Performance Of Thermal Energy Storage System Research Articles

Novel protic ionic liquids-based phase change materials for high performance thermal energy storage systems

Phase change materials (PCMs) are an important class of innovative materials that considerably contribute to the effective use and conservation of solar energy and wasted heat in thermal energy storage systems (TES). The performance of TES can be improved by using environmentally friendly PCMs called ionic liquids (ILs) based on ethanolamines and fatty acids. The 2-hydroxyethylammonium, bis(2-hydroxyethyl)ammonium, and tris(2-hydroxyethyl)ammonium palmitate ILs, which function is in the temperature range of 30–100 °C and provide a safe and affordable capacity, are introduced in this study for the first time as PCMs. PCMs' chemical composition and microstructure were examined using fourier transformation infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM), respectively. DSC was used to evaluate the ILs' latent heat of fusion and specific heat capacity, while TGA was used to establish their thermal stability. Finally, a home-made device with a PCMs (synthesized ILs) container cell and a commercial thermoelectric generator device to record the real-time voltage (V) was used to convert thermal energy into electrical energy.

Subcooling Effect on PCM Solidification: A Thermostat-like Approach to Thermal Energy Storage

Choosing the right phase change material (PCM) for a thermal energy storage (TES) application is a crucial step in guaranteeing the effectiveness of the system. Among a variety of PCMs available, the choice for a given application is established by several key factors, e.g., latent heat, stability, and melting point. However, phenomena such as subcooling—for which PCM cools in a liquid state below its solidification point—can lead to a reduction in the amount of energy stored or released, reducing the TES overall effectiveness, and also in some inaccuracies when modeling the problem. Thus, understanding the effects of subcooling on PCM performance is crucial for modeling and optimizing the design and the performance of TES systems. To this end, this work analyzes the PCM discharging phase in a cold thermal energy storage coupled to a chiller system. A first conduction-based predictive model is developed based on enthalpy–porosity formulation. Subcooling phenomena are encompassed through a control variable formulation, which takes its cue from the operation of a thermostat. Then, thermal properties of the PCM, i.e., the phase change range and specific heat capacity curve with temperature, are evaluated by using differential scanning calorimetry (DSC), in order to derive a second predictive model based on these new data, without including subcooling, for the sake of comparison with the first one. Experimental results from the storage tank confirm both model reliability and the fact that the PCM suffers from subcooling. Between the two numerical models developed, the first one that considers subcooling proves it is able to predict with satisfactory accuracy (RMSE < 1 °C) the temperature evolution on different tank levels.

A 1D Reduced-Order Model (ROM) for a Novel Latent Thermal Energy Storage System

Phase change material (PCM)-based thermal energy storage (TES) systems are widely used for repeated intermittent heating and cooling applications. However, such systems typically face some challenges due to the low thermal conductivity and expensive encapsulation process of PCMs. The present study overcomes these challenges by proposing a lightweight, low-cost, and low thermal resistance TES system that realizes a fluid-to-PCM additively manufactured metal-polymer composite heat exchanger (HX), based on our previously developed cross-media approach. A robust and simplified, analytical-based, 1D reduced-order model (ROM) was developed to compute the TES system performance, saving computational time compared to modeling the entire TES system using PCM-related transient CFD modeling. The TES model was reduced to a segment-level model comprising a single PCM-wire cylindrical domain based on the tube-bank geometry formed by the metal fin-wires. A detailed study on the geometric behavior of the cylindrical domain and the effect of overlapped areas, where the overlapped areas represent a deviation from 1D assumption on the TES performance, was conducted. An optimum geometric range of wire-spacings and size was identified. The 1D ROM assumes 1D radial conduction inside the PCM and analytically computes latent energy stored in the single PCM-wire cylindrical domain using thermal resistance and energy conservation principles. The latent energy is then time-integrated for the entire TES, making the 1D ROM computationally efficient. The 1D ROM neglects sensible thermal capacity and is thus applicable for the low Stefan number applications in the present study. The performance parameters of the 1D ROM were then validated with a 2D axisymmetric model, typically used in the literature, using commercially available CFD tools. For validation, a parametric study of a wide range of non-dimensionalized parameters, depending on applications ranging from pulsed-power cooling to peak-load shifting for building cooling application, is included in this paper. The 1D ROM appears to correlate well with the 2D axisymmetric model to within 10%, except at some extreme ranges of a few of the non-dimensional parameters, which lead to the condition of axial conduction inside the PCM, deviating from the 1D ROM.

Experimental and computational study of melting phase-change material for energy storage in shell and tube heat exchanger

The preservation of thermal energy is a significant problem that needs to be resolved. In the present energy demand situation, effective storage of available energy will be a technique to manage the gap between the demand and supply. Solar energy is one such energy, which is an easily obtainable and renewable source of energy. Still, the maximum employment of solar energy is not readily possible without the implementation of the thermal energy storage system (TESS). This TESS can be an integral part of solar applications, thus TESS acts as a reservoir of energy to deliver the energy requirement at all times. Hence, it is an appropriate system to unite the energy source, i.e., the sun and the applications. Thus, the TESS is very vital in solar applications. The main objective of this work is to design and analyze the vertical shell and tube type TESS using water as heat transfer fluid (HTF) and paraffin wax RT58 as phase change material (PCM), which is subjected to change its phase at 46 °C to 68 °C from solid to liquid when exposed to higher temperatures, and its operating temperature range is 60 °C to 80 °C. Here, PCM (RT58) is enclosed in the tubes of the shell and tube storage unit. The property of the high specific heat capacity of PCM is tested for utilization of latent heat storage that can be used any time as required. Temperature measurements are taken using a T-type thermocouple along with an indicator and a series of experiments were conducted to study the effects of PCM in TESS. PCM encapsulated heat exchanger performance is investigated for both charging and discharging cycles with 85 °C inlet water temperature and a constant flow rate of 0.04 kg/s. To analyze the melting characteristics of PCM, simulation is carried out using ANSYS Fluent in the workbench, and comparative simulation studies are carried out for quadrilateral and triangular node shapes for better accuracy with experimental data. The heat energy stored in the charging cycle is 302.327 kJ for 100 min duration, and in discharging cycle is 295.8 kJ for 56 min duration. The HTC and Nusselt number were also calculated for both the charging and discharging cycle. It is evident from the present work, that the PCM can act as a better alternate for energy storage medium compared with the present solar energy storage methods, ensuring the correct adaptability of PCM for the particular application. The heat transfer rate can be increased further by using more amounts of different PCM or PCM composite with higher latent heat and also by providing better tank insulation. In future work, emulsification of nanomaterial in the PCM or in the HTF, and implementation of finned tube profile can increase the overall performance of TESS.

Numerical model development for the prediction of thermal energy storage system performance: CFD study

A latent heat storage system to store available energy, to control excess heat generation and its management has gained vital importance due to its retrieve possibility. The design of geometry parameters for the energy storage system is of prime interest before experimentation. In the present study, a numerical investigation of 2D square enclosure filled with phase change material and discrete heating (Ld = 0.2 L, 0.4 L, 0.6 L, and 0.8 L) from the bottom while maintaining heater at constant heat flux has been carried out using the finite volume method. The enthalpy- porosity method was employed to model the phase change material melting process and optimum heater location predicted by solving fluid flow and heat transfer governing equations. Validation studies were conducted for two different geometries square and rectangular subjected to different boundary conditions. The results of the present work are depicted in terms of isotherms, liquid fraction, local phase change material temperature, and average phase change material temperature. It is observed both Ld = 0.2 L and 0.4 L locations have ensured the complete melting rate than other Ld = 0.6 L, 0.8 L locations. Moreover, energy stored by phase change material while heater at Ld = 0.4 L, 0.6 L, 0.8 L is decreased by 9.33%, 50.16%, and 53.05% respectively than compared to Ld = 0.2 L. Thus, the developed numerical model predicts that the enclosure type latent heat storage system is sensitive to the heater location for a given boundary condition.

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.

Experimental investigation on latent heat thermal energy storage system for stationary C.I. engine exhaust

Increasing gap between demand and supply of energy has resulted in raised prices of conventional energy sources, and concern for clean environment has led to increase in use of renewable energy and focus on increased energy efficiency. In context of increased energy efficiency, energy storage and waste heat recovery play an important role as these systems improve overall efficiency of systems. In this study, a latent heat thermal energy storage system (LHTESS) for stationary C.I. engine exhaust heat was developed and integrated with engine. A shell and tube type heat exchanger having 346mm diameter and 420mm height with 45 numbers of tubes with 18mm diameter was developed to store thermal energy, in which Erythritol (C4H10O4) was used as a phase change material. The engine performance and the thermal energy storage system performance parameters such as amount of heat stored, and charging efficiency were evaluated. Slight decrease in the engine performance was observed when latent heat thermal energy storage system was integrated to engine but amount of energy which could be recovered was significant. At a load of 4.4kW the maximum charging efficiency, recovery efficiency and percentage energy saved was 69.53%, 38% and 11.33% respectively.

Adsorption Prediction and Modeling of Thermal Energy Storage Systems: A Parametric Study

Thermal energy storage allows for the storage of energy from intermittent sources to correct for the variable supply and demand. The current work investigates adsorption technology for thermal energy storage through the development of a theoretical model, which describes the material and energy transfers in the system. The theoretical model was used to conduct a parametric study which examines the effect of column dimension, particle diameter, adsorption activation energy, flow rate, column void fraction, and adsorbent heat of adsorption on the thermal energy storage system performance. It was found that an optimal column length to column diameter ratio of 1.4, a column diameter to particle diameter ratio of 14.7, a flow rate of 24 LPM, and a void fraction of 0.4 gave the best thermal energy performance for a column volume of 6.275 × 10–5 m3. Also, a low activation energy and a high heat of adsorption represent the best adsorption parameters for optimal temperature outputs, breakthrough behavior, and ener...

The Effect of PCM Capsule Material on the Thermal Energy Storage System Performance

Phase change material (PCM) based thermal energy storage (TES) systems are gaining increasing importance in recent years in order to reduce the gap between energy supply and demand in solar thermal applications. The present work investigates the effect of PCM capsule material on the performance of TES system during charging and discharging processes. The TES unit contains paraffin as PCM filled in spherical capsules and is integrated with flat plate solar collector. Water is used as sensible heat material as well as heat transfer fluid (HTF). The PCM capsules are of 68 mm diameter and are made using three different materials, namely, (i) high density polyethylene (HDPE), (ii) aluminum (Al), and (iii) mild steel (MS). The experimental investigation showed that the charging and recovery of stored energy are less affected by the spherical capsules material. The variables, like charging time and discharging quantity, are varied around 5% for the different capsule materials. Even though aluminum thermal conductivity is much higher than HDPE and mild steel, its influence on the performance of TES system is very low due to the very high internal heat resistance of PCM material stored in the spherical capsules.

Numerical analysis and evaluation of an open-type thermal storage system using composite sorbents

A family of composite sorbents has been acknowledged as promising thermal storage materials for low grade thermal energy storage owing to its high specific storage capacity and low regenerating temperature. This paper reports a simplified numerical model aiming to determine the dynamic characteristics of the composite sorbents and evaluate the specific capacity and COP of the open-type thermal energy storage system. The computational results were validated with the experimental measurements carried out on an open-type thermal energy storage set-up filled with composite sorbents. By using the simplified numerical model, the dynamic characteristics of the composite sorbents in the thermal energy storage process were determined. The effects of the composite sorbents and the operating parameters on thermal energy storage system performance were also evaluated.

Use of thermal energy storage for sustainable buildings

Thermal energy storage (TES) systems are examined from the perspectives of energy, exergy, environmental impact, sustainability and economics, with a focus on how they can help make buildings more sustainable. Reductions in energy use and environmental emissions through TES are discussed in detail and highlighted with a case study. The case study demonstrates that TES exergy efficiencies normally are lower than energy efficiencies due to exergy destructions and losses. The importance of using exergy analysis to obtain more realistic and meaningful assessments than are obtained with energy analysis of the efficiency and performance of TES systems is demonstrated. The results indicate that TES can play a significant role in achieving more efficient, environmentally benign, sustainable and economic energy use in buildings, and appears to be an appropriate technology for addressing the mismatches that often occur between times of energy supply and demand.

Economic Analysis of Thermal Energy Storage Systems

This paper describes an investigation of the economic performance of a central chiller plant with thermal energy storage (TES) operating in a hotel in California and an office building in Wisconsin for the following three conventional control strategies: chiller-priority, constant-proportion, and storage-priority control. Since all control strategies depend on properly selected design parameters, the storage and chiller capacities as the primary design parameters were varied over a wide range, and the life-cycle economic feasibility of each possible design was assessed over a 20-year economic life using the net present value method. Measured cooling and weather data for both buildings were used in annual calculations to determine annual utility cost savings on the basis of one of four artificial rate structures. Trends in the life-cycle economic performance of TES systems as a function of chiller and storage capacities were revealed, and design guidelines were developed.

Energetic, environmental and economic aspects of thermal energy storage systems for cooling capacity

Thermal energy storage (TES) systems for cooling capacity and their applications are examined from the perspectives of energy savings, environmental impact and economics. Reductions possible through TES in energy use and environmental pollution levels are discussed in detail and highlighted with illustrative case studies of actual systems. The importance of using exergy analysis to obtain more realistic and meaningful assessments than the conventional energy analysis of the efficiency and performance of TES systems is demonstrated. The results indicate that cold TES can play a significant role in meeting society’s preferences for more efficient, environmentally benign and economic energy use in various sectors, and appears to be an appropriate technology for addressing the mismatches that often occur between the times of energy supply and demand.