Energy Discharge Processes Research Articles

Thermodynamic and Exergoeconomic Analysis of a Novel Compressed Carbon Dioxide Phase-Change Energy Storage System

As an advanced energy storage technology, the compressed CO2 energy storage system (CCES) has been widely studied for its advantages of high efficiency and low investment cost. However, the current literature has been mainly focused on the TC-CCES and SC-CCES, which operate in high-pressure conditions, increasing investment costs and bringing operation risks. Meanwhile, some studies based on the phase-change CO2 energy storage system also have had the disadvantages of low efficiency and the extra necessity of heat or cooling sources. To overcome the above problems, this paper proposes an innovative compressed CO2 phase-change energy storage system. During the energy charge process, molten salt and water are used to store heat with a smaller temperature difference in heat exchangers, and high-pressure CO2 is reserved in liquid form. During the energy discharge process, throttle expansion is applied to realize the evaporation at room temperature, and CO2 absorbs the reserved heat to improve the power capacity in the turbine and the system energy storage efficiency. The thermodynamic and exergoeconomic studies are performed firstly by using MATLAB. Then, the parametric study based on energy storage efficiency, system unit product cost, and exergy destruction is analyzed. The results show that energy storage efficiency can be improved by lifting liquid CO2 pressure as well as compressor and turbine isentropic efficiencies, and CO2 evaporation pressure has the optimal pressure point. The system unit product cost can be reduced by decreasing liquid CO2 pressure and compressor isentropic efficiency, while CO2 evaporation pressure and turbine isentropic efficiency both have optimal points. Finally, the optimization of two performances is performed by NSGA-II, and they can reach 75.30% and 41.17 $/GJ, respectively. Moreover, the optimal energy storage efficiency is obviously higher than that of other energy storage technologies, indicating the great advantage of the proposed system. This study provides an innovative research method for a new type of large-scale energy storage system.

Multi-port DC circuit breaker based on dual capacitor current limiting and energy sink

The coupling relationship between the removal of the fault current and the dissipation of the residual energy of the fault line during the fault isolation process of the multi-port capacitive current-limiting DC circuit breaker prolongs the breaking time of the fault to a certain extent. In order to solve the above problems, this paper proposes a multi-port DC circuit breaker based on dual-capacitor current limiting and energy sinking on the premise of decoupling the fault current attenuation and removal process and the fault line residual energy discharge process. Further reduce the fault breaking time and equipment withstand voltage requirements of the DC circuit breaker. Firstly, the topology and working principle of the proposed circuit breaker are analyzed, and then the parameters in the topology are designed. Then a simulation model is built in the PSCAD/EMTDC simulation platform to verify the fault breaking performance and breaking time. The effect of decoupling the fault current clearing process and the fault line residual energy discharge process on the effectiveness of shortening the fault breaking time is verified.

Effect of fins and nanoparticles in the discharge performance of PCM thermal storage system with a multi pass finned tube heat exchange

This work studies the heat exchange process of a latent heat thermal energy storage (LHTES) system equipped with a compact finned tubes heat exchanger (HE) as this is one of the most important aspects of the storage system, the capacity for effectively delivering its stored energy. This work fills in a literature gap for 3D, transient heat transport fluid (HTF) flow models concerning storage systems with phase change materials (PCMs) with fins and nanoparticles allowing for an evaluation on the quality of heat delivered by the system. Numerical simulations, for full turbulent conditions of the HTF flow, were developed to access the influence of the fin pitch and the PCM thermal properties in the performance of the energy discharge process. Samples of commercial paraffin-wax A53 doped with graphene based nanoplatelets were tested and characterised. Different types of nanoplatelets were employed in the range of 0.5% to 6% weight. Measured data of the thermal conductivity, specific heat and fusion latent heat are presented. The simulations were developed for three fin pitch values 5 , 10 and 20 mm and for 1%wt and 6%wt nanoparticles loads. The effect of fins and combination of fins and nanoparticles in the outlet temperature and liquid fraction distribution inside the LHTES unit during the discharge process in a 3D full scale model was analysed. The system performance was evaluated based of off the outlet temperature of HTF to ascertain both the quantity and quality of the heat provided. The results show that the PCM thermal conductivity is significantly enhanced by the addition of graphene nanoparticles with a high aspect ratio. The addition of only 1%wt doubled the solid phase PCM thermal conductivity and for a 6%wt load the thermal conductivity increased by a factor of 3.5. Meanwhile, specific and latent heat values of the samples are relatively unaffected. The numerical results further show that applying thin fins is an effective approach to enhance LHTES systems discharge performance. Increasing the fin number significantly enhances the heat transfer rate and the HTF discharge temperature during solidification and has a positive impact in the useful discharge heat capacity, providing better quality heat. Combining fins and nanoparticles improves the discharge process, nevertheless the role of nanoparticles becomes secondary as the fins number increases. The results demonstrate that standardised compact finned heat exchangers ubiquitously used in the HVAC industry can successfully overcome the low thermal conductivity of common PCMs without compromising the useful heat discharge capacity or resorting to nanoparticles decreasing the discharge time between 60 and 77% with adequate fin number. • Samples of paraffin-wax with graphene nanoplatelets employed and characterised. • The paraffin thermal conductivity improved via nanoparticles with a high aspect ratio. • The paraffin specific and latent heat marginally affected by the nanoparticles. • 3D simulations, for full turbulent conditions of the heat transport fluid (HTF) flow. • Results showed employing HE fins with proper fin pitch replaces need of particles.

Combined enhancement of thermal and chemical performance of closed thermochemical energy storage system by optimized tree-like heat exchanger structures

The full exploitation of thermochemical energy storage potential is still hampered by several technical limitations, such as the poor heat and mass transfer in the reactive beds. This paper addresses the need of enhancement of heat and mass transfer in solid-gas reactive beds by proposing, investigating and optimizing the use of branched fins made of high conducting material. Two reactor configurations in closed-system operation are examined, and the optimal fin designs are derived by means of surrogate models based on high-fidelity finite elements predictions. The results indicate the use of optimized branched fins to increase the amount of discharged energy up to +9.1% compared to literature benchmarks. However, in the instance of reactive beds where transfer of heat and mass mainly occur along the same predominant direction, fins bifurcations might also hamper the transfer of reactants, leading to diminished thermal performance particularly during the latest stages of the energy discharge process. As a consequence, the optimal number of bifurcations is smaller for reactors that require to operate for longer discharge time. Conversely, such trend is not present in the instance of reactive beds where transfer heat and mass mainly occur along district and mutually orthogonal directions. In this case the typical heat transfer maximization design guidelines can be adopted, indicating a negligible influence of mass transfer on the optimal fins architecture.

CFD-Guided Evaluation of Spark-Assisted Gasoline Compression Ignition for Cold Idle Operation

A closed-cycle, three-dimensional (3D) computational fluid dynamics (CFD) analysis campaign was conducted to evaluate the performance of using spark plugs to assist gasoline compression ignition (GCI) combustion during cold idle operations. A conventional spark plug using single-sided J-strap design was put at a location on the cylinder head to facilitate spray-guided spark assistance. Ignition was modeled with an L-type energy distribution to depict the breakdown and the arc-to-glow phases during the energy discharge process. Several key design parameters were investigated, including injector clocking, number of nozzle holes, spray inclusion angle, number of fuel injections, fuel split ratio, and fuel injection timings. The study emphasized the region around the spark gap, focusing on flame kernel formation and development and local equivalence ratio distribution. Flame kernel development and the ignition process were found to correlate strongly with the fuel stratification and the flow velocity near the spark gap. The analysis results showed that the flame kernel development followed the direction of the local flow field. In addition, the local fuel stratification notably influenced early-stage flame kernel development due to varying injection spray patterns and the fuel injection strategies. Among these design parameters, the number of nozzle holes and fuel injection timing had the most significant effects on the engine combustion performance.

Thermodynamic Analysis of a Hybrid Electrical Energy Storage System Integrating High‐Temperature Thermal Energy Storage and Compressed Air Energy Storage for Mitigating Renewable Energy Curtailment

Compressed air energy storage (CAES) system can smooth out the fluctuated power from renewable energy and allow it to meet electricity demands reliably. It further enables the mitigation of renewable energy curtailment. A hybrid energy storage system integrating high‐temperature thermal energy storage (HTTES) and CAES is proposed. In the energy charging process, the high‐ and low‐quality power from renewable energy are stored separately in the air storage reservoir (ASR) and HTTES system. In the energy discharging process, the compressed air is heated to high temperature in the HTTES system and finally expands in air turbines for electricity generation. Thermodynamic analysis reveals that a round trip efficiency of 64.36% with an energy storage density of 5.38 kWh m−3 can be achieved. It is found that the proposed hybrid system has the beneficial effect of improving the system performance. The largest exergy destruction takes place in the HTTES system. The round trip efficiency is mostly sensitive to the output temperature of HTTES, the isentropic efficiency of compressors and turbines, and the maximum and minimum operating pressures of ASR.

G-equation based ignition model for direct injection spark ignition engines

To accurately model the Direct Injection Spark Ignition (DISI) combustion process, it is important to account for the effects of the spark energy discharge process. The proximity of the injected fuel spray and spark electrodes leads to steep gradients in local velocities and equivalence ratios, particularly under cold-start conditions when multiple injection strategies are employed. The variations in the local properties at the spark plug location play a significant role in the growth of the initial flame kernel established by the spark and its subsequent evolution into a turbulent flame. In the present work, an ignition model is presented that is compatible with the G-Equation combustion model, which responds to the effects of spark energy discharge and the associated plasma expansion effects. The model is referred to as the Plasma Velocity on G-surface (PVG) model, and it uses the G-surface to capture the early kernel growth. The model derives its theory from the Discrete Particle Ignition (DPIK) model, which accounts for the effects of electrode heat transfer, spark energy, and chemical heat release from the fuel on the early flame kernel growth. The local turbulent flame speed has been calculated based on the instantaneous location of the flame kernel on the Borghi-Peters regime diagram. The model has been validated against the experimental measurements given by Maly and Vogel,1 and the constant volume flame growth measurements provided by Nwagwe et al.2 Multi-cycle simulations were performed in CONVERGE3 using the PVG ignition model in combination with the G-Equation-based GLR4 model in a RANS framework to capture the combustion characteristics of a DISI engine. Good agreements with the experimental pressure trace and apparent heat-release rates were obtained. Additionally, the PVG ignition model was observed to substantially reduce the sensitivity of the default G-sourcing ignition method employed by CONVERGE.

Experimental study on thermal performance of a mobilized thermal energy storage system: A case study of hydrated salt latent heat storage

A mobilized thermal energy storage (M-TES) system was proposed to utilize the renewable energy or recover the industrial waste heat in this paper. The M-TES system with 215 kg sodium acetate trihydrate as phase change material (PCM) filled into a tube-in-tank heat exchanger was designed and experimental tests were conducted to evaluate its thermal performance. The charging-discharging behavior of M-TES system was observed and temperature evolutions of PCM and heat transfer fluid (HTF) were monitored. The charging process of the M-TES system could be accomplished in 1200 s; while for energy discharging process, it took a little longer (1400 s) due to the prevention of natural convection. It also revealed that the non-uniform temperature distribution of PCM would result in asynchrony of melting/solidification. The PCM in the middle part of the tubes would melt at first, whereas the temperature of the PCM close to the end of the tubes increased much slower. The temperature difference was much more significant during the discharging process. Super cooling could be found easily for the PCM in the ends of tubes. The released heat in the discharging process was applied to heat the cooling water to mimic the domestic consumption of hot water to investigate the thermal performance of energy discharging. With perfect performance of our indirect contact heat exchanger, the thermal efficiency was estimated of 79.4%. Our tests would provide an experimental basis for the application of the low-temperature mobilized LHTES system.

Role of metal foam in solidification performance for a latent heat storage unit

The current latent heat storage (LHS) units are usually poor in energy charging and discharging efficiency. Given this, a two dimensional (2D) numerical model of the energy discharging process is presented and comprehensively analyzed to predict the role of metal foam in the solidification performance of LHS units. In the model, the fractal geometry reconstructed by the fractal Brownian motion is utilized for the pore characterization of the metal foam. The proposed model is validated through a melting experiment in copper foams from the reference. The temperature dynamic response and the solidification front evolution in metal foam are analyzed and compared to those in a corresponding cavity. The roles of the fractal dimension and porosity in the solidification behaviors are quantitatively analyzed. The results report that the presence of metal foam enhances the solidification performance. For the main goal of maximizing the latent storage, the appropriate porosity of an LHS unit is dependent on the duration time for the heat discharging process in the real application of thermal energy storage. Even if the porosity is the same, the fractal dimension also affects the solidification performance. A decrease in the fractal dimension (lower degree of disorder for pore distribution) provides greater access to heat flow through the phase change material-foam composite and thus leads to improvement in the interstitial heat transfer, which in turn accelerates the rate of heat release. The fractal dimension is expected to be less than 1.5 for superior solidification performance.

Solidification behaviors and parametric optimization of finned shell-tube ice storage units

The present paper reports on a numerical study of the ice storage process in finned shell-tube ice storage (STIS) units, with a focus on the special solidification behavior using water as the phase-change material (PCM). The proposed model is experimentally verified using an energy-discharging process in an ice storage unit. The effects of natural convection and buoyancy reversal on the solidification behavior are examined and investigated. Moreover, the Taguchi method is utilized to optimize the fin geometry of STIS units. The results indicate that the natural convection and buoyancy reversal are negatively correlated with the ice storage performance. An increase of the superheat factor leads to the enhancement of the buoyancy reversal intensity, which is not conducive to the acceleration of the solidification rate. In addition, the increases in fin height, fin width, and fin number are positively correlated with ice storage performance. It is demonstrated that the fin height is the dominant factor affecting the overall ice storage performance, and it is independent of the superheat factor. From the perspective of trade-off between ice storage rate and ice storage capacity, the optimal fin parameters for the STIS unit are fin height H = 40 mm, fin number N = 10, fin width Δ = 3 mm for engineering applications.

Analysis of a phase change material-based unit and of an aluminum foam/phase change material composite-based unit for cold thermal energy storage by numerical simulation

Thermal energy storage systems have increasingly attracted researchers and engineers in the last decades, not only because they permit to overcome the mismatch between thermal energy supply and demand in solar thermal systems, but also because they have the potential to improve the power grid reliability and flexibility, as they allow, for example, to store the heat or the cooling energy produced by residential electric heat pumps for a later use. In this context, this work analyses the cooling energy charging and discharging of two different cold thermal energy storage units, based on the use of a phase change material (PCM), by numerical simulation. One unit consists of an aluminum cylindrical container partially filled with a biological PCM. The other one is similar, with the only difference that it also includes an aluminum metal foam immersed in the PCM. Two different 2D axisymmetric models have been developed, both based on the effective heat capacity method in order to account for the PCM latent heat, for the numerical simulation of the two different units, and the numerical results are compared with experimental ones, obtained by means of a climatic chamber, in order to assess, in each case, the best set of the model arbitrary parameters among the considered ones. Moreover, for each simulation model, a sensitivity analysis is performed to show the effects of two arbitrary model parameters on the numerical results. Simulation results relative to the unit with only the PCM indicate that the cooling energy charging is most affected by conductive heat transfer, and in particular by the thermally resistive layer of solid PCM that forms at the lateral wall of the aluminum container, and that heat transfer by free convection plays a crucial role in the cooling energy discharging process. As concerns the unit with the PCM and the aluminum foam, numerical results show that the charging process and the discharging one are about four times and two times faster than without the foam, respectively. Furthermore, numerical results show that, with the employed aluminum foam, free convection within the PCM is negligible in both the cooling energy charging and discharging process.

Structure Optimization of Fast Discharge Resistor System for Quench Protection System

In the quench protection (QP) process of superconducting fusion devices, the operating reliability and efficiency of power components are affected by stray parameters of the fast discharge resistor (FDR) system, especially the system stray inductance. In this paper, the fundamental condition of the QP operating process and the large power FDR system structure layout for the Large-scale Superconductor Test Facility (LSTF) are presented. The negative affected VCB, CPC circuit action, and the energy discharging process are further analyzed. The stray inductance optimization method of the resistor module and structure connections are proposed to reduce the stray inductance value. Finally, the optimization results are presented by the Q3D module of the FEA software.

Thermodynamic analysis of a novel hybrid thermochemical-compressed air energy storage system powered by wind, solar and/or off-peak electricity

In this paper, a hybrid energy storage system based on integrated thermochemical and compressed air energy storage is proposed. This hybrid system can store energy from wind, solar and/or off-peak electricity simultaneously. In the energy charging process, the concentrated solar heat is used to provide heat for the endothermal reduction of tricobalt tetroxide to cobalt monoxide. Meanwhile, wind energy is employed to drive a series of compressors for compressing the input ambient air. The products (cobalt monoxide and compressed air) are then stored temporarily. In the energy discharging process, the compressed air is released to oxidize cobalt monoxide which generates high-quality heat. The superheated and pressurized air is then used for electricity generation via air turbines. In conventional compressed air energy storage, natural gas is employed for superheating the compressed air. By contrast, the proposed hybrid system completely eliminates the use of fossil fuel and replaces it with the exothermal oxidation heat of cobalt monoxide, which originates from solar energy. Moreover, cobalt oxides can be stored at ambient temperature for an extended period without any insulation requirements, which renders the system to be a long-term energy storage solution. According to the thermodynamic analysis, the hybrid system can achieve a round trip efficiency of 56.4% with an energy storage density of 3.9 kWh/m3. Meanwhile, the proposed system has an overall exergy efficiency of 75.6% with the largest exergy destruction process taking place in the intercoolers. The sensitivity analysis demonstrates that the round trip efficiency is mostly sensitive to the reactivity of cobalt monoxide, the isentropic efficiency of compressors and turbines, and cavern operating pressures.

Bubble agitation as a new low-intrusive method to crystallize glass-forming materials

Xylitol is a very promising Phase Change Material (PCM) for long-term thermal energy storage at low to medium temperatures due to its low melting temperature, its high energy density, its high and stable undercooling allowing long-term storage with reduced thermal losses. However, the activation of the energy discharge process (crystallization activation) is difficult and the subsequent crystallization rates (discharge powers) are very low.To overcome such problems, a simple and efficient technique is proposed. It consists in bubbling undercooled Xylitol using a gas until it crystallizes and the recalescence phase is completed. The mechanisms activating the crystallization are presented in this paper and the bubbling performances are experimentally investigated.

Development of the C-dump topology-based four-level converter for switched reluctance motor drives

Demand for brushless and commutator-less motor drives is increasing day by day for industrial applications. So improvement in switched reluctance motor (SRM) drives remains an important research area due to cheap and maintenance free motor construction. In literature, various converter topologies and their control strategies are described, however the C-dump-based converter topologies have gained interest owing to its ability to retrieve the stored magnetic energy. This can be further utilised to achieve desirable performance characteristics. The paper addresses the development of a drive system for a four phase, 8/6 switched reluctance motor. The drive system consists of a power circuit and its control scheme. The power components are designed for a 4-phase, 3 hp, 8/6 SRM which is used to obtain the experimental results. A simple PI-based speed control scheme is implemented which is suitable for low power applications such as cooling fan, blower, etc. A C-dump-based converter is proposed which allows reduction of switching losses, especially in the low speed range and it eliminates energy intake from the source during energy discharge process.

Integrated chemisorption cycles for ultra-low grade heat recovery and thermo-electric energy storage and exploitation

The integrated chemisorption technology driven by ultra-low grade heat for simultaneous electrical power and thermal energy storage has been investigated in this work. A resorption cycle employing low and high temperature salts, with the coupling between manganese chloride (MnCl2), calcium chloride (CaCl2) and sodium bromide (NaBr), was evaluated and compared in the proposed integrated system for a heat source temperature of between 30°C and 100°C. During the energy charging process, working fluid compression was introduced to convert mechanical power or electricity into chemical energy so that it could be stored during the adsorption process at the same time as utilising low temperature heat energy. During the energy discharging process, mechanical power could be generated via the expansion of the desorbed high pressure working vapour from the low temperature salt reactor, there is also a by-product potential of the cooling energy which can be extracted from the cold expansion exhaust. In addition to the power generation and the potential cold energy, upgraded heat could also be provided by the exothermic adsorption process in the high temperature salt reactor. The performance of this integrated cycle in terms of energy and exergy efficiency and energy density have been discussed. With the help of ultra-low grade heat, a 100% round-trip electricity storage efficiency of the proposed system has been found to be achievable using CaCl2–NaBr and MnCl2–CaCl2 pairs when the heat source temperature was higher than 50°C and 60°C, respectively. Also a temperature lift of the heat by 15–33°C and 22–68°C respectively was possible using these two adsorbent pairs.

Optimization on Performance of the Latent Heat Storage Unit (LHSU) in Telecommunications Base Stations (TBSs) in China

Abstract A latent heat storage unit (LHSU) that combined with energy storage modules and an air cooler was investigated to save the space cooling energy consumption in telecommunications base stations in China. A mathematical model was employed to simulate the heat transfer processes within energy storage modules and air cooler. In addition, an experiment was carried out in an enthalpy difference laboratory. The relative errors of air and temperature difference between the simulation and experimental results were calculated for energy charging process and energy discharging process, respectively. It was shown that the relative error for the water circulation system was reported to 1.33% and 5.23% under the charging and discharging process and 7.27% and 8.00% for air circulation system. The annual energy saving ratio was presented as a function of PCM melting temperature for varied pump flow rates and fan flow rates. To obtain the maximum annual energy savings ratio, a genetic algorithm was developed, considering fan flow rate, pump flow rate and melting temperature of PCM. Annual energy savings ratios were improved by 6.48%, 4.39%, 3.48%, and 3.51%, in Shenyang, Zhengzhou, Changsha, and Kunming, respectively.

Thermal evaluation of laminated composite phase change material gypsum board under dynamic conditions

Thermal evaluation of non-deform laminated composite phase change material (PCM) gypsum board has been carried out. The theoretical studies covered the analysis of different thicknesses of PCM layers and their corresponding heat transfer rates during energy storage and discharge processes. A simply approach was also provided for determining the appropriate thicknesses of PCM layer under various conditions. For the purpose of experimental study and validation, a laminated gypsum board consisting of a 4 mm PCM layer was evaluated in a naturally ventilated condition. It achieved a maximum heat exchange of 15.6 W/m2 and a maximum energy storage of 363.7 kJ/m2. A model room built with the laminated PCM gypsum boards was also evaluated and achieved a maximum temperature reduction of 5 °C as compared with 1.8 °C for the one with ordinary gypsum board. Even though about 25% of the energy stored could not be released within the targeted period, the overall thermal performance of the PCM gypsum board was quite remarkable. Further heat transfer enhancement mechanism may therefore be necessary for the energy discharge process.

Improved C‐dump converter for switched reluctance motor drives

This study presents a new power converter for switched reluctance motor-based drives. In comparison with the standard power systems, the proposed one allows reduction of the switching losses, especially in a low-speed range, as well as eliminates energy intake from source during the energy discharge process. As a result the efficiency of drive can be increased in the entire range of rotational speed. The computer model of the drive and a detailed study on relationships between the output power and the rotational speed for a sample two-phase motor-based drive are presented. To verify simulations results the measurements on the laboratory test-stand are carried out.

Eddy-Current Losses of a HT-SMES Coil Under Controlled and Uncontrolled Discharge Conditions

This paper presents an analytical approach of eddy-current loss in a high temperature superconducting magnetic energy storage coil based on finite-element algorithm and numerical circuit analyses. A solenoidal coil with step-shaped cross-sectional shape is introduced to reduce the total eddy-current loss of the whole coil by diminishing the eddy-current losses located at two coil ends. Two coil-current-dependent formulas are derived to estimate the eddy-current loss, e.g., 294 J for a 1.2 H/880 A Bi-2223 solenoidal coil in a 1000 V/500 kW constant-power energy discharge process and then the uncontrolled energy discharge process.