Humidity Ratio Differences Research Articles

Direct/indirect optimal use of shallow geothermal energy in floor radiant cooling system coupled with air-handling system

Shallow geothermal energy is used as the cold source of floor radiant cooling systems (RFCSs). A physical simulation test bench was built to study the cooling and dehumidification characteristics of an RFCS to evaluate its feasibility. Three operation modes involving direct-supply chilled water, direct-supply chilled-water coupling air-handling system (AHS), and indirect cascade utilization were evaluated. Results showed that the room could be cooled rapidly in the direct-supply chilled-water mode. When the direct-supply chilled-water temperature was lower than 16 °C, condensation occurred on the floor surface. The AHS effectively avoided condensation, and the maximum humidity ratio difference between the air inlet and outlet was 13.61 g/kg. However, the temperature difference between the chilled water and the floor surface was reduced. Under the cascade utilization mode, the cooling efficiency of the system improved, and the indoor temperature and relative humidity were maintained at 20.6–28.1 °C and 57.0–76.1 %, respectively. This study provides valuable insights into the direct application of geothermal energy in RFCSs.

Are water bodies effective for urban heat mitigation? Evidence from field studies of urban lakes in two humid subtropical cities

Blue infrastructure is considered a nature-based solution to address urban heat problems. However, the potential nocturnal urban heat island (UHI) effect of water bodies has not been investigated and elaborated thoroughly. Besides, existing studies have primarily focused on surface temperature and are limited to individual cities during summer daytime. In this study, screen-height air temperature/humidity measurements at lake, residential, and rural sites in two humid subtropical cities of China (Nanjing, Guangzhou) were conducted continuously for one year. The UHI intensity (UHII), urban moisture island intensity (UMII, evaluated by humidity ratio difference), and heat index (HI) were analyzed across different weather conditions and time scales. Results demonstrated that (1) compared to residential sites, urban lakes can help mitigate urban heat excess, while this capability at night is not as significant as during daytime; compared to rural sites, urban lakes exhibit diurnal thermal behavior of daytime cooling and nighttime warming, with a weak urban cool island intensity (UCII) of 0.1–0.6 °C during the day but a significant UHII of 1.2–1.3 °C at night during warm months (May–September), which is comparable to residential sites with UHII of 1.1–1.3 °C; (2) urban moisture excess tends to occur during nighttime and cold seasons; (3) compared to rural sites, lakes contribute to a good relief on heat stress during daytime, but a significant negative effect at night due to the intensified warming and humidifying. Findings of this study can provide empirical evidence to help better understand the effectiveness of water bodies for urban heat mitigation.

Preferred Mode of Atmospheric Water Vapor Condensation on Nanoengineered Surfaces: Dropwise or Filmwise?

Condensing atmospheric water vapor on surfaces is a sustainable approach to addressing the potable water crisis. However, despite extensive research, a key question remains: what is the optimal combination of the mode and mechanism of condensation as well as the surface wettability for the best possible water harvesting efficacy? Here, we show how various modes of condensation fare differently in a humid air environment. During condensation from humid air, it is important to note that the thermal resistance across the condensate is nondominant, and the energy transfer is controlled by vapor diffusion across the boundary layer and condensate drainage from the condenser surface. This implies that, unlike condensation from pure steam, filmwise condensation from humid air would exhibit the highest water collection efficiency on superhydrophilic surfaces. To demonstrate this, we measured the condensation rates on different sets of superhydrophilic and superhydrophobic surfaces that were cooled below the dew points using a Peltier cooler. Experiments were performed over a wide range of degrees of subcooling (10-26 °C) and humidity-ratio differences (5-45 g/kg of dry air). Depending upon the thermodynamic parameters, the condensation rate is found to be 57-333% higher on the superhydrophilic surfaces compared to the superhydrophobic ones. The findings of the study dispel ambiguity about the preferred mode of vapor condensation from humid air on wettability-engineered surfaces and lead to the design of efficient atmospheric water harvesting systems.

Experimental study of vacuum-based membrane dehumidifier for HVAC system: Parametric analysis and dehumidification performance

Vacuum-based membrane dehumidifiers (VMDs), an emerging air dehumidification technology, use selective membranes to remove moisture from humid air; these dehumidifiers have the potential to save energy and enhance thermodynamic efficiency through decoupled latent and sensible cooling. In this study, a prototype of a vacuum-based membrane dehumidifier at the scale of an air-conditioning system was developed, and its dehumidification characteristics were experimentally evaluated under various test conditions (i.e., air temperature, air humidity, air velocity, and pressure on the permeate side). Four indices were selected to examine the dehumidification performance of the system: the humidity ratio difference, moisture removal rate, dehumidification efficacy, and performance coefficient. The results revealed that the humidity ratio difference ranged 3.38–14.76 g/kg, moisture removal rate 0.16–1.09 kg/h, dehumidification effectiveness 35.4 %–82.7 %, and COP 0.16–0.86. In the parametric analysis of the prototype, the airflow rate and humidity ratio are identified as essential parameters for the dehumidification performance. Moreover, the thermodynamic perspective in the Psychrometric chart and transient operation were utilized to evaluate the operational characteristics of the prototype unit. All experimental results showed that the difference in temperature between the inlet and outlet air was less than 1.5 °C, and the saturation time is shown to be within 5 min.

Sensitivity Analysis of Frost Deposition in Turbulent Flow over a Cold Plate using Direct Numerical Simulation

We use a recently-developed, turbulence-informed physics-based model to study frost buildup over a flat plate subjected to turbulent flow. The plate temperature is held fixed but varied across simulations in the range −10∘C≤Tp*≤−5∘C. Similarly, the free-stream temperature and humidity ratio are also held fixed but varied across simulations in the range 0∘C≤T∞*≤20∘C and 3.77×10−3≤w˜∞≤1.47×10−2, respectively. We find the Nusselt and Sherwood numbers, when scaled by the frost surface to free-stream temperature and humidity ratio difference, respectively, to become independent of free-stream temperature and humidity ratio as well as plate temperature. The scaling can be used in conjunction with the frost layer conservation equations of mass and energy to provide an estimate for the temporal evolution of the frost layer under turbulent flow conditions for a range of plate temperatures as well as free-stream temperatures and humidity ratios. For an initial shear Reynolds number of Re=180, we find the heat and mass transfer rates to be enhanced by about a factor of three compared to laminar flow conditions. We additionally conduct a sensitivity analysis on the frost layer density and thermal conductivity empirical correlations, as well as the frost layer Schmidt number. The latter parameterizes lateral frost diffusion and must be obtained from experiments. For the cases considered, we find the frost buildup to be only sensitive to the frost density correlation.

Role of ternary hybrid nanofluid in the thermal distribution of a dovetail fin with the internal generation of heat

The primary goal of this research is to examine the thermal variance in a dovetail fin under fully wet conditions with ternary hybrid nanofluid ZnFe 2 O 4 + MnZnFe 2 O 4 + NiZnFe 2 O 4 - H 2 O taking temperature and humidity ratio differences into account as driving forces for heat and mass transfer systems, respectively. The consequences of surface convection, radiation, and internally generated heat on the heat exchange of the fin have been taken into account. The mathematical modeling involves dimensionless transformation to convert the balanced energy equation to ordinary differential equation, and the problem is then solved numerically as well as analytically using the fourth-fifth order Runge-Kutta-Fehlberg's (RKF) methodology and DTM-Pade approximant. The significance of major thermal parameters such as radiation-conduction, wet factor, heat generation, and the ambient temperature variable on the temperature profile is explored graphically, contributing to an analysis of thermal performance. As the main outcome, ternary hybrid nanoliquid exhibits higher thermal response compared to mono and binary hybrid nanoliquid. Also, the thermal dispersion is higher for the lower values of wet parameter and radiative variable.

Experimental study of the effect of high humidity on the phase change plate thermal storage under natural convection

Phase change materials (PCMs) play a significant role in energy storage and temperature regulating, but the traditional researches failed to accurately consider the influence of high humidity on the thermal performance of phase change-unit, leading to a fallible guide, especially in humid region. This research fills in an important research gap by designing a novel experiment to clarify the influence of high humidity environment on the melting process of phase change plate (PCP). Further, the improved momentum equation and the similarity number Grd caused by humidity ratio difference are derived in theoretical method. Finally, the heat and mass transfer criterion equations of PCP under natural convection are fitted. The results show that the melting process of PCP is significantly affected by high temperature and humidity environment in different ways. Under the experimental condition, the melting rate of PCP can be increased by more than 60%, and the proportion of latent heat transfer can reach more than 80%. The heat and mass transfer criterion equations adopt the same form but different coefficients. The results would provide valuable design guidelines for engineers regarding the estimation of the energy release process of phase change-unit used in a humid climate zone at the design stage.

Non-equilibrium thermodynamic analysis of coupled heat and moisture transfer across a membrane

Non-equilibrium thermodynamics theory is used to analyze the transmembrane heat and moisture transfer process, which can be observed in a membrane-type total heat exchanger (THX). A theoretical model is developed to simulate the coupled heat and mass transfer across a membrane, total coupling equations and the expressions for the four characteristic parameters including the heat transfer coefficient, molar-driven heat transfer coefficient, thermal-driven mass transfer coefficient, and mass transfer coefficient are derived and provided, with the Onsager’s reciprocal relation being confirmed to verify the rationality of the model. Calculations are conducted to investigate the effects of the membrane property and air state on the coupling transport process. The results show that the four characteristic parameters directly affect the transmembrane heat and mass fluxes: the heat and mass transfer coefficients are both positive, meaning that the temperature difference has a positive contribution to the heat transfer and the humidity ratio difference has a positive contribution to the mass transfer. The molar-driven heat transfer and thermal-driven mass transfer coefficients are both negative, implying that the humidity ratio difference acts to reduce the heat transfer and the temperature difference works to diminish the mass transfer. The mass transfer affects the heat transfer by 1%–2% while the heat transfer influences the mass transfer by 7%–14%. The entropy generation caused by the temperature difference-induced heat transfer is much larger than that by the humidity difference-induced mass transfer.

Filmwise Condensation From Humid Air on a Vertical Superhydrophilic Surface: Explicit Roles of the Humidity Ratio Difference and the Degree of Subcooling

Abstract The process involving heat and mass transfer during filmwise condensation (FWC) in the presence of noncondensable gases (NCG) has great significance in a large variety of engineering applications. Traditionally, the condensation heat transfer is expressed in the literature as a function of the degree of subcooling—reckoned as the difference between the ambient dry bulb temperature and the condenser wall temperature. However, in the presence of NCG, there exists a finite gradient of vapor mass fraction near the condenser plate, which directly influences the vapor mass flux to the condenser surface, thus limiting the condensation rate. The effects of both these influencing thermodynamic parameters, i.e., the degree of subcooling and the difference of humidity ratio (between the freestream environment and on the condenser plate), have been characterized in this work both experimentally and through a mechanistic model. The vapor mass flux during condensation on a subcooled vertical superhydrophilic surface under free convection regime is experimentally measured in a controlled environment (temperature and humidity) chamber. The mechanistic model, based on the similarity of energy and species transports, is formulated for the thermogravitational boundary layer over the condenser plate and tuned against the experimental results. Further, the model is used to obtain comprehensive data of the condensate mass flux and condensation heat transfer coefficient (CHTC) as functions of the salient thermal operating conditions over a wide parametric range. Results indicate that humidity ratio difference has a more pronounced influence on the condensation mass transfer rather than the degree of subcooling. Regime maps of condensate flux and CHTC show how these can be explicitly identified in terms of the degree of subcooling and humidity ratio difference, regardless of the prevailing thermal and humidity conditions at the freestream and the condenser plate. The mechanistic model thus lends to the development of empirical correlations of condensate mass flux and CHTC as explicit functions of these two parameters for easy use in practical FWC configurations.

Effect of air parameters on ice sublimation in ice rink

This study aims at the sublimation rate of ice and analyzes the influence of supply air and ice parameters on ice surface smoothness. The sublimation rate can be expressed as the product of the sublimation coefficient and humidity ratio difference. COMSOL Multiphysics is adopted as a simulation tool to calculate the associated ice sublimation rate. In ice rinks, the sublimation coefficient is 0.1–0.2 mm/(h·kPa), and the sublimation rate is 0–0.07 mm/h. The results show that the main factor affecting the sublimation coefficient is air velocity over the ice surface. Therefore, the ice sublimation can be decreased by reducing air velocity and decreasing humidity ratio difference between the air and ice surface. The aforementioned findings were applied in an actual curling venue to reduce the sublimation of the ice surface, and the accumulated sublimation within 10 h was reduced from 0.24 mm to 0.016 mm, which effectively solved the problem of ice surface unevenness. This study working on the ice sublimation rate provides inspiration for maintaining the ice stability in ice rinks and improving the quality of venues.

Exergy analysis of dehumidification systems: A comparison between the condensing dehumidification and the desiccant wheel dehumidification

This paper presents a framework of exergy analysis to evaluate the performance of the condensing dehumidification and the desiccant wheel dehumidification. The performance of two dehumidification methods are compared for pure dehumidification purposes based on proposed dehumidification systems that undertake only the latent load. An air-source heat pump is adopted in the dehumidification systems to serve as the cold and heat source. Based on thermodynamic perfectibility theory and the laboratory experiment of the desiccant wheel, the power requirements and exergy efficiency of the two dehumidification methods are analyzed and compared under 17 sets of operating conditions. The coefficient of performance of the heat pump in the condensing dehumidification system is 2–3 times higher than in conventional air-conditioning systems. Environmental factors that influence the performance of each dehumidification method are identified. The performance of condensing dehumidification is affected mostly by the humidity ratio of indoor air, whereas the outdoor air temperature has a minor impact. The humidity ratio difference between indoor and outdoor air is the most influential factor in the desiccant wheel dehumidification, followed by the outdoor air temperature. Under the operating conditions examined in the present study, the condensing dehumidification shows 3–4 times higher exergy efficiency than the desiccant wheel dehumidification. Therefore, the condensing dehumidification should be preferred over the desiccant wheel dehumidification for typical indoor dehumidification applications.

Performance of heat pump driven internally cooled liquid desiccant dehumidification system

Internally cooled liquid desiccant dehumidification systems are considered to be compact and efficient dehumidification systems in the air-conditioning field. A theoretical method is proposed to investigate the ideal maximum coefficient of performance and current gaps between actual and ideal working conditions of the heat pump driven internally cooled liquid desiccant dehumidification system. It is found that the maximum coefficient of performance is determined by the inlet humidity ratio of process air and regeneration air, and the required humidity ratio difference. It has been proved to be not significantly affected by the solution concentration. Furthermore, three perfectness are identified and deduced to explain gaps between the actual and ideal working condition. They include heat–cold offset perfectness due to heat–cold offset of the circulating solution between dehumidifier and regenerator, transfer capacity perfectness due to the limited transfer capacity of the internally cooled/heated dehumidifier/regenerator, and heat pump thermodynamic perfectness. The results indicate that actual coefficient of performance generally only corresponds to 10%–35% of the maximum coefficient of performance. With aid of the aforementioned perfectness, reasons for limited coefficient of performance of the current system are analyzed to illustrate future improvement direction and potential.

A Full-Solid-State Humidity Pump for Localized Humidity Control

Summary Humidity management is essential and widely needed in various fields. Traditional moisture removal technologies are mostly based on cooling condensation or desiccant dehumidification. These methods come with inherent defects of energy efficiency and are bulky and complex for small-space humidity-control applications. Here, we demonstrate a full-solid-state humidity pump using the advantages of commercial thermoelectric coolers and silica gel materials. Silica gel is coated on the surface of heat sinks to minimize contact thermal resistance between the material and heat source while maintaining enough heat and mass transfer area between the desiccant and the air. The humidity pump device exhibits 0.61 g W−1 h−1 humidity transfer efficiency, corresponding to a humidity transfer rate of 28.38 g h−1. The proposed device neither uses refrigerant nor introduces liquid water. It opens up the possibility of use in localized humidity control applications with higher efficiency and a wider ambient temperature range than traditional technologies.

Experimental investigation and parametric studies on structured packing chamber based liquid desiccant dehumidification and regeneration systems

In this paper, an experimental investigation carried out for estimating the overall energy exchange between the liquid desiccant and the ambient air is presented. The influence of air humidity ratio on the performance of the liquid desiccant dehumidification/regeneration system is studied in detail. A correlation for humidity ratio difference between the process air inlet and exit of the packed chamber in terms of known operating parameters is developed for both dehumidification and regeneration systems. A two–dimensional thermal model is proposed for predicting the performance of the adiabatic counter – flow liquid desiccant dehumidifier/regenerator. The proposed model is validated with the experimental data and found accurate. Influences of air humidity ratio, L/G ratio and Lewis number on the latent heat ratio of both the dehumidification and the regeneration systems are analysed. Using Derringer's desirability function method, for a given range of operating parameters, optimal inlet condition for the maximum humidity ratio difference between the inlet and the exit of the dehumidification/regeneration system is obtained.

Experimental and numerical analysis on total heat recovery performance of an enthalpy wheel under high temperature high humidity working conditions

Enthalpy wheels (EWs) have been extensively used for total heat recovery from ventilation air. Despite there being many studies reporting on their performances under common working conditions with a small temperature and humidity ratio difference (ΔT < 20 °C, Δω < 15 g kg−1), there are few studies reporting on their performances under high temperature high humidity (HTHH) working conditions. This study aims to investigate the total heat recovery performance of an EW under HTHH working conditions. A mathematical model is established and validated to predict the performance. The effects of rotation speed, air inlet velocity, air inlet temperature, and air inlet humidity ratio on the total heat recovery performance are numericallyinvestigated, and performance comparisons between HTHH working conditions and other common working conditions are also carried out. The comparison results show that the EW performs better for total heat recovery under HTHH working conditions, with εtotal improving by 6% compared to common working conditions. The reasons behind the discrepancies are revealed by analyzing the distributions of temperature and humidity in the EW. This study is expected to be beneficial to design EWs to recover total heat from HTHH waste gas.

Performance investigation of regenerative total heat exchanger with periodic flow

This study proposes a new regenerative type of the periodic total heat exchanger system, which consists of four centrifugal fans and two fixed wheels. The total heat exchanger, which operates periodically to replace the rotation of the fixed wheel in conventional design, can save more energy from motors. The fixed wheels alternately store and release energy using the heat and moisture storage materials of aluminum and silica gel. The operation periods are 2 min, 3 min, 4 min, 6 min, and 8 min. The results show that under the temperature and humidity ratio differences of 10 °C and 4g/kg between indoors and outdoors, the operating period of 3 min has the best performance of sensible heat effectiveness, latent heat effectiveness, and total heat effectiveness. When the temperature and humidity differences increase, the optimal operating period becomes shorter. In order to increase heat effectiveness, adding another high temperature and low moisture air for regeneration in the total heat exchanger increases the latent heat effectiveness, but the sensible heat effectiveness decreases.

Theoretical analysis of heat and mass transfer characteristics of a counter-flow packing tower and liquid desiccant dehumidification systems based on entransy theory

Abstract Dehumidifier and regenerator are the key components of a liquid desiccant dehumidification system. This paper examines the heat and mass transfer processes of a counter-flow packing tower using entransy analysis. Under ideal thermodynamic conditions, if NTUm of the counter-flow packing tower is infinite, the reversible process only exists when the inlet states of air and solution lie on the same iso-relative humidity line. In actual heat and mass transfer processes, the relative position of the inlet parameters of air and solution in the psychrometric chart will influence the entransy dissipation in the packing tower. Based on the analytic solution of temperature difference and humidity ratio difference between air and solution, minimum entransy dissipation is determined when the states of inlet air and solution lie on the same iso-relative humidity line. The influence of inlet characteristics can be quantified using an unmatched coefficient of inlet parameter (γ). This parameter is an indicator of the flow-path design and the system optimization. Furthermore, a practical application is presented by comparing three typical flow paths. The flow path with the smallest γ value reduces the required temperature of heat sources from 60 °C to 45 °C. The realization of such low-grade heat sources not only means high/efficient utilization of heat or solar energy, but also results in less waste heat.

Experimental and theoretical investigation of regenerative total heat exchanger with periodic flow for air-conditioning systems

This study experimentally investigates the theoretical performance of the periodic total heat exchanger. The total heat exchanger has a double flow-circuit heat exchange function for periodic positive- and reverse-directional pumping. The operating period parameters are 2 minutes, 3 minutes, 4 minutes, 6 minutes, and 8 minutes. Under the temperature and humidity ratio differences, 10 °C and 4 g kg−1, the operating period of 3 min results in optimal performance of sensible heat effectiveness, latent heat effectiveness, and total heat effectiveness. This study also proposes the lumped system, with dimensionless analysis assumption applied to the whole, and solves it in a one-dimensional model involving the heat and mass transport equations. The dimensionless operating period dominates latent heat effectiveness, while the latent heat effectiveness decreases when the dimensionless operating period increases.

Performance investigation and exergy analysis of air-handling processes using liquid desiccant and a desiccant wheel

Air-dehumidification systems using liquid desiccant and a desiccant wheel are common approaches using desiccant for air humidity control. This article focuses on the performance of air-dehumidification systems using liquid desiccant and a desiccant wheel. Based on basic air-handling processes using desiccants, the characteristics of the driving forces are investigated with the help of an unmatched coefficient ξ and exergy analysis. The performace of liquid desiccant and desiccant wheel systems is investigated, and approaches for improvement are proposed. It is indicated that the temperature difference ΔT or humidity ratio difference Δω in the liquid desiccant process is influenced by the inlet state of the air and solution. The liquid desiccant process heating solution, rather than air, for regeneration is optimal from the perspective of lowering ξ and reducing unmatched destruction. The ξ is approaching 1 between air and solid desiccant, i.e., the driving force is relatively uniform with the same dehumidification and regeneration regions. A multi-stage process is supposed to be effective for optimizing the desiccant wheel system, helping to reduce the exergy destruction arising from the unmatched coefficient. The present study is beneficial to cast light on the relationship between air-dehumidification systems using liquid desiccant and a desiccant wheel.

Performance analysis of ventilation systems with desiccant wheel cooling based on exergy destruction

This paper investigates the performances of ventilation systems with desiccant wheel cooling from the perspective of exergy destructions. Based on the inherent influencing factors for exergy destructions of heat and mass transfer and heat sources, provide guidelines for efficient system design. First, performances of a basic ventilation system are simulated, which is operated at high regeneration temperature and low coefficient of performance (COP). Then, exergy analysis of the basic ventilation system shows that exergy destructions mainly exist in the heat and mass transfer components and the heat source. The inherent influencing factors for the heat and mass transfer exergy destruction are heat and mass transfer capacities, which are related to over dehumidification of the desiccant wheel, and unmatched coefficients, which represent the uniformity of the temperature or humidity ratio differences fields for heat and mass transfer components. Based on these findings, two improved ventilation systems are suggested. For the first system, over dehumidification is avoided and unmatched coefficients for each component are reduced. With lower heat and mass transfer exergy destructions and lower regeneration temperature, COP and exergy efficiency of the first system are increased compared with the basic ventilation system. For the second system, a heat pump, which recovers heat from the process air to heat the regeneration air, is adopted to replace the electrical heater and cooling devices. The exergy destruction of the heat pump is considerably reduced as compared with heat source exergy destruction of the basic ventilation system and the first system, leading to a great enhancement of COP and exergy efficiency.