Solar panels are simply a P-N junction of a semiconductor, constructed in such way that the maximum junction area is exposed to light. Tempered glass is used to cover the cells so that they are protected from the environment. The textured cell surface and the tempered glass together reduce the reflection of the light. A single P-N junction cell does not produce much voltage. Hence, a solar module consists of several solar cells connected in series to achieve the desired output voltage and power.
Figure 2: A typical solar cell structure
The solar panels can be broadly categorized as bulk silicon solar cells and thin film solar cells. The bulk solar cells are made from 180-240 μm thick self supporting silicon wafers. The wafers are processed to make a P-N junction and then connected together to form a solar cell module. Bulk solar cells are also referred as crystalline solar cells. They can be further categorized into mono-crystalline and poly-crystalline form. Mono-crystalline wafers are cut from cylindrical ingots of silicon crystal; hence, they are expensive, but make highly efficient solar modules. Poly-crystalline silicon cells are made from cast square ingots, making them less expensive to produce than mono-crystalline. However, polycrystalline silicon cells are less efficient.
Thin film solar cells are made by depositing a thin layer of photovoltaic material on a substrate. The layer thickness can vary. The processing cost for the thin film is much lower than the bulk silicon solar cell's hence they are lower in cost. On the flip side, they are less efficient. The various materials used in thin film silicon cell deposition have different light absorption properties. Such layers can be stacked up to form a multilayer thin film solar cell, which are often referred as tandem cells. With the multilayer structure, the overall efficiency can be increased. Thin film cells can also be formed in various kinds of surfaces. This has opened up a new application area of solar panels like in roof tiles and other uneven surfaces.
Inverters and chargers
The electricity generated from solar panels cannot be directly used to power appliances, as the current voltage profile is not constant over different times of the day. It also varies widely with applied loads. Electricity generated from the solar panels is DC. Most of the appliances we use require AC as input supply. Hence, in most of the applications, inverters are used with solar electricity to convert DC to AC with fixed frequency and voltage.
Sunlight is available in the daytime only. Hence, the solar system cannot generate energy at night. In some applications, a battery is used to store energy during daytime so that it can be used at night. These applications require a battery charger. The battery charger used for this purpose is different than ordinary domestic chargers available, as the charger input is DC generated from solar panels. In either the case of a battery charger or inverter, the best unit is one that uses the benefits of the Maximum Power Point (MPP) of the solar module.
A solar module can be assumed to be a DC voltage source whose internal impedance varies over temperature or amount of light striking its surface. The condition of light can be like daylight and evening light, partial shadow, etc.
Basic energy transfer theory dictates that for maximum power transfer, the source impedance and the load impedance have to be equal. In the case of solar panels, the source impedance varies with environmental conditions. Hence, to get the most out of a panel, the load connected to it must have varying impedence and should always be equal to the panel's impedence. The inverter and charger which fulfill these conditions as a load to the solar panel is called Maximum Power Point Tracking (MPPT) device. MPPT is very important to utilize solar panel efficiently on various environmental conditions.