• Energy Research

Abstract Thermochemical energy storage (TCES) has attracted significant attention in recent years due to some unique features of the technology such as very high energy density and negligible heat loss during storage. The TCES, however, is still at its early stage of development currently at a technology readiness level of 1–3. Major technical challenges of the TCES include materials stability, charge/discharge kinetics and limited temperature lift. Here we firstly studied the application of shell-and-tube thermochemical reactor with silica gels as heat storage material in open TCES system by experimental method. And then validated model (the maximum root mean square percentage error of 13.62% between the modeling and experiments) of single tube reactor containing 0.29 kg silica-gel was established to numerically investigate the discharging behavior of the thermochemical reactor under different operating conditions and flow directions of air and water. The numerical simulation results showed inverse heat transfer occurred for a counter-flow of air-water. The problem could be solved by changing the counter-flow of air and water to the parallel-flow. Thus, the water outlet maximum temperature limit was broken through. The total heat uptake increased by at least 24.14% when water flow rate was less than 0.36 kg/h and 11.93% when air flow rate was more than 1.07 kg/h, respectively. By increasing the inlet temperature of air and water from 23 °C to 38 °C, the maximum temperature lift could be significantly increased by 79.94% for air and 80.81% for water, respectively. Meanwhile, the total heat uptake increased by 107.44%. For a completely charging and discharging process, the discharging rate of parallel-flow was faster than that of counter-flow.

Abstract Thermochemical energy storage (TCES) has attracted significant attention in recent years due to some unique features of the technology such as very high energy density and negligible heat loss during storage. The TCES, however, is still at its early stage of development currently at a technology readiness level of 1–3. Major technical challenges of the TCES include materials stability, charge/discharge kinetics and limited temperature lift. Here we firstly studied the application of shell-and-tube thermochemical reactor with silica gels as heat storage material in open TCES system by experimental method. And then validated model (the maximum root mean square percentage error of 13.62% between the modeling and experiments) of single tube reactor containing 0.29 kg silica-gel was established to numerically investigate the discharging behavior of the thermochemical reactor under different operating conditions and flow directions of air and water. The numerical simulation results showed inverse heat transfer occurred for a counter-flow of air-water. The problem could be solved by changing the counter-flow of air and water to the parallel-flow. Thus, the water outlet maximum temperature limit was broken through. The total heat uptake increased by at least 24.14% when water flow rate was less than 0.36 kg/h and 11.93% when air flow rate was more than 1.07 kg/h, respectively. By increasing the inlet temperature of air and water from 23 °C to 38 °C, the maximum temperature lift could be significantly increased by 79.94% for air and 80.81% for water, respectively. Meanwhile, the total heat uptake increased by 107.44%. For a completely charging and discharging process, the discharging rate of parallel-flow was faster than that of counter-flow.

Molten alkali metal salt effectively promotes the performance of calcium looping (CaL). Deep insight into the nonequilibrium phase-transition characteristic of alkali metal salt is better for the control of the temperature in CaL, which not only ensures the complete melting of metal salt but also prevents the reaction from inhibiting caused by higher temperatures. In this work, therefore, the molecular dynamics simulation method is used to explore the nonequilibrium phase-transition characteristic of Na2SO4. The results show that the equilibrium melting temperature of nanosodium sulfate on the calcium oxide surface is 810 K, which is lower than the macroscopic melting temperature. Meanwhile, the high heating rates led to the atoms in Na2SO4 unable to break through the thermal stability limit, resulting in overheating of the crystal. Both the surface premelting and overheating melting temperature of the crystal are increased. When the heating rates are 0.25, 0.5, and 1.0 K/ps, the overheating melting temperatures are 845, 885, and 930 K, respectively. More than that, the surface defects enhance the interaction between CaO and Na2SO4 because of the surface being charged. The increases in the interaction not only effectively break the stability of the crystal lattice of Na2SO4 on the defective surfaces but also promote the energy transport inside Na2SO4. Therefore, as the defect concentration increases from 0 to 3% and 5%, the overheating melting temperature of Na2SO4 gradually decreases from 845 to 836 and 815 K.

Molten alkali metal salt effectively promotes the performance of calcium looping (CaL). Deep insight into the nonequilibrium phase-transition characteristic of alkali metal salt is better for the control of the temperature in CaL, which not only ensures the complete melting of metal salt but also prevents the reaction from inhibiting caused by higher temperatures. In this work, therefore, the molecular dynamics simulation method is used to explore the nonequilibrium phase-transition characteristic of Na2SO4. The results show that the equilibrium melting temperature of nanosodium sulfate on the calcium oxide surface is 810 K, which is lower than the macroscopic melting temperature. Meanwhile, the high heating rates led to the atoms in Na2SO4 unable to break through the thermal stability limit, resulting in overheating of the crystal. Both the surface premelting and overheating melting temperature of the crystal are increased. When the heating rates are 0.25, 0.5, and 1.0 K/ps, the overheating melting temperatures are 845, 885, and 930 K, respectively. More than that, the surface defects enhance the interaction between CaO and Na2SO4 because of the surface being charged. The increases in the interaction not only effectively break the stability of the crystal lattice of Na2SO4 on the defective surfaces but also promote the energy transport inside Na2SO4. Therefore, as the defect concentration increases from 0 to 3% and 5%, the overheating melting temperature of Na2SO4 gradually decreases from 845 to 836 and 815 K.

Abstract Traditional air conditioning (AC) faces low energy efficiency and thermal comfort challenges. This study explores the integration of thermal energy storage (TES) containing a phase change material (PCM) with a conventional AC unit (PCM-AC) to meet the challenge. A PCM based TES device was designed and fabricated and an experimental system was established. Comparisons are made between AC and PCM-AC scenarios in terms of spatial temperature changes at the initial transient stage, spatial temperature fluctuations at the steady-state operations, relative humidity, coefficient of performance (COP), energy savings, and emergency ventilation/cooling. A developed model was used to simulate the room temperature fluctuations with and without PCM under the Matlab Simulink environment. The experimental results showed that, compared with the AC, the testing space temperature fluctuation of the PCM-AC was reduced significantly to ∼2.56 °C (compared with 4.31 °C for the AC case); the ON-OFF frequency of the compressor of the PCM-AC was reduced by 27%; the overall COP was increased by 19.05%; and the emergency ventilation/cooling time was prolonged by almost 9 times. The results also showed the potential of the use of PCM-AC to significantly narrow down the relative humidity fluctuations and hence the potential for enhancing the thermal comfort. The simulation results agree well with the experimental data. The economic analysis showed that the electrical cost of the PCM-AC could be reduced by ∼17.82%, leading to a payback period between 1.83 and 3.3 depending on the grade the PCM used and the scale of operations.

Abstract Traditional air conditioning (AC) faces low energy efficiency and thermal comfort challenges. This study explores the integration of thermal energy storage (TES) containing a phase change material (PCM) with a conventional AC unit (PCM-AC) to meet the challenge. A PCM based TES device was designed and fabricated and an experimental system was established. Comparisons are made between AC and PCM-AC scenarios in terms of spatial temperature changes at the initial transient stage, spatial temperature fluctuations at the steady-state operations, relative humidity, coefficient of performance (COP), energy savings, and emergency ventilation/cooling. A developed model was used to simulate the room temperature fluctuations with and without PCM under the Matlab Simulink environment. The experimental results showed that, compared with the AC, the testing space temperature fluctuation of the PCM-AC was reduced significantly to ∼2.56 °C (compared with 4.31 °C for the AC case); the ON-OFF frequency of the compressor of the PCM-AC was reduced by 27%; the overall COP was increased by 19.05%; and the emergency ventilation/cooling time was prolonged by almost 9 times. The results also showed the potential of the use of PCM-AC to significantly narrow down the relative humidity fluctuations and hence the potential for enhancing the thermal comfort. The simulation results agree well with the experimental data. The economic analysis showed that the electrical cost of the PCM-AC could be reduced by ∼17.82%, leading to a payback period between 1.83 and 3.3 depending on the grade the PCM used and the scale of operations.

Abstract Phase change materials (PCMs) based thermal energy storage (TES) has proved to have great potential in various energy-related applications. The high energy storage density enables TES to eliminate the imbalance between energy supply and demand. With the fast-rising demand for cold energy, cold thermal energy storage is becoming very appealing. In this paper, a review of TES for cold energy storage consisting of various liquid-solid low-temperature PCMs has been carried out. The classification of the PCMs is briefly introduced. Recent approaches to optimizing the properties of PCMs, particularly to remedy the poor thermal conductivity, leakage of liquid PCMs and the high degree of super-cooling, which limits the cold applications of TES, have also been reviewed. Methods for increasing the thermal performance including using composite PCMs and solid mesh are compared. Both modelling and experimental research on cold energy storage devices have been examined. The current cold energy storage applications including air conditioning, free cooling, etc. have been summarised. Compared with previous reviews, this work emphasises the cold energy storage applications instead of the materials aspects. The main challenges and approaches to cold thermal energy storage from the perspective of the engineering applications have been identified. Recommendations for future low charging rates and device design methodology are proposed.

Abstract Phase change materials (PCMs) based thermal energy storage (TES) has proved to have great potential in various energy-related applications. The high energy storage density enables TES to eliminate the imbalance between energy supply and demand. With the fast-rising demand for cold energy, cold thermal energy storage is becoming very appealing. In this paper, a review of TES for cold energy storage consisting of various liquid-solid low-temperature PCMs has been carried out. The classification of the PCMs is briefly introduced. Recent approaches to optimizing the properties of PCMs, particularly to remedy the poor thermal conductivity, leakage of liquid PCMs and the high degree of super-cooling, which limits the cold applications of TES, have also been reviewed. Methods for increasing the thermal performance including using composite PCMs and solid mesh are compared. Both modelling and experimental research on cold energy storage devices have been examined. The current cold energy storage applications including air conditioning, free cooling, etc. have been summarised. Compared with previous reviews, this work emphasises the cold energy storage applications instead of the materials aspects. The main challenges and approaches to cold thermal energy storage from the perspective of the engineering applications have been identified. Recommendations for future low charging rates and device design methodology are proposed.