• Energy Research

Abstract The objective of this study is to investigate the performance characteristics of an R600a domestic refrigerator-freezer (RF) adopting a condenser outlet split (COS) ejector cycle. Experiments are conducted to measure the performances of conventional and COS ejector-based domestic RFs using R600a. A test bench is used to analyze the pressure lifting effect, mass flow rate variation, and coefficient of performance (COP) improvement with respect to the entrainment ratio (ER). For entire cycle operation at similar cooling capacity condition, the overall COP improvement of the test bench adopting COS ejector cycle over the baseline cycle is 11.4% at the ER of 0.18. Moreover, the COS ejector-based domestic RF is tested to examine its feasibility in actual applications. The COS ejector-based domestic RF with a compressor speed of 1450 rpm exhibits a temperature profile in the freezer compartment that is similar to that of the baseline domestic RF. At similar cooling capacity condition, the energy consumption of the COS ejector-based domestic RF with the compressor speed of 1450 rpm is 10.9% lower than that of the baseline domestic RF, owing to the pressure lifting effect.

Abstract In a liquid injection heat pump, it is very essential to control the compressor discharge temperature without wet-compression problems at extreme outdoor conditions. The objective of this study was to optimize the injection-hole geometries of a liquid injection heat pump in order to prevent the risk of wet-compression while reducing compressor discharge temperature at overload cooling conditions. In this study, a simulation model for predicting the performance of a liquid injection heat pump was developed and validated. The optimum injection-hole geometries were determined to obtain the maximum multiplication ratio, which led to a lower instant injection mass flow rate in terms of R- and θ-directional positions. In addition, the injection-hole diameter was minimized to prevent wet-compression while obtaining the target injection mass flow rate. The discharge temperature of the optimized compressor was decreased by 9.2 °C over the baseline compressor while maintaining the same risk for wet-compression at the overload cooling test condition.

This study aims to prepare a stable caprylic acid (CA) and cetyl alcohol (CAL) organic binary mixture as a solid–liquid phase change material (PCM) with a phase transition temperature in the range for exotic chilled refrigeration. A detailed study was carried out on the thermo-physical properties, thermal reliability and corrosion analysis of the prepared binary mixture. The result showed that the binary mixture of caprylic acid–cetyl alcohol (CA–CAL) with a eutectic point at 85:15 molar mass ratio is suitable for medium-range refrigeration application. The determined onset melting/freezing temperature with differential scanning calorimetry (DSC) was 10 °C/8.9 ± 0.1 °C with a phase transition enthalpy of 154.1/153.3 ± 1% J/g. The binary mixture thermal conductivity measured in the solid phase (at 0 °C) and liquid phase (at 20 °C) was (0.288 ± 0.028) and (0.156 ± 0.007) W/(m⋅K), respectively. Moreover, the thermal reliability test result of the prepared binary mixture under accelerated thermal cycling for 500 melting/freezing cycles showed a maximum of 10.1% deviation in thermal properties, which was in the acceptable range for organic binary PCM. The prepared PCM was found to be compatible with stainless steel and aluminum over an extended period of time, based on corrosion tests conducted on aluminum, copper and stainless steel over a period of 84 days. According to this study, the binary combination CA–CAL as PCM is a potential candidate for cold chain food transportation, supermarket cold cabinets, and other refrigeration applications.

Abstract A pulsating heat pipe (PHP) is an excellent cooling device based on the phase change of a working fluid. However, the performance of the PHP can be degraded by nonuniform heating conditions in the evaporator section. The objective of this study is to investigate the thermal performance characteristics of a PHP at various nonuniform heating conditions. The thermal performance of the PHP is measured by varying the dimensionless heat difference from 0 to 0.3, heat input from 30 to 100 W, and filling ratio from 50% to 70%. As a result, the optimal filling ratios for the best PHP performance and reliability are determined to be 50%, 60%, and 70%, at the dimensionless heat differences of 0, 0.2, and 0.3, respectively. In addition, the thermal resistance and evaporator temperature difference of the PHP increase with an increase in the dimensionless heat difference owing to the decreased driving force. Finally, contour maps for the effective thermal conductivity are proposed to provide design guides of PHPs.

Abstract The use of daylight in buildings to save energy while providing satisfactory environmental comfort has increased. Integration of the daylighting and thermal energy systems is necessary for environmental comfort and energy efficiency. In this study, an integrated meta-model for a daylighting, heating, ventilating, and air conditioning (IDHVAC) system was developed to predict building energy performance by artificial lighting regression models and artificial neural network (ANN) models, with a database that was generated using the EnergyPlus model. The design of experiments (DOE) method was applied to generate the database that was used to train robust ANN models without overfitting problems. The IDHVAC system was optimized using the integrated meta-model and genetic algorithm (GA), to minimize total energy consumption while satisfying both thermal and visual comfort for occupants. During three months in the winter, the GA-optimized IDHVAC model showed, on average, 13.7% energy savings against the conventional model.

Abstract Even though a dual evaporator ejector cycle (DEEC) offers several advantages over a standard two-phase ejector cycle, few experimental investigations of the performance of the DEEC are available in the literature. This study presents the performance characteristics of an R410A air-conditioner adopted with a DEEC under various operating conditions and ejector geometries. The COP of the DEEC decreased with an increase in entrainment ratio (ER) due to the decrease in pressure lifting ratio. For the optimum ER, the effectiveness of the DEEC increased with an increase in compressor speed with a larger total mass flow rate. The optimum mixing section diameter was determined to be 5 mm based on the cooling seasonal performance factor (CSPF) and CSPFbin of the DEEC. The maximum allowable limit for the ER was also suggested to be 0.3. In addition, the CSPF of the DEEC was 6.3% higher than that of the baseline cycle at an ER of 0.1.

Thermal energy storage (TES) systems have been verified to be a promising solution for reducing the imbalance between energy supply and demand; however, sensible and latent thermal storage systems have certain limitations. To minimize the issues of rapid drop of temperature in sensible storage and poor heat transfer in latent storage, novel thermal energy storage should be designed. Considering this, combined sensible-latent TES system was developed to mitigate the drawbacks of individual sensible and latent storage. This study aims to investigate experimentally the impact of various volume fractions of phase change material (PCM) in the proposed system on thermal performance and also in terms of economics. The results demonstrated that 60% PCM volume fraction had 50.03% lower capacity cost, 165.4% higher storage capacity, 168.8% higher storage density, and 26.47% efficiency than the sensible TES. Furthermore, 60% volume fraction of PCM had a 13.09% lower installation cost, 4.77% and 11.69% lower charging/discharging time, and 12.64% higher efficiency than the latent storage system. This concludes that under the assumptions of the current study, a 60% volume fraction of PCM is the optimal design parameter for a combined sensible-latent TES system for achieving the best performance in terms of cost and thermal performance.

Abstract Even though a condenser outlet split (COS) ejector cycle offers several advantages over a standard two-phase ejector cycle, few experimental investigations on the performance of the COS ejector cycle are available in the literature. The objective of this study is to investigate the effects of the various ejector geometries on the performance of a small-sized household refrigeration cycle by using the refrigerant R600a under various operating conditions. The performance of the COS ejector cycle was measured and analyzed by varying the compressor speed, entrainment ratio (ER), nozzle position, and mixing section diameter. The enhancement in COP of the COS ejector cycle over that of the baseline cycle increased with the decrease in ER and increase in compressor speed, owing to the reduction in expansion loss. At a compressor speed of 45 Hz and an ER of 0.3, the maximum COP improvement of the COS ejector cycle with the optimum mixing section diameter over that of the baseline cycle was 6.8%. In addition, the optimum mixing section diameter for a given ER and compressor speed was proposed.