Index of content:
Volume 8, Issue 5, September 2016
- Bioenergy and Biofuels
Stochastic analysis of wind energy uncertainty impact on ISO risk-taking in joint energy and reserve markets using conditional value at risk8(2016); http://dx.doi.org/10.1063/1.4962413View Description Hide Description
Uncertainty management of the large scale wind power resources is mainly known as one of the most challenging aspects in future electricity market operations, where network operation may put at risk due to the wind power forecasting errors. Therefore, it is necessary to evaluate the impact of the wind power uncertainty on the operator risk-taking by using validated tools. This paper proposes a new mixed integer linear programming model for joint energy and reserve market clearing taking into account the co-optimization of the market clearing and operation cost of the thermal units. Also, wind power uncertainty is handled by means of two-stage stochastic programming and risk concept is introduced and formulated via a well-known measure to account for operator risk taking level. The proposed model is formulated as a two-stage stochastic programming where it clears the joint energy and reserve markets considering different penetration and forecasting accuracy levels of the wind power. Furthermore, supplementary terms are added to the objective function and constraints of the problem in order to measure the risk related to the wind power uncertainty.
8(2016); http://dx.doi.org/10.1063/1.4962414View Description Hide Description
Meeting the goals set by the Energy Independence and Security Act requires evaluation of all potential feedstock sources including arid and semi-arid portions of the western United States (U.S.). The objective of this study was to assess the lignocellulosic feedstock potential in stream buffers of the inland Pacific Northwest. A 3-yr (2010–2012) experiment was conducted at two sites within each of the three precipitation zones (low, mid, and high). At each site, barley (Hordeum vulgare L.), wheat (Triticum aestivum L.), alfalfa (Medicago sativa L., cultivar Ladak), tall wheatgrass (Agropyron elongatum Podp. cultivar Alkar) (TWG), and a mix of alfalfa and tall wheatgrass (MIX) were planted in a randomized complete block experimental design. Productivity followed precipitation; in the high and mid precipitation zones, the MIX and TWG treatments showed potential production of 3,079 ± 262 l ha−1 and 3,062 ± 235 l ha−1. Productivity in the low zone was inadequate or unreliable as a source of feedstocks. A geographic information system was then used to identify the area available for stream buffers with soil resources that matched the experimental results within each precipitation zone. In 3.7 × 106 ha of dryland cropland, 44 656 ha (1.5%) available within the mid and high precipitation zones is capable of producing 147 million liters of ethanol. This potential contribution is 0.3% of the lignocellulosic ethanol production expected by the year 2022. Though not a substantial contribution, the added benefit of producing energy for on-farm consumption might provide an additional incentive for landowners and managers to install conservation buffers.
Hydrocracking of bio-alkanes over Pt/Al-MCM-41 mesoporous molecular sieves for bio-jet fuel production8(2016); http://dx.doi.org/10.1063/1.4962561View Description Hide Description
Bi-functional catalysts consisting of platinum on aluminosilicate MCM-41 materials with Si/Al ratios between 10 and 30 were prepared via direct mixed-gel synthesis. The catalysts were tested in the hydrocracking of bio-alkanes, produced from biodiesel hydrodeoxygenation and composed of n-hexadecane and n-octadecane, for the production of bio-jet fuel. The effects of temperature, pressure, weight hourly space velocity (WHSV), and H2/n-paraffin weight ratio on bio-alkanes conversion and product distribution were examined. The conversion was found to be dependent on the acid strength of the catalyst supports which were proportional to the Al content. However, the catalyst selectivity decreased with the increasing Al content. The optimal Si/Al ratio, temperature, pressure, WHSV, and H2/n-paraffin weight ratio were determined to be 20, 330 °C, 2 MPa, 1 h−1, and 0.20, respectively. Under these conditions, the bio-alkanes conversion and kerosene/gasoline ratio in the product reached 65.62% and 1.96, respectively.
- Wind Energy
8(2016); http://dx.doi.org/10.1063/1.4963240View Description Hide Description
A parametric study of vertical axis turbines of the H-Darrieus type is conducted using state-of-the-art Computational Fluid Dynamics (CFD) and the k-ω Shear Stress Transport RANS model in its unsteady form. Although most parameters have previously been investigated individually, the effect of solidity, number of blades, tip speed ratio, Reynolds number, fixed blade pitch angle, and blade thickness on the aerodynamic efficiency of the turbine is evaluated using the same performance evaluation set-up in order to determine what would be the best aerodynamic configuration and operation parameter in a given application. The quantitative impact of 3D effects associated with the blade aspect ratio and the use of end-plates is also investigated. For high-Reynolds applications, optimal radius-based solidity is found to be around , while higher solidities show a lower maximum efficiency than what was previously published using simpler streamtube based methods. In 3D, a small blade aspect ratio () leads to a relative efficiency drop of nearly 60% compared to the 2D prediction. Longer blades improve the 3D efficiency greatly. End-plates are found to have a positive effect on power extraction performances, as long as their size and thus their drag are limited.
8(2016); http://dx.doi.org/10.1063/1.4963688View Description Hide Description
The Savonius vertical axis wind turbine is a simple device, easy to manufacture, has good starting characteristics, and rotates with wind from any direction; nevertheless, it has a lower efficiency than the other wind turbines. The aim of this paper is to numerically explore the non-linear unsteady flow over a conventional Savonius using three dimensional computations with emphasis on the placement of these turbines in a linear array and the effect of an obstacle that acts as a wind deflector. First, an infinite array of turbines is used to study the gap distance between the wind turbine axis rotors. This investigation is conducted via numerical simulations based on the computational fluid dynamics computer program Fluent 14.5. It is found that a gap distance L = 1.4R gives a very good performance. Second, four farms with different number of turbines—from 3 to 21 turbines—are studied. The effect on the power coefficient of the number of turbines in each farm is reported and analyzed. Third, a new arrangement that includes an obstacle at one end of the array of turbines is presented. The best configuration explored in this work increases the power coefficient of each Savonius wind turbine by 82% compared to a single turbine. Finally, the effect of the wind direction for the best configuration is presented and the range of wind angles for which the farm outperforms isolated turbines is calculated.
Experimental study of surface curvature effects on aerodynamic performance of a low Reynolds number airfoil for use in small wind turbines8(2016); http://dx.doi.org/10.1063/1.4963236View Description Hide Description
This paper presents the wind tunnel experimental results to investigate the effects of surface gradient-of-curvature on aerodynamic performance of a low Reynolds number airfoil Eppler 387 for use in small-scale wind turbines. The prescribed surface curvature distribution blade design method is applied to the airfoil E387 to remove the gradient-of-curvature discontinuities and the redesigned airfoil is denoted as A7. Both airfoils are manufactured with high precision to reflect the design. Low-speed wind tunnel experiments are conducted to both airfoils at chord based Reynolds numbers 100 000, 200 000, and 300 000. Surface pressure measurements are used to calculate the lift and pitching-moment data, and the wake survey method is applied to obtain the drag data. The experimental results of E387 are compared with NASA Low Turbulence Pressure Tunnel (LTPT) results for validation. The gradient-of-curvature discontinuities of E387 result in a larger laminar separation bubble which causes higher drag at lower angles of attack. As the angle of attack increases the separation bubble of the airfoil E387 moves faster towards the leading edge than that of A7, resulting in a premature bubble bursting and earlier stall on E387. The impact of the gradient-of-curvature distribution on the airfoil performance is more profound at higher angles of attack and lower Reynolds number. The aerodynamic improvements are integrated over the 3D geometry of a 3 kW small wind turbine, resulting in up to 10% increase in instantaneous power and 1.6% increase in annual energy production. It is experimentally concluded that an improved curvature distribution results in a better airfoil performance, leading to higher energy output efficiency.
Equivalent inertial time constant of doubly fed induction generator considering synthetic inertial control8(2016); http://dx.doi.org/10.1063/1.4963239View Description Hide Description
The integration of large-scale doubly fed induction generators (DFIGs) into power system leads to the deterioration of the dynamic frequency characteristics in case of power shortage. One effective way to address this deterioration is through synthetic inertial control. However, a quantitative representation of the synthetic equivalent inertial time constant of DFIG (Heq) is greatly needed for research on the dynamic frequency characteristics of wind power-incorporating systems. This study introduces the traditional synthetic inertial control technique of DFIG, and reveals that the dynamic inertial response process is influenced by both the inertial controller and the speed controller. Expressions of Heq in the frequency and time domains were mathematically derived, and the accuracy of Heq was verified by comparing the calculated and simulated values. The 3-stage time varying characteristics of Heq are detailed, and the resulting mechanisms are analyzed.
8(2016); http://dx.doi.org/10.1063/1.4964310View Description Hide Description
Unlike horizontal axis turbines, the Darrieus-type wind turbines have less efficiency and suffer from the self-starting inability. The effects of fixed and variable blade pitch angle as an idea for improving the performance of Darrieus turbine have been investigated using the CFD analysis, and a pitching system (variable pitch Darrieus-type wind turbine) has been proposed that can reduce both the blades oscillating motion and the magnitude of angle of attack in one revolution compared to that of the Darrieus-type wind turbines. In this study, the method of computational fluid dynamics with moving mesh has been used for analyzing the unsteady two-dimensional flow simulation. The numerical results show that the SST k-ω turbulence model matches well with the experimental results and can capture the flow separation phenomenon at low tip speed ratios. Also, it was observed that a small negative fixed pitch angle of −3° can delay the separation and improve the performance of wind turbine. The numerical simulation also showed that the variable-pitch blade turbine can reduce or eliminate the flow separation on its blades at a lower tip speed ratios than that of the fixed pitch blades. This result increases the starting torque and obtaining high efficiency with decreasing in torque ripple on blades during the turbine operation compared to that of the fixed-pitch blade Darrieus turbine.
8(2016); http://dx.doi.org/10.1063/1.4964311View Description Hide Description
Increased power production is observed in downstream vertical-axis wind turbines (VAWTs) when positioned offset from the wake of upstream turbines. This effect is found to exist in both laboratory and field environments with pairs of co- and counter-rotating turbines, respectively. It is hypothesized that the observed production enhancement is due to flow acceleration adjacent to the upstream turbine due to bluff body blockage, which would increase the incident freestream velocity on appropriately positioned downstream turbines. A low-order model combining potential flow and actuator disk theory captures this effect. Additional laboratory and field experiments further validate the predictive capabilities of the model. Finally, an evolutionary algorithm reveals patterns in optimized VAWT arrays with various numbers of turbines. A “truss-shaped” array is identified as a promising configuration to optimize energy extraction in VAWT wind farms by maximizing the performance enhancement of downstream turbines.
Fatigue damage characteristics of a semisubmersible-type floating offshore wind turbine at tower base8(2016); http://dx.doi.org/10.1063/1.4964366View Description Hide Description
Fully coupled analysis of a semisubmersible-type floating offshore wind turbine is carried out for the fatigue damage investigation at tower base by using the code FAST. By combining the axial force and the bending moment, the stress around the circumference can be calculated. This study investigates the effect of second order wave force, wind load, mooring model, and tower elasticity on the stress response at tower base. There are three peaks in the stress amplitude spectrum, corresponding to pitch resonance, wave frequency, and first-order natural mode of the tower, respectively. The first and third peaks are mainly induced by wind load. The effect of the second order wave force and mooring model on the stress seems to be neglectable based on the numerical results. By using the rainflow counting method, the fatigue damage is calculated. The obtained fatigue damage under several environmental conditions indicates that the misalignment and coupling effect of wind and wave loads may significantly affect the total fatigue damage. The wind and wave induced responses are calculated separately (uncoupled method). Next, two superposition methods for fatigue damage calculation are analyzed: fatigue damage superposition and stress superposition. Compared with the fully coupled analysis considering wind and wave together, the former gives lower prediction but the latter seems to provide acceptable results. Finally, considering the long-term wind and wave distribution, the fatigue damage under different environmental conditions is calculated, and the fatigue damage obtained from stress superposition method shows reasonable agreement with that obtained from a fully coupled analysis.
- Photovoltaic Energy
A comparative study of the impact of horizontal-to-tilted solar irradiance conversion in modelling small PV array performance8(2016); http://dx.doi.org/10.1063/1.4964363View Description Hide Description
The capabilities of modelling the performance of photovoltaic (PV) systems depend both on the available PV models and on the accuracy of the input information that the models need. The meteorological input to any model consists mainly of the solar irradiance incident in the module as well as other influencing variables (temperature, wind speed, etc.). Since incident solar irradiance on characteristic inclined surfaces is not frequently available, the performance models use to incorporate specific models for computing tilted solar irradiance from the other components (global horizontal and direct normal) with higher availability. In this work, System Advisor Model (SAM), a performance model, has been used to analyze the contribution of some well-known transposition models to the uncertainty in modelling the system performance. The study has been performed for a small-scale photovoltaic array of CdTe placed at the roof of a building in Madrid. The performance predicted by SAM is rather good with around 3% of root mean squared error in the daily AC power when the input irradiance is taken from experimental measurements at the modules tilt angle. A 3% of an additional error increase compared to the results with the experimental tilt irradiance was observed for using all of the transposition models included in SAM.
8(2016); http://dx.doi.org/10.1063/1.4966229View Description Hide Description
This work reports on the development of Linear Quadratic Regulator (LQR) based maximum power point tracking method for standalone photovoltaic system. The advantages of the method are faster tracking ability, transfer of maximum deliverable power and its implementation with reduced complexity. The tracking performance of the proposed LQR method is compared with that of the conventional Perturb and Observe (P&O) method in MATLAB/Simulink environment for both constant and varying irradiation conditions. The improved performance of the LQR method in terms of tracking speed and tracking efficiency is confirmed through experimental results obtained using dSPACE controller.
- Solar Energy
Short-term forecasting of solar photovoltaic output power for tropical climate using ground-based measurement data8(2016); http://dx.doi.org/10.1063/1.4962412View Description Hide Description
This paper highlights a new approach using high-quality ground measured data to forecast the hourly power output values for grid-connected photovoltaic (PV) systems located in the tropics. A case study using the 1-year database consisting of PV power output, global irradiance, module temperature, and other relevant variables obtained from Universiti Teknikal Malaysia Melaka is used to develop forecast models for three typical weather conditions—clear, cloudy, and overcast sky conditions. A machine learning method (Support Vector Regression—SVR) and an Artificial Neural Network method (nonlinear autoregressive) are used to produce the models and the results are compared with a benchmark model using the persistence method. Comparison with all the variables suggests that tilted global horizontal irradiance (GHItilt) and module temperature (Tmod) are the essential input variables to forecast the PV power output. It has also been observed that SVR performs well across all types of sky conditions, with the forecasting skill values between 0.65 and 0.79 when compared to the benchmark persistence method.
Life cycle production and costs of a residential solar hot water and grid-connected photovoltaic system in humid subtropical Texas8(2016); http://dx.doi.org/10.1063/1.4963238View Description Hide Description
Hot and humid climates with high solar radiation have the potential to offset residential building energy consumption with the application of solar hot water and photovoltaic electricity generation. However, costs, lack of incentives for the systems, and the need for proof-of-concepts continue to limit market penetration. The surplus of natural gas in certain areas of the United States, particularly Texas, continues to keep gas and electricity production economical compared to solar alternatives. However, trends that demand lower energy homes, a desire for local energy independence, and the lowering of carbon dioxide emissions continue to fuel solar energy systems penetration. To support solar use, this research was performed to evaluate and analyze the real-world life cycle energy and costs of a solar photovoltaic and solar hot water system (SHWS) installed on a high-end residential home in Houston, TX (IECC Zone 2). The house was a well-insulated, large urban home with two renewable energy systems installed: a 3.5 kW solar photovoltaic system (SPVS) and a 1.71 kW solar hot water system. Analyses were part of a larger study performed to investigate the contributions of the solar energy offsets on the operational energy of the building over a life cycle period of 30-years. Field measurements of energy production were compared to solar energy simulations based on the typical meteorological year and the National Solar Radiation Database (NSRDB) data. NSRDB provided the basis for a probabilistic interpretation of annual energy production in terms of probability measures, P50/P90. It was found that field estimates were within simulation uncertainties and P90 predictions were within 2.5% of TMY3 (typical meteorological year data 3) results for both the solar photovoltaic system (SPVS) and solar hot water system (SHWS). Additionally, optimizations in the system design and life cycle costs were investigated to determine the annual optimal performance for the solar energy systems. The SHWS was installed at a less than optimum azimuth of 270° instead of 180° (corresponding to a 16% reduction in annual output). The SPVS was installed at optimal design conditions of 180° azimuth and 42° tilt. Payback and levelized cost of energy (LCOE) could have been minimized with the addition of another solar hot water collector with a minimal impact to overall cost. Cost sensitivity analysis on the LCOE and net-present value (NPV) were also performed and over a 30-year life cycle period, TMY3 based simulations predicted a NPV of $796 (21.8-year non-discounted payback) and -$1246 (29.2-year non-discounted payback) for the SHWS and SPVS, respectively. The SHWS achieved a LCOE of 8.1 ¢/kWh, while the value for the SPVS was 12.29 ¢/kWh. For the SPVS, the photovoltaic module and collector costs were the largest life cycle costs, and of special note, a reduction in the module cost by 67% reduces the LCOE to the market average-high electricity price of 10 ¢/kWh. Finally, the combined renewable energy systems generated an estimated 30-year life-cycle energy production of 184 814 kWh, with auxiliary gas provided for supplemental hot-water heating. On average, a Texas residential home utilizes 420 000 kWh of electrical energy, 19% of which is domestic hot water demand.
8(2016); http://dx.doi.org/10.1063/1.4965252View Description Hide Description
Parabolic solar troughs are amongst the most widely studied solar thermal technologies available today. Methods for improving efficiency include the use of selective coatings on the absorber which reduces dominant thermal radiation losses. In this paper, we model a different approach of reducing radiation losses: the glass cover around the absorber can be coated with a hot mirror film, which reflects infrared radiation back onto the absorber. In order to describe such a mechanism, it becomes necessary to model theoretically the long range thermal radiation interactions inside the receiver unit. Our model uses discretization of the active surfaces to account for all the dominant radiation interactions, and can be used in a simulation to establish a temperature profile for the receiver unit, from which thermal properties can be inferred. The results of the simulation are compared to existing simulations and experimental data, wherever possible.
- Energy Storage
8(2016); http://dx.doi.org/10.1063/1.4964309View Description Hide Description
Micro grids are inclined to use renewable energy sources within the availability limits. Energy storage improved the management of hybrid energy resources used in parallel with the grid. This study models the hybrid usage of wind and solar energies as a support for the grid with the availability of batteries. A mixed integer mathematical model is proposed to schedule the use of different resources minimizing the cost following the power market. Capacity, energy balancing and demand constraints are taken into account for this purpose. This model can be used as a decision support system for micro grid load planning.
- Marine and Hydroelectric Energy
8(2016); http://dx.doi.org/10.1063/1.4963237View Description Hide Description
This paper presents an investigation on air compressibility in the air chamber and its effects on the power conversion of oscillating water column (OWC) devices. As it is well known that for practical OWC plants, their air chambers may be large enough for accommodating significant air compressibility, the “spring effect,” an effect that is frequently and simply regarded to store and release energy during the reciprocating process of a wave cycle. Its insight effects on the device's performance and power conversion, however, have not been studied in detail. This research will investigate the phenomena with a special focus on the effects of air compressibility on wave energy conversion. Air compressibility itself is a complicated nonlinear process in nature, but it can be linearised for numerical simulations under certain assumptions for frequency domain analysis. In this research work, air compressibility in the OWC devices is first linearised and further coupled with the hydrodynamics of the OWC. It is able to show mathematically that in frequency-domain, air compressibility can increase the spring coefficients of both the water body motion and the device motion (if it is a floating device), and enhance the coupling effects between the water body and the structure. Corresponding to these changes, the OWC performance, the capture power, and the optimised Power Take-off (PTO) damping coefficient in the wave energy conversion can be all modified due to air compressibility. To validate the frequency-domain results and understand the problems better, the more accurate time-domain simulations with fewer assumptions have been used for comparison. It is shown that air compressibility may significantly change the dynamic responses and the capacity of converting wave energy of the OWC devices if the air chamber is very large.
- Energy Conversion
Piezoelectric type acoustic energy harvester with a tapered Helmholtz cavity for improved performance8(2016); http://dx.doi.org/10.1063/1.4962027View Description Hide Description
This paper reports an improved acoustic energy harvester with a tapered Helmholtz cavity. The harvester consists of a bimorph piezoelectric composite plate and a Helmholtz resonator (HR) with a tapered cavity. The architecture, operational mechanism, fabrication, and characterization of the harvesters are described. The harvesters are tested under sinusoidal sound pressure levels (SPLs) inside a lab as well as random SPLs in a real ambient acoustical environment. When a harvester with a tapered HR and without proof mass attached to its piezoelectric plate is characterized at a sinusoidal SPL of 130 dB, a maximum power of 90.6 μW is delivered to 1 kΩ load. In comparison, a similar harvester with a cylindrical shape HR produced a maximum power of 51.4 μW under the similar acoustic conditions. It is found that 76.26% increase in power is achieved with the tapered cavity for the HR. Furthermore, due to the attachment of a proof mass (0.84 g) with the harvester, its power production capability is further increased by 103.3%, from 90.6 to 184.18 μW. Moreover, in a real environment, the maximum voltage amplitudes of about 260 and 280 mV are produced by the harvester when placed in the surrounding of a motorbike and domestic electric generator, respectively.
8(2016); http://dx.doi.org/10.1063/1.4962416View Description Hide Description
Using a sample of 18 prefecture-level cities in Henan province, this study explored the regional allocation of energy intensity reduction targets from the following three viewpoints: equity principle with common but differentiated responsibilities; intensity reduction target fulfillment; and economic differences and reduction potential among regions. Based on a preliminary decomposition model, an analytic hierarchy process (AHP) and Ward's hierarchical clustering, an intensity allocation method is proposed. First, the preliminary regional decomposition scheme is presented via the preliminary decomposition model. Then, a multi-criteria evaluation system consisting of four layers and covering 13 evaluation indicators is developed via the AHP method, and the evaluation results are analyzed via the cluster method to further improve the preliminary scheme. As decision makers may have different preferences when allocating the reduction burden, we allocate different weights to the indicators and analyze the results using a sensitivity analysis. The clustering results indicate that the 18 regions of Henan are divided into five categories, and each category has its own significant characteristics. Regions with high obligation and potential should share the largest reduction burden. The allocation results show that seven regions, including Zhengzhou and Luoyang, are expected from 2016 to 2020 to exceed the provincial average decrease rate of 16%.
8(2016); http://dx.doi.org/10.1063/1.4963314View Description Hide Description
In this paper, a novel maximum power point (MPP) tracking technique for photovoltaic system (PV) with fast convergence speed and reduced range for the MPP search operation is presented. The characteristic of this method is the limited searching area/range for the tracking. The adaptable variable duty step used in the proposed method instantaneously brings the operating point close to the MPP, thus bounding the searching area. The value of duty gets updated according to the panel temperature and irradiance, and the operating point always remains close to the MPP. By bounding the search operation, the overall tracking speed and efficiency of the tracking increase. Further enhancement of the tracking speed is obtained by varying the step size of duty ratio of the DC-DC converter used; this is done in such a manner that the size of variable duty step is large for the points far away from MPP and becomes very small at or near MPP. The projected tracking algorithm is compared with conventional Perturb and Observe MPPT method in diverse irradiance and temperature conditions, and evaluation of the proposed tracking method is reported. Finally, field performance of the proposed method has been done by using a 250 W PV system. Arduino Uno microcontroller board is used for controlling the duty of the DC-DC converter. Results obtained from the hardware implementation have been presented and is concluded that the method has fast tracking capability and better efficiency. To sum up, overall performance of the proposed Fast Mutable Duty MPP Tracking technique is appreciable.