Introduction of Solar Street Lighting System

In today's world where energy shortage and environmental pollution are becoming increasingly serious, fully developing and utilizing solar energy is an energy strategic decision for sustainable development by governments around the world. Solar street lights are becoming more and more popular because they do not require special management and control, require a one-time investment for installation without future electricity expenses, do not require the installation of transmission lines or trenching for laying cables, and can be easily installed in squares, campuses, parks, streets, etc.
Solar
street lighting systems are usually composed of solar cell modules, batteries, light sources, controllers (AC light sources also require inverters), etc. This article designs and optimizes the solar street lighting system running in Zhuhai from the following aspects: ① Light source selection; ② Control circuit; ③ Determination of the optimal tilt angle of solar panels; ④ Determination of the capacity of solar cell modules and batteries.

Solar street lights are different from ordinary street lights. They use solar cells as the only power supply. Because the cost of solar cell components is still relatively high, in order to reduce system costs, efficient light sources must be used. LED is a semiconductor light-emitting device that can convert electrical energy into visible light. In recent years, LED technology has made key breakthroughs, and its performance-price ratio has also been greatly improved. Compared with traditional street light sources, LED light sources have high light efficiency, nearly twice that of incandescent lamps, and long life, which can reach more than 105 hours; traditional light sources have relatively large power consumption, and most of them work under high voltage. The use of boost inverter links reduces energy utilization, while LEDs use low-voltage DC power supply, which is safe and has low light source control costs, making it possible to adjust brightness and frequent switching.

Solar street light systems are generally small photovoltaic systems. The World Bank standard is that the self-consumption current of small photovoltaic system controllers must be less than 1% of the rated working current. Therefore, the design of the controller circuit and the selection of low-power devices are very important. The voltage comparator composed of integrated operational amplifiers is used as the control circuit in solar street lights. This circuit is a control system composed entirely of hardware. It is simple and reliable, easy to maintain, low cost, and the circuit itself has extremely low power consumption. It is a circuit with good matching. The key to this circuit is to design a better voltage difference for the battery's charge and discharge characteristics. At the same time, the device selection should be reliable. In addition, the charge and discharge status indication circuit composed of light-emitting diodes becomes a practical controller circuit with the function of preventing the battery from over-discharging and over-charging. The system adopts a direct-coupled charge and discharge controller circuit. According to the matching characteristics of LED and battery, it can achieve power adaptation. In the months when solar radiation is insufficient, the battery's charging state is usually low, so the terminal voltage is also low when the battery is discharged. In this way, the load working current is small and the power is small, and the system can work longer. On the contrary, when the solar radiation is sufficient, the load working current is large, the power is large, and it will be brighter.

Determine the optimal tilt angle of solar panels

In the design of independent photovoltaic systems, the plane of solar cell modules is usually oriented toward the equator, with a certain inclination angle relative to the ground. Since the amount of solar radiation varies with the seasons and climate, the inclination angle is different, and the solar radiation received in each month varies greatly. In addition, since the battery is limited by its rated capacity when charging and the discharge depth when discharging, in the optimization design of solar street lights, the optimal inclination angle should be determined according to the load conditions, local climate conditions, and geographical conditions, so that the solar radiation on the plane of the square array can meet the requirements of continuity, uniformity, and maximum as much as possible, and reduce the system cost.

Load conditions

Solar street light products mainly have two working modes: timing and light control. In fact, they are balanced load mode and seasonal load mode. Timing control: It is not affected by the outside world. The light is turned on and off at a fixed time. There is a problem that the light is not on at night or is still on at dawn. The light-controlled solar street light automatically turns on when the outdoor light is dark to a certain degree (200 lx) and automatically turns off when the day breaks. The advantage is that the light source can be automatically controlled according to the lighting conditions. There is no situation where the light source works before dark or does not work after dark. It can work normally all year round. The working time of the light control method
is related to the local latitude and the solar declination of the day. According to the hour angle formula of sunrise and sunset:
COS = COS L - tan ~tan8o] (1),
j6 is latitude; - declination, the declination angle of the day is the angle between the light and the equatorial plane at noon when the sun is shining.
Under normal circumstances, there is no sunshine 0.5 h before sunrise and 0.5 h after sunrise, but there is still residual light in the sky, so there is no need to turn on the lights for lighting. Therefore, the working time of the light-controlled solar street light can be reduced by
1 h. The working time of the street light can be calculated by the following formula:
T = 23-2/15cos-[tan~tan8] (2)
By calculating with formula (2), we can get the annual working time variation of the light-controlled solar street light installed in Zhuhai (Figure 1). It can be seen from the figure that the longest working time of the light-controlled solar street light is 12.35 h at the winter solstice, and the shortest working time is 9.65 h at the summer solstice. Due to the low latitude of Zhuhai, the working time of the light-controlled solar street light does not change much throughout the year. Therefore, although the light-controlled solar street light in this area is a seasonal unbalanced load, the load change is not very large, and the load condition is not the main factor affecting the optimal tilt angle of the solar cell module. Considering the reliability of the control switch light and the cost of the controller,
the solar street light designed in Zhuhai adopts the light-controlled mode.

Climate

Zhuhai is located south of the Tropic of Cancer and has a subtropical marine monsoon climate with abundant sunshine. The total annual solar radiation is 4651.6MJ/n~, making it one of the regions with relatively abundant solar energy resources in Guangdong Province. As can be seen from Figure 2, the period with the least total radiation in Zhuhai is spring, when there are more rainy days, poor atmospheric transparency, and low clouds often cover the sky. During this period,
the proportion of scattered radiation in the total radiation is very high. As can be seen from Table 2, the proportion of scattered radiation in February is the highest value of the year, reaching 65.6%; in summer, it is mainly sunny and hot weather with abundant sunshine. The solar radiation on the horizontal plane
is the highest throughout the year, and the proportion of direct radiation in the solar radiation is very high; in autumn, Zhuhai is clear and sunny, and the sky is clear. Although the solar altitude angle gradually decreases, the solar radiation is still
relatively high; in winter, there are many sunny days, especially in the winter, and the rainfall is scarce.
Calculation of solar irradiance on inclined
surfaces The calculation of solar irradiance on inclined surfaces is generally based on meteorological data from the past 10 to 20 years. Based on the provided horizontal surface solar irradiance data, the model of sky scattered radiation anisotropy proposed by Hay_I can be used to calculate the solar irradiance received by the array surface with different inclinations towards the equator.
Its expression is:
HT: HR8+HoI_R H}H +0. s, ~HB|Ho)
(1+cosf1)J+0.5pH (1-cosp) (3)
R: cos0/cos0
Incident angle: =COS [cos0cosfl+sin0sinflcos
(y-y JJ
Zenith angle: =c0s [sinasin~+c0sc0sc0s∞][3
Solar azimuth: ys = d * dy. +
fl1* *180。_3heat
n [ ={.}'
fl ≯ (≯-01 fl ∞ 01
I—l other'' I—l other''
=ar~cos(tan3/tan~)Lu is the inclination angle of the slope, f0 is the reflectivity of the surface, is the incident angle, mortar is the solar zenith angle; a is the solar altitude angle;), is the surface azimuth, y is the solar azimuth. Through the calculation of the Hay model above, the monthly average solar radiation changes of planes with different inclination angles in Figure 3 are obtained. It can be seen from Figure 3 that February to April is the period when each inclination plane receives the lowest solar radiation, and the solar irradiances received by several inclination planes during this period are not much different. The main reason for this is that February to April is the rainy season in Zhuhai, with more cloudy and rainy weather, and the proportion of scattered radiation in the total radiation is very high. It can be seen from Table 2 that the proportion of scattered radiation in these three months is all above 55%. Since the change of inclination angle has a greater effect on the direct radiation received by solar cell modules, but has little effect on scattered radiation, the difference in solar radiation received by planes with different inclination angles in the three months of 2-4 is not very large; in the summer months of 5-8, the plane with an inclination angle of 0 receives the largest solar radiation, and the planes with inclination angles of 15. and 22. receive the largest solar radiation. The solar irradiance is very close, but the solar irradiance received by the 40° plane is much lower than that of the other three inclination angles; Zhuhai area has a clear sky in autumn, although the solar altitude angle gradually decreases, the solar irradiance is still relatively high, and the solar irradiances received by the 15°, 22°, and 40° inclination angle planes are very close, and after September, the solar irradiance received by the 0° plane decreases rapidly, with a large drop; in winter, there are many sunny days, especially in the first winter, with little rainfall. In January and December of winter, the altitude angle is the lowest period of the year. From Figure 3, it can be seen that the solar irradiance varies greatly on the inclined planes with different angles. The maximum solar irradiance can be obtained at 40°, and the solar irradiance on the horizontal plane is seriously reduced. At the same time, the working time of the light control solar lighting system before and after the winter solstice is the longest in the whole year, so the inclination angle should take proper care of the solar irradiation in winter.

Through the analysis of the monthly average solar radiation changes in different inclination planes in Figure 3, combined with our load conditions, when designing the solar street light system, we must first consider
the normal working requirements of the system under low solar radiation conditions from February to April. The light-controlled solar street lights used in Zhuhai are seasonal loads with high power consumption in winter, but the load trend does not change much. The load condition is not
the main factor affecting the optimal inclination of the solar panel. Combining several factors, the solar cell assembly layout inclination angle of Zhuhai solar street lights is based on the local latitude of 22. This not only properly takes care of the increase in load power consumption in winter, but also has the best output in spring from February to March. After entering the summer, it can produce more electricity than 40. f hectares, quickly restore the battery and extend the battery life.

Determine
the capacity of solar panels and batteries The combination of solar cells and battery capacity is to determine the minimum solar cell components and battery capacity under the premise of ensuring the reliability of street lamp load, so as to achieve the best combination of reliability and economy through optimal design. Most of the reliability is measured by the load loss rate (LOLP) at home and abroad. It is defined as the ratio of system power outage to the actual power consumption time. The LOLP value is between 0 and 1. The smaller the value, the higher the reliability. There should be a certain limit to the reliability requirements. In the past, domestic photovoltaic systems were often designed to achieve no power outages regardless of their purpose, that is, IJ(】IJP=0. In fact, the AC power grid can only provide power to large cities at the order of IJ(】LP=10I3. Therefore, it is obviously unreasonable to require the relatively expensive photovoltaic system to achieve 100% reliability. Therefore, when designing solar street lights, while meeting the reasonable reliability of the load, it is necessary to have the best economy. According to the recommendation of Table 3, the reliability L0LP value of Zhuhai solar street lights is 0.1, which can guarantee power supply for 4-5 rainy days. According to this reliability design, the system configuration is shown in Table 4, using 8 1W high-power white light LEDs as lamp loads, 2 20Wp solar cell modules (arranged at a 220° tilt angle facing south), and 12V, 40AH maintenance-free lead-acid batteries. In

summary.

In the process of photovoltaic power generation application, many products are not designed according to the working characteristics and operating rules of photovoltaic power generation, which often makes the system unable to operate normally for a long time. According to the
local Climate conditions, combined with the characteristics of solar photovoltaic power generation, it is very meaningful to optimize the design of the solar street lights running there, which not only reduces the cost of the solar street light system, but also improves the reliability of the system. After nearly one year of testing and observation of the solar semiconductor lighting street light system, the results basically meet the design requirements. The solar street light controller can accurately control the entire system and can correctly protect the battery from overcharge and overdischarge. After 4 consecutive cloudy days, the street lights still work normally.