A solar road lighting design

1 Introduction

Solar road lighting is a lighting device that uses solar energy as energy. It has attracted widespread attention because it is not affected by the mains power supply, does not require trenching or overhead wires, does not consume conventional electricity, and can be installed on-site as long as there is sufficient sunlight. The main application of solar road lighting is solar street lights. It is widely used in rural tourism roads, urban-rural combined roads, remote mountainous areas, etc. It is especially suitable for installation in remote mountainous areas with inconvenient transportation and areas where it is not convenient to access the mains power, and has broad market prospects.

2 Solar Street Light Design

2.1 Site Investigation

Since solar street lights use solar radiation to generate electricity, they have special requirements for the specific location of the street lights. Before installing solar street lights, the installation site must be surveyed on site. The survey mainly includes:

a Check whether there are trees, buildings, etc. on both sides of the installation section (mainly on the south side or the east and west sides) to block the lighting. If there are trees or buildings blocking the lighting that may affect the lighting, measure their height and distance from the installation site, and calculate whether they affect the lighting of the solar cell modules; the general requirement for solar energy lighting is that solar energy lighting can at least ensure that there is no blocking that affects the lighting between 9:00 am and 3:00 pm.

b. Observe whether there are cables, wires or other facilities that affect the installation of solar lamps above the installation location (Note: it is strictly forbidden to install solar lamps under high-voltage lines);

c. Understand whether there are cables, optical cables, pipelines or other facilities that affect the construction under the foundation and battery compartment of the solar street light, and whether there are signs prohibiting construction. Try to avoid the above facilities during installation. If it is really impossible to avoid them, please contact the relevant departments and negotiate and agree before construction.

d. Avoid installing in low-lying areas or areas prone to water accumulation;

e. Take photos of the installation site;

f Measure the width, length, height of obstructions and distance of the road section, record the direction of the road and provide it together with photos and other information to the scheme designer for reference.

2.2 Installation layout

(1) Choose the lighting installation method based on the road width and lighting requirements:

a. Single-side layout

b. Bilateral symmetrical arrangement;

c. Double-sided staggered arrangement

Figure 1 Three ways to arrange solar street lights

(2) The cantilever length of the lamp should not exceed 1/4 of the installation height, and the elevation angle of the lamp should not exceed 15°;

(3) The relationship between the installation height (H), spacing (S) of the lamps and the road width (W) and layout is as follows:

Table 1 Relationship between the installation height (H), spacing (S) of lamps and the road width (W) and layout method

2.3 Light source selection

The principle of selecting solar street light sources is to choose light sources that are suitable for environmental requirements, have high light efficiency and long life. At the same time, in order to improve the efficiency of solar power generation, try to choose DC input light sources to avoid power loss caused by the introduction of inverters (because the efficiency of small inverters is relatively low, generally less than 80%).

Commonly used light sources include: tri-color energy-saving lamps, high-pressure sodium lamps, low-pressure sodium lamps, LEDs, ceramic metal halide lamps, electrodeless lamps, etc. Now we analyze and compare the most widely used solar lamp light sources:

The following table shows the characteristics of common DC input light sources:

Table 2 List of common DC input light source characteristics

When making specific selections, refer to the road conditions and customer requirements. It should be noted that various light sources have certain power limits and common specifications. When choosing power, try to choose the commonly used light source power. In recent years, some new light sources have also appeared, such as mixed light energy-saving street lamps, which integrate two light source systems, metal halide lamps with high color rendering and high color temperature and high-efficiency light source low-pressure sodium lamps, into the electrical compartment of the lamp, which significantly improves the overall light efficiency and color rendering and color temperature, and also improves the lighting quality to a certain extent.

2.4 System Configuration Calculation

The calculation of solar street light configuration is generally carried out according to the design method of independent photovoltaic system. Special design software can be used for design. In recent years, the RetScreen software jointly developed by the Canadian Department of Environment and Resources and NASA has been widely used. The following is a simple estimation method for solar street light configuration:

2.4.1 Calculation of peak sunshine hours

The formula is as follows:

Peak sunshine hours = A/(3.6×365)

A is the total radiation of the inclined surface in the previous year, in MJ/m2

For example, if the annual radiation on the square array surface of a certain place is 6207 MJ/m2, then the annual peak sunshine hours are:

6207÷3.6÷365=4.72 hours

2.4.2 Determination of system voltage

a The DC input voltage of the solar street light source is used as the system voltage, which is generally 12V or 24V. In special cases, AC load can also be selected, but an inverter must be added to work;

b. When selecting AC load, the DC voltage of the system should be increased as much as possible to reduce line loss if conditions permit;

c The selection of system DC input voltage must also take into account the selection of electrical components such as controllers and inverters.

2.4.3 Calculation of solar panel capacity

For solar street lights, the overall system configuration calculation formula is as follows:

P = light source power × light source working time × (17/12) ÷ peak sunshine hours ÷ (0.85 × 0.85)

Where P is the power of the battery module, in W;

The unit of light source working time is h;

The unit of peak sunshine hours is h;

0.85 represents the comprehensive coefficients of the battery's Coulomb efficiency, battery component attenuation, array combination loss, and dust shielding;

For example: if the light source power is 18W, working 8 hours a day, and the local annual sunshine hours are 4.0, the required solar panel power is 18×8×(17/12)÷4÷(0.85×0.85)=70.5W

When choosing the power of the solar panel, just select it according to the specifications of the solar panel.

2.4.4 Battery capacity calculation

First, determine the type of battery and the number of days the battery should be stored based on the local rainy and cloudy weather conditions. Generally, the number of days selected for storage in the north is 3 to 5 days, 2 days can be selected in the western region with little rainfall, and the number of days can be appropriately increased in the southern region with heavy rainfall. The capacity calculation formula is as follows:

Battery capacity = load power × daily working time × (storage days + 1) ÷ discharge depth ÷ system voltage

Where: the unit of battery capacity is Ah;

The unit of load power is W;

The unit of daily working time is h;

The unit of storage days is d;

Discharge depth, generally around 0.7

The system voltage unit is V

For example: the light source power is 18W, working 8 hours a day, and the battery storage days are 3 days, then the required battery capacity is: 18×8×4÷0.7÷12=68Ah

Then select the battery according to the system voltage and capacity requirements.

The above calculation does not take into account the influence of temperature. If the minimum operating temperature of the battery is lower than -20℃, the discharge depth of the battery should be corrected. For the specific correction coefficient, please consult the battery manufacturer.

2.4.5 Average illumination calculation

When designing road lighting, the calculation of contrast, brightness and uniformity is essential. Generally, it can be calculated using road lighting design software or lighting calculation tables, or it can be simply calculated based on the light distribution curve of the lamp. The following is a calculation formula for the average illumination of common roads. Readers can perform illumination calculations or calculate parameters such as street lamp spacing and light source power based on illumination for their own reference.

Calculation formula

Where: F---total luminous flux of the light source (lm);

U---Utilization coefficient (checked from the lamp utilization coefficient curve);

K---maintenance factor;

W---Road width (m);

S---Street lamp installation spacing (m);

N is a value related to the arrangement method. When the street lights are arranged on one side or staggered, N=1; when they are arranged in a rectangular shape, N=2.

2.4.6 Light pole design

Solar street lights are usually made of steel conical poles, which are beautiful, strong, durable, easy to make into various shapes, simple processing technology, and high mechanical strength. The cross-sectional shapes of common conical poles are circular, hexagonal, octagonal, etc. The taper is mostly 1:90, 1:100, and the wall thickness is generally selected at 3-5mm according to the stress of the pole.

Since solar street lights work outdoors, in order to prevent the light poles from rusting and corroding and reducing the structural strength, the light poles must be treated with anti-corrosion. The anti-corrosion method is mainly to take preventive measures against the causes of rust. Anti-corrosion should avoid or slow down the influence of factors such as moisture, high temperature, oxidation, chlorides, etc. The commonly used methods are as follows:

a Hot-dip galvanizing: The process and method of immersing the pre-treated parts into molten zinc liquid to form a zinc and zinc-iron alloy coating on their surface. The thickness of the zinc layer is 65-90um. The zinc layer of the galvanized parts should be uniform, smooth, without burrs, nodules and excess lumps. The zinc layer should be firmly combined with the steel rod, and the zinc layer should not peel off or bulge.

b. Plastic spraying: Plastic spraying is carried out after hot-dip galvanizing. The powder for plastic spraying should be outdoor-specific powder, and the coating should not peel or crack. Plastic spraying can further improve the anti-corrosion performance of the steel pole and greatly improve the aesthetics and decoration of the lamp pole. There are also many colors to choose from.

In addition, since the solar light poles are equipped with electrical components such as controllers (some batteries are also installed in the light poles), in addition to meeting the requirements of strength and shape, the design of solar light poles must also pay attention to the waterproof and anti-theft performance of the light poles to prevent rainwater from entering the light poles and causing electrical failures; the maintenance door should not be opened with conventional tools (such as hexagon socket bolts, panel screws, pliers, etc.) to prevent human damage or theft.

3 Special cases in solar street light design

3.1 Centrally powered solar street lights

In the case where solar street lights need to be installed in some obstructed areas, the lights can be powered centrally, that is, all solar panels required by solar lights are concentrated on a bracket that does not affect lighting, and then the system supplies power to each street light. See Figure 2.

Figure 2 Photo of a company's solar-powered street light

3.2 Switching from AC power to solar street lights

Solar street lights installed in government compound: Because there are buildings around to block the light, but the installation location of the street lights has been determined, and there must be no difference in the opening time of the street lights in the compound, they must be turned on and off at the same time. Independent solar street lights obviously cannot meet the requirements of the system.

At the same time, street lamps cannot fail to light up on rainy days. To address this situation, a solar-to-mains power complementary system + centralized power supply can be used. The solar panels required by the system are centrally installed on the roof, and the entire street lamp system is centrally controlled to achieve simultaneous lighting. When the solar power supply is insufficient, the mains power can be used to supplement it. See Figure 3.

Figure 3 Photo of a company's solar-to-grid switching system

4 Summary

In short, solar street lights involve knowledge in many fields, and certain professional knowledge is required for the design optimization of system configuration and the selection of accessories. Of course, there are many factors that affect the quality of solar street light projects. In addition to the design being as perfect as possible, project construction is also a very important link. The professional knowledge related to construction will not be repeated here.