In order to further studies on thermoelectric generation, an experimental thermoelectric generator unit incorporating the commercially available thermoelectric modules with the parallel-plate heat ...exchanger has been constructed. The experiments are carried out to examine the influences of the main operating conditions, the hot and cold fluid inlet temperatures, flow rates and the load resistance, on the power output and conversion efficiency. The two operation parameters such as the hot fluid inlet temperature and flow rate are found to significantly affect the maximum power output and conversion efficiency. A comparison of the experimental results with those from the previously published numerical model is also presented. The meaningful results obtained here may serve as a good guide for further improving the numerical model and conducting a system level optimization study in the next step. Also, the present study shows the promising potential of using this kind of thermoelectric generator for low-temperature waste heat recovery.
This paper presents a generalized theoretical model for the optimization of a thermoelectric cooling (TEC) system, in which the thermal conductances from the hot and cold sides of the system are ...taken into account. Detailed analyses of the optimal allocation of the finite thermal conductance between the cold-side and hot-side heat exchangers of the TEC system are conducted by considering the constraint of the total thermal conductance. The analysis results show that the maximum coefficient of performance (COP) and the maximum cooling capacity of the TEC system can be obtained when the finite total thermal conductance is optimally allocated. Furthermore, the effects of the total thermal conductance and the heat capacity rate of the cooling fluid on the performance of the TEC system and the optimal thermal conductance allocation ratio are also examined.
► A generalized optimization model of a thermoelectric cooling system is developed. ► The model considers the thermal conductances of two sides' heat exchangers. ► Optimization analyses are conducted with a constraint of total thermal conductance. ► Maximum coefficient of performance and cooling capacity can be obtained.
This paper presents a numerical model to predict the performance of thermoelectric generator with the parallel-plate heat exchanger. The model is based on an elemental approach and exhibits its ...feature in analyzing the temperature change in a thermoelectric generator and concomitantly its performance under operation conditions. The numerical simulated examples are demonstrated for the thermoelectric generator of parallel flow type and counter flow type in this paper. Simulation results show that the variations in temperature of the fluids in the thermoelectric generator are linear. The numerical model developed in this paper may be also applied to further optimization study for thermoelectric generator.
•Optimization of a thermoelectric cooler system is presented.•The total heat transfer area of heat exchangers is considered as a constraint.•The best performances are characterized by different ...optimal area allocation ratios.•Optimal area allocation ratios are mainly affected by hot side thermal conductance.
In this paper, the theoretical analyses are conducted to explore the optimization problems of thermoelectric cooler (TEC) systems applied in electronic cooling. The study mainly focuses on the optimal heat exchanger configuration of a TEC system. The effects of total heat transfer area allocation ratio, thermal conductance of the TEC hot and cold side and TEM element material properties on the cooling performance of the TEC are investigated in detailed based on the developed mathematical model. The analysis results indicate that the highest coefficient of performance (COP), highest heat flux pumping capability of the TEC and lowest cold side temperature can be achieved by selecting an optimal heat transfer area allocation ratio. The optimal heat transfer area allocation ratio mainly depends on the relevant objective functions, the hot and cold side thermal conductance, total heat exchanger size and the TEM element material properties. These results reveal that the heat transfer area allocation ratio is an applicable characteristic of optimum design for TEC systems. It is hoped that the considerations and analysis results may provide guides for the design and application of practical thermoelectric cooler system in electronic cooling.
•Double ejectors are applied in a two-stage transcritical CO2 heat pump cycle.•The performance of the new cycle with double ejectors is evaluated theoretically.•The new cycle exhibits higher ...performance compared to the basic two-stage cycle.•The heating COP can be increased by 10.5–30.6% under given operation conditions.
In this study, two ejectors are proposed as expansion devices for a two-stage transcritical CO2 heat pump cycle to enhance the cycle performance. The two ejectors are arranged at the low- and high-pressure stages, respectively, to recover more available expansion work, and significantly reduce the throttling loss at each stage. The performance of the improved two-stage cycle is evaluated by using the developed mathematical model, and then compared with those of the basic two-stage cycle with a flash tank. The simulation results show that the improved two-stage cycle exhibits higher heating COP and volumetric heating capacity compared to the basic two-stage cycle. By further incorporating an internal heat exchanger, the heating COP can be increased by 10.5–30.6% above that of the baseline cycle when the subcooling degree varied from 0 to 15°C under given operation conditions of −15°C evaporating temperature, 10MPa gas cooler pressure and 35°C outlet temperature. Additionally, the effects of the gas cooler pressure and intermediate pressure on the maximal heating COP are also discussed.
The pre-bored grouted planted (PGP) pile is a composite pile consisting of a precast concrete pile and the cemented soil around the pile. Thus, the PGP pile shaft capacity is affected by the ...pile–cemented soil interface and the strength of the cemented soil formed around the pile. In this paper, a series of shear tests were conducted to investigate the frictional capacity of the precast concrete pile–cemented soil interface, and the strength of cemented soil and its effect on the interface frictional capacity. The test results show that the frictional capacity of precast pipe pile–cemented soil interface is largely dependent on the cemented soil strength, and the peak skin friction of the interface increases from 7.58 to 204 kPa with the cemented soil strength increasing from 65 to 1500 kPa. An adhesion factor is proposed to correlate the peak skin friction of the precast pipe pile–cemented soil interface and cemented soil strength. The adhesion factor is found to range from 0.116 to 0.141 corresponding to the range of cemented soil strength from 65 to 1500 kPa. It was also noted the failure at the pile–cemented soil interface is brittle. The scale effect also exists in the pile–cemented soil interface shear test, and the scale effect factor is 1.9 when the pile diameter decreases from 85 to 37 mm. Accordingly, the design of PGP pile should avoid approaching the frictional capacity of the pile–cemented soil interface.
One of the most challenging issues that restrict the biomass/waste-based nanocarbons in supercapacitor application is the poor structural inheritability during the activating process. Herein, we ...prepare a class of activated carbon fibers by carefully selecting waste cotton glove (CG) as the precursor, which mainly consists of cellulose fibers that can be transformed to carbon along with good inheritability of their fiber morphology upon activation. As prepared, the CG-based activated carbon fiber (CGACF) demonstrates a surface area of 1435 m
2
g
−1
contributed by micropores of 1.3 nm and small mesopores of 2.7 nm, while the fiber morphology can be well inherited from the CG with 3D interconnected frameworks created on the fiber surface. This hierarchically porous structure and well-retained fiber-like skeleton can simultaneously minimize the diffusion/transfer resistance of the electrolyte and electron, respectively, and maximize the surface area utilization for charge accumulation. Consequently, CGACF presents a higher specific capacitance of 218 F g
−1
and an excellent high-rate performance as compared to commercial activated carbon.
EVRCs (ejector-expansion vapor-compression refrigeration cycles) applied in domestic refrigerator-freezers have been concerned duo to their potentials of improving cycle performance. However, the ...previously presented EVRCs with various cycle configurations are still limited to favorable performance improvement. Therefore, further development is required to promote the use of EVRCs. This paper summarizes the existing EVRCs for promising applications in domestic refrigerator-freezers. Considering the limited capacity of the existing EVRCs to enhance cycle performance, we further present a novel MEVRC (modified EVRC), in which the use of the two-phase ejector to more efficiently recover the expansion work would significantly enhance the overall system performance. A mathematical model is developed to carry out comparative simulation studies between different EVRCs. According to the results of the simulation for the EVRCs using the refrigerant R600a, the MEVRC can reach a highest pressure lift ratio of the ejector and give most excellent performance improvements in the COP (coefficient of performance) and the volumetric refrigeration capacity compared with the other EVRCs. Therefore, the potential use of MEVRC deserves further experimental validation. The present study aims to provide a deep insight into EVRCs and contribute to the development of ejector expansion refrigeration technologies in domestic refrigeration.
•Four EVRCs (ejector-expansion vapor-compression refrigeration cycles) are summarized.•A novel MEVRC (modified EVRC) with a two-phase ejector is proposed.•A mathematical model is developed to carry out comparative studies for mentioned EVRCs.•The results show the performance of MEVRC outperforms that of other EVRCs.
In this paper, an ejector enhanced high-temperature heat pump with dual-pressure condensation and evaporation is proposed to improve the system performance. Theoretical analyses of the system ...operation characteristics are conducted using energetic and exergetic methods. The performance comparisons among the basic cycle, parallel compression cycle, and ejector enhanced cycle are conducted with six different refrigerants, including R245fa, R600a, R1234ze(Z), R1336mzz(Z), R1224yd(Z), and R1233zd(E). The results demonstrate that environmentally-friendly refrigerant R1234ze(Z) would be a promising alternative refrigerant. Compared with the basic cycle and parallel compression cycle at selected operation conditions, 29.5% and 12.6% improvements in COP, and 16.7% and 11.1% higher system exergy efficiency are achieved in the ejector enhanced cycle on average. The volumetric heating capacity of the ejector enhanced cycle is increased by 15.7%–21.7%. The ejector enhanced cycle outperforms the other two cycles in high-temperature heat pump applications at the large temperature lift and temperature rise in the heat sink. The assessment offers an option to improve the energy utilization efficiency of the high-temperature heat pumps.
In this work, the fullerene C60 nano-oil is proposed as a promising lubricant to enhance the performance of domestic refrigerator compressors. The stability of fullerene C60 nanoparticles dispersed ...in a mineral oil and the lubrication properties of the nano-oil were investigated experimentally. It was confirmed that the nanoparticles steadily suspend in the mineral oil at stationary conditions for a long period of time. The friction coefficients of the nano-oil significantly decrease with increasing the concentration of nanoparticles in the mineral oil, especially at the lower applied loads. The friction coefficients of the nano-oil with the concentration of 1–3 g L−1 are 12.9–19.6% lower than that of pure mineral oil. The applications of the nano-oil with the specific concentration of 3 g L−1 to two domestic refrigerator compressors were examined by compressor calorimeter experiments. The results shows the COPs of two compressors were improved by 5.6% and 5.3%, respectively, when the nano-oil was used instead of pure mineral oil.
•Fullerene C60 nano-oil is proposed to enhance refrigerator compressor performance.•The stability of fullerene C60 nano-oil has been investigated for three years.•The lubricity of fullerene C60 nano-oil has been evaluated by friction coefficient.•Cops of two compressors with fullerene C60 nano-oil are improved by average 5.45%.
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