Three principles of photovoltaic off-grid system design
1. Confirm the power of the off-grid inverter according to the user's load type and power.
Household load is generally divided into inductive load and resistive load, washing machine, air conditioner, refrigerator, water pump, range hood with motor load is inductive load, motor starting power is 5-7 times the rated power, in the calculation of the power of the inverter, to take these load starting power into account. The output power of the inverter is greater than that of the load. For
monitoring station, communication station and other occasions with strict requirements, the output power is the sum of all the load power. However,
for general poor families, considering that all the loads can not be turned on at the same time, in order to save costs, the sum of the load power can be multiplied by the coefficient of 0.7-0.9. Not every customer will be clear about the load power.
2. Confirm the power of the component according to the daily power consumption of the user.
The modules are designed to meet the load's daily demand for electricity under average weather conditions, which means that the solar modules need to generate as much electricity as the load does in a year. Because the weather conditions are lower than or above the average, the solar cell module design basically meets the needs
of the worst season of light, even in the worst season of light the battery can be fully charged every day. But
in some areas, the worst season is far below the annual average, and designing solar modules for the worst can waste much more electricity than is needed at other times of the year. At
this time, we can only consider appropriately increasing the design capacity of the battery, increasing the storage of electric energy, so that the battery is in a shallow discharge state, to make up for the damage caused by the lack of power
generation in the worst lighting season. Component power generation can not be completely converted to electricity,
but also consider the efficiency of the controller and the loss of the machine and the loss of the battery, the solar controller has TWO types of PWM and MPPT, PWM controller efficiency of about 85%, the input voltage range is relatively narrow,
but the price is low, MPPT controller efficiency of about 95%, the price is relatively high. Battery in the process
of charging and discharging, there will be 10-15% loss. Available power of the off-grid system = total power
of the module * average hours of solar power generation * controller efficiency * battery efficiency.
There are some off-grid users, not installed electricity meter, is not very clear about their electricity consumption, and some off-grid system, is a new, then it is necessary to estimate the daily electricity consumption, for light bulbs, electric fans, hair dryers such a load, electricity consumption is equal to power times time; But air conditioning, refrigerator such load, it is gap sex work, TV, computer, acoustics such load, working rarely in full
power state, when calculating electric quantity, it is necessary to consider comprehensively.
Air conditioning is one of the biggest household appliances power consumption load, air conditioning of electric power is 735 W, 1 that is 1 hour at full capacity operation consumes 0.735 KWH, air conditioning and a index "capacity", the unit is W, 1 air conditioning refrigerating capacity of about 2300 W, the power consumption of air conditioning and indoor and outdoor temperature difference, room area, air conditioning can efficiency has a lot to do, A 1P air conditioner, 8 hours in the evening, power consumption 1-5 degrees.
3. Determine the battery capacity according to the user's evening power consumption or expected standby time.
The task of the battery is to ensure the normal power consumption of the system load when the solar radiation is insufficient. For critical loads, to be able to maintain the normal operation of the system within a few days, consider consecutive rainy days. For general loads such as solar street lamps, it can be selected in 2 ~ 3 days according to experience or need. Important loads such as communications, navigation and hospital treatment are selected within 3 to 7 days. Also consider the location of the photovoltaic system installation, if in a remote location, the battery capacity should be designed to be large, because maintenance
personnel can take a long time to reach the site. In practical application, some mobile communication base station
because of the high mountain road far, to a very inconvenient, in addition to the normal configuration of battery, but also equipped with a group of backup battery, for the general poor families, the main consideration of the price, do not have
to consider the rainy day, the sun is good when multi-purpose. Use less when the sun is bad, not when there is
no sun. When choosing the load, try to use energy-saving equipment, such as LED lights, frequency conversion
air conditioning. The design of battery mainly includes the design and calculation of battery capacity and the
design of battery series and parallel combination. In the photovoltaic power generation system, most of the lead
acid batteries are used. Considering the life of the battery, the discharge depth is generally between 0.5-0.7. Battery
design capacity = (Load daily power consumption x Consecutive rainy days)/Battery discharge depth.
Off-grid users' needs are diverse, and photovoltaic systems are designed according to users' requirements. For
example, there is a customer who has six 260W modules in his home and wants to make a set of photovoltaic off-grid system. The customer has mains electricity in his home, but there are frequent power outages. The total load is 10KW, and the daily
electricity consumption is 20-30 degrees. There is no way to satisfy clients' demand for electricity use, can
consider to meet the needs of customer part load, by calculating the component can send a day 5 degrees, use 3.0 KVA off-grid inverter, 4 pieces of 12 v150ah battery, flashboard output by one, at ordinary times take home lighting bulbs, computer,
washing machine, such as load at night if there is electricity, You can also connect a separate air conditioner.
For large projects aimed at specific groups of people, due to the different situation of each user, can not meet all the needs, at this time to consider comprehensively. Under normal circumstances, it is to take an average value, such as the photovoltaic project to solve the problem of electricity
consumption in the western regions of China. The average herdsman family uses two 250W components, a 500VA off-grid inverter integrated machine, and two 12V150AH batteries, which can generate 2.5 KWH of electricity every day. The cost of raw materials
is about 6000 yuan. It can meet the electricity demand of TV sets, small refrigerators, DVD players, energy saving
lamps and other electrical appliances. Relatively light weight, easy to move. In a photovoltaic poverty alleviation project in Ganzi Prefecture, Sichuan province, medium power off-grid system is adopted, including 8 250W components,
3KVA off-grid inverter integrated machine, and 4 batteries of 12V200AH, which can generate 8-10 KWH of electricity every day and meet the electricity demand of TV sets, refrigerators, DVD players, computers, 1P air conditioners, energy-saving
lamps and other electrical appliances. Due to the high overall power generation, the system can support more
household appliances and improve the quality of life.
Common problems of photovoltaic off-grid system:
When designing photovoltaic off-grid system, we should be flexible and not stick to a fixed formula. Photovoltaic
off-grid system can not solve all the electricity problems, encounter a number of continuous cloudy and rainy days, can only save electricity. There
is no unified standard for off-grid inverters, and there is no compulsory certification. The products on the market are uneven, and the quality and price of products differ greatly. Please look for the brand when choosing off-grid inverters.
If you encounter fake or inferior products, you should complain or report to the police in time.
1, components, inverter, battery design to match, any one can not be too large or too small, novice design, often calculate the power consumption is too large, such as 1P air conditioning 12 hours, calculated as 10 KWH electricity, 300W refrigerator 24 hours, calculated as 7.2 KWH electricity, resulting in battery capacity is too large, the system cost is too high. When designing battery capacity, it is best to give 2 days to be able to fill.
2, photovoltaic off-grid system output connection load, each inverter output voltage and current phase and amplitude are not the same, some inverter manufacturers do not support the output end parallel, do not connect the inverter output end together.
3, the load is such as the elevator load can not be directly connected to the inverter output end, because when the elevator is down, the motor reverses, will produce a back electromotive force, into the inverter, the inverter is damaged. If off-grid system must be used, it is recommended to add a frequency converter between the inverter and the elevator motor.
4. For photovoltaic microgrid system with complementary input of municipal power, the insulation of the components should be done well. If there is leakage current of the components to the ground, it will be transmitted to the municipal power and cause the leakage switch of the municipal power to trip.
5, the voltage of the component and the voltage of the battery to match, PWM controller between the solar module and the battery through an electronic switch connected, there is no inductive device in the middle, the voltage of the component is between 1.2-2.0 times the voltage of the battery, if it is 24V battery, the input voltage of the component is between 30-50V, MPPT controller, There is a power switch tube and inductor circuit in the middle, the voltage of the component is between 1.2-3.5 times of
the voltage of the battery, if it is a 24V battery, the input voltage of the component is between 30-90V.
6, the output power of the component and the power of the controller should be similar, such as a 48V30A controller, the output power is 1440VA, the power of the component should be about 1500W. When selecting the controller, first look at the battery voltage, and then divide the component power by the battery voltage,
is the output current of the controller.
7, battery charging current is generally 0.1C-0.2C, the maximum is not more than 0.3c, for example, 1 lead-acid battery 12V200AH, charging current is generally between 20A and 40A, the maximum can not exceed 60A; The discharge current of batteries is generally 0.2C-0.5C, and the maximum is not more than 1C. The
maximum output power of one 12V200AH lead-acid battery is not more than 2400W. Different manufacturers, different models, and specific values are different.