Performance characteristics of a two-stage transcritical N2O refrigeration cycle with vortex tube

ABSTRACT

The thermodynamic analysis has been presented in this article using nitrous oxide as the refrigerant in a two-stage transcritical cycle with the vortex tube (TSTCVT) instead of the expansion valve and its results are compared with the two-stage transcritical cycle with the expansion valve (TSTCEV). The evaporator and the gas cooler temperature ranges in both the cycles have been considered between ?55°C to 5°C and 35°C to 60°C for the analysis. Gas cooler and intercooler pressures are simultaneously optimised to obtain the maximum cooling coefficient of performance (COP). The COP of the TSTCVT improves by 1.97–27.19% in comparison to TSTCEV. A decrease in evaporator temperature and an increase in gas cooler exit temperature reduce the COP of TSTCVT. The comparison of refrigerants N₂O and CO₂ in TSTCVT shows that N₂O exhibits higher cooling COP, higher second law of efficiency and lower optimum gas cooler pressure under the considered operating conditions.     

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.     

Potential nitrous oxide (N2O) emission from aquaculture in Nepal

Nitrous oxide has a greenhouse potential 310 times that of carbon dioxide (CO₂) over a 100-year lifespan. N₂O has been produced in the aquaculture system via nitrification and denitrification. This study mainly concerns estimation of the annual N₂O emission from aquaculture in Nepal and potential emission by the years 2020 and 2030. Theoretically, this study estimates emission of N₂O-N from aquaculture in Nepal in 2020 and 2030 to be 1.1 × 10⁸ g N₂O-N (±5.2%) and 2.24 × 10⁸ g of N₂O-N (±2.5%) of 95% confidence limit, respectively, with the emission factor 1.69 g of N₂O-N/kg of fish. In terms of carbon dioxide equivalent (CO₂eq), these values are 5.36 × 10¹⁰ g and 1.1 × 10¹¹ g, respectively, in 2020 and 2030, which accounts for 0.57% and 0.70% of Nepal’s CO₂ emission in 2020 and in 2030, respectively. This study also briefly deals with the parameters that stimulate N₂O emission and some potential technologies likely to reduce the N₂O emission in aquaculture. Greenhouse gas emission from aquaculture should be included by all countries in their estimation of global warming and climate change matters.     

Combined air conditioning and tap water heating plant using CO2 as refrigerant

A combined air conditioning and tap water heating plant using carbon dioxide (CO₂) as refrigerant has been investigated theoretically and experimentally. The system is suitable for countries with year around cooling demand, such as Indonesia or Singapore, and a need for hot tap water. A unique CO₂ transcritical cycle characteristic for heating process can afford an improvement to a CO₂ air conditioning system when rejected heat from the system is recovered. Some parameters affecting performance of the combined system are discussed.     

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%.     

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。     

Sorbents with the structure of layered double hydroxides for aquatic media treatment from U(VI)

Sorbents representing the layered double hydroxides of composition [Mg₄Fe₂(OH)₁₂] ? CO₃ ?nH₂O, [Zn₄Al₂(OH)₁₂] ? CO₃ ? nH₂O, and also the products of their thermal treatment at 400°C intended for the aquatic media treatment from U(VI) have been investigated. It is shown that the high sorption capacity of these sorbents makes it possible to recommend them for the effective removal of U(VI) from waters having different mineral composition.     

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.     

Performance of a CO2 sorbent for indoor air cleaning applications: Effects of environmental conditions,sorbent aging,and adsorption of co-occurring formaldehyde

Indoor air cleaning systems that incorporate CO₂ sorbent materials enable HVAC load shifting and efficiency improvements. This study developed a bench-scale experimental system to evaluate the performance of a sorbent under controlled operation conditions. A thermostatic holder containing 3.15 g sorbent was connected to a manifold that delivered CO₂-enriched air at a known temperature and relative humidity (RH). The air stream was also enriched with 0.8-2.1 ppm formaldehyde. The CO₂ concentration was monitored in real-time upstream and downstream of the sorbent, and integrated formaldehyde samples were collected at different times using DNPH-coated silica cartridges. Sorbent regeneration was carried out by circulating clean air in countercurrent. Almost 200 loading/regeneration cycles were performed in the span of 17 months, from which 104 were carried out at reference test conditions defined by loading with air at 25°C, 38% RH, and 1000 ppm CO₂, and regenerating with air at 80°C, 3% RH and 400 ppm CO₂. The working capacity decreased slightly from 43-44 mg CO₂ per g sorbent to 39-40 mg per g over the 17 months. The capacity increased with lower loading temperature (in the range 15-35°C) and higher regeneration temperature, between 40 and 80°C. The CO₂ capacity was not sensitive to the moisture content in the range 6-9 g/m³, and decreased slightly when dry air was used. Loading isothermal breakthrough curves were fitted to three simple adsorption models, verifying that pseudo-first-order kinetics appropriately describes the adsorption process. The model predicted that equilibrium capacities decreased with increasing temperature from 15 to 35°C, while adsorption rate constants slightly increased. The formaldehyde adsorption efficiency was 80%-99% in different cycles, corresponding to an average capacity of 86 ± 36 µg/g. Formaldehyde was not quantitatively released during regeneration, but its accumulation on the sorbent did not affect CO₂ adsorption.     

Window/door opening‐mediated bedroom ventilation and its impact on sleep quality of healthy,young adults

This work examined window/door opening as means of bedroom ventilation and the consequent effect upon occupants’ sleep, using data from 17 healthy volunteers. Bedroom CO₂ level, temperature, and relative humidity were measured over 5 days, for two cases: open window or door (internal, bedroom door), and closed window and door. Participant filled questionnaires and sleep diary provided subjective measure of sleep quality. Actigraphy objectively monitored the participants during sleep. Additionally, a FlexSensor, placed under pillows of participants, detected movement during sleep. Average CO₂ level for the Open conditions was 717 ppm (SD = 197 ppm) and for Closed conditions was 1150 ppm (SD = 463 ppm). Absolute humidity levels were similar for both conditions, while Open conditions were slightly cooler (mean = 19.7°C, SD = 1.8°C) than Closed (mean = 20.1°C, SD = 1.5°C). Results showed significant correlations (P < .001) between actigraphy data and questionnaire responses for: sleep latency (r = .45), sleep length (r = .87), and number of awakenings (r = .28). Of all analyzed sleep parameters, questionnaire‐based depth of sleep (P = .002) and actigraphy‐based sleep phase (P = .003) were significantly different between Open and Closed conditions.     

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 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%.     

太阳能辅助CO_2热泵系统回热器设计及实验研究

在CO2跨临界循环系统中,节流损失相对较大。通过增加回热器的方法以提高跨临界CO2热泵系统的COP。根据相关的换热关联式对CO2热泵系统的回热器进行了设计计算并且进行了相关的实验研究。计算得到的回热器,长度为1.27 m,内管内径6 mm,外管内径13 mm,壁厚1 mm。并且在相同环境工况下,利用自行搭建的CO2热泵系统实验台,分别在有回热器与无回热器时进行了实验研究。实验结果表明:当系统引入回热器后,压缩机排气压力、温度上升,压缩机耗功上升速率加快,制热量增加速率亦加快,且制热量增加速率大于压缩机耗功增加速率,系统制热COP增加,带回热器时系统平均COP达到3.34,无回热器时系统平均COP为3.03,提高约10.2%。     

Source identification of nitrous oxide on autotrophic partial nitrification in a granular sludge reactor

Emission of nitrous oxide (N₂O) during biological wastewater treatment is of growing concern since N₂O is a major stratospheric ozone-depleting substance and an important greenhouse gas. The emission of N₂O from a lab-scale granular sequencing batch reactor (SBR) for partial nitrification (PN) treating synthetic wastewater without organic carbon was therefore determined in this study, because PN process is known to produce more N₂O than conventional nitrification processes. The average N₂O emission rate from the SBR was 0.32 ± 0.17 mg-N L⁻¹ h⁻¹, corresponding to the average emission of N₂O of 0.8 ± 0.4% of the incoming nitrogen load (1.5 ± 0.8% of the converted NH₄⁺). Analysis of dynamic concentration profiles during one cycle of the SBR operation demonstrated that N₂O concentration in off-gas was the highest just after starting aeration whereas N₂O concentration in effluent was gradually increased in the initial 40 min of the aeration period and was decreased thereafter. Isotopomer analysis was conducted to identify the main N₂O production pathway in the reactor during one cycle. The hydroxylamine (NH₂OH) oxidation pathway accounted for 65% of the total N₂O production in the initial phase during one cycle, whereas contribution of the NO₂⁻ reduction pathway to N₂O production was comparable with that of the NH₂OH oxidation pathway in the latter phase. In addition, spatial distributions of bacteria and their activities in single microbial granules taken from the reactor were determined with microsensors and by in situ hybridization. Partial nitrification occurred mainly in the oxic surface layer of the granules and ammonia-oxidizing bacteria were abundant in this layer. N₂O production was also found mainly in the oxic surface layer. Based on these results, although N₂O was produced mainly via NH₂OH oxidation pathway in the autotrophic partial nitrification reactor, N₂O production mechanisms were complex and could involve multiple N₂O production pathways.     

Human responses to high levels of carbon dioxide and air temperature

In this study, 30 subjects were exposed to different combinations of air temperature (T_a: 24, 27, and 30°C) and CO₂ level (8000, 10 000, and 12 000 ppm) in a high-humidity (RH: 85%) underground climate chamber. Subjective assessments, physiological responses, and cognitive performance were investigated. The results showed that as compared with exposure to T_a = 24°C, exposure to 30°C at all CO₂ levels caused subjects to feel uncomfortably warm and experience stronger odor intensity, while increased mental effort and greater intensity of acute health symptoms were reported. However, no significant effects of T_a on task performance or physiological responses were found. This indicated that subjects had to exert more effort to maintain their performance in an uncomfortably warm environment. Increasing CO₂ from 8000 to 12 000 ppm at all T_a caused subjects to report higher rates of headache, fatigue, agitation, and feeling depressed, although the results were statistically significant only at 24 and 27°C. The text typing performance and systolic blood pressure (SBP) decreased significantly at this exposure, whereas diastolic blood pressure (DBP) and thermal discomfort increased significantly. These effects suggest higher arousal/stress. No significant interaction effect of T_a and CO₂ concentration on human responses was identified.     

Emission of CO2 and N2O from soil cultivated with common bean (Phaseolus vulgaris L.) fertilized with different N sources

Addition of different forms of nitrogen fertilizer to cultivated soil is known to affect carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions. In this study, the effect of urea, wastewater sludge and vermicompost on emissions of CO₂ and N₂O in soil cultivated with bean was investigated. Beans were cultivated in the greenhouse in three consecutive experiments, fertilized with or without wastewater sludge at two application rates (33 and 55 Mg fresh wastewater sludge ha^− 1, i.e. 48 and 80 kg N ha^− 1 considering a N mineralization rate of 40%), vermicompost derived from the wastewater sludge (212 Mg ha^− 1, i.e. 80 kg N ha^− 1) or urea (170 kg ha^− 1, i.e. 80 kg N ha^− 1), while pH, electrolytic conductivity (EC), inorganic nitrogen and CO₂ and N₂O emissions were monitored. Vermicompost added to soil increased EC at onset of the experiment, but thereafter values were similar to the other treatments. Most of the NO₃⁻ was taken up by the plants, although some was leached from the upper to the lower soil layer. CO₂ emission was 375 C kg ha^− 1 y^− 1 in the unamended soil, 340 kg C ha^− 1 y^− 1 in the urea-amended soil and 839 kg ha^− 1 y^− 1 in the vermicompost-amended soil. N₂O emission was 2.92 kg N ha^− 1 y^− 1 in soil amended with 55 Mg wastewater sludge ha^− 1, but only 0.03 kg N ha^− 1 y^− 1 in the unamended soil. The emission of CO₂ was affected by the phenological stage of the plant while organic fertilizer increased the CO₂ and N₂O emission, and the yield per plant. Environmental and economic implications must to be considered to decide how many, how often and what kind of organic fertilizer could be used to increase yields, while limiting soil deterioration and greenhouse gas emissions.     

Strength characterization of cohesionless soil treated with cement and polyvinyl alcohol

Lime, cement, and bitumen are well-known traditional binders for improving the bearing capacity of soils. However, the production of these binders results in a massive impact on the environment due to the emission of greenhouse gases, such as carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). In this study, a novel cement–polyvinyl alcohol (PVA) mixture is proposed to fabricate strong composite geomaterials. The advantage of the proposed materials is that they can increase the unconfined compressive strength (UCS) and, combined with cement hydration, producing PVA glue, can be used to fill up the soil pores. Laboratory tests indicate a threefold increase in UCS with the cement–PVA-combined mixture compared to a cement-stabilized one. The results of scanning electron microscope (SEM) observations suggest that the cement–PVA composite can ameliorate the pore structure that is more solid than the cement-stabilized one. Moreover, by curing at 80 °C, the strength of the cement–PVA stabilized soil decreases by threefold, plateauing at the same strength as the non-PVA stabilized soil. In addition, the results of cyclic thermal exposure tests suggest that, with the increase in the number of heating/cooling cycles, the UCS gradually decreases compared to the initial one. However, the loss of UCS is less than 25 % under the three cycles of exposure. Thus, these composites have the potential to promote urban renewal projects in an ecofriendly manner.     

Summer fluxes of atmospheric greenhouse gases N2O, CH4 and CO2 from mangrove soil in South China

The atmospheric fluxes of N₂O, CH₄ and CO₂ from the soil in four mangrove swamps in Shenzhen and Hong Kong, South China were investigated in the summer of 2008. The fluxes ranged from 0.14 to 23.83 μmol m⁻² h⁻¹, 11.9 to 5168.6 μmol m⁻² h⁻¹ and 0.69 to 20.56 mmol m⁻² h⁻¹ for N₂O, CH₄ and CO₂, respectively. Futian mangrove swamp in Shenzhen had the highest greenhouse gas fluxes, followed by Mai Po mangrove in Hong Kong. Sha Kong Tsuen and Yung Shue O mangroves in Hong Kong had similar, low fluxes. The differences in both N₂O and CH₄ fluxes among different tidal positions, the landward, seaward and bare mudflat, in each swamp were insignificant. The N₂O and CO₂ fluxes were positively correlated with the soil organic carbon, total nitrogen, total phosphate, total iron and NH₄⁺-N contents, as well as the soil porosity. However, only soil NH₄⁺-N concentration had significant effects on CH₄ fluxes.     

N2O emission hotspots at different spatial scales and governing factors for small scale hotspots

Chronically nitrate-loaded riparian buffer zones show high N₂O emissions. Often, a large part of the N₂O is emitted from small surface areas, resulting in high spatial variability in these buffer zones. These small surface areas with high N₂O emissions (hotspots) need to be investigated to generate knowledge on the factors governing N₂O emissions. In this study the N₂O emission variability was investigated at different spatial scales. Therefore N₂O emissions from three 32 m² grids were determined in summer and winter. Spatial variation and total emission were determined on three different scales (0.3 m², 0.018 m² and 0.0013 m²) at plots with different levels of N₂O emissions. Spatial variation was high at all scales determined and highest at the smallest scale. To test possible factors inducing small scale hotspots, soil samples were collected for slurry incubation to determine responses to increased electron donor/acceptor availability. Acetate addition did increase N₂O production, but nitrate addition failed to increase total denitrification or net N₂O production. N₂O production was similar in all soil slurries, independent of their origin from high or low emission soils, indicating that environmental conditions (including physical factors like gas diffusion) rather than microbial community composition governed N₂O emission rates.