Low-temperature Thermal Energy Research Articles

Preparation and thermal properties investigation of pentaerythritol based phase change microcapsules for low and medium temperature thermal energy storage

As one of classic organic compounds, pentaerythritol (PE) has been reported to have two phase-change processes with one solid-solid transition and other solid-liquid transition. To utilize these two phase-change enthalpies and avoid the liquid leakage over heat storage process, this work for the first time proposes to encapsulate PE with use of silicon dioxide (SiO2) as a shell substance through a so-called so-gel technology. The microstructure characterization and thermoproperties as well as the cycling performance of the PE- SiO2 composite are detailedly investigated. The results showed that PE could be successfully covered by SiO2 to form a spherical core-shell structure. Such an encapsulation has a mean size of 0.3 μm and an encapsulated rate of over 77.8 %. Due to the involvement of two phase-transition stages, the latent heat of the PE- SiO2 composite reached to 255 kJ/kg, which is higher than other organic microencapsulations. In addition, the thermal conductivity is largely enhanced by adjusting the ratio of SiO2. For a given SiO2 ratio of 77.8 %, the thermal conductivity of PE-SiO2 composite is 0.93 W/m·K, 36.8 % higher than that of pristine PE. Apart from these, the results also indicated that the composite achieves excellent thermal cycling performance. After one hundred repeated heating-cooling cycles, the latent heat of the PE- SiO2 composite decreases by less than 5 %, demonstrating that this composite material has a large potential for application in the field of thermal energy storage at low and medium temperatures.

High power density charging-free thermally regenerative electrochemical flow cycle for low-temperature thermoelectric conversion

Low-temperature thermal energy widely exists in nature and is sustainable. The thermally regenerative electrochemical cycle (TREC) based on the Seebeck effect has attracted widespread attention due to its high thermoelectric efficiency and good cycle stability. However, it has a low power density and cannot sustain continuous operation for extended periods. This work proposes a continuous charging-free thermally regenerative electrochemically cycled flow battery (TREC-FB) system, which can discharge continuously at both high and low temperatures without the assistance of external power source. By adding guanidine hydrochloride (GdmCl) and inorganic salt ions (Cs+, Ca2+, Mg2+, etc.) to the K3Fe(CN)6/K4Fe(CN)6 and I2/KI electrolytes respectively, precipitates are formed in the electrolytes at low temperature and dissolve at high temperature, which creates the concentration gradient of ions. As a result, the temperature coefficient of the full cell increases from −2.24 mV/K to −3.6 mV/K, and both voltage and power density are doubled. Theoretical analysis of the temperature coefficient enhancement is conducted based on the Nernst equation, and the mechanism of the temperature coefficient enhancement is verified using UV–vis spectra. The discharging capacity and energy of the charging-free TREC-FB with high temperature coefficient reach 5 Ah/L and 1.9 kJ/L, respectively, and the thermoelectric efficiency reaches 8.9 % under sufficient heat recuperation. Two TREC-FBs at high and low temperatures are connected to carry out continuous operation experiments of the charging-free TREC-FB system. The use of charge-reinforced ion-selective membrane can effectively inhibit the migration of I− and I3− ions, achieving about 36 h of continuous operation, and the average power density achieves 0.6 W/m2.

Solid–Liquid Phase Equilibrium of the n-Nonane + n-Undecane System for Low-Temperature Thermal Energy Storage

The current article presents an exploration of the solid–liquid phase diagram for a binary system comprising n-alkanes with an odd number of carbon atoms, specifically n-nonane (n-C9) and n-undecane (n-C11). This binary system exhibits promising characteristics for application as a phase change material (PCM) in low-temperature thermal energy storage (TES), due to the fusion temperatures of the individual components, thereby motivating an in-depth investigation of the solid–liquid phase diagram of their mixtures. The n-nonane (n-C9) + n-undecane (n-C11) solid–liquid phase equilibrium study herein reported includes the construction of the phase diagram using Differential Scanning Calorimetry (DSC) data, complemented with Hot–Stage Microscopy (HSM) and low-temperature Raman Spectroscopy results. From the DSC analysis, both the temperature and the enthalpy of solid–solid and solid–liquid transitions were obtained. The binary system n-C9 + n-C11 has evidenced a congruent melting solid solution at low temperatures. In particular, the blend with a molar composition xundecane = 0.10 shows to be a congruent melting solid solution with a melting point at 215.84 K and an enthalpy of fusion of 13.6 kJ·mol–1. For this reason, this system has confirmed the initial signs to be a candidate with good potential to be applied as a PCM in low-temperature TES applications. This work aims not only to contribute to gather information on the solid–liquid phase equilibrium on the system n-C9 + n-C11, which presently are not available in the literature, but especially to obtain essential and practical information on the possibility to use this system as PCM at low temperatures. The solid–liquid phase diagram of the system n-C9 + n-C11 is being published for the first time, as far as the authors are aware.

Operation strategy optimization and heat transfer characteristic analysis of photovoltaic/thermal module series connected with flat plate solar collector: System experimental study

Flat plate solar collector (FPSC) produces only thermal energy without electricity, whereas the photovoltaic thermal (PV/T) module produces electricity and low-temperature thermal energy. This paper presents a low-cost method to strengthen the functional uniqueness of individual PV/T and FPSC components through connecting a PV/T module in series with the other component. Under various water volume conditions, an experimental test on high mass flow rate was carried out. Results showed that the FPSC produced a better heating contribution and that reverse heat transfer of the PV/T module might be reduced by changing the operation strategy. The water volume is a crucial factor that influences the system's available heating time, a system that operates in the optimal water volume range will produce greater energy savings. Furthermore, the maximum values of the thermal, electrical, exergy, and primary energy-saving efficiency were 51.91 %, 13.52 %, 19.13 %, and 87.20 %, respectively. The optimal water volume distribution range of the system is 160–180 L. With an average annual decrease of 2.96 t in CO2 emissions, the system provides a 4.28-year payback period and saves 5285.71 USD in electricity costs throughout its life cycle. The novel system performs extremely well in terms of energy and cost efficiency.

Three-dimensional topology-optimized structures for enhanced low-temperature thermal energy storage

In this study, a three-dimensional topologically-optimized structure was developed to enhance the thermal energy storage performance of low-temperature phase change materials. The topology of the structure employed in the thermal energy storage device was developed using COMSOL Multiphysics by maximizing heat diffusion in a design domain with a constant temperature plate and adiabatic boundary conditions. The optimized thermal energy storage device was additively manufactured, and its thermal performance was experimentally characterized and compared against two conventional structures as baselines, viz., a plate fin and a pin fin structure. For the first time, this study seeks to determine the sole influence of fin topology on thermal energy storage performance by designing the fins with the same physical parameters, viz., surface area, volume, base plate size, and material. The fin structure volumes were set at approximately 5% of the simulated domain volume and were fabricated by Selective Laser Melting, a metal additive manufacturing technique. The fin structures were experimentally tested under three different constant plate temperatures (65 °C, 70 °C, and 75 °C) using two different phase change materials (RT35 and PEG1000). Their performances were evaluated by comparing the total charging time, melt fraction, and base plate temperature. Our results show that the topology of the optimized fin structure can reduce charging times by up to 9.1% when a constant plate temperature of 65 °C is applied. The topology of the optimized fins also achieved base plate temperatures that were up to 4 °C lower than conventional fins while having a more uniform distribution of heat to the phase change material within the housing. Additionally, by fixing the critical physical parameters of the fin structures, this work also shows that the fin topology plays a significant role in enhancing the melting performance of thermal storage devices.

Study Outlines Challenges of Repurposing Oil and Gas Wells for Geothermal Applications

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 215751, “Challenges for Repurposing Oil and Gas Wells for Geothermal Applications,” by Livio Santos, SPE, and Arash Dahi Taleghani, SPE, The Pennsylvania State University. The paper has not been peer reviewed. _ This work provides insights into the technical and economic viability of repurposing oil and gas wells for geothermal energy production for direct heating. The potential benefits—especially for areas that are highly dependent on fossil fuels for heating—and challenges of this approach are assessed, while technical, market, and policy barriers for future development are identified. Repurposing oil and gas wells for geothermal energy requires a multidisciplinary approach that involves expertise in geology, engineering, and regulatory affairs. Introduction Repurposing abandoned wells for power generation in geothermal energy projects can present challenges because of the low temperatures associated with these wells. The low temperature gradients and thermal energy available in such wells have been found to limit overall power-generation potential. The same low-temperature geothermal resources can still be used effectively, however, for community heating applications. Community heating systems have lower temperature requirements compared with those necessary for power generation and are compatible with available heat from low-temperature geothermal sources. In particular, closed-loop geothermal systems, specifically designed for community heating, offer several advantageous features. The closed-loop design ensures that the fluid remains contained within the system. These systems are environmentally compatible because they do not extract or discharge water from natural sources. Additionally, with better control over the working fluid and heat-exchange process, closed-loop systems deliver optimized system performance, energy efficiency, and cost savings. They also require less maintenance and feature lower operational risks because of the closed circuit. Closed-loop systems offer fluid-selection flexibility and streamlined regulatory compliance. Not all abandoned wells are suitable for repurposing, however. Thorough feasibility assessments and well-integrity evaluations are necessary to ensure that the abandoned well is structurally sound, has the appropriate depth and geological characteristics, and can extract geothermal heat effectively. The condition of the well casing, cementing, and any potential wellbore damage also must be evaluated. An economic analysis is performed to assess the financial viability and cost-effectiveness of the geothermal community heating system. Finally, an environmental impact assessment evaluates the environmental consequences of the geothermal community heating system.

Solid–Liquid Phase Equilibrium: Alkane Systems for Low-Temperature Energy Storage

The thermal characterization of two binary systems of n-alkanes that can be used as Phase Change Materials (PCMs) for thermal energy storage at low temperatures is reported in this work. The construction of the solid–liquid binary phase diagrams was achieved using differential scanning calorimetry (DSC) and Raman spectroscopy. The solidus and liquidus equilibrium temperatures were determined using DSC for thirty-nine different samples, three for the pure n-alkanes and the remaining for binary mixtures at selected molar compositions and used to acquire the corresponding solid–liquid phase diagrams. The two binary systems of n-octane/n-decane (C8/C10) and n-decane/n-dodecane (C10/C12) are characterized by a eutectic behavior at low temperatures. The eutectic temperature for the system C8/C10 was found at 211.95 K and the eutectic composition appeared at the molar fraction xoctane = 0.87. For the system C10/C12, the eutectic temperature was found at 237.85 K, and the eutectic composition appeared for the molar fraction xdecane = 0.78. This work aims to fulfill the lack of available data in the existing literature, considering the potential application of these systems for low-temperature thermal energy storage. Raman spectroscopy was used to complement the DSC data for the construction of the solid–liquid phase equilibrium diagrams, enabling the identification of the solid and liquid phases of the system. Additionally, the liquidus curve of the phase diagram was successfully described using a modified freezing point depression curve as fitting equation, the absolute root mean square deviation for the data correlation of the C8/C10 and C10/C12 systems being 2.56 K and 1.22 K, respectively. Ultimately, the fitting procedure also enabled a good prediction of the eutectic point for both studied systems.

An experimental study on the performance of TEGs using uniform flow distribution heat exchanger for low-grade thermal energy recovery

Low-temperature thermal energy (<100 °C) is widely present in energy systems and nature, and its utilization is of great significance for energy conservation and carbon reduction. Thermoelectric generator (TEG), without refrigerant and moving parts, can directly convert low-temperature thermal energy into high-grade electricity through the Seebeck effect. One of the biggest defects of TEG is the low efficiency from thermal to electricity. To improve the efficiency of TEG, a staggered fin was arranged in the main flow area for the heat exchanger. Meanwhile, a header with pin-finned and variable triangle geometry is applied in the inlet and outlet of heat exchange to uniform fluid flow. A TEG experimental system containing the heat exchanger whose header with pin-finned and variable triangle geometry was built to evaluate the output performance at variable load resistance, hot and cold source temperature, flow rate, flow direction, and electrically connected array configurations. Moreover, experimental results were compared with other literature. When the inlet hot and cold source temperatures are 40.8–81.0 °C and 20.0–30.0 °C, the TEG system could generate around 0.6–18.8 W with a flow rate of 5 L/min. The TEG system has relative high output performance and a low flow resistance compared to other literatures, and it is the potential to recover low-grade thermal energy recovery.

Performance investigation and evaluation of a low-temperature solar thermal energy storage system under dynamic weather conditions

Designed and operational parameters matter greatly in how successful an energy storage system performs. However, a current challenge in matching suitable these parameters lies in the poor understanding of such systems’ performance under dynamic weather conditions. In this paper, we investigated a phase change material (PCM) storage unit that is particularly aimed for poor-solar areas, and connected the unit to a flat plate solar collector to establish a complete solar thermal energy storage system (STESS). To optimize the system performance, different grades of solar radiation and outdoor ambient temperatures are introduced, and the effects of heat transfer fluid (HTF) flow rates and solar collecting areas are investigated numerically through multiple performance evaluation parameters. The results indicate that the solar collecting area has a significant effect on the performance of the STESS compared to that of the water flow rate. And different collecting areas of 1 m2, 1.5 m2, and 3 m2 with an optimal water flow rate of 0.06 m3/h are matched to different average daily solar radiation of 203.4 W/m2, 155.2 W/m2, 92.7 W/m2, respectively. Further for the heat release process, different heat release efficiencies from 88.32 % to 97.12 % with different energy consumption of the fan from 0.003 kWh to 0.063 kWh are obtained when the air flow rate is increased from 30 m3/h to 90 m3/h. One of the air flow rates of 60 m3/h is superior for the STESS considering higher heat release efficiency and lower energy consumption of the fan. The findings of this paper will contribute to optimizing the performance of a STESS in practical applications.

Enhancing thermal energy storage in buildings with novel functionalised MWCNTs-enhanced phase change materials: Towards efficient and stable solutions

Phase change materials (PCMs) are a promising panacea to tackle the intermittency of renewable energy sources, but their thermal performance is limited by low thermal conductivity (TC). This pioneering work investigates the potential of organic PCM-enriched surface-modified and un-modified multi-walled carbon nanotubes (MWCNTs) for low-temperature thermal energy storage (TES) applications. The functionalised and un-functionalised MWCNTs enhanced PCM have demonstrated a TC enhancement of 158 % and 147 %, respectively, at 25 °C. However, the TC value of the unmodified MWCNTs-based PCM dropped by 52.5 % after 48 h at 25 °C, while that of the functionalised MWCNTs-based PCM remained stable. A DSC analysis of up to 200 thermal cycles confirmed that the surface-modified and un-modified MWCNTs had no major effect on the peak melting and cooling temperatures of the nano-enhanced PCMs although a minor decrease of 7.5 % and 7.7 % in the melting and crystallisation enthalpies, respectively, was noticed with the inclusion of functionalised MWCNTs. Moreover, functionalised MWCNTs incorporated PCMs have led to increases in specific heat capacity by 23 % with an optimal melting enthalpy value of 229.7 J/g. In addition, no super-cooling, no phase segregation, and a small phase change temperature were noticed with these nano-enhanced PCMs. Finally, no chemical interaction from nano-PCMs was seen in the FT-IR spectra with the incorporation of both functionalised and un-treated MWCNTs. It is evident that the functionalised MWCNT-based PCM has better thermal stability and it offers a promising alternative for improving thermal storage and management capabilities in buildings, contributing to a sustainable and energy-efficient building design.

Experimental analysis of one and two-stage thermoelectric heat pumps to enhance the performance of a thermal energy storage

This experimental study demonstrates the possibility to enhance the performance of a low-temperature thermal energy storage system (∼160 °C) based on airflow heating using electrical heaters by including thermoelectric technology. An improvement of the 17 % on COP is reached by using an optimized thermoelectric heat pump system to preheat the airflow, consisting of three one-stage and three pyramidal two-stage thermoelectric heat pumps sequentially installed along the airflow that is heating.This research experimentally analyses and compares the COP of three different configurations of thermoelectric heat pumps: one-stage, square two-stage, and pyramidal two-stage thermoelectric heat pumps. The experimental study aims to characterize the operation of each configuration for heating an airflow of 16.5 m³/h at 25 °C as ambient temperature. To that purpose, the airflow inlet temperature, voltage supply, and voltage ratio between stages have been modified.The experimental results show that for 25 °C as inlet temperature the one-stage thermoelectric heat pump has the best performance with a maximum generated heat of 78 W. Whereas, a two-stage thermoelectric heat pump is required when the inlet temperature increases. At 40 °C as inlet temperature, the square two-stage configuration provides the best performance with a voltage ratio of 2, which reaches a COP of 3.29 generating only 20 W of heat. However, the pyramidal two-stage configuration is able to achieve the maximum heat outputs with a voltage ratio of 1, generating 172; 161; 149 and 138 W, with corresponding COP values of 1.17; 1.16; 1.14 and 1.11 for inlet temperatures of 25; 40; 55 and 70 °C. This configuration is the one that achieves the greatest COP values with high inlet temperatures.

Preparation and characterization of quinary nitrate salt based composite phase change material with low melting point for low and medium temperature thermal energy storage

This work concerns the development of a shape-stable molten salt based composite phase change material (PCM) for low and medium temperature thermal energy storage. The composite is fabricated by using a cold compression and hot sintering method with the employment of a eutectic quinary nitrate salt of NaNO3-NaNO2-KNO3-KNO2-LiNO3 as PCM, halloysite nanotube (HNT) as skeleton supporting material and natural graphite as thermal conductivity enhancement additive. A sequence of characterizations is performed to investigate the composite microstructure, chemical and physical compatibility, thermal stability, phase change behaviour, and thermal conductivity as well as cycling performance. The results indicate that an excellent chemical compatibility has been achieved among the ingredients of quinary salt, HNT and graphite within the composite. A mass concentration of 50 wt% HNT endows the composite with the optimal formulation in which 10 wt% graphite can be successfully accommodated and a thermal conductivity around 1.31 W/m·K can be acquired. Moreover, in such a formulation, the composite presents a considerably low melting temperature of 72.4 °C and a high thermal decomposition temperature of 530 °C, which achieves the composite a relatively high energy storage density nearly 500 kJ/kg at a temperature range of 25–510 °C. The results presented in this work demonstrate that the quinary salt-HNT-graphite composite with fairly low phase transition temperature and a splendid combination of thermal properties and cycling performance could be a promising candidate to replace the conventional organic based PCMs utilized in low temperature thermal energy storage fields.

Fabrication and investigation of paraffin based shape-stable composite phase change materials with styrene ethylene butylene styrene as supporting skeleton for low temperature thermal energy storage and management

Paraffin based composite phase change materials (PCMs) have gained intensive attentions in low temperature thermal energy storage (TES) and management application domains because of its suitable melting temperature range and advanced thermoproperties. Seeking decent structural supporting materials (SSM) and the associated manufacturing approaches that capable of fabricating paraffin composites at appropriate temperatures to avert the thermal decomposition of the paraffin particularly for low-molecule ones are in high demand. Herein, we developed a low-molecule paraffin (RT18HC) composite through a simple blending approach by using styrene-ethylene/butylene-styrene (SEBS) as SSM, and showed that such SEBS-paraffin composites could be able to fabricate at a low operation temperature of 80 °C, significantly lower than that required for the preparation of conventional high density polyethylene (HDPE)-paraffin composites. To enhance the composite effective thermal conductivity, a micro-sized graphite power as enhancer was also involved over the fabrication process. Owing to the desirable solubility of paraffin in SEBS, a crosslinking structure inlaying paraffin and graphite was formed, endowing the composite with ability to maintain the structure stabilization and eliminate the graphite sedimentation over the repeated thermal cycles. 10 wt% of SEBS gives the composite optimum formulation at which 10 wt% graphite can be involved, and also an excellent shape stability and thermal adjustment ability can be achieved.

Bubble-injection and seeding enabled crystallization of erythritol/xylitol eutectic phase change material

The utilization of Erythritol/xylitol eutectic material for low-temperature thermal energy storage and thermal management holds great promise. However, its inability to undergo spontaneous nucleation and crystallization poses a significant obstacle. This research paper introduces a novel nucleation technique that combines gas injection agitation and seeding to address this issue. The study aims to investigate the impact of gas injection tube diameter, thermal environment, and gas injection duration on the nucleation and crystallization characteristics of the material. Experimental analysis reveals that, under optimal conditions, the solidification enthalpy reaches 203.1 J/g, equivalent to 85% of the melting enthalpy. Additionally, the material exhibits excellent thermal cycle stability, with no observed phase separation or deterioration after 100 heating and cooling cycles. Furthermore, the influence of Multi-Walled Carbon Nanotube (MWCNT) nanoparticle additives on nucleation is explored. Different MWCNT additive mass fractions are compared. The obtained results underscore the significance of understanding the effects of gas injection parameters and nanoparticle additives on nucleation and crystallization processes. The enhanced solidification enthalpy, remarkable thermal cycle stability, and insights into MWCNT effects pave the way for broader applications of this promising phase change material in thermal energy storage and thermal management.

Experimental investigation on photothermal conversion performance of MWCNT-DW/EG nanofluids for low-temperature direct absorption solar thermal energy systems

Solar collector is the core equipment of solar solar thermal utilization system, but its efficiency of solar thermal conversion is low, and it has not yet met the needs of practical applications.Direct absorption solar collector (DASC) serves as a promising technology to harvest solar energy, but the performance of DASCs is largely affected by the working liquid. In this paper, the effects of MWCNT-DW/EG nanofluid mass fraction, pH value, mass velocity and liquid level height on the stability, optical properties, thermal conductivity and the performance of direct absorption solar thermal conversion were investigated. The potential of MWCNT nanofluid for direct absorption solar collector was evaluated comprehensively. The results showed that the 0.01 wt% MWCNT-DW/EG nanofluid with pH of 7 can achieve the best photothermal conversion that increased up to 38.5 % compared to the base fluid, and the efficiency of DASC was significantly higher than that of evacuated tube solar collector with selected-coating after being exposed under a solar intensity of 700 W/m2 for 2400 s. This study provides scientific data and important reference for the numerical simulation and practical application of the direct suction solar collector.

Design optimization of low-temperature latent thermal energy storage for urban cooling applications

This study investigates a novel design optimization method for a low-temperature latent thermal energy storage (LTES) in a shell-and-tube heat exchanger configuration for a district/urban cooling network. An ordered hexagonal tube-packing method is first developed for the inner configuration of the LTES followed by the implementation of a differential evolutionary multi-objective optimization scheme for key design variables. Using a numerical model, transient characterisation of the optimized designs from a prior work confirmed that the method was capable of producing improved outcomes of up to 54.7% increased storage capacity. Key observations from the optimization revealed that heat transfer maximization scales costs by up to 10 times while minimizing costs translates to a mere three-fold decrease in heat transfer characteristics. Further characterisation of the optimized LTES designs under district cooling application showed that optimizing for discharge exergy efficiency in addition to cost and heat transfer efficiency was crucial in maximising discharge efficiency by up to 30%, as well as producing 33% and 20% greater capital cost savings compared to the single- and double-objective optimization cases respectively.

Low-grade thermal energy utilization through using organic Rankine cycle system and R1233zd(E) at different heat source temperatures

The organic Rankine cycle (ORC) is a promising thermal-electric conversion technology for low-grade thermal energy utilization, however, its performance is significantly varied by the heat source condition. In this study, a compact ORC system using R1233zd(E) was built in Harbin, China for low-grade thermal energy utilization, and the thermal performances of the system at different heat source temperatures (HST, 80, 100, and 120 °C) and heat-source mass flow rates (mhs, 0.011 ∼ 0.346 kg/s) were investigated and presented. The results showed that the heat source conditions (HST and mhs) primarily influenced the ideal bubble point temerature difference (BPTD), outlet pressure, and super-heating degree (SHD) of the evaporator. With the increases in HSTs and mhs, the net output power, thermal efficiency, electricity generation and electricity generation efficiency varied in the ranges of 0.04 ∼ 0.77 kW and 0.9 % ∼ 7.1 %, 0.02 ∼ 0.51 kW and 0.5 % ∼ 4.7 %, respectively. The biggest thermal efficiency (7.1 %) and electricity generation efficiency (4.7 %) of the ORC were obtained at the HST of 120 °C, mhs of 0.09 kg/s. The results also showed that the disadvantages of low-temperature thermal energy could be compensated by improving the mhs, providing more possibilities for the efficient utilization of ORC systems driven by low-grade thermal energies.

Carnot Battery Based on Brayton Supercritical CO2 Thermal Machines Using Concentrated Solar Thermal Energy as a Low-Temperature Source

Carnot batteries store surplus power as heat. They consist of a heat pump, which upgrades a low-temperature thermal energy storage, a high-temperature storage system for the upgraded thermal energy, and a heat engine that converts the stored high-temperature thermal energy into power. A Carnot battery is proposed based on supercritical CO2 Brayton thermodynamic cycles. The low-temperature storage is a two-tank molten salt system at 380 °C/290 °C fed by a field of parabolic trough collectors. The high-temperature storage consists of another two-tank molten salt system at 589 °C/405 °C. Printed circuit heat exchangers would be required to withstand the high pressure of the cycles, but shell and tube heat exchangers are proposed instead to avoid clogging issues with molten salts. The conventional allocation of high-temperature molten salt heat exchangers is then modified. Using solar energy to enhance the low-temperature thermal source allowed a round-trip efficiency of 1.15 (COP of 2.46 and heat engine efficiency of 46.5%), thus increasing the stored power. The basic configuration has a levelised cost of storage of USD 376/MWh while replacing the shell and tube heat exchangers with hybrid printed circuit heat exchangers is expected to lower the cost to USD 188/MWh.

A numerical model for a thermally regenerative electrochemical cycled flow battery for low-temperature thermal energy harvesting

Low-temperature thermal energy (<130 °C) recycling and utilization can significantly increase energy efficiency and reduce CO2 emissions. Among various technologies for heat-to-electricity conversion, thermally regenerative electrochemical cycle (TREC) has garnered significant attention for remarkable efficiency in thermal energy utilization. The thermally regenerative electrochemical cycled flow battery (TREC-FB) in this paper offers several advantages, including continuous power output and operating without an external power supply. The goal of this investigation is to enhance the understanding of how various parameters affect system performance through simulation, thus optimizing cell performance. In this work, based on the conservation equations and electrochemical equations, the two-dimensional steady models coupled with the flow field and electrochemical field of high-temperature cell and low-temperature cell are constructed separately by COMSOL Multiphysics. The diffusion coefficient and kinetic parameters in the model were obtained by cyclic voltammetry (CV), chronoamperometry (CA) and Tafel electrochemical measurements for subsequent application in the models. Experimental results have confirmed the validity of this model. The main focus of this work is to examine how the system performance is impacted by various factors including current density, electrolyte flow rate, temperature coefficient, porous electrode geometry, heat recuperation efficiency, and temperature difference between hot and cold cells. The results indicate that a larger electrolyte flow rate leads to larger power density, but reduces system efficiency. Smaller porous electrode thickness, higher temperature coefficient, higher heat recuperation efficiency and larger temperature difference between the cells can enhance the system performance. This work offers a new guide for further enhancing TREC-FB performance.

Predictive control of low-temperature heating system with passive thermal mass energy storage and photovoltaic system: Impact of occupancy patterns and climate change

With the increasing energy prices and growing concerns over energy security, an accelerated transition to net zero carbon built environment has never been more important. Many studies have shown the capabilities of advanced control strategies such as model predictive control (MPC) to achieve energy efficiency, balance with thermal comfort and air quality. It has also shown its capability to provide demand flexibility, minimising peak load demands and maximising the production of renewable energy sources in buildings. This work investigates the potential of integrating price-responsive MPC with a low-temperature heating system and passive structural thermal energy storage (STES). Integration with a photovoltaic (PV) system is also explored. The system performance under future climate conditions is evaluated considering different design and operating conditions, including different thermal mass, occupancy patterns and internal heat gains, setpoint strategies and operation temperatures of the low-temperature heating system. The coupled model developed has been verified and validated with numerical and experimental data, and good agreement is observed. The results showed that mediumweight thermal mass and a medium-temperature (45 °C) under-floor heating inlet temperature provided a higher load shifting ability, based on a realistic occupancy profile for a residential building and a high tolerance setpoint strategy during unoccupied periods. The result also showed that higher low-price energy usage and lower heating energy usage could be achieved in future climate conditions. Finally, an increase in the load shifting ability was observed after the integration of rooftop solar PV system.