查看更多>>摘要：Evaporative cooling equipment has advantages of energy and water conservation and compact structure, is widely used in various industrial sectors. To better characterize the heat transfer capacity of the evaporative cooling process, the falling-film evaporation heat transfer process outside horizontally closed arranged tubes (horizontal tubes plate) was analyzed. A theoretical model was established to divide the total heat transferred by the tube wall into two parts: the convective heat transfer of the air and that of the liquid film. The latter was divided into three parts: the heat transferred by the convective heat transfer between the liquid film and the air, the mass and latent heat transfer between the liquid film and the air, and the heat absorbed by the rise in temperature of the liquid film. Experiments were performed with both smooth and ribbed tubes. Correlations of the heat transfer coefficients between the tube wall surface and the air and between the tube wall surface and the liquid film and the coverage rate of the liquid film were obtained. The experimental results indicated that using ribbed tubes effectively enhanced the liquid film coverage on the tubes, resulting in a maximum reinforcement of 122.73% compared with that of smooth tubes. Moreover, the experimental results verified the correlation equations, with the deviation between the calculated and experimental results was within ±10%.
查看更多>>摘要：Before any nanolubricant is being applied in a refrigeration system, its thermo-physical properties shall be investigated. In this paper, hybrid nanolubricant is prepared by dispersing SiO2-TiO2 nanoparticles at 50:50 composition ratio into the polyvinyl ether (PVE) compressor lubricant using a two-step method. The investigation was done for volume concentrations from 0.01 to 0.10% under temperature range of 303 to 353 K. The Newtonian behaviour of the nanolubricant was obtained, and relative thermo-physical enhancement was determined by comparing its performance to the pure lubricant. It was observed that the maximum increment viscosity does not exceed 3% from the base fluid, while thermal conductivity for 0.1% concentration increases up to 1.6%. Overall observation also reveals that both rheological and thermal properties increase by increasing concentrations, but the same properties decrease with temperature. An interesting finding is the nanolubricant had viscosity decrement than the pure lubricant specifically at 303 K. New regression models were suggested for thermo-physical properties with high accuracy R-squared values of 0.9989 and 0.9920 for viscosity and thermal conductivity, respectively. As a conclusion, SiO2-TiO2/PVE nanolubricant is recommended in refrigeration systems with a volume concentration of less than 0.10%.
查看更多>>摘要：This work studies two binary mixtures of R1243zf + R1234yf and R1243zf + R245cb which can be possibly used as a long-term alternative refrigerant for heat pumps. We present critical property (critical temperature, pressure, density) and vapor liquid equilibrium (VLE) of R1243zf + R1234yf and R1243zf + R245cb two binaries. The maximum extended uncertainties (k=2) are estimated as U(T) = 0.21 K, U(p) = 0.048 MPa, and U(ρ) = 6.4 kg/m3 for critical property measurement, and U(T) = 0.06 K and U(p) = 0.015 MPa for bubble point measurement. The uncertainty of liquid-phase mole fraction is estimated less than 1.8 × 10?4. The critical property is investigated by a novel method that determines the critical point via modeling vapor-liquid coexistence points to the combined equations of the scaling law and rectilinear diameters law. The VLE of R1243zf + R1234yf and R1243zf + R245cb binary mixtures in the range of 293.45 to 353.55 K are studied by the static-synthetic method which determines the bubble point via the variable volume cell technique. The experimental bubble points are correlated by the Peng-Robinson (PR) equation of state (EoS) with vdW and MHV2 mixing rules. The modeling results are in good agreement with the measured data. The high temperature VLE of the two binary systems is predicted up to the critical point line by PR-MC-MHV2 model with the optimal binary parameters.
查看更多>>摘要：Incorporating low-temperature renewable energy sources such as geothermal energy, solar energy, and waste heat into district heating and cooling systems is expected to be an effective solution for reducing fossil energy consumption and carbon emissions. This study aims to propose an optimal intelligent control strategy for the water-source heat pump coupled with an ice storage district cooling system, which can fully maximize the economic potential of renewable energy and ice storage systems. The proposed control strategy treats the ice melt cooling rate, water supply temperature, and cooling ratio of the water source heat pump as three independent control variables. The minimum operating cost per unit of cooling supply is identified as the optimal objective function of the genetic algorithm. An integrated heat transfer model of the water-source heat pump coupled with an ice storage district cooling system was developed, and experimental test data verified its accuracy. Compared with the ice melting priority control strategy and the water-source heat pump priority control strategy, the GA-based optimal control strategy saved 8.7–9.3% of the operating cost while maintaining a relatively good energy-saving performance. This proposed intelligent control strategy can stimulate the tremendous economic potential of the ice storage district cooling system coupled with renewable energy sources.
查看更多>>摘要：A general scroll compressor model with high accuracy, fast simulation speed and good numerical robustness is proposed in this study. The governing ordinary differential equations derived from mass and energy conservation use pressure and specific enthalpy as state variables, which can be applied to both single-phase compression and two-phase compression. Both the governing equations for inner compression and the nonlinear equations for internal leakages are explicit in pressure and specific enthalpy to avoid severe numerical stiff problems. In addition, the detailed scroll wrap temperature distribution is calculated based on the proposed second order differential equation considering heat conduction and periodic heat convection. Validation against experimental data shows that cooling capacity errors are less than 3.5% and power consumption errors are less than 2.8% in non-injection and liquid injection conditions. Evaluation of model speed and robustness shows that the model has a convergence rate of 100% and an average calculation speed of 12.4 s per case in various conditions.
查看更多>>摘要：The heat pump heating system has been widely accepted to provide space and water heating in commercial and residential buildings. However, the operating energy performance in practice conditions is limited due to the traditional control strategy. An efficient-cost advanced control strategy is desired to improve the overall operating performance. This paper presents a model predictive control (MPC) strategy for an air source heat pump (ASHP) heating system to optimize the overall operating performance. Based on the control-oriented model and objective function, MPC can provide the optimized compressor frequency and water mass flow rate in real-time, which can lead to the optimized real-time variable water temperature difference. Thus, the total power consumption can be minimized for the heating system by coordinating the compressor power consumption and water pump power consumption in the real-time operating condition. A high-fidelity physical model is first built to produce the required data source, and then the control-oriented model is derived based on the machine learning technique. MPC is designed to minimize the total power consumption providing that the heating capacity is well maintained. The MPC strategy is evaluated and compared with the traditional PI control method under three operating cases.
查看更多>>摘要：This paper presents the results of refrigerant mass distribution measurements carried out in an invertible air-to-water heat pump. The unit works with refrigerant R32 and produces a nominal cooling capacity equal to 60 kW. Experimental tests have been performed during both cooling and heating operations using the quick-closing valves technique to isolate the charge within the components of the system. A mathematical model, developed to predict the refrigerant charge in the heat exchangers of the heat pump and its performance, is described and validated. The model includes a physical description of the heat transfer mechanisms in the condenser and in the evaporator. The model employs void fraction correlations to calculate the refrigerant mass in the two-phase regions of the heat exchangers. In the finned coil heat exchanger, the model can account for the effects of the airflow maldistribution, which has been also experimentally measured. When the not uniform air velocity is considered, the refrigerant charge in the heat exchangers is modelled within ±10%. The results of the model showed that the airflow maldistribution can increase the total refrigerant charge in chiller mode by 12.7% and decrease the performance of the system by 11%, compared to the case of uniform air velocity.
查看更多>>摘要：A regenerative Ericsson refrigeration cycle using electrocaloric (EC) materials as the working medium is established. In accordance with the thermodynamic properties of EC materials, the mathematical expressions of the coefficient of performance (COP) and cooling rate of the regenerative EC Ericsson refrigeration cycle are derived. The cooling rate is selected as an objective function and optimized with respect to one of the two isothermal process temperatures in the Ericsson refrigeration cycle, Subsequently, the optimal ranges and bounds of the COP and dimensionless cooling rate are determined. Furthermore, the impacts of the heat leakage, finite heat capacities of heat reservoirs at high and low temperature sides, the Curie temperature of EC materials, the electric field intensity difference in the EC Ericsson refrigeration as well as the regenerator efficiency, internal irreversibility resulting from the thermal hysteresis of EC materials, etc. on the optimal performance of the regenerative EC Ericsson refrigeration cycle are revealed. The conclusions obtained are beneficial to the optimal parameter design of EC refrigerators.
查看更多>>摘要：The main purpose of this work is to focus on comparing a FAM Z05 Zeolite-water and silica gel-water for designing two adsorption beds and evaluating the performance of the system. Two adsorption beds are tested in this study, the first one is using Polyvinyl acetate (PVA) glue for coating Zeolite with a weight of 1.2 kg, the second one is using polyvinylpyrrolidone (PVP) glue for coating Zeolite with a weight of 1.4 kg. The cooling water temperature of 25 °C and the chill water temperature of 20 °C are fixed during the experimental work, whereas three different hot water temperatures of 60, 70 and 80 °C are tested and changed, and five cycle’ times of 20, 25, 30, 35 and 40 minutes are changed to investigate their effects on the performance of the system. The test results show that the Coefficient of Performance (COP) on PVA glue at hot water of 70 °C is higher than that on PVP glue and this can be explained by the fact that the PVP glue blocks the porosity of the Zeolite and then deactivates the active nucleation sites which results in decreasing the adsorption of the water vapor. It is also observed that COP increases with increasing the cycle time. In addition, comparing the performance of the Zeolite adsorption bed with PVA glue to the silica-gel adsorption bed, the desorption temperature is much lower, and SCP at hot water of 80 °C is better than other types of coating glues.
查看更多>>摘要：The consumption of low temperature refrigeration systems goes up dramatically due to the fast development of cold chain. In this regard, developing the absorption-compression hybrid cooling systems and employing the low-grade heat to save the compressor work is effective to address the above-mentioned issue. However, one of the main obstacles associated with such solution lies in small scale heat source vs. large heat consumption, which remarkably lowers the amount of energy saving. Consequently, a novel layout, absorption-compression hybrid cooling system with parallel subcooling and recooling, is proposed to deal with the above-mentioned problem. Comparative study is performed to display the advantage of the proposed facility at first. Subsequently, exergy analysis and parametric analysis of the proposed layout are implemented to derive design and operation criteria. Finally, the economic analysis of proposed system is carried out to assess the feasibility. It is found that the compressor power of the novel system is 9.3% and 10.45% less than that of two-stage vapour compression chiller and absorption-compression cascade layout, respectively. Moreover, the payback period of proposed layout is 3.67 years. The paper is favorable to enhance the energy saving of refrigeration system which is associated with small scale low-grade heat source and low temperature application.
NSTL
Elsevier