How robust is the ring stain for evaporating suspension droplets?

The ring stain is commonly seen when droplets containing particles, such as coffee, are left to dry on a surface: a pinned contact line leads to outward radial flow, which is enhanced by the diverging evaporative flux at the contact line. As shown by Deegan et al. (1997) particles are swept outwards in this flow and create a ring which grows according to a simple power law with time. The final dried width and height of the ring should also be given by power laws of concentration, with both exponent equal to 0.5 provided all particles are in the ring, and the packing factor and ring profile are constant. We use suspensions of polystyrene particles in water with sizes ranging from 200 to 500 nm and initial concentrations c 0 from 0.009% to 1% deposited on glass substrates to investigate these scaling predictions. We vary the drying rate from 0.5 to 5 nl/s using humidity and reduced pressure, use a range of substrates to vary the initial contact angle between 5° and 35°, and invert the droplets to change the direction of gravity. We find that for all but the very lowest pressures, the ring height follows the predicted power law, with exponent equal to 0.50 ± 0.04 and the ring width having an exponent of 0.33 ± 0.05. The discrepancy between the measured and predicted width exponent is accounted for by an observed variation of droplet radius with concentration, and the presence of particles in the center of the droplet. In addition, for low pressures (fast evaporation) the scaling laws no longer hold: the ring is much narrower and there is significant deposition in the center of the droplet, possibly due to reduced particle-enhanced pinning

Financing long-term generation capacity in a reference price oriented capacity market

Many power markets around the world have been facing inadequacy with generation capacity investment to meet the growing demand. Among various frameworks directed towards this problem, capacity markets have emerged in major eastern US power markets. In this paper, a prototype capacity market is discussed which is consistent with the trend of convergence of market design. Based on this, the critical role of the reference capacity price is brought up, followed by a detailed explanation of its economic rationale and concerns. Noted with the necessity of a systematic pricing scheme to determine the value of the reference capacity price, a pricing model based on the general Black-Scholes contingent claim framework is proposed. In this model, the capacity value is treated as a path-dependent derivative with electricity prices and natural gas prices as underlyings. Numerical study is conducted to prove model validity with a lattice approach adopted. © 2010 IEEE.published_or_final_versionThe 43rd Hawaii International Conference on System Sciences (HICSS 2010), Honolulu, HI., 5-8 January 2010. In Proceedings of the Annual Hawaii International Conference on System Sciences, 2010, p. 1-

Wind power forecasting and integration to power grids

This is a summary of the presentation in the special session: "Digital Signal Processing for Green Power Systems and Delivery". In recent years, wind power penetration level in power systems has increased significantly. Grid integration has become one of the major issues for wind power growth due to the intermittent characteristics of wind power. The uncertainty of power generation from wind farms may result in power system stability and security problems. Accurate wind power forecasting could reduce the uncertainty to generation scheduling to certain extent, hence increase the wind power penetration level in the system. © 2010 IEEE.published_or_final_versionThe 1st International Conference on Green Circuits and Systems (ICGCS 2010), Shanghai, China, 21-23 June 2010. In Proceedings of ICGCS, 2010, p. 555-56

Wind power investment in thermal system and emissions reduction

This paper presents an analytical model for wind power investment. Most generation planning problems are formulated in multiperiod mixed integer programming with cost minimization as objective. We try to resort to finance literature for models able to systematically characterize return and risk. Real option theory is chosen. A primitive function is defined for the fuel cost able to be saved as the revenue of a wind power project. Subsequently the real project is described as a contingent claim on the stochastic fuel prices. Theoretical valuation of the project is thus given by the solution of a partial differential equation derived by Ito lemma. This formulation avoids the ambiguity in analyzing wind power investment based on non-market-based tariffs, but focuses on the welfare to the system as a whole. Finally a hypothetical scenario of carbon emission price is included to demonstrate the incentive it could offer to renewable generation. ©2010 IEEE.published_or_final_versionThe IEEE Power and Energy Society (PES) General Meeting, Minneapolis, MN., 25-29 July 2010. In Proceedings of PES, 2010, p. 1-

Voltage collapse mechanism based on system circuit and its solution manifolds

The problem of voltage collapse in the power system occurs when the load parameter exceeds some critical value. The critical operation point is known as the nose point in P-V curve. In this paper, the mechanism of voltage collapse is explained based on system circuit and solution manifolds. The corresponding mathematical model is derived. The proven theorem shows that the essential reason for voltage collapse is that the solution manifold of injection branch equations being not transversal with that of linear network equations. And the coincidence of the non-transversal of solution manifolds with voltage collapse and static bifurcation is proved.published_or_final_versio

Available transfer capability evaluation by decomposition

A central issue of running a successful electric power market is the evaluation of the associated available transfer capability (ATC) representing the room available for trading. Due to the need to post and update ATC values at regular intervals, the underlying calculation method should be moderately fast with acceptable accuracy. This paper proposes the evaluation of ATC by using Benders decomposition. The problem is first broken up into a master problem expressing the steady state operating condition and subproblems for the contingent conditions. Each subproblem is solved independently and a linear constraint using Lagrange multipliers of the subproblem is generated and added to the master problem. The proposed decomposition scheme is applied to IEEE 30 bus system with satisfactory results as compared with the distribution factors method.published_or_final_versio

Design of fuzzy damping controller of UPFC through genetic algorithm

The addition of a PSS-like supplementary controller to the UPFC main control can provide effective damping to the low frequency oscillation on the heavily loaded tie lines of interconnected power system. However, the conventional controller designed based on a linearized model cannot provide satisfactory performance over a wide range of operation point and under large disturbances. In this paper, the design of the Fuzzy Damping Controller (FDC) for UPFC as a substitute of the conventional supplementary controller is presented. Scaling factors are inserted to the membership functions of both inputs and output for easy design of fuzzifier and defuzzifier. Genetic Algorithm (GA) is applied to optimize the performance of the FDC through fine-tuning of the scaling factors. A 4-machine interconnected power system is used as the test system with a UPFC installed in one of the tie lines. Comparisons between the performances of both the fuzzy and the conventional supplementary controllers are studied. Computer test results show that FDC is very effective in damping the oscillation and in the meantime has a better robustness as compared with its conventional counterpart.published_or_final_versio

Transfer capability computations in deregulated power systems

With the recent trend towards deregulating power systems around the world, transfer capability computation emerges as the key issue to a smoothly running power market with multiple transactions. Total Transfer Capability (TTC) is the basic measure for evaluating available transfer capability (ATC). The paper presents the calculation of TTC through an optimal power flow approach. The objective function is to maximize the sum of the sending end-end generation and receiving-end load of specified buses. The constraints are ac power flow equations and system operation limits. The sequential quadratic programming (SQP) method is used for the optimization process. The IEEE 30 bus system is used for testing the proposed algorithm and the results compared favorably with that from the Continuation Power Flow (CPF) method.published_or_final_versio

Market Allocation Between Bilateral Contracts and Spot Market without Financial Transmission Rights

In the electricity market, it is very important for Generation Companies (Gencos) to decide how to sell energy among different transaction markets in order to maximize profits with relatively low risk. In this paper, two energy transaction markets are considered: spot markets and bilateral contract markets. An energy selling allocation approach with network congestion consideration is established based on modern portfolio theory. Analytical solution for the optimal allocation is derived with given bilateral contract prices and statistical characteristics of the spot market prices. The numerical simulation for energy selling allocation is demonstrated based on the actual data of the USA California power market.published_or_final_versio

A study of fuzzy logic based damping controller for the UPFC

The supplementary control of the UPFC can significantly enhance the damping of interconnect power systems. However, the conventional controller designed based on a linearized model cannot provide adequate damping signal over a wide range of operation points. In this paper, a fuzzy controller is designed as the substitute of the conventional controller. The parameters of the fuzzy controller are optimized through gradient descent training. Transient stability simulation and small signal analysis are conducted to test the performance of the controller. Computer results show that the fuzzy controller is very effective in damping the oscillation and has a better robustness.published_or_final_versio