Recently, the demand for residential energy storage batteries has been gradually increasing worldwide. As a new market, both products and markets are still in the exploration stage. Considering the high cost of residential energy storage batteries, it is crucial to understand how to use residential storage systems more efficiently and economically.
1. Structure of Residential Solar Energy Storage Battery System
The diagram above illustrates a typical residential energy storage battery system, mainly composed of solar panels, the grid, inverters, batteries, and electrical appliances. If the input end has only solar panels, it's called an off-grid system. If both solar panels and the grid input simultaneously, it's called a hybrid system. If only grid input is available, it's generally used as a backup power system.
2. User Needs Analysis
2.1. First, understand the types and power of the user's electrical appliances.
List the user's electrical appliances and their maximum power, then calculate the total power, P0.
P0=P1+P2+P3+....+Pn
2.2. Understand the number of electrical appliances that may be used simultaneously and calculate the total power value, P1.
The purpose of this step is to prepare for a more reasonable configuration of the inverter.
3. Inverter Selection
3.1. Power Selection
Based on the user's needs analysis results, select an inverter with a rated power higher than P0. If the customer's budget is limited, the requirement can be lowered, but aim to select an inverter with a rated power higher than P1. The maximum output power of the inverter determines the maximum power of devices that users can use simultaneously, making this choice crucial. If the configuration is too small, it can lead to overload situations, resulting in overload protection triggering or even damaging the inverter.
3.2. Type Selection
Inverters come in high-frequency and low-frequency types, with the following differences:
3.2.1. In terms of reliability, low-frequency inverters are superior to high-frequency ones. Low-frequency inverters use silicon-controlled rectifier (SCR) rectifiers, a technology that has been matured over half a century, with strong resistance to current shocks. Since SCR is a semi-controlled device, faults such as bypass and mis-operation are less likely to occur. High-frequency inverters use IGBT high-frequency rectifiers, which, although operating at a higher switching frequency, have stricter voltage and current working areas, resulting in lower shock resistance compared to SCR, hence lower reliability.
3.2.2. In terms of environmental adaptability, high-frequency inverters are superior to low-frequency ones. High-frequency inverters use microprocessors as the control center, embedding complex hardware analog circuits into the microprocessor and controlling UPS operation through software programs. Therefore, they have significantly reduced volume, weight, and noise, and have a smaller impact on space and environment. Hence, high-frequency inverters are commonly used in low- to medium-power applications with less stringent reliability requirements, such as portable power supplies.
3.2.3. In terms of load zero-ground voltage requirements, low-frequency inverters are superior to high-frequency ones. In high-power three-phase high-frequency inverters, the neutral line introduces rectifiers and serves as the neutral point of the positive and negative busbars. This structure inevitably couples the rectifier and inverter high frequency on the neutral line, raising the zero-ground voltage at the load end, making it difficult to meet the requirements of zero-ground voltage less than 1V for server manufacturers such as IBM and HP. In addition, during the switch between mains and generators, high-frequency inverters often have to switch to bypass operation due to the absence of the neutral line, which may cause significant faults such as load flashing under specific conditions. Low-frequency inverters do not require the neutral line to participate in rectifier operation. When the neutral line is disconnected, the UPS can maintain normal power supply.
Therefore, in the specific application of residential energy storage battery systems, the inverter also needs to be optimally configured according to the customer's specific application scenarios and requirements.
3.3. Voltage Selection
Inverters come in low-voltage and high-voltage systems. Low voltage generally refers to systems with input voltages below 48V, while high voltage refers to systems with input voltages of 96V and above. Generally, single high-power output inverters are high-voltage systems, such as those with a power output of over 10KW. If there is a high-power output requirement but a low-voltage system is selected, parallel connection mode is required to provide sufficient power.
4. Residential Energy Storage Battery Energy Configuration
4.1. Battery System
In the field of residential energy storage batteries, there are various types of batteries, including lead-acid and lithium iron phosphate batteries. The characteristics of these two types of batteries are as follows:
4.1.1. Lead-acid batteries: Single-cell voltage is 12V, can be freely connected in series and parallel without requiring BMS management. Advantages include standard products, easy replacement, relatively low price, safety, strong environmental adaptability; disadvantages include relatively short lifespan and environmental pollution.
4.1.2. Lithium iron phosphate batteries: Single-cell voltage is 3.2V, can be combined in series and parallel to achieve the required voltage and capacity, currently mainly 24V and 48V, and require strict BMS management. Advantages include safety, environmental protection, long lifespan, and high cost-performance ratio.
Traditional energy storage is mainly lead-acid, and it is currently the most widely used. Lithium iron phosphate batteries are mainly used in residential energy storage battery systems and industrial energy storage fields.
4.2. Residential Energy Storage Battery Voltage Matching
The voltage of residential energy storage batteries is mainly determined by the inverter and needs to match the input voltage of the inverter. For example, a 48V inverter should be paired with a 48V battery, and a 96V inverter should be paired with a 96V battery. Mismatching is not allowed.
4.3. Residential Energy Storage Battery Energy Configuration
This is determined by the user's budget. Look at how long the user wants to use the battery at full power. For example, if the total power of the user's devices is P, and the time needed for full-power usage is h, and the inverter conversion efficiency is β, then the battery energy Q=P*h/β.
4.4. Output Power
It is generally determined by the design of the residential energy storage battery system, including the discharge rate of the battery core, the maximum current that the system structure can withstand, the maximum current of the BMS, heat dissipation performance, etc. For example, for a 48V system, if the design maximum output current is 100A, then its output power is 4800W, etc.
5. Photovoltaic Component Configuration
5.1. Maximum Allowable Installation Power
This is determined by the area of available land where users can install solar panels, generally rooftops. This area limits the maximum total power of solar panels allowed to be installed. If the available area is S square meters, the area of each solar panel is S1, and the power is Pw, then the maximum allowable installed solar panel power is P=S/ S1*Pw.
5.2. Actual Solar Panel Power
This is determined by the user at the initial stage of system design. If the user needs T to fill the battery, assuming the effective sunlight time at the location is t, then the solar panel power P = battery energy / T / t = Q / T / t. P should be less than or equal to the maximum allowable installed power Pw. This value is influenced by the budget. If the budget is sufficient, then P = Pw.
6. Other Accessories
6.1. Installation Brackets
Mainly refers to the installation brackets for solar panels, which are determined by the number of solar panels and the installation method.
6.2. Cables
Various connecting wires and harnesses are determined during system design.
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