1. Value of off-grid systems

From a long-term summary, off-grid systems are mainly based on the following starting points:

(1) There is no way to use the electricity (grid), such as mountains, boats, cars, the field and so on. This is the most typical off-grid system requirement; More than 80% of users are not convenient to use the mains, want to achieve power supply through the off-grid system.

(2) there is mains electricity, but often irregular power outage areas. In case of power failure, the photovoltaic off-grid system can continue to supply the load, to meet the continuous demand.

(3) Have mains power, but want to reduce the electricity bill through off-grid system. The off-grid system can not only charge the battery by photovoltaic modules, but also charge the battery by setting mains power when the rate is low, and then supply the load by discharging the battery when the rate is high.

/2. Composition of typical off-grid system

The picture

Off-grid system diagram

The diagram above shows the composition of a typical off-grid system. These points should be noted:

(1) The off-grid system refers to the system output does not need to be connected to the mains power grid like the grid-connected photovoltaic system. The output of the off-grid system is the supply load.

(2) The off-grid system provides a V-F source, which acts as a power grid.

(3) The main equipment in the off-grid system includes: photovoltaic modules, off-grid inverters, battery packs, confluence devices, distribution boxes and electrical loads.

(4) The most streamlined off-grid system can only contain two parts: off-grid inverter, battery pack.

/3. Off-grid system design process

(1) Confirm requirements

Making an off-grid system is a "discretionary" process, because in most cases different users need to carry the same load, and the duration of electricity is different. The right way is to first identify the requirements: what loads are going to be carried? How big is the load? How long is it used in a day? And then figure out how big the system needs to be. For example, a customer in North China needs to build a set of off-grid power station. The main load is a 3500W blower, which uses electricity for 3 hours every day and can provide backup power for 1 day. Based on this information, the power of the load is 3.5KW and the power consumption is 10.5kWh per day.

(2) Off-grid machine selection

Early off-grid system control part by MPPT controller +DC/AC converter to perform, with the development of technology, the current mainstream manufacturers are advocating the use of control inverter integrated machine, one device can achieve functions at the same time, such as Gurewatt's SPF ES series off-grid inverter.

After confirming the demand, we will choose the appropriate off-grid inverter. The output of the selected off-grid machine must be able to cover the rated power of the load, and try to leave some margin, in order to make the system more reliable. The load in the above figure is used as an example. The rated power is 3.5kW. The SPF 5000 ES off-grid unit can be selected, and the output power is 5kW.

The picture

Under the premise of covering the load power, how many inverters are needed depends on the backup power demand, which is the so-called "continuous rainy days". Because the power in the off-grid system is generated jointly by the components and the off-grid machine, if the backup power demand is large, one off-grid machine and its matched components may not be generated, so it needs to be multiplied.

(3) Selection of photovoltaic modules

As long as it is a photovoltaic system, the energy source is photovoltaic modules (of course, off-grid systems and mains power as input). The selected off-grid machine has its set acceptable component capacity, here can be directly according to the off-grid machine specifications to match the components. Adopt the current mainstream 182 assembly 550Wp*8 blocks, rated power 4.4kWp, daily average generation of about 16kWh. It can also be matched to the top or even to the top, which needs to be matched according to specific needs.

The combination of components is actually the same as the principle of grid-connected inverter. The group string of off-grid machine has the maximum allowable access voltage value, which can be connected according to the parameters of the selected components. The maximum PV input voltage of the SPF 5000ES off-grid machine is 450V, and the MPPT voltage ranges from 120V to 430V. If 550Wp components are used, the general open-circuit voltage is about 49V. You can directly press 8 blocks in a series to connect to the PV input port of the off-grid machine.

(4) Battery selection

The battery is a major feature of the off-grid system that is different from the grid-connected system. It is the buffer zone of the system, otherwise the system will become an island.

The picture

In the choice of battery, or according to the user's demand for electricity to do. For example, the customer has a 10.5kWh backup power requirement, which means that the battery needs to release 10.5kwh of power through the off-grid machine to meet the load when there is no light.

However, both lead-acid batteries and lithium-ion batteries have a concept or characteristic of Depth of Discharge (DoD=Depth of Discharge), which must be taken into account. That is to say, the electricity stored in the battery cannot be released 100% each time. Only a part of it can be released. Otherwise, the battery will be damaged and the battery will fail prematurely.

Typical lead-acid batteries discharge to a depth of around 60%, while lithium batteries can discharge to a depth of 90%. When selecting the battery capacity, the discharge depth also affects the total capacity of the battery. Take the above example again, choose the lead-acid battery, 10.5kWh of electricity is calculated according to 60% discharge depth, need to configure a total 10.5kwh / 0.6=17.5kWh battery set, these batteries are like a container, the system to store the electricity, when the load needs to release.

Do not need so complex to calculate the capacity or specifications of a single battery, 5kW off-grid machine requirements configuration of the battery should be more than 200Ah specifications, know this, just through the number of adjustment, with the same or close to the total battery capacity.