Thermal Energy Storage Charging Research Articles

Exploration of phase change material melt front during latent heat thermal energy storage charging

Latent Heat Thermal Energy Storage (LHTES) is a technology that utilises the phase change energy of a material to store energy and is used in multiple engineering disciplines. The use of this technology at high temperatures presents new material challenges that need to be considered. The exploration of Phase Change Material (PCMs) melting phenomena was completed with finite volume methods to model advection-diffusion interactions of different PCM classes undergoing pure substance melting. This study modelled a two-dimensional system with heat flux elicited by an isothermal wall – producing a melt front in the PCM. The investigation focussed on retention of a Solid Salt Barrier (SSB) between the liquid PCM and the storage tank wall. The importance of such a barrier is to mitigate the corrosion between the liquid PCM and the containment vessel. Results reveal the possibility of SSB retention through the adaptation of a LHTES system aspect ratio. A thermal storage inefficiency via unmelted PCM has been demonstrated and quantified when an SSB is maintained. Through varying geometry and PCM choice (carbonate, fluoride, and chloride eutectics), it was also shown that SSB retention is possible; however, when optimising the rate of internal enthalpy change, the corresponding aspect causes the SSB protected wall to be insignificant compared to exposed surfaces at the top and base. This paper provides insight into the implications of aspect ratio choice for thermal energy storage tank designs; as well, suggestions are made regarding exploration of divergent geometries to greater utilise the PCM energy storage capacity available in SSB systems.

An Experimental Demonstration of the Effective Application of Thermal Energy Storage in a Particle-Based CSP System

Tests were performed at the particle-based CSP test facility at King Saud University to demonstrate a viable solution to overcome the limitations of using molten salt as a working medium in power plants. The KSU facility is composed of a heliostat field, particle heating receiver (PHR) at the top of a tower, thermal energy storage (TES) bin, a particle-to-working fluid heat exchanger (PWFHX), power cycle (microturbine), and a particle lift. During pre-commissioning, a substantial portion of the collected solar energy was lost during particle flow through the TES bin. The entrained air is shown to be the primary cause of such heat loss. The results show that the particle temperature at the PHR outlet can reach 720 °C after mitigating the entrained air issue. Additionally, during on-sun testing, a higher temperature of the air exiting the PWFHX than that of the air entering is observed, which indicates the effective solar contribution. Half-hour plant operation through stored energy was demonstrated after heliostat defocusing. Lastly, a sealable TES bin configuration for 1.3 MWe pre-commercial demonstration unit to be built in Saudi Arabia by Saudi Electric Company (SEC) is presented. This design modification has addressed the heat loss, pressure build-up, and contamination issues during TES charging.

Optimal day-to-day scheduling of multiple energy assets in residential buildings equipped with variable-speed heat pumps

The largest parts of energy demand within the residential sector are used for space heating (SH), domestic hot water (DHW) and space cooling (SC). The thermal and electrical needs can be mainly covered by energy systems that consist of Photovoltaics (PVs), solar thermal collectors (STCs), heat pumps (HPs) and flexibility sources, e.g. battery energy storage (BES) and thermal energy storage (TES). In this paper, various control strategies for the optimal day-to-day operation of residential energy systems that consist of a variable-speed HP, STCs, PVs, as well as BES and TES units are examined. The proposed strategies are based on the concepts of self-consumption maximization (SCM) and time-of-use arbitrage (ToUA) considering priorities on BES and TES charging. A typical Greek single-family building is assumed examining multiple criteria for the assessment of the rule-based approaches. The results demonstrate that priority on TES charging can lead to higher system performance than on BES charging. Regardless of the number of ToU tariff zones, during off/mid-peak periods the upper energy needs of the SH/SC trajectory should be fulfilled only in case of excess PV power, otherwise the cost of imported energy increases significantly. Both SCM and ToUA strategies can lead to rates of self-consumption and self-sufficiency that deviate in average per about 4% from the optimal values. Moreover, the ToUA-based strategies can approach the minimum cost of imported energy with a relative increase of 22% compared to the optimum. However, the minimum thermal discomfort due to space temperature cannot be achieved without the prediction of the daily SH/SC demand. Furthermore, the proposed strategies can reach the minimum thermal discomfort due to DHW losses, while leading to lower BES degradation rates than the optimization. Finally, from the multi-objective optimization for the first year of system operation, it is verified that the optimal dimensioning of PV, BES and TES is considerably affected by the type of rule-based strategy selected.

Modeling of Combined Lead Fast Reactor and Concentrating Solar Power Supercritical Carbon Dioxide Cycles to Demonstrate Feasibility, Efficiency Gains, and Cost Reductions

Solar power has innate issues with weather, grid demand and time of day, which can be mitigated through use of thermal energy storage for concentrating solar power (CSP). Nuclear reactors, including lead-cooled fast reactors (LFRs), can adjust power output according to demand; but with high fixed costs and low operating costs, there may not be sufficient economic incentive to make this worthwhile. We investigate potential synergies through coupling CSP and LFR together in a single supercritical CO2 Brayton cycle and/or using the same thermal energy storage. Combining these cycles allows for the LFR to thermally charge the salt storage in the CSP cycle during low-demand periods to be dispatched when grid demand increases. The LFR/CSP coupling into one cycle is modeled to find the preferred location of the LFR heat exchanger, CSP heat exchanger, sCO2-to-salt heat exchanger (C2S), turbines, and recuperators within the supercritical CO2 Brayton cycle. Three cycle configurations have been studied: two-cycle configuration, which uses CSP and LFR heat for dedicated turbocompressors, has the highest efficiencies but with less component synergies; a combined cycle with CSP and LFR heat sources in parallel is the simplest with the lowest efficiencies; and a combined cycle with separate high-temperature recuperators for both the CSP and LFR is a compromise between efficiency and component synergies. Additionally, four thermal energy storage charging techniques are studied: the turbine positioned before C2S, requiring a high LFR outlet temperature for viability; the turbine after the C2S, reducing turbine inlet temperature and therefore power; the turbine parallel to the C2S producing moderate efficiency; and a dedicated circulator loop. While all configurations have pros and cons, use of a single cycle offers component synergies with limited efficiency penalty. Using a turbine in parallel with the C2S heat exchanger is feasible but results in a low charging efficiency, while a dedicated circulator loop offers flexibility and near-perfect heat storage efficiency but increasing cost with additional cycle components.

Near-Optimal Receding Horizon Control of Thermal Energy Storage

Abstract Efficient plant operation can be achieved by properly loading and sequencing available chillers to charge and discharge thermal energy storage (TES) reservoirs at optimal rates and times. TES charging sequences are often determined by heuristic rules that typically aim to reduce utility costs under the time-of-use rates. However, such rules of thumb may result in significantly suboptimal performance on some days. Rigorous optimization, on the other hand, is computationally expensive and can be unreliable if not carefully implemented. A novel receding-horizon control (RHC) algorithm is developed to reliably compute near-optimal control for charging and discharging the stratified sensible cool storage reservoir of a chiller plant. The algorithm provides a constant coefficient-of-performance (or cost-per-ton-hour) 24 h dispatch plan under which chillers operate at higher capacity during more favorable weather conditions. The algorithm uses a one-dimensional search requiring at most N2/2 evaluations per day of chiller performance where N is the number of planning horizon time-steps and chiller performance is modeled as a function of capacity fraction and ambient wet- or dry-bulb temperature. Analysis of four hot climates, ranging from dry to humid, indicates 2.4–2.6% energy savings under a flat electricity rate relative to the same plant operating without TES. Annual cost savings from 6% to 9% were found for electricity billed under a simple (10 a.m.–10 p.m.) time-of-use rate with no demand or ratchet components.

Charging optimization of multi-tube latent heat storage comprising composite aluminum foam/nano-enhanced coconut oil

The fast charging of thermal energy storage (TES) systems is a requirement for the practical application of these systems. The thermal energy should be stored in a unit within a reasonable time. The present research aims to minimize the thermal charging time of a latent heat TES unit by using aluminium foams, nanoparticles (copper oxide), and geometrical optimization of the unit. The melting phase change was simulated in the TES unit by using FEM. The Taguchi optimization method was invoked to maximize the melting rate during two hours of thermal charging. The results showed that the geometrical design of the unit and the porosity of foam are the most influential design parameters on the thermal energy storage and melting rate. The variation of the design parameters could improve the melting rate by 41%. The presence of nanoparticles could only improve the melting rate by less than 2%. The optimal design of the TES unit can be fully charged in two hours. Such fast-charging time could be advantageous in solar systems and transient heat recovery.

Combined-cycle gas turbine power plant integration with cascaded latent heat thermal storage for fast dynamic responses

The combined-cycle gas turbine (CCGT) power plants are often required to provide the essential fast grid balance service between the load demand and power supply with the increase of the intermittent power generation from renewable energy sources. It is extremely challenging to ensure CCGT power plants operating flexibly and also maintaining its efficiency at the same time. This paper presents the feasibility study of a CCGT power plant combined with the cascaded latent heat storage (CLHS) for plant flexible operation. A 420 MW CCGT power plant and a CLHS dynamic models are developed in Aspen Plus based on a novel modelling approach. The plant start-up processes are studied, and large amount of thermal energy can be accumulated by CLHS during the start-up. For load-following operation, extensive dynamic simulation study is conducted and the simulation results show that the extracted exhaust gas can be used for thermal energy storage charging, and the stored heat can be discharged to produce high temperature and high pressure steam fed to the steam turbine. Besides, the stored heat can also be used to maintain the heat recovery steam generator (HRSG) under warm condition to reduce plant restart-up time. The simulation results demonstrate that the integration of CLHS with CCGT power plant is feasible during the start-up, load-following and standstill operations.

Feasibility study of Combined Cycle Gas Turbine (CCGT) power plant integration with Adiabatic Compressed Air Energy Storage (ACAES)

The paper presents the research outcome on integration of an Adiabatic Compressed Air Energy Storage system with a Combined Cycle Gas Turbine power plant to increase its operation flexibility. The study demonstrates the novel hybrid CCGT-ACAES plant including an extended operational load level range and increased operation flexibility which supports the power grid to allow more power generation from connected intermittent renewable energy sources. It is also shown that this new hybrid power plant will avoid the Combined Cycle plant gas turbine operating under the low load level. Lowering the minimum hybrid plant operational load level during air and Thermal Energy Storage charging process to the required minimum Heat Recovery Steam Generator load level eliminates the need for Combined Cycle Gas Turbine plant cycling operation increasing the lifetime of the plant components. Adiabatic Compressed Air Energy Storage plant concept is based on proved and well established direct two-tank Thermal Energy Storage technology used in Concentrated Solar Power plants. Improved hybrid plant flexibility is occupied by slight decrease (2%) in the plant efficiency. Further investigation into alternative advanced Thermal Energy Storage systems based on latent heat and chemical reaction heat would offer better hybrid plant round-trip efficiency across operational plant load level range.

Technical Feasibility Study of Thermal Energy Storage Integration into the Conventional Power Plant Cycle

The current load balance in the grid is managed mainly through peaking fossil-fuelled power plants that respond passively to the load changes. Intermittency, which comes from renewable energy sources, imposes additional requirements for even more flexible and faster responses from conventional power plants. A major challenge is to keep conventional generation running closest to the design condition with higher load factors and to avoid switching off periods if possible. Thermal energy storage (TES) integration into the power plant process cycle is considered as a possible solution for this issue. In this article, a technical feasibility study of TES integration into a 375-MW subcritical oil-fired conventional power plant is presented. Retrofitting is considered in order to avoid major changes in the power plant process cycle. The concept is tested based on the complete power plant model implemented in the ProTRAX software environment. Steam and water parameters are assessed for different TES integration scenarios as a function of the plant load level. The best candidate points for heat extraction in the TES charging and discharging processes are evaluated. The results demonstrate that the integration of TES with power plant cycle is feasible and provide a provisional guidance for the design of the TES system that will result in the minimal influence on the power plant cycle.

Transient local entropy generation analysis for the design improvement of a thermocline thermal energy storage

Second-law methodologies for the design improvement of thermal energy storage (TES) systems are complicated by the fact that TES charging and discharging are highly dynamic (i.e. unsteady) in nature and most of the works done in the field consist of black-box parametric studies, where no information at the local level can be obtained by the analyst.In the present paper, we aim at filling this gap by the introduction of three novel indicators, namely the cumulated local exergy destruction, the characteristic time and the lifespan of entropy generation. These figures of merit provide the designer with a powerful tool that facilitates the identification of the temporal and spatial location of criticalities and the development of design improvements. To test the effectiveness of this innovative methodology, we focused on the design improvement of a molten salts thermocline TES tank for concentrated solar power applications.We found that analyzing the cumulated exergy destruction only is a blind way to proceed because very limited insights can be obtained on the physical phenomenon. In this case, the total irreversibilities can be reduced only by 7% and 12% with the help of a porous and a solid baffle respectively. On the other hand, the simultaneous examination of the three parameters allows to generate a far better design by using a compounded baffle able to reduce the total entropy generation of more than 60% compared to the initial configuration.

Investigation on the dynamic characteristics of a direct contact thermal energy storage charging process for use in conventional air-conditioning systems

This study proposes a 3-dimensional (3D) mathematical model to simulate the heat transfer process in a direct contact latent heat thermal energy storage (TES) tank for conventional air-conditioning systems. The thermal performance, including the congealing point and congealing latent heat of a new phase change material (PCM), which is developed for use in conventional air-conditioning systems with a direct contact TES tank, is studied. To improve the charging performance of direct contact TES system and further understand the freezing process, a 3D numerical model was developed in ANSYS FLUENT. The volume of fluid (VOF) method tracked the volume fraction of each of the phases. The Navier-Stokes equations were solved using a finite-volume formulation, and energy equation was modeled by using an enthalpy-based formulation. The method provided a comprehensive model of the dynamic and thermal aspects of the impact process. To validate the analytical model an experimental cool storage air-conditioning system with a direct contact TES tank was designed and setup. The effects of the heat transfer fluid (HTF) inlet temperature, the flow rate of the liquid PCM, the complete charging time, cold storage capacity of the direct contact storage tank and distribution of temperature in the direct contact cold storage device were investigated. The results indicate that the charging capacity increases more rapidly when the PCM flow rate is greater. The complete charging time can be reduced by increasing the flow rate of the liquid PCM and decreasing inlet temperature of the HTF when charging the same quality of PCM. The charging capacity is larger with a lower HTF inlet temperature, however, changing the HTF inlet temperature does not appreciably change the total storage capacity.

Assignment Model for Charging of Thermal Energy Storage by Electric Chillers Using Zero-one Programming Approach

Assignment Model for Charging of Thermal Energy Storage by Electric Chillers Using Zero-one Programming Approach

Preparation and thermal energy storage properties of paraffin/expanded graphite composite phase change material

The paraffin/expanded graphite (EG) composite phase change material (PCM) was prepared by absorbing liquid paraffin into EG, in which paraffin was chosen as the PCM. EG was produced by microwave irradiation performed at room temperature. It was found that the EG prepared at 800 W irradiation power for 10 s exhibited the maximum sorption capacity of 92 wt% for paraffin. Scanning electron microscopy images showed that paraffin was uniformly dispersed in the pores of EG. Differential scanning calorimeter analysis indicated that the melting temperature of the composite PCM was close to that of paraffin, and its latent heat was equivalent to the calculated value based on the mass fraction of paraffin in the composite. X-ray diffraction analysis showed that the composite PCM was just a combination of paraffin with EG, and no new substance was produced. Thermal energy storage performance of the composite PCM was tested in a latent thermal energy storage (LTES) system. Transients of axial and radial temperature profiles were obtained in the LTES for the composite PCM and paraffin. The thermal energy storage charging duration for the composite PCM was reduced obviously compared to paraffin.

Evaluation of Reinforcement Learning for Optimal Control of Building Active and Passive Thermal Storage Inventory

This paper describes an investigation of machine learning for supervisory control of active and passive thermal storage capacity in buildings. Previous studies show that the utilization of active or passive thermal storage, or both, can yield significant peak cooling load reduction and associated electrical demand and operational cost savings. In this study, a model-free learning control is investigated for the operation of electrically driven chilled water systems in heavy-mass commercial buildings. The reinforcement learning controller learns to operate the building and cooling plant based on the reinforcement feedback (monetary cost of each action, in this study) it receives for past control actions. The learning agent interacts with its environment by commanding the global zone temperature setpoints and thermal energy storage charging∕discharging rate. The controller extracts information about the environment based solely on the reinforcement signal; the controller does not contain a predictive or system model. Over time and by exploring the environment, the reinforcement learning controller establishes a statistical summary of plant operation, which is continuously updated as operation continues. The present analysis shows that learning control is a feasible methodology to find a near-optimal control strategy for exploiting the active and passive building thermal storage capacity, and also shows that the learning performance is affected by the dimensionality of the action and state space, the learning rate and several other factors. It is found that it takes a long time to learn control strategies for tasks associated with large state and action spaces.