Alternative Heat Transfer Fluids Research Articles

Thermal stress in externally irradiated tubes of solar central receivers with a gas–particle fluidized dense suspension

One of the objectives of the new generation of concentrating solar power plants is to increase the actual temperature limit (565 °C) up to or near 1000 °C. An alternative heat transfer fluid that could help to reach this objective is the use of a fluidized dense gas–particle suspension ascending through a vertical tube. Since the predominant cause of rupture in solar receivers is the creep-fatigue damage process, which is mostly influenced by the maximum temperatures and stress experienced along the tube, the main objective of this work is to numerically study the thermomechanical behavior of a non-uniform externally irradiated tube where particles are fluidized and moves upwards. The heat transfer problem in the two media present (i.e. dense suspension of particles and tube wall) was solved separately: a Computational Particles Fluid Dynamic model solves the heat transfer in the particles, whereas a three dimensional Finite Volume model simulates the heat conduction through the tube wall to obtain the temperature profile, which serves as input to calculate the thermal stress of the tube with an analytical method. Higher thermal stresses were obtained for an absorbed power of 250 kW/m2 (σVM,max=219MPa) compared to that for an absorbed power of 500 kW/m2 (σVM,max=72MPa) due to the lower temperature difference between the front and rear sides of the tube caused by the more pronounced increase of the radiative losses in the front side of the tube compared to that at the rear side. The thermomechanical behavior of a particle receiver was compared to that of a molten salt receiver. For an incident solar flux of 500 kW/m2, the particle receiver reduced the maximum thermal stress by 73% compared to the molten salt receiver. However, the tube’s maximum temperature exceeded the working limit for stainless steel tubes. In order for the receiver to survive heat fluxes characteristic of solar power plants, research into technological solutions that allow to improve the effectiveness of the heat transfer rate from the tube to the particle flow is necessary.

Design of the LIMELIGHT Test Rig for Component Testing for High-Temperature Thermal Energy Storage with Liquid Metals

Thermal energy storage systems for high temperatures >600 °C are currently mainly based on solid storage materials that are thermally charged and discharged by a gaseous heat transfer fluid. Usually, these systems benefit from low storage material costs but suffer from moderate heat transfer rates from the gas to the storage medium. Therefore, at the Karlsruhe Liquid Metal Laboratory, liquid metals are investigated as alternative heat transfer fluids for such heat storage systems, making use of the broad temperature range, in which the metals are in a liquid state, and their efficient heat transport capabilities. In this work, the design and construction of a high-temperature test rig using liquid lead is presented. The goal of the experiments is to demonstrate the operability of a pump, valves and measurement equipment at 700 °C in a challenging corrosive environment. Based on material pre-tests in stagnant lead at 700 °C, which are also shown in this study, aluminizing and pre-oxidation of the pipes and components are applied for enhanced corrosion protection.

Magnetohydrodynamics Ag-Fe3O4-Ethylene Glycol Hybrid Nanofluid Flow and Heat Transfer with Thermal Radiation

The potential of hybrid nanofluid as an alternative heat transfer fluid is undoubted and the insightful research on enhancing its thermal conductivity is crucial. This study accentuates the influence of magnetic field and thermal radiation on the ethylene glycol base hybrid nanofluid with a combination of argentum and magnetite nanoparticles. The mathematical equations of the hybrid nanofluid model are derived with the suitable similarity transformations and then solved numerically with the execution of bvp4c codes in Matlab software. Graphical results show that an upsurge in magnetic parameter reduces the momentum boundary layer thickness while the higher thermal radiation enlarges the thermal boundary layer thickness. The effects of suction and nanoparticles concentration are also presented graphically. Stability analysis reveals that the first solution obtained in this study is stable, and conversely, the second solution is not.

Characterization and testing of solid particles to be used in CSP plants: Aging and fluidization tests

The main limitation of Concentrated Solar Power (CSP) is its maximum operating temperature, since the main fluid used as heat transfer fluid (HTF) and thermal storage media (TES) is molten salts, which degrade around 565 °C. Therefore, there is great interest in developing HTFs that allow the maximum working temperature to be increased to more than 565 °C. This increase in the maximum operating temperature of the HTFs will, in turn, allow CSP plant efficiency to be enhanced. Different studies have proposed using particles as a medium to capture and store solar energy, but very little is known about the effect on the particles after resisting several cycles of cooling and heating and what happens to the particles’ physical properties when they are fluidized. It is true that a variety of solid particles permit high performance to be achieved at high temperatures and with low material costs, but not all particles are valid for this purpose. This work focuses on different materials considered promising candidates for use as alternative HTF and storage materials in CSP plants, which were characterized and tested: sand, carbo Accucast ID50 and silicon carbide. The properties of the particles before and after being fluidized (assessing their resistance to abrasion) and before and after high temperature (900 °C) thermal treatments (aging treatment) are the main finding of this paper. This work highlights the considerable advantages of SiC and carbo over sand. This study also determines that the combination of high temperature and several hours of fluidization does not affect the diameter of the particles: However, the fluidized samples vary considerably in diameter. In this way, SEM and DSC techniques highlight the problem of the fine appearance associated with sand during fluidization.

A Simple Approach for Heat Transfer Enhancement of Carbon Nanofluids in Aqueous Media.

Nanofluids are considered alternative heat transfer fluids because of their excellent thermal and electronic conductivities. Recently, carbon nanomaterials such as carbon nanotubes and graphene have been considered to fabricate enhanced heat transfer nanofluids, but using them to prepare stable nanofluids remains challenging because of their hydrophobicity. Herein, a stable aqueous graphene and carbon nanotube dispersion was prepared using nanostructured cellulose without any additional chemicals. The dispersibility of graphene in cellulose was compared with that in conventional surfactants such as sodium dodecyl benzene sulfonate, sodium dodecyl sulfate, and lauryl betaine. In addition, the optimal mass ratio for the carbon material to cellulose was determined and improvement in the thermal and electrical conductivity of the nanofluid was investigated. The dispersion ability of cellulose was more significant than that of surfactants, and it played a major role in improving the thermal and electrical conductivity. The highest thermal conductivity obtained for the graphene-cellulose nanofluid was 615.23 W/m*K for a mass ratio of 2:1 at 20 °C. The electrical conductivity of the nanofluids increased remarkably with an increase in the cellulose content. Furthermore, the obtained nanofluid improved the heat transfer performance dramatically. It can be assumed that our proposed system can be used to ensure numerous economic and environmental benefits in the domain of heat transfer fluids.

Simplified heat transfer model for parabolic trough solar collectors using supercritical CO2

The main Concentrated Solar Power (CSP) plants type installed worldwide corresponds to parabolic trough (PT) technology transforming solar energy into thermal energy on the receiver tubes. The use of alternative Heat Transfer Fluids (HTF) in order to increase the solar-to-electric efficiency by means of either higher HTF temperatures or the use of supercritical cycles have opened a new line of research. In this framework, super-critical carbon dioxide (sCO2) seems to be a good candidate to replace current HTFs in PTs. This work implements a one-dimensional heat transfer model for a PT solar collector considering different HTFs such as synthetic oil, sub-critical carbon dioxide, and sCO2. The numerical results of the outlet fluid temperature are compared to experimental data and numerical results published in the literature, obtaining maximum temperature deviations of 0.9%, 2.9% and 2.7% respectively. Finally, a sensitivity analysis was conducted to evaluate the influence of the solar irradiation, mass flow rate and HTF inlet temperature on the thermal performance of a PT working with sCO2 showing that the solar irradiation produces the greatest variations.

Electrical conductivity and pH modelling of magnesium oxide–ethylene glycol nanofluids

Nanofluids as new composite fluids have found their place as one of the attractive research areas. In recent years, research has increased on using nanofluids as alternative heat transfer fluids to improve the efficiency of thermal systems without increasing their size. Therefore, the examination and approval of different novel modelling techniques on nanofluid properties have made progress in this area. Stability of the nanofluids is still an important concern. Research studies on nanofluids have indicated that electrical conductivity and pH are two important properties that have key roles in the stability of the nanofluid. In the present work, three different sizes of magnesium oxide (MgO) nanoparticles of 20, 40 and 100 nm at different volume fractions up to 3% of the base fluid of ethylene glycol (EG) were studied for pH and electrical conductivity modelling. The temperature of the nanofluids was between 20 and \(70^{\circ }\hbox {C}\) for modelling. A genetic algorithm polynomial neural network hybrid system and an adaptive neuro-fuzzy inference system approach have been utilized to predict the pH and the electrical conductivity of MgO–EG nanofluids based on an experimental data set.

Investigation of using molten salt as heat transfer fluid for dry cooled solar parabolic trough power plants under desert conditions

This research aims mainly at further investigate technical options to reduce the cost of electric power production from concentrating solar power plants in hot arid regions. The work also aims to minimize water consumption due to its scarcity in such regions by means of investigating the utilization of dry cooling option. However, dry cooling system increases the parasitic power consumption of the power plant and this is bound to reduce the overall system efficiency. In arid regions, this loss in efficiency is compensated by the abundance of solar radiation that results in a better solar field efficiency. Furthermore, this study investigates the effect of using alternative heat transfer fluid in the solar circuit instead of synthetic oil. For this purpose, an optimization of a solar parabolic trough power plant coupled with dry cooling system has been performed using the simulation software “SAM”. The optimization is carried out by varying the solar field size, the thermal energy storage system, the size of the power block (steam turbine) and the type of the heat transfer fluid. This was done in order to find out the best combination that gives lowest power production cost. The power plant was simulated in two steps with different combinations; while “Therminol VP I” was used in the first step, as heat transfer fluid, in the second step, “Therminol VPI” was replaced by “Molten Salt”. The obtained results prove that plants with large solar fields (solar multiple of 2) and large thermal energy storage systems (7.5 full load hours) perform better and can generate power at lower costs compared with smaller plants. The investigation of alternative heat transfer fluid shows that molten salt is more cost effective than synthetic oil in parabolic trough power plants. The obtained Levelized Cost Of Electricity of the best power plant optimized configuration is 17.24 c€/kWhe and 13.8 c€/kWhe, for plants with “Therminol VP I” and “Molten Salt” as heat transfer fluid, respectively. The optimization results with “Molten Salt” as heat transfer fluid show excellent results regarding the dry cooled solar power plants. This show good technical and economic potentials in future designs.

Jatropha curcas crude oil as heat transfer fluid or thermal energy storage material for concentrating solar power plants

Valorisation of local and low cost eco-materials has become an imperative for the sustainable development of Concentrating Solar Power (CSP) in West Africa. In this study, Jatropha curcas crude oil is studied as alternative heat transfer fluid or thermal energy storage material, particularly as a thermal oil substitute. The thermal stability of Jatropha curcas crude oil was experimentally investigated. The crude oil was aged by through thermal treatments, using galvanised steel and 316L stainless steel reactor under steady-state and dynamic conditions up to 210°C. The change in physico-chemical parameters of Jatropha curcas crude oil, such as viscosity, flash point, acidity number, water content, iodine value, peroxide value and chemical composition was monitored. The results indicate a relative stability of the total acid number during the dynamic and pseudo-static tests both in galvanised steel and in 316L stainless steel reactors. The results also show that the measured viscosity at 40°C remains practically constant after tests in steady-state conditions. This is also the case of the total acidity number. The evolution of iron and zinc contents of the oil shows that the use of 316L stainless steel material highly limits the degradation process of Jatropha curcas crude oil. Therefore, the main benefits of Jatropha curcas crude oil are its sustainable character, wide availability, good energy storage density, low cost and absence of use conflict. The oil can, therefore, be considered a suitable candidate for thermal applications up to 210°C, such as small scale CSP plants.

Techno-economic evaluation of modular hybrid concentrating solar power systems

This paper assesses the influence on techno-economic performance of modularising hybrid Concentrating Solar Power (CSP) systems with fossil fuel backup for both a Hybrid Solar Receiver Combustor (HSRC), which integrates a combustor into a solar cavity receiver, and a Solar Gas Hybrid (SGH) system with a similar cavity receiver and a back-up boiler. It was found that the energy losses in a system of small-sized modules, which employs molten salt as its Heat Transfer Fluid (HTF), are dominated by trace heating owing to the increased piping over their larger receiver counterpart. However, this can be reduced significantly by using alternative HTFs with a lower melting point such as sodium. In addition, for modularisation to be cost effective requires it to also enable access to alternative, lower-cost manufacturing methods. That is, the benefit of standard learning rates is insufficient to lower the Levelized Cost of Electricity (LCOE) on its own. For a plant with 30 units of 1 MWth modules the LCOE is competitive, relative to a single unit of 30 MWth, after ∼10 plants are installed if the modularised components (i.e. heliostats, receivers and towers) can be decreased by >80% and >40% for molten salt and sodium as the HTF, respectively.

Experimental study on thermal performance of optimum PG brine as a radiator coolant

AbstractThe present study deals with the experimental impact of an alternative heat transfer fluids for overall performance improvement for radiators. Water and water mixed with anti‐freezing agents such as ethylene glycol (EG) and propylene glycol (PG) are the traditional coolants for an automotive radiator. Comparison of experimental and numerical analysis of optimum brine solution, that is 25% of propylene glycol and water as coolant for the rectangular fin radiator, has been well discussed. A closed loop test rig was designed, and fabricated with a wind tunnel section to achieve uniform velocity at the test section of the rectangular radiator and was tested for performance. Experimental runs were conducted at varying operating temperatures which included the runs for water, and an optimum propylene glycol brine solutions at 70 °C and 80 °C with various flow rates. Results show the energy performance of an optimum brine solution was nearly similar to that of water at high temperatures. The Nusselt number, heat transfer coefficient, and heat transfer rate for an optimum propylene glycol brine is nearly the same as water at 80 °C with a maximum deviation of 15%, 5.7%, and 6.6%, respectively, for theoretical and experimental result comparisons. Air side and coolant side pressure drops had a maximum deviation of 3.66% and 6.6%, respectively. Air and coolant exit temperatures had a deviation of 5% and 3.5%, respectively, with an air frontal velocity of 4.6 m/s in a rectangular fin radiator for an optimum brine solution used as coolant for the automotive radiator. The optimum propylene glycol brine may be environmentally beneficial.

Rheological Properties of Heat Transfer Liquids for Solar Thermal Systems

This article discusses the rheological properties of heat transfer fluids used in all year round solar systems. Four commercially known media were measured. Two of them were glycols based media and two of them alternative heat transfer fluids based on glycerol. Parameters of viscosity and density were measured in a wide range of temperature. Measured dependencies were mathematically approximated by Vogel-Fulcher-Tammanovou equation that very accurately describes their shape. The temperature coefficient of volumetric expansion β is another parameter of heat-transfer fluids described in this article. β plays a very important role in the construction of the solar system. The last parameter determining the year round operation is the freezing point. Freezing point was measured by the cryoscopic methods.

Miniaturization of automobile radiator by using zinc-water and zinc oxide-water nanofluids

High performing fluids for energy conservation and energy efficiency replace conventional heat transfer fluids. This study relates to the development of an alternative heat transfer fluid called as nanofluid. Nanofluid is a dispersion of solid nanoparticles in a base fluid having enhanced thermal properties compared to base fluid. Zinc and Zinc oxide, being eco-friendly and having easy nanoparticle production processes, are considered for the synthesis of nanofluids of different volume fractions. In this experimental study related to heat transfer, the preparation of Zinc-water (Zn-H2O) nanofluid involves the single step method, while the preparation of Zinc oxide-water (ZnO-H2O) uses the two-step method. Six nanofluids comprising of three Zn-H2O and three ZnO-H2O in different volume fractions are tried for this study. Conduct an experimental study to calculate the enhancement of heat transfer coefficient and pressure drop compared to water. Apply the performance evaluation criterion to assess the heat transfer performance of the considered nanofluids. Amongst the six nanofluids, Zn-H2O nanofluid of 0.5% volume fraction proves to have the best heat transfer performance. Then, assess this high performing fluid theoretically in an automobile radiator to get benefits of its use. If by replacing the water with Zn-H2O nanofluid of 0.5% volume fraction it is estimated that the size of the radiator, inventory of the fluid, and pumping power is reduced, thus, making this nanofluid an energy efficient fluid for the engine cooling system.

Experimental study of natural convective heat transfer of water-ZrO2 nanofluids in vertical sub channel

The conventional technique for increasing heat dissipation is to increase the surface area for exchanging heat with a heat transfer fluid. However, the conventional enhanced surface technique has reached their limit with regard to improving heat transfer. Meanwhile, a performance of the convective heat transfer depends on the characteristics of the heat transfer fluid. Therefore, researches on nanofluids heat transfer are innovative ways to find alternative heat transfer fluid (coolants) with better performances. This paper presents an experimental study on natural convective heat transfer of water-ZrO2 nanofluids in a triangular and rectangular array of uniformly heated vertical cylinders with pitch to diameter ratio (P/D) of 1.16. The nanofluids were used in this experiment is a colloidal water-ZrO2 and the concentration of nano particles in the solution used by 0.05 %. The study seeks for a new correlation for natural convective heat transfer of water

Heat Transfer Fluids for Optimal Year-Round Operation of Solar Thermal Systems in Central Europe

In our work, we focused on the problems of heat transfer fluids, these systems are used commercially, and also to alternative heat transfer fluids, which could have the potential to succeed in solar thermal systems for optimal year-round operation. We focused on both the thermal and physical properties of these substances, and the environmental and economic aspects and all other important properties, such that the liquid should ideally have. All measurements and evaluation was conducted with a focus on Central Europe.

Heat transfer and pressure drop characteristic of zinc–water nanofluid

Development of alternative working fluids with enhanced thermal properties is very much needed to replace conventional fluids. Colloidal solution of some base fluid with solid nanoparticles dispersed in it, which is called as nanofluid, is emerging as a promising alternative heat transfer fluid. Zinc, being ecofriendly material, is selected as dispersed phase in water to develop zinc–water (Zn–H2O) nanofluid. Zn–H2O nanofluid is synthesized by single step method and characterized. Thermophysical properties are estimated by available theoretical models. Estimated properties proved that nanofluid is having enhanced thermophysical properties compared to the base fluid due to which nanofluid can become potential working fluid for heat exchanging devices. Synthesized nanofluid is circulated through heat transfer loop to assess its performance in turbulent flow regime and at constant wall temperature condition. Heat transfer coefficient and pressure drop are estimated from experimental results and both are considered as performance evaluation criteria for heat transfer performance assessment. 83 % increase in Nusselt number with 9 % increase in pressure drop is observed for the nanofluid compared to water.

Impact of the heat transfer fluid in a flat plate phase change thermal storage unit for concentrated solar tower plants

Thermal energy storage allows improved dispatch-ability of power from a concentrated solar power plant and increases its annual capacity factor. The selection of an appropriate heat transfer fluid (HTF) is important for designing a cost-effective thermal storage system and to improve the cycle efficiency of the power plant. The current state-of-the-art HTF for tower power plants is molten salts, which have the drawback of having low degradation temperature and high melting temperatures respectively. Alternative HTFs under investigation allow for a much larger range of operation, and can offer other cost and performance advantages. In this study, a comparison of six gaseous and liquid HTFs was carried out to determine their suitability for use in a high temperature thermal storage unit with flat slabs of phase change materials. The comparison is in terms of their thermo-physical properties, heat transfer characteristics between the flat plates and the total delivered electrical energy to the grid. Using a validated mathematical model of phase change material in thin slabs, the HTF outlet temperature, heat transfer rate and liquid fraction profiles were predicted when using different HTFs at a constant heat capacity rate for both charging and discharging processes. For the capacity rate considered, liquid sodium was identified as the best HTF, delivering the highest electrical energy to the grid, achieving 99.4% relative to the ideal case. Solar salt achieved a value of 93.6%, while the gaseous fluids of atmospheric air, air at 10bar, s-CO2 at 100bar and steam at 10bar achieved between 87.9% and 91.3% of the ideal delivered electricity. Gaseous fluids have the advantage of being able to be used as the working fluid in the power block. This study shows that gaseous fluids are comparable to liquid HTFs in PCM storage facilities.

Improving high temperature heat capture for power generation in gasification plants

As part of the gasification process, hot syngas is cooled to low temperatures for gas cleaning by generating high pressure steam in a radiant heat exchanger. The steam, in turn, is used to generate electric power. However, the very large temperature difference that exists between the syngas and generated high pressure steam results in significant losses during the heat transfer process. These losses can be reduced, and hence the overall plant efficiency improved, by choosing materials and working fluids that reduce the temperature difference in the syngas heat exchanger. A set of alternatives to high pressure steam is liquid metals or molten salts. These liquids can be kept at a pressure equal to the syngas pressure, reducing the tensile hoop stress in the heat exchanger tubes. Since the strength of steel decreases with temperature, reducing the hoop stress allows tubes (and the liquids inside them) to reach higher temperatures. The liquid metals or molten salt can then be used in a secondary heat transfer loop to transfer heat to a steam generator. This paper describes a cost-effective radiant heat exchanger that can be used to transfer heat from high temperature syngas to these alternative heat transfer fluids, and also quantifies the increase in overall IGCC plant efficiency by doing so. A capital cost assessment shows a small payback time and improved revenue is achieved by using this technology, making this a realistic option for gasification processes.

Use of Ultrasound Thermometry to Study Natural Convection in Opaque Nanofluids

Abstract Nanofluids are being evaluated as alternative heat transfer fluids, and thus their behavior during storage or low velocity applications in which natural convection can be significant has to be known and documented. Buoyancy induced flows in rectangular enclosures using nanofluids can be studied experimentally using thermocouples, thermistors, surface heat flux sensors, and ultrasound thermometry. The effects of the mass fraction concentration of nanoparticles, the enclosure aspect ratio, and the inclination have been studied experimentally, but more could be done. The opacity of nanofluids does not permit the use of particle image velocimetry, laser induced fluorescence, or any other means of flow visualization or visual temperature measurement of the local fluid temperature. However, the temperature field can be observed using a non-invasive method such as ultrasound thermometry. The experimental enclosure here was validated using water as the initial fluid; measured values of the local fluid temperature were compared with numerical simulations utilizing commercial software. Nanofluid mass fractions of 10% and 25% were used for comparative purposes to study the effects of concentration on the temperature field. Buoyancy force reversal effects were witnessed in both 10% and 25% concentrations. The nanofluid also prolonged the multicellular effects that occur in buoyancy inversion flows. A Rayleigh number inversion was observed with the 25% mass fraction nanofluid. The multicellular regime transitions to a boundary layer regime at about Ra = 1 × 107 when the aspect ratio is 2.625 and at about Ra = 2 × 108 when the aspect ratio is 1.000 for different concentrations of nanofluid. The observations could be physically explained. The current work confirms that temperature measurements of the flow field can be made to assess convective regimes and flow phenomena that induce significant temperature variations. The use of ultrasound thermometry is successfully demonstrated for opaque nanofluid.

Sensitivity Analysis of Saturated Steam Production in Parabolic Trough Collectors

Thermal and synthetic oil is the most common fluid used as heat transfer fluid in parabolic trough collectors for steam generation or producing high volumes of hot water. The solar field configuration using oil is the unfired-boiler configuration. The use of water as alternative heat transfer fluid is not new but is not extended. This paper presents numerical results related to the thermal-hydraulic behavior of solar fields producing saturated steam for industrial process heat applications considering two different types of small sized parabolic-trough collectors which are available in the market. A sensitivity analysis was conducted considering different design criteria and working conditions. The effect of inlet water pressure, temperature and DNI is analyzed. One of the conclusions extracted from the numerical results is that low working pressures can originate high pressure losses in the solar field depending on the absorber pipe diameter and steam pressure and, consequently, steam temperature in the solar field outlet can vary a lot even under small transients.