Packed Bed Thermal Storage System Research Articles

Multi-factor analysis and optimization design of a cascaded packed-bed thermal storage system coupled with adiabatic compressed air energy storage

In an adiabatic compressed air energy storage (A-CAES), one of the key components is the heat storage system, in which the packed bed filled with encapsulated phase-change capsules has been widely investigated because of its excellent thermal performance. In this paper, a packed-bed thermal energy storage model with three layers of phase change materials (PCM) is proposed in the context of an A-CAES. Four factors primarily affecting the thermal performance of the packed bed thermal storage system, namely the mass flow rate of heat transfer fluid (HTF), the inlet temperature of HTF, the cascade situation of PCM, and the arrangement of particle sizes, are critically analyzed by conducing the orthogonal experiments. On this basis, the machine learning method is applied to predict and optimize the thermal performance of the packed bed aiming to find the optimal results. The comparison of results between the optimized and original models expressively shows a reduction of 8.46% in PCM mass, an increase of 92.18%/116.82% in the overall heat storage/release quantity, and an increase of 19.23% in the overall efficiency. This work outlooks a guideline for the potential application of packed bed in A-CAES.

Heat transfer and storage characteristics of a hexagonal close structured packed-bed thermal storage system with molten salt phase change materials

The phase change material (PCM) spheres using 316SS as the ball shell and solar salt (NaNO3-KNO3, 60–40 wt%) as the PCM were arranged in an orderly pebble-bed arrangement. The charging performance, energy and exergy of PCM spheres in the packed ball thermal energy storage system (PBTES) were investigated using CFD simulation in order to solve the problem of heat storage at 300 °C–500 °C. The results show that the total melting time of PCM spheres slightly decreases from 1455 s to 1319 s with the inlet velocity increasing. The average heat storage efficiency of PCM spheres and PBTES are improved with the inlet temperature increasing. When the inlet flow rate increases from 0.01 m/s to 0.10 m/s, the energy efficiency of PBTES increases approximately 5.06 times, and the exergy efficiency increases from 17.91 % to 99.55 %. However the heat energy storage and exergy efficiency of PCM spheres are not affected by the inlet velocity changing. The energy efficiency of PBTES increases about 2.09 times, and the exergy efficiency increases from 30.29 % to 70.93 % with the inlet temperature grows from 300 °C to 500 °C. It provides an idea for the design of the medium-high temperature and high-efficiency molten salt PCM heat storage system.

A high-temperature thermal stability and optical property study of inorganic coatings on ceramic particles for potential thermal energy storage applications

Ceramic-based packed bed solutions are becoming more common in the energy fields as both thermal energy storage and heat exchanger. Such solutions are usually designed for the working temperature ranges above 600 °C, thus thermal radiation becomes significant and even acts as the dominant heat transfer mechanism. Therefore, applying high-temperature coatings with different thermal properties could be an efficient way in enhancing the performance of these applications. In this work, the high-temperature long residency and cyclic thermal stability of six inorganic coatings applied on a ceramic substrate are investigated. Both qualitative and quantitative assessments are performed. The results show that HIE-Coat 840MX and Pyropaint 634 ZO exhibit excellent thermal stability performance both at high-temperature testing (1000 °C) and under thermal cycle testing (400 °C–800 °C). TiO 2 based coatings could be a viable solution if the powder is pre-treated to avoid polymorph transition during the operation. Stainless steel 304 powder-based coating could also be a possible solution, since the adhesive curbs the oxidation and hinders the coating from deterioration. Contrarily, Pyromark 2500 and MgO based coating show different degradation problems that limit their exploitation in high-temperature applications undergoing thermal cycles. The investigated coatings show a wide range of thermal emissivity (between 0.6 and 0.9), with stable or decreasing trends with temperature. This enables a potential 20% change of the effective thermal conductivity for the packing structure. This work is a stepping-stone towards further detailed experimental studies on the influence of coatings on various packed bed thermal storage systems, and thus offer a new option in improving the performances of the energy equipment with packed bed systems. • Thermal stability of six inorganic coatings on ceramic particles tested at 1000 °C. • Pyro-Paint 634-ZO and HIE-Coat 840MX show excellent thermal stability. • MgO coating on ceramic particles shows cracks after 30 thermal cycles (400–800 °C). • Optical properties of selected coatings are studied in the λ range 250–26000 nm. • Effective thermal conductivity increases of 27% at 1000 °C can be attained.

충전층 축열시스템의 성능 특성

An analytical model is presented, for packed-bed thermal storage systems. The model is based on a mathematical solution for packed-bed storage systems, subject to arbitrary periodic inlet temperature conditions and heat loss to ambient. The model is validated via comparison with well-known theoretical and experimental data in the literature. It is also demonstrated that the model can be used to analyze the experimental results, and to predict the performance of packed beds in general. The model is simple and flexible, so that it can facilitate design and analysis of the packed-bed systems, subjected to various boundary conditions in the field.

Modeling, Simulation and Optimal Operation of Multi-Extraction Packed-Bed Thermal Storage Systems

Solar thermal power technologies require storage systems to mitigate the natural variability of solar irradiation. Packed bed thermal storage systems (PBTES) offer a cost-effective solution using air as heat transfer fluid and rocks as a storage medium. Compared to its alternatives, however, PBTES presents a limited flexibility of operation due to the conventional unidirectional flow, which involves the progressive reduction of the outlet temperature during discharge and thus lowers the thermodynamic efficiency of the power cycle. The present study summarizes the progress on the design and optimal operation of a novel multi-extraction PBTES, a project that aims at mitigating its typically poor operational flexibility for solar power applications. To this end, a one-dimensional model with a high spatial resolution of a PBTES was developed, which includes four intermediate outlet points along the axial direction to investigate the benefits of optimal extraction operation. In order to reduce the computational burden, a coarser model of the storage system is used in combination with non-linear model predictive control (NLMPC). Through the optimal manipulation of the extraction valves, the output temperature is maintained close to a prescribed temperature throughout the discharge. The control admits not only constant temperature targets, but also time-varying scheduled profiles. This work describes the limitation of such a design and control approach and sets the direction for the future, more detailed analyses needed to demonstrate its applicability.

Spray-type packed bed concept for thermal energy storage: Liquid holdup and energy storage characteristics

The performance of a thermal energy storage system strongly influences the overall efficiency of a central solar power plant. Therefore, low-cost and high-efficiency thermal energy storage technologies have attracted considerable attention in recent years. A thermal energy storage concept using a spray-type packed bed is proposed in the present study. In addition, a small-scale semi-transparent spray-type packed bed thermal storage system was set up, using thermal oil as a transfer fluid and spherical particles as the storage media inside the packed bed. An experimental study on the liquid holdup and heat storage characteristics of the proposed spray-type packed bed system was conducted. The results indicate that the total liquid holdup increases as the volume flow rate increases, and the static liquid holdup inside the packed bed shows no relationship with the flow rate or height of the packed bed. Herein, a comparison of the unit cost for different thermal energy storage technologies is presented.

Analytical formulation of effective heat transfer coefficient and extension of lumped capacitance method to simplify the analysis of packed bed storage systems

Analysis of the transient temperature evolution during charging or discharging of the packed bed thermal storage systems is immensely simplified with the formulation of an effective heat transfer coefficient between the solid storage materials and the heat transfer fluid. It can cut significant computational cost which is otherwise required for a complete numerical simulation. The lumped capacitance method, the simplest of the available options, is restricted only to the low Biot number scenarios and hence is seldom applicable to any real system. The formulation of the effective heat transfer coefficient allows the extension of the lumped capacitance method for moderate Biot numbers. The present work develops such a formulation to simplify the analysis of packed bed storage systems through analytical route. Earlier attempts in this direction were made through weighted average time method which is inherently restricted to the simple one-dimensional heat conduction problems. On the other hand, we find out the effective heat transfer coefficient starting from a general three dimensional analytical solution of the transient heat conduction and proceed with the well-known one term approximation which acts as the basis of the Heisler charts. The method is not dimensionally restricted and hence we can include realistic three dimensional shapes such as cuboid, short cylinder etc. in the formulation. We validate our method by comparing the resulting temperature profiles for the one-dimensional geometries which have been attempted earlier by the weighted average time method. We also provide the accuracy estimation (as a function of increasing Biot number) against the full numerical simulation for the geometries where the weighted average time is not applicable. Therefore, the current study provides the tool for an inexpensive theoretical estimation for the transient heat transfer behaviour in the thermal storage tanks which has long term design implications particularly for the large scale concentrated solar thermal power plants.

Experimental investigation of cycling behaviour of pilot-scale thermal oil packed-bed thermal storage system

Extensive experimental study of dual media thermocline thermal energy storage using rock and sand as solid matrix and thermal oil as heat transfer fluid is presented. The work is devoted to the establishing of repeatable behaviours observed with multiple charge/discharge cycles and to the influences of operating parameters. Experimental results show that established behaviours are independent of most of operating parameters, notably tank initial state, mass flow rate and temperature ranges, and mainly depend on thermocline control strategy, i.e. part of the thermal gradient which is pulled out of the tank in charge and discharge. Experimental performances have been determined for a large range of control strategies and show tank utilisation rate ranging from about 10% to more than 80%. Thermocline response to perturbations has been studied and shows that partial charge perturbations lead to performances reduction with rapid return to established behaviours. These results may strongly enhance dual media thermocline understanding and may help design and integration of such TES in a given process.

Cyclic behaviors of the molten-salt packed-bed thermal storage system filled with cascaded phase change material capsules

A transient, one-dimensional dispersion-concentric model to numerically study the cyclic behaviors of the molten-salt packed-bed thermal energy storage system filled with cascaded phase change material (PCM) capsules is presented. Three different storage systems are investigated which include the non-cascaded system, the cascaded systems with 3 and 5 cascaded phase change temperatures (PCTs). Detailed characteristics of heat transfer between molten salt and the packed PCM capsules are discussed, and various numerical results are presented, including the temperature distributions of molten salt and PCM capsules, the variations in the molten-salt outlet temperature, the accumulated efficiency of each cycle. The results show that the non-cascaded system suffers from a low charging ratio and a long charging time due to the constrains of PCT while the cascaded systems especially with 5 cascaded PCTs are found to have both a fast discharging rate and a fast charging rate. The cyclic process of the cascaded system with 5 cascaded PCTs can reach to a repeatable state after some cycles, at which high accumulated efficiencies can be achieved. It is also found that the practical storage capacity of the storage system with cascaded PCM capsules depends highly on the threshold temperatures to stop the charging/discharging process.

Experimental Research of the Heat Transfer Characteristics using a Packed-bed of Honeycomb Ceramic for High Temperature Thermal Storage System

Abstract Due to intermittent nature of solar energy, the thermal energy storage (TES)is vital for the concentrated solar power (CSP) technologies. This paper reports on an experimental investigation of the heat transfer characteristic of a packed-bed using honeycomb ceramic for high temperature thermal storage system. The experimental results showed that the honeycomb ceramic material can be used as the thermal storage material for high temperature packed-bed thermal storage system. After 8 hours of charging, the temperatures of the thermal storage module were 600 °C, 500 °C and 400 °C. And the air outlet temperature is above 483 °C after two hours of discharging when the air flow rate is 150m 3 /h.

Experimental and numerical investigation of a pilot-scale thermal oil packed bed thermal storage system for CSP power plant

A pilot-scale oil/rock thermocline thermal energy storage (TES) system, consisting of a packed bed of two characteristic sizes rocks as storage material and thermal oil as heat transfer fluid, is experimentally studied. Exhaustive temperature instrumentation, with more than 250 imbedded thermocouples, shows that no significant inhomogeneity or by-pass exists in the rock bed and that the heat transfer process is mainly one-dimensional. Investigation of multiple charge/discharge cycles exhibits established behaviour quite different from single charge/discharge cycle and directly influenced by operating control of the TES system. A predictive approach of the packed bed pressure drops, based on hydraulic characteristics inside the rock-bed, has been developed and show very good agreement with the experimental measurements. A one dimensional numerical model based on two energy balances has been developed and validated with experimental data for both single and multiple charge/discharge. These experimental and numerical results confirm that dual-media thermocline storage is controllable and predictable and can be used as efficient storage solution for CSP plant.

Thermal analysis and exergy evaluation of packed bed thermal storage systems

Thermal energy storage in a packed bed is analyzed for dynamic temperature response during cyclic storage and recovery. A robust system-scale heat transfer model for the packed bed is developed and accounts for wall heat transfer and intra-particle diffusion effects. Based upon the temperature field modeling, recovered and lost exergy are calculated. The analysis shows that, for packed beds, sensible heat storage systems can provide much higher exergy recovery as compared to phase change material (PCM) storage systems under similar high temperature storage conditions. The roles of axial dispersion and wall heat losses are considered.

Extending the validity of lumped capacitance method for large Biot number in thermal storage application

In a typical thermal energy storage system, a heat transfer fluid is usually used to deposit/extract heat when it flows through a packed bed of solid thermal storage material. A one-dimensional model of the heat transfer and energy storage/extraction for a packed-bed thermal storage system has been developed previously by the authors. The model treats the transient heat conduction in the thermal storage material by using the lumped capacitance method, which is not valid when the Biot number is large. The current work presents an effective heat transfer coefficient between the solid and fluid for large Biot numbers. With the corrected heat transfer coefficient, the lumped capacitance method can be applied to model the thermal storage in a wide range of Biot numbers. Four typical structures for the solid thermal storage material are considered. Formulas for the effective heat transfer coefficient (and effective Biot number) are presented. To verify the prediction by the lumped capacitance method using the effective heat transfer coefficient, we compare the results to the corresponding analytical solutions. The results are in very good agreement. The effective heat transfer coefficient extended the validity of the lumped capacitance method to large Biot numbers, which is of significance to the analysis of thermal energy storage systems.

Performance and efficient design of packed bed thermal storage systems. Part 1

A simple model is presented which provides a qualitative understanding of axial dispersion in packed beds. In Part 1 of this paper the model is verified by comparison with results in the literature. Experimental verification will be given in Part 2. Temperature wave front velocities, storage tank geometry, the significance of natural convection, and convection coefficient instabilities are also discussed. The efficient design of packed bed thermal storage systems is emphasized.

Packed bed thermal storage systems

Experiments were conducted on a vertical flow packed bed sensible thermal storage system. The packing consisted of hollow high density polyethylene spheres filled approximately 95% with water, and water was also used as the working fluid. Experimental results show the effect of altering D E (the equivalent sphere diameter of the packing) on the degree of axial dispersion in thermally short packings. The significance of thermally short versus thermally long packings to designers of sensible storage packed bed systems is discussed. Experimentally measured average temperature wave velocities in the packing are presented. The significance of natural convection during heat exchanger operation is also demonstrated experimentally. Natural convection may explain instabilities in packed bed convection coefficients reported in the literature. It is shown that one-dimensional temperature profiles in the packing can be obtained using rectangular storage tanks in conjunction with flow distributors.

Performance and efficient design of packed bed thermal storage systems. Part 1

Performance and efficient design of packed bed thermal storage systems. Part 1

Non-Archimedean models of mixing—II. A nearly dispersional model for packed bed thermal storage systems

A non-Archimedean mathematical model for simulating the transient characteristics of heat transfer in a packed bed thermal storage system is presented. The use of Hermite polynomials is proposed for the analysis of systems which exhibit many scales of heat transfer. The obtained solution is an expansion in which the first term corresponds to the axial dispersion. A comparison of experimental data reported from literature with the numerical prediction shows a good correlation.

Transient analysis of packed-bed thermal storage systems

A model, including longitudinal thermal dispersion and intraparticle conduction, is developed to describe the transient response of a packed column with a time-varying inlet temperature. The model can be applied to a diabatic column with uniform and constant ambient temperature, or to a perfectly insulated system. The expression for the temperature response within the column as a function of time and axial position is obtained by expanding the solution to an impulse of heat at the inlet in terms of either Hermite or Laguerre polynomials. The response to an arbitrary inlet temperature transient, found via a convolution integral, is expressed as a single quadrature. The response to a step input is an algebraic expression.

Dynamic response of a packed bed thermal storage system—a model for solar air heating

A mathematical model for simulating the dynamic temperature response of a packed column to an arbitrary time-dependent inlet air temperature is developed. The model includes axial thermal dispersion as well as intra-particle conduction, features that have usually been neglected but can be important in solar energy applications. Solutions, presented in terms of moments of the temperature response to an impulse of heat at the inlet, can be evaluated by simple numerical quadrature. Results of the model compare favorably with experimental data found in the literature. The model is used to optimize heat storage in a rock bin system subject to a realistic transient inlet temperature.

Transient analysis of packed-bed thermal storage systems

A one-dimensional single-phase conductivity model of packed beds in which air and rock are at the same temperature is used to develop closed-form analytical solutions for the transient responses produced by time-varying air inlet temperatures. In particular, the single-blow or step response of this simple conductivity model is compared with the classical Schumann two-phase model which ignores axial conductivity. The close agreement exhibited at sufficiently large time justifies the development of approximate equivalent models that combine the effects of air rock heat transfer and axial conductivity.