Study of humid air turbine cycle with external heat source for air humidification

In this paper, through introducing an external heat source to the conventional humid air turbine (HAT) cycle, we have studied the performances of the improved humid air gas turbine cycle mainly by exergy analysis method. In order to attain the performance of the humid air gas turbine with external heat source, we compare it with the conventional HAT cycle in detail with different factors such as the pressure ratio, turbine inlet temperature (TIT) and the external circulating water mass flow. The results showed that the specific work of the new system and the humidity ratio of saturator are all increased in some degree. For example, in the same pressure ratio and TIT, when the ratio of the external circulating water mass flow rate with that of the internal water is 0.2, the specific work increases more than 15.2 kJ kg⁻¹a, and the humidity raises at least 2.0 percent points. By introducing the external circulating water into the system, though thermal efficiency of the new HAT cycle is lower than that of the conventional HAT cycle, the exergy efficiency exhibits different results. Generally, when the pressure ratio is over 8, the exergy efficiency for the proposed HAT cycle is higher than the conventional HAT cycle; while less than 8, whether or not the exergy efficiency increases will mainly depend on TIT. In addition, the exergy destructions of components in systems were investigated. Through the comparison of the new system with the conventional HAT cycle, it was found that the exergy loss proportion in combustion declines for the new system, and the proportion of exhaust loss increases. From the viewpoint of total energy system, the HAT cycle with utilization of external heat source is a beneficial way to improve the overall performances of energy utilization. Copyright © 2009 John Wiley & Sons, Ltd.     

Performance analysis of humid air turbine cycle with solar energy for methanol decomposition

According to the physical and chemical energy cascade utilization and concept of synthesis integration of variety cycle systems, a new humid air turbine (HAT) cycle with solar energy for methanol decomposition has been proposed in this paper. The solar energy is utilized for methanol decomposing as a heat source in the HAT cycle. The low energy level of solar energy is supposed to convert the high energy level of chemical energy through methanol absorption, realizing the combination of clean energy and normal chemical fuels as compared to the normal chemical recuperative cycle. As a result, the performance of normal chemical fuel thermal cycle can be improved to some extent. Though the energy level of decomposed syngas from methanol is decreased, the cascade utilization of methanol is upgraded. The energy level and exergy losses in the system are graphically displayed with the energy utilization diagrams (EUD). The results show that the cycle’s exergy efficiency is higher than that of the conventional HAT cycle by at least 5 percentage points under the same operating conditions. In addition, the cycle’s thermal efficiency, exergy efficiency and solar thermal efficiency respond to an optimal methanol conversion.     

Proposal of humid air turbine cycle incorporated with absorption heat transformer

A detailed graphical exergy study based on the energy‐utilization diagram (EUD) is applied to humid air turbine (HAT) cycle incorporated with a modified two‐stage absorption heat transformer (TAHT). Employing the sensible and latent heat exchange modes in this TAHT and then introducing to the HAT cycle have clarified that this method can intensively recover the waste heat of HAT cycle and improve the system performance. Compared to a conventional HAT cycle, the overall cycle efficiency can be increased by 2 per cent points and the specific work can be increased by 7.3 per cent. Copyright © 2000 John Wiley & Sons, Ltd.     

Thermal performance of solar air heaters—Experimental correlation

A mathematical model and a solution procedure for predicting the thermal performance of four types of single-pass flat-plate solar air collectors were presented in an earlier paper by Ong (1995). Instead of resorting to complicated algebraic manipulations to solve the energy equations a matrix-inversion technique was employed. In this paper, theoretical predictions of surface and air temperatures were obtained for the Types II, III and IV collector designs. In addition, the Type II collector was considered with and without bottom insulation. Experimental data from previous studies were obtained and compared with the present theoretical predictions. Satisfactory qualitative and quantitative agreement was obtained between theoretical predictions and experimental data.     

Experiments in a solar simulator on solid desiccant regeneration and air dehumidification for air conditioning in a tropical humid climate

This paper presents an indoor and analytical study to evaluate the performance of a desiccant cooling system that uses silica gel as desiccant, electric light bulbs to simulate solar radiation, and forced flow of air through an IDC (integrated Desiccant/Collector). In the regeneration process, the rate at which water is removed from the desiccant increases with irradiation and decreases with air flowrate. In the air dehumidification process, the adsorption rate decreases with irradiation and increases slightly with flowrate. Comparisons between analytical calculations and experimental data show good agreement, and the calculations show that it should be possible to operate this system in tropical humid climates using the regeneration process in the day and the air dehumidification in the night time.     

Effect of internal partitioning on room air quality with mixing ventilation—statistical analysis

In designing modern office buildings, building spaces are frequently zoned by introducing internal partitioning, which may have a significant influence on the room air environment. This internal partitioning was studied by means of model test, numerical simulation, and statistical analysis as the final stage. In this paper, the results produced from the statistical analysis are summarized and presented.     

Thermodynamic investigation and comparison of selected production processes for hydrogen and hydrogen-derived fuels

The results are reported of comparisons based on energy and exergy analyses of a wide range of production processes for hydrogen and hydrogen-derived fuels (HDFs). A commercial process-simulation computer code, previously enhanced by the author for exergy analysis, is used in the analyses. Depending on the process and the efficiency definition used, overall efficiencies are determined to range widely, from 21 to 92% for energy efficiencies and from 19 to 83% for exergy efficiencies. The losses for all processes are found to exhibit many common factors. Energy losses associated with emissions account for 100% of the total energy losses, while exergy losses associated with emissions account for 4 to 11% of the total exergy losses. The remaining exergy losses are associated with internal consumptions. It is anticipated that the results will prove useful to those involved in the improvement of existing and design of future production processes for hydrogen and HDFs.     

Process model-free analysis for thermodynamic efficiencies of sulfur–iodine processes for thermochemical water decomposition

Material, energy, and entropy balances, which depend only on stream conditions and flows entering and leaving a system, have been used to evaluate different scenarios for thermochemical decomposition of water to manufacture hydrogen using the Sulfur–Iodine cycle. Energy efficiencies have been found for idealized systems with variable stream amounts, as well as for a common flowsheet, to locate the greatest effects on energy requirements and inefficiencies. Aspen Plus^®, OLI Engine, and ProSimPlus property models have been used on the sections of a General Atomics process to reveal the effects of differences in computed energies and entropy generation. While the calculated efficiencies are generally consistent with those of the literature, differences in stream properties and phase behaviors suggest that optimal process configurations from simulations may have significant uncertainties.     

A thermodynamic analysis of a novel high efficiency reciprocating internal combustion engine—the isoengine

A novel concept for a high efficiency reciprocating internal combustion engine (the isoengine) is described and its cycle is analysed. The highly turbocharged engine configuration, which is intended primarily for on-site and distributed power generation, has a predicted electrical output of 7.3 MW. It has the option for co-generation of up to 3.2 MW of hot water at 95 °C supply temperature. The maximum net electrical plant efficiency is predicted to be about 60% on diesel fuel and 58% on natural gas. The key to the high electrical efficiency is the quasi-isothermal compression of the combustion air in cylinders, which are separate from the power cylinders. This achieves a significant saving in compression work and allows the recovery of waste heat back into the cycle, mainly from the exhaust gas by means of a recuperator. The construction of a first 3 MW_e prototype isoengine has been completed and its testing has begun. Relevant test results are expected in the near future.     

Experimental techniques for determining heat and mass transfer due to condensation of humid air in cooled,open cavities

An experimental investigation was conducted to examine combined buoyancy driven heat and mass transfer in open cavities of different aspect ratios. Test cavities were constructed with calorimeter plates bonded to Styrofoam insulation. The inside of the cavities was cooled and then exposed to ambient air for approximately 17–30 min. Measurements were conducted at three initial cavity temperatures (5 °C, 1 °C, and −5 °C) each for a range of ambient relative humidity from 60% to 75%. The mass of retained condensate accumulated on the inside cavity walls and the transient cavity temperatures were measured. The mass flux of retained condensate inside the cavity was compared to the mass flux determined utilizing the heat/mass transfer analogy. The results between the two methods were in good agreement. The effects of the retained condensate on the mass flux measurements due to the water’s thermal resistance and surface emittance were also investigated and shown to have a negligible impact.     

Thermal comfort standards for air conditioned buildings in hot and humid Thailand considering additional factors of acclimatization and education level

This paper presents the result of a large thermal comfort survey conducted using 1520 Thai volunteers from different climatic regions of Thailand. The survey was conducted using different types of air-conditioned buildings from the private and public sectors.Apart from common thermal comfort factors such as air dry bulb temperature, relative humidity and air velocity, two non-quantifiable factors were considered. These are the acclimatization to the use of air conditioner at home and the education level, i.e., post graduate, graduate and scholar. A general database for thermal comfort studies in Thailand was created, and different thermal comfort standards were developed for the three climatic regions of Thailand. Twenty six degree Celsius and 50–60% relative humidity could be used as a comfortable environment condition for the whole country. The data was then used to generalize an earlier concept we developed for setting thermal comfort standard using data from non air-conditioned buildings.     

Dynamic response of a packed bed thermal storage system—a model for solar air heating

A mathematical model for simulating the dynamic temperature response of a packed column to an arbitrary time-dependent inlet air temperature is developed. The model includes axial thermal dispersion as well as intra-particle conduction, features that have usually been neglected but can be important in solar energy applications. Solutions, presented in terms of moments of the temperature response to an impulse of heat at the inlet, can be evaluated by simple numerical quadrature. Results of the model compare favorably with experimental data found in the literature. The model is used to optimize heat storage in a rock bin system subject to a realistic transient inlet temperature.     

Energy and economic assessment of desiccant cooling systems coupled with single glazed air and hybrid PV/thermal solar collectors for applications in hot and humid climate

This paper presents a detailed analysis of the energy and economic performance of desiccant cooling systems (DEC) equipped with both single glazed standard air and hybrid photovoltaic/thermal (PV/t) collectors for applications in hot and humid climates. The use of ‘solar cogeneration’ by means of PV/t hybrid collectors enables the simultaneous production of electricity and heat, which can be directly used by desiccant air handling units, thereby making it possible to achieve very energy savings. The present work shows the results of detailed simulations conducted for a set of desiccant cooling systems operating without any heat storage.System performance was investigated through hourly simulations for different systems and load combinations. Three configurations of DEC systems were considered: standard DEC, DEC with an integrated heat pump and DEC with an enthalpy wheel. Two kinds of building occupations were considered: office and lecture room. Moreover, three configurations of solar-assisted air handling units (AHU) equipped with desiccant wheels were considered and compared with standard AHUs, focusing on achievable primary energy savings.The relationship between the solar collector’s area and the specific primary energy consumption for different system configurations and building occupation patterns is described. For both occupation patterns, sensitivity analysis on system performance was performed for different solar collector areas. Also, this work presents an economic assessment of the systems. The cost of conserved energy and the payback time were calculated, with and without public incentives for solar cooling systems. It is worth noting that the use of photovoltaics, and thus the exploitation of related available incentives in many European countries, could positively influence the spread of solar air cooling technologies (SAC). An outcome of this work is that SAC systems equipped with PV/t collectors are shown to have better performance in terms of primary energy saving than conventional systems fed by vapour compression chillers and coupled with PV cells.All SAC systems present good figures for primary energy consumption. The best performances are seen in systems with integrated heat pumps and small solar collector areas. The economics of these SAC systems at current equipment costs and energy prices are acceptable. They become more interesting in the case of public incentives of up to 30% of the investment cost (Simple Payback Time from 5 to 10 years) and doubled energy prices.     

Integration of gas switching combustion in a humid air turbine cycle for flexible power production from solid fuels with near-zero emissions of CO2 and other pollutants

This work presents a novel plant configuration for power production from solid fuels with integrated CO₂ capture. Specifically, the Gas Switching Combustion (GSC) system is integrated with a Humid Air Turbine (HAT) power cycle and a slurry fed entrained flow (GE-Texaco) gasifier or a dry fed (Shell) gasifier with a partial water quench. The primary novelty of the proposed GSC-HAT plant is that the reduction and oxidation reactor stages of the GSC operation can be decoupled allowing for flexible operation, with the oxygen carrier serving as a chemical and thermal energy storage medium. This can allow the air separation unit, gasifier, gas clean-up, CO₂ compressors and downstream CO₂ transport and storage network to be downsized for operation under steady state conditions, while the reactors and the power cycle operate flexibly to follow load. Such cost-effective flexibility will be highly valued in future energy systems with high shares of variable renewable energy. The GSC-HAT plant achieves 42.5% electrical efficiency with 95.0% CO₂ capture rate with the Shell gasifier, and 41.6% efficiency and 99.2% CO₂ capture with the GE gasifier. An exergy analysis performed for the GE gasifier case revealed that this plant reached 38.9% exergy efficiency, only 1.6%-points below an inflexible GSC-IGCC benchmark configuration, while reaching around 5%-points higher CO₂ capture rate. Near-zero SO_x and NO_x emissions are achieved through pre-combustion gas clean-up and flameless fuel combustion. Overall, this flexible and efficient near-zero emission power plant appears to be a promising alternative in a future carbon constrained world with increasing shares of variable renewables and more stringent pollutant (NO_x, SO_x) regulations.     

Liquid organic hydrogen carriers: Development of a thermodynamic and kinetic model for the assessment of hydrogenation and dehydrogenation processes

Liquid organic hydrogen carriers (LOHCs) are drawing interest as a viable storage technology because of many favourable characteristics, such as high gravimetric and volumetric energy density. However, technological readiness is still limited, and very few kinetic and thermodynamic models are currently available. This paper aims to provide a complete and reliable model for the catalytic hydrogenation and dehydrogenation of a 0H-NEC/12H-NEC system following a general chemical thermodynamics approach that could easily extend to other LOHCs. The insurgence of already documented high-temperature phenomena has been taken into account by introducing two novel modifications to the base model, making the model reliable over a wide temperature range. First, the model identifies a maximum rate constant reached for a threshold temperature; secondly, it evaluates thermodynamic equilibrium conditions to account for a maximum H₂ uptake.     

Defects in boiler evaporator tubes—A detailed case study concerning defect initiation and extension processes

The present report attempts to describe a detailed failure analysis that was conducted on a series of full and partial through-thickness tube wall defects. Essentially two different defects were observed. One was a full through-thickness defect, the shape of which strongly suggests that it was driven by an active corrosion-dominated process. The second type of defect was partial through-thickness in nature and exhibited a common development route inasmuch as it extended through the evaporator tube wall by at least four discrete events or defect extension processes, viz.

1. (i) semi-elliptical pit formation, 0·3 mm deep;

2. (ii) thick porous-like magnetite growth (up to 400 μm thick) at the base of the pit;

3. (iii) heavy or active corrosion tunnelling, 1.3 mm in diameter; and

4. (iv) an environmentally assisted crack (EAC) fatigue crack propagation region which varied between 0.5 and 1.4 mm in size.

In some instances, a fifth defect extension event, which was corrosion dominated, was observed at the end of the fatigue-dominated event (iv). This event was similar to event (iii). Detailed fractography demonstrated that pit formation was the result of a mixed transgranular and intergranular corrosion process while the corrosion tunnelling was caused by a predominantly intergranular corrosion activity. The fatigue-dominated (event (iv)) fracture surface region contained a mixture of flat, cleavage-like facets which are indicative of an EAC growth process, and a fissured, ductile striated, failure mode which is commonplace in the case of pure, mechanically driven, fatigue processes. In the case of the full through-thickness defect, only an intense intergranular corrosion process was evidenced. The likelihood of these discrete events being caused by on-load or off-load process is discussed, while the presence of localised concentrations of sulphur and chlorine were detected on the corrosion-tunnelled and EAC fatigue growth surface regions.     

Experimental study of free convection heat transfer and condensation of vapor of humid air over an inclined cold tube

In this study condensation heat transfer on a cold inclined circular cylinder due to natural convection for various conditions is investigated experimentally. The cylinder is placed in an isolated test room to permit pure natural circulation of ambient air. Ambient temperature and humidity of the test room are controlled by a refrigeration cycle and humidifying. The ambient relative air humidity changed in the range of 30 to 50% and temperature from 25 to 35 °C. The ethylene‐glycol/water solution is used as a refrigerant to control and keep the temperature of the test section at a constant value. The cold surface temperature is varied from 2 to 6 °C. The condensation rate and heat flux are found to depend mainly on time, temperature difference between ambient air and cold surface, ambient relative humidity, and tube inclination. Results are plotted for various conditions with respect to time. The experimental results are used to propose a correlation to predict the condensate mass flow rate for free convection heat transfer. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21015     

Numerical investigation of coupled natural convection and radiation in a differentially heated cubic cavity filled with humid air. Effects of the cavity size

A numerical study of natural convection with surface and air/H₂O mixture radiation in a differentially heated cubic square cavity is presented. The coupled flow and heat transfers in the cavity are predicted by coupling a finite volume method with a spectral line weighted sum of gray gase model to describe gas radiative properties. The radiative transfer equation is solved by means of the discrete ordinate method. Simulations are performed at Ra?=?10⁶, considering different combinations of passive wall and/or gas radiation properties and different cavity length. It was found that in presence of a participative medium representative of building, cavity length has a strong influence on temperature and velocity fields which affect the global circulation and heat transfers in the cavity. For each steady-state solution, the convective and radiative contributions to the global heat transfer are discussed. More specifically, boundary layer thickness, thermal stratification parameter, and three-dimensional effects are compared to pure convective case results. The results suggest that radiative effects, often considered as negligible in view of the relatively low optical thickness, may not be neglected when trying to predict regime transitions.     

Analysis of heat transfer and frost layer formation on a cryogenic tank wall exposed to the humid atmospheric air

In this paper heat transfer characteristics and frost layer formation are investigated numerically on the surface of a cryogenic oxidizer tank for a liquid propulsion rocket, where a frost layer could be a significant factor in maintaining oxidizer temperature within a required range. Frost formation is modeled by considering mass diffusion of water vapor in the air into the frost layer and various heat transfer modes such as natural and forced convection, latent heat, solar radiation of short wavelength, and ambient radiation of long wavelength. Computational results are first compared with the available measurements and show favorable agreement on thickness and effective thermal conductivity of the frost layer. In the case of the cryogenic tank, a series of parametric studies is presented in order to examine the effects of important parameters such as temperature and wind speed of ambient air, air humidity, and tank wall temperature on the frost layer formation and the amount of heat transfer into the tank. It is found that the heat transfer by solar radiation is significant and also that heat transfer strongly depends on air humidity, ambient air temperature, and wind speed but not tank wall temperature.