A reliable process for compressing hydrogen and for removing all contaminants
is that of the metal hydride thermal compression. The use of metal hydride
technology in hydrogen compression applications though, requires thorough
structural characterization of the alloys and investigation of their sorption
properties. The samples have been synthesized by induction - levitation melting
and characterized by Rietveld analysis of the X-Ray diffraction (XRD) patterns.
Volumetric PCI (Pressure-Composition Isotherm) measurements have been conducted
at 20, 60 and 90 oC, in order to investigate the maximum pressure that can be
reached from the selected alloys using water of 90oC. Experimental evidence
shows that the maximum hydrogen uptake is low since all the alloys are
consisted of Laves phases, but it is of minor importance if they have fast
kinetics, given a constant volumetric hydrogen flow. Hysteresis is almost
absent while all the alloys release nearly all the absorbed hydrogen during
desorption. Due to hardware restrictions, the maximum hydrogen pressure for the
measurements was limited at 100 bars. Practically, the maximum pressure that
can be reached from the last alloy is more than 150 bars.
Flow characteristics of an adiabatic capillary tube in a transcritical CO2 heat pump system have been investigated employing the homogeneous model. The model is based on fundamental equations of mass, energy and momentum which are solved simultaneously. Two friction factor empirical correlations (Churchill, Lin et al., Int. J. Multiphase Flow 1991; 17(1):95–102) and four viscosity models (Mcadams, Cicchitti, Dukler and Lin) are comparatively used to investigate the flow characteristics. Choked condition at the outlet is also investigated for maximum mass flow rate. Subcritical and supercritical thermodynamic and transport properties of CO2 are calculated employing a precision property code. Choice of viscosity model causes minor variation in results unlike in chlorofluorocarbons (CFCs) refrigerants.
Relationships between cooling capacity with capillary tube diameter, length and maximum mass flow rate are presented. A lower evaporating temperature yields a larger cooling capacity due to the unique thermodynamic properties of CO2. It is also observed that an optimum cooling capacity exists for a specified capillary tube. Copyright
This study sets out to determine whether agricultural consumers of natural gas are responsive to changes in the relative prices of different types of energy. A demand model is specified and estimated. The conclusions strongly suggest that not only is the price of natural gas a factor impacting the quantity of natural gas demanded by agriculture, but that other types of energy are substitutes for natural gas and that income and weather (measured by heating degree days) likewise affect natural gas demand.
There has been considerable interest in producing fuels from biomass and wastes since the oil crises of the last two decades which has been reinforced by subsequent environmental concerns and recent political events in the Middle East. This project was undertaken to provide a consistent and thorough review of the full range of processes for producing liquid fuels from biomass to compare both alternative technologies and processes within those technologies in order to identify the most promising opportunities that deserve closer attention.
Hydrogen is the most promising alternative fuel for spark-ignited engines. This paper experimentally investigated the performance of a pure hydrogen-fueled SI engine at idle and lean conditions. The experiment was carried out on a four-cylinder gasoline-fueled SI engine equipped with an electronically controlled hydrogen port-injection system and a hybrid electronic control unit which was used to govern the hydrogen injection duration. The engine original electronic control unit was used to adjust the opening of idle bypass valve and spark timing to enable the engine to be run around its target idle speed. The test results showed that the fuel energy flow rate was reduced with the increase of excess air ratio for the pure hydrogen-fueled engine at idle and lean conditions. When excess air ratio increased from 2.08 to 3.2, the hydrogen energy flow rate was decreased from 11.79 to 9.97MJ/h. Both the flame development and propagation periods were increased with the increase of excess air ratio. Because of the increased opening of idle bypass valve and dropped cylinder temperature, both pumping and cooling losses were reduced when the engine was leaned out. NOx and CO emissions were negligible, but HC and CO2 were still existed for the pure hydrogen-fueled SI engine due to the possible burning of the blow-by lubricant oil gas. Copyright (c) 2013 John Wiley & Sons, Ltd.
Process conditions for the direct solar decomposition of sulfur trioxide have been investigated and optimized by using a
receiver–reactor in a solar furnace. This decomposition reaction is a key step to couple concentrated solar radiation or
solar high-temperature heat into promising sulfur-based thermochemical cycles for solar production of hydrogen from
water. After proof-of-principle a modified design of the reactor was applied. A separated chamber for the evaporation
of the sulfuric acid, which is the precursor of sulfur trioxide in the mentioned thermochemical cycles, a higher mass flow
of reactants, an independent control and optimization of the decomposition reactor were possible. Higher mass flows of
the reactants improve the reactor efficiency because energy losses are almost independent of the mass flow due to the
predominant contribution of re-radiation losses. The influence of absorber temperature, mass flow, reactant initial
concentration, acid concentration, and residence time on sulfur trioxide conversion and reactor efficiency has been
investigated systematically. The experimental investigation was accompanied by energy balancing of the reactor for
typical operational points. The absorber temperature turned out to be the most important parameter with respect to
both conversion and efficiency. When the reactor was applied for solar sulfur trioxide decomposition only, reactor
efficiencies of up to 40% were achieved at average absorber temperature well below 1000°C. High conversions almost
up to the maximum achievable conversion determined by thermodynamic equilibrium were achieved. As the reradiation
of the absorber is the main contribution to energy losses of the reactor, a cavity design is predicted to be the
preferable way to further raise the efficiency.
The properties of the components of a membrane electrode assembly in a polymer electrolyte fuel cell (PEFC) determine its efficiency and performance. This paper aims at demonstrating the importance of nanoscale properties of PEFC membranes and electrodes and discussing the information obtained by various experimental techniques. The nanostructure and conductivity of freshly prepared as well as artificially degraded Nafion membranes and Pt/C electrodes are investigated by contact atomic force microscopy (AFM), conductive AFM, pulsed force mode (PFM)-AFM, in situ scanning tunnelling microscopy (STM), and scanning electron microscopy. The different techniques can provide complementary information on structure and conductivity. With in situ STM on Pt catalyst covered graphite, a layer of very small Pt particles between the catalyst particles is imaged, which is probably not visible with TEM and can explain a systematic discrepancy between TEM and XRD in particle size distribution. Conductive AFM is used to investigate the conductivity of Nafion. The images show a quite inhomogeneous distribution of current at the surface.
The percentage of conductive surface increases with humidity, but regions without any current still present up to 80% of relative humidity (RH). Comparison with PFM-AFM images, where differences in adhesion forces are measured,
indicates that hydrophobic regions are present at the surface with comparable dimensions, which are attributed to
non-conductive PTFE-like polymer backbone. The changes in hydrophilic and hydrophobic parts after artificial degradation by plasma etching in air plasma can be imaged by PFM. High-resolution current images of the membrane were used to directly compare the measured nanostructure of the single conductive channels with model predictions from the literature. Recent models in the literature propose the formation of water-filled inverted micelles, with a mean
diameter of 2.4 nm, and their agglomeration into clusters agrees well with the current images
A two-step thermochemical cycle for solar production of hydrogen from water has been developed and investigated. It is based on metal oxide redox pair systems, which can split water molecules by abstracting oxygen atoms and reversibly incorporating them into their lattice. After successful experimental demonstration of several cycles of alternating hydrogen and oxygen production, the present work describes a thermodynamic study aiming at the improvement of process conditions and at the evaluation of the theoretical potential of the process.
Ejector refrigeration systems can use low grade thermal energy, at temperatures as low as 60°C, to provide space cooling. Since this waste energy is widely available and the cost of its supply is negligible in many cases, cooling costs can be lower than conventional systems, which makes the method very attractive. The present study describes a computer simulation model for ejector refrigeration systems that was developed using an existing ejector theory. This model allows for internal irreversibilities within the ejector to be included and caters for the addition of a regenerator and a precooler for improving the system coefficient of performance. The study shows that HCFC-123 is a suitable replacement for CFC-11 in this application. Results also indicate that the use of a variable geometry ejector can maintain the optimum performance of refrigeration systems when operating conditions change.
The performance of a vapour-compression system was examined with both R12/mineral oil and R134a/mineral oil charges. The mineral oil was then removed from the system using a multiple flushing method and the experiments repeated using a charge of R134a and an ester-based lubricant to establish the effect of the oil on the performance of the system. Results were compared with theoretical data for R134a and R12 refrigerants.
A 1700 m2 solar pond was constructed in the desert of Kuwait where severe weather conditions prevail in all seasons. The paper describes in detail a diffuser design for the gradient establishment, gradient stability, and thermal performance of the pond. The main problem encountered in operating the pond was mixing between the upper zone and the gradient zone, even when the wind speed was as low as 5 m/s. No mixing between the gradient and the lower connective zone was observed. The wind effect was severe in causing mixing even when the upper convective zone increased to 0.90 m.
This paper discusses the design and construction philosophy of the Kuwait solar-pond/multistage-flash system. The present work came about to study the performance of a solar pond as a main source for energy collection and storage and to use the collected heat in producing fresh water, which is difficult to obtain in remote areas. The pond is built with 1700 m2 of surface area and 45° sloping sides. Taking the natural surroundings and the nature of the climate into account, the pond is estimated to perform at 18% efficiency. The collected energy, which is estimated at 1800 kWh/day, will be used to produce 25 m3 of fresh water daily.
A heat pump can be used to recover, upgrade and recycle heat from the condenser of the distillation column to its reboiler. In recent years there has been a growing interest in the various methods of using heat pumps to recycle energy in distillation columns. More than one hundred and fifty references for heat pump assisted distillation systems have been listed and classified under the following categories: heat pump assisted distillation (1) with an external working fluid, (2) using one of the distillation components as the working fluid, (3) overall assessments and (4) experimental data. The classification of more than one hundred and fifty references reported in this paper was made in order to help people to make better use of existing data and to encourage further research in heat pump assisted distillation systems.
The generation, consumption and pricing of electricity in the member states of the European Community is modelled over the period from 1953–1986. Theoretical demand functions are developed and optimality conditions for equilibrium are established.
The paper reports the results of a simple cointegration analysis applied to bivariate causality models and quarterly data on crude oil consumption, GDP and inflation in Thailand to investigate the long-term relationships in the sense of Granger between oil and these two major macroeconomic aggregates. For the period from January 1966 to January 1991, the empirical evidence indicates that oil consumption, output growth and inflation rate, as formulated in our models, are not random walks. In addition, oil consumption is significantly cointegrated with economic growth and, unfortunately, inflation rate.
The decade of the 1980s was marked by very large changes in all factors controlling oil demand, supply and price. On the demand side, in the OECD nations, the early 1980s saw an apparent break or discontinuity in long-established relations between economic growth and energy demand. On the supply side, the apparent "disaggregation' of OPEC in its cartel aspects, massive debt problems for most non-OECD oil exporters, changes in OECD oil demand, and the explosive growth of market and trading as the major price fixer, all contributed to downward price trends. These reached their peak in the price collapse of 1986. This is taking place in a vigorous upward cyclic economic context, where economic recovery and growth in the OECD is being led by industry and manufacturing. Overall, the implications are that oil price growth will not fall to the rate of general inflation before 1991. -from Author
This paper reports the results of energy analysis of two 210 MW coal-fired thermal power stations located a good distance apart. A new and simple method for evaluation of thermal efficiency has been presented. Measures for improvement in plant performance in the coal, air, water and steam circuits as well as auxiliary power and secondary oil have been depicted, based on the analysis and existing field conditions.
In the present work, exergy analysis of a coal-based thermal power plant is done using the design data from a 210 MW thermal power plant under operation in India. The entire plant cycle is split up into three zones for the analysis: (1) only the turbo-generator with its inlets and outlets, (2) turbo-generator, condenser, feed pumps and the regenerative heaters, (3) the entire cycle with boiler, turbo-generator, condenser, feed pumps, regenerative heaters and the plant auxiliaries. It helps to find out the contributions of different parts of the plant towards exergy destruction. The exergy efficiency is calculated using the operating data from the plant at different conditions, viz. at different loads, different condenser pressures, with and without regenerative heaters and with different settings of the turbine governing. The load variation is studied with the data at 100, 75, 60 and 40% of full load. Effects of two different condenser pressures, i.e. 76 and 89 mmHg (abs.), are studied. Effect of regeneration on exergy efficiency is studied by successively removing the high pressure regenerative heaters out of operation. The turbine governing system has been kept at constant pressure and sliding pressure modes to study their effects.
In this paper, a performance evaluation of a newly developed blend of HFCs and HCFCs, as a substitute for CFC-12, CFC-502, and HCFC-22, is presented. The blend's performance has been evaluated using water/air, air/air and water/water heat pumps, as well as a domestic refrigerator. The test results showed that the blend is not only environmentally sound, but it also enhances the performance by 7% to 30%, depending upon the application.
This paper presents a periodic analysis of a three zone solar pond as a solar energy collector and long term storage system. We explicitly take into account the convective heat and mass flux through the pond surface and evaluate the temperature and heat fluxes at various levels in the pond during its year round operation by solving the time dependent Fourier heat conduction equation with internal heat generation resulting from the absorption of solar radiation in the pond water. Eventually, an expression, for the transient rate at which heat can be retrieved from the solar pond to keep the temperature of the zone of heat extraction as constant, is derived. Heat retrieval efficiencies of 40.0 per cent, 32.1 per cent, 28.3 per cent and 25.5 per cent are predicted at collection temperatures of 40, 60, 80 and 100°C, respectively. the retrieved heat flux exhibits a phase difference of about 30 to 45 days with the incident solar flux; the load levelling in the retrieved heat flux improves as the thickness of the non-convective zone increases. the efficiency of the solar pond system for conversion of solar energy into mechanical work is also studied. This efficiency is found to increase with collection temperature and it tends to level around 5 per cent at collection temperatures about 90°C.
Combined cycle power plants (CCPPs) have an important role in power generation. The objective of this paper is to evaluate irreversibility of each part of Neka CCPP using the exergy analysis. The results show that the combustion chamber, gas turbine, duct burner and heat recovery steam generator (HRSG) are the main sources of irreversibility representing more than 83% of the overall exergy losses. The results show that the greatest exergy loss in the gas turbine occurs in the combustion chamber due to its high irreversibility. As the second major exergy loss is in HRSG, the optimization of HRSG has an important role in reducing the exergy loss of total combined cycle. In this case, LP-SH has the worst heat transfer process.
The first law efficiency and the exergy efficiency of CCPP are calculated. Thermal and exergy efficiencies of Neka CCPP are 47 and 45.5% without duct burner, respectively. The results show that if the duct burner is added to HRSG, these efficiencies are reduced to 46 and 44%. Nevertheless, the results show that the CCPP output power increases by 7.38% when the duct burner is used. Copyright
Numerical calculations have been carried out to explain the effect of self absorption on the nature of the emitted intensity of the Na-5890 Å line of the sodium doublet in water gas combustion plasma. The self absorption is purely an effect caused by the presence of cold wall boundary layers in an MHD duct. The effect of seed concentration, core temperature, wall temperature, boundary layer thickness and profile parameter on line shapes has been studied. It has been found that seed percentage is the major factor influencing the line shape parameters and core temperature is the major factor affecting the maximum radiated monochromatic line intensity.
A nonlinear programming optimization problem is considered for the case in which the decision vector is the vector of zonal irradiation while the objective function is the equilibrium fuel feed rate with a CANDU-600 reactor subject to certain restrictions regarding the reactivity excess and the maximum powers per channel and bundle. The objective and restriction functions of the system are estimated within a time average approximation of a three-dimensional two-group diffusion type, with the control systems uniformly immersed within the reactor, and also considering the newest increments of the parasitic absorbents using FMDP program. The optimization problem is solved by means of the SUMT method, modified so as to become a technique of sequential solving for certain subproblems of linear optimization through the application of LPROG program. The results are presented as optimal vectors of zonal irradiation and as optimal values of fuel feed rate, with some two-zone cases, which establish the best zonal configuration. This configuration is then extended to a four-zone case which is analysed in a similar manner, evidencing a saving potential which gives the possibiliy of increasing the reactor performance.
Twenty-six absorbent—refrigerant combinations, holding good promise as fluid systems, have been considered for single stage absorption air conditioning system. These fluids have been compared on the basis of solution characteristics, life expectancy characteristics and refrigeration cycle characteristics. The mass flow rates of rich and poor solutions per ton of refrigeration capacity and the coefficient of performance (CP) were compared for an evaporator temperature of 5°C, absorber and condenser temperatures of 35°C and a generator temperature of 120°C (low grade energy sources). More than half of the waste energy available in industry happens to be at a temperatures below 200°C. Other types of low grade thermal energy such as solar energy and geothermal energy can be used in operating vapour absorption refrigeration and air-conditioning systems.
This paper presents an investigation on using an ammonia refrigerant with liquid/solid absorbents in an absorber heat recovery cycle where heat released during the absorption process is used to heat up the strong solution coming out of the absorber, thereby reducing the generator heat input and hence improving the coefficient of performance. A comparative thermodynamic study is made with NH3-H2O and NH3-LiNO3 pairs as working fluids for both conventional absorption and absorber heat recovery systems. It is found that an improvement of about 10 per cent in COP for the absorber heat recovery cycle is achieved over the conventional absorption cycle and the NH3-LiNO3 system yields a higher COP than for NH3-H2O over a wide range of generator temperatures and condenser/absorber temperatures. A detailed parametric study is also presented in this paper.
Spray-pyrolysed selective cobalt-oxide (CoOx) coatings were prepared on the surface of a bright nickel-plated copper tubular absorber (α = 0.89–0.91 and ϵ100°C = 0.18) for operation in conjunction with a prototype linear Fresnel reflector solar concentrator (LFRSC). Some preliminary tests were conducted to study the optical and thermal performance characteristics of the selective cobalt-oxide coated absorber in the concentrated solar flux. The tests conducted included determination of the overall heat loss coefficient UL of the absorber at temperatures from 50 to ∼ 120°C, and the optical efficiency ηo of the concentrator-absorber system, and measurement of the stagnation temperature of the absorber with the prototype solar concentrator. Based on the results of UL and ηo measurements, the thermal efficiency η of the concentrator-absorber system at a working temperature of 115°C has been determined for a typical beam radiation Ib of 600 W/m2. Further, comparison of the results of this study with those obtained using a dimensionally identical black-painted absorber indicates that the performance of the selective cobalt-oxide coated absorber is considerably superior to that of an ordinary black-painted absorber.
Experiments have been carried out to determine the effect of absorber reflux on the performance of a water-lithium-bromide absorption cooler. The reflux ratio was varied for two mass flow rates of solution leaving the absorber and two absorber temperatures. It has shown that absorber reflux can be used to decrease both the temperature and the concentration in the generator. The coefficient of performance and heat loads tend to have optimum values at a particular reflux ratio.
Two different approaches for designing a linear Fresnel reflector solar concentrator (LFRSC) with a flat horizontal absorber are described. The performance characteristics of both the designs are studied in detail. The distribution of local concentration ratio on the surface of the absorber, for each design, is investigated using the ray trace technique. Results of some typical numerical calculations are presented graphically and discussed.
The aim of this study is to provide a remedy for the low thermophysical properties of air, which is used as a fluid of transfer in solar collectors. A fully developed flow needs to be created by the use of staggered fin rows soldered under the absorber plate. The fluid flow undergoes contractions followed by expansions, which creates a fully developed turbulent flow, and increases the thermal heat transfer between the absorber plate and the air. The fins increase the heat transfer surface, from which an appreciable improvement of the thermal heat performance of solar air heaters has been found in comparison to those of solar air heaters with a plane absorber. In this work we have tested the influence of the dimension of the fins and the influence of the space between consecutive fin rows mounted in staggered rows.