Some farmers in remote areas of Yunnan are very suitable for household small-scale solar photovoltaic power systems. The characteristics of these farmers are:
(1) Residing on high altitude and remote mountain tops or halfway up, with sparse population, scattered residential areas, and low electricity load;
⑵ The transportation in the place of residence is extremely inconvenient and relatively far from the power grid;
Lack of renewable micro hydropower and micro wind power resources around residents;
The solar energy resources in the residential area are relatively good.
Therefore, fully utilizing the unique solar energy resources in these areas without electricity, relying on the poverty alleviation policies of the provincial government, and promoting the application of small-scale solar photovoltaic power systems for household use in a planned and step-by-step manner, tailored to local conditions, is a way to solve the electricity consumption problems of farmers in these areas more quickly, efficiently, and economically. It is also an ideal way with relatively small investment and easy construction and management.
We have received support from the Yunnan Provincial Development and Reform Commission, and since 2000, we have gradually promoted the application of small solar photovoltaic power systems for household use to farmers in areas without electricity, enabling some farmers to realize their desire to light up and watch TV for decades. Introduce this system to everyone as shown in Figure 1.
1. Configure battery capacity, solar cell power, and inverter
The household small-scale solar photovoltaic power supply system can use simple calculation methods to configure solar cell power and battery capacity based on local sunshine time and geographical environment. We first standardize the electricity consumption of a household, and then calculate the battery capacity, solar cell power, and inverter power based on this electricity consumption.
1.1 Calculation of electricity consumption
We have unified regulations for the electrical appliances of a household as shown in the diagram. Each household can install and use three energy-saving lamps of around 10W, and a 21 inch color TV (with a power of about 70W) can be inserted into the socket. The total power of all appliances is about 100W. Using a single 12V lead-acid battery to store electrical energy, the 12V DC power is converted into 220V AC power through an inverter, which is used normally by a household for 4 hours every day; The DC input current of the inverter (5% inverter loss) is 8.75A; The daily electricity consumption of 100W appliances is 35Ah.
1.2 Configure battery capacity
When designing the capacity of lead-acid batteries, in addition to considering the number of days that lead-acid batteries can be used in continuous cloudy and rainy days, it is also necessary to consider that each time the lead-acid battery is discharged, it should not be fully discharged at 100% depth, and a portion of the capacity should be left behind. This is conducive to the physical and chemical conversion of internal energy in lead-acid batteries, and the service life of lead-acid batteries is relatively extended. However, it should also be noted that the larger the capacity of lead-acid batteries, the more self discharge they will have. If the capacity is chosen too much, it will consume some electrical energy and increase unnecessary investment; If the capacity is chosen too small, the excess electricity cannot be stored when the electricity consumption is low. Therefore, we first design the number of days a lead-acid battery can be used continuously in cloudy and rainy weather as 3 days, and set the daily consumption of 35Ah of electricity for 4 hours as 25% of the capacity of this lead-acid battery. Therefore, the capacity of the lead-acid battery is 140Ah. In practical applications, we choose a single 200A, 12V fully sealed maintenance lead-acid battery.
1.3 Power of solar cells
The power of the solar cell must be appropriately greater than the actual power consumption by 100W, because on the one hand, it is necessary to consider charging efficiency and line losses, and on the other hand, it is necessary to consider that lead-acid batteries can still provide electricity for three days of load use in the event of continuous cloudy and rainy days. There is approximately 4.5 hours of optimal sunlight exposure per day in the local area. We hope that the solar cell can complete the charging of lead-acid batteries within 4.5 hours with a capacity greater than 35Ah, and the charging capacity at other times will be considered as loss. Therefore, the charging current of solar cells should be greater than the quotient of 35Ah per day consumed by the load divided by 4.5 hours, which is greater than 7.8A. We actually chose two 75W polycrystalline silicon solar cell modules, each module outputting a working current of 4.5A, and the two modules outputting a working current of 9A and a power of 150W in parallel.
1.4 Selection of inverters
⑴ Input voltage and waveform
Because the small solar photovoltaic power supply system uses 12V lead-acid batteries to provide electricity, energy-saving lamps and televisions are the main loads. Therefore, the first choice is to choose an inverter with an input of DC 12V, a capacity of 550VA, and an output of AC sine wave 50HZ and 220V, which is beneficial for load operation.
⑵ Output power
According to the principle that the TV generates a demagnetization current that is 3-5 times greater than the working current at the moment of startup, and the need to increase other electrical appliances in the future, an inverter with an output power greater than 350W should be selected. The inverter works in a low load state for a long time, which is beneficial for the inverter’s operation.
⑶ Function
In addition to overload, undervoltage, and reverse protection functions, it should also have intelligent management and control functions for the charging and discharging capacity of solar cells, which is beneficial for protecting lead-acid batteries and extending their service life.
⑷ Reliability
Inverter is one of the core and durable components of the entire solar photovoltaic power supply system. When choosing an inverter, one should not be tempted by cheap prices. It is necessary to choose an inverter that is easy to operate, of good quality, and durable and reliable, in order to improve the credibility of the entire solar photovoltaic power supply system.
Based on the above four selection relationships, we have chosen a wall mounted inverter that can be hung on the wall, is easy to operate (with only one switch), has good quality, and is durable and reliable.
2. Issues to be noted during actual installation and operation
1) The location for installing solar panels is shown in Figure , and the location for installing solar panels should be chosen as much as possible in the front and back of the user’s house, where they receive the most sunlight every day. It is worth noting that besides not being shaded, this place cannot be a must pass place for cattle and horses.
2) The installation of solar panels is only when the sunlight is perpendicular to the surface of the solar panel for the longest time, the maximum power generation of the solar cell will be achieved. In the actual installation process, for the sake of simplicity and practicality, the solar panels are fixed and installed facing south with an inclination angle of the local latitude.
3) The selection and installation of cables should be as short as possible, with sufficient current carrying cross-sectional area, and good copper core quality when selecting cables connecting the solar cell to the inverter and the battery to the inverter. We generally stipulate that the cable length between the solar cell and inverter shall not exceed 20m, the copper core wire diameter shall not be less than 6mm2, the cable length between the battery and inverter shall not exceed 1.2m, and the copper core wire diameter shall not be less than 10mm2. As shown in Figure 1, the cable from the solar panel to the farmer’s home needs to be reinforced with thin steel cables.
4) Installing DC miniature air switches and AC miniature air switches is difficult to purchase fuses or fuses in mountainous rural areas. To facilitate farmers and better protect inverters, we have installed a 30A-50A DC miniature air switch between the battery and inverter, and a 1A AC miniature air switch at the inverter’s AC output port.
5) During the installation process, four things must be done well: ① Invite knowledgeable and cultural farmers in the village to participate in the installation, and designate 1-2 maintenance personnel based on their thinking, reaction, and hands-on abilities to help other farmers in the village solve some simple problems in the future; ② Indicate the correctly installed positive and negative terminal connection ports to prevent future maintenance from reversing the positive and negative terminals of the power supply and damaging the inverter; ③ Try to tighten all screws connecting cable heads as much as possible, minimize contact resistance, and minimize energy loss; ④ Try to tighten all the screws for adjusting the azimuth angle, inclination angle, and other screws on the bracket as much as possible to reduce damage to the solar panel caused by strong winds.
6) After installation and operation, please do not press the switch on the inverter at will, do not connect or pull wires randomly, and use energy-saving lamps for lighting. Do not overpower or use electrical appliances for extended periods of time. Use electrical appliances according to regulations and adjust the daily usage time according to weather conditions. For example, if today is a sunny day with sunlight shining from morning till night, the usage time of electrical appliances tonight can be extended. Conversely, the usage time of electrical appliances can be shortened, which is beneficial for the cyclic operation of the solar photovoltaic power system.
3.Daily maintenance work
1) The surface of the solar panel should be kept clean regularly to prevent it from being shaded by tall trees and newly built houses, in order to prevent dust or shading on the surface of the solar panel, which may cause a decrease in daytime power generation and insufficient electricity consumption at night.
2) Regularly check all screws on the solar panel bracket for looseness or oxidation damage to prevent damage to the solar panel due to strong winds.
3) Regularly inspect the cable terminals inside the junction box on the back of the solar panel and the terminals between the inverter and the cable for looseness or oxidation, to prevent faults caused by poor cable contact or increased oxidation resistance that may not charge the battery or cause minor charging issues.
4) When the wires, switches, and lamp caps do not light up, the lamp caps, switches, and wires should be checked for poor contact, short circuits, and open circuits. Some farmers connect their own wires without using adhesive tape to wrap the joints, which can easily cause short circuit faults.
5) Fully sealed and maintenance free lead-acid batteries, as long as attention is paid to keeping the surroundings of the battery dry and the surface clean; The maintenance of open end lead-acid batteries is more troublesome. In addition to keeping the battery dry and surface clean, it is also important to regularly add distilled water to the battery. Adding too much distilled water can cause corrosion of the connection column head due to electrolyte leakage, increasing the contact resistance of the cables connected to the column head, resulting in a fault where the current emitted by the solar cell cannot be charged and cannot be discharged when using electricity.
6) Inverter ① should be regularly kept clean and wired firmly. If smoke or rain is found to corrode the inverter, the suspension position of the inverter should be adjusted as soon as possible, otherwise it will damage the inverter. ② Although a sine wave inverter is configured and has no impact on inductive loads, it is advisable not to bring high-power motors to avoid faults that may damage the inverter due to surge current and voltage.