Why is adiabatic compressed air energy storage yet to become a viable energy storage option?

Recent theoretical studies have predicted that adiabatic compressed air energy storage (ACAES) can be an effective energy storage option in the future. However, major experimental projects and commercial ventures have so far failed to yield any viable prototypes. Here we explore the underlying reasons behind this failure. By developing an analytical idealized model of a typical ACAES design, we derive a design-dependent efficiency limit for a system with hypothetical, perfect components. This previously overlooked limit, equal to 93.6% under continuous cycling for a typical design, arises from irreversibility associated with the transient pressure in the system. Although the exact value is design dependent, the methodology we present for finding the limit is applicable for a wide range of designs. Turning to real systems, the limit alone does not fully explain the failure of practical ACAES research. However, reviewing the available evidence alongside our analytical model, we reason that underestimation of the system complexity, difficulty with the integration of off-the-shelf components, and a number of misleading performance claims are the primary reasons hindering ACAES development.</p

EXPERIMENTAL STUDY TO DETERMINE THE LOCAL CONDENSATION HEAT TRANSFER COEFFICIENTE FOR R134A FLOWING THROUGH A 4.8 MM INTERNAL DIAMETER SMOOTH HORIZONTAL TUBE

Refrigerant fluid R134a is commonly one of the most utilised invapour compression cycles wordlide, wheter in dommestic HVAC orautomotive regrigeration systems. This paper’s goal is toexperimetnally determine the fluid local condensation Heat TransferCoefficient (HTC), in several flor regimes. In this work, the mass fluxwas equal to 200, 250 and 300 kg/(m2s) and the fluid flowedthrough a smooth, horizontal 4.8 mm internal diameter aluminiumpipe, during which its vapour quality varied along the entire qualityrange. A purpose built test rig was developed, in which fluidconditions were constantly monitored and controlled. Throughmeasurements in temperature and pressure, an energy balance wasused to calculate experimentally the local heat transfer coeeficient.Average results for the unit quality range equalled to 3781 , 3459 and3944 W/m2K for saturation temperature equal to 30 C and theaforementioned mass velocities. Likewise, at 35C the averages HTCfound were 2903, 3141 and 3898 W/m2K at the same mass fluxrates. Later on, the experimental results were compared to tencommonly used HTC correlation found in relevant references,with Chato’s correlation returning the best fitting

EXPERIMENTAL STUDY OF THE PRESSURE DROP OF THE ECOFLUID R1234YF COMPARED TO THE FLUID R134A IN SMOOTH TUBES WITH 4.8MM INTERNAL DIAMMETER

The refrigerant fluid R1234yf is a hydrofluorefine with zero potential for degradation of the ozone layer and low potential for global warming. It is one of the potential substitutes for the currently used R134a in automotive systems. In this work, the pressure drop suffered by the fluids R134a and R1234yf when flowing in a test section through a pipe with a 4.8 mm internal diameter was measured. The pressure drop was plotted as a function of the void fraction at the exit of the test section and the values were compared concerning the change in mass flux, change in saturation temperature, and comparatively between R1234yf and R134a. A significant increase in pressure drop was observed by the increases of the mass flux, showing an increment of 155.46% of the pressure loss from 200 to 300 kg·m-2·s-1 for R1234yf at 35ºC and 161.07% for R134a in the same conditions. Despite being high, those values are expected since increasing mass flux also increases the friction between both phases. On the other hand, by increasing the saturation temperature, the pressure drop is slightly lower once the differences between the densities of the liquid and vapor phases are reduced. Compared with R134a, the R1234yf ecofluid presents less pressure drop, showing a reduction of 24% for 300 kg·m-2·s-1

RF spectroscopy in a resonant RF-dressed trap

We study the spectroscopy of atoms dressed by a resonant radiofrequency (RF) field inside an inhomogeneous magnetic field and confined in the resulting adiabatic potential. The spectroscopic probe is a second, weak, RF field. The observed line shape is related to the temperature of the trapped cloud. We demonstrate evaporative cooling of the RF-dressed atoms by sweeping the frequency of the second RF field around the Rabi frequency of the dressing field.Comment: 7 figures, 8 pages; to appear in J. Phys.

EXPERIMENTAL STUDY OF A DOMESTIC SOLAR HEATER WITH IRON ORE AS THE ABSORBER

In Brazil's households, the use of electricity to heat water prevails, but its replacement by solar collectors can be justified due to the country's high level of solar radiation, average temperature and number of sunny days. A drawback preventing the broad spread of this technology its elevated inicial cost that, in spite of the short return on investment time, hinders its insertions on the population lower classes. Thereby, this study's goal is to search a cheaper alternative to one of the collector's most expensive component, by proposing the replacement of the aluminium sheets, used as absorber, by iron ore. This material was chosen due to its abundance in Minas Gerais, low cost and for being dark colored, ensuring high levels of absortance (0,4 < λ< 0,7 µm). A finely grained hematitic () ore was used to achieve an uniform compaction throughout its area, reducing this way, the thermal contact resistance between the absorber and the copper tubes. The collector was built and tested in similar conditions it would have faced during its lifespan, and the result shows that the maximum thermal efficiency was 58%

AVERAGE TOTAL HEMISPHERIC EMISSIVITY MEASUREMENT IN THE LWIR SPECTRUM FOR ADHESIVE TAPES USED IN THE THERMOGRAPHY TAPE TEST

Thermographers often use comparative methods to estimate surfaceemissivity. Among the most used is the tape method. In this method a knownemissivity tape in the LWIR (Long Wavelength Infrared) spectrum is placedon the surface to be inspected. After thermal equilibrium, the temperature ofthe tape and the surface under inspection must be the same. In this case, thetemperature observed on the tape is the reference temperature. The emissivityof the surface must then be changed until the reference temperature isreached. It is common practice to admit the value of the emissivity of theadhesive tape as 0.95, there are few studies that present these data withmetrological rigor, which leads to doubts about the emissivity of thecommercial tapes. In this work, experiments were performed on Tekbond,Double A, 3M 101, Rapix, Altape, adhesive tapes for temperatures of 50, 55,60, 65 and 70ºC. An experimental apparatus was developed through which itwas possible to estimate surface reflection, transmission and atmosphericemission for one and two layers of tapes, in order to make emissivitymeasurements possible. Through the data it was possible to statisticallyestimate the LWIR average total hemispheric emissivity as well as theacceptance range to 95% certainty, being therefore equal to ε=0.94±0.03. Itis possible to arm, therefore, that the value of 0.95, usually used as emissivityof the adhesive tape, is extremely reasonable because it is 0.01 of the averagevalue of the Gaussian distribution calculated by this work

How pressure affects costs of power conversion machinery in compressed air energy storage; Part I:Compressors and expanders

This study addresses a critical economic aspect in compressed air energy storage that has not been discussed much in existing literature: the impact of operating pressure on machinery capital cots. It aims to answer whether the cost per unit of power for power conversion systems changes with the maximum storage pressure. Considering that higher storage pressures are associated with greater energy density, enhanced energy storage capabilities and improved system efficiency. This paper helps clarify uncertainties in initial cost estimations for power-generation plants. Effects of operating pressure on the components and overall sizes and consequently costs of power conversion machinery are individually investigated in two parts. Part I encompasses the compressor and expanders, and part II comprehensively discusses the effects of the operating pressure on the costs of heat exchangers. The analysis employs a conceptual engineering approach, revealing that higher intake pressure reduces overall compressor/expander size, leading to cost savings. Additionally, increasing the number of compression stages for higher storage pressures enhances exergy storage cost-effectiveness. To establish an advanced adiabatic CAES plant with a storage pressure of 200 bar instead of 50 bar, there is potential for a 6 % reduction in $/kW expenditure.</p

How pressure affects costs of power conversion machinery in compressed air energy storage; part II:Heat exchangers

In the field of compressed air energy storage, a critical economic aspect that has been overlooked in existing literature relates to the influence of storage pressure on the capital cost of power conversion system. In Part I, a comprehensive study was conducted to address this question focusing on compressors and expanders. This part is devoted to the heat exchangers and basically assesses the engineering rationale behind the relationship between the cost per kW for HXs and operating pressure. Based on the performed analysis, the operating pressure of a HX impacts two crucial cost-related factors: the heat transfer area and required tube thicknesses. Higher operating pressures are associated with the smaller heat transfer area tending to lower costs, but increasing pressure raises tube thickness requirements, tending to increase costs. Below approximately 200 bar, the former effect prevails over the latter, leading to cost reductions with rising pressure. Conversely, at higher pressures, the latter effect outweighs the former, resulting in cost increases with increasing pressure. On the other hand, as the number of compression stages is increased to attain higher storage pressures, there is a noteworthy variation in the cost contribution of HXs. Specifically, the contribution of HX costs within the PCS machinery escalates from 10 % at a storage pressure of 30 bar to approximately 35% at a storage pressure of 350bar. This cost increase is accompanied by a substantial reduction in costs associated with other PCS machinery components (compressors and expanders), ultimately justifying the advantages of operating at higher storage pressures.</p

How pressure affects costs of power conversion machinery in compressed air energy storage; Part I:Compressors and expanders

This study addresses a critical economic aspect in compressed air energy storage that has not been discussed much in existing literature: the impact of operating pressure on machinery capital cots. It aims to answer whether the cost per unit of power for power conversion systems changes with the maximum storage pressure. Considering that higher storage pressures are associated with greater energy density, enhanced energy storage capabilities and improved system efficiency. This paper helps clarify uncertainties in initial cost estimations for power-generation plants. Effects of operating pressure on the components and overall sizes and consequently costs of power conversion machinery are individually investigated in two parts. Part I encompasses the compressor and expanders, and part II comprehensively discusses the effects of the operating pressure on the costs of heat exchangers. The analysis employs a conceptual engineering approach, revealing that higher intake pressure reduces overall compressor/expander size, leading to cost savings. Additionally, increasing the number of compression stages for higher storage pressures enhances exergy storage cost-effectiveness. To establish an advanced adiabatic CAES plant with a storage pressure of 200 bar instead of 50 bar, there is potential for a 6 % reduction in $/kW expenditure.</p

Improving the performance of a shell and tube latent heat thermal energy storage through modifications of heat transfer pipes:A comprehensive investigation on various configurations

The modification of the geometric configurations of heat transfer pipes in shell and tube Latent Heat Thermal Energy Storage (LHTES) systems not only enhances the melting process of the phase change material (PCM) but also improves the overall performance of these systems. This study aims to investigate ways to enhance the performance of LHTES systems by employing heat transfer pipes with various fin and twisted tape arrangements in a horizontal orientation. The Finite Volume Method and Enthalpy-Porosity method are employed to simulate the melting process. Stearic acid is used as the PCM material, while water serves as the heat transfer fluid. Eight different geometric configurations are modelled in the LHTES system: base case, horizontal fins, vertical fins, helical fins, horizontal tape, vertical tape, twisted tape and helical fins with twisted tape. The results show that within the time range of 0 and 29 min, the combined configuration of helical fins with twisted tape consistently demonstrates the fastest melting process. After 29 min, the configuration with vertical fins exhibits a marginally faster melting process than the combined configuration of helical fins with twisted tape. The configurations involving tapes also contribute to accelerated melting, although to a lesser extent than those with fins. Particularly, twisted tape proves highly effective in facilitating faster melting. The complete melting process times for configurations with vertical fins, helical fins, and combined helical fins with twisted tape are 38.7 %, 23.5 % and 32.7 % faster compared to the base case which is ∼69 min. Among the configurations, using tapes results in higher flow resistance and surface area compared to the base case. The attractive features of these configurations make them ideal for creating efficient and space-saving energy storage systems. This study provides crucial insights into essential heat and mass transfer processes, which can be leveraged to develop advanced LHTES systems for enhanced performance and sustainable energy solutions.</p