How to Create a Solar Power Solution

1. Introduction

How to design a solar energy application?

Today, with energy becoming increasingly scarce, the utilization of natural energy has become the focus of people's attention. Among various natural energies, the endless solar energy is highly favored for its ubiquitous advantage.

Virtually all energy on Earth comes from the sun: the formation of coal and oil, the growth of plants, the movement of water and wind. However, due to the dispersion and instability of solar energy, directly harnessing it is not easy. Only in recent years, with the increasing shortage of energy and the improvement of solar energy utilization technology, direct use of solar energy has become a reality.

There are many ways to utilize sunlight, and using silicon photovoltaic cells to generate electricity is one of the most convenient. Currently, the main application area of ​​silicon photovoltaic cells is to solve the lighting problem in areas with limited power supply.

On a clear summer day, sunlight provides less than 1,000 watts per square meter. This suggests that while solar energy is vast, it is also highly dispersed. Commercial silicon solar cells combined with solar LED lighting systems offer significant advantages.

A complete solar lighting system mainly consists of the following five parts:

1. Solar panels

Solar panels generate electricity when there's sunlight. The power generated is calculated based on the lighting power and duration. For example, if a 2-watt lighting fixture is required to last 10 hours in the absence of sunlight, and considering the conversion losses in the converter circuit, the solar panel's power generation must be around 3 watts.

2. Battery

The battery stores the electricity generated by the solar cell during periods of sunlight for use when the sun is out. The battery capacity depends on the power of the solar panel, the power of the LED light, and the desired lighting duration. For example, if you use a 2-watt LED light and a 3-watt solar panel, and require 10 hours of continuous lighting in the absence of sunlight, a 12V/2.2AH battery would be suitable.

3. Solar charging control circuit

The function of this part of the circuit is to control the charging level when there is sufficient sunlight and the illumination time is long. Charging will stop when the battery is full to prevent the battery from being damaged by overcharging, thereby protecting the battery and extending its service life.

4. LED driver

This is the core control circuit of the system. It has three functions:

①. Complete the constant current drive control of the light-emitting diode so that the current flowing through the light-emitting tube does not change with the voltage of the battery.

②. It has light control function, which automatically turns off the light when it is daylight and turns on the light when it is dark.

③. Low voltage protection: When the battery voltage drops to 10.8V, the output is shut down to prevent over-discharge and damage to the battery.

2. Design and Implementation

For this design, I chose a relatively low-power solar panel or photovoltaic (PV) system: 20W is enough to provide our system with an output of 12V or 6V, with a current of less than or approximately 1.5A. Unfortunately, we can't simply connect the PV to the end device. While most systems today are equipped with internal voltage regulators, the PV's voltage will fluctuate depending on the amount of sunlight it receives. A 12V PV can fluctuate between 0.65V (without sunlight) and 15V; this could damage the target end device if it is rated for 12V.

Therefore, we need to include a control circuit that can provide us with an ideal or constant DC voltage to power the system and ensure that any fluctuations in the input voltage are removed. This can be achieved very easily by using a low-dropout regulator (LDO) with a sufficiently wide input voltage range. In this example, I used the TI LM317 with an adjustable output to provide the application with maximum flexibility, as shown in Figure 1. Using a lead-acid battery as the terminal system preserves the harvested power, as both voltage and current drop at night or when sunlight is low.

Figure 1: Control circuit schematic

3. Power consumption and voltage drop

In this design, the total power managed will be limited by the thermal resistance of the selected LDO. If high currents are required or the ambient temperature is too high, a heat sink can be applied as needed to further cool the system. If using the LM317, we need to keep the temperature below a maximum of 125°C. Using a TO-220 package means we also need to limit power dissipation to ~4.5W (if using 0.5A as the output). This precaution ensures sufficient bandwidth to operate the system within an ambient temperature range of 40°C. Fortunately, the LM317 has an internal thermal shutdown feature that prevents damage if the device overheats.

Considering the worst-case scenario, Equation 1 calculates the power of the 12V PV applied to the 6V battery through the control circuit:

In this case, the solar panel will never exceed the selected 4.5 W. However, we can achieve higher current output by applying a heat sink.

When the battery is fully charged, the battery voltage will be high but the current will be very low, reducing the dropout voltage and making the open circuit of the PV effective. I chose a Schottky rectifier to reduce the dropout voltage requirement and protect the battery from discharging when the voltage at the battery node is higher than the required dropout voltage.

The advantages and disadvantages of this circuit include:

    (+) Standard, off-the-shelf equipment; small board; inexpensive.

    (+) Can be designed with adjustable voltage output; battery discharge is zero when there is no sunlight.

    (-) High voltage dropout (depending on the LDO); depending on the output voltage, the dropout voltage may be too high. (This can be easily solved using a switching regulator or a low-dropout LDO.)

    (-) Not many bells and whistles; no indicator lights or LEDs. Functionality will depend entirely on the LDO.

We can also apply this circuit to battery chemistries that require a constant DC voltage. Applying additional LDOs and replacing the battery ensures reduced power consumption and improved overall system efficiency. We can also apply a boost converter to utilize the PV's power output in low-light conditions.