High Temperature Heat Transfer Fluid Research Articles

Corrosion Suppression with Redox Control in KCl-MgCl2 Under Flow

Solar power systems must be competitive with conventional energy sources in in terms of cost to make up a significant portion of the electric power grid. Solar power can likely more rapidly grow and make-up a larger portion of the US power grid if systems have energy storage capability that could continue to provide power for extended periods without sunlight and vary the power as needed (i.e. – be dispatchable). Concentrated solar power tower systems with thermal energy storage using advanced high operating temperature heat transfer fluids and advanced thermodynamic cycles can potentially meet the twin goals of low cost and high reliability while being dispatchable. Molten salts are a promising candidate for a high operating temperature heat transfer fluids. Due to the decomposition of molten nitrates at temperatures above about 550 °C, molten fluorides or chlorides are expected to be needed for higher temperature operation. KCl-MgCl2 is a promising heat transfer fluid due to its low cost and good thermal transport properties. Corrosion in molten chloride salts can be severe if salt chemistry is not well controlled. Research has shown that the main corrosion mechanism in Fe-Ni-Cr alloys exposed to molten salts is selective oxidation of Cr and that the corrosion mechanism is mass transfer limited. In the research to be presented, thermosiphons were operated at three different temperature ranges to add flow to corrosion tests and evaluate changed corrosion rates. Four alloys were tested in the thermosiphons with varying temperature and with and without Mg redox control. The most extreme case tested had the hot region at ~930 °C and cold region at ~880C. The research found that naturally convective flow could increase up to 5 times the corrosion compared to static tests for Haynes-230 without corrosion inhibition. Corrosion inhibition using Mg worked well to decrease corrosion and mass transfer reactions for Fe, Co, and Ni based superalloys.

Electrochemical Study of Corrosion in High Temperature Molten Salts

Concentrating solar power systems are projected to need to produce energy with a cost of $0.06/kWh when implemented on a very large scale. It is generally believed that temperatures of the heat transfer fluids will need to be higher than 800oC to drive an advanced Brayton cycle or Rankine cycle with superheated steam to reach this target. However, current state-of knowledge of corrosion science indicates a lack of published data on the impact of molten salts at temperatures above 800°C on corrosion of molten salt contacting materials such as steels or high temperature alloys and ceramics. This work investigates the corrosion mechanism of selected alloys and ceramics in chloride and fluoride molten salts used for high operating temperature heat transfer fluids using a combination of electrochemical techniques, theoretical calculations, and thermodynamic predictions. The electrochemical characterizations included linear polarization resistance scans, electrochemical impedance spectroscopy, and cyclic potentiodynamic polarization. Results show that the Fe-Ni-Cr alloys tested have high general corrosion and require corrosion inhibition. Advanced materials, such as SiC-SiC and TZM, have been shown to have much lower corrosion and good passivation behavior. Experimental tests using coupon immersion in the molten salts support the mechanism determined from electrochemical testing that the main element being oxidized is Cr. Analysis of samples also indicates selective Cr oxidation concentrates along grain boundaries and creates porous networks in the sample. The electrochemical impedance tests suggest that the corrosion is diffusion controlled and this assumption correlates well with corrosion modeling studies. The electrochemical testing data combined with thermodynamic modeling led to the development of a reaction mechanism for high temperature corrosion in molten salts.

Interplay between structure and transport properties of molten salt mixtures of ZnCl2-NaCl-KCl: A molecular dynamics study.

Molten mixtures of network-forming covalently bonded ZnCl2 and network-modifying ionically bonded NaCl and KCl salts are investigated as high-temperature heat transfer fluids for concentrating solar power plants. Specifically, using molecular dynamics simulations, the interplay between the extent of the network structure, composition, and the transport properties (viscosity, thermal conductivity, and diffusion) of ZnCl2-NaCl-KCl molten salts is characterized. The Stokes-Einstein/Eyring relationship is found to break down in these network-forming liquids at high concentrations of ZnCl2 (>63 mol. %), while the Eyring relationship is seen with increasing KCl concentration. Further, the network modification due to the addition of K ions leads to formation of non-bridging terminal Cl ions, which in turn lead to a positive temperature dependence of thermal conductivity in these melts. This new understanding of transport in these ternary liquids enables the identification of appropriate concentrations of the network formers and network modifiers to design heat transfer fluids with desired transport properties for concentrating solar power plants.

A Parametric Investigation of a Concentrating Photovoltaic/Thermal System With Spectral Filtering Utilizing a Two-Dimensional Heat Transfer Model

A 2D heat transfer model of a hybrid photovoltaic/thermal (PV/T) system has been created. This paper investigates the impact of ideal filters to best accommodate for a nonuniform PV temperature along the length of the receiver. The proposed configuration consists of a GaAs cell laminated to an aluminum extrusion. The working fluid, a transparent high-temperature heat transfer fluid with suspended nanoparticles, flows through the hollow extrusion where it cools the PV cell before it is redirected in front of the cell acting as an optical filter. The model accounts for PV cell efficiency, temperature, and bandgap dependence, the details often neglected in prior works.

A new solar fuels reactor concept based on a liquid metal heat transfer fluid: Reactor design and efficiency estimation

A new reactor concept for two-step partial redox cycles is presented and evaluated by transient simulation that considers heat and mass transfer along with reaction kinetics. The major difference between the reactor described herein and previous designs is that the conversion from solar to chemical energy is divided into two steps: sunlight-to-thermal energy conversion accomplished with a liquid metal based receiver, and the thermal-to-chemical conversion accomplished with a separately optimized array of reaction chambers. To connect these two conversion steps, liquid metal is used as a high temperature heat transfer fluid that feeds the solar energy captured in the receiver to the reactor. The liquid metal also facilitates efficient heat recuperation (∼80%) between the reaction chambers. The overall thermal-to-chemical efficiency from the thermal energy in the liquid metal to the chemical energy in the hydrogen fuel is estimated to be 19.8% when ceria is employed as the reactive oxygen storage material. This estimated efficiency is an order of magnitude higher than previous designs and the reactor concept discussed herein identifies important insights that apply to solar–fuel conversion in general.

Numerical investigations of high temperature packed bed TES systems used in hybrid solar tower power plants

Packed bed thermal energy storage systems with air as high-temperature heat transfer fluid are a cost effective technology for air-based hybrid solar tower plants. A scheme of this plant has been proposed and an optimal mathematical heat transfer model has been formulated to investigate the cyclic thermal behavior of two packed bed TES systems within this plant. The first TES system stores energy at high-temperature (around 1000°C) and at high pressure (around 10bars), while the second one stores energy at 650°C and under atmospheric pressure. The developed model in this study is a dynamic 1-dimensional two-phase heat transfer model, which incorporates variable thermophysical properties for both fluid and solid phases, and hence, enables an accurate prediction of the TES system dynamic. A numerical method for solving the model equations is described in this effort, as well as the required physical correlations. Based on this model a simulation tool was developed in the environment of MATLAB R2011b, and was validated with previous experimental and simulation studies. This calculation tool is fast compared to previous models; thus, it enables the prediction of the cyclic performance of packed bed TES technology at high temperature and pressure working ranges in a relatively short simulation time.

Corrosion of Hastelloy in Molten Salt Used As High Temperature Heat Transfer Fluids

Introduction When one mole of tetrahedral zinc dichloride (ZnCl2) and cubic potassium chloride or sodium chloride, a thermally stable complex forms. The corrosion of metal in molten salt, its physical stability and heat transfer properties are needed to determine if metal can house these salts used as heat transfer fluid (HTF) for concentrating solar power (CSP) to generate electricity. Preliminary results show that some ternary mixtures, like NaCl-KCl-ZnCl2 in molar fractions of 13.4%-33.7%-52.9% melt near 200oC and have very low vapor pressures (~ 7psig) at 800oC and has satisfactory thermal and transport properties for collecting solar energy up to 800oC [1]. High corrosion rates of metals in this molten salt are found for aerated salt, but metal corrosion rates are low when the salt is anhydrous and deoxygenated. Electrochemical testing with atmosphere control in cell A working and a counter electrode of C-276 Hastelloy and a silver/silver chloride reference electrode (0.2 V vs NHE) are housed in a quartz electrochemical cell isothermally heated in a furnace. The C-276 Hastelloy is polished with 600 grit SiC paper, rinsed with DI water and acetone. Linear polarization [2] was used to electrochemically characterize the rate of metal oxidation by scanning the working electrode from -30 mV vs. OCP to +30 mV vs. OCP at a scan rate of 0.2 mV/s. The atmosphere of salt in the cell is controlled by heating to 300°C to melt the salt; sparging salt for about 30min with Argon flowing at 150 sccm. The metal sample is inserted into salt and gas passed over salt during the polarization study. Gas is bubbled into the salt when heating to a new temperature, then over salt during polarization at the new temperature. For Fig. 1, the Hastelloy C276 metal area is 5.15cm2 in 150 grams of KCl-NaCl-ZnCl2 salt. Figure 1 Table 1 Gravimetric determination of metal weight loss in anaerobic molten salt Acell with two caps is used to gravimetrically determine corrosion rates. One cap has a long tube to bubble gas into the salt; the other has a short tube to keep gas over the salt. This decreases the salt lost while flowing Argon to maintain the anaerobic condition. For example, the initial weight of metal in one experiment was 10.5830 gm (58 cm2); the mass of salt 300 gm; the salt temperature 800oC, and immersion time 50 hours (2.08 days). The salt was heated at 250°C for 1 hour, then 30 minutes at 350°C without flowing Argon; heated at 500°C for 1h with Argon flowing at 175 sccm without metal in the salt toremove water and oxygen. Argon gas is admitted from a side tube at about 450 sccm to prevent air entering cell as the metal coupon is added and the cap switched from the long tube to short tube. The Argon was kept flowing through the cell cap at 175 sccm and the side tube at 450 sccm. The temperature was 800°C for 50 hours. The metal sample was removed from the cell at 300°C by pouring all contents into a quartz beaker. The metal was sonicated in DI water, then in dilute HCl, then in DI water, 15min each at RT. The sample was re-weighed. The corrosion rate, CR, was 53.34 mpy. This test was repeated with a metal coupon, 5.205 cm2 in area with all other conditions constant. The corrosion rate was 30.29 mpy at 800oC. Conclusion The anaerobic immersion experiments give low corrosion rates because water and oxygen are removed from the salt. The comparison between the small and large coupons indicates sample size does not limit corrosion. The CR from the gravimetric method is consistent with that from the electrochemical method. The C276 metal in the selected salt appears intrinsically stable at elevated temperatures in absence water and oxygen. Acknowledgement The authors gratefully acknowledge the US Dept of Energy for support of this work under Sunshot MURI Program grant number DE-EE0005942. DOE Technical Manager, Levi Irwin. References Halide and Oxy-halide Eutectic Systems for High Performance High Temperature Heat Transfer Fluids, DoE MURI DE-EE0005942, Principal Investigator, Peiwen Li (2014).M. STERN AND A. L. GEARY, “Electrochemical Polarization, I. A Theoretical Analysis of the Shape of Polarization Curves”, JOURNAL OF THE ELECTROCHEMICAL SOCIETY Vol. 104, pp. 56 -63 (1957). Figure 1

Multidimensional Modeling of Nickel Alloy Corrosion inside High Temperature Molten Salt Systems

Concentrated solar power (CSP) systems in the presence of high operating temperature heat transfer fluids for power production processes has attracted more attention in recent years. The challenge with these systems is the corrosion of alloys at high-temperature (700-1000°C) in receivers and heat exchangers causing a reduction in heat transfer efficiency and influencing the apparent durability [1-3]. In this work, a corrosion model that includes thermal gradients and fluid flow is developed to predict corrosion rates and mechanisms observed in a nickel base alloy in contact with a molten salt heat transfer system. The model takes into account the electrochemical kinetics in addition to mass transfer-limited corrosion coupled with a computational fluid dynamic (CFD) model that can predict the local electrochemical environment and corrosion rates in a system with temperature and fluid flows. The kinetic and mass transfer parameters used in the model are based on the experimental coupon studies were conducted between 700-1000 °C within molten salt in a thermosiphon reactor that designed to allow exposure the coupons to the non-isothermal condition. The thermal gradients between top and bottom of the reactor could induce natural convection of the salts [4-5]. Fig. 1 shows an example of the model prediction of corrosion rate distribution, i (A/cm2) at coupon surfaces at both hot and cold zones in addition to the temperature gradient near the wall and streamlines that show the flow pattern of molten salt inside the thermosiphon. the This corrosion model accounts for the effects of different operational conditions such as temperature and component concentrations in addition to the alloy surface properties on the corrosion rate. Acknowledgements The authors gratefully acknowledge the financial support for this work from the DOE EERE SunShot Initiative (DE-AC36-08GO28308) under a subcontract from SRNL to the University of Alabama and to the University of South Carolina. The authors also thank the University of South Carolina Center for Fuel Cells and the CD-adapco group for their support. R eferences C. Forsberg, P.F. Peterson, and H.H. Zhao, J. Sol. Energy Eng. Trans.-ASME, 129, 141-146, 2007.C. Forsberg, Progress in Nuclear Energy, 47, 32-43, 2005.D. Ludwig, L. Olson, K. Sridahran, M. Anderson, and T. Allen, Corrosion Engineering Science and Technology, 46, 360-364, 2011.B. L. Garcia-Diaz, L. Olson, M. Martinez-Rodriguez, R. Fuentens, J. Gray. paper #741, 2014 ECS and SMEQ Joint International Meeting, Cancun, Mexico, Oct. 08, 2014.R. Fuentens, L. Olson, M. Martinez-Rodriguez, J. Gray, B. L. Garcia-Diaz. paper #743, 2014 ECS and SMEQ Joint International Meeting, Cancun, Mexico, Oct. 08, 2014. Figure 1. Corrosion rate distribution, i (A/cm2) at coupon surfaces at both hot and cold zones in addition to the temperature gradient near the wall and streamlines inside the thermosiphon. Figure 1

Cooling Characteristics of Water Based Nanofluids With Agitation

Abstract A series of quenching experiments was performed according to the ISO 9950, ASTM 6200-01, and ASTM D6482-06 standards. Water based nanofluids with 0.01, 0.1, and 0.2 g/L TiO2 were prepared using a two-step method and ultrasonic homogenization. The nanofluids were tested to see the possibility of their application as high temperature heat transfer fluids. Nanoparticles used in this series of experiments were 50 nm in diameter. All of the cooling curves were recorded and analyzed using the ivf SmartQuench system. The first set of experiments was conducted under still conditions according to the ISO 9950 standard. Pure water and nanofluids were investigated. The addition of nanoparticles showed an effect on the full film phase duration, an increase in the maximum cooling rate, and a change in transition temperature. A second series of quenching experiments was conducted according to ASTM D 6482-06. For this purpose, a quenching bath with agitation was built at the QRC. Three levels of agitation were tested: one according to the standard, 1000 rpm, one below, and one above the specified speed. Experiments with pure water and all three nanofluids were conducted and cooling curves recorded. The cooling curves were analyzed using software ivf SQintegra ver4.0. The effects of agitation and nanoparticles contents on cooling characteristics were compared.

Investigation of heat transfer characteristics and energy balance analysis of FLiNaK in turbulent boundary layers of pipe flow

The transport of energy in turbulent boundary layers is characterised by two complimentary transport mechanisms, vis-a-vis conduction and turbulent advection. These processes in the inertial sub-layer of fully developed turbulent flow in a round pipe at two Reynolds numbers are investigated to obtain conductive and advective heat fluxes. CFD simulations are carried out at these Reynolds numbers using Large Eddy Simulations (LES) to generate high quality transient velocity and temperature data. A fluoride salt with a eutectic composition of 46.5%LiF–11.5%NaF–42%KF (mol %) commonly known as FLiNaK (Prandtl number = 3), a potential candidate for high temperature heat transfer fluid is used as a fluid for simulations. Simulation results have been validated for the mean velocity profile using Direct Numerical Simulation data (DNS). Transient velocity information is used to identify and characterise turbulent structures using a wavelet transform based methodology called Wavelet Transform Modulus Maxima (WTMM) for obtaining the advective heat flux. Conductive flux is calculated using temperature gradients. Energy balance analysis with respect to input heat flux, was performed using conductive and advective heat fluxes. WTMM analysis is also used for visualising near wall flow structures, and to further find out the heat transfer coefficients. The predicted Nusselt numbers have shown good agreement with the available experimental data.

Electrochemical Study of Corrosion in High Temperature Molten Salts

Concentrating solar power systems are projected to need to produce energy with a cost of $0.06 / kWh when implemented on a very large scale. It is generally believed that temperatures of the heat transfer fluids will need to be higher than 800oC to drive an advanced Brayton cycle or Rankine cycle with superheated steam to reach this target. However, current state-of knowledge of corrosion science indicates a lack of published data on the impact of molten salts at temperatures above 800°C on corrosion of molten salt contacting materials such as steels or high temperature alloys and ceramics. This work investigates the corrosion mechanism of selected alloys and ceramics in chloride and fluoride molten salts used for high operating temperature heat transfer fluids using a combination of electrochemical techniques, theoretical calculations, and thermodynamic predictions. The electrochemical characterizations included linear polarization resistance scans, electrochemical impedance spectroscopy, and cyclic potentiodynamic polarization. Results show that the Fe-Ni-Cr alloys tested have high general corrosion and require corrosion inhibition. Advanced materials, such as SiC-SiC and TZM, have been shown to have much lower corrosion and good passivation behavior. Experimental tests using coupon immersion in the molten salts support the mechanism determined from electrochemical testing that the main element being oxidized is Cr. Analysis of samples also indicates selective Cr oxidation creates porous layers in the sample. The electrochemical impedance tests suggest that the corrosion is diffusion controlled.

Use of metallic nanoparticles to improve the thermophysical properties of organic heat transfer fluids used in concentrated solar power

One of the approaches to enhance the efficiency, and consequently, reduce costs to produce electricity from concentrated solar power (CSP) is by the development of advanced high temperature heat transfer fluids (HTFs). Incorporation of metallic nanoparticles into conventional heat transfer fluids could significantly improve the thermal transport properties of the HTFs. This study reports on the synthesis and investigation of copper nanoparticles synthesized in-house and dispersed in two synthetic HTFs Therminol 59 (TH59) and Therminol 66 (TH66). Liquid phase reduction of a copper salt was used to produce copper nanoparticles. Suspensions with various copper nanoparticle loadings (0.5–2vol.%) were prepared. Characterizations such as the thermal conductivity, dynamic viscosity, mass specific heat capacity, and fluid stability were performed on the suspensions. Thermal conductivity enhancements over the base fluids were as high as approximately 20% at a 2vol.% particle loading. These enhancements in the thermal conductivity are higher than the predictions based on the Effective Medium Theory (EMT). Dynamic viscosity measurements showed that if good dispersion of nanoparticles is achieved, the composite fluids behave in a Newtonian manner and the dynamic viscosity increases over the base fluid are minor at temperatures 125°C and above. Stability of the suspensions with time was also investigated. Based on the measured properties of the suspensions, a figure of merit for heat transfer was calculated to evaluate the viability of the suspensions.

Survey of Properties of Key Single and Mixture Halide Salts for Potential Application as High Temperature Heat Transfer Fluids for Concentrated Solar Thermal Power Systems

In order to obtain high energy efficiency in a concentrated solar thermal power plant, more and more high concentration ratio to solar radiation are applied to collect high temperature thermal energy in modern solar power technologies. This incurs the need of a heat transfer fluid being able to work at more and more high temperatures to carry the heat from solar concentrators to a power plant. To develop the third generation heat transfer fluids targeting at a high working temperature at least 800 ℃, a research team from University of Arizona, Georgia Institute of Technology, and Arizona State University proposed to use eutectic halide salts mixtures in order to obtain the desired properties of low melting point, low vapor pressure, great stability at temperatures at least 800 ℃, low corrosion, and favorable thermal and transport properties. In this paper, a survey of the available thermal and transport properties of single and eutectic mixture of several key halide salts is conducted, providing information of great significance to researchers for heat transfer fluid development.

Liquid metal technology for concentrated solar power systems: Contributions by the German research program

Concentrated solar power (CSP) systems can play a major role as a renewable energy source with the inherent possibility of including a thermal energy storage subsystem for improving the plant dispatchability. Next-generation CSP systems have to provide an increased overall efficiency at reduced specific costs and they will require higher operating temperatures and larger heat flux densities. In that context, liquid metals are proposed as advanced high temperature heat transfer fluids, particularly for central receiver systems. Their main advantages are chemical stability at temperatures up to 900 ℃ and even beyond, as well as largely improved heat transfer when compared to conventional fluids like oil or salt mixtures, primarily due to their superior thermal conductivity. However, major issues here are the corrosion protection of structural materials and the development of technology components and control systems, as well as the development of indirect storage solutions, to circumvent the relatively small heat capacity of liquid metals. On the other hand, using liquid metals might enable alternative technologies like direct thermal-electric conversion or use of solar high-tem­perature heat in chemical processes. This article aims at describing research areas and research needs to be addressed for fully evaluating and subsequently utilizing the potential of liquid metals in CSP systems. A second aim of the article is a brief overview of the liquid metal research capabilities of Karlsruhe Institute of Technology (KIT), their background and their relation to CSP and the aforementioned research pathways.

Survey of Properties of Key Single and Mixture Halide Salts for Potential Application as High Temperature Heat Transfer Fluids for Concentrated Solar Thermal Power Systems

In order to obtain high energy efficiency in a concentrated solar thermal power plant, more and more high concentration ratio to solar radiation are applied to collect high temperature thermal energy in modern solar power technologies. This incurs the need of a heat transfer fluid being able to work at more and more high temperatures to carry the heat from solar concentrators to a power plant. To develop the third generation heat transfer fluids targeting at a high working temperature at least 800 ℃, a research team from University of Arizona, Georgia Institute of Technology, and Arizona State University proposed to use eutectic halide salts mixtures in order to obtain the desired properties of low melting point, low vapor pressure, great stability at temperatures at least 800 ℃, low corrosion, and favorable thermal and transport properties. In this paper, a survey of the available thermal and transport properties of single and eutectic mixture of several key halide salts is conducted, providing information of great significance to researchers for heat transfer fluid development.

Nanofluids with encapsulated tin nanoparticles for advanced heat transfer and thermal energy storage

SUMMARY Novel high-temperature heat transfer fluids (HTFs) with incorporated phase change nanomaterials were synthesized and tested for heat transfer and thermal energy storage. The advanced thermal properties were achieved by preparing a nanofluid consisting of core/shell silica encapsulated tin (Sn/SiO2) nanoparticles dispersed in a synthetic HTF Therminol 66 (TH66) at loadings up to 5 vol%. Tin nanoparticles were synthesized by modified polyole reduction method followed by sol–gel silica encapsulation process. The measured increase in thermal conductivity of the nanofluid (~13% at 5 vol%) was in agreement with Maxwell's effective medium theory. Latent heat of phase change during melting of Sn core added ~11% increase to the volumetric thermal energy storage of the nanofluid when cycled in between 100°C and 270°C. The value could be further improved if thermal cycling is conducted in a narrower temperature range. The experimental results demonstrated dual functionality of the engineered nanofluids as desired for Concentrated Solar Power systems. Viscosity and stability of the nanofluids as well as thermal stability of core/shell nanomaterials) were investigated in a wide temperature range to obtain a perspective on any additional pumping power requirements for the nanofluid over the base fluid. Copyright © 2013 John Wiley & Sons, Ltd.

Packed-bed thermal storage for concentrated solar power – Pilot-scale demonstration and industrial-scale design

A thermal energy storage system, consisting of a packed bed of rocks as storing material and air as high-temperature heat transfer fluid, is analyzed for concentrated solar power (CSP) applications. A 6.5MWhth pilot-scale thermal storage unit immersed in the ground and of truncated conical shape is fabricated and experimentally demonstrated to generate thermoclines. A dynamic numerical heat transfer model is formulated for separate fluid and solid phases and variable thermo-physical properties in the range of 20–650°C, and validated with experimental results. The validated model is further applied to design and simulate an array of two industrial-scale thermal storage units, each of 7.2GWhth capacity, for a 26MWel round-the-clock concentrated solar power plant during multiple 8h-charging/16h-discharging cycles, yielding 95% overall thermal efficiency.

Thermal and Transport Properties of Six Ionic Liquids: An Experimental and Molecular Dynamics Study

Experimental measurements and molecular dynamics simulations are used to determine the density, heat capacity, self-diffusivity, shear viscosity, and thermal conductivity of six ionic liquids over a range of temperatures. The ionic liquids examined are 1-butyl-3-methylimidazolium bis[(perfluoroethyl)sulfonyl]imide ([bmim][Pf2N]), 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([bmim][Tf2N]), 1-butyl-2,3-dimethylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([bmmim][Tf2N]), 1-butyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide ([bmpyr][Tf2N]), N-butyl-N,N,N-trimethylammonium bis[(trifluoromethyl)sulfonyl]imide ([N4111][Tf2N]), and N,N,N-trimethylammonium-N-butanoic acid bis[(trifluoromethyl)sulfonyl]imide ([N4111][COOHTfN]). The results of this work suggest that several of these ionic liquids have properties that would enable them to be successful high temperature heat transfer fluids. In particular, their energy storage densities and thermal conductivities are quite favorable when compared to conventional heat transfer fluids. The low temperature viscosities of the ILs are significantly higher than conventional fluids, but the viscosities drop rapidly with increasing temperature. The simulations, which are purely predictive, agree quantitatively with the experimental data for density and qualitatively for other properties. It is shown that the simulated thermal conductivity can be adequately correlated with density and molecular weight of the [Tf2N]-based ionic liquids.

Large scale magnesium hydride tank coupled with an external heat source

The experimental study of a large scale magnesium hydride tank (10 kg) is presented. In order to enhance thermal exchanges and improve storage time, the MgH2 powder has been compacted with 10 wt.% of Expanded Natural Graphite. An efficient heat exchanger has been designed to transfer the heat from the endo/exothermic sorption reactions to an external heat source/sink through a high temperature heat transfer fluid. An improvement of the loading and discharging time along the first hydrogenation cycles is observed owing to material thermal conductivity modifications. The properties of the cycled material have been studied in order to understand this evolution of the tank behavior.

Concentrated heat storage for solar heating

The Independent Solar System (ISS) project was initiated in 2008 in order to develop a solar thermal heating system capable of covering the full-year heat and hot water load of a single family house using high temperature thermal storage. A team of European solar thermal experts, research institutions and industrial partners developed the system by combining innovative technologies (high temperature heat transfer fluid, super-insulated storage tanks and a specialized control system) with standard off-the-shelf components. The aim was to create an efficient, affordable and reliable solar thermal heating and hot water system operating at high temperatures for small and medium sized buildings. In 2009, the first prototype system was produced and installed on a pilot building. Over the 2009-2010 heating period actual performance of the prototype system was monitored using web-based applications and then evaluated against system performance simulations. The data shows promising potential for this renewable energy solution for homes in south and central Europe.