Thermal Storage Fluid Research Articles

An improved laser flash method for thermal conductivity measurement of molten salts

Thermal conductivity measurement of high-temperature heat transfer fluids provides a crucial basis for designing utility-scale thermal systems. Molten salts are promising heat transfer and thermal storage fluids in high-temperature thermal energy storage systems, while the molten salt thermal conductivity obtained in existing studies exhibits large deviations due to the high experimental complexity and unstandardized test procedures. In this work, we improve the conventional laser flash analysis method by proposing a theoretical heat transfer model for multi-layer heat conduction and providing a near-optimal molten salt container design. With water as a test sample, the relative error of thermal conductivity measurement using the improved method is 6.3%. The thermal conductivity of Solar Salt from 250 to 400 ​°C, and of Hitec Salt from 160 to 250 ​°C are measured and compared with the previous work. Both results show that the thermal conductivity increases with the temperature rising. This work will promote the technology standardization for accurately acquiring the thermal conductivity of molten salts or other similar high-temperature heat transfer fluids.

A Review on Recent Progress in Preparation of Medium-Temperature Solar-Thermal Nanofluids with Stable Dispersion.

Direct absorption of sunlight and conversion into heat by uniformly dispersed photothermal nanofluids has emerged as a facile way to efficiently harness abundant renewable solar-thermal energy for a variety of heating-related applications. As the key component of the direct absorption solar collectors, solar-thermal nanofluids, however, generally suffer from poor dispersion and tend to aggregate, and the aggregation and precipitation tendency becomes even stronger at elevated temperatures. In this review, we overview recent research efforts and progresses in preparing solar-thermal nanofluids that can be stably and homogeneously dispersed under medium temperatures. We provide detailed description on the dispersion challenges and the governing dispersion mechanisms, and introduce representative dispersion strategies that are applicable to ethylene glycol, oil, ionic liquid, and molten salt-based medium-temperature solar-thermal nanofluids. The applicability and advantages of four categories of stabilization strategies including hydrogen bonding, electrostatic stabilization, steric stabilization, and self-dispersion stabilization in improving the dispersion stability of different type of thermal storage fluids are discussed. Among them, recently emerged self-dispersible nanofluids hold the potential for practical medium-temperature direct absorption solar-thermal energy harvesting. In the end, the exciting research opportunities, on-going research need and possible future research directions are also discussed. It is anticipated that the overview of recent progress in improving dispersion stability of medium-temperature solar-thermal nanofluids can not only stimulate exploration of direct absorption solar-thermal energy harvesting applications, but also provide a promising means to solve the fundamental limiting issue for general nanofluid technologies.

Evaluating the Corrosion Resistance of Inconel 625 Coatings, Processed by Compact Plasma Spray, for Applications in Concentrating Solar Power Plants

Thermal energy storage (TES) systems have paramount importance in the design of Concentrating Solar Power (CSP) plants. TES systems allow storing the energy collected from solar radiation as heat energy in a thermal fluid and, in that way, extending the energy duration period of the plant and making the produced electricity dispatchable, depending on the actual demand and not only on the availability of the sun. The thermal fluids, synthetic oils, or molten salts, usually operate at temperatures from 500°C up to 800°C. The harsh operative conditions bring out issues related to the compatibility with the construction materials of CSP components, i.e., carbon and stainless steel. Coating of low-alloy structural steel with high-resistant materials has been addressed as a promising solution for mitigating the corrosion in TES system components. Compact plasma spray process was used to deposit Inconel 625 alloy onto T22 carbon steel coupons. Nitrate salts mixture, 60%NaNO3-40KNO3, commonly employed in CSP systems as operative and thermal storage fluid was used as corrosion medium. The tests were conducted by immersing coated and uncoated samples in molten salts at 500°C for 1, 3 7, and 14 days to assess the corrosion behavior of the In625 coatings. After 24 hours of exposition to molten nitrate salts, the T22 surface showed a pronounced oxidized layer having a thickness of approximately 20 µm. This layer is mainly composed of oxygen, iron, and chromium, which are the main constituents of carbon steel, with a few traces of sodium and potassium derived from the reaction of salts with the steel. Inconel 625, on the other hand, showed the formation of very thin scales of corrosion products localized only on the surface of the sample. Longer exposition is expected to produce a more pronounced degradation of uncoated steel, but barely affect the Inconel 625 coating

Design and development of novel LiCl–NaCl–KCl–ZnCl2 eutectic chlorides for thermal storage fluids in concentrating solar power (CSP) applications

Molten chloride mixtures are of great scientific and technical interest for sensible heat storage in solar power generation. In this work, the condensed phases in a LiCl–NaCl–KCl–ZnCl 2 multicomponent system were critically evaluated and predicted via literature review and simulations of LiCl–KCl and KCl–ZnCl 2 systems. Two innovative LiCl–NaCl–KCl–ZnCl 2 quaternary chloride systems with low melting points were proposed and developed on the basis of a thermodynamic design. The thermal–physical properties of these two eutectic compositions, including melting point, mixing enthalpy, specific heat capacity, density, thermal conductivity, and mass loss, were then experimentally determined. The experimental results were consistent with the theoretical predictions. The novel quaternary eutectic chlorides with excellent thermophysical properties developed herein are potential candidates for thermal energy storage and transfer materials in concentrating solar power plants. • Thermodynamic prediction of LiCl–NaCl–KCl–ZnCl 2 system was performed via CALPHAD. • Two novel quaternary eutectic chlorides were successfully designed for thermal energy storage and transfer. • Thermophysical properties of the system were determined by experiment and classical prediction method. • Thermodynamic performance of the novel chlorides was substantially improved compared with other potential materials. • The novel quaternary eutectic chlorides were proposed as potential candidate materials in CSP applications.

Electrical energy storage using a supercritical CO2 heat pump

This work proposes a new Pumped Thermal Energy Storage (PTES) configuration that works with supercritical CO2 as the working fluid and molten salts as the thermal storage fluid. The net work generated by this novel proposal is 12.46 MW in the load and 10 MW in the discharge, reaching an efficiency of 80.26%. In addition, a techno-economic optimization is carried out to quantify the discounted cost per unit of energy discharged from this storage technology with a value of 0.116 €/kWh. This technology places this technology as a strong candidate for the future energy solution due to its low wear per duty cycle and easy power scalability. It presents a lower initial cost than other technologies with low LCOS.

Nuclear renewable hybrid energy system assessment through the thermal storage system

In support of the use of more efficient and low-carbon nuclear energy along with solar energy in power generation systems, the present paper presents a technical-economic analysis of a nuclear renewable hybrid energy system (NR-HES) case study for a given electricity and residential heat demand profiles of a typical remote community in Canada. The studied hybrid energy system is composed of a small modular nuclear reactor (SMR), a concentrated solar tower (CST), and a thermal energy storage (TES) unit. A fossil fuel backup energy source is also considered to generate excess electricity as needed. The CST and nuclear reactor both share the same TES system. Five different thermal storage fluids are examined: Therminol, Dowtherm, solar salt, Hitec salt, and Hitec XL salt. A transient thermodynamic mathematical model is developed in order to determine the overall configuration, performance, and operation strategy for the NR-HES. The operational strategy of the NR-HES is based on the steady-state full-power operation of the SMR, storing the excess energy in the TES system when the combined nuclear and solar power generation exceeds the power demand of the electrical grid and using the stored thermal energy during the periods of peak demand A detailed economic analysis of the NR-HES is conducted for each operational scenario, including the annualized cost of the TES system and the NR-HES plant, as well as annual revenue and profit generated by the TES system. The developed model is used to obtain the most economical thermal storage fluid and to optimize the size of TES system in order to minimize the use of fossil fuels and maximize profit. Parametric studies of reactor thermal power and CST's field area are conducted to determine their effect on the performance and configuration of the NR-HES. Based on the current case study in terms of demand profiles, the results revealed that Therminol is the most economical thermal storage fluid for the present NR-HES with a charging capacity and round-trip storage efficiency of 237 268 MWh/year and 96%, respectively. The optimum Therminol storage tank size is evaluated to be 14 m in height and 10 m in diameter with a yearly average power cycle efficiency of ~36% and economic profit of 1 158 30 CAD/year. The parametric studies indicated that a larger TES size may decrease the usage of fossil fuel energy sources; however, it increases system total capital cost and decreases the power cycle efficiency and TES profit at given load. The results of this study indicate that an NR-HES is a promising low-carbon power generation system with the potential to meet the electricity and residential heat demand of remote communities.

The Cooling Potential of Sky Radiation With Variations in System Parameters

This study evaluated the building cooling capacity of sky radiation, which was previously identified to have the greatest cooling potential among common ambient sources for climates across the U.S. A heat pipe augmented sky radiator system was simulated by a thermal network with nine nodes, including a thin polyethylene cover with and without condensation, white (zinc oxide) painted radiator plate, condenser and evaporator ends of the heat pipe, thermal storage fluid (water), tank wall, room, sky and ambient air. Heat transfer between nodes included solar flux and sky radiation to cover and plate, wind convection and radiation from cover to ambient, radiation from plate to ambient, natural convection and radiation from plate to cover, conduction from plate to condenser, two-phase heat transfer from evaporator to condenser, natural convection from evaporator to water and from water to tank wall, natural convection and radiation from tank wall to room, and overall heat loss from room to ambient. A thin layer of water was applied to simulate condensation on the cover. Nodal temperatures were simultaneously solved as functions of time using typical meteorological year (TMY3) weather data. Auxiliary cooling was added as needed to limit room temperature to a maximum of 23.9 °C. For this initial investigation, a moderate climate (Louisville, KY) was used to evaluate the effects of radiator orientation, thermal storage capacity, and cooling load to radiator area ratio (LRR). Results were compared to a Louisville baseline with LRR = 10 W/m2 K, horizontal radiator and one cover, which provided an annual sky fraction (fraction of cooling load provided by sky radiation) of 0.855. A decrease to 0.852 was found for an increase in radiator slope to 20 deg, and a drop to 0.832 for 53 deg slope (latitude + 15 deg, a typical slope for solar heating). These drops were associated with increases in average radiator temperature by 0.73 °C for 20 deg and 1.99 °C for 53 deg. A 30% decrease in storage capacity caused a decrease in sky fraction to 0.843. Sky fractions were 0.720 and 0.959 for LRR of 20 and 5, respectively. LRR and thermal storage capacity had strong effects on performance. Radiator slope had a surprisingly small impact, considering that the view factor to the sky at 53 deg tilt is less than 0.5.

The effects of multiple covers with condensation and optical degradation of a polyethylene windscreen on the performance of a sky cooling system

ABSTRACTThis study evaluates the performance of a heat-pipe-augmented sky radiator system modelled using MATLAB. It was developed to simulate thermal networks with the following nodes: polyethylene cover(s), radiator plate with selective surface, heat pipe condenser end, heat pipe evaporator end, thermal storage fluid, fluid container wall, and room. Applying a windscreen of polyethylene that is mostly transparent to long-wave radiation, a drawback is its susceptibility to optical property degradation. Sky fractions of 100% were possible in cities with small cooling loads (Rock Springs, Seattle, San Diego and Denver). Sky fractions of over 50% were achieved in New Orleans and Houston and over 40% in Miami. A second study examined the degradation of polyethylene cover material. Louisville and two challenging climates (Miami and New Orleans) were simulated. In the Louisville, Miami and New Orleans climate, performance was reduced by 2.7, 14.1 and 9.0%, respectively, due to degradation of the cover’s material.

EXPERIMENTAL STUDY ON OPTIMISED COMPOSITION OF WATER BASED NITRATE SALT SOLUTION FOR SENSIBLE HEAT STORAGE

Objective: To find out factors like specific heat, heat capacity and heat loss. Method/Analysis: Sodium nitrate (NaNO3) is mixed in water with different weight quantity like 100g, 200g, 300g last 600g and potassium nitrate (KNO3 ) is mixed in water with weight quantity 100g, 200g, 300g and 400g. Findings: Total heat holding storage is more in NaNO3 than KNO3 , but KNO3 has better energy holding capacity. But KNO3 is reached saturation point in 1 litter water while NaNO3 went up 600g without reaching saturation. Novelty/Improvement: Results shows big molecule has low heat capacity and more Total heat or energy holding storage. Keywords: Energy Holding Capacity (EHC), Potassium Nitrate (KNO3 ), Sodium Nitrate (NaNO3 ), Sustainable Development, Thermal Capacity (TC), Thermal Storage Fluid System

Experimental Test of Properties of KCl–MgCl2 Eutectic Molten Salt for Heat Transfer and Thermal Storage Fluid in Concentrated Solar Power Systems

The eutectic mixture of MgCl2–KCl molten salt is a high temperature heat transfer and thermal storage fluid able to be used at temperatures up to 800 °C in concentrating solar thermal power systems. The molten salt thermophysical properties are reported including vapor pressure, heat capacity, density, viscosity, thermal conductivity, and the corrosion behavior of nickel-based alloys in the molten salt corrosion at high temperatures. Correlations of the measured properties as functions of molten salt temperatures are presented for industrial applications. The test results of tensile strength of two nickel-based alloys exposed in the molten salt at a temperature of 800 °C from 1-week length to 16-week length are reported. It was found that the corrosion and strength loss is rather low when the salt is first processed to remove water and oxygen.

Thermodynamic and economic investigation of a screw expander-based direct steam generation solar cascade Rankine cycle system using water as thermal storage fluid

Solar electricity generation system (SEGS) which employs cascade steam-organic Rankine cycle (SORC) and steam screw expander (SE) is promising due to the high efficiency at moderate heat source temperature. This paper puts a special emphasis on heat storage and thermo-economic evaluation. Preferable operating temperature of the system is first clarified on the basis of SE characteristics. The temperature-dependent permissible stress of steam accumulator is modelled and the capital cost is investigated. Comparison between the direct steam generation (DSG) SEGS and an indirect one using thermal oil is made at a power capacity of 1MW and storage of 6.5h. The results indicate the DSG system has both thermodynamic and economic superiorities. The hot side temperature (TH) of SORC generally does not exceed 250°C to achieve an optimum solar thermal power efficiency. Given radiation of 750W/m2, the maximum efficiency (ηT,m) is 14.3% with a corresponding TH around 240°C. The material cost of pressure vessels is 2.55 million RMB. For the indirect system, the optimal TH is about 230°C and ηT,m approximates to 13.2% and the estimated oil cost is 7.92 million RMB. It is recommended to adopt steam accumulators in the SE-driven SEGS.

Survey and evaluation of equations for thermophysical properties of binary/ternary eutectic salts from NaCl, KCl, MgCl2, CaCl2, ZnCl2 for heat transfer and thermal storage fluids in CSP

Abstract Considered as promising candidates of composing eutectic chloride molten salts for high temperature heat transfer fluid (HTF) with low corrosivity to nickel-based alloys, NaCl, KCl, MgCl2, CaCl2, and ZnCl2 are formulated into proper binary and ternary mixtures to satisfy the property requirement for HTFs. Experimental and predicted thermophysical properties of 16 selected eutectic binary and ternary molten salts are provided. The molten salts can work in a temperature range from after eutectic melting to at least 800 °C with low vapor pressure and acceptable low corrosivity. Surveyed equations and correlations for thermophysical properties of salt mixtures are used to predict heat capacity, density, thermal conductivity, and viscosity. The results are compared to experimental data from the author’s tests as well as reported in literature. Evaluated equations and correlations that can better predict the transport properties of the salts are selected and recommended. Authors expect that more experimental tests will be conducted to provide data for application in CSP industry.

Partitioning of MCHP-TES system run time to incorporate transient system behaviour: a comprehensive exergy-based analysis using simulation

By combining heat and power generation, mini-combined and micro-combined heat and power systems (MCHP) provide an efficient, decentralised means of power generation that can complement the composition of the electricity generation mix. Dynamic tools capable of handling transient system behaviour are required to assess MCHP efficiency beyond a mere static analysis based on steady-state design parameters. Using a simulation of a cogeneration system, we combine exergetic definitions for different operational system states to quantify the overall system efficiency continuously over the whole period of operation. The concept of exergy allows direct comparison of different forms of energy. A sensitivity analysis was performed where we quantified the effect on MCHP overall performance under varying engine rotational speed, thermal energy storage size and fluid storage temperature in a range of MCHP simulations. We found that the exergetic quantity of natural gas used by the MCHP decreased slightly at higher engine speeds (−2% to −4%). While the total amount of electricity generated is almost constant across the range of different engine output, more thermal exergy (up to +21%) can be recovered when the engine is operating at elevated speeds. Furthermore, selection of specific optimal thermal storage fluid temperatures can aid in improving system efficiency. Copyright © 2016 John Wiley & Sons, Ltd.

Use of encapsulated zinc particles in a eutectic chloride salt to enhance thermal energy storage capacity for concentrated solar power

Concentrated Solar Power (CSP) is considered as a viable large-scale renewable energy source to produce electricity. However, current costs to produce electricity from CSP are not cost competitive as compared to the traditional energy generation technologies based on fossil fuels and nuclear. It is envisioned that development of high efficiency and high heat capacity thermal storage fluids will increase system efficiency, reduce structural storage volume, and hence, contribute to reducing costs. Particularly, with respect to CSP, current high temperature energy storage fluids, such as molten salts, are relatively limited in terms of their thermal energy storage capacity and thermal conductivity. The current work explores possibility of boosting the thermal storage capacity of molten salts through latent heat of added phase change materials. We studied the advantage of adding coated Zn micron-sized particles to alkali chloride salt eutectic for enhanced thermal energy storage. Zinc particles (0.6 μm and 5 μm) obtained from commercial source were coated with an organo-phosphorus shell to improve chemical stability and to prevent individual particles from coalescing with one another during melt/freeze cycles. Thermal cycling tests (200 melt/freeze cycles) showed that coated Zn particles have good thermal stability and are chemically inert to alkali chloride salt eutectic in both N2 and in air atmospheres. Elemental mapping of the cross-sectional view of coated Zn particles from the composite after thermal cycles showed no signs of oxidation, agglomeration or other type of particle degradation. The measured enhancement in volumetric thermal storage capacity of the composite with just ∼10 vol% of coated Zn particles over the base chloride salt eutectic varies from 15% to 34% depending on cycling temperature range (ΔT = 50°C–100 °C).

Effects of Multi-Walled Carbon Nanotubes on Enhancing the Thermodynamics Characteristic of Alkali Nitrate and Chlorate Salt for Concentration Solar Plants

Solar thermal power generation technology is the most feasible technology to compete with fossil fuels in the economy, and is considered to be one of the most promising candidates for providing a major share of the clean and renewable energy needed in the future. The appropriate heat transfer fluid and storage medium is a key technological issue for the future success of solar thermal technologies. Molten salt is one of the best heat transfer and thermal storage fluid for both parabolic trough and tower solar thermal power system. It is very important that molten salt heat transfer mechanisms are understood and can be predicted with accuracy. But studies on molten salts heat transfer are rare. This study will lay a foundation for the application of carbon nanotubes in molten salt which can remarkably improve the stability and capacity of thermal storage. Thermal analysis methods and scanning electron microscope (SEM) are utilized to provide a review of thermophysical properties and thermochemical characteristics of the MWCNTs-salt composite materials.

Theoretical study on thermal stability of molten salt for solar thermal power

Molten salt (HTS) composed of 53% KNO3, 40% NaNO2 and 7 wt.% NaNO3 has been used as heat transfer media and thermal storage fluid in the solar thermal power, but thermal decomposition will occur at higher temperature because of the oxidation of nitrite to nitrate in the air. In this paper, the reaction mechanism of NO2− oxidation is researched by quantum mechanical method. The results show that two components of the transition state (O2NO2−) and intermediate ([NO4−]) are found in the reaction. This reaction is an exothermic reaction and the activation barrier is 94.0 kJ mol−1. The energy difference of this reaction is very large, so the reaction rate is very slow.

Lifetime of Imidazolium Salts at Elevated Temperatures

Abstract This report summarizes progress to date on the thermal stability of imidazolium salts being considered for application as heat transfer and thermal storage fluids in solar parabolic trough power systems. Imidazolium salts are a subset of the general class of molten salts. They are termed ionic liquids because many have freezing points at or below room temperature. This class of salts was selected for initial study because there were many examples that were reported to be stable at high temperatures. These reports were usually based on the results of standard thermal gravimetric analysis (TGA) methods. Work by our subcontractor at the University of Alabama and at NREL showed that slow heating rates or when the temperature is held constant for long times resulted in decomposition temperatures that are much lower than those found with the usual TGA methods. We have used a TGA technique that allows calculation of the rates of thermal decomposition as a function of temperature. The results lead us to the conclusion that the imidazolium salts known to be the most thermally stable would not have useful lifetimes above about 200°C. At present this determination is based on the rough approximation that the fluid in a solar trough system experiences a constant, high temperature. Better estimates of the useful lifetime will require a system model that takes into account the time at temperature distribution of a fluid moving through the different components in a solar plant.