Thermal and mechanical properties of nitrate thermal storage salts in the solid-phase

Implementation of molten salt compounds as the heat transfer fluid and energy storage medium provides specific benefits to energy collection and conversion. Nitrate salts have been identified as a strong candidate for energy transfer and storage and have been demonstrated for use in these applications over time. As nitrate salts have solidification temperatures above ambient, concern for recovery from salt freezing events has instigated efforts to understand and predict this behavior. Accurate information of salt property behavior in the solid-phase is necessary for understanding recovery from a freeze event as well as for phase change thermal energy storage applications. Thermal and mechanical properties for three representative salts (solar salt, HITEC salt, and a Na–K–Li–Ca nitrate salt; spanning the range of liquidus temperatures from approximately 90–240 °C), have been obtained. These properties include: specific heat, coefficient of thermal expansion, thermal conductivity, latent heat of fusion, compressive strength, tensile strength, Young’s modulus and Poisson’s ratio. Specific heat, thermal conductivity and latent heat of fusion were measured using differential scanning calorimetry. Temperature was not observed to have a significant effect on tensile strength using an indirect tensile test (Brazilian test). Peak stress and Young’s modulus (both from unconfined compressive strength testing) were shown to decrease while Poisson’s ratio increased with increasing temperature.     

Preparation and thermal energy properties of paraffin/halloysite nanotube composite as form-stable phase change material

Phase change materials (PCMs) have attracted extensively interests in solar storage. In the study, we prepared a new kind of composite PCM by impregnating paraffin (P) into halloysite nanotube. The as-prepared composite PCM was characterized by TEM, FT-IR and DSC analysis techniques. The composite can absorb paraffin as high as 65 wt.% and maintain its original shape perfectly without any paraffin leakage after subjected to 50 melt–freeze cycles. The melting temperature and latent heat of composite (P/HNT: 65/35 wt.%) were determined as 57.16 °C and 106.54 J/g by DSC. Graphite was added into the P/HNT composite to improve thermal storage performance, and the melting time and freezing time of the composite were reduced by 60.78% and 71.52% compared with the composite without graphite, respectively. Due to its high adsorption capacity, high heat storage capacity, good thermal stability and simple preparation method, the composite can be considered as cost-effective latent heat storage material for practical application.     

A case study of underground thermal storage in a solar-ground coupled heat pump system for residential buildings

This paper presents a case study of underground thermal storage in a solar-ground coupled heat pump system (SGCHPS) for residential buildings. Based on the experimental results, the operation performance is simulated by the unit modelling. The results show that the performance of underground thermal storage of SGCHPS depends strongly on the intensity of the solar radiation and the matching between the water tank volume and the area of solar collectors. Compared with the solar radiation, the variations of the water tank temperature and the ground temperature raise lag behind and keep several peaks during the day time. In the present study, the experimental efficiency of underground thermal storage based on the absorbed solar energy by the collectors reaches 76%. For the similar design of SGCHPS, it is suggested that the optimal ratio between the tank volume and the area of solar collectors should range from 20 to 40 L/m².     

Liquid sodium versus Hitec as a heat transfer fluid in solar thermal central receiver systems

Selection of an appropriate HTF is important for minimising the cost of the solar receiver, thermal storage and heat exchangers, and for achieving high receiver and cycle efficiencies. Current molten salt HTFs have high melting points (142–240 °C) and degrade above 600 °C. Sodium’s low melting point (97.7 °C) and high boiling point (873 °C) allow for a much larger range of operational temperatures. Most importantly, the high temperatures of sodium allow the use of advanced cycles (e.g. combined Brayton/Rankine cycles). In this study, a comparison between the thermophysical properties of two heat transfer fluids (HTFs), Hitec (a ternary molten salt 53% KNO₃ + 40% NaNO₂ + 7% NaNO₃) and liquid sodium (Na), has been carried out to determine their suitability for use in high-temperature concentrated solar thermal central-receiver systems for power generation. To do this, a simple receiver model was developed to determine the influences of the fluids’ characteristics on receiver design and efficiency. While liquid sodium shows potential for solar thermal power systems due to its wide range of operation temperatures, it also has two other important differences – a high heat transfer coefficient (～an order of magnitude greater than Hitec) and a low heat capacity (30–50% lower than Hitec salt). These issues are studied in depth in this model. Overall, we found that liquid sodium is potentially a very attractive alternative to molten salts in next generation solar thermal power generation if its limitations can be overcome.     

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 6.5 MWh_th 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 7.2 GWh_th capacity, for a 26 MW_el round-the-clock concentrated solar power plant during multiple 8 h-charging/16 h-discharging cycles, yielding 95% overall thermal efficiency.     

High-temperature phase change materials for thermal energy storage

The development of energy saving technologies is very actual issue of present day. One of perspective directions in developing these technologies is the thermal energy storage in various industry branches. The review considers the modern state of art in investigations and developments of high-temperature phase change materials perspective for storage thermal and a solar energy in the range of temperatures from 120 to 1000 °C. The considerable quantity of mixes and compositions on the basis of fluorides, chlorides, hydroxides, nitrates, carbonates, vanadates, molybdates and other salts, and also metal alloys is given. Thermophysical properties of potential heat storage salt compositions and metal alloys are presented. Compatibility of heat storage materials (HSM) and constructional materials have found its reflection in the present work. Data on long-term characteristics of some HSMs in the course of repeated cycles of fusion and solidification are analyzed. Article considers also other problems which should be solved for creation of commercial high-temperature heat storage devices with use of phase change materials.     

Effect of solid volume fraction and tilt angle in a quarter circular solar thermal collectors filled with CNT–water nanofluid

Solar thermal collectors have significant importance due to its wide use in solar thermal technology. Augmentation of heat transfer is a key challenge for solar thermal technology. A quarter circular solar thermal collectors is investigated throughout the paper introducing carbon nanotube (CNT)–water nanofluid in the cavity. Tilt angle of this type of collector plays a vital role and heat transfer can be maximized for a particular tilt angle and solid volume fraction of the nanofluid. Galerkin weighted residual of FEM has been applied for the numerical solution of the problem. Grid independency test and code validation have been assessed for the accuracy of numerical solution. In this paper a wide range of solid volume fraction (δ = 0 to δ = 0.12) and tilt angle (ϕ = 0 to ϕ = 60°) has been investigated for Rayleigh number (Ra = 10⁵–10⁸) with varying dimensionless times. It has been found that both solid volume fraction and tilt angle play vital roles for the augmentation of heat transfer and a good heat transfer characteristic can be obtained by compromising between these two parameters. The results are shown using streamline, isotherm contour and related graph and chart.     

Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material

This study aimed determination of proper amount of paraffin (n-docosane) absorbed into expanded graphite (EG) to obtain form-stable composite as phase change material (PCM), examination of the influence of EG addition on the thermal conductivity using transient hot-wire method and investigation of latent heat thermal energy storage (LHTES) characteristics of paraffin such as melting time, melting temperature and latent heat capacity using differential scanning calorimetry (DSC) technique. The paraffin/EG composites with the mass fraction of 2%, 4%, 7%, and 10% EG were prepared by absorbing liquid paraffin into the EG. The composite PCM with mass fraction of 10% EG was considered as form-stable allowing no leakage of melted paraffin during the solid–liquid phase change due to capillary and surface tension forces of EG. Thermal conductivity of the pure paraffin and the composite PCMs including 2, 4, 7 and 10 wt% EG were measured as 0.22, 0.40, 0.52, 0.68 and 0.82 W/m K, respectively. Melting time test showed that the increasing thermal conductivity of paraffin noticeably decreased its melting time. Furthermore, DSC analysis indicated that changes in the melting temperatures of the composite PCMs were not considerable, and their latent heat capacities were approximately equivalent to the values calculated based on the mass ratios of the paraffin in the composites. It was concluded that the composite PCM with the mass fraction of 10% EG was the most promising one for LHTES applications due to its form-stable property, direct usability without a need of extra storage container, high thermal conductivity, good melting temperature and satisfying latent heat storage capacity.     

Utilization of phase change materials in solar domestic hot water systems

Thermal energy storage systems which keep warm and cold water separated by means of gravitational stratification have been found to be attractive in low and medium temperature thermal storage applications due to their simplicity and low cost. This effect is known as thermal stratification, and has been studied experimentally thoughtfully. This system stores sensible heat in water for short term applications. Adding PCM (phase change material) modules at the top of the water tank would give the system a higher storage density and compensate heat loss in the top layer because of the latent heat of PCM. Tests were performed under real operating conditions in a complete solar heating system that was constructed at the University of Lleida, Spain. In this work, new PCM-graphite compounds with optimized thermal properties were used, such as 80:20 weight percent ratio mixtures of paraffin and stearic acid (PS), paraffin and palmitic acid (PP), and stearic acid and myristic acid (SM). The solar domestic hot water (SDHW) tank used in the experiments had a 150 L water capacity. Three modules with a cylindrical geometry with an outer diameter of 0.176 m and a height of 0.315 m were used. In the cooling experiments, the average tank water temperature dropped below the PCM melting temperature range in about 6–12 h. During reheating experiments, the PCM could increase the temperature of 14–36 L of water at the upper part of the SDHW tank by 3–4 °C. This effect took place in 10–15 min. It can be concluded that PS gave the best results for thermal performance enhancement of the SDHW tank (74% efficiency).     

Numerical study on performance of molten salt phase change thermal energy storage system with enhanced tubes

Based on enthalpy method, numerical studies were performed for high temperature molten salt phase change thermal energy storage (PCTES) unit used in a dish solar thermal power generation system. Firstly, the effects of the heat transfer fluid (HTF) inlet temperature and velocity on the PCTES performance were examined. The results show that although increasing the HTF inlet velocity or temperature can enhance the melting rate of the phase change material (PCM) and improve the performance of the PCTES unit, the two parameters will restrict each other for the fixed solar collector heat output. Then three enhanced tubes were adopted to improve the PCTES performance, which are dimpled tube, cone-finned tube and helically-finned tube respectively. The effects of the enhanced tubes on the PCM melting rate, solid–liquid interface, TES capacity, TES efficiency and HTF outlet temperature were discussed. The results show that compared with the smooth tube, all of the three enhanced tubes could improve the PCM melting rate. At the same working conditions, the melting time is 437.92 min for the smooth tube, 350.75 min for dimpled tube which is reduced about 19.9% and 320.25 min for cone-finned tube which is reduced about 26.9% and 302.75 min for helically-finned tube reduced about 30.7%. As a conclusion, the thermal performance of PCTES unit can be effectively enhanced by using enhanced tube instead of smooth tube. Although, the HTF pressure drops for the enhanced tubes are also larger than that of the smooth tube, the largest pressure drop (1476.2 Pa) is still very lower compared with the working pressure (MPa magnitude) of the dish solar generation system. So, the pressure drops caused by the enhanced tubes could almost be neglected.     

A comparison of solar absorption system configurations

Solar thermal driven cooling systems for residential applications are a promising alternative to electric compression chillers, although its market introduction still represents a challenge, mainly due to the higher investment costs. The most common system configuration is an absorption chiller driven by a solar thermal system, backed up by a secondary heating source, normally a gas boiler. Heat storage in the primary (solar) circuit is mandatory to stabilize and extend the operation of the chiller, whereas a cold storage tank is not so common.This paper deals with the selection of the most suitable configuration for residential cooling systems with solar energy. In Spain, where cooling needs are usually higher than heating needs, the interest of a reversible heat pump as auxiliary system and a secondary cooling storage are analyzed.A complete TRNSYS model has been developed to compare a configuration with just hot storage (of typical capacity 40 L/m² of solar collector surface) and a configuration with both, hot and cool storages. The most suitable configuration is very sensible to the solar collector area. As the collector area increases, the advantages of a cool storage vanish. Increasing the collector area tends to increase the temperature of the hot storage, leading to higher thermal losses in both the collector and the tank. When the storage volume is concentrated in one tank, these effects are mitigated. The effect of other variables on the optimal configuration are also analyzed: collector efficiency curve, COP of the absorption chiller, storage size, and temperature set-points of the chillers.     

Lauric and palmitic acids eutectic mixture as latent heat storage material for low temperature heating applications

Palmitic acid (PA, 59.8 °C) and lauric acid (LA, 42.6 °C) are phase change materials (PCM) having quite high melting temperatures which can limit their use in low temperature solar applications such as solar space heating and greenhouse heating. However, their melting temperatures can be tailored to appropriate value by preparing a eutectic mixture of the lauric and the palmitic acids. In the present study, the thermal analysis based on differential scanning calorimetry (DSC) technique shows that the mixture of 69.0 wt% LA and 31 wt% PA forms a eutectic mixture having melting temperature of 35.2 °C and the latent heat of fusion of 166.3 J g⁻¹. This study also considers the experimental determination of the thermal characteristics of the eutectic mixture during the heat charging and discharging processes. Radial and axial temperature distribution, heat transfer coefficient between the heat transfer fluid (HTF) pipe and the PCM, heat recovery rate and heat charging and discharging fractions were experimentally established employing a vertical concentric pipe-in-pipe energy storage system. The changes of these characteristics were evaluated with respect to the effect of inlet HTF temperature and mass flow rate. The DSC thermal analysis and the experimental results indicate that the LA–PA eutectic mixture can be a potential material for low temperature thermal energy storage applications in terms of its thermo-physical and thermal characteristics.     

The particle thermal conductivity influence of nanofluids on thermal performance of the microtubes

The paper presents the numerical analysis on microchannel laminar heat transfer and fluid flow of nanofluids in order to evaluate the suitable thermal conductivity of the nanoparticles that results in superior thermal performances compared to the base fluid. The diameter ratio of the micro-tube was D_i/D_o = 0.3/0.5 mm with a tube length L = 100 mm in order to avoid the heat dissipation effect. The heat transfer rate was fixed to Q = 2 W. The water based Al₂O₃, TiO₂ and Cu nanofluids were considered with various volume concentrations ϕ = 1,3 and 5% and two diameters of the particles d_p = 13 nm and 36 nm. The analysis is based on a fixed Re and pumping power Π, in terms of average heat transfer coefficient and maximum temperature of the substrate. The results reveal that only the nanofluids with particles having very high thermal conductivity (λ_Cu = 401 W/m K) are justified for using in microcooling systems. Moreover, the analysis is sensitive to both the comparison criteria (Re or Π) and heat transfer parameters (h_ave or t_max).     

Analysis of low temperature solar thermal electric generation using regenerative Organic Rankine Cycle

The innovative configuration of low temperature solar thermal electric generation with regenerative Organic Rankine Cycle (ORC) is designed, mainly consisting of small concentration ratio compound parabolic concentrators (CPC) and the regenerative ORC. Advantages of the innovative configuration such as effectively reducing heat transfer irreversibility and permitting the use of thermal storage with phase change materials (PCMs) are outlined. The numerical simulation of the heat transfer and power conversion processes are carried out based on distributed parameters. The effects of regenerative cycle on the collector, ORC, and overall electricity efficiency are then analyzed. The results indicate that the regenerative cycle has positive effects on the ORC efficiency but negative ones on the collector efficiency due to increment of the average working temperature of the first-stage collectors. Thus, it is necessary to evaluate the overall electricity efficiency when regenerative cycle is adopted. Further investigation shows that there are maximum efficiencies for both the ORC and the system electric generation on conditions of constant irradiance, evaporation temperature, and environment temperature. And the regenerative temperature at which the system electricity efficiency reaches its maximum is smaller than that at which the ORC efficiency reaches its maximum by 12–21 °C. Thus, the regenerative cycle optimization of the solar thermal electric generation differs from that of a solo ORC. The system electricity efficiency with regenerative ORC is about 8.6% for irradiance 750 W/m² and is relatively higher than that without the regenerative cycle by 4.9%.     

High-temperature carbonate/MgO composite materials as thermal storage media for double-walled solar reformer tubes

The composite materials of molten alkali-carbonate/MgO-ceramics are examined as thermal storage media in a tubular reformer using a double-walled reactor tube of a laboratory scale. The concept of a double-walled reformer tube is proposed as a solar tubular reformer and involves packing a molten salt/ceramic composite material in the annular region between the internal catalyst tube and the exterior solar absorber wall. The composite materials of Na₂CO₃, K₂CO₃, and Li₂CO₃ with magnesia are tested as thermal storage media. The reforming performances of the composite materials are tested in the cooling mode of the double-walled reactor tube. The experimental result obtained under feed gas mixture of CH₄/CO₂ = 1:3 at 1 atm shows that the use of 80 wt%Na₂CO₃/20 wt%MgO composite material successfully delayed the cooling time of the catalyst bed by 5–19 min in comparison to the case without a composite material. In addition, the Li₂CO₃/MgO and Na₂CO₃/MgO composite materials relatively revealed good performances: they prolonged the cooling time by over 10 min in the gas hourly space velocity (GHSV) range of 5000–12,500 h^?1. The application of the reactor tubes to solar tubular reformers is expected to realize stable operation of the solar reforming process under fluctuating insolation during cloud passage.     

Carbon nanotube thermal interfaces on gadolinium foil

We report the thermal behavior of gadolinium foils to be used in magneto thermoelectric generator cells. Magneto thermoelectric generator cell technology exploits the ferromagnetic phase transition of gadolinium to drive the movement of a diaphragm ‘shuttle’ whose mechanical energy is converted to electrical form and which enhances heat transfer through both conduction and convection. Efficient heat transfer at mechanical interfaces is critical to increase shuttle speed and the commensurate rate of heat transfer. The synthesis and characterization of carbon nanotube thermal interfaces for the gadolinium foils are described. The total thermal interface resistance of the carbon nanotube coated gadolinium was measured using a one-dimensional reference calorimeter technique. Improvement of carbon nanotube growth based on parametric process variations is described, and the effect of hydrogen embrittlement on the magnetic properties of the gadolinium foils during carbon nanotube growth is quantified. The samples generated in this study were consistently measured with total thermal interface resistances in the range of 65–105 mm² K/W, a reduction of 55–70% compared to bare gadolinium (R_int ≈ 230 mm² K/W). The addition of carbon nanotube arrays did not alter the magnetic properties of the gadolinium foils and only a slight decrease in the magnetic moment of the gadolinium samples (8–13%) was measured after growth.     

A novel thermal water pump for circulating water in a solar water heating system

This research purpose was to perform a parametric study of a novel thermal water pump well fitted in a simulated solar water heating system (SWHS). The SWHS was composed of a heating tank (HT), a hot water storage tank (ST) and an overhead tank (OT). The HT together with a specially designed valve act as a novel thermal water pump that gets power from hot water vapor and air pressure produced by a built-in electric heater in order to transfer heat from the HT to ST. The general operation of this pump has four stages for each cycle: heating, water circulating, vapor circulating and water supplying. The discharge water heads were varied with an increment of 0.25 m from 0.75 to 3 m. According to the experiment, it was found that the pump could operate at an average HT temperature of about 80–95 °C leading to 70–80 °C ST temperatures and 20–35 pumping cycles and consumed 17 MJ energy input during 9-h period. The overall thermal efficiency of the SWHS was 33–42% and the mean pump efficiency was about 0.005–0.011% depending upon the discharge heads.     

Experimental investigation and development of new correlation for thermal conductivity and viscosity of BioGlycol/water based SiO2 nanofluids

Nanofluids are a new class of engineered heat transfer fluids which exhibit superior thermophysical properties and have potential applications in numerous important fields. In this study, nanofluids have been prepared by dispersing SiO₂ nanoparticles in different base fluids such as 20:80% and 30:70% by volume of BioGlycol (BG)/water (W) mixtures. Thermal conductivity and viscosity experiments have been conducted in temperatures between 30 °C and 80 °C and in volume concentrations between 0.5% and 2.0%. Results show that thermal conductivity of nanofluids increases with increase of volume concentrations and temperatures. Similarly, viscosity of nanofluid increases with increase of volume concentrations but decreases with increase of temperatures. The maximum thermal conductivity enhancement among all the nanofluids was observed for 20:80% BG/W nanofluid about 7.2% in the volume concentration of 2.0% at a temperature of 70 °C. Correspondingly among all the nanofluids maximum viscosity enhancement was observed for 30:70% BG/W nanofluid about 1.38-times in the volume concentration of 2.0% at a temperature of 70 °C. The classical models and semi-empirical correlations failed to predict the thermal conductivity and viscosity of nanofluids with effect of volume concentration and temperatures. Therefore, nonlinear correlations have been proposed with 3% maximum deviation for the estimation of thermal conductivity and viscosity of nanofluids.     

Reducing cold-start emission from internal combustion engines by means of thermal energy storage system

Increasing environmental pollutions is an important problem appearing at cold start of internal combustion engines. Developments of new devices that solve this problem are an extremely urgent need especially for cold regions. In this study, a developed experimental sample of thermal energy storage system (TESS) for pre-heating of internal combustion engines has been designed and tested. The development thermal energy storage device (TESD) works on the effect of absorption and rejection of heat during the solid–liquid phase change of heat storage material (Na₂SO₄ · 10H₂O). The TESS has been applied to a gasoline engine at 2 °C temperature and 1 atm pressure. Charging and discharging time of the TESD are about 500 and 600 s, respectively and temperature of engine is increased 17.4 °C averagely with pre-heating. Maximum thermal efficiency of the TESS system is 57.5 % after 12 h waiting duration. CO and HC emissions decrease about 64% and 15%, respectively, with effect of pre-heating engine at cold start and warming-up period.     

Preparation,electrical, mechanical and thermal properties of composite bipolar plate for a fuel cell

A novel composite bipolar plate for a polymer electrolyte fuel cell has been prepared by a bulk-moulding compound (BMC) process. The electrical resistance of the composite material decreases from 20 000 to 5.8 mΩ as the graphite content is increased from 60 to 80 wt.%. Meanwhile, the electrical resistance of composite increases from 6.5 to 25.2 mΩ as the graphite size is decreased from 1000 to 177 μm to less than 53 μm. The thermal decomposition of 5% weight loss of composite bipolar plate is higher than 250 °C. The oxygen permeability of the composite bipolar plate is 5.82×10⁻⁸ (cm³/cm² s) when the graphite content is 75 wt.%, and increases from 6.76×10⁻⁸ to 3.28×10⁻⁵ (cm³/cm² s) as the graphite size is longer or smaller than 75 wt.%. The flexibility of the plate decreases with increasing graphite content. The flexural strength of the plate decreases with decrease in graphite size from 31.25 MPa (1000–177 μm) to 15.96 MPa (53 μm). The flexural modulus decreases with decrease of graphite size from 6923 MPa (1000–177 μm) to 4585 MPa (53 μm). The corrosion currents for plates containing different graphite contents and graphite sizes are all less than 10⁻⁷ A cm⁻². The composite bipolar plates with different graphite contents and graphite sizes meet UL-94V-0 tests, and the limiting oxygen contents are higher than 50. Testing show that composite bipolar plates with optimum composition are very similar to that of the graphite bipolar plate.