How to design an off-grid photovoltaic energy storage system

  A common off-grid energy storage system is a backup power system (UPS), which is widely used in areas with frequent power outages and unstable power grids, or loads that require a high power supply guarantee rate from the power grid. Its core component is a charging and inverter integrated machine with dual functions of charging and inverter. When the mains power is normal, the charging inverter can directly connect the mains power to supply power to the load, and at the same time convert the alternating current into direct current and charge the battery, and ensure that the battery will automatically stop charging after the battery is fully charged. The charging inverter will automatically cut off the connection between the mains and the load, start the inverter instantly, and switch to the battery to supply power to the load through the inverter.

                          Diagram of off-grid photovoltaic energy storage system with mains complementation

  The off-grid photovoltaic energy storage system is a photovoltaic energy storage off-grid system composed of photovoltaic power generation, energy storage system and inverter. It can directly use photovoltaic modules to charge the battery to meet the demand of electricity load. The off-grid photovoltaic power generation system is mainly suitable for some special and some electricity loads far away from the power grid. There is still a large market demand space for some communication, border checkpoints, islands or field operations and crowds, especially underdeveloped countries such as Africa. Countries and regions still have very large development space and markets.

                                       Philippine 5KW household off-grid energy storage project case

  Off-grid photovoltaic energy storage power stations can also be installed in places with frequent power outages or unstable grid power supply. The battery system can not only be charged by the photovoltaic power generation system, but also be charged by conventional electricity when the conventional power grid is energized to ensure the power supply.

How to make Off-grid photovoltaic energy storage power station

1. Leave enough margin for the inverter

Usually off-grid photovoltaic power generation is a power supply system for a specific load. During the design process, the rated power of the load connected to the system should be calculated and checked, and the output power of the inverter used should be determined according to the total power of the load. Considering the inrush current, starting power and reactive power required by some special loads during startup, the possible influence and demand on the inverter, a sufficient margin should be reserved for the inverter. Once the capacity of the inverter is determined, the capacity of the connected load cannot be expanded or increased at will, so the off-grid photovoltaic power generation system should leave enough margin for the inverter.

2. Fully consider the matching of various loads and inverters

The off-grid photovoltaic power generation system should fully consider the matching of various loads and inverters, especially if there are inductive loads such as washing machines, refrigerators, air conditioners and other inductive loads with motors, sine waves must be selected and equipped. Or a quasi-sine wave inverter to meet the demand of the inductive load on the inverter output waveform.

3. Calculate the daily average working time and electricity consumption of each load

The capacity of the photovoltaic array and the capacity of the battery are designed based on the customer's needs, so as to meet the user's electricity demand and control the overall investment cost.

4. The design is based on the amount of solar radiation in the worst month of the year

The off-grid system needs to ensure that the month with the least amount of radiation, the photovoltaic system can still guarantee power supply (usually the worst month of the year is in winter. This is different from the grid-connected system. The design of the grid-connected power station is based on the total amount of solar radiation throughout the year. ) Therefore, the off-grid photovoltaic power generation system usually designs the inclination angle of the photovoltaic array according to the optimal inclination angle of the worst month. That is to say, the array inclination angle of the off-grid photovoltaic power generation system is usually 2° to 8° larger than the local geographic latitude angle to take into account the weak solar radiation in winter.

5. The photovoltaic array capacity Qp can be calculated by the following formula

Pi: load power

hi: load working time

H: daily peak hour of solar radiation in the worst month

ηb: battery charge and discharge efficiency

ηc: controller efficiency

ηi : Inverter efficiency

6. The battery capacity Qb can be calculated by the following formula

Qb = (Ah)

D: days of electricity storage

A: battery depth of discharge

Summary: Only by doing a good job of system design can the power generation of the power station be maximized. In specific projects, it is recommended to refer to and apply it flexibly, and not blindly design.