Research and application of molten salts for sensible heat storage

熔融盐具有液体温度范围宽,黏度低,流动性能好,蒸汽压小,对管路承压能力要求低,相对密度大,比热容高,蓄热能力强,成本较低等诸多优点,已成为一种公认的良好的中高温传热蓄热介质.本文对熔融盐显热蓄热技术原理和发展现状进行了简要概述,包括熔融盐的种类,熔融盐显热蓄热技术的原理,关键技术,研发现状及其在太阳能热发电和间歇性余热利用中的应用.认为开展高温熔融盐传热蓄热介质制备,热性能表征和熔融盐流动与传热性能研究,进而完善整个熔融盐蓄热系统,提高蓄热效率,降低管路腐蚀性,提高系统可靠性仍将是未来熔融盐蓄热技术的研究重点.     

Performance analyses of sensible heat storage systems for thermal applications

In this paper, sensible heat storage (SHS) systems and performance evaluation techniques are studied. A detailed investigation is presented of the availability of SHS techniques for solar thermal applications, selection criteria for SHS systems, the economics of SHS systems, the main issues in evaluating SHS systems, the viability of SHS systems, the environmental impacts of SHS systems and criteria for SHS feasibility studies, as well as energy saving options. In addition to energy and exergy analyses, several definitions of energy and exergy efficiency for the performance of SHS systems are provided with an illustrative example. © 1997 John Wiley & Sons, Ltd.     

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.     

Experimental results of a sensible heat storage system for residential and light commercial application

This paper presents the performance results for a sensible heat storage system. The system under study operates as an air source heat pump which stores the compressor heat of rejection as domestic hot water or hot water in a storage tank that can be used as a heat source for providing building heating. Although measurements were made to quantify space cooling, space heating, and domestic water heating, this paper emphasizes the space heating performance of the unit. The heat storage system was tested for different indoor and outdoor conditions to determine parameters such as heating charge rate, compressor power, and coefficient of performance (COP). The thermal storage tank was able to store a full charge of heat. The rate of increase of storage tank temperature increased with outdoor temperature. The heating rate during a charge test, best shown by the normalized rate plots, increased with evaporating temperature due to the increasing mass flow rate and refrigerant density. At higher indoor temperature during the discharge tests, the rate of decrease of storage tank temperature was slower. Also, the discharge heating rate decreased with time since the thermal storage tank temperature decreased as less thermal energy became available for use. Copyright © 1999 John Wiley & Sons, Ltd.     

Comparison of energy and exergy efficiencies of an underground solar thermal storage system

In this experimental study, solar energy was stored daily using the volcanic material with the sensible heat technique. The external heat collection unit consisted of 27 m² of south‐facing solar air collectors mounted at a 55° tilt angle. The dimensions of the packed‐bed heat storage unit were 6 × 2 × 0.6 m deep. The packed‐bed heat storage unit was built under the soil. The heat storage unit was filled with 6480 kg of volcanic material. Energy and exergy analyses were applied in order to evaluate the system efficiency. During the charging periods, the average daily rates of thermal energy and exergy stored in the heat storage unit were 1242 and 36.33 W, respectively. Since the rate of exergy depends on the temperature of the heat transfer fluid and surrounding, the rate of exergy increased as the difference between the inlet and outlet temperatures of the heat transfer fluid increased during the charging periods. It was found that the average daily net energy and exergy efficiencies in the charging periods were 39.7 and 2.03%, respectively. The average daily net energy efficiency of the heat storage system remained nearly constant during the charging periods. The maximum energy and exergy efficiencies of the heat storage system were 52.9 and 4.9%, respectively. Copyright © 2004 John Wiley & Sons, Ltd.     

Sensible heat storage in a single tank using molten salt and associated natural convection heat transfer

熔融盐双罐显热储热技术已较为成熟,但储热成本较高。为了降低储热成本,本文提出在储热罐内布置浸没式换热器实现熔盐单罐储热和释热新方法,分析了熔融盐单罐储热基本原理,在此基础上模拟分析了垂直布置交错排列两根水平圆柱表面熔融盐自然对流传热的规律。研究发现,为了增强管排表面自然对流传热,两根圆柱的垂直间距不宜过小,而水平间距不宜过大,这些研究结果为熔融盐单罐储热设计提供理论基础。     

显热储热材料的制备及性能研究

采用水泥作为材料的胶凝剂,添加热容、热导率大的物质作为骨料来制备混凝土储热材料。研究表明:当铝酸盐水泥含量为10%时,材料的抗压、抗折强度能满足工业需求;材料的比热容随温度的升高先增大,在500℃时达到最大,后随着温度的升高反而降低;材料的热导率随着石墨粉含量的增加几乎成直线上升,当石墨含量为5%时材料的热导率大于1.7W/(m·K)。     

Advances in thermal energy storage development at the German Aerospace Center(DLR)

万方数据知识服务平台-中外学术论文、中外标准、中外专利、科技成果、政策法规等科技文献的在线服务平台。     

Latent heat storage

The investigations of materials presumably suitable as storage media for latent heat indicate that water, some salt hydrates and eutectic mixtures of water and salt hydrates possess extreme heats of fusion. Their melting points, ranging from about -50° to + 130°C, fit well for storing low grade heat in residential energy systems. Detailed experimental investigations on a large number of these media show, however, that only a few of them satisfy the quality requirments for practical application in storage units. Flexible flat-plate storage containers especially developed for selected salt hydrates which expand on melting also show satisfactory performance over long periods of operation. In the case of water and selected water-salt hydrate eutectics the volume increases on solidification, and the expansion of solid storage material, being very inhomgeneous, breaks even flexible containers after only a few storage cycles. This ruinous local expansion can be avoided, however, by adding a small amount of special, lower melting salt hydrate eutectics which homogenize the crystallization and solidification of the storage medium.     

Application characteristics of heat storage material system

电储热技术是当前适应煤改电环保需求和清洁供暖发展趋势的一项新技术,各种温度级别的储热材料均存在,根据需求选择并制备材料,进而设计电热储能的系统结构成为储热研究关键内容。本文针对蓄热材料的热性能进行了应用特性研究,分析并提出一种蓄热材料温度体系,并对高温蓄热材料应用特点进行了系统设计分析与比较,提出一种基于特制氧化镁的电阻式蓄热系统设计方法,并分析其特点。     

Latent heat storage for solar energy systems: Transient simulation of refrigerant storage

This paper presents a brief review of the available latent heat storage systems for solar energy utilization. A new concept of latent heat storage of solar energy via the refrigerant-absorbent mass storage in absorption cycle heat pump systems used for solar space heating/cooling has been proposed and assessed thermodynamically. A computer modelling and numerical simulation study shows that the concept of refrigerant storage is fundamentally sound, technically feasible and yields the following advantages over other storage methods: (i) the storage capacity per unit volume is high as the latent heat of vaporization of the refrigerant is high; (ii) the heat loss from the storage to the surroundings is minimum as the storage temperature is near the ambient; (iii) prolonged energy storage is possible with no degradation in system performance and hence suitable for combined solar heating and airconditioning. The effects of operating parameters on the energy storage concentration and storage efficiency have been studied in detail.     

Augmentation of solar-assisted humidification-dehumidification water desalination system using heat recovery and thermal energy storage system

In previous investigations, humidification-dehumidification (HDH) solar-assisted desalination systems were designed produce the daily fresh water during sun hours which lead to big sizes and unsteady systems. In the present study, integration of solar-assisted HDH desalination system with heat recovery and thermal energy storage unit is developed to enhance system productivity, reduces auxiliary power consumptions and system size and assure system continuous operation. The mathematical modelling based on energy and mass conservation equations is presented and solved using iterative techniques by C++ and engineering equation solver software. Detailed parametric study of the developed system is conducted for wide ranges of operating conditions and design parameters to study the effects of integrating the HDH system with solar collectors, heat recovery and thermal energy storage units on the system performance. The results revealed that (i) this integration improves system productivity and reduces operating cost, (ii) increasing air to water mass ratio and sea water temperature and decreasing ambient humidity decrease water productivity and gained output ratio (GOR) and increase operating cost parameter (OCP) and (iii) increasing air inlet temperature and sea water flow rates increase GOR and decrease OCP. Comparison with previous systems showed that the proposed system reduces the electric heating power of the system at solar noon by 37% at MR = 0.5 and gives daily fresh water productivity (123.7 kg/h) two times more than previous systems with comparable OCP (0.099 $/kg).     

Thermal performance of a solar-assisted heat pump drying system with thermal energy storage tank and heat recovery unit

Thermal performance parameters for a solar-assisted heat pump (SAHP) drying system with underground thermal energy storage (TES) tank and heat recovery unit (HRU) are investigated in this study. The SAHP drying system is made up of a drying unit, a heat pump, flat plate solar collectors, an underground TES tank, and HRU. An analytical model is developed to obtain the performance parameters of the drying system by using the solution of heat transfer problem around the TES tank and energy expressions for other components of the drying system. These parameters are coefficient of performances for the heat pump (COP) and system (COPs), specific moisture evaporation rate (SMER), temperature of water in the TES tank, and energy fractions for energy charging and extraction from the system. A MATLAB program has been prepared using the expressions for the drying system. The obtained results for COP, COPs, and SMER are 5.55, 5.28, and 9.25, respectively, by using wheat mass flow rate of 100 kg h⁻¹, Carnot efficiency of 40%, collector area of 100 m², and TES tank volume of 300 m³ when the system attains periodic operation duration in fifth year onwards for 10 years of operation. Annual energy saving is 21.4% in comparison with the same system without using HRU for the same input data.     

A review on theory and application of chemical heat pump in thermal energy storage

化学热泵是高效,环保的新型能源技术,在余热回收,储热,可再生能源等领域具有广泛的应用前景.本文综述了化学热泵系统的一般理论和在储热技术中的应用,介绍了化学热泵系统技术在反应与工质对选择,传热强化以及工业研究与应用等方面的发展.     

Economic evaluation of latent heat thermal energy storage using embedded thermosyphons for concentrating solar power applications

An economic evaluation of a latent heat thermal energy storage (LHTES) system for large scale concentrating solar power (CSP) applications is conducted. The concept of embedding gravity-assisted wickless heat pipes (thermosyphons) within a commercial-scale LHTES system is explored through use of a thermal network model. A new design is proposed for charging and discharging a large-scale LHTES system. The size and cost of the LHTES system is estimated and compared with a two-tank sensible heat energy storage (SHTES) system. The results suggest that LHTES with embedded thermosyphons is economically competitive with current SHTES technology, with the potential to reduce capital costs by at least 15%. Further investigation of different phase change materials (PCMs), thermosyphon working fluids, and system configurations has the potential to lead to designs that can further reduce capital costs beyond those reported in this study.     

Study on key parameters design and economic evaluation of the electric heating and solid sensible heat thermal storage device

电制热固体储热系统对可再生能源消纳、能源清洁化利用具有重要意义。电制热固体储热装置的关键参数设计以及经济性分析是提高经济效益的重要手段。因此,本文提出了电制热固体储热装置投资运行费用计算方法。通过对比不同供暖方式所需费用分析了电制热固体储热装置的经济性。同时研究了谷电利用系数对电制热固体储热装置经济性的影响。最后,采用案例分析验证本文所提经济性评估方法的合理性与正确性。本文的研究内容为用户对电制热固体储热装置的选择提供参考。     

Review on heat transfer analysis in thermal energy storage using latent heat storage systems and phase change materials

Thermal energy storage (TES) is a technology that stocks thermal energy by heating or cooling a storage medium so that the stored energy can be used later for heating and cooling applications and for power generation. TES has recently attracted increasing interest to thermal applications such as space and water heating, waste heat utilisation, cooling, and air conditioning. Phase change materials (PCMs) used for the storage of thermal energy as latent heat are special types of advanced materials that substantially contribute to the efficient use and conservation of waste heat and solar energy. This paper provides a comprehensive review on the development of latent heat storage (LHS) systems focused on heat transfer and enhancement techniques employed in PCMs to effectively charge and discharge latent heat energy, and the formulation of the phase change problem. The main categories of PCMs are classified and briefly described, and heat transfer enhancement technologies, namely dispersion of low‐density materials, use of porous materials, metal matrices and encapsulation, incorporation of extended surfaces and fins, utilisation of heat pipes, cascaded storage, and direct heat transfer techniques, are also discussed in detail. Additionally, a two‐dimensional heat transfer simulation model of an LHS system is developed using the control volume technique to solve the phase change problem. Furthermore, a three‐dimensional numerical simulation model of an LHS is built to investigate the quasi‐steady state and transient heat transfer in PCMs. Finally, several future research directions are provided.     

能量地下蓄存及其传热效能分析

储能技术是实现能源可再生化和高效利用的一种有效途径,提高其综合利用率和实现能源的实时补充。着重论述地下蓄能技术发展状况和面临的研究问题,并通过实验和模拟计算,对蓄能的传热作用进行了分析和探讨,指出蓄能改变地下蓄能体的能位,并表现为蓄能体温度和分布的变化,这种变化随时间而改变。建议进一步开展完善地下蓄能理论研究,推动中国地下蓄能技术的发展。     

Experimental investigation of thermal performance of a solar assisted heat pump system with an energy storage

An experimental solar assisted heat pump space heating system with a daily energy storage tank is designed and constructed, and its thermal performance is investigated. The heating system basically consists of flat plate solar collectors, a heat pump, a cylindrical storage tank, measuring units, and a heating room located in Gaziantep, Turkey (37.1°N). All measurements are automatically collected as a function of time by means of a measurement chain feeding to a data logger in combination with a PC. Hourly and daily variations of solar radiation, collector performance, coefficient of performance of the heat pump (COP_HP), and that of the overall system (COP_S) are calculated to evaluate the system performance. The effects of climatic conditions and certain operating parameters on the system performance parameters are investigated. COP_HP is about 2.5 for a lower storage temperature at the end of a cloudy day and it is about 3.5 for a higher storage temperature at the end of a sunny day, and it fluctuates between these values in other times. Also, COP_S turns out to be about 15–20% lower than COP_HP. Copyright © 2004 John Wiley & Sons, Ltd.     

Inverse design methodology to optimize sensible thermal energy storage systems working as rectifiers

The main goal of this paper is to present a methodology to achieve the optimal design of a sensible thermal energy storage system (T3S) working as a thermal rectifier. The system was composed by the heat storage material (HSM), distributed in a set of flat parallel plates, and the working fluid, both modeled by a simplified lumped element model (LEM). The ratio of operational outlet temperature range to source temperature oscillation is defined as the designed rectifying duty. Optimization procedure combines a one‐factor‐at‐a‐time (OFAT) and line search strategies in order to find optimal T3S design that satisfies the designed rectifying duty with the minimum HSM mass. The inverse design philosophy is applied to the optimal results to generalize the T3S dynamic behavior as functions fitting curves of the number of transfer unit (NTU) and the time constant τ . These fitting curves can be used to identify T3S geometric parameters, HSM thermal properties, fluid inlet conditions, among others, which guarantee the outlet fluid temperature to be found within the operational range with the minimum HSM mass. A three step‐by‐step sequence design methodology is presented and detailed, based on design charts from the NTU and τ correlations. The proposed design methodology is able to find the optimal plate length L, plate thickness e_s , and plate distance e_f that satisfies the designed rectifying duty for three test cases. These optimal T3S designs are simulated in a computer fluid dynamics (CFD) code, with deviations below 1.5% between the designed rectifying duty and the one simulated. With the proposed approach, several design solutions or configurations can be found for T3S operating as a thermal rectifier based on NTU and τ fitting curves submitted to a sinusoidal cyclic temperature input and with constant and uniform HSM and fluid properties.