Cut-off Temperatures Research Articles

Influence of Cell Temperature on Theoretical Properties of InGaP/ InGaAs/Ge Triple-Junction Concentrated Solar Cells

Aims: This study aims to analyze the accuracy of single- and double-diode models in predicting the electrical parameters of InGaP/InGaAs/Ge triple-junction solar cells as described in relevant patents under various operating conditions. Methods: This study obtained and analyzed experimental and theoretical values of the relevant electrical parameters of solar cells through a combination of experimental research and theoretical model calculations. Results: The results indicated that the root mean square error of the short-circuit current decreased from 0.21 at 400 W/m² to 0.11 at 1000 W/m². The temperature of the two precision cut-off points for the open-circuit voltage in the single- and double-diode models increased from 34°C and 64°C at 400 W/m² to 39°C and 72°C at 1000 W/m². Additionally, for peak power and conversion efficiency, the precision cut-off temperatures of the single- and double-diode models were 56°C, 68°C, and 77°C at 400 W/m², 600 W/m², and 800 W/m², respectively. Conclusion: The theoretical values of the short-circuit current exceeded the corresponding experimental values. The single- and double-diode models for open-circuit voltage exhibited two accuracy cut-off points, with the single-diode model demonstrating greater accuracy within this temperature range. Similarly, the peak power and conversion efficiency models for single- and double-diodes have an accuracy cut-off point, with the double-diode model performing better at higher temperatures.

Utilizing Ragone framework for optimized phase change material-based heat sink design in electronic cooling applications

This study explores the latent thermal energy storage potential of an organic phase change material with porous copper foam and its applicability in electronic cooling under varying heat load conditions. The organic phase change material, n-eicosane, is known for its inherently low thermal conductivity of 0.15 W/mK, rendering it vulnerable during power spikes despite its abundant latent heat energy for phase transition from solid to liquid. Porous copper foams are often integrated into n-eicosane to enhance the composite's thermal conductivity. However, the volume fraction of the phase change material in the porous foam that optimally improves the thermal performance can be dependent on the boundary condition, the cut-off temperature, and the thickness. A finite difference numerical model was developed and utilized to ascertain the energy consumption for the composite of n-eicosane with two kinds of porous copper foam with varying porosity under different heat rates, cut-off temperatures, and thickness. In addition, the results are compared with a metallic phase change material (gallium), a material chosen with a similar melting point but significantly high thermal conductivity and volumetric latent heat. For validation of the numerical model and to experimentally verify the effect of boundary condition (heat rate), experimental investigation was performed for n-eicosane and high porosity copper foam composite at varying heat rates to observe its melting and solidification behaviors during continuous operation until a cut-off temperature of 70 °C is reached. Experiments reveal that heat rate influences the amount of latent energy storage capability until a cutoff temperature is reached. For broad comparison, the numerical model was used to obtain the accessed energy and power density and generate thermal Ragone plots to compare and characterize pure gallium and n-eicosane - porous foam composite with varying volume fractions, cutoff temperature, and thickness under volumetric and gravimetric constraints. Overall, the proposed framework in the form of thermal Ragone plots effectively delineates the optimal points for various combinations of heat rate, cut-off point, and aspect ratio, affirming its utility for comprehensive design guidelines for PCM-based composites for electronic cooling applications

Hedging temperature risk with CDD and HDD temperature futures

AbstractThis paper is concerned with managing risk exposure to temperature using weather derivatives. We consider hedging temperature risk using so‐called HDD‐ and CDD‐index futures, which are instruments written on temperatures in specific locations over specific time periods. The temperatures are modelled as continuous‐time autoregressive (CARMA) processes and pricing of the hedging instrument is done under an equivalent pricing measure. We develop hedging strategies for locations, cutoff temperatures, and time periods different to the ones in the traded contracts, allowing for more flexibility in the hedging application. The dynamic hedging strategies are expressed explicitly by the term structure of the volatility. We also provide numerical case studies with temperatures following a CAR(3)‐process to illustrate the temporal behaviour of the hedge under different scenarios.

한국 수도권 지역 열대야 현상의 온도 기준과 사망 위험

The effects of extreme climate events associated with global warming on health have become a major public health concern. Previous studies on the mortality risk of high night-time temperatures, called “hot nights”, demonstrated that the current hot-night warning system in South Korea may not be sufficient because the warning system does not report the risks of night-time temperatures lower than 25°C (the current cut-off point for hot nights) and the threshold points were heterogeneous among subpopulations. Therefore, this study aimed to investigate the mortality risk of hot nights using various cut-off points and subpopulation analysis to suggest effective health risk-based warning systems.BR We collected time-series data on daily mortality and minimum temperature for 66 districts (si-gun-gu) in the capital area (Seoul, Gyeonggi, and Incheon) of South Korea during the summer period (Jun. ~ Sep.) from 2011 to 2017. Cut-off point temperatures were set from 20°C to 25°C, and the days with a daily minimum temperature above the cut-off point were defined as hot nights. Through two-stage time-series analysis with a distributed lag model, the association between hot nights and mortality was estimated for each district. We then calculated the pooled risk estimates using a meta-regression model.BR In the capital areas as a whole, positive mortality risk was detected around at 22°C, and the risk was highest at 25°C. In general, across all cut-off temperatures, Seoul showed higher risk compared to Gyeonggi and Incheon, and the cut-off value at which the risk was significant was the lowest (22°C). In addition, risk disparities between subpopulations (sex, age, education status, and marital status) were prominent at the highest cut-off point (25°C).BR In conclusion, this study provides evidence for health-risk-based hot night warning systems, which should be expanded to lower night-time temperatures and should account for regions and subpopulations.

Identification of thermal signature and quantification of charcoal in soil using differential scanning calorimetry and benzene polycarboxylic acid (BPCA) markers

Abstract. Black carbon (BC) plays an important role in terrestrial carbon storage and can sustainably improve soil fertility. However, the accurate quantification of BC remains critical to fully unravelling the functions and dynamics of BC in soil. In this study, we explored the potential of differential scanning calorimetry (DSC) to identify, characterize and quantify charcoal in the soil of pre-industrial charcoal kiln sites from various forest and cropland areas in Belgium and Germany. Pre-industrial charcoals and uncharred soil organic matter (SOM) demonstrated a distinct thermal signature that could be used to distinguish between them, with charcoal being more thermally stable than SOM. The DSC pattern of charcoals was characterized by one to three specific exothermic peaks, varying in size and position depending on soil conditions. Our data suggest that the thermal moieties within charcoal depend on the strength of chemical bonds of C atoms (increasing with the degree of aromatic condensation and decreasing with weathering) and on the activation energy required to initiate combustion. Despite the specific thermal features of charcoal, its decomposition spans a wide range of temperatures that overlaps with the thermal signature of uncharred SOM. This stresses the challenge of BC quantification in soil and hinders the use of cut-off temperatures to accurately quantify charcoal in soil. Therefore, charcoal-C content was estimated from the relative height of exothermic peaks, attributed either to the combustion of charcoal or SOM. For a selection of 45 soil samples, charcoal-C content estimated by DSC was compared to benzene polycarboxylic acid (BPCA) abundance, a widely used method to quantify BC in soil. The two methods correlated strongly (R2=0.97), with BPCA C representing about one-fifth of DSC-derived charcoal C. This reinforces the view that operationally defined BC content has an absolute quantitative value only if the recovery rate is controlled, which is very complicated for many case studies. Overall, our results demonstrate that dynamic thermal analysis is largely under-exploited despite providing quantitatively interpretable information across the continuum of SOM.

Heat Generation and Temperature Rise Characteristics of Single Overcharged Lithium-Ion Batteries

It’s critical to quantitatively investigate the thermal characteristics of single overcharged lithium-ion batteries to realize security alert before thermal runaway occurs. In this work, various (LiCoO2 + LiMn2O4)/graphite soft pack cells overcharged under different cut-off voltages, temperatures and C-rates are tested electrochemically to calculate the heat generation rate and distinguish the dominating heat resource. The results show that overcharged cells with higher cut-off voltage, overcharge temperature and the lower overcharge C-rate exhibit higher heat generation and temperature rise rate as well as poorer state of healthy. Among nonexplosive tested cells, the cell overcharged to 4.8 V at 0.1 C rate and 40 °C exhibits the highest heat generation and temperature rise rates of 9.17 W·l−1 and 4.60 °C·h−1 during 1 C charging at 25 °C. For overcharged cells, lithium plating, increased resistance and gas generation are observed, which are the reason for the accelerated total heat generation rate compared to baseline cells. Comparing with reversible heat, the irreversible heat resulting from diffusion overpotential and the sum of ohmic and charge transfer overpotential is dominating for overcharged cells working under high current. It’s recommended to comprehensively monitor the temperature change of each cell of battery pack.

Cost performance of encapsulated phase change material-based thermal energy storage systems

Abstract The cost of thermal storage is crucial to the economic viability of concentrated solar power plants. The aim of this study was to investigate ways to reduce the cost of latent heat thermal energy storage systems, in particular encapsulated phase change material technology. A design approach based on the heat transfer modelling, with consideration of the cut-off temperatures, was established. The model was solved using a control-volume methodology with an implicit scheme. Various configurations were investigated, considering a 100-MW power plant with 6 hours of backup capacity. The results indicate that there is a possibility to significantly reduce the capacity cost to about $9/kWht when the internal insulation of the storage tank along with a low-cost ceramic shell and chloride-based salts are used.

Exploiting Gaussian steering to probe non-Markovianity due to the interaction with a structured environment

We put forward a measure based on Gaussian steering to quantify the non-Markovianity of continuous-variable (CV) Gaussian quantum channels. We employ the proposed measure to assess and compare the non-Markovianity of a quantum Brownian motion (QBM) channel, originating from the interaction with Ohmic and sub-Ohmic environments with spectral densities described by a Lorentz-Drude cutoff, both at high and low temperatures, showing that sub-Ohmic, high temperature environments lead to highly non-Markovian evolution, with cyclic backflows of Gaussian steerability from the environment to the system. Our results add to the understanding of the interplay between quantum correlations and non-Markovianity for CV systems, and could be implemented at the experimental level to quantify non-Markovianity in some physical scenarios.

Numerical Study on the Dynamic Performance of Combined Sensible-Latent Heat Packed-Bed Thermocline Tank

Abstract Thermal energy storage (TES) using thermocline technology with phase change materials (PCM) is a promising technique for peak-shaving operation in cogeneration units. One of the disadvantages of this connotation is the use of the highly cost PCM capsules in a water tank. To circumvent this issue, a new thermocline tank connotation is proposed. The tank is packed with a mixture of solid filler pills and PCMs capsules, forming a multi-layered packed-bed system. A transient concentric-dispersion model is developed to assess the dynamic performance of a solid-PCM multi-layered packed-bed (SPMLPB) tank. The influences of the PCMs volume fraction (VF) and the charge and discharge dimensionless cutoff temperatures criterion on the dynamic performance have been investigated. The results show that the VF of PCMs influences the system's behavior, in terms of both energy storage and release. As the PCMs volume fraction increases from 10% to 40%, the amount of energy storage, energy release, and latent utilization ratio increased by 82.65%, 73.94%, and 55%, respectively, while the overall exergy efficiency drops by 6.3%. Besides, increasing both the charge and discharge cutoff temperatures criterion (STch*/STdisch*) enhances the total utilization ratio and energy recovered. When STch* increases from 0.27 to 0.7, the total utilization ratio and energy recovered increased by 63.63% and 28.67%, respectively, by maintains STdisch*=0.26.

A fast multi-dimensional magnetohydrodynamic formulation of the transition region adaptive conduction (TRAC) method

We have demonstrated that the transition region adaptive conduction (TRAC) method permits fast and accurate numerical solutions of the field-aligned hydrodynamic equations, successfully removing the influence of numerical resolution on the coronal density response to impulsive heating. This is achieved by adjusting the parallel thermal conductivity, radiative loss, and heating rates to broaden the transition region (TR), below a global cutoff temperature, so that the steep gradients are spatially resolved even when using coarse numerical grids. Implementing the original 1D formulation of TRAC in multi-dimensional magnetohydrodynamic (MHD) models would require tracing a large number of magnetic field lines at every time step in order to prescribe a global cutoff temperature to each field line. In this paper, we present a highly efficient formulation of the TRAC method for use in multi-dimensional MHD simulations, which does not rely on field line tracing. In the TR, adaptive local cutoff temperatures are used instead of global cutoff temperatures to broaden any unresolved parts of the atmosphere. These local cutoff temperatures are calculated using only local grid cell quantities, enabling the MHD extension of TRAC to efficiently account for the magnetic field evolution, without tracing field lines. Consistent with analytical predictions, we show that this approach successfully preserves the properties of the original TRAC method. In particular, the total radiative losses and heating remain conserved under the MHD formulation. Results from 2D MHD simulations of impulsive heating in unsheared and sheared arcades of coronal loops are also presented. These simulations benchmark the MHD TRAC method against a series of 1D models and demonstrate the versatility and robustness of the method in multi-dimensional magnetic fields. We show, for the first time, that pressure differences, generated during the evaporation phase of impulsive heating events, can produce current layers that are significantly narrower than the transverse energy deposition.

Thermodynamics of lattice vibrations in non-cubic crystals: the zinc structure revisited.

A phenomenological model of anisotropic lattice vibrations is proposed, using a temperature-dependent spectral cutoff and varying Debye temperatures for the vibrational normal components. The internal lattice energy, entropy and Debye-Waller B factors of non-cubic elemental crystals are derived. The formalism developed is non-perturbative, based on temperature-dependent linear dispersion relations for the normal modes. The Debye temperatures of the vibrational normal components differ in anisotropic crystals; their temperature dependence and the varying spectral cutoff can be inferred from the experimental lattice heat capacity and B factors by least-squares regression. The zero-point internal energy of the phonons is related to the low-temperature limits of the mean-squared vibrational amplitudes of the lattice measured by X-ray and γ-ray diffraction. A specific example is discussed, the thermodynamic variables of the hexagonal close-packed zinc structure, including the temperature evolution of the B factors of zinc. In this case, the lattice vibrations are partitioned into axial and basal normal components, which admit largely differing B factors and Debye temperatures. The second-order B factors defining the non-Gaussian contribution to the Debye-Waller damping factors of zinc are obtained as well. Anharmonicity of the oscillator potential and deviations from the uniform phonon frequency distribution of the Debye theory are modeled effectively by the temperature dependence of the spectral cutoff and Debye temperatures.

Subliquidus rheology of basalt from the 2018 Lower East Rift Zone Kilauea eruption: isothermal vs. dynamic expression

The dynamic effects of temperature and strain rate on rheology of crystal-bearing magma are investigated. We conducted high-temperature rheometry experiments in both isothermal and dynamic crystallization regimes and recovered textural data for the isothermal runs. We propose a framework for the parameterization of magma rheology, via an equation describing how the rheological cutoff temperature (the temperature at which magma stops flowing) varies as a function of cooling rate and strain rate. This equation may be used to inform rapid response in effusive crises. Cooling rate has the larger effect, with higher cooling rates yielding lower cutoff temperatures; higher strain rates yield higher cutoff temperatures. Textural analyses reveal differences in crystal aspect ratios, such that higher cooling rates produce only subequant crystals, whereas lower cooling rates also produce a second, higher aspect ratio crystal population. We identify this textural variation as the physical cause for the dependence of cutoff temperature on cooling rate. Plain language summaryAs lava cools, it crystallizes. Eventually, this crystallinity becomes so high that the lava can no longer advance. The temperature at which the crystals “lock up” the lava is called the “rheological cutoff temperature.” This depends, in principle, on the crystallization pathway, which is influenced by both the cooling rate and the strain rate of the lava flow. We conducted rheological experiments on 2018 Kilauea lavas along different crystallization pathways. We determined that higher cooling rates (5 °C/min) yield cooler cutoffs (983–1058 °C), and higher strain rates (8 s−1) yield hotter cutoffs (1058–1093 °C). Moreover, the cooling rate affects the cutoff temperature more than the strain rate. Through a complementary set of experiments, we found that the physical cause for the dependence of the cutoff temperature on the cooling rate is crystal aspect ratio (length/width). At any given crystallinity, crystals with a higher aspect ratio interact more, and lock at higher temperatures. Higher cooling rates produce only crystals with an aspect ratio of 1, whereas lower cooling rates produce higher aspect ratio crystals as well. Therefore, lava which has cooled more slowly, which crystallizes higher aspect ratio crystals, has a higher rheological cutoff temperature.

An accurate method of measuring shoulder sling compliance: a validation study

BackgroundThe effect of postoperative shoulder sling compliance on surgical outcomes is unknown. The goal was to determine an accurate method to measure sling compliance. We compared volunteer recorded sling wear time with temperature-based sensors to monitor sling compliance.MethodsData loggers sutured at three locations measured heat generated in 15-minute intervals. Slings wearers logged sling wear to accurately cross-reference with temperature sensors. Secondary experiments analyzed whether surrounding ambient temperature can be discerned from actual sling wear. We created an algorithm to describe actual sling wear time as a function of heat recorded and calculated percent wear accuracy.ResultsThe modified sling was worn for 172 h. The algorithm modeled sling on/off times by analyzing cutoff temperatures. Diagnostic accuracy was >99 % for the three locations, with no statistically significant differences among them. Compared with sling wear, ambient temperature took longer to reach critical temperature values determined by the algorithm, helping distinguish compliance from false positives.ConclusionsThe described algorithm can effectively quantify shoulder sling wear time based on heat-generated sensor readings. False positives from ambient temperature are minimal. This measurement method could be used to study the relationship between postoperative sling use and functional outcomes after shoulder surgery.

Constant-rate heating-induced thermal runaway in 18650-type Li-ion cells charged/discharged at 1 °C: Effect of undischargeable Li at anode

Charge/discharge cycling tests of lithium-ion batteries were carried out at 1 °C. During cycling, the delivered capacity decreased substantially, just 30% of the capacity at 25 °C is retained after 45 cycles. The exothermic behavior of fresh and aged cells is analyzed by an accelerated rate calorimeter (ARC) via two different heating protocols. In the heat-wait-search (HWS) experiments, aged cells show an unstable self-heating process from the start of the test, which is aggravated by the depth of discharge (DOD). This is not observed in fresh cells. In the constant-rate heating method, the aged cells show different thermal behavior at different cutoff temperatures, with a high risk of thermal runaway (TR) above 160 °C. Most importantly, in aged cells heated to 140 °C, TR could be averted by external cooling. Analyses of the differential capacity profiles reveal that thermal stability degradation of aged cells is closely related to the undischargeable Li accumulated in the graphite anode during cycling at 1 °C.

Parametric analysis and optimization of a latent heat thermal energy storage system for concentrated solar power plants under realistic operating conditions

High-temperature latent heat thermal energy storage (LHTES) systems are currently being considered for integration into concentrated solar power (CSP) plants; however, the challenge is to properly design the LHTES system under real-world operating conditions. Thus, this numerical investigation studied the effects of the LHTES system's design parameters on its performance under periodic steady-state with charging and discharging ‘cutoff’ temperatures to mimic its real-world operation. The study found that with the incorporation of cutoff temperatures, the system's specific energy and storage effectiveness decreased by 74% and 68%, respectively, due to lower useful charging and discharging times. Furthermore, the study demonstrated that the system's useful charging and discharging time could be augmented by increasing the shell radius (R) or length (L) of the system, or by decreasing the system's tube radius (ro) or the velocity of the heat transfer fluid (um) that flows through the system. The system's geometrical parameters (R, L, and ro) and um also substantially influenced its performance, but in a different manner than their influence on charging-discharging times. For example, increasing R deteriorated the system's performance substantially. Thus, we proposed optimized designs that achieved high charging-discharging times as well as good performance levels, using the response surface methodology.

The performances and degradation mechanisms of a commercial artificial graphite/LiNi0.5Co0.2Mn0.3O2 pouch cell at different cutoff voltages and temperatures

Tremendous efforts have been made in improving the electrochemical properties of LiNi0.5Co0.2Mn0.3O2 (NCM523)-based lithium-ion batteries in lab-scale coin cells. However, two questions remain to be answered, including what is the practical performance of high-energy-density based pouch cells in all-climate and why has a charge cutoff voltage of ≥4.4 V still not been commercialized in NCM523-based cells. Herein, an ~2 Ah artificial graphite/LiNi0.5Co0.2Mn0.3O2 (AGr/NCM523) pouch cell was developed, and the cell behaviors at 4.2 and 4.4 V under cold, room and hot temperature conditions were researched in detail. When charged to 4.4 V, a specific capacity of 185.7 mAh g−1 was achieved, along with gravimetric and volumetric energy-densities of 261.9 Wh kg−1 and 640.1 Wh L−1, respectively; additionally a similar rate and low-temperature (0, −10 and −20 °C) discharge capabilities were maintained at compared with those of the cell at 4.2 V. However, the practical applications of this 4.4 V AGr/NCM523 cell are hindered by its low storage stability at 70 °C and short service life upon cycling at 0, 25 and 45 °C. Various characterizations were employed to differentiate the contributions of the cutoff voltage to the cycling degradation mechanism. Results reveal that side reactions, especially electrolyte breakdown, increased at both the cathode and anode interphases, which notably increased the impedance and became the major reason for the capacity fade of the cell at 4.4 V. Notably, the damage to the interior structure of NCM523 was similar to that of NCM523 at 4.2 V. This fundamental and comprehensive understanding of NCM523 in a full cell will pave the way for optimizing state-of-the-art NCM523 materials and high-voltage LIBs.

Short-term effects of heatwave and PM2.5 exposure on the risk of preterm birth in Guangzhou, China

PDS 72: Environmental health issues in LMIC, Exhibition Hall (PDS), Ground floor, August 28, 2019, 10:30 AM - 12:00 PM Background: Extreme temperatures and air pollution both have significant burden on birth outcomes, however, the effects of short-term exposure to extreme heat and its interaction with air pollution on preterm birth (PTB) remain unclear. Objectives: To investigate the short-term effects of different definitions of heatwaves (n=12) and PM2.5 exposure on PTB and further explore interactive effects on the PTB risk. Methods: Using birth certificate data from Guangzhou, China, we included 249,489 singleton live births that occurred in the warm season (May 1-October 31) between January 2015 and July 2017. Daily average concentrations of PM2.5 from 11 monitoring stations and meteorological variables from five weather stations were used to estimate district-specific exposures. A series of cut-off temperatures and durations were used to define heatwaves. Distributed lag models (DLMs) using Cox proportional hazard models were used to estimate the effects of heatwave and PM2.5 exposure during the last week before delivery, and their additive interactive effect on PTB was assessed by using the relative excess risk due to interaction (RERI). Results: For most heatwave definitions used, an increased risk of PTB was observed for the women experienced at least one heatwave event in the last gestational week, with hazard ratios (HRs) of PTB ranging from 1.24 (95% CI: 1.01-1.52) to 1.78 (1.48-2.15). The effect estimates increased with the intensity of heatwave and the effects on later PTB (weeks 35-36) were stronger than those on earlier PTB (weeks 28-34). We observed weak effects of PM2.5 during the last week, and a negative additive interaction between heatwaves and PM2.5 exposure on PTB, with the RERIs<0. Conclusions: Exposure to heatwaves late in gestation is associated with a greater risk of PTB in Guangzhou. The finding of the negative interaction between short-term heatwaves and PM2.5 exposure on PTB warrants further investigation about the potential underlying mechanisms.

Shell-and-tube or packed bed thermal energy storage systems integrated with a concentrated solar power: A techno-economic comparison of sensible and latent heat systems

A detailed techno-economic comparison—using annual, transient integrated system modelling—was conducted for sensible and latent heat thermal energy storage (TES) systems. As the most viable near-term competitors, thermocline/packed-bed and shell-and-tube configurations were compared to the conventional two-tank molten salt system. Concrete and a range of phase change materials (PCMs) were considered as the storage mediums in this study. All analyses were conducted for 15 h of storage capacity for the 19.9 MWe Gemasolar concentrated solar power (CSP) plant as a real-world case study.It was found that when the CSP plant is constrained to the operational within the tight bounds of the standard two-tank system, the dual-media thermocline (DMT) system with concrete or encapsulated PCM shows the best performance. Imposing rigid operational boundaries significantly disadvantages shell-and-tube (ST) systems (e.g. resulting in up to ∼50% less annual electricity production than a CSP plant with two-tank system). However, the results of this study reveal that extending the TES charge and discharge cut-off temperatures closer to the plant’s maximum and minimum operating range (i.e. 800 K and 650 K) can maximize the potential of dual-media TES alternatives. Under these loose operational conditions, the CSP plant will be required to occasionally operate at conditions which are far from its nominal design point (i.e. up to 75% variation). This flexibility allows all the TES alternatives considered in this study to achieve similar CSP annual electricity output as the two-tank system. In this case (e.g. all TES systems achieve the same annual output), the TES alternatives can be compared based on their specific costs rather than their levelized cost of electricity. Compared to the specific cost of two-tank molten salt systems, ∼24.5 US$ kWhth−1, a 62% reduction of specific storage cost was found to be achievable with concrete storage a dual-media thermocline (DMT) system, representing the best techno-economic option. This was followed by 49% cost reduction for a pipeless shell-and-tube (ST) system incorporating concrete or a PCM with 1 mm pipe thickness. Without minimizing the capsule thickness to 0.1 mm, the packed bed system with PCM would never have economic justification. Overall, this study reveals that if CSP systems can be designed to have more flexibility in their operational temperature range, a significant cost savings is available in moving to alternative TES systems.

System-level performance optimization of molten-salt packed-bed thermal energy storage for concentrating solar power

Molten-salt packed-bed thermal energy storage using thermocline technology is more cost-competitive than the conventional two-tank thermal energy storage, due to its integrated design and the employment of a low-cost packed-bed. However, such a storage configuration suffers the main drawback of a low capacity factor when applied to concentrating solar power because of the adoption of conservative cut-off temperatures. The present work evaluates the feasibility of taking less conservative cut-off temperatures to improve the utilization of the packed-bed thermal energy storage from the perspectives of a system-level operation and storage economy. The investigations are carried out on two levels. The first-level investigation reveals the effects of both the charging and discharging cut-off temperature on the thermal performance of the packed-bed thermal energy storage under ideal operating conditions. Three typical packed-bed configurations are involved. The results show that the capacity factor of the packed-bed thermal energy storage increases as the charging cut-off temperature increases and the discharging cut-off temperature decreases, especially for the configurations using latent-heat when the adopted cut-off temperatures jump over the phase change points of the encapsulated phase change materials. The second-level investigation discusses the impacts of different levels of deep charges (using high charging cut-off temperatures) on the scale design of the packed-bed thermal energy storage, the daily operation of the low temperature molten-salt pump (LT-pump) and the central receiver of a 100 MWe conventional concentrating solar power tower plant. The results indicate that a deeper charge operation is always accompanied with a smaller required packed-bed size as well as a higher required delivery capacity and higher pressure head of the LT-pump and that it always results in a larger daily pumping consumption, a higher peak inlet temperature ramping rate and a higher receiver pressure drop. The maximum allowable charging cut-off temperature is identified to be 500 °C for each packed-bed configuration, according to the operating limitations on the pump and receiver. Moreover, a cost analysis is carried out to obtain the optimum charging cut-off temperature for each packed-bed configuration. The obtained results show that performing deep charges with the cost-optimized charging cut-off temperatures can effectively improve the cost competitiveness of the molten-salt packed-bed TES integrated into concentrating solar power plants.

Experimental performance assessment of a hermetic reciprocating compressor operating in a household refrigerator under on–off cycling conditions

This paper reports a study on the performance of a hermetic reciprocating compressor operating in a household refrigerator. The refrigerator was provided with sensors to measure the mass flow rate and evaporating and condensing pressures, while the compressor was instrumented with thermocouples in selected components and pressure transducers to measure the p-V diagram. Tests were conducted at ambient temperatures of 32 °C, 25 °C and 16 °C with different freezer cut-off temperatures (−10 °C, −16 °C and −19 °C). The phenomena that affect the mass flow rate and power consumption were evaluated during typical on-periods. It was observed that the thermodynamic efficiency of the compressor varied by 5–10% during the first few minutes after the compressor start-up, but remained virtually constant throughout the remainder of the on-period. The results showed that the volumetric and thermodynamic efficiencies tend to increase at lower ambient temperatures, while the overall efficiency of the compressor remains practically constant. Additionally, the compressor average temperatures and efficiencies collected from the application tests were compared to data obtained with calorimeters, revealing that the two sets of data do not differ much from each other.