Application Research On A Stand-alone PV System With Ultracapaticor Battery Hybrid Energy Storage

Rapid economic development has brought high energy consumption and the energy crisis is increasing. Therefore, the new energy development has become the world's imminent major issue. Solar energy is the most promising new energy and photovoltaic power generation is one of the most main way to use it. However, due to the randomness of photovoltaic power generation, the energy storage device must be additional to ensure the stability of stand-alone photovoltaic system. The ultracapacitor is a new energy storage component and has excellent energy storage potential. This paper researches the key issues of ultracapacitor- battery hybrid energy storage system used in stand-alone photovoltaic system.It firstly introduces the present circuit structures of stand-alone photovoltaic system. Because of the battery power output capacity is insufficient, so it is necessary to provide greater capacity to meet the short-term high-power load but it lead to waste. This paper proposes a kind of ultracapacitor- battery hybrid energy storage system structure. It improves power output capacity of energy storage system, reduces the battery capacity allocation, extends battery life. This structure uses bi-direction converter for ultracapacitor and battery respectively. It makes ultracapacitor and battery can separate choice in power and voltage level, not to meet strict matching relation with load and dc bus voltage. It is advantage for converter design. The charging and discharging of ultracapacitor and battery can control separately and more flexibly.This paper established the ultracapacitor- battery hybrid energy storage system equivalent model and analyses how to improve system performance in theory. It designs the system control part. The direction converter has three modes-MPPT, constant voltage and stop working, the bi-direction converter works in Boost, Buck and stop mode, they switch three modes between. For charging and discharging characteristics of battery, this paper proposes a novel bi-direction voltage and current loop control strategy in order to realize battery voltage and current limiting in both direction, this can guarantee the response speed and eliminate voltage difference.For the characteristics of photovoltaic cells, ultracapacitor and battery, it presents the corresponding energy management strategy. The core is to ensure three converters work in set mode and three power sources coordinating and stable work according to the photovoltaic cells and hybrid energy storage components working condition. The control strategy is verified through simulation method, the results show that the hybrid energy storage system can increase the power output capacity, reduce battery capacity allocation, prolong the system working life.