Hybrid systems present a new dimension to the time correlation of intermittent renewable energy sources. The paper considers the daily energy consumption variations for winter and summer weekdays and weekends in order to compare the corresponding fuel costs and evaluate the operational efficiency of the hybrid system for a 24-h period. Previous studies have assumed a fixed load and uniform daily operational cost. A load following diesel dispatch strategy is employed in this work and the fuel costs and energy flows are analysed. The results show that the photovoltaic-diesel-battery model achieves 73% and 77% fuel savings in winter and 80.5% and 82% fuel savings in summer for days considered when compared to the case where the diesel generator satisfies the load on its own. The fuel costs obtained during both winter and summer seasons for weekdays and weekends show substantial variations which should not be neglected if accurate operation costs are to be achieved. The results indicate that the developed model can achieve a more practical estimate of the fuel costs reflecting variations of power consumption behavior patterns for any given system. Keywords: optimisation algorithm, hybrid system, control strategy, economic dispatch, energy efficiency, operation efficiency * Corresponding author. Tel.: +2712 420 2068.Email address: henerica.tazvinga@up.ac.za. (Henerica Tazvinga)Preprint submitted to Solar Energy August 8, 2013Nomenclature P 1 (t) control variable representing energy flow from the diesel generator to the load at any hour (kW ) P 2 (t) control variable representing energy flow from the PV array to the load at any hour (kW ) P 3 (t) control variable representing energy flow from the PV array to battery at any hour (kW ) P 4 (t) control variable representing energy flow from the battery to the load at any hour (kW ) P L (t) control variable representing the load at any hour (kW ) T A the ambient temperature ( 0 C) NT standard and nominal cell operating temperature conditions A c the PV array area (m 2 ) P pv the hourly energy output from a PV generator of a given array area (kW h/m 2 ) η R the PV generator efficiency at reference temperaturethe ratio of beam irradiance incident on a tilted plane to that incident on horizontal plane SOC the state of charge B C (t) the state of charge of the battery bank at any hour B C (t − 1) the state of charge of the battery bank at the previous hour η C the battery charging efficiency η D the battery discharging efficiency B C (0) the initial state of charge of the battery B min C the minimum allowable battery bank capacity (kW h) B max C maximum allowable battery bank capacity (kW h) DOD the depth of discharge a, b fuel cost coefficients P DG generator rated power output (kV A) 2

In the electricity market, customers have many choices to reduce electricity cost if they can economically schedule their power consumption. Renewable hybrid system, which can explore solar or wind sources at low cost, is a popular choice for this purpose nowadays. In this paper optimal energy management for a grid-connected photovoltaic-battery hybrid system is proposed to sufficiently explore solar energy and to benefit customers at demand side. The management of power flow aims to minimize electricity cost subject to a number of constraints, such as power balance, solar output and battery capacity. With respect to demand side management, an optimal control method (open loop) is developed to schedule the power flow of hybrid system over 24 hours, and model predictive control is used as a closed-loop method to dispatch the power flow in real-time when uncertain disturbances occur. In these two kinds of applications, optimal energy management solutions can be obtained with great cost savings and robust control performance.

In this paper, an energy dispatch model that satisfies the load demand, taking into account the intermittent nature of the solar and wind energy sources and variations in demand, is presented for a solar photovoltaic-wind-diesel hybrid power supply system. Model predictive control techniques are applied in the management and control of such a power supply system. The emphasis in this work is on the co-ordinated management of energy flow from the battery, wind, photovoltaic and diesel generators when the system is subject to disturbances. The results show that the advantages of the approach become apparent in its capability to attenuate and its robustness against uncertainties and external disturbances. When compared with the open loop model, the closed-loop model is shown to be more superior owing to its ability to predict future system behavior and compute appropriate corrective control actions required to meet variations in demand and radiation. Diesel consumption is generally shown to be more in winter than in summer. This work thus presents a more practical solution to the energy dispatch problem. Keywords: energy management, disturbance, intermittent nature, hybrid energy system, optimization scheme

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This paper presents an optimal energy management strategy for a grid tied photovoltaic-wind-fuel cell hybrid power supply system. The hybrid systems meets the load demand consisting of an electrical load and a heat pump water heater supplying thermal load. The objective is to minimize energy cost and maximize fuel cell output, taking into account the time-of-use electricity tariff. The optimal control problem is solved using a mixed integer linear program with the supply switch to the heat pump water heater and the power from the grid, power to/from inverter, power to electrolyzer and from fuel cell the control variables. The hot water temperature inside the water storage tank and the hydrogen in the storage tank are the state variables. The performance of the proposed control strategy is tested by simulating different operating scenarios: with and without renewable energy feed-in, and the results confirm the effectiveness of the proposed control strategy, as it increases the supply reliability of the system. Keywords