Talking about how to design off-grid photovoltaic system
01Basic data that must be understood before installation
First of all, we must understand the user voltage phase. Is it single-phase AC 220V or three-phase AC 380V? This determines the output characteristics of the inverter;
The second is the load type, is it inductive load or resistive load? This determines the load power and output waveform of the inverter.
The third is the full-load operating time of the load, that is, how many degrees do we need for the average daily power consumption? If it is a photovoltaic grid-connected power station, because there is no energy storage device, only reasonable photovoltaic module power is required; if it is a photovoltaic off-grid power generation system, the battery capacity must also be calculated, including when there is no photovoltaic power generation condition in continuous rainy days. The self-reserved power of the system.
02 Photovoltaic off-grid power generation case
We take the photovoltaic off-grid power station of a small lake farmer as an example. Due to the high cost of long-distance power supply, the power loss and voltage loss of the transmission line are large, and the typhoon hits, resulting in unstable power consumption, frequent unexpected power outages, and affecting production and living power. For this reason, it is planned to use photovoltaic off-grid power generation. The sunlight radiation intensity is high during the day, and the photovoltaic power generation is directly inverted and output. The power supply works and charges the battery at the same time.
The voltage is AC220V 50Hz, and electrical equipment mainly includes:
10 sets of oxygen pumps for fish ponds (300W)
TV + satellite receiver (200W) 1 set
1 rice cooker (750W)
Induction cooker (2000W) 1
1 refrigerator (100W)
Lighting (100W)
The load is not used at the same time. The oxygen pump works during the day when the sun is shining and does not work at night; the power of other electrical appliances is about 3000W, and the daily power consumption is about 10 degrees. Since the lake has sufficient sunlight, the self-reserved power in cloudy and rainy days is not considered.
03Photovoltaic inverter
According to the above data provided by the user, in this system design, SAKO Sanke? Off-grid photovoltaic inverter integrated machine is selected, the power is 48V 6KVA, the power factor is 0.9, the inverter conversion efficiency>88%, and the actual loadable power is up to 5000W, can meet the user's electrical equipment output power requirements.
04Battery capacity
The energy storage device used in the photovoltaic off-grid power generation system is a commonly used lead-acid battery with large capacity and high cost performance.
The reserve capacity of the battery is 10KWh. Since the DC input voltage of the photovoltaic inverter is DC48V, the theoretical capacity of the battery is calculated:
10000VAh/48V=208Ah
According to the relevant technical standards of the battery, the discharge rate of the battery is more economical and reasonable at 0.5C2, which can ensure the battery's cycle charge and discharge times and effectively extend the service life. Because the lake has sufficient sunshine, the photovoltaic output is directly inverted during the day, and there is no need to go through the repeated discharge procedure of the battery. The electric power consumption at night is small and the discharge time is short. Therefore, the battery discharge rate is appropriately increased to 0.6C2 in the design of this scheme. Actual capacity calculation:
208Ah/0.6=347Ah
The value here is 400Ah, that is, the total capacity is: 48V 400Ah
The specification of the lead-acid battery is 12V 200Ah/pc, the connection mode is 4 strings and 4 parallels, and a total of 8 batteries are required.
05 PV module power
After the battery configuration capacity is obtained through the above calculation, the power configuration of the photovoltaic module is calculated.
The geographical location of the lake has strong solar radiation, and the effective sunshine time is as long as 6 hours. The use of polycrystalline silicon photovoltaic modules has a photoelectric conversion efficiency of 16%, which meets the standards set by the National Energy Administration.
The calculation formula for photovoltaic power generation is: system power generation = photovoltaic module power × sunshine time × comprehensive coefficient. The comprehensive coefficient refers to the loss coefficient caused by factors such as temperature change, line loss, and controller (or inverter) conversion efficiency. The value is generally 0.5-0.7, and the value is 0.6 this time. Therefore, the photovoltaic module power calculation:
48V×400Ah/(6h×0.6)=5333W
The specification of the module is 36V 275W, the size is 1900×980×45mm, and the area is about 2 square meters. The connection method is that every 2 pieces (72V) are connected in series, and then 10 strings are connected in parallel. A total of 20 pieces of photovoltaic modules are required, with a total power of 72V 5500W and a photovoltaic module array area of 40 square meters.
