Enthalpy Recovery Research Articles

Performance investigation and exergy analysis of enthalpy recovery device using liquid desiccant

Enthalpy recovery device using liquid desiccant is regarded as an appropriate approach for energy saving in air-conditioning system. This kind of enthalpy recovery device with a cross-flow pattern is investigated in the present study and its simulation model is built. Exergy analysis is utilized as the theoretical tool for optimizing the handling process. Reasons leading to exergy destruction are clarified. In addition to solution mixing process, limited transfer ability and unmatched coefficient are two main factors accounting for exergy destruction. Recovery efficiencies varying with mass flow ratio between air and solution are analyzed and the optimal flow ratio is obtained. A multi-stage device is proposed as an improvement to reduce the negative influence due to the unmatched coefficient and solution mixing process. Both recovery efficiencies ηh (ηm) and exergy efficiency ηex improve with the increase of stage number, while exergy destruction caused by unmatched coefficient or solution mixing process decreases. The multi-stage process also helps to reduce the required NTU for a certain recovery efficiency. Efficiency of a 3-stage or 4-stage device is quite close to that with infinite stages. The present research is beneficial to design an optimized enthalpy recovery device using liquid desiccant.

Memory Effect Manifested by a Boson Peak in Metallic Glass.

We explore the correlation between a boson peak and structural relaxation in a typical metallic glass. Consistent with enthalpy recovery, a boson peak shows a memory effect in an aging-and-scan procedure. Single-step isothermal aging produces a monotonic decrease of enthalpy and boson peak intensity; for double-step isothermal aging, both enthalpy and boson peak intensity experience, coincidently, an incipient increase to a maximum and a subsequent decrease toward the equilibrium state. Our results indicate a direct link between slow structural relaxation and fast boson peak dynamics, which presents a profound understanding of the two dynamic behaviors in glass.

Enthalpy recovery and its relation to shear response in an amine cured DGEBA epoxy

A description of enthalpy recovery for an amine-cured, digylcidyl ether of bisphenol A (DGEBA) epoxy (Epon 828/Jeffamine T403) was acquired through a series of experiments targeting the effects of the individual parameters: cooling rate, heating rate, aging temperature, and aging time. From these experiments, the peak temperature, Tp, and fictive temperature, Tf, were tracked as measures of the extent of structural relaxation. The KAHR formalism provides a standard for comparison to similar epoxies and was parametrized using the peak shift method. The resulting parameterization was then used to assess the relation between volumetric and shear relaxation. In particular, the KAHR model was used to predict volumetric relaxation time-temperature shift factors that are directly compared to measured time-temperature shift factors in shear. This unique methodology to assess equivalence between volumetric and shear relaxation shift factors further establishes the basis for this key assumption of continuum constitutive models based on rheological simplicity.

Time-temperature superposition in the enthalpy relaxation study of polystyrene

Enthalpy recovery measurement was used to study the physical aging behavior of a polystyrene. The aging experiments at temperatures (TA) near Tg were conducted with aging times (tA) ranging from several minutes to several days. The recovered enthalpy, ΔH(tA, TA), increased with tA, and then a plateau value of ΔHplate appeared in the long time aging experiment. The master curve was successfully constructed concerning the normalized relaxation isotherm of ϕ for the temperature range of Tg−16 < TA < Tg−8 if ΔHplate was used to calculate the normalized term. This implies that the enthalpy relaxation evolves with the same molecular dynamics in respective TA. The shift factor in the master curve was interpreted in terms of the WLF (Williams–Landel–Ferry) equation to determine the coefficients for the superposition of ϕ–tA curves. A smaller value was found in c1 in comparison with those reported in the literature on polystyrene viscoelastic data, which was discussed in terms of temperature dependence of the relaxation rate in the enthalpy decay. POLYM. ENG. SCI., 56:561–565, 2016. © 2016 Society of Plastics Engineers

Enthalpy Relaxation of Polystyrene at Different Molecular Weight Using Fast Calorimetry

The enthalpy recovery of polystyrene materials with narrow molecular weight distribution is studied by means of fast calorimetry. Two different molecular weights, Mw, have been investigated, namely 2000K and 212k. Intrinsic isotherm experiments at aging temperatures ranging from above to below the nominal Tg have been reported. The effect of the cooling rate on the glass transition temperature is also exanimated and it is showed that the cooling rate-dependence of the Tg is Vogel-Fulcher and not Arrhenius. The main finding is that the enthalpy recovery kinetics is not affected by the molecular weight showing that the contribution to free volume related due to chain terminals is negligible in the molecular weight range (212k-2000k) analysed in this paper.

Biomass fast pyrolysis energy balance of a 1kg/h test rig

The present paper offers a methodological approach towards the estimation and definition of enthalpies constituting an energy balance around a fast pyrolysis experiment conducted in a laboratory scale fluid bed with a capacity of 1 kg h-1 operating with pure N2 as fluidization medium at atmospheric pressures and temperatures ~ 500oC. An effort was made to achieve a satisfying 92.5% retrieval of products (dry basis mass balance) with the differences mainly attributed to loss of some bio-oil constituents into the quenching medium, ISOPARTM. The chemical enthalpy recovery for bio-oil, char and permanent gases is calculated 64.6%, 14.5% and 7.1%, respectively. All the energy losses from the experimental unit into the environment, namely the pyrolyser, cooling unit etc. are discussed and compared to the heat of fast pyrolysis that was calculated at 1123.5 kJ per kg of beech wood. This only represents 2.4% of the biomass total enthalpy or 6.5% its HHV basis. For the estimation of some important thermo-physical properties such as heat capacity and density, it was found that using data based on the identified compounds from the GC/MS analysis is very close to the reference values despite the small fraction of the bio-oil components detected. The methodology and results can help as a starting point for the proper design of fast pyrolysis experiments, pilot and/or industrial scale plants.

On Decentralized Air-conditioning for Hot and Humid Climates: Performance Characterization of a Small Capacity Dedicated Outdoor Air System with Built-in Sensible and Latent Energy Recovery Wheels

In large commercial buildings facing high cooling loads, conventional heating, ventilation, and air-conditioning (HVAC) systems are typically based on the use of one, or few, large-capacity air handling units (AHUs) to supply conditioned air throughout a centrally-managed air distribution network. The alternative concept of ‘decentralized’ HVAC typically entails the deployment of multiple, small capacity fan coil units (FCUs) operating as dedicated outdoor air systems (DOAS) and located closest to the individual thermal zones they must condition. For this study, the authors commissioned the development of a small capacity, packaged cooling coil with built-in enthalpy recovery devices. This customised unit, intended for use in a cooling-only, decentralized DOAS configuration, includes: two rotary wheels for sensible and latent energy recovery, a single cooling coil, and both supply and exhaust fans. The unit has been intended for building air conditioning use in the Singaporean climate. This paper provides energy performance data based on laboratory testing of the unit with independent control of the following variables: outdoor air conditions, return air conditions, and chilled water flow rate. It is intended that empirical data offered by this study can be adapted for use in future building energy models of decentralized HVAC systems.

Physical aging and glass transition of hairy nanoparticle assemblies

ABSTRACTMatrix free assemblies of polymer‐grafted, “hairy” nanoparticles (aHNP) exhibit novel morphology, dielectric, and mechanical properties, as well as providing means to overcome dispersion challenges ubiquitous to conventional polymer‐inorganic nanocomposite blends. Physical aging of the amorphous polymer glass between the close‐packed nanoparticles (NPs) will dominate long‐term stability; however, the energetics of volume recovery within the aHNPs is unknown. Herein, we compare glass transition temperature (Tg) and enthalpy recovery of aHNPs to NP‐polymer blends, across different nano‐silica loadings (0–50 v/v%) and canopy architecture of polystyrene (PS) grafted silica. For aHNPs, the grafting of PS to silica imposes an additional design constraint between silica volume fraction, graft density, and graft molecular weight. At low and intermediate silica volume fraction, the Tg of blended nanocomposites is independent of silica content, reflecting a neutral polymer‐NP interface. For aHNPs, the Tg decreases with silica content, implying that chain tethering decreases local segment density more than the effect of molecular weight or polymer‐NP interactions. Additionally, the Tg of the aHNPs is higher than a linear matrix of comparable molecular weight, implying a complementary effect to local segment density that constrains cooperativity. In contrast, enthalpy recovery rate in the blend or aHNP glass is retarded comparably. In addition, a cross‐over temperature, Tx, emerges deep within the glass where the enthalpy recovery process of all nanocomposites becomes similar to linear unfilled matrices. Differences between structural recovery in aHNP and blended nanocomposites occur only at the highest silica loadings (∼ 50 v/v%), where enthalpy recovery for aHNPs is substantially suppressed relative to the blended counterparts. The absence of physical aging at these loadings is independent of brush architecture (graft density or molecular weight of tethered chains) and indicates that the impact of chain tethering on effective bulk structural relaxation starts to appear at particle‐particle surface separations on the order of the Kuhn length. Overall, these observations can be understood within the context of how three separate structural characteristics impact local segment density and relaxation processes: the dimension and architecture of the tethered polymer chains, the separation between NP surfaces, and the confinement imposed by chain tethering and space filling within the aHNP. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 319–330

The energy saving potentials of zone-level membrane-based enthalpy recovery ventilators for VAV systems in commercial buildings

Enthalpy recovery ventilators exchange heat and mass between exhausted air and outdoor ventilation air to reduce mechanical ventilation loads for HVAC systems in buildings. This study demonstrates the energy saving potential of integrating membrane-based energy recovery ventilators at zone-level into the conventional variable air volume (VAV) systems for commercial buildings. Building energy simulations are used to evaluate various operation strategies and potential energy savings for four selected climates including Minneapolis, Atlanta, Baltimore, and Miami. A medium-size DOE reference commercial building model with conventional VAV systems was used as a baseline. Three cases implemented with energy recovery ventilators were simulated using EnergyPlus and their results were compared with baseline cases in compliance with ASHRAE standard 90.1. The annual HVAC energy saving potential can reach up to 18–49% with this technique for various climates. The challenges of implementing and operating the zone-level energy recovery ventilators are also discussed in this paper.

Experimental evaluation of enthalpy efficiency and gas-phase contaminant transfer in an enthalpy recovery unit with polymer membrane foils

Experimental studies were conducted in a laboratory setting to investigate the enthalpy efficiency and gas-phase contaminant transfer in a polymer membrane enthalpy recovery unit. One commercially available polymer membrane enthalpy recovery unit was used as a reference unit. Simulated indoor air and outdoor air by twin chambers was connected to the unit. Three chemical gases were dosed to the indoor exhaust air to mimic indoor air contaminants. Based on the measurements of temperature, humidity ratio, and contaminant concentrations of the indoor exhaust air and outdoor air supply upstream and downstream of the unit, the temperature efficiencies, humidity efficiencies, enthalpy efficiencies, and contaminant transfer ratios were calculated. The results showed that over 60% of enthalpy recovery efficiency could be achieved and that the contaminant transfer ratios were in the range of 5.4% to 9.0%. The enthalpy efficiency in cold–dry climate conditions was slightly higher than in hot–humid climate conditions.The contaminant transfer ratio were independent of any hygrothermal difference between indoor and outdoor air and was unrelated to its molecule size or water solubility. The conclusion indicated that the polymer membrane enthalpy recovery unit may be a viable choice for energy recovery in ventilation systems.

Characterization of catalytic partial oxidation of methane with carbon dioxide utilization and excess enthalpy recovery

The characteristics of catalytic partial oxidation of methane (CPOM) under CO2 addition and excess enthalpy recovery are investigated where a rhodium-based catalyst is employed. The influences of O2/CH4 and CO2/O2 molar ratios, in the ranges of 0.4–0.7 and 0–2, respectively, on CPOM performance are emphasized. The energy efficiency of the Swiss-roll reactor is also studied. The results reveal that the O2/CH4 ratio plays a crucial role in methane conversion, whereas it is insensitive to the CO2/O2 ratio. The H2 contributed by steam reforming is pronounced at higher O2/CH4 ratios; on the other hand, H2 produced from dry reforming is significant at lower O2/CH4 ratios and high CO2/O2 ratios. The H2/CO ratio in the product gas is between 1 and 2, and the values depends on the O2/CH4 and CO2/O2 ratios. Increasing CO2/O2 ratio substantially increases CO2 consumption, but leads to a decrease in CO2 conversion. Within the investigated ranges of O2/CH4 and CO2/O2 ratios, at least 18.2% and up to 77.0% of CO2 in the feed gas is converted to CO. The energy efficiency of the reaction system with considering CH4 conversion is between 83.5% and 89.9%. Overall, CPOM performed at O2/CH4=0.6 is recommended in that it provides higher CH4 conversion, syngas production, CO2 consumption, and system energy efficiency.

Reaction phenomena of catalytic partial oxidation of methane under the impact of carbon dioxide addition and heat recirculation

The reaction phenomena of CPOM (catalytic partial oxidation of methane) in a Swiss-roll reactor are studied numerically where a rhodium-based catalyst bed is embedded at the center of the reactor. CO2 is added into the feed gas and excess enthalpy recovery is performed to evaluate their influences on CPOM performance. In the study, the mole ratio of O2 to CH4 (O2/CH4 ratio) is fixed at 0.5 and the mole ratio of CO2 to O2 (CO2/O2 ratio) is in the range of 0–2. The results reveal that CO2 addition into the influent has a slight effect on methane combustion, but significantly enhances dry reforming and suppresses steam reforming. The reaction extents of steam reforming and dry reforming in CPOM without heat recovery and CO2 addition are in a comparable state. Once CO2 is added into the feed gas, the dry reforming is enhanced, thereby dominating CH4 consumption. Compared to the reactor without excess enthalpy recovery, heat recirculation drastically increases the maximum reaction temperature and CH4 conversion in the catalyst bed; it also intensifies the H2 selectivity, H2 yield, CO2 conversion, and syngas production rate. The predictions indicate that the heat recirculation is able to improve the syngas formation up to 45%.

Dynamic CO2 adsorption performance of internally cooled silica‐supported poly(ethylenimine) hollow fiber sorbents

The dynamic adsorption behavior of CO2 under both nonisothermal and nearly isothermal conditions in silica supported poly(ethylenimine) (PEI) hollow fiber sorbents (Torlon®‐S‐PEI) is investigated in a rapid temperature swing adsorption (RTSA) process. A maximum CO2 breakthrough capacity of 1.33 mmol/g‐fiber (2.66 mmol/g‐silica) is observed when the fibers are actively cooled by flowing cooling water in the fiber bores. Under dry CO2 adsorption conditions, heat released from the CO2‐amine interaction increases the CO2 breakthrough capacity by reducing the severity of the diffusion resistance in the supported PEI. This internal resistance can also be alleviated by prehydrating the fiber sorbent with a humid N2 feed. The CO2 breakthrough capacity of prehydrated fibers is adversely affected by the release of the adsorption enthalpy (unlike the dry fibers); however, active cooling of the fiber results in a constant CO2 breakthrough capacity even at high CO2 delivery rates (i.e., high adsorption enthalpy delivery rates). In full RTSA cycles, a purity of 50% CO2 is achieved and the adsorption enthalpy recovery rate can reach ∼72%. Studies on the cyclic stability of uncooled fiber sorbents in the presence of SO2 and NO contaminants indicate that exposure to NO at 200 ppm over 120 cycles does not lead to a significant degradation of the sorbents, but SO2 exposure at a similar high concentration of 200 ppm causes 60% loss in CO2 breakthrough capacity after 120 cycles. A simple amine reinfusion technique is successfully demonstrated to recover the adsorption capacity in poisoned fiber sorbents after deactivation by exposure to impurities such SO2. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3878–3887, 2014

Fast scanning calorimetry study of the structural relaxation in a random propylene-co-1-octene copolymer

The structural relaxation (or physical aging) has been analyzed by means of fast scanning calorimetry for a random propylene-co-1-octene copolymer, in a wide range of annealing temperatures and times. For annealing temperatures well below Tg only the structural relaxation is observed, and both the rate of enthalpy recovery and the peak temperatures related to the relaxation show a remarkable reduction with decreasing annealing temperatures. When the glass transition temperature is approached, although the structural relaxation is observed first, mesophase ordering is obtained at higher annealing times. The analysis of this mesophase ordering suggests homogeneous nucleation, and, in agreement with prior research in this field, the main feature is that the ordering in the glassy state requires prior completion of the enthalpy relaxation.

The effect of temperature gradients on the parameters of Adam–Gibbs model

The most commonly used method for studying relaxation processes in polymers is monitoring the enthalpy recovery by measuring differential scanning calorimetry (DSC) heat capacity data. Because of its great influence on heat capacity data, the effect of temperature gradients in the sample on relaxation dynamics study should be investigated. A model for calculating temperature profile in a DSC sample was proposed previously by combining Fourier's law of heat conduction with the Tool–Narayanaswamy–Moynihan (TNM) model. This model was examined using PS samples with different thicknesses. It was found that the temperature gradient influenced optimized values of the kinetic parameters. The thermal history dependence of model parameters is still prominent, however. Here the Scherer–Hodge (SH) equation (based on the Adam–Gibbs model) instead of the TNM equation, is used for describing the relaxation time–structure–temperature relationship in the temperature profile calculation. It is found that the influence of temperature gradients on the thermal history dependence of the Adam–Gibbs (AG) model parameters is greater than that of the TNM model parameters. But the thermal history dependence of the AG model parameters still exists when the thermal lag effect was incorporated.

Elevating the fracture toughness of Cu49Hf42Al9 bulk metallic glass: Effects of cooling rate and frozen-in excess volume

A major challenge for the structural applications of bulk metallic glasses (BMGs) is to improve their fracture toughness. Here we demonstrate that by increasing the cooling rate during the casting of liquid Cu49Hf42Al9 into BMG, using a mixed argon and helium atmosphere, the notch toughness of the resultant BMG can be tripled relative to that obtained at slower cooling rates. The much elevated toughness is attributed to a ten-fold increase in the size of the plastic zone at crack tip, due to the proliferation of shear banding facilitated by enhanced propensity for shear transformations. The latter propensity is explained by the reduced shear modulus and microhardness, as well as increased enthalpy recovery, all of which are rooted in structural disorder as reflected by the lowered density and increased frozen-in excess volume. Such a structure-property correlation is systematically demonstrated by monitoring all these properties over a range of diameters of the as-cast BMG rods that correspond to cooling rate levels from 40 K/s to 103 K/s.

Direct Evidence of Two Equilibration Mechanisms in Glassy Polymers

We investigated the kinetics of enthalpy recovery of several glass-forming polymers at temperatures significantly below the glass transition temperature (Tg) and for aging times up to one year. We find a double-step recovery at relatively low aging temperatures for the longest investigated aging times. The enthalpy recovered after the two-step decay approximately equals that expected by extrapolation from the melt. The two-step enthalpy recovery indicates the presence of two time scales for glass equilibration. The equilibration time of the first recovery step exhibits relatively weak temperature dependence, whereas that of the second step possesses pronounced temperature dependence, compatible with the Vogel-Fulcher-Tammann behavior. These results, while leaving open the question of the divergence of the relaxation time and that of a thermodynamic singularity at a finite temperature, reveal a complex scenario of glassy dynamics.

Performance comparison between enthalpy recovery wheels and dehumidification wheels

A mathematical model of desiccant wheels is established and validated by experimental results. The different effects of substrate, air mixing, and rotation speed on the performance of enthalpy recovery wheels and dehumidification wheels are analyzed. Substrate with high heat capacity and low heat conductivity is recommended for enthalpy recovery wheels, while substrate with low heat capacity and low heat conductivity is suitable for dehumidification wheels. Air mixing should be considered in evaluating recovery wheels' performance but can be ignored for dehumidification wheels. For recovery wheels without considering air mixing, higher the rotation speed, higher the recovery efficiency; when considering air mixing, the optimal rotation speed is lower than the corresponding rotation speed at a 5% air mixing rate. There also exists an optimal rotation speed to obtain better dehumidification performance for dehumidification wheels. Meanwhile, the reasons behind the discrepancies between recovery wheels and dehumidification wheels are discussed.

Enthalpy Recovery of Polystyrene: Does a Long-Term Aging Plateau Exist?

A glass is not in thermodynamic equilibrium below its glass transition temperature (Tg), and consequently, its properties, such as enthalpy, volume, and mechanical properties, evolve toward equilibrium in a process known as structural recovery or physical aging. Several recent studies have suggested that the extrapolated liquid line is not reached even when properties have ceased to evolve. In this work, we present measurements of the enthalpy recovery of polystyrene at an aging temperature 15 °C below the nominal Tg, for aging times up to 1 year. The results indicate that the equilibrium liquid enthalpy line can indeed be reached for aging 15 K below Tg. The results are analyzed in the context of the TNM model of structural recovery.

Enhancement of heat recirculation on the hysteresis effect of catalytic partial oxidation of methane

The hysteresis characteristics of catalytic partial oxidation of methane (CPOM) in a Swiss-roll reactor are predicted numerically by varying Damköhler number. Particular attention is paid to the influences of heat recirculation, gas hourly space velocity (GHSV), and atomic O/C ratio on the hysteresis loop and performance of CPOM. The reactions of methane combustion, steam reforming, and CO2 or dry reforming are simultaneously considered. The results reveal that preheating reactants through excess enthalpy recovery is conducive to the ignition of CPOM and extending its extinction limit, so the ignition and extinction Damköhler numbers are lowered. The analysis also suggests that steam reforming is more sensitive to the heat recovery than methane combustion and dry reforming. An increase in GHSV reduces the residence time of reactants in the catalyst bed, thereby enlarging the ignition and extinction Damköhler numbers of CPOM. A higher O/C ratio facilitates the ignition of CPOM, stemming from more oxygen supplied, but the ratio should be controlled below 1.2. From the hysteresis phenomena, hydrogen can be produced from methane at a lower Damköhler number to save more energy for performing CPOM.