Hybrid Diesel Generator – Battery Systems For off- grid rural applications K. Kusakana* and H.J. Vermaak Department of Electrical Engineering and Computer System Central University of Technology, Free State Bloemfontein, South Africa kkusakana@cut.ac.za , hvermaak@cut.ac.za Abstract—This paper discusses the advantages of adding a battery storage system to isolated load not served by the grind and continuously running under a Diesel Generator. Even though adding batteries means increasing the Initial Cost of the system, the resulting Cost of Energy produced as well as the total Net Present Costs are sensibly being reduce while using the hybrid Diesel generator-battery system. HOMER is used to compare the technical, economical and environmental characteristics of the hybrid system to the stand alone diesel generator. Finally, the Cost of Energy produced as well as the total Net Present Costs can be further reduced by optimally sizing and applying optimal control strategies to the hybrid system. The decision variables as well as the objective function for the optimal sizing and operation control of the hybrid diesel generator–battery system have been proposed. Keywords - Hybrid Diesel Generator–Battery, Distributed generation, Optimal sizing, Operation control, Homer. I. INTRODUCTION The lack of reliable electrical power supply, the high cost of AC grid extension and rough topography are some of the severe challenges faced in the rural electrification of a good number of developing countries. In most of the cases, loads in those rural areas are powered by small Diesel Generators (DGs) running continuously [1]. Compared to other supply option such as renewable energy sources, DGs have low initial capital costs and generate electricity on demand. Some major disadvantages of DGs are the high operation and maintenance costs, transport and storage costs, and noise as well as pollution emission in the environment [2]. The overall cost resulting of the use of the diesel generator is very high. This includes some of the following costs:  Operating cost which comes mainly from the direct fuel cost;  Cost of the transportation of the fuel. This can be high depending on how the area to supply is remote or isolated;  The maintenance and replacement cost of the generator. In the specific case of a small generator running continuously, the lifespan may be around two years. Battery storage system are often used as back-up when the generator runs out of fuel, during the start up or to cover up the load when the generator is shut-down for maintenance. By optimally designing the hybrid generator-battery system and applying optimal operation control strategies, significant savings can be archived in the overall running costs. In this type of operation the generator runs only for a short period of the time to recharge the battery, and the load is mainly supplied by the battery while the generator is switched off. This paper discusses the optimal sizing as well as the optimal operation control of a standalone hybrid DG-battery system to save initial, operating and maintenance costs at remote and isolated sites not connected to the grid. II. HYBRID SYSTEM DESCRIPTION AND OPERARTION 2.1. Diesel generator DGs which are being used to supply rural loads are most of the time sized to be able to carry the load even during peak power demand. For safety reasons, they are being sized to leave room for upgrades, and they never run at full load (always around 30% to 60%) in order to avoid undue mechanical stress [3]. The following is the typical consumption figures (according to typical manufacturer specifications): TABLE 1. DIESEL GENERATOR CONSUMPTION Generator loading Fuel consumption (L/h) Fuel consumption (L/kWh) 75% 1.98 0.26 50% 1.56 0.31 30% 1.04 0.52 It is evident that most of the DG supplying rural loads is not used efficiently because running every time at low loading. 2.2. Hybrid DG/Battery system Using the DG in conjunction with battery storage in a hybrid system can change the consumption figure presented above by pushing the DG to always run at close to full load. In this case, the DG will only be running to charge the batteries, and then turned off as soon as the battery is fully charged [4]. Below are the typical different steps in the hybrid system operation cycle: • Step1: The hybrid system is initiated with the batteries 100% charged. • Step 2: The DG is turned off and the load is supplied only by the battery storage system. • Step 3: As soon as the battery storage system has discharged to a preset level, the DG is turned on to fully recharge the batteries. ＹＷＸＭＱＭＴＶＷＳＭＴＵＶＹＭＹＯＱＳＯＤＳＱＮＰＰ＠ﾩＲＰＱＳ＠ｉｅｅｅ ＸＳＹ