Theoretical study on a frost-free household refrigerator-freezer

This study deals with a frost-free household refrigerator-freezer, in which frosting can be retarded by dehumidifying the air before it enters the evaporator of the refrigerator-freezer via a desiccant-coated heat exchanger. Because the desiccant can be regenerated via the condensation heat of the refrigerant (which is exhausted into ambient air in conventional household refrigerator-freezers), the proposed system can achieve high energy efficiency. Calculations show that the coefficient of performance of this system (COP) is within the range 1.5–2.5 at an ambient temperature of 15–35 °C. Moreover, it is found that the relative humidity of the refrigerator air (RH_RA) and the temperature of freezer air (T_FA) have a significant effect on COP: COP decreases by about 13% when RH_RA varies from 0.4 to 0.8 and by 10% when T_FA runs from −18 to −23 °C.     

Experimental investigation of solar driven desiccant air conditioning system based on silica gel coated heat exchanger

A silica gel coated heat exchanger based air conditioning system driven by the evacuated tube solar water heater has been experimentally investigated. The system has been operated for two different modes namely cooling with dehumidification mode and heating with humidification mode in summer and winter season respectively. The system performance is analyzed in terms of regeneration rate, dehumidification rate and thermal coefficient of performance (COP_th). Experimental results demonstrated that, for cooling and dehumidification mode, the process air is cooled by an average temperature of 8.5 °C. A better dehumidification rate can be achieved by using pre-cooling before dehumidification process. Post-cooling after dehumidification process is found to be advantageous for cooling capacity and COP_th. For heating with humidification mode, the process air is heated by an average temperature of 13.3 °C with an average increment in humidity ratio of 1.9 g/kg. It is found that the average COP_th of the system is 0.45 and 0.87 for cooling and heating mode respectively.     

Experimental and theoretical study on a novel double evaporating temperature chiller applied in THICS using R32/R236fa

A novel chiller with double evaporating temperatures is proposed in this paper, which can be applied in temperature and humidity independent control system (THICS). A zeotropic mixture R32/R236fa is selected as the refrigerant, and chilled water with two different temperatures is produced. The experimental coefficient of performance (COP_exp), theoretical coefficient of performance (COP_th), and second law efficiency (η) of the chiller are studied. The performance of the chiller is studied by varying the mass fraction of R32 in the R32/R236fa (W(R32)), chilled water temperature, and the flow rates of the heat transfer media (chilled water and cooling water). The results show that the high temperature chilled water (T_H,out) can be at 15–18 °C, and the low temperature chilled water (T_L,out) can be at 6–8 °C. When T_H,out is 17 °C and T_L,out is 7 °C, the maximum COP_th and COP_exp are 4.73 and 3.97, respectively. Second law efficiency, η, increases to 31% as W(R32) increases from 0.3 to 0.6.     

Optimisation of a desiccant cooling system design with indirect evaporative cooler

Solar desiccant-based air-conditioning has the potential to significantly reduce cost and/or greenhouse gas emissions associated with cooling of buildings. Parasitic energy consumption for the operation of supply fans has been identified as a major hindrance to achieving these savings. The cooling performance is governed by the trade-off between supplying larger flow-rates of cool air or lower flow-rates of cold air. The performance of a combined solid desiccant-indirect evaporative cooler system is analysed by solving the heat and mass transfer equations for both components simultaneously. Focus is placed on varying the desiccant wheel supply/regeneration and indirect cooler secondary/primary air-flow ratios. Results show that for an ambient reference condition, and 70 °C regeneration temperature, a supply/regeneration flow ratio of 0.67 and an indirect cooler secondary/primary flow ratio of 0.3 gives the best performance with COP_e > 20. The proposed cooling system thus has potential to achieve substantial energy and greenhouse gas emission savings.     

Theoretical analyses of a new two-stage absorption-transcritical hybrid refrigeration system

In this context, a two-stage absorption-transcritical hybrid refrigeration system is proposed. R744 is chosen as a refrigerant for the transcritical heat pump subsystem and LiBr-H₂O working pair for the two-stage absorption refrigeration subsystem. Based on the mathematical and physical models, theoretical investigation is carried out on its performance. The main effects are discussed on COP_net (the ratio of cooling capacity powered by low-grade heat to the low-grade heat consumption for the hybrid system) and COP_mt (the ratio of cooling capacity powered by mechanical work to the mechanical work consumption for the hybrid system). Comparing with the normal two-stage absorption refrigeration system, theoretical results show that COP_net could be improved up to about 55% when the refrigeration temperature is 7 °C. In addition, COP_mt are more than 50% higher than that of the conventional transcritical refrigeration system. It is also found that both 45–55 °C low-grade heat and condensing heat could be used as actuating heat of the two-stage absorption refrigeration subsystem.     

Experimental investigation and analysis of composite silica-gel coated fin-tube heat exchangers

Desiccant coated heat exchanger provides a promising option for desiccant cooling system, since it can handle sensible load and latent load simultaneously within one component. It is fabricated by coating desiccant material on the surface of conventional fin-tube heat exchanger. In order to enhance the performance of conventional silica gel coated heat exchanger (SGCHE), a novel composite silica gel coated heat exchanger (CCHE) is proposed and fabricated. An experimental setup is built to test and compare the dynamic performance of SGCHE and CCHE. Influences of main operation parameters including water temperatures and inlet air conditions on system performance are analyzed in terms of average dehumidification capacity (D_avg) and thermal coefficient of performance (COP_th). Optimization of cycle switch modes is also discussed. Experimental results show that CCHE has better dehumidification performance compared with SGCHE. In addition, pre-cooling before dehumidification process is found to be advantageous to both D_avg and COP_th.     

Development of an ejector cooling system with thermal pumping effect

This paper presents a feasibility study of an ejector cooling system (ECS) that utilizes a multi-function generator (MFG) to eliminate the mechanical pump. The MFG serves as both a pump and a vapor generator. The MFG is designed based on the pressure equilibration between high and low pressures through heating and cooling process. In this design, an ECS that contains no moving components and is entirely powered by heat can be practicable. A prototype using refrigerant R141b as working fluid was constructed and tested in the present study. The experimental results showed that the system coefficient of performance (COP_o) was 0.218 and the cooling capacity was 0.786 kW at generating temperature (T_G) 90 °C, condensing temperature (T_C) 32.4 °C and evaporating temperature (T_E) 8.2 °C. While taking into account the extra heat needed for the MFG operation, the total coefficient of performance (COP_t) is 0.185. It is shown that a continuous operation for the generation of cooling effect in an ECS with MFG can be achieved. This cooling machine can be very reliable since there is no moving part.     

Potential of a desiccant-evaporative cooling system performance in a multi-climate country

In this paper, a desiccant-evaporative cooling system introduced and applied to a Ventilation and Makeup mode operating cycle. Desiccant part of hybrid cooling system is a heat driven component and effective in area where the use of thermal energy is more economical than electrical power. First, mathematical model of desiccant component based on transient and coupled heat and mass transfer derived. Then the hybrid system model applied to predict the system performance under various operational conditions. The numerical results validated using experimental measurements. The effects of various outdoor design conditions on COP and output of hybrid cycle temperature presented in contour plot forms. Based on these contour plots, COP and output cycle temperature can easily obtain under various ambient conditions. In addition, the potential of presented hybrid desiccant-evaporative cooling system to provide thermal comfort in various outdoor design conditions evaluated and compared with direct and direct-indirect evaporative cooler. The results show these systems are more effective than direct and direct-indirect evaporative cooling systems and provide a better thermal comfort even in hot and humid area. Moreover, introduced systems successfully provide better thermal comfort condition in a multi-climate country (Iran) especially in the area where the evaporative coolers are not applicable.     

Growth conditions of CuAlO2 films — Thermodynamic considerations

CuAlO₂ (CAO) is a metastable phase. However, it can be obtained at room temperature in air under certain conditions. p(O₂), T, and heating and cooling rate upon annealing of as-deposited CAO in pulsed laser deposition and other deposition methods act simultaneously on its formation (and disintegration upon cooling) and survival of crystallites contacting each other. Conversion of CAO into crystallized CuAl₂O₄ and CuO below the phase boundary at 1003 °C (at 0.21 atm O₂) is a very slow process helping to maintain the compound under ambient conditions. Conductivity is a function of the annealing parameters. With thick films on sapphire substrates, annealing can lead to an interconnected network of crystallites which assures electrical conductivity.     

Investigation on capacity matching in a heat pump and hollow fiber membrane-based two-stage liquid desiccant hybrid air dehumidification system

The heat pump and hollow fiber membrane-based two-stage liquid desiccant hybrid air dehumidification system is promising recently because solution droplets can be prevented from crossing over into the process air. The quasi-isothermal processes are realized by two-stage dehumidification processes and the system performance is improved. In this study, a novel capacity matching index (CMI) is introduced to evaluate the energy capacity matching of the system through modeling study. It is found that CMI is usually lower than 1 under the typical hot and humid weather condition like South China and the demand and supply of energy in the system is mismatching. As inlet air temperature rises, the dehumidification rates, CMI, EER and COP all decrease. But CMI is almost constant with different inlet air humidity. The influence of air inlet temperature to dehumidifiers and regenerators on the system performance is also investigated. The higher the inlet temperature of dehumidifiers is, the larger the CMI, EER and COP are. The dehumidification rates and CMI both grow with an increase in the inlet temperature of regenerators. It is beneficial for energy balance of the system and high moisture loads, but the side effect is that the EER and COP both decrease.     

Study of magnetocaloric effect in LaFe11.5Si1.5 alloys prepared by different cooling methods

LaFe_11.5Si_1.5 alloys are annealed at 1503 K for 5 h and cooled down to room temperature by furnace cooling, air cooling and quenching in ice water, respectively. The main phases are 1:13 phases in those alloys. The impurity phases are α-Fe and the amount of LaFeSi phase is so small that it is hard to observe in their XRD patterns. The powder X-ray diffraction patterns and SEM show that the three cooling methods have little influence on the phase relation and microstructure of those LaFe_11.5Si_1.5 alloys. But the lattice constant of LaFe_11.5Si_1.5 alloy prepared by quenching in ice water is lesser than those of the other two alloys, respectively. For studying the influence of different cooling processes on magnetic properties, the Curie temperature, thermal and magnetic hysteresis, magnetocaloric effect and relative cooling power are investigated. The result shows that the Curie temperature of LaFe_11.5Si_1.5 prepared by quenching in ice water is 197.6 K, about 4 K lesser than those of the other two LaFe_11.5Si_1.5 alloys. The maximum ΔS _M (T, H) of LaFe_11.5Si_1.5 prepared by furnace cooling and quenching in ice water is the most and the least under the field of 0–2 T, respectively.     

Experimental study on cooling performance of air conditioning system with dual independent evaporative condenser

Evaporative condenser is an energy efficient and environmentally friendly air conditioning equipment. This paper proposed an air conditioning system using dual independent evaporative condenser and investigated the cooling performance. Many factors, such as evaporator water inlet temperature, compressor frequency, air dry-bulb temperature, air velocity and water spray rate, which influenced the cooling performances of air conditioning system with evaporative condenser have been investigated. The results indicated that cooling capacity and coefficient of performance (COP) increased significantly with the increasing of evaporator water inlet temperature (12–25 °C), the air velocity (2.05–3.97 m s⁻¹) and the water spray rate (0.03–0.05 kg m⁻¹ s). However, COP decreased with the increasing ambient air dry-bulb temperature (31.2–35.1 °C) and the compressor frequency (50–90 Hz). Furthermore, the heat transfer coefficient (K₀) was 232–409 W m⁻² K⁻¹ in different air velocity and water spray rate.     

Performance analysis of ammonia absorption GAX cycle for combined cooling and hot water supply modes

An ammonia Generator–Absorber heat eXchange (GAX) absorption cycle with combined cooling and hot water supply modes is developed in this study. This paper proposes new multi-modes GAX cycles which function in three different modes (case 1, case 2 and case 3) of cooling and hot water supply with one hardware (ammonia/water GAX absorption heat pump), and finds the best cycle for performance improvement by the parametric analysis. The key parameters are the outlet temperature of hot water and the split ratio of the solution. It is found that the COP_c values for case 1, case 2 and case 3 are 60%, 42% and 87% of COP_c for case 0, respectively, which is the standard cooling mode for the conventional GAX cycle. From the viewpoints of hot water supply, case 1 gives the best performance. However, during the summer season when the cooling mode is the primary purpose rather than the hot water supply, case 3 is the most desirable cycle. The split ratio of the solution should be carefully determined depending on the primary application of the modified GAX cycle; cooling or hot water supply applications. It is also recommended that the optimum design values of UA_SCA and UA_HCA for case 3 should be less than those for case 1.     

Experimental investigation on heating performance of gas-injected scroll compressor using R32, R1234yf and their 20wt%/80wt% mixture under low ambient temperature

In this paper, an integrated gas-injected scroll compressor heat pump system using R1234yf, R32 and its binary mixtures as working fluid was developed and their heating performances under low ambient temperature were quantitatively evaluated. A composite test system consisting of second-refrigerant calorimeter and water-cooled condenser was used to test the system working performance. The condensing temperature, evaporating temperature, compressor power input and other variables were analyzed to evaluate the system heating capability and energy efficiency. Test results showed that the R1234yf system can run at an evaporating temperature of −25 °C. R1234yf/R32 mixture can run at an evaporating temperature of −20 °C and it has the highest heating COP value among other refrigerants; R1234yf/R32 gas injection system provided very significant performance improvements for heating performance, compared with no gas injection, the heating capacity and heating COP can improve 16%~20% and 13%~16%, respectively.     

Experimental investigation on vapor injection heat pump with a newly designed twin rotary variable speed compressor for cold regions

A newly designed twin rotary variable speed compressor was adopted to improve the heating performance of a vapor injection heat pump air conditioner (HPAC) at cold regions in this study. The performances of a R410A HPAC were measured and investigated. The results showed that an optimum intermediate pressure existed to achieve the maximum heating capacity and system $\text{COP}$ for the injection cycle, and the heating capacity was enhanced effectively by increasing compressor frequency, however, at the cost of $\text{COP}$ decrease. Comparative research on system performances between injection and non-injection cycles indicated that the newly designed compressor could improve the heating performance of the HPAC at low ambient temperatures. Specifically, the system's heating capacity was enhanced by 5.6%-14.4%, and $\text{COP}$ improvement reached up to 3.5%. Vapor injection cycle was not suggested at high ambient temperatures due to its lower $\text{COP}$ compared with non-injection cycles.     

Critical analysis of a biogas powered absorption system for climate change mitigation

The importance of biogas as a renewable alternative is being studied because of an increase in the cost of conventional fuels. The present article suggests a numerical study of a biogas powered NH₃–H₂O absorption refrigeration system where biogas is used to heat the water which serves as an energy input to generator of an absorption system. A computational model has been developed for the analysis which involves the determination of effect of generator temperature on various performance parameters, i.e., exergy losses in the different components, COP_cooling, COP_heating and the exergy efficiency. The results indicate that COP_cooling and COP_heating lies in the range of 0.159–0.33 and 1.16–1.33, respectively, whereas exergetic efficiency lies in the range of 0.29–0.80 for the same variation in generator temperature ranging from 50 to 70 °C. The highest exergy loss is found in the generator while the lowest is found in the condenser and it is also found that with an increase in the evaporator as well as absorber and condenser temperature, the COP increases and decreases, respectively. The effect of ambient temperature on exergy loss in the different components is also studied. Exergy analysis is an excellent tool to pin point the losses in the system due to irreversibility which are the basis for the further improvement in the system components.     

Solar heating and cooling system with absorption chiller and low temperature latent heat storage: Energetic performance and operational experience

Absorption cooling systems based on water/lithium bromide (LiBr) solution typically require an open wet cooling tower to transfer the reject heat to the ambient. Yet, water consumption, the need for water make-up and cleaning, formation of fog, and the risk of Legionella bacteria growth are hindering factors for the implementation of small solar cooling systems. The application of a latent heat storage supporting the heat rejection of the absorption chiller in conjunction with a dry cooling system allows eliminating the wet cooling tower. By that means heat rejection of the chiller is shifted to periods with lower ambient temperatures, i.e. night time or off-peak hours.The system concept and the hydraulic scheme together with an analysis of the energetic performance of the system are presented, followed by a report on the operation of a first pilot installation.     

Irreversible refrigerators with isothermal heat exchanges: Réfrigérateurs irréversibles avecéchanges thermiques isotherms

A maximum for the coefficient of performance (COP_f) is verified for a temperature difference between reservoirs and the hot isotherm of a closed irreversible cyclical refrigerator working at steady-state conditions. A maximum also exists when the COP_f is considered as a function only of a parameter depending on the thermal characteristics of the heat exchangers. The influence of the parameter and entropy generation on the COP_f maxima is described.     

Portable forced-air tunnel evaluation for cooling products inside cold storage rooms

Freezing process efficiency is affected by the required conditions to keep the air flow and temperature at the product surface. The objective of this work was to obtain results on comparative studies with air exhaustion and blowing using an experimental portable forced-air freezing tunnel. The device was designed to improve cooling rates inside storage room without the need for a cooling/freezing tunnel. A heterogeneity factor was proposed for air circulation evaluation and compared with convective heat transfer coefficient (h_ef) values. Lower modules of heterogeneity factor values represent smaller temperature differences among samples. Comparing two different air flow processes, heterogeneity factor values were similar for regions where the cooling air could flow without obstructions. However, larger differences were observed for regions with hampered air circulation. Results indicated that the air distribution, as well as the heat transfer, occurs more uniformly around the products in the exhausting process than in the blowing system.     

Room-temperature facile synthesis of hexagonal NaYF4 and NaYF4: Yb,Er powder without any organic additives and its upconversion fluorescence properties

Based on the structure difference between cubic and hexagonal phases, a facile method was designed for the synthesis of pure hexagonal phase β-NaYF₄ and β-NaYF₄: Yb, Er powder at room temperature and ambient pressure. In the whole process, neither any organic additives, nor high temperature and high pressure were involved that indicated an environmentally friendly technology. The optimal synthesis conditions for hexagonal phase at room temperature is the pH value between 5.0 and 6.5, the amorphous Y (OH) ₃ as Y precursor and the F^?/Y³⁺ molar ratio equal to 6:1. A possible preparation mechanism based on the difference between the coordination structure of the hexagonal and the cubic phases was also proposed. It is found that the as prepared powder of the hexagonal phase can emit red upconversion luminescence with a high R/G ratio, which is significantly different from the light emitted by the usually synthesized hexagonal phase. This method can also provide a new perspective for studying the sustainable synthesis of rare earth compounds.