Optimization of the economic dispatch model for off-grid hybrid power systems with renewable energy in Uganda.

Abstract

Economic Dispatch (ED) involves serving the least cost energy to the load. The subject of economic dispatch is a challenging task which gets more complicated with the variability of the inputs or determinants. The traditional ED is slowly implemented using the Automatic Generation Control (AGC) in the tertiary loop. In the traditional power systems, the slow changing nature of the load makes implementation of the ED a relatively simple, day a head planning task, supported by dispatched generation. The integration of intermittent renewable energy onto power systems introduces uncertainty in the power generation which is a new challenge to the whole process of ED. In this context, this work presents a model for optimum economic operation of a solar Photo Voltaic (PV), diesel generators hybrid with energy storage system, demonstrated by means of a case study of Kalangala Infrastructure Services. The focus was to optimize battery storage system (charging and discharging), photovoltaic generation and diesel generators while considering short term hourly data. The network considered was comprised of 0.6 MW solar PV System, 0.6 MW battery energy storage system and a combination of diesel generators (0.4 MW, 0.25 MW and 0.18 MW). The ED problem was formulated as a typical optimization problem and solved using mixed integer linear programming method. The model employed datasets such as: hourly irradiance data, generation prices (ramping, startup and shutdown), charging and discharging costs. The study revealed that there was poor utilization of the PV energy for low load conditions of the grid as compared to when the load was scaled above 0.5 MW. The flexible role of the energy storage system during charging and discharging cycles was highlighted in ensuring power systems stability. Finally, upward load scaling and load shifting further reduced the marginal cost from USD 6,000 per MWh to USD 1,600 per MWh. The silent costs which included ramping costs, startup and shut down costs usually associated with flexible generation complementing renewable energy sources were seen to limit the full utilization of the PV energy for plant capacity factor below 50%. The above results were based on hybrid network with low plant capacity factor (below 45%). Therefore, there is need to improve the plant capacity factor of the network and also to determine the optimal load point for such operations.