maize, hydrolyzed using a genetically modified mold fungus, Trichoderma reesei [44], [49], [50]. Since maize is a food-commodity, substituting it with inedible lignocellulosic biomass like straw also lowers the market competition on the raw materials.
Erythritol can be produced using high-temperature chemical synthesis of dialdehyde starch, using nickel catalysts [46]. However, due to very low yields, this is not commercialized [46]. Another chemical synthesis method decarbonizes ribonic acid or arabinonic acid (reactant) by electrolysis to obtain erythrose, from which erythritol is produced [51]. Here, the reactants could also be obtained from hexose sugars such as, allose, altrose, glucose, fructose or mannose [51]. This process produces higher erythritol yields at a shorter time, compared to the biotechnological processes which use fermentation [52]. Therefore, erythritol apparently has great potential in reaching higher yields and hence cheaper production, with cheaper raw materials as well as with shorter production times.
Erythritol can also be fermented from crude glycerol, as an alternative to glucose, using the yeast Yarrowia lipolytica [48]. Then, the starch extraction and hydrolysis are no longer needed. As mentioned in section 1.1, crude glycerol is a by-product of biodiesel, bioethanol, or soap production, or fats hydrolysis. Erythritol is mostly fermented in batch or fed-batch cultures, while repeated batch cultures (RBC) give rise to a more efficient production [48]. The yeast Yarrowia lipolytica Wratislavia K1, from crude glycerol, has produced considerable erythritol productivities (erythritol per glycerol per hour), of 0.54 g/(l·h) in RBC [48], or 1 g/(l·h) in fed-batch cultures [53]. The batch production of erythritol from crude glycerol using Yarrowia lipolytica produced the high erythritol yields of 109 g/l [54]. Thus, this cost-effective by product, crude glycerol, has the potential to greatly economize the erythritol production. Fig. 1 summarizes all these opportunities to alter the erythritol production process, to reduce costs.
Straw
Starch separation
Starch
Enzymetic hydrolysis
Glucose Fermentation using yeast
Sterilized fermentation
broth
Purification, crystallization
Pure Erythritol Crystals Biodiesel or Bioethanol
Production Soap Prodcution Fats or Oils Hydrolysis
Crude
Glycerol Fermentation using yeast
Ion exchange resins, Activated
carbon,
Ultrafiltration < 99.5%
purity GM mold
fungus Trichoderm
a reesei
E.g.
Fungus Yarrowia lipolytica
Fig. 1. Erythritol production process adaptation opportunities for cost-reduction
3.2. Simplified erythritol production cost assessment
Detailed cost assessments of erythritol production are not found in open literature. Hence, here, a simple estimate of erythritol production cost is performed. This is done by assuming a batch operation employing crude glycerol as
the carbon course, and using an economic analysis of a batch production of ethanol [55] as the basis. The corresponding cost evaluation, accounting the investment, utilities and process operations, is summarized in Table 5.
The batch production of ethanol involves primarily a fermentation step, followed by distillation and dehydration [55].
Fermentation is also common for erythritol production, while herein, the distillation and dehydration expenses [55]
were approximated to the erythritol crystallization and purification process costs. Crude glycerol use by-passes the starch separation and hydrogenation (c.f. section 3.1). The ethanol production cost estimates made for the year 2000 [55], were projected to 2017 by considering the average inflation rates per year for 2000-2016 [56] in combine.
Similar to O’Brien et al. [55], a batch produced per every 48 hours, and a plant operation through the whole year were considered. Considering an investment payback period of 9 years, the projected capital cost was annualized for 9 years. An erythritol yield of 109 g/l from crude glycerol, for a batch process [54] was used to calculate the required amount of crude glycerol. The glycerol price was used as 800 USD/ton [57]. The annual batch production cost is the sum of the annualized capital cost, annual utilities, and annual operational costs. The ‘other costs’, specific for the erythritol production, were assumed as 60% of the annual batch production cost. This was to include the costs for crude glycerol transportation, further purification of erythritol (to reach above 99% purity) and other miscellaneous costs. The total annual cost is the sum of the annual costs of batch production and other costs. A total production volume of 50,000,000 gallons per year [55] was considered the same in the erythritol case, which is equivalent to 280 kilo tons of erythritol (for solid density of 1.48 g/cm3 [37]). Thereby the unit costs were calculated, accounting for a total annual production cost of 14.75 USD/kg of erythritol (c.f. Table 5).
Table 5. Simplified economic analysis of a batch process of erythritol production (adapted based-on the ethanol production [55]) Batch fermentation costs Fermentation,
$/yr
Crystallization and purification, $/yr
total annual cost, $/yr
Unit costs
$/Gal
unit cost
$/kg
Total capital cost 27 591 826 19 519 985 47 111 811 0.942 0.168
Annualized capital cost (over 9 years) 3 066 348 2 168 459 5 234 807 0.105 0.019
Steam --- 3 342 621 3 342 621 0.067 0.012
Cooling water 230 947 430 094 661 042 0.013 0.002
Electric power 210 083 85 760 295 843 0.006 0.001
Annual utilities 441 030 3 858 476 4 299 505 0.086 0.015
Plant labour 1 102 216 778 458 1 880 674 0.038 0.007
Supplies 690 252 488 227 1 178 479 0.024 0.004
Glycerol cost 2 569 912 585 --- 2 569 912 585 51.398 9.174
General operational expenses 276 273 194 255 470 528 0.009 0.002
Annual operational costs 2 571 981 326 1 460 940 2 573 442 266 51.469 9.187
Annual cost, batch production 2 575 488 704 7 487 874 2 582 976 579 51.660 9.221
Other costs 1545293222 4 492 725 1 549 785 947 30.996 5.533
Total annual cost 4 120 781 927 11 980 599 4 132 762 526 82.655 14.754
Considering the cheapest market prices of erythritol, e.g. 1-10 USD/kg [58], the unit production cost found here:
14.75 USD/kg is still within the same price range. However, these market prices (c.f. [58]) correspond to the commercial-grade erythritol. Whereas, in the present work, the process costs were allocated for a higher purity (above 99%), and hence are more reasonable to be compared with the laboratory-grade prices, e.g. 1900 USD/kg [41]. The identified unit cost of 14.75 USD/kg is 129 times lower than the laboratory-grade erythritol price. Therefore, overall, even after a large profit margin, it is quite palpable that pure erythritol market price can be effectively lowered by e.g.
erythritol production using crude glycerol. Although crude glycerol has experienced price fluctuations over the years [20], the glycerol price used here (800 USD/ton) still is representative of the higher prices. The production cost of erythritol can be lowered even further by using e.g. 80% pure crude glycerol from biodiesel production, costing only 44 USD/ton [20]. This results in an erythritol production cost of merely 1.04 USD/kg. This results in a pure erythritol production cost: the same as the cheapest of the commercial-grade prices today; and up to 1821 times cheaper than
the laboratory-grade price today (1900 USD/kg). Even though the cost assessment here is rather simplified, it still indicates a great potential in obtaining very cost-effective, pure erythritol for TES. This analysis is restricted to the use of crude glycerol as a raw material. It will be interesting to see the cost reduction opportunities in using straw instead of e.g. maize, incorporated with special fungi as detailed in section 3.1. This is however, excluded in this work.
3.3. Glycerol and erythritol-glycerol blend as PCMs
The melting temperatures and enthalpies of both erythritol and glycerol are known (c.f. Table 1). Despite their attractive thermal properties, glycerol succumbs to glass transition, becoming vitreous at around -89 °C [20], while erythritol undergoes thermally activated change, becoming a browned thicker material [15], [36]. The system erythritol-glycerol is novel in the PCM-context, with potential to work as a cost-effective and renewable PCM alternative. To find an ideally suitable PCM composition in a blend, i.e., having a sharp and reversible phase change, its phase diagram must be known. Thereby, if the system contains congruent melting compositions or eutectics (in the absence of supercooling), these can be chosen as PCMs [59]. Hence in this study, the first assessment of the erythritol- glycerol systems’ phase change behavior is done, to explore the existence of PCM-ideal compositions.
In the evaluations conducted, from the pre-study liquid samples of 10, 40, 60 and 90 mol% Er, after melting in the oven at 135 °C, only the 90 mol % Er crystallized at room temperature. These samples were then kept in the freezer at -17 °C for 20 hours. Thereby, the 60 mol% sample also crystallized, while the 40 mol% Er was a mixture of both solid and liquid, and the 10 mol% Er remained a thick, glassy liquid (as shown in Fig. 2 (a)). These were re-melted in the oven by heating from room temperature to 135 °C, allowing isothermal 1 hour ramps at every 10 °C. Thereby, the approximate melting temperatures of 40, 60 and 90 mol% Er were found to be at around 60, 100 and 130 °C respectively.
Fig. 2. (a) The 10, 40, 60 and 90 mol % Er compositions (from left-to-right), after the pre-heating followed by pre-cooling test; (b) The summarized T-history of the evaluated (0-93 mol% Er) samples, where 0-30 mol% Er underwent glass-transition.
In the T-history evaluations, 70-93 mol% Er compositions underwent melting and freezing, as shown in Fig. 2 (b), indicating a melting-point decreasing trend towards glycerol-rich compositions. During freezing, a very large supercooling (between 33-52 °C) was observed. In contrast, pure glycerol, and the 10-30 mol% Er compositions underwent glass transition (with continuous heating and cooling curves, Fig. 2 (b)). They had no phase change at all, even after seeding small amounts of erythritol. Hence, deriving the erythritol-glycerol phase diagram was not possible.
The melting of the 40-93 mol% Er blends imply a possible melting point minimum in the system, which could be a PCM-suitable congruent melting or a eutectic composition. However, this cannot be characterized without realizing phase change on all the blends. Seeding erythritol, or stirring (e.g. of the supercooled 10 mol% Er) failed to initiate crystallization in the glassy compositions. The addition of small amounts of water [60], or bubbling with air [61] on
these could be explored in future, to initiate crystallization. However, even if crystallization is achieved, the practical application of a PCM derived from this system is questionable. This is owing to the extremely glassy compositions 0- 40 mol% Er, and the significantly supercooling remainder. In addition, the final T-history products have browned, indicating the thermally activated change similar to erythritol. Therefore, it appears that neither glycerol nor the erythritol-glycerol system are suitable as PCMs, until fast-crystallization is realized while avoiding supercooling and thermally activated change.
3.4. Olive oil as a PCM
Olive oil is a multicomponent blend primarily made of triacylglycerols. Triacylglycerols are a combination of glycerol and fatty acids [22]. Olive oil composition depends on the region of olive crops, and differs even between batches of the same producer. Olive oil composition can be analyzed using techniques such as High-Pressure Liquid Chromatography (HPLC). However, this was excluded in this work. A sample of Spanish olive oil was found to comprise of the fatty acids: linoleic; palmitic; oleic; margaric; and stearic, in the concentrations 84.51; 33.05; 1327.23;
5.76; 89.48; and 1540.03 in μg/ml oil, respectively [62]. These are total amount of fatty acids bound in TAGs as well existing as free fatty acids. It is reasonable to assume that the olive oil evaluated in this study, produced in Spain, also contains these fatty acids in rather similar proportions. The price of olive oil as a food-commodity is moderate, with an average price over 2013-2016 of 4.2 USD/kg, and the price predicted for 2017-2018 being 3.9 USD/kg [63]. Despite such price fluctuations, in overall, large-scale olive oil production could result in cost-reductions, if suitable for TES.
During the four T-history cycles, both the olive oil samples S1 and S2 displayed quite consistent phase change.
Their specific heat (cp) profiles during the heating and cooling cycles are summarized in Fig. 3.
Fig. 3. Specific heat (cp) characteristics of olive oil for two evaluated samples S1 and S2, during (a) melting of S1; (b) freezing of S1; (c) melting of S2; and (d) freezing of S2
There in Fig. 3, two distinct phase changes can be identified per cycle, during both heating and cooling. In heating this is more prominent. The secondary peak, occurring before melting, or after freezing is the larger of the two peaks (Fig. 3). This phase change behavior with two peaks is consistent with what Ferrari et al. [39] also observed, using Differential Scanning Calorimetry (DSC). Ferrari et al. [39] saw similarities between these two peaks and the polymorphism followed by melting in triolein, a TAG which accounts for more than 30% of olive oil. As olive oil is a multicomponent blend, to confirm if this secondary peak is indeed a polymorph, or e.g. is an indication of a multicomponent near-eutectic composition, further evaluations are necessary. These e.g. could be calorimetric, crystallographic and microstructural evaluations in combine, on compositions around the olive oil composition.
A slight supercooling of around 2 °C was observed during freezing, as the temperature profiles indicate in Fig. 4.
The average phase change enthalpies and temperatures of S1 and S2 over the four heating and cooling cycles are summarized in Table 6. As a whole, the total melting and freezing enthalpies of the evaluated olive oil are 105±11 and 97±8 kJ/kg, over the temperature ranges -4.5 to 10.4 °C and -8 to -11.9 °C (0.4 °C uncertainty with 0.95 confidence), respectively. Hence, the melting occurs over a wider temperature range, with a slightly larger enthalpy (about 10 kJ/kg) than with freezing. The results here are comparable with what Ferrari et al. [39] obtained in the DSC, melting around -15 to 10 °C, with a total melting enthalpy of around 90 kJ/kg. As the origin of the olive oils here and in their study [39] are different, moderate disparities are inevitable. The samples evaluated here undergo considerable hysteresis, between the start of melting and freezing, around 3.5 °C, and with a much wider temperature difference between the end of melting and freezing (i.e., ~ 22 °C).
Fig. 4. Temperature profiles of olive oil during cooling, indicating supercooling of: (a) S1; and (b) S2
Table 6. Average phase change temperature ranges (T, °C) and enthalpies (ΔH, kJ/kg) of olive oil, over the four evaluated cycles (S1 and S2- two samples of olive oil, Avg.- average, T-temperature, Mel- melting, Frz- freezing)
Secondary phase change Melting/Freezing Total phase change
ΔH (kJ/kg)a
T,start (°C)b
T,end (°C)b ΔH (kJ/kg)a
T,start (°C)b
T,end (°C)b
ΔH (kJ/kg)a
T,start (°C)b
T,end (°C)b
S1 Avg. Mel 76 -4.5 1.6 27.5 1.6 10.3 103.5 -4.5 10.3
Avg. Frz -75 -8.0 -9.6 -18 -9.6 -11.8 -93 -8.0 -11.8
S2 Avg. Mel 81 -4.4 2.0 26 2.0 10.5 107 -4.4 10.5
Avg. Frz -81 -8.1 -10.3 -19 -10.3 -12.0 -100 -8.1 -12.0
Avg. Mel 78.5 -4.5 1.8 27 1.8 10.4 105 -4.5 10.4
Frz -78 -8.0 -10.0 -19 -10.0 -11.9 -97 -8.0 -11.9
a10% expanded uncertainty with 0.95 confidence, b0.4 °C expanded uncertainty with 0.95 confidence
As the results indicate, olive oil either has a polymorphic phase change closely followed by melting (or freezing), or, is a near-eutectic composition in a multicomponent system. The freezing temperature of olive oil, between around
-8 to -12 °C, is attractive e.g. for pre-cooling in chilling applications, with a moderate enthalpy of around 100 kJ/kg.
Therefore, the results encourage future systematic explorations of the olive oil multicomponent blend system, to e.g.
find a congruent melting composition or a eutectic near the evaluated composition. The thermal conductivity of olive oil (c.f. Table 1) was not found within open literature. Hence it is another crucial parameter to be determined to realize an olive-oil based renewable, safe and cost-effective PCM. The refinement of olive oil by e.g. activated carbon and other filters [64] could yield temperature and enthalpy adjustments more favorable for chilling. The fatty acids in olive oil, e.g. Oleic acid, can also serve as renewable PCMs. They can be extracted from the olive oil TAGs e.g. via the fat hydrolysis at low temperatures, with solid acid catalysts [65], or lipases [66]. Here, a by-product is glycerol, which can be linked to a cost-effective erythritol production.
4. Concluding remarks
To reach sustainability with TES using PCMs, in addition to the energy saving opportunities, the PCMs themselves must also be environmental friendly, non-toxic, and cost-effective. This study shows that, the food-grade and renewable organic materials: erythritol, glycerol and olive oil, are such sustainable candidates.
Glycerol and olive-oil are already cost-effective, while erythritol, is rather expensive. However, by means of a production process and cost analysis, this study shows that even erythritol has great potential to be a cost-effective PCM. By using crude glycerol as the source of carbon in fermentation, a batch production process can approximately yield high-purity erythritol, e.g. at a production cost that is, similar to the current commercial-grade prices, and 130- 1820 times lower than the laboratory-grade erythritol prices. For TES design, erythritol has been characterized rather well as a PCM, while glycerol, erythritol-glycerol blend system and olive oil are novel. However, the thermal cycling using the T-history method, has shown that glycerol as well as the erythritol-glycerol blends up to 30 mol% Er compositions are extremely influenced by glass transition, avoiding phase change even with seeding. The compositions above 60 mol% Er melted indicating decreasing melting points towards the glycerol-rich compositions.
However, their freezing accompanied very large supercooling. Due to the absence of phase change in up to 30 mol%
Er, deriving the erythritol-glycerol phase diagram was not possible, to find suitable blend compositions. The system in addition underwent thermally activated change. Hence, it appears that the applicability of glycerol or its blends with erythritol as PCMs can be realized only if a fast-crystallization initiation mechanism is found, and thermally activated change is avoided.
Olive oil is a multicomponent blend, primarily made of triacylglycerols, formed by the esterification of glycerol with fatty acids. A commercial-grade virgin olive oil (from Spain) was chosen for the study here, which was assumed to contain the fatty acids: linoleic; palmitic; oleic; margaric; and stearic, in the concentrations 84.51; 33.05; 1327.23;
5.76; 89.48; and 1540.03 in μg/ml oil. The T-history evaluations of this olive oil shows that it melts and freezes consistently for four cycles, at the temperatures -4.5 to 10.4 °C and -8 to -11.9 °C, with the enthalpies 105 and 97 kJ/kg respectively. The melting temperature and enthalpy are comparable with the literature values, produced using DSC, at around -15 to 10 °C, and 90 kJ/kg respectively. Minor disparities are possibly due to the compositional difference of the used olive oils between the studies. In the evaluated olive oil, minor supercooling (~2 °C) and considerable hysteresis (3.5 -22 °C) were identified. The T-history results also indicated two separate phase changes during melting and freezing in olive oil. This could be an indication of a polymorph, or a near-eutectic composition.
In either case, it is worth to further evaluate compositions close to the evaluated exact olive oil composition, to find a pure-material like olive-oil-based PCM. As a whole, olive oil has a potential to be tailored into a cost-effective, environmental friendly, and safe PCM, apt for instance, in pre-cooling in chilling applications. Its thermal conductivity is an important TES design parameter to be evaluated as future work. Furthermore, the fatty acids extracted from e.g.
olive oil hydrolysis, could also work as sustainable PCMs.
In conclusion, this study shows that erythritol has great potential as a cost-effective and sustainable PCM for low- temperature heating applications. Glycerol or erythritol-glycerol blends are questionable as PCMs due to extremely glassy behaviors. Olive oil has attractive phase change temperatures and enthalpies, while, after refining it into a more suitable composition, also has potential to serve as a sustainable PCM for chilling applications.
Acknowledgements
The authors’ gratitude is expressed to the Swedish Energy Agency for funding the particular research project:
34948-1, and to Ms. Sofia Kumova (KTH, and Gubkin Russian State University of Oil and Gas- Russia) for her experimentation assistance as a summer intern.
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