查看更多>>摘要：Condensation heat recovery is a way to save energy and reduce carbon emissions. For the auto-cascade refrigeration cycle system, increasing the heat recovery temperature is contradictory to ensuring the refrigerate performance of the system. Here is a nested cascade refrigeration cycle system with a heat recovery system is designed to increase the heat recovery temperature. Carry out energy, economy, and carbon emissions analysis of the system. In the economic analysis, the impact of carbon tax trading policies was considered. The analysis results show that when cooling 1 kgR23 consumes the same energy and obtains the same heat supply when the recovery temperature of the auto-cascade refrigeration cycle reaches 50°C, the recovery temperature of the heat recovery system is 67°C, which increases the recovery temperature by 17 °C. Without considering carbon tax transactions, when the recovery temperature is lower than 100 °C, the economy of the heat recovery system is better than that of standard coal; and when the recovery temperature is higher than 100 °C, the economy of standard coal is better. When considering the impact of carbon tax transactions, when the heat recovery temperature is below 110 °C, and the heat recovery system is the most economical. Among them, when the condensation temperature of R744 rises from 50 °C to 120 °C in the heat recovery system, the carbon dioxide emissions increase from 20.8 kg/GJ to 45.84 kg/GJ. In the temperature range of 50–120 °C, it is lower than electric heating, standard coal heating, and natural gas heating.
查看更多>>摘要：Steam ejector plays a key role in the refrigeration system. The water has been used as a working fluid because water is more economical and non-toxic. The operating fluid often enters the evaporator as a water liquid and droplets evaporate by absorbing heat. Cooling load changing causes the vapor in the outlet of the evaporator to be accompanied by droplets. Therefore, there are droplets at the inlet of secondary nozzle and flow should be simulated as a wet steam flow with injection droplets at the secondary flow. In this paper, the effects of droplets injection at secondary flow have been investigated on the performance of the refrigeration cycle. For this purpose, the Eulerian-Eulerian method has been used and the results have been validated with experimental data. Six cases are considered with different wetness and number of droplets in the secondary flow. The results of case 6 show COP, Tˉoutlet, and ER decrease about 22.93%, 15.66%, and 22.93%, respectively. In addition, the steam accompanies by droplets at the outlet of ejector. Therefore, it is essential for designers and operators to model and consider the effects of the droplets on the wall erosion to optimize the wet steam ejector and refrigeration cycle performance.
查看更多>>摘要：The cascade refrigeration systems become the preferred choice in the field of cold storage due to its excellent performance under the condition of low evaporation temperature. Hence, a set of accurate performance prediction and energy management framework is essential for energy conservation and emission reduction. In this paper, a library of refrigeration components is established, based on which, two methods taking the data-driven model and the knowledge-driven model into account are presented to obtain optimal intermediate pressure in the cascade-condenser from the perspective of power and COP prediction respectively. Then, the effectiveness of the proposed optimal energy management strategy is demonstrated taking an actual CO2/NH3 cascade refrigeration system as a case study. Ten parameters are used to train GA-LSSVM model and 13014 on-site testing data points are randomly divided into the training set and the testing set. Two empirical formulas are used to calculate the isentropic efficiency of the NH3 and the CO2 compressors. A thermodynamic model is used as the knowledge-driven model to connect the calculation of the high-temperature stage with that of the low-temperature stage. The results show that these two methods are accurate enough for power and COP prediction. Based on the proposed methods, a set of data (1440 data points) from a typical workday is used to show the improvement by applying the optimal energy management strategy and the results show that the power consumption can be reduced by 10.52%.
查看更多>>摘要：Latent Thermal Energy Storage (LTES) systems adopting Phase Change Materials (PCMs) have been proposed to be implemented along the cold chain over the last years. Hence, in this work, a novel insulated wall concept for refrigerated truck is proposed. A 2D transient numerical model of the truck cell is developed. Results are obtained considering the varying hourly solar radiation from 6 AM to 4 PM of a typical summer day in Vicenza (Italy). The innovative composite wall consists of a traditional polyurethane (PU) insulation layer wrapping a layer of PCM with different thickness from 0.5 cm to 2 cm. Three paraffin waxes with different reference melting temperature of 2 °C, 4 °C, and 5 °C are studied. The results confirm the validity of our hypothesis because even the minimum thickness layer of PCM considered here can ensure an adequate temperature inside the cell for almost the entire simulated period.
查看更多>>摘要：The formation of joints is critical to the long-term reliability with leak-free operation of heating, ventilation, air conditioning, and refrigeration (HVAC&R) systems. Leakages commonly occur due to fatigue failure developing in joined materials from continuous pressure and temperature cycling with mechanical vibration and refrigerant under pressure inside the system. In particular, rapid pressure and temperature changes happen frequently (e.g., multiple times per day) when the system is switched on and off. Therefore, it is important to have an automatically controlled fast-response pressure and temperature cycling (PTC) test stand available to test the performance of refrigeration joints to evaluate new bonding technologies. An innovative PTC test stand with hot-gas bypass control was designed, built, and demonstrated that eliminated the need for an evaporator and ensures rapid transition between different operating conditions. Tests were performed to demonstrate test stand functionality using R410A as the refrigerant to provide pressure and temperature cycles from 600 to 4500 kPa and 5 to 80 °C. A 50-cycle, 5-hour demonstration test was performed with both adhesively bonded and brazed joints following standardized joint testing guidelines. Both joint types survived the test without leaking, suggesting that the adhesive joints have sufficient thermal fatigue resistance along with the conventional brazed joints. Throughout the demonstration, the test stand accurately controlled the setpoint temperatures and pressures while switching the test section between these conditions. The test stand serves as a new approach for pressure and temperature cyclic fatigue testing of joints in HVAC&R systems.
查看更多>>摘要：Rapid growth of data centers has caused a huge increase in energy consumption. Utilizing natural cold resources and harvesting waste heat are two promising ways to reduce the total power consumption of data centers. This paper proposes an integrated system of free cooling and heat recovery. The system has four working modes: mechanical cooling, mechanical heat recovery, pump free cooling and pump heat recovery. Experimental results show that pump free cooling mode can achieve an energy efficiency ratio of 7.99 when outdoor temperature is 2 °C, and different modes can be switched smoothly. The applicability of this system has been evaluated in five different cities in China. The results show that annual average energy efficiency ratio can reach 6.18 and free cooling time can take up as high as 72.9% during operation in Xining. The system can harvest the most heat in Harbin, 7930.96 kW? h per year, while the least in Guangdong, 645.31 kW? h per year. This means that this new system has relatively great application prospects in northern cold cities of China, while is moderately limited in southern cities.
查看更多>>摘要：Long duration manned space travel is projected to bring a need for large cooling capacities in microgravity for typical freezing and refrigeration temperatures. Among possible cooling technology alternatives, the vapor compression cycle has the highest coefficient of performance but the confidence for microgravity applications is very low. This research effort experimentally investigated the effects of hyper and microgravity on a vapor compression cycle during parabolic flights. A total of 122 parabolas were flown over four days with a repeating pattern of 5 sequential parabolas. Transient data of the evaporation temperature and cooling capacity for selected parabolas are presented as the test stand experienced alternating hyper and microgravity levels. Most measurements and performance indicators showed mild effects on the cycle operation during the varying g-forces. For example, the refrigerant side cooling capacity fluctuated on average within a band of 15% (+/-7.5%) through all sets of parabolas. A loss of superheat due to gravity changes was observed for one set of parabolas but the set was started with an unusually low superheat of only 3 K. For all other sets of parabolas, superheat and subcooling were maintained. Flight results were compared with inclination testing in a laboratory using the same test stand. Inclination changes from -90° to + 90° impacted both the liquid and suction line mass flow rate while varying the gravity level between 0 and 1.8 g affected the suction line mass flow rate but not the liquid line mass flow rate.
查看更多>>摘要：An NH3/CO2 cascade refrigeration system with ejector subcooling (ESCRS) is proposed in this paper. Based on the basic principle of thermodynamics, the mathematical model of the refrigeration system is established and thermodynamic analysis is carried out to obtain the influence of main working parameters on system performance. Compared with the conventional cascade refrigeration system (CCRS), it is concluded that the performance of ESCRS is better than that of the CCRS. Within a certain range, the greater the subcooling degree, the more obvious the decrease of power consumption of low-temperature compressor, the greater the discharge temperature of NH3 compressor decreases, and the more obvious the increase of system COP and exergy efficiency. Under typical operating conditions, the COP and exergy efficiency of the system proposed in this paper is about 5.4% and 4.8% higher than that of the CCRS.
查看更多>>摘要：This paper presents a mechanistic model that predicts two-phase refrigerant distribution in brazed plate heat exchangers. The modeling of two-phase distribution is based on the flow visualization in the inlet header, in which the flow regime is identified to be periodic and one cycle includes two or three flow stages, given a fully developed stratified/stratified-wavy flow is present at the heat exchanger entrance. To predict the vapor refrigerant distribution, this model considers two different flow stages in the inlet header: for the top corner vapor flow, the vapor refrigerant distribution is predicted by the balance between the suction force (radial pressure gradient) and the axial momentum of each phase; for the vapor jet flow, the vapor refrigerant is assumed to be distributed evenly among the channels. The time ratio of the two flow stages is estimated by assuming the irregular large-amplitude rolling waves in the feeding tube, which lead to the vapor jet flow in the header, to have the most dangerous wavelength and their traveling time can be related to the liquid slug frequency. The apparent vapor refrigerant distribution is the time average of the vapor distributions at the two stages. For the liquid refrigerant distribution, it is predicted by imposing an equal total pressure drop for each flow path. The proposed model is validated against the experimental results: quantified two-phase refrigerant distribution, heat exchanger capacities, pressure profiles in the headers, and surface temperature of the sidewalls.
查看更多>>摘要：The article presents an original solution to the problem of selecting an energy-optimal refrigeration system for a typical supermarket application. In this optimization problem carbon dioxide (CO2) and ammonia (NH3) have been assumed as natural refrigerants and the objective function is the minimum of the annual electricity demand. In the presented methodology, the refrigeration system is described by a vector whose coordinates are constant parameters and decision variables. A set of acceptable variants was determined using system analysis and matrix calculus as a tool, based on the limiting conditions. The following systems were determined as acceptable variants: classical CO2 booster (1′ generation), 2′ generation CO2 booster - with auxiliary compressors for parallel compression of flash gas from the liquid receiver, a 3′ generation CO2 booster - with multiejectors in cooperation with parallel compression and flooded evaporators, and an indirect expansion (IX) system with ammonia. The objective function was determined from simulation models of the operation of the acceptable systems throughout the year. For the humid continental climate (Poznan, Poland) the energy-optimal variant is the 3′ generation booster system, in relation to which the annual electricity demand for the 1′ and 2′ generation CO2 booster and the ammonia indirect expansion system is higher by 33.4%, 29.0% and 12.9%, respectively. For the Mediterranean climate (Rome, Italy) the energy-optimal system is the indirect expansion ammonia system, in relation to which the annual electricity demand for 1′, 2′ and 3′ generation CO2 booster is higher by 30.4%, 23.1% and 1.4%, respectively.
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Elsevier