Stand-alone photovoltaic (PV) power generation systems are isolated systems, as opposed to grid-connected systems. These systems are primarily used in remote, unpowered areas, and their main purpose is to solve the problem of lack of electricity. Their power supply reliability is greatly affected by factors such as weather conditions and load, and their power supply stability is relatively poor, often requiring the installation of energy storage and energy management equipment.
I. Classification of Independent Photovoltaic Power Generation Systems
Stand-alone photovoltaic (PV) systems, also known as off-grid PV systems, mainly consist of solar cell modules, a controller, and batteries. If AC loads are to be supplied, an AC inverter is also required. Stand-alone PV systems can be broadly classified into two categories: DC PV systems and AC PV systems.
(I) DC Photovoltaic Power Generation
1. Battery-free DC photovoltaic power generation system
A battery-free DC photovoltaic (PV) power generation system is characterized by a DC load, meaning the load has no specific usage time requirement and is primarily used during the day. The solar cells are directly connected to the load, generating electricity to power the load when there is sunlight and ceasing operation when there is no sunlight. The system does not require a controller or battery storage. The advantage of a battery-free DC PV power generation system is that it eliminates energy losses during controller operation and the storage and release processes in batteries, thus improving solar energy utilization efficiency. The most typical application of this system is a solar photovoltaic water pump.
2. DC photovoltaic power generation system with storage battery
A DC photovoltaic power generation system with a storage battery consists of solar cells, a charge and discharge controller, a storage battery, and a DC load. When sunlight enters, the solar cells convert light energy into electrical energy to power the load and simultaneously store electrical energy in the storage battery. At night or on cloudy or rainy days, the storage battery supplies power to the load. This type of system has a wide range of applications, from small solar lawn lights and courtyard lights to large mobile communication base stations and microwave relay stations far from the power grid, as well as power supply to rural areas in remote regions. When the system capacity and load power are large, it is necessary to equip the system with a solar cell array and a storage battery bank. [3]
(II) Alternating Current Photovoltaic Power Generation
1. AC and AC/DC hybrid photovoltaic power generation systems
Compared to DC photovoltaic (PV) systems, AC PV systems, including hybrid AC/DC PV systems, have an additional AC inverter to convert DC power into AC power, providing energy to AC loads. Hybrid AC/DC PV systems can supply power to both DC and AC loads.
2. Grid-supplemented photovoltaic power generation system
A grid-complementary photovoltaic (PV) power generation system is a stand-alone PV system that primarily uses solar photovoltaic power generation, supplemented by regular 220V AC power. This allows for smaller capacity designs for both solar cells and batteries. Essentially, electricity generated on sunny days is used, while grid power is used to supplement it on cloudy or rainy days. Given that most parts of my country experience more than two-thirds of sunny weather annually, this approach reduces the initial investment in solar PV systems while offering significant energy savings and emission reductions. It represents a good transitional solution for the current promotion and widespread adoption of solar PV power.
II. Application of Independent Photovoltaic Power Generation Systems
Independent photovoltaic (PV) power stations, also known as isolated PV power stations, are suitable for establishing in villages, towns, and islands without electricity where there is good sunlight and relatively high load demand, or in areas with a relatively concentrated user base within a few kilometers. Currently, the capacity of independent PV power stations ranges from several kilowatts to tens of kilowatts. A power station consists of PV panel arrays, batteries and converters, energy processors, and power distribution and transmission systems.
The power generation system charges the batteries during the day and supplies electricity to photovoltaic water pumps, processing machines, etc., for pumping, water storage and processing operations. At night, it completes the inverter discharge control of the batteries to supply power to the loads. When designing an independent power station, it is very important to consider the rational use of batteries, especially for nighttime electricity use or motor-type dynamic loads with a high electricity consumption ratio.
