Thermal Energy Storage Device Research Articles

Optimal rule-based control for the management of thermal energy storage in a Canadian solar district heating system

Solar district heating systems are a promising solution to facilitate the adoption of large-scale, solar energy-based technologies. Thermal energy storage devices could also play an instrumental role to manage the mismatch between solar energy resources and community heating loads; however, controlling such systems is no easy task. In this paper, a novel heuristic reactive control strategy is developed to properly manage the thermal energy storage capacity, at both the short-term and the long-term, in a Canadian solar district heating system (the Drake Landing Solar Community). This strategy targets the optimization of: (a) the temperature difference across solar collectors and (b) the flow rate modulation of the variable speed pump linking the short-term and the seasonal thermal energy storage devices. A simulation is used to compare this new control approach with the conventional control strategy currently in place. Results are assessed for the year 2017–18 in terms of energy, cost and greenhouse gas emissions. Despite a 10% increase in natural gas consumption, electricity use was decreased by 43%; reductions of 34% and 29% were respectively achieved in terms of energy cost and GHG emissions.

Investigation on charging enhancement of a latent thermal energy storage device with uneven tree-like fins

The charging intensification of latent thermal energy storage (LTES) devices has an important meaning for solar energy applications. For a more uniform temperature and faster melting rate of LTES devices, uneven tree-like fins are applied and optimized here. Numerical research of melting behaviors in tree-like finned LTES devices is performed with a particular focus on fin layouts. The melting interface transformation and transient temperature distribution in four LTES devices are examined. Moreover, the roles of fill angle and central-angle gradient in melting performance are studied for the optimization of uneven tree-like fins. The results indicate that uneven tree-like fins considerably improve energy charging performance since the heat conduction enhancement exceeds the free convection suppression. Compared with uniform tree-like fins, the uneven tree-like fins lead to a more uniform temperature distribution and consistently faster melting rate by the synergetic improvement of free convection and heat conduction in LTES devices. More significantly, the optimal fill angle and the central-angle gradient are suggested to be 300° and 8° for maximizing the melting performance. Correspondingly, the complete melting time of the optimal LTES devices with heterogeneous and gradient tree-like fins respectively decrease by 49% and 46% compared to LTES devices with uniform tree-like fins.

Study on Optimal Operation for Concentrating Solar Power Plants Considering Wind Power Reduction

Concentrating solar power (CSP) generation units, which can generate electricity without any pollutant emissions, is one of the most attractive alternatives to fossil fuels. However, the main disadvantage of the CSP device is the high investment cost. In order to improve the economic feasibility, it is necessary to properly design the thermal energy storage (TES) device to optimize its economic benefits. In this paper, considering the operation requirements of power system, the optimal co-allocation model of solar energy field and TES device is established to weigh the investment cost and wind power reduction. Through a case study, the effectiveness of using energy storage to improve wind power consumption is verified on the basis of considering the constraints of the actual operation of the system, solar radiation and wind energy uncertainty.

Virtual testbed for model predictive control development in district cooling systems

Recently, with increasing cooling demands, district cooling has assumed an important role as it is more efficient than stand-alone cooling systems. District cooling reduces the environmental impact and promotes the use of renewable sources. Earlier studies to optimise the production plants of district cooling systems were focused primarily on plants with compressor chillers and thermal energy storage devices. Although absorption chillers are crucial for integrating renewable sources into these systems, very few studies have considered them from the cooling perspective. In this regard, this paper presents the progress and results of the implementation of a virtual testbed based on a digital twin of a district cooling production plant with both compressor and absorption chillers. The aim of this study, carried out within the framework of INDIGO, a European Union-funded project, was (i) to develop a reliable model that can be used in a model predictive controller and (ii) to simulate the plant using this controller. The production plant components, which included absorption and compressor chillers, as well as cooling towers, were built using the equation-based Modelica programming language, and were calibrated using information from the manufacturer, together with real operation data. The remainder of the plant was modelled in Python. To integrate the Modelica models into the Python environment, a combination of machine learning techniques and state-space representation models was used. With these techniques, models with a high computational speed were obtained, which were suitable for real-time applications. These models were then used to build a model predictive control for the production plant to minimise the primary energy usage. The improvements in the control and the resultant energy savings achieved were compared with a baseline case working on a standard cascade control. Energy savings up to 50% were obtained in the simulation-based experiments.

Discharging process and performance of a portable cold thermal energy storage panel driven by embedded heat pipes

By recent interest in overcoming heat transfer limitations of phase change materials (PCMs), latent thermal energy storage devices that assisted by embedded heat pipes (HPs) have received much attentions. Due to coupling with HPs, phase change process of PCMs can be enhanced and be better managed. In this study, a cold thermal energy storage panel for portable applications that embedded HPs into PCM is investigated experimentally. An effective way to control the cold energy supply of the panel is to control air temperature and air velocity. However, air properties varied with temperature as well as layout of HPs in the airside have influence on energy supply, which is implicit to the control of the panel. Therefore, the average effectiveness-NTU method is utilized to indicate the connection between air properties to the cold energy supply performance. The model validation results show that the effectiveness-NTU technique can describe the variation of refrigeration capacity with inlet air temperature and air velocity. The calculation error of the model is below ±9.0%. This method can be used as a design tool and is expected to provide a framework for designing portable cold thermal energy storage device with manageable cold energy supply.

Development of reduced order thermal dynamic models for building load flexibility of an electrically-heated high temperature thermal storage device

A control-oriented model of an Electrically heated Thermal Energy storage device (ETS) is presented. The ETS consists of bricks heated up to 871 °C with electric coils; heat is discharged with an airflow passing through channels in the bricks. The device stores energy during off-peak periods to meet future on-peak building heating loads. While its rationale is understood, improvement regarding operation strategies is needed. The charging/discharging could be adjusted according to predicted heating loads to reduce bills, and enhance building energy flexibility in its interaction with the grid. The model, to facilitate implementation of model predictive control (MPC), is based on the following features: (a) calibration of grey-box resistance-capacitance models with a heat exchanger model; (b) periodic state updates from sensors; (c) “effective” brick conductivity. A 1-capacitance model with state updates and “effective” brick conductivity was comparable to a detailed 140-capacitance model. Therefore, a simple model with modifications gives adequate predictions and thus can aid with MPC. Finally, strategies are evaluated to study energy flexibility potential. Using the ETS during the morning critical demand hours and anticipating the rise in setpoint temperature from nighttime setback, peak reduction 11% is obtained. A reduction of 73% is found when HVAC heating coil is limited.

Experimental Work on Cold Starting Emission of CI Engine using Engine Waste Heat Energy

In the present day, emission by internal combustion engines causes several problems like acid rain, depletion of ozone layer, and global warming for which it has become a prior concern. The hazards further increases during cold weather conditions. In this paper, emission analysis has been carried out using single cylinder, 4 stroke, direct injection and water cooled variable compression ratio diesel engine. A thermal energy storage device (TESD) containing phase changing material (PCM) has been designed and tested for storing the waste energy of cooling water from engine and reutilizing it for pre-heating. The working principle of TESD is based upon the principle of absorbing and rejecting heat during phase change of PCM material. The test condition is 15° C and 1 atm pressure at which, the experiments are carried out using TESD. A significant reduction in CO (23.72%), HC (2.03%) and smoke opacity (6.05%) after 900secs and an increase in engine temperature upto 61ºC after 840secs of engine running is observed.

Performance enhancement of a phase-change-material based thermal energy storage device for air-conditioning applications

This work concerns performance enhancement of phase change material (PCM) based thermal energy storage (TES) devices for air-conditioning applications. Such devices have numerous potential applications in the building environment. The TES device often uses air as the heat transfer fluid and, as a result, its performance is often limited by heat transfer in either or both of the PCM and the air sides. This paper aims to overcome the heat transfer limitations through intensifying heat transfer using two methods of extending heat transfer surfaces (fins) in both the PCM and air sides and adding heat transfer enhancement materials in the PCM. First, a TES device with different configurations (no fins; offset strip fins on the air side only; straight fins on the PCM side only; offset strip fins on air side and straight fins on PCM side) and PCM with different thermal conductivities were modelled and compared. The comparison leads to an advantageous utilization of fins instead of adding thermal conductive particles. The results also indicated a significant extent of performance enhancement of the TES device due to the use of fins with the charging and discharging times reduced respectively by ~85% and ~74%. The airside fins were found to be more effective than the PCM side fins. The results also showed that the heat transfer enhancement due to the PCM side fins could be achieved by increasing PCM thermal conductivity. The modelling results were then validated by experimental data. Finally, a PCM-based TES device was designed and manufactured based on the above results. The device was integrated into an air-conditioning system and experimentally tested. The results showed that, for both charge and discharge processes, the stabilization of the outlet temperature of the air conditioning system was significantly improved, leading to an increased extent of thermal comfort and decreased outlet temperature fluctuations. The obtained results can be used as the design guideline of the compact TES device for air conditioning.

Development of a heat transfer coefficient based design method of a thermal energy storage device for transport air-conditioning applications

We studied the heat transfer characteristics of a phase change material (PCM) based thermal energy storage (TES) device for transport air conditioning applications. The charging and discharging times and the transient heat flux of the TES device were measured. These data allow quantification of the transient and overall heat transfer coefficients, thus a quantitative comparison between the overall heat transfer coefficients of the charging and the discharging processes. An attempt was made to process the overall heat transfer coefficient data to give the Nusselt number, the Reynolds number, and the Prandtl numbers for both the discharging and charging processes. An empirical relationship between these non-dimensionless numbers was then obtained, which was validated by further experiments. It was shown that the empirical expressions agree with the further experimental data within 2.33% and 1.65% respectively for the charge and discharge processes. To our best knowledge, this study represents the first effort to measure the transient behavior of a PCM based TES device, leading to a set of experimentally validated empirical expressions for the overall heat transfer coefficients.

Two-stage scheduling for island CPHH IES considering plateau climate

In this paper, an island integrated energy system (IES) combining power, heat and hydrogen (CPHH) is built, and the optimal power-heat-hydrogen scheduling is studied under actual plateau meteorological data. Considering the comprehensive demands of residents, an IES including renewable energy sources (RESs), fuel cell, electrolyzer, electric boiler, thermal energy storage device and new energy vehicles is established and operated. In the first stage of operation, according to the prediction of RESs and demands, the day-ahead optimization is carried out which aims to minimize the operation cost, while the electric vehicles (EVs) are considered as special demand response (DR) loads. As for the second stage of operation, the ultra-short-term prediction is implemented to provide prediction data for model predictive control (MPC), realizes the real-time operation. As CPHH system, fuel cell and electrolyzer work to meet various needs of the plateau residential area with high efficiency. In addition, the method and IES structure proposed in this paper is compared with other options.

Ragone plots and discharge efficiency-power relations of electric and thermal energy storage devices

Abstract Ragone plots (energy-power relations) and discharge efficiency-power relations are important for characterizing energy storage (ES) devices, as they contain the information on the maximum power and the available energy. In this theoretical study, these two characteristics and the losses per energy are derived in the framework of endoreversible thermodynamics for ideal electric and thermal ES systems with different dependencies of their intensive thermodynamic variables (potentials, temperatures) of the reservoirs on the extensive state variables (electric charge, heat) that quantify their state of charge. While for battery and latent heat storage device, the normalized Ragone plot is equal to the discharge efficiency-power relation, the two characteristics differ for electric capacitor and sensible heat storage device due to their intrinsically limited depth of discharge at constant power. Due to the decrease of the exergy of heat at decreasing temperature, the discharge efficiency of the ideal sensible heat storage device exhibits a local maximum at a finite power value.

Performance comparison of thermal energy storage system for indirect solar dryer with and without finned copper tube

A novel numerical model is developed to identify the effect of fins on thermal energy storage (TES) device of indirect type solar dryer (ITSD). Two models have been developed to study the above behavior, one being TES device without fins (case – I) and the second one with fins (case – II). CFD simulations were carried out for four air velocity conditions and a comparative study was performed in both cases. It was found that both cases work fine with an air velocity of 1 m/s compared to higher air flow velocities (2 to 4 m/s). Both systems can work up to 10.00 pm with the help of TES device. The average outlet temperature of case – I from 4.00 to 10.00 pm is 5.47 K higher than the inlet temperature and it is better (9.1 K) for case – II during the air velocity at 1 m/s. The average phase change material (PCM) temperature and melting fraction of case – II is 3.7 K and 6.44%, respectively, higher than case – I at 1 m/s. Maximum heat gained by the air for case – II is 55.2% more compared to case – I at an air velocity of 1 m/s.

Thermodynamic Analysis of Packed Bed Thermal Energy Storage System

A packed-bed thermal energy storage (PBTES) device, which is simultaneously restricted by thermal storage capacity and outlet temperatures of both cold and hot heat transfer fluids, is characterized by an unstable operation condition, and its calculation is complicated. To solve this problem, a steady thermodynamics model of PBTES with fixed temperatures on both ends was built. By using this model, the exergy destruction, thermocline thickness, thermal storage capacity, thermal storage time, and other key parameters can be calculated in a simple way. In addition, the model explained the internal reason for the change of thermocline thickness during thermal storage and release processes. Furthermore, the stable operation of the PBTES device was analyzed, and it was found that higher inlet temperature of hot air, and lower temperature difference between cold and hot air can produce less exergy destruction and achieve a larger cycle number of stable operation. The work can be employed as the basis of the design and engineering application of PBTES.

THE FUNCTIONING MODEL OF INTEGRATED ENERGY SUPPLY SYSTEM WITH CO-GENERATION UNITS OPERATION, TAKING INTO ACCOUNT PROSPECTS OF BIOENERGY DEVELOPMENT IN UKRAINE

In conditions of renewable energy sources development and implementation of modern technologies in the energy sector, energy from biomass in the structure of power supply systems is an actual topic. The development of bioenergy is an important part of ensuring the energy security of many countries, as it enables to reduce fossil fuel consumption, dependence on imported energy sources and ensure sustainable local energy supply. The article presents the functioning model of integrated energy supply system (energy hub) with co-generation units operation, taking into account prospects of bioenergy development in Ukraine. This paper studies the optimal operation problem of an energy hub with multiple energy sources to serve electricity and heat loads in the presence prices. The main task was solved to make full use of its own energy sources to meet the needs of consumers in energy carriers with the most effective financial performance. The availability of electric and thermal energy storage devices will allow more efficient usage of available energy resources and equipment. Usage of a cogeneration plant in the structure of the energy hub gives the opportunity to provide energy resources and the possibility of generated electricity trading. A multicriteria approach to the planning and optimization of the operation of such energy hub is proposed. According to the simulation results using the example of an energy hub, it is shown that this method is suitable for planning hourly energy consumption with a compromise in terms of the minimum cost of these energy resources and CO2 emissions.

Discharging performance enhancement of a phase change material based thermal energy storage device for transport air-conditioning applications

A compact thermal energy storage device containing a phase change material has been designed and experimentally investigated for smoothing cooling load of transport air conditioning systems. The phase change material based device used two different types of fins, serrated fins in the air side and perforated straight fins in the phase change material side, for enhancing the device performance. The focus of the work was on the discharging process of the compact device, which is more important for transportation applications. An experimental rig was designed and constructed for measuring the discharging behaviour of the device. The processing of the measurement data gave the discharging time, discharging depth, discharging power, accumulated discharging energy, thermal efficiency and exergy efficiency under different inlet air temperatures and velocities over the working period defined on the basis of the cooling requirement. The results show a fast response with the hot air flow cooled down to the desired temperature range of 16–20 °C in seconds. The thermal energy storage device showed a high thermal and exergy efficiency at 99.8% and 43.3%, respectively. The discharging depth was higher than 97%, indicating good heat transfer performance of the device. The PCM based device also showed flexibility for adjusting the output cooling capacity, leading to a great potential to resolving frequent fluctuations of cooling loads of transport air conditioning systems.

Design and validation of MEMS based micro energy harvesting and thermal energy storage device

An abundant source of energy is available in solar radiation which could be used for the generation of electrical energy. This research paper investigates a new method for the conversion of solar thermal energy into electrical energy. This method uses the Thermoelectric Generator (TEG) coordinated with the Phase Change Material (PCM). To enhance the performance of solar TEG, Solar radiation is focused on the Fresnel lens and solar concentrator. The output obtained from the solar concentrator is more than from the Fresnel lens. From the concentrator, 1.38 V is generated with a hot side TEG temperature of 195 °C and a cold side temperature of 62 °C in the day time and the off radiation maximum potential of 134 mv. The proposed system can harvest a maximum voltage of about 1.38 volts and 0.17Watts of power during day time.

Numerical analysis of heat transfer characteristics for air in a latent heat thermal energy storage using flat miniature heat pipe arrays

In the present study, the thermal performance of a latent heat thermal energy storage device based on flat miniature heat pipe arrays with straight rectangular fins during the charging process is numerically investigated using porous media to reduce computational resources and time. Air is selected as the heat transfer fluid (HTF). The influence of a thermal storage unit (TSU) with one heat transfer component (HTC) on the melting rates of the phase change material (PCM) is simulated at different inlet temperatures and flow rates of HTF. The heat transfer between two HTCs that form a tandem is analyzed and compared, and the effect of different arrangements of the two HTCs on the charging process is then studied. Results indicate that the inlet temperature and the volume flow rate of the TSU influence the charging process. The average outlet temperature and charging power, which grow in a power function relationship with the volume flow rates, increase linearly with the inlet temperatures. The average charging power of the HTF through HTC-2 is reduced by approximately 63.71% compared with HTC-1 when the two HTCs form a tandem. The average charging power of the TSU with two HTCs connected in tandem is higher than that of the parallel TSU at the same inlet temperature and volume flow rate.

On the Melting Process of the Phase Change Material in Horizontal Rectangular Enclosures

Phase change material (PCM) is one of the most important ways to store and manage energy. The melting process of PCM in a rectangular enclosure with the different aspect ratio is frequently related to some thermal energy storage devices. In this work, the melting of PCM in the horizontal rectangular enclosures heated from the different sides and the influence of aspect ratio of the rectangle are carefully studied. The enthalpy porosity technique and the finite volume method (FVM) are used to simulate the melting process numerically. The results show that the melting process of PCM can be dominated by conduction or natural convection due to the different heated sides. The melting of PCM in the enclosure heated from the bottom side is firstly affected by conduction and then mostly influenced by convection. In addition, the aspect ratio of the rectangular enclosure is found to play an important role in the melting process. Finally, a series of fitting correlations of the liquid fraction, Nusselt number and the energy storage are presented with the influence of aspect ratios in order to provide the reference for designing the rectangular container of PCM. This study is helpful for the selection of an appropriate aspect ratio and heating method to achieve the desired energy storage performance of encapsulated PCM.

Biomass retrofit for existing solar organic Rankine cycle power plants: Conceptual hybridization strategy and techno-economic assessment

This study aims to investigate techno-economic benefits of biomass retrofit for practical concentrated solar power (CSP) organic Rankine cycle (ORC) power plants. In this regard, technical parameters of an existing CSP-ORC plant currently running in Ottana (Italy) have been adopted for analyses. The plant consists of linear Fresnel collectors solar field, coupled with a two-tank oil-based thermal energy storage (TES) device, feeding a 630 kWe ORC plant. Fixed and modular biomass hybridization approaches were proposed for retrofit. In the fixed approach, biomass furnace constantly supplies thermal energy to ORC such that a pre-determined minimum power production is guaranteed, with or without solar thermal energy availability. In the modular approach, biomass furnace is regulated to supply balance thermal energy needed to continuously operate the ORC at its full capacity. Furthermore, beyond the biomass retrofit case study, a newly integrated design case study was introduced, with modified solar field and TES capacity. By using the meteorological conditions of Ottana, results of techno-economic performance for the retrofit case study showed that annual net electrical efficiency increases by 4–5 percent points, relative to the solar-only ORC plant. Marginal levelized cost of electricity (LCOE) of between 103 €/MWh and 109 €/MWh were obtained. Also, marginal net present value (NPV) ranges from 1.83 M€ to 3.22 M€. For the newly integrated design case study, results showed that design case with modular hybridization approach represents the highest annual net electrical efficiency, at 19.8%. For this approach, economic results showed that LCOE ranges between 130 €/MWh and 146 €/MWh. Beyond current state-of-the-art, the results obtained in this study show that biomass retrofit for existing CSP-ORC plants can lead to higher full-load operation hours and more prospective plant economics. Moreover, the implemented modular approach enables scheduled power profile to be followed, thereby enhancing plant dispatchability.

A comparative analysis of waste heat recovery systems in vehicles and their viability in real-world applications

In motor vehicles, an average of 60-70% of the overall fuel energy is dissipated primarily through the heated exhaust gases and engine coolant, which accounts for 90% of an engine’s thermal output. The fuel efficiency and environment impact of vehicles can therefore be improved by implementation of systems designed to recover this wasted energy. This meta- study explores the current methods for waste heat recovery (WHR) currently in production and research phases. A comparison is also made between the thermodynamic viability of each proposed system, from which future strategies to maximise the efficiency of WHR systems can be obtained. These include the use of the organic Rankine cycle (ORC), thermoelectric generators (TEG), and regenerative braking. The purpose of this paper is to analyse the current state of research for waste heat recovery in vehicles and therefore provide a basis for further research and investigation. The results indicate a promising future for further study of ORCs in the field of WHR for internal combustion engines (ICE) in vehicles. This is due to the various design opportunities that ORCs offer, including multiple loop configurations, different working fluids and integration of thermal energy storage devices. Current research for TEGs indicate a high cost to efficiency ratio for the materials required for production, meaning that TEGs are not as viable of a solution for WHR in vehicles relative to ORCs. This paper concludes that a fuel savings of 8-19% can be achieved through the integration of multiple energy recovery systems.
 Keywords: Waste Heat Recovery; Vehicles; Thermodynamics; Organic Rankine Cycle; Thermoelectric Generators; Regenerative Braking; Exergy