Simple thermal decomposition reactions for storage of solar thermal energy

Simple thermal decomposition reactions have been investigated for the purpose of solar thermal energy storage. Ten criteria regarding the thermodynamics and kinetics of the reaction and the physical properties of the components of the reaction have been established. One particular reaction, the decomposition of ammonium hydrogen sulfate, has been evaluated in a preliminary manner and appears to satisfy all of the established criteria. The efficiency of storage is high and the decomposition occurs in the vicinity of 500°C. Other compounds such as ammonium halides, alkali and alkaline earth metal hydroxides, carbonates, sulfates and oxides have also been examined.     

Preparation and thermal stability of ternary sulfate molten salt

以硫酸钠、硫酸钾和硫酸镁为原料,采用在硫酸钠-硫酸钾二元共晶盐中加入硫酸镁的方法制备三元硫酸熔盐。应用TG-DSC联用分析仪、热常数分析仪、X射线衍射仪以及热循环法对复合熔盐的熔点、相变潜热、热导率、比热容、分解点以及热稳定性进行表征。结果表明：所制备的三元硫酸熔盐熔点分布在667.5～669.7 ℃之间,较二元熔盐熔点降低了160 ℃左右,硫酸镁含量为30%（质量分数）的三元硫酸熔盐相变潜热值最大为94.3 J/g,比热容最大为1.13 J/(g·K)（720℃≤T≤800℃）,导热系数为0.41 W/(m·K),分解温度为1070 ℃,经50次热循环后,相变潜热值降低约4.34%,熔点和物相保持基本恒定,具有良好的热稳定性。该研究为硫酸盐作为高温传热蓄热介质提供了依据。     

Thermal design of a solar hydrogen plant with a copper-chlorine cycle and molten salt energy storage

In this paper, the operating temperature ranges of various solar thermal energy technologies are analyzed, with respect to their compatibility with solar hydrogen production via thermochemical cycles. It is found that the maximum temperature of 530 °C required by the oxygen production step in the Cu-Cl cycle can be supplied by current solar thermal technologies. The heat requirements are examined for the Cu-Cl cycle and it is found that the heat source must be sufficiently high and above the maximum temperature requirement of the Cu-Cl cycle, in order to match the heat requirements of the cycle. The quantity of molten salt and solar plant dimensions for capturing and storing solar heat for an industrial hydrogen production scale are also estimated for 24 h operation per day. The flow characteristics and heat losses of molten salt transport in pipelines are studied while considering the influences of pipeline diameter, heat load and weather conditions. The heat loss from a solar salt storage tank is also calculated based on various tank diameters and heights. The intermediate product of molten salt produced in the oxygen production step gives the Cu-Cl cycle a significant advantage of linkage with current high temperature solar thermal technologies. This allows flexibility for integration of the Cu-Cl cycle and solar thermal plant. Using a thermal network analysis of the Cu-Cl cycle, the layout options for the integration of a Cu-Cl cycle with various solar thermal technologies are presented and discussed in this paper.     

Economic implications of thermal energy storage for concentrated solar thermal power

Solar energy is an attractive renewable energy source because the sun's energy is plentiful and carbon-free. However, solar energy is intermittent and not suitable for base load electricity generation without an energy backup system. Concentrated solar power (CSP) is unique among other renewable energy options because it can approach base load generation with molten salt thermal energy storage (TES). This paper describes the development of an engineering economic model that directly compares the performance, cost, and profit of a 110-MW parabolic trough CSP plant operating with a TES system, natural gas-fired backup system, and no backup system. Model results are presented for 0–12 h backup capacities with and without current U.S. subsidies. TES increased the annual capacity factor from around 30% with no backup to up to 55% with 12 h of storage when the solar field area was selected to provide the lowest levelized cost of energy (LCOE). Using TES instead of a natural gas-fired heat transfer fluid heater (NG) increased total plant capital costs but decreased annual operation and maintenance costs. These three effects led to an increase in the LCOE for PT plants with TES and NG backup compared with no backup. LCOE increased with increasing backup capacity for plants with TES and NG backup. For small backup capacities (1–4 h), plants with TES had slightly lower LCOE values than plants with NG backup. For larger backup capacities (5–12 h), plants with TES had slightly higher LCOE values than plants with NG backup. At these costs, current U.S. federal tax incentives were not sufficient to make PT profitable in a market with variable electricity pricing. Current U.S. incentives combined with a fixed electricity price of $200/MWh made PT plants with larger backup capacities more profitable than PT plants with no backup or with smaller backup capacities. In the absence of incentives, a carbon price of $100–$160/tonne CO_2eq would be required for these PT plants to compete with new coal-fired power plants in the U.S. If the long-term goal is to increase renewable base load electricity generation, additional incentives are needed to encourage new CSP plants to use thermal energy storage in the U.S.     

A literature review on some engineering issues of high temperature molten salt thermal energy storage systems

高温熔盐蓄热单元对平衡太阳能热发电过程中的能量供求和延长日落后的发电时间有着不可替代的作用,并已经成为现代太阳能热发电站中的一个不可或缺的子系统。本文作者结合所在重点实验室的850 ℃的二元碳酸优态盐高温熔盐蓄热实验系统的安装与操作经验,讨论了高温熔融盐蓄热系统中常见的一些工程问题,包括熔盐输送管道的连接、熔盐回路的预防凝固和加热保温、熔盐的充装与排放、熔盐长轴泵、事故工况的研究和预防等。     

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.     

Control of a hybrid solar/electric thermal energy storage system

A controller for operating a hybrid thermal energy storage system (HTESS) is presented. The storage system accumulates solar energy during sunny days and releases it later at night or during cloudy days and, simultaneously, it stores electric energy during off-peak periods and releases it later during on-peak periods. The control of the system rests on an anticipatory strategy and on a regulation strategy. The anticipatory strategy is based on a fuzzy logic and feedforward controller (FLFFC) that can handle simultaneously the storage and retrieval of both electricity and solar energy. It takes into account the weather forecasts for solar radiation and outside air temperature, and optimizes the off and the on-peak periods for electrical heating. The regulation strategy depends on a PID controller which regulates the air flow from an electric fan in order to maintain the room temperature at the set point. Numerical simulations were conducted over one to three-month winter periods to test the response of the controller. Results indicate that the proposed control system is far superior to traditional control systems. It remains robust and reliable even for cases in which the weather forecasts are of poor reliability and accuracy (5-day horizon weather forecasts with reliability of 50%, ?10 K temperature accurate and ?50% solar radiation accurate). The performance of the HTESS as well as the thermal comfort of the room is maintained in all situations and at all times. Moreover, the electricity consumption for space heating is minimized and 95% of this electricity is consumed during off-peak hours.     

Coal gasification by CO2 gas bubbling in molten salt for solar/fossil energy hybridization

Coal gasification with CO₂ (the Boudouard reaction: C+CO₂=2CO, Δ_rH°=169.2 kJ/mol at 1150 K), which can be applied to a solar thermochemical process to convert concentrated solar heat into chemical energy, was conducted in the molten salt medium (eutectic mixture of Na₂CO₃ and K₂CO₃, weight ratio=1/1) to provide thermal storage. When CO₂ gas was bubbled through the molten salt, higher reaction rates were observed compared to the case without CO₂ gas bubbling (CO₂ gas was streamed over the surface of the molten salt). Thus the coke formed by coal pyrolysis was well suspended in the molten salt by CO₂ gas bubbling. When the CO₂ flow rate was increased from 15 to 60 μmol/s, the CO evolution rate was increased (15 to 26 μmol/s). However, CO₂ conversion efficiency was decreased (50 to 22%). Based on the maximum CO evolution rate (26 μmol/s), solar thermal energy from a solar farm (300×300 m²) could be converted to chemical energy at a rate of 50,000 kJ/s by the coal (23 ton as C) gasification process studied here. This assumes 50% solar heat to chemical energy conversion efficiency which can be generally obtained by the actual solar experiments.     

Technical assessment of solar thermal energy storage technologies

Solar energy is recognized as one of the most promising alternative energy options. On sunny days, solar energy systems generally collect more energy than necessary for direct use. Therefore, the design and development of solar energy storage systems, is of vital importance and nowadays one of the greatest efforts in solar research. These systems, being part of a complete solar installation, provide an optimum tuning between heat demand and heat supply. This paper reviews the basic concepts, systems design, and the latest developments in (sensible and latent heat) thermal energy storage. Parameters influencing the storage system selection, the advantages and disadvantages of each system, and the problems encountered during the systems operation are highlighted.     

太阳能热发电的显热蓄热技术进展

介绍了太阳能热发电显热蓄热的3种技术:单一流体蓄热,直接接触蓄热和间接接触蓄热。单一流体蓄热,主要有导热油、熔融盐和蒸汽3种选择。直接接触蓄热,使用廉价材料作为蓄热介质,节约了成本。间接接触蓄热使模块化安装成为可能,进一步克服了直接接触蓄热的缺点,为蓄热技术发展提供了便利。     

Pebble bed-oil thermal energy storage for solar thermo-electric power systems

Results of a study to examine the operating characteristics of a 100 kWh thermal energy storage (TES) system suitable for solar thermo electric applications is described. The system chosen consisted of a pebble bed as the primary storage medium and oil as the heat transfer cum storage medium. The operating temperatures considered were between 230 and 250°C with a 20 deg C swing. A full-size unit consisting of a steel tank of volume 10 m³ with 50 mm pebbles, suitable instrumentation and facility for heating the oil was built. The important operating variables and characteristics of the system studied included the transient behaviour of the bed, namely the thermal wave front characteristics. Results of the theoretical analysis of the transient bed behaviour were compared with the experimental data on the wave front propogation characteristics and the comparisons are discussed. The uniformity of flow distribution is also examined.     

Decay of thermal stratification in a water body for solar energy storage

An experimental study of the temperature decay in a thermally stratiffied water body is carried out. An enclosed water body is initially stratified by the recirculating flow of hot water discharge and also, statically, by the addition of hot water at the top of cold fluid. The time dependent temperature profiles are measured for various initial temperature distributions and ambient conditions. The results obtained indicate the temperature field to be largely one-dimensional and a simple analytical model is formulated to study the basic nature of the process. A fairly good agreement with experiments is obtained. The study discusses the basic mechanisms that arise and considers the decay process in terms of these mechanisms. Several interesting features, with respect to energy storage in stratified fluids, are brought out.     

Evaluation and selection of energy storage systems for solar thermal applications

Energy storage is one of the key technologies for energy conservation and therefore is of great practical importance. One of its main advantages is that it is best suited for solar thermal applications. This study deals with a comprehensive discussion of the evaluation and the selection of sensible and latent heat storage technologies, systems and applications in the field of solar energy. Several issues relating to energy storage are examined from the current perspective. In addition, some criteria, techniques, recommendations, checklists on the selection, implementation and operation of energy storage systems are provided for the use of energy engineers, scientists and policy makers. Copyright © 1999 John Wiley & Sons, Ltd.     

Review of latent thermal energy storage systems for solar air-conditioning systems

Solar air conditioning is an important approach to satisfy the high demand for cooling given the global energy situation. The application of phase-change materials (PCMs) in a thermal storage system is a way to address temporary power problems of solar air-conditioning systems. This paper reviews the selection, strengthening, and application of PCMs and containers in latent thermal storage system for solar air-conditioning systems. The optimization of PCM container geometry is summarized and analyzed. The hybrid enhancement methods for PCMs and containers and the cost assessment of latent thermal storage system are discussed. The more effective heat transfer enhancement using PCMs was found to mainly involve micro-nano additives. Combinations of fins and nanoadditives, nanoparticles, and metal foam are the main hybrid strengthening method. However, the thermal storage effect of hybrid strengthening is not necessarily better than single strengthening. At the same time, the latent thermal storage unit has less application in the field of solar air-conditioning systems, especially regarding heat recovery, because of its cost and thermal storage time. The integration of latent thermal storage units and solar air-conditioning components, economic analysis of improvement technology, and quantitative studies on hybrid improvement are potential research directions in the future.     

Microencapsulated phase change slurries for thermal energy storage in a residential solar energy system

Phase change materials (PCMs) are attractive for use in thermal energy storage applications and thermal regulation/control due to their high-energy storage density over a small temperature range. The direct use of phase change materials for energy storage and/or heat transfer applications has been limited due to the low thermal conductivity of the PCM particularly when solidifying on the heat transfer surface. A Phase change slurry (PCS) consists of small micro-encapsulated PCM particles suspended in a carrier fluid which enhances the heat transfer to the PCM. The PCS can serve not only as the thermal storage media but also as the heat transfer fluid, and hence may have many potentially important applications including in the field of heating, ventilation and air-conditioning (HVAC), refrigeration, solar energy and heat exchangers. A test system to examine PCS performance in residential thermal energy storage applications has been developed to both observe and characterise the thermal processes that occur in a thermal store with a helical coil heat exchanger. These test results will be used to improve the system design and identify limitations when used for intermittent application.     

Independent thermal storage power station with molten salt:Technology and evaluation

储能电站可解决可再生能源间歇性和不稳定性的问题,满足常规电力系统和区域能源系统效率,安全性和经济性的迫切需要.本文提出了一种新的蓄热电站技术----独立熔盐蓄热电站,进行了独立熔盐蓄热电站原理的介绍,概念设计和技术经济评价.结果表明:独立熔盐蓄热电站初期投资很低,仅为6152.88元/kW,投资回收期短,在3年以内.独立熔盐蓄热电站占地面积小,可建在城市中实现热电联供,总能效率可由单纯发电的30%提高到80%以上.     

太阳能对不同介质储热性能的影响比较

为了确定太阳能对不同介质储热性能的影响,对等体积的水、细砂和卵石进行了初步的闷晒试验,随后分别将三者与全玻璃真空太阳集热管连接组成储热系统进行比较.     

Phase change performance assessment of salt mixtures for thermal energy storage material

The phase transition performance of the CaCl₂ · 6H₂O–Ca(NO₃)₂ · 4H₂O composite salt system with nucleating and thickening agents was investigated in this paper. The CaCl₂ · 6H₂O–Ca(NO₃)₂ · 4H₂O composite salt system was prepared by adding Ca(NO₃)₂ · 4H₂O (12 wt%) to CaCl₂ · 6H₂O. Different nucleating agents including SrCl₂ · 6H₂O, SrCO₃, BaCl₂, BaCO₃, Na₂B₄O₇ · 10H₂O, H₃BO₃ and NH₄Cl were used to address the problems of phase segregation and supercooling phenomena during the phase change process. The results show that the single nucleating agent SrCl₂ · 6H₂O or the mixture of nucleating agents with 2 wt% SrCl₂ · 6H₂O, 1 wt% BaCl₂ and 0.5 wt% of thickening agent carboxyl methyl cellulose is the most suitable for this system. The latent heat remained constant at about 116 J/g before and after adding the agents. Density functional theory was used to further investigate the microstructure‐related reason for the salt–water separation and supercooling phenomena. It can be deduced that the hydrogen bond is the vital factor involved during the phase transition. The aim of adding thickener was to form more hydrogen bonds which encapsulated the hydrated species and made it difficult to lose the hydrated waters. The main purpose of adding nucleating agent was to break the metastable state among microscopic species. The results of this work indicate that the CaCl₂ · 6H₂O–Ca(NO₃)₂ · 4H₂O salt mixture has potential as a thermal energy storage material. Copyright © 2017 John Wiley & Sons, Ltd.     

Verification of hydrothermal stability of adsorbent materials for thermal energy storage

This paper presents an experimental protocol for the cycling stability of adsorbent materials for thermal energy storage (TES) applications under hydrothermal conditions. Two different aging conditions were identified, namely, cycle and shelf test. The former one mimicking the cycling between desorption and adsorption conditions, while the latter one keeping a constant temperature for long time under constant water vapor pressure. A flexible experimental setup was then designed and realized to contemporarily perform both aging condition under selectable operating conditions. The protocol defines different characterization methods to compare the fresh and the aged samples. The measurement of the water vapor adsorption equilibrium isobars represents the main parameter to directly highlight possible degradation phenomena. Subsequently, X‐ray diffraction patterns (XRD), nitrogen physisorption, and scanning electron microscopy coupled to energy dispersive x‐ray (SEM–EDX), are used to evaluate structural, textural, morphological, and elemental composition variation that can help in identifying the causes of possible degradation. The proposed protocol was employed to validate the stability of a commercial adsorbent, AQSOA Z02, that proved a quite stable behavior both under cycle and shelf investigated conditions.