Scholarly article on topic 'Stability Testing of Thermal Oil in Direct Contact with Rocks used as Filler Material for Thermal Energy Storage in CSP Power Plants'

Stability Testing of Thermal Oil in Direct Contact with Rocks used as Filler Material for Thermal Energy Storage in CSP Power Plants Academic research paper on "Materials engineering"

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{TES / "filler material" / stability / HTF / "Thermal oil."}

Abstract of research paper on Materials engineering, author of scientific article — H. Grirate, N. Zari, A. Elmchaouri, S. Molina, R. Couturier

Abstract A Thermocline Thermal Energy Storage (TES) system potentially offers a cheap and simple way of achieving dispatchability of energy after sunset or during intermittent cloudy weather conditions. The system performance depends on the properties of the suitable media selected. As very promising solid TES material, natural rocks can be chosen as filler material for energy storage system owing their availability and many advantages such as high volumetric heat capacity and stable physical and chemical performance. In previous tests, quartzite rock was identified as a potential filler material for TES. In the present paper, the compatibility of the heat transfer fluid (HTF) with rocks and silica sand was tested in laboratory under defined conditions. Physicochemical and thermal characterizations of the studied materials were performed in order to evaluate their sustainability after extended time over a range of temperature. It has been concluded that no significant deterioration of tested oil aged in contact with quartzite and silica sand can affect the performance of the thermocline thermal energy storage system.

Academic research paper on topic "Stability Testing of Thermal Oil in Direct Contact with Rocks used as Filler Material for Thermal Energy Storage in CSP Power Plants"

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Energy Procedía 69 (2015) 860 - 867

International Conference on Concentrating Solar Power and Chemical Energy Systems,

SolarPACES 2014

Stability testing of thermal oil in direct contact with rocks used as filler material for thermal energy storage in CSP power plants

H. Grirateab, N. Zaria *, A. Elmchaourib, S.Molinac, R. Couturierc

a Moroccan foundation for Advanced Science Innovation and Research MASclR, Avenue Mohamed El Jazouli,Rabat, Morocco b Faculty of Science and Technology, Laboratory of Physical Chemistry and Bio-organic Chemistry, Mohammedia, Morocco c CEA LITEN, National Institute of Solar Energy, Solar Technologies Department, Thermal Systems Laboratory,Grenoble,France

Abstract

A Thermocline Thermal Energy Storage (TES) system potentially offers a cheap and simple way of achieving dispatchability of energy after sunset or during intermittent cloudy weather conditions. The system performance depends on the properties of the suitable media selected. As very promising solid TES material, natural rocks can be chosen as filler material for energy storage system owing their availability and many advantages such as high volumetric heat capacity and stable physical and chemical performance. In previous tests, quartzite rock was identified as a potential filler material for TES.

In the present paper, the compatibility of the heat transfer fluid (HTF) with rocks and silica sand was tested in laboratory under defined conditions. Physicochemical and thermal characterizations of the studied materials were performed in order to evaluate their sustainability after extended time over a range of temperature. It has been concluded that no significant deterioration of tested oil aged in contact with quartzite and silica sand can affect the performance of the thermocline thermal energy storage system.

© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG Keywords: TES, filler material, stability, HTF, Thermal oil.

* Corresponding author. E-mail address: n.zari@mascir.com

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG doi:10.1016/j.egypro.2015.03.109

1. Introduction

Concentrated Solar Power (CSP) is considered one of the most important technologies in the field of renewable energy. Due to its many advantages, CSP has attracted more attention during the last decades. It presents a significant potential to supply several process, such as heat for industry, cooling and heating power, co-generation and water desalination [1].It can also be used as part of a hybrid energy source into the electricity production mix, improving then grid integration and economic competitiveness [2, 3]. Furthermore, CSP demonstrated high capacity factor and dispatchability after sunset or during intermittent cloudy weather conditions using Thermal Energy Storage (TES).

The thermal energy storage has the potential to correct the mismatch between supply and energy demand [4] by shifting electrical loads from high-peak to off-peak hours. In order to reach high efficiency, there is different energy storage material dividing TES system into various categories "Thermochemical heat storage", "latent heat storage" and "sensible heat storage". Thermochemical reaction storage is based on using the stored heat to release a reversible chemical reaction while the latent heat storage consists of changing medium phase during the energy storage process. The sensible heat storage is the most economic technique in TES [5]. The energy is exchanged between two materials (HTF and storage material) without phase transition in an isolated system [6]. There are different ways to store energy such as TES system operating with two tanks, this technique is the most widely used in the field. There are also thermocline systems which are based on hot and cold temperature regions superposed in one tank. Compared to the two-tank system, the single tank TES system can save up to 35% of investment cost [7]. Moreover, the performance of a thermocline system based on sensible heat storage is the ability to use low cost material replacing the expensive storage fluid and at the same time avoiding the fluid's thermal mixing. The solid material maintains the stratification in a single tank which is occurring by density differences in the medium. The solid materials used in thermocline system are metals, sand, bricks, rocks, concrete and ceramic [8]. These kinds of materials will not freeze or boil at high temperature. The difficulties of the high vapor pressure of liquid materials can be limited by storing thermal energy in solid media. In order to store the energy, a heat transfer fluid (HTF) should be used to carry heat through pipes or a heat exchanger to the thermal energy storage tank. However, the separation will limit heat transfer characteristics. To resolve this problem, a direct contact between HTF and solid media is recommended. Thus, a thermocline system with direct contact materials is simpler in design. Also, it offers the capacity to operate at relatively low temperature gradient with limited losses. To ensure cost reduction effectively, rocks can be chosen as filler material in direct contact with HTF. They are relatively inexpensive, long lasting, safe and readily available in many areas. Pacheco et al. have tested a small pilot-scale thermocline TES system with molten salt as HTF and rocks and sand as filler materials [9].They reported that quartzite and silica sand are the most suitable filler materials in combination with molten salt. Brosseau et al. have also tested isothermal and thermal cycling experiments of several types of rocks (limestone, taconite, marble, quartzite rock, and silica sand). It was concluded that no significant deterioration was observed in the case of quartzite rock and silica sand [7]. These materials are considered as the appropriate filler materials to withstand the molten salt environment quite well.

Molten salts (mixture NaNO3/KNO3 (60/40% wt)) can reach a high temperature of about 550°C. Nevertheless, using this fluid as HTF may lead to complications in the solar field due to its high freezing point [10].

Otherwise, synthetic oil offers high exchange coefficient with a maximum temperature range from 390 to 400 °C, best flow freezing point (<25°C) and chemical neutrality in contact with stainless steel absorbers [11]. Synthetic oil presents the currently generation of commercial fluids [12]. It provides the best combination of low freezing point.

The aim of this work is to evaluate the compatibility of the quartzite selected from a previous study according to several criteria (shape, surface characteristics, porosity, density, calorific capacity...) with synthetic oil. Physicochemical and thermal characterisations of rocks have demonstrated that quartzite can be used in direct contact with thermal oil as the most suitable filler material for thermal energy storage. However, the compatibility of filler materials in hot thermal oil environments was unknown. Thus, a design of experiments was developed to test the stability of quartzite rock in contact with synthetic oil and then perform rigorous testing to fully assess the sustainability of rocks and the thermal oils after long term use at high temperature.

2. Methodology

2.1. Samples description

The thermal oil tested is Therminol 66, it was obtained from its commercial source. This sample is a blend of partially hydrogenated terphenyls (74-87%), terphenyls (3-8%), higher polyphenyls, quaterphenyl and their hydrogenated products (18%). Some of the many physical properties reported for the commercial fluid tested (Therminol66) are listed in table 1. For more properties, see reference [13].

Pebbles were sampled from a river near from Rabat in Morocco. These rocks are rounded stones, worn smooth by the action of water. The size sampled is of about 3cm.

Table l.studied fluid properties

Therminol 66 Properties

T°Max (°C) 345

Kinematic Viscosity mm2/s at 40°C 29,64

Density (kg/m3) at 15 °C 1011

Flash Point (°C) 170

Fire Point (°C) 216

Auto-ignition T° (°C) 399

Pour point (°C) -32

Boiling point (°C) at 1013 mbar 359

Molecular Weight (g/mol) 252

2.2. Experimental set up

The experiments were performed in specific equipment installed in laboratory for ageing tests (Fig. 1). It can be described as a controlled stainless steel recipient connected on the top with a purge inert gas (Argon) inlet/outlet to keep oxygen out (1,2 bar). Two recipients were loaded with 150 mL of tested oil (Therminol 66) and other two recipients were filled with pebbles of quartzite, silica sand and 120 mL of oil (Therminol 66).

__- manometer

., inlet valve

Release valve Cooling system

thermocouples

cooling manometer Glass wool

recipient

Fig.1. Recipient Mounting

All ageing cells were fully sealed before inert gas sparging. Afterwards, the cells were heated using a programmable temperature controller in temperature ranging from 250 to 350°Cfor a total of 500 hours. The temperature rise was carried out by an offset of about 50°C until reaching the desired temperature (250/350°C).

2.3. Characterization methods

Resulting aged oils were characterized by a number of analytical techniques in order to evaluate their sustainability after long term use in contact with rocks.

The Gas chromatography-mass spectrometry (GC-MS) analyses were performed on a HP-5 MS in order to separate molecules. Helium was used as carrier gas at a flow rate of 0.8 mL/min. Chemical characterization by Fourier transformed infrared spectroscopy from 500 to 4000 cm-1 was carried out using ABB Bomem Spectac FT-IR coupled to Attenuated total reflectance (ATR). Ultra violet/visible (UV/vis) spectroscopy was made in quartz cells. The wavelength range investigated from 200 nm to 1000nm. Oil viscosity was measured at 40°C using a Rheometer MCR 500. Other physical parameters were determined using different techniques.

All experiments were conducted at least three times in order to ensure the reproducibility of the measurements, the average value being recorded.

3. Result analysis and discussion

3.1. Samples appearance

In Figs. 2-3 and Table.2 it can be seen that the color of rocks and the transparency of the synthetic oil brought into contact with quartzite and silica sand drastically changes with the ageing time at both temperatures (250°C and 350°C).

■fiVA y f K - "

Fig.2. Samples oils aged at 250°C

(a) T66 initial state; (b) T66 aged at 250°C /500h; (c) T66+Qzt+sand at 250°C/500h

Fig.3. Samples oils aged at 350°C

(a) T66 initial state; (b) T66 aged at 350°C /500h; (c) T66+Qzt+sand at 350°C/500h

Table 2. Samples rocks before and after ageing

The color change can inquire about different phenomenon such as oxidation. For this purpose, many physicochemical analyses were made in order to assess the oils stability in contact with rocks and sand.

3.2. Chemical composition analysis

Representative Infrared spectra of tested oils are shown in Fig. 4. Under the current ageing conditions, Therminol 66 does not show any detectable oxidation. The hydroxyl and the carbonyl bands are absent in the analyses. The curves show only the oils structure in both oils (oil aged without rocks and oil aged in the presence of rocks).

.<2 S0

4000 3500 3000 2500 2000 1500 Frenquency (cm-1)

Fig. 4.Therminol 66 Infrared spectra

Fig 5 presents the chromatograms obtained from the GC-MS analysis. The result has shown that the chromatograms of tested oils are superimposed on each other. Peaks are related to the Therminol 66 composition which is comprised of blend of three molecules, namely: 230, 236 and 242g/mol. Thus, the synthetic oils chemical

However, the color change is observed in the case of both oils (oils aged in presence of rocks at 250°C and 350°C). Therefore, a UV/Vis spectroscopy was made to observe any change in the absorption. Fig. 6 shows the result obtained. Ageing at high temperature causes the ageing to be accelerated; as a result, the oils clouded more quickly and the absorption edge for any given oil is shifted further to the right.

wavelength (nm)

Fig. 6.Therminol 66 UV/Vis spectra

3.3. Physical measurements

Table 3 shows the behavior of the aged oils which is somewhat similar to their initial state at a temperature of about 250°C. Acid number has revealed that no oxidation has occurred during the ageing process which confirms the result obtained by the FTIR analysis. Also, this can be seen in values of viscosity which tend to increase after oxidation phenomenon. Indeed, the viscosity tends to decrease as the temperature rise, as observed in the case of the oil heated to 350°C. When the molecules are subjected to a temperature rise, they accumulate a kinetic energy allowing movements (phenomenon of thermal expansion) which cause a volume increase and a decrease in the density, the viscosity and the flash point [14]. It is worth to note that the measurement of the acid number is the most representative to assess the ageing condition of the oil. Indeed, the acidity of operating oil results from acid products such as carboxylic acids. The acid-base titration demonstrated that the tested oils have not undergone oxidation during the ageing process. Also, it can be seen that oils tested in the presence of rocks and sand behave the same as oils heated at high temperature without materials.

Table 3. Physical Measurements of Tested Oils

Synthetic oils Viscosity at 40 °C (mPa.s) Flash Point (0C) Density (g/cm3) Acid number (mgKOH/g) Water Content (%)

T66 initial state 32 178 1,04 0 0

Aged T66 33 178 1,05 0 0

2500C Aged T66 +Qzt+sand 33 173 1,04 0 0

Aged T66 23 78 1,05 0 0

3500C Aged T66 +Qzt+sand 21 58 1,05 0 0

4. Conclusion

In this study, Therminol 66 samples were thermally aged in the presence of quartzite and silica sand chosen as material storage at temperatures of T = 250°C and T=350°C during 500 hours under argon atmosphere into stainless steel recipients hermetically closed.

In order to assess the oils behavior after ageing process and also to observe any change in the physicochemical properties of studied oil in the presence of rocks and silica sand, several analytical techniques (FTIR, UV/vis, GC-MS...) were performed. It appears from analyses that oils tested in the presence of rocks and silica sand have shown the same behavior as the oils aged without materials, except the optical properties which have demonstrated a change of the absorption depending only on the temperature rise.

In conclusion, this experience has revealed that no significant deterioration of thermal oil has occurred in the presence of the quartzite rock and silica sand after long time of ageing at high temperature.

Acknowledgement

We would like to thank all members of laboratory for their help to use technical methods for characterization. References

[1] Hoel, M , "Depletion of fossil fuels and the impacts of global warming", Resour. Energy Econ 1996., vol. 18, no. 0928, pp. 115-136, .

[2] Narbel, P. A et al, "A Review of Solar Energy Markets, Economics and Policies," A Review of Solar Energy, October, 2011.

[3] IEA, Renewable Energy Technology Roadmaps, Paris, 2010.

[4] Dincer, Rosen, M.A., Thermal Energy Storage, Systems and Applications, Wiley, NewYork,2002.

[5] Harmeet Singh et al, A review on packed bed solar energy storage systems, Renewable and Sustainable Energy Reviews 2010, vol 14, pp. 1059-1069.

[6] Myamya. K. Encapsulation of phase change materials for heat storage, Master's Theses. 2003.

[7] Brosseau D et al,Testing of thermocline filler material sand molten-salt heat transfer fluids for thermal energy storage systems in parabolic trough power plants. J Solar Energy Eng- Trans ASME 2005, 127: 109-16.

[8] Hasnain, S.M., Review on sustainable thermal Energy Storage technologies, Part I: heat storage material sand techniques. Energy Conversion and Management 2011, 39:1127-1138.

[9] Pacheco, J. E., Showalter, S. K. and Kolb, W. J., "Development of a Molten-Salt Thermocline Thermal Storage System for Parabolic Trough Plants", Journal of Solar Energy Engineering 2002, 124: 153-159.

[10] Kearney D, Herrmann U, Nava P, Kelly B, Mahoney R, Pacheco J, et al. Assessment of a molten salt heat transfer fluid in parabolic trough solar field. Trans ASME 2003, 125:170-6.

[11] Blake D. M, New Heat Transfer and Storage Fluids for Parabolic Trough Solar Thermal Electric Plants, National Renewable Energy Laboratory, 11th Solarpaces, September 4-6, 2002, Zurich, Switzerland.

[12] Valkenburg M. E. V., Vaughn R. L., Williams. M., and Wilkes J. S., "Thermochemistry of ionic liquid heat-transfer fluids," Thermochim. Acta, 2005, 425: 181-188.

[13] Solutia, http://www.szsolutia.com

[14] Course, liquid state, Chicoutimi University.