One feedwater heater taken out of service as a strategy to maintain full load and its effect on steam power cycle parameters and performance

The aim of this study is to analyze the suitability of one heater removal as a strategy for maintaining full load operation of steam power cycles when superheated and/or reheated temperatures (T_SS, T_RS) decrease and the effect on the net heat rate (HR_Net). For this purpose, three regenerative cycles with different numbers of closed feedwater heaters were chosen. The cycles were analyzed at different steady states with Thermoflex software. Removing a heater has an important influence on the cycle operation and performance, leading to the redistribution of extraction mass flows, with the heater immediately downstream being the most affected. This may make it necessary to reduce the load of the cycle. However, when the highest pressure heater (highest PH) is removed from service, the changes are not so significant. When T_SS and/or T_RS decrease, the plant may not achieve full load operation. Nevertheless, if the highest PH is removed from service, it can help to recover full load. This is due to the decrease in the water/steam mass flow through the steam generator, which produces an increase in T_SS and/or T_RS. On the one hand, this measure leads to higher HR_Net in comparison to that of the nominal conditions. On the other hand, there are certain conditions at which HR_Net is lower than when all the heaters are in service and the values of T_SS and/or T_RS are low. Thus, for maintaining full load, the highest PH removal can be applied and cycle parameters optimized in order to reach a HR_Net closer to its nominal value. The higher the number of closed feedwater heaters, the more adequate is the application of this strategy.     

Measured shortwave sky radiance in an urban atmosphere

An extensive data set of measurements of the shortwave sky radiance at Toronto, Canada, was compiled by the authors in 1984. A pyroelectric radiometer with a half-angle field of view of 5 degrees was used to make a total of more than 100,000 sky radiance measurements. This paper presents an analysis of almost 40,000 of these measurements in the form of a set of plots showing means and percentiles of sky radiance as a function of position in the sky; atmospheric clearness index, k_t; diffuse fraction, k; and solar zenith angle, θ_z. The data show (i) that, in the circumsolar region, the highest observed values of sky radiance occur for partly cloudy sky conditions characterized by intermediate values of k and k_t, (ii) that, in the regions of the sky away from the solar position, sky radiance tends to increase monotonically both with increasing k and with increasing k_t, (iii) that there is a progressive transition with increasing k, from the horizon brightening for skies with low diffuse fractions to horizon darkening for skies with diffuse fractions approaching unity, (iv) that the shape of the sky radiance distribution is a strong function of the elevation of the sun in the sky, and (v) that the sky radiance is highly variable, with a positively skewed probability density function.     

Investigation of pollutants dispersion from power stations

Theoretical investigations were conducted of pollutants dispersion, NO_x, SO₂ and Particulate Matter (PM), from stacks of arbitrary four power plants in Libya, e.g. North Benghazi, South Tripoli, Zweitina, Khoms. The first four stations are gas power plants, while the last one is gas and steam station. Gaussian plume model has been used to identify ground‐level NO_x concentrations profile downwind and crosswind of the chimneys through urban regions and also the location of maximum pollutant concentrations. The study is based on the worst‐case emission conditions of Pasquill stability categories (class D). Results indicate that maximum ground‐level NO_x impacts for all plants locate at a distance of approximately 1.0–2.5 km from stacks. The site most critical to ambient air NO_x impact is Zweitina, where the plant site is in direct vicinity to residential areas. Khoms electric station exhibits the maximum emitted NO_x, SO₂, and PM intensity, about 305, 48, and 0.7 µg/m³, respectively, that is lower than allowable concentrations recommended by World Health Organization. Copyright © 2006 John Wiley & Sons, Ltd.     

Stochastic approach in the optimization of A nuclear power system

The paper deals with a CANDU-CANDU (Th/Pu)-LMFBR (PuO₂) nuclear power system which evolves in a finite time interval. Its initial evolution is only in the CANDU variant, and subsequently in the variants CANDU (Th/Pu) and LMFBR (PuO₂) by the use of Pu produced in the system. It is assumed that the fuel burn-up in the LMFBR (PuO₂) reactors is a random value, as it is governed by an a priori determined field of probability. The resources of natural uranium and Pu which severely influence the development of the system are also random, as they cannot be definitely known, and moreover they are actually governed by another field of probability already known. Under these conditions, the set of optimal solutions and associated optimal values represented by the nuclear electric powers released in the system at the end of the considered time interval have to be derived. Concomitantly, the distribution of the optimal value, its average value and standard deviation can be evaluated. This type of stochastic approach to nuclear power system optimization is much more valid than the deterministic approach, as it supplies information of interest for the decision-makers engaged in the solution of a nuclear power policy.     

A time series model for Kt with application to global synthetic weather generation

This article describes a general mathematical procedure for generating synthetic daily solar irradiation values. The procedure should be useful for simulating solar energy systems, requiring only the 12 monthly means ( ), as input. The procedure is based on a time series analysis of the daily K_t values, described in the article. It incorporates the well-known probability distribution function for K_t, by using a transformed variable, rather than K_t, as the fundamental random quantity.     

Antireflection nanocomposite coating on PV panel to improve power at maximum power point

A thorough investigation was performed on various chemicals and nanocomposite materials to apply antireflection coating on the PV panel intending to improve the solar cell efficiency by generating more voltage, current, and power at maximum power point (MPP). The improvement in solar cell efficiency would indicate the efficient way of utilizing solar energy and converting into electrical energy which in turn saves lot of cost and improves the power generation. To improve the efficiency by the coating approach, the nanocomposite materials mixing with some composition of efficient chemicals demonstrates extremely encouraging change in surface inactivity, antireflection, and wanted vitality band hole of the materials. After a detailed experimentation, it is been identified that the multiwall carbon nanotube, titanium dioxide, and silicon dioxide materials demonstrate elevated efficiency in increasing the V_MPP, I_MPP, and P_MPP when compared with uncoated cells. The investigational inferences in detailed are analyzed along with scanning electron microscopy findings and finally characterized the poly-Si solar cell with coating. Overall, there is a relative improvement of 36.45% of efficiency when compared with the uncoated cells.     

Design and optimization of GaN‐based betavoltaic cell for enhanced output power density

In this work, we designed and optimized a gallium nitride (GaN)‐based betavoltaic (BV) cell using an AlGaN back‐barrier layer and finger structure for improving the output power density. A short‐circuit current density (J_SC) and an open‐circuit voltage (V_OC) of the BV cells associated with an output power density were investigated by using electron‐beam (e‐beam) irradiation. The device with the Al_0.25Ga_0.75N back‐barrier layer exhibited an enhanced J_SC because the potential barrier with a high height reduced excess carriers moving to the substrate region. The finger structure of the proposed BV cells was optimized by changing parameters such as the width of the intrinsic GaN region (W_i‐GaN) and heights of the p‐GaN and n‐GaN regions (H_p‐GaN and H_n‐GaN). The optimized BV cell with a W_i‐GaN of 100 nm, a H_n‐GaN of 100 nm, and a H_p‐GaN of 200 nm obtained a higher J_SC compared to that of the conventional p‐i‐n BV cell because an optimum structure resulted in a wide depletion area, which was involved in the improved charge collection. As a result, the output power density of the proposed BV cell was enhanced by 14.8% than that of the conventional BV because of the improved J_SC. The proposed structure shows a high potential for BV cells with a high‐power conversion efficiency.     

Two-dimensional resolution of minority carrier diffusion constants in different silicon materials

Hall measurements are a common method to determine the majority charge carrier diffusion constant. But the diffusion constant of the minority carriers D_n, the more interesting parameter in photovoltaics, is rather hard to detect. In this paper we introduce a method to determine D_n locally resolved and mapped in two dimensions. For that purpose the local diffusion length L_diff, which can be calculated from LBIC (laser beam induced current) measurements, has been combined with the local bulk lifetime τ_b received by μ-PCD (microwave-detected photo conductance decay) measurements. We evaluated the diffusion constants of the minority charge carriers D_n for different p-type silicon materials with a resolution of 100 μm. The measurements were carried out on solar cells before and after remote plasma hydrogen passivation in order to get an impression of the diffusion constant dependency on hydrogen incorporation.     

Experimental study on blade pitch angle effect on the performance of a three‐bladed vertical‐axis Darrieus hydro turbine

Because of higher concerns about increasing global warming, energy consumption and reduction of conventional energy resources and growing attention are given to renewable energy and to cross flow turbines such as the Darrieus turbine to harness water energy (water currents, reservoirs, rivers, and oceans). The aim of the experimental investigation presented in this paper is to evaluate the effect of hydro Darrieus turbine blades fixation pitch angle “i_g” on its performance. Four main sets of experimental tests were conducted for the same vertical‐axis hydro turbine model (VAHT) with four blade fixation pitch angles (i_g = ?1.75°, ?4.5°, 1.75°, and 4.5°), at various water flow velocities (V = 0.3‐0.64 m/s corresponding to a water free flow Reynolds number of 2.5 × 10⁴ to 4.36 × 10⁴). A comparison between the results of the present work and with those of a previous experimental study for i_g = 0° shows that the best performance of the tested turbine is obtained when the blades are set at a pitch angle of 1.75°. In fact, the corresponding optimum mechanical power and power coefficient relative increases are respectively as much as 82% and 67% with respect to i_g = 0° at V = 0.37 m/s and as much as 65% and 77% at V = 0.46 m/s. The worst performance is obtained for the negative blades fixation pitch angle of ?4.5°; at the water flow velocity of 0.37 m/s, this leads to power, and power coefficient relative decreases respectively about 75% and 81% with respect to the results obtained for i_g = 1.75° and respectively about 54% and 68% of those obtained in the previous work for i_g = 0°.     

Characterization of high open-circuit voltage double sided buried contact (DSBC) silicon solar cells

The double sided buried contact (DSBC) silicon solar cells have consistently shown high open-circuit voltages (V_oc) than its single sided buried contact counterpart because of better rear surface passivation. The rear surface passivation which is provided by the rear floating junction is effective only when there is no leakage in the rear floating junction. However, the partial shunting of the rear floating junction can cause a drop in the fill factor of the cell which has been the only parameter limiting the realization of the structure's potentials. In this paper, LBIC (light beam induce current), spectral response, dark I-V and J_sc-V_oc measurements for DSBC cells have been carried out to help explain some of the experimentally observed attributes of this structure. The partly shunted rear floating junction has been identified by LBIC measurement as low current regions near the rear metal contacts.     

4.2 V Stack of metal oxide-polypyrrole-based composite electrodes and their power management

Polypyrrole (PPy)-based composites containing nanostructured VO _x, PbO _x, and MnO _x are galvanostatically synthesized on exfoliated graphite papers (1.0 and 6.25 cm²) formed as a result of the intercalation with their metals. Both intercalation and synthesis are performed in the synthesis solution containing metal(II)tetrafluoroborate salts, pyrrole monomer, HBF₄, H₂O, Triton-X 100 (TX100), and carboxymethyl cellulose (CMC). The mass loading of the composite at the electrode is maintained at 10 mg cm⁻². Symmetric and asymmetric unit cells are prepared in polyvinyl alcohol (PVA)/H₂SO₄ (1:1) gel electrolyte using composites synthesized on the treated graphite paper electrode. The electrical properties of these cells are examined by electrochemical impedance spectroscopy and their capacitive properties by cyclic voltammetry and galvanostatic-charge discharge test. The asymmetrical cell, prepared with PPy/PbO _x/CMC composite having energy density and cycle life better than others, exhibits 7.77 Wh kg⁻¹ and 0.51 kW kg⁻¹ at a working voltage of 1.4 V, considering the total mass of all sub-components. The three-cell asymmetric stack design, which is prepared using these composite-coated electrodes, achieves a coulombic efficiency of 76% at 4.2 V. Finally, an electronic control system (DC/DC buck-boost converter) is designed and implemented to manage the charge and discharge cycles of single-cell and bipolarly connected stack to operate the asymmetric supercapacitor arrays with high performance.     

ACCURATE UPPER BOUND EFFICIENCY FOR SOLAR THERMAL POWER GENERATION

A class of accurate upper bounds for the efficiency of converting solar energy into work was derived in this paper by taking into account (i) the irreversibilities associated with the heat transfer inside the heat engine and (ii) details about the system considered, as the geometric (view) factor of the Sun, the dilution factors of solar and ambient radiation and the optical properties of the converter (concentration ratio, absorptance, reflectance and transmittance for both transparent cover and absorber), A simpler (but less accurate) upper bound efficiency is: where T _s, and T₀ are Sun and ambient temperatures, respectively. This upper bound is still more accurate than the upper limit efficiencies usually cited in literature.     

Band-gap engineering in CuIn(Se,S)2 absorbers for solar cells

Thin films based on CuInSe₂ have become very successful as absorber layers for solar cells. It is only in the recent past that gallium (Ga) and sulfur (S) were incorporated into CuInSe₂ in order to increase the energy band gap of the film to an optimum value with the ultimate aim of producing more efficient devices. This paper focuses on the incorporation of S into partly selenized CuInSe₂ films in order to produce CuIn(Se,S)₂ films with varying S/Se+S ratios, resulting in different band-gap energies. This was achieved by varying the conditions when selenizing Cu/In alloys in H₂Se/Ar, and then exposing these various partly selenized films to H₂S/Ar under identical conditions.     

Outdoor performance of amorphous silicon and polycrystalline silicon PV modules

The daily watt-hour efficiency (η_Wh) and daily integrated output power (P_Wh) of the a-Si and poly-Si module have been used to examine the performances of both modules on the basis of two years' data accumulated at outdoor conditions. Results from the analysis of experimental data taken under incident solar energy higher than 3.0 kWh/m² per day show that the annual average of η_Wh of the a-Si module is about 95% and 92.5% of its efficiency at STC condition at the first and second year, respectively, while the values are nearly unchanged at about 89% for the poly-Si module. During a one year period, the average P_Wh of the a-Si and poly-Si module was about 60% and 56%, respectively, of their calculated output power at STC condition, so that the P_Wh for each watt-peak (W_p) of the maximum power of a-Si module is about 11% higher than that of the poly-Si module.     

Comparison of microstructures and thermal insulation capability of plasma sprayed nanostructured and traditional YPSZ coatings

Abstract

Microstructural features and thermal insulation capability of plasma sprayed nanostructured and traditional yttria partly stabilised zirconia (YPSZ: ZrO₂–8Y₂O₃) coatings were investigated. Both nanostructured YPSZ coating and traditional YPSZ coating were mainly composed of non-transformable t-ZrO₂ phases. The detailed microstructures of the nanostructured YPSZ coating were observed using FESEM, which presented three types of microstructures: columnar grains, equiaxed grains and nanosized zirconia particles embedded in the so called matrix formed by melted powders. However, the traditional YPSZ coating only contained columnar and equiaxed grains. The average porosities of nanostructured and traditional YPSZ coatings are 12·5 and 9·7% respectively. Compared with the traditional YPSZ coating, the nanostructured coating contained finer microcracks. The nanostructured YPSZ coating has higher thermal insulation capability than the traditional YPSZ coating. For YPSZ coating of 200 μm in thickness, the temperature drop ΔT (thermal insulation capability) of the nanostructured coating at 1350°C increased by 27% compared with that of the traditional coating.     

Effect of lignocellulose-derived inhibitors on growth and hydrogen production by Thermoanaerobacterium thermosaccharolyticum W16

In the process of producing H₂ from lignocellulosic materials, inhibitory compounds could be potentially formed during pre-treatment. This work experimentally investigated the effect of lignocellulose-derived inhibitors on growth and hydrogen production by Thermoanaerobacterium thermosaccharolyticum W16. Representative compounds presented in corn stover acid hydrolysate were added in various concentrations, individually or in various combinations and subsequently inhibitions on growth and H₂ production were quantified. Acetate sodium was not inhibitory to T. thermosaccharolyticum W16, rather than it was stimulatory to the growth and H₂ production. Alternatively, furfural, hydroxymethylfurfural (HMF), vanillin and syringaldehyde were potent inhibitors of growth and hydrogen production even though these compounds showed inhibitory effect depending on their concentrations. Synergistic inhibitory effects were exhibited in the introduction of combinations of inhibitors to the medium and in hydrolysate with concentrated inhibitors. Fermentation results from hydrolysates revealed that to increase the efficiency of this bioprocess from corn stover hydrolysate, the inhibitory compounds concentration must be reduced to the levels present in the raw hydrolysate.     

Modeling and experimental analysis of a self-excited six-phase induction generator for stand-alone renewable energy generation

This paper presents a simple d–q model of a saturated multi-phase (six-phase) self-excited induction generator (SP-SEIG). Performance equations for this machine are given which utilize the saturated magnetizing inductance L_m=(λ_m/i_m) and its derivative (dL_m/di_m) rather than dynamic inductance L=(dλ_m/di_m). In the analytical model, the effects of common mutual leakage inductance between the two three-phase winding sets have been included. A detailed experimental investigation about the voltage build-up, collapse of voltage, and various performance including loading and unloading characteristic, power capability and reliability of six-phase self-excited induction generator is also presented in the paper. Experimental results are recorded: (a) with capacitor bank connected across both the three-phase winding sets, and (b) with capacitor bank connected across only one three-phase winding set. Loading and unloading transients are recorded with independent three-phase resistive loads at each of the two three-phase winding sets, and measured steady-state characteristics for various load and/or capacitor bank configurations. Experimentations were also carried out to judge the performance of the SP-SEIG when three-phase load was connected via an interposed Y−Δ/Y six-phase to three-phase transformer.     

Analysis and cost optimization of the triple‐pressure steam‐reheat gas‐reheat gas‐recuperated combined power cycle

Increasing the inlet temperature of gas turbine (TIT) and optimization are important methods for improving the efficiency and power of the combined cycle. In this paper, the triple‐pressure steam‐reheat gas‐reheat recuperated combined cycle (the Regular Gas‐Reheat cycle) was optimized relative to its operating parameters, including the temperature differences for pinch points (δT_PP). The optimized triple‐pressure steam‐reheat gas‐reheat recuperated combined cycle (the Optimized cycle) had much lower δT_PP than that for the Regular Gas‐Reheat cycle so that the area of heat transfer of the heat recovery steam generator (HRSG) of the Optimized cycle had to be increased to keep the same rate of heat transfer. For the same mass flow rate of air, the Optimized cycle generates more power and consumes more fuel than the Regular Gas‐Reheat cycle. An objective function of the net additional revenue (the saving of the optimization process) was defined in terms of the revenue of the additional generated power and the costs of replacing the HRSG and the additional fuel. Constraints were set on many operating parameters such as the minimum temperature difference for pinch points (δT_PPm), the steam turbines inlet temperatures and pressures, and the dryness fraction at steam turbine outlet. The net additional revenue was optimized at 11 different maximum values of TIT using two different methods: the direct search and variable metric. The performance of the Optimized cycle was compared with that for the Regular Gas‐Reheat cycle and the triple‐pressure steam‐reheat gas‐reheat recuperated reduced‐irreversibility combined cycle (the Reduced‐Irreversibility cycle). The results indicate that the Optimized cycle is 0.17–0.35 percentage point higher in efficiency and 5.3–6.8% higher in specific work than the Reduced‐Irreversibility cycle, which is 2.84–2.91 percentage points higher in efficiency and 4.7% higher in specific work than the Regular Gas‐Reheat cycle when all cycles are compared at the same values of TIT and δT_PPm. Optimizing the net additional revenue could result in an annual saving of 33.7 million US dollars for a 481 MW power plant. The Optimized cycle was 3.62 percentage points higher in efficiency than the most efficient commercially available H‐system combined cycle when compared at the same value of TIT. Copyright © 2007 John Wiley & Sons, Ltd.     

The first pilot demonstration: solar updraft tower power plant in Jordan

The high cost of importing heavy fuel oil from neighbouring countries of Jordan stimulated the need to consider renewable energy as an alternative to imported power. The paper describes a systematic experimental study on the pilot solar updraft tower power plant model system. Particular attention is given to measurements of air velocity, temperature, solar radiation and voltage difference. This is the first pilot solar updraft power plant that has been built in Jordan to evaluate such technology in Mutah Area. In this paper, all the experimental data were taken from a very small pilot project (compared with other prototypes that we constructed which were placed in the backyard of our faculty). For the experimental part, the solar updraft power plant was a pilot project with an updraft power plant height H=4(m), updraft power plant radius R _c=0.29(m), plastic solar collector area A _coll=36(m²), collector roof height=1(m), wind turbine that was a compensation of the suction fan with a diameter D _fan=0.5(m), and a small generator (6 V) The experimental results in this paper show that the maximum height (H _max) gradually increases with the solar irradiation (G). Also, in the same way, with the pressure difference (Δ p) increases with solar irradiation.