1. From solar cells to solar panels
Photovoltaic solar panels or modules are formed by connecting multiple solar cells in series or parallel to achieve a specific voltage or current output. The connected solar cells are generally covered with glass on the upper layer so that light is incident into the cells, connected with EVA (ethylene-vinyl acetate copolymer), and the bottom layer is attached with polyvinyl fluoride sheets and then fixed on the aluminum frame.
For solar cells connected in series, the short circuit current is the same and the voltage is the sum of the voltages of all cells. However, the efficiency of this weatherproof structure is lower than that of a single cell, as the EVA film also reduces the transmission coefficient due to the increased reflection of light, the effects of the connections between cells, and the aging of the encapsulant under long-term operation. The lifespan of solar panels produced by manufacturers is generally 25 years. The structure of the solar panel and the series-parallel structure of the solar cells are shown in Figure 1 and Figure 2. The output voltage of solar panels on the market is generally 6V, 12V, 24V, 48V. The voltage of the rechargeable battery should be higher than the voltage of the battery being charged, and the current flows from high potential to low potential.
Most low power standalone systems are typically 12V. For a 12V panel, usually 36 silicon solar cells are connected in series, the open circuit voltage of a single cell is about 0.5V (0.5~0.55V), and the total open circuit voltage is about 20V. The short-circuit current is determined by the size of the battery. A lead-acid battery marked “12V” will display 14V when fully charged.
The charging current depends on the power of the solar panel. In order for the battery to have a long service life, the maximum current should not exceed the current marked by the manufacturer.
Solar cells are connected together to form solar panels, and solar panels are connected together to form solar arrays.
The solar panels can be formed in series or parallel structure as required. For solar panels connected in parallel, a blocking diode should be provided to eliminate the influence of reverse current of the solar panels.
2. I-V characteristics of solar cell modules
Ideally, the I-V characteristics of a solar cell module or array are similar to the I-V characteristics of a solar cell. However, the scaling up multiplication factor is determined by the number of cells in series and parallel. If n solar cells are connected in series, m solar cells are connected in parallel, the open-circuit voltage of each cell is VOC, and the short-circuit current is ISC, then the short-circuit current of the formed solar cell module is mISC, the open-circuit voltage is nVOC, and the IV curve of the formed module is As shown in Figure 5.
3. The size of the solar panel
The efficiency of solar cells and solar panels is defined at an irradiance of 1000W/m² and an ambient temperature of 25°C. The electric power generated by a panel with an efficiency of 15% (monocrystalline silicon cell) and an area of 1m² is 1000W/m²×0.15m²=150W. This shows that the size and technical level of the solar panel are the primary indicators of the amount of electrical power generated by the battery. The short-circuit current and open-circuit voltage of the panel are generally determined by the specific application of the panel.
4. Positioning of solar panels
In order for a solar panel to generate as much electrical power as possible, we need to orient the solar panel perpendicular to the incoming rays of the sun. At the equator the solar radiation is perpendicular to the ground, so the solar panels need to be placed parallel to the ground. At other latitudes, the angle between the solar radiation angle and the horizontal plane is the same as the latitude value there, so in order for the solar panel to receive enough sunlight, the inclination of the solar panel should be the same as the latitude value (see Figure 6), It faces north in the southern hemisphere and south in the northern hemisphere.
However, the orientation of the solar panels should also be oriented throughout the year, taking into account seasonal changes caused by the tilt of the Earth’s axis of rotation. In order for solar panels to maximize the use of solar energy, the orientation of the panels should be adjusted regularly. Devices such as sun trackers are often used in places with low solar irradiance in order to obtain maximum power output throughout the day. For static and permanently installed solar panels, the orientation should match the latitude value.