Bed Thermal Energy Storage System Research Articles

An advanced wall treatment for a 1D model of packed bed thermal energy storage systems

Abstract Packed bed thermal energy storages are used to store the thermal energy conveyed by a heat transfer fluid within a porous media. The container wall plays an important role in the thermodynamic behaviour of these systems, because of its thermal inertia and the heat losses towards the ambient. Within the many numerical models existing in the literature for these energy storage systems, the wall is either solved, resulting accurate but expensive in computational resources, or modelled, resulting light and fast but often limited to particular configurations. In the present work a quasi-1D model, called athens , is developed using an advanced wall treatment with the aim of being faster than the most complex models and, at the same time, more accurate than the simplest. The wall behaviour is then coupled to the fluid and the packed bed equations, using equivalent properties computed from the solution of the conduction equation in the radial direction only. This new approach is validated and compared with a more common two-dimensional solution of the wall. The results show a notable improvement in the computational time and only a small decrease in accuracy, to the point that particular cases had to be specially prepared in order to highlight the differences between the models.

An analysis of a packed bed thermal energy storage system using sensible heat and phase change materials

Thermal energy storage is an important subsystem of a solar thermal power station. Compared with the two-tank storage system, the packed bed storage system uses a single tank to store thermal energy temporarily and release it when the energy is needed. The temperature distribution in the packed bed is generally in the form of the thermocline. This study analyzed a thermocline storage system packed with sensible heat material and phase change material. These materials were characterized separately in terms of temperature difference, outlet temperature profile, total energy storage, and charging time. Results demonstrated the presence of a maximum temperature difference in the middle of the thermocline. The maximum value of the temperature difference decreased following an increase in the charging time. The density and conductivity of the sensible material and the inlet velocity of the fluid considerably affected the temperature difference. In the phase-change thermal storage system, the temperature difference between the material and the fluid exhibited two peaks, wherein the first peak was higher than the second peak. The total storage charging time and storage energy increased following an increase in latent heat. The total storage charging time was reduced following a decrease in melting point, although the final thermal energy storage remained the same.

Numerical analysis of thermal energy storage system using packed-bed

ABSTRACTSolar energy offers an affordable solution to cooking and other household problems of day-to-day life. However, its intermittent nature limits its applications and hence storage is required for the same. In this paper, the numerical simulation of a thermal energy storage system (TES) has been presented. In this analysis, a model is built to estimate the charging and discharging time of packed bed thermal energy storage (PBTES) system. The model consists of a cylindrical container filled with a concrete pebble. The heat transfer fluid used is air passed through the packed-bed. Several parameters are modified to study the impact of pebble diameter and void fraction over the charging and discharging time of PBTES system. The system is built to store energy for18 h with a temperature range between 300 and 120°C, so that it delivers 2.5 kW after charging.

A porous medium based heat transfer and fluid flow model for thermal energy storage in packed rock beds

Thermal energy storage in packed rock beds helps to reduce energy costs and carbon footprint’s on an industrial, commercial and residential scale. Fluid flow and heat transfer in large-size packed rock beds used in mining applications such as heating/cooling of mine intake air or ventilation of block-caved mines have recently received significant attention. Understanding the porous structure of such packed rock beds is a necessity in the design of such systems. The pressure drop across a rock bed directly affects its heat exchange performance, as it requires additional fan power to circulate air during periods of storage/extraction. In this study, the fluid flow behavior inside a packed rock bed thermal energy storage system is investigated by developing a computational fluid dynamics model and a heat transfer model. The model offers useful information for evaluating the performance of rock beds packed with large rocks or in caved zones. Finally, the main goal of this study is to perform a practical energy saving analysis for porous media composed of large particles by changing the physical properties of the porous medium, such as porosity and permeability. The findings of this study also show that while the total thermal energy storage capacity of the system is not significantly affected by the mass flow rate, a lower mass flow rate can provide a longer working period for thermal energy storage systems.

Evaluation of the thermal performance of the air-rock bed solar energy storage system

The packed bed of rocks using air as a heat transfer fluid represents the suitable thermal energy storage system for solar air heaters. The aim of this study is to describe the thermal behaviour and to assess the thermal performance of the air/rock bed thermal energy storage system during the discharge process. A transient two-phase numerical model is developed and successfully validated with experimental data. A detailed study was carried out to investigate the effect of mass flow rate and the number of consecutive thermal cycles charge/discharge on the thermal behaviour of the rock bed, the outlet temperature of air and recovered energy during the discharge process. The obtained results help to understand the thermal behaviour and to evaluate the performances of the storage system during the discharge process which has not been studied carefully as the charging phase despite its great importance.

Investigation on the thermal performance of a high temperature packed bed thermal energy storage system containing carbonate salt based composite phase change materials

This paper concerns the thermal performance of a high temperature packed bed thermal energy storage (TES) system containing carbonate salt based composite phase change materials (CPCMs) that made of a eutectic carbonate salt of NaLiCO3 (phase change material, PCM), MgO (ceramic skeleton material, CSM) and graphite flakes (thermal conductivity enhancement material, TCEM). A rectangular packed bed configuration containing CPCMs bricks is built and a three-dimensional computational model is established to study the thermal performance of the system. The enthalpy-porosity approach and surface-to-surface (S2S) radiation model are respectively adopted to model the phase change process and the radiation heat transfer inside the system. A ferric oxide is also used as the sensible heat storage material to compare with the CPCMs based system. The numerical model is first compared with the published experimental data and reasonably good agreements are obtained, indicating the confidence of the model. Extensive modelling is then performed under different conditions to investigate the effects of various parameters including the radiation heat transfer, TCEM mass loading and heat transfer fluid (HTF) operation conditions on the system performance. The results indicate that the system containing CPCMs shows better charging and discharging performance in comparison with the system containing ferric oxide due to the large energy storage density and high thermal conductivity. The thermal radiation has an important influence on the system performance. The system heat transfer efficiency is apparently enhanced when the radiation heat transfer influence is taken into consideration. When the emissivity is at δ = 1, the total charging period of the system is respectively shortened by 10.6% and 25.7% than that of the emissivities at δ = 0.5 and δ = 0. The use of TCEM in the CPCMs significantly enhances the heat transfer performance of the system. An increase in the TCEM loading from 0% to 30% respectively leads to the reduction in charging and discharging processes by almost 30.3% and 29.2%. The results also indicate that, for a fixed charging/discharging power, both the overall charging and discharging periods of the system decrease with the increase of Re number or decrease of Ste number since an increase in the Re number (decrease in the Ste number) leads to an overall enhancement of the heat transfer between the HTF and the CPCMs bricks and hence an overall improvement in the charging and discharging rates.

Thermodynamic modeling of a solar energy based combined cycle with rock bed heat storage system

In the current study, a combined system which consists of gas and steam turbines supported by concentrating solar receiver is studied through a thermodynamic approach using energy and exergy methods. Because of the intermittent nature of solar energy, a rock bed thermal energy storage system is incorporated into the system. The rock bed storage allows the secondary cycle to operate independently in order to generate electricity at night or once the solar energy is not available. Charging capacity of the rock bed is 9 MW for an 8 h period of time. The rock bed stores thermal energy for 4 h and thereafter, discharges it to run the steam cycle for 12 h. The energy and exergy efficiencies of the system are investigated through specific parametric studies to examine the effects of changing state properties and working conditions on energy and exergy efficiencies. According to the performance evaluation, the gas turbine could generate 3.87 MWe power while 1.76 MWe power is generated by the steam turbine. The overall energy and exergy efficiencies are obtained to be 69.2% and 37.3%, respectively.

Performance analysis of a solar dryer integrated with the packed bed thermal energy storage (TES) system

This study presents the energy and exergy-based performances of a solar dryer integrated with packed bed (TES) as thermal energy storage medium. As a sample application, drying kinetics of orange slices was determined. The aim of this study is to evaluate the thermal storage potential of the packed bed by focusing on energy consumption and exergy-sustainability indicators. Experiments were repeated twice a day (sunshine hours and off-sunshine hours). The results indicated that solar dryer integrated with packed bed reduced the moisture content of orange slices from 93.5% to 10.28% (at the first experiment) and 10.76% (at the second experiment), respectively. Total useful energy consumption for both the two status were detected as 89.892 MJ and 88.11 MJ, respectively. The exergy efficiency for the drying system during the sunshine hours ranged from 50.18 to 66.58%.The exergy efficiency of the drying process, in case of using the stored thermal energy, also changed between 54.71 and 68.37%. Moreover, a mathematical model was developed to predict the change of the moisture ratio of orange slices during time. According to the results of the model, Modified Henderson and Pabis Model presented the optimum parameters for determining the drying kinetics of orange slices.

Shell-and-tube or packed bed thermal energy storage systems integrated with a concentrated solar power: A techno-economic comparison of sensible and latent heat systems

A detailed techno-economic comparison—using annual, transient integrated system modelling—was conducted for sensible and latent heat thermal energy storage (TES) systems. As the most viable near-term competitors, thermocline/packed-bed and shell-and-tube configurations were compared to the conventional two-tank molten salt system. Concrete and a range of phase change materials (PCMs) were considered as the storage mediums in this study. All analyses were conducted for 15 h of storage capacity for the 19.9 MWe Gemasolar concentrated solar power (CSP) plant as a real-world case study.It was found that when the CSP plant is constrained to the operational within the tight bounds of the standard two-tank system, the dual-media thermocline (DMT) system with concrete or encapsulated PCM shows the best performance. Imposing rigid operational boundaries significantly disadvantages shell-and-tube (ST) systems (e.g. resulting in up to ∼50% less annual electricity production than a CSP plant with two-tank system). However, the results of this study reveal that extending the TES charge and discharge cut-off temperatures closer to the plant’s maximum and minimum operating range (i.e. 800 K and 650 K) can maximize the potential of dual-media TES alternatives. Under these loose operational conditions, the CSP plant will be required to occasionally operate at conditions which are far from its nominal design point (i.e. up to 75% variation). This flexibility allows all the TES alternatives considered in this study to achieve similar CSP annual electricity output as the two-tank system. In this case (e.g. all TES systems achieve the same annual output), the TES alternatives can be compared based on their specific costs rather than their levelized cost of electricity. Compared to the specific cost of two-tank molten salt systems, ∼24.5 US$ kWhth−1, a 62% reduction of specific storage cost was found to be achievable with concrete storage a dual-media thermocline (DMT) system, representing the best techno-economic option. This was followed by 49% cost reduction for a pipeless shell-and-tube (ST) system incorporating concrete or a PCM with 1 mm pipe thickness. Without minimizing the capsule thickness to 0.1 mm, the packed bed system with PCM would never have economic justification. Overall, this study reveals that if CSP systems can be designed to have more flexibility in their operational temperature range, a significant cost savings is available in moving to alternative TES systems.

Operation strategies guideline for packed bed thermal energy storage systems

Operation strategies guideline for packed bed thermal energy storage systems

A novel effective thermal conductivity correlation of the PCM melting in spherical PCM encapsulation for the packed bed TES system

Packed bed thermal energy storage (TES) system with phase change material (PCM) encapsulation is one of the potential TES technologies for the solar thermal application. In present study, the constrained melting process of PCM in spherical encapsulation is simulated by a validated natural convection included model. This model is then used to simulate the melting processes of NaNO3 with different conditions. Meanwhile, the same melting processes are also calculated by the conduction controlled model with available effective thermal conductivity correlations reported in the literature. It is found that the changes of liquid fraction obtained from the natural convection included model and the conduction controlled model with the reported thermal conductivity correlations show obvious difference. Then, based on the simulating results of the cases using the natural convection included model, a new correlation for the effective thermal conductivity is proposed by linear regression. It shows that the conduction controlled model with the proposed effective thermal conductivity correlation gives better results on the change of liquid fraction than that with correlations reported in the literature. Therefore, the proposed correlation can be used to calculate the melting process of PCM in a capsule by the conduction controlled model, which is in urgent need for modeling of packed bed latent TES systems.

English

English

Investigation of the dynamic characteristics of a thermal energy storage unit filled with multiple phase change materials

In order to improve the thermal performance of thermal energy storage systems, a packed bed thermal energy storage systems unit using spherical capsules filled with multiple phase change materials (multi-PCM) for use in conventional air-conditioning systems is presented. A 3-D mathematical model was established to investigate the charging characteristics of the thermal energy storage systems unit. The optimum proportion between the multi-PCM was identified. The effects of heat transfer fluid-flow rate and heat transfer fluid inlet temperature on the liquid phase change materials volume fraction, charging time and charging capacity of the thermal energy storage system unit are studied. The results indicate that the charging capacity of multi-PCM units is higher than that of the conventional single-PCM (HY-2). For proportions 0:1:0, 2:3:3, 3:2:3, 3:3:2, 4:1:3, and 4:2:2, the charging capacity decreases by approximately 24.84%, 14.69%, 6.47%, 3.82%, and 1.13%, respectively, compared to the 4:2:2 proportion. Moreover, decreasing the heat transfer fluid inlet temperature can obviously shorten the complete charging time of the thermal energy storage systems unit.

Applicability of the local thermal equilibrium assumption in the performance modelling of CSP plant rock bed thermal energy storage systems

Gas-solid packed beds have been widely studied as a cost-effective means of thermal energy storage in concentrating solar power (CSP) plants. Typically, the operation of packed beds in such systems is modelled by accounting for a finite rate of heat transfer between the fluid and solid media. This approach requires the coupled solution of the fluid- and solid-phase energy equations, which is computationally-costly, especially for year-long performance simulations. The local thermal equilibrium assumption, which assumes an infinite inter-phase heat transfer rate, can be applied to reduce the complexity and thus computational cost of packed bed models. However, the implications of making such an assumption in the context of CSP thermal energy storage system performance modelling is poorly understood. In fact, the application of the approach in long-term simulations has not been investigated before. This work addresses the topic by comparatively evaluating the performance of local thermal equilibrium and local thermal non-equilibrium models in the annual simulation of an air-rock packed bed, hypothetically operating in an open volumetric receiver CSP plant. The level of inter-model agreement is assessed in terms of annual bed exergy yield, bed blowing work, and plant power generation time. In addition, solution times are compared to establish the extent of computational cost savings. A parametric study examining the effect of variations in key bed design parameters on inter-model agreement is also conducted. The results obtained provide a clear indication of the strengths and weaknesses of either modelling approach, as well as of the suitability of the local thermal equilibrium assumption in general.

CFD investigation of a sensible packed bed thermal energy storage system with different porous materials

CFD investigation of a sensible packed bed thermal energy storage system with different porous materials

Analysis of an integrated packed bed thermal energy storage system for heat recovery in compressed air energy storage technology

Compressed air energy storage (CAES) represents a very attracting option to grid electric energy storage. Although this technology is mature and well established, its overall electricity-to-electricity cycle efficiency is lower with respect to other alternatives such as pumped hydroelectric energy storage. A meager heat management strategy in the CAES technology is among the main reasons of this gap of efficiency. In current CAES plants, during the compression stage, a large amount of thermal energy is produced and wasted. On the other hand, during the electricity generation stage, an extensive heat supply is required, currently provided by burning natural gas. In this work, the coupling of both CAES stages through a thermal energy storage (TES) unit is introduced as an effective solution to achieve a noticeable increase of the overall CAES cycle efficiency. In this frame, the thermal energy produced in the compression stage is stored in a TES unit for its subsequent deployment during the expansion stage, realizing an Adiabatic-CAES plant. The present study addresses the conceptual design of a TES system based on a packed bed of gravel to be integrated in an Adiabatic-CAES plant. With this objective, a complete thermo-fluid dynamics model has been developed, including the implications derived from the TES operating under variable-pressure conditions. The formulation and treatment of the high pressure conditions were found being particularly relevant issues. Finally, the model provided a detailed performance and efficiency analysis of the TES system under charge/discharge cyclic conditions including a realistic operative scenario. Overall, the results show the high potential of integrating this type of TES systems in a CAES plant.

Modeling of the drying process of apple slices: Application with a solar dryer and the thermal energy storage system

In this study a solar air heater was developed to determine the drying rate of apple (Golden Delicious). In order to provide the drying process continuously a packed bed thermal energy storage system was designed and manufactured. Also, a recuperator unit was used for waste heat recovery. In the recuperator unit, it is provided the mixing of the fresh air with the drying air whose moisture content increases, at a specified rate. So at the rate of 50–60% waste heat is recovered and reutilized in the drying system. The advantage of the system is that it consumes less energy at the rate of 76.8% than other drying technologies. Experiments on the drying system were repeated twice a day. In the experiments the drying kinetics of apple slices (5 ± 2 mm thick) was determined at constant temperatures ranging from 50−60°C. Also, Diffusion Approximation model was used for drying of apple slices in constant temperature. A mathematical model was developed to calculate the prediction of the moisture ratio during time and it is observed that the model is complied with the studies in the literature.

Experimental study using different heat transfer fluid of a packed bed thermal energy storage system during charging process

Heat transfer analysis of a packed bed-PCM latent heat thermal energy storage system is presented in this article. The cylindrical packed bed is filled with spherical capsules of PCM (water) that is used for an air conditioning commercial application. In the study, the experimental model is tested to analyse the thermal performance of packed bed-PCM latent. The results obtained are used for the thermal behaviour of charging process. The main objective is to compare the loading time using as heat transfer fluid (HTF) ethylene glycol and ethanol. The effects of mass flow rate, freeze rate and inlet HTF temperature are investigated. The results of the study indicate that changing the HTF type and inlet temperature have different effects on total time in charging process.

Study of the Performance of Paraffin Wax as a Phase Change Material in Packed Bed Thermal Energy Storage System

The present work deals with an experimental investigation of charging and discharging processes in thermal storage system using a phase change material PCM. Paraffin wax was used as the PCM which is formed in spherical capsules and packed in a cylindrical packed column which acted as an energy storage system. Air was used as the heat transfer fluid HTF in thermal storage unit. The effect of flow rate and inlet temperature of HTF on the time of charging and discharging process were studied. The results showed that the faster storage of thermal energy can be made by high flow rate of heat transfer fluid HTF and high inlet temperature of heat transfer fluid. It was found that at 65°C HTF inlet temperature, the melting and solidification processes accelerated by 27.9% and 57.14% respectively, when the flow rate was increased from 9 to 24 L/s. Also, when the HTF inlet temperature changed from 65°C to 80°C, the time needed to complete melting process decreased by 38.8%.

Thermodynamic analysis of an improved adiabatic compressed air energy storage system

Energy storage technology is a cutting-edge research in the field of new and renewable energy application. In this paper we introduce the concept of an energy storage based on adiabatic compressed air energy storage (A-CAES) combined with packed bed thermal energy storage (PBTES) system. First, the system thermodynamic performance of a typical single cycle is discussed and the effect of PBTES heights is analyzed. The results show that an overall efficiency in excess of 49% is achievable and the PBTES heights have significant influence on the thermal behavior of PBTES, as well as the overall efficiencies. Because there is still heat energy remaining in the packed bed until the discharge process is terminated, an improved A-CAES system with a heat recuperator is further proposed. It is found that this improved system shows a promotion of ∼5% compared with the first present A-CAES system. The cycle efficiency of the improved system increases with the increase of continuous cycles, and then reaches a stable value of 56.74% after around 25 cycles. The main conclusions drawn from this work will be helpful for future development of a high-efficiency A-CAES system combined with PBTES.