Investigation of Environmental Effects Based on Exergetic Irreversibility for Display Cases’ Units in Commercial Cooling

This study examines energetic and exergetic performances of display cases’ units used in market applications depending on different refrigerants. Besides CO₂ emission potential of each refrigerant based on exergetic irreversibility obtained from analyses is calculated by the method of Total Equivalent Warming Impact (TEWI). In this study, 1 kW cooling capacity and vapor compression cooling cycle is taken as reference and refrigerants of R-22, R-134a, R-404A, and R-507 together with alternative refrigerant R-407C and R152a are examined separately. According to analyses, R-404A gas, used widely in market applications, has low performance with average COP 3.89 and average exergy efficiency 55.20%. R-152a gas has the best performance by the thermodynamics parameters including COP 4.49, exergy efficiency 63.79%, and 0.23 kW power consumption and emission parameter 14097.490 ton CO₂/year. Although COP is used as a criterion to evaluate the systems, this study finally emphasizes the importance of exergy analysis and TEWI method which are important methods to determine irreversibility and emission potential of the systems.     

Optimal heat rejection pressure for transcritical CO2 refrigeration cycle with an expander

Heat rejection pressure plays an important role in designing a transcritical CO₂ refrigeration system, and it has an optimal value to maximize the system’s coefficient of performance (COP). With a thermodynamic simulation model, the optimal heat rejection pressure is studied in the paper for an expander cycle, as well as conventional throttle valve cycle. The effects of compressor efficiency, expander efficiency, gas cooler outlet temperature, and evaporation temperature on the optimal heat rejection pressure are analyzed. It is the first time for a transcritical CO₂ expander cycle that the optimal heat rejection pressure is correlated with the gas cooler outlet temperature and the evaporation temperature at given compressor efficiency and expander efficiency. The average deviation from the correlation to simulation results is less than 1.0%. The correlation provides a guideline to system development and performance optimization of a transcritical CO₂ expander cycle.     

Performance of a small-scale solar-powered adsorption cooling system

In this study, an experimental investigation on the performance of a small-scale residential-size solar-driven adsorption (silica gel-water) cooling system that was constructed at Assiut University campus, Egypt is carried out. As Assiut area is considered as hot, arid climate, field tests for performance assessment of the system operation during the summer season are performed under different environmental operating conditions. The system consists of an evacuated tube with a reflective concentration parabolic surface solar-collector field with a total area of 36 m², a silica gel-water adsorption chiller of 8 kW nominal cooling capacity, and hot and cold water thermal storage tanks of 1.8 and 1.2 m³ in volume, respectively. The results of summer season field test show that under daily solar insolation varying from 21 to 27 MJ/m², the solar collectors employed in the system had high and almost constant thermal efficiency. The daily solar-collector efficiency during the period of system operation ranged from about 50% to 78%. The adsorption chiller performance shows that the chiller average daily coefficient of performance (COP) was 0.41 with the average cooling capacity of 4.4 kW when the cooling-water and chilled-water temperatures were about 31°C and 19°C, respectively. As the chiller cooling water is cooled by the cooling tower in the hot arid area, the cooling water is at a higher temperature than the design point of the chiller. Therefore, an experiment was carried out using the city water for cooling. The results show that an enhancement in the chiller COP by 40% and the chilling power by 17% has been achieved when the city water was 27.7°C.     

Design and performance of a dynaniic gas flux chamber

Chambers are commonly used to measure the emission of many trace gases and chemicals from soil. An aerodynamic (flow through) chamber was designed and fabricated to accurately measure the surface flux of trace gases. Flow through the chamber was controlled with a small vacuum at the outlet. Due to the design using fans, a partition plate, and aerodynamic ends, air is forced to sweep parallel and uniform over the entire soil surface. A fraction of the air flowing inside the chamber is sampled in the outlet. The air velocity inside the chamber is controlled by fan speed and outlet suction flow rate. The chamber design resulted in a uniform distribution of air velocity at the soil surface. Steady state flux was attained within 5 min when the outlet air suction rate was 20 L/min or higher. For expected flux rates, the presence of the chamber did not affect the measured fluxes at outlet suction rates of around 20 L/min, except that the chamber caused some cooling of the surface in field experiments. Sensitive measurements of the pressure deficit across the soil layer in conjunction with measured fluxes in the source box and chamber outlet show that the outflow rate must be controlled carefully to minimize errors in the flux measurements. Both over- and underestimation of the fluxes are possible if the outlet flow rate is not controlled carefully. For this design, the chamber accurately measured steady flux at outlet air suction rates of approximately 20 L/min when the pressure deficit within the chamber with respect to the ambient atmosphere ranged between 0.46 and 0.79 Pa.     

Investigation of activated carbon/ethanol for low temperature adsorption cooling

Commercially available adsorption cooling systems use water/silica gel, water/zeolite and ammonia/ chloride salts working pairs. The water-based pairs are limited to work above 0°C due to the water high freezing temperature, while ammonia has the disadvantage of being toxic. Ethanol is a promising refrigerant due to its low freezing point (161 K), nontoxicity, zero ozone depletion, and low global warming potential. Activated carbon (AC) is a porous material with high degree of porosity (500–3000 m²/g) that has been used in wide range of applications. Using Dynamic Vapour Sorption (DVS) test facility, this work characterizes the ethanol adsorption of eleven commercially available activated carbon materials for cooling at low temperature of ?15°C. DVS adsorption results show that Maxsorb has the best performance in terms of ethanol uptake and adsorption kinetics compared to the other tested materials. The Maxsorb/ethanol adsorption process has been numerically modeled using computational fluid dynamics (CFD) and simulation results are validated using the DVS experimental measurements. The validated CFD simulation of the adsorption process is used to predict the effects of adsorbent layer thickness and packing density on cycle uptake for evaporating temperature of ?15°C. Simulation results show that as the thickness of the Maxsorb adsorbent layer increases, its uptake decreases. As for the packing density, the amount of ethanol adsorbed per plate increases with the packing density reaching maximum at 750 kg/m³. This work shows the potential of using Maxsorb/ethanol in producing low temperature cooling down to ?15°C with specific cooling energy reaching 400 kJ/kg.     

Exergy analysis of the transcritical N2O refrigeration cycle with a vortex tube

In this article, a comparative study is presented for the transcritical cycle with expansion valve (TCEV) and transcritical cycle with vortex tube (TCVT) mainly based on the second law of thermodynamics. Natural refrigerant nitrous oxide (N₂O) is used in both the cycles for analysis. The evaporator and gas cooler temperatures are varied from ?55°C to 5°C and 35°C to 60°C, respectively. The effects of various operating and design parameters on the optimum heat rejection pressure, coefficient of performance (COP), exergy loss (irreversibility), and the exergetic efficiency are studied. Exergy analysis of each component in TCEV and TCVT is performed to identify the amount and locations of irreversibility. It is observed that the use of the vortex tube in place of the expansion valve reduces the total exergy losses and increases the exergetic efficiency as well as COP. The exergetic efficiency and COP of the TCVT are on average 10–12% higher compared to TCEV for the considered operating conditions. The computed values of the exergetic efficiency for TCVT using refrigerant N₂O are the highest at an evaporator temperature of ?55°C, and the corresponding values of exergetic efficiency and exergy losses varies between 25.35% and 15.67% and between 74.65% and 84.33%, respectively. However, COP at the same evaporator temperature of ?55°C varies between 0.83 and 0.51. Furthermore, the optimum heat rejection pressure in TCVT is lower compared to that in TCEV. The results offer significant help for the optimum design and operating conditions of TCVT with refrigerant N₂O.     

改性活性炭脱除硫化氢工艺参数研究

针对改性活性炭脱除硫化氢过程,研究了空速、温度、原料气浓度、颗粒分布孔径4个主要工艺参数对脱硫效率的影响。结果表明:空速在1 500～4 200h-1时穿透硫容随着空速的降低而增加,当空速继续降低为1 200h-1时穿透硫容基本不变;当0～40℃时,随着温度的升高,穿透时间增加,脱硫效率提高,当温度超过40℃时,随着温度继续升高脱硫效率降低;相同空速下原料气硫化氢浓度变化只改变穿透时间;改性活性炭脱硫剂发挥脱硫作用的微孔结构范围是1～5nm。     

A novel radiant floor system: Detailed characterization and comparison with traditional radiant systems

ABSTRACT

Radiant floor systems have the potential to reduce energy consumption and the carbon footprint of buildings. This study analyzed a novel radiant panel configuration comprising a metal plate with small spikes that can be pressed into cement board or wood. The behavior of this configuration was simulated for different materials for the metal plate, spike dimensions, and varying spacing between spikes. An annual energy simulation model compared the radiant panel configuration with the traditional concrete-based system. Simulations were run under heating dominant, cooling dominant, and neutral conditions; significant cost savings and greenhouse gas emission reduction were seen across all scenarios.     

Research on distribution performance of desert sand for heat storage in downcomer

In this research, desert sand is used as the sensible heat storage medium, which exchanges heat with air in the downcomer to realize heat storage and heat release. The desert sand distribution uniformity has a significant impact on the heat exchange performance and efficiency between desert sand and air for the process of convection in the downcomer. Given the superiority of sensible heat storage in convective heat transfer between desert sand and air, distributors with cylinder or conical bore solid particles and homogeneity performance testing device are designed and manufactured on the basis of convection system equipped with solid particle–air downcomer. Then, the convection experiment between solid sand and air is researched. The greater the desert sand flow rate and higher the volume density, the larger the variance of regional mass flow rate and the worse the homogeneity performance. For the cylinder bore distributor, the smaller the sand particle size is, the greater affected the sand groups can be. The sand homogeneity performance is preferable with the two particle size ranges: 0.18-0.25 mm and 0.15-0.18 mm. The total sand flow rate decreases, but the uniformity improves with the increase of the air flow velocity, and the best distribution performance is achieved at an air velocity of 0.6 m/s. However, the distribution performance declines with the air flow velocity persistently increasing because the sand groups are pushed to one pipe side close to the wall. The sand groups deflect seriously with the air flow velocity increasing.     

RIPARIAN MICROCLIMATE AND STREAM TEMPERATURE RESPONSE TO FOREST HARVESTING: A REVIEW1

Forest harvesting can increase solar radiation in the riparian zone as well as wind speed and exposure to air advected from clearings, typically causing increases in summertime air, soil, and stream temperatures and decreases in relative humidity. Stream temperature increases following forest harvesting are primarily controlled by changes in insolation but also depend on stream hydrology and channel morphology. Stream temperatures recovered to pre‐harvest levels within 10 years in many studies but took longer in others. Leaving riparian buffers can decrease the magnitude of stream temperature increases and changes to riparian microclimate, but substantial warming has been observed for streams within both unthinned and partial retention buffers. A range of studies has demonstrated that streams may or may not cool after flowing from clearings into shaded environments, and further research is required in relation to the factors controlling downstream cooling. Further research is also required on riparian microclimate and its responses to harvesting, the influences of surface/subsurface water exchange on stream and bed temperature regimes, biological implications of temperature changes in headwater streams (both on site and downstream), and methods for quantifying shade and its influence on radiation inputs to streams and riparian zones.     

Integrated energy management strategy of powertrain and cooling system for PHEV

ABSTRACT

Energy management strategy (EMS) is crucial in improving the fuel economy of plug-in hybrid electric vehicle (PHEV). Existing studies on EMS mostly manage powertrain and cooling system separately which cannot get the minimum total energy consumption. This paper aims to propose a novel EMS for a new type of dual-motor planetary-coupled PHEV, which considers cooling power demand and effect of temperature on fuel economy. Temperature-modified engine model, lithium-ion battery model, two motors, and cooling system models are established. Firstly, the separated EMS (S-EMS) is designed which manages powertrain and cooling system separately. Sequentially, after the analysis of thermal characteristics of the powertrain and cooling system, the thermal-based EMS (T-EMS) is then proposed to manage two systems coordinately. In T-EMS, cooling power demand and the charging/discharging energy of motors are calculated as equivalent fuel consumption and integrated into the object function. Besides, a fuzzy controller is also established to deicide the fuel-electricity equivalent factor with consideration of the effect of temperature and state of charge on powertrain efficiency. Finally, the hardware-in-loop experiment is carried out to validate the real-time effect of EMS under the New European Driving Cycle. The result shows that cooling power demand and temperature can significantly affect the fuel economy of the vehicle. T-EMS shows better performance in fuel economy than S-EMS. The equivalent fuel consumption of the cooling system of T-EMS decreases by 27% compared with that of S-EMS. The total equivalent fuel consumption over the entire trip of PHEV using T-EMS is reduced by 9.7%.     

Grey relational analysis of an integrated cascade utilization system of geothermal water

With the drastic decrease in fossil resources and rapid deterioration of the global environment, the utilization of geothermal resources has been strongly advocated. The combination of heat, power, and cold utility generation is commonly used to increase the utilization efficiency of geothermal resources. In this study, an integrated cascade utilization system of waste geothermal water (ICUWGW) from a flash geothermal power plant in China is established to increase the utilization efficiency of geothermal water. The waste geothermal water leaving the power plant is proposed for further use in cascade for two-stage LiBr/H₂O absorption cooling, agricultural product drying, and residential bathing. Twelve candidate temperature schemes showing different inlet and outlet temperatures of every subsystem are proposed for the ICUWGW. Several criteria are selected for the evaluation and screening of the candidate schemes. Grey relational analysis incorporating analytic hierarchy process is conducted to screen the optimal temperature scheme for the ICUWGW to meet the comprehensive criteria of thermodynamics and economics. Results show that the optimal scheme features significant improvement in energy efficiency, exergy efficiency, and equivalent electricity generation efficiency compared with those of the current geothermal power plant. The investment payback time of the additional subsystems for cooling, drying, and bathing is 1.85 years. Exergy analysis is also conducted to determine the further optimization potential of the optimal ICUWGW. Sensitivity analysis of electricity price on the performance of the optimal ICUWGW is also performed.     

Refrigeration waste heat utilization for drying applications: a review

ABSTRACT

Cold chain industry has a vast potential for waste heat recovery. It is a matter of importance for energy efficiency point of view, as global energy demand is increasing day by day. Ample amount of low-grade energy is either unutilized or underutilized. The heat rejected by a Heat pump or refrigeration system emerged as a promising solution for dehydration by utilizing low-grade waste heat despite higher investment. As compared to solar drying technology, heat pump drying evolved as a reliable method regarding better process control, energy efficiency, and quality of the product to be dried. Energy utilized through the refrigeration system’s waste/exhaust heat recovery in combination with or without renewable energy source enhances the overall efficiency of the system and also reduces the cost. This useful review investigated and compared the research findings of waste heat utilization through heat pump and from condenser of refrigeration system on laboratory, pilot as well as industrial scale for drying of various fruits, vegetables, and agro products. Various drying parameters like drying rate, moisture content, Specific Moisture Extraction Rate (SMER), Coefficient of Performance (COP), Exergy efficiency, and temperature as well as humidity conditions inside the drying chamber were also reviewed to promote the technological advancement of energy utilization by commercial cold storage waste heat recovery.     

Analysis of an intermediate-temperature proton-conducting SOFC hybrid system

The performance of an intermediate-temperature proton-conducting solid oxide fuel cell (pSOFC) hybrid system is investigated in this work. The hybrid system consists of a 20-kW pSOFC, a micro gas turbine (MGT), and heat exchangers. Heat exchangers are used to recover waste heat from pSOFC and MGT. The performance of the system is analyzed by using Matlab/Simulink/Thermolib. Flow rates of air and hydrogen are controlled by assigning different stoichiometric ratio (St). St considered in this study is between 2 and 3.5 for air, and between 1.25 and 1.45 for hydrogen. Results show that the combined heat and power (CHP) efficiency increases as the fuel St decreases or air St increases. This is because lowering fuel St means fewer fuel will be wasted from the fuel cell stack, so the CHP efficiency increases. On the other hand, as air St increases, the amount of recovered waste heat increases, so does the CHP efficiency.     

Experimental study of new solar air heater design

The suitable design is the most important key to a cost-effective solar air heater. Although there are many techniques that have been proposed to improve the solar air heaters’ performance by means of different turbulence promoters, they cannot ensure a compromise between the cost and the effectiveness. The aim of this study is to find simple and tolerable solution to get rid of the inconvenience resulting from the widely adopted heat-transfer-enhancement techniques by providing an optimized solar air heater design. The proposed design consists of a slightly curved smooth flow channel with an absorber plate of convex shape. A prototype of a curved solar air heater of 1.28 m² collector area was built and tested under summer outdoor conditions in Biskra (Algeria). The performance was evaluated in terms of thermal and effective efficiency for mass flow rates of 0.0172, 0.029, and 0.0472 kg/sm². It is observed that the overall efficiency of this solar air heater is considerably higher in comparison with the efficiency range of the conventional smooth flat plate heaters reported in the literature for similar operating conditions.     

Experimental investigation on the phase change material-based modular heat exchanger for thermal management of a building

A phase change material (PCM)-based flat plate modular heat exchanger for free cooling application, suitable for the diurnal temperature variation that prevails during the summer months of Bangalore city, India is designed and experimentally investigated. The flow and other parameters selected in the present study are meant to suit the accelerated charging of the PCM in the modular heat exchanger during the early morning hours, and to provide cool energy to the room during the daytime, by circulating the ambient air through the heat exchanger at a lower velocity. It is observed from the charging experiments that the decrease in the inlet air temperature has more influence in reducing the solidification time than the increase in the inlet air velocity. The heat exchanger designed in the present investigation is capable of maintaining the room temperature around 30°C for a longer duration of 8 hr when the heat load is 0.5 kW. It is suggested to design the modular heat exchanger with a surface area proportionate to the present heat exchanger size when the room heat load increases beyond 0.5 kW, in order to maintain a minimum comfortable temperature of 30°C in the room.     

Configuration influence in relation to fluid flow of venturi system

Due to its simplicity and accurately measuring the flow rate, the venturi system is a special kind of pipe that is widely used in various applied fluid mixtures. One of the venturi system's important applications is ejectors devices that accurately facilitate adding air to water to sustain oxygen demand target levels in many waterworks engineering systems. This study aims to improve venturi system measurement accuracy through experimental investigation and analytical analysis for the venturi system conditional configuration parameters effect on target aeration operational efficiency. In the experiment work, different runs are implemented to characterize the performance of such aerators by describing the impact of venturi characteristics and configurations, including water flow rate, air inlets orifices diameters, inlet velocities, throat lengths, inlet angles, outlets angles, and outlet diameters on aeration efficiency. Results show that the venturi air vent diameter is an important governing parameter for determining aeration performance value. Additionally, an indicated increase in aeration performance with an increasing throat length to its diameter ratio. Meanwhile, the results revealed a varying noted effect of the venturi system characteristics and configurations on aeration performance. Moreover, the equations that relate venturi system configuration and Reynolds numbers with the aeration operational performance are developed to facilitate the target accurate aeration efficiency estimation.     

Auto-adaptive Air Distribution and Structure Optimization of Ejector Burner for Biomass Alcohol Fuels

A plenty of studies on the utilization of biomass alcohol fuels have been conducted, but combustion efficiency and stability of this fuels still need to be improved. Based on biomass alcohol fuels (bio-methanol and bio-ethanol), this paper studied auto-adaptive air distribution characteristics and optimum structure parameters of an ejector burner by numerical simulation method. Also, an experiment was conducted to verify the numerical results. The results show that the mole air entrainment ratio (MAER) keeps almost constant when the ejector fuel nozzle exit locates at the segment between the ejector throat and the suction chamber entrance, but a bigger ratio α would lead to a higher MAER till the α is bigger than 8.5 for bio-methanol and 11.5 for bio-ethanol. The bio-ethanol fuel is more beneficial for air carrying role because of its big molecular weight. Operation pressure (P_w) has a little impact on MAER of the two fuels, but the rise of back pressure (P_b) would lead to rapid decrease of MAER for the two fuels. For the optimum structure burners, the MAER can be maintained at the value of theoretical complete combustion. Its changing rate is less than 2.3% for bio-methanol and 2.5% for bio-ethanol when the burner load changes from 30% to 120%, which is highly consistent with the experimental results. The optimum burner can distribute air supply automatically with the changing of burner load.     

Studies on the Drying Characteristics of Zingiber Officinale Under Open Sun and Solar Biomass (Hybrid) Drying

Drying characteristics of Zingiber officinale (Ginger) under the open sun and direct type natural convection solar biomass (hybrid) drying were studied. It has been observed that under open sun drying conditions, the drying rate depends on the product thickness and climatic conditions. The results have been drawn for both the summer (April-May, 2004) and winter (November-January, 2003–04) months of Delhi, in India. In the hybrid drier, the ginger, with a thickness of 0.008 m, dried in 33 hours in comparison to 96 hours in open-sun drying. The overall drying efficiency of the hybrid drier was found to be 18% and 13% under summer and winter climatic conditions respectively. The loss of volatile oil content of the ginger is less in hybrid drier in comparison to open sun drying. It was found that the average drying air temperature of 60°C with average air velocity of 0.6m/sec was sufficient for the drying of ginger in the hybrid drier. Ginger quality after drying is better and drying time is less in the hybrid drier in comparison to open-sun drying. The hybrid drier is a simple device, which can be manufactured with locally available materials and can be used for drying of other spices, vegetables and fruits etc.     

Effect of pipes in heat pump system on electric vehicle energy saving

ABSTRACT

Refrigerant pressure drop and temperature change in pipes are normally ignored in the thermodynamic analysis of traditional vehicle air conditioning system, this will result in serious errors. In this Paper, pressure drop and temperature difference are simulated in different pipes of electric vehicle (EV) heat pump system to analysis the effects of pipes in the actual EV heat pump system. The results indicate that the greater the mass flow, the faster pressure drop increases, the temperature difference decreases. Pressure drop of saturated liquid refrigerant is smaller than that of saturated gas refrigerant at the same saturation pressure and mass flow rate. The higher the refrigerant pressure (no phase change), the slower pressure drop decreases, the faster the temperature difference decreases. Pressure drop decreases with the increment of bending angle of the pipe. For EV heat pump system, suitable valves and less branches are helpful for energy saving of the system. Shortening the pipe between compressor and condenser can reduce temperature change obviously. Pressure drop per unit length in the pipe between evaporator and compressor is large especially in heating mode because of lower refrigerant density. It even reaches to over 100 times of that in the pipe between condenser and throttle valve in heating mode and has negative effects on the performance of the system. If the evaporator is closer to the compressor and the number of branches is less, then pressure drop will decrease a lot, which will be advantageous for energy saving of the heat pump system.