Design and implementation of solar lighting cabinet brick system in Tibetan area

1 Introduction

As a sustainable clean energy, solar energy is increasingly valued by countries around the world. In Tibet, where the land is vast and the population is sparse, solar photovoltaic power generation has become the first choice. The most urgent problem in Tibet is not to build photovoltaic power stations extensively. The specific reasons are: first, the construction cost of photovoltaic power stations is high. At present, its cost is about 10 times that of coal-fired power generation; on the other hand, maintenance costs must be taken into account. If the equipment is maintained by the supplier for life after delivery, it is bound to pay high maintenance costs, especially when the equipment is installed in remote mountainous areas, where maintenance is inconvenient. The high cost makes it difficult to promote solar photovoltaic power generation equipment in Tibet. This system is a solar photovoltaic power generation lighting system designed for Tibetan buildings in Tibet. It can be embedded in building materials and is easy to use.

2 Design ideas

Through the investigation, it was found that the Tibetans most wanted to solve the problems of lighting and mobile phone charging. It is not difficult to see that the key to solve the immediate problems of Tibetans is to use a miniaturized photovoltaic power generation system. There is sufficient sunlight in Tibet, and most Tibetan buildings are oriented north-south, with balconies or windows facing south. If a "brick" in the shape of a closet is designed on the south wall, and a solar photovoltaic panel is installed on the outside of the "brick" against the wall, and a lighting load, mobile phone charging connector and control panel are installed on the inside of the "brick", then this closet lighting "brick" can solve the problems of lighting and mobile phone charging at the same time. The photovoltaic charging system, battery, system control panel, solar photovoltaic charging controller and other electronic equipment are all installed in the closet "brick". At the same time, in order to facilitate the replacement of batteries, the solar closet brick is designed to be opened like a closet through hinges (the installation height should be 1.5 m for user operation).

In order to make the system acceptable to the majority of Tibetans, the following points must be considered: (1) Since the end users are the vast Tibetan areas, the system control interface must consider three functions: Tibetan interface, Chinese character interface and Tibetan prompt, so that the human-computer interaction is more humane and convenient for Tibetans to use; (2) In remote mountainous areas, the indoor light of Tibetan buildings is very weak at night, so a remote control function must be added; (3) When the system is charging, two-level protection measures must be considered, namely: hardware circuit protection and software circuit protection, to prevent the system from failing to work properly in the event of misoperation; (4) In order to protect the lead-acid battery, a solar panel with slightly higher power must be used to improve the charging efficiency and avoid the lead-acid battery being in a loss state, thereby extending the battery life.

3 System Hardware Composition

The system hardware mainly consists of SPCE061A main control board, solar panel, solar charge and discharge controller, and battery, as shown in Figure 1.

In Figure 1, the LCD uses the SPLC501 module, which is directly controlled by SPCE061A and cooperates with keyboard input to set and display relevant information of the system; the status indicator directly displays the current status of the system (such as charging status); the keyboard design is relatively simple, mainly imitating the operation of the mouse to simplify the user's system setting and operation, and a total of 3 keys are set: up, down and confirm keys; the voice prompt uses the DA channel of SPCE061A and is directly output to the speaker through the audio amplifier circuit composed of SPY0030; the open circuit voltage of the solar panel is 20 V, and considering the protection of the battery, an 8 W model is selected; the battery is 20AH, DC12V; finally, multiple forced switches and load output interfaces are led out to the controller.

4 Hardware Circuit Principle

4.1 Working principle of the main charging circuit

Figure 2 is the schematic diagram of the main charging circuit. J1 is the solar panel access terminal and J10 is the 12 V battery lead terminal. The system main control microcontroller operates at a voltage of 5.0 V, and the control circuit operates without 12.0 V, which is provided by the battery (directly drawn from J10). J1 is connected to the solar panel, and the entire main circuit must form a charging circuit through MOSFETV1→L1→VDI(2545)→F1( RF 30)→J10→R14→ground. The core of the charging is to control V1. The circuit composed of R11, VQ3(8050), R7, R3 and circuit IRF9540 is the key to realize the control of MOSFET V1.

The base of the NPN transistor is connected to PWM through R11, and the PWM wave outputs the control signal through the SPCE061A main control board. When the PWM output is low, the main circuit of the NPN transistor VQ3 is in the cut-off state. It can be judged that the MOSFET is in the cut-off state, and the charging circuit of the solar panel and the battery is equivalent to the open circuit state; when the PWM output is high, VQ3 is turned on, and the positive pole of the solar panel → R3 → R7 → VQ3 → ground forms a loop. Due to the voltage divider effect of R3, the MOSFET is turned on, the main charging circuit is turned on, and the battery is in the charging state.

The core of the entire circuit control is the control of the PWM wave on and off of the MOSFET. The design adopts a three-stage charging method. The main control board SPCE061A has a PWM output function to ensure that the system implements a three-stage charging algorithm, thereby effectively protecting the battery.

4.2 Working Principle of Hardware Protection Circuit

Overload protection is achieved by collecting the state value of the charge and discharge circuit (current of the main circuit). When the charge and discharge circuit is abnormal, it is assumed that the discharge current is large, and the circuit reduces the potential of I_DET through the main circuit. In Figure 2, from the main circuit of B+→+F1→J10→I_DET→R14→grounding, it can be seen that I_IDE is actually close to the ground potential. In the analysis of the charging process, it can be approximately equivalent to the ground potential.

Figure 3 is an overload protection circuit. From VDD→R30→R35→ground, it can be calculated that the same-direction end of IC1B (LM358A) is 0.15 V; and the feedback closed loop formed by R24 and R25 makes the equation (Vo-V_)/R25=(V_-0)/R24 valid, and then the equation: Vo=(V_R25)/R24+V-. Since the op amp is in a deep negative feedback state, V_=V+=0.15 V. The potential change of I_DET is amplified by IC1B, that is, if the potential becomes positive, the potential will be higher after amplification. If it is a negative change (such as discharge), the potential change to the negative direction will be more obvious. The final output is the I_AD signal with a larger potential change.

The I_AD signal is compared with the reference voltage V_REF (1.4V) through the IC1A open-loop voltage comparator. When not overcharged, the output potential of the I_AD signal should be 2.1075 V, which is higher than the reference voltage 1.4 V of the same-direction end of IC1A. The circuit output is low, that is, the ground potential. At this time, this state will not affect the control loop composed of VQ1, VQ2, VQ5, and VQ6, that is, the branch formed by R21 and VD9 is equivalent to being disconnected.

If it is an over-discharge situation, the current of the main circuit increases, the potential of I_DET decreases → the potential of IC1B output I_AD is lower (less than the reference voltage V_REF of the comparator) → the potential of the comparator output changes from low to high (INT1 is high level). At this time, VQ2 and VQ6 (another control circuit) are connected to the ground through VD9 (or VD5) → VQ6 (or VQ2) under the action of INT1 high level, and the collectors of VQ2 and VQ6 are connected to the positive pole of the power supply through resistors. Therefore, VQ2 and VQ6 are unconditionally forced to turn on, V1_Driver, V2_Driver are forced to be pulled to a low level, and the lighting output load will be forced to turn off, thus avoiding over-discharge (i.e. load short circuit).

5 Software design

The system has Tibetan and Chinese interfaces, which users can switch according to actual needs. The system parameters are flexible to set, and the settings of system parameters such as the minimum charging voltage and the minimum discharging voltage must be considered; in order to make the operation more user-friendly, the system has other auxiliary functions such as automatic light off setting and alarm number setting. Considering that the system is mainly used in remote mountainous areas in Tibet, simple operation and friendly human-computer interaction interface are the primary considerations. The input of the system uses 3 buttons, namely: up key, down key and confirmation key. This keyboard design is similar to the buttons of a mouse, which is easy to operate and set.

5.1 System Programming

Figure 4 shows the main program flow. This program is an infinite loop, during which functions such as system settings, charging control, and discharging control are completed.

(1) The system setting subroutine provides an interface for human-computer interaction. First, the subroutine initializes the system and then enters the main interface of the system setting. Before entering the secondary interface (such as charging voltage setting, discharging voltage setting, etc.), you must select the secondary interface menu to enter by pressing the up or down key. At the same time, you must press the confirmation key to enter the secondary interface menu. This subroutine provides the function of automatically returning after 20 seconds of no operation.

(2) The charging control sub-function determines whether the battery needs to be charged by sampling the output voltage of the solar panel and the A/D conversion value of the battery voltage, and performs corresponding processing in the charging mode adjustment sub-function. At the same time, the three-stage charging algorithm is also implemented in this sub-function; the charging control sub-function also detects overcharging; by using the R14 resistor to collect the current loop current, the amplifier accurately amplifies it, and the A/D channel calculates the voltage value, so that the current loop current value can be reversely deduced.

(3) Discharge control subfunction The discharge control subfunction first collects the current value of the discharge circuit to determine whether the system is in an overcurrent state. If it is overcurrent, it is forced to shut down using software; otherwise, it further detects whether the battery is in a low voltage state. If it is lower than the preset threshold voltage, it activates an alarm and shuts down the output circuit; if it is greater than the preset threshold voltage, it detects whether the output circuit is already open and determines the timing time to achieve the function of timing off the lighting load.

5.2 Software protection part

Overcharge and overdischarge protection consists of two levels of protection. The first level is software protection, which is achieved by collecting the circuit status of the charge and discharge circuit and shutting down the external circuit by software; the other level is hardware forced shutdown protection as shown in Figure 3, that is, when the circuit network is overcharged or overdischarged, the hardware circuit automatically determines the abnormal state of the circuit and forcibly shuts down the external circuit. The software protection includes three parts: sudden circuit abnormality, battery overdischarge, and battery overcharge.

The software protection in these three cases is achieved by collecting system data, further analyzing the system status, and controlling the system through the microcontroller to protect the system; while the hardware forced shutdown will automatically respond and turn off the load output to protect the system when the system is in an overcurrent discharge state (such as a load short circuit), regardless of whether the software protection is started.

6 Test Results

Combined with the actual situation in Tibet, considering the needs of lighting and mobile phone charging in remote Tibetan areas, and combining the characteristics of Tibetan architecture, Tibetan solar lighting cabinet bricks were designed. After the design was completed, field tests were carried out in the suburbs of Lhasa, agricultural and pastoral areas in Shannan, and other areas, and the results were good. Table 1 shows the test results of equipment indicators. Therefore, this design has a certain novelty in application and is a relatively classic solution to the lighting and mobile phone charging needs of Tibetan-style buildings in Tibet.