Theoretical study on a frost-free refrigerated display cabinet

This study deals with a frost-free refrigerated display cabinet, in which frosting can be retarded by dehumidifying the air before it enters the evaporator of the display cabinet 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 refrigerated display cabinets), the proposed system can achieve high energy efficiency. Calculation shows that the coefficient of performance (COP) of this system is 3.1 and the daily electricity consumption of the compressor is 10.5 kWh when the temperature (T_RA) and relative humidity (RH_RA) of the return air entering the evaporator are 12 °C and 0.6, respectively. Furthermore, it is found that T_RA and RH_RA significantly affect the COP: COP increases by about 115% when T_RA varies from 8 to 14 °C, and increases by 35% when RH_RA varies from 0.5 to 0.8.     

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.     

Cellulose-pad water cooling system with cold storage

The present study develops a cooling system using water as the working medium which is cooled at night by cellulose-pad cooling tower (CWCT) and stored for cooling application at daytime. That is, it utilizes the natural energy drawn from diurnal ambient air temperature difference. A cooling system was built and tested. It is found that the coefficient of performance of CWCT for heat dissipation of water at night, COP_nt, is between 3.8 and 11 and varies linearly with the evaporation temperature glide D_G (difference between cold water temperature in the storage tank and wet-bulb temperature of ambient air). The COP for room cooling at daytime run with air cooler in a room, COP_day, is between 8.8 and 12.6. For day cycle operation, the measured overall cooling COP_o is 5.1. COP_o is expected to reach 9.4 at room temperature 45 °C.     

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.     

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.     

High-temperature heat capacity and vibrational spectra of Eu<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Eu₂Sn₂O₇ compound has been prepared by solid-state reaction (by sequentially firing a stoichiometric mixture of Eu₂O₃ and SnO₂ in air at 1273 and 1473 K) and its heat capacity has been determined by differential scanning calorimetry in the temperature range 370–1000 K. The heat capacity data have been used to evaluate the thermodynamic properties of europium stannate: enthalpy increment H°(T)–H°(370 K), entropy change S°(T)–S°(370 K), and reduced Gibbs energy Ф°(T). Raman spectra of Eu₂Sn₂O₇ polycrystals with the pyrochlore structure have been measured in the range 200–1200 cm^–1.     

Mixed conductivities of A-site deficient Y,Cr-doubly doped SrTiO3 as novel dense diffusion barrier and temperature-independent limiting current oxygen sensors

A-site-deficient Y, Cr doubly doped SrTiO₃ ((Y_0.08Sr_0.92)_1-xTi_0.8Cr_0.2O_3−δ (x = 0.01, 0.03, 0.05)) powders were synthesized via sol–gel method, followed by sintering at 1450 °C at ambient condition. The phase composition, mixed conductivities, and sensing performance are characterized to identify the influence of A-site deficiency on the Y- and Cr-doubly doped SrTiO₃. The ionic conductivity and total conductivity of (Y_0.08Sr_0.92)_1-xTi_0.8Cr_0.2O_3−δ clearly increase and decrease upon an increase in the A-site deficiency, respectively. The enlarged saddle point and decreased relaxation time are responsible for the augmentation of ionic conductivity. The oxygen sensor with (Y_0.08Sr_0.92)_1-xTi_0.8Cr_0.2O_3−δ dense diffusion layer show superior sensing performance with A-site deficiency level increasing. The relationship between logI_L and 1000/T is obtained and the charge compensation mechanism is systematically discussed. The obtained results demonstrated that limiting current is nearly independent of temperature at high operating temperature. This paper provides a chemical strategy to enhance the mixed conductivity of oxygen sensors through Y- and Cr-double doping and via a simple, low cost, and traditional sol–gel technique.     

Thermodynamic properties of (TeO<Subscript>2</Subscript>)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>(MoO<Subscript>3</Subscript>)<Subscript>1–<Emphasis Type="Italic">n</Emphasis></Subscript> glasses

The thermal behavior of (TeO₂) _n (MoO₃)_1–n (n = 0.75, 0.85, 0.90) tellurite glasses has been studied by differential scanning calorimetry in the range from T = 300 to T = 850 K and heat capacity has been measured in the temperature range. The thermodynamic characteristics of the devitrification process and glassy state have been determined. The experimental data obtained have been used to evaluate the standard thermodynamic functions of the system in glassy and supercooled liquid states: heat capacity C _p °(T), enthalpy H°(T)–H°(320), entropy S°(T)–S°(320), and Gibbs function G°(T)–G°(320) in the temperature range 320–630 K. The composition dependences of the glass transition temperature and thermodynamic functions for the glasses have been obtained. The thermal and thermodynamic properties of the tellurite glasses have been compared to those of previously studied (TeO₂) _n (WO₃)_1–n and (TeO₂) _n (ZnO)_1–n glasses.     

High-temperature heat capacity and thermodynamic properties of Tb<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>

Tb₂Sn₂O₇ has been prepared by solid-state reaction in air at 1473 K over a period of 200 h and its isobaric heat capacity has been studied experimentally in the range 350–1073 K. The C _p(T) data for this compound have no extrema and are well represented by the classic Maier–Kelley equation. The experimental C _p(T) data have been used to evaluate the thermodynamic properties of terbium stannate (pyrochlore structure): enthalpy increment H°(T)–H°(350 K), entropy change S°(T)–S°(350 K), and reduced Gibbs energy Ф°(Т).     

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.     

Synthesis and High-Temperature Heat Capacity of Neodymium Titanate

Neodymium dititanate, Nd₂Ti2O₇ (monoclinic structure, sp. gr. P2₁), has been prepared by solid-state reaction in air at temperatures from 1673 to 1773 K using the Nd₂O₃ and TiO₂ oxides as starting materials. The high-temperature heat capacity of the resultant polycrystalline Nd₂Ti₂O₇ samples has been determined by differential scanning calorimetry. The experimental C_p(T) data have been used to evaluate the thermodynamic functions of neodymium dititanate (enthalpy increment H°(T)–H°(320 K), entropy change S°(T)–S°(320 K), and reduced Gibbs energy Ф°(T)) in the temperature range 320–1053 K.     

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.     

Thermoelectric power of superionic conductor Ag7I4VO4

Thermoelectric power studies on Ag₇I₄VO₄. superionic conductor have been carried out in the temperature range 25 to 60°C. Thermo-electric power is found to vary linearly with the inverse of the absolute temperature, and can be expressed by the equation −θ = [(0.182 × 10³/T) + 0.276] mV/K. The heat of transport is nearly equal to the activation energy of Ag⁺ ion migration calculated from the conductivity plots indicating that the material has an average structure.     

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.     

Theoretical study of R32 in an oil flooded compression cycle with a scroll machine

R32 is regarded as a potential alternative for R410A, but it has a low slope of isentropic line, high superheat inside a compressor and thus a high discharge temperature. These disadvantages limit its wider adoption. In order to improve the performance of R32 air conditioner, oil flooded compression with regenerator has been suggested. A single stage oil flooded compressor model is developed to obtain a more accurate system-level improvement. In the compressor model, the heat transfer losses between shell and ambient, suction gas and motor, and high-pressure and low-pressure cylinders are considered. By means of parametric studies, it was found that the novel cycle resulted to be beneficial to increase the compressor internal superheating, to decrease the compressor heat losses and to improve its overall isentropic efficiency while cooling capacity or heating capacity is degraded. COP_h improvement can reach up to 16.4% for an evaporating and condensing temperatures of −25 °C and 45 °C, respectively. The discharge temperature resulted to be lower than 110 °C. In addition, a thorough comparison between R32 and R410A with both novel and baseline systems has been carried out. The results indicate that the novel cycle has potential benefits for applications in R32 air conditioners.     

Heat capacity of the MVO<Subscript>4</Subscript> (M = Al,Ga, In,Tl) orthovanadates

The heat capacity of InVO₄ has been determined by differential scanning calorimetry in the temperature range 339–1089 K. The experimental C_p(T) data have been used to evaluate the thermodynamic functions of indium orthovanadate: enthalpy increment H°(T)–H°(339 K), entropy change S°(T)–S°(339 K), and reduced Gibbs energy Ф°(Т). The specific heats of GaVO₄ and TlVO₄ have been evaluated.     

Synthesis and high-temperature heat capacity of Gd<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>

Gd₂Sn₂O₇ gadolinium stannate with the pyrochlore structure has been prepared by solid-state reaction and its high-temperature heat capacity has been determined by differential scanning calorimetry in the temperature range 350–1020 K. The C_p(T) data are shown to be well represented by the classic Maier–Kelley equation. The experimental C_p(T) data have been used to evaluate the thermodynamic functions of gadolinium stannate: enthalpy increment H°(T)–H°(339 K), entropy change S°(T)–S°(339 K), and reduced Gibbs energy Ф°(Т).     

Ambient effect on damping peak of NiTi shape memory alloy

We report the effects of ambient condition and loading rate on the damping capacity of a superelastic nickel-titanium shape memory alloy during stress-induced martensitic phase transformation with release and absorption of latent heat. The damping capacity was measured via a tensile loading-unloading cycle in the strain-rate range of 10^− 5-10^− 1/s and three ambient conditions: still air and flowing air with velocities of 2 m/s and 17 m/s. It is found that, for each ambient condition, the maximum damping capacity (damping peak) is achieved at the strain rate whose loading time (t_T) is close to the characteristic heat-transfer time (t_h) of the ambient condition.     

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.     

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.