Performance analysis of a transcritical N2O refrigeration cycle with vortex tube

In the present study, vortex tube is used in transcritical vapour compression cycle as expansion device to improve the coefficient of performance (COP). The thermodynamic analysis has been performed using nitrous oxide in transcritical cycle with vortex tube (TCVT) and its results are compared with those of a transcritical cycle with expansion valve (TCEV). The evaporator and the gas cooler temperatures have been varied between ?55°C and 5°C and between 35°C and 60°C, respectively, for the analysis. The COP of the TCVT improves by 1.72–27.01% compared to TCEV. A decrease in evaporator temperature and an increase in gas cooler exit temperature result in a decrease in COP. The increase in cold mass fraction brings a negligible increase in maximum COP. The performance comparison of N₂O and CO₂ in TCVT shows that maximum cooling COP for N₂O is higher than for CO₂, but the optimum pressure required for N₂O is lower than for CO₂.     

Operation costs exergoeconomic of transcritical refrigeration cycle with carbon dioxide

ABSTRACT

In this work, the thermodynamic and exergoeconomic studies are performed to a single vapour compression refrigeration cycle which uses CO₂ as a refrigerant in transcritical conditions. This analysis establishes the behaviour of the compressor discharge pressure based on the gas cooler temperature, when it is operating at ambient conditions greater than 20°C. The effect of varying the cooling chamber temperature is also shown. In the analysis, the coefficient of performance (COP), exergetic efficiency and exergoeconomic operation costs for each equipment are obtained, considering a refrigeration capacity of 5 TR. For instance, at a gas cooler temperature of 45°C and a cooling chamber temperature of ?5°C, the maximum COP is 1.78 to a gas cooler pressure of 120 bar and the exergetic efficiency is 0.1556; however, operating at these conditions, it generates an exergoeconomic operation cost of 1.74?€?h^?1.     

A thermodynamic analysis of a transcritical cycle with refrigerant mixture R32/R290 for a small heat pump water heater

In this study, a thermodynamic analysis on the performance of a transcritical cycle using azeotropic refrigerant mixtures of R32/R290 with mass fraction of 70/30 has been performed. The main purpose of this study is to theoretically verify the possibility of applying the chosen refrigerant mixture in small heat pumps for high temperature water heating applications. Performance evaluation has been carried out for a simple azeotropic mixture R32/R290 transcritical cycle by varying evaporator temperature, outlet temperature of gas cooler and compressor discharge pressure. Furthermore, the effects of an internal heat exchanger on the transcritical R32/R290 cycle have been presented at different operating conditions. The results show that high heating coefficient of performance (COP_h) and volumetric heating capacity can be achieved by using this transcritical cycle. It is desirable to apply the chosen refrigerant mixture R32/R290 in small heat pump water heater for high temperature water heating applications, which may produce hot water with temperature up to 90 °C.     

融冰过冷二氧化碳空调循环高压参数分析

常规跨临界二氧化碳空调循环的性能系数对气体冷却器出口工质温度十分敏感，该温度越低，同等条件下性能越好。为了提高制冷量和性能系数，在常规循环基础上引入了融冰过冷器以大幅度降低气体冷却器出口工质温度。基于压缩机等熵效率实验数据对融冰过冷二氧化碳空调循环进行了计算，重点分析了循环高压对系统性能的影响。计算结果表明：引入融冰过冷能大幅度提高系统性能系数，并且在循环高压较低时更为明显。在典型的应用范围内，融冰过冷循环不存在使性能系数最大的优化高压。当其他参数一定时，高压越高，性能系数越低。与常规无融冰过冷循环不同，中间换热器的引入不利于性能系数的提高，因此在进行多运行模式的实际系统设计时应予折中考虑。本文的分析结果可为实际系统设计和运行提供指导。     

Advanced control of condensing temperature for enhancing the operating efficiency of air-cooled chillers

Air-cooled chillers are commonly used to provide cooling energy in commercial buildings in the subtropical climate. These chillers have long been considered inefficient because they operate under head pressure control where the condensing temperature is kept high in the refrigeration cycle. This paper considers a novel method to strategically lower the condensing temperature to enhance chiller efficiency at any outdoor temperature. An experiment is carried out on an air-cooled reciprocating chiller to confirm that an electronic expansion valve enables refrigerant to be adequately fed into the evaporator in all operating conditions, even when the condensing temperature falls to around 20 °C. By enhancing the heat rejection airflow of the condenser, both the condensing temperature and chiller power can decrease considerably with less fluctuation. According to the algorithm of staging condenser fans, this enhancement corresponds to resetting the set point of the condensing temperature based on any outdoor temperature. The potential and benefits of implementing this reset strategy are discussed.     

Optimisation of electronic device coupled LiBr–H2O absorption heat pump system working at ambient temperature using Taguchi approach

The study attempts to optimise the parameters of a lithium bromide–water (LiBr–H₂O) miniature absorption refrigeration system using Taguchi approach for electronic cooling working at ambient conditions. Thermodynamic optimisation was performed to obtain the optimum coefficient of performance (COP) for heat removal of 100?W by using the Taguchi approach. Three factors were considered: generator temperature (T_g), condenser temperature (T_c) and absorber temperature (T_a), at three different levels. The result showed that the percentage contribution of generator temperature is more on COP. Optimisation of three significant heat exchangers: evaporator, condenser and absorber, was performed by using the Taguchi approach. The study was carried out individually for all the components by varying the hydraulic diameter and aspect ratio at three different levels. The results showed that the hydraulic diameter contributes more than the aspect ratio.     

Effect of mass recovery evaporator temperature and condenser temperature on the performance of a solar adsorption-based desalination plant

Water scarcity increases alarmingly as the population increases. Over the years, a number of salt water desalination techniques have been proposed and reached limitations. The requirement of minimum energy is very well satisfied by an adsorption system, since it can operate with low-grade energy and waste heat exhaust from most industries. The first part of this work discusses the effect of condenser and evaporator temperatures on the performance of silica-gel adsorption cycle mathematically. The second part discusses the performance variations due to mass recovery in the two-bed adsorption system mathematically. It was found that the reduction in condenser temperature and increase in the evaporator temperature both increase the fresh water productivity and cooling capacity of a plant. A desalination plant with mass recovery assistance is superior in performance than the conventional plant. Portable water productivity of 8?m³/day/ton is achieved with the condenser temperature of 15°C and the evaporator temperature of 30°C.     

CO_2跨临界水——水热泵循环系统的实验研究

在作者设计和建立的实验台上进行了CO2 跨临界水—水热泵多种工况的循环性能实验研究 ,系统运行稳定、调节方便。实验结果表明 ,系统运行时的最大制冷 (热 )系数取决于系统的蒸发温度和气体冷却器出口温度。CO2 跨临界循环热泵较之传统工质热泵具有较大优势 ,尤其是在高温热泵领域。指出了系统的改善措施     

CO2跨临界水—水热泵循环系统的实验研究

在作者设计和建立的实验台上进行了CO2跨临界水-水热泵多种工况的循环性能实验研究,系统运行稳定、调节方便,实验结果表明,系统运行时的最大制冷（热）系数取决于系统的蒸发温度和气体冷却器出口温度,CO2跨临界循环热泵较之传统工质热泵具有较大优势,尤其是在高温热泵领域,指出了系统的改善措施。     

Development and testing of two-stage evaporative cooler

Evaporative cooling has been proved as an effective method of storage of fruits and vegetables of moderate respiration rates. Therefore, a modified evaporative cooler named two-stage evaporative cooler (TSEC) has been developed to improve the efficiency of evaporative cooling for high humidity and low temperature air conditioning. Two-stage evaporative cooler consists of the heat exchanger and two evaporative cooling chambers. The performance of cooler has been evaluated in terms of temperature drop, efficiency of the evaporative cooling and effectiveness of TSEC over single evaporation. The temperature drop through TSEC ranged from 8 to 16 °C. With the several observations for diurnal runs, it was observed that TSEC could drop the temperature up to wet bulb depression of ambient air and provided the 90% relative humidity. Efficiency of single evaporation was 85–90%. Effectiveness of the two-stage evaporative cooling was found to be 1.1–1.2 over single evaporation. The two-stage evaporative cooler provided the room conditions as 17–25 °C temperature and 50–75% relative humidity, which can enable to enhance the shelf-life of wide range of fruit and vegetables of moderate respiration rates.     

The fate of chlorine and chloramines in cooling towers Henry's law constants for flashoff

Chlorine and chloramines are volatile compounds which are stripped (“flashed off”) from recirculating cooling water systems by the large volumes of air which flow through the water in the cooling tower. The fraction of a volatile gas, such as hypochlorous acid (HOCl), which is removed by stripping is determined by Henry's constant H for that gas: H = X_G/X_L, where X_G is the mole fraction of the gas in the air and X_L is the mole fraction of the gas in the water. We have measured H for HOCl, OCl^?, NH₃, NH₂Cl, NHCl₂ and NCl₃ at 20 and 40°C. We found H = 0.076 for HOCl, compared to 0.71 for NH₃, at 20°C. At 40°C, H was about 2.5-fold larger for HOCl. This means that 10–15% of the HOCl is stripped from cooling water on each passage through a typical cooling tower. The measured flashoff of free available chlorine (HOCl + OCl^?) was markedly pH-sensitive with a pK of 7.5, exactly as expected if HOCl is volatile but OCl^? is not. The data permit a quantitative understanding of the fate of chlorine in cooling systems. The values of H at 40°C for NH₂Cl, NHCl₂ and NCl₃ were 1.28, 3.76 and 1067. This means that all of the chloramines are quickly stripped in a cooling tower.     

Effect of reduced evaporator airflow on the high temperature performance of air conditioners

Two residential sized air conditioners were tested in psychrometric rooms at reduced evaporator airflows ranging from 0 to 50% below that recommended by the manufacture of each of the units. Outdoor temperatures ranged from 35 to 49°C. One of the units used a thermal expansion valve for flow control while the other unit used a short tube orifice. Performance of the units was quantified by the capacity, power, coefficient of performance, and sensible heat ratio. Results at 35°C indicated that the reduction in air produced a larger drop in capacity and coefficient of performance for the orifice controlled unit than the thermal expansion valve controlled unit. The power showed less than a 4% reduction for either unit as the airflow was reduced by 50%.     

A novel control logic for fan coil unit considering both room temperature and humidity control

This paper proposes a novel fan coil unit (FCU) control logic, which controls fan speed level and water valve duty ratio to make both temperature and humidity match the set-points. The performance of the proposed control logic is compared with two other control logics using simulation. One of them is to open/close water valve to make room air temperature match the set-point, which is the usually used control logic. Another control logic is to open/close water valve according to a duty ratio decided by a fuzzy logic. The simulation results show that the proposed control logic can achieve control performance: the temperature is t _set ± 0.5°C and humidity is d _set ± 3% when duty ratio period is 300 s, and the temperature is t _set ± 1°C and humidity is d _set ± 7% when duty ratio period is 600 s. The control performance of proposed control logic is better than the valve on/off control logic and the fuzzy control logic.     

跨临界CO2汽车空调系统研究进展

综述了近年来这种空调系统在理论和实验研究方面取得的主要进展,包括系统各主要部件(压缩机、换热器和节流机构)的发展状况及研究的发展趋向。已有的研究表明,系统的COP存在最大值,而且与气体冷却器出口温度几乎呈正比关系;气体冷却器的出口温度和压缩机的效率是影响系统性能的主要因素;安装内部换热器可以提高系统的性能,尤其是在高压侧压力不高时;当高压侧压力较高时,与气体冷却器和压缩机相比,吸气换热器的影响几乎可以忽略不计。     

Bin weather data for five climatic zones of Morocco

ABSTRACT

The Bin method is one of the well-known and accurate steady-state approach to predict heating and cooling energy consumption of buildings. The application of this method requires detailed ambient temperature bin data. In this paper, the dry-bulb temperature bin data for five climatic zones of Morocco was generated. It was calculated from ?12°C to 45°C with 3°C increments in six daily 4?h shifts (1–4, 5–8, 9–12, 13–16, 17–20 and 21–24?h) for Marrakech, Ifrane, Fez, Errachidia and Agadir. The bin data given in this paper may have the positive impact on building energy efficiency in Morocco.     

Performance analysis of a solar-assisted ground-source heat pump system for greenhouse heating: an experimental study

This study investigates the performance characteristics of a solar-assisted ground-source (geothermal) heat pump system (SAGSHPS) for greenhouse heating with a 50 m vertical 32 mm nominal diameter U-bend ground heat-exchanger. This system was designed and installed in the Solar Energy Institute, Ege University, Izmir (568 degree days cooling, base: 22 °C, 1226 degree days heating, base: 18 °C), Turkey. Based upon the measurements made in the heating mode from the 20th of January till 31st of March 2004, the heat extraction rate from the soil is found to be, on average, 57.78 W/m of bore depth, while the required borehole length in metre per kW of capacity is obtained as 11.92. Design practices in Turkey normally call for U-bend depths between 11 and 13 m/kW of heating. The entering water temperature to the unit ranges from 8.2 to 16.2 °C, with an average value of 14 °C. The greenhouse air has a maximum day temperature of 31.05 °C and night temperature of 14.54 °C with a relative humidity of 40.35%. The heating coefficient of performance of the heat pump (COP_HP) is about 2.00 at the end of a cloudy day, while it is about 3.13 at the end of sunny day and fluctuates between these values in other times. The COP values for the whole system are also obtained to be 5–20% lower than COP_HP. The clearness index during experimental period is computed as average 0.56. At the same period, Cucumus sativus cv. pandora F₁ was raised, and product quality was improved with the climatic conditions in the designed SAGSHPS. However, experimental results show that monovalent central heating operation (independent of any other heating system) cannot meet the overall heat loss of the greenhouse if the ambient temperature is very low. The bivalent operation (combined with other heating system) can be suggested as the best solution in Mediterranean and Aegean regions of Turkey.     

CO2跨临界水源热泵两级供热系统理论比较

在供热系统中，需要的水温较高时，由于回水温度高而导致CO2跨临界水源热泵能效比下降。如果降低回不温度，就要求不同温度的热水分级处理。CO2跨临界水源热泵两级供热系统可以实现能量的分级利用。分析对比了3种形工的CO2跨临界水源热泵两级供热系统，发现它们各有特点，可根据不同的使用要求采用适宜的两组系统。     

直接蒸发冷却器在热管机房空调中的应用研究

研究了一种直接蒸发冷却器与热管机房空调耦合的空调系统,直接蒸发冷却器的使用降低了冷凝器进口温度,可延长热管的年运行时间,同时,提高热管的COP。实验中利用焓差实验室对机组进行测试,实验结果表明,在小温差(室内外温差5~10℃)下,热管的COP在加直接蒸发冷却器前后由2.86~5提升到4.5~5.36。     

An estimate of the correction applied to radiant flame measurements due to attenuation by atmospheric CO2 and H2O

A narrow band statistical model has been used to estimate the uncertainty introduced into radiative heat flux measurements from fires which is attributable to attenuation by atmospheric H₂O and CO₂. The flames were assumed to be soot-dominated with blackbody emission characteristics. The ambient surroundings near the flames were assumed to be homogeneous with the total pressure being fixed at one atmosphere. Atmospheric CO₂ concentrations were held constant at 0.04 kPa and the water vapor concentrations varied between 0.55–5.63 kPa based on temperature and relative humidity. The remaining partial pressures were accounted for by O₂ and N₂. Correlations to estimate atmospheric attenuation are given over a range of conditions that include path length (10–200 m), ambient temperature (19–35°C), source temperature (1000–1600°C) and relative humidity (0.25–1.0) as parameters. The results of these calculations indicate that, over this range of conditions, the radiant flux can be attenuated by as much as 42%.     

How sulfates and increased temperature affect delayed ettringite formation (DEF) in white cement mortars

In this study,white cement CEM I and white limestone cement CEM II-LL A and Β with 15%, 25% and 35% limestone substitution were studied. The way delayed ettringite is forming due to exposure to increased temperature (50 °C) and external sulfate attack was examined in mortar samples which were immersed for 90 days in three different solutions: (a) saturated solution Ca(OH)₂ at 50 °C, (b) saturated solution of Ca(OH)₂ at 20 °C and (c) 5% w/w Na₂SO₄ solution at 50 °C. During this period mortar samples were visually observed regularly while their expansion was estimated on a weekly basis by measuring the change of length with a micrometer. At the end of the 90-days period the compressive strength of the mortars was determined and the deterioration products were identified through means of X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and thermogravimetric analysis (DTG). The results of this study show that DEF occurred in two forms. Samples cured at increased temperature contained DEF type I, which caused mediocre expansion and damage. Samples cured at increased temperature in the presence of sulfates produced DEF type II, which caused significant damage on the surface and exhibited high expansion.