Analysis of the design of solar high power LED street light

1 Introduction

In recent years, with the development of solar photovoltaic technology and LED lighting technology, solar LED street lamps have entered the field of urban lighting. LED is the abbreviation of Light Emitting Diode. LED as illumination source has DC low voltage drive compared with traditional illumination source; low power consumption; anti-vibration; long life; nanosecond response speed; large design space ;Environmental protection; continuous switching flash, easy to achieve 0-100% dimming function and other outstanding advantages, is considered a new generation of green lighting. Solar LED street lights use solar energy as an energy source. Each street lamp is independent, easy to install, no need to lay cables and wires, no need to exchange electric energy and electricity, DC power supply, light control timing control, safe and reliable, energy saving, economical, environmentally friendly, practical.

In this paper, for the purpose of practical application, the principle of solar cell power selection and the matching method of battery capacity are briefly introduced. The LEDs are analyzed and configured in terms of chip power selection, combination mode and driving mode, and designed to meet the road lighting use in Taiyuan. Solar LED street lights required.

2. Selection of solar panels and batteries

2.1 Solar panel selection

At present, silicon-based solar cells are classified into three types: single crystal silicon solar cells, polycrystalline silicon solar cells, and amorphous silicon thin film solar cells. At present, the photoelectric conversion efficiency of monocrystalline silicon solar cells is about 15%, and the highest is 24%. Currently, all kinds of solar cells have the highest photoelectric conversion efficiency and the most mature technology. The service life is generally up to 15 years and up to 25 years. The photoelectric conversion efficiency of polycrystalline silicon solar cells is much lower than that of monocrystalline silicon solar cells. The photoelectric conversion efficiency is about 12%, and the service life of polycrystalline silicon solar cells is also shorter than that of monocrystalline silicon solar cells. The photoelectric conversion efficiency of amorphous silicon thin film solar cells is low. At present, the international advanced level is about 10%, and it is not stable enough. As time goes by, its conversion efficiency is attenuated, which directly affects its practical application. Therefore, solar cells are currently used in monocrystalline silicon solar cells.

According to the principle of solar radiation, the amount of radiation obtained on the solar array is related to many factors: local latitude, altitude, atmospheric pollution or transparency, changes in the four seasons of the year, changes in one day, reaching the ground. The ratio of the direct and scattered components of the solar radiation, the reflection coefficient of the ground surface, the operation mode of the solar cell array or the inclination of the fixed square matrix, and the cleanliness of the surface of the solar cell array. The charging and discharging efficiency of the solar lighting system is 0.75, the loss correction coefficient of the solar battery module group is 0.95, and the dust shielding and other loss correction coefficient is 0.90. After the query data and unit conversion and simplified processing, the formula for calculating the total solar cell consumption P(Wp) can be obtained [1]

Where, QL is the daily power consumption (Wh) of the load, HL is the annual average daily radiation amount of the horizontal plane (KJ/m2·d), Kop is the optimal radiation coefficient of the slope radiation, and A is the safety factor, generally taking A=1.1-1.3 .

2.2 Battery selection

Since the input energy of the solar photovoltaic power generation system is extremely unstable, it is generally necessary to configure the battery system to work. The selection of the battery capacity generally follows the following principles: First, the energy of the daytime solar battery module is stored as much as possible while meeting the night illumination. At the same time, it is also necessary to be able to store the electrical energy needed to meet the nighttime lighting of a continuous rainy day (seven days). If the battery capacity is too small, it can not meet the needs of night lighting. The battery capacity is too large. On the one hand, the battery is always in a deficient state, which affects the battery life and causes waste. At present, the valve-controlled sealed lead-acid battery is often used in conjunction with solar photovoltaic cells. Because of its low maintenance and reliable performance, it is also called "maintenance-free battery", which has the superiority that other batteries can't match.

The capacity of the battery should be determined according to the power of the solar panel and the power of the LED street lamp and the lighting time. The battery should be matched with the solar cell and the LED street lamp. A simple way to determine the relationship between them. The solar cell power must be more than 4 times higher than the load power for the system to work properly. The voltage of the solar cell should exceed 20~30% of the working voltage of the battery to ensure normal storage of the battery. The battery capacity must be more than 6 times higher than the daily load of the load.

The calculation of the capacity Bc of the battery can be calculated by the following formula:

Bc=(PL*10*D)/(Kb*V)

Where: PL - average daily power consumption

D—the number of rainy days

Kb—safety factor = 1.1-1.4 (including temperature correction factor To = 0 for 1 degree, -10 for 1.1, -10 for 1.2, and depth of discharge for cc = 0.75)

V—operating voltage

According to the above formula, the capacity of the battery can be estimated, and the charging efficiency of the battery depends on the charging method. For charging the battery, the method is usually to charge quickly in the initial and middle stages to restore the capacity of the battery. At the end of charging, a small current is used to supplement the battery for the loss of self-discharge. After the battery reaches the state, PWM (pulse width modulation) is adopted. Charging method to increase the conversion efficiency of solar energy and ensure the charging effect of the battery. During the initial discharge of the battery, the voltage drops rapidly; the mid-term voltage drops slowly and lasts for a long time; at the end of the discharge, the discharge voltage drops sharply, and the discharge stops immediately, otherwise the battery will be irreversibly damaged. In the charging and discharging process of the battery, in addition to setting a proper charging and discharging threshold, it is necessary to perform appropriate temperature compensation on the charging and discharging threshold and perform necessary overcharge and overdischarge protection. According to the system requirements and the verification of various indicators, 12V/100Ah valve-regulated sealed lead-acid batteries are selected.

3. LED chip selection

According to the input power of current market products, the input power is several tens of mW of LED, which is called traditional low-power chip; the LED whose input power is less than 1W is called power LED; for input power equal to 1W or more than 1W LEDs are called W-class power (high power) LEDs. At present, 1W, 3W, 5W, 8W, and 10W are common in high power. 1W and 3W LEDs have been used in large quantities, which are moving toward high current (300mA~1.4A), high efficiency (60~120lm/W), and adjustable brightness [3]. High-power LED street lamps use a single LED chip with a power greater than 1 watt. A plurality of LED chips are integrated on a printed circuit board and arranged as a planar light source, and combined into a high-power LED single module, which is incorporated into the street light fixture to increase the amount of light by increasing the chip area.

When multiple LEDs are grouped together for road lighting design, in addition to sufficient luminous flux and reasonable optical design to ensure a reasonable light distribution, more important is the heat dissipation problem. Because the street lamp has the advantages of outdoor nighttime use, the heat dissipation surface is located on the side, and the body shape is restricted, which is beneficial to the natural convection heat dissipation of the air, the LED street lamp selects the natural convection heat dissipation mode, and at the same time, the whole lamp adopts the high thermal conductivity aluminum as the heat dissipation body. Solved the heat dissipation problem of LEDs.

4. LED combination and driving method

There are three common combinations of LEDs: parallel, series, and hybrid.

Parallel mode: The LED driver is required to output a large current with a low load voltage. The voltages distributed across all LEDs are the same. When the LEDs have a large difference in consistency, the current through each LED is inconsistent and the brightness of the LEDs is different.

Series mode: The LED driver is required to output a higher voltage. When the consistency of the LEDs is large, the voltages across the different LEDs are different. The current through each LED is the same, and the basic brightness of the LEDs is the same.

Hybrid mode: When more LEDs are needed, if all LEDs are connected in series, the LED driver will need to output a higher voltage. If all LEDs are connected in parallel, the LED driver is required to output a large current. Connecting all or three LEDs in parallel not only limits the amount of LEDs used, but also increases the load current of the parallel LEDs and increases the cost of the drivers. The number of LEDs in the hybrid mode is evenly distributed. The voltages distributed on one string of LEDs are the same. The current on each LED through the same string is also basically the same, and the brightness of the LEDs is the same. At the same time, the current through each string of LEDs is also similar.

Through the above analysis, high-power LEDs are used in the field of illumination. Since the current and voltage parameters of LEDs have typical PN junction volt-ampere characteristics, small changes in forward voltage drop cause large forward current changes. Unstable operating currents can affect LED life and light decay. Therefore, the brightness of the high-power LED is completely controlled by the current, and the drive circuit must provide a constant current.

In the driver circuit, we use the high-power LED driver chip XLT604 to drive the LED. The XLT604 is a PWM high-efficiency LED driver control chip designed by BICMOS technology. It effectively drives high-brightness LEDs from input voltages from 7VDC to 450VDC. The chip can drive an external MOSFET at a fixed frequency of up to 300KHz, the frequency of which can be programmed by an external resistor. The externally highlighted LED string can be controlled in a constant current mode to maintain constant brightness and enhance the reliability of the LED. The constant current value is determined by the external sampling resistor value, ranging from a few milliamps to 1 amp. The XLT604-driven LED can adjust the brightness linearly through the external control voltage, and can also adjust the brightness of the LED string through the external low-frequency PWM mode. Therefore, according to the actual needs of the solar LED street lamp design, the LED hybrid mode is selected, and the XLT604 driver chip is selected for the drive circuit design. Drive the LEDs.

5 Solar LED street light design in Taiyuan area (street light is 30w system)

The total annual sunshine hours in Taiyuan is 2360 hours to 2796 hours. The total annual solar radiation is 5442.8 MJ/m2 to 5562.18 MJ/m2, and the peak sunshine hours are 4.8320532, which is a national high-illumination category. Has good solar energy utilization conditions.

Taking the 30w LED street light system in Taiyuan as an example

(1) Determine local meteorological and geographical conditions

After the data inquiry Taiyuan area:

Latitude: north latitude 37°27′-38°25′

Longitude: East longitude 111°30'-113°09'

Altitude: 800m

The longest rainy day: 7 days

(2) Determination of daily consumption of load

The power of the street lamp is 30w, and the working time is 10 hours per day. Therefore, the daily power consumption is: QL=30w×10h=300Wh

(3) After data query, the inclination angle of Taiyuan area is 38°, the best inclination angle is 38°+10°=48°, and the solar panel faces south.

(4) Calculation of total energy of solar cells

Through data inquiry, we learned that Taiyuan area Kop=1.1

From the local latitude, longitude and altitude, and the data released by the local meteorological department: the daily average solar radiation in the Taiyuan area is 15MJ/m2d, so the average annual solar radiation in the horizontal plane: HL=15000KJ/m2dP=5618×A×QL/( Kop×HL)=5618×1.1×300/(1.1×15000)=112WP

Solar cell optional components:

Working voltage: 17.2V

Working current: 3.49A

Open circuit voltage: 21.6V

Short circuit current: 3.9A

Peak power: 60Wp,

Solar panels with a peak power of 60Wp are used. The total number of solar modules used is 2, and the solar cells are designed to charge 12V batteries.

(5) The calculation of the capacity Bc of the battery can be calculated by the following formula:

Bc=(30W*10*7)/(1.1*12)=159 Ah

Therefore, two sets of 12V100Ah valve-regulated sealed lead-acid batteries can be selected.

(6) At present, high-power LED street lamps are basically white light with a color temperature of about 5000K. As a road illumination source, the visual sense is too cold and cold, and the observing ability will decrease when it is farsighted. The warm white light of about 3000K is more suitable for road lighting. At the same time, the wind-proof design of the battery pack bracket and the wind-resistant design of the pole are also considered. The working voltage of the LED street lamp is generally 12V or 24V, which is a safe voltage and does not make electrical protection grounding. However, LED street light metal poles should be grounded and the grounding resistance should be no more than 10 ohms.

6 Conclusion

This paper briefly introduces and analyzes the realization and application of solar high-power LED street lamps in Taiyuan. There are still many problems in the practical application of solar LED street lamps, including: First, the problem of centralized management, if you do not use electrical lines, it is difficult to achieve centralized management and remote monitoring that is more important for street lamps. Second, solar energy Battery life, battery life, and other control component life are often lower than LED life. To achieve long life of the system, many problems need to be solved, as well as higher maintenance costs. Third, general installation The problem of the weight of a relatively high-solar solar cell brings about an increase in the wind-proof capability of the lamp post design, and the like, and these problems are fundamentally unsolvable problems of solar LED street lighting. Therefore, the application of solar high-power LED street lamps still has a long way to go.

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