With the rapid development of the photovoltaic industry in the past few years, its comprehensive cost of power generation has significantly dropped, gradually approaching the grid parity. At present, international authorities hold the expectation of a large increase in the proportion of renewable energy represented by photovoltaics in the future energy consumption structure. Among them, ExxonMobil believes that by 2040, the total proportion may triple; Bloomberg New Energy Finance (BNEF) predicts that by 2050, the proportion of global wind-power and photovoltaic power capacities are expected to be close to 50%. In the future, it has become a consensus at home and abroad that the proportion of photovoltaic power capacity in the global energy structure will continue to significantly increase. However, due to the instability of photovoltaic power generation, the growth in photovoltaic systems merging into the power grid is bound to have a great impact on the stability of the power grid. Energy storage technology can smooth the output of photovoltaic power and make new energy power stations merge them friendly. Therefore, the “photovoltaic+energy storage” mode will make it possible for us to truly enter the renewable energy era.
The function of the energy storage system is to store the photovoltaic energy into the battery timely, and then input the electric energy into the power grid timely. Compared with AC-side energy storage system, the DC-side energy storage system, for its higher efficiency, has more advantages in the application of photovoltaic power generation side. It is very important for DCDC bidirectional converter, the most critical power conversion center for energy storage on DC-side, to adapt to the diversity of batteries and to own functional diversity.
Can DCDC adapt to the diversity of batteries?
1. Lithium battery
Currently, most electrochemical energy storage power stations are mainly dominated by lithium batteries, which have high power density and long cycle life. Due to the rapid development of electric vehicles, the cost of lithium batteries has been greatly reduced, and the cost per watt of ternary lithium-iron batteries is less than 1 yuan. Therefore, lithium batteries have been widely used in energy storage power stations. However, the safety problem of lithium batteries is also very serious, and this asks strict requirements of power devices. DCDC shall, thus, have adjustable bidirectional voltage with gradual onset, and need to communicate with BMS of batteries, so as to protect batteries.
2. Flow battery
The flow battery is now called the “best match” battery for energy storage in photovoltaic system. It is characterized by high reliability, long service life, high residual value and ease of maintenance, well-matched with photovoltaic power stations that can generate electricity for 25 years. However, there is no obvious breakthrough in the problems of low density and low conversion efficiency of flow battery, so most of the flow battery projects are still pilot demonstration projects. But DCDC bidirectional energy storage converter is supposed to be compatible with flow battery. DCDC has to run at low voltage and be able to limit current and precharge flow battery, since the voltage of flow battery is low and there is no voltage at its first use.
3. Lead-acid battery
The lead-acid battery also has been applied in certain energy storage systems, especially the lead-carbon battery with long cycle life, high cost performance and outstanding stability. DCDC bidirectional energy storage converter is supposed to be compatible with lead-acid batteries.
4. Echelon battery
The echelon battery, also known as the secondary battery, belongs to the battery of secondary utilization. Nowadays, with such a large scale of electric vehicles, a large number of echelon batteries will appear continuously. The consistency of the echelon battery is poor, which asks very high requirements for DCDC and BMS. Therefore, in order to match the echelon battery, DCDC is supposed to have two-stage topology to solve the consistency problem well.
In short, as the center of power conversion, DCDC bidirectional converter ought to have the ability to match all kinds of batteries.
Can DCDC meet the requirements of different functions under different circumstances?
1. Application of constant power mode
At present, the common application mode of DCDC is mainly constant power mode, that is, DCDC connects the battery with the DC bus, as shown in the following figure. In this application, DC-DC bidirectional converter is supposed to achieve remote control, accept the scheduling of EMS, and charge and discharge under certain circumstances.
2. Application of MPPT mode
In some areas with high demand for frequency modulation, DCDC has an innovative application whose topology is shown in the following figure. This application requires DCDC to have MPPT function, which can track the maximum power point of photovoltaic panels to generate electricity, so as to achieve the highest power generation. Besides, the application system can well assist the work of power grid.
Based on a deep understanding of the development trend of energy storage, Kehua has developed the DCDC bidirectional converter product that can meet the needs of multiple conditions, and has been applied in southern India for two years. After these two years of continuous verification and upgrading, Kehua’s DCDC bidirectional converter has been already equipped with various functions.